Electromigration in WLCSP solder bumps

Electromigration in two different bumping technologies for WLCSPs has been investigated. The UBM and the structure of the pads on the PCBs were found to have a major influence on the failure mode. A Ni metallization layer serving as a diffusion barrier drastically reduces the intermetallic growth in the solder ball compared to samples assembled on Cu-OSP pads. In the former the principal failure mode is electrical open due to voiding at the Ni–solder interface. In the latter Cu diffuses into the solder at a high rate creating Cu–Sn intermetallics and eventually leading to an electrical open in the Cu track on the PCB being the principal failure mode. Additional experiments showed that the lifetime of the Cu-OSP mounted samples can be extended by using via-in-pad structures.