Investigation of Thermal Performance of Various Thermal Interface Materials Used in Top-Side-Cooled MOSFETs

Thermal Interface Materials (TIM) are crucial in power electronics systems for automotive applications. Wide band-gap (WBG) power devices operating at high voltages and capable of enduring high-temperature swings pose particularly high challenges for the top-side TIM, which needs to enable the cooling of the transistors and their electrical isolation from the metal heatsink. A Typical arrangement of top-side-cooled transistors is depicted in Fig. 1. During operation, the differences in the coefficients of thermal expansion (CTE) of the transistor, TIM, and heatsink can lead to pump-out effects or mechanical fatigue and delamination. This degrades the thermal path for heat dissipation, and increases the junction temperatures (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{T}_{\boldsymbol{j}}$</tex>) of the transistor and the thermal resistance <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\left(\boldsymbol{R}_{\boldsymbol{t h}}\right)$</tex> to the heat sink as well as impairs the electrical insulation during thermal cycling. The mechanical stresses during the temperature cycles can cause cracks within the TIM that can easily lead to the dielectric breakdown in it. Currently, there are two main kinds of silicone materials in use as TIM: grease and adhesive, respectively. The latter mainly fail due to delamination, cracks and fractures at the interfaces and through the TIM film due to the thermomechanical mismatch. In contrast, the former shows pumpout and dry-out effects, where the continuous expansion and contraction during thermal cycling cause the grease to migrate out of the interface. Both failure modes are more likely in configurations with top-side cooling of the individual component than for cooling through the large system bond at the bottom of the module substrate, due to the larger change in warpage curvature and higher temperature swings that occur at the TIM directly placed on the discrete component. The high temperatures lower the viscosity of the grease so that it can be pushed out of the interfaces even faster, caused by the cyclic warpage, which also is the reason for delamination and fractures of silicone adhesives. In today's power electronics, thermal grease-based TIMs are preferred for their high conductivity and ease of application; however, they are more prone to pump-out failure compared to solid TIMs. In general, TIM failures are a key concern due to the thermal and electrical consequences. As there are various options available for TIMs from both groups of materials, there is a need for optimum TIM selection. In addition, implementing a scheme of condition monitoring is advisable for assuring long-term reliability and stable efficiency.