Thermal Interface Materials Thermal Management for SiC Semiconductors: Hybrid Approach for Top Side Cooling

Thermal Interface Materials ensure efficient heat dissipation from the heat source (semiconductors) to the heat sink (heat sinks). Increasingly, very high electrical insulation strength is also required. Hybrid materials offer an interesting alternative.

The increasing demand for powerful and compact power electronics places high demands on the thermal management of modern semiconductor technologies. In particular, semiconductors based on silicon carbide (SiC) and gallium nitride (GaN) offer significant advantages over conventional silicon solutions, including higher voltage resistance, lower switching losses, and improved efficiency at high temperatures. However, this higher power density also comes with an increased cooling requirement.

Traditionally, heat dissipation occurs via the bottom side of the semiconductor (Bottom Side Cooling), where the waste heat is transferred through the substrate and solder connection to the housing. However, multiple thermal interfaces limit the efficiency of this method.

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A promising alternative is top side cooling, where the heat is dissipated directly via the top of the chip to a heatsink. This reduces thermal resistance and improves temperature homogeneity within the semiconductor, which in turn increases the lifespan and efficiency of the entire system.

A key factor for the successful implementation of top side cooling is the selection of a suitable thermal interface material (TIM), which enables efficient heat transfer.

Challenges of Conventional TIMs for Top Side Cooling

To optimize heat dissipation in top side cooling, TIMs must meet several requirements: they should have high thermal conductivity, minimize thermal interface resistance, electrically insulate in most applications, and simultaneously offer high mechanical stability and reliability.

Thermal greases have a very low thermal resistance due to their small "BLT" (bond-line thickness), but they are not electrically insulating. Additionally, some greases available on the market tend to exhibit the so-called "pump-out" effect, particularly during intensive "power cycles."

In general, applying the paste via screen or stencil printing is very time-consuming. Additionally, pastes are not designed for tolerance compensation; cross-linking systems like gap filler liquids are used in such cases.

Phase Change Materials (PCM) melt at higher operating temperatures, thereby reducing their contact resistance. However, they are mechanically less stable over longer periods or higher cycle counts and can therefore lose performance. This material group is also not suitable for applications requiring electrical insulation. Additionally, applying a PCM requires specialized and time-consuming assembly methods, such as screen printing.

The use of gap filler liquids (GFL) has now become a frequently chosen approach, especially for higher quantities. The combination of high thermal conductivity, low material costs, and simple automation through dispensing systems offers several advantages. Unlike the previously mentioned product groups, gap filler liquids are also electrically insulating.

A disadvantage is the longer process time in production due to the required curing process of the material itself. In the case of complex geometries, the precise control of material distribution is somewhat complicated by dependency on the dispensing process and component tolerances. Nevertheless, gap filler liquids are a very popular approach, as they offer significant advantages in tolerance compensation through "wet-on-wet" assembly and exert very low mechanical pressure on the assemblies.

Insulation Strength: Hybrid Approach in Thermal Interface Materials

The trend in the automotive sector to switch from 400-V systems to an 800-V architecture presents new challenges, particularly in the area of electrical insulation strength. For safety reasons, a two-layer structure of the TIM is required, for example, to ensure electrical insulation even in the event of contamination by metallic particles or air inclusions.

Keramold is a specially developed, highly thermally conductive, and electrically insulating granulate based on TPE (thermoplastic elastomer), which combines the advantages of various material groups in the field of TIMs. This special material combines 3D heat transfer with high electrical insulation of electronic components.

It also expands the possibilities of thermal management as it goes beyond the classic 2D heat conduction paths of films or pads.

The granulate can be applied using conventional injection molding techniques or through overmolding. In overmolding, the molten granulate is applied directly to the circuit board or semiconductor chip, creating a precise, thermally conductive, and electrically insulating protective layer that does not require a cross-linking process.

Comparison to TIMs Made from Traditional Plastics

Top-side cooling, in particular, brings numerous mechanical, electrical, and thermal challenges. The power loss of the SiC semiconductor consists of conduction and switching losses and can, depending on the application, amount to several hundred watts per module (e.g., for SiC MOSFETs in inverters for electric vehicles). Additionally, the demand for insulation strength is increasing, with the electrical insulation of the TIM often needing to withstand several kilovolts, which can be achieved with the newly developed material.

Through overmolding, the contact resistance between the TIM and the semiconductor can be minimized. This is also supported by the low hardness and high ductility of the granulate, which, unlike traditional plastics, lies in the Shore A range.

Thanks to special fillers, products in the Keramold series achieve a thermal conductivity of over 2.5 W/mK in the z-direction (through-plane) and over 3.2 W/mK in the x/y-direction (in-plane), which is significantly higher compared to conventional plastics. A material with 3.5 W/mK (Keramold 35) is already in development. The improved heat dissipation and additional heat spreading allow hot spots to be cooled more effectively, and heat sinks to be designed smaller.

The 3D shape of the TIM not only improves heat transfer itself but also makes it easier to handle creepage currents, as it encapsulates not just the contact surface of the semiconductor but also, for example, the pins. The material utilization with this method is also very high and reproducibly achievable. The TIM has a CTI (Comparative Tracking Index) value of more than 600 V for creepage resistance, categorizing it in insulation class I, and it can help keep component spacing small.

Overmolding with the soft and elastic TPE additionally protects the semiconductor chip from mechanical stresses such as vibrations or "CTE mismatch" (thermal mismatch due to differing coefficients of thermal expansion in material composites), moisture, and environmental influences, thereby extending the lifespan of the electronic components.

Overmolding semiconductors can be fully integrated into existing production processes, resulting in reduced processing time during manufacturing. Curing of 2K materials such as potting or gap fillers takes significantly longer compared to injection molded parts. The manufacturing quality and repeatability in overmolding are very high.

Through the complete encapsulation of the circuit board and SiC semiconductors, other materials and process steps such as potting or conformal coating can be eliminated, as the material is designed not only for heat transfer but also for protecting the electronic assembly.

Conclusion: Key Technology for Top Side Cooling

The requirements in the field of power electronics are very complex and also individual. Therefore, it is not possible to generally determine which TIM group should be used, as it depends on many factors. Nevertheless, Keramold definitely represents a new approach and provides developers with new degrees of freedom.

By directly overmolding the semiconductor with a thermally conductive protective layer, both heat dissipation, electrical insulation, and mechanical stability are optimized while reducing production process time. Their greatest advantage is evident when, in addition to protecting the electronics, an improvement in thermal performance is also desired. The properties will continue to be further improved, which is still necessary for some applications.

Hybrid materials may initially be more complex, but they will play an increasingly important role due to the rising demands on insulation strength. (kr)

*Wolfgang Höfer is the Head of Thermal Management at Kerafol in Eschenbach.