Thermal fatigue properties and grain boundary character distribution of lead-free Sn-1.2Ag-0.5Cu solder interconnects on WLP

Both drop reliability and thermal fatigue properties of modified-LF35, a commercial low-silver type solder, and Sn-xAg-0.5mass%Cu (x: 1 and 3 mass%) lead-free solders on Casio's wafer level packages were discussed. LF35 had superior drop reliability than SAC105 or SAC305. However, LF35 of a standard type and SAC105 showed the shorterr thermal fatigue life than SAC305. Hence, thermal fatigue properties of LF35 was improved by modifying its chemical composition. Both cracks, which were initiated by thermal strain, and grain-boundary damage due to grain-boundary sliding degraded the thermal fatigue lives. From a microstructural observation using orientation imaging microscopy (OIM), both modified-LF35 and SAC305 suppressed coarsening of tin grains due to thermal strain as compared with SAC105. Moreover, both modified-LF35 and SAC305 showed a larger amount of coincidence site lattice (CSL) boundaries than SAC105. It is suggested that not only smaller tin grains but also larger amount of CSL boundaries observed in modified-LF35 and for SAC305 suppressed crack propagation and grain-boundary damage.