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60 DESIGN007 MAGAZINE I APRIL 2020 These are low-cost items that are readily available; all that you need to do is match the screen mesh diameter to the product you are using and the required thickness you wish to apply. Your TIM supplier should be able to as- sist with this. How you apply a thermal inter- face material, or thermal paste, is important for the effectiveness and reliability of the product. For the best results, the entire surface must be covered using a thin, uniform layer. For the manual application of a thermal paste using a screen, we recommend starting with a screen thread count of around 70 threads per cm. The thread count of the screen controls the thickness of paste applied and may need to be adjusted to suit your particular application. Lay the cleaned heat sink or substrate, face- up on a flat work surface. Place the screen evenly over the surface and apply a thick line of thermal paste at the far end of the screen us- ing a spatula. Using a squeegee that is wider than the substrate, slowly and carefully drag the paste over the surface, ensuring all areas are completely covered. Apply high pressure to the squeegee when in use and only pass over the surface once. Gently lift the screen, slowly applying pressure so that you can see the screen separating from the surface. Do not lift the screen off quickly or unevenly as this may affect the distribution of the paste left on the substrate. If any errors do occur, clean the product from the screen and substrate by wiping off the ex- cess with a dry cloth and then clean. Once re- moved, you should be left with a uniform lay- er of thermal paste on the substrate. Assem- ble the parts together immediately to avoid any contamination migrating onto the applied paste. 2. Why is less often more when it comes to the application of thermal management products? Thermal management products are designed to remove air from between a heat-conducting component or device and its heat sink. This could be a standard heat sink, or it could be the device housing designed to offer both pro- tection and heat dissipation. As such, the air gaps could be very small (in the case of ther- mal interface applications) or up to a few mm in the case of a gap between component and casing. In all cases, air is the worst conductor of heat, and the heat sink or casing will be the best heat conductor. To ensure the most effi- cient heat transfer, all air must be displaced without adding extra thermal management material to widen the gaps further; therefore, keep the quantity of thermal management material to a minimum. 3. Why does a thicker layer of thermal paste not enhance the performance of the paste? The thermal management product is de- signed to be flexible in terms of its application and should, to some degree, conform to the contours of the interface in a way that a sol- id metal material could not. However, a solid metal heat sink will have a much higher bulk thermal conductivity than a thermal manage- ment material, such as a paste or phase change product, for example. Therefore, if you apply too much of the ther- mal management material, you are not only displacing air, you are also increasing the bond line thickness between the heat-generating component and the heat sink. By pushing the heat sink further away from the component and putting a lesser thermally conductive ma- terial at the interface, the overall thermal resis- tance will increase, consequently reducing the efficiency of thermal transfer. 4. What are the potential issues to be faced by not employing correct thermal management? If the temperature of a device or component is not controlled, it could lead to inconsisten- cy of performance, reduced efficiency, or—ul- timately—failure. The actual results will de- pend on each individual design requirements, hence the need to correctly assess which type of thermal management is most suited to your application.

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