Thermally conductive plastic – How PlastFormance technology can replace existing metal products

A thermally conductive plastic is abrasive, expensive and difficult to process?

 

The solution to these challenges lies in the appropriate technology!

 

Here we show, how a state-of-the-art thermally conductive plastic with electrical insulation provides the ideal combination of performance, injection moldability and system cost.

 

Thermally conductive plastic and its applications

The general trend in electronics is the miniaturization of electronic components and assemblies with even higher computing power at the same time. The power density is increasing, which means that heat generation in the component becomes the limiting factor for service life. As a result of the increased heat generation, thermal management is an increasingly important issue for the designer of assemblies, which require cooling. A thermally conductive plastic only makes sense if the assembly can be cooled efficiently.

 

Generally, current state-of-the-art heat sinks are still largely made of metals, such as aluminum. The disadvantage is the lack of insulating properties. This requires the use of interface materials for electrical insulation, which are expensive and have a poor thermal conduction.

 

A thermally conductive plastic with electrical insulation capabilities at the same time, can significantly reduce system complexity. By selecting suitable fillers, thermal conductivity and electrical insulation can be combined in one material. Thus, most of the components (e.g. of a heat sink) can be integrated in a single assembly. The innovation in the PlastFormance technology consists in the realization of extreme filling degrees in the thermally conductive plastic, while at the same time maintaining the injection moldability. The selected isometric particle morphology enables thermal conductivity in all spatial directions.

 
heatsink

Material requirements for heat sink components

The main task of a heat sink is to dissipate heat from heat-producing components. For this purpose, the heat flux from the emitter must first reach the heat sink via the entire heat guide path. Both the internal thermal resistance and the heat transfer resistances are adding up in the process. The dissipated heat is finally released through convection to the ambient air. A higher temperature difference between the heat sink and the discharge medium (air, water) is resulting in more efficient heat dissipation.

Consequently, the internal thermal resistance, the heat transfer resistance and the total number of required intermediate layers have a crucial role in the entire assembly. However, component geometry and orientation are also non-negligible parameters in the efficiency of heat sinks.

 

Thermal conduction in electronic assemblies

Optimal heat dissipation can be achieved, among other things, by minimizing the components of the heat sink. Each component has a specific thermal resistance, which influences the overall heat conduction path (see Figure 1).

Thermal Interface Materials (TIMs) also affect heat dissipation in a counterproductive way. As electrical insulators, they usually have very low thermal conductivity and form the “eye of the needle” in the heat conduction path.
heat conduction path of an LED heatsink

Figure 1: Schematic representation of the heat conduction path of an LED heatsink

(based on [1])

The total thermal path is composed of all added thermal resistances as follows. [1]
heat conduction path as a sum of all thermal resistances

Currently used materials

 
Due to its low density and high heat capacity, mainly aluminum finds its application in today's heat sinks. Although copper has a higher thermal conductivity, aluminum is used because of its simpler and cheaper production compared to copper.
thermal conductive plastic

Figure 2: PlastFormance heatsink

The heat sinks are usually manufactured by extrusion or aluminum die casting. Both processes have restrictions on the design freedom of the components. While only linear geometries can be realized in the extrusion process, undercuts of the cast parts in aluminum die casting must be dispensed with. The necessary molded parts for aluminum die casting are expensive and require a lavish production. Usually, additional surface treatment is necessary to prevent corrosion. Post-processing methods, such as machining, brazing or welding, also make the entire component production more expensive. [2]
Despite its disadvantages, this production chain is established in the industry. However, less well-known, optimized materials and manufacturing processes already exist. A thermally conductive plastic can already compete with aluminum, through suitable filler selection. Due to simplified production cycles, the combination of electrically insulating properties and thermal conductivity, as well as increased design freedom in component construction, a thermally conductive plastic represents a suitable alternative to aluminum (see Figure 2).
 
thermal conductive plastic

Thermally conductive plastic as heat dissipation material

A thermally conductive plastic compound combines the properties of the filler with those of the used polymer. A wide variety of materials, such as ceramics or metals, are suitable as fillers. The properties of the filler, for example excellent thermal conductivity, are transferred to the resulting compound. Thus, a plastic can be produced, that is e.g. electrically insulating and thermally conductive.

PlastFormance technology in thermally conductive plastic

 
With increasing filling level, the character of the filler in the compound increases. The problem with conventional compounds is, that processability in injection molding also decreases at the same time.
processability in injection molding with increased filler concentration

Figure 3: Processability in injection molding with increased filler concentration (based on [3])

Here, PlastFormance technology sets new standards. This is because PlastFormance compounds can contain a filler concentration of up to 80% by volume (see Fig. 3) and are still producible by injection molding. This production method allows greater freedom of design, than conventional processes for manufacturing aluminum heat sinks.
uniform heat dissipation

Figure 4: Uniform heat dissipation through powdered fillers

Another advantage is the homogeneous distribution of isotropic fillers. Platelet-shaped fillers, such as hexagonal boron nitride (hBN), arrange themselves in the direction of flow, resulting in anisotropic heat dissipation (see Figure 4). A thermally conductive plastic from PlastFormance guarantees uniform heat dissipation due to its spherical filler morphologies.
 

Unification of individual component parts

Conventional heat sinks consist of several components to ensure isolation of the aluminum component from the electronic product. A thermally conductive plastic for example, makes it possible to refrain from thermal interface materials (TIMs).
integration of individual components

Figure 5: Integration of individual components using PlastFormance technology

The combination of thermally conductive and electrically insulating properties allow the attaching of LED chips including their conductor tracks, directly to the injection-molded compound (Figure 5).

Why thermal conductivity like aluminum is not needed

 
Typical aluminum alloys for heat sinks achieve thermal conductivities of about 140 W/mK. While unfilled plastics have thermal conductivities of 0.2 W/mK, a thermally conductive plastic from PlastFormance is in the range between 3 and 15 W/mK.
comparison of the thermal conductivity of several products

Figure 6: Contact temperature as a function of the heat sink thermal conductivity (based on [1])

This circumstance may seem technologically disadvantageous, but from a thermal conductivity of about 10 W/mK, only small temperature reductions can be observed (cf. [1], see Figure 6).
 

On the one hand, this is because heat sink geometry and convection are not enough, to dissipate the heat efficiently. On the other hand, the electrically insulating thermal interface materials (TIM) with their very low thermal conductivities (0.5 - 1.5 W/mK) are also a limiting factor.

Conclusion

 
The current use of aluminum as a material for heat sinks does not exploit the full potential in terms of thermal management of electronic components. A thermally conductive plastic based on PlastFormance technology, offers numerous advantages, which are resulting in increased heat dissipation efficiency. These include:
  • Combination of electrical insulation and thermal conductivity in one material
  • High freedom of design, due to injection molding processing
  • Integration of different components in one device
  • Simplification in manufacturing, due to elimination of post-processing procedures
  • Cost savings in the overall system
  • Corrosion-free

References

[1] Heinle, C.: „Simulationsgestützte Entwicklung von Bauteilen aus wärmeleitenden Kunststoffen“; TU Erlangen-Nürnberg (2012)

[2] Dietrich, J.: „Praxis der Umformtechnik“; Springer Verlag (2018)

[3] Griesinger, A.: „
Wärmemanagement in der Elektronik: Theorie und Praxis“; Springer Verlag (2019)
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