EMI Shielding Theory & Gasket Design Guide - Sealing

SECTION CONTENTS PAGE

Theory of shielding and gasketing 192

Conductive elastomer gasket design 196

Gasket junction design 196

Corrosion 198

Selection of seal cross section 202

General tolerances 204

Gasket mounting choices 205

Fastener requirements 206

Designing a solid-O conductive elastomer gasket-in-a-groove 209

Mesh EMI gasketing selection guide 214

Glossary of terms 218

Part number cross reference index 220

EMI Shielding Theory& Gasket Design Guide

191

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EMI Shielding Theory

Theory of Shieldingand GasketingFundamental Concepts

A knowledge of the fundamentalconcepts of EMI shielding will aidthe designer in selecting the gasketinherently best suited to a specificdesign.

All electromagnetic waves consistof two essential components, amagnetic field, and an electric field.These two fields are perpendicularto each other, and the direction ofwave propagation is at right anglesto the plane containing these twocomponents. The relative magnitudebetween the magnetic (H) field andthe electric (E) field depends uponhow far away the wave is from itssource, and on the nature of thegenerating source itself. The ratioof E to H is called the waveimpedance, Zw.

If the source contains a largecurrent flow compared to its potential,such as may be generated by aloop, a transformer, or power lines,it is called a current, magnetic, orlow impedance source. The latterdefinition is derived from the factthat the ratio of E to H has a smallvalue. Conversely, if the sourceoperates at high voltage, and onlya small amount of current flows, thesource impedance is said to behigh, and the wave is commonlyreferred to as an electric field. Atvery large distances from thesource, the ratio of E to H is equalfor either wave regardless of itsorigination. When this occurs, thewave is said to be a plane wave,and the wave impedance is equalto 377 ohms, which is the intrinsicimpedance of free space. Beyondthis point all waves essentially losetheir curvature, and the surfacecontaining the two componentsbecomes a plane instead of asection of a sphere in the caseof a point source of radiation.

The importance of waveimpedance can be illustrated byconsidering what happens when anelectromagnetic wave encounters adiscontinuity. If the magnitude of the

wave impedance is greatly differentfrom the intrinsic impedance of thediscontinuity, most of the energy willbe reflected, and very little will betransmitted across the boundary.Most metals have an intrinsicimpedance of only milliohms. Forlow impedance fields (H dominant),less energy is reflected, and moreis absorbed, because the metalis more closely matched to theimpedance of the field. This is whyit is so difficult to shield againstmagnetic fields. On the other hand,the wave impedance of electricfields is high, so most of the energyis reflected for this case.

Consider the theoretical caseof an incident wave normal tothe surface of a metallic structureas illustrated in Figure 1. If theconductivity of the metal wall isinfinite, an electric field equal andopposite to that of the incidentelectric field components of thewave is generated in the shield.This satisfies the boundary conditionthat the total tangential electric fieldmust vanish at the boundary. Underthese ideal conditions, shieldingshould be perfect because the twofields exactly cancel one another.The fact that the magnetic fields arein phase means that the current flowin the shield is doubled.

Shielding effectiveness of metallicenclosures is not infinite, becausethe conductivity of all metals is finite.They can, however, approach verylarge values. Because metallicshields have less than infiniteconductivity, part of the field is

transmitted across the boundaryand supports a current in the metalas illustrated in Figure 2. Theamount of current flow at any depthin the shield, and the rate of decayis governed by the conductivity ofthe metal and its permeability. Theresidual current appearing on theopposite face is the one responsiblefor generating the field which existson the other side.

Our conclusion from Figures 2and 3 is that thickness plays animportant role in shielding. Whenskin depth is considered, however,it turns out that thickness is onlycritical at low frequencies. At highfrequencies, even metal foils areeffective shields.

The current density for thin shieldsis shown in Figure 3. The currentdensity in thick shields is the sameas for thin shields. A secondaryreflection occurs at the far side ofthe shield for all thicknesses. Theonly difference with thin shields isthat a large part of the re-reflectedwave may appear on the frontsurface. This wave can add to orsubtract from the primary reflectedwave depending upon the phaserelationship between them. For thisreason, a correction factor appearsin the shielding calculations toaccount for reflections from thefar surface of a thin shield.

A gap or slot in a shield will allowelectromagnetic fields to radiatethrough the shield, unless thecurrent continuity can be preservedacross the gaps. The function of anEMI gasket is to preserve continuityof current flow in the shield. If thegasket is made of a materialidentical to the walls of the shielded

Figure 1 Standard Wave Pattern of aPerfect Conductor Illuminated by aNormally Incident, + X Polarized PlaneWave

z

Er

Hr

Hi

Ei

E

H

PerfectlyConductivePlane z=0

y

x

Figure 2 Variation of Current Densitywith Thickness for Electrically ThickWalls

Et

Jt

Jo

Ei



193

( ( () ) )(6)

enclosure, the current distribution inthe gasket will also be the sameassuming it could perfectly fill theslot. (This is not possible due tomechanical considerations.)

The flow of current through ashield including a gasket interface isillustrated in Figure 4. Electromagneticleakage through the seam can occurin two ways. First, the energy canleak through the material directly.The gasket material shown inFigure 4 is assumed to have lowerconductivity than the material in theshield. The rate of current decay,therefore, is also less in the gasket.It is apparent that more current will

appear on the far side of the shield.This increased flow causes a largerleakage field to appear on the farside of the shield. Second, leakagecan occur at the interface betweenthe gasket and the shield. If an air

gap exists in the seam, the flow ofcurrent will be diverted to thosepoints or areas which are in contact.A change in the direction of the flowof current alters the current distributionin the shield as well as in the gasket.A high resistance joint does notbehave much differently than openseams. It simply alters the distributionof current somewhat. A currentdistribution for a typical seam isshown in Figure 4. Lines of constantcurrent flow spaced at larger intervalsindicate less flow of current.

It is important in gasket designto make the electrical properties ofthe gasket as similar to the shieldas possible, maintain a high degreeof electrical conductivity at theinterface, and avoid air, or highresistance gaps.

Shielding andGasket Equations 1

The previous section was devotedto a physical understanding of thefundamental concepts of shieldingand gasketing. This section is devotedto mathematical expressions usefulfor general design purposes. It ishelpful to understand the criteriafor selecting the parameters of ashielded enclosure.

In the previous section, it wasshown that electromagnetic wavesincident upon a discontinuity will bepartially reflected, and partly trans-mitted across the boundary and intothe material. The effectiveness of theshield is the sum total of these twoeffects, plus a correction factor toaccount for reflections from the backsurfaces of the shield. The overallexpression for shielding effectivenessis written as:

S.E. = R + A + B (1)

where

S.E. is the shielding effectiveness2 expressed in dB,

R is the reflection factor expressed in dB,

A is the absorption term expressed in dB, and

B is the correction factor due to reflections fromthe far boundary expressed in dB.

The reflection term is largelydependent upon the relativemismatch between the incomingwave and the surface impedance ofthe shield. Reflection terms for allwave types have been worked outby others.3 The equations for thethree principal fields are given bythe expressions:

where

RE, RH, and RP are the reflection terms for theelectric, magnetic, and plane wave fieldsexpressed in dB.

G is the relative conductivity referred tocopper,

f is the frequency in Hz,

µ is the relative permeability referred tofree space,

r1 is the distance from the source to theshield in inches.

The absorption term A is thesame for all three waves and isgiven by the expression:

A = 3.338 x 10–3 x t µfG

where

A is the absorption or penetration lossexpressed in dB, and t is the thicknessof the shield in mils.

The factor B can be mathematicallypositive or negative (in practice it isalways negative), and becomesinsignificant when A>6 dB. It isusually only important when metalsare thin, and at low frequencies (i.e.,below approximately 20 kHz).

B (in dB) = 20 log10

1 –(K – 1)2

10 –A/10 e–j.227A

(K + 1)2

where

A = absorption losses (dB)

K = ZS /Z H = 1.3(µ/fr 2G)1/ 2

ZS = shield impedance

Z H = impedance of the incidentmagnetic field

Figure 3 Variation of Current Densitywith Thickness for Electrically Thin Wall

Et

JtJo

Ei

Rereflectionfrom rear wall

Current onfront wall dueto reflectionfrom rear wall

Et

Ei

Gasket

Metal Shield

Lines ofconstant current

σg < σm

Figure 4 Lines of Constant CurrentFlow Through a Gasketed Seam

RE= 353.6 + 10 log10 G (2)

f 3µr12

RH= 20 log10 0.462 µ + 0.136r1 fG +0.354 (3)r1 Gf µ

RP= 108.2 + 10 log10 G x 106

(4)µf

√√( )

(5)

References1. Much of the analysis discussed in this section was performed by Robert B. Cowdell, as published in

“Nomograms Simplify Calculations of Magnetic Shielding Effectiveness” EDN, page 44, September 1, 1972.

2. Shielding Effectiveness is used in lieu of absorption because part of the shielding effect is caused byreflection from the shield, and as such is not an absorption type loss.

3. Vasaka, G.J., Theory, Design and Engineering Evaluation of Radio-Frequency Shielded Rooms,U.S. Naval Development Center, Johnsville, Pa., Report NADC-EL-54129, dated 13 August, 1956.

√




The preceding equation wassolved in two parts. A digital computerwas programmed to solve for B witha preselected value of A, while Kvaried between 10–4 and 103. Theresults are plotted in Figure 9.

The nomograph shown in Figure8 was designed to solve for K inequation (6). Note that when ZH

becomes much smaller than ZS

(K>1), large positive values of B mayresult. These produce very large andunrealistic computed values of S.E.,and imply a low frequency limitationon the B equation. In practical cases,absorption losses (A) must be cal-culated before B can be obtained.1

A plot of reflection and absorptionloss for copper and steel is shown inFigure 5. This illustration gives agood physical representation of thebehavior of the component parts ofan electromagnetic wave. It alsoillustrates why it is so much moredifficult to shield magnetic fieldsthan electric fields or plane waves.Note: In Figure 5, copper offers moreshielding effectiveness than steel inall cases except for absorption loss.This is due to the high permeabilityof iron. These shielding numbers aretheoretical, hence they are very high(and unrealistic) practical values.

If magnetic shielding is required,particularly at frequencies below14 kHz, it is customary to neglect allterms in equation (1) except theabsorption term A. Measurements ofnumerous shielded enclosures bearsthis out. Conversely, if only electricfield, or plane wave protection isrequired, reflection is the importantfactor to consider in the design.

The effects of junction geometry,contact resistance, applied forceand other factors which affectgasket performance are discussedin the design section which follows.

Polarization EffectsCurrents induced in a shield flow

essentially in the same direction asthe electric field component of theinducing wave. For example, if theelectric component of a wave isvertical, it is known as a verticallypolarized wave, and it will cause a

current to flow in theshield in a verticaldirection. A gasketplaced transverse tothe flow of current isless effective thanone placed parallelto the flow of current.

A circularlypolarized wavecontains equalvertical andhorizontal compo-nents, so gasketsmust be equallyeffective in bothdirections. Wherepolarization isunknown, gasketedjunctions must be designedand tested for the worse condition;that is, where the flow of current isparallel to the gasket seam.

NomographsThe nomographs presented in

Figures 6 through 9 will aid thedesigner in determining absorptionand magnetic field reflection lossesdirectly1. These nomographs arebased on the equations describedin the previous section.

Absorption Loss – Figure 6:Given a desired amount of absorptionloss at a known frequency, determinethe required thickness for a knownmetal:

a. Locate the frequency onthe f scale and the desiredabsorption loss on the A scale.Place a straight-edge acrossthese points and locate a pointon the unmarked X scale(Example: A = 10 dB,f =100 kHz).

b. Pivot the straight-edge aboutthe point on the unmarked Xscale to various metals notedon the G x µ scale. A lineconnecting the G x µ scaleand the point on the unmarkedscale will give the requiredthickness on the t scale.(Example: for copper t = 9.5 mils,cold rolled steel t = 2.1 mils).

Some care must be exercisedin using these charts forferrous materials because µvaries with magnetizing force.

Magnetic Field Reflection –Figure 7: To determine magneticfield reflection loss RH:

a. Locate a point on the G/µscale for one of the metalslisted. If the metal is not listed,compute G/µ and locate apoint on the numerical scale.

b. Locate the distance betweenthe energy source and theshield on the r scale.

c. Place a straight-edge betweenr and G/µ and locate a pointon the unmarked X scale(Example: r =10 inches forhot rolled steel).

d. Place a straight-edge betweenthe point on the X scale andthe desired frequency on thef scale.

e. Read the reflection loss fromthe RH scale. (For f = 10 kHz,RH = 13 dB).

f. By sweeping the f scale whileholding the point on the Xscale, RH versus frequencycan be obtained. (Forf = 1 kHz, RH = 3.5 dB).

(Note that thickness is not a factorin calculating reflection losses.)

EMI Shielding Theory continued

References1. Robert B. Cowdell, “Nomograms Simplify Calculations of Magnetic Shielding Effectiveness” EDN, page 44, September 1, 1972.

100Hz 1kHz 10kHz 100kHz 1MHz 10MHz 100MHz 1GHz 10GHz

0

50

100

150

200

250

300

5

4

3

6

72 8

1

IronCopper

FREQUENCY

LOS

S (

dB)

See text details and correction for thin sheetsShielding effectiveness = absorption + reflection loss

1 2Copper σ = 1 µ = 1

3 4

5 6

7 8

Absorption loss per milthickness

Reflection loss – Electric fields

Reflection loss – Plane waves

Reflection loss – Magnetic fields

Iron σ = .17 µ = 200

Figure 5 Shielding Effectiveness of Metal Barriers



195

Magnetic Field Secondary Reflec-tion Losses K Figures 8 and 9:To determine the magnetic fieldsecondary reflection loss factor Kto solve for B:

Given: r = 2 inches for 0.0162 in.thick copper and A = 1.3 dB.

Find B at 1 kHz.a. Draw a line between copper

on the G/µ scale and r = 2inches on the “source to shielddistance scale.” Locate a pointon the X scale.

b. Draw a line from the point onthe X scale to 1 kHz on thef scale.

c. At its intersection with the Kscale, read K = 2.2 x 10–2.

d. Proceed to Figure 9.e. On Figure 9, locate K = 2.2 x

10–2 on the horizontal scale. f. Move vertically to intersect the

A = 1.3 curve (interpolate),and then horizontally to findB = –8.5 dB.

Figure 7 Magnetic Field Reflection Loss Nomograph, RH1

Figure 8 Magnetic Field Secondary ReflectionLoss Factor Nomograph1

A = 6.0 dBA = 5.0 dB

A = 4.0 dB

1 kHz

A = 3.0 dB

A = 2.0 dB

A = 1.5 dB

A = 1.0 dB

A = .8 dB

A = .6 dB

A = .4 dB

A = .2 dB

10 -4 10 -3 10 -2 10 -1 1-25

-20

-15

-10

-5

0

B in

dB

|K|

|K| = 1.3[µ/fr2G]1/2

|K| = 2.2 x 10-2

Figure 6 Absorption Loss Nomograph1

Figure 9 Solving for Secondary Reflection Loss (B)1




Conductive Elastomer Gasket Design*

* Complete solid-O gasket design information starts on page 209.

Gasket Junction DesignThe ideal gasketing surface is

rigid and recessed to completelyhouse the gasket. Moreover, itshould be as conductive aspossible. Metal surfaces matingwith the gasket ideally should benon-corrosive. Where reaction withthe environment is inevitable,the reaction products should beelectrically conductive or easilypenetrable by mechanical abrasion.It is here that many gasket designsfail. The designer could not, or didnot treat the mating surface with thesame care as that given to theselection of the gasketing material.

By definition, a gasket is necessaryonly where an imperfect surfaceexists. If the junction were perfect,which includes either a solidlywelded closure, or one with matingsurfaces infinitely stiff, perfectly flat,or with infinite conductivity acrossthe junction, no gasket would benecessary. The more imperfect themating surfaces, the more critical isthe function of the gasket. Perfectsurfaces are expensive. The finalsolution is generally a compromisebetween economics and performance,but it should not be at the expenseof neglecting the design of theflange surfaces.

The important property thatmakes a conductive elastomergasket a good EMI/EMP seal is itsability to provide good electricalconductivity across the gasket-flange interface. Generally, the betterthe conformability and conductivity,the higher the shielding effectivenessof the gasket. In practice, it hasbeen found that surface conductivityof both the gasket and the matingsurfaces is the single most importantproperty that makes the gasketedseam effective; i.e., the resistancebetween the flange and gasketshould be as low as possible.

At this stage of the design everyeffort should be given to choosing aflange that will be as stiff as possibleconsistent with the construction usedand within the other designconstraints.

1. Flange MaterialsFlanges are generally made of the

same material as the basic enclosurefor reasons of economy, weldability,strength and resistance to corrosion.Wherever possible, the flangesshould be made of materials with thehighest possible conductivity. It isadvisable to add caution notes ondrawings not to paint the flangemating surfaces. If paint is to beapplied to outside surfaces, be surethat the contact surfaces are wellmasked before paint is applied, andthen cleaned after the masking tapeis removed. If the assembled unitsare subject to painting or repaintingin the field, add a cautionary note ina conspicuous location adjacent tothe seal that the seal areas are to bemasked before painting.

Ordinarily, the higher the conduc-tivity of a material, the more readily itoxidizes – except for noble metalssuch as gold and silver. Gold isimpervious to oxidation, and silver,although it oxidizes, forms oxidesthat are soft and relatively conductive.

Most oxides, however, are hard.Some of the oxide layers remain thinwhile others build up to substantialthickness in relatively short time.These oxides form insulating, orsemi-conducting films at theboundary between the gasket andthe flanges. This effect can beovercome to a degree by usingmaterials that do not oxidize readily,or by coating the surface with aconductive material that is lesssubject to oxidation. Nickel plating isgenerally recommended foraluminum parts, although tin has

become widely accepted. Zinc isprimarily used with steel. Consult theapplicable specifications beforeselecting a finish. A good guide tofinishing EMI shielded flanges foraerospace applications has beenpublished by SAE Committee AE-4(Electromagnetic Compatibility)under the designation ARP 1481. Adiscussion of corrosion controlfollows later in this guide.

2. Advantages of Grooved DesignsAll rubber materials are subject to

“Compression Set,” especially ifover compressed. Because flangesurfaces cannot be held uniformlyflat when the bolts are tightened(unless the flanges are infinitelystiff), gaskets tend to overcompressin the areas of the bolts. Propergroove design is required to avoidthis problem of over compression. Agroove also provides metal-to-metalcontact between the flange members,thereby reducing contact resistanceacross the junction.

A single groove will suffice for mostdesigns. Adding a second grooveparallel to the first adds approximately6 dB to the overall performance ofa single-groove design. Addingmore grooves beyond the seconddoes not increase the gasketingeffectiveness significantly.

3. Flange Design ConsiderationsMost designers fight a space

limitation, particularly in the vicinityof the gasketed seam. Complexfasteners are often required to makethe junctions more compact.

The ideal flange includes agroove for limiting the deflection of agasket. The screw or bolt fastenersare mounted outboard of the gasketto eliminate the need for providinggaskets under the fasteners. Amachined flange and its recom-mended groove dimensions areshown in Figure 10. The gasket may



197

be an “O” or “D”-shaped gasket,either solid or hollow.

Solid conductive O-rings arenormally limited to a deflection of25 percent. Therefore, the minimumcompressed height of the O-ring(also the groove depth) is relatedto the uncompressed height (ordiameter) by the expression H = 0.75W, where W is the uncompresseddiameter. The width of the groove, G,should be equal to 1.1 W. Allowsufficient void in the groove area toprovide for a maximum gasket fill of95 percent. Conductive elastomergaskets may be thought of as“incompressible fluids.” For thisreason, sufficient groove crosssectional area must be allowed forthe largest cross-sectional area ofthe gasket when tolerances are takeninto account. Never allow grooveand gasket tolerance accumulationsto cause an “over-filled” groove(see gasket tolerances in sectionwhich follows).

When a seal is used to isolatepressure environments in addition toEMI, the bottom of the gasket grooveshould have a surface finish of 32-64 µin. (RMS) to minimize leakagealong the grooves. Avoid machining

methods that produce longitudinal(circumferential) scratches or chattermarks. Conversely, a surface that istoo smooth will cause the gasket to“roll over” or twist in its groove.

The minimum distance from theedge of the groove to the nearestterminal edge, whether this terminalbe the edge of a casting, a changein cross section, or a fasteningdevice, should be equal to thegroove width, G.

Bolts should be located a minimumdistance, E (equal to one-half thediameter of the washer used underthe head of the bolt) from the edge ofthe flange.

Square or rectangular crosssection gaskets can be used in thesame groove provided sufficientvoid is allowed for displacement ofthe rubber. A good design practiceis not to allow the height of thegasket to exceed the base width. Abetter, or a more optimum ratio is aheight-to-width ratio of one-half. Tallgaskets tend to roll over when loaded.

The thickness of a flange isgoverned by the stiffness required toprevent excessive bowing betweenfastener points. Fewer, but largerbolts, require a thicker flange toprevent excessive deflections. Forcalculations of elastic deformation,refer to pages 206 and 207.

O-shaped and D-shaped gasketsmay also be used in sheet metalflanges. The gaskets can be retainedin a U-channel or Z-retainer, and aredeflection-limited by adjusting thechannel or retainer dimensions withrespect to gasket height. Suggestedretainer configurations are shown inFigures 11a and 11b.

A basic difference between flangesconstructed from sheet metal andthose which are machined fromcastings is that the bolts cannot belocated as close to the edge of thepart when the flange is made ofsheet metal. Note, in Figure 11a, F isrecommended to be 1.5 D, where Dis the diameter of the washer.

Flat gaskets are ordinarily usedwith sheet metal or machinedflanges as typically illustrated inFigure 12. Bolt holes in the flangesshould be located at least 1.5 timesthe bolt diameter from the edge ofthe flange to prevent tearing whenthe metal is punched. If the holesare drilled, the position of theholes should be not less than thethickness of the gasket material fromthe edge of the flange. If holes mustbe placed closer to the edge thanthe recommended values, ears or

��

��

GG G

H

EE = D2

D = Diameter of WasherW = Uncompressed Diameter of O-RingH = Groove Depth = 0.75-0.90 WG = 1.1 W

Figure 10 Machined Flange withGasket Groove

��

D

G

WH

Cover

OverlappingSpot Welds

F = 1.5 D

Gasket Cavity

"Z" Retainer

Sheet Metal Housing

F

Figure 11a Shaped Sheet MetalContainer

Figure 11b Z-Retainer Forms GasketCavity

F

t

F = t

��

Figure 12 Flat Gasket on SheetMetal Flange




Conductive Elastomer Gasket Design continued

slots should be considered asshown in Figure 13. Holes in flatgaskets should be treated in asimilar manner.

4. Dimensional TolerancesGrooves should be held to a

machined tolerance of ±0.002 in.Holes drilled into machined partsshould be held to within ±0.005 in.with respect to hole location. Locationof punched holes should be within±0.010 in. Sheet metal bendsshould be held to +0.030 and–0.000 in. Gasket tolerancesare given in the “Selection of SealCross Section,” later in this guide.

5. Waveguide FlangesThe three concerns for waveguide

flanges are to ensure maximumtransfer of electromagnetic energyacross the flange interface to preventRF leakage from the interface, and tomaintain pressurization of the wave-guide. Conductive elastomericgaskets provide both an electrical anda seal function. For flat cover flanges,a die-cut sheet gasket (CHO-SEAL1239 material), incorporating expandedmetal reinforcement to control gasketcreep into the waveguide opening,provides an excellent seal. Raisedlips around the gasket cut-outimprove the power handling andpressure sealing capability of thegasket. Choke flanges are best

sealed with molded circular D-Sectiongaskets, and contact flanges withmolded rectangular D-gaskets in asuitable groove (both in CHO-SEAL1212 material).

The peak power handling capa-bilities of waveguide flanges arelimited primarily by misalignment andsharp edges of flanges and/or gaskets.Average power handling is limited bythe heating effects caused by contactresistance of the flange-gasketinterface (“junction resistance”).

CorrosionAll metals are subject to corrosion.

That is, metal has an innate tendencyto react either chemically or electro-chemically with its environment toform a compound which is stable inthe environment.

Most electronic packages mustbe designed for one of four generalenvironments:

Class A. ControlledEnvironment Temperature andhumidity are controlled. Generalindoor, habitable exposure.

Class B. UncontrolledEnvironment Temperature andhumidity are not controlled. Exposedto humidities of 100 percent withoccasional wetting. Outdoor

exposure or exposure inuncontrolled warehouses.

Class C. Marine EnvironmentShipboard exposure or landexposure within two miles of saltwater where conditions of Class Aare not met.

Class D. Space EnvironmentExposure to high vacuum and highradiation.

■ FinishesTable I shows the minimum finish

necessary to arrest chemicalcorrosion and provide an electricallyconductive surface for the commonmetals of construction. Only theClass A, B, and C environments areshown in the table because thespace environment is not a cor-rosive one (i.e., metals are notgenerally affected by the spaceenvironment).

Some metals require finishingbecause they chemically corrode.These are listed in Table I, andshould be finished in accordancewith the table. To select a properfinish for metals not given in Table I,refer to the material groupings ofTable II. Adjacent groups in Table IIare compatible. Another excellentsource of information on corrosion-compatible finishes for EMI shielded

ENVIRONMENTMetal Class A Class B Class CCarbon and 0.0003 in. cadmium plate 0.0005 in. cadmium 0.003 in. nickelAlloy Steel 0.0005 in. zinc plate 0.001 in. zinc 0.001 in. tin

0.0003 in. tin 0.0005 in. tin

Corrosion- No finish required No finish required; No finish required;Resistant Steels 0.0005 in. nickel to 0.001 in. nickel to

prevent tarnish prevent tarnish

Aluminum 2000 Chromate conversion Chromate conversion coat 0.001 in. tin& 7000 series coat (MIL-C-5541, Class 3) (MIL-C-5541) plus conductive

epoxy or urethane

Aluminum 3000, No finish required, unless Chromate conversion coat Chromate conversion5000, 6000 series shielding requirements coat plus conductiveand clad are high (see above) epoxy or urethane

Copper and 0.0003 in. tin 0.0005 in. tin 0.003 in. nickelCopper Alloys 0.001 in. tin

Magnesium 0.0003 in. tin 0.0005 in. tin 0.001 in. tin

Zinc Base Castings No finish required 0.0003 in. tin 0.0005 in. tin

MINIMUM FINISH REQUIREMENTS FOR STRUCTURAL METALS

Table I

Figure 13 Ears or Slots in Sheet Metal Flanges or Flat Gaskets



199

flanges is ARP 1481, developed andpublished by SAE’s Committee AE-4(Electromagnetic Compatibility).

When a finish is required to maketwo mating metals compatible, finishthe metal which is found in the lowernumbered grouping of Table II. Metalsgiven in Table II will, because oftheir inherent corrodibility, alreadybe finished and the finish metalwill be subject to the same rule.For example, to couple metalsseparated by two or more groups(e.g., 4 to 2), find a finish whichappears in Group 3 and 4. TheGroup 3 metal should be platedonto the Group 2 metal to makemetals 2 and 4 compatible. Thereason for this is, if the finish metalbreaks down, or is porous, its areawill be large in comparison to theexposed area of the Group 2 metal,and the galvanic corrosion willbe less.

On aluminum, chromate conversioncoatings (such as Iridite) can beconsidered as conductive finishes.MIL-C-5541 Class 3 conversioncoatings are required to have lessthan 200 milliohms resistance whenmeasured at 200 psi contact pressureafter 168 hours of exposure to a5 percent salt spray. RecommendedMIL-C-5541 Class 3 coatings areAlodine 600, or Alodine 1200 and1200S dipped.

Organic FinishesOrganic finishes have been used

with a great deal of success toprevent corrosion. Many organicfinishes can be used, but none willbe effective unless properly applied.The following procedure has beenused with no traces of corrosionafter 240 hours of MIL-STD-810Bsalt fog testing.

Aluminum panels are cleanedwith a 20% solution of sodiumhydroxide and then chromateconversion coated per MIL-C-5541Class 3 (immersion process). Theconversion coated panels are thencoated with MIL-C-46168 Type 2urethane coating, except in theareas where contact is required. Formaximum protection of aluminumflanges, a CHO-SHIELD 2000 seriesconductive coating and CHO-SEAL1298 conductive elastomer gasketmaterial are recommended. Foradditional information, refer toDesign Guides for CorrosionControl on page 201.

The finish coat can be anysuitable urethane coating thatis compatible with the MIL-C-46168coating. It is important to note thattest specimens without the MIL-C-46168 coating will show some signsof corrosion, while coated test speci-mens will show no traces of corrosion.

CHO-SHIELD® 2000Series Coatings

When using CHO-SHIELD 2000series conductive urethanecoatings, not enough can be saidabout surface preparation to attainmaximum adhesion. The easilymixed three-component system allowsminimum waste with no weighingof components, thus eliminatingweighing errors. Because of the fillerloading of the 2000 series coatings,it is recommended that an air agitatorcup be incorporated into the spraysystem to keep the conductiveparticles in suspension during thespraying sequence. It is recom-mended that approximately 7 milsof wet coating be applied. Thisthickness can be achieved byspraying multiple passes, with aten minute wait between passes.

A 7-mil wet film coating willyield a dry film thickness of 4 mils,which is the minimum thicknessrequired to attain the necessarycorrosion and electrical valuesreferenced in Chomerics’ TechnicalBulletin 30. The coating thicknessplays an important role in the electricaland corrosion properties. Thinnercoatings of 1-3 mils do not exhibitthe corrosion resistance of 4-5 milcoatings.

The coating will be smooth to thetouch when cured. It is recommendedthat the coating be cured at roomtemperature for 2 hours followedby 250°F +/-10°F for one-half hourwhenever possible. Alternate curecycles are available, but withsignificant differences in corrosionand electrical properties. Twoalternate cure schedules are twohours at room temperature followedby 150°F for two hours, or 7 days atroom temperature.

Full electrical properties areachieved at room temperature after7 days. It should be noted that the250°F cure cycle reflects theultimate in corrosion resistance properties. The 150°F/2 hour androom temperature/7 day cures willprovide less corrosion resistance

Group Material Groupings*

1 Gold – Platinum – Gold/Platinum Alloys – Rhodium – Graphite – Palladium – Silver – SilverAlloys – Titanium – Silver Filled Elastomers – Silver Filled Coatings

Rhodium – Graphite – Palladium – Silver – Silver Alloys – Titanium – Nickel – Monel – Cobalt –2 Nickel and Cobalt Alloys – Nickel Copper Alloys – AISI 300 Series Steels – A286 Steel –

Silver Filled Elastomers – Silver Filled Coatings

Titanium – Nickel – Monel – Cobalt – Nickel and Cobalt Alloys – Nickel Copper Alloys – Copper –

3 Bronze – Brass – Copper Alloys – Silver Solder – Commercial Yellow Brass and Bronze – Leaded Brass and Bronze – Naval Brass – Steels AISI 300 Series, 451, 440, AM 355 and PH hardened – Chromium Plate – Tungsten – Molybdenum – Certain Silver Filled Elastomers

Leaded Brass and Bronze – Naval Brass – Steels AISI 431, 440, 410, 416, 420, AM 355 and

4 PH hardened – Chromium Plate – Tungsten – Molybdenum – Tin-Indium – Tin Lead Solder –Lead – Lead Tin Solder – Aluminum 2000 and 7000 Series – Alloy and Carbon Steel –Certain Silver Filled Elastomers – CHO-SHIELD 2000 Series Coatings

Chromium Plate – Tungsten – Molybdenum – Steel AISI 410, 416, 420, Alloy and Carbon –5 Tin – Indium – Tin Lead Solder – Lead – Lead Tin Solder – Aluminum – All Aluminum Alloys –

Cadmium – Zinc – Galvanized Steel – Beryllium – Zinc Base Castings

6 Magnesium – Tin

* Each of these groups overlaps, making it possible to safely use materials from adjacent groups.

METALS COMPATIBILITY

Table II





than the 250°F cure, but are wellwithin the specification noted inTechnical Bulletin 30.

1091 PrimerBecause of the sensitivity of

surface preparation on certainsubstrates and the availability ofequipment to perform the etching ofaluminum prior to the conversioncoating, Chomerics has introduced1091 primer, which is an adhesionpromoter for CHO-SHIELD 2000series coatings. When used inconjunction with an alkaline etch orchemical conversion coating perMIL-C-5541 Class 3, the 1091primer will provide maximumadhesion when correctly applied.(See Technical Bulletin 31.) Thisprimer is recommended only for the2000 series coatings on properlytreated aluminum and is notrecommended for composites.

For further assistance on theapplication of CHO-SHIELD 2000series coatings on other metallicand non-metallic substrates, contactChomerics’ Applications EngineeringDepartment.

■ Galvanic CorrosionThe most common corrosion

concern related to EMI gaskets isgalvanic corrosion. For galvaniccorrosion to occur, a unique set ofconditions must exist: two metalscapable of generating a voltagebetween them (any two unlikemetals will do), electrically joined bya current path, and immersed in afluid capable of dissolving the lessnoble of the two (an electrolyte). Inshort, the conditions of a batterymust exist. When these conditionsdo exist, current will flow and theextent of corrosion which will occurwill be directly related to the totalamount of current the galvanic cellproduces.

When an EMI gasket is placedbetween two metal flanges, the firstcondition is generally satisfiedbecause the flanges will probablynot be made of the same metal asthe gasket (most flanges are

aluminum or steel, and most EMIgaskets contain Monel, silver, tin,etc.). The second condition issatisfied by the inherent conductivityof the EMI gasket. The last conditioncould be realized when the electronicpackage is placed in service, wheresalt spray or atmospheric humidity, ifallowed to collect at the flange/gasketinterface, can provide the electrolytefor the solution of ions.

Many users of EMI gaskets selectMonel mesh or Monel wire-filledmaterials because they are oftendescribed as “corrosion-resistant.”Actually, they are only corrosion-resistant in the sense that they donot readily oxidize over time, evenin the presence of moisture.However, in terms of electrochemicalcompatibility with aluminum flanges,Monel is extremely active and itsuse requires extensive edge sealingand flange finish treatment toprevent galvanic corrosion. Mostgalvanic tables do not includeMonel, because it is not a commonlyused structural metal. The galvanictable given in MIL-STD-1250 doesinclude Monel, and shows it to havea 0.6 volt potential difference withrespect to aluminum – or almost thesame as silver.

A common misconception isthat all silver-bearing conductiveelastomers behave galvanically assilver. Experiments designed toshow the galvanic effects of silver-filled elastomer gaskets in aluminumflanges have shown less corrosionthan predicted. Silver-plated-aluminum filled elastomers exhibitthe least traces of galvanic corrosionand silver-plated-copper filledelastomers exhibit more. (SeeTable III).

Tables of galvanic potential donot accurately predict the corrosivityof metal-filled conductive elastomersbecause of the composite nature ofthese materials. Also, these tablesdo not measure directly two importantaspects of conductive elastomer“corrosion resistance”: 1) thecorrosion of the mating metal flange

and 2) the retention of conductivityby the elastomer after exposure toa corrosive environment.

Instead of using a table ofgalvanic potentials, the corrosioncaused by different conductiveelastomers was determined directlyby measuring the weight loss of analuminum coupon in contact with theconductive elastomer (after exposureto a salt fog environment). Theelectrical stability of the elastomerwas determined by measuring itsresistance before and after exposure.Figure 14a describes the test fixturethat was used. Figure 14b shows thealuminum weight loss results forseveral different silver-filledconductive elastomers. Thealuminum weight loss shows a twoorder of magnitude difference be-tween the least corrosive (1298silver-plated-aluminum) and mostcorrosive (1215 silver-plated-copper) filled elastomers. For silver-containing elastomers, the filler

*Standard Calamel Electrode. Aluminum Alloysapproximately –700 to –840 mV vs. SCE in 3% NaCl.Mansfield, F. and Kenkel, J.V., “Laboratory Studies ofGalvanic Corrosion of Aluminum Alloys,” Galvanic andPitting Corrosion – Field and Lab Studies, ASTM STP576, 1976, pp. 20-47.

Ecorr vs. SCE*Material (Millivolts)

Pure Silver –25

Silver-filled elastomer –50

Monel mesh –125

Silver-plated-copperfilled elastomer

–190

Silver-plated-aluminumfilled elastomer

–200

Copper –244

Nickel –250

Tin-plated Beryllium-copper –440

Tin-plated copper-cladsteel mesh

–440

Aluminum* (1100) –730

Silver-plated-aluminum filledelastomer (die-cut edge)

–740

CORROSION POTENTIALS OF VARIOUSMETALS AND EMI GASKET MATERIALS

(in 5% NaCI at 21°Cafter 15 minutes of immersion)

Table III



201

substrate that the silver is plated onis the single most important factor indetermining the corrosion caused bythe conductive elastomer.

Figure 14c shows the weight lossresults for nickel and carbon-filledelastomers compared to 1298. Thenickel-filled materials are actuallymore corrosive than the silver-plated-aluminum filled elastomers.The carbon-filled materials areextremely corrosive.

Figure 14d compares theelectrical stability of severalconductive elastomers before andafter salt fog exposure. In general,silver-containing elastomers aremore electrically stable in a salt fogenvironment than nickel-containingelastomers.

Design Guides forCorrosion Control

The foregoing discussion is notintended to suggest that corrosionshould be of no concern whenflanges are sealed with silver-bearingconductive elastomers. Rather,corrosion control by and largepresents the same problem whetherthe gasket is silver-filled, Monel wire-filled, or tin-plated. Furthermore, thedesigner must understand the factorswhich promote galvanic activity andstrive to keep them at safe levels. By“safe”, it should be recognized thatsome corrosion is likely to occur(and may be generally tolerable)at the outer (unsealed) edges of aflange after long-term exposureto salt-fog environments. This isespecially true if proper attentionhas not been given to flangematerials and finishes. The objectiveshould be control of corrosion withinacceptable limits.

The key to corrosion control inflanges sealed with EMI gaskets isproper design of the flange andgasket (and, of course, properselection of the gasket material). Aproperly designed interface requiresa moisture-sealing gasket whosethickness, shape and compression-deflection characteristics allow it tofill all gaps caused by uneven or unflat flanges, surface irregularities,

bowing between fasteners andtolerance buildups. If the gasket isdesigned and applied correctly, itwill exclude moisture and inhibitcorrosion on the flange faces andinside the package.

Bare aluminum and magnesium,as well as iridited aluminum andmagnesium, can be protected byproperly designed conductiveelastomer gaskets. It is important tonote that magnesium is the least noblestructural metal commonly used, anda silver-filled elastomer in contactwith magnesium would theoreticallyproduce an unacceptable couple.

Some specific design suggestionsfor proper corrosion control at EMIflanges are:

1. Select silver-plated-aluminumfilled elastomers for best overallsealing and corrosion protection.CHO-SEAL 1298 material offersmore corrosion resistance than anyother silver-filled elastomer (seeFigure 15, next page).

2. For aircraft applications,consider “seal-to-seal” designs, withsame gasket material applied toboth flange surfaces (see Figure 16).

3. To prevent corrosion on outsideedges exposed to severe corrosiveenvironments, use dual conductive/non-conductive gaskets (see page55) or allow the non-conductiveprotective paint (normally applied tooutside surfaces) to intrude slightlyunder the gasket (see Figure 17).

0

50

100

150

200

Volu

me

Resi

stiv

ity (m

ohm

-cm

)

CHO-SEAL1298

1.50" Dia.

1.25" Dia.

1.75" Dia.

1/4-20 CRES Fastener

Non-ConductiveSealing Gasket

Non-ConductiveSealing Gasket

Upper Delrin Block

LowerDelrin Block

Conductive Gasket

Aluminum AlloyCoupon

1.00"

1.00"

1298 1287 1285 1350S6304 1224 1215

Wei

ght L

oss

(mg)

50

0

300

2.2

19 22

167

37

237281

0

10

20

30

40

Wei

ght L

oss

(mg)

CHO-SEAL1298

Nickel Powder Filled

NickelFiber Filled

CarbonFilled

NickelPowder Filled

NickelFiber Filled

CarbonFilled

2.2

25.5

10.3

35.2

Figure 14a Test Fixture

Figure 14b Average WeightLoss of CHO-SEALElastomers

Figure 14c Weight Loss of 6061-T6Aluminum Coupons in Contactwith Conductive ElastomersDuring 168 hr. Salt Fog

Figure 14d Volume Resistivity (mohm-cm) of Conductive Elastomers Before and After 168 hr. Salt Fog Exposure

7.3 7.3

102.3

445.1

84.8

132.4 126.7

1052.2

CHO-SEAL Material

Before

After

(for composition, see Specifications Table, pgs. 32-34.)

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CHO-SEAL1287

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sealantNon-conductive

sealant

Figure 16 “Seal-to-seal” designincorporating CHO-SEAL® 1287conductive silver-aluminum fluorosiliconegaskets on both mating flange surfaces.Gaskets are bonded and edge sealed toprevent moisture from entering the gasket/flange area.





4. If moisture is expected to reachthe flange interfaces in Class C(marine) environments, flangesurfaces should be coated or platedto make them more compatible withthe EMI gasket material. Chomerics’CHO-SHIELD 2000 series coatingsare recommended for silver-filledelastomer or Monel wire gaskets,and tin plating for tin-plated gaskets.

5. Avoid designs which createsump areas.

6. Provide drainage and/or drainholes for all parts which wouldbecome natural sumps.

7. Provide dessicants for partswhich will include sumps but cannotbe provided with drain holes. Dessi-cant filters can also be provided forair intake.

8. Avoid sharp edges orprotrusions.

9. Select proper protectivefinishes.

The definition of a “safe“ level ofgalvanic activity must clearly beexpanded to include the requirementsof the design. If all traces of corrosionmust be prevented (e.g., airframeapplications) the structure must beproperly finished or must be made

of materials which will notcorrode in the useenvironment. In thesecases, the outside edges ofEMI-gasketed flanges mightalso require peripheralsealing as defined in MIL-STD-1250, MIL-STD-889 orMIL-STD-454. MIL-STD-1250 deserves specialmention. Although it wasdeveloped many yearsprior to the availabilityof CHO-SEAL 1298conductive elastomer

and CHO-SHIELD 2000series conductive coatings,it offers the following usefulcorrosion control methodsapplicable to electronic

enclosures:1. Bonds made by conductive

gaskets or adhesives, and involvingdissimilar contact, shall be sealedwith organic sealant.

2. When conductive gaskets areused, provision shall be made indesign for environmental andelectromagnetic seal. Wherepractical, a combination gasket withconductive metal encased in resinor elastomer shall be preferred.

3. Attention is drawn topossible moisture retention whensponge elastomers are used.

4. Because of the serious lossin conductivity caused by corrosion,special precautions such as environ-mental seals or external sealantbead shall be taken when wire meshgaskets of Monel or silver are usedin conjunction with aluminum ormagnesium.

5. Cut or machined edges oflaminated, molded, or filled plasticsshall be sealed with imperviousmaterials.

6. Materials that “wick” or arehygroscopic (like sponge core meshgaskets) shall not be used.

7. In addition to suitability for theintended application, nonmetallicmaterials shall be selected whichhave the following characteristics:

a. Low moisture absorption;b. Resistance to fungi andmicrobial attack;

c. Stability throughout thetemperature range;d. Freedom from outgassing;e. Compatibility with othermaterials in the assembly;f. Resistance to flame and arc;g. For outdoor applications,ability to withstand weathering.

Selection ofSeal Cross Section

Selection of the proper con-ductive elastomer gasket crosssection is largely one of application,compromise, and experience withsimilar designs used in the past.Some general rules, however, canbe established as initial designguidelines in selecting the classof gasket to be used.

1. Flat GasketsWhen using flat gaskets, care

must be taken not to locate holescloser to the edge than the thicknessof the gasket, or to cut a webnarrower than the gasket thickness.This is not to be confused with thecriteria for punching holes in sheetmetal parts discussed earlier.

Keep in mind also that flatgaskets should not be deflectedmore than about 10 percent,compared with 15 to 30 percent formolded and solid extruded gasketsand 50% for hollow gaskets. Standardthicknesses for flat gaskets are0.020, 0.032, 0.062, 0.093 and0.125 in. (see General Tolerances onpage 204.)

Where possible, the flangeshould be bent outward so that thescrews or bolts do not penetrate theshielded compartment (see Figure18a). If the flange must be bentinward to save space, the holes inthe gasket must fit snugly aroundthe threads of the bolts to preventleakage along the threads anddirectly into the compartment. Thiscalls for closely toleranced holesand accurate registration betweenthe holes in the flange and the holesin the gasket, and would requiremachined dies (rather than ruledies) to produce the gasket. Analternate solution can be achievedby adding an EMI seal under theheads of bolts penetrating the

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EMI Gasket

Paint Paint

Figure 17 Non-Conductive PaintIntrudes Slightly Under Gasket toProvide Edge Protection

Figure 15 Comparison of corrosion results obtained fromCHO-SEAL® 1298 conductive elastomer (left) and puresilver-filled elastomer (right) mated with aluminum after168 hours of salt fog exposure.



203

enclosure, or by using an insertsimilar to an acorn nut that has beeninserted in the flange and flared tomake the joint RF-tight. “Blind nuts”can also be welded or attached witha conductive epoxy adhesive(see Figure 18b).

2. Shaped or Molded GasketsGroove designs for O- or D-

shaped configurations are effectivebecause gasket deflection can becontrolled and larger deflectionscan be accommodated. O-ringcross sections are preferredbecause they can be deflectedmore easily under a given load.D-shapes or rectangular crosssections are excellent for retrofitapplications because they can bemade to accommodate almost anygroove cross section. Groovedesigns also provide metal-to-metalflange contact, and require fewerfasteners, thereby minimizing thenumber of paths where directleakage can occur.

Fasteners should be located suchthat pressure distribution is uniformat the corners (see Figure 19).

3. Hollow GasketsHollow gasket configurations are

very useful when large gaps areencountered, or where low closureforces are required. Hollow gasketsare often less expensive, and theycan be obtained with or withoutattachment tabs. Hollow gasketswith tabs are referred to in the textand in the tables as “P-gaskets”. Theminimum wall thickness of hollowgaskets is 0.020 in. dependingon material. Contact Chomerics’Applications Department for details.Hollow gaskets will compensate fora large lack of uniformity betweenmating surfaces because they canbe compressed to the point ofeliminating the hollow area.

4. Compression LimitsWhen compression cannot be

controlled, compression stopsshould be provided to preventgasket rupture caused by over-compression. Grooves providebuilt-in compression stops. Figure20 gives nominal recommendedcompression ranges for CHO-SEALand CHO-SIL materials, assumingstandard tolerances.

5. ElongationThe tensile strength of conductive

elastomer gaskets is not high. It isgood practice to limit elongation toless than 10 percent.

6. SplicingWhen grooves are provided for

gasket containment, two approachesare possible. A custom gasket can

be molded in one piece and placedinto the desired groove, or a stripgasket can be spliced to length andfitted to the groove. To properly seata spliced solid “O” cross sectiongasket, the inner radius of thegroove at the corners must be equalto or greater than the gasket crosssection width. Other cross sectionsneed greater inner radius and maynot be practical due to twistingwhen bent around corners. Splicescan be simply butted (with noadhesive) or bonded with aconductive or non-conductivecompound. If it has been decidedthat a spliced gasket will provide asatisfactory seal, the decisionbetween splicing and moldingshould be based on cost. When astandard extrusion is available,splicing is generally recommended.For custom extrusions, splicing isgenerally more cost effective inquantities over 500 feet.

7. Gasket Limitations Imposedby Manufacturing MethodsCurrent manufacturing tech-

nology limits conductive elastomergasket configurations to thefollowing dimensions and shapes :■ Die-cut Parts

Maximum Overall Size: 32 in. longx 32 in. wide x 0.125 in. thick(81 cm x 81 cm x 3.18 mm)Minimum Cross Section: Width-to-thickness ratio 1:1 (width is notto be less than the thickness ofthe gasket).

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Figure 18a External Bolting PreventsEMI Leakage

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Figure 18b Insert Pressed-In andFlared Makes EMI Tight Joint(Alternate: Weld or Cement withConductive Epoxy)

X/2

X/2

X

X

Figure 19 Fastener Location NearCorners

Figure 20 Gasket Deflection Ranges (mm dimensions in parentheses)

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H

W ��

T

A

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Deflection W Deflection Deflection DeflectionRange Dia. Range H Range T Range A

0.007-0.018 0.070 0.006-0.012 0.068 0.001-0.002 0.020 0.025-0.080 0.200(0.178-0.457) (1.778) (0.152-0.305) (1.727) (0.025-0.051) (0.508) (0.635-2.032) (5.08)

0.010-0.026 0.103 0.008-0.016 0.089 0.001-0.003 0.032 0.030-0.125 0.250(0.254-0.660) (2.616) (0.203-0.406) (2.261) (0.025-0.076) (0.813) (0.762-3.175) (6.35)

0.013-0.031 0.125 0.012-0.024 0.131 0.003-0.006 0.062 0.075-0.250 0.360(0.330-0.787) (3.175) (0.305-0.610) (3.327) 0.076-0.152) (1.575) (1.905-6.35) (9.144)

0.014-0.035 0.139 0.014-0.029 0.156 0.003-0.009 0.093(0.356-0.889) (3.531) (0.356-0.737) (3.962) (0.076-0.229) (2.362)

0.016-0.032 0.175(0.406-0.813) (4.445)



■ Molded PartsCurrently available in any solidcross section, but not less than0.040 in. in diameter. The outerdimensions of the gasket arelimited to 34 inches in anydirection. Larger parts can bemade by splicing. Molded partswill include a small amount offlash (0.008 in. width and 0.005 in.thickness, maximum).

■ Extruded PartsNo limitation on length. Minimumsolid cross-section is limited to0.028 in. extrusions. Wall thicknessof hollow extrusions varies withmaterial but 0.020 in. can beachieved with most materials.

8. General TolerancesThe following tables provide generaltolerances for conductive elastomergaskets. It is important to note that allflat die-cut, molded, and extrudedgaskets are subject to free-statevariation in the unrestrained condition.The use of inspection fixtures to verifyconformance of finished parts iscommon and recommended whereappropriate.

Also note that “Over-all Dimensions”for flat die-cut gaskets and moldedgaskets includes any feature-to-featuredimensions (e.g., edge-to-edge, edge-to-hole, hole-to-hole).

9. Gasket Cross SectionBased on Junction GapsGasket geometry is largely deter-

mined by the largest gap allowedto exist in the junction. Sheet metalenclosures will have larger variationsthan machined or die castings. Theultimate choice in allowable gaptolerance is a compromise betweencost, performance and the reliabilityrequired during the life of the device.When a value analysis is conducted,it should be made of the entirejunction, including the machiningrequired, special handling, treatmentof the surfaces and other factorsrequired to make the junctionfunctional. Often, the gasket is theleast expensive item, and contributesto cost-effectiveness by allowingloosely-toleranced flanges to bemade EMI-tight.

The maximum gap allowed to existin a junction is generally determinedby the minimum electrical perfor-mance expected of the seal. Asecondary consideration must be

given to the barrier as a pressureseal if gas pressures of significantmagnitude are expected. The gasketwill blow out if the pressure is toohigh for the gap.

The minimum gap allowed inthe junction is determined by theallowable squeeze that can betolerated by the gasket material.Deflection of conductive elastomergaskets was given in Figure 20. Flatgaskets may be deflected as muchas 6-10 percent (nominal), dependingon initial thickness and applied force.O-shaped and D-shaped gasketsare normally deflected 10 to 25percent; however, greater deflectionscan be achieved by manipulatingcross section configuration.

Determination of the exact gasketthickness is a complex probleminvolving electrical performance,flange characteristics, fastenerspacing and the properties of thegasket material. However, an initialestimate of the necessary thicknessof a noncontained gasket can bedetermined by multiplying thedifference in the expected minimumand maximum flange gaps by afactor of 4, as illustrated in Figure21. A more detailed discussion, anda more accurate determination ofgasket performance under loadedflange conditions, can be found inthe Fastener Requirements section,page 206.

204


Figure 21 Gasket Deflection Along aFlange

G max G min t o

tO = 4 (Gmax – Gmin)(tO = Original thickness of gasket)

EXTRUDED STRIPGASKETS inch (mm) TOLERANCE

Cut Length<1.000 (25.40) ±0.010 (0.25)1.0 to 30.000 (25.40 to 762) ±0.062 (1.58)> 30.000 (762) ±0.2% Nom. Dim.

Cross Section< 0.200 (5.08) ±0.005 (0.13)0.200-0.349 (5.08-8.86) ±0.008 (0.20)0.350-0.500 (8.89-12.70) ±0.010 (0.25)> 0.500 (12.70) ±3% Nom. Dim.

FLAT DIE-CUT GASKETSinch (mm) TOLERANCE

Overall Dimensions≤10 (254) ±0.010 (0.25)>10 to ≤15 (254 to 381) ±0.020 (0.51)>15 (>381) ±0.20% Nom. Dim.

Thickness0.020 (0.51) ±0.004 (0.10)0.032 (0.81) ±0.005 (0.13)0.045 (1.14) ±0.006 (0.15)0.062 (1.57) ±0.007 (0.18)0.093 (2.36) ±0.010 (0.25)0.125 (3.18) ±0.010 (0.25)>0.125 (>3.18) Contact a Chomerics

Applications or Sales Engineer

Hole Diameters>0.060 (1.52) dia. if sheet thickness is...

≤0.062 (1.57) ±0.005 (0.13)>0.062 (1.57) ±0.008 (0.20)

MOLDED GASKETSinch (mm) TOLERANCE

Overall Dimensions0.100 to 1.500 (2.54 to 38.10) ±0.010 (0.25)1.501 to 2.500 (38.13 to 63.50) ±0.015 (0.38)2.501 to 4.500 (63.53 to 114.30) ±0.020 (0.51)4.501 to 7.000 (114.33 to 177.80) ±0.025 (0.64)>7.000 (>177.80) ±0.35%

Nom. Dim.Cross Section0.040 to 0.069 (1.02 to 1.75) ±0.003 (0.08)0.070 to 0.100 (1.78 to 2.54) ±0.004 (0.11)0.101 to 0.200 (2.57 to 5.08) ±0.005 (0.13)0.201 to 0.350 (5.11 to 8.89) ±0.008 (0.20)

Flash Tolerance 0.005 (0.13) Max.Thickness0.008 (0.20) Max. Extension



205

Our various EMI gasket mounting techniques offer designers cost-effective choices in both materials and assembly.These options offer aesthetic choices and accommodate packaging requirements such as tight spaces, weight limits,housing materials and assembly costs. Most Chomerics gaskets attach using easily repairable systems. Our ApplicationsEngineering Department or your local Chomerics representative can provide full details on EMI gasket mounting. The mostcommon systems are shown here with the available shielding products.

Pressure-Sensitive AdhesiveQuick, efficient attachment strip■ Conductive Elastomers■ SOFT-SHIELD ■ POLASHEET■ SPRING-LINE ■ POLASTRIP

Adhesive CompoundsConductive or non-conductivespot bonding■ Conductive Elastomers■ MESH STRIP

FramesExtruded aluminum frames and stripsadd rigidity. Built-in compression stopsfor rivets and screws.■ Conductive Elastomers■ MESH STRIP

Friction Fit in a GroovePrevents over-deflection of gasketRetaining groove required ■ Conductive ■ MESH STRIP

Elastomers ■ POLASTRIP■ SOFT-SHIELD ■ SPRINGMESH

Metal ClipsTeeth bite through painted panelsRequire knife edge mounting flange■ Conductive Elastomers■ METALKLIP■ SPRING-LINE

Rivets/ScrewsRequire integral compression stopsRequire mounting holes on flange■ Conductive ■ SHIELDMESH

Elastomers■ SPRING-LINE

■ COMBO STRIP

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Spacer GasketsFully customized, integral conductiveelastomer and plastic spacer provideeconomical EMI shielding and groundingin small enclosures. Locator pinsensure accurate and easy installation,manually or robotically.

Robotically Dispensed Form-in-Place Conductive ElastomerChomerics’ Cho-Form® automatedtechnology applies high qualityconductive elastomer gaskets to metalor plastic housings. Manufacturingoptions include Chomerics facilities,authorized Application Partners, andturnkey systems.

Friction Fit on TangsAccommodates thin walls,intricate shapes■ Conductive Elastomers

Gasket Mounting Choices




Fastener Requirements1. Applied Force

Most applications do not requiremore than 100 psi (0.69 MPa) toachieve an effective EMI seal.Waveguide flanges often provideten times this amount. Hollow stripsrequire less than 10 pounds per in.Compression deflection data formany shapes, sizes and materials isincluded in the Performance Datasection of this handbook.

The force required at the point ofleast pressure, generally midwaybetween fasteners, can be obtainedby using a large number of smallfasteners spaced closely together.Alternatively, fasteners can bespaced further apart by using stifferflanges and larger diameter bolts.Sheet metal parts require morefasteners per unit length thancastings because they lack stiffness.

To calculate average appliedforce required, refer to load-deflectioncurves for specific gasket materialsand cross sections (see PerformanceData, page 80).

2. Fastener Sizes and SpacingFastener spacing should be

determined first. As a general rule,fasteners should not be spacedmore than 2.0 inches (50 mm) apartfor stiff flanges, and 0.75 inch (19 mm)apart for sheet metal if high levels ofshielding are required. An exceptionto the rule is the spacing betweenfasteners found in large cabinetdoors, which may vary from 3 inches(76.02 mm) between centers tosingle fasteners (i.e., door latches).The larger spacings are compen-sated for by stiffer flange sections,very large gaskets, and/or somereduction in electrical performancerequirements.

The force per bolt is determinedby dividing the total closure force bythe number of bolts. Select a fastenerwith a stress value safely below theallowable stress of the fastener.

3. Flange DeflectionThe flange deflection between

fasteners is a complex probleminvolving the geometry of the flangeand the asymmetrical application offorces in two directions. The one-dimensional solution, which treatsthe flange as a simple beam on anelastic foundation, is much easier toanalyze1 and gives a good firstorder approximation of the spacingsrequired between fasteners, becausemost EMI gaskets are sandwichedbetween compliant flanges.

Variation in applied forcesbetween fasteners can be limitedto ±10 percent by adjusting theconstants of the flange such that

βd = 2,

where

β =

wherek = foundation modulus of the seal

Ef = the modulus of elasticity of the flangelf = the moment of inertia of the flange and seald = spacing between fasteners

The modulus of elasticity (Ef) forsteel is typically 3 x 107. The modulusfor aluminum is typically 1 x 107, andfor brass it is about 1.4 x 107.

The foundation modulus (k) ofseals is typically 10,000 to 15,000 psi.

The moment of inertia (lf) ofrectangular sections, for example,may be obtained from the followingexpression2:

lf = bh3

12where

b is the width of the flange in contactwith the gasket (inches) and

h is the thickness of the flange (inches).

ExampleCalculate the bolt spacings for

flanges with a rectangular cross-section, such as shown in Figure 22,

whereh is the thickness of the flange.b is the width of the flange.d is the spacing between fasteners.

Assume the flange is to be madeof aluminum.

To maintain a pressure distributionbetween bolts of less than ±10percent, ßd must be equal to 2(see Figure 23 and discussion).

Assume an average foundationmodulus (k) of 12,500 psi for theseal. If the actual modulus is known(stress divided by strain), substitutethat value instead.

The bolt spacings for aluminumflanges for various thicknesses andwidths have been calculated for theprevious example and are shown inFigure 24.

The previous example does nottake into account the additionalstiffness contributed by the box towhich the flange is attached, so theresults are somewhat conservative.

√4 k4 EfIf

h

b

d

Figure 22 Bolt Spacings for Flanges

References1. Galagan, Steven, Designing Flanges and Seals for Low EMI, MICROWAVES, December 1966.

2. Roark, R.J., Formulas for Stress and Strain, McGraw-Hill, 4th Ed., p. 74.

–0.4

–0.2

0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8

ßd = 1

ßd = 4

ßd = ∞

ßd = 2

ßd = 3

ßx (+)(–)

Arr

ay F

acto

r

N –

1

N =

0∑

Anß

d –

ßx

Figure 23 Array Factor vs. Spacing




207

Actual deflection vs. distancebetween fasteners may be computedfrom the following expression:

N –1

y =βp

Anβd–βx2k

n = 0

where p is the force applied by the fastener,and β and k are the constants of the flange asdetermined previously. N represents the numberof bolts in the array.

The array factor denoted by thesummation sign adds the contri-bution of each fastener in the array.The array factor for various boltspacings (βd) is shown in Figure 23.Although any value can be selectedfor βd, a practical compromisebetween deflection, bolt spacing andelectrical performance is to select abolt spacing which yields a value βdequal to 2.

For βd = 2, the flange deflectionfluctuates by ±10 percent. Minimumdeflection occurs midway betweenfasteners and is 20 percent lessthan the deflection directly under thefasteners. The variation in deflectionis approximately sinusoidal.

Table IV lists a few recommenda-tions for bolts and bolt spacings invarious thin cross section aluminumflanges.

Bolt spacings for waveguideflanges are fixed by Military and EIAStandards. Waveguide flangesnormally have bolts located in themiddle of the long dimension of theflange because the flow of current ismost intense at this point.

4. Common FastenersMany different types of fasteners

are available, but bolts are the mostwidely used fastening devices. Theapproximate torque required to applyadequate force for mild steel bolts isshown in Table V.

These values are approximateand will be affected by the type oflubricants used (if any), plating, thetype of washers used, the class andfinish of the threads, and numerousother factors.

The final torque applied to thefasteners during assembly shouldbe 133 percent of the design valueto overcome the effect of stress-relaxation. When torqued to thisvalue, the gasket will relax over aperiod of time and then settle to thedesign value.

Torque may be converted totension in the bolts by applyingthe formula

Tension =Torque

0.2 x Bolt Dia.

Frequently the rule of thumb valueof 0.2 for the coefficient of friction canresult in torque and bolt estimateswhich may be seriously in error.Excessive bolt preload may lead toRF leakage. Therefore, if lubricants

are used for any reason, refer to theliterature3 for the proper coefficientvalues to be applied.

In soft materials, such asaluminum, magnesium andinsulating materials, inserts shouldbe provided if the threads are“working threads.” A thread isconsidered a “working thread” if itwill be assembled and disassembledten or more times.

Torque loss caused by elongationof stainless steel fasteners shouldalso be considered. High tensilestrength hardware is advised whenthis becomes a problem, but caremust be taken of the finish specifiedto minimize galvanic corrosion.

Thermal conductivity of hightensile strength hardware is lowerthan most materials used in electro-mechanical packaging today, so

MAX. TORQUE TOPREVENT STRIPPING

SCREW -TO- THICKNESS FOR UNC-2A THREADSIZE (in.) (in.) (in.-lbs.)

#2 3⁄8 0.062 4.5

#4 3⁄4 0.125 10.0

#6 1 0.125 21.0

#8 11⁄4 0.156 37.5

#10 13⁄8 0.156 42.5

Σ

CL CL

0.00.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

0.4

0.2

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

Calculated for aluminum flanges with arectangular cross section where:

Ef = 10,000,000k = 12,000 (average modulus for conductive silicones)h = thickness

h = 1/32"

h = 1/16"

h = 3/32"

h = 1/4"

Fas

tene

r S

paci

ng, d

(in

ches

)

Width of Flange, b (inches)

Figure 24 Fastener Spacing

Table IV

3. Roehrich, R.L., Torquing Stresses in Lubricated Bolts, Machine Design , June 8, 1967, pp. 171-175.

Max. Recommended

Size Threads Torque Tension* Basic Pitchper in. (in.-lbs.) (lbs.) Dia.(inches)

#4 40 43/4 0.095848 6 0.0985

#5 40 7 0.108844 81/2 0.1102

#6 32 83/4 0.117740 11 0.1218

#8 32 18 0.143736 20 0.1460

#10 24 23 0.162932 32 0.1697

1/4" 20 80 1840 0.217528 100 2200 0.2268

5/16" 18 140 2530 0.276424 150 2630 0.2854

3/8" 16 250 3740 0.334424 275 3950 0.3479

7/16" 14 400 5110 0.391120 425 5250 0.4050

1/2" 13 550 6110 0.450020 575 6150 0.4675

9/16" 12 725 7130 0.508418 800 7600 0.5264

5/8" 11 1250 11,040 0.566018 1400 11,880 0.5889

* Tension =Torque

0.2 x Diameter of Bolt †

† Basic Pitch Diameter

RECOMMENDED TORQUE VALUESFOR MILD STEEL BOLTS

Table V




that the enclosure expands fasterthan the hardware and usually helpsto tighten the seal. Should theequipment be subjected to lowtemperatures for long periodsof time, the bolts may requiretightening in the field, or can bepretightened in the factory undersimilar conditions.

Under shock and vibration, astack up of a flat washer, split helicallockwasher and nut are the leastreliable, partly because of elongationof the stainless steel fasteners,which causes the initial loosening.The process is continued undershock and vibration conditions.Elastic stop nuts and locking insertsinstalled in tapped holes haveproven to be more reliable undershock and vibration conditions,but they cost more and are moreexpensive to assemble.

5. Electrical Performance as a Function of Fastener SpacingThe electrical performance

(shielding effectiveness) providedby a gasket sandwiched betweentwo flanges and fastened by boltsspaced d distance apart is equivalentto the shielding effectivenessobtained by applying a pressurewhich is the arithmetic mean of themaximum and minimum pressureapplied to the gasket under thecondition that the spacing betweenfasteners is considerably less thana half wavelength. For bolt spacingsequal to or approaching one-halfwavelength at the highest operatingfrequency being considered, theshielding effectiveness at thepoint of least pressure is thegoverning value.

For example, assume that agasket is sandwiched betweentwo flanges which, when fastenedtogether with bolts, have a value ofβd equal to 2. Figure 23 shows thata value of βd = 2 represents adeflection change of ±10 percentabout the mean deflection point.Because applied pressure is directlyproportional to deflection, the appliedpressure also varies by ±10 percent.

Shielding effectiveness values fortypical silver-plated-copper filled,die-cut gaskets as a function ofapplied pressure are shown inFigure 25. The curves show thatthe shielding effectiveness variesappreciably with applied pressure,and changes as a function of thetype of field considered. Plane waveattenuation, for example, is moresensitive to applied pressure thanelectric or magnetic fields.

Thus, in determining the perfor-mance to be expected from ajunction, find the value for anapplied pressure which is 10percent less (for βd = 2) than thevalue exerted by the bolts directlyadjacent to the gasket. For example,examine a portion of a typical gasketperformance curve as shown inFigure 26.

The average shielding effectivenessof the gasketed seam is a functionof the mean applied pressure, pm.

For spacings which approach orare equal to one-half wavelength,the shielding effectiveness is afunction of the minimum pressure,p1. Therefore, the applied pressuremust be 20 percent higher toachieve the required performance.For this condition, the spacebetween the fasteners can beconsidered to be a slot antennaloaded with a lossy dielectric. Ifthe slot is completely filled, thenthe applied pressure must be 20percent higher as cited. Conversely,if the slot is not completely filled(as shown in Figure 27), the openarea will be free to radiate energythrough the slot.

The cut-off frequency forpolarizations parallel to the longdimension of the slot will bedetermined by the gap height, h.The cut-off frequency for thepolarization vector perpendicularto the slot will be determined by thewidth of the slot, w. The attenuationthrough the slot is determined bythe approximate formula

A(dB) = 54.5 d/λ c

where

d = the depth of the slot,andλc is equal to 2w or 2h, depending upon thepolarization being considered.

This example also illustrates whyleakage is apt to be more for polari-zations which are perpendicular tothe seam.

For large values of βd, thepercentage adjustments must beeven greater. For example, the

20000

50

100

150

400

Magnetic Fields

Plane Waves

Electric Fields

Shi

eldi

ng E

ffect

iven

ess

(dB

)

Applied Pressure (psi)

10 kHz to 10 MHz

200 kHz

100 kHz

14 kHz

18 MHz

1 GHz

10 GHz

400 MHz

Figure 25 Shielding Effectiveness vs.Applied Pressure

Figure 26 Typical Gasket PerformanceCurve

p1 = Minimum Pressurepm = Mean Pressure p2 = Maximum Pressure

Pressure p1 pm p2

Max.Mean

Min.

Shi

eldi

ng E

ffect

iven

ess

Figure 27 Unfilled Slot is Free toRadiate When Spacing is Equal to 1/2Wavelength

w h




209

percentage increase required tosatisfy βd = 3 is 64 percent. It isdesirable, therefore, that βd shouldbe kept as small as possible. Thiscan be achieved by using stiff flangesor spacing bolts closer together.

Designing a Solid-OConductive ElastomerGasket-in-a-Groove

The solid-O profile is the mostoften specified conductive elastomerEMI gasket for several key reasons.Compared to other solid crosssections, it offers the widest deflectionrange to compensate for poorlytoleranced mating surfaces and toprovide reliable EMI shielding andpressure sealing. It can be installedin a relatively small space, and isthe most easily installed and manu-factured. It also tends to be lessprone to damage, due to the absenceof angles, corners or other crosssection appendages.

The “gasket-in-a-groove” designoffers five significant advantagesover surface-mounted EMI gaskets:

1. Superior shielding , due tosubstantial metal-to-metal contactachieved when the mating surfacesare bolted together and “bottomout”. (Flat die-cut gaskets preventmetal-to-metal contact betweenmating flange members, whichreduces EMI shielding performance– especially in low frequencymagnetic fields.)

2. Positive control over sealingperformance . Controlling the size ofthe gasket and groove can ensurethat required shielding and sealingare achieved with less carefulassembly than is required for flatgaskets. In other words, the gasket-in-a-groove is more foolproof.

3. Built-in compression stopprovided by the groove eliminatesthe risk of gasket damage due toexcessive compression.

4. A gasket retention mechanismcan be provided by the groove,eliminating the need for adhesivesor mounting frames.

5. High current-handlingcharacteristics of the metal-to-metal flange design improves theEMP and lightning protection offeredby an enclosure.

This section presents the methodfor calculating groove and gasketdimensions which will permit theshielding system to function underworst-case tolerance conditions.Adherence to these generalguidelines will result in optimumshielding and sealing for typicalelectronics “boxes”. It should beunderstood that they may not besuitable for designing shielding forsheet metal cabinets, doors, roomsor other large, unconventionalenclosures.

Important Notes: The guidelinespresented here are intended toconsider only “solid O” gasket crosssections. The calculations for hollowO, solid and hollow D, and customgasket cross sections differ fromthese guidelines in several key areas.

Chomerics generally does notrecommend bonding solid O gasketsin grooves. If for some reason yourdesign requires gasket retention,contact Chomerics’ ApplicationsEngineering Department for specificrecommendations, since the use ofadhesives, dove-tailed grooves or“friction-fit” techniques requirespecial design considerations notcovered here.

Extreme design requirements orunusually demanding specificationsare also beyond the scope of theguidelines presented here. Exampleswould include critical specificationsfor pressure sealing, exceptionallyhigh levels of EMI shielding, excep-tional resistance to corrosion, harshchemicals, high temperatures,heavy vibration, or unusual mountingand assembly considerations.

Mechanical ConsiderationsCauses of Seal Failure

In order to produce a gasket-in-a-groove system which will not fail, the

designer must consider threemechanical causes of seal failure:

gasket over-deflection andassociated damage (see Figure28d)gasket under-deflection andloss of seal (see Figure 28f)groove over-fill, which candestroy the gasket (see Figure28e).

Designing to avoid these problemsis made more complicated by theeffects of:

worst-case tolerance conditions

deformation of the cover(cover bowing)poor fit of mating surfaces.

The key to success involvesselection of the appropriate gasketsize and material, and careful designof the corresponding groove.Deflection Limits

In nearly every solid-O appli-cation, Chomerics recommends aminimum deflection of 10% of gasketdiameter. This includes adjustmentsfor all worst-case tolerances of boththe gasket and groove, coverbowing, and lack of conformitybetween mating surfaces. Werecommend a maximum gasketdeflection of 25% of gasket diameter,considering all gasket and groovetolerances.

Although sometimes modifiedto accommodate application pecu-liarities, these limits have beenestablished to allow for stressrelaxation, aging, compression set,elastic limits, thermal expansion, etc.Maximum Groove Fill

Solid elastomer gaskets (asopposed to foam rubber gaskets)seal by changing shape to conformto mating surfaces. They cannotchange volume. The recommendedlimit is 100% groove fill under worst-case tolerances of both gasket andgroove. The largest gasket crosssectional area must fit into the smallestcross sectional groove area.




Figure 28cSection A-A of AssembledEnclosure Flange and Gasket(Sectioned midway through gasketand groove)

��

��

��

Bolt outboardof groove

Cover thickness (T)

Conductive elastomergasket in groove

Cover bowing (CB)

B

B

C

C

Bolt spacing (L)

Analyzing Worst-CaseTolerances

Figures 28a-c illustrate the issuesof concern, and identify the para-meters which should be consideredin developing an effective design.

Figures 28d and e illustrate twodifferent cases which can result ingasket damage in the area oftorqued bolts. In Figure 28d, therelationship between groove depthand gasket diameter is critical inavoiding over-deflection. In Figure28e, sufficient groove volume mustbe provided for a given gasketvolume to permit the gasket todeflect without over-filling the groove.

As shown in Figure 28f, coverdeformation and groove sizing mustbe controlled to make sure thegasket is sufficiently deflected toseal the system.

Since a single gasket and grooveare employed for the entire perimeter,the design must be optimized foreach of the worst-case examplesillustrated in Figures 28d-f.

Figure 28aExploded View ofElectronic Enclosure View for

section A–A

SolidO-profileEMI gasket

Groovefor gasket

Flange

Enclosure

Cover

��

��

View forsection A–A

Bolt Spacing (L)Lack ofconformity(LOC)

Flange width (FW) “O” Stripconductive elastomerin rectangular groove

Cover

Flange

��

��

Figure 28bCut-away View of Assembly




211

Figure 28d Section B-B from Figure 28c –Worst Case Maximum Deflection (Maximumgasket diameter, minimum groove depth)

Problem: Gasket too tall for minimum groovedepth (deflection beyond elastic limit).Results in gasket damage or fracture.

Figure 28e Section B-B from Figure 28c –Worst Case Maximum Groove Fill (Maximumgasket diameter in minimum groove depth andwidth)

Problem: Minimum groove dimension cannotaccommodate maximum gasket diameter,resulting in gasket damage.

Figure 28f Section C-C from Figure 28c – WorstCase Minimum Deflection (Minimum gasketdiameter in maximum depth groove, aggravatedby cover bowing and lack of conformity betweenmating surfaces)

Problem: Gasket will not be deflected therecommended 10% minimum. Combinedeffects of tolerances, cover bowing, and lackof conformity can result in complete loss ofcover-to-gasket contact over time, andconsequent seal failure.

Solution: Under-deflection avoided withlarger minimum gasket diameter and/orshallower maximum groove depth.

Maximumdiameter gasket Bolt

Optional washer

Minimumgroovedepth

Nominalgroove depth

Gasket deflectedmore than 25%

Gasket damage

Minimumgroovedepth

Nominalgroove depth

Gasket deflected25% or less

Maximumdiameter gasket

Minimumgroove

width

Minimumgroovedepth

Nominalgroove depth

Nominalgroove

width

Nominalgroove

width

Gasket damage

Nominalgroove depth

Maximumgroove depth

Gasketdeflected

less than 10%

Minimum diameter gasket

Nominalgroove depth

Maximumgroove depth

Gasketdeflected

10% or moreNominalgroove depth

Solution: Over-deflection avoided withsmaller maximum gasket diameter and/ordeeper minimum groove depth.

Solution: Groove over-fill avoided withsmaller maximum gasket diameter and/orgreater minimum groove depth and/or width.




Calculating the Dimensionsand Tolerances for theGroove and EMI Gasket

Figure 29 diagrams the calcula-tion and decision sequence requiredto determine the dimensions for aproperly designed solid-O gasket/groove system. Because therelationship between groove depthand gasket diameter is central toseal performance, groove depth isselected as the key variable todetermine first.

Start by making an educatedguess as to reasonable values forgroove and gasket sizes andtolerances, based on desirednominal gasket deflection of 18%.For example, if 0.025 in. of gasketdeflection is desired, start with anominal gasket diameter of 0.139 in.This is calculated by dividing thedesired total gasket deflection by0.18 to estimate the required gasketsize. (Total Gasket Deflection ÷ 0.18= Approx. Nominal Gasket Size.)This relationship is an alternate formof Formula 1. Final groovedimensions can only be determinedafter completing all of thecalculations called for in Figure 29,and arriving at values which remainwithin the recommended limits forgasket deflection and groove fill.

Figure 29 Procedure for Calculating Gasket and Groove Dimensions

SELECT A REASONABLE GASKET DIAMETER

CALCULATE NOMINAL GROOVE DEPTH (Formula 1)

ESTABLISH TOLERANCES

CALCULATE MAXIMUM GASKET DEFLECTION (Formula 2)Adjust parameters if this value is more than 25%

CALCULATE NOMINAL GROOVE WIDTH (Formula 4)

CALCULATE MINIMUM GASKET DEFLECTION (Formula 3)Adjust parameters if this value is less than 10%

VERIFY THAT FINAL GROOVE DIMENSIONS SA TISFY BOTH MIN.AND MAX. GASKET DEFLECTION AND GROOVE FILL LIMITS

UNDER WORST-CASE TOLERANCE CONDITIONS




213

Formulas (see definition of terms at right)

1. Nominal Groove DepthGrDnom = 0.82 GaDnom

2. Maximum Gasket Deflection(Worst Case, expressed as a % of gasket diameter)

GaDfmax = 100[ (GaDnom + GaT) – (GrDnom – GrDT)](GaDnom+ GaT)

3. Minimum Gasket Deflection(Worst Case, expressed as a % of gasket diameter)

a. GaDfmin = 100[ (GaDnom – GaT) – (GrDnom + GrDT) – CB – LOC ](GaDnom– GaT)

where

b. CB = GDF x L4

max

FWmin x T3min x E x 32

(Note: Formula must be adjusted when using metric units)

and

c. LOC = 0.001 in. for machined surfaces with surface roughness of 32-64 µin. RMS.

(For discussion, see Terms.)

4. Nominal Groove Width

a. GaAmax = 0.7854* (GaDnom + GaT)2

b. GrWmin =GaAmax

GrDmin

c. GrWnom= GrWmin + GrWT

*Note: 0.7854 =π4

TermsAll values may be calculated in inches or mmunless otherwise indicated.GaAmax – Maximum gasket cross section area(in2 or mm2)GaDnom – Nominal gasket diameterGaT – Gasket tolerance (difference between max.and nom. or min. and nom.)GrWmin – Minimum groove widthGrWT – Groove width toleranceGrWnom – Nominal groove widthGrDmin – Minimum groove depthGrDnom – Nominal groove depthGrDT – Groove depth tolerance (differencebetween max. and nom. or min. and nom.)GaDfmax – Maximum gasket deflection (%)GaDfmin – Minimum gasket deflection (%)Lmax – Maximum bolt spacingFWmin – Minimum flange widthTmin – Minimum cover thicknessGDF – Gasket deflection force (ppi or Newtonsper meter).Note: For the purpose of this guide, the GDF valueshould represent the worst-case minimum gasketdeflection arising from cover bowing. For example,the GDF is taken at 10% deflection for thecalculation in Formula 3b.E – Young’s modulus. (For aluminum, use 1 x 107

psi, or 7 x 105 kg/cm2.)CB – Cover bowing, generally calculated bymodeling the elastic deformation of the cover as auniformly loaded beam with two fixed supports.(The moment of inertia of the cover is modeled as arectangular beam, the “height” of which is taken tobe equal to the cover thickness, while “width” isconsidered equal to flange width. The moment ofinertia can be adjusted for cover configurationsother than flat. Refer to an engineering handbookfor the necessary revisions to Formula 3b.) Anassumption is made that one side of a cover/flangeinterface is infinitely stiff, typically the flange. If thisis not essentially true, elastic deformation of each iscomputed as though the other were infinitely stiff,and the two values combined.LOC – Lack of conformity, the measure of themismatch between two mating surfaces whenbolted together, expressed in inches. Experiencehas shown that machined surfaces with a surfaceroughness of 32-64 µin. RMS exhibit an LOC of0.001 in. It is left to the engineer’s judgment todetermine LOC for other surfaces. LOC can bedetermined empirically from measurements madeof actual hardware. In this guide, LOC applies onlyto the surfaces which form the EMI shieldinginterface.




EMI ShieldingPlus Environmental/Pressure Sealing

Some gasket applications requireonly restoration of the shieldingintegrity of an enclosure, and canbe satisfied with Chomerics’ simpleMESH STRIP gasketing. In thesecases, the use of MESH STRIP withElastomer Core provides additionalresiliency. Elastomer cored stripsoffer limited environmental sealingby positive blocking of dust and rain.

Additional environmental sealing orexclusion of ventilating air or vaporrequires a gasket such as COMBOSTRIP, which incorporates a smooth,easily compressed, elastomersealing strip in parallel with the EMIshielding strip. When an appreciablepressure differential must bemaintained between the interior andexterior of an enclosure, in additionto EMI protection, materials such asCHO-SEAL conductive elastomers orPOLA gaskets should be used.

Gasket Attachmentand Positioning

Substantial cost savings canresult from the careful choice ofgasket attachment or positioningmethod, which often determinesthe final choice of material.

A. Groove Capture This method isstrongly recommended if a groovecan be provided at relatively low cost,such as die-casting. (Caution: POLA-STRIP gaskets are essentiallyincompressible, although they seemto compress because the materialflows while maintaining the samevolume. Extra space must be allowedto permit the solid elastomer materialto flow (see Figure 30).B. Pressure-Sensitive AdhesiveThis is often the least expensiveattachment method for mesh EMIgasket materials. Installation costsare dramatically reduced with only aslight increase in cost over gasketing

without adhesive backing. In manycases customers purchase COMBOSTRIP or COMBO Gasket materialsfor applications which don’t requireenvironmental sealing, but utilize theadhesive-backed rubber portion asan inexpensive, temporary attach-ment method (“third hand”) duringinstallation.

C. Bond Non-EMI Portionof Gasket Non-conductiveadhesives may be employed tobond an EMI gasket in position byapplying adhesive to the portion thatis not the EMI gasket (and whichcan be insulated from the matingsurfaces by a non-conductivematerial).

Note: When specifying non-conductive adhesive attachment,applicable drawings and standardprocedures for production personnelshould emphasize that the adhesiveis to be applied only to the portionof the gasket which is not involvedwith the EMI shielding function. Theassumption that the gasket “willhold better if all of it is bonded ratherthan half of it” will result in seriousdegradation of EMI shieldingeffectiveness.1. Figure 31a illustrates this method

used for COMBO STRIP andCOMBO Gaskets, in which onlythe elastomer portion is bondedto one of the mating surfaces.

2. “Combo“ forms of POLASTRIPmay be bonded if, as in Figure31b, the adhesive is restricted to

the non-conductive portion. Spotapplications to the conductivearea are permissible.

3. MESH STRIP – The all metal andelastomer core versions of thesewith attachment fins can be heldin position with non-conductiveadhesive or epoxy if it is restrictedto the mounting fins (see Figure31c).

4. Frame Gasketing can be attachedwith a non-conductive adhesive orepoxy restricted to the aluminumextrusion (see Figure 31d).However, most Frame Gasketsare attached mechanically withfasteners.

5. Dry Back Adhesive for NeopreneSponge COMBO Gaskets –Factory-applied solvent-activatedadhesive is recommended forseveral reasons: a) controlledapplication guarantees restrictionof the adhesive to the non-conductive portion; b) controlledadhesive thickness assuresreliable bonding without reducingcompressibility; and c) theadhesive provides a permanentbond.

D. Bolt-Through Holes This is acommon, inexpensive means to holdgaskets in position (see Figure 32).For most Chomerics metal shieldingproducts, providing bolt holesinvolves only a small tooling charge,with no additional cost for the holes

Figure 30 Allowing for Solid ElastomerFlow in Groove Capture AttachmentMethod

Good DesignPoor Design

��

��

��

(a)*

(b)*

(c)*

(d)*

(c)*

*Areas where non-conductive adhesives can be used

Figure 31 a-d Application of Non-Conductive Adhesive

Mesh EMI Gasketing Selection Guide



215

in the unit price of the gasket. Bolt-holes can be provided in the finportion of MESH STRIP, or inrectangular cross section MESHSTRIP if these are wide enough,(minimum width 3/8 in. (9.52 mm).

E. Special Attachment MeansKnitted mesh fins provided on someversions of MESH STRIP, andextruded aluminum strips on FrameGasketing are designed for attach-ment (see Figure 33). Attachmentfins can be clamped under a metalstrip held down by riveting or spotwelding, or can be bonded with astructural adhesive or epoxy. Thealuminum extrusions in FrameGaskets can also be fastened byriveting or bolting.

Friction, Abrasion andImpact Considerations

EMI gaskets should be positionedso that little or no sliding or shearoccurs when compressed. In Figure34a, the EMI gasket is subject tosliding as the door is closed, whichmay lead to tearing, wearing out, ordetachment. Figure 34b illustratesthe preferred position, in which theEMI gasket is subjected almostentirely to compression forces.

Mesh Gasketing MaterialsA. Knitted Wire MeshKnitted wire mesh can be producedfrom any metal which can be drawninto wire form. However, the greatmajority of shielding requirementsare readily satisfied with a choice oftwo materials – monel or Ferrex –both of which are standard produc-tion materials for Chomerics’ meshgaskets.

Two design considerations shouldinfluence the choice of EMI gaskets:

■ required shielding performancein E-, H- and Plane Wave fields,

■ required corrosion resistanceof the gasket.

Additional considerations includethe mechanical strength, durability,

resiliency and compression set ofthe gasket material.

Monel

This good all-purpose nickel-copper alloy resists oxidation(thereby maintaining its conductivity),has good EMI qualities, and verygood mechanical strength andresiliency. In controlled or protectedatmospheres, it may be used incontact with aluminum; but wheresalt spray environments areencountered, galvanic corrosionis a problem.

Note: In salt spray environments,monel is corrosion-resistant, butwhen in contact with aluminumflanges, electrolytic currents willcause corrosion of the aluminumflange.

Ferrex ®

Chomerics’ Ferrex tin-platedcopper-clad steel wire offers thebest EMI/EMP performance of thestandard mesh materials, especiallyfor H-field shielding. Its mechanicalproperties are very close to monel,and it is more compatible withaluminum, but it has poorer intrinsiccorrosion resistance than monel.

With this understanding ofmaterial characteristics, gasketmetal is usually chosen using thefollowing guidelines:

For low frequency magneticfield shielding: recommendedgaskets are Ferrex versions ofknitted mesh gasketing (providedcorrosion resistance requirementsare not severe).

For high frequency electric fieldshielding: recommended gasketsare monel or Ferrex.

For best corrosion resistance(except in contact with aluminum insalt spray environments wherecorrosion will occur): monel isrecommended, preferably embedded

Figure 32 Bolt-Through GasketMounting

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Figure 33 Rivet or Spot Welding

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Cabinet

Cabinet

Door

Door

Frame Gasketing

Box

Box

Cover

Cover

Rivet or spotweld aluminumextrusion tocover or cabinet.

Rivet or spotweld strip overmounting fin.

EMI Mesh Strip Gasketing

Figure 34 a-b Sliding Motion vs.Straight Compression

��

Door

Cabinet

(a) Poor design, door slides on EMI gasket

��

Door

Cabinet

(b) Good design, door compresses on EMI gasket




in elastomer (e.g., POLA). Aluminummesh is sometimes selected whenequipment specifications permitno other metal to be used againstaluminum mating surfaces, forgalvanic corrosion compatibility.However, it must be understoodthat aluminum mesh oxidizes readily,and shielding effectiveness there-fore degrades.

Chomerics Knitted WireMesh Products

MESH STRIP is available asresilient, single and dual all-metalstrips or compressed shapes,with optional mounting fins. Bothrectangular and round profiles areoffered in a large range of standarddimensions for use as EMI gasketswhere no environmental sealing isrequired (see Figure 35). Note: Seealso SPRINGMESH highly resilientwire mesh gaskets made from tin-plated steel wire, page 111.

Wire Mesh Frame Gaskets offercombinations of one or two round-profile mesh strips, or one mesh/one pressure-seal strip (round orrectangular) with a metal mountingframe (see Figure 36). METALKLIPclip-on strips consist of wire meshover elastomer core gasketsattached to metal mounting clips.

COMBO and COMBO STRIPGaskets combine a low-profile, solidor sponge elastomer strip in parallelwith one or two rectangular meshstrips (see Figure 37). With solidelastomers, the mesh strip has ahigher profile than the elastomer, toallow for compression of the mesh.

MESH STRIP with Elastomer Coreis available in round or rectangularprofiles, with solid or hollowelastomer, with an optional meshmounting fin (see Figure 38).

Compressed Mesh Gasketsare jointless units made by die-compressing knitted metal mesh,usually in round or rectangularforms, with a constant rectangularcross section. Standard waveguidetypes are available, and Chomericsmaintains a large selection ofexisting tooling for other annulartypes.

B. Oriented Wire in SiliconePOLASTRIP/POLASHEET arecomposite mesh and elastomermaterials in which wire isembedded in part or all of thesilicone elastomer. The mesh isin the form of individual wiresoriented perpendicular to the jointmating surfaces, for maximum EMIshielding (see Figure 39).

C. Woven Metal MeshMETALASTIC Gasketing is formed ofwoven aluminum mesh, filled withsilicone or neoprene for pressuresealing. It is produced in 8 in. (20.3cm) wide sheets in random lengths,in thicknesses of 0.016 in. (0.40 mm)and 0.020 in. (0.51 mm). The 0.016in. (0.40 mm) material is the thinnestavailable for EMI plus pressure sealgasketing. It can be obtained insheets, standard connector gaskets,

or custom die-cut configurations. Itshould only be used where jointunevenness is less than 0.002 in.(0.05 mm).

D. Expanded Metal MeshPORCUPINE METALASTICgasketing is a material composed ofexpanded Monel metal mesh, and isavailable with optional silicone filling.It is produced in sheets of con-tinuous length, 12 in. (30.4 cm) by0.020 in. or 0.030 in. (0.51 mm or0.76 mm) thick. PORCUPINEMETALASTIC gasketing is easily cutinto intricate shapes with inexpensiverule dies, has high uniformity inthickness, ±0.004 in. (0.010 mm),and withstands high compressionforces without damage. Availableas sheets and standard connectorgaskets, it can also be supplied incustom die-cut configurations. Itshould only be used where jointunevenness is less than 0.003 in.(0.08 mm) (see Figure 40).

Figure 39 POLA Materials Profiles��

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Figure 35 MESH STRIP GasketingProfiles

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Figure 36 Frame Gasketing Profiles

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Figure 37 Normal and High PressureCOMBO STRIP Gaskets

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Figure 40 PORCUPINEMETALASTIC Die-Cut Gaskets(fully dimensioned drawingsrequired)

Figure 38 MESH STRIP with Elastomer Core Profiles

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A

B

B AA

AD

C

C

A

E

sponge sponge

solid solid

Mesh EMI Gasketing Selection Guide continued



217

Wire Mesh EMI Gasket Selection Guide

Sche

mat

ic C

ross

Sec

tion

Form

ed K

nitte

dFo

rmed

Kni

tted

Wire

Wire

Stri

psOr

ient

ed W

ire in

Mat

rix o

fCo

nstru

ctio

nKn

itted

Wire

Strip

s in

Par

alle

l with

Clam

ped

inEx

pand

edSi

licon

e El

asto

mer

Form

ed o

r Com

pres

sed

Mes

h Ov

erEl

asto

mer

Stri

ps;

Alum

inum

Met

alW

oven

Wire

(ava

ilabl

e w

ith p

ress

ure

Knitt

ed W

ire M

esh

Elas

tom

er S

trips

or D

ie-C

ut G

aske

tsEx

trusi

ons

in E

last

omer

in E

last

omer

sens

itive

adh

esiv

e)

Strip

s,St

rips,

Gas

kets

Strip

s,Di

e-Cu

tSt

rips,

Fab

.Ga

sket

s M

ade

Join

tless

Rin

gsM

ade

byGa

sket

s M

ade

Elas

tom

er w

ithLe

ngth

s, F

ram

esSh

eets

,Sh

eets

,St

rips,

Gas

kets

Shee

ts,

Avai

labl

e Fo

rms

by J

oini

ngor

Rec

tang

ular

Join

ing

Strip

s,by

Joi

ning

Join

ed E

MI

with

Joi

ned

Die-

Cut

Die-

Cut

Mad

e by

Die-

Cut

Strip

sGa

sket

sCl

ip-O

n St

rips

Strip

sSt

rips

EMI S

trips

Gask

ets

Gask

ets

Join

ing

Strip

sGa

sket

s

14 k

Hz (H

)>2

0- >

30 d

B>2

5- >

30 d

B>2

5- >

35 d

B>2

0- >

30 d

B>2

0- >

30 d

B>3

5 dB

>35

dB>4

6 dB

>35

dB

18 M

Hz (E

)>1

02 d

B>1

02 d

B>1

02 d

B>1

02 d

B>1

02 d

B>1

02 d

B>1

02 d

B>1

02 d

B>1

02 d

B

1.0

GHz

(P)

>83-

>93

dB

>93

dB>9

3 dB

>83-

>93

dB

>93

dB>8

5 dB

>40

dB>9

3 dB

>93

dB

30-4

0%30

%30

-50%

30%

30%

15%

10%

`20

%20

%

25-3

0%25

%25

-40%

30%

25%

10%

7%17

%17

%

20-2

5%20

%20

-30%

25%

25%

10%

7%17

%17

%

Min

imum

/Max

imum

Hei

ght

Inch

es0.

062/

0.50

00.

040/

0.37

50.

125/

0.75

00.

062/

0.37

50.

093/

0.25

00.

020/

0.03

00.

016/

0.02

00.

062/

0.31

20.

030/

0.25

0(m

m)

(1.5

7/12

.70)

(1.0

2/9.

53)

(3.1

8/19

.05)

(1.5

7/9.

53)

(2.3

6/6.

35)

(0.5

1/0.

76)

(0.4

1/0.

51)

(1.5

7/7.

92)

(0.7

6/6.

35)

Min

. Wid

th (G

reat

er o

f Act

ual

Inch

es0.

062/

1 /2H

0.06

2/1 /2

H0.

62/1 /2

H0.

125/

11 /2H

0.43

70.

140

0.12

50.

093/

1 /2H

0.12

5Di

m. o

r Por

tion

of H

eigh

t)(m

m)

(1.5

7/1 /2

H)(1

.57/

1 /2H)

(1.5

7/1 /2

H)(3

.18/

11 /2H)

(11.

0)(3

.56)

(3.1

8)(2

.36/

1 /2H)

(3.1

8)

Reco

mm

ende

d Co

mpr

essi

onps

i5-

100

5-10

05-

100

20-1

005-

100

20-1

0020

-100

20-1

0020

-100

Pres

sure

(kg/

cm)

(0.3

5-7.

03)

(0.3

5-7.

03)

(0.3

5-7.

03)

(1.4

1-7.

03)

(0.3

5-7.

03)

(1.4

1-7.

03)

(1.4

1-7.

03)

(1.4

1-7.

03)

(1.4

1-7.

03)

In S

lot

Exce

llent

Exce

llent

Exce

llent

Exce

llent

NoNo

NoGo

odPo

ssib

le

Pres

sure

Sens

itive

N/A

N/A

N/A

Exce

llent

N/A

N/A

N/A

Spec

ial

Exce

llent

Adhe

sive

Bond

Non

-EM

I Gas

ket

Vers

ions

with

*Ve

rsio

ns w

ithGo

od-E

xcel

lent

Poor

(3)

Spec

ial

Spec

ial

Com

bo V

ersi

onN/

APo

rtion

(4)

Fins

Onl

y(2)

Fins

Onl

y(2)

Only

Cond

uctiv

eUs

e Si

licon

e Ba

se A

dhes

ive

Adhe

sive

Poor

to G

ood

Poor

to G

ood

Poor

to G

ood

N/A

N/A

NoNo

See

Note

7.

Bolt

thru

Poss

ible

with

N/A

Poss

ible

with

Exce

llent

Exce

llent

(3)

Exce

llent

Exce

llent

Exce

llent

Exce

llent

Bolt

Hole

sFi

n Ve

rsio

ns(2

)Fi

n Ve

rsio

ns(2

)

Neop

rene

Ver

sion

N/A

N/A

–30°

F to

150

°F–3

0°F

to 1

50°F

–30°

F to

150

°FN/

A–4

0°F

to 2

25°F

Spec

ial

Spec

ial

–34°

C to

66°

C–3

4°C

to 6

6°C

–34°

C to

66°

C–4

0°C

to 1

07°C

Silic

one

Vers

ion

N/A

N/A

–80°

F to

400

°F–8

0°F

to 4

00°F

–80°

F to

400

°F–8

0°F

to 4

00°F

–65°

F to

500

°F–7

0°F

to 5

00°F

–80°

F to

400

°F–6

2°C

to 2

04°C

–62°

C to

204

°C–6

2°C

to 2

04°C

–62°

C to

204

°C–5

3°C

to 2

60°C

–57°

C to

260

°C–6

2°C

to 2

04°C

Stan

dard

Met

als

Avai

labl

e in

EM

IM

onel

Mon

elM

onel

Mon

el,

Alum

inum

Mon

el,

Mon

el,

Porti

on (o

ther

s al

so a

vaila

ble)

Fe

rrex

(1) ,

Ferr

ex(1

) ,Fe

rrex

(1) ,

Mon

el, F

erre

x(1) ,

Alum

inum

Mon

el, F

erre

x(1)

Alum

inum

Only

Alum

inum

Alum

inum

Alum

inum

Alum

inum

Alum

inum

EMI

Ratin

g(6)

Clas

s A

–Pe

rman

ently

Clos

ed

Clas

s B

–Op

en-C

lose

inSa

me

Posi

tion

Clas

s C

–Co

mpl

etel

yIn

terc

hang

eabl

e

Atta

chm

ent

orPo

sitio

ning

Elas

tom

erTe

mpe

ratu

reRa

nge

Max

imum

Join

tUn

even

ness

,%

of

Gask

etHe

ight

(1) F

erre

x®is

Cho

mer

ics’

trad

enam

e fo

r tin

-pla

ted,

cop

per-

clad

ste

el E

MIg

aske

ting.

(2) T

wo

vers

ions

,an

dha

ve fi

ns e

spec

ially

des

igne

d fo

r eas

y at

tach

men

t.(3

) The

alu

min

um e

xtru

sion

is in

tend

ed a

s a

conv

enie

nt m

eans

of a

ttach

men

t.(4

) Mos

t pro

duct

s fo

r whi

ch th

is m

etho

d is

sui

tabl

e ar

e av

aila

ble

with

“dr

y ba

ck”

(sol

vent

-act

ivat

ed) a

dhes

ives

alre

ady

appl

ied.

(5) A

vaila

ble

with

out e

last

omer

in m

etal

form

onl

y.

(6) T

hese

EM

I rat

ings

are

bas

ed o

n M

IL-S

TD-2

85 te

st m

etho

ds a

nd a

re u

sefu

l for

mak

ing

mea

ning

ful q

ualit

ativ

eco

mpa

rison

s be

twee

n pr

oduc

ts in

this

tabl

e si

nce

all t

ests

wer

e co

nduc

ted

unde

r sim

ilar c

ondi

tions

. The

yca

nnot

be

used

to c

ompa

re to

oth

er E

MI g

aske

t dat

a un

less

thos

e da

ta w

ere

obta

ined

by

the

sam

e m

etho

ds.

(7) N

on-c

ondu

ctiv

e RT

V yi

elds

exc

elle

nt re

sults

, but

use

spa

ringl

y. If

mor

e ad

hesi

ve s

urfa

ce is

nee

ded,

use

cond

uctiv

e ad

hesi

ve.

*Pre

ssur

e se

nsiti

ve a

dhes

ive

is a

vaila

ble

for c

erta

in m

esh

over

cor

e ga

sket

s. C

onta

ct C

hom

eric

s fo

r det

ails

.

SHIE

LDM

ESHTM

MES

H ST

RIPTM

COM

BO®

AND

PORC

UPIN

E(5

)

MES

H ST

RIPTM

COM

PRES

SED

(ELA

STOM

ER C

ORE)

AND

COM

BO®

STRI

PCO

MBO

®FR

AME

MET

ALAS

TIC®

MET

ALAS

TIC®

POLA

STRI

P®PO

LASH

EET®

PROD

UCT

TRAD

E NA

ME

(ALL

-MET

AL)

MES

H GA

SKET

SM

ETAL

KLIP

® G

ASKE

TING

GASK

ETIN

GGA

SKET

SGA

SKET

ING

GASK

ETIN

GGA

SKET

ING

GASK

ETIN

G

GASK

ETIN

G




ElectricalAbsorption Loss: Attenuation of anelectromagnetic wave or energyencountered in penetrating a shieldcaused by the induction of currentflow in the barrier and the resultingI2R loss. Usually stated in dB (decibels).

Ambient ElectromagneticEnvironment: That electromagneticfield level existing in an area andemanating from sources other thanthe system under test.

Attenuation: A reduction in energy.Attenuation occurs naturally duringwave travel through transmissionlines, waveguides, space or amedium such as water, or may beproduced intentionally by inserting anattenuator in a circuit or a shieldingabsorbing device in the path ofradiation. The degree of attenuationis expressed in decibels or decibelsper unit length.

Attenuator: An arrangement of fixedand/or variable resistive elementsused to attenuate a signal by adesired amount.

Cross Coupling: Coupling of thesignal from one channel to anotherwhere it becomes an undesired signal.

Conductivity: Capability of a materialto conduct electrical currents.

Decibel (dB): A convenient methodfor expressing voltage or power ratiosin logarithmic terms. The number ofsuch units of attenuation, N is

where

P1/P2 is a unitless power ratio. N can also beexpressed in terms of a voltage ratio E1/E2 asfollows:

Degradation: An undesired changein the operational performance of atest specimen. Degradation of theoperation of a test specimen doesnot necessarily mean malfunction.

Depth of Penetration: Distancewhich a plane wave must travelthrough a shield to be attenuated 1/e,or approximately 37 percent of itsoriginal value. (Also called “skindepth”). It is a function of the shield’sconductivity and permeability and thewave’s frequency.

Electrical or E-Field: A field inducedby a high impedance source, such asa short dipole.

Electromagnetic Compatibility(EMC): A measure of an equipment’sability to neither radiate nor conductelectromagnetic energy, or to besusceptible to such energy fromother equipment or an externalelectromagnetic environment.

Electromagnetic Interference (EMI):Undesired conducted or radiatedelectrical disturbances, includingtransients, which can interfere withthe operation of electrical or electronicequipment. These disturbances canoccur anywhere in the electromagneticspectrum.

Emanation: Undesired electromag-netic energy radiated or conductedfrom a system.

Gasket-EMI: A material that is insertedbetween mating surfaces of anelectronic enclosure to provide lowresistance across the seam andthereby preserve current continuityof the enclosure.

Ground: A reference plane commonto all electronic, electrical,electro-mechanical systems and connectedto earth by means of a ground rod,ground grid, or other similar means.

Hertz: An international designationfor cycles per second (cps).

Insertion Loss: Measure ofimprovement in a seam, joint orshield by the addition of a conductivegasket. Usually stated in dB.

Interference: Any electromagneticphenomenon, signal or emission,man-made or natural, which causesor can cause an undesired response,malfunctioning or degradation ofperformance of electrical or electronicequipment.

Internal Loss: Attenuation of electro-magnetic energy by the reflectionand re-reflection of electromagneticwaves within a shield or a barrier.Usually stated in dB.

Magnetic or H-Field: An inductionfield caused predominantly by acurrent source. Also called a lowimpedance source, such as may begenerated by a loop antenna.

Malfunction: A change in theequipment’s normal characteristicswhich effectively destroys properoperation.

Permeability: The capability of amaterial to be magnetized at a givenrate. It is a non-linear property of boththe magnetic flux density and thefrequency of wave propagation.

Plane Wave: An electromagneticwave which exists at a distancegreater than a wavelength fromthe source, where the impedanceof the wave is nearly equal to theimpedance of free space – 377 ohms.

Radio Frequency (RF): Any frequencyat which coherent electromagneticradiation of energy is possible.Generally considered to be anyfrequency above 10 kHz.

Radio Frequency Interference (RFI):Used interchangeably with EMI. EMIis a later definition which includes theentire electromagnetic spectrum,whereas RFI is more restricted tothe radio frequency band, generallyconsidered to be between the limits10 kHz to 10 GHz.

Reflection Loss: Attenuation of theelectromagnetic wave or energycaused by impedance mismatchbetween the wave in air and the wavein metal.

Glossary of Terms

N (dB) =10 log P1

P2

N (dB) =20 log E1

E2



219

Relative Conductivity: Conductivityof the shield material relative to theconductivity of copper.

Relative Permeability: Magneticpermeability of the shield materialrelative to the permeability of freespace.

Shield: A metallic configurationinserted between a source and thedesired area of protection which hasthe capability to reduce the energylevel of a radiating electromagneticfield by reflecting and absorbing theenergy contained in the field.

Shielding Effectiveness: A measureof the reduction or attenuation inelectromagnetic field strength at apoint in space caused by the insertionof a shield between the source andthat point. Usually stated in dB.

Shielding Increase: The differenceof an electromagnetic field amplitudeemanating through a seam (measuredunder fixed test conditions) with andwithout the gasket in the seam, withthe force joining the seam remainingconstant. The difference is expressedin dB based on voltage measurements.

Skin Effect: Increase in shieldresistance with frequency because ofcrowding of current near the shieldsurface because of rapid attenuationof current as a function of depth fromthe shield surface.

Surface Treatment: Coating or platingof mating surfaces of a junction.

Susceptibility: Measure of thedegradation of performance of asystem when exposed to anelectromagnetic environment.

Total Shielding Effectiveness: Thedifference of an electromagneticamplitude emanating from a sourcewithin an enclosure, and that from asource in free space. The differenceis expressed in dB based on voltagemeasurements.

Wave Impedance: The ratio ofelectric field intensity to magneticfield intensity at a given frequencyexpressed in ohms.

MechanicalAbrasion Resistance: The resistanceof a material to wearing away bycontact with a moving abrasivesurface. Usefulness of standard testsvery limited. Abrasion resistance is acomplex of properties: resilience,stiffness, thermal stability, resistanceto cutting and tearing.

Cold Flow: Continued deformationunder stress.

Compression Set: The decrease inheight of a specimen which has beendeformed under specific conditionsof load, time, and temperature.Normally expressed as a percentageof the initial deflection (rather than asa percentage of the initial height).

Durometer: An instrument formeasuring the hardness of rubber.Measures the resistance to thepenetration of an indentor pointinto the surface of the rubber.

Elasticity: The property of an articlewhich tends to return to its originalshape after deformation.

Elastic Limit: The greatest stresswhich a material is capable ofdeveloping without a permanentdeformation remaining after completerelease of the stress. Usually thisterm is replaced by various loadlimits for specific cases in which theresulting permanent deformationsare not zero but are negligible.

Elastomer: A general term for elastic,rubber-like substances.

Elongation: Increase in lengthexpressed numerically as a fractionor percentage of initial length.

Hardness: Relative resistance ofrubber surface to indentation by anindentor of specific dimensions undera specified load. (See Durometer).Numerical hardness values representeither depth of penetration orconvenient arbitrary units derivedfrom depth of penetration. Devicesfor measuring rubber hardness are

known as durometers and plasto-meters. Durometers are used mostcommonly. The higher the durometernumber, the harder the rubber, andvice versa.

Hardness Shore A: Durometerreading in degrees of hardness usinga Type A Shore durometer. (Shore Ahardness of 35 is soft; 90 is hard).

Permeability: A measure of the easewith which a liquid or gas can passthrough a material.

Permanent Set, Stress and StrainRelaxation: Permanent Set is definedas the amount of residual displacementin a rubber part after the distortingload has been removed. StressRelaxation, or Creep, is a gradualincrease in deformation of an elastomerunder constant load with the passageof time, accompanied by acorresponding reduction in stresslevel.

Resilience: The ratio of energy givenup on recovery from deformation tothe energy required to produce thedeformation – usually expressedin percent.

Tear Strength: The force per unit ofthickness required to initiate tearingin a direction normal to the directionof the stress.

Tensile Strength and Elongation:Tensile Strength is the force per unitof the original cross sectional areawhich is applied at the time of therupture of the specimen duringtensile stress. Elongation is definedas the extension between benchmarksproduced by a tensile force appliedto a specimen, and is expressedas a percentage of the originaldistance between the marks. Ultimateelongation is the elongation at themoment of rupture. Tensile Stress,more commonly called “modulus,” isthe stress required to produce acertain elongation.




19-11-XXXX-XXXX Conductive Elastomer Die-Cut Parts 5819-12-XXXX-XXXX Conductive Elastomer Molded Parts 6119-13-XXXX-XXXX Molded-In-Place Cover Seals 77-7919-18-XXXX-XXXX Conductive Elastomer Extrusions 35-5819-24-XXXX-XXXX (see detailed index on pages 87-90)19-41-XXXX-XXXX Conductive Elastomers, Fabric Reinforced 7620-01-XXXX-XXXX Conductive Elastomer Waveguide Gaskets 67-7120-02-XXXX-XXXX20-03-XXXX-XXXX20-11-XXXX-XXXX30-01-XXXX-XXXX Conductive Elastomer Connector Gaskets 72-7530-02-XXXX-XXXX30-03-XXXX-XXXX30-XX-XXXX-XXXX40-XX-XXXX-XXXX Conductive Elastomer Sheet Stock 58-6041-XX-XXXX-XXXX43-XX-XXXX-XXXX50-XX-XXXX-XXXX Conductive Compounds 133-14251-XX-XXXX-XXXX52-XX-XXXX-XXXX53-XX-XXXX-XXXX54-XX-XXXX-XXXX55-XX-XXXX-XXXX70-2X-XXXX-XXXX CHO-SHRINK® Heat Shrinkable Tubing 186-188

71-XX-XXXX-XXXX CHO-SHRINK® Heat Shrinkable 186-188Molded Parts80-10-XXXX-XXXX CHO-DROP® EMI Absorbers 183

81-XX-XXXX-XXXX SPRING-LINE® Beryllium-Copper Gaskets 126-132(fingerstock, card cage and D connector gaskets)

82-XX-XXXX-XXXX SOFT-SHIELD® 5000 93-97Low Closure Force Foam Gaskets83-XX-XXXX-XXXX CHO-SORB® EMI Ferrites 176-18286-XX-XXXX-XXXX CHO-BUTTONTM EMI Grounding Contacts 156CAD-XX-XXX-XXXX CHO-FOIL® EMI Shielding Tapes 146-149CCD-XX-XXX-XXXXCCE-XX-XXX-XXXXCCH-XX-XXX-XXXXCCJ-XX-XXX-XXXXCCK-XX-XXX-XXXXCBL-XX-XXXX-XXXX EMI Shielding Laminates 150-151CHO-EMI-TAPE-BOX EMI Shielding Tapes Kit 149CFT-XX-XXX-XXXX CHO-FABTM EMI Shielding Fabric Tape 146-149

CMT-XX-XXXX-XXXX CHO-MASK® II EMI Foil Tape with 144-145Peel-Off MaskCJ-XXX-XX CHO-JAC® Cable Jacketing 149CWA-XX-XXXX-XXXX SOFT-SHIELD® 4000 Low Closure 98-100CWF-XX-XXXX-XXXX Force Foam GasketsE-01-XXXXX EmiClareTM GP 70 EMI Shielded Windows 171-172

FPCV-XXXXX-XXXXXX STREAMSHIELDTM EMI Shielded 158-160Vent/Airflow Panels

G-01-XXXXX WIN-SHIELDTM Shielded Window 172-173(Glass Assembly)L-XXXX-XX CHO-STRAP® Grounding Straps 153

P-01-XXXXX WIN-SHIELDTM Shielded Window 172-174(Plastic or AgF8 Film Assembly)

Use this table to identify product groups by Part Number and locate them in this Handbook.

Chomerics Part Number Cross Reference Index

CHOMERICSPART NO.

DESCRIPTION REFERTO PAGE

CHOMERICS DESCRIPTION REFERPART NO. TO PAGE

01-0101-XXXX MESH STRIP TM All Metal Gaskets 81-8201-0104-XXXX01-0199-XXXX SPRINGMESH® Highly Resilient Gaskets 11101-02XX-XXXX COMBO® STRIP Mesh/Rubber Gaskets 112-11401-03XX-XXXX01-06XX-XXXX01-07XX-XXXX01-04XX-XXXX MESH STRIP TM With Elastomer Core 109-11001-05XX-XXXX

01-09XX-XXXX SOFT-SHIELD® 1000 103-104Low Closure Force Foam Gaskets01-1292-XXXX SOFT-SHIELD® 2000 101-10201-1392-XXXX Low Closure Force Foam Gaskets

02-XXXX-XXXX SHIELDMESHTM Compressed 115Mesh Gaskets04-XXXX-XXXX METALASTIC ® EMI Gasketing 12105-XXXX-XXXX-XX SHIELD WRAPTM Knitted Wire Mesh Tape 19006-01XX-XXXX-XX Mesh Frame Gaskets and Strips 116-11706-02XX-XXXX-XX06-21XX-XXXX-XX06-03XX-XXXX-XX SHIELD CELL® Shielded Vent Panels 16206-05XX-XXXX-XX06-09XX-XXXX-XX06-X7XX-XXXX SLIMVENTTM Shielded Air Vent Panels 16506-XX15-XXXX-XX Steel Honeycomb Shielded Vents 16406-07XX-XXXX-XX SHIELDSCREEN® Shielded Air Filters 16606-13XX-XXXX-XX06-14XX-XXXX-XX06-XX14-XXXX-XX Brass Honeycomb Shielded Vents 16406-1010-XXXX-XX06-1014-XXXX-XX06-11XX-XXXX-XX OMNI CELL® Shielded Vent Panels 16206-12XX-XXXX-XX06-15XX-XXXX-XX VIP Shielded Air Filters 16506-16XX-XXXX-XX

07-XXXX-XXXX POLASHEET® and POLASTRIP®

118-120Composite Gasketing08-XXXX-XXXX PORCUPINE METALASTIC® EMI Gaskets 9710-00-XXXX-XXXX Conductive Elastomer O-Rings 6410-01-XXXX-XXXX Conductive Elastomer D-Rings 6310-02-XXXX-XXXX Conductive Elastomer Flat Washers 6610-03-XXXX-XXXX10-04-XXXX-XXXX Conductive Elastomer Extrusions 35-5810-05-XXXX-XXXX (see detailed index on pages 87-90)

10-06-XXXX-XXXX10-07-XXXX-XXXX10-08-XXXX-XXXX10-09-XXXX-XXXX14-XXXX-XXXX-X ZIP-EX-2® Zippered Cable Shielding 184-18517-03XX-XXXX-XX METALKLIP® Clip-On EMI Gaskets 12319-04-XXXX-XXXX Conductive Elastomer Extrusions 35-5819-05-XXXX-XXXX (see detailed index on pages 87-90)

19-06-XXXX-XXXX19-07-XXXX-XXXX19-08-XXXX-XXXX19-09-XXXX-XXXX

EMI Shielding Theory & Gasket Design Guide - Sealing

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