1972-02-01

Use of Conductive Elastomers in Hard Back Printed Circuit Switches 720513

Hard back printed circuit switches, also known as on-circuit switches, offer an opportunity for switch designs which are reduced in complexity and enhanced in reliability in single switch mode concepts. On-circuit switching also offers the opportunity for the consolidation of two or more switching modes into one assembly which can have reliability, cost, convenience, and styling advantages; as well as leading to simplification of connecting wiring harnesses.
While having the advantages noted, the on-circuit switching concept also has limitations. Failure of the printed circuit due to mechanical wear and/or electrical erosion is a major concern. Careful selection of switching modes must be made so that electrical load and endurance requirements do not exceed the ability of the device.
The Achilles Heel of the concept is at the interface between the movable switch contact and the stationary contact, which is integral with the printed circuit board. Cold filament current inrush in lighting circuits on “make”, and surges from inductive circuits on “break”, cause electrical erosion which combines with mechanical wear accumulated during endurance to tax the ability of the designer and manufacturer to produce an acceptable device for the user.
The limitations can be minimized and virtually eliminated by employing a new contact material known as Pressex. Pressex is a rubber-like compound which can be formulated to be an insulator in its free state and a highly efficient conductor when compressed.
This paper will discuss Pressex as a switching material, and describe how its use can overcome the limitations of conventional on-circuit switch design.

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