Wettability and Calculated Work of Adhesion of Bi-Containing SAC305 and SAC405 Solders on ENIG and Cu-OSP Finishes

Abstract

Reliable solder joint formation in electronic packaging depends on the coupled effects of solder composition and pad finish on molten solder wetting, yet direct comparisons of Bi-modified SAC305 and SAC405 solders on common surface finishes remain limited. In this study, nominal SAC305- and SAC405-based solders with Bi additions up to 3 wt.% were evaluated on ENIG and Cu-OSP finishes at 250 °C under forming gas. Sessile drop testing was used to measure contact angles, and atomic force microscopy and surface free energy measurements were used to characterize the substrates. The work of adhesion was calculated using the Young–Dupré relationship from contact angle and liquid surface tension values. For both alloy series, contact angle generally decreased with increasing Bi content, indicating improved spreading, with a stronger effect for SAC305 on Cu-OSP. ENIG produced lower contact angles and higher calculated work of adhesion than Cu-OSP. SAC405-based solders generally showed higher calculated work of adhesion than SAC305-based solders because of their higher liquid surface tension. However, the lowest contact angle did not always correspond to the highest calculated work of adhesion. These results show that Bi content, Ag level, and pad finish jointly influence the apparent wettability and calculated solder–substrate interaction of SAC-Bi solders.