In the field of switching technology, the insulated-gate bipolar transistor (IGBT) is the most mature solution. It is a power semiconductor which is built on a standard silicon wafer and can handle voltages as high as 1700 V. The Si-IGBT has some significant advantages like reliability and cost efficiency. This technology has been successfully used for both industrial and automotive applications since many years. The production costs of Si-IGBTs have been decreased over the years by focussing on a highly automated production and achieving the highest quality level. Even if the IGBT technology is the most mature, this field is still under development which will lead to a higher power density and a smarter energy efficiency. The challenges are to develop IGBTs above 1700 V‑on 300 mm wafers (which saves costs) and having devices with a junction temperature of 200 °C. In the long-term this can allow both a 20 % higher power density and a lifetime which can be up to 50 % longer.
New IGBTs, new requirements for Thermal Management
Of course, this new kind of IGBTs will need a high performance and temperature stable Thermal Interface Material for the connection to the heat sink, as the lifetime of electronic components can be dramatically affected by temperature. Therefore, the German manufacturer Kerafol is focussing on materials with low thermal resistance due to an optimal connection of the joining partners as well as a high breakdown voltage to ensure a safe and stable operation for long-term applications. Furthermore, it is planned to reduce the losses of e.g., new inverter systems by implementing new approaches with new kind of IGBTS and consequently also new kind of Thermal Interface Materials which can handle the high voltage and high temperature applications.
SiC and GaN are a game changer for automotive applications
The switching speed of the Si-IGBTs is limited to around 15 kHz which means higher losses and bigger passive components. New materials with a wider band Gap like SiC and GaN can be used at higher frequencies and the size of other electric components (like…) can be reduced, which saves space. The power losses of e.g., SiC-MOSFETS, both switching and conduction losses, are very low and the total efficiency is higher in comparison to Si-IGBTs. SiC has a wide energy band gap which enables to withstand high voltages and to deal with a high-power environment and high temperatures.
The SiC is already used for more than 25 years in power converters for solar, wind energy and industrial applications. At the same time, more and more automotive applications like inverters, on board Chargers and DCDC Converters are following this trend. It can be assumed, that SiC Semiconductors increase the range of Electric Vehicles by 6 %. The advantages in comparison to Si-IGBT have been described above, but now also the costs are getting lower and the production process gets more and more stable which makes it interesting for the automotive industry.
Figure 3: SiC is one of the most promising semiconductors
For Thermal Management, these trends in the field of semiconductors are a new starting position.
Despite the semiconductor material, the power density in power electronic components is continuously increasing. This high level of generated heat and temperature requires smart thermal transfer from the electronics to the heat sink (e.g., cooling plate or alumina housing). Next to the thermal parameters, the voltage level in the used concepts of the above-described automotive applications is also continuously increasing which means, a reliable electrical isolation between electronics and housing is crucial.
To combine these two counter-rotating properties of thermal conductivity an electrical isolation is the job of Thermal Interface Materials. The success of reaching the target comes from usage of Tapes, Gap Pads, double sided adhesives, graphite tapes and liquid solutions like Pastes or Gap Filler Liquids (GFL).
Whereas in the past it was mainly Tapes and Pads that were used, these days liquid materials are being considered more often. Tapes and Pads have the great advantage, they can be applied easily by hand which is very comfortable especially in the prototyping stage. Those materials are approved, widely used and, what`s most important, reliable solution for IGBTs.
The trend goes to Gap Filler Liquids
For higher quantities GFL are becoming state of the art. The material price and in the end the price per part is lower due to the faster production time and higher material utilization as there is, despite production start and stop, no waste material.
The thermal conductivity of Pads and GFL is in a similar range of up to 6 W/mK, but the total thermal resistance in the application is lower for the GFL, due to the better compensation of the surface roughness. When it comes to the question, how to apply the materials in the serial production line, the GFL have an additional advantage. The dispensing equipment ensures a reliable and cost-effective solution by having a short cycle time. In many applications there are different electronic parts with different gaps and tolerances that must be connected. This means it can be possible to use one GFL instead of different Pads with various geometries.
Experience has shown that two-component low volatile silicone elastomers are the best solution to meet the requirements of automotive applications. These kinds of materials are characterized by high temperature resistance, long lifetime and a very good behavior against vibrations and mechanical stress which is caused by the different kind of thermal expansion factors of the involved materials.
Conclusion
More and more electronic devices are becoming established in the automotive market and of course one big part of that are the applications for electric and hybrid vehicles like Battery and Power Conversion (Inverter, DC/DC Converter, on board charger).
To reach the ambitious targets of the industry, the challenges for semiconductors and Thermal Interface Materials are becoming more complex.
As the applications gets more and more complex, it is important to focus on overlapping R&D. It is not only about one single part, it is about the whole concept which means the interaction of semiconductor, Thermal Interface Material and automation in the production line which determines the final outcome.
The GFL are getting more and more the favorite solution for the automotive industry, but there are still plenty solutions where new Tapes and Pads are required. Another trend is, that the percentage of customized solutions becomes higher due to the special requirements of the customer.
Wolfgang Höfer, Business Unit Leader Thermal Management Kerafol Keramische Folien GmbH & Co. KG
