Evaluation of Ag sintering die attach for high temperature power module applications

Ag sintering technology has become a potential candidate to replace solder technology due to its well known high melting point (961°C), excellent electrical and thermal conductivity. In this paper, we report the evaluation of die attach using Ag sinter technology on substrates with different top metallizations (Au, Ag and Cu). Low pressure nano-Ag paste (Alpha Argomax) was chosen for this evaluation. Sintering is carried out in air using parameters suggested by the supplier. Bonding qualities and bonding mechanism were characterized by cross-section scanning electron microscopy, transmission electron microscopy and energy dispersive X-Ray spectroscopy. Bonding strength was evaluated by bend test and shear test. The experimental results indicated the bonding quality of Ag sinter to a Cu surface is not as good as bonding to Au and Ag surfaces. Oxidation of Cu is shown at the bond interface, leading to a poor bonding interface. However, Ag sintering to Cu surface without plating of the Au or Ag is of significant benefit to reduce cost. The benefit will be more important for the interconnection between substrate and baseplate due to the large bonding area. These results suggest Ag sintering to a Cu surface needs further development in terms of modifying the Ag paste composition to resist Cu oxidation or alternatively sinter in a N₂ atmosphere. Temperature cycling tests indicate that sintered substrates exhibit better temperature cycling capabilities than soldered substrates for high temperature applications.