Tin Whisker Growth on Electronic Assemblies Soldered with Bi-Containing, Pb-Free Alloys

Abstract

With the introduction of environmental legislation, lead (Pb)-free materials have made their way into the electronics supply chain. As a result, tin whiskers may initiate and grow under specific conditions. Sufficiently long whiskers may cause in-field short circuit failures, which are a serious concern in many high-reliability applications such as aerospace, automotive, and medical. There are efforts in developing third generation Pb-free solder alloys to replace tin-silver-copper (Sn-Ag-Cu, or SAC). Bismuth (Bi) has been considered for inclusion in SAC alloys to reduce melting point and improve reliability. In this study, we examine whisker formation on small outline transistor (SOT) components assembled with SAC-Bi alloys after ambient temperature (25°C) or high temperature (85°C), high humidity (85% RH) storage. Four alloys were considered—Senju M42 (Sn-2.0Ag-0.7Cu-3.0Bi), Violet (Sn-2.25Ag-0.5Cu-6.0Bi), Sunflower (Sn-0.7Cu-7.0Bi), and SAC305 (Sn-3.0Ag-0.5Cu). The assemblies were finished with immersion tin (ImmSn) or electroless nickel immersion gold (ENIG), and some were intentionally contaminated prior to exposure. It was found that SAC305 tended to grow longer whiskers than SAC-Bi alloys, though more whiskers were found on the latter. Particle Stimulated Nucleation (PSN) of recrystallization of the β-Sn matrix, influenced by Bi precipitation, appears to influence dynamic recrystallization (DRX), the mechanism for whisker growth.