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Short between power and ground

David Engineer
Intellectual 340 points

I have populated a custom board and was ohming out the connections and noticed a short between my power plane and ground plane (24 ohms between planes). I went through the process of removing components connected to power and re-testing for the short. I eventually removed the MSP and found that my short disappeared (>10 Mohm between planes). Is this normal? Is the MSP bad? I am using an MSP430F5638IPZ.

  • 0
    Mastermind 7395 points

    I think that it is not normal, but it doesn't necessarily mean that the MSP is bad. There could be many other causes, for example:

    1) Bad soldering almost shortcutting two pins of the MSP

    2) An error on the PCB design (schematics bug) that is making the MSP shortcut the power planes. For example, let's say that an I/O is shortcut to GND. Through the ESD diode of that I/O pin, you would be getting a very low impedance path to Vcc. Is only an example, you should check (or post) your schematic. BTW, You would get (in this hipothetic case) low impedance only when testing in one direction.

    3) Do you have other boards to test? If yes, is the problem appearing in all of them?

    4) It could be that the MSP is bad. If you can, replace with another one and retest.

    Good luck with the debugging.

  • 0 in reply to Avi Chami1
    Expert 1495 points

    Additionally, you may check for solder bridges under decoupling capacitors. I'm sure there are quite a few of them on your board, and a visual check under a magnifying lens will help.

    All the best !

  • 0 in reply to Avi Chami1
    Intellectual 340 points

    Thanks Albert_83 and Jayant Deshpande for the responses, I am a student at a University working on a custom "one-off" and only have one board to tinker with. I have checked for solder bridges under a microscope and found none. I do have one of my I/O pins connected directly to ground, however when I tested the resistance, I got the same value in both directions. At this point, I will try to swap MSP's and see if that fixes my problem. If not, then I have an issue in by board that I have yet to identify.

  • 0 in reply to David Engineer
    Guru 35545 points

    Remember that a resistance meter works by applying a voltage, measuring the resulting current, and thence computing resistance by Ohm's law.

    If the circuit under test test contains semiconductors, it is quite likely that something could be forward-biassed by the test voltage - so the resistance calculation becomes erroneous. The "something" could be a "normal" junction, or some "parasitic" effect in a chip.

    Basically, trying to check for shorts this way in a circuit containing semiconductors is seriously flawed...

    Some test meters have a special low-voltage mode to attempt to avoid this effect.

    What happens if you reverse the test leads?

     

  • 0 in reply to Andy Neil
    Intellectual 340 points

    If I reverse the test leads, I get the exact same behavior.

  • 0 in reply to David Engineer
    Intellectual 340 points

    I found the problem, it was the MSP after all. I replaced the MSP with a second one, and my short went away. Thanks for all of the help everyone!

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