Experimental investigation of the effect of reflow cooling rate on the IMC growth of SAC lead-free solder alloy

The present research is conducted to investigate a critical issue for Lead-free solder alloy. Near-ternary eutectic Sn-Ag-Cu alloys have been studied since they are the leading Lead-free candidate solders for various applications. There are three main phases in the near-ternary eutectic alloys: β-Sn rich phase, Ag₃Sn and Cu₆Sn₅. Cooling rate is an important processing factor that affects the microstructure of these alloys and then significantly influences mechanical behavior of the Sn-Ag-Cu solder joints. It is demonstrated that the amount and size of large Ag₃Sn plates increase with decreasing of the cooling rate. When large Ag₃Sn plates present in the solder joints at the lower cooling rate, they may affect the mechanical integrity of the joints by providing a crack initialization during the mechanical testing under room-temperature condition. In the present paper, the effects of cooling rate on the microstructure and mechanical properties are studied on Sn-3.8Ag-0.7Cu solder ball, including shear strength and ball pull test. There are two kinds of fracture mode for Ag₃Sn plates preformed mechanical loading in room-temperature condition. One is brittle fracture inside Ag₃Sn plate itself; the other is interfacial fracture of Ag₃Sn plates and the IMC layer. Moreover, the fractures of large Ag₃Sn plates induce the decrease of mechanical properties on Sn-3.8Ag-0.7Cu solder ball. The critical cooling rate of large Ag₃Sn plate formation is also investigated.