Removing Solder Mask question

[kane333]

There have been certain pc boards that I've noticed the grounding plane to be gold foil. When I removed the solder mask using NaOH and water over low heat, I noticed that the nice golden color of the grounding plane areas had turned almost a silvery looking color. Is this because I used too much NaOH to the water or is this a common occurrence? In watching LazerSteves video on removing solder mask, it seemed the test board also lost some of its rich golden color. I'm wondering if some of the gold dissolved into solution if too much NaOH was used. Has anyone else had this happen or notice this?

Edited for spelling

 

[denim]

I experienced this same situation. I have not processed the boards yet for pm's so I don't know if it merely discolored the gold or if the gold disappeared. I'll find out soon though.

edited for spelling.

 

[butcher]

Elemental gold will not dissolve will not dissolve in NaOH, so your gold will still be there.

But gold can form hydroxides:
If gold is in solution, it can be precipitated using NaOH, or some other strong hydroxide like KOH, (this would normally be done with only pure gold, as other metals also would form insoluble hydroxides), Now if we added an excess of sodium hydroxide (or KOH), we can redissolve the gold, and put the gold back into solution as a soluble hydroxide in the concentrated caustic solution (there are several metals that will do this).

Some metals also reach with ammonium hydroxide with these Amphoteric properties.

This Amphoteric properties of certain metals is very useful in separating metals, or testing for metals in solutions.

 

[skippy]

So you could have discoloured the gold with a layer of gold hydroxide, which might rub off or fall off? Hmmm.

I wouldn't think gold would form hydroxides from its elemental form without extra help.

Maybe the hydroxide eats away at enough nickel underneath the gold to change the appearance or to allow the gold to deplate.

 

[kane333]

I put one of the boards into an AP mix and found it to be a copper substrate beneath the foils. Not sure if nickel would be used but I could be wrong. But there is also small amounts of solder left on some of the boards. Could the Tin that's in most solders dissolve into solution and cement back to the gold foil?