In-situ investigation of discontinuous precipitation in Sn-Bi low temperature solders

The mechanisms of precipitation in near-eutectic Sn-Bi alloys have been studied with in-situ electron backscatter diffraction (EBSD) and compared with microstructures in solder balls. Time-lapse SEM imaging revealed the occurrence of discontinuous precipitation (DP) with a two-phase front advancing into supersaturated β-Sn as either columnar or equiaxed cells. In-situ EBSD provided evidence that the migrating front changes the orientation of the tin phase and establishes a specific orientation relationship (OR) between the (Bi) lamellae and advancing β-Sn. In solder balls that solidified with multiple tin dendrites, DP initiated at tin grain boundaries and grew into the dendrites, changing part of their orientation. In solder balls with a single tin dendrite, the tin orientation change involved a change in OR or orientation variant. Thus, DP can increase the number of tin orientations and create tin grain boundaries which are known sites of stress localisation during thermal cycling of electronic components.