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 β-Sn phase and establishes a specific orientation relationship (OR) between the (Bi) lamellae and advancing β-Sn. In solder balls that solidified with multiple β-Sn dendrites, DP initiated at β-Sn grain boundaries and grew into the dendrites, changing part of their orientation. In solder balls with a single β-Sn dendrite, the β-Sn orientation change involved a change in OR variants. Thus, DP can increase the number of β-Sn orientations and create tin grain boundaries which are known sites of stress localisation during thermal cycling of electronic components.