Thermal management design considerations

The current pressure on traditionally lower-powered components and devices – typically 20 watts or less – to increase their performance has placed greater emphasis on the need to extract heat, to avoid malfunction or even damage.

As demand increases for more sophisticated electronic devices, so too does the speed and functionality required; causing more heat to be generated. In addition to this, with the shrinking in size of electronic devices, the more tightly packed components and increased wattage per square centimetre creates a denser thermal environment, further increasing the heat generated and increasing the risk of overheating.

Other contributing factors to greater heat generation include the expectation for devices such as smartphones and tablets to operate longer, as well as the widespread adoption of LED lighting, in which the surface area to manage thermal transfer is very small.

In turn, this has spurred research and development into new and more efficient thermal interface materials (TIMs). The end goal is pretty simple: to reduce the risk of creating a void in the interface between two surfaces by ensuring good contact (‘wetting’) and to have the highest possible level of thermal conductivity along with the thinnest possible interface. There are a range of TIMs available, all with relative pros and cons, but there is an option available for pretty much any design need imaginable today.

We’ve put together some design considerations for finding the optimum thermal management solution:

Power density
One of the biggest issues to consider is power density. 100 watts over a square centimetre as opposed to 100 watts over a square inch have very different levels of power concentration and the higher the power density, the greater the need for thermal management.

Tip: Greases and liquid adhesives usually perform best in high power density applications.

Thermal impedance
As well as thermal conductivity, equally important is thermal impedance, which depends on the ‘wet-out’ or ‘wetting’, thermal conductivity and the thickness of the TIM. For instance, a metal foil has excellent thermal conductivity, but could allow air to be caught in the interface between two surfaces, thus reducing effective removal of heat.

Tip: There is a wide variety of TIMs available to designers using either lower-power components or creating high-powered devices such as LEDs.

Thermal tapes
Thermal management tapes have developed a great deal in the past decade and are often multi-purpose. They incorporate a fixing adhesive, meaning no additional fixing is required, and some contain EMI properties too. Tapes typically present a very cost effective option and can be die-cut to suit rapid assembly. Tapes do have a limit on the amount of power they can handle, though they will be sufficient for many lower-power density applications.

Tip: To achieve a sufficient degree of ‘wetting’, always seek out tapes that have good conformability.

Thermal pads
Pads have evolved considerably in the past few years. Previously viewed as gap fillers, pads are now a good substrate with low thermal resistance and are excellent for providing the close conformity needed to ensure no air enters the interface between two surfaces.

As well as traditional silicone pads – which have excellent softness – acrylic pads are also available and have superior heat transfer properties, as well as eliminating oil swelling and the risk of silicone outgassing, which can lead to corrosion. While acrylic pads are not as soft as silicone, they are suitable for many powered design needs. Thermal pads can also be die-cut into custom shapes, to achieve a range of benefits, including rapid assembly.

Tip: Thermal pads require additional fixing materials, so this needs to be included in the design process.

Thermal greases and liquid adhesives
Greases and liquid adhesives are usually the best option where an extremely thin interface layer is required. They both offer excellent wetting ability and liquid adhesives offer a high bond strength. An additional attachment method is required where using a thermal grease and these products can be messy in their application.

Tip: Greases and liquid adhesives are usually the best option for higher powered devices – e.g. CPUs.