Seam Design

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IMPORTANT INFORMATION AND DISCLAIMERS

1. INTEL CORPORATION (AND ANY CONTRIBUTOR) IS PROVIDING THIS INFORMATION AS A CONVENIENCE AND

ACCORDINGLY MAKES NO WARRANTIES WITH REGARD TO THIS DOCUMENT. IN PARTICULAR, INTEL (AND ANY

CONTRIBUTOR) DOES NOT WARRANT OR REPRESENT THAT THIS DOCUMENT OR ANY PRODUCTS MADE IN

CONFORMANCE WITH IT WILL OPERATE IN THE INTENDED MANNER. NOR DOES INTEL (OR ANY CONTRIBUTOR)

ASSUME RESPONSIBILITY FOR ANY ERRORS THAT THE DOCUMENT MAY CONTAIN.

2. NO REPRESENTATIONS OR WARRANTIES ARE MADE THAT ANY PRODUCT BASED IN WHOLE OR IN PART ON THE

ABOVE DOCUMENT WILL BE FREE FROM DEFECTS OR SAFE FOR USE FOR ITS INTENDED PURPOSE. ANY PERSON

MAKING, USING OR SELLING SUCH PRODUCT DOES SO AT HIS OR HER OWN RISK.

3. INTEL DISCLAIMS ALL LIABILITY ARISING FROM USE OF OR IN CONNECTION WITH THE INFORMATION PROVIDED

IN THIS DOCUMENT, INCLUDING LIABILITY FOR INFRINGEMENT OF ANY PROPRIETARY RIGHTS RELATING TO THE

INFORMATION OR THE IMPLEMENTATION OF INFORMATION IN THIS DOCUMENT. INTEL DOES NOT WARRANT OR

REPRESENT THAT SUCH DEVICES OR IMPLEMENTATION WILL NOT INFRINGE SUCH RIGHTS. INTEL IS NOT

OBLIGATED TO PROVIDE ANY SUPPORT, INSTALLATION OR OTHER ASSISTANCE WITH REGARD TO THE

INFORMATION.

4. THE INFORMATION REFERRED TO IN THIS DOCUMENT IS INTENDED FOR STANDARD COMMERCIAL USE ONLY.

CUSTOMERS ARE SOLELY RESPONSIBLE FOR ASSESSING THE SUITABILITY OF THE INFORMATION FOR USE IN

PARTICULAR APPLICATIONS. THE INFORMATION IS NOT INTENDED FOR USE IN CRITICAL CONTROL OR SAFETY

SYSTEMS, MEDICAL OR LIFE SAVING APPLICATIONS, OR IN NUCLEAR FACILITY APPLICATIONS.

5. NO LICENSE, EXPRESS OR IMPLIED, BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS

IS GRANTED HEREIN.

Copyright 2001 Intel Corporation. All rights reserved.

Version 1.0, August 2001

Third-party brands and names are the property of their respective owners.

Revision History

Revision Document Update Date

1.0 Initial release August 2001


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Contents

1 Scope.................................................................................................................. 5

1.1 Limitations of Document ........................................................................................................51.2 Reference Documents...........................................................................................................6

1.3 Terminology...........................................................................................................................7

2 Overview............................................................................................................. 8

2.1 Basic U-Seam Operation.......................................................................................................9

2.2 U-Seam Design Types ..........................................................................................................10

3 Creating a U-Seam Design................................................................................ 11

3.1 EMI Performance of a U-Seam .............................................................................................11

3.2 Cost Benefit of U-Seams versus EMI Gaskets......................................................................11

3.3 U-Seam Design Considerations ............................................................................................11

3.3.1 Total Linear Distance of Seam .................................................................................11

3.3.2 Gap Geometry (U-Seam Width, Thickness of L Section) .......................................14

3.3.3 Depth of Overlap.......................................................................................................17

3.4 Manufacturing Tolerance Considerations..............................................................................20

3.4.1 Evaluation of Worst Case U-Seam Configurations ..................................................20

3.5 General Design Considerations .............................................................................................27

3.6 Cost/Performance Tradeoff ...................................................................................................28

4 Design Requirements........................................................................................ 29

4.1 Compliance with EMC Requirements....................................................................................29

4.2 Material Options.....................................................................................................................29


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Figures

1. U-Seam Geometry Definition .............................................................................................................7

2. U-Seam Example in Chassis .............................................................................................................8

3. Basic U-Seam Operation ...................................................................................................................9

4. Standard U-Seam Design ..................................................................................................................10

5. Standard U-Seam Design with Additional Overlap ............................................................................10

6. U-Seam Length..................................................................................................................................12

7. Effect of Changing Total U-Seam Length ..........................................................................................13

8. U-Seam Width and Gap.....................................................................................................................14

9. U-Seam Width (non-parallel case).....................................................................................................14

10. Measured Effect of U-Seam Gap Size on SE ..................................................................................15

11. Predicted Effect of U-Seam Gap Size on SE...................................................................................16

12. Overlap for Standard U-Seam Design .............................................................................................17

13. Diminished Overlap because of Lead-In at U-seam Design............................................................17

14. Measured Effect of U-Seam Depth of Overlap on SE .....................................................................18

15. Predicted Effect of U-Seam Depth of Overlap on SE ......................................................................1916. U-Seam Configuration A Tolerance Analysis (1 of 2) ......................................................................21

17. U-Seam Configuration A Tolerance Analysis (2 of 2) ......................................................................22

18. U-Seam Configuration B Tolerance Analysis (1 or 2) ......................................................................24

19. U-Seam Configuration B Tolerance Analysis (2 or 2) ......................................................................25

Tables

1. Reference Documents .......................................................................................................................6

2. Tolerance Analysis of Configuration A...............................................................................................23

3. Tolerance Analysis of Configuration B...............................................................................................26


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1 Scope

The purpose of this document is to provide an overview of the Electromagnetic

Compatibility (EMC) design and implementation of U-seams for use in chassis

enclosures. This study focuses on U-seam design recommendations above1 GHz. Limited measurements have been carried out on U-seam panel performance

below 1 GHz and resulted in similar performance as to that predicted at 1 GHz.

1.1 Limitations of Document

The equations and trends contained in this document have been verified to be

applicable to chassis sized structures. Due to the small size of the enclosure used,

there is an interaction between the noise source and the enclosure itself (because of

near field effect). Although no extensive studies were carried out of this effect, it is

believed that this has negligible affect on the design guidelines contained within this

document.

The bulk of the analysis leading to this document was performed with no direct

contact between the two halves of the assembled panels. Any incidental contact

between the panels will increase the Shielding Effectiveness of a design.

Even though some recommendations for gap and clearance for U-seams are given,

the fit and tolerance of a particular design are the responsibility of the chassis

designer.

The EMI performance of U-seams has been experimentally verified up to 10 GHz.

Theoretical extrapolations have been used to predict performance up to 12 GHz.


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1.2 Reference Documents

Table 1. Reference Documents

Document Title Document Number or Author

Field and Wave Electromagnetics David K. Cheng

Controlling Radiated Emissions by Design Michel Mardiguian

Noise Reduction Techniques in Electronic Systems

(second edition)

Henry W. Ott


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1.3 Terminology

EMC Electromagnetic Compatibility

EMI Electromagnetic Interference

SE Shielding Effectiveness

U-seam U-shaped seam designed where chassis panels join together for the purpose of

containing EMI in a chassis enclosure without the aid of metalized gaskets.

Throughout this document, U-seam geometry is defined as shown in Figure 1:

Gap

Gap

T

Overlap

U-SeamWidth

L-Panel

U-Panel

Gap

Gap

T

Overlap

U-SeamWidth

L-Panel

U-Panel

Figure 1. U-Seam Geometry Definition

U-seam width Width of U-panel slot

T Thickness L-panel

Gap Average gap between panels, represented as (Width-T)/2.

Overlap Depth of overlap between U-panel and L-panel

Note: All dimensions throughout this document are in millimeters (mm) unlessotherwise specified.


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2 Overview

The primary purpose of U-seams is to improve the ability of a chassis enclosure to

contain very high frequency EMI at its seams without the need for expensive EMIgaskets. Where U-seams can be implemented, significant cost savings can result. A

good design can also take advantage of the snap together nature of U-seams to

improve manufacturability, serviceability, and enclosure structural integrity. Figure 2

shows an example of a U-seam design at the perimeter of a chassis enclosure.

Figure 2. U-Seam Example in Chassis


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2.1 Basic U-Seam Operation

A U-seam is essentially a method of attaching two sheet metal edges together such that

one edge bends tightly around the other. This tight u-shaped bend traps EMI inside the

enclosure. Its EMI performance is governed by the geometry of the seam including thesize of the gap formed by the seam, the amount of overlap between the two panels, and

the total length of the seam (seeFigure 3).

Section 3 describes how U-seam geometry can be varied to obtain the desired EMI

performance.

Significant EMI Internal to Chassis

Reduced EMI External to Chassis

L-Pane

l

U-Panel

Figure 3. Basic U-Seam Operation


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2.2 U-Seam Design Types

U-seams can be designed in a variety of different ways. Two such designs are shown

below. The first type shown in Figure 4 is considered worst case and was used

throughout U-seam evaluations when developing this document. The second typeshown in Figure 5 creates additional overlap and is expected to add to the capacitive

and reflective losses thus improving the shielding effectiveness of the U-seams. Both

types are expected to be common U-seam implementations, but evaluating using worst

case construction ensures adequate shielding effectiveness.

L-Panel

U-Panel

L-Panel

U-Panel

Figure 4. Standard U-Seam Design

L-Panel

U-Panel

Figure 5. Standard U-Seam Design with

Additional Overlap


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3 Creating a U-Seam Design

3.1 EMI Performance of a U-SeamThe metric of U-seam EMI performance is determined by its Shielding Effectiveness

(SE). The Shielding Effectiveness of a U-seam represents the amount of EMI

attenuation that the U-seam offers at a given frequency. This depends on several factors

that include the gap size of the seam, the amount of overlap between the two panels,

and the total length of the seam.

3.2 Cost Benefit of U-Seams versus EMI Gaskets

A U-seam design can present a significant cost advantage compared to that of a seam

utilizing EMI gasketing. As frequencies continue to increase (and wavelengthsdecrease) grounding points need to be so closely spaced to provide adequate shielding

that the use of continuous gaskets at seams becomes increasingly prevalent.

Continuous soft gasketing and labor to install it can add significant cost, especially in

larger chassis with multiple access panels.

3.3 U-Seam Design Considerations

The following sections provide details regarding the effect of the basic geometrical

design parameters on U-seam EMI performance.

3.3.1 Total Linear Distance of Seam

The total length of a U-seam (total perimeter distance) does not have a significant

impact on the Shielding Effectiveness. A decrease in the total length by a factor of

four, for example, only decreases the Shielding Effectiveness by approximately 3 dB.

Note: When estimating the shielding effectiveness of a U-seam design for an enclosure,

the total perimeter length on an exposed side (the total length exposed to an antenna at

one time, for example) should be considered to take the additive affect of EMI leakage

through all exposed seams (relative to measurement antenna) into account.


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Figure 6 shows how the U-seam length is measured.

Length

L-Panel

U-Panel

Length

L-Panel

U-Panel

Figure 6. U-Seam Length


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The plot in Figure 7 shows the predicted effect of changing the total length by a factor

of four.

Option 1 detail:

U Width 1.5 mm; L Thickness 1 mm; Avg. Gap 0.25 mm; Overlap 7 mm; Total Seam

Length 250 mm

Option 2 detail:


Length 1000 mm

Comparative Shielding Effectiveness Plot

0

5

10

15

20

25

30

35

40

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Frequency (MHz)

ShieldingEffectiveness(dBuV/m)

250mm U-Seam Length

1000mm U-Seam Length

Figure 7. Effect of Changing Total U-Seam Length


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3.3.2 Gap Geometry (U-Seam Width, Thickness of L Section)

The gap size is the average distance between the internal wall of the U-panel and the

wall of the L-panel. A decrease in the average gap size from 0.5 mm to 0.25 mm

increases the Shielding Effectiveness by approximately 3 dB.

Figure 8 shows the U-seam gap geometry where the average gap is determined as

(Width T)/2.

Gap

Gap

TU-SeamWidth

L-Panel

U-Panel

Gap

Gap

TU-SeamWidth

L-Panel

U-Panel

Figure 8. U-Seam Width and Gap

For the case where non-parallel U-seam sides exist, the worst case U-width should be

considered as shown in Figure 9 below.

U-SeamWidth

U-Panel

L-Panel

Figure 9. U-Seam Width (non-parallel case)


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Figure 10 shows the measured effect of varying U-seam width on Shielding

Effectiveness in the 1-4 GHz range.

Measured Effect of U-Seam Width on SE

0

5

10

15

20

25

30

35

40

45

1000 2000 3000 4000

Frequency (MHz)

Shielding

Effectiveness(dBuV/m

1.56mm U-seam

1.76mm U-seam

1.96mm U-seam

2.76mm U-seam

3.26mm U-seam

Figure 10. Measured Effect of U-Seam Gap Size on SE


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The plot in Figure 11 shows the predicted effect of changing the U-seam gap size and is

based on empirical data.

Option 1 detail:


Length 1000 mm

Option 2 detail:


Length 1000 mm


0

5

10

15

20

25

30

35

40

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Frequency (MHz)


1.5mm U-Seam Width (0.25mm gap)

2.0mm U-Seam Width (0.50mm gap)

Figure 11. Predicted Effect of U-Seam Gap Size on SE


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3.3.3 Depth of Overlap

Depth of overlap is the distance that the U-panel and L-panel overlap each other when

assembled. An increase in depth of overlap increases the Shielding Effectiveness. An

increase in the overlap from 6 mm to 7 mm, for example, increases the Shielding

Effectiveness, but varies with frequency. At lower frequencies, overlap has more effect,but as frequencies increase, this effect diminishes.

Figure 12 shows the U-seam overlap geometry where the overlap variable is determined

as the distance of overlap between the assembled L-panel and U-panel.

OverlapL-Panel

U-Panel

OverlapL-Panel

U-Panel

Figure 12. Overlap for Standard U-Seam Design

However, when a U-seam design includes a lead-in or chamfer, the overlap is calculateddifferently because the lead-in will reduce EMI performance of the U-seam. Figure 13

shows how Overlap is measured for a U-seam with a lead-in design. In this case, the

distance is measured starting at approximately the center of the bent metal edge.

Overlap

L-Panel

U-Panel Lead-in or Chamfer

Figure 13. Diminished Overlap because of Lead-In at U-seam Design


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Figure 14 shows the measured effect of varying the depth of overlap on Shielding

Effectiveness in the 1 4 GHz range.

Measured Effect of U-Seam Depth of Overlap on SE

-20

-15

-10

-5

0

5

10

1000 2000 3000 4000

Frequency (MHz)

Impa

ctonSE(dBuV/m)

5.2 mm overlap

4.4 mm overlap

3.6 mm overlap

2.8 mm overlap

2.0 mm overlap

1.2 mm overlap

0.4 mm overlap

Figure 14. Measured Effect of U-Seam Depth of Overlap on SE


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The plot in Figure 15 shows the predicted effect of changing the U-seam Depth of

Overlap and is based on empirical data.

Option 1 detail:


Length 1000 mm

Option 2 detail:


Length 1000 mm


0

5

10

15

20

25

30

35

40

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Frequency (MHz)


7mm Overlap

6mm Overlap

Figure 15. Predicted Effect of U-Seam Depth of Overlap on SE


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3.4 Manufacturing Tolerance Considerations

When designing a U-seam for a particular application, it is important to design to worst-

case tolerances that may exist in the chassis to ensure that a U-seam design is robust

under all expected tolerance variations. While a quick CAD file review may show thata design is adequate, the actual formed sheet metal dimensions may prove to be very

different and possibly inadequate. Although some guidelines will be provided in this

section, it is important that a tolerance analysis be performed on your own U-seam

design.

3.4.1 Evaluation of Worst Case U-Seam Configurations

The following figures show two basic U-seam configurations. Depending upon the

configuration used, different tolerances can be expected. The sections below will show

typical tolerance expectations for each case and the corresponding maximum

U-seam gap and minimum U-seam overlap. Each case is analyzed in the followingsections.


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3.4.1.1 Analysis of Configuration A

Figure 16. U-Seam Configuration A Tolerance Analysis (1 of 2)


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Figure 17. U-Seam Configuration A Tolerance Analysis (2 of 2)


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Table 2. Tolerance Analysis of Configuration A

Component Nominal (in)Cover 18.8860 0.0150

Nominal Gap -0.0260 0.0000

U-Seam Width 0.0830 0.0060Material Thickness 0.0320 0.0010

Chassis (Inside Wall to Inside Wall) -19.0630 0.0200

Material Thickness 0.0320 0.0050

U-Seam Width 0.0830 0.0060

TOLERANCE DIMENSION= 0.0270 RSS (3)= 0.0269

Min Gap= 0.0001

Max Gap= 0.0539

Min Width= 0.0770

Max Width= 0.0890

Worst Case U-Seam Width (in) 0.0890

Worst Case U-Seam Width (mm) 2.26

Component Nominal (in)I length (Cover) 0.3890 0.0060

Material Thickness 0.0320 0.0010

U-Seam Depth -0.4210 0.0060

Overlap Dimension 0.3640 0.0060

TOLERANCE DIMENSION= 0.3640 RSS (3)= 0.0104

Min Dim= 0.3536

Max Dim= 0.3744

Worst Case Overlap (in) 0.3536

Worst Case Overlap (mm) 8.98

Tolerance (in)

U-WIDTH AND GAP ANALYSIS

OVERLAP ANALYSIS

Tolerance (in)


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3.4.1.2 Analysis of Configuration B

Figure 18. U-Seam Configuration B Tolerance Analysis (1 or 2)


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Figure 19. U-Seam Configuration B Tolerance Analysis (2 or 2)


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Table 3. Tolerance Analysis of Configuration B

Component Nominal (in)Cover 18.1030 0.0300

Chassis (Right Hand "I" feature) 0.3930 0.0060

Chassis (Inside Wall to Inside Wall) -19.0200 0.0200Chassis (Left Flat Wall) 0.4690 0.0060

U-Seam Width 0.0790 0.0060

TOLERANCE DIMENSION = 0.0240 RSS (3)= 0.0375

Min Gap= 0.0000

Max Gap= 0.0615

Min Width= 0.0730

Max Width 0.0850

Worst Case U-Seam Width (in) 0.0850

Worst Case U-Seam Width (mm) 2.16

Horizontal Overlap 0.3620

Minimum (Inches) 0.3245

Component Nominal (in)I length (Cover) 0.3920 0.0060

Material Thickness -0.0320 0.0010

U-Seam Depth -0.3610 0.0060

Overlap Dimension 0.3350 0.0060

TOLERANCE DIMENSION = 0.3340 RSS (3)= 0.0104

Min Dim= 0.3236

Max Dim= 0.3444

Worst Case Overlap (in) 0.3236

Worst Case Overlap (mm) 8.22

Tolerance (in)

Tolerance (in)

U-WIDTH AND GAP ANALYSIS

OVERLAP - HORIZONTAL ANALYSIS

OVERLAP - VERTICAL ANALYSIS


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3.5 General Design Considerations

Avoid discontinuities in U-seams, such as breaks or interruptions as much as

possible. These may result in resonance and leakage that adversely affect the

overall Shielding Effectiveness.

In most chassis designs, a U-seam needs to transition across a number of edges

and corners. This can make it very difficult to create a continuous, unbroken U-

seam when designing sheet metal components. It is therefore necessary to include

gaps or breaks in the U-seam typically in corners where the U-seam transitions

from one edge to another. These should be designed to minimize the break

created. Based on containment requirement of 10 GHz, a maximum break of

1 cm would typically not degrade U-seam performance, but should be kept to

0.5 cm in the corner areas due to the higher concentration of currents.

In a similar manner, the edge that fits into the U-seam may require breaks in the

corners to accommodate manufacturing requirements. These should be designed

to minimize the breaks created. Based on containment requirement of 10 GHz, a

maximum break of 1 cm at the corner would typically not degrade U-seam

performance.

Some designs require breaks in a U-seam edge to accommodate a tab, mount

bracket or clearance for a subassembly. These should be designed to minimize the

breaks created. Based on containment requirement of 10 GHz, a maximum break

of 1 cm would typically not degrade U-seam performance.

Care should be taken when incorporating tabs or brackets in a U-seam. The tab or

bracket can become a very efficient EMI radiator if its length approaches , , orfull wavelength of any frequencies radiating from the chassis enclosure.

U-seams may need to have a lead-in or chamfer (refer to Figure 13) to assist

alignment and assembly of the panel edge into the U-seam. Care should be taken

not to compromise the Waveguide gap by incorporating a chamfer or angling one

side of the U-seam to improve panel fit. Where used, a bent lead-in edge of a

U-seam is not considered when calculating the overlap of a U-seam because this

will diminish EMI performance.

Avoid paint at the U-seam interface. Paint will reduce the shielding effectiveness

and may cause assembly issues due to the added material thickness.

When designing U-seams on large metal covers, ensure that the cover is rigid

enough such that the U-seam will not partially disengage under normal conditions

due to bowing.


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3.6 Cost/Performance Tradeoff

A deeper U-seam design may prove to be slightly more expensive (due to slightly more

total material and possible tooling costs) than a more shallow design. However, the

deeper design provides more Shielding Effectiveness and perhaps enables the chassis to

contain higher frequencies for future applications. The deeper and narrower the U-seamdesign, however, the higher the potential for mechanical fit issues to occur because of

mechanical tolerances. Avoiding mechanical issues because of EMI over-design is an

important consideration in optimizing a U-seam design for overall quality and

performance.


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4 Design Requirements

The following section describes the basic U-seam design requirements for a system.

These requirements can be met with a number of design variations.

4.1 Compliance with EMC Requirements

EMC is regulated by different regulatory bodies in different geographies. For example,

the Federal Communication Commission (FCC) regulates emissions for products sold

in the United States. Products must comply with the EMC regulations in the

geographic areas where the product will be marketed and sold. It is therefore

imperative to ensure the system complies with the EMC regulations. Refer to your

product regulations group for current information about regulations. Typically, Intel

products comply with the following EMC regulations:

FCC, CFR 47, Part 15

CISPR 22 / EN55022

EN55024

4.2 Material Options

U-seams must be made of electrically conductive materials. Examples include zinc,

nickel, steel, and aluminum. Compatible metals must be used in the enclosure;

otherwise, galvanic corrosion may occur.Materials of various conductivity have not been evaluated at this time. Predictive

equations are based on data collected using zinc-plated sheet metal (electro-galvanized

cold-rolled steel) test panels. It is expected that metals with similar or higher

conductivity would result in similar performance; however, for materials that are

considerably less conductive, Shielding Effectiveness is likely to decrease.

Seam Design

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