Electromigration (EM), which describes the mass transport due to 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, synchrotron radiation real-time imaging technology is used to in situ study the interfacial reaction and atom migration in Cu/Sn-37Pb/Cu solder interconnects undergoing L-S EM at 185 ℃ with a current density of 1.0×10⁴ A/cm². In the heating stage, Pb atoms directionally migrate from the cathode toward the anode, resulting in the growth of Pb-rich phase at the anode. In the dwelling stage, Pb atoms diffuse backward, then equilibrium phase is obtained. In the cooling stage, Pb atoms directionally migrate from the cathode toward the anode again until the solder solidified, three phases is obtained: the Pb-rich phase, the Sn-Pb phase and the Sn-rich phase. The abnormal migration behavior of Pb atoms in different stages is determined by the combined effect of the chemical potential gradient flux (J_chem) and EM-induced flux (J_em). J_em is determined by the effective charge number (Z^*) of Pb atoms, which was calculated to be -3.20 at 185 ℃ based on the growth kinetics of the Pb-rich layer model.