| Quality data is shown below for the following Spira products: |
Why We Use Transfer Impedance Testing
There are two industry-accepted methods for testing the shielding capability of EMI gaskets. These are: (1) Shielding Effectiveness (as defined in MIL-G-83528), and (2) Transfer Impedance as defined in SAE ARP-1705 Rev A.
The Shielding Effectiveness test was designed by one of the EMI gasket manufacturers where there exists built in test anomalies, which provide extremely high levels of shielding for the products under test. As an example, tests performed at 2 GHz illustrated “Shielding Effectiveness” test results in excess of 120 dB using no gasket between the enclosure and cover specified in the test procedure. Placing a sheet of non-conductive newspaper as a gasket between the cover and shielded enclosure provided 93 dB of shielding.
“Transfer Impedance” as a method of testing EMI gasket was derived from the academic community. Transfer Impedance measures the impedance of a gasket and gasketed joint. The procedure for measuring transfer impedance of gaskets (or gasketed joints) is detailed in SAE, ARP-1705 Rev A dated January 1, 2006.
The test data that follows has been done using Spira’s ZT-1000 Transfer Impedance Test Fixture, a spectrum analyzer, and a printer.
Shielding Quality
Shielding quality as defined in SAE, ARP-1705 Rev A is the theoretical “Shielding
Effectiveness” of the gasket and gasketed joint using the conditions
and constraints of shielding theory and MIL-STD-285. The definition as
listed in ARP-1705 Rev A is illustrated below. The shielding quality data
illustrated in the pages that follow assume that the incident wave is a
plane wave having an impedance of 377 ohms.
|
 Zw
using constraints consistent with MIL-STD-285 |
* We use 377 as the wave impedance for all calculations to simplify computation.
Joint
Surfaces Are Important
In the data shown on the following pages we have illustrated the transfer
impedance (and subsequent Shielding Quality) of the gaskets against two
surfaces: tin plated aluminum and 6061-T6 aluminum. This is presented to
illustrate the importance of selecting a gasket that will provide the required
shielding using the specific joint surfaces of concern.
Test Conditions
All test data consists of new gaskets and freshly cleaned joint surfaces
in a controlled temperature, low humidity office environment. Aging and/or
environmental conditions may deteriorate the level of shielding provided
by a gasket in conjunction with specific joint surfaces.
Shielding Quality of Spira-Shield Gaskets
| This data applies to Spira-Shield tin plated beryllium copper gaskets, or other gaskets which include it, such as the following standard configurations: | |
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1. Standard Force Spira-Shield, Tin Plated Beryllium Copper Gasket,
Example: SS-04, IWSS-04 2. Moderate Force Spira-Shield, Tin Plated Beryllium Copper Gasket, Example: MS-04, IWMS-04 3. Low Force Spira-Shield, Tin Plated Beryllium Copper Gasket, Example: LS-04, IWLS-04 |
Shielding Quality
of Quick-Shield Gaskets
| This data applies to Quick-Shield stainless steel gaskets, or other gaskets which include it, such as Quick-Strip. |
 |
1. Standard Force Quick-Shield, Stainless Steel Gasket, Example:
NI-04 |
Shielding Quality
of Ultra Quick-Shield Gaskets
| This data applies to Ultra Quick-Shield tin plated stainless steel gaskets, or other gaskets which include it, such as Quick-Strip with Ultra Quick-Shield. |
 |
1. Standard Force Ultra Quick-Shield, Tin Plated Stainless
Steel Gasket, Example: TNI-04, IWNI-04 |
Shielding Quality
of Flexi-Shield Gaskets
| This data applies to Flexi-Shield
or other gaskets which include it, such as Flexi-Shield O-Rings (see graph, line 1). |
 |
1. Standard Force Flexi-Shield, Tin Plated Beryllium Copper
Gasket, Example: ST-06, IWST-06 |
This data is typical of
the data we have obtained. Results will vary depending on the application.
Shielding Quality
of Endur-o-Shield Gaskets
| This data applies to Endur-o-Shield or other gaskets which include it, such as the following standard configurations: | |
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 |
1. Standard Force Enduro-Shield, Tin Plated Beryllium Copper
Gasket, Example: SISS-04, IWSISS-04 |
This data is typical of
the data we have obtained. Results will vary depending on the application.
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