In electronics, you often need to transfer heat from one flat metal surface to another. A good example of this is in an LED light, where the heat from the LED module needs to be transferred to a heat sink located behind the module. However, it turns out that simply placing one flat piece of metal on top of another is a very poor way of transferring this heat. On a microscopic level, the metal planes will not be perfectly smooth. Each of the contacting surfaces will have micro-imperfections such as bumps, dips, peaks, and cracks. When two of these ‘rough’ surfaces are placed on top of each other, there is surprisingly little direct contact. In a typical application, usually less than five percent of the surface is actual metal-to-metal contact. The parts of the surfaces that are not in direct contact with each other will have small air gaps between them. This “interstitial air” is a very poor thermal conductor and can keep a lot of heat from flowing from the hotter plate to the colder one. The solution to improving heat flow is to put a thin, soft, thermally conductive layer between the two surfaces. This Thermal Interface Material (TIM) will drive out the interstitial air and replace it with something that is a much better conductor of heat. Many substances have been used as a TIM, such as oily pastes, grease, soft metals, and now flexible graphite.
NeoGraf’s Compressible vs. Non-compressible Flexible Graphite TIMs
The choice of which material to use depends on several factors:
- The needed lifespan of the TIM is critical. Many materials, such as paste or grease, will dry out, pump out, or leak over time. When this happens, the dry TIM becomes a thermal barrier.
- The material must withstand whatever temperature the metal surfaces will experience without performance degradation.
- The material must not outgas anything that could harm the device.
- The TIM may need to be reused, or reworked, without reducing its effectiveness.
NeoGraf Solutions developed and marketed two lines of eGRAF® HITHERM™ Thermal Interface Materials. These are the compressible and non-compressible versions. Non-compressible flexible graphite is ideal for when both surfaces are parallel with each other. The soft flexible graphite easily rearranges itself to fill in the micro surface roughness features. This creates an easy path for the heat to flow from the hotter plate to the cold. For some applications, both metal surfaces are not parallel. One surface may even be concave or convex. This can happen when the metal surfaces are very large, such as in an IGBT module, or if the surfaces are only screwed down at the edges. In cases such as this, compressible flexible graphite TIM is ideal. The flexible graphite will compress at the higher-pressure areas, such as the screw holes, yet still make solid thermal contact at the lower pressure areas of the plate.
Common Applications for TIMs
A recent example of compressible flexible graphite being used to solve a tricky thermal interface problem is with the overclocked home computer industry. When taking a home computer to the limits of the processor, massive amounts of heat is generated. If the heat is not removed as fast as it is generated, the processor can easily be damaged. Soft, compressible flexible graphite is used as the TIM between the computer processor and the liquid cooled heat sink. The flexible graphite easily conforms to both the top of the processor and the heat sink, even if they are not always completely parallel. This material has allowed the overclocked computer enthusiast community to apply a graphite TIM, then be confident it will last as long as the device itself.
A recent example of where non-compressible graphite was ideal was in a recent series of space communication satellites. These satellites needed a permanent TIM between the processors and the heat sinks. For this application, it was vital that the TIM did not outgas anything that could damage the sensitive satellite components. Graphite TIMs are perfect for this type of application because they don’t outgas and are listed on the NASA outgassing page as an approved material. It was also critical that the material didn’t dry out or otherwise be affected by the harsh temperature and pressure conditions of space. Since graphite is a solid material, which cannot dry out and is temperature insensitive, it easily met both these challenges.
As a world-leading North American manufacturer, NeoGraf Solutions is a global supplier that provides consistent, high quality, high-value flexible graphite products that meet the most stringent global environmental sustainability standards. Contact us today to learn more about NeoGraf’s graphite Thermal Interface Materials and our diverse line of additional high-quality flexible graphite products.