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I need help please with repairing metal backed dielectric pcb

Discussion in 'Troubleshooting and Repair' started by aajlh, Jan 22, 2013.

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  1. aajlh

    aajlh

    4
    0
    Dec 22, 2011
    Hi

    My son has a remote control boat that got water inside on the PCB. The water could have been mildly salty. We cleaned the board with isopropyl alcohol. When we reattached the battery pack there was an electrical burning smell and smoke coming from one end of the PCB. We removed the batteries immediately.

    The board has a metal backing and (from a google search) I assume that there is a dielectric layer between the board and the metal. At the end where the smoke is coming from there appears to be black residue (perhaps burnt salty residue) in the layer between the board and the dielectric layer. I assume the residue is causing a short circuit.

    We cannot get into that area to try to clean it. I am wondering if we could disassemble the layers, ie prise off the board from the dielectric layer with a screwdriver and the dielectric layer from the metal backing and clean it all up and somehow put it back together (duct tape?). Is it possible this could work without killing the PCB? Would appreciate any suggestions anyone may have.

    Thanks

    Anne
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,449
    2,809
    Jan 21, 2010
    Dome pictures would probably help.

    It's almost certain that you need to get into where the burning happened.

    I'm not sure what you mean by dielectric layer. Perhaps you mean insulating layer.
     
  3. aajlh

    aajlh

    4
    0
    Dec 22, 2011
    Thanks Steve

    Here are some photos. Hope that makes it clearer. By dielectric layer I do mean insulating layer. I don't know what I'm talking about here, just looked up google and found the term dielectric. I mean the layer between the board and the metal backing. It looks like a piece of cream coloured rubber.

    In the photos I've shown one side which is all clean as a comparison to the other side which is blackened.

    I know the board looks 'fried' but I'm hoping I can open it all up and clean off the residue and that it might still work. I just don't know if separating the three layers will destroy it completely or if it's worth a try.
     

    Attached Files:

  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,449
    2,809
    Jan 21, 2010
    At first glance, I'd say you have quite a bit of death and destruction.

    The metal plate on the back looks like a heatsink, and the rubber layer is possibly an adhesive thermal layer.

    There's a bit of corrosion and some components on the top layer look damaged.

    Added to that, the chips have lots of closely spaced leads and all sorts of contamination can hide under them. They may also be hard oto locate, and not a job for a beginner to replace.

    It may be cheaper to get a new board.

    Looking at the design of the boat, there's not much to prevent this getting wet. I would recommend you find some way to house the electronics in a sealed box with goo (like a silicone sealer) around the wires so water can't get in.
     
  5. aajlh

    aajlh

    4
    0
    Dec 22, 2011
    Hi Steve

    Well I guess I've got nothing to lose so I'll try to separate the layers and clean it and see if by some miracle it still works.

    Thanks
     
  6. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    My advice, unsolder all the wires AFTER YOU MARK WHERE THEY ALL GO and get down to the bare board... Also remove the big capacitor on the edge of the board and mind the polarity of said capacitor before removal...

    Go to the hardware store and get some acetone, get an airtight food container (PP or PE won't melt and is what most Tupperware like containers are made of) put some acetone in the container and submerge the board... Agitate for awhile, remove the board and using a tooth brush give it a good scrubbing, repeat until clean...

    Once the board is clean you will want to try to get some of the oxidation off it... Phosphoric acid is best for this as it doesn't attack metals as fast as other acids, now for me I get pure phosphoric acid, but for your one off try the old home remedy of Coca Cola :) Let is sit for several hours or even over night (possibly longer) submerged in Coke, check every so often and watch the progress... Too long and you can cause more damage, so if you see metal vanishing stop right there...

    Once you get the oxidation off, run it under clean water, and possibly even scrub it down with some dish soap and your tooth brush... Then once again give it a quick bath in the acetone and let dry... The last dip in the acetone is just to aid in evaporation, water can linger behind acetone evaporates in seconds...

    At that point take new pictures, we will be better able to see where you are at, it looks ugly but who knows...

    FYI the heat sink might release in the acetone, and that is not a bad thing as it will give you access to that side of the board for inspection...
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    10,810
    2,438
    Nov 17, 2011
    Since you "have nothing to loose", you can try cleaning the PCB in the dishwasher (right!). Of cours, after removing the wires as CocaCola explained. Just don't put dishes in there, run it with only the PCB for cleaning. Dishwasher detergent is remarkably efficient at cleaning.
     
  8. CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    Dishwashers do, do a great job of cleaning but there is that lead thing, not a 'huge' issue but one that has to at least be mentioned...

    If you use a dishwasher use the powdered detergents as they are slightly abrasive vs the gel detergents that are mostly chlorine... And on that note at least in the US phosphates have been pretty much banned for household consumer detergents, phosphates simply work better then almost all the alternatives... So my advise go to the hardware store and get some REAL TSP (aka Trisodium Phosphate) be aware that a large majority of the places at least in the US are selling bogus TSP alternatives verify that you get the real stuff... If your local hardware store doesn't have the real deal visit a household paint store, as it's the de-facto wall cleaner used by pros before painting... Anyway when you get some TSP add a heaping tablespoon to the pre-wash and wash detergent compartments, the end result is a supercharged clean :)

    And you can use in for regular dishes as well, it will help eliminate the white film on glass and will make cleaning just that much better... Two words of caution don't use the TSP with fine china or fine crystal or glassware it will dull the surface with extended use, there are better phosphates for that use... And if you have older sewer lines or a septic, it's advisable you dose you lines regularly with root killer as the phosphates are steroids to plants :)
     
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