Effect of reflow profiles on the microstructure and shear properties of non-eutectic Sn-35Bi-1Ag solders

Due to the low cost and low melting temperature, Sn-Bi-based solder alloys are widely utilized, including eutectic Sn-58Bi, and non-eutectic Sn-35Bi-1Ag. The traditional rules for setting soldering temperatures originated from eutectic alloys, yet selecting the appropriate soldering temperature for non-eutectic alloys with two melting peaks remains to be further investigated. Here, we investigate the microstructures, voids, and shear properties of Sn-35Bi-1Ag at reflow temperatures ranging from 175 °C to 205 °C and reflow time duration ranging from 0 to 120 s. The results reveal that higher reflow temperature or extended reflow duration time effectively improve the rough surface morphology of the solder joints, enhance the homogeneous dispersion of Bi phase in the solder matrix, and increase the interfacial intermetallic compound (IMC) thickness varying from 0.9 to 3.0 μm. To control voids rate, adjustments to the reflow temperature and time are necessary to achieve the intermediate temperature value and time, such as the combination of 60 s at 175 °C with a minimum void ratio (0.6%). Furthermore, this sample exhibits the optimal performance under both low and high shear rates, potentially attributed to its relatively thin IMC and reduced voids. This work offers guidance for optimizing the reflow profile for the non-eutectic solder alloys.