Low-pressure (< 5 MPa) Low-temperature Joining of Large-area Chips on Copper Using Nanosilver Paste

Abstract:
The low-temperature joining technology (LTJT), either the conventional high-pressure version using micro-size silver powders or the low-pressure version using nanosilver paste, has been widely demonstrated for attaching power chips on silver- or gold-metallized substrates. In this work, we investigated the processing conditions for bonding large-area chips by sintering nanosilver paste directly on plain copper surface. We developed a double-print process for making this type of bonding: started by printing an initial layer of silver paste; followed by drying in pure nitrogen at temperature up to 180 °C; applied a thin second print to ensure complete "wetting" of the paste on the bonding surface of the chip; and then heated in a hot press to 275 °C under pressure ranging from zero to 12 MPa. Ceramic mechanical chips measured at 6 mm x 6 mm were bonded by the double-print process on direct-bond-copper (DBC) substrate. Die-shear test of the attached chips showed an average bonding strength of over 40 MPa at applied pressure of 3 MPa and over 77 MPa under 12 MPa sintering pressure. SEM imaging of the failure surface showed a much denser microstructure of sintered silver layer when applying pressure. X-ray imaging showed a bonding layer free of voids. Since the samples were sintered in air, the DBC surface showed some oxidation. To test the wire-bondability of the oxidized surface, aluminum wires of 250 microns in diameter were wedge-bonded at different locations of the oxidized surface. Pull test results of the bonded wires showed a minimum pull-strength of 400 gram-force, which exceeds the minimum of 100-gf required by the IPC-TM-650 test standard.