solder mask removal via NaOH

[rewalston]

I have been removing the solder mask following lazersteve's great tutorial, the only deviation is that I am unable to heat the solution as he so I had to let it sit for about 6 hours to complete. I have a bit of a conundrum. I have run some finger boards with no problems, but some of the gold pads on the fingers are missing. From what I understand, this shouldn't happen at this stage. So I'm assuming that the "missing" gold is now in the sludge in the bottom of my mask bucket. Now this is a very basic solution, how would I go about recovering any gold that might have been inadvertently remove? I'm thinking of incineration to remove organics and then running everything through AP to finish the recover, but I'm not exactly sure. Any ideas are welcome.

Rusty

 

[g_axelsson]

What do you mean with missing? Is it a silvery colored finger?

If it is then it's probably only tin that have been dissolved somewhere and then plated back on top of the gold finger. All those metals in the electrolyte can play some tricks on us.
Just go on and process it as nothing have happened.

I don't know if the sludge could contain any gold. Take a small sample, add some AR and test with stannous and then you will know.

Göran

 

[goldsilverpro]

I never was worried too much about the super thin gold under the solder mask. I did play with it somewhat, though. I found that soaking it in concentrated sulfuric at room temperature popped it loose quite nice. The sulfuric could then probably be reused.

 

[rewalston]

Göran, no the fingers weren't silver colored, they were gold. The whole pad is gone as if it were in AP, instead of NaOH. It isn't much but was just curious. The NaOH bucket is now quite thick, so in order to filter everything out for disposal I'm going to have to add water to it to thin it out enough to filter, then I'll just take the filter and residue and let it dry out and I'll put it in a can for disposal the next time I'm down at the land fill where they accept toxic waste.

Rusty

 

[Geo]

Rinse the loose solids with water and dissolve with hcl/Cl. If any loose copper is present, it will color the solution. This small amount of copper shouldn't effect the gold recovery. Filter the solution to remove the trash and heat to drive off free chlorine and let cool before precipitating.

 

[rewalston]

Thanks Geo, still need to dilute it in order to filter. It's way to concentrated right now, I would guess that it's a combination of the trash in it as well as water loss due to evaporation (although I kept the bucket covered to some extent).

Rusty

 

[butcher]

It seems pointless to me to create a large volume of waste to get a flake of gold.

NaOH waste solutions can be used to treat the acidic waste solutions, using these two wastes to adjust the pH of each.

The highly caustic solution, heat generated during the reaction, and friction of the boards rubbing together may have loosened the (glue) bond of the copper trace ( finger plated with gold), to the circuit board.

I wouldn't worry about a missing flake, but if it showed up later when I was using the acidic waste to neutralize the caustic waste, I would surely grab that small flake of missing gold.

 

[Geo]

I agree with butcher if we are talking about a few missing finger foils. Depending on the amount we are talking about. You may be better off to save and add together these smaller bits until its enough to warrant the refining. I save stuff like that and add it to my filters. I did answer the question literally, if I were going to refine it in a lot and was concerned with every flake, I would do exactly what I described.

 

[rewalston]

Thanks guys, I guess it's a case of the "fever". With what I have I'm not expecting a whole lot, was just thinking of getting as much as I can. But as they say "calmer heads prevail". I'll bow to the majority.

Rusty