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.
Analysis of the formation mechanism of coarse-dense structure of silver paste in die bonding
