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Thermal Interface Material Chemistry

There is no denying that silicone is the dominant chemistry in thermal interface material technology today. But it is slowly giving ground to other chemistries like polyurethanes and even epoxy.

Silicone Thermal Interface Materials

Silicone is and always has been the preferred thermal interface medium and for good reason. The most obvious reason being that it does the best at the highest heats. We are after all trying to dissipate heat, so it would only follow we’d select the chemistry that handles it the best. Silicones can handle temperatures well in excess f 200°c and not sacrifice their performance.

Silicones are also inherently soft. Many applications benefit from this reality. As things heat up and cool down, they expand and contract. A phenomenon known as CTE mismatch occurs and softer, more resilient materials like silicone handle this well. Not to mention the advantages low durometer materials like this bring to the table in terms of assembly stress. You will eventually need to push things together, so the softer the material, the better. Sometimes thermal interface materials are called on to cover mixed topography – drawing heat from several heat producing components of differing heights. Ultra soft pads and of course liquid dispensed materials are able to compensate.

Polyurethane Thermal Interface Materials

There is a steady rise in the use of polyurethane thermal materials for a host of reasons. But it’s probably pertinent to start with the one major weakness polyurethanes have in relation to silicones and that is their ability to deal with extreme heat. While silicones are good for over 200°c, polyurethanes are maxing out at around 140°c in this use case. But we’ve found that many thermal interface applications are not asking the material to hold up to temperatures over 140°c.

With that out of the way, onto the advantages and there are a few.

1. Cost. Polyurethanes are simply cheaper than silicones. Sometimes by as much as 50%. If you can get away with polyurethane thermal interface materials, then you should, as they don’t give up much else in the way of performance.
2. Versatility. Polyurethanes are versatile polymers. They can be made to be relatively hard, or very soft, reaching the Shore 00 scale for durometer. The options are virtually endless.
3. Outgassing. We have an entire learning on this. But silicone outgassing is a real phenomenon, and many designers and factories are looking to avoid it at all costs. While all materials will outgas, silicones put our volatile siloxanes, which are no laughing matter. Additionally, silicone oil likes to migrate. It will stick to surfaces, cause shorts and find its way to optics. There are factories that won’t allow them in the door for these reasons. Polyurethanes benefit, as they mimic many of the advantages silicones bring.
4. Adhesion. Modern designers are finding more and more that for true long-term reliability, they need some stick in order to keep things in their place long term. Polyurethanes are naturally more of an adhesive than silicone thermal interface materials that often offer only a natural tackiness.

Epoxy Thermal Interface Materials

Here we run into a bit of a nomenclature problem. There really aren’t many options for epoxies in traditional thermal interface material world, mostly because of hardness. Epoxies are, by nature, very hard when cured and they vast majority of thermal interface applications cannot account for this.

But there are an emerging number of thermally conductive adhesives becoming available and several different suppliers are throwing their hats in the ring. In many cases, other design features are freed up, because some heat can be dissipated by the adhesive that had to be there anyway.

Polyimide Thermal Interface Materials

These are better known as phase change materials. While any polyimides are known for their heat resistance, these ones are engineered to phase from solid to liquid at given temperatures, and have long been a reliable replacement for thermal greases and pastes. They are covered in detail here.