Microstructural studies and bonding strength of pressureless sintered nano-silver joints on silver, direct bond copper (DBC) and copper substrates aged at 300 °C

Abstract Sintered Ag joint is a potential Pb-free die attach materials for power electronics because of its high operating temperature, high electrical and thermal conductivity as well as its thermo-mechanical reliability. While the long term reliability of the pressure assisted nano-Ag joint has been studied extensively, the performance of pressureless nano-Ag joint during long-term reliability remains a question. In this paper, we addressed the reliability gaps in this area by characterizing the evolution of porosity and microstructures of the pressureless sintered nano-Ag joints on Cu, direct bond copper (DBC) and Ag plated substrates during their high-temperature storage in air at 300 °C. The die shear strength of the nano-Ag joint on DBC substrate maintained above 5 MPa for 1000 h of thermal aging while those formed on Cu substrate fell below 5 MPa after 50 h of thermal aging. The microstructure of the sintered nano-Ag joint showed Cu oxide accumulated at the sintered Ag and Cu interface, and upon reaching the critical thickness, they would weaken the sintered nano-Ag joints for the Cu substrate. This microstructure differed from those formed on the Ag plated substrate which showed void-less diffusion band to improve or maintain the die-shear strength. Nano-Ag joints on the DBC substrate also shared similar resilience because of their matching coefficient of thermal expansions with the Cu oxides. The initial increase of die shear strength could be attributed to the densification of the sintered silver joints in terms of the total volume of porosity, pore size and pore shape distribution. The relationship between the oxidation kinetics of Cu, aging time and shear strength were also established to predict the reliability strength of sintered nano-Ag joints on DBC and Ag substrates. In the case of DBC, its rough surface provided additional anchoring to the sintered Ag joint, resulting in a higher die shear strength despite prolonged aging. These favourable results suggested that pressureless nano-Ag joint could perform as well as pressure assisted nano-Ag joint during the long-term reliability test.