Unified viscoplastic constitutive model for Sn3.8Ag0.7Cu solder

The time-independent plastic and time-dependent creep deformations of Sn3.8Ag0.7Cu solder are considered in a unified framework by using a state variable approach. A new viscoplastic constitutive model that includes the kinematic hardening behavior of solder is proposed in this paper. Back stress, which is used for characterizing the kinematic hardening behavior, is selected as the state variable for the proposed constitutive model. The model constants are determined from curve fitting constant strain-rate experiment results. A numerical model based on the new unified viscoplastic constitutive model was applied to simulate various loading histories and compared to experimental test results, and to numerical predictions obtained by using the Anand viscoplastic model.