How Does Thermal Gel Work?

Thermal gel is a type of thermal interface material (TIM) used to improve heat transfer between two surfaces in electronic devices. A thermal conductive gel flows into small gaps and surface irregularities that would otherwise trap air, helping create more consistent contact between the heat source and the heatsink or cooling plate. Because it is highly conformable, thermal gel is commonly used around CPUs, GPUs, power electronics and automotive components where gap dimensions or surface profiles are difficult to accommodate with a rigid interface material.

In simple terms, thermal gel works by replacing the air trapped between uneven mating surfaces with a conformable, thermally conductive material. Once dispensed into the gap, it conforms to the component and cooling surface, reducing air voids and creating a more continuous path for heat to move from the heat source to the heat sink, heat spreader or cooling plate. Depending on the formulation, the gel may remain in a dispensable state or cure after application.

How Thermal Gel Works

1. Filling gaps and surface irregularities: Surfaces in electronics, even if they appear smooth, have microscopic imperfections. Larger assembly tolerances can also leave gaps between power devices and heatsinks or cooling plates. Because air transfers heat poorly, these gaps can increase interface resistance. A thermal interface gel conforms to the available space and displaces much of the trapped air, improving physical contact across the interface. By displacing these air pockets, the gel increases the effective thermal contact area between the component and the cooling surface.

   
   

2. High Thermal Conductivity: The gel is designed to have higher thermal conductivity than air. Thermal conductivity comes largely from thermally conductive fillers dispersed throughout the gel matrix. In silicone-based thermal gels, the silicone provides softness and conformability, while fillers such as thermally conductive ceramic particles form the main heat-transfer path through the material. Heat moves from the component surface into the gel and through this filled material toward the heat sink or cooling plate. By ensuring efficient heat transfer, the gel helps maintain the temperature of the component within safe limits, extending its lifespan and preventing overheating.

   
   

3. Conformability: Unlike thermal pads, liquid gap filler is highly conformable. It can spread into thin layers to conform to irregular surfaces. This feature makes it ideal for complex assemblies or when surfaces are uneven. This is especially useful where component heights vary or where a single cooling plate must bridge several uneven surfaces. The material can fill the available gap without requiring the same compression force as a preformed pad.

   
   

4. Low Pump-out and Dry-out: Unlike some other TIMs, thermal gel has a lower tendency to dry out or "pump out" under the stress of temperature cycling, ensuring long-term reliability in applications like automotive electronics, where conditions may be extreme.

What Happens After Thermal Gel Is Dispensed?

Not all thermal gels behave in the same way after application. The final state depends on whether the material is supplied as a one-part or two-part system.

A one-part dispensable thermal gel is supplied ready to use. Depending on the formulation, it may be designed to remain soft without an additional curing step. This simplifies dispensing and eliminates A/B mixing during production.

A two-part thermal gel or liquid gap filler contains separate A and B components. After the two parts are mixed and dispensed, the material cures to form a soft, compliant interface between the heat source and cooling surface.

The heat-transfer principle is the same in both cases: the material fills the gap, displaces air and maintains contact between the two surfaces. The main difference is how the material reaches and maintains its final state after dispensing.

What Determines Thermal Gel Performance in a Real Assembly?

Thermal conductivity is important, but it does not determine the performance of a thermal gel by itself. In an actual assembly, heat must travel through both the gel and the interfaces where the gel contacts the component and cooling surface.

Four factors are especially important:

This is why a thermal gel should not be selected from its W/m·K value alone. A higher-conductivity material can still perform poorly if the bond line is unnecessarily thick or if the material does not make good contact with the mating surfaces.

For gap-filling applications, engineers should evaluate thermal conductivity together with the expected gap, dispensing behavior, interface resistance and mechanical stress on the components.

For a deeper explanation of the relationship between conductivity and resistance, see AOK's guide to thermal resistance and thermal impedance.

Typical Applications

● Automotive Electronics: Used to manage heat in high-power components like power inverters, electric motors, and battery management systems.

● CPUs/GPUs: Helps to dissipate heat from processors, improving cooling performance.

● LED Lighting: Used to enhance heat transfer from high-power LEDs to their heat sinks.

Thermal gels are most useful where the interface has uneven surfaces, variable gaps or low allowable assembly pressure. Their dispensable form also makes them suitable for automated production where the application volume and pattern need to be controlled.

To compare dispensable thermal interface materials for different gap, conductivity and curing requirements, view AOK's liquid gap filler solutions or review the thermal interface material datasheets in the AOK catalogue.

For application-specific material selection, provide the expected gap, contact area, operating temperature, electrical insulation requirements and dispensing process when contacting AOK.