Intermetallic compounds (IMCs) form at the solder/substrate interface during soldering, providing metallurgical bonding. However, cracks tend to initiate within IMCs during loading making the joint easy to fail at low shear strength. This inconvenience can be eliminated if nano-sized multi-principal-element alloy (MPEA) layers are induced to form at the interface. The nano-grained MPEAs show synergetic strength and ductility; they are able to release local stress concentration and to distribute the strain localization when the joints are subjected to shear stresses. So, the bonded CuNi/Au-Sn/CuNi joint achieves shear strength of 93 MPa. This study provides a new approach to improve the mechanical reliability of the bonded joint thanks to the improvement of shear strength.