Surface Precipitation and Microstructural Characterization of SnBi solder alloys

The transition to lead-free (Pb-free) solders has increased the demand for reliable alloys compatible with the solder reflow process. Bismuth (Bi) is a common Pb substitute in Pb-free solders, as it lowers the alloy’s melting temperature and enhances joint strength. This study investigates the microstructural characterization and evolution of hypoeutectic and hypereutectic tin-bismuth (SnBi) alloys.

The presence of Cu₆Sn₅ and Ag₃Sn intermetallic compounds (IMCs) provides critical context for understanding Bi interactions with various phases during solidification. This study presents a systematic investigation of Bi precipitation in Sn7Bi and SAC-Bi solder alloys, utilizing advanced microstructural characterization techniques. Electron backscatter diffraction (EBSD) and energy-dispersive spectroscopy (EDS) were employed alongside time-resolved imaging with scanning electron microscopy (SEM). The findings demonstrate the correlation between Bi precipitation and diffusion mechanisms at grain boundaries, Cu₆Sn₅-Sn and Ag₃Sn-Sn interfaces, as well as in the bulk alloy.

Our findings reveal that the commonly reported Bi particle phenomenon is precipitation driven by surface diffusion, where Bi particles nucleate and precipitate on exposed surfaces. Building on this understanding, we examined the precipitation mechanisms in binary Sn7Bi alloys, focusing on different Bi particle morphologies. Globular and plate-like Bi precipitates nucleate at distinct interfaces, such as particle/matrix boundaries or grain boundaries, and exhibit unique coarsening behaviors. Thermal annealing induces redistribution of these precipitates, which correlates with significant grain growth. We extend this analysis to the Sn70Bi system, integrating insights from the hypoeutectic and hypereutectic SnBi alloys.