Last year I introduced the subject of thermal management materials – why they are necessary, what materials and methods are available to us, how best to apply them and so on – basing each blog on our customers’ frequently asked questions. Up until now, my colleague, Alistair Little, has discussed resins in greater detail in each of his columns; now, I’ve been invited back to pick up from my last series – this time concentrating more closely on the practicalities of thermal management with this five-point guide to material choice and application.
Understanding the dimensions – interface or gap filler?
Knowledge of the dimensions of your application is critical to the selection of appropriate thermal management materials. A thermal interface is the space between a component and its heat sink, and the thermally conductive media used in this space are referred to as thermal interface materials (TIMs). This space is usually very small, i.e. on the micron scale. A gap filling application, on the other hand, is more to do with the distance between a component and the metal housing that encloses an electronics assembly and is typically measured in millimetres. In this case, a thermally conductive material is used to help minimise the chances of hot spots within the unit itself while the casing is used as the heat sink.
The difference between a few microns and a few millimetres could be critical to the performance of the thermal medium chosen. For example, if you place a TIM in a gap filling application, it is likely to be unstable in the thicker layer: with vibration, or following a period of temperature cycling, it could easily be displaced. Likewise, if a gap filling material is used in a thermal interface application, it will be very difficult to achieve a thin, even film, creating a higher thermal resistance at the interface and consequently reduced heat transfer efficiency.
Bonding or non-bonding – paste or pad?
There are many different types of thermally conductive materials, and choosing between them will be dictated by production requirements and application design, as well as critical performance factors that must be achieved. For example, choosing between a bonding or non-bonding material may depend on whether the heat sink needs to be held in place by the interface material, in which case a bonding compound is the better choice. Alternatively, a compound that is fixed (does not move) may be required, in which case it may be appropriate to choose a thermal pad, which has the additional benefit of being pre-cut to size for ease of application.
However, both of these options may result in a thicker interface layer and therefore a higher thermal resistance. The trade-off then comes from the performance requirements of the chosen compounds and understanding the conditions of the application.
Maximising heat transfer efficiency across a wide temperature range.
Thermal changes are common within heat dissipation applications because most devices are switched on and off, or have varying power requirements in use. In addition, environmental temperature changes can lead to extremes within the device – automotive applications are a good example, as these must also operate after being powered down in conditions well above and below what we would consider a standard ambient temperature.
It is therefore critical that the chosen thermal dissipation media operates within the temperature limits defined for the device, while maintaining performance during changeable conditions. A typical problem is ‘pump-out’ whereby the stresses exerted by the minute changes in dimensions of the interface substrates can cause a non-curing interface material to move over time. The ability of a TIM to resist these stresses will improve the performance of the device over its lifetime and will be dependent upon the interfacial spacing, as well as the type and amount of TIM applied.
Where thermal effects are significant, it might be worth considering the use of phase change materials – non-curing, non-bonding products that change to a slightly softer material above their phase change temperature. The properties of these materials allow them to conform to the contours of the interface and provide a much lower thermal resistance than a cured product, whilst minimising the effects of pump-out, typically associated with non-curing products. However, if a phase change material is used in a device which typically operates below the phase change temperature, the material will remain in its solid form and will not provide the desired low thermal resistance. You can read more about these Phase change materials in a previous blog of mine Here.
Environmental conditions – is protection required?
Aside from thermal changes, there may be other environmental factors to consider. A thermal interface material or gap filler must also be resistant against other environmental conditions such as high humidity, salt mist, corrosive gases and so on. It is important to consider at the design stage whether or not these external factors could impact on the performance of the thermal compound. As a TIM is usually applied in a very thin layer between two substrates, it is unlikely to be fully exposed to such conditions; however, a gap filling material could be subject to more challenging environments, in which case the better approach would be to switch from a gap filler to a full protection compound, such as a thermally conductive encapsulation resin.
How to apply thermal interface material?
Application technique will depend upon the type of product. For both curing and non-curing products, the method of application may be screen printing or automated dispensing, the only difference being the available working time of a curing material. For example, if the product quickly becomes touch dry, it may not be suitable for stencil printing as the cured product may block the screen. In most cases, a minimum amount of material must be applied for thermal interface and gap filling applications to ensure maximum heat transfer. For a thermal interface, the layer must achieve uniform coverage over the entire interface; and when using a gap filler, the material must be applied while ensuring that all air is expelled, as air is a poor conductor of heat and may cause additional hotspots.
If an encapsulation resin is deemed to be the best choice, it is likely that the entire PCB will need to be covered. The amount of resin applied will have to strike a balance between achieving the desired protection level and minimising any weight and volume gains contributed by the resin.
Hopefully, the foregoing has provided a useful introduction to thermal management materials. Look out for my next contribution, where I will be continuing to explore thermal management further.
