The ageing physics of electrical contacts subjected to DC current

The ageing behaviour has been studied on a population of over 3000 connectors subjected to 1 kA DC for 10/sup 5/ hours. The metal combination in the contact interfaces was Aluminium to Nickel-plated (t=3 /spl mu/m) Copper. The contact resistances of the connectors were measured, and statistically evaluated. A number of contact surfaces were selected for metallographic examination and analysis. The findings were the following: The DC ageing behaviour of the connector type studied includes a slow ageing rate when the contact resistance is below 40 /spl mu//spl Omega/. Between 40 and 60 /spl mu//spl Omega/, the ageing rate increases dramatically for cathodic contacts, whereas it decreases for the anodic ones. Above 60 /spl mu//spl Omega/, cathodic contacts age even faster, including self-healing events. In this region, the anodic ageing rate approaches zero. In the transition region, the current density and temperature have increased to levels, for which onset of electromigration in cathodic a-spots by thermal diffusion can take place. In the high resistance region, electromigration will further increase the ageing rate. Due to the large differences in resistivity between Al/Ni intermetallics with very large and very small amounts of Ni, electromigration will only be triggered in cathodic contacts.