Abstract: An unified viscoplastic constitutive Anand model was utilized to describe the inelastic deformation behavior for SnAgCu solder alloy in PBGA package and to analyze the stress-strain response in the solder joints under thermal cyclic loading.The results demonstrated that the stresses and strains of corner solder joints were higher than those of inner solder balls.The equivalent plastic strain of the solder alloy near the IMC layer increased dynamically due to the properties of the brittle IMC layer.The strain energy represented in the stress-strain hysteresis loop of the IMC solder joints in high stress concentration region was higher than that of non-IMC solder.The simulation results revealed that the IMC layer significantly affected thermal fatigue reliability of the solder joints.The thermal fatigue lifetime using the Anand model was basically consistent with experimental data.