"How to find" / "where is" shorted-to-ground on my PCB?

Discussion in 'General Electronics Chat' started by booboo, Aug 27, 2015.

  1. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    Hi everybody
    A couple days ago, I received my PCB. but I have a problem. I have shorted-to-ground problem on my PCB. errrrrrrr!:mad: such problems are sooo tough!
    I'm diging whole of the net to find a way to locate the shorted point. via google video I found these:

    www.youtube.com/watch?v=F-xI4ceQCco[who has such a device at home!?]
    www.youtube.com/watch?v=JAOV_erLpYw[not bad but a bit complicated]
    www.youtube.com/watch?v=JTrb3FYsXQM[it's not available to me]
    www.youtube.com/watch?v=XhqwC3BFBug[not bad but I don't have this spray]

    Via google web found these:

    http://electronics.stackexchange.com/questions/16398/find-short-location-on-pcb
    http://www.edaboard.com/thread31322.html

    In the E.se page, Russell's answer is nice (How about it? is it good in your opinion?). my DMM is VC9805. Is it enough to use this method? and for making current, How to make it and how much? a simple and temporary way(circuit) would be appropriate I think. this is the circuit:

    [​IMG]

    Can you see any shorted circuit?
    I have these regulators & ICs in stock:
    L7805CV, L7905CV, AMS117 3v3 and 5v, LM317T

    Can I make a temporary circuit for current?
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    You are more familiar with your circuit/pcb than anyone here so its much more work for us to find the error for you..

    Can't you run net-net error checking in your PCB program?
    Got a schematic?
    Did you compare the schematic to the PCB layout (again another error check function that's in any decent pcb program)?
    Did the PCB manufacturer run any etest/flying probe?
    Can we assume blue traces are 1 layer and red are another? But what about green traces at P4/P5/Q2 (I'll assume they are on the red trace layer)?
     
  3. MrChips

    Moderator

    Oct 2, 2009
    12,414
    3,353
    Is your PCB fully populated with all components soldered on the board?
    What is S1? Is it a push button?
    Double check the pinout of S1.
    Remove S1 and see if the short goes away.
     
    DerStrom8 likes this.
  4. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    Thank you guys
    I apologize because I forgot to say that My PCB isn't fully populated. this is the PCB:

    [​IMG]

    1- My PCB program is Altium. when I'm seeing the PCB and put the cursor of the mouse on for example GND line, I cannot see any problem on it(short circuit) because just GND line getting bold.
    2- yes. but there is no problem in schematic.
    3- Not completely. a glance.
    4- No, I don't think so
    5- Yes. it's just a two layer PCB. the green traces is because of overlapping components' marks.
    Yes, it's a push button. but I had the problem before mounting it too.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,414
    3,353
    You need to provide more information.
    Post your circuit diagram.
    What is the part number on the MCU?
    Show sharp high resolution photos of both side of your board.
     
  6. JWHassler

    Member

    Sep 25, 2013
    201
    33
    Is the short on your populated board or on the naked PCB? Big difference in the required effort.
    If it's on the unpopulated PCB, you might just need a microscope and a whole day.
     
  7. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    I attached the schematic PDF(it it enough?).
    it's STM32F103RBT6.

    [​IMG]
    Front:
    [​IMG]
    [​IMG]

    And back:

    [​IMG]

    [​IMG]

    Is this images good?
     
  8. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    Several of your links describe using a current source and a volt meter. This is a very good method. If you do not have a power supply with current adjustment, then put a resistor in series with your power supply and adjust the voltage. The lower the voltage the closer you are to the short.
     
    booboo likes this.
  9. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    I don't know if it was on the naked-PCB because when I soldered the components, I thought there is no problem.
    I have a PSU at home. could it help?
     
  10. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    OK, so you have a computer power supply. That will work. I would run the test at around .5 amps.

    - Use the 5 volt and ground outputs of your power supply.
    - Put a 10 ohm 3 watt resistor in series with the 5 volts. (Or something close.)
    - Connect the lines to the shorted path.
    - Probe the board with your volt meter, looking for the lowest voltage. You are looking for very small voltage.
     
    booboo likes this.
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Yes your PSU could help, and here is how:

    A short is a very low resistance connection and thus hard to localize with just an ohmmeter. The voltmeter part of your DVM is much more sensitive but you need to push thru some current to see the rEsistance. So you current limit your supply and connect it across the short. With your voltmeter pins you poke to find the lowest voltage difference, which is the short. This method does need a little luck to work.

    There have been times where the short could be fused open by zapping a large current thru it. Usually one has to add a largest cap it up the zap current available. Don't touch a live probe to your board, make the probe connections first, then complete the circuit away from the board. This insures you don't leave a scorch mark on the board itself.

    Obviously zapping in a method of last resort.
     
    booboo likes this.
  12. dl324

    Distinguished Member

    Mar 30, 2015
    3,211
    619
    Have you visually inspected the soldering around the LQFP package?

    Regarding the freeze spray; buy a can and keep it on hand. Also keep an eye out for an HP547A current tracer probe. When used in conjunction with a current source, you be able to isolate shorts more quickly (over the voltmeter or ohmmeter approaches). They're a bit expensive now, but the prices wax and wane. I picked up a spare set (of HP545A, HP546A, HP547A, and HP548A) for under $75 4-5 years ago on eBay; they're several times that currently... This seems to be the best deal on eBay.
     
    booboo likes this.
  13. Ancel UnfetteredOne

    Member

    Jul 3, 2015
    36
    14
    If you have access to a FLIR camera like an E8 model or better....apply current limited power, perhaps a 1/2A. The 'short' will produce a heat bloom fairly quickly and you have found it.

    I use this method to trace faulty components as well , such as caps with poor ESR and transistors going bad.
     
    booboo likes this.
  14. booboo

    Thread Starter Member

    Apr 25, 2015
    165
    2
    Thank a lot guys
    Why 5v? it has 3v3. I'm worry that 5v would hurt my MCU and other 3v3 components on the board. this is the specs of my PSU:

    [​IMG]

    And for 3wat resistor, can I hook up four 10 ohm 0.5watt in parallel?
     
  15. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    I would simply use a multimeter on continuity mode/ohms (beeper turned on).
    Stick one probe on ground and then probe about till you find a pin/trace that isn't supposed to be connected..

    This board looks so simple there is no need for anything "fancier" (FLIR camera..current source,etc... there is no need).

    Judging by the very poor quality solder job I see already I'm gonna say the issue is more than likely due to that.. Start with the fine pitch devices first.. Like that micro..

    Of course the PCB looks to be of low quality too so I wouldn't be surprised if that could be an issue too..
     
    ronv and booboo like this.
  16. dl324

    Distinguished Member

    Mar 30, 2015
    3,211
    619
    3.3V is fine; some PC supplies (old ones) won't have 3.3V.
    That would give you 2.5 ohms at 2W. 5V with 10 ohms would have given you 500mA max; with 3.3V, you'd want a 6.2 ohm 2W resistor.
     
    booboo likes this.
  17. nerdegutta

    Moderator

    Dec 15, 2009
    2,514
    784
    I make my own PCBs, and before I even consider to start soldering, I'm checking the PCB for shorts. I put my DMM in "beep mode", and have one probe on GND and then I probe around. Sometimes I need jumper wires to connect the different ground planes. Those are the first thing I solder on, and then I probe again.

    I guess with a board that's professional made, it could be hard. Especially if the board has conformal coating.
     
  18. dl324

    Distinguished Member

    Mar 30, 2015
    3,211
    619
    I make my own boards too; so I know where the likely places for bridges are and make sure they're clear before I stop etching.
     
  19. Ancel UnfetteredOne

    Member

    Jul 3, 2015
    36
    14
    Well, the FLIR method is a near instant locator of heat generating issues such as a short. If the OP has access to one, it is a superior method of locating heat generating internal board faults, component faults, heatsink bonding faults, current overload faults, or even shorted faults hidden beneath multi pin components due to solder balls etc. Poking about fine pitch parts for faults can be very tedious.

    I had a fault once that wasn't a dead short.....incomplete solder paste remelt bridging UNDER a 0805 part loading the LM317 regulator into thermal shutdown. So the resistance was a few ohms. The FLIR method identified the problem w/o having to de-solder anything to check.
    It showed the traces leading to the 0805 part a few degrees warmer than others.
     
    booboo likes this.
  20. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    If using it on a populated board; I'd suggest a silicon diode shunting the current source output to protect any sensitive active circuitry.

    It'll give a standing conduction reading, but should still easily distinguish a short.
     
    booboo likes this.
Loading...