Information about Thermal Management
Most of the time one would like to derive the resulting heat loss as well and quickly as possible to the heat sink (cooling element). For this purpose, the component is coupled as tightly as possible and over a short distance.
In the case of space problems (very tight, no room for large cooling elements at the location of heat generation, even distribution of heat over a large area, e. g., backlight illumination of screens), it may be necessary to use so-called "heat spreaders". These allow little heat through the material, but spread heat very quickly and well to a larger area. So one can direct the heat flow in the direction of a heat sink. Graphite foils achieve up to 1,500 W / mK thermal conductivity in the surface, the electrically insulating Temprion™ OHS™ up to 50W / mK.
Heat transfer as a function of thermal conductivity and thickness of the material
There is a direct, linear connection between the material thickness and the heat flow that can be transported through the material. If you want to transport the same amount of heat through a material of double thickness, the thermal conductivity must also be doubled.
Thermal resistance = (material thickness * 1000) / (thermal conductivity * surface)
[mm * 1000 / (W / m * K * mm²) = K / W
Advantage for Kapton MT +: very good thermal conductivity due to low material thickness and very good dielectric strength
Important: this statement refers only to the transport within the material - the heat transfer resistance is not taken into account (see above "How important are interface coatings")
Equiation:
(assumtion: area is constant)
Kapton MT = 0,45 W/m*K and 25µm
a gap-pad of 1,00 mm needs 18 W/m*K
a gap-pad of 2,00 mm needs 36 W/m*K
to transport the same quantity of heat per time!
How to measure thermal conductivity?
According to ASTM D5470, the thermal conductivity of a test material is defined as follows:
A heated metal block supplies the heat source. Closely above and below the heat-conducting material temperature sensors are mounted. Below is a metal block that represents the heat sink (cooled if required).
After a stable heat flow has set, the temperature difference generated by the test material is determined. From this, the thermal conductivity is calculated.
Advantage of this method over the laser flash method: the roughness on the surface of the material is also measured. Because in the real situation of installation, the contact resistance at the interfaces of the individual materials will always play a role.
Thermal conduction depends not only on the materials used
The heat transfer e. g. from the case of a power transistor to an aluminum heat sink can be quite bad. Surface roughness reduces the direct contact area necessary for heat conduction by about 60-80%.
Even a good heat-conducting, but "hard" insulation material can not completely fill the resulting air pockets. Especially high performance films like Kapton® MT have disadvantages.
By using thermally conductive waxes (PCM, Phase Change Material) one can avoid this disadvantage. The wax, which is solid at room temperature, melts during initial startup and fills the cavities. This results in a continuous heat path without interruptions by air inclusions. The thermally conductive phase change material is so thin that it hardly affects the heat conduction.
Total heat resistance = (resistance housing) + (resistance transition) + (resistance TIM) + (resistance transition) + (resistance heat sink)
The solution: Reduction of the heat transfer resistance through thermally conductive coatings!
Example: Thermal pad made of filled silicone material
