Improvement of Thermal Fatigue Properties of Sn-Ag-Cu Lead-Free Solder Interconnects on Casio's Wafer-Level Packages Based on Morphology and Grain Boundary Character

Thermal fatigue properties of commercial LF35 (Sn-1.2Ag-0.5Cu-0.05Ni), SAC105 (Sn-1Ag-0.5Cu), and SAC305 (Sn-3Ag-0.5Cu) solders on Casio's wafer-level packages are discussed from the viewpoints of both morphology and grain boundary character. Orientation imaging microscopy revealed that both LF35 and SAC305 resisted the coarsening of tin grains during thermal fatigue as compared with SAC105, correlating with their greater fraction of coincidence site lattice boundaries. This seems to explain why LF35 has superior thermal fatigue life in spite of its lower silver content.