Andre M. Delhaise¹, Zohreh Bagheri¹, Stephan Meschter², Jeffrey Kennedy¹, Polina Snugovsky¹; ¹Celestica, ²BAE Systems

With the introduction of environmental legislation, lead (Pb)-free materials have made their way into the electronics manufacturing industry. Pb-free solder alloys can initiate and grow tin whiskers under specific conditions, which may cause in-field failures in many high-reliability applications such as aerospace, automotive, and medical.Bismuth (Bi) has been considered for inclusion in Pb-free solder alloys to replace the current tin-silver-copper (Sn-Ag-Cu, or SAC) industry standard. In this study, we discuss whisker formation after ambient temperature, high humidity (25°C/85% RH) storage, on small outline transistor (SOT) and chip capacitor components assembled with Bi-bearing alloys. Four alloys were considered – SAC 305 (Sn-3.0Ag-0.5Cu), Violet (Sn-2.25Ag-0.5Cu-6.0Bi), Sunflower (Sn-0.7Cu-7.0Bi), and Senju M42 (Sn-2.0Ag-0.7Cu-3.0Bi). The boards were finished with either electroless nickel immersion gold (ENIG) or immersion tin (ImmSn), and half of assemblies were intentionally contaminated with sodium chloride (NaCl) to study the impact of corrosion on Sn whisker formation.