High Thermal Stability of SiC Packaging with Sintered Ag Paste Die-attach combined with Imide-based Molding

Abstract

We report that ultra-high thermal reliability can be achieved by combining Ag paste sintering die-attach and injection transfer molding with imide-based mold composite. Test specimens of SiC SBD in TO247 discrete-package are prepared; SiC dies bonded on Cu lead frame, Al ribbon wiring from electrode of SiC Schottky barrier diode (SBD) chip to the Cu leads, and injection transfer molding process. The hybrid nano+micro flake Ag paste realizes a low temperature bond process of 250°C in air, die-shear stress recorded 23 MPa without intensive pressure. The thermo-mechanical parameters like glass transition temperature, Young's modulus and CTE of the thermosetting composite are carefully tuned for both the manufacturing process targeting the device operating temperature of 250°C. The test pieces are tested by harsh thermal cycling between −50°C to 250°C, and compared with the other specimens made with usual Pb-5Sn soldering die-attach with the same molding composite. Interface failures are investigated by scanning acoustic tomography (SAT), and the electric properties of SBD are checked by I-V measurements. The sintered Ag die-attach survived over 500 cycles without any failures around the interfaces of SiC die, Cu lead-frame, and mold materials, and no obvious change is found in electronic properties. However, Pb-Sn die-attach joints are gradually damaged with increasing thermal cycles causing degraded I-V property. The SEM images indicate the microstucture of sintered Ag layer is unchanged after the thermal cycles, because of the impregnated imid-matrix into the porous Ag. Our results demonstrate that the combination of Ag sinter paste die-attach and imide-based thermosetting composite can be a massproduction-ready packaging technology to realize the high operation temperature of post-silicon power devices.