Fatigue life estimation of SAC solder based on inelastic strain analysis using stepped ramp wave loading

The fatigue life of solder alloys for the slow-fast cyclic loading condition is much shorter than that for the fast-slow cyclic loading condition, despite the two loading conditions having almost the same inelastic strain amplitudes. This means that the inelastic strain amplitude is not the best parameter used for estimating the fatigue life which shows the strain rate dependency. Then, the authors have previously proposed a fatigue life estimation method for Sn-3.0Ag-0.5Cu (SAC) solder using the creep strain amplitude obtained from the inelastic strain analysis using the stepped ramp wave (SW) loading. In this study, the fatigue tests for SAC solder under 16 loading conditions at RT and the SEM observations for the fatigue failure surfaces were conducted to give experimental supports to both the strain analysis method and the life estimation method. The inelastic strain analyses for all the tests showed that the analysis results were well correlated with the aspects of the fracture surfaces, and that the creep strain amplitudes obtained from the inelastic strain analysis were highly correlated with the strain-rate-dependent fatigue lives of SAC solder at RT. Also it was found that the relational expression between the creep strain amplitude and the fatigue life formulated at RT could be used for estimating the fatigue lives at 398 K.