Microstructure and Mechanical Properties of Sn-8.55Zn-1Ag-XAl Solder Alloys

Abstract

The microstructure and mechanical properties of as cast lead-free solders including eutectic Sn-9Zn, Sn-8.55Zn-1Ag and Sn-8.55Zn-1Ag-XAl (X=0.01∼0.45 mass%) alloys were investigated. Microstructures of the Sn-Zn-Ag-XAl alloys consist of AgZn₃ compound, Zn-rich phase, Al-rich segregation and hypoeutectic phase. The addition of Al dramatically improves the 0.2% offset yield stress, Vickers hardness, ultimate tensile stress (UTS) and total tensile strain of the Sn-8.55Zn-1Ag-XAl alloys. The increase in the Al content of the Sn-8.55Zn-1Ag-XAl alloy from 0.01 to 0.45 mass% increases the average yield stress from 49.9 to 54.0 MPa (∼47.6 MPa for eutectic Sn-9Zn) and the Vickers hardness from 17.0 to 18.3 Hv (∼16.8 Hv for eutectic Sn-9Zn). The average values of UTS of the Sn-8.55Zn-1Ag-XAl alloys with X=0.01, 0.1, 0.25 and 0.45 mass% Al are 55, 58, 55 and 60 MPa, respectively, while those of the tensile strain are 47%, 52%, 58% and 45%, respectively (52 MPa and 50% for the eutectic Sn-9Zn alloy). Fracture mechanisms of the Sn-8.55Zn-1Ag-XAl alloys are correlated with the Zn-rich phase and Al segregation.