Mechanism of the Delayed Growth of Intermetallic Compound at the Interface between Sn-4.0Ag-0.5Cu and Cu-Zn Substrate

A Cu-Zn wetting layer was very effective to depress the excessive growth of intermetallic compound (IMC). The effect of Zn addition to the Cu layer on the IMC growth and microvoid formation in the solder interface was similar to the effect of Zn addition into the Sn-Ag-Cu (SAC) solders. In this study, the mechanism of slow IMC growth at the SAC/Cu-Zn interfaces was investigated. As the aging time increased, Zn atoms accumulated at the Cu6Sn5/Cu interface and formed a Zn-rich layer. By adding Zn into the Cu wetting layer, the IMC growth was delayed due to the retardation of the formation of the Cu3Sn layer. Since the low driving force for the formation of Cu3Sn became smaller by adding Zn, the diffusion of Cu in Cu-Zn into SAC solder was delayed. Also, the CuZn phase formed at the Cu6Sn5/Cu-Zn interface plays a role as a diffusion barrier of interdiffusion of Cu and Sn.