Effect of primary creep and plasticity in the modeling of thermal fatigue of SnPb and SnAgCu solder joints

It has been conventional to simplify the thermo-mechanical modeling of solder joints by omitting the primary (transient) contributions to total creep deformation, assuming that secondary (steady-state) creep strain is dominant and primary creep is negligible. The error associated with this assumption has been difficult to assess because it depends on the properties of the solder joint and the temperature–time profile. This paper examines the relative contributions of plasticity, primary and secondary creep in Sn40Pb and Sn3.8Ag0.7Cu solders using the analysis of a trilayer solder joint structure with finite elements and a newly developed finite difference technique. The influences of temperature amplitude and ramp rate have been quantified. It was found that for the thermal profiles considered, the role of plasticity was negligible for trilayer assemblies with SnPb and SnAgCu solder interlayers. Furthermore, when primary creep was included for SnAgCu, the temperature-dependent yield strength was not exceeded and no plastic strains resulted. Neglect of primary creep can result in errors in the predicted stress and strain of the solder joint. Damage metrics based on the stabilized stress vs. strain hysteresis loop, for symmetric 5min upper/lower dwell periods, differ widely when primary creep is considered compared to the secondary-only creep model. Creep strain energy density differences between the secondary-only and primary plus secondary creep models for SnPb were 32% (95°C/min–Δ165°C thermal profile), 32% (95°C/min–Δ100°C) and 35% (14°C/min–Δ100°C); similarly for SnAgCu, the differences were 29% (95°C/min–Δ165°C), 46% (95°C/min–Δ100°C) and 58% (14°C/min–Δ100°C). Accumulated creep strain differences between the secondary-only and primary plus secondary creep models for SnPb were 21% (95°C/min–Δ165°C), 25% (95°C/min–Δ100°C) and 25% (14°C/min–Δ100°C); similarly for SnAgCu the differences were 82% (14°C/min–Δ100°C), 89% (95°C/min–Δ100°C) and 100% (95°C/min–Δ165°C). In turn, these discrepancies can lead to errors in the estimation of the solder thermal fatigue life due to the changing proportion of primary creep strain to total inelastic strain under different thermal profiles, particularly for SnAgCu.