Mitigation of Tin Whisker Growth by Dopant Addition

Due to world wide Pb-free regulation for electroplated tin, whisker formation has returned as a long term reliability problem for tin coated electronic components. In addition the exact mechanism(s) responsible for Sn whisker growth mitigation by Pb were never clearly indentified, which makes the search for an replacement of Pb a difficult process. In this work the effects of In doping on tin whisker growth were investigated. In order to maintain Sn as a single phase material only small additions of In were incorporated, approximately 5–10 wt.% In. Indium was incorporated into Sn using a 100 nm over-plate of In on 1 μm thick Sn followed by heat treatments at 125°C and 160°C to permit diffusion of In into Sn. Control samples of pure Sn in the as-plated as well as 125 C heat treated conditions were also used. Whisker density results show a dramatic decrease of almost two orders of magnitude for the 160°C HT Sn-In sample. This is a new result in whisker mitigation studies, and we interpret it as a real effect of In, although further verification including the use of control samples are required. The segregation of dopants at the grain boundaries (GB) of tin, which might lead to reduced self diffusion of Sn, was investigated by performing molecular dynamic (MD) simulations on randomly added Pb atoms. Segregation of Pb clusters to GB was observed. While simulations with In dopant has not been conducted as yet, nevertheless such segregation may be one of several mechanisms responsible for the In effect.