Abstract: Cu/In-Sn-20 Cu / Cu solder joints were prepared using Cu particle composite solder In-Sn-20 Cu, and the influence of thermal cycling from -20 ℃ to 120 ℃ on the microstructure and mechanical properties of Cu/In-Sn-20Cu/Cu solder joints was investigated. The growth of intermetallic compounds (IMC) in the interface reaction zone, evolution of IMC in in-situ reaction zone and change of shear performance of the solder joint during thermal cycling were analyzed. The morphological characteristics of the solder joint fractures were thoroughly explicated. The results show that with the increase of thermal cycling cycle, the thickness of interfacial IMC continues to increase, and the main phase in the interfacial (diffusion) reaction zone is Cu₃ (In, Sn) phase. During 200-400 cycles of thermal cycling, the quantity of gray island-like Cu₃ (In, Sn) phases in the in-situ reaction zone of the solder joint increases, accompanied by the appearance of voids in the in- situ reaction zone. After 600 cycles, the volume of Cu₃ (In, Sn) phases increases and the voids increase. After 800 and 1000 cycles of thermal cycling, peeling of Cu₃ (In, Sn) phases and extensive void formation occur. With the increase of thermal cycles, the shear strength of the Cu/In-Sn-20Cu/Cu solder joint gradually decreases. The shear strength of the solder joint without thermal cycling is 28.8 MPa, decreasing by 23.2% to 22.1 MPa after 400 cycles. After 1000 cycles, the shear strength is the lowest of 12.4 MPa, which is 56.9% lower than that of the initial value. The fracture mode of the solder joint transforms from ductile fracture to brittle fracture.