# recovering gold "dissolved" in tin



## vgecas (Nov 23, 2014)

hi,

i have been reading this forum for a while, but didn't succeeded to find an answer...
lets say we have a flip chip, that has gold plated circles and triangles at the corners (like in attached image). on the underside it has solder balls. i suspect that there is gold plating under these solder balls. because gold plating is covered with solder, i suspect that gold is "dissolved" in tin, thou it can not be visible after mechanical separation. could someone confirm my suspections? and if i am right, how that gold could be recovered? if i would run these chips through AP, solder and copper base will get dissolved, what will happen to "tin dissolved gold"? maybe adding excess peroxide will dissolve that gold to, and later it could be precipitated with SMB?


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## solar_plasma (Nov 23, 2014)

Tin would foul your AP, bad choice. A possibility would be to dissolve it in HCl alone, a tiny amount of gold, some copper, some silver chloride and lead chloride would make the main part of undissolved solids. Tin chloride would stay in solution, but it would be contaminated with dissolved lead chloride. Another possibility would be a tin parting cell.

There are more professional solvents, too, that are used to recover tin, but I have no clue about those.

All in all it might not be worth the effort, if not for the educational interest. A lot of toxic waste to handle, a pain to refine and a tiny yield.


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## g_axelsson (Nov 23, 2014)

My advice : Forgettaboutit!

It is so little gold compared to the amount of trouble of getting it off the board. There is a high possibility that the solder is silver based (no lead in that case) and then it would probably be more values in the solder than in the gold... not that it is wort going after it either.

This is one type of scrap I never touch, I just toss it in with power supplies and other trash boards and sell it for 0.4 euro / kg.

Göran


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## bswartzwelder (Nov 23, 2014)

Just out of curiosity, how many of these beasts do you have? One or two, even a dozen would not be a feasible amount to try to recover from.

Welcome to the board. It's welcome relief to see that someone new has done their homework and started by reading. Keep up with the reading because there is a wealth of good information to be found.


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## Geo (Nov 23, 2014)

That's a BGA (ball grid array) and the solder balls are tin/silver with a low percentage of silver. the only gold is what visible gold you see. They are not worth the time or effort. I incinerated ten pounds and dissolved the whole lot of them in AR. I didn't recover any appreciable gold out of ten pounds.


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## vgecas (Nov 24, 2014)

hi,

thanks for adding clearance. i could gather couple of kilos of such chips, so i thought there could be reasonable amount of valuables to recover...


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## g_axelsson (Nov 24, 2014)

I think there will be gold "under" the solder balls (alloyed in the solder after the balls are applied and soldered).
It will be a tiny amount as the gold is only protecting the board until it can be soldered, the process is called ENIG, Electroless Nickel Immersion Gold, and they just dip the boards in a gold solution at the end to protect the surface. The thickness is just a tenth of normal gold plating on pins and fingers and it is only applied on exposed conductors.
On cutouts and trims, the parts of the board that is cut off after manufacturing and could have 100 % coverage you usually only see around 0.7 g Au / kg. (like this) It looks impressive but it's really thin gold.

It wouldn't surprise me to see assays in the vicinity of 0.03 - 0.1 g Au per kg for this material.

There is probably more value in the tin than in gold for this material.

... but then, the best way to know is to send some material for assay.

Göran


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## Anonymous (Nov 24, 2014)

Goran do you get a lot of gear?


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## g_axelsson (Nov 24, 2014)

spaceships said:


> Goran do you get a lot of gear?


I get a couple of tons of equipment and scrap per year, resulting in 100-200 kg of medium and high quality circuit boards, a lot of steel, some aluminum, brass, copper, stainless and cables that I sell. (Doing it for fun! 8) )

I have some plans on expanding but my current job takes too much time! :shock: 
... and I might have a jeweller as a customer, refining their gold scrap. First he wants to test me with a batch of polishing waste. If that takes off I will slow down the scrap business.

My knowledge of electronic components comes from having worked and designed electronic circuits all my life.

Göran


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## Anonymous (Nov 25, 2014)

Thanks for that, I was almost thinking that you were getting tonnes per month there! It's amazing how much you can learn isn't it?


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