The rise of lead-free (Pb-free) solders has driven the need for reliable alloys suitable for the solder reflow process. Bismuth (Bi) is a typical Pb replacement in Pb-free solders to reduce the solder alloy’s melting temperature and enhance joint strength. This work focuses on the microstructural characterization and evolution of hypoeutectic and hypereutectic tin-bismuth (SnBi) alloys.

 

Our work shows that the commonly reported Bi particle phenomenon is precipitation driven by surface diffusion, in which the bismuth particles nucleate and precipitate at the exposed surface. We have continued this work on binary Sn-7Bi to investigate the precipitation mechanisms of different morphologies of Bi particles. Globular and plate-like Bi precipitates nucleate at different interfaces (particle/matrix or grain boundaries) and have vastly different coarsening mechanisms. These precipitated particles undergo redistribution after thermal annealing, which induces grain growth. We also present an extension of our study to the Sn70Bi system, where we combine our findings in the hypoeutectic SnBi alloy with those in the hypereutectic alloy.

 

With the presence of Cu6Sn5 and Ag3Sn intermetallic compounds (IMCs), we can better understand how Bi interacts with different phases during solidification. In this talk, we report on a systematic observation of Bi precipitation in Sn-7Bi and SAC-Bi solder alloys. The microstructure was characterized with electron backscatter diffraction (EBSD) and energy-dispersive spectroscopy (EDS), focusing on time-resolved imaging using scanning electron microscopy (SEM). Correlation of bismuth precipitation to the diffusion mechanisms at the grain boundaries, Cu6Sn5-Sn and Ag3Sn-Sn boundaries, and in the bulk will be discussed.