Employing Single-Crystal Cobalt Substrates to Control $\beta\text{Sn}$ Grain Orientations in Solder Interconnections

Lead-free solder joints on the traditional Cu substrates usually contain few ${\beta \text{Sn}}$ grains with random orientations. Due to the strong anisotropy of ${\beta \text{Sn}}$ , some common reliability issues of solder joints such as electromigration are related to ${\beta \text{Sn}}$ grain orientations. In this paper, we proved that ${\beta \text{Sn}}$ grain orientations can be effectively controlled using single-crystal Co substrates through adjusting the interfacial ${\alpha \text{CoSn}_{3}}$ morphologies. Two single-crystal Co substrates, ${(11\overline{2}0)\text{Co}}$ and ${(10\overline{1}0)\text{Co}}$ , were used in this study. The textures of interfacial ${\alpha \text{CoSn}_{3}}$ were observed by selective etching. The grain orientations of the interfacial ${\alpha \text{CoSn}_{3}}$ and ${\beta \text{Sn}}$ were examined by electron backscatter diffraction (EBSD). The result indicated that interfacial ${\alpha \text{CoSn}_{3}}$ 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) $\mathbf{CoSn}_{3}$ were analyzed to understand the mechanism of the orientation selection of ${\alpha \text{CoSn}_{3}}$ . On ${(11\overline{2}0)\text{Co}}$ , there are only 20 ${\beta \text{Sn}}$ orientations including both single and twinned grains. On ${(10\overline{1}0)\text{Co}}$ , there was no [001]Sn perpendicular to the substrate plane, which ought to improve the reliability of solder interconnections