Thermal evaluation of metalized ceramic substrates for use in next-generation power modules toward international standardization

Abstract

Wide bandgap (WBG) semiconductors such as SiC and GaN have opened their market as the next generation of high power modules. As advanced electronic power modules must deal with increasing power density, power modules with an insulating ceramic substrate are subjected to extremely high temperature due to high current and voltage. Currently, the junction temperature of Si power devices is kept low enough, around 150 ℃, to maintain the module durability. Nevertheless, the temperature will be soon beyond 200 ℃ for WBG devices. Thus thermal dissipation performance has one of the key technologies in designing modules. Directly bonded copper (DBC) or aluminum (DBA) substrates have been widely used as two of the typical ceramic insulating substrates for high power modules. If the extensive heat from a junction is kept inside a module, the module will be easily damage due to the increasing temperature resulting in severe thermal stress inside. Designing a proper module structure requires not only each material property but also complex component shape/structure/layout including each interface heat transfer. An accurate measurement method of precise thermal properties for such a complex metallized ceramic substrates is currently still missing. To meet the urgent requests for the next generation of high power modules, a new and simple measurement method of the thermal properties has been proposed for complex metallized ceramic substrates. First we have designed a micro heater SiC chip as an accurate and controllable heat source instead of a SiC active die. The heater chip was die-attached on metallized ceramic substrate mimicking real module packaging structure. The thermal resistance of the metallized ceramic substrate was evaluated. Due to the high power of the chip, i.e. 1 kW/cm2, the obtained thermal resistance has an excellent accuracy within a few percent error, when adequate cooling system is used. The thermal resistance includes those of die-attach material and thermal interface materials (TIM). The developed method thus enables precise comparison in thermal properties of high power modules, which was proposed as an ISO standard.