Thermal Interface Materials

Thermal grease has been around since the early days of manufacturing electronics and trying to control the unwanted heat that is built up in the components. Thermal grease is applied between 2 surfaces where heat needs to be transferred between the hot component and its heatsink or heat spreader. It works by removing air voids present microscopically on the mating surfaces. Air has a very poor thermal conductivity and will greatly reduce the thermal transfer if it is present within the interface.

Pads provide a flexible method of thermally joining 2 mating surfaces and often delivering electrical isolation also. Also materials such as Bondline can also mechanically bond as well. They remove air voids as they are able to flow.

Thermal management is vital to the performance and safety of your assembly. A simple 10C rise in temperature within your semiconductor has been said to half its operating life. 10 Degrees C change in temperature can happen with a day/night temperature excursion. In the case of an LED, it is the same plus the light outputs are greatly reduced and even the colour of the light can be altered with heat build-up. If you manage your heat you can often drive the components harder achieving greater efficiencies and even reducing the cost of your application. For instance, you may be able to reduce your component count thereby making very big savings. Also, smaller heat sinks can be possible just by choosing a greater performing Thermal interface material.

Traditional Thermal Interface materials often work by means of the material “cold flowing” and removing or reducing the micro air voids. To get this to happen the material must have mechanical pressure applied by means of a fixing method (commonly screws or clips).
A more recent material (last 10 years or so) called “Phase Change” can be used on pressure-sensitive applications such as ceramic or CPU type applications. The material softens when it reaches a threshold temperature typically 55-60C to “wet-out” the surfaces to remove micro-air voids. The material only has to see this temperature once and then it is bedded in. In the case of Gap Pads (pads thicker than 0.5mm thickness) then the surfaces are often visco-elastic and have a residual stickiness to them which means they are “auto-flowing” without a great deal of pressure. The material feels sticky because it is “flowing” into your finger print making it feel tacky but there is NO adhesive present!

Most thermal interfaces are simply applied by assembling them between mating surfaces and clamping. Others such as Bondline are thermal conductive double-sided tapes. Gap pads are again designed to be installed between mating surfaces with mechanical fixings. It can be a good idea sometimes if the pad is thin to make either a visual tab to protrude so that it can be seen by the assembly operative or inspection department as this will indicate the presence of the TIM. (Sometimes they are missed out of the assembly and cannot be see because they are too thin leading to latent failures in the field).

You can select a phase change or soft gap pad instead of a TIM needing higher pressures.

If you need to replace a TIM perhaps for servicing or warranty then ideally you should see material on both surfaces as you part them. This means the “wet out” has happened well and thermally you are getting the best performance. Surfaces should be cleaned down and a new TIm should be applied.

TIM’s are required wherever there is a need to manage the unwanted heat building up in any electronic assembly. A very common application for Materials Direct is Electric vehicle battery systems. Fast charging and fast discharging lead to very high temperatures needing to be controlled often with liquid cooling plates. Those plates need to be connected to the cells via conductive paths being the most efficient. There are also often very high voltages and currents in these systems so high-reliability materials are required.

A typical TIM consists of a “carrier” which can be a glass fibre fabric or polymer to which a “binder” is coated to both sides. The “binder” has in it a blend of ceramic particles called the “filler”. The assembled TIM is often soft to aid “cold Flow” when under pressure to “wet-out” the surfaces and remove micro-air voids. Some TIM’s are however a homogenized material and consist of “binder” and “filler” only such as UniGap products. Bondline tapes also are Acrylic adhesive and filler only without a carrier.

The double-sided adhesive tapes such as 3M and Bondline are some of the highest thermal conductive tapes available on the market. The bond with high strength electrically isolates and thermally conducts. They remove the need for unsightly screws and fixings in LED lighting applications. Ideal for use with bonding Aluminium backed PCBs or FR4 PCBs to metal heat sinks. However, they should not be used on powder coated surfaces.