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100 DESIGN007 MAGAZINE I JANUARY 2018 allowing for minimal bond line thickness and minimal thermal resistance at that interface. From this it can be deduced that the best per- formance will only be achieved with a phase change material when it is used above its phase change temperature. Are phase change materials restricted to a single method of application or are there more options? There are, in fact, numerous options for apply- ing phase change materials. Some products can be screen or stencil printed, so it is possible to apply a thin layer to many different substrates and unit designs. When considering which type of application is appropriate for a phase change material, it is essential to understand the tem - peratures that the device will be subjected to during operation. For example, while a device subjected to thermal cycling or operation at a continually stable temperature will dictate what type of thermal management product is likely to achieve the most efficient performance, with phase change materials there is the additional factor of the phase change temperature to con - sider. If the continuous operating temperature of the device is below the phase change tem- perature, the product will not perform at the le vel normally expected of it. Why is the stability of a phase change product important? Phase change materials provide greater sta- bility than traditional thermal interface mate- rials, such as thermal pastes. This is because most devices will go through some form of thermal cycle, even if it is just as simple as when the device is switched on and off. When changes in temperature occur, all the materials in the unit will expand or contract to a certain degree, depending upon the tempera- ture the device reaches in operation and, ulti- mately, the temperatures that the individual components reach. The coefficient of thermal expansion will vary from component to compo- nent due to the different materials from which they are made, so contraction and expansion can happen at different rates, and effects such as pump-out can occur as a result. Pump-out happens at the interface where the mating substrates alter with the tempera- ture changes, producing a shear type action at the surface that can lead to changes in the rhe- ology of the interface material and movement of the thermal product from its original appli- cation position. Phase change materials alter their state above and below the phase-change temperature, so they can resist the effects of pump-out and remain more stable over many thermal cycles. Are there other, more traditional thermal management solutions that feature a high level of stability? Companies have introduced some novel products that offer the benefits of traditional thermal management solutions while com- bining the stability required for high thermal cycling applications. These offer a surface cure only and can be easily removed if any rework is required. Other traditional products that pro- vide a complete cure while also featuring a high level of stability include single-part silicones or two-part epoxies, for example. However, with these products, rework is much more difficult, and they are unlikely to achieve the low ther- mal resistance of a traditional thermal paste. Why would you select a phase change material over a more traditional thermal solution? A phase change material is chosen mainly for its stability coupled with its ability to main- tain a low thermal resistance. As noted above, there are other options that offer good stability in changing thermal conditions, but few pro- vide as good a compromise as phase change materials with regards to balancing these two desirable properties. As with all new projects and applications, it is advisable to get some expert advice before you settle on any material or solution and then test it thoroughly before you commit. DESIGN007 Jade Bridges is global technical support manager for Electrolube.

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