Study on low-Ag content Sn-Ag-Zn/Cu solder joints

The eutectic Sn-Ag-Cu solder is the most popular lead free solder. But reliability and cost issues limit its application. On the other hand, Sn-Ag-Zn system has many advantages comparing with Sn-Ag-Cu. In this paper, interfaces of Sn-xAg-1Zn/Cu and Sn-2Ag-xZn/Cu (x = 1, 2, 3), Sn-2Ag-2.5Zn/Cu and Sn-1.5Ag-2Zn/Cu solders joints were studied to understand effects of Ag and Zn contents. Results show that shearing strength of as-reflowed Sn-2Ag-2Zn/Cu and Sn-1.5Ag-2Zn/Cu joints is higher than other joints. Because of the strong Cu-Sn reaction and the formation of Ag3Sn, the Sn-Ag-Zn series solder joints are not suitable for use above 150 °C temperature. After 250 °C soldering for 4 h, while the Zn content increased from 1 wt% to 2 wt%, the interfacial IMC of Sn-Ag-Zn/Cu altered from Cu6Sn5 to Cu5Zn8. The Cu5Zn8 interface has higher shearing strength than Cu6Sn5 interface. Relationships among microstructure, strength and aging condition are discussed.