Impact of tensile strength on thermal fatigue properties and failure modes of Sn-Ag-Cu-Ni solder joints

With the requirements of higher board level reliability, researchers gradually turned attention to complex composition solders like Sn-Ag-Cu-Ni-Bi solders. This quinary system solder has lower melting point and higher tensile strength than traditional Sn-Ag-Cu solder, which may good for the reliability. In this research, the effects of tensile strength on thermal fatigue properties of Sn-2Ag-0.5Cu-0.05Ni and Sn-4Ag-0.5Cu-0.05Ni-3Bi solders were studied. After 1000 thermal cycles, SEM observation and nanoindentation test were used to characterize the failure mechanism and mechanical properties. The results demonstrated that Sn-4Ag-0.5Cu-0.05Ni-3Bi solder joints had better thermal fatigue properties than Sn-2Ag-0.5Cu-0.05Ni solder joints. The tensile strength of Sn-4Ag-0.5Cu-0.05Ni-3Bi solder was higher, so it can endure more stress and have less deformation in thermal cycling. And with the diffusion of Cu/Sn couples, the modulus and hardness of Sn-2Ag-0.5Cu-0.05Ni solder near the IMC interface decreased a lot, which promoted the crack formation and finally caused the fracture at the bulk solder. While for Sn-4Ag-0.5Cu-0.05Ni-3Bi solder, it didn't decrease too much due to the Bi concentration effect at the interface, which led to the crack formation in IMC. Hence, solders with higher tensile strength, like Sn-4Ag-0.5Cu-0.05Ni-3Bi solders, may have better thermal fatigue properties.