seam design
TRANSCRIPT
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August 2001
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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.
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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
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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:
U Width 1.5 mm; L Thickness 1 mm; Avg. Gap 0.25 mm; Overlap 7 mm; Total Seam
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:
U Width 1.5 mm; L Thickness 1 mm; Avg. Gap 0.25 mm; Overlap 7 mm; Total Seam
Length 1000 mm
Option 2 detail:
U Width 2.0 mm; L Thickness 1 mm; Avg. Gap 0.5 mm; Overlap 7 mm; Total Seam
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)
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:
U Width 1.5 mm; L Thickness 1 mm; Avg. Gap 0.25 mm; Overlap 7 mm; Total Seam
Length 1000 mm
Option 2 detail:
U Width 1.5 mm; L Thickness 1 mm; Avg. Gap 0.25 mm; Overlap 6 mm; Total Seam
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)
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.