Investigation of small Sn-3.5Ag-0.5Cu additions on the microstructure and properties of Sn-8Zn-3Bi solder on Au/Ni/Cu pads

The formation of intermetallic compounds and the shear strength of Sn-Zn-Bi solder alloys with various (0, 1, 3, 5 and 7 wt%) weight percentages of Sn-Ag-Cu were investigated on Au/Ni metallized Cu pads depending on the number of reflow cycles. In Sn-Zn-Bi solder joints, scallop-shaped AuZn3intermetallic compound (IMC) particles were found at the interfaces and in the solder ball regions, fine Bi- and needle-shaped Zn-rich phase were observed in the Sn matrix. After Sn-Ag-Cu additions, an additional Ag-Zn intermetallic compound layer was adhered to the top surface of the AuZn3layer at the interface and fine spherical-shaped AgZn3intermetallic compound particles were detected in the solder ball regions together with Bi- and Zn-rich phase volumes. After the addition of Sn-Ag-Cu, the shear strength of Sn-Zn-Bi solder joints increased due to the formation of the fine AgZn3intermetallic compound particles. The shear strengths of Sn-Zn-Bi and Sn-Zn-Bi/7 wt% Sn-Ag-Cu solder joints after one reflow cycle were about 44.5 and 53.1 MPa, respectively and their shear strengths after eight reflow cycles were about 43.4 and 51.6 MPa, respectively.