Thermal interface material is crucial to every thermal management solution. It's important to know the characteristics of various thermal interface material types so you're well equipped to make the right choice for your application.
Selecting the Right Thermal Material
Depending upon your application, you may want to use one type of TIM over another to help facilitate better performance. Some are rigid, others flexible. Some TIMs are solid and others can change between phases. There's a wide range of thermal interface material types available for improving heat transfer between surfaces, but it's crucial to spec in the right type.
So let's get into them!
Thermal Interface Material Type List:
For anyone who's built their own PC, thermal grease is probably the first of the thermal interface material types that comes to mind. Thermal grease, as you can guess, is a grease specially designed to have a high thermal conductivity. Most thermal greases are silicone based with tiny thermally conductive filler particles that increase the overall conductivity of the mixture. There are silicone-free greases on the market for applications that are sensitive to silicone. Applications that are concerned with the wettability and adhesion of surfaces that may come in contact with a thermal grease would benefit from using a silicone-free compound.
Widely Available and Common Application Choice
Thermal grease is easy to acquire, making it popular for DIY projects or smaller quantity prototype or production runs. For applications that require consistency from one product to the next, it's relatively simple to create a template for screening on thermal grease. This makes application specific grease patterns simple and cost effective. Other thermal interface materials require die cutting to produce custom shapes, which are typically more expensive than a grease screen.
Low Interface Resistance, Ideal for Flat Surfaces
Since greases are a pseudo type of fluid, applying pressure to thermal grease between two surfaces forces the grease to shear and spread thin between those surfaces. This works to our advantage in thermal management. The thinner the material between the surfaces you're trying to transfer heat between, the less resistance your interface material will impose on heat transfers. This makes thermal grease ideal for flat and smooth surfaces. Rougher or more detailed surfaces with different heights have small pockets where the grease can't quite fill, which is why other thermal interface materials like gap fillers were designed.
Thermal greases require spring-loaded mounting forces. As the thermal grease heats up, it can flow a little bit and thin out. To make sure that both surfaces are consistently in contact and compressing the grease, it's best to use a springy force to mount greased surfaces.
Greases, What a Mess
We joke in the test lab about the uncanny ability of thermal grease to get anywhere and everywhere. Look at it wrong and it gets on your shirt. Like any other grease, thermal greases can be tricky to clean up and keep contained well. In smaller quantities, grease is kept in tubes and syringes, allowing for more control over application. Larger quantities of thermal greases come in larger containers with big lids and applying grease from a open tub can make a mess.
For some applications, this ability of thermal grease to seemingly travel on its own accord rules it out as a potential TIM. If it's safely applied and tucked away from any regular areas of maintenance, thermal grease could be a viable option for your application. Otherwise, look to less liquid-like substances for the right thermal interface material.
Grease is Non-Reusable
While there are many great things about thermal grease with is flexibility and ease of application, it comes with the drawback of not being reusable. After the grease is compressed and thins out, there isn't a surefire way to gather the grease back up into the original thickness you applied without introducing air pockets that negate the whole point of a thermal interface material. This is why manufacturers like Aavid offer thermal solutions with thermal grease pre-screened on heat sinks in order to provide a consistent amount of grease along with a grease cover, to protect the grease until ready for installation.
After extended periods of use, thermal grease has a tendency for more volatile chemicals in the mixture to outgas and dry out. The chemicals that outgas are to decrease viscosity and simplify the application process, so it's not a problem for the product years down the road. It is a problem when it comes to rework. The grease is left as a crumbly mess and unable to be reapplied. The only way to get the same performance as before is to reapply fresh thermal grease.
Gap fillers are another popular interface material type. Gap fillers are elastomeric sheets, typically made from silicone, that contain specialized thermal filler material to increase the overall thermal conductivity of the material. These materials come with a wide range of options, so it's fairly easy to find a suitable gap filler for a specific application. Gap fillers are typically cut to standard device sizes or customized shapes for specific applications.
Wide Range of Gap Filler Material Options
Gap fillers are probably the most diverse major thermal interface material type. All gap fillers have a base elastomer and a thermal filler mixed in, which include silicone and silicone free materials. These are just a fraction of options available when selecting a gap filler. Within the same elastomer and filler mix, there are multiple sheet thicknesses, tacky or adhesive options for each side of the sheet, reinforcement materials like fiberglass, and carrier options for protecting the material before application. Some materials can electrically isolate hot devices. Other gap fillers have the ability to absorb electromagnetic interference (EMI). Between all of these options, you could have a hundred options with one material type. This range of options is what makes gap fillers a popular selection when it comes to thermal interface material.
Accommodating Tolerance Stack-Up and Multiple Devices with Gap Fillers
Since gap fillers are made from an elastomeric base material, it has a springy quality to it. This means it can be compressed and can apply pressure proportional to its deflection against the surfaces pressing against it. But instead of an axial spring, it's an elastic surface that can be compressed varying amounts across its entire surface. This is why gap fillers are so effective in accommodating tolerance stack ups and multiple devices. Gap fillers will yield to varying heights, so if there's a particular device where the tolerances stack up and have a a bit of variance, the gap filler can still effectively connect a device to a heat sink. And it doesn't need to be just one device, it could be multiple devices that need to connect to a single heat sink. With gap fillers, it's possible.
Somewhat Reusable Thermal Interface Material
Gap fillers have some level of reusability. Since they're elastomeric, these thermal interface material have the ability to spring back into place. It's when we push too much that we create plastic deformation in the gap filler where it wouldn't be able to fully recover its original thickness. So if we stay within that range, we can use gap fillers again. If there is an adhesive side to the gap filler, it may not peel off well, which would also limit its ability to be reused. Adhesive or tacky surfaces also have the uncanny ability to find any and every particle floating around, so the surface may get dirty if the de-installation and re-installation of the gap filler is not in a clean and controlled environment.
Thermal interface material in the form of hardware is typically employed for it's high thermal conductivity and electrical insulation properties. Some hardware is also used as bushings or bearing surfaces. Thermal interface insulating hardware is known for mechanical stability and higher temperature resistance compared to other TIMs.
Insulating Hardware Materials
Thermally conductive ceramics like aluminum oxide, aluminum nitride, and beryllium oxide are commonly used since they're affordable and relatively easy to manufacture into discrete hardware components. Mica, a naturally occurring mineral, has a sheet like structure with excellent conductivity through the plane of the sheets. Since it's widely available and easy to process, mica is also a popular material choice when it comes to electrically insulating yet thermally conductive hardware components. Hardware can also be manufactured from plastics such as nylon, PTFE-filled acetal, or diallyl phthalate. Plastics that are used for insulating hardware require high dielectric strength, and good thermal and chemical stability.
Inflexible to Modification and Customization
Hardware needs to be fabricated in the shape that it will be used in, especially ceramic TIM hardware. Most hardware is device specific or created to fit very specific dimensions.
The ceramics need to shaped for the device it will be used for and include proper lead and mounting holes before it's fired. Otherwise, post machining would be required. Machining ceramics can be difficult and potentially hazardous. Ceramics are brittle and require delicate handling when machining and in the case of beryllium oxide, special precautions must be utilized to protect against inhalation of any particulates. Small particles of beryllium oxide are toxic when inhaled into the lungs. So all in all, it's much easier to make ceramics the shape you need right from the get go and not after.
Plastics are a little more forgiving than ceramics when it comes to machining. Since most of these pieces are injection molded or extruded and cut to length, post processing typically isn't required. But you may have an application that needs something a little different. In these cases, it tends to be easier to fabricate plastic insulating hardware in the final shape required since machining plastic can come with its own complications. Most machining processes heat up the plastic and cause unwanted deformation and warping. Some plastics can catch fire. Others may be too brittle and snap. In general, it's best to get the shape you need for your plastic insulating hardware right when you purchase them.
Plays Well With Others
Since most hardware has very little elasticity or compressibility, they're used in conjunction with other thermal interface materials. In cases where the surface isn't extremely smooth, a more compliant material can be used with insulating hardware to remove air gaps between surfaces. Thermal grease, which may not have the level of electrical isolation required for a particular application, would be placed on both sides of the hardware to minimize any air gaps. This provides the application with both high thermal conductivity and high electrical isolation between surfaces. In some cases, a material like a gap pad or gap filler should be used with hardware, especially if the application may have shock and vibration concerns. In these situations, insulating hardware thermal interface materials are best used with another thermal interface material type.
Hardware, so long as it's not broken, can be reused like any other hardware. Simply remove from one application carefully and install the hardware into a new assembly. Though if you're using hardware in conjunction with another thermal interface material like grease, the grease would need to be cleaned off and replaced for the new application of the hardware..
Thermal Pads & Films
Thermal pads and films are thin materials used to conduct heat from one surface to another. These interface materials are also ideal for heat dissipation away from hot spots. All but a few thermal pads and films are flexible materials. Like gap fillers, thermal pads, films, and foils are typically cut to standard device sizes or customized shapes for specific applications.
Thermal Pads are typically made from a higher durometer silicone based material than gap fillers. Like gap fillers, silicone pads also doped with more conductive materials like aluminum oxide or boron nitride. Additionally, thermal pads are reinforced by fiberglass or another material to increase tear resistance of the material. This makes thermal pads a robust and compliant replacement for thermal interface hardware.
Thermal films are commonly made of polyimide, a transparent thermoset polymer that has great electrical isolation properties. You'll also hear it referred to as its brand name Kapton. Films can be made from other materials, like graphite, which we'll get into in a moment.
Not all thermal pads and films are considered electrically insulating. For those that are electrically insulating, they're becoming a more popular option than insulating hardware because of their flexibility, light weight and extremely thin properties. This is especially advantageous for consumer electronics that continue to pack more power and components into thinner devices. If you've got high power applications and require some level of flexibility or compliance with your thermal interface material, thermal pads or thermal films are the preferred choice.
Graphite Pads & Films
Graphite films are the exception to the general rules of a thermal pad or film. Graphite pads are made of a stack of graphene sheets on top of each other, so heat and electricity travel easily between the carbon atoms bonded together within a sheet. Graphene sheets do not bond strongly together since the carbon bonds are already made in the graphene planes and not between the While excellent when it comes to spreading heat along its plane, graphite films are relatively delicate and brittle compared to other films and pads. They are also non-electrically insulating since electrons can easily travel through the graphite structure.
While that being said, graphite interface materials can effectively replace thermal pastes or greases. While we'll get into phase change materials in a little bit, graphite materials are a useful material when your application doesn't reach melting temperatures of phase change materials. Graphite materials have high temperature resistance, so they can be used in applications at temperatures beyond 200°C. At extreme temperatures, beyond 200°C, placing graphite films in a vacuum environment should be considered. This will prevent oxidation of the carbon within the graphite films. graphite films to be used for EMI shielding up to the GHz range, with superior attenuation.
These films are not reusable since the pressure can make the graphite stick to each of the surfaces it contacts. When the surfaces connected by a graphite pad are removed from each other, the graphite pad sticks to the surfaces but peels apart along the individual layers of graphene of the pad.
Reusability of Thermal Pads and Films
Since some pads contain silicone, they're similar to gap filler thermal interface materials where if you compress them past a certain point, they stay slightly compressed. Fortunately, most thermal pads have a high durometer and take large amount of force to deform the shape and thickness of the pad. This makes thermal pads an ideal reusable thermal interface material.Films are more like hardware when we consider reusability. Generally, if the thermal film is in tact and hasn't been creased, it's just as reusable as thermal hardware is. If the film has been creased, it may generate unwanted air pockets.
Thermal Tape is a common interface material. Thermal tapes are adhesive on one or both sides so can either stick to just one surface, or join two surfaces together. This is typically done with a pressure sensitive adhesive that you need to compress between surfaces to get the mechanical bond a thermal tape can provide. The biggest difference between thermal tapes and your everyday, run-of-the-mill double sided tapes is that they are specially formulated using high thermal conductivity fillers and polymers.There are thermal tapes that are just an adhesive. They're placed on a liner or carrier to keep them in a sheet or roll before they make they're way to their application. These baseless thermal tapes can be tricky to manipulate and apply correctly if not cut and handled properly. If any part of the thermal tape starts sticking to something, it can be difficult to remove the tape without stretching the adhesive. This is why a good portion of thermal tapes have a base material. These carriers are usually a thermally conductive film with the adhesive applied to one or both sides.
Don't Worry About that Lose Screw
Thermal tape can reduce the need for mounting hardware for smaller devices and heat sinks, which you would be typically be mounting together. This is especially helpful if you find a small device on your board needs some thermal management after you've designed and spun your board. You may not have room for mounting hardware, but you still have the ability to stick on a heat sink to your device with some thermal tape.
The Weight of the World
You may want to reconsider thermal tape for your application if you have a heavy duty application or heat sink you're trying to apply to a surface. The weight of larger heat sinks may overpower the mechanical strength of the thermal tape. High level of vibration or shock may also overpower the bonding power of the thermal tape. While thermal tapes have a certain level of mechanical strength and are good in a pinch, they're typically not the best thermal interface choice of rough applications. Stick to smaller applications with thermal tape.
I'm Stuck on You
Thermal Tapes are not reusable if they're doing their job. They want to stick to what ever you stick them to and not let them go. There are tricks like heating up the adhesive and then trying to pull surfaces apart to disassemble an assembly with thermal tape. Or some adhesives need some sort of solvent or cleaning agent to help remove any residue from the surfaces, which ruins the adhesive power of the tape. Peeling thermal tapes away from their applied surface typically renders them uneven and ineffective as a thermal interface material for future applications. Surfaces need to be cleaned and new tape needs to be applied. But thermal tape is fairly easy to apply, so it doesn't necessarily rule out thermal tape for applications that require rework or maintenance.
Phase Change Material
Phase change material is an interesting thermal interface material type. It's composed of a wax substance that has a specific melting temperature, typically between 50-65°C. While the material is transitioning from a solid to a liquid, the temperature of the material stays consistently at its melting temperature as is absorbs heat. This provides excellent temperature control between surfaces. Once the phase change material absorbs its latent heat of fusion, the energy it takes to completely melt the solid, then the phase change material will start to increase temperature while in its liquid state.
Many phase change material are deposited onto a highly thermally conductive base material that is also installed in the application. Some use a thermal film or aluminum foil to hold the material before and while it's installed. Other phase change materials have films on both sides so as you install the waxy material, the films from both sides are removed, leaving just the phase change material between the surfaces.
Infiltrating Every Nook and Cranny
When phase change material is heated up past a specific temperature, it melts and flows into any existing nooks and crannies between the surfaces it's between. Phase change thermal interface materials remove even the tiniest air pockets and provide really low interface resistance between surfaces. So after the first time the phase change melts, you can count on a consistently low thermal resistance between the surfaces you're transferring heat between.
We'll Get Over This Rough Patch
Because phase change materials turn into liquids, it can get into some tight spaces that other thermal interface materials can't quite get at. This also means that it can handle rougher surfaces easily. Surfaces with imperfections, rough spots, or any surfaces less than perfect could benefit from using phase change material for heat transfer. Gap fillers are still the best choice for huge height disparities though. A large amount of phase change material would need to be added to take up the same volume that gap fillers can.
Must Love Springs
Like thermal greases, phase change materials thin out after it's first applied between surfaces. As the wax melts and fills up any available voids, that material is now surface imperfections and no longer adding to the thickness of the material. This is why phase change materials should be used with spring-loaded mounting methods. A spring force will compress the phase change material while it's in a liquid state. The force helps thin out the material, which also decreases your interface resistance. All of this helps improve our thermal transfer between surfaces.
Easy Clean Up When Replacing Phase Change Material
Like thermal grease, phase change material is not reusable, but also not a mess to clean off like grease. Unlike greases, phase change materials revert to a more solid form when cool, making it easier to scrape off surfaces. Typical cleaning fluids like isopropyl alcohol can also be used to clean off the wax like phase change material without needing to otherwise treat the surfaces.
Thermal epoxy is the most robust thermal interface material. What sets thermal epoxy apart from other epoxies is the thermally conductive fillers mixed in with the resins. Some epoxies use thermally conductive ceramic particles and others use small metallic particles. Like other epoxies, there are one part and two part resins that can be mixed and applied to join surfaces together. The type of epoxy used is typically dependent on the materials being joined together.
With the Strength of Many Materials
Epoxy does something that most other thermal interface materials don't; Thermal epoxies create a strong mechanical bond between the surfaces it cures between. This allows thermal epoxy to both be a thermal interface material and a mounting method. In some cases, this can help reduce the amount of mounting hardware utilized in a product or application. This is why we can make heat sinks with epoxy, which we refer to as epoxy bonded heat sinks.
Potential Shipping Limitations
Thermal epoxy is not reusable. Like any other epoxy, once you set thermal epoxy, the polymeric bonds that form and attach to surfaces will not easily break. This is why you should consider the amount of rework you may need to do on your product before you determine thermal epoxy is right for you. If you need to conduct maintenance on your device that has epoxied surfaces, you're going to have a hard time getting around your heat sink and an even harder time removing it.
While there are solvents on the market that can remove particular cured resins, they're specialized products that you typically wouldn't have in the lab or workshop. Sometimes you just need to take your epoxy bond to the show and just saw or mill off what you've epoxied together. That's certainly undesirable when you have a delicate circuit board on one side of that epoxy bond.
You do have some give and wiggle room to reorient and remount epoxy bonded surfaces before the epoxy cures. This amount of time depends upon the temperature, humidity, and cure time of the epoxy you're using. It's usually easier to clean up uncured epoxy if you need to redo something, so you need to be sure about what you're doing before you even mix your epoxy and certain by the time the epoxy starts to cure.
With the Strength of Many Materials
Resins and hardeners that make up thermal epoxy can have some fairly volatile chemicals in them. That's why there can be some shipping restrictions when it comes to shipping uncured epoxy. Uncured thermal epoxy may need to be shipped by ground since air shipping can carry risk that air freight companies to not want to manage.
Thermal Interface Wrap Up
Whew! That's a lot of information about thermal interface materials. So don't feel overwhelmed by all the options and nuances of each material. Boyd has a bunch of engineers well versed in thermal interface material selection and application.