Question

A last ditch method for identifying connections in a old multilayer circuit board that I have come up with, when you have a spare, is to put the board in salt (NaCl) solution and connect a power supply to it. One pole goes via a wire to the circuit you are trying to find (AA), the other to either the board ground (GND), or an electrode in the tank.

In the tank, you get streams of large bubbles from every pad connected to AA, and smaller fainter ones from the ground or other electrode. It is great help as it tests the entire board at once.

When doing this as an experiment, I have noticed that every ground terminal went a mid-grey colour, and that pads on the AA pole went black. I believe I was getting hydrogen at AA, and chlorine at the ground (but cannot be 100% sure on supply polarity).
This process does take the solder off the AA pads on the circuit, so I guess some copper was involved as well.

I did a quick search, but cannot find any salts that would have these colours likely to exist. The only black salt I have found is Cu(II)O, but cannot believe that is being formed in a solution- especially at both pads (more at AA).

The solder appears to be lead free (board is from 2017-2019 so is mostly tin. There may a tiny bit of silver, but it is a tiny percentage.

Would anyone like to suggest what the chemicals or mixtures of chemicals are? Electrolysing salt can give NaClO, NaOH, as well as the two gases mentioned, so there are many options.

Answer 1

The darkening of circuit board pads during salt solution electrolysis can be attributed to the oxidation of chloride ions to chlorine gas at the anode and the formation of metal hydroxides at the cathode. Discoloration and the removal of solder may be due to associated chemical reactions including hydrogen gas production.

Step-by-step explanation:

When a salt solution such as NaCl is electrolyzed, various chemical reactions can occur at the electrodes. At the anode (positive electrode), chloride ions are oxidized to chlorine gas, which can be represented by 2Cl(aq) → Cl₂(g) + 2e.

The darkening of the pads associated with the anode, where the AA wire was connected, likely indicates the production of chlorine and possibly other reactions involving the oxidation of metals such as tin and maybe copper.

The discoloration to a mid-grey at the cathode (negative electrode) could be due to the formation of metal hydroxides or other tin or lead-free solder oxidation products. For example, hydrogen gas and hydroxide ions are formed when water molecules pick up electrons and undergo reduction, which can be represented by 2H₂O(l) + 2e → H₂(g) + 2OH−(aq). This process might also explain why solder is removed from the circuit pads.