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