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Pressureless sintering of nanosilver paste has been widely demonstrated for bonding power semiconductor devices on silver- or gold-coated substrates. In this paper, we extended the application of nanosilver bonding to bare copper surface by controlling sintering atmosphere in an effort to prevent surface oxidation of copper during sintering. Three different atmospheres were evaluated: 1) pure nitrogen (N2); 2) nitrogen with 1 vol% oxygen (1% O2/N2); and 3) a forming gas consisting of 4 vol% hydrogen and 96 vol% nitrogen (4% H2/N2). The bonding strengths of sintered joints were evaluated by die-shear test. The joints sintered in 1% O2/N2 had the lowest average bonding strength of approximately 15 MPa due to the formation of copper oxide clearly visible on the failure surface of the substrate. The joints sintered in 4% H2/N2 had the highest average bonding strength of over 40 MPa. Microstructure characterization of the sintered joints showed that the bond-line density was strongly affected by the sintering atmosphere. The joints sintered in the forming gas had a denser microstructure than those sintered in pure nitrogen. It is believed that the higher sintered density is a result of catalytic hydrogenation and hydrogenolysis of organic binder in the paste, which occurred at the sintering temperature with nanoparticles of silver as catalyst. This increase in microstructure density and prevention of copper surface oxidation led to the high bonding strength of the sintered joints formed on copper in the forming gas.