The polarity of electromigration at the interface between solder joints and finishes was studied with the interconnects structure of Ni(P)/Au finishes-SnPb solder joint-Ni(P)/Au finishes. The mechanism of microstructural evaluation of interconnects was investigated in terms of directional diffusion of atoms under electrical and chemical potential. The IMC layers on the interface between Ni(P)/Au finishes and solder joints have only a little change in thickness after the annealing of 120 ℃ for 100 h because the velocity of wetting reaction is faster than the solid state reaction. But under the condition of electromigration, the directional diffusion of Sn atoms along the electron flow direction induces the abnormal growth of IMC on the anode interface, and the restraint at cathode. The cathode contact is on the upside, where the electron current enters the solder bump, and locates at the triple point. The solder would melt at this point by Joule heating, resulting in the reaction between the Cu wire and solder.