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Heat Transfer and Thermal Conductivity: The Facts

In my first two columns in this series, I presented a broad introduction to the subject of thermal management of electronic circuits. I hope my “quick start guide” and “problems and solutions” items have sparked readers’ interest in this elemental but essential aspect of electronic system design.

This month I’m taking a closer look at thermal interface materials—how they can be applied to achieve efficient heat transfer, and the significance of bulk thermal conductivity in relation to heat transfer and thermal resistance. I’ll also be touching on the influence that product miniaturisation is having on thermal management techniques, and I will return to this recurring question: Why do I have to spread my thermal interface material so thinly?

As in previous columns, my approach will be based on the frequently asked questions that our customer support teams field every day on the phone, at exhibitions and when visiting customer premises. I've selected five of the most common questions, followed by their respective responses, and I hope they offer useful guidance for readers who may be experiencing design problems related to achieving a satisfactory heat transfer performance in their electronic assemblies.

What would be your top suggestions for achieving the most efficient heat transfer? 

First and foremost, read the thermal interface material manufacturer’s advice about optimum application methods. They’ve been around the block a few times and they know what they’re talking about! Remember, more is not necessarily better; uniform and thin applications of the heat transfer medium always work best. Most products are likely to have an optimum thickness of application, as determined by the manufacturer, and you should try to achieve this for best results.

It is important to understand your application. What are the environmental conditions under which it must operate, and how variable are they? Will the chosen heat transfer compound perform consistently as environmental conditions swing from one extreme to the other? Consider the heat sink; is this the actual casing surrounding the assembly or a separate heat sink placed on top of a component? Where cases are concerned, is there a gap between the case and the component?

What key issues must be considered when trying to achieve efficient heat transfer? 

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 filling material being used. These compounds provide a medium for improving the conditions under which heat transfer takes place and thereby 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.

To read this entire column, which appeared in the November 2017 issue of The PCB Design Magazine, click here.