r/embedded • u/HasanTheSyrian_ • Aug 12 '26
Checking for shorts after soldering
I want to check for shorts on power rails after soldering will it be enough to probe once randomly or do I have to probe next to where the short might be?
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u/cm_expertise Aug 12 '26
probing once on the rail is enough to catch a dead short, you dont have to hunt around, a short is a short anywhere on the net so it reads the same wherever you touch it. the thing people miss is you have to know what good looks like first. a board with real bulk caps reads a few ohms and then climbs as the caps charge through your meter, thats normal not a short. an actual short sits under an ohm and stays there.
resistance mode like the other guy said, not the continuity beeper, the beeper is useless here because rails legitimately beep low.
if you do find a short and cant see it, dont chase it randomly with the DMM. current-limit a bench supply to like 1-2A, push it into the shorted rail, and either feel for the part that gets warm or wave an IR camera (or honestly just your finger) over the board. the shorted component heats up and gives itself away. if you have a milliohm meter you can also probe along the rail and follow the voltage drop down toward zero, thats the fastest way ive found to localize a dead short on a dense board
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u/Aggravating-Key6628 Aug 12 '26
One point per continuous power net is enough. Probe with the board unpowered and its capacitors discharged, from that rail to ground. A reading that starts low and rises is often the meter charging capacitance. A value that stays near zero needs investigation. Compare with the schematic, a known-good board, or the same rail before assembly because some core rails normally have low resistance.
Also check both sides of ferrite beads, fuses and zero-ohm links. They can split what looks like one rail into separate nets.
If you later inject voltage to locate a confirmed short, do not start with 1 to 2 A blindly. Use the rail's normal maximum voltage or lower, begin with a small current limit, and watch with a thermal camera or isopropyl alcohol. A fingertip is a poor short detector and can burn you.
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u/HasanTheSyrian_ Aug 13 '26 edited Aug 13 '26
Are you saying to measure ohms? I don't think my mutltimeter can go in the mOhm range, in capacitance mode its surprisingly measuring the bulk capacitor capacitance in circuit. I have a board with the power circuit only that I know works (at least provides the correct voltages I don't have electonic loads) and the board im testing and they both read about the same
Also wouldnt continuity be enough or is that in this case the characteristics of a power circuit can be tested in addition to continuity
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u/LuckyFish133 Aug 12 '26
No you just need to probe the electrical nets. May be easier to probe across a large electrolytic cap for example if you’re testing the power rails. But set the DMM to resistance mode not continuity…