Nonlinear finite element simulations on the mechanical response of the isothermally aged lead-free solders

The mechanical response, material properties, and mechanical failures of the lead-free solder interconnects continuously evolves and changes due to the isothermal ageing process and during thermal cycling. Therefore, it is very important to include such evolution behavior in the finite element-based evaluations of the solder thermal fatigue life analysis. The current article aims to investigate the effect of the pre-isothermal ageing on the mechanical response of the thermally cycled SAC305 lead-free interconnects utilizing finite element analysis and literature-based material parameters. Additionally, this article explores the solder creep response using two common constitutive laws, Anand and Garofalo models. The simulation results showed that the solder mechanical behavior due to thermal cycling is strongly driven by the ageing process especially for shorter time of ageing. Nonetheless, this effect becomes less effective for prolonged ageing periods. Moreover, the numerical computations proved that the solder response becomes mainly dominated by secondary creep rather than viscoplasticity when the solders are exposed to longer ageing durations. Finally, a detailed discussion on the effect of the pre-ageing process on solder plastic strains and inelastic strain energy density and hence the thermal fatigue life is presented.