The effect of liquid-solid electromigration (EM) on the interfacial reaction in Ni/Sn-9Zn/Ni interconnects was investigated under a current density of 5×10³ A/cm² at 230 ℃. A reverse polarity effect was revealed, i.e., the interfacial intermetallic compounds (IMC) at the cathode grew continuously and was remarkably thicker than those at the anode. This results from the directional migration of Zn atoms from the anode toward the cathode, which is induced by the positive effective charge number (Z ^*) of Zn atoms but not the back-stress. A thin Ni₅Zn₂₁ layer formed at each interface after soldering. The initial Ni₅Zn₂₁ interfacial IMC gradually transformed into [Ni₅Zn₂₁+(Ni, Zn)₃Sn₄] after liquid-solid interfacial reaction for 8 h, due to the local equilibrium at the interface changed with decreasing of Zn atoms content. The interfacial IMCs at both anode and cathode were identified as Ni₅Zn₂₁, and no IMC transformation occurred undergoing liquid-solid EM, because the Zn atoms content at the cathode was enough under electron current stressing, and the diffusion of Zn atoms toward anode was inhibited. The reverse proving was proposed to explain the positive value Z ^* of Zn atoms. The abnormal directional migration of Zn atoms toward the cathode prevented the dissolution of cathode substrate, which is beneficial to improving the EM reliability of micro-bump solder interconnects.