Electromigration (EM), which describes the mass transport due to the momentum exchange between conducting electrons and diffusing metal atoms under an applied electric field, has become a serious reliability issue in high-density packaging. With the increasing demands for miniaturization, liquid-solid (L-S) EM will pose a critical challenge to the reliability of solder interconnects. In this work, The interfacial reactions and diffusion behaviors of In, Sn and Cu atoms in Cu/Sn-52In/Cu interconnects during L-S EM under a current density of 2.0×10⁴ A/cm² at 120 and 180 ℃ have been in situ studied by using synchrotron radiation real-time imaging technology. During L-S EM, since there was no back-stress, the In atoms directionally migrated toward the anode due to the negative effective charge number (Z^*) of In, which is different from the In atoms directionally migrated toward the cathode due to the back-stress induced by the preferential migration of the Sn atoms over the In atoms toward the anode during the solid-solid (S-S) EM. Furthermore, a modified expression for calculating the effective charge number Z^* of liquid metals was proposed based on the enthalpy changes of melting process. The Z^* of In atoms was calculated to be -2.30 and -1.14 at 120 and 180 ℃, respectively, which was consistent with the migration behavior of In atoms. The model provides a theoretical basis for determining the direction of the EM. The polarity effect, evidenced by the IMC layer at the anode growing continuously while that at the cathode was restrained, was resulted from the directional migration of In and Cu atoms toward the anode during L-S EM, which was more significant at high temperature. The consumption of cathode Cu during L-S EM followed a parabolic relationship with the EM time, and the consumption rate was magnitude higher at high temperature. The migrations of In atoms was discussed in terms of diffusion flux.