Examining Ag-Ag Direct Bonding on Chemically Coated DBC Substrates as a Pasteless Die-Attach Approach

Inhalt:
In this paper Ag-Ag direct bonding as a pasteless high temperature die-attach technology for power electronic applications on ceramic substrates is examined as conceivable alternative to silver sintering. To this end, sputtered Ag metallization on silicon chips and chemically coated direct bonded copper (DBC) substrates were bonded with varying parameters using design of experiments. Factors were pressure, bonding time and surface treatment. For the best parameter set average die-shear strengths of 29 MPa (out of 4 samples) have been reached processing non-polished silver coated DBC substrates at 250deg C with 20 MPa pressure and 60 minutes of process time. Microsections and images from a scanning acoustic microscopy (SAM) showed connected and non-connected areas at the joining interface. Lower shear strengths were found for polished surfaces than for the non-polished counterparts. By scanning electron microscopy (SEM) analysis the diffusion-based development of solid interconnection was confirmed, demonstrating feasibility of Ag-Ag direct bonding on grinded DBCs. The overall results affirm the viability and point out challenges of this die-attach process.