Project 1
Basic investigation for effectiveness
of heat conducting materials
On the behalf of a manufacturer of automotive electronics, various thermal interface materials (TIM) were compared with regard to their effectiveness in a basic investigation.
Background to the investigation
With increasing power density in electronics, the thermal interface between heat-generating electronic components and the heat sink is becoming increasingly important. Here it is necessary to increase the heat flow, only in this way can sufficient heat dissipation be achieved as a prerequisite for guaranteeing the service life expectations of electronic modules.
Thermal Interface Materials
At the beginning, a market study compared the different technologies used for thermal interface materials.
Technologies considered
The use of adhesives as thermal interface materials offers advantages. Thus further mechanical fastening is not necessary and the risk of the heat conducting material flowing out of the gap, e.g. due to high temperatures or vibrations, is avoided. Within the scope of the study, various heat-conducting adhesives were investigated, some of which differ significantly in composition and price compared to heat-conducting pastes and gap fillers.
Experimental setup
A diode serving as a heat source is located on a printed circuit board which is connected to a heat sink by means of the investigated thermal interface material (TIM). In addition to an FR4 printed circuit board, the performance of a metal core printed circuit board was also investigated.
From the voltage (U2) determined in the test, the temperature at the diode (T1) and at the heat sink (T2) as well as the applied voltage (U1) and the current intensity (I), the thermal resistance was calculated for 3 adhesive layer thicknesses.
Results FR-4 printed circuit board
As expected, the thermal resistance increases with increasing adhesive layer thickness and, with the tested adhesive tape (#7) with a thickness of 0.5 mm, it is even higher than the value achieved without heat conducting material. Otherwise, the tests show that there are considerable differences between the different materials used.
Results FR-4 printed circuit board
This is also noticeable in a material-dependent ΔT between the diode temperature and the almost constant heat sink temperature over the test series. With a good thermal interface material (e.g. #5) a reduction of the diode temperature by up to 10 °C can be achieved under these test conditions. However, an adhesive (#3) that is not optimized with regard to heat conduction also shows a positive effect.
Results insulated metal substrate (IMS) circuit board
The comparison between FR4 and IMS curcuit boards on TIM #5 (1K moisture-curing Al2O3 filled silicone) clearly shows the not unexpected reduction in diode temperature, ΔT between diode and heat sink temperature, and thermal resistance.
Conclusion
There is no such thing as the ideal heat-conducting adhesive, which is equally suitable for all applications. Besides the desired high thermal conductivity the fillers used to increase thermal conductivity unfortunately have disadvantages. They are very hard and therefore abrasive. They are also significantly more expensive than the fillers commonly used in adhesives. The high filler content required to achieve good thermal conductivity leads not only to a high adhesive price, but also to a high material viscosity. In conjunction with the abrasive behavior, the final result is that high demands are placed on the application technology.
Therefore, it should be checked individually for each individual case which thermal conductivity is actually required.