Choosing the right thermal interface material: paste, pad or foil

Choosing the right thermal interface material

Knowledge of the requirements of your application are
the key to selecting the correct thermal interface materials

When it comes to choosing the right thermal material, many people are unsure of exactly what to look for. In this article, we would like to give you everything you need to know about thermal interface materials.
As soon as the performance of the components increases, the cooling requirement also increases accordingly. As a rule of thumb, the failure rate doubles for every 10°C increase in junction temperature. As a result, the heat from the components must be dissipated to the flow of ambient air.
The demand is great, so that a large number of new thermal management systems have been developed. Almost all of these still use thermal interface materials (TIMS), which are designed to allow the effective flow of heat across the appropriate interfaces of cooling systems.
The main task of TIMs is to guarantee effective heat transfer to dissipation devices such as heat sinks or manifolds. As heat flows, it repeatedly encounters resistance that complicates and impedes the overall heat transfer. TIMs help to overcome the most problematic resistance. We are talking about the contact resistance between the counterparts (heat source-heat sink).
This is because air gaps greatly reduce the flow of heat from the hot component to the cool component. Effective TIM replaces the existing gaps created by the non-smooth mating surfaces. This is done with the help of special material, the thermal conductivity of which is significantly greater than that of air. Here, the poor conduction of point contacts and air is replaced by significantly improved conduction through solids.
Most TIMs are polymer-based composites. These are filled with filler particles, which are thermally conductive. The common fillers are aluminum oxide, boron nitride, aluminum nitride and magnesium oxide. If galvanic isolation is not required, metal fillers such as silver can also be used. In order to reduce the contact resistance, a certain amount of pressure between the interfaces is required. This pressure then compresses the filler particles, allowing the material to flow into the surface irregularities. Once the material is in place, the effective thermal resistance of a TIM includes the bulk resistivity of the material and the contact resistance between the TIM and its interfaces.

Problems of application for thermal interface materials

Although thermal interfaces and TIMs are typically considered early in the design process, one should then consider some specific factors when actually selecting the thermal interface material:

• The most important specification is definitely the thermal impedance, which is measured in degrees Kin2 / W. This is an application specific measure of the ratio
  the temperature difference between two mating surfaces to the steady-state heat flow through these surfaces. Due to the additional mounting pressure and the size of the area, the
  thermal impedance typically decreases while increasing with TIM thickness.
• The ability of a material to conduct heat regardless of its thickness is called thermal conductivity and is measured in W/mK. While TIMs can be compared to thermal conductivity values, this value does not indicate how good the material's ability is at minimizing contact resistance.
• The distance (gap) between the heat source and the heat distributor is also important. Normally, the thinner the TIM, the better. However, the interfaces are never perfect, so a minimal thickness of material is required to compensate for the irregularities.
• In order to select the type of material, the surface flatness of the cut surfaces is a decisive factor. For example, if both surfaces are smooth, grease or thin films are a very good option, but this is rarely the case. This is because plastic ICs are usually concave in the middle. If the heat sink is very smooth, the contact surface is reduced, especially at the edge. This leaves an air pocket in the middle.
• Sometimes electrical insulation, measured in kV, is required. TIMs that have a silicone base share this property with thicker materials such as thermal pads (gap filler).
  However, thinner phase change materials and greases are not necessarily reliable electrical insulators. Graphite itself is electrically conductive.
• When working with irregular surfaces, compressibility is a crucial factor. A good example is when you want to cover a whole range of components. For example, if heat and excess pressure are applied to a silicone-based TIM, the silicone can escape and migrate along the circuit board. If the pressure is insufficient, there will be excessive thermal resistance at the interface.
• The temperature range in which the material can be used is also important. For example, silicone-based TIMs can withstand higher temperatures than non-silicone options.
• UL flame class rating is required for most TIM applications. The majority of the materials to choose from are available with V0 values ​​that meet typical requirements.
• In general, silicone is an excellent thermal material with a high temperature range. Nevertheless, there are areas in which silicone-free variants have to be used. For example, the use of silicone in space is not possible due to outgassing.
• You should also pay attention to a simple application. After all, the type of attachment is a clear cost and performance decision. For example, small heatsinks are usually simply attached with double-sided thermal tape, while larger heatsinks require additional hardware. Adhesives can be applied to both or one side of the thermal material as required. It should be noted here that the thermal impedance increases with each layer of adhesive.
• One should also ask how easy it is to handle the chosen materials in a manufacturing environment. Can the materials simply be reworked if, for example, the heat sink has to be removed? Phase change materials and thermal paste must be completely replaced, while some thermal pads can be reused.
• And finally, the long-term stability of the material should also be considered. This depends on factors such as service temperature, time, application and material properties.

Thermal interface material options

Phase Change Materials (PCM)

What is unique about phase change materials (PCMs) is that they undergo a transition from a solid to a semi-solid phase using heat from the operational processor and light clamping pressure. The semi-solid phase has the property of adapting very easily to both surfaces. The ability to completely fill interfacial air gaps and surface voids under light clamping pressure allows this material to perform on par with thermal paste.
PCMs is much less liquid than fat. However, PCMs contain waxes and at the moment the melting temperature is reached, the PCMs can flow out of narrow areas.

Thankfully, recently introduced PCMs are no longer wax based, so they don't drip. PCMs are very easy to handle at normal room temperature because they are solid. This provides more control when applying the solid pads to the heatsink surface. Many phase change pads create a very durable adhesive bond between the heatsink and the processor. Because of this, care must be taken when removing the heatsink from the processor. A small twisting motion usually helps with removal. Applying too much force can damage the processor.

Thermal Compound

Thermally conductive pastes are usually silicone enriched with thermally conductive fillers. Hardening is usually not necessary and they can flow and adapt perfectly to the interfaces. The thermal interfaces can easily be reworked. However, it must be ensured that sufficient paste or grease has been applied before installing the heatsink. Too little grease can result in gaps between the heatsink and the processor. On the other hand, too much grease can also be counterproductive as it can lead to air gaps and leakage outside the interface. It should also be noted that with prolonged use and generally over time, some greases can deteriorate or dry out. Of course, this has a negative effect on the heat transfer performance. Thermally conductive pastes as interface materials (TIM) are nevertheless the
first choice in applications with high-performance processors - despite the disadvantages mentioned above. This is mainly because the thermal conductivity of thermal greases is in the order of 10 W/mK, which is clearly superior to other TIMs.

gap filler

One of the largest market segments for TIMs are gap fillers. These can be supplied in different strengths. These efficient, soft and highly thermally conductive materials can cover gaps up to 15mm. The practical gap fillers can cover multiple components of different heights and then transfer the heat into a common heat spreader. These pads are popular and often used when low compression forces are required. The relatively high compressibility is thus an important feature of this type of TIM. The gap fillers can also be shaped individually and the new Formin-Place gap-filler connections in particular are a popular option for automating large ones
volumes.

thermal foils

Thermally conductive foils not only do the heat transfer, but also provide electrical insulation. In terms of tear resistance and puncture resistance, the thermal films offer excellent durability. Silicone and non-silicone (e.g. ceramic-filled polyurethane) thermal interface foils and graphite materials fall into this category. The range of thermal conductivities and price ranges is wide, so that everyone can find a good solution.

gap pads

Thermal pads typically consist of molding unreinforced silicone with conductive fillers. Thermal pad reinforcements are typically woven glass, metal foils, or polymeric films. The handy thermal pads are usually pre-cut in different sizes to accommodate different sized components. While phase change materials and thermal paste are clearly superior in terms of conduction, thermal pads have the advantage of being a cheap and convenient option for applications with lower cooling requirements.

graphite foils

This cost-effective option has been used for a long time. The foils are electrically conductive and can be used at very high temperatures of up to 500 ºC. Some suppliers align the fibers horizontally. This results in very different thermal conductivity measurements. There is material that shows 7,0 W / mK on the x-axis and 150,0 W / mK on the yz-axis - a clear difference.

Double-sided thermal tapes

A thermal tape can consist of a finely woven, nickel-plated copper mesh that closely conforms to irregular mounting surfaces. To attach small heatsinks to components, thermally conductive double-sided adhesive tapes made of PSA are very often used. Important factors here are peel strength, lap and punch shear strength, holding power and thermal resistance. As far as the performance in terms of heat conduction of the double-sided adhesive tapes is concerned, it is in the middle range. Although you save on additional assembly parts, the tapes have problems with irregular surfaces of the components and are therefore only of limited use. Plastic ICs are typically concave in the middle and the heatsink surfaces vary, which can cause air gaps in the interface.

thermal adhesives

Thermal adhesives - also called heat adhesives - can be both one- and two-component systems. These are equipped with conductive fillers. The application is usually done by dosing or stencil printing. Curing of the adhesive is necessary to enable safe crosslinking of the polymer, which provides the adhesive property. The fact that the thermal adhesives provide structural support, eliminating the need for mechanical clamping, is certainly the biggest benefit of this TIM.

thermal gels

Gels are a similar material to fats that is slightly cross-linked. The behavior is correspondingly similar, with the bleeding of the material being reduced.

TIMs made of metal

Metal thermal interface materials can be made in all sorts of shapes and are currently no longer limited to soldering applications. In numerous applications, the TIMs made of metal can be reworked very well and can also be recycled without any problems.
Recently, the need for high-performance TIMs in special devices such as power amplifiers and IGBT modules has increased, leading manufacturers to explore other types of metal TIMs.
Good examples are: liquid metals, phase change metals and SMA-TIMs (soft metal alloys).
Certainly the easiest to use is the soft or compressible metal thermal interface material (SMA-TIM). Metal TIMs are highly thermally conductive, reliable, and easy to use with compressible metals.

Recently, a hybrid material has also been developed, which consists of a heat-conducting silicone film on one side and a copper film on the other. This material is especially good for making flex circuits and protecting against EMI and RFI noise.

Conclusion

Unfortunately, thermal interfaces are often only considered quite late in the design phase of cooling systems. This is most definitely not the best course of action. After all, TIMs are clearly a key factor in the cost of thermal management design. Today, there is typically more and more excess heat to manage, so there is clearly a need for high-performance TIMs.
When used judiciously, thermal interface materials are sure to help reduce the size of heatsinks and the need for larger and larger fans. Also, a good TIM is an easier, faster, and clearly cheaper option than changing heatsinks or redesigning the case entirely.