A thermal interface material may show high thermal conductivity in the datasheet, yet the assembled unit can still retain more heat than expected. The reason is rarely the material property alone, but rather the interface itself: surfaces only touch at isolated points, small gaps remain in between, and the material layer that actually remains after assembly often differs from what was originally planned. Anyone selecting a material should therefore first describe the installation situation and only then compare individual properties. This turns a simple datasheet search into a technical selection process based on the real design and helps avoid later surprises during operation.
Why the Datasheet Value Alone Is Not Enough
Thermal conductivity in W/m·K describes a material property: it indicates how well a given material conducts heat when it is in uniform, full-surface contact. On its own, however, it does not show what thermal resistance a specific connection between component, interface material, and heat sink will actually achieve. Other factors also contribute, including the remaining layer thickness after installation, the condition of both contact surfaces, and the mounting process. Even surfaces that appear smooth have microscopic irregularities; air collects in the resulting voids, conducts heat poorly, and impedes heat transfer. The interface material is intended to fill exactly these gaps and create continuous contact.
In application note AN 18-001, Semikron Danfoss describes factors influencing thermal resistance in power modules. For the application of thermal paste, these include the heat sink surface, the applied layer thickness, the mounting force, and thermal cycling. The note refers specifically to power modules, but it illustrates a general assessment principle: a single material property cannot be interpreted independently of the overall design. Even a particularly high conductivity value does not automatically compensate for unsuitable geometry or poor contact.
Start by Assessing Gap and Contact Surfaces
Before looking for a specific material, it is worth examining the actual distance between the heat source and the heat sink across the entire relevant area. What matters is not only a nominal dimension, but also manufacturing tolerances, component heights, possible warpage, and differences between individual mounting points within a series. A large or uneven gap places different demands on a material than a tightly defined, consistent interface gap. The condition of the surfaces themselves is equally important: flat, rough, textured, or coated. These details show whether a material mainly needs to compensate for unevenness or whether it is primarily intended to form a thin, defined layer between two already well-matched surfaces.
“Thinner is better” does not apply across the board. A layer that is as thin as possible can shorten the heat path, but it must fit the geometry, the chosen material format, and the mounting process. If the gap is not the same everywhere, a material that is too rigid or specified too thin may fail to make sufficient contact in some areas and create higher resistance there. Conversely, material should not be installed thicker than the design requires without good reason. What ultimately matters is the layer that actually remains after assembly, not the initial thickness of the unused material.
Choose the Material Format to Match the Design
The gap, surface geometry, and production sequence usually narrow down the range of suitable material formats. Thin films or pads are suitable where defined, largely flat contact surfaces and controlled assembly with consistent contact pressure are possible. Gap fillers and softer pads can compensate for larger gaps or tolerance-related variations, but they require sufficient compressibility and defined mechanical contact pressure. Phase change materials alter their consistency under heat and pressure, so the intended operating temperatures and mounting conditions must match the product design. Technical ceramics combine thermal conductivity and electrical insulation in a rigid component form, but they place different demands on contact, fracture resistance, and geometry than soft, flexible materials.
ICT Südwerk offers various thermal interface materials for electronic applications in its portfolio, including films, gap fillers, silicone-free variants, and technical ceramics based on aluminum oxide and aluminum nitride. This range illustrates why a suitable format is better derived from the specific installation situation than from any general ranking: a soft pad may compensate for assembly deviations, while a thin film may better suit another contact geometry. Which format is actually appropriate depends on the component in question, the application requirements, and the manufacturer’s specifications.
Consider Insulation, Contact Pressure, and Production Together
Besides pure heat transfer, electrical isolation between component and heat sink often also matters, for example when the cooling plate is at a different potential or when several components are cooled through a shared heat sink. For such cases, suppliers such as ICT Südwerk offer materials that can provide not only heat transfer but also electrical insulation. Whether this insulation is sufficient in a specific case depends on the application, the electrical design, and the manufacturer’s specifications for the specific product and cannot be generalized across all material forms.
Mechanical contact pressure, which holds a material in place during operation, is just as important. The Semikron Danfoss application note already mentioned explicitly identifies mounting force and process parameters as factors influencing the resulting thermal resistance of a connection. If the pressure is too low, a material may not fully flow into surface irregularities; if it is too high, sensitive components or thin ceramic plates may be damaged. The permissible and useful pressure range depends on the specific assembly, the selected material, and the intended production process, not on a universal limit.
Evaluate Datasheet and Assembly Together
A comparison between several materials is only useful if the underlying test conditions are comparable, for example similar layer thickness, similar contact pressure, and comparable test temperature. If properties are determined under different conditions, they can only be compared to a limited extent. In this sense, application note AN 18-001 distinguishes between the pure material property of a paste and the resulting thermal resistance in the assembled state, which can vary from module to module and from application to application.
For this reason, validation on a real sample or on the actual assembly is useful before a material is finally specified. This makes it possible to check whether the post-assembly layer actually turns out as intended, whether the component stays within the expected temperature range during operation, and whether the mechanical connection remains stable over time. Such a practical test does not replace the manufacturer’s specifications, but it complements them with the perspective of your own design and makes the selection more robust than simply choosing the highest conductivity value in the datasheet.
A Practical Evaluation Sequence for Selection
For practical use, this leads to a clear sequence based on the specific assembly rather than on a single property:
- document the gap, tolerances, and condition of the contact surfaces
- define electrical insulation requirements and permissible mechanical load
- narrow down the appropriate material format based on installation space, assembly process, and operating conditions
- compare datasheet values of different candidates under comparable test conditions
- validate the selection on a sample or on the real assembly before releasing it for series production
This sequence prevents an attractive figure from driving the decision too early, even though the basic boundary conditions of the installation situation have not yet been clarified. It can be applied regardless of the specific supplier and is suitable both for new developments and for reviewing existing assemblies where requirements or production volumes have changed.
Conclusion: Base the Decision on the Assembly, Not on a Single Value
A high thermal conductivity value is a useful first indication, but it does not replace analysis of the real contact interface. Gap size, surface condition, electrical requirements, contact pressure, and production process together determine which material format is even suitable and how a datasheet value will actually perform in operation. Anyone who clarifies these factors before comparing materials and then verifies the selection on a sample or on the assembly will make a decision that fits the design itself—regardless of which supplier ultimately provides the right components.