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I had to create a printed circuit board for a project, using the home method (PCB developer solution, iron chloride.) At first, after the PCB was etched, I cleaned it very well using paper towels and isopropyl alcohol.

The PCB looked like this:

enter image description here

Then, I applied a thin layer of flux, hoping the soldering process will be better and the board will be protected a little. I used a flux dispensing pen because I didn't have any other kind of flux.

Here's the flux dispenser pen type I used:

enter image description here

Here's how the board looked after I applied the thin layer of flux:

enter image description here

I let it dry over night.

The next day, when I wanted to solder the components, I noticed that the soldering material wouldn't stick to copper, so I applied some more flux. Apparently, it worked, but some minutes later, the copper surface where I applied the flux turned green, with a dirty aspect:

enter image description here

Can someone help me with an answer, why did this happen? I assume it's the flux, but I'm not sure why.

Will the board be affected and in the end the traces will be destroyed?

Is there a solution to clean the dirt and protect the existing board?

Later edit: I tried to fix the damage already done by cleaning the PCB with isopropyl alcohol as well as I could, and it turned out pretty well, in my opinion. Here's a photo of the cleaned PCB:

enter image description here

It's been over a month and the board works well, I had no problem with the connections. The project I used the PCB for is an UV exposure box. I've put the PCB inside a plastic case, and the plastic case is inside a bigger wood case, also protected with extruded polystyrene. I hope this way the PCB will be protected from humidity. Also, the humidity in the laboratory where the PCB is located is controlled, always under 40%.

Thank you for all your help and good advice, I learned a lot from your answers.

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    \$\begingroup\$ Incidentally, you don't need to remove the copper from where holes are going to be drilled through. That would stop the outline from becoming a bit thin if it get slightly over-etched. You could make a very small hole to help centre the drill bit. \$\endgroup\$ – Andrew Morton Oct 13 '20 at 10:11
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    \$\begingroup\$ The flux should be applied shortly before soldering. It should still be wet when you solder. Then a cleaner should be used afterward. \$\endgroup\$ – Justin Oct 13 '20 at 14:51
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    \$\begingroup\$ Flux is not a board treatment. You do not apply it in advance. You also do not apply it everywhere. \$\endgroup\$ – DKNguyen Oct 13 '20 at 15:13
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    \$\begingroup\$ @DKNguyen I didn't know that. Some work colleagues taught me that and I took it for granted. I really appreciate the information you gave me, thank you! \$\endgroup\$ – Victoria Oct 13 '20 at 16:57
  • \$\begingroup\$ Many thanks for your advice! @Justin \$\endgroup\$ – Victoria Oct 13 '20 at 16:59
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If you look at the datasheet, you'll see a few important tips: enter image description here

enter image description here

This flux must be cleaned after you apply it, since its corrosive, and shouldn't be used on bits that you're not soldering. The latter is true for most (all?) fluxes. The reason that water solubility is important for this flux is so that you can clean the pcb with water, and dont have to use other chemicals to clean it. If you dont want to have to clean the flux, you could try no-clean flux, which is less corrosive and doesnt absolutely need to be removed after soldering (although you still should remove it). In order to remove this kind of flux, you will typically need chemicals (like isopropyl alcohol).

I'm not sure theres a way to fix the damage already done to the board, but you could try cleaning the corrosion off with alcohol and scrubbing and then protecting the board with some type of conformal coating.

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