In-situ Observation of the Failure Induced by Thermomigration of Interstitial Cu in Pb-free Flip Chip Solder Joints

Abstract

Thermomigration in Pb-free SnAg solder joints is investigated during accelerated electromigration tests under 9.7 x 103 A/cm(2) at 150 degrees C. 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 400 degrees C/cm. Copper atoms may migrate against current flow and become the dominant diffusion species. Also, the Q* can be estimated around I I kJ/mole. On the other hand, a control experiment shows that Ni-Sn IMCs did not migrate even tinder a huge thermal gradient over 1400 degrees C/cm.