The addition of Co and/or Ni resulted in obvious changes
in both the microstructural and mechanical properties of
Sn–3.0mass%Ag–0.5mass%Cu (SAC305) bulk solder. The
single addition of Co in SAC305 solder formed CoSn2
IMCs and suppressed the undercooling during the solidi-
fication process, while the single addition of Ni only
increased the formation of the rod-shaped (Cu,Ni)6Sn5
precipitates and very slightly suppressed the undercooling.
The single addition of Ni or Co could not refine the b-Sn
grains effectively. The dual addition of Co–Ni resulted in
not only refined b-Sn grains but also suppression of the
undercooling, as well as the formation of CoSn2 IMCs.
During the tensile tests, Co and/or Ni additives had little
effect on the UTS of SAC305 solder; however, the ductility
deteriorated notably. The existence of thin plate-like CoSn2
IMCs led to easy initiation of cavities or micro-cracks
during tensile deformation, which were responsible for the
decrease of ductility