Thermomigration in SnPb composite solder joints and wires

Composite flip chip solder joints and composite solder wire samples were used to study thermomigration. The composite solder joint consists of 95Pb5Sn on the Si side and eutectic 37Pb63Sn on the substrate side. Composite wire samples were made by joining Cu (wire) - 95Pb5Sn (ball) - eutectic 37Pb63Sn solder (ball) - Cu (wire) together with the size of the wire and ball of 300 /spl mu/m in diameter. We have observed thermomigration induced composition redistribution of Sn and Pb, in which Sn moved to the hot end and Pb to the cold end. A threshold temperature gradient, which is about 1000 /spl deg/C/cm, is needed to induce the thermomigration.