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Design007-Nov2020

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24 DESIGN007 MAGAZINE I NOVEMBER 2020 fore, may not behave in the same way as a material that is applied in production. For this reason, the most appropriate choice on paper might not be the best overall choice when tak- ing in these additional factors. 2. What is thermal resistance? Thermal resistance doesn't just rely on the bulk thermal conductivity alone; it considers the product's performance in the actual use conditions by also factoring in the bond line thickness and the contact resistance at the two interface surfaces. The most common way to evaluate the performance of a thermal man- agement product is to check the thermal resis- tance between the component and heat sink with and without the thermal management product applied. Another common method is to measure the device or component operat- ing temperature with and without the thermal management product applied. 3. Why does the viscosity of a thermal management material affect processing parameters and eventual performance of the product in use? The initial viscosity of a thermal manage- ment material can impact the application method. For instance, with screen printing, the mesh size controls the thickness of paste applied. If the paste is too viscous, it may not spread very well over the screen; therefore, the desired thickness may not be applied. Simi- larly, with dispensing applications, if the prod- uct is too low in viscosity, it might spread too well and flow into unwanted areas. When in use, the way a material responds to shear will indicate how the product may behave under changing temperature condi- tions, such as with the pump-out effect. If the thermal management material is shear thin- ning, it will reduce in viscosity with increasing shear. The effect of shear can occur between two surfaces, which expand and contract with changes in temperature, illustrated by their CTE. If you have large CTE values for the sur- faces or big differences in CTE between the two surfaces, the effect of shear may impart a viscosity change in the product and hence affect its stability in end-use. 4. How significant a problem is vibration when considering the choice of thermal management material? Vibration can be a significant problem in certain applications. Like the pump-out effect, vibration is a physical change that happens in the surrounding environment. With such movement, a change in the position of the ther- mal management product may occur. This can lead to reduced efficiency of thermal transfer. In gap-filling applications, the effects of vibration can be much greater, especially if the product is a non-curing paste or putty. Gap fill- ing applications generally have significantly more product applied in one area; therefore, the movement from vibration can cause large shifts in the placement of the gap filling prod- uct if it is not contained by the structure of the PCB casing. If vibration tests are required for the PCB, these tests should definitely be repeated with the chosen thermal manage- ment product present to ensure no significant changes are seen during the expected lifetime of the device. 5. What are your top five tips for success? One factor I cannot stress enough is to adopt the "test before you buy" approach. If a thermal management product is not tested before use, the end performance of your product might be very different from what you expected. The following points are essential for a suc- cessful thermal management process: 1. Consider all influential external conditions (temperature, vibration, etc.). 2. Decide on the ideal production process for the volumes being produced. 3. Look at the board design. For TIM applications, review if the interface gap can be minimised and ensure it is not excessively wide. 4. Consider the materials present on the PCB/unit. Are there any sensitive materials present or high CTE values?

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