Removing solder from printed circuit boards

[VolkD]

I have a lot of circuits of this type. You can see from the photo that there is gold plating under the solder. Since my modified AP method refuses to cooperate with the solder, I have to remove the solder first.
Any advice would be welcome.
Ideally, I would remove the solder to the point where the gold color is visible.

 

[haveagojoe]

I use Hydrochloric acid but dilute Sulfuric would also work, probably faster. The problem would be that Sulfuric acid would also attack the solder mask, so when you get to the Copper Chloride etching stage there will be a lot more copper exposed.
For boards like that I chop them up with snips, cutting off all the bits with gold and no solder, and then process the soldered bits seperately. It depends how many you have to process I guess.

Edit to add: on the plus side for Sulfuric is that it won't attack the copper so there isn't a risk of releasing the foils prematurely, which can happen with HCl if you leave it too long.

Edit 2: an advantage with using HCl is that you can recycle it by dropping the tin out as tin oxide by bubbling air through it. The tin oxide is a whitish powdery precipitate which can easily be filtered out.

 

[VolkD]

Better than HCl alone is to add a little citric acid. Even better, instead of citric acid, add ammonium chloride. A little of each is best.Unfortunately, I don't have ammonium chloride. I know that it is a fertilizer used in agriculture, but unfortunately I can't find a connection between the chemical and commercial name.
I was hoping to find out about some other method.

I like turbo methods

 

[haveagojoe]

One of the problems is that during soldering the gold alloys slightly with the tin solder, so attempts to clean the solder off will never be completely successful, it won't be as good as the unsoldered pads and some tin will carry over into the Copper Chloride etch. There the bubbler will make it fall out as tin oxide, which gets mixed up with the foils.

You may have seen my recent post about an experiment of how to deal with that using Zinc:
https://goldrefiningforum.com/threa...-etches-experiment-writeup.35821/#post-388888

 

[Marcel]

I have made good experience using dilute sulfuric acid. It takes some time but will dissolve Tin and Lead. I do not use HCl because of the formation of Silver Chloride and the fact that it may also attack the copper.

 

[VolkD]

One of the problems is that during soldering the gold alloys slightly with the tin solder, so attempts to clean the solder off will never be completely successful, it won't be as good as the unsoldered pads and some tin will carry over into the Copper Chloride etch. There the bubbler will make it fall out as tin oxide, which gets mixed up with the foils.

Ammonium chloride and citric acid are supposed to solve this problem.

 

[VolkD]

I have made good experience using dilute sulfuric acid. It takes some time but will dissolve Tin and Lead. I do not use HCl because of the formation of Silver Chloride and the fact that it may also attack the copper.

Actually, it doesn't even bother me, unless the copper is attacked so badly that the gold foils fall off.

 

[Yggdrasil]

I have made good experience using dilute sulfuric acid. It takes some time but will dissolve Tin and Lead. I do not use HCl because of the formation of Silver Chloride and the fact that it may also attack the copper.

Do really Sulfuric dissolve Lead?

 

[haveagojoe]

Actually, it doesn't even bother me, unless the copper is attacked so badly that the gold foils fall off.

Well yes, that is the risk, because as soon as any copper is attacked it forms Copper Chloride which then dissolves more copper more quickly. So if HCl is used, in my opinion it should be done cold so that it's not too fast, so that it can be stopped at the right moment if copper is attacked, before it starts to release foils. But that means a slow process, so if you want something faster, Sulfuric might be a better choice since it will leave the copper and the foils intact.

 

[VolkD]

I don't mind if it eats copper and if the foils are removed. But I'm afraid of problems with filtering. Problems could arise if I had a lot of various chlorides between the foils.

 

[Yggdrasil]

Well yes, that is the risk, because as soon as any copper is attacked it forms Copper Chloride which then dissolves more copper more quickly. So if HCl is used, in my opinion it should be done cold so that it's not too fast, so that it can be stopped at the right moment if copper is attacked, before it starts to release foils. But that means a slow process, so if you want something faster, Sulfuric might be a better choice since it will leave the copper and the foils intact.

Sulfuric attacks Copper, particularily dilute.

 

[VolkD]

Do really Sulfuric dissolve Lead?

I've tried it. It works well, but I don't like it. You have to add some peroxide and heat it to around 100°C.

 

[haveagojoe]

Sulfuric attacks Copper, particularily dilute.

I stand corrected. In that case there is little advantage and HCl seems to be the only realistic approach.

 

[Yggdrasil]

I've tried it. It works well, but I don't like it. You have to add some peroxide and heat it to around 100°C.

We were discussing dilute Sulfuric for dissolving Tin.
Then it came up that it might dissolve Lead as well.
That goes against my understanding of Lead and Sulfuric.

Piranha soultion is a big no no in refining and even worse hot.

 

[Marcel]

Do really Sulfuric dissolve Lead?

Well, I should clearify, the lead not go into the diluted sulphuric solution, but the tin alloy that contains lead will be dissolved and the lead seperated mechanically not chemically dissolved from the alloy, so it will fall down to the bottom.

 

[haveagojoe]

I don't mind if it eats copper and if the foils are removed. But I'm afraid of problems with filtering. Problems could arise if I had a lot of various chlorides between the foils.

Exactly, although its oxides which tend to be the problem, particularly tin oxide. Chlorides should stay in solution so will pass through the filter, except copper (i) chloride which has to be converted back to copper (ii) chloride to become soluble. But the oxides like tin oxide which are insoluble will be mixed with the foils and can't be separated by filtering.

 

[Yggdrasil]

Exactly, although its oxides which tend to be the problem, particularly tin oxide. Chlorides should stay in solution so will pass through the filter, except copper (i) chloride which has to be converted back to copper (ii) chloride to become soluble. But the oxides like tin oxide which are insoluble will be mixed with the foils and can't be separated by filtering.

Have you had problems with this?
I have never encountered any significant filter issues with AP.

 

[Marcel]

Sulfuric attacks Copper, particularily dilute.

One must consider the proportions: The vast amount of the exposed material is tin alloy, which is more attracted due to the lower position in metal/reactivity series to go into solution. Your case assumes pure copper. But the cement effect and the ratio of exposed surface of the metals will keep the copper mainly undissolved until most of the tin has been dissolved. So I take this observation from the practical point of view of my experiences with this method.

 

[Marcel]

Have you had problems with this?
I have never encountered any significant filter issues with AP.

Copper(I) has been a painful filtering experience for me as well sometimes. You should have added more HCl and bubbling is a must.

 

[Yggdrasil]

Copper(I) has been a painful filtering experience for me as well sometimes. You should have added more HCl and bubbling is a must.

Yes but that is easily mitigated by HCl and air.
My post was aimed at the Tin Oxide part of the discussion.