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PCBD-Aug2016

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58 The PCB Design Magazine • August 2016 Your traces have hot spots. At least, those that carry a moderate current do. Surprised? Well, I was a little surprised, too, when I looked at this a little more closely. One chapter in my recent book [1] focuses on fusing current. It contains the image (Figure 1), captured on video, which shows a 20 mil wide trace that had been heated for about 15 minutes, just at the moment of fusing. There are several interesting things in this image, es- pecially how the smoke is blown out from un- der the trace at certain points with considerable pressure. But note that the trace fuses at a point, not everywhere along the trace. It is clear from observation that the trace is much hotter at some points than at others. Figure 2 appeared in a separate article pub- lished in 2010 [2] . (It has been enhanced after some collaboration with the author of that ar- ticle.) The image shows a trace being heated to the melting point. At this stage, the hottest por- tions of the trace are over 600°C, but other areas remain in the 200°C temperature range. The reasons for the temperature variation at high temperatures is not too hard to under- stand. There may be minor contamination un- der the trace or in the copper that accounts for it. Certainly, at higher temperatures (say above about 300°C) the board may begin to delami- nate, severely disrupting its cooling character- istics. There may be small variations in trace width or thickness that help account for the delam, and these effects would be randomly distributed along the length of the trace. But in a variety of lower-temperature stud- ies [1] , I personally took trace temperature mea- by Douglas G. Brooks, PhD Your Traces Have Hot Spots! BROOKS' BITS Figure 1: A 20 mil wide trace at the moment of fusing.

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