The current density distribution in a line-to-bump structure as a function of cross-sectional area ratio of the electrical conductor was investigated, as well as the effects of current crowding on electromigration and interfacial reaction in lead-free solder joints. Finite element analysis shows that the crowding factor is directly proportional to the cross-sectional area ratio between Cu line and contact opening at the cathode side. solder joints with Cu line in different widths were designed and tested under at for . The experiment results show that big voids induced by electromigration are only formed at the structure with a narrower line. Moreover, the growth of intermetallic compound layers, as well as dissolution of Cu at the cathode side, is accelerated by smaller current crowding, whereas impeded by a bigger one.