Robust bonding and thermal-stable Ag–Au joint on ENEPIG substrate by micron-scale sinter Ag joining in low temperature pressure-less

Abstract

Robust bonding and thermal-stable sinter Ag joining on an Au finished substrate was first time achieved for use in SiC power modules. Five kinds of Ag paste, including different solvents and Ag fillers, and four kinds of Au plating processes (ENIG, ENIGEG, ENEPIG and ENEPIGEG) were used to optimize the initial shear strength of Ag–Au joints. Here, EN means electroless Ni plating, EP means electroless pure palladium plating, IG means immersion gold plating process and EG is the process of electroless gold plating. Micro-scaled Ag flake paste showed the best die shear strength at 33.9 MPa by in-suit formation of Ag nanoparticles, accomplished with a sintering temperature of 250 °C without pressure in air. With the use of Electron backscatter diffraction (EBSD) analysis, the ENEPIG plating process exhibited an Au surface with greater Au (111) grain orientation and larger grain size, which both of that benefits bonding with Ag paste. The thermal-stable sinter Ag joining on ENEPIG was evaluated by aging at 250 °C up to 1000 h. The shear strength had slightly increased to 36.5 MPa after 1000 h. The mechanism of robust bonding and thermal-stable during high temperature aging were systematically analyzed via Scanning Electron Microscope (SEM), Energy dispersive X-ray spectroscopy (EDS), Transmission Electron Microscope (TEM) and X-ray diffraction (XRD), which was attributed to the large Au grains size, the favorable surface condition of the ENEPIG, and the Pd barrier layer which prohibited Ni diffusion into the Au layer during sintering and aging.