The Effects of Mechanical, Thermal, and Electrochemical Pretreatments for a Copper Substrate on the Porosity of a Gold Electrodeposit

Summary

An electroformed copper substrate was used to investigate the effects of pretreatment sequences which, while having the common feature of altering surface roughness, differed in the other changes of surface state produced. Pretreatments were mechanical abrasion and polishing, scratching and wire brushing, electrolytic polishing and etching, bright dipping, and chemical polishing. The basis metal was annealed at various stages in some process sequences. It was found that smoothing the surface by removing macro-asperities had a large effect in reducing porosity, whereas the effect of brightening by removing micro-asperities was relatively minor. Annealing reduced porosity, mainly, it seemed, through recrystallization increasing the substrate grain size on the surface. Although the causes of some effects were incompletely resolved, relief of residual stress by annealing after mechanical treatments is probably less important in reducing porosity than is substrate grain growth. Finally, it must be re-emphasized that sheet metal rolled from slab or ingot is, metallurgically, a much more complex material, on which the effects described here can be over-ridden with any treatment which removes metal. In particular, mechanical or electrochemical polishing can increase porosity when applied to such substrates, even though the surface becomes smoother.