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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 column on our customers’ frequently asked questions. 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 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.
To read this entire article, which appeared in the June 2018 issue of Design007 Magazine, click here.