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Design007-Sept2020

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50 DESIGN007 MAGAZINE I SEPTEMBER 2020 Accounting for the resistance change of metal with temperature can have a big impact on the system. If you ignore copper—like the circuit board, big com- mon-mode chokes, or inductors—you might be cutting a lot of power dissipation and heat out of your analysis. Even if you include them but use room-temperature copper calculations, there can be big differences between analysis and reality. Thermal foam can be a very useful way to get heat to a metal case when a closed system is used (e.g., no fan). It's critical to have the foam in compression, otherwise, it won't achieve the desired low thermal resistance. The ambient temperature is very important, and "ambient" is often not considered prop- erly. Just because the air outside your product is 50°C doesn't mean inside is 50°C. For most electronics, the actual ambient temperature in which they're operating is the steady-state tem- perature inside the enclosure. Question #2 For the most part, the most effective way of getting the heat out/dissipating heat from one side to the other is to use filled vias. The best filled-via is a stitching via with solid copper plug matching the CTE of the copper being used on the PCB. The next best solution would be copper epoxy-filled vias. After that, a silver-filled via works okay. Lastly, epoxy fill can get some of the heat out. All work better than a standard stitching via that is unfilled. Air—particularly stagnant air—is a poor ther- mal conductor, so any material filling the via will lower that thermal pathway's resistance. A smaller via filled with solid copper and many such stitching vias in an array would be the best if using vias to dissipate heat. Also, note that many fabricators have a size limitation on the size of filled vias; most say 0.008"–0.020." Question #3 Generally, not increasing the copper area via a ground or power plane and even add- ing higher copper weight internally is not the answer. This approach may be quite beneficial for transient thermal excursions, but for most products, steady-state power dissipation is the concern. Here, I would recommend a basic analysis through rough equations would be of a greater benefit than adding copper layers internally. But, as Kevin said, producing a prototype that can be tested in a thermal chamber would really tell you where you are. If you are looking to dissipate heat and are considering adding copper, the best layers would be the outer layers with copper pour. This helps EMI but can dissipate heat through the surface. Again, as Kevin said, "There is no magic copper layer that will be the best way to get the heat out." Regarding heat in an enclosure, what is the best way to get the heat out when a board is in an enclosure? The first and oldest way is to simply have a fan in the enclosure. Not being able to do that, Kevin mentioned a trick he learned when he was designing for DVRs that cannot have a fan. Another good method is to utilize compressed foam in the enclosure, but why compressed? Common sense would say uncompressed foam is porous. The idea behind the foam is primarily spread- ing the heat to a chassis or other system com- ponent, which can hopefully get the heat out. The compression helps achieve lower thermal resistance, improving the effectiveness of the thermal pathway. Question #4 A simple operating test in a thermal chamber using a prototype design can help make sub- stantial leaps in understanding product risks, missed aspects of analysis, and correlating cal- culations. Question #5 Question #5 is probably the best and tough- est question to answer. The short version is to Kevin Carrington

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