Emi Form Shield

8/7/2019 Emi Form Shield

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RH Technical Industries

RH Technical Industries Ltd - 8 Regents Court, Walworth Industrial Estate, Andover, Hampshire. SP1

Tel: 01264 721 450 •

Fax: 01264 721 510 •

email: [email protected]•

www.rhti.co.uk of 6

EMI Form Shield

“The green replacement for conductive paint and platingfor plastic housings”

Editor’s notes

The authors of this article are; Chris Hills, Managing Director, RH Technical Industries, Reed Niederkorn, VP Engineering and Development with APM, Inc. The article gives an overview of

shielding solutions but concentrates on the recent and ongoing developments of Form-Shield ™ by APM and RH Technical Industries Limited. {RHTI}

APM and RHTI are both members of the Laird Group PLC. APM is based in Missouri, USA where development and manufacturing is established. It also has manufacturing sites in Germany, UK and Taiwan as well as sales offices in Japan and Singapore. They are the manufacturers of a wide range of EMI protection products and have grown very rapidly over the last 5 years. APM has recently received an award for “Customer Service Innovation” from an independent market analysis firm.

RHTI is an industrial graphics printer based in the UK and has gained very wide experience over the last 20 years of printing conductive materials with precision and in the last two years undertaking extensive work in the field of In Mould Decoration for the telecommunications market.RHTI is qualified to ISO 9001 and has recently been awarded the Environmental Management Standard ISO 14001.

The joint efforts of these two companies has brought together the needs of the customer, a thorough understanding of the needs of EMI solutions, and expertise in print and forming to develop a new and exciting solution to the market.

SummaryA new product, named Form-Shield™, has been developed to replace the painting and platingsteps historically used to shield plastic enclosures. APM have submitted a patent application forvarious aspects of this product and process. Form-Shield™ consists of a thin thermo-formable

plastic sheet, onto which highly conductive de-formable ink is printed. This conductive sheet isthen formed into the desired shape, including complexgeometry, penetrations, and multiple “can” shields.This finished component is then assembled into thefinal product by friction or spot adhesives, therebyeliminating the need for the costly and complex stepsof painting or plating the plastic housing at themoulding or assembly step. Additionally, a conductivegap-filling gasket may be applied at the forming stepto eliminate the need for further EMI shielding at thetime of assembly. This exciting new product savestime and cost for the OEM, plus when the productreaches end-of-life the shield is easily removed to

allow recycling of the plastic enclosure. It is alsopossible to mould the shield into the housing, whichcompletely eliminates a separate shielding step at anyphase of the manufacture. Figure 1 gives examples ofthe complex forming possibilities with Form-Shield™.

BackgroundIn order to achieve the small size, lightweight and complex geometry in handsets, notebook PC’s,PDA’s and other small electronic devices, many of these devices are made using plasticenclosures. While meeting the needs of physical containment, plastic enclosures are generally

Figure 1Form-Shield™; The possibilities areonly limited by your imagination

InnovativeSpecialist

PrintSolutions

For over 40 years



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transparent to electromagnetic waves, so significant efforts are needed in order to achieveelectromagnetic compatibility (EMC). There are two issues regarding shielding plastic enclosures:the planar part of the enclosure and the gaps between sections of the enclosure.

Planar shield

There is a long history of technologies used to render plastic enclosures impermeable toelectromagnetic interference (EMI), including:

¾ Painting the enclosure with conductive paint

¾ Plating the enclosure with various metals by electrolysis plating or electroplating

¾ Use of metal inserts or board-mounted “cans”

¾ Use of metal foil or metallized fabric laminates

In the case of shielding paint, a metal-filled paint is applied to the plastic enclosure, generally aftermasking and priming the plastic part. This requires the moulder or secondary converter having thenecessary infrastructure for this process; including masking equipment, spray booths, off-gasincineration system and pressure painting equipment as well as the necessary skills. Using forcedhot air or an extended period of ambient drying is a requirement, adding to the space needed forthis operation. The metallic paint is strongly adhered to the plastic, so removing this at the end of

the product life cycle to allow recycling of the plastic is very difficult or impossible.

Plating the enclosure is generally more complexthan painting, but a highly conductive surface and avery thin coating may be achieved. In order for thesurface of the plastic to accept the plating, it mustfirst be subjected to at least one pre-treatment tomake the surface catalytic to the electrolysisdeposition. Then the catalytic surface is contactedwith an electrolysis plating bath and the conductor(generally copper or nickel) is deposited on thesurface. The complexity of plating operationsrequires a skilled workforce and significant

investment in capital and space, and as withconductive paints, the resulting metal layer is verydifficult or impossible to remove when the productreaches the end of its life. Plating is generally moreexpensive than painting, but the EMI shieldingperformance is typically superior to painting.Disposal of waste in the recycling process should bea key consideration. Both the painting and platingprocesses require that the entire enclosure isdisposed of while with Form-Shield™, only theshielding laminate itself needs to be removed anddiscarded.

In addition, there has been continual R&D for many years on modifying the plastic by filling it withmetal particles or fibres as well as use of conductive plastics including Polypyrole or Polyaniline.However, none of these are used to a significant extent today.

Gap shield

There is a well-established industry supplying shielding gaskets for gaps, including the followingtypes:

¾ Conductive elastomers (extruded, moulded, or dispensed directly on the housing)

¾ Metal clips and fingers

¾ Resilient foams covered with conductive metallized fabrics

Figure 2Form-Shield™; Simple to remove anddiscard at end-of-life



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¾ Knitted wire over elastomers

The basic requirement of a gap shield is that it must provide a highly conductive contact along theentire length of the gap, making the electrical connection between the two conductive planarshields.

APM have been working to develop a new product that removes the limitations of the existingproducts. The objective is to shield the plastic enclosure at the time of product assembly byinserting a single shielding component rather than creating the shielding as part of the moulding orsecondary operations. This would allow the plastic enclosure shields to be made in very largevolumes at strategic local manufacturing plants and shipped to the assembly point – in just thesame way as other components are handled.

Form-Shield™Recent developments in formulation of formable inks have made it possible to print conductive inkon a thermo-formable film, then form the film using conventional thermoforming equipment toproduce a complex-shaped highly conductive shield. This conductive ink may be applied usingstandard high-speed screen printing methods in both web and sheet. A photo of a printing pressmaking Form-Shield™ is shown in Figure 3.

The film used as the base for Form-Shield™ maybe any formable film, including Polycarbonate,Polyester and Hybrids, and the thickness of thefilm can be as low as 0.075mm. UL rated fireretardant films can also be used. The choice offilm composition and thickness is determined bythe size, shape and extent of draw in the formingstep – enough film thickness must exist afterforming to be structurally adequate. Themaximum percentage elongation that is feasible islimited by the conductive ink and not the thermo-formable film, and is generally 35-50%, althoughdeeper draws are possible by application of

additional printed layers in selective areas.

The ink is generally silver-based, but silver platedcopper, tin/copper and nickel/copper inks areunder development.. The ink thickness isdetermined by the shielding effectiveness requiredof the finished part, and Figure 4 demonstratesthe range of ink thickness and the resulting

shielding effectiveness.

S E ( d B ) v s . In k W e ig h t ( g /f t ^ 2 )

R2

= 0 .6 8 1 4

3 0

4 0

5 0

6 0

7 0

8 0

9 0

0 2 4 6 8 1 0

In k W e ig h t o f S a m p le (g /f t ^ 2 )

Shielding Eff. of Sample

(dB)

Figure 3A roll-form screen press at RHTI,Andover, UK

Figure 4



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The inherent flexibility of the screen printingprocess allows the designer a wide range ofoptions that can be achieved by printing beforeforming. These options include the developmentof any conductive pattern from a total flood to holeor grid as shown in Figure 5. It is easy to print onboth sides of the material in close register to dealwith specific or difficult shielding needs. Wherethere is a need for insulation of the conductive filmthis can be achieved by selective printing of asuitable dielectric and finally it is possible to printselective adhesive to provide fixing of the partduring assembly. All these options are availableto the end user at marginal cost increase, as theyare all achieved during the high-speed web-printing phase of manufacture. They are designedto minimize the level of additional work orcomponents required during assembly andtherefore to achieve the lowest additional cost.

Cost comparison with other methods

Figures 6 & 7 show a cost comparison between Form-Shield™ and other methods. As the totalcost of the shielded enclosure will depend on the scale of operation, labour cost, shieldingeffectiveness and other factors, this is given as an example only.

Figure 6 Figure 7

Table 1 summarises Form-Shield™ with other products used to provide shielding of cases.

Conductive Paint, Conductive Plating & Metal Shield

Aspect Form-Shield™ ConductivePaint

ConductivePlating

Metal ShieldInsert

Recyclability Good Poor Poor Good

ShieldingEffectiveness Very Good Very Good Excellent Excellent

Logistics Simple Involved Involved Simple

Process stages Simple Involved Involved Simple

Additional Processing Ready to fit High prep work High prep work Ready to fit

Cost Overall Lower Overall High High High

Buy in Cost Higher Low Low High

Tooling Cost Low Medium Medium Very High

Insulation Low cost Difficult Difficult Extra Cost

Fixing Low cost N/A N/A Simple

Design option testing Simple Difficult Difficult Difficult

Complex Shapes Simple Simple Simple Difficult

Overall Environmental Low Medium High Low

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

0 H W D O & D Q

) R U P 6 K L H O G

& D Q

& R V W 6 F D O H

0 0.5 1 1.5 2

0 H W D O L V H G

+ R X V L Q J

) R U P 6 K L H O G

+ R X V L Q J

& R V W 6 F D O H

Figure 5Form-Shield™; An innovative solutionto difficult shielding needs



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Impact

End User CapitalInvestment

Low High Very High Low

Moulder skilldemands

None High High Low

Table 1

On board shielding with Form-Shield™

The lightweight and versatile forming properties of Form-Shield™ provide a new range of optionsfor the on board shielding of components. The ability to form multiple cavities in a single piece ofForm-Shield™ means the entire board may be shielded with a single component, versus multiplemetal cans. As well as simplicity of assembly the designer has the freedom that comes fromknowing that the top surface of Form-Shield™ is an excellent dielectric therefore allowing eventighter packaging of PCB and components. New developments in conductive elastomers appliedat the time of printing will allow for earthing using a simple mechanical fixing and therefore simplere-work or access for maintenance.

Form-Shield™ cans will offer a further advantage to the user and the environment. Metal cansneed to be supplied in bandoleers, which involves the manufacturer of the cans in the cost of

loading these as well as their purchase. The populator then needs to dispose of this material aswaste. Form-Shield™ can be produced in a continuous web with each can being held by break-out legs and the waste material utilized as the logistics carrier thus saving waste and time.

Table 2 shows a comparison between Form-Shield™ and metal cans to provide on boardshielding.

Metal Cans

Aspect Form-Shield™ Metal Cans

Recyclability Excellent Good

SE Very Good Excellent

Multiple Cans in single part Simple Difficult

Weight Low Medium

Insulation Simple Difficult

Rework & repair access Simple Complex

Tooling Cost Low High

Transport Logistics Simple Extra Cost

Fixing to PCB Adhesive Solder

Table 2

Additional embodiments of Form-Shield™

Combination of gap shielding with planar shielding

Independent of the method of making the enclosure perimeter conductive, the gap between theenclosure typically must be shielded in order for the finished product to meet EMC requirements.For small devices, form-in-place (FIP) shielding has been gaining in acceptance. In FIP, a flexibleelastomer that is highly loaded with conductive particles is dispensed through a nozzle anddeposited directly on the part to be shielded. This process requires expertise, floor space, andcapital investment. By dispensing the FIP compound directly onto the formed conductive film, thisprocess step is removed from the moulding site, and can be done efficiently at very large scale.What comes out of the shipping container at the assembly plant is a finished part that is directlyinserted into the plastic housing, eliminating all extra steps and ensuring a consistent and highquality EMI shield.

Developments of new printable substrates will allow for the application of conductive gaskets andconductive adhesives prior to forming and will therefore reduce the end cost of the parts oncethese techniques and materials are ready for market.



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In-Mould-Shielding – “IMS”

Another use of Form-Shield™ is to mould the shield directly into the plastic part. This may seemto sacrifice one of the key attributes of Form-Shield™– recyclability, but this is not the case. A specialcoating used with Form-Shield™ for IMS provides astrong bond to hold the shield in place, but a firm pullmay remove the shield at end-of-life. In this case,

there is not even a separate film to handle inassembly but the product is recyclable.

Form-Shield™ has been designed in direct responseto customer demand for cost effective, highperformance, and environmentally friendly EMIshielding. With manufacturing facilities in NorthAmerica, Europe and Asia, APM is uniquelypositioned to deliver EMI solutions in each major areaof consumption and support customer requirementswith fully trained local staff.

Authors:

Reed Niederkorn Email: [email protected] Engineering and Development Tel: + 1 314 344 - 9339APM Inc.

Chris Hills Email: [email protected] Director Tel: + 44 (0)1264 721 450RH Technical Industries

To submit application questions: Email: [email protected]

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) R U P 6 K L H O G � ) U H H G R P R I ' H V L J Q

Emi Form Shield

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