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54 The PCB Design Magazine • September 2017 Thermal pastes are often designed to be ap- plied in as thin a layer as possible. They improve the contact between the device and its heatsink by eliminating air gaps and ensuring that the full surface contact area is available for heat trans- fer. There is, however, a critical thickness which determines maximum thermal transfer with minimal thermal resistance, and while this will depend on the roughness of the substrates and required spacing, it is generally between 30 and 100 microns. A thermally conductive heat transfer material will naturally have a lower thermal conductivity than the metallic heat sink material, so keeping the thickness of the film at the interface as low as possible will decrease the thermal resistance and, in turn, lower the operating temperature of the device. While interface materials should be ap - plied at minimum thickness to achieve low ther- mal resistance, the resulting bond line may also be affected by the substrate's smoothness and spacing, i.e., components and heatsink surfaces at the interface. If a non-curing thermal interface material is applied more thickly due to spacing/ materials considerations, the greater the pump- out effect will be. So, while the general rule is to minimise the thickness of non-curing thermal interface mate - rials, this must not be to the detriment of their stability in use. For example, it is particularly important to ensure that a lower film thickness does not result in air gaps forming in the film between the device and its heatsink as this will increase thermal resistance and the device will not be cooled as efficiently as desired. Consider using alternatives to non-curing thermal interface pastes such as the new phase change materials that are now coming on to the market, or for those wide operating tempera - ture range applications, a surface-cure silicone thermal paste; both alternatives will minimise pump-out whilst keeping thermal resistance low. The low phase change temperature of phase change materials allows low thermal resistance over a wide temperature range, ensuring mini- mal bond line thickness with improved stability and pump-out resistance. And while a surface cure thermal paste doesn't set entirely (thus al- lowing for easy rework), it is specifically formu- lated to resist pump-out, particularly for those applications that are exposed to rapid and fre- quent changes in temperature. As well as phase change materials and surface curing silicone thermal pastes, other alternatives to non-curing thermal pastes include thermal gap filler pads, which are available as silicone and non-silicone based sheet materials that can be cut to size and applied by hand. These are highly thermally conductive, but they do have a higher thermal resistance than thermal pastes. Another approach is to use a room tempera - ture vulcanized (RTV) product, which becomes a flexible rubbery material on curing, combin- ing the properties of silicone gap filler pads with those of a conventional heat transfer paste. RTVs can be used to bond the heatsink to the com- ponent while also offering a flexible heat trans- fer medium. A much less flexible bond can be achieved between the device and its heatsink by using a two-component epoxy resin which cures to a tough solid bond. A promising new material is the thermal gel which is a silicone-based formulation that offers the low thermal resistance of a non-curing ther - mal paste minus the latter's pump-out problems. Gels are highly conformable and, even better than the softest of silicone gap filling pads, they impart minimal mechanical forces on delicate components during application and in use. For certain types and designs of heat generat- ing circuitry, it may be more beneficial to encap- sulate the device in a heatsink enclosure using a thermally conductive encapsulation compound. Silicone, polyurethane and epoxy resins provide both heat dissipation and environmental protec- tion all in one. A variety of thermal management products are available now, but as I hope I've made clear in the foregoing, it's complicated deciding on the right choice of material and/or application technique. I strongly recommend you get some expert advice before you settle on any particular material or method. PCBDESIGN Jade Bridges is the global technical support manager for Electrolube Ltd. THERMAL MANAGEMENT: PROBLEMS AND SOLUTIONS

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