Relative rates of nickel diffusion and copper diffusion through gold

Abstract

The low‐temperature diffusion of nickel through gold has been evaluated using Auger electron spectroscopy. Samples used were printed‐wiring‐board fingers having a nickel ’’diffusion barrier’’ plated between the copper substrate and the cobalt‐hardened gold contact surface. A coefficient was determined for diffusion via the gold defect structure at 150°C using both first‐arrival and surface‐accumulation approaches. Both methods yield comparable results with a coefficient D′ of (2–7) ×10⁻¹⁵ cm²/sec. This is between one and two orders of magnitude smaller than that determined for copper under nearly identical conditions. Results of other investigations are compared and contrasted to the present findings. The surface reaction of nickel with air and chlorine‐containing environments is sufficiently rapid that diffusion is the rate‐limiting step in the accumulation of nickel at the gold surface.