IEEE Antennas and Propagation Society International Symposium, Boston, MA, July 8-13, 2001

Prediction of Shielding Degradation Arising from Variation in Contact Impedance of Inter-metallic Junctions

Lin Li and Omar M. Ramahi
University of Maryland
College Park, MD 20742


A major task of EMI shields is to maintain shielding effectiveness through proper enclosure design. In most cases, separable connections are necessary in shielding enclosure design, and consequently, inter-metallic junctions are introduced as a result of the use of gaskets, connectors, internal compartments, and ventilation panels in an enclosure. When two dissimilar metals are brought in contact with each other, there is a risk of corrosion. Since the shielding effectiveness of an enclosure depends on good electrical conductance and low-impedance paths across the metallic junctions, corrosive residues at these junctions could change the impedance, and subsequently, degrade the shielding performance of enclosure. The surface current flowing along the enclosure surface could be diverted and forced to flow through a different and unintended path that may have a larger loop area. As a result, higher emissions can be produced. Any slots or seams located in this unintentional current path may also be excited and produce higher emissions. This phenomenon is called corrosion-induced electromagnetic interference (CI-EMI). Furthermore, the increased impedance at junctions due to the corrosive residue increases the voltage drop at these junctions resulting in higher external radiation.

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