Thin sections on wide power traces: Bad? - Page 1
Forum section: electronics / projects

author: Starlord
author_rank: Frequent Contributor
timestamp: January 12, 2018, 11:55:34 am
content: I have a board where I need to run a power trace between some through hole pads. Because it's a two layer design, I can't run a complete trace on both sides of the board and stitch it together. And the widest I can make the trace with the pads there is 20 mils.I don't have a specific current limit I want to achieve in mind, just as much as possible, with the bare minimum being 2A, which the trace I have calculated is able to handle with a 20C temperature rise.The thing I'm wondering though is what if I made the trace wider, but allowed a dozen through hole pads to cut into it? It would look like a sawtooth basically. It would be narrow between the pads, but much wider in the spaces between.I am guessing this will act as a heat sink, and will improve the current carrying capabilities of the trace despite the thinner sections between each pair of pads.But at the same time I am given pause at the idea of being able to run large currents through very short narrow traces. That screams "fuse" to me!Still, with the overall width being 100 mils with short 20 mil wide sections, it seems like with this method I might be able to run say 5A through that same trace and still maintain a 20C rise, though I have no way to calculate this.What say you? Good idea? I really don't have any other options. Going to a four layer PCB is too expensive, and I have to run traces across this trace so I can't run it on both sides of the PCB and stitch it together with vias. Nor can I increase my copper thickness from 1oz to 2oz or more. And I don't want to resort to that trick where you leave the copper exposed and tin it. Nor do I want to run jumper wires across the board.What I really want to know is is there some downside to doing this that I'm not aware of. Will those short sections heat up to 100C rapidly at 5A? Or will the heat sink function as expected? I mean, if I were desperate I could do SOME via stitching to boost the performance more, but I'd like to keep this simple. I definitely don't need more than 5A. And I probably don't need more than 3A. And it's not likely to be constant even if I do need 3-5A for short stints.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 12, 2018, 12:12:04 pm
content: Test it with a thermal camera.And remember that 2 oz. copper can be an option to help

author: ovnr
author_rank: Frequent Contributor
timestamp: January 12, 2018, 12:31:07 pm
content: How long are the narrow sections? As a quick rule of thumb, I've found that lengths above approximately 5x the width of the narrow section will act as fuses; shorter will usually be fine. That's with somewhat exaggerated ratios (1mm vs 0.16mm, for instance).

author: Starlord
author_rank: Frequent Contributor
timestamp: January 12, 2018, 06:00:17 pm
content: Quote from: ovnr on January 12, 2018, 12:31:07 pmThat's hard to say. What I've got are two rows of through hole pads on a 0.1" grid. The pads themselves are just large enough that with the 6 mil clearance there is 20mils of space to run a trace between them. But the plan is not to run a trace between them, but to put down a power plane that is 100 mils wide. Thus the width between the pairs of pads will vary with the pad curvature, and only sliver between each pair of pads will actually be only 20 mils wide.So if the trace were a constant 20 mils, then the length between pads would be 100 mils, which is 5x the width. But in reality the width will vary constantly along the length of the "trace".But based on what you're saying it sounds like it will be okay. And if it's not, well, I'll found out the hard way!

author: DaJMasta
author_rank: Super Contributor
timestamp: January 12, 2018, 06:26:06 pm
content: Easy improvements: leave the solder mask off and tin the top, or just use a decent gauge wire jumper in place of the trace. Thicker copper will also do the trick, and sometimes the difference between regular 1oz and 2oz isn't too bad.Trace resistance is measured per square (the math works out so that as long as the thickness and material is uniform, the size of the square does not matter). Imagine you have a trace 10mils by 100 mils, so it's resistance is 10 square of the material's resistance. If you take the middle 50mils worth of distance and split it in half to go around something, each 50 mil segment is now 10 square in resistance (5 mil squares vs. 10mil of before), though you have two in parallel, so you get half that resistance, back to 5 square and the trace ends up being the same resistance. That said, if there is even the slightest variation in the resistance of either path (and there ALWAYS is), one side's resistance is slightly higher, and if you approach the current limit of the trace, you'll likely burn out one side and have a cascading failure to take down the second quickly.So the way it works out, even if the risk is marginal, splitting a single trace into two equally sized ones is not a good idea. And that's only if you can match the width at the narrowest point - even if you can make your 20 mil trace 50 mil wide, if your hole cutouts reduce any part of the split below 20 mil of total width, that part is going to be the highest resistance point and will act like a fuse under high currents.Running traces hot can still work, but you run into all sorts of issues: cooking other components, traces delaminating from the board, potential thermal runaway (the resistance actually increases slightly with increasing temperatures, which under a constant current load will generate even more heat), and so on. Just find yourself a nice jumper lead and take the easy road

author: ovnr
author_rank: Frequent Contributor
timestamp: January 12, 2018, 06:51:06 pm
content: Sounds like it'll be quite fine indeed.I did some testing on a normal 35µm copper board, with a 0.16 mm wide and 0.5 mm long trace between two 1 x 1 mm pads. I dumped 15A into it. Didn't pop, even after 10s or so. Didn't even discolor the soldermask. Do note that longer traces - a few mm or so - poppedinstantly. The heatsinking effect you get to larger planes is quite significant.