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what causes the black corrosion /deposit on the pcb?

Discussion in 'Repairing Electronics' started by kat_80, Dec 14, 2017.

  1. alec_t

    alec_t Well-Known Member Most Helpful Member

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    The stuff seems to be primarily on soldered ('tinned') surfaces.
    I'm showing my chemical ignorance here, but could it be lead sulphide; from atmospheric sulphur dioxide reacting with any lead content of the solder?
     
  2. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    If you look at the first picture with the kapton(?) tape, that is not a soldered surface, nor are the vias.
     
  3. alec_t

    alec_t Well-Known Member Most Helpful Member

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    The tape seems to be doing a good job of masking, but some black stuff has crept over its edges and even underneath in places. I'm not seeing much black on the vias themselves; more on their immediate surroundings.
    Is a Pb + SO2 reaction feasible?
     
  4. dave miyares

    Dave New Member

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  5. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Creeping over and under what is probably kapton tape would be more characteristic of a mold than a chemical oxide. Albeit, tin whiskers can grow quite long, but that is not a likely consideration here.

    What product are you proposing to be formed from lead and sulfur dioxide?
     
  6. alec_t

    alec_t Well-Known Member Most Helpful Member

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    PbS?
     
  7. kat_80

    kat_80 New Member

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    not yet, I can try that next. I dont' have any acid, will vinegar works? if it is I will put some on the black spot and see if it can be easily removed.
     
  8. dave miyares

    Dave New Member

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  9. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Vinegar may be OK. If that doesn't work, try a little diluted battery acid (e.g., 4 parts water plus 1 part battery acid from a charged battery). Hydrochloric acid ("muratic acid," HCl)) would be best. Observe whether it dissolves and the color.

    Battery acid should give a blue solution. Hydrochloric acid should give a dark green or olive drab-colored solution.
     
  10. kat_80

    kat_80 New Member

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    tested it with vinegar, it came off quite easily , much quicker than isopropyl alcohol. I would say it is soluble under acid. I used cleared tape and able to tape off some sample. once I got my microscope delivered ,I will post some image what it look like under 50x zoom.

    I also tested it with bleach. it does not come off easily like vinegar.
     
    Last edited: Dec 19, 2017
  11. hyedenny

    hyedenny Member

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    Google "pcb creep corrosion." Absent the lack of additional detail, I suspect that ozone or H2S might be the culprit.
     
  12. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    What color is the vinegar solution? Is it perfectly clear or cloudy?
     
  13. kat_80

    kat_80 New Member

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    I don't have the proper tube or chemistry testing tools like high school lab. so I used q-tip and sprayed some vinegar on the blacken part, waited 30 sec, and gently scrubbed the surface and the black stuff just came off really easily. I did the same using bleach and alcohol but had no effect to black stuff. the pcb creep corrosion image I googled fit really well. I got a feeling its the sulfur + moisture in the house acting up. my basement was not fully finished before so the furnace room air contain small amount of sulfur from burning natural gas may escaped and mixed with the air moisture. that maybe explaining why the computer motherboard did not have any creep corrosion because the fan kept the air circulated and while the electronic without the fan got broken down after a year of use.
     
  14. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Your natural-gas furnace should be room-sealed, i.e. air for combustion and flue gases should respectively enter and leave the combustion chamber via ducts leading outside the building. Without that arrangement your life could be in danger :eek:.
     
  15. kat_80

    kat_80 New Member

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    hello,
    happy new year! finally got my 200x zoom digital microscope. I have attached a few more pictures.
    1812195256594.jpg
    this was what come off the pcb after I use the clear tape to tape it off.
    181220855319.jpg
    this is what the board look like after cleaning.
    1812195747168.jpg
    this is what remain in the acid solution. its not soluble.
    1812201032566.jpg
    here is a close up of the "creep" corrosion
     
  16. hyedenny

    hyedenny Member

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    Nice pictures. What kind of microscope is it?
    If you don't have the resources or facilities, I would take it to the local high school and ask if they can have them try to figure out what it is. Or mail a sample to me, and I'll assay it.
     
  17. GromTag

    GromTag Active Member

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    Chem,
    SO2, Sulfur Dioxide
    NOx, Nitrogen Oxide
    CO, Carbon Monoxide
    Primary, Sulfur Dioxide with some help from the LPG fuel source, the heater.

    : Edit : changed, it's LPG not LGP. I'll get it initially right one day....
     

    Attached Files:

    Last edited: Jan 2, 2018
  18. GromTag

    GromTag Active Member

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    Ohh, and don't forget, added abnormality as there is current flowing with the addition of thermal differences on the boards in various areas when they are in operation. Some areas hotter than the others, and current flow with frequency such as clock or data causing the oddness of the patterns.
     
  19. GromTag

    GromTag Active Member

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    Odd concept in thought... Thermal operation of an electronic device can cause the term usage that changes the areas that produce the most heat breaking down the boards protective coating casing microscopic pores to result if not present all ready there.

    Wear of the device when used.

    Or just the process used. Have some WD drives here that have turned greyish bronze and have stopped working, no natural gas system here.

    So if the circuit is used for a bit then placed on a shelf for a few years
    those areas could show the result verses if a new or very well sealed board set next to for the same time would likely show next to no issues as that in intensity.

    Tho, many circuits are tested before shipping to be sold, thus the results could even exist for new boards as well if placed in an similar environment as the corroding board.
     
  20. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    Is the finish on the copper silver or gold? Are the devices "lead free?"
     
  21. GromTag

    GromTag Active Member

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    If referring to the mentioning of the WD drives? Or just for relevance, Copper plate, are RoHs compliant , lead free.
     
  22. kat_80

    kat_80 New Member

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    I used "Celestron 5 MP Handheld Digital Microscope Pro" from amazon
     

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