The mechanical reliability of the future miniaturized interconnects is mainly governed by the intermetallic compounds such as Cu6Sn5. Alloyed Cu6Sn5 with various elements, including Co and In, have been introduced and attracted attention for different reasons, such as the enhancing mechanical reliability and lowering the bonding temperature. Hence, this work aimed to evaluate the microstructural and mechanical properties of Cu6Sn5-, Cu6(Sn,In)5-, (Cu,Co)6Sn5-, and (Cu,Co)6(Sn,In)5-interconnects. The grain size, grain orientation, and crystal structure of the pure and alloyed Cu6Sn5 phases were analyzed using electron backscatter diffraction. The results revealed that all the joints contained monoclinic and hexagonal crystal structures arbitrarily formed across the bond-line. Furthermore, the Cu6Sn5 grains exhibited random grain orientation and there was no discernible difference between the pure and alloyed Cu6Sn5 interconnects other than Cu6(Sn,In)5 grains elongated along the perpendicular direction to the bonding interface. However, it was found that alloying elements altered the grain sizes. In alloying refined and elongated the Cu6Sn5 grains while the Co alloying enlarged the Cu6Sn5 grains. The mechanical properties of the interconnects were examined using nanoindentation test. The results indicated that the hardness (H) and Young's modulus (Ei) values of Cu6Sn5 is increased with the alloying elements. (Cu,Co)6(Sn,In)5 showed the highest Ei/H value which indicates its highest plasticity.