Abstract: Electromigration in solder joints is unique in two aspects due to geometry and composition. Geometrically, there is a very large cross-sectional change from an Al or Cu interconnecting line to a solder bump. The large cross-section change means a large current density change from line-to-bump or vise versa, and it induces current crowding at the contact between the interconnect and the bump. The current crowding has caused a new failure mode of electromigration. Compositionally, solder are eutectic alloys. In a eutectic alloy below the eutectic temperature, there is no chemical potential gradient as a function of composition at constant temperature and pressure. For example, there is no interdiffusion or homogenization between 70Pb30Sn and 30Pb70Sn in a diffusion couple annealed below the eutectic temperature, except ripening. Hence, electromigration can induce a very large compositional gradient in the eutectic SnPb alloy by driving Sn (near room temperature) or Pb (near 150 °C) from cathode to anode. Yet there is no reverse atomic flux to oppose electromigration due to the large concentration gradient induced by electromigration. It is different from the classic Soret effect. This eutectic effect may explain why a large amount of Cu or Ni can be dissolved from the UBM at cathode and driven to anode to from intermetallic compounds. This is because Sn-Cu and Sn-Ni also form eutectic structures of Sn and intermetallic compounds. In this talk, experimental observations of the current crowding effect and the eutectic effect will be presented.