Formation Mechanisms of Thermal Fatigue Crack Networks in Sn-Ag-Cu Die-attach Joint in High Speed Thermal Cycling

抄録

To investigate the mechanisms by which fatigue crack networks form in die-attach joints in power semiconductors, high-speed thermal cycling test was performed using a Si/Sn-Ag-Cu/Si specimen and the formation process of fatigue crack networks in the solder layer was observed. Fatigue cracks were found to emerge around intermetallic compounds in the β-Sn dendrite boundaries or from the high-angle (high-Σ) grain boundaries of β-Sn generated by continuous dynamic recrystallization. In all of these cases, the subsequent cycles caused the individual cracks to propagate in a cross shape and to become connected to each other, resulting in the formation of fatigue crack networks. Finite element method (FEM) analysis confirmed that the solder layer was in a state of equibiaxial tensile and compressive creep in the direction parallel to the joint surface of the solder layer during high-speed thermal cycling. FEM analysis also indicated that equibiaxial tensile creep is the driving force behind fatigue fractures. FEM analysis results for the cross-shaped micro-cracks confirmed that equibiaxial tensile creep in the period of decreasing temperature caused the cross-shaped cracks to open and propagate. Further, these propagated cracks became connected to each other to form fatigue crack networks.