Mechanical properties degradation of Sn–37Pb solder joints caused by interfacial microstructure evolution under cryogenic temperature storage

The interfacial microstructure, shear property and fracture behaviors of Sn–37Pb joints after cryogenic temperature storage for different durations were investigated in this study, storage test at room temperature (298 K) and thermal aging test at 423 K were also carried out as comparison. The morphology of interfacial Cu₆Sn₅ IMCs for the joints stored under 77 K and 173 K transformed from scallop-like to column-like, moreover, the IMC layer thickness slowly increased with prolonged storage time. The growth rate of Cu₆Sn₅ layer in Sn–37Pb joint stored at 77 K was slightly faster as compared with that stored at 173 K. However, the morphology and thickness of Cu₆Sn₅ IMC layer still remained roughly unchanged after storage at 298 K for up to 40 days. The Cu₆Sn₅ IMC growth of Sn–37Pb joint stored at 77 K and 173 K was expected to be caused by Cu atom diffusion from Cu pad towards Sn–37Pb solder, which was induced by the stress gradient. Additionally, the shear force declined and fracture position changed from in Sn–37Pb solder to primarily in Sn–37Pb solder and partly along the interface with the prolonging of storage time at 77 K and 173 K, which were induced by IMC layer growth.