Abstract: On the basis of the thermal-elastic-plastic theory, an ANSYS finite element numerical method was employed to investigate the mechanical behaviors of PBGA SnAgCu lead-free solder joints under reflow soldering cooling process.The models and methods of transient thermal-structural coupling function and the finite element of life and death cell technology were performed to analyze transient temperature field and stress field.The large difference in the thermal properties led to the non-uniform stress and plastic deformation.The equivalent stress increased rapidly in the initial solidification of the solder balls.The peak equivalent stress occurred when the temperature of solder balls was reduced to room temperature.The simulation results showed that the crack failure is most likely to occur during solidification at the cooling stage of the soldering process.The results provided theoretical knowledge for the optimization of reflow soldering cooling process and the control of soldering stress to eliminate crack failure.