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

Failure induced by thermomigration in Pb-free SnAg flip chip solder joints has been investigated by electromigration tests under 9.7×103 A/cm2 at 150 °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 °C/cm, theoretical calculation indicates that the thermomigration force is greater than the electromigration force at 9.7×103 A/cm2 stressing.