Effect of oxygen on microstructural coarsening behaviors and mechanical properties of Ag sinter paste during high-temperature storage from macro to micro

This study systematically investigates the effect of oxygen on the microstructural evolution as well as the macro/micromechanical characterization of Ag sinter paste during the aging process at 250 °C for 1000 h. The sintered Ag joint is separated into two groups and, respectively, kept in air and vacuum (10⁻⁴ Pa) atmospheres. The microstructure of sintered Ag paste become clearly coarsened after aging at 250 °C in air for 100 h, while it remains almost the same as in the initial state in vacuum, even after aging for 1000 h. The different microstructural evolutions are investigated in detail by transmission electron microscopy, which reveals that coarsening are involved in the Ag-O reaction and form a large number of Ag nanoparticles around sintered Ag particles, as well as enhance coalescence. Furthermore, the macro/micromechanical properties of sintered Ag paste, after aging in air and vacuum atmospheres, are evaluated by a die shear test and a nano-indenter system with a spherical diamond tip. Shear strength and fracture mode were systematically analyzed. The micromechanical properties of sintered Ag paste, including Young's modulus and hardness, depend on its microstructure and are largely influenced by its porosity, which decreases from initiation to 250 h and recovers after aging for 500 h in air. However, there is almost no change during the aging process in the vacuum atmosphere. The results of this study contribute to a better understanding of the microstructural evolution and the mechanical properties of sintered Ag joints, including the change in bonding quality and Young's modulus during aging for actual power modules, which are packaged by a resin.