Pressureless Silver Sintering on Nickel for Power Module Packaging

Die attach by low-temperature silver sintering has been widely used in power electronics packaging. Most of the reported work was done on direct-bond-copper (DBC) substrates metallized with silver or gold. There is a lack of studies of sintered-silver bonding on nickel (Ni), a low-cost metallization on DBC. In this study, we fabricated a power module using pressureless in-air sintered-silver die attach on a Ni-plated DBC substrate. Strong die-shear strength of over 40 MPa was achieved. It was also found the static and switching characteristics, and the transient and steady-state thermal performance of the modules using Ni metallization was as good as that using Au metallization. Cross-sectional microstructure and chemistry analyses of the sintered-Ag/Ni interface revealed limited Ni oxidation. We believed that the high-density packing of silver particles and outgassing of the organic molecules from the paste during the bonding process helped to lower the partial pressure of oxygen in the bondline, which, in turn, prevented rapid Ni oxidation and gave rise to extensive formation of strong Ag–Ni metallic bonds at the interface. The findings of this study show the low-cost potential of the die-attach technology for power module packaging since the bonding process can be done pressureless in air on low-cost Ni metallization.