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Design007-Nov2023

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26 DESIGN007 MAGAZINE I NOVEMBER 2023 e top-layer pad on the le heats to about 90°C, while the plane on the right lowers the temperature of the top-layer right-hand pad to about 46°C. Now suppose we have the opposite situa- tion and want to lower the pad temperature. One might think this was an application where thermal vias might be used. But thermal vias have to go somewhere, typically a plane (or at least another pad) on a lower layer, and here there is no such resource. So, thermal vias are not an option. But since there is a plane under the right-hand pad, thermal vias are an option there. But here is the problem. Figure 3 also shows the thermal pattern on the bottom layer. What most of us fail to realize is that the thermal profile of a bottom layer of a board looks a lot like the thermal profile on the top layer. at is because the boards are typically very thin (compared to other dimensions). In this simu- lation the bottom board area under the top le pad is only about 3°C cooler than the pad on the top layer. e board area under the right- hand pad, even though this layer is a solid cop- per plane, is only about 7°C cooler than the pad on the top layer. is is typical of most boards. So, any thermal vias we drop from the top pad to the bottom plane will be mostly inef- fective because the temperature difference between them is already so small. e addition of the plane has already lowered the pad tem- perature as much as practical. Figure 4: Thermal profile of top pad with two thermal vias.

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