Electromigration Characteristics of Flip Chip Sn-3.5Ag Solder Bumps under Highly Accelerated Conditions

The effects of severe current crowding and Joule heating on the damage morphology and the electromigration parameters were evaluated for flip chip Sn-3.5Ag solder bumps with Cu under-bump metallurgy, in-situ electromigration testing in a scanning electron microscope was performed to correlate the statistical lifetimes with the detailed microstructural characteristics. The highly accelerated test conditions used led to an activation energy of 1.63 eV and a current density exponent of 4.6, which can be attributed to severe current crowding and Joule heating effects. Real-time microstructural analysis revealed that interfacial voids nucleated around corners of the under-bump metallurgy layer, where electrons entered from the chip or the substrate interconnect line to the solder bump and then grew along the Cu6Sn5/solder interface, irrespective of the current flow's direction. Accelerated growth of Kirkendall voids was also observed at the Cu3Sn/Cu interface and within Cu3Sn. However, electrical failure of the bump resulted front electromigration-induced interfacial void propagation along the Cu6Sn5/solder interface, which can be explained in terms of the large difference in Cu diffusion flux between the Cu-Sn intermetallic compound layer and the solder itself.