Everything You Need to Know about Adhesive Converting

Your Guide to Thermal Interface Material

Posted by Sue Chambers Jul 16, 2020 10:30:10 AM

Thermal Interface Materials (TIMs) are considered a crucial part of any efficient thermal management system. Their job is to transfer heat between two or more solid surfaces.

Today, thermal interface materials include an array of products, including phase change materials, gap fillers, and thermal grease. There are also less compliant materials, such as thermally conductive hardware, adhesive films, and thermal rubber pads.

Better understanding the options can help ensure you choose the right material for your application. It is crucial that you fully understand the factors that impact what material is right for your needs. Keep reading to learn more about this here.

Types of Thermal Interface Materials: Flavors and Categories

There are several thermal interface materials used today. The most common are described here.

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 are usually used to adhere LED lights and semiconductor packages to heat sink surfaces.

Unlike some of the other materials discussed below, tapes only require pressure to achieve their full potential. You could also replace mechanical attachment hardware like springs and screws, which reduces your costs and installation time.

Pastes, Putties, Gels, and Greases

Thermal pastes, putties, gels, and greases are produced in the fluid state at room temperature. They usually have a consistency that is like toothpaste or soft butter.

While some products are designed to harden or cure into a rubbery consistency after applied, some will remain fluid throughout their entire working lives.

Grease is usually sold as a non-curing product and applied in thin layers to help with the thermal transfer between surfaces that are typically smooth and flat. 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.

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 should not be connected.

Unfortunately, depending on the application, several issues may arise. In most cases, a better option is to use pads, tapes, adhesives or roll goods, as they eliminate many of the most common issues. 

Phase-Change Materials

The phase-change materials or PCMs will change from a solid-state to a viscous liquid at lower temperatures. This usually is in the range of 131 to 149 degrees Fahrenheit.

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.

These are relative to grease-state products, but PCMs offer a few more appealing benefits. They are not as messy during a manual assembly process, they do not typically suffer from dry-out issues, and can be pre-applied for assembly in the future.

All this is possible while providing thermal performance that is similar to the grease-state products mentioned above. While this is true, PCMs have a lower tolerance for misapplication, and they cost more.

PCMs can be found in several forms. These include bulk rolls or sheets, shapes that are pre-cut from the same form to fit certain device packages, dispensable fluids, and deodorant-like sticks to provide a rub-on application. The dispensable fluids are designed to harden or dry to a solid phase-changing state after the initial application.

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 even a reliable electrical insulation characteristic. These products are classified as “thermal pads” or something like that.

Potting Compounds and Liquid Adhesives

Thermal adhesives are actually a specialized type of glue that is 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. This occurs while allowing heat transfer from the innards of a system to the outer shell. They can be purchased in fluid form.

Products that are sold as fluids can have several adhesive and mechanical properties once they are cured. Some will barely stick to a surface when they are applied and can be peeled away easily. Others provide moderate adhesion, and some will even offer permanent adhesion.

Some 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.

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. The properties of the substance vary based on the direction through the material when they are measured.

Through the thickness of these sheets, it is possible to conduct heat just as well as any of the other TIMs listed. The base material is very flexible and provides a moderate level of 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.

Dimension Considerations: Gap Filler or Interface

Understanding your application’s dimensions is critical for selecting the right thermal management materials. The thermal interface is the space present between the heat sink and 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 is probably going to 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, it is probably going to be challenging to achieve an even, thin film. This creates higher thermal resistance at the interface. It will also result in minimal heat transfer efficiency.

Non-Bonding or Bonding: Pad or Paste

There are several options when it comes to thermally conductive materials. Choosing between these is determined by application design and production design, along with critical performance factors that have to be achieved.

For example, deciding between non-bonding or bonding materials depends on if the heat sink needs to be held in place by the interface material. If so, a bonding compound is the better option.

On the other hand, a compound that is fixed (this means it will not move) may be necessary. In this situation, it may be necessary to choose a thermal pad. This offers the extra benefit of being pre-cut to size to ensure a smooth application.  

However, you need to understand both 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.

Maximizing the Heat Transfer Efficiency Through Wide Temperature Ranges

Thermal changes are frequent in heat dissipation applications because most devices are turned off and on. They usually have varying power requirements while being used, too.

Also, environmental temperature changes may lead to extremes in the device. A good example of this is in automotive applications. These have to operate after powering down in conditions that are below or above what is considered a standard ambient temperature.

This means it is essential that the selected thermal dissipation media can operate within the temperature limits that have been defined for the device. This must be done while maintaining performance during changing conditions.

A common problem is “pump-out.” This occurs when stresses exerted by changes in the interface substrates cause non-curing interface material to begin moving.

The ability a TIM has to resist these stresses will help improve the performance of the device over its life. However, it is dependent on interfacial spacing and the amount and type of TIM you apply.

When thermal effects are an important consideration, it is worth considering the use of phase change materials. These include non-bonding, non-curing materials that change to a softer material over the phase change temperature.

The properties of the materials let them conform to the interface’s contours and offer a lower thermal resistance than the cured product, all while minimizing the effects of the pump-out, which is usually seen with the non-curing products.

Is Protection Required for Environmental Conditions?

Along with thermal changes, there could be other environmental factors you must consider. A gap filler or thermal interface material may also be resistant against other environmental conditions like corrosive gases, salt mist, high humidity, and more.

It is also crucial for you to consider, at the design stage, regardless of if these external factors may impact the performance of the thermal compound you have used.

Since TIM layers are usually very thin and between two substrates, it is not likely to be completely exposed to these conditions. However, with gap filling material, it may be more likely to be impacted by challenging environments. What this means is that it may be best to switch from the gap filler to a compound that offers full protection, like a thermally conductive encapsulation resin.

Applying Thermal Interface Material

The application technique you use is dependent on the product. For non-curing and curing products, the application method may be automated dispensing or screen printing. 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 a suitable option 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 a maximum level of heat transfer. For thermal interface, the layer has to achieve uniform coverage on the whole interface.

When gap filler is used, the material has to be applied while making sure 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 needs to strike a balance between the desired level of protection and to reduce any volume or weight gains from the resin’s application.

Considering the Compatibility of the Material Selected

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 electronics applications where it is used.

There are some situations where it could be a problem, though. For example, some humidity and gas sensors may be damaged if they are exposed to silicone vapors. This means you must choose a TIM that is silicone-based.

Also, plastic optics and silicone-encapsulated LEDs may be damaged when they are exposed to certain organic vapors. This is why it is a good idea to verify the compatibility of the materials with the thermal adhesives you are thinking about using.

Understanding Thermal Interface Material: Now You Know

When it comes to thermal interface material, there is a lot to know. Understanding the options and getting to know what factors to consider will help you make the right selection for your application.

In most cases, choosing the more seamless application offered by pads, tapes, and adhesives will be best. This reduces the mess, application hassle, and other issues that occur 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 need these materials or help to decide what is right for your application, contact us, or request a quote. We are here to help with your TIM needs, regardless of what you are building.

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Sue Chambers

Written by Sue Chambers

President and CEO of Strouse