Heat sinks made from plastics: thermal conductivity by Boron Nitride

3M™ Boron Nitride powders facilitate customized solutions in many electronics application areas.

Large part counts, low costs and considerable design freedom: with good reason, plastics are a favorite material with modern designers. However, in the electronics field they are quickly bumping up against the stops: plastic is not heat-conductive. Electronic components require materials that can dissipate heat quickly and effectively in the smallest space. As a filler for polymers, 3M™ Boron Nitride solves this dilemma very simply.

The hexagonal boron nitride produced by 3M Technical Ceramics in Kempten opens up a novel class of material—in other words, that of heat-conductive polymers. This offers very different opportunities for innovative and installation-friendly solutions to take the heat away from power electronics modules.

Thermal conductivity comparable with that of copper

Because boron nitride has a thermal conductivity that is comparable with that of copper, in a mixture with a thermoplastic, this can achieve thermal conductivities in excess of 10 W/m*K. The filler also offers a unique combination of application properties. In plastic processing—compounding, extrusion, injection molding—it can be used without any problems. It has a low dielectric constant, a very good electrical insulation capacity, and a high resistance to friction; it is non-toxic and has good resistance to temperature.

According to the state of the art, metallic heat sinks made from aluminum or copper were used formerly to dissipate heat in electronics. As these are also electrically conductive at the same time, the structural elements to be cooled and the heat sink must, for example, be isolated from each other by means of insulating films. In order to interconnect the individual elements, these are bolted, clamped or bonded. All in all, power electronics manufacture goes hand in hand with a multitude of individual components and high installation costs.

It would be much simpler to produce cooling and functional elements from a heat-conducting plastic. Using plastic instead of metal reduces tool costs, as no cutting machining is required; even an electrically insulating intermediate layer is superfluous. And finally, the installation steps can also be reduced and automated owing to a reduced number of components.

Printed circuit board plus injection molding = LED flashlight

As this can be implemented practically in the design, 3M Technical Ceramics demonstrates a novel LED flashlight as an example. In contrast to conventional LED flashlights, there are only two components: a printed circuit board and a plastic body, which is melted directly around the PCB by injection molding.

The technical heart of the flashlight is a PCB from Häusermann, bent at an angle of 90 degrees. Thanks to its three-dimensional design, in this case a powerful LED and the electronic control unit are incorporated in a single PCB.

The material for the heat-conducting plastic body is based on a Luvocom compound from Lehmann+Voss. With special expertise in the compounding field, the company makes customized materials for quite special customer requirements.

The heat sink for the demonstration LED flashlight is made from 60% PET plastic and 40% 3M™ Boron Nitride cooling filler. Boron nitride turns the non-heat-conducting plastic into a reliable heat conductor.

Flexible forming

In forming matters, 3M Technical Ceramics approached RF Plast GmbH, a specialist in the development and production of sophisticated plastic shaped parts. As the forming of the compound material is really flexible, a single-part design was achievable, which united the cooling function and the ergonomic shape of the flashlight. A complex cooling rib structure can also be produced in the injection molding, which enlarges the surface of the device to a dimension necessary for cooling. The PCB is molded directly with the compound for production of the LED torch. No further assembly steps are required.

The heat generated by the LED is simply diverted via the plastic body connected firmly with it. The LED flashlight, with the light output of a 40-watt incandescent bulb is heated to a maximum and uncritical 73°C at the LED semiconductor, and 40°C at the housing. Without boron nitride, the temperature for the same light output would rise to well above 150°C. The LED would become overheated and therefore destroyed.

Plastic heat sink with boron nitride added to the mix

Adding boron nitride to the mix makes polymers heat-conducting, and at the same time, they act as electrical insulators. Polymers with boron nitride added to the mix are outstandingly suited to injection molding: they fill even fine cavities at low pressure and are not abrasive. This means that injection-molding tools do not wear out. In a nutshell, boron nitride enables completely new paths in electronics design to be trodden with conventional technologies.

A decisive advantage of heat sinks made from electrically insulating, but heat-conductive, plastics is that the electronics unit, which generates the heat, can be mounted directly on the heat sink. Additional insulation measures are not required.

Conventional heat sinks spread the heat—compared with the structural element—over a large area and thus dissipate it in the air. A complex shape such as the body of an LED flashlight would be possible only with very high energy consumption. Heat sinks made from heat-conducting plastic, in contrast, can be produced in freely selectable shapes, with any structure you wish for enlarging the surface dissipating the heat.

Simplified production process

A further argument is the simple production process in comparison with using conventional metal cooling elements: fitting individual cooling elements or installing PCBs on cooling elements by bonding or clamping requires additional, mainly manual and thus expensive assembly steps. In addition, the different thermal expansions with large area bonded connections of PCB and metal heat sink can lead to mechanical stresses. Problems that can be solved by using plastic cooling elements.

There are numerous opportunities to use heat-conducting plastics in electronics: As the example of the LED flashlight demonstrates, polymers can be used to remold electronic functional elements and thus fix them mechanically at the same time.

The advantages of heat-conducting polymers can be used in many fields: In automobile electronics, they can be used for sensors, heating systems, LED lighting or electric motor components. A further large example of use is consumer electronics, from LED televisions through smartphones to tablets.

The advantages of thermally conductive heat sinks at a glance

• short development time and low development costs through the combination of simple technologies

• high thermal conductivity exactly where it is needed

• weight reduction compared with metal components

• reduced production and installation costs

• no limit to design options

• heat dissipation without additional insulation expense