Thermal conductivity is an important consideration in potting or casting electronic modules containing heat generating components. The heat being generated in operation by the various devices must be dissipated in order not to exceed the maximum operating temperature limit of the most sensitive component in the circuit.
Heat energy flows from hot to cold meaning that a temperature differential must exist between the device and the surrounding air (ambient) in order to transfer heat. The larger the temperature differences the higher the possible heat transfer and conversely the smaller the difference the less heat will be transferred.
Another consideration is the total surface area available to dissipate the heat to the surrounding ambient air. Here, the larger the surface area the better. Commercially available heat sinks often incorporate "fins” to increase the surface area available for heat transfer. A large surface area will remain cooler in operation and will result in a larger difference in temperatures between the heat source and the dissipating surface.
Since air is very poor in conducting heat, the majority of electrical and electronic modules are filled or encased in a "thermally conductive” epoxy or urethane compound. In addition to improved heat transfer, these materials also provide excellent mechanical and electrical protection for the device.
Material Considerations
Consideration must be given to all the "layers” of different materials present in the system from the heat source to the dissipating surface. Particular attention must be paid to the thermal conductivity of the potting or casting compound and the case material if any. Generally speaking, specifically formulated plastic or metallic cases are best.
Epoxy or Urethane compounds, specifically formulated for improved thermal conductivity, are the best choices because they have excellent electrical insulating as well as very good thermal conductivity properties. Thermally conductive compounds contain special filler combinations to enhance heat transfer.
Both Epoxy and Urethane compounds, without fillers, have excellent insulating properties, both from the electrical and thermal standpoint, therefore unfilled systems are not particularly suitable for heat dissipation. Since most of the heat will be transferred through the filler particles, the higher the filler content the better.
The best thermal conductivity is obtained by the use of metallic fillers. Unfortunately metallic fillers impart electrical conductivity which is not a desirable side effect in electrical and electronics applications therefore other types of fillers must be used.
Air entrapment must be minimized during the formulating process as well as during application. Trapped air, since it is a poor conductor of heat, will impede heat transfer.
The Trade-offs
As with any manufacturing process, there are a number of trade-offs to be considered to obtain the best combination of handling and performance. For example, a highly filled system is best for heat transfer but it is difficult to mix, pour and de-air during processing. The following table provides a summary of key properties to consider:
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The idea is to obtain the required heat transfer for the given circuit with the minimum amount of filler loading to allow for the best possible handling characteristics and cured properties.
In electronic circuits the maximum allowable internal temperature is determined by the device with the lowest maximum operating temperature in the circuit. For example, the maximum junction temperature of semi conductor devices or the maximum operating temperature for capacitors etc. have to be considered. The maximum operating temperature of the finished module must be such that a temperature gradient is still maintained from the internal source to the outside of the case.
The case itself must also be manufactured from a product that has reasonable thermal conductivity. A metal case is best for optimum heat dissipation or alternately an appropriately sized heat sink should be used to insure adequate dissipation.
A handy formula to calculate the maximum allowable ambient temperature for a set of operating conditions:
| RTH case - ambient T case T ambient P dissipation P in P out Efficiency |
Thermal resistance of the case to ambient Case temperature The ambient temperature Internal losses Power in Power out Efficiency under the expected operating conditions |
RTH case - ambient = T case - T ambient
P dissipation
P dissipation = P in - P out = P out - P out
Efficiency
