Keeping static discharge out of membrane switches

Aug. 4, 2005
Electric membrane switches often have pressure-sensitive adhesives securing an overlay on top of the switch.

But to get the most out of the adhesive, engineers usually call for a rough surface, giving the adhesive plenty of sites to form mechanical bonds. Unfortunately, these rough surfaces create networks of microscopic fissures, an open road for electric static discharge (ESD) to find its way to the switch and compromise its integrity. The usual fix is to add an ESD shield above the switch. It acts as an antenna, "grabbing" static charges and routing them to ground. But such shields add costs and production time.

At The Bergquist Co., Chanhassen, Minn. (www.bergquistcompany.com), engineers have developed a lower-cost alternative that relies on a polyester resin to seal the switch against ESD as well as thermal stress, solvents, and changes in pressure. Dubbed HeatSeal technology, the extra layers of resin leave no air gaps, microscopic fissures, or any other path into the circuitry. The HeatSeal resin withstands 135 kV of ESD without breaking down. Conventional PSA switches, in comparison, only withstand 15 kV.

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