The paper focused on the changes in microstructure and mechanical properties of the full Cu41Sn11 solder joint (Cu/Cu41Sn11/Cu) during isothermal aging at 420 °C. It was motivated by potential applications of Cu–Sn intermetallic compounds (IMCs) solder joint in third-generation wide bandgap semiconductor devices. Experimental results revealed that the Cu41Sn11 phase was unstable under high-temperature conditions, the full Cu41Sn11 joint transformed into the full α(Cu) joint (Cu/α(Cu)/Cu) joint at 150 h during thermal aging. The formed α(Cu) phase was a Cu solid solution with inhomogeneous Sn atomic concentration, and its crystal structure and orientation were consistent with the original Cu plate. The conversion of the Cu41Sn11 to α(Cu) was accompanied by the formation of voids due to the volume shrinkage effect, predominantly near the middle of the solder joint interface. The α(Cu) solder joint presented a decrease in strength but an increase in strain rate sensitivity index compared to the Cu41Sn11 solder joint. Furthermore, the strain rate sensitivity index of α(Cu) and Cu41Sn11 is lower than that of ordinary Sn solders. After the shear test, the fractures that occurred in Cu41Sn11 grains were brittle, while the fractures in α(Cu) grains were ductile.