Interfacial reactions and reliability of Sn-Zn-Bi-XCr solder joints with Cu pads

In this paper, the interfacial reactions of Sn-8Zn-3Bi and Sn-8Zn-3Bi-0.3Cr solders with Cu pads and the growth of IMCs during long-term aging treatment were presented and discussed. Less than 3 mum Cu₅Zn₈ IMC layer was found at the Sn-8Zn-3Bi-(0.3)Cr/Cu during the reflow at 230degC for 1 min. During solid state aging treatment at 150degC for 4, 9, 16, 25 days, the Cu₅Zn₈ IMC grown up with the extension of aging time, some of the IMCs were scattered into the bulk solders, Zn-poor layer was observed in the Sn-8Zn-3Bi solder joints. In the case of Cr-containing solders, the thickness of Cu₅Zn₈ IMC growth was much thinner than that of in the Sn-8Zn-3Bi solder joints, and no significant Zn-poor layer was found in Cr-containing solders. The growth rate of IMCs at the Sn-8Zn-3Bi-0.3Cr/Cu interface was about 1/2 times than that of in the Sn-8Zn-3Bi solder. During long-term aging, solid phase transition happened in Cr-containing solders, which indirectly and effectively controlled the diffusion of Zn atom to the interface, and thus slowed down the growth rate of Cu-Zn IMCs. It was supposed that Cr-containing Sn-Zn-Bi solders performed better long-term solder joint reliability.