Whiskers, hillocks, and film stress evolution in electroplated Sn and Sn-Cu films

Abstract

The growth of surface defects on lead-free tin electroplated films is believed to be a stress relief phenomenon. These observed defects include hillocks and whiskers that grow spontaneously after deposition. Previous work has shown that it is possible to plate pure tin and observe only hillock growth. Whisker growth, however, is observed with the addition of copper contamination to the electrolyte. In this work, pure tin and tin-copper films of increasing copper content were electroplated on phosphor bronze substrates. The stresses associated with plating these films and the evolution of stress over time were measured using cantilever beam deflection. The density and morphologies of hillocks and whiskers were measured as a function of time and are related to cantilever beam stress measurements. Results show an increase in long-term stress with increasing copper content. An initial decrease in the plating stress over the first 24 to 48 hours and an increase in relaxation rate with copper amounts of less than 0.0018 mol / L in the electrolyte was also observed. A transition in film behavior observed in copper concentrations between 0.0018 and 0.0057 mol / L, as characterized by an increasing as- plated plating stress, decreasing relaxation rate, and the appearance of whiskers. A change in hillock appearance was also observed in these concentrations. Results support a growth model where a source feeds material into a defect while the ratio of incoming material to the rate of grain boundary movement determines the types of defects observed.