High bond strength Cu joints fabricated by rapid and pressureless in situ reduction-sintering of Cu nanoparticles

•Rapid and pressureless in situ reduction-sintering of Cu nanoparticles has been proposed.•High joint shear strength of over 30 MPa was achieved at 220 °C for only 5 min.•The level of oxide film that hinders sintering was determined.•The consolidation behavior of Cu nanoparticles during sintering was observed.