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Design007-Jun2018

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54 DESIGN007 MAGAZINE I JUNE 2018 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 require- ments and application design, as well as criti- cal 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 appropri- ate to choose a thermal pad, which has the additional benefit of being pre-cut to size for ease of application. However, both 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, environmen- tal 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 tempera- ture limits defined for the device, while main- taining performance during changeable condi- tions. 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 mate- rial 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 typi- cally operates below the phase change temper- ature, the material will remain in its solid form and will not provide the desired low thermal resistance. Environmental Conditions: Is Protection Required? Aside from thermal changes, there may be other environmental factors to consider. A ther- mal interface material or gap filler must also be resistant against other environmental con- ditions such as high humidity, salt mist, cor- rosive gases, and so on. It is important to con- sider at the design stage whether these exter- nal factors could impact on the performance of the thermal compound. As a TIM is usu- ally applied in a very thin layer between two substrates, it is unlikely to be fully exposed to such conditions; however, a gap filling mate- rial could be subject to more challenging envi- ronments, 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? Application technique will depend upon the type of product. For both curing and non-cur- ing products, the method of application may be screen printing or automated dispensing, the only difference being the available work-

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