Experimental Investigation of the Effect of Reflow Cooling Rate on the IMC Growth of SAC Lead-Free Solder Alloy

Abstract

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.