Low-Temperature Sintering of Nanoscale Silver Paste for Large-Area Joints in Power Electronics Modules

Abstract

Today, reflow soldering is a commonly used technique to establish large-area joints in power electronics modules. These joints are needed to attach large-area (>1 cm2) power semiconductor chips to the substrate, e.g., a direct-bond copper substrate, and the multichip module substrate to a copper base plate for heat spreading. Thermal performance, specifically thermal conductivity and thermomechanical reliability, of these large-area joints are critical to the electrical performance and lifetime of the power modules. Soft solder alloys, including the lead-tin eutectic and lead-free alternatives, have low thermal conductivities and are highly susceptible to fatigue failure. As demands mount for higher power density, higher junction temperature, and longer lifetime out of the power modules, reliance on solder-based joining is becoming a barrier for further advancement in power electronics systems. Recently, we successfully demonstrated lowtemperature sintering of nanoscale silver paste as a lead-free solution for achieving highperformance, high-reliability, and high-temperature interconnection of small devices (<0.09 cm2). In this paper, we report the results of our study to extend the low-temperature sintering technique to large-area joints. The study involved redesigning the organic and inorganic components of the nanoscale silver paste, analyzing the burnout kinetics of the various organic species sandwiched between large-area plates, and developing desirable temperature-time profile to improve sintering and bonding strength of the joints.