Microscopic observation of creep-fatigue crack propagation in lead-free solder

Abstract

Creep-fatigue crack propagation tests of lead-free solder were conducted under various loading waveforms. The waveforms adapted in displacement-controlled tests were four triangular waveforms: pp waveform having fast loading-unloading, cc waveform having slow loading-unloading, cp waveform with slow loading followed by fast unloading, and pc waveform with fast loading followed by slow unloading, and three waveforms with hold time: cc-h waveform having a hold time under tension and compression，cp-h waveform having a hold time under tension, and pc-h waveform having a hold time under compression. In load-controlled conditions, three waveforms are adapted: pp, cc-h and cp-h. Microscopic observation using SEM and EBSD was conducted near the crack tip region and on fracture surfaces. Plenty of microcracks were observed near the main crack tip on the surfaces of specimens, while almost all of them disappeared after removing the surface layer of about 0.4mm. On the removed surfaces, microcracks were observed near the main crack tip for unsymmetrical waveforms, cp, pc, cp-h, while no microcracks for symmetrical waveforms, pp, cc, cc-h. The existence of microcracks is responsible for crack acceleration under unsymmetrical waveforms. EBSD observation showed the formation of subgrains within original grains of Sn and eutectic phases near the crack tip, and the grain size decreased with increasing crack propagation rates. The grain average of GROD also decreased with increasing crack propagation rate, while that of KAM was nearly constant without respect to the crack propagation rate. Striations, fragmentation, and intergranular fracture facets are three main features of creep-fatigue fracture surfaces. The features of striations were clear for the cases of pp, cc, pc, while vague for the other cases. The spacing of striations was nearly equal to the crack propagation rate, supporting the linear summation rule of creep and fatigue crack propagation rates. Plenty of intergranular facets were observed for cp, cp-h. Fragmentation was abundant for waveforms including creep contribution.