Abstract: Effect of Bi addition into Sn-1.0Ag-0.5Cu (SAC105) Pb-free solder on the interfacial microstructural evolution in BGA solder joints was studied, and the shear properties of solder joints during isothermal aging and thermal cycling were analyzed The results showed that 2wt.% Bi addition into SAC105 depressed the growth intermetallic compound (IMC) thickness at the interface of solder joints during thermal cycling, and also depressed the breaking of interfacial IMC layer caused by thermal mismatch of solder joints during the thermal cycle. However, Bi addition can also increase the shear strength of SAC105 solder joints during thermal cycling or isothermal aging, while the effect of thermal cycling treatment on shear properties on SAC105-2Bi solder joints is little. Compared with SAC105 solder joints, Bi addition changed the failure mode from ductile to brittle mode of SAC105-2Bi solder joints during thermal conditions. Therefore, Bi addition decreased the thermal cycling reliability of SAC105 solder joints.