High-strength pressure-free bonding is investigated using Cu nanoparticles as an alternative to conventional solders. Focus is placed on the morphology of Ni-Sn intermetallic nanoparticles, an additive to a paste of Cu nanoparticles, for improvement of sinterability. The shear strength increases from 23.2 (Cu nanoparticles only) to 31.8 MPa, when 10 wt% of the newly synthesized 15-nm Ni3Sn2 nanocubes is mixed with the Cu nanoparticle paste. This is the first example of the use of base metal nanoparticles under pressure-free conditions to achieve the bonding strength of an ordinary Pb-free solder (Sn-Ag-Cu). The addition of smaller Ni3Sn2 nanocubes 8 nm in size or irregularly shaped Ni3Sn2 nanoparticles (25.0 MPa) results in a limited increase in shear strength (26.6 MPa), while the addition of micrometer-sized Ni3Sn2 particles results in a decrease in shear strength (21.5 MPa). The effects of the size and shape of the added Ni3Sn2 particles on the shear strength are discussed based on SEM observation of the sintered layers.