A novel viscoplastic model was developed for simulating the constitutive behavior of SAC305 (Sn3.0Ag0.5Cu) solder. The model incorporates dynamic recovery functions in the kinematic hardening rule for modeling the cyclic-softening behavior of the solder. The unified viscoplastic model was discretized by following the backward Euler integration scheme and implemented as a user-defined material subroutine (USERMAT) in ANSYS. Validation of the numerical model was conducted by simulating the solder rod responses under either strain-or stress-controlled cycling and compared to experimental measurements. It was shown that the numerical simulation is capable of predicting the softening response under cyclic straining and the ratcheting response under cyclic stressing. An ANSYS model for wafer-level package (WLP) under board-level temperature cyclic condition was also developed to simulate the ball grid array (BGA) solder joint response. From the simulation, the viscoplastic strain energy density accumulation over one temperature cycle was identified as a feasible parameter for evaluating the thermomechanical reliability of the of solder joints in electronic assembly.