Thermal interface materials (or TIMs) are a necessary link in the chain for thermal management solutions for heat-sensitive devices. If you see a heat sink in your system, by necessity there’s a TIM underneath it to improve and enable thermal transfer between the heat sink and the part being cooled. Silicone TIMs are common, popular, and widely used, but can suffer from silicone oil bleeding during extended use that can cause problems in some designs. Can this tendency be eliminated?
The answer is that this oil bleed tendency cannot necessarily be eliminated, but it can absolutely be mitigated.
The first question that folks ask is whether silicone-free TIM, such as Laird’s A18841-08, are free from the effects of oil bleed. While silicone-free parts, by definition, are free of silicone oil bleed, it doesn’t necessarily follow that they’re free of material bleed altogether. Oil bleed in TIMs is generally a factor of the material’s base fabrication.
These materials are formed from cross-linked or entangled elastomer chains molded into shape with a working material such as alumina mixed into the elastomer to provide thermal conductivity. Despite seeming to be solid when formed into a pad, TIM exists across a range of solid to liquid product determined by how much cross-linking occurs within the polymers. The more cross-linking the manufacturer causes, the firmer and more solid the material is. Softer, less firm/solid materials are easier to work with, more effectively fill gaps between component and heat sink to improve thermal transfer, and require less mechanical stress to compress, but they also generally have a lot more unlinked polymers. These unlinked polymers are the source cause of oil bleed in TIMs, as compressing a TIM to properly gap fill beneath a heat sink will eventually squeeze this unlinked material free of the thermal pad.
To minimize oil and other material bleed, one should first check for products specifically calling out low bleed or low volatility. Products that do not mention any sort of control on bleed/volatility can be presumed to not have taken any especial measures to control or reduce this phenomenon. There’s no single specific hard-and-fast rule, but if forced to guess based on little other information, firmer materials are somewhat less likely to evince bleed than softer ones. As well, the less compression you use to hold the heat sink, the thermal pad, and the device being cooled together, the less bleeding you experience and the longer that bleed will take - the balance being that firmer materials with less compression will not offer the same thermal performance as a more typical TIM assembly. Though considered distinctly old-fashioned and unable to match the thermal transfer performance of modern polymers, mica pads such as those available from DigiKey do not bleed any oil or other residues as they do not have any unbound elastomers to bleed if not paired with thermal greases or pastes.
For more information and some tips on controlling silicone bleed in applications where the use of silicone is unavoidable, see Laird’s application note on controlling silicone bleed..
For more information on TIM itself, please check our guide to Thermal Interface Materials