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82 DESIGN007 MAGAZINE I JUNE 2022 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 mate- rials. e application method and applied thick- ness of heat transfer compounds will depend on the type of interface or gap filling material used and both are crucial to achieving effi- cient heat transfer. ese compounds provide a medium for improving the conditions under which heat transfer takes place and thereby maximise 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, and remember, high thermal conductivity doesn't necessarily mean high heat transfer. Measuring thermal resis- tance at the interface is the most effective way to ensure the thermal compound is improving the efficiency of heat transfer and these results can differ depending on the thickness and quality of application. roughout our discussions, you will see both points come up time and again. ey are crucial to ensuring effective and efficient heat transfer with reliable performance from each device as well as over the lifetime of the product. 2. Bonding or non-bonding? ere are many different types of ther- mally conductive materials, and choosing between them will be dictated by produc- tion requirements and application design, as well as critical performance factors that must be achieved. Due to the variety of products available, this question is oen one of the first to arise and can sometimes be answered very simply. 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. ere are also many other factors that can affect this choice and it is oen better to review the requirements as a whole first. is allows a holistic view of the requirements from application through to performance in use. Usually by taking this approach, the decision of bonding or non-bonding oen becomes clear. 3. Increasing efficiency across a wide temperature range ermal 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 aer being powered down in conditions well above and below what we would consider a standard ambient temperature. It is therefore critical that the chosen ther- mal dissipation media operates within the tem- perature 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 sub- strates can cause a non-curing interface mate- rial to move over time. e ability of a ther- mal interface material (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. These compounds provide a medium for improving the conditions under which heat transfer takes place and thereby maximise efficiency.