Processing fingers with nitric

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miroman

Member
Joined
Dec 6, 2017
Messages
19
Hi all,

I had to proceed few kilos of RAM (mainly SDRAM, also some DDR and even old EDO and FPM sticks). By advice of a friend (he doesn't read forums :) ) I decided to treat them with diluted nitric. From 6 kg. sticks I collected 490 gr. very close cut fingers. Put them in diluted 1:1 nitric. Every 2 hours stirred them and let them sit overnight. At the morning most of the foils were on the bottom of the beaker. No air, no bubbler, just simple nitric. I'd say that it's much easier and faster than AP process.
But when I trimmed the fingers, I noticed some of the modules had a row of gold-plated holes just above the fingers:
IMG_20180715_092248.jpg
As seen, the foils are gone, but most of the holes are untouched. Also there were few Dummy modules for RDRAM sockets with gold holes:
IMG_20180715_095217.jpg
So I decided to try to take this gold too. I mixed a small quantity of AR (20 gr. HCl + 5 gr HNO3), which I intended to use later for the foils. Before foils, I put the proper pieces in AR for a minute (only few pieces at a time, because the AR was too little). It took me maybe half an hour extra. But it worth it, as the yield was 2.9 gr.
Meanwhile, since I was threatinig "the holes", AR little by little began to became classical orange. I expected a greenish, as I thought the holes are too copper with gold plate. Unfortunately I forgot to take a picture. Maybe the holes are with thicker plate, or without a copper (only gold)? Anyway, when I put the foils, AR became little greenish.

So, if you have access to nitric, I belive treatment with diluted nitric is a better process than AP. Also processing the holes above the fingers adds some more yield. I think 2.9 gr. from 490 gr. fingers is good :)

P.S. Now I'm beginning to collect chips and MLCCs :)

Regards, Miro.
 
I think the reason the aqua regia didn't dissolve so much copper is that the nitric acid had already dissolved the easily reached copper. The only metal left that was easily dissolved was the gold.
You can see the shadow of copper inside the board in the first picture as a darker area and around the holes it is already dissolved. In the rambus dummy module you can see the same effect along the edges where the copper is bright under the solder mask and along the edge it's darker because the copper is gone.

Once I also did fingers in nitric acid but the problem to source nitric acid is a big problem for most persons, the red cloud of NOx is also a problem when going up in scale. Copper chloride does the same thing cheaper but slower. It isn't so sensitive to pieces with solder too, nitric acid and tin forms metastannic acid and that might create problems further on when filtering the gold chloride.

It's all about personal preferences.

Göran
 
Oh, now I see. You're right, the copper base is almost gone with nitric.
When that happens to a foil, it fells. But a hole gold-plate can't fall because of the geometry of the plate.
About the tin - you're right too. You can process this way only absolutely tin-free areas. That's why it's important to cut the fingers close.

Regards, Miro.
 
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