A Study on Creep-Fatigue Crack Propagation in Sn-3.0Ag-0.5Cu Solder

Abstract

The crack propagation in Sn-3.0Ag-0.5Cu lead-free solder has been studied at creep-fatigue conditions. Strain-controlled push-pull low cycle crack propagation tests were performed using the fast-fast (pp), the fast-slow (pc), the slow-fast (cp) and the slow-slow (cc) triangular strain waveforms at 313K. Crack propagation rates in the pp strain waveform were well correlated with the cyclic J-integral range and fatigue J-integral range and those in the pc, cp and cc strain waveforms with the creep J-integral range. No evidence of creep-fatigue interaction was found in the correlation of the crack propagation rate with the J-integral ranges. The correlations are classified into two groups depending on the strain waveform. One is the cycle-dominant crack propagation in the pp strain waveform. The other is the time-dominant crack propagation in the pc, cp and cc strain waveforms. A fatigue-creep J-integral range parameter applicable to all the test conditions in this study has been proposed for correlating the crack propagation rates. In the FE analyses, larger equivalent creep strain was found in the time-dominant condition (cp strain waveform) as the crack propagates. Larger normal stresses at the crack tip are found in the cycle-dominant condition (pp strain waveform) in comparison with those in the time-dominant condition (cp strain waveform). This difference in the normal stress is increased as the crack propagates.Transgranular cracking was found in the cycle-dominant crack propagation, and intergranular cracking in the time-dominant crack propagation. The time-dominant crack propagation rate was higher than the cycle-dominant crack propagation rate when compared with J-integral ranges which resulted from the creep damage at the crack tip in the time-dominant conditions. Clear recrystallization around the crack was found in the cycle-dominant and the time-dominant conditions. The recrystallized grain size was smaller in the time-dominant crack propagation than in the cycle-dominant crack propagation. The cycle-dominant crack propagated in the direction of maximum normal stress macroscopically, but the time-dominant crack propagated in the direction of maximum shear stress which was discussed in relation to micro shear cracks formed at the crack tip.