Contact resistance changes of silver, silver alloys, and gold plated silver coupons exposed to ozone

Abstract

The goal of this research program was to establish the effects of ozone on logic level switching performance and to select materials that would enhance the performance of these switches in the presence of ozone. Initially, four different material systems were examined through a series of experiments using a computer-controlled contact resistance probe. An electronically-controlled ozone chamber was specially designed for this research project. The four different material systems investigated were 85% silver 15% nickel, 90% silver 10% nickel, fine grain silver, and fine silver (.999 pure). Fine grain silver is produced by adding a minute amount of nickel to fine silver. These material systems were exposed to three different ozone levels (0.1 PPM, 7.0 PPM, and 20.0 PPM) and contact resistance data was collected. The ozone level of 0.1 corresponds to approximate ambient levels in switching devices. Higher concentrations of ozone are likely in panels where arcing occurs or in sealed devices. The higher levels of 7.0 and 20.0 PPM ozone used in this project are for accelerated testing purposes. The results of the experiments at ozone levels of 0.1 PPM were not conclusive within the test period, but ozone appeared to have some effect on the contact resistance of the coupons with a higher level of nickel content. Experimentation at 7.0 and 20.0 ozone accelerated the effects of ozone, observed during the experimentation at 0.1 PPM ozone, At 7.0 and 20.0 PPM ozone, the contact resistance data increased as the nickel content in the coupon increased. Although the coupons containing the higher nickel content were more susceptible to an increase in ozone, even the fine silver coupons were attacked. After the experiments on silver and silver-alloys were completed, a study was made to see if soft gold electroplating could offer protection from ozone damage. A set of contact resistance data was collected at 7.0 PPM ozone on fine silver and 85% silver 15% nickel coupons that were plated with 10 and 60 /spl mu/in thick gold. As expected, the contact resistance data for the coupons with a layer of electroplated soft gold were lower than the contact resistance data collected on the uncoated silver coupons. In addition, the contact resistance averages for the coupons with 10 /spl mu/in of gold plating are higher than the contact resistance averages for the coupons with 60 /spl mu/in of gold plating. Therefore, a noble metal protects the surface from the adverse oxidation effects due to ozone and thicker is better.