Failure induced by thermomigration of interstitial Cu in Pb-free flip chip solder joints

Abstract

Failure induced by thermomigration in Pb-free SnAg flip chip solder joints has been investigated by electromigration tests under 9.7 x 10(3) A/cm(2) at 150 degrees C. The fast interstitial diffusion of Cu atoms from underbump metallization into Sn matrix caused void formation at the passivation opening on the chip side. The Cu diffusion was driven by a large thermal gradient and led to void formation even in the neighboring unpowered bumps. When the thermal gradient is above 400 degrees C/cm, theoretical calculation indicates that the thermomigration force is greater than the electromigration force at 9.7 x 10(3) A/cm(2) stressing. (C) 2008 American Institute of Physics.