Interface-Materials / TIM
Interface Material / TIM
With a thermal conductivity of only 0.0263 W/mK, air is an exceptionally poor conductor of heat. Therefore, various materials are available to bridge the gap between heat sources and heatsinks. These contact materials, available as pastes, adhesives, pads, films, or specialized phase-change materials, have a very low thermal resistance and are applied directly between components. Depending on surface quality and expected temperatures, specific thicknesses and materials are required.
System Importance
The performance of a system is influenced not only by the cooling power of the components but also by the interfaces between each part. Thermal Interface Materials (TIMs) ensure efficient heat dissipation and prevent local overheating. These materials fill even the smallest gaps or surface irregularities on contact surfaces, facilitating heat transfer between each component. Thermal paste, a relatively low-cost solution, is widely used in electronics, engine construction, and heating/cooling devices with a thermal conductivity of 2-10 W/mK. For more demanding applications, materials with higher thermal conductivity, such as films or adhesives, are required.
Types and Properties
There are numerous ways to compensate for tolerances, uneven surfaces, or roughness, connecting contact surfaces almost airtight with interface materials. However, not all materials are equally suited for all applications.
Generally, TIMs can be divided into three types:
Thermal pastes are usually ceramic-based, enriched with microscopic particles of aluminum oxide, boron nitride, and zinc oxide. Typically silicone-free and formulated with various synthetic oils, they offer long-term stability and exceptional performance.
Upon initial heating, the TIM’s viscosity decreases, filling in tiny surface irregularities and displacing air pockets. Within the first 20-30 hours of operation, the paste hardens, reducing expansion from temperature fluctuations. With a temperature range of -40 to 180°C, thermal paste is suitable for many applications, such as between microchips and heatsinks.
Thermal Films / Specialty Films
In power electronics, higher demands are often placed on heat dissipation and dielectric strength. Thermal films are available in various formulations for different applications, with materials like polyamide, silicone, or carbon fibers offering a suitable solution for almost any need.
The most notable are graphite thermal films, usable in temperatures from -200 to +600°C. They have excellent electrical and thermal conductivity, with thermal conductivities of 5-16 W/mK across the fiber and 155-470 W/mK along the fiber direction, efficiently directing heat away. However, they require a certain level of compression.
Thermally conductive adhesives are based on resins filled with metal particles like silver or graphite, ensuring high thermal conductivity while securely bonding the heatsink. To prevent electrical conductivity, ceramic or mineral fillers can be used instead. With a thermal conductivity of 1-4 W/mK, these adhesives can handle temperatures up to 250°C.
Compared to welding or soldering, adhesive bonding has the advantage of allowing diverse materials to be joined. These adhesives cure at room temperature, reducing mechanical tension or deformation.
These adhesives are used in microelectronics, power electronics, sensor technology, energy technology, and the automotive industry, providing durable, thermally conductive bonds between components.
There are also specialized types beyond these main categories, including unique materials such as:
Phase-Change Material (PCM) / PCM Screen Printing
If conventional thermal paste is unsuitable, PCM may be an alternative. At room temperature, PCM is dry and formable. When installed, it changes phase under pressure (>20N/cm²) and temperatures around 45°C, becoming liquid to fill gaps and voids, ensuring optimal heat transfer without the need for traditional thermal paste.
A special screen-printing process also allows mass production of coated components, with phase-change material (PCM) applied directly to individual parts.
Ceramic / Thermal Interface Discs
Ceramic interface discs are primarily used as insulators due to their low inductance. They enable galvanic isolation while offering excellent thermal conductivity along with corrosion and wear resistance.
In addition, aluminum oxide ceramics are pressure-resistant up to 4,000 MPa and can withstand temperatures from 1,000 to 1,500°C. Fiberglass-reinforced silicone insulating discs are self-adhesive, flame-retardant, and can be installed without thermal paste, although they offer only moderate thermal conductivity. These discs can bridge large surface irregularities due to their elasticity but are not suitable for high power densities or Peltier technology due to their lower thermal conductivity and necessary minimum thickness.
Thermally Conductive Epoxy
Component irregularities on a circuit board often prevent the attachment of a flat heatsink. Thermally conductive epoxy allows heat from uneven heat sources to be uniformly transferred to enclosures or cooling solutions. Potting compounds typically consist of standard epoxy mixed with thermally conductive metals, such as aluminum. It is important to insulate electronics adequately, for example with PU varnish, before applying epoxy.
Due to the metal content, thermally conductive epoxy is electrically conductive; however, this can be countered by adding ceramic fillers or other non-conductive additives. While epoxy compounds are less heat-resistant, silicone-based products can handle temperatures above 180°C. Thermal conductivity varies depending on the composition and ranges from 1 to 7.5 W/mK.
The variety of materials and compositions within these primary groups ensures an optimal solution for nearly any application involving heat-conducting components. These materials differ in thickness, thermal conductivity, and electrical insulation properties.
Which interface material is best suited for a specific application depends on the following properties:
- Thermal conductivity
- Thermal resistance
- Tolerances between contact pairs
- Temperature sensitivity
- Thermal impedance
- Environmental compatibility
Advantages and Disadvantages
Electronic components generate heat depending on the application, and this waste heat must be dissipated efficiently. Not all heat transfer methods are suitable for every application, which is why various methods exist to connect components mechanically and thermally, ensuring effective heat dissipation. TIMs vary in thermal conductivity and thickness. Some materials are also more reliable, better at filling gaps, and can be modified or reshaped as needed.
Correcting an Inadequate Cooling Solution
After setting up a passive cooling solution, it may become apparent that it is underpowered. There are two solutions:
- The heatsink can be enlarged to increase the area available for convection, or additional fans can be installed.
- A Peltier element can be added. This complex solution is used when enlarging the heatsink is not feasible due to space constraints. It’s important to note that a Peltier element requires energy input to operate. This energy further heats the already burdened heatsink, increasing the temperature gradient. In practice, this means that the ratio of dissipated energy to operating energy must be greater than 1 for effective cooling.
Important:
If a heatsink is underpowered, it is generally not physically possible to correct this with Peltier elements.
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Overview of Interface Materials: |
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