In-situ observation of the failure induced by thermomigration of interstitial Cu in Pb-free flip chip solder joints

Thermomigration in Pb-free SnAg solder joints is investigated during accelerated electromigration tests under 9.7times10³ A/cm² at 150degC. Different from Pb-containing solders, it is found that Cu-Sn intermetallic compounds (IMCs) migrate toward the cold end on the substrate end and thus voids accumulate in the passivation opening for the bump with current flowing from the substrate end to the chip end. Theoretical calculation indicates that the thermomigration force is greater than the electromigration force at a thermal gradient above 400degC/cm. Copper atoms may migrate against current flow and become the dominant diffusion species. Also, the Q* can be estimated around 11 kJ/mole. On the other hand, a control experiment shows that Ni-Sn IMCs did not migrate even under a huge thermal gradient over 1400degC/cm.