Life test and electromagnetic shielding performance evaluation of EMI gaskets

MIL-G-83528B, ASTM 4935-89 and JSS 52100 are some of the specifications used in evaluating the performance of EMI/RFI/EMP gaskets. These gaskets are used to maintain the electromagnetic (EM) shielding integrity in shielded enclosures. A number of test methods are being developed and evaluated to measure accurately EM shielding or volume resistivity of the test samples before and after life test. The measurement methods for volume resistivity given in specifications are standard and accurate. Unfortunately, measurement methods for EM shielding has its own quirks and there is a large variation in shielding performance data from method to method for the same type of EMI gaskets. Besides, measurements procedures and the interpretation of test results have their own problems. It is very difficult to get accurately repeatable results using the same EMI gasket not only in various techniques but also in the same technique in the same test conditions and configurations. Therefore, the use of an EMI gasket on the basis of measurement results in a practical application may cause severe interference problems. In this communication, an attempt has been made to critically analyze all available methods for shielding effectiveness (SE) measurement of EMI gaskets. The comparative experimental results of SE for various types of EMI gaskets being tested using the methods given in modified MIL-STD-285 (MIL-G-83528B)/ JSS 52100 and ASTM D-9,12,14 (co-axial holder) are presented in this paper. The error which may arise in the case of conducting plastic or similar materials due to lack of surface contact is highlighted. The requirement for a different type of SE measurement technique for the EMI gaskets is emphasized and suggested.