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70 The PCB Design Magazine • November 2017 Again, think variability of environmental conditions. Just because the heat transfer is efficient under standard ambient conditions, doesn't mean that it will remain stable over the lifetime of the product. Accelerated testing might be able to reveal related design issues; however, in-application testing is more likely to provide definitive information about the long- term performance of heat transfer materials. Application of the heat transfer compound will depend on the type of interface or gap fill- ing material being used. These compounds pro- vide a medium for improving the conditions under which heat transfer takes place and there- by maximise its efficiency. Compounds should not be applied in excess amounts in the belief that they will achieve the thermal conductivity of a solid metal heat sink. Remember, high thermal conductivity doesn't necessarily mean high heat transfer. This will depend on the thermal resistance achievable with a particular thermal compound and how effectively it can be applied in an opti- mally thin film. Bulk thermal conductivity can be measured in lots of different ways. Nowadays, the equip- ment available for measuring this parameter is very accurate, but the methods of measurement are likely to differ from one laboratory to an- other. If a true comparison of bulk thermal con- ductivity is required, it is advised that the same equipment and conditions are used for compar- ison purposes. Bulk thermal conductivity is only the trans- fer of heat through the thermal management material itself. The measurement is usually tak- en using much greater thicknesses of the ma- terial than would be used in a real application and it will not be representative of the thermal resistance that would be expected for that appli- cation. My company always advises that ther- mal management products are applied in a real- world application so that the efficiency of heat transfer can be tested under the specific condi- tions of use. Product miniaturisation is increasing the need for efficient thermal management within devices. As devices get smaller, the spacing be- tween components also gets smaller and any heat generated by these components is concen- trated at specific areas of the PCB. Components that generate a lot of heat could therefore ad- versely affect other nearby components which, due to miniaturisation, are much closer to one another. Excessive heat can age PCBs and com- ponents, consequently shortening their lifes- pan. By using effective thermal management techniques, excessive heat can be dissipated, en- abling densely populated designs to be realised without compromising the life expectancy of the device. HEAT TRANSFER AND THERMAL CONDUCTIVITY: THE FACTS What key issues must be considered when trying to achieve efficient heat transfer? How has the trend for product miniaturisation affected the thermal management materials market? What tips can you give about measuring bulk thermal conductivity and making effective comparisons between different thermal management products?

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