Over the past few months, I have been exploring the subject of thermal management materials and solutions – why we use them in the first place, what materials and methods are available to us, how we apply them and what the limitations are of these materials, as we put them to the test in real world applications. In these columns you may have noticed more than a passing reference to a new generation of thermal management solutions – Phase Change Materials – which offer a number of advantages over more traditional, non-curing thermal pastes, such as the achievement of minimal bond line thickness with improved stability and pump out resistance. Phase change materials are also attracting the attention of specialist electronics sectors such as the LED lighting industry, where they are increasingly becoming the product of choice for larger sized assemblies destined for exterior and heavy-duty interior LED lighting applications.
So, this month, I’m going to look more closely at Phase Change Materials and I will use some of the ‘frequently asked questions‘ that our customer support teams field every day on the telephone, at exhibitions and when visiting customers’ premises. I’ve selected five of the most common questions and hope the answers offer useful guidance for those readers who are keen to find out more about these innovative new thermal management products.
Can you please explain exactly what ‘changes’ take place in the properties of phase change materials during their use?
A phase change material is designed to alter its state at the phase change temperature, meaning that it will transition from a solid material to a softer material above the phase change temperature. This allows the product to ‘flow’ into all the voids and small gaps that are present at an interface between two substrates, just like a paste would do on application, thus allowing for minimal bond line thickness and minimal thermal resistance at that interface. From this it can be deduced that the best performance 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 applying phase change materials. For example, Electrolube’s 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 temperatures 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 consider. If the continuous operating temperature of the device is below the phase change temperature, the product will not perform at the level normally expected of it.
Why is the stability of a phase change product important?
Phase change materials provide greater stability than traditional thermal interface materials, 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 of the materials in the unit will expand or contract to a certain degree, depending upon the temperature the device reaches in operation and, ultimately, the temperatures that the individual components reach. The coefficient of thermal expansion will vary from component to component 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 temperature changes, producing a shear type action at the surface that can lead to changes in the rheology of the interface material and movement of the thermal product from its original application position. Phase change materials alter their state above and below the phase-change temperature, so they are able to 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?
Electrolube have introduced some novel products that offer the benefits of traditional thermal management solutions while combining 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 provide 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 thermal 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 maintain a low thermal resistance. As noted above, there are other options that offer good stability in changing thermal conditions, but few provide as good a compromise as phase change materials with regards to balancing these two desirable properties.
Electrolube has a substantial portfolio of thermal management products and has invested extensively in product research and development. The company’s new high-performance phase change materials complement this portfolio and provide alternative approaches to thermal management, when it is appropriate to specify them. As with all new projects and applications, it is advisable to get some expert advice before you settle on any particular material or solution and then test it thoroughly before you commit.