An improved unified creep-plasticity model for SnAgCu solder under a wide range of strain rates

Based on the unified creep and plasticity theory, an improved constitutive model is proposed in this study to describe the uniaxial mechanical behaviour of Sn3.0Ag0.5Cu (SAC305) solder alloy subjected to a wide range of strain rates. In the usual service condition of electronic devices, the strain rates of solder material are far less than 1.0 s(-1) at which the creep deformation is dominant, especially at higher working temperatures. However, the strain rate could range from 1.0 to 300 s(-1) under drop impact in electronic packaging structures, which is drawing more attention due to lack of experimental data, especially on dynamic mechanical properties of lead-free solder alloys. In extreme impact conditions, the solder material may experience even higher strain rates. As different mechanisms dominate the respective regime of strain rates, the developed constitutive model is calibrated to be applicable to most of the strain rate regimes by properly considering the coupled effect of creep and plasticity. Moreover, the parameters in the proposed model are defined with clear physical meanings and reasonably determined by regression to the published experimental studies. Lastly, the developed model is compared with other constitutive models from the literature, including the power-law equation for creep deformation at low strain rates and the Johnson-Cook model for plastic deformation at high strain rates. It is concluded that the proposed model is more generalized and capable of predicting uniaxial mechanical behaviour of SAC305 solder at low, medium and high strain rates with reasonable accuracy.