Thermally conductive adhesives are a category of Thermal Interface Materials (TIM) that provide a combination of low thermal resistance and high bond strength across an interface. These polymer products are typically utilized in such applications as heat sink mounting, die bonding, electronic encapsulation, coil potting, and more. However, researching these types of products on the web and identifying the best thermally conductive adhesive for your application can be a daunting task.
This post is dedicated to simplifying the research and selection process by explaining some TIM basics, giving some concept to typical values and properties, and providing some actionable tips for researching thermally conductive adhesives on the web.
Recognize The Alternatives
It is first important we mention the alternatives to weed out any applications that may not even be best suited with an adhesive for their thermal management application in the first place. If you’re sure a thermally conductive adhesive is what you’re looking for, feel free to skip this section.
There are several types of thermal interface materials, and each has their own place in heat management assemblies. TIMs include:
Thermal Greases (Thermal Compounds)
- Typical Thermal Resistance Kcm^2/W: 0.2 - 1.0
- Description: Silicone or hydrocarbon greases, loaded with thermally conductive filler materials to provide low thermal resistance. Thermal greases are simple but messy thermal interface solution.Thermal Pads
- Typical Thermal Resistance Kcm^2/W: 1.0 - 3.0
- Description: Pre-cured thermally conductive elastomeric pads are easy to apply but aren’t able to provide thermal resistance as low as greases due to their thickness. Typically require high pressure clamping to achieve reasonable thermal.Phase Change Materials
- Typical Thermal Resistance Kcm^2/W: 0.3 - 0.7
- Description: Solid materials that melt between 50-80C. Combine pad convenience and grease thermal performance. Provide some adhesion and are not rework able like greases and thermal pads.Thermal Tapes
- Typical Thermal Resistance Kcm^2/W: 1.0 - 4.0
- Description: Thermally conductive tapes are like thermal pads but provide more substantial adhesion across the interface.Gels
- Typical Thermal Resistance Kcm^2/W: 0.4 - 0.8
- Description: Like greases but partially cure to reduce mess. Comparable to grease.Solder
- Typical Thermal Resistance Kcm^2/W: 0.05
- Description: Traditional heat transfer method. Requires high heat, electrically conductive (most other options are electrically insulating), not rework able, and not suitable for large bond areas.Thermally Conductive Adhesives
- Typical Thermal Resistance Kcm^2/W: 0.15 - 1.0
- Description: The topic of our discussion. Thermosetting polymers provide substantial long-term strength across the bond interface, and 2nd to only soldering in ultimate heat transfer capabilities. Requires a more robust manufacturing process.
If you're at the stage in your research process where you're not sure which thermal interface material is best for your application, reach out to us and tell us a little bit about your application. We're always happy to provide free advice when we can.
Know The Typical Chemical Families
If you’re researching thermally conductive adhesives on the web, one thing that will become immediately apparent is that there are really only three chemical families that adhesive formulators use for this type of product. Thermal interface adhesives are generally epoxy, polyurethane, or silicone-based systems.
- Epoxies are best for applications that require high levels of mechanical strength across the bond interface. These products are able to carry large filler quantities, allowing them to achieve some of the highest levels of thermal conductivity. Cure mechanisms include two-part room temperature curing systems or one-part heat curing systems. Some frozen one-part systems exist as well that will cure at room temperature.
- Polyurethanes aren’t able to achieve thermal conductivity values quite as high as epoxies, however, they can be formulated as low modulus materials and perform exceptionally in potting applications. Polyurethanes are generally chosen when the priority is impact or physical stress relief rather than thermal dissipation. Cure mechanisms are limited to two-part curing systems.
- Silicones can provide thermal conductivity comparable to epoxy systems, however, aren’t able to provide nearly as much mechanical strength. These products, however, perform exceptionally well at high temperatures and can be formulated as low modulus materials.
Don’t Be Fooled By Listed Thermal Conductivity Values
As you peruse the web searching for the most thermally conductive product available, you will be tempted to dedicate all your attention to a single metric, bulk thermal conductivity. At a glance this makes sense. We’re trying to achieve thermal conductivity; lets find the product that lists the highest value. However, in practice a product’s thermal resistance tends to differ greatly from the theoretical value estimated by its thermal conductivity.
Calculating theoretical thermal resistance from thermal conductivity is easy. The formula is shown below.
So what goes wrong in practice?
- Thermal conductivity doesn’t account for minimum bond line thickness: An adhesive’s bond line thickness is limited by the choice of filler material. In many cases choosing an adhesive that uses a smaller particle size but lower bulk thermal conductivity may ultimately result in a bond line with a lower thermal resistance. Make sure to check an adhesive’s maximum particle size. Exception – potting and encapsulation applications will not suffer from a large particle size. Their “bond line” is generally limited by how thick the material is applied, not particle size. In these applications choosing a large particle material may be beneficial.
- Thermal conductivity is often measured under perfect conditions: When formulators determine the bulk thermal conductivity for their product they will (for obvious reasons) try and create perfect conditions. Materials will be mixed and thoroughly de-aired prior to polymerization – preventing air voids in the material – and clamps will be used to insure excellent substrate contact. However, these practices set up for conditions that are generally not realistic for the end user to recreate. Lower viscosity products will inherently have fewer issues associated with air voids and substrate contact. With this in mind, choosing a lower viscosity but lower thermal conductivity product may ultimately result in a bond line with a lower thermal resistance.
Some Applications May Benefit From Electrically Conductive Adhesives
Some of the best thermally conductive fillers are adamantly avoided by adhesive formulators because they also increase the electrical conductivity of the resulting polymer material – a property unacceptable in many electrical applications.
However, for those few applications that can tolerate electrical conductivity in their thermal material, engineers should instead consider looking at products listed as electrically conductive adhesives. Electrically conductive silver filled epoxies for example can display exceptional levels of thermal conductivity.
Don’t Be Afraid Of Pursuing A Custom Product
Particularly for thermally conductive adhesives, pursuing a custom product may make the most sense. In some cases an entirely novel product can be created, however, in many cases an existing product simple needs to be tweaked. All the different properties we have discussed so far – thermal conductivity, viscosity, and particle size – are values that can easily be changed to meet your application specifications. Many adhesive formulators offer customization free of charge.
If you’re not sure who to contact, let us know! Tell us about your application! We can help! At Cova Scientific we help engineers identify and connect with formulators across the country. We can help you take the next steps forward.