A New Development of Direct Bonding to Aluminum and Nickel Surfaces by Silver Sintering in air Atmosphere

Owing to the superb properties of silver such as high melting temperature, high thermal and electrical conductivity, low temperature silver sinter technology has attracted growing attention in recent years especially for the applications required high power and high operating temperature. Current silver sinter technology required plating of precious metal finishing on the substrates prior to sintering process in order to form a strong sinter joint. Direct bonding on non-precious metal surfaces by silver sintering is therefore of great interest, since the precious metal finishing on substrate is no longer necessary, which will lead to the reduction of manufacturing cost. This paper explores the development of a safe-to-use micro-silver sinter paste for pressure sintering on aluminum and nickel surfaces. In this study, Ag metallized Si dies were attached on nickel-plated direct copper bonding substrates and high purity aluminum plates by silver sintering process at 250 °C with a pressure of 10 MPa for 3 min in air atmosphere. The cross-sectional SEM images of sintered samples indicate that a dense sintered layer was formed on Ni and Al surface. After die shear test, SEM-EDX was conducted on the fracture surface of Ni and Al substrates and the results confirmed that silver sintered joint was created on Ni and Al surface. The EDX analysis results further illustrate an interdiffusion of Ag/Ni and Ag/Al occurred at the interface located between sintered layer and substrates. High bonding strength of silver sintered joint was created on Ni and Al surfaces and the average die shear strength remained above 30 N/mm² after 500 h storage at 250 °C. Cohesive break in the sintered layer was obtained for both Ni and Al samples before and after high temperature storage where silver sintered layer can be found on both the die backside and the substrate surface indicating that good adhesion on Ni and Al surfaces was achieved with the newly developed silver sinter paste.