Download Transfer Function Analysis Of EM Shielded Enclosures

Transfer Function Analysis
Of
EM Shielded Enclosures
W. Michael King
Systems Design Advisor
www.SystemsEMC.com
1
Rev. 7/19/04
IEEE/EMC 2004 –Transfer Functions Analysis of EM Shielded Enclosures – All rights reserved
Prior to discussion of what occurs when currents
propagate onto and within EM shielded enclosures
(which are characterized by conducted impedances) it is
useful to recall the concepts of the EM wave impedance
structures as the sources of current excitation.
2
Observing The Impedance Structures of EM Waves #1
Graphics from “EMCT: Electromagnetic Compatibility Tutorial”
Used with permission from W. Michael King,
E. Pavlu, and Elliott Laboratories
3
Observing The Impedance Structures of EM Waves #2
Graphic from “EMCT: Electromagnetic Compatibility Tutorial”
Used with permission from W. Michael King,
E. Pavlu, and Elliott Laboratories
4
Evaluating EM Wave Structures To Practical Distances
Graphics from “EMCT: Electromagnetic Compatibility Tutorial”
Used with permission from W. Michael King,
E. Pavlu, and Elliott Laboratories
5
Reviewing The Applied Currents Into Shield Boundaries
Review of interposing media
impedances
Graphics from “EMCT: Electromagnetic Compatibility Tutorial”
Used with permission from W. Michael King,
E. Pavlu, and Elliott Laboratories
6
What Occurs In “Real” EM Shielded Enclosures…..
…..Once the Current As Sourced From EM Waves
Conducts Into And Through
The Enclosure Structure?
7
Infinite Sheet with Aperture
- Incident EM fields cause
currents I1 to flow on sheet
- Current flows around small
aperture inducing voltage V due
to resistance of a skin depth of
metal and inductance and
capacitance of U-shaped path
- Voltage V causes current I2 to
flow in local areas on other side
inducing electric field E2 and
magnetic field H2 (not shown)
- E2 maximum at center of slot,
H2 (Magnetic field) maximum
at ends of slot.
8
Apertures In Products Form “Arrays”
Graphics from “EMCT: Electromagnetic Compatibility Tutorial”
Used with permission from W. Michael King,
E. Pavlu, and Elliott Laboratories
9
Small Enclosure Behind Aperture
- Incident fields, external current I1
and Voltage V similar to Infinite
Sheet with Aperture (page 8)
- Enclosure reduces impedance across
rear of aperture and increases I2
- E2 smaller than for Infinite Sheet
- I2 larger than for Infinite Sheet
10
Impedance of Gasket Segments
Impedance of Gasket
Segments with 7” spacing
as measured by Transfer
Impedance Test Fixture
- Seven (7) inches is approximately ¼ wave length of 470 MHz.
- The measured impedance at 470 MHz was 22 Ohms.
- The voltage can be estimated (through extrapolation) across a slot that is ¼ wave
length long.
- The contents of the graph can also be used to estimate the voltage across any slots
or holes as a function of frequency (generated by the slot or hole) and induced
field strength.
11
Maintenance Cover
 Relatively high levels of shielding can be obtained without the
use of EMI gaskets.
 Gaskets can be cost effective by reducing the number of screws
needed to obtain the required shielding.
 Gaskets are generally required (or very cost effective) in shielding
for frequencies of 100 MHz and higher.
 Corrosion control should be an important consideration in the
selection of the EMI gasket.
12
Air Vent Filters
 Air Vent Filters are used successfully to allow air to penetrate an enclosure
and attenuate the EM Waves

The materials used for this purpose are (1) conductive screening, (2) perforated metal,
and (3) honeycomb panels
 Honeycomb panels are often preferred because of the high level of shielding
and small resistance to air flow

The panels are manufactured from brass, steel and copper foils, which are soldered,
welded or brazed together, and offer excellent levels of shielding (up to 140 dB)
 Aluminum honeycomb panels are often used because of the low cost for the
aluminum paneling.

The level of shielding offered by the basic aluminum honeycomb panels can vary from
very poor (less than 20 dB) to as much as 70 dB.

Non-conductive epoxy is used in manufacturing to hold the foil segments together.

The basic conductivity across the epoxy joints is obtained through incidental bridging
of the epoxy, which occurs when the honeycomb loafs are cut into panels.
13
Shielding of Aluminum Honeycomb Panels
 The shielding offered by aluminum honeycomb panels is a result of the
conductivity which occurs across the epoxy joint of the foils used to make
the panels.
 The plating (and processing) used to enhance the shielding offered by
basic Aluminum Panels is as follows:
1. Electroless Nickel plating (.002 inches thick)
- The panels offer as much shielding as the soldered or welded panels.
However, the cost to plate is extremely high.
2. Tin plating of the panels
- The process can add to the shielding supplied by the basic panel.
However, the cleaning process used during the plating process can significantly
reduce the level of shielding.
3. Chemical Film plating of the panels
- The use of the plating cannot improve the shielding and can result in a significant loss
of shielding due to the cleaning process used in plating the filters.
4. Patented Blending Process
- The blending process supplies a high reliable level of shielding to the basic panel
where the 2 panel versions offer as much shielding as the soldered or welded panels.
14
Shielding Effectiveness Test Results
of Shielded Aluminum Honeycomb Panels
 The level of shielding offered by aluminum honeycomb panel
material is a function of the conductivity across the epoxy joint
used to hold the panel foil segments together.
 The data below illustrates the level of shielding offered by 1/8 inch
cell by ¼ inch thick honeycomb panels, and a microscopic picture
of the foil to foil joint of the panel under test.
Epoxy Joint
Incidental Contact Points
Incidental contact points across the epoxy
joint are created when the honeycomb is
cut into panels.
Basic Aluminum Honeycomb Panel
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Shielding Effectiveness Test Results
of Shielded Aluminum Honeycomb Panels
Tin Plated Aluminum Honeycomb Panels
Blended Aluminum Honeycomb Panel
The tin plating process can remove the
incidental contact points.
Blending creates a continuous conductive
path across the epoxy joint.
16
Shielding Effectiveness Test Results
of Shielded Aluminum Honeycomb Panels
Solder provides excellent conductivity
between the brass foils.
Brass Honeycomb Panel
The nickel plating totally bridges the
epoxy joint.
Electroless Nickel Plated Honeycomb Panel (.002 inches thick)
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Performance Approximation of Waveguides “In Cutoff”
fc = 5910/W
fc = 6920/W
where,
fc = cutoff frequency in megahertz (MHz)
where,
fc = cutoff frequency in megahertz (MHz)
W = largest inside dimension of the
rectangular aperture of the
Graphics from “EMCT: Electromagnetic Compatibility Tutorial”
waveguide (in inches)
Used with permission from W. Michael King,
E. Pavlu, and Elliott Laboratories
W = inside diameter (in inches) of the
circular waveguide
18
Power and Signal Line Penetration of Enclosure
 Power and signal lines brought into a system are usually protected
by shields on the cables or terminated into an EMI filter at the
penetration.
 When EMI gaskets are used to reference the shields or filters to the
equipment chassis, care must be exercised to illuminate the loss
of the EM bond due to corrosion.
 The amount of shielding provided by a shield in a cable can vary
as much as 30 dB depending upon the conductivity of the gasket
used to reference the shield to the equipment chassis.
 When the lines are not shielded or terminated into EMI filters,
extreme care must be exercised to ensure EM coupling to and from
the wires is not excessive.
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Equipment Box with Shielded Cables
Gaps except at screws
(rubber gasket or anodized
aluminum surface)
- This design (without gasket) induces large magnetic fields H2 due to
concentrating the current in the bolts and large electric field E2 due to the voltage
drop along the inductance and contact resistance of the bolts.
20
Shielding Materials
For Optically Transparent Sheet Materials
 Copper Sheeting with etched square holes:

Often used on Airplane windshields for EMI and Lightning protection.
 Screening:

Usually .001 thick wire on .010 to .020 inch spacing.

It is recommended to fuse the wires together with tin plating.
 Conductive coatings:

Gold, Indium, and Indium Tin Oxide are the usual coating materials.
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Termination of Optically Transparent Sheet Materials
 Best Option:

The EMI shield is protected on both sides with glass.

The EM bond between the shield and chassis is positive and inexpensive to obtain.
EMI Shield
Glass
Chassis
EMI Gasket
 Good Option:

The EMI shield is inside the enclosure and not subject to scratching & abrasive cleaning.

The EM bonding block will require fairly close tolerance and is relatively expensive.
EMI Shield
Glass
EM bonding block
Chassis
EMI Gaskets
22
Termination of Conductive Glass
 Poor Option:

For Electrostatic Discharge Only.

The coating can be subject to scratching & abrasive cleaning causing a loss of shielding.
EMI Shield
Glass
Chassis
EMI Gasket
 Not Recommended:

The EMI screen shielding material is referenced to chassis using silver epoxy.

The silver can cause pitting of the chassis and loss of shielding due to galvanic corrosion.
EMI Shield
Chassis
Glass
Silver Epoxy
EMI Gasket
23
Loss of Surface Conductivity Due to Oxidation
Contact Resistance
(Average of Several Test Sites)
Results of a test conducted by IBM
and presented by G. Roessler of
Technit in a paper entitled
"Corrosion and the EMI / RFI
Knitted Wire Mesh Gasket."
Metal
Aluminum
Initial
Resistance After
Resistance (W ) One Year (W )
0.01 to 100
> 100
Nickel-Silver
0.01 to 100
> 100
Phosphor Bronze
0.01 to 100
> 100
Brass
0.01 to 100
> 100
Nickel
0.01 to 100
> 100
Beryllium Copper
0.001 to 0.01
> 100
Copper
0.001 to 0.01
> 100
Red Gold
0.001 to 0.01
0.01 to 100
Green Gold
0.001 to 0.01
0.01 to 100
Silver
< 0.001
0.01 to 100
Silver-Cadmium Oxide
< 0.001
> 100
Tin
< 0.001
0.001 to 0.01
Tin-Lead
< 0.001
0.001 to 0.01
Rhodium
< 0.001
0.001 to 0.01
Platinum
< 0.001
0.001 to 0.01
Platunum-Iridium
< 0.001
0.001 to 0.01
Gold
< 0.001
< 0.001
24
Loss of Surface Conductivity Due to Oxidation
Resistance
Measurements of
Selected Materials
Resistance (mW )
Material
Initial
400 HR
95% RH
1000 HR
95% RH
clad/clad
1.3
1.1
2.0
2024
clean only/clean only
0.11
5.0
30.0
6061
clean only/clean only
0.02
7.0
13.0
2024
light chromate conversion/same
0.40
14.0
51.0
6061
light chromate conversion/same
0.55
11.5
12.0
2024
heavy chromate conversion/same
1.9
82.0
100.0
6061
heavy chromate conversion/same
0.42
3.2
5.8
cadmium/cadmium
1.8
2.8
3.0
1010
cadmium-chromate/same
0.7
1.2
2.5
1010
silver/silver
0.05
1.2
1.2
1010
tin/tin
0.01
0.01
0.01
Copper
clean only/clean only
0.05
1.9
8.1
Copper
cadmium/cadmium
1.4
3.1
2.7
Copper
cadmium-chromate/same
0.02
0.4
2.0
Copper
silver/silver
0.01
0.8
1.3
Copper
tin/tin
0.01
0.01
0.01
Alum 2024
Steel 1010
Results of testing extracted
from paper “Corrosion
Control in EMI Design” by
Earl Grosshart of Boeing
Aerospace Company.
Finish
25
None
Mil-C-5541 Class 1A
Mil-C-5541 Class 3
Electroless Nickel
Cadmium Plated Bare
Cadmium Colored Chromate
Cadmium Clear Chromate
Chromium
Mil-C-5541 Class 1A
Mil-C-5541 Class 3
Electroless Nickel
Cadmium Bare
Cadmium Colored Chromate
Cadmium Clear Chromate
Chromium
Tin
Cadmium Bare
Cadmium Colored Chromate
Cadmium Clear Chromate
Nickel
Electroless Nickel
Chromium
Tin
Lead
Silver
Passivated
Cadmium (Passivated)
Tin
Passivated
Cadmium (Passivated)
Tin
Tin
Silver
Gold
Solder (Lead-Tin)
Silver Paint
Zinc Paint
Silver Adhesive
Carbon Adhesive
None
Nickel
GASKET
MATERIALS
Aluminum
Tin Plated
Monel
Silverelastomer
Stainless Steel
Beryllium Copper
A
A
C
E
C
C
A
A
D
C
C
C
A
A
D
C
C
C
D
D
A
D
A
D
A
A
D
C
C
C
A
A
D
C
A
C
A
A
D
C
A
C
A
A
A
A
A
D
A
A
D
C
D
C
A
A
D
C
C
C
D
D
A
D
A
D
D
A
D
E
D
C
A
A
D
C
A
C
A
A
D
C
A
C
A - Compatible
D - Compatible in controlled temperature
and humidity only.
A
A
A
A
A
D
A
A
D
A
A
C
A
A
D
C
C
C
A
A
D
C
A
C
A
A
D
C
A
D
D
D
A
D
A
D
D
D
A
D
A
D
A
A
A
A
A
C
A
A
A
A
A
C
D
A
D
X
D
D
X
A
A
A
A
D
C
A
A
D
A
C
A
A
D
E
D
C
A
A
A
D
A
D
C
A
A
D
A
C
A
A
D
E
D
C
A
A
A
D
A
C
A
A
A
D
A
C
X
A
D
A
A
C
X
A
D
A
A
C
A
A
A
D
A
C
X
D
D
A
A
D
X
A
D
X
X
C
D
D
D
A
A
C
Titanium
Miscellaneous
Copper Alloys
Corrosion
Resistant
Steel
High Nickel
and PH Steels
Carbon and
Alloy Steel
AISI-410
Aluminum 2000,
7000 Series
Aluminum
Clad, 1000
3000, 5000
6000 Series
Casting 356
Material Compatibility
Extracted from SAE Standard: “SAE, ARP-1481"
MATERIALS
FINISHES
D
D
D
A
D
C
D
D
A
D
A
C
C - Requires sealing if exposed to humid environment.
E - Requires sealing regardless of exposure.
X - Not usable.
D
D
A
D
A
C
26
EMF Potentials: Key To Stability
Graphic from “EMCT: Electromagnetic Compatibility Tutorial”
Used with permission from W. Michael King,
E. Pavlu, and Elliott Laboratories
27
Galvanic Corrosion of Mesh Gasket
 Galvanic Corrosion caused by Tin Plated Mesh Gasket against
Aluminum MCGS Frame.
System in Field Environment for 6 months. Photograph by Rockwell International.
28
Loss of Conductivity Due to Corrosion
 Silver Filled Elastomeric
Gasket loses conductivity
due to Sulfer Sulfide Coating
of Silver Particles
 Transfer Impedance Test Data of
EMI Gaskets Against Chemical
Film Plated Aluminum
New
Aged 1 Year
(stored on shelf)
Silver Filled Silicon Elastomer
30 lb. force
Reading taken by Peter Madle at
TRW and reported at 1978 IEEE
EMC Symposium Atlanta, GA.
29
Adequate compression
is essential!
Graphics from “EMCT: Electromagnetic Compatibility Tutorial”
Used with permission from W. Michael King,
E. Pavlu, and Elliott Laboratories
30
Summary
 Recognition of the fundamental processes of EM wave structures
is essential background, required to evaluate enclosure
performance.
 Shielding mechanisms are found in the form of responses to
conducted currents that are propagated in the enclosure
materials and structures, as sourced from EM waves.
 Shielding performance of EM shielded enclosures is related to the
transfer functions and impedance structures of the materials and
construction.
 Impedances of seams, gaps, and apertures typically constitute
the limitation of performance of EM shielded enclosures,
assuming appropriate selection of material.
 Stability of performance for gaps and seams is dramatically
impacted by compression pressures and EMF compatibility.
31
Selected References
1.
King, Michael W., EMCT: High Speed Design Tutorial (ISBN 0-7381-3340-X), Module 3,
Sections A and B, Published by Elliott Laboratories, Sunnyvale California, August, 2002.
2.
Archambeault, Bruce and Thibeau, Richard “Effects of Corrosion on the Electrical Properties of
Conducted Finishes for EMI Shielding,” 1989 IEEE, EMC Symposium, Denver, CO.
3.
Denny, Hugh W. & Shouse, Kenneth R., EMI “Shielding of Conductive Gaskets in Corrosive
Environments,” 1990 IEEE, EMC Symposium, Washington D.C.
4.
Groshart, E., “Corrosion Control in EMI Design,” 1975 IEEE, EMC Symposium, Texas.
5.
Kunkel, George, “Corrosion Effects on Field Penetration through Apertures,” 1978 IEEE, EMC
Symposium, Atlanta, Georgia.
6.
Kunkel, George, “Corrosion Effects on EMI Gasketed Joints” 1979 IEEE, EMC Symposium,
San Diego, CA.
7.
Lachmar, E.B. “Corrosion of MGCS Frame at EMI Gasket Interfaces” Rockwell International
Internal Letter, April 12, 1983.
8.
Lin, W.W; Reilly, J.J., Lee, H.J. & Brown, R.J. “Preliminary Corrosion Study of EMI Gaskets for
Avionic Systems,” Naval Air Development Center, September 1987.
9.
Reinhold Publishing Corporation “CORROSION, Special Report,” M/DE Magazine, January 1963.
10. Roesseler, George D., “Corrosion and the EMI/RFI Knitted Wire Mesh Gasket,” Frequency
Technology, March 1969.
11. Cain, Bruce L., “EMI/RFI Gasketing for Tactical Shielded Door Seam Applications”, U.S. Army
Construction Engineering Research Laboratory.
32
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