Thermal Interface Materials and Gap Fillers

What a Thermal Interface Material Does

Two surfaces that look flat are not, at the microscopic scale. When a heat sink is placed on a component, the contact is made at a few high points, and the air in the gaps between them is an almost perfect insulator. A thermal interface material fills those gaps so that heat can travel from the component into the heat sink instead of being blocked by the air.

The material is not a substitute for metal; it is a filler for the roughness between two metal surfaces. Its job is to remove the air, and the best material is one that fills the gaps completely while adding as little thickness as possible. A thick layer of a good conductor often performs worse than a thin layer of a modest one, because the layer itself becomes the resistance.

Interface materials also serve mechanical purposes. They accommodate the small dimensional differences between a component and a heat sink, absorb vibration, and in some designs provide electrical isolation. Those requirements interact with the thermal performance and should be considered together.

Types of Material

Thermal grease is the classic option. It is applied as a thin layer, conforms to the surfaces under pressure, and offers the lowest thermal resistance of the common materials. Its drawbacks are that it can pump out under thermal cycling, migrate, and be difficult to apply consistently in volume production.

Thermal pads are pre-cut sheets that are placed between the surfaces. They are clean, consistent, and easy to handle, and they can be supplied with an adhesive or a fiberglass carrier. Their thermal resistance is higher than grease because a pad is thicker, but the repeatability is much better and there is no pump-out risk.

Gap fillers are dispensed liquids or pastes that cure in place. They are used where the gap is large or varies across the assembly, and they can be dispensed to a controlled thickness before the parts are assembled. Phase change materials sit between grease and pad, becoming soft at operating temperature and improving the contact when it matters most.

Thickness, Pressure and Contact Resistance

Thermal resistance of an interface has two parts: the bulk resistance of the material, which grows with thickness, and the contact resistance at each surface, which falls as the material fills the surface texture. Total resistance is minimised at the thickness where the two effects balance, which is usually the thinnest layer that still fills all the gaps.

Pressure improves contact by pushing the material into the surface texture and reducing the bond line thickness. The mounting hardware therefore matters as much as the material: a heat sink held with insufficient or uneven force will leave thick regions where the contact is poor. The compression should be measured or specified, not left to feel.

Surface flatness and finish also set the achievable result. A rough or warped surface requires a thicker layer and gives worse performance. Where the component and the heat sink cannot be made flat enough, a gap filler that conforms is used rather than forcing a thin bond line that will not be achieved.

Dispensing and Placement

Grease is applied as a dot, a line, or a screen-printed pattern, and the pressure of assembly spreads it across the surface. The volume is critical: too little leaves areas with no coverage, and too much squeezes out around the edges and can contaminate nearby parts. Thermal grease is conductive in some formulations, so any squeeze-out onto a circuit must be prevented.

Pads are placed by hand or by machine and rely on the assembly pressure to make contact. Their thickness must match the gap so that the pad is compressed by a defined amount; a pad that is too thin will not fill the gap, and one that is too thick will hold the surfaces apart.

Gap fillers are dispensed to a pattern and thickness and then allowed to cure after the parts are assembled. The dispense volume, pattern, and cure schedule all affect the result, and the process should be validated by measuring the achieved bond line rather than by inspecting the appearance.

Curing and Aging

Curing changes the material’s properties and the process has to account for it. A gap filler that cures in place will shrink slightly, which can reduce the contact pressure, and a material that cures too quickly may not flow into the gap before it sets. The cure schedule should be defined by the supplier’s data and verified on the actual assembly.

Aging is the long-term concern. Materials can dry out, migrate, or pump out of the bond line under repeated thermal cycling, and the interface that was adequate at the start can degrade over years. Silicone-based materials are generally stable at temperature but can migrate, while hydrocarbon materials can evaporate. The choice should reflect the operating temperature and the life of the product.

Vibration and mechanical shock can also change the interface by shifting the parts relative to each other. Where the product sees those conditions, a cured gap filler that holds the parts in place is often preferred over grease, even at a small thermal cost.

Selection for the Application

The starting point is the gap. A small, well-controlled gap suits grease or a phase change material, which give the lowest resistance. A moderate gap with a need for handling consistency suits a pad. A large or variable gap where conformability matters suits a dispensed gap filler.

The next consideration is the process. A high-volume line needs a material that can be applied repeatably, which usually favours pads or automated dispense over hand-applied grease. A prototype or a low-volume product can use grease without the handling concerns that a production line would have.

Finally, the electrical and mechanical requirements should be checked. Some applications need an electrically insulating interface, some need the material to be non-migrating, and some need a defined compression under the mounting force. The material should be selected against all of those requirements rather than on thermal conductivity alone.

FAQ

Which thermal interface material is best? There is no single best. Grease gives the lowest resistance for a thin, well-controlled gap; pads give repeatability; gap fillers conform to large or variable gaps. The choice follows the gap and the process.

Is a thicker layer of thermal material better? No. Thermal resistance grows with thickness, so the best result is the thinnest layer that still fills all the gaps between the surfaces.

Does mounting pressure matter? Much more than most people expect. Pressure reduces the bond line and improves contact, and uneven pressure leaves thick, poorly conducting regions.

Can thermal grease cause electrical problems? Some formulations are electrically conductive or contain metal particles. Squeeze-out should be controlled, and the material should be chosen for the electrical requirement of the assembly.

Why does a thermal interface degrade over time? Pump-out, migration, drying, and shrinkage reduce the contact over repeated thermal cycles. The selection should account for the product’s operating temperature and expected life.

Conclusion

A thermal interface material earns its place by removing the insulating air between two surfaces, and its performance depends on thickness, pressure, and stability as much as on thermal conductivity. Matching the material to the gap and the process, controlling the applied volume and compression, and accounting for aging keeps the thermal path intact for the life of the product. For related assembly topics, read our notes on PCB assembly, PCB capabilities, quality management, and PCBA testing for how thermal design is realised in 2026.