Low-temperature diffusion of copper through gold

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Abstract
Low‐temperature (75–150 °C) diffusion of copper through gold is studied with Auger electron spectroscopy. Grain‐boundary or pipe diffusion measurements and calculations are discussed with boundary‐value‐problem assumptions. Electroplated cobalt‐hardened gold on electroplated copper printed‐wiring‐board fingers is used. In addition, contact‐resistance data is presented for different heating times and temperatures and a plot of CR values versus amount of surface film is shown. Chlorine, which evolves from the board material upon heating, reacts with copper which has diffused to the surface. This reaction provides a much better sink for the copper than oxidation. Diffusion coefficients are estimated which are several orders of magnitude larger than those previously reported for the Au‐Cu system and reasons are discussed. Because of the chlorine emanating from the board, data from accelerated tests, in which the printed wiring board is also heated to elevated temperatures, may give erroneous predictions when extrapolated using an Arrhenius plot. Results demonstrate that diffusion via the defect structure is not the rate‐controlling step in the accumulation of copper and/or copper compounds on the gold surface when exposed to normal atmospheres. The surface accumulation is limited by the growth kinetics of the surface film.