The electromigration-induced failure in the composite solder joints consisting of 97Pb-3Sn on the chip side and 37Pb-63Sn on the substrate side was studied. The under-bump metallization (UBM) on the chip side was 5 mum thick electroplated Cu coated on sputtered TiW/Cu and on the substrate side was electroless Ni/Au. It was observed that failure occurred in joints in a downward electron flow (from chip to substrate), while those joints having the opposite current polarity showed only minor changes. During electromigration, in addition to the compositional change by the moving of Pb atoms in the same direction as the electrons, current crowding was observed inside the UBM and it enhanced the phase transformation of Cu to Cu3Sn and to Cu6Sn5 at the UBM/solder interface. Due to the growth of Cu6Sn5, the Cu UBM was consumed rapidly, resulting in void formation-induced failure at the cathode side. The Cu6Sn5 intermetallic compound and void were first initiated from the upper left-hand side corner of the contact window which matches the current crowding region. The sequence of Cu UBM consumption and void formation is presented. The current crowding has been confirmed by simulation. The mechanism of electromigration-induced failure in the composite solder joint structure is discussed. (C) 2003 American Institute of Physics.