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.

The minimum pitch for flip-chip bonding is limited by the difference in the thermal expansion coefficient between the chip and the substrate, it is approximately 40 µm when solder is used for the bumps. This constraint can be significantly alleviated by applying a conductive paste to the bumps and bonding the chips together at low temperatures. Bumps made of conductive paste are typically formed via screen printing. By using the imprinting method, bumps and wirings can be formed with a narrow pitch and high aspect ratio. In addition, replica molds can be used repeatedly to reduce processing costs. To use the replica mold repeatedly, the silver paste embedded in the recessed pattern of the replica mold during imprinting must be completely transferred to the substrate and the initial state of the replica mold must be kept constant. Therefore, we investigated the transferability of the imprint technology by changing the process conditions during imprinting. Using a UV-curable silver paste as an imprinting process, we confirmed that UV-curing the silver paste before separating the replica mold from the substrate improved the transferability.