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I've recently bought a used motherboard and while cleaning it, I noticed these white stains on the back side of the PCB:

enter image description here

I've managed to perfectly clean the front side with pure isopropyl alcohol and a soft brush, which was mostly just dusty, but I can't easily remove the stains on the back. When soaking in alcohol, they can be smeared around somewhat with a cotton pad, but they never seem to clean up fully.

I've worked with used hardware before and remember having seen similar stains on other electronics. They also seem to have a certain formation to them, but I can't make out a logical pattern.

What are they and is this a sign of.. anything particular?

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  • \$\begingroup\$ Can't be just marks where it got stuck to the parts of the base plate it was mounted on before? \$\endgroup\$
    – Eugene Sh.
    May 30, 2022 at 21:52
  • \$\begingroup\$ Possible. The case in which I bought the hardware does not feature such patterns and there's a gap between the PCB and the metal plate (stand-offs), but I can't exclude that it has been mounted to something else before. But what could cause this? If it was mounted to something conductive, wouldn't the components short? \$\endgroup\$ May 30, 2022 at 22:37

2 Answers 2

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The lines seem to be surrounding all areas with exposed PTH leads sticking out, to the exclusion of areas with only SMD parts.

I think they mark the openings of something called a "selective wave soldering pallet" that is used to protect previously assembled SMD parts during wave soldering of the PTH parts. I'm not sure how the stains are formed. I can see some references to flux residue being trapped under the side walls of the pallet mask.

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  • \$\begingroup\$ I've gotten similar pollution from (bad) flux residue when cleaning boards in water and when it happens it's hard to get rid of even with isopropyl. Suppose a selective pallet like you suggest is used - It could then perhaps be flux residue from the wave soldering flux? Imagine they use one flux for SMD and another for TH and then clean the boards - after which one kind of flux gives a reaction and the other doesn't. This would explain why the areas with many TH joints in the upper part of the picture are more polluted. \$\endgroup\$
    – Lundin
    May 31, 2022 at 14:00
  • \$\begingroup\$ Or optionally the boards were never cleaned at all and the wave solder flux just reacted with something else out of the field. Either way it's quite likely harmless. \$\endgroup\$
    – Lundin
    May 31, 2022 at 14:03
  • \$\begingroup\$ I suppose this is the most likely answer. I also suspected that it was from some sort of template, but I wasn't sure what the substance of the stains could be. So it's likely just insufficient cleaning? This is quite unfortunate (albeit harmless), as it is an Asus workstation motherboard, of which one would expect a higher level of quality. \$\endgroup\$ Jun 2, 2022 at 9:51
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Possibly water damage (the appearance suggests that) but the “smearing” suggests possible sloppy application of thermal compound, which is typically oily.

In the first case, the board may be damaged beyond realistic repair.

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  • \$\begingroup\$ The board works perfectly fine (as of yet). I've tested it and all components for days before removing and noticing the stains. I've never seen any TIM that would look or behave this way. I know viscous ones that will smear and wipe off relatively easily and old, dry ones that don't wipe off but can be removed by scrubbing, this does not really. Water damage is possible, but as mentioned it's only found on the back side, the front side looks like new now, which would be strange. \$\endgroup\$ May 30, 2022 at 22:33

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