You think you’ve finally built the perfect sandwich when you suddenly take a bite and realize there’s nothing inside.
Often, the component between two similar materials allows the whole unit to succeed. In an electronic device, the thermal interface material ensures the safety and functionality of the other parts. It’s not as delicious as your perfect sandwich, but it’s equally important.
Having manufactured thermal interface materials in the past, we can confidently recount some of the more common questions we’ve received over the years.
Understanding the different thermal interface options helps ensure you choose the right material for your application. Today, you’re going to fully understand the factors determining which (if any) type of thermal interface material suits your needs.
What are Thermal Interface Materials?
Thermal Interface Materials (TIMs) are crucial to efficient thermal management. The purpose of thermal interface material is to transfer heat between two or more solid surfaces.
Today, they include an array of products, including phase change materials, gap fillers, and thermal grease, as well as less compliant materials like thermally conductive hardware, adhesive films, and thermal rubber pads.
What is Thermal Interface Material Made of?
There are many different types of thermal interface materials, so we will cover the main categories to give you a general idea of the advantages of each TIM.
As you learn about these thermal interface materials, remember how they might suit your application needs. Afterward, we’ll cover how they’re applied and what you should consider when making your final choice.
MATERIAL #1: ADHESIVE TAPES
Thermally conductive adhesive tapes have the benefit of being both a part of the thermal management system and the mechanical stability of the structure. Adhesive tapes typically adhere LED lights and semiconductor packages to heat sink surfaces.
Unlike several of the alternative thermal interface materials you’ll see, tapes only require pressure to function. You could also replace mechanical attachment hardware like springs and screws, which reduces your costs and installation time.
MATERIAL #2: PASTES, PUTTIES, GELS, AND GREASES
Thermal pastes, putties, gels, and greases are produced in the fluid state at room temperature. While some products are designed to harden or cure into a rubbery consistency after application, some will remain fluid throughout their working lives.
Putty is a material that is applied to increase thicknesses. It is used for filling in gaps that are present between irregular or unusual surfaces or those that do not touch one another.
Grease is usually sold as a non-curing product and applied in thin layers to help reduce thermal resistance between surfaces that are typically smooth and flat.
These products are electrically non-conductive. However, some can be designed to be conductive and may cause issues if the bridge nodes in the circuit shouldn’t be connected.
Unfortunately, depending on the application, several issues may arise. You might wonder, “What can I use instead of thermal paste?” In most cases, a better option is to use pads, tapes, adhesives, or roll goods, as they eliminate the most common problems.
MATERIAL #3: PHASE-CHANGE MATERIALS
Phase-change materials (PCMs), will change from solid to viscous liquid at lower temperatures (usually in the range of 131 to 149°F).
This phase-changing properly allows you to handle and process the materials as a solid at room temperature and maintain the wetting and comfortability properties of fluids at elevated operating temperatures.
PCMs offer attractive benefits compared to grease-state products. They are neater during a manual assembly process, don’t typically suffer from dry-out issues, and can be pre-applied for assembly in the future.
All this is possible while providing thermal performance similar to grease-state products. Yet, PCMs have a lower tolerance for misapplication, and they cost more.
PCMs can be found in several forms, including bulk rolls, sheets, pre-cut shapes to fit device packages, dispensable fluids, and deodorant-like sticks to provide a rub-on application. After the initial application, the dispensable liquids are designed to harden or dry to a solid phase-changing state.
The shape- and sheet-type products will combine the chosen PCM with other materials to offer desirable properties, including improved structural integrity for easy handling or a reliable electrical insulation characteristic. These products are often classified as “thermal pads.”
MATERIAL #4: POTTING COMPOUNDS AND LIQUID ADHESIVES
Thermal adhesives are a specialized glue designed to handle heat transfer while holding items together. You can purchase these as pressure-sensitive tapes or sheets and as curing fluids.
A thermal potting compound is mainly designed to provide protective encapsulation, which occurs while allowing heat transfer from the innards of a system to the outer shell.
Thermal potting compounds can be sold as fluid and have several adhesive and mechanical properties once cured. Some will barely stick to a surface when applied and can be peeled away easily. Others provide moderate adhesion, and some will even offer permanent adhesion.
Certain thermal potting compounds or liquid adhesives will have a rubbery and soft texture when fully cured and are easy to damage or tear, similar to a firm cheese. Others will cure almost in an inflexible state.
MATERIAL #5: ADVANCED MATERIALS
A newer type of thermal material is based on a type of carbon called pyrolytic graphite. These have the unusual property of being anisotropic, meaning the properties and direction of the substance vary when measured.
Through the thickness of these sheets, it is possible to conduct heat just as well as any other TIMs listed. The base material is very flexible and provides moderate electrical conductivity.
Because of the unique characteristics of these graphite-based materials, they can move heat around in the tight confines of modern consumer devices. They also work in applications where a quality thermal management solution is needed.
Gap Filler: Dimension Considerations
Understanding your application’s dimensions is critical for selecting the right thermal management materials. The thermal interface is the space between the heat sink and the component, and the thermally conductive media used here is the TMI.
The space is typically small (usually on the micron scale). A gap-filling application has more to do with the distance between a component and the metal housing enclosing the electronics assembly. This is usually measured in millimeters.
In this situation, the thermally conductive material helps minimize the possibility of hot spots in the unit, while the casing works as the heat sink.
The difference between a few millimeters and a few microns is critical to the performance of the thermal material you choose. For example, if you place the TIM in a gap-filling application. It will likely be more unstable in the thicker layer with vibrations or thermal cycling causing displacement.
Also, if a gap-filling material is used for a thermal interface application, achieving a thin, even film will likely be challenging. This creates higher thermal resistance at the interface, resulting in minimal heat transfer efficiency.
WHAT MATERIAL IS USED FOR GAP FILLING?
Gap-filling material must be conductive and narrow enough to fit its intended purpose. Many gap fillers are made of silicone because of its variability and ability to function as a thermally conductive material, but thermal pads might also have ceramic powder mixed in for the sake of better conduction.
IS EPOXY OR RUBBER THERMALLY CONDUCTIVE?
Epoxy and rubber (or silicone rubber) typically have very low thermal conductivity, so they are typically used for bonding devices during electrical insulation or heat dissipation.
Thermal Pad vs. Paste
There are several options when choosing between thermally conductive materials. This decision is determined by your application and production design, along with critical performance factors that have to be achieved.
For example, deciding between a thermal pad or thermal paste material will depend on if the heat sink needs to be held in place by the interface material. If so, a bonding compound, like the thermal pad, is the better option. This offers the extra benefit of being pre-cut to size to ensure a smooth application.
However, both of these options will provide a thicker interface layer and higher thermal resistance. The trade-off comes from the performance requirements of the selected compounds, along with an understanding of the application’s conditions.
What is the Best Thermal Interface Material?
When it comes to choosing the best TIM, the issue of material compatibility is one that is often overlooked. That’s because it is not considered a significant problem in many of the modern electronic applications where it is used.
Yet, there are some situations where it could be a problem. For example, some humidity and gas sensors may be damaged if exposed to silicone vapors, meaning you’ll want to choose a silicone-based TIM.
Another example is plastic optics and silicone-encapsulated LEDs, which might be damaged when exposed to certain organic vapors. These commonly unforeseen issues are why it’s a good idea to verify the compatibility of the materials with the thermal adhesives you are considering.
How To Apply Thermal Interface Material
The application technique you use is dependent on the product. The application method may be automated dispensing or screen printing for non-curing and curing products. The main difference is the working time of the curing material.
For example, if the product dries to the touch quickly, it may not be suitable for stencil printing. This is because the cured product could block the screen.
Usually, a small amount of the material should be applied for gap filling and thermal interface applications to help ensure maximum heat transfer. The layer must achieve uniform coverage on the whole interface.
When gap filler is used, the material must be applied while ensuring all the air is expelled. This is because air is a poor conductor of heat and may result in additional hotspots.
If you choose encapsulation resin, you will likely have to cover the entire PCB. The amount of resin you need to apply must strike a balance between the desired level of protection and reduce any volume or weight gains from the resin’s application.
Understanding Thermal Interface Material: Now You Know
When it comes to thermal interface material, there is a lot to know. Understanding the options and knowing what factors to consider will help you make the proper selection for your application.
In most cases, the more seamless application offered by pads, tapes, and adhesives will be best. This reduces the mess, application hassle, and other issues with gels, putties, and greases.
Remember, though, issues can arise. Working with a professional service provider may be best to ensure you get the desired outcome for your application. Each thermal interface material is unique, and understanding this is the first step to finding the right option.
If you’re interested in finding out more about thermal interfaces and other materials, check out our learning center linked above. Otherwise, if you need materials or help to decide what is right for your application, feel free to contact us at Strouse. We are here to help with your TIM needs, regardless of what you’re building.
Originally published: July 16, 2020