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I gave a motherboard circuit board to local a repairing professional. He traced and soldered some chips but it is not still working and advised to show motherboard to manufacturer service center.

But my motherboard now has yellow stains after marks of soldering or maybe soldering flux paste. I am attaching photographs of the soldering marks for your reference.

solder flux paste after marks image1

solder flux paste after marks image2

Before showing to service center I want to clean those yellow marks as some service center technicians make excuses if it has been soldered by local technicians - they may refuse to even touch or find the problem.

I have tried to clean by isopropyl alcohol, but the marks are still there.

Can someone advise or suggest how and by which chemical can I clean the yellow after marks solder or solder flux paste?

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  • \$\begingroup\$ There are flux cleaners available but the last ones I used were highly toxic. Did you use a stiff brush when you tried to clean it? \$\endgroup\$
    – Ron Beyer
    Commented Feb 26, 2020 at 15:33
  • \$\begingroup\$ @RonBeyer What was in them? Mine is mostly ethanol. It might actually be drinkable if not for the 5% isopropanol in it \$\endgroup\$
    – DKNguyen
    Commented Feb 27, 2020 at 15:13
  • \$\begingroup\$ @DKNguyen I don't remember but we had to bring the boards outside and run a fan behind us when we did it (we weren't a board house, just making a product). They switched to something like this after I switched projects but it was still pretty nasty. \$\endgroup\$
    – Ron Beyer
    Commented Feb 27, 2020 at 15:45
  • \$\begingroup\$ @RonBeyer MSDS for that stuff says 70% trans-dichloroethylene and 25% Tetrafluoroethane so yeah that would be nasty. \$\endgroup\$
    – DKNguyen
    Commented Feb 27, 2020 at 15:47

3 Answers 3

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What percentage isopropyl alcohol? 70% is not enough. Even 90% is a bit low. I use >99.9% which can sometimes be found in drug stores. Sometimes even that feels low. Ethanol works maybe 50-100% better and smells better too but costs 4x. You don't really need ethanol as long as you can scrub though.

It takes rubbing with FRESH areas of a paper towel. Flux saturated yellow/brown areas of paper towel won't rub away flux anymore. Move around to clean areas of the towel. You can rub a thick, sticky flux spot and it will look like nothing has changed, but flip the paper towel over and see its colour. If it is bright yellow or dark brown then you did remove some residue but it was so thick you couldn't tell. So move to a fresh piece of towel. It can take 5 passes or more for really thick residue. Just keep flipping the towel over, looking at it, and moving to a clean spot soaked in alcohol.

Brushes help too but mainly as a substitute for your fingers to press the paper towel down into nooks and crannies. I never found a brush (stiff like a toothbrush or even stiffer) alone to be very effective unless submerged since it can scrub off residue but can't pick it up. Helps keep the brush clean too if you use it through a towel, especially if you are using a $30 ESD brush.

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  • \$\begingroup\$ Isopropyle I used sticker written contents shows "each 100ml isopropyle alchohol I.P 70v/v Purifies water I.P QS to 100ml color brilliant blue FCF is its okay ....for my knowledge in want to ask what alchohal value should be there some one recommended me Hand sanitizer with more alchohal for cleaning....but what's the recommended alcohols value should I purchase \$\endgroup\$ Commented Feb 26, 2020 at 16:19
  • \$\begingroup\$ @Androidquery See edit \$\endgroup\$
    – DKNguyen
    Commented Feb 26, 2020 at 16:35
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Use pure (better than 90 percent) isopropyl alcohol (or ethanol) and a brush.

When I did that kind of thing often, I had a small (1 cm or 3/8 inch wide) paint brush. I cut the bristles to about 1 cm long as well.

Dunk the brush in a container of alcohol, scrub the flux spots with the brush. Repeat until clean.

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The mildest approach is 99% or better isopropanol. Drug store rubbing alcohol often contains additives as well as water that leave a nasty white residue.

It's possible to use stronger solvents such as lacquer thinner but they can attack plastic and remove markings if they get on the solder side.

Use the solvent and one of those free toothbrushes that the dentist gives you on every visit- the bristles are just stiff enough. Obviously you don't use it as a toothbrush after that.

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