Effect of copper addition on the microstructure and mechanical properties of lead free solder alloy

The microstructure and shear strength of a transition joint consisting of Sn-Ag-Cu solder alloy and Cu substrate were evaluated in reflowed condition. The results were compared with those obtained from a similar joint made with conventional eutectic Sn-Pb solder alloy. The reaction products at the interface are mainly Cu6Sn5 and Cu3Sn along with Cu6Sn5 for Sn-Pb and Sn-Ag-Cu to Cu joint, respectively. The shear strengths of the two systems are ∼55 and ∼68 MPa, respectively. The fracture takes place through the Cu6Sn5 intermetallic phase as well as through the solder alloy and the strength depends on the thickness of the reaction layer. The width of the brittle reaction layer at the interface for Sn-Ag-Cu to Cu transition joint is thinner with respect to Sn-Pb to Cu joint; hence the shear strength of the former is superior to the latter.