The PCB Design Magazine

PCBD-Dec2017

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December 2017 • The PCB Design Magazine 19 DR. JOHANNES ADAM SOUNDS THE ALARM FOR THERMAL DESIGN that the EEs look at before getting the parts for the board? Adam: What I claim is that the electrical en- gineering team should be able to do the ther- mal simulation, because they have the design under control. They know about the schemat- ics and what the board should do at the end. It's the better way of thermal design rather to hand over this part to a mechanical engineer. Of course, they should talk to each other about how to add a proper heat sink, fans, air grills, and so on. But the electrical engineer should be able to do in what he's interested in regarding board thermal issues himself. I can tell you a funny story. As a consultant, I had to calculate the effect of a high current pulse in a 14-layer board, and it was more than 100 amps. I asked the client, "What the hell? A hundred amps? I can tell you it will melt." "Not necessarily, it's just for a second." The detailed calculation then showed that within that sec- ond, the inner trace heated up to maybe 150°C, but you saw only little temperature signature on the outer layers. This was because a second was too short in time for the heat to penetrate the PCB. An infrared camera could never have detected what happened inside the PCB. Shaughnessy: Well, maybe this will help edu- cate some people, and get EEs talking more about thermal. Adam: It's also an educational mission, yes! As I told you earlier, temperature is not a topic in the curriculum of an electrical engineer. They just learn, "Don't get hotter than 100°C and this is the thermal resistance, more or less." All the rest is more or less unknown to them. The electrical engineer will have to educate himself, 10 years or so. It didn't even get much coverage at conferences. And suddenly the big EDA com- panies all offer some kind of thermal analysis capabilities. Adam: It is a matter of convenience. My TRM software can do very high-resolution multi- layer calculations going down to sub-millime- ter resolution in XY space. That's better than many thermal imaging cameras and some- times they look very realistic. And with simula- tion you can look inside the PCB and see what happens there. I believe that, with respect to big commer cial EDA and FEM tools, my tool is easier to use for both designers and developers. You could also treat a PCB model from scratch without any external data. By the grace of its design it is also an educational tool. By the way, it also calculates the voltage drop, option- ally taking into account the local trace temper- ature. Shaughnessy: Let's say you have a board that you know is going to have some thermal chal- lenges. Other than putting in heat sinks and fans, is it possible to just design it for better thermal operation? Adam: This can become difficult, because heat is also looking for its path from a hot spot to some cold spot. If there is complete coverage of the board with heating components, it is more or less hot throughout the entire volume of the PCB. The only way to make it cooler is to make it bigger and to separate the heat sources. No- body likes to do that. Best is to add heat sinks or apply fan cooling, but there will be little im- provement by making layers thicker, using a high-cost better conducting board material or drilling thermal via. On the other hand, if heating is by few com- ponents or traces, then you can try to move the components or to make the traces thicker or wider and go down with temperature then. The software can be used to optimize the design and cost. I also know cases where traces were over- designed. Shaughnessy: So, you think that it should be taken into account just like all the other data " An infrared camera could never have detected what happened inside the PCB. "

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