Evaluation Technique for EMI Seams

Abstract

For metallic enclosures electromagnetic inter­ ference (EMI) leakage through the walls of the enclo­ sure is negligible compared with that via discontinui­ ties in the enclosure, particularly via the enclo­ sure's nonpermanent seams. The EMI characteristics of seams can be quantitatively specified in terms of the transfer impedance of the seams from which the shielding effectiveness (SE) of the seam may be calcu­ lated. Unfortunately, for the EMI design engineer, there is a lack of available data on transfer imped­ ance of seams. Some manufacturers of EMI gaskets are beginning to supply typical transfer impedance data on their gaskets. However, the gasket is only part of the seam; quantitative data is not supplied on the important effects of metal-to-metal contact in the final seam, or the effects of metal surface finish/ plating, area of joining surfaces, bonding pressure, fastener spacing, etc.