Pore corrosion model for gold-plated copper contacts

The research goal presented here is to model the electrical response of gold plated copper electrical contacts exposed to a mixed flowing gas stream consisting of air containing 10ppb H/sub 2/S at 30/spl deg/C and a relative humidity of 70%. This environment accelerates the attack normally observed in a light industrial environment (similar to, but less severe than, the Battelle class 2 environment). Corrosion rates were quantified by measuring the corrosion site density, size distribution, and the electrical resistance of a probe contact with the aged surface, as a function of exposure time. A pore corrosion numerical model was used to predict both the growth of copper sulfide corrosion product which blooms through defects in the gold layer and the resulting electrical contact resistance of the aged surface. Assumptions about the distribution of defects in the noble metal plating and the mechanism for how corrosion blooms affect electrical contact resistance were needed to close the numerical model. Comparisons are made to the experimentally observed corrosion-bloom number density, bloom size distribution, and the cumulative probability distribution of the electrical contact resistance. Experimentally, the bloom site density increases as a function of time, whereas the bloom size distribution remains relatively independent of time. These two effects are included in the numerical model by adding a corrosion initiation probability proportional to the surface area and a probability for bloom-growth extinction proportional to the bloom volume, due to Kirkendall voiding. The cumulative probability distribution of electrical resistance becomes skewed as exposure time increases. While the resistance increases as a function of time for a fraction of the bloom population, the median value remains relatively unchanged. In order to model this behavior, the resistance calculated for large blooms is heavily weighted by contributions from the halo region.