The viscoplastic constitutive properties of Sn3.9Ag0.6Cu lead-free alloy are presented and compared with baseline data from eutectic Sn63Pb37 solder. Steady-state creep models are obtained from creep and monotonic tests at three different temperatures for both solders. Based on steady-state creep results and creep test data, a transient creep model is developed for both Pb-free and Sb37Pb solders. One-dimensional incremental model of the test setup is developed to simulate constant-load creep, monotonic, and isothermal cyclic mechanical tests performed over various temperatures, strain rates and stresses using a thermo-mechanical-microstructural (TMM) test system, developed by the authors. By fitting simulation results to monotonic testing data, plastic models are also achieved. The devoloped viscoplastic constitutive models are evaluated in a two-dimensional nonlinear Finite element analysis of a PBGA352 package under a −55°C∼125°C thermal cycling environment. The viscoplastic behavior of Pb-free solder is compared with that of eutectic Sn37Pb solder.

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