2025 年 18 巻 p. E24-006-1-E24-006-7
Commercial solder alloys with Bi additions are increasingly used in electronics manufacturing due to their low melting point and strengthening effects. However, the solubility of Bi in Sn is highly temperature-dependent, and partial or complete dissolution of Bi precipitates at elevated temperatures may impact solder joint reliability. To investigate the thermal stability and mechanical response of Bi-containing solders at service temperatures, two alloys—Sn-1.5wt%Bi-0.7wt%Cu-0.05wt%Ni (S1) and Sn-3.5wt%Ag-3wt%Bi-2wt%Sb-0.8wt%Cu (S2)—were characterized by in-situ scanning electron microscopy (SEM), in-situ synchrotron powder X-ray diffraction (PXRD), and tensile testing over a range of temperatures. During heating, Bi phase dissolution was observed in the alloy with higher Bi content, leaving voids that remained upon cooling. Consequently, the alloy with the higher Bi concentration exhibited a pronounced decrease in ultimate tensile strength after heating. In contrast, the alloy with lower Bi concentration maintained a relatively stable microstructure and ultimate tensile strength (UTS) throughout the heating cycle. The effect of the changing concentration of Bi on the Sn phase crystal lattice was examined using synchrotron PXRD.
