  The presence of bifurcation of the Kirkendall marker plane, a very special
phenomenon discovered recently, is found in a technologically important Cu Sn
system. It was predicted based on estimated diffusion coefficients; however,
could not be detected following the conventional inert marker experiments. As
reported in this study, we could detect the locations of these planes based on
the microstructural features examined in SEM and TEM. This strengthens the
concept of the physicochemical approach that relates microstructural evolution
with the diffusion rates of components and imparts finer understanding of the
growth mechanism of phases. The estimated diffusion coefficients at the
Kirkendall marker planes indicates that the reason for the growth of the
Kirkendall voids is the nonconsumption of excess vacancies which are generated
due to unequal diffusion rate of components. Systematic experiments using
different purity of Cu in this study indicates the importance of the presence
of impurities on the growth of voids, which increases drastically for greather
than 0.1 wt percent impurity. The growth of voids increases drastically for
electroplated Cu, commercially pure Cu and Cu(0.5 at percent Ni) indicating the
adverse role of both inorganic and organic impurities. Void size and number
distribution analysis indicates the nucleation of new voids along with the
growth of existing voids with the increase in annealing time. The newly found
location of the Kirkendall marker plane in the Cu3Sn phase indicates that voids
grow on both the sides of this plane which was not considered earlier for
developing theoretical models.
