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Design007-June2020

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JUNE 2020 I DESIGN007 MAGAZINE 71 degree, depending on the temperature the de- vice reaches in operation and, ultimately, the temperatures that the individual components reach. The coefficient of thermal expansion will vary from component to component, so contraction and expansion can happen at dif- ferent rates with adverse effects such as pump- out occurring as a result. To overcome this issue, a product that is sta- ble during the thermal shock cycle is required. Pump-out can occur when a device, such as an IGBT, is subject to temperature changes, re- sulting in relative motion between the conduc- tor and its heatsink between which a non-cur- ing thermal paste has been applied. This mo- tion can cause such pastes to be squeezed or pumped out from the interface gap, reducing the thermal transfer performance. To tackle issues with pump-out, it is first im- portant to understand the conditions and ma- terials involved. Temperature extremes and rate of change of temperature are important factors that will determine the choice of ther- mal interface material; for example, if operat- ing temperatures are likely to range between -50°C and 200°C, a silicone-based thermal paste would be the preferred option. In addition to these temperature consid- erations, the materials that are being used may affect the interface material, particular- ly regarding the spacing between the device and its heatsink (otherwise known as the bond line thickness). While interface mate- rials should be applied at a minimum thick- ness to achieve low thermal resistance, the re- sulting bond line may also be affected by the substrate's "smoothness" and spacing (i.e., components and heatsink surfaces at the in- terface). If a non-curing thermal interface ma- terial is applied more thickly due to spacing/ materials considerations, the pump-out effect will be greater. 5. What are the potential problems associated with thermal shock and thermal management material performance? As described previously, a thermal manage- ment product at an interface could be affected by the pump-out effect. This will result in the product moving and possibly allowing air to creep in between the interface. By having an uneven surface of thermal management mate- rial, hot spots can form, and the dissipation of heat will be affected. If a cured product is used, the flexibility of this material must be able to withstand the changes in the coefficient of expansion of all the different materials in- volved. The cured TIM should not crack or lose adhesion to the interface substrates during the thermal shock cycle. Phase change materials are highly suited to the thermal management of electronic assem- blies for a number of reasons. They offer ef- ficient thermal transfer, along with enhanced performance with thermal shock cycles and greater thermal protection where temperature spikes can occur due to their ability to store and release thermal energy (latent heat) dur- ing the phase change process. Providing an al- ternative to traditional thermal greases, once heated above their phase change temperature, the new phase change materials become high- ly thixotropic liquids that perform equally as well as, but often superior to, a thermal grease. The consistency and performance of these new materials avoids possible application and mi- gration problems that can be associated with thermal greases. Help is available for thermal management material selection and/or application tech- nique, so always seek expert advice. We thrive on challenges, so if you have a particular ther- mal management issue or question, please reach out. No matter how great the challenge may seem, we always aim to achieve a stream- lined production process and efficient heat transfer both in the initial application and final use. DESIGN007 Jade Bridges is global technical support manager at Electrolube. To read past columns from Electrolube, click here. Download your free copy of Electrolube's book, The Printed Circuit Assembler's Guide to… Conformal Coatings for Harsh Environments, and watch the micro webinar series "Coatings Uncoated!"

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