Comparing Test Methods for EMI Gaskets

In this article, we compare the measuring shielding capability of EMI gaskets using the transfer impedance test method with the results obtained by using current shielding effectiveness test methods. This includes accuracy and the ability to test for the salient variables associated with the ability of an EMI gasket to seal electrical/electronic enclosures.

Overview

EMI gaskets are used by the electrical/electronic industry to create a “Faraday Cage” out of boxes (containers) made up of individual pieces of metal. The gaskets perform this task by electronically bonding the various metal pieces together. The ability of the gaskets on the market to bond the pieces together can vary by as much as 110 dB (800,000 times). The variables associated with the ability of a gasket to bond the metal pieces together are: 1) the conductivity of a gasket; 2) the surface conductivity of the gasket material; 3) the conductivity of the joint material; 4) the surface conductivity of the joint plating; 5) the force of the gasket against the joint surface; and 6) the environment in which the gasketed joints are expected to operate.

There are two basic methods used to grade EMI gaskets. These are transfer impedance and shielding effectiveness. Transfer impedance has been shown to be accurate to within +/‑2 dB over the frequency range of 10 kH_z to 18 GH_Z. The method can be used to grade all of the variables listed above, and has a dynamic range of greater than 110 dB (see Figure 1). Shielding effectiveness testing has a fairly large number of methods employed to grade EMI gaskets. Some of the methods are more accurate than others.

The test method employed by the EMC gasket manufacturing industries is contained in MIL‑DTL‑83528C. When this method of test was first proposed in MIL‑G‑83528, members of the IEEE EMC Society TC‑4 Technical Committee on Interference Control performed a test on a newspaper at 2GHz. The results (along with the results from other test laboratories) indicate that the results of the test method can be inflated by as much as 40 dB (see Figure 2).

Transfer Impedance Testing

The transfer impedance test method is contained in SAE ARP‑1705 Rev B. Figures 3a and 3b show the test fixtures. As can be observed from Figure 3a, a known level of current enters the cavity between the base and contact plate. The current flows outward on the contact plate to the gasket under test. It then flows across the gasket and back to the input connector on the base plate. The voltage across the gasket (difference in voltage between the contact and base plate) is measured by a calibrated spectrum analyzer connected to the output connector. The impedance of the gasket is equal to the voltage across the gasket divided by the current through the gasket. The transfer impedance in Ω ‑m is equal to the impedance times the length of the gasket in meters. The variables listed above can be tested in the following manner:

1. The conductivity and surface conductivity of a gasket material can be measured by the selection of the gasket.
2. The conductivity and surface conductivity of the base and contact plates can be measured by the selection and use of the material and finish of the plates.
3. The effect force of a gasket against a specific joint surface has on the conductivity of a gasket and joint surfaces can be measured by knowing the force versus deflection of the gasket material under test. Controlling the deflection of the gasket by the height of the spacers used to control the comparison will yield the force information.
4. The effects an environment can have on a gasketed joint can be obtained by subjecting a set of Plates containing a gasket under test to the environment(s) of concern.

Shielding Effectiveness Testing

The shielding effectiveness test method of choice by the EMI gasket industry is contained in MIL‑DTL‑83528C. This method of test is a modified MIL‑STD‑285 test. The modifications are:

1. MIL‑STD‑285 stipulates that the transmitting antenna be aimed at a seam (EMI gasket under test). The modification has the transmitting antenna aimed at the center of a largo (26” x 26” x 3/8” thick) aluminum plate.
2. MIL‑STD‑285 stipulates that the receiving antenna interrogate the area behind the seam (gasket under test) for maximum field strength. The modification has the receiving antenna centered behind the large plate.

The modification of having the receiving antenna behind the large plate can result in an inflated level of shielding of as much as 40dB. The modification of aiming the transmitting antenna at the middle of the large plate instead of at the gasket under test obviously has an impact on the measured shielding.

Another area of concern is the force of the gasket on the joint surfaces. The shielding effectiveness test uses ¼‑20 bolts on 2.0 inch centers to clamp the test plate down on the gasket. This force can exceed 400 pound per inch versus a small fraction of that amount in actual application. This excess force can result in a significant error, producing an inaccurate dynamic range reading of 30 dB which greatly reduces the value and usability of the data.

A “slot aperture” shielding effectiveness test method that has proven to be accurate (see Figure 4) has a dynamic range of over 100 dB over the frequency range of 20 MHz to 18 GHz. The method can be used to test a gasket against any desired finish and to any compression force. There is a new shielding effectiveness test contained in SAE ARP‑6248 that shows promise for testing from 1.0 to 40 GHz. The dynamic range appears to be over 100 dB where the gaskets can be tested against any desired finish and force.

Summary

Transfer impedance testing of EMI gaskets is an accurate and inexpensive method of testing EMI gaskets. It has the ability to test for all of the variables the design engineering community needs for properly selecting an EMI gasket for a specific application.

There are three shielding effectiveness test methods that should be of interest. The shielding effectiveness test contained in MIL‑DTL‑83528C is the current test method of choice by the EMI gasket industry. Its test is from 20 MHz to 10 GHz and was originally designed to result in the most inflated levels of shielding of any of the test methods at the time of its use. Because of its limitations, it is considered by many design engineers to result in nearly worthless data. The slot aperture test produces accurate data over the frequency range of 20 MHz to 18 GHz. The method is not used by the EMI gasket industry as the data is not considered high enough to be used for promotional purposes. The new test method contained in SAE ARP‑6248 can be used to test over the frequency range of 1.0 to 40 GHz, should have excellent dynamic range and is relatively inexpensive to use.

Selected Bibliography

Freyer, Gustave J., “Comparison of Gasket Transfer Impedance and Shielding Effectiveness Measurements, Part I”, IEEE EMC International Symposium, Anaheim 1992

Hatfield, Michael O., “Comparison of Gasket Transfer Impedance and Shielding Effectiveness Measurements, Part II”, IEEE EMC International Symposium, Anaheim 1992

Kunkel, George M., “Measuring the Shielding Quality of Various Gaskets and Gasketed Joints”, A Demonstration, IEEE EMC Society on Product Compliance, Santa Clara, 1994 

IEEE, 1302 Standard, “IEEE Guide for The Electromagnetic Characterization of Conductive Gaskets in the Frequency Range of DC to 18GHz.”

MIL‑G‑83528, “Gasketing Material, Conductive, Shielding, Gasket, Electronic, Elastomeric, EMI/RFI

MIL‑DTL‑83528C, Gasketing Material, Conductive, Shielding Gasket, Electronic, Elastomer, EMI/RFI, General Specification

SAE, ARP‑1705, “Coaxial Test Procedure to Measure the RF Shielding Characteristics of EMI Gasket Materials.”

SAE, ARP‑6248 Draft, “Stripline Test Method to Characterize the Shielding Effectiveness of Conductive EMI Gaskets up to 40GHz.