Effect of Zn content on Cu-Ni cross-interaction in Cu/Sn-xZn/Ni micro solder joints

The effect of Zn content on Cu-Ni cross-interaction in Cu/Sn-xZn/Ni (x = 1, 5, 9) micro solder joints during reflow at 250 A degrees C was investigated. The cross-sectional microstructure, top-view morphology and phase composition of interfacial intermetallic compounds (IMCs) were examined to reveal IMC evolution. At the Ni side, Cu-Ni cross-interaction occurs in all the solder joints. However, the Cu-Ni cross-interaction is obviously weakened when the Zn content is higher than 5 wt% by forming Ni5Zn21-type or Ni3Sn4-type IMCs instead of Cu6Sn5-type IMC which is a typical reaction product under Cu-Ni cross-interaction. At the Cu side, Cu-Ni cross-interaction occurs in all the Cu/Sn-1Zn/Ni solder joints and the Cu/Sn-5Zn/Ni and Cu/Sn-9Zn/Ni solder joints after the initial Cu5Zn8-type IMC transforms into CuZn-type IMC. Zn can strongly retarded the Cu-Ni cross-interaction by dominating interfacial reaction. The reaction path at each interface is discussed based on the Sn-rich corner of Sn-Zn-Cu, Sn-Zn-Ni and Sn-Ni-Cu ternary isotherms.