Effect of the combination of electromigration and thermomigration on phase migration and partial melting in flip chip composite SnPb solder joints

Composite SnPb flip chip solder joints are used to study the combined effect of electromigration and thermomgration. The study showed that electromigration have led to the redistribution of Sn-rich phase and Pb-rich phase and displaced the low melting Sn-rich phase to the downward electron current crowding region in the contact area on the chip side. A pancake-type void forms and propagates at the contact interface and is accompanied by the migration of the Sn-rich phase followed by the partial melting of the solder joint. The migration of the Sn-rich phase was unexpected, and the location of the melting was unpredicted. We propose here that the migration of Sn-rich phase in the contact area is caused by thermomigration in the lateral direction owing to the existence of a localized hot spot at the current crowding region. The melting behavior is time dependent because it requires an incubation time for void formation, void propagation, and phase migration. Finally, the partial melting at the Sn-rich region occurs due to a large Joule heating at the end of the void growth before an electrical opening takes place. (c) 2006 American Institute of Physics.