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.