Contact Resistance Behavior of the 60 Pd-40 Ag Alloy in Tarnishing Environments

Bell Laboratories has recently approved the use of the 60% palladium-40% silver alloy as a replacement for pure palladium on the general purpose wire spring family of relays. In reaching this decision, many people at the Columbus, Ohio, location carried out exhaustive tests to compare Pd and this particular Pd-Ag alloy with respect to all relevant failure modes of electrical contacts. This paper reports on the tests relating to the tarnishing characteristics of this alloy and the resulting contact resistance behavior. Because the Bell System has many years of field experience with Pd that show generally satisfactory results, emphasis is placed on the relative performance of Pd-Ag and Pd in these tests. The results reported here are attributable to many individuals within Bell Labs who will be credited, but their findings are consolidated in one document in the interest of clarity. Coupons of Pd and Pd-Ag, as well as telephone type wire spring relays using various combinations of Pd, Pd-Ag and 22 karat Au, were exposed to tarnishing environments. Ellipsometry and Auger electron spectroscopy show limited sulfiding with slightly greater thickness on the Pd. Contact resistance measurements were made on the coupons using a Au probe and on the relays using a 22 karat overlay on one of the contact pairs. These measurements show no significant difference between the two materials. Relays from a telephone central office in New York City which were equipped with Pd, Pd-Ag and pure Ag in 1943 for a trial were studied in 1973. Data are presented which show that the Pd and Pd-Ag contacts are essentially equivalent.