The advantages of insulation foils are the low material thickness (usually only 0.025....0.05 mm), the very high dielectric strength and the short distance required for heat transport. This completely compensates for the disadvantage of low specific thermal conductivity in many cases (rule of thumb: for the same desired heat flow, a material with twice the specific thermal conductivity must be used with twice the material thickness).
Conventional insulation foils are pressure-resistant, non-flexible materials. Unlike other materials (silicone mats, gap fillers, casting compounds, heat-conducting paste), they cannot penetrate the fine unevenness of surfaces. As a result, 30-40% of the direct contact area between the heat source and the heat sink can be lost. At 0.024 W/mK, air is a very poor conductor of heat. Such air inclusions (i.e. no direct contact) considerably impair the thermal conductivity of thermally conductive but rigid insulation foils.
This disadvantage is compensated for by TIM coatings. This generally refers to coatings that are suitable for displacing air pockets between microscopically rough surfaces (interfaces). For this purpose, these surface finishes of thermally conductive insulation foils must be soft enough to penetrate such unevenness and create a form fit. CMC Klebetechnik offers various solutions for this task, which are described below:
Phase-change coating; these coating compounds are solid at room temperature, but melt quite early (55-60°C). They then flow into the unevenness of the surfaces and fill them up. This creates a positive connection between the heat source and heat sink.
Thermally conductive acrylate adhesives combine heat conduction and adhesive strength. CMC Klebetechnik offers two variants: an adhesive that is already quite mobile below 100°C with moderate adhesive strength and a high-performance acrylate adhesive that can be used up to 180°C.
Thermally conductive silicone coatings that can be either adhesive or non-adhesive. They can withstand temperatures above 200°C and are highly resistant to ageing.
Measurements were carried out to get a feel for how strong the influence of TIM coatings from CMC adhesive technology is. For this purpose, the bare film and coated films with non-thermally conductive and thermally conductive TIM coatings were compared with each other:
Exemplary measured values (measurement method ASTM D5470, NOT laser flash):
Kapton® MT+ without coating 0.28 W/m*K
Kapton® MT+ with non-thermally conductive adhesive 0.36 W/m*K
Kapton® MT+ with thermally conductive adhesive 0.73 W/m*K
Kapton® MT+ with phase change coatings 0.84 W/m*K
As can be clearly seen, with the same base film, heat conduction is significantly improved when a coating of thermal interface material is applied. The thermally conductive variant (same layer thickness) doubles the heat transfer and is at approximately the same level as the theoretical specific thermal conductivity value of Kapton® MT+ (0.85 W/m*K). However, it is only with the TIM coating that the very good Kapton® MT+ can fully exploit its advantages.
The reason for this is easy to explain: when calculating the heat transfer in a complete system, the temperature difference between the component and the cooling element is proportional to the sum of all individual thermal resistances - including the interface resistances between the surfaces in contact. And these interface resistances can easily be significantly higher than the thermal resistances of the heat conducting materials used. If these interface resistances are eliminated, the total calculation improves considerably.
Better heat dissipation leads to a longer service life of the electronic components and thus to greater reliability of your devices!
Notes:
Thermal conductivity measurements are dependent on pressure, method, layer thicknesses and measuring equipment.
Interface materials cannot replace metal-to-metal contact. Metals are always dramatically more thermally conductive!
Interface materials are also better the better they wet the (rough) surface.
The shorter a heat path is, the higher the possible heat flow. The larger the cross-sectional area, the more heat energy can be transferred (fins of heat sinks).
Interface materials (TIM) should not be electrically conductive (unless it does not matter, e.g. graphite foil).
TIM products should be resistant to ageing and should not tend to "pump out" due to thermal movements (thermomechanical stress).
How important are coatings on thermally conductive films?