Current Carrying Capability of Sn0.7Cu Solder Bumps in Flip Chip Modules for High Power Applications

The primary objective of this work was to characterize the current carrying capability of Sn0.7Cu solder bumps for use in high power flip chip module applications. The factors to be considered in using modules as test vehicles for current carrying capability studies are explored. Experimental data on the current carrying capability of Sn0.7Cu solder bumps in a module test vehicle is complemented with thermal simulations to understand the phenomena occurring the first level solder interconnects. The associated electromigration characteristics of the Sn0.7Cu on plated Cu was also evaluated. Current densities of 6.0x10³, 9.42x10³, 1.06x10⁴, and 1.18x10⁴ A/cm² were investigated at 115C, 125C and 135C. No failures were observed for up to 2550 hours with the lowest current level of 6.0x10³ A/cm² at 135C. However, a variety of failure modes were seen at the other current levels. In this work a thermocouple was used to approximate the temperature of the solder bump. While this method has produced excellent results in obtaining the electromigration of solder bumps in single die applications, failure analysis and thermal simulation indicated that this is not the case with flip chip modules. A Computational Fluid Dynamics (CFD) tool was employed and thermal modeling confirmed that the bump temperature was significantly higher than the temperature measured on the surface of the mold compound. Temperature gradients played a significant role in determining the electromigration characteristics and extra considerations are needed to examine the multiple effects resulting from it.