Mass diffusion in polycrystalline copper/electroplated gold planar couples

Abstract

The Au/Cu system is common to electrical connectors and recent trends toward higher ambient temperatures and thinner gold electroplates focuses attention on a new failure mode. This mode is degradation of thin gold electroplates by mass diffusion of the base metal (copper) to the surface of the electroplate at low temperatures (less than 250°C). Therefore, diffusion experiments were conducted for polycrystalline copper/electroplated gold planar couples over the temperature range of 50° to 750°C. The chemical interdiffusion coefficients, ~D, were calculated using the Boltzmann-Matano solution on the concentration-distance profiles which were determined using an electron microprobe. Results of this study show that ~D is a small function of concentration, generally with a variation of less than a factor of 3 to 5. The correlation of the temperature dependence of ~D between 250° and 750°C with existing data is excellent. The data conform to an Arrhenius equation: ~D= 1.5×10⁻⁵ e^−23,600/RT At temperatures below 250°C the values of ~D deviate from this equation and below 150°C are significantly greater than would be predicted by extrapolating the high temperature Arrhenius equation. No correlation was found between electroplate thickness and diffusion rate.