The effects of minor additives, that is, Co and Ni, on the microstructural and mechanical properties of Sn–3.0 mass%Ag–0.5 mass%Cu (SAC305) bulk solder were investigated. The addition of Co and/or Ni resulted in microstructural changes of the SAC305 solder, such as the formation of new intermetallic compounds (IMCs) and the refinement of grain size, as well as the suppression of undercooling. The single addition of Co in SAC305 solder resulted in the formation of CoSn2 IMCs and undercooling suppression, whereas the single addition of Ni accelerated the appearance of rod-shaped (Cu,Ni)6Sn5 IMCs inside the β-Sn dendrites during the solidification process. The dual addition of Co–Ni resulted in refined β-Sn grains and suppression of undercooling, as well as the formation of CoSn2 IMCs. In tensile tests, Co and/or Ni additives had little effect on the tensile strength of SAC305 solder, but obviously suppressed the elongation ratio and reduction of area. During tensile deformation in samples with existing thin plate-like CoSn2 IMCs, micro-cracks or cavities were easily initiated through the interface between CoSn2 and the solder matrix, which was responsible for the decrease of ductility.