Comparison of high-speed shear properties of low-temperature Sn-Bi/Cu and Sn-In/Cu solder joints

To study the high-speed shear properties of low-temperature solders, we select four groups of samples of Sn-xBi (x = 22, 30 wt%) and Sn-xIn alloys (x = 18, 22 wt%) with the melting temperature ranging from 190 to 200 degrees C. High-speed shear test results show that the shear strength of Sn-Bi/Cu solder joints is much higher compared to Sn-In/Cu solder joints, but their displacement is much lower. Consequently, the shear energy of Sn-Bi solders is less than half of the shear energy of Sn-In solders. This variation can be attributed to various fracture modes controlled by alloying element's characteristics. Sn-xBi/Cu solder joints containing beta-Sn, intrinsic brittle Bi phase and Cu6Sn5 exhibit brittle interfacial fracture, while Sn-xIn/Cu solder joints containing single gamma-InSn4 phase and Cu6(Sn, In)5 show ductile fracture (e.g., Sn-22In/Cu) and part of mixed fracture (e.g., Sn-18In/Cu). This reveals that toughness of soft gamma-InSn4 phase is much better than that of microstructure containing beta-Sn and intrinsic brittle Bi phases. Furthermore, for Sn-Bi solders, the quantity of brittle Bi-rich phase in the fracture surface is proportional to Bi content in alloys, leading to lower strength of Sn-30Bi/Cu solder joints. While for Sn-In solders, compared to 18In/Cu, 22In/Cu solder joints contain more Cu6(Sn, In)5 IMCs in the matrix and thicker interfacial IMCs, it still exhibits higher shear energy under high-speed shear tests. This may be attributed to the fact that increasing In content not only makes the matrix softer but also coordinates with the deformation of interfacial IMC. The study provides a guidance for the development of low-temperature solder alloys with superior toughness.