How to pressureless sinter-bond power chips with bare copper substrates using nanosilver paste?

Nanosilver paste pressureless sintering for bonding power semiconductor devices have been widely used. However, most of the substrates used are silver- or gold-coated substrates. Additional coatings will increase the manufacture cost, reduce the production efficiency, and cause problems with the coating lamination. The bonding process was carried out by sintering nanosilver paste on bare copper DBC substrates in different reducing atmospheres in this study. Three different reducing atmospheres were used as comparison: i.e., (1) sintering in pure nitrogen (N2), (2) sintering in 4% hydrogen and 96% nitrogen (4%H2/N2), and (3) two-step sintering profile: drying in the air in the first step and then sintering in 4%H2/N2 in the second step. The bonding strength of sintered joints was evaluated by the shear test. The highest bonding strength can be obtained by the two-step sintering. The average shear strength obtained by the two-step sintering method is more than 40 MPa at the peak temperature of 310℃. It can be seen that the sintering atmosphere has a critical effect on the density of the joints. The sintered joints with the highest density were obtained by the two-step sintering method. Many necks were formed between the silver particles during the sintering.