A new unified creep-plasticity constitutive model coupled with damage for viscoplastic materials subjected to fatigue loading

A new unified creep-plasticity (UCP) constitutive model coupled with damage is proposed for Sn-3.0Ag-0.5Cu (SAC305), one of the most successfully commercialized Pb-free solders. By calibrating the proposed damage model based on entropy generation, the fatigue issues are elucidated for SAC305 solder at different temperatures and strain rates. After implementing a user-defined material subroutine, finite element simulations with the proposed UCP-damage model are capable of predicting the strength deterioration as the macroscopic response in terms of load-deformation curves. More importantly, the UCP-damage model reveals the damage accumulation and evolution of viscoplastic materials sensitive to fatigue deformation at different temperatures and strain rates. By comparing the predictions with and without damage, the equivalent plastic strains are obviously different when evaluating the thermal cycling fatigue life of electronic packaging structures, which confirms the significance of the UCP model reasonably coupled with the intrinsic description of damage by the concept of entropy generation.