Choosing the Right Thermal Interface Material

Thermal resistance describes the heat conducting performance of one or more materials and is defined by the temperature difference between both sides. Where there is lower thermal resistance there is better heat transfer.

Thermal interface materials (TIMs) are used to improve the heat transfer from electronic components to heatsinks. By replacing the insulating air gaps resulting from the microscale roughness of the surface, TIMs can improve the heat flow between component and heat sink. The overall thermal resistance depends on:

• Thermal conductivity (TC) of the thermal interface material
• Bond line thickness (BLT) - the amount of thermal interface material used between the substrates
• Contact resistance between different types of material at contacting interfaces (electronic component, TIM, heat sink)

Managing thermal interface materials is critical to maintaining the reliability and the longevity of electronic devices

Three ways to improve heat transfer are:

• Increase the thermal conductivity of the Thermal Interface Material
• Reduce BLT between the electronic component and heat sink
• Reduce contact resistance by increasing the mounting pressure for solid TIMs or use a liquid-dispensed TIM

Lower bond lines reduce the distance heat must travel to be removed from the heat source. Therefore, a thin bond line is preferred to reduce thermal resistance. The BLT is determined by the part design and depends on the tolerances of the substrate and assembly stack. In general:

• BLT ranges for adhesives: 80 μm to 300 μm
• Gap filler: 70 μm to 2 mm
• Phase change and grease: 15 μm to 50 μm

Additional constraints, such as part design or electrical insulation, can require a higher BLT. To achieve precise bond line thickness during production, glass beads with precise sizes can be added to increase the minimum achievable BLT. A pressure bond line thickness test can also be performed to better understand assembly conditions.

Contact resistance generally depends on the TIM’s ability to conform to the surface of the heat source and heat sink. A higher contact resistance typically leads to less heat transfer.

• For a solid material (e.g. foil or cured silicone sheet), contact resistance can be improved by using more pressure.
• Liquid TIMs conform easily without high pressure, helping to limit stress on the electronic components.

Due to the heavily filled, highly abrasive nature of TIMs, specialized equipment is required for transferring and dosing, and not all pumps can be used.

Pump types that dispense TIMs particularly well include piston pumps and positive displacement pumps or screw pumps.

Piston pumps forward materials through the pump with an up/down movement to provide positive displacement. Creating little to no friction during dispensing, minimizing wear and tear on the equipment.

Screw pumps (progressive cavity pumps) dispense material by rotating a screw. This action also creates little to no friction, helping to reduce equipment wear. Unlink regular pumps, these pumps do not use rubber seals which can become damaged by the abrasiveness of the material.

The least complex dispense pattern to achieve the required performance is best. More complex dispensing patterns can take longer to produce, and in turn decrease overall cycle time. Each application is unique and there is no set standard for where a particular pattern should be used.

It’s important to know which type of dispensing needle is appropriate for your application as there are many types of fluids with different properties. Using the wrong one can lead to broken machinery, poor part quality, and an inefficient production process.

The most common dispensing needle is the general-purpose tip as it can be used with nearly any TIM; however, this does not mean that it is the best choice for your application.

A good rule to follow is that the diameter of your needle opening should be at least 7 to 10 times bigger than the largest particle in the material (e.g. 80 μ max particle size multiplied times 10 means needle diameter needs to be at least 800 μ).

Tapered tips are generally the best option for thicker viscosity fluids because they promote flow and help dispense a greater number of precise, consistent deposits in the shortest amount of time. Tapered tips allow you to lower the pressure on your pneumatic fluid dispenser, which is important when dispensing filled materials, because high pressure can cause them to separate in the syringe barrel.

Due to low viscosity and the density difference between the polymer and the filler (polymer 1g/cm3 vs. filler 4g/cm3 AI2O3), potting materials can separate over time, allowing the filler to settle to the bottom. Therefore, it is important to fully mix potting materials before use. Mixing procedures include spatula mixing, drill mixing, shaking and rotational mixing. Each material has recommended procedures.

Silicone can absorb air, so degassing low viscosity thermal materials in a vacuum mixer is recommended before use. Otherwise, air may remain in the cured silicone and cause potential issues, including visible defects and dielectric breakdown. It is important to degas all low viscosity materials, not just the top layer.

It is extremely important to thaw silicone that has been stored in a refrigerator at 7°C (44°F) or below before opening it. Warming up the silicone to room temperature 22°C (71.6°F) can take up to 24 hours depending on the packaging and amount of material. If you open it before it reaches room temperature, ambient humidity will immediately infiltrate the silicone, and bubbles can form during vulcanization at higher temperatures. This may lead to poor adhesion performance.

The performance of any adhesive is dependent on proper surface preparation:

• When preparing a surface for bonding, make sure that it is clean; contaminants can weaken a bond.
• Make sure that the surface is dry; a wet surface poses the same problems as a dirty one.
• Effective solvent cleaning substances are isopropyl alcohol for plastics, and MEK for metals, but compatibility needs to be verified beforehand.
• Depending on the substrate, it may be necessary to anneal the parts to get rid of the humidity inside the plastic; the substrate must return to room temperature before applying adhesive.
• For difficult-to-bond substrates, primers or additional plasma treatment should be used to help promote adhesion.

Proper adhesion is highly dependent upon bond line thickness. In most cases, adhesion force increases with lower bond line thickness. To maintain the correct bond line thickness, thermal adhesives can be formulated with precisely sized glass beads that act as a spacer and help ensure bond line thickness control.

The Thixotropic Index (TI) is a ratio of a material’s viscosity at two different speeds. It is obtained by taking measurements at low and high shear rate. Because thixotropic materials drop in viscosity as shear stress increases, the TI value for a thermal interface material indicates the material’s ability to hold its shape when exposed to stresses, such as gravity. A high TI indicates that a thermal interface material will resist sagging or slumping out of place after dispensing.

It is critical to thermal interface performance that it remains where it is applied. A thermal interface material cannot dissipate heat from a component if it does not stay in contact with that component.