Employing Single-Crystal Cobalt Substrates to Control betaSn Grain Orientations in Solder Interconnections

Lead-free solder joints on the traditional Cu substrates usually contain few beta Sn grains with random orientations. Due to the strong anisotropy of beta Sn, some common reliability issues of solder joints such as electromigration are related to beta Sn grain orientations. In this paper, we proved that beta Sn grain orientations can be effectively controlled using single-crystal Co substrates through adjusting the interfacial alpha CoSn3 morphologies. Two single-crystal Co substrates, (11overline20)Co and (10overline10)Co, were used in this study. The ures of interfacial alpha CoSn3 were observed by selective etching. The grain orientations of the interfacial alpha CoSn3 and beta Sn were examined by electron backscatter diffraction (EBSD). The result indicated that interfacial alpha CoSn3 presented 2 or 4 dominant orientations related to single Co with fixed orientation relationships (ORs). The interfacial atomic mismatches and crystal growth kinetics represented by the angle between the substrate plane and (100) CoSn3 were analyzed to understand the mechanism of the orientation selection of alpha CoSn3. On (11overline20)Co, there are only 20 beta Sn orientations including both single and twinned grains. On (10overline10)Co, there was no [001]Sn perpendicular to the substrate plane, which ought to improve the reliability of solder interconnections.