Practice: Selecting, using and assessing thermal pads correctly – measurements instead of myths

1 - Introduction and Basics of Pad Selection 2 - We are delving deeper (and thicker). 3 - Measurement: Thin and Hard Pads 4 - Measurement: Thin and Soft Pads 5 - Measurement: Thickness, Ultra-Soft Pads 6 - Measurement: Thermal Putty, Pad Stacks, Recommendation, and Conclusion

Thermal interface pads (also known as thermal interface materials or TIM) are widely used solutions for transferring heat between uneven or different height components in electronic devices. Their use offers both advantages and potential disadvantages, depending on the choice of material, the quality of workmanship and the specific conditions of use. The purpose of thermal pads lies in their flexibility and ability to compensate for uneven surfaces or larger gaps. The layer thickness and the Shore value play a central role here. If we consider, as we do today, the use on graphics cards and mainboards, then pads with a soft Shore value (typically below 35 Shore 00 to a maximum of 70 Shore 00) are the material of choice (in addition to thermal putty), as they enable better adaptation to surfaces, reduce contact pressure and minimize mechanical stress on sensitive components (expansion!). This is particularly important with thinner pads, because a harder material could cause insufficient contact at a short distance and, if the thickness is not suitable, too much pressure. We will of course be investigating all of this today.

Why it is not possible to make a general statement about thermal conductivity in W/m-K

The specification of thermal conductivity in W/mK for thermal pads is misleading and technically not directly meaningful, as it reflects neither the actual performance of the pad nor the complex conditions in practice. The thermal conductivity only describes the material property in an idealized, homogeneous state, which is not the case with thermal pads due to the mixture of polymer matrix and fillers. Furthermore, this specification does not take into account the actual contact with the surfaces or the effects of air inclusions, unevenness or contact pressure.

More decisive for the function of a thermal pad is its behavior when pressed together and the resulting thermal interface resistance, as these factors determine the actual heat transfer at the interface. Only with sufficient pressure and deformation can the pad compensate for unevenness, minimize air pockets and create a larger contact surface. A high W/mK value alone does not guarantee good performance if the interface resistance remains high due to poor adaptation. This is exactly what I’m about to prove with exact measurements, because this packaging label of a 1 mm pad is simply wrong (I’ll show you why later):

Spreading myths about W/mK values misleads customers, as these are often misunderstood as the sole indicator of quality. As thermal conductivity in real applications depends on many parameters, the focus should be on practical performance parameters such as the thermal resistance of the overall system. Manufacturers or suppliers who make unrealistic promises risk not only disappointing customers, but also damaging the credibility of the entire industry in the long term. Silence is actually better here than spreading misinformation with simplified and inaccurate statements. This also applies to various forums, where such statements are repeatedly demanded by “experts”, which unfortunately drift miles away from reality. But I am also happy to provide this proof.

The degree of hardness as a so-called Shore value

I don’t want to exaggerate the theory now, but unfortunately some basic knowledge is necessary: The so-called Shore values indicate the hardness of a material, measured according to standardized procedures, and are crucial for thermal pads, as they influence their adaptability and mechanical behavior. For graphics cards and mainboards, where thermal pads typically have to bridge uneven surfaces and components of different heights, Shore values in the range of 25 to 70 Shore 00 are relevant. But here, too, we need to differentiate once again.

Lower Shore values (around 25 to 50 Shore 00) allow good adaptation to irregular structures and require less contact pressure, which reduces mechanical stress on sensitive components that can also expand. This is particularly important for GPU and VRAM modules, as they often have large height differences. Harder pads (around 50-70 Shore 00), on the other hand, can be advantageous for flat and even surfaces, as they remain more stable and do not deform as easily with thinner layer thicknesses. The choice of Shore value must therefore take into account the mechanical conditions and the contact pressure of the respective application! I would definitely not buy a thermal pad whose hardness is not specified if I did not know it.

Which pad for which requirement? A generalization

In contrast to thermal putty, where such considerations are rather superfluous, you really have to be careful when choosing the right pad. The appropriate degree of hardness (Shore value) of a thermal pad depends very much on the type of surfaces and the requirements of the respective components. Different priorities apply to active and passive components, as both the thermal requirements and the mechanical load capacity vary.

Softer thermal pads with a Shore value of around 25-50 Shore 00 are well suited for uneven surfaces or sensitive components such as MOSFETs and memory modules (VRAM) with larger gaps of more than 1 mm. These components also often vary in height or are located in mechanically critical areas, so a softer pad provides a better fit and minimizes mechanical stress on solder joints or components. MOSFETs also generate high heat flows, which is why an optimum contact surface is particularly important to reduce thermal resistance.

Higher Shore values of 50-70 Shore 00 are preferred for flat and even surfaces and gap dimensions of 1 mm and less, as found in passive components such as coils or large heat sinks, where VRAM can of course also be connected. Harder pads retain their shape better and prevent the material from deforming laterally under contact pressure, which is important for large components or even contact surfaces. This is particularly useful for thin pads, where a soft material could lose its function due to compression.

The surfaces of heat sinks vary greatly in their roughness and texture. High-quality heat sinks often have polished or very smooth surfaces that improve direct contact with the thermal pad. Cheaper heatsinks, on the other hand, can have rough or textured surfaces that need to be compensated for by soft thermal pads. Smooth surfaces favor heat transfer because there are fewer air pockets, while rough surfaces place higher mechanical demands on the pad to fill in unevenness. Therefore, uneven, rough or sensitive components require softer pads for good adaptation and minimal stress. Smooth, even surfaces, on the other hand, benefit from harder pads that ensure a stable and durable contact surface. The choice should always take into account the mechanical and thermal requirements of the specific application.