Lye doesn’t leach gold, so where is it!

[Juan-2-many]

Hi friends,
I’ve got hundreds of these little data stacking cable ends. 90% of them are gold plated under the solder mask. I used the good ‘ol lye solution to remove the mask and let it sit longer (hourish) than intended.

I rinsed them off and noticed it looks like the gold plating leached off?! After reading more threads, it’s clear lye doesn’t dissolve gold, but it sure looks like it especially when the fingers start to dull out (right in pic). So is it possible other metals are plating out? Should I soak them in HCl to reveal gold again?

Picture description:
Left: original (scratched to reveal gold)
Middle: mask off (starting to “leach”)
Right: gold has left the building

Thanks in advanced!

 

[Golddigger76]

Hi friends,
I’ve got hundreds of these little data stacking cable ends. 90% of them are gold plated under the solder mask. I used the good ‘ol lye solution to remove the mask and let it sit longer (hourish) than intended.

I rinsed them off and noticed it looks like the gold plating leached off?! After reading more threads, it’s clear lye doesn’t dissolve gold, but it sure looks like it especially when the fingers start to dull out (right in pic). So is it possible other metals are plating out? Should I soak them in HCl to reveal gold again?

Picture description:
Left: original (scratched to reveal gold)
Middle: mask off (starting to “leach”)
Right: gold has left the building

Thanks in advanced!

Have you tried to scrub or scrape what was solder mask, or where the gold was plated ?
It might still be there under oxidized solder mask, I would definitely check for that.

 

[Juan-2-many]

Have you tried to scrub or scrape what was solder mask, or where the gold was plated ?
It might still be there under oxidized solder mask, I would definitely check for th

Just tried it (dry) and it didn’t make a difference. I also rubbed an HCl soaked q-tip on it for a min or so and also no change. Q-tip was white as when it started too. I was hoping for something, other than white. Lol

Edit: when I scraped it like I would the mask, it made the silver shiny that’s disconcerting. Haha

 

[Deano]

Despite the claims of many people on this forum, a look at a Pourbaix diagram for gold shows that gold may be solubilised in a strong caustic solution if the Eh is high enough.
The reaction is fairly slow but provided the conditions for Eh and pH are met then solubilising will take place.
It is unlikely that distilled water will have a high Eh and it is unlikely that caustic soda will be manufactured with impurities which will give a solution a high Eh.
It is more likely that the water you are using is not distilled and contains base metal hydroxide convertible species which are soluble in strong caustic solution.
Your options are to use distilled water or to only use the minimum contact time to remove your solder.
Deano

 

[Juan-2-many]

Despite the claims of many people on this forum, a look at a Pourbaix diagram for gold shows that gold may be solubilised in a strong caustic solution if the Eh is high enough.
The reaction is fairly slow but provided the conditions for Eh and pH are met then solubilising will take place.
It is unlikely that distilled water will have a high Eh and it is unlikely that caustic soda will be manufactured with impurities which will give a solution a high Eh.
It is more likely that the water you are using is not distilled and contains base metal hydroxide convertible species which are soluble in strong caustic solution.
Your options are to use distilled water or to only use the minimum contact time to remove your solder.
Deano

That’s correct, I used tap water. So, if pH is dropped to around 7 or 8, sodium metabisulfite should *precipitate the gold?

 

[anarxi]

Lye doesn’t leach gold, so where is it!​

gold is in a jar with a solder mask
in tin...

 

[Yggdrasil]

That’s correct, I used tap water. So, if pH is dropped to around 7 or 8, sodium metabisulfite should *precipitate the gold?

No, SMB needs low pH for the SO₂ to evolve.

 

[Martijn]

I rinsed them off and noticed it looks like the gold plating leached off?!

That is the question and as far as i understand, not clear yet.

So before you start looking for gold, you need to know if it's worth looking.
Because my second point is: it's usually ENIG plating, and that disappears very fast into nothing. Nothing will precipitate that can bee seen.

To learn how thick gold plating is, try using a pen eraser to erase the gold off. Enig will come off much easier than gold on plated contact fingers.
Something can look nice, but not worth it to ga after.
These traces will come out in the copper cell.

There is not much there, so you have not lost anything of significance. move on.

 

[Alondro]

HCl + copper chloride is the way to go for leaching away solder and base metals. Even the thinnest of gold plating remains, though it turns into fine brown-purple powder.

It only doesn't work well for higher percentage silver solder (because of the silver choride buildup), but that isn't encountered often unless you process older, really high-end electronics that demanded corrosion resistance.

 

[Slochteren]

Despite the claims of many people on this forum, a look at a Pourbaix diagram for gold shows that gold may be solubilised in a strong caustic solution if the Eh is high enough.
The reaction is fairly slow but provided the conditions for Eh and pH are met then solubilising will take place.
It is unlikely that distilled water will have a high Eh and it is unlikely that caustic soda will be manufactured with impurities which will give a solution a high Eh.
It is more likely that the water you are using is not distilled and contains base metal hydroxide convertible species which are soluble in strong caustic solution.
Your options are to use distilled water or to only use the minimum contact time to remove your solder.
Deano

This is why I like GRF, such a piece off information that pop up in a random post. Don't think you will find this information anywhere else....

 

[Juan-2-many]

That is the question and as far as i understand, not clear yet.

So before you start looking for gold, you need to know if it's worth looking.
Because my second point is: it's usually ENIG plating, and that disappears very fast into nothing. Nothing will precipitate that can bee seen.

To learn how thick gold plating is, try using a pen eraser to erase the gold off. Enig will come off much easier than gold on plated contact fingers.
Something can look nice, but not worth it to ga after.
These traces will come out in the copper cell.

There is not much there, so you have not lost anything of significance. move on.

I remember an old post from you talking about ENIG plating. I’ll try the eraser trick to see. These come off of newer Cisco switch (3850 and 9300 series) data stacking cables. The fact that it’s so new means cheaper materials so…
Thanks!

 

[g_axelsson]

Hi friends,
I’ve got hundreds of these little data stacking cable ends. 90% of them are gold plated under the solder mask. I used the good ‘ol lye solution to remove the mask and let it sit longer (hourish) than intended.

I rinsed them off and noticed it looks like the gold plating leached off?! After reading more threads, it’s clear lye doesn’t dissolve gold, but it sure looks like it especially when the fingers start to dull out (right in pic). So is it possible other metals are plating out? Should I soak them in HCl to reveal gold again?

Picture description:
Left: original (scratched to reveal gold)
Middle: mask off (starting to “leach”)
Right: gold has left the building

Thanks in advanced!

Fastest answer you could get is to put it in HCl for a couple of hours. If the gold appears again it was only covered by something, probably tin.

I'm convinced that the lye dissolve some tin and then it is deposited on top of the gold again. The driving force is electrical currents created by the different metals on the material. You have a copper board with a nickel strike and gold on top. At the end there are soldered cables so there are tin and possibly lead too. All this in an electrolyte made of lye in water.

So there are a lot of different electrical potentials that can drive electrolytical reactions. Another argument is that on the piece with gold "disappearing" the areas close to the top which isn't electrically connected to the solder, the gold is "still there".

I've seen this several times but I never investigated it more, just assuming the gold would still be there and carried on with the process.

Göran

 

[Juan-2-many]

To learn how thick gold plating is, try using a pen eraser to erase the gold off. Enig will come off much easier than gold on plated contact fingers.
Something can look nice, but not worth it to ga after.
These traces will come out in the copper cell.

Just gave this a try. No difference.

Fastest answer you could get is to put it in HCl for a couple of hours. If the gold appears again it was only covered by something, probably tin.

I'm convinced that the lye dissolve some tin and then it is deposited on top of the gold again.

This a fascinating. Giving it an HCl soak now.

Thanks everyone for the help!

 

[Juan-2-many]

Fastest answer you could get is to put it in HCl for a couple of hours. If the gold appears again it was only covered by something, probably tin.

Well after a 24 hour soak in HCl (not AP), this is how they look. I’m starting to be convinced gold (dust perhaps?) is in the lye solution.

I’ll put these aside for now and filter off the lye solution. Check the solids out. Maybe acidify the solution to see if SMB will drop anything.

Question: Stannous chloride works with acidic solutions only, correct?