Diffusion Barrier Effect of Ni-W-P and Ni-Fe UBMs during High Temperature Storage

The high temperature storage test (HTST) was conducted on the SnAgCu/Ni-W-P and Ni-Fe solder joints. While the conventional Ni-P solder joints were used as comparison to study the diffusion barrier effect of Ni-W-P and Ni-Fe under bump metallization (UBM). Both cross section and top view for the microstructural evolution of solder joints during 150°C aging were observed by the scanning electron microscope (SEM). After reflow, (Cu, Ni)6Sn5 in the forms of chunky and rod-like was formed with an average thickness of around 1µm in SAC/Ni-P solder joint. During the HTST, bulky (Cu, Ni)6Sn5 grains were formed with a 5µm in diameter due to the interconnections of multiple (Cu, Ni)6Sn5 grains. In terms of SAC/Ni-Fe solder joints, during the reflow process, FeSn2 layer and rod-like (Cu, Ni)6Sn5 grains were formed. During the aging at 150°C, rod-like dispersed (Cu, Ni)6Sn5 grains started to interconnect with each other which finally progressed into an outer IMC layer upon FeSn2 phase. In Ni-W-P solder joints, the morphology and composition of IMCs is similar to it in Ni-P solder joints. The thickness of (Cu, Ni)6Sn5 was much thicker during reflow but turned out to be below it in Ni-P solder joints after 120h aging. Experimentally, both Ni-W-P and Ni-Fe UBM show an excellent diffusion barrier effect to retard the Kirkendall voids formation compared to the conventional Ni-P UBM. Specifically, (Cu, Ni)6Sn5 were formed at the SnAgCu/ Ni-W-P interface with a total thickness around 2µm, while only a 1µm thick FeSn2 layer accompanying with several dispersing (Cu, Ni)6Sn5 grains outside were formed at the SnAgCu/Ni-Fe interface. The addition of Fe elements can dramatically supress the diffusion of Ni and the formation of Ni3Sn4, which shows superior diffusion barrier compared to Ni-P UBM. The addition of W into Ni-P significantly decreases the growth rate of the interfacial IMCs during the aging process, which shows potential for electronic devices operated under long-term aging process.