Synchrotron radiation real-time imaging technology was used to in situ study the liquid-solid interfacial reactions in Cu/Sn-9Zn/Ni solder interconnects with and without temperature gradient. It was clarified that a Cu5Zn8 intermetallic layer adhering to the interfaces of the solder interconnects acted as an effective barrier against Cu or Ni diffusion, and thus retarded the consumption of the underneath Cu or Ni substrate and suppressed the coupling effect of Cu-Ni cross-interaction and thermomigration. However, both Cu6Sn5-type and Ni3Sn4-type intermetallic compounds had little effect on protecting Cu or Ni substrate. The barrier mechanism of the Cu5Zn8 intermetallic layer has been discussed. In addition, the massive spalling of interfacial CusZns was in situ observed during reflow with Cu as the hot end. The massive spalling was caused by the Zn depletion in the liquid solder, the fast dissolution of hot end Cu or Cu6Sn5-type intermetallic compound, and the poor-soluble nature of Cu5Zn8 layer.