Remove/Strip only Tin from PCB traces.

[rajhlinux]

How can I remove tin from PCB without affecting any bit of copper traces?

I read many professional PCB fabrication factories uses a "nitric acid" based solution to strip away the Tin which also does not affect the copper circuit trace quality.

It's almost impossible to find anything that I can buy a "commercial product" online that can remove the Tin from the PCB. Seems like I need to make my "own tin stripper".

So every time I google something related to removing tin, I always end up in this website. But all the things I read here to remove tin is really confusing, its like people using 10 different types of acids and refining it for gold.

Anyone here knows some magic formula that can be easily made to strip tin?
I'm prototyping my own PCB and will need to Tin the copper traces so that it will act as an etch resist when using an etchant to etch away the unwanted copper while the tin protects the circuit traces.
The "tin etch resist" will then need to be stripped away after etching the unwanted copper. The reason why I want to remove the "tin etch resist" is so that I want to avoid "tin whiskers" down the road which can cause shorts. Also I obviously don't want to wait more than an hour or so to completely remove the tin. I read people using common acids and waiting 24 hours to have the tin removed.

Thanks for any helpful info.

 

[voidforged]

Nitric acid will dissolve the copper and convert the tin to a white goopy mess. Use hydrochloric (you can buy as muriatic acid from a hardware store) to dissolve the tin without dissolving the copper.

 

[rajhlinux]

Thank you for your quick reply.

I also read that hydrochloric acid is a "slow" process. I only need to remove/strip "Tin" out, no extra stuff like a "tin/lead solder mix". If using "hydrochloric acid", will it be a long slow process just to strip away the "Tin" from the copper traces... slow as in more than 1 hour?

If it helps or matters, this is the exact specific Tin electroless plating solution I'll be using to plate the copper traces as an etch resist: "MG Chemicals 421A Liquid Tin".
Which is free of co‑deposited organics, has no lead.
The MDS data sheet states that it is based of the following chemicals:
thiourea - 10%
tin(II) methanesulphonate - 5%
methanesulphonic acid - 4%

Also, just 30%-37% of hydrochloric or muriatic acid will only be required, no other acids or chemicals are needed? (Seems too good to be true. :shock: ) It's somewhat weird since all the professional PCB fabrication factories never mentioned of using hydrochloric acid based solution to remove the "Tin etch resist". They always mention some kind of nitric acid solution, when you stated that the nitric acid will attack the copper, is really counterintuitive in the PCB fabrication industry, I'm really stumbled

 

[voidforged]

You could also use lye, although I tend to avoid that as the wastes are a little more difficult to deal with and its typically considered to be more hazardous than HCl. Assuming there is nothing on the boards at the point you're at other than the tin mask, either route would work. Nitric converts the tin into metastannic acid which is horrible to deal with, as well as it dissolves the copper so just forget that entirely for your application.

EDIT: Scratch that, the lye would likely react with the PCB board itself to some extent. Best to just use HCl.

 

[g_axelsson]

As you understand, this is a precious metal refining forum. We seldom pay any regards to metals like tin so most of the posters doesn't care to selectively remove tin from a board.

I've worked with electronics for most of my life and I've made many boards myself. What I don't understand is how you are applying tin and use it as an etch resist, I've never heard about that before. I've used standard UV exposed etch resists and foils to make my traces. In the beginning I used tape, then ink and pen plotters and finally OH-films in laser printers. Nowadays I just order cheap prototype boards from China. For a quick and dirty way I've even used permanent markers on blank copper boards to do some rough circuits in just hours.

But using tin as a resist I've never heard about that before. What etch do you use that doesn't affect tin but etches copper?

Hydrochloric acid should in theory dissolve tin and leave the copper undamaged. But real life is a bit messier, if you got dissolved oxygen it might oxidize the copper a bit and then HCl will dissolve the copper oxide, creating copper chloride which will dissolve more copper as long as there is oxygen dissolved in your liquid. If you leave the solution too long it will absorb oxygen from the air and start a copper chloride etch, etching away your copper.
So it's a matter of timing.

I don't think you will find many exact recipes here, electronic scrap isn't a well defined product and as various processes progress the ratio and concentration of the chemicals involved will vary. A lot of the processes we use are managed by eying the reaction and making educated guesses of what is going on inside our beakers.

Göran

 

[g_axelsson]

Ok, it seems you are using alkaline copper chloride etch. That's something I've never used and I've only read about it being used in the industry.

I used iron chloride at first, then HCl and hydrogen peroxide and finally a specially made etch based on some salt you bought and mixed with water yourself. I don't remember exactly what it was so I will not make a guess either.

The tin strippers uses nitric acid but (I guess) at a low concentration together with inhibitors that protects the copper from attach by the nitric acid. The tin stripper probably have other advantages over using a chloride based stripper. Maybe hydrochloric acid have long term effects on corrosion of electronic components. The exact reason might be a trade secret or hidden within some obscure scientific report.

For reference, here is a company selling chemicals for etching and stripping. Doesn't say a lot about the finer details though.
https://www.bece-chemie.de/en/products/etching-stripping/alkaline-etching/

And this looks like an interesting article that might have more insight in what the tin stripping solutions contains.
https://www.emerald.com/insight/content/doi/10.1108/cw.2004.21730aab.010/full/html

Göran

 

[rajhlinux]

Thanks for your helpful reply.

It's hard to believe a pro like yourself never heard of "tin etch resist", it's actually an industry standard for high quality professional PCB manufacturing. The reason why PCB manufactures goes through the extra hassle to tin plate the PCB before etching is so that the "etchant" which removes unwanted copper will not further attack the wanted copper traces. It provides a better quality etch.

But anyhow I'll provide some reference link for legitimacy of tin being used as an etch resist:

A professional PCB fabrication video, scroll through the video time at 19:20, and listen what the narrator says, for some reason they literally don't show you the tin plating but they mention about it:
[youtube]https://www.youtube.com/watch?v=sIV0icM_Ujo&t=1204s[/youtube]

Another PCB manufacturer mentions about the important reason for the "tin etch resist":
https://www.pcb.co.uk/news/stage-4-plate-strip-etch

Yes, indeed the etchant to remove the unwanted copper is not your traditional etchant since it has to be an etchant that does not attack the "tin etch resist" while its stripping the unwanted copper.

You seem to understand that "nitric acid" is used in the PCB fabrication industry to remove "tin etch resist", I'm sure you know many people who makes PCB at home actually add liquid "Tin" to their PCB boards for extra "circuitry protection", while this is all hoax at the professional electronics level. You will never find military, medical or aerospace equipment that has tinned traces. Since "Tin" causes the growth of "Tin Whiskers" and can cause a short between traces (which is a phenomena and scientists/engineers can not explain why it occurs).

So my thread is not so irrelevant with this forum, I have a common issue just like anyone in this forum, I have a PCB which needs "Tin" to be removed. I'm sure some folks around here knows a thing or two about stripping tin with "nitric acid" but does not attack the copper traces.
Hope they can share some legitimate working tried and true formula.

 

[rajhlinux]

Alright I found a legitimate source which uses "nitric acid" as a "tin stripper" and completely exposes it's ideal "tried and true" formula... (yes, you guessed it, information extracted from a patent document):
https://patents.google.com/patent/EP0559379B1/en

Here are three variations of the magic "tin stripping" nitric acid based formula (this will not remove copper traces):

EXAMPLE I:
An aqueous solution was prepared containing 400 g/l of nitric acid (anhydrous), 40 g/l of ferric nitrate (anhydrous), 10 g/l of hydrochloric acid (anhydrous) and 1 g/l of a surfactant mixture containing a quaternary ammonium compound to serve as an inhibitor. The solution, at room temperature, was used to strip tin-lead (7.62 µm - 0.3 mils)-coated copper panels in an immersion process. The time needed for producing a clean copper surface tree of tin-lead and copper-tin intermetallic was on the order of about 1-3 minutes. The solution was employed in stripping a multitude of panels to a point where the solution contained 150 g/l (20 oz./gallon) of metals, at which stage no sludge or precipitate were evident in the solution. Attack of the copper surface of the panels was negligible.

EXAMPLE II:
An aqueous solution was prepared containing 300 g/l of nitric acid (anhydrous), 60 g/l ferric nitrate (anhydrous), 20 g/l of sodium chloride, and 1 g/l of the same surfactant mixture as used in Example I. The solution was used at room temperature to strip tin-lead (7.62 µm - 0.3 mils) coated copper panels in an immersion process. The time needed for producing a clean copper surface, free of tin-lead and copper-tin intermetallic, was on the order of about 1 to 3 minutes. The solution was employed in stripping a multitude of panels to a point where the solutio n contained about 127 g/l (about 17 oz./gallon) of metals, at which stage no sludge or precipitate was evident in the solution. Attack of the copper surface of the panels was negligible.

EXAMPLE III:
An aqueous solution was prepared containing 250 g/l of nitric acid (anhydrous), 40 g/l of ferric nitrate (anhydrous), 10 g/l of hydrochloric acid (anhydrous), and 1 g/l of the same surfactant mixture as used in Example I. The solution was used at room temperature to strip tin-lead (7.62µm - 0.3 mils) coated copper panels in a spray process. The time needed for producing a clean copper surface, free of tin-lead and copper-tin intermetallic, was on the order of about 30 seconds. The solution was employed in stripping a multitude of panels to a point where the solution contained 161 g/l (21.5 oz./gallon) of metals, at which stage no sludge or precipitate was evident in the solution. Attack of the copper surface of the panels was negligible.

 

[GoIdman]

Rajhlinux,

I am a butvconfused with what you want to do, it seems to me you want to do things backward.
I do have 20+ years in electroniscs but never done tin stripping.
The way pcb are manufactured even at home is that firs you make the traces on a plated fresh board then you etch away the unwanted copper to expose only the needed traces then use reflow technique to cover and protect the traces with a tin/pb/ag mixture.
These are the most common and basic pcb manufacturing techniques.
What seems to me is that you want to reclaim the solder to process it for Ag.
Correct me if i misunderstood your post.

Be safe.

Pete.

 

[Marcel]

Cold dilute H₂SO₄ will slowly dissolve tin while keeping Cu mostly intact. Rinse with lots of water afterwards.

 

[Geo]

Cold dilute H₂SO₄ will slowly dissolve tin while keeping Cu mostly intact. Rinse with lots of water afterwards.

Dilute sulfuric acid will readily attack copper. It is actually the etchant used in large copper heap leaching operations.
I have not tried it yet as I haven't found a good formulation for it but a solution of sodium nitrate and citric acid and an oxidizer such as H2O2 should selectively dissolve tin solder.

 

[Marcel]

I have used dilute sulfuric acid to depopulate boards. It is used as etchant for copper, yes, but the parts fell off, long before the copper was dissolved. Also, dilute (<5%) sulfuric acid which is at room temperature has a hard time etching copper and the tin acts like some sort of victim anode. In addition, on PCBs most traces are covered by solder resist, so the acid won't reach them anyway. But no matter what, this is probably not ideal, since the purpose was different and there was no problem with sacrificing a bit of copper while depopulating the PCBs. Still sulfuric acid might be too strong, and it requires watching the process to catch the optimal moment.
Your suggested mix sounds better.

Btw: Someone once told me he had a good experience using simple sodium chloride solution (NaCl=table salt) to dissolve tin and depopulate PCBs. It takes a few days, though.

EDIT 2: I just remembered another non-chemical treatment: Tin plague or tin pest!

Allotropic transformation​

At 13.2 °C (about 56 °F) and below, pure tin transforms from the silvery, ductile metallic allotrope of β-form white tin to the brittle, nonmetallic, α-form grey tin with a diamond cubic structure. The transformation is slow to initiate due to a high activation energy but the presence of germanium (or crystal structures of similar form and size) or very low temperatures of roughly −30 °C aids the initiation. There is also a large volume increase of about 27% associated with the phase change to the nonmetallic low temperature allotrope. This frequently makes tin objects (like buttons) decompose into powder during the transformation, hence the name tin pest.[3]

The decomposition will catalyze itself, which is why the reaction speeds up once it starts; the mere presence of tin pest leads to more tin pest. Tin objects at low temperatures will simply disintegrate.

Tin pest - Wikipedia