Brooks' Bits: Your Traces Have Hot Spots!


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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 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, especially how the smoke is blown out from under the trace at certain points with considerable pressure. brooksbits_fig1.jpgBut 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 published in 2010. (It has been enhanced after some collaboration with the author of that article.) The image shows a trace being heated to the melting point. At this stage, the hottest portions 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 are not too hard to understand. There may be minor contamination under the trace or in the copper that accounts for it. Certainly, at higher temperatures (say above about 300°C) the board may begin to delaminate, severely disrupting its cooling characteristics. 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.brooksbits_fig2.jpg

But in a variety of lower-temperature studies, I personally took trace temperature measurements using a small thermocouple. I noticed that if I moved the thermocouple slightly, I would get a different temperature reading. Not by much, maybe 1.0° or 1.5°C. This is more than the resolution of the thermocouple, but not enough for me to be satisfied that the differences were real. So I began to wonder if these variations in temperature appeared at lower trace temperatures, say in the 40°C. range.

To read this entire article, which appeared in the August 2016 issue of The PCB Design Magazine, click here.

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