Immersion tin plating issues

A. The object of this process is to create an electroless tin plate using stannous chloride as a source of the tin and magnesium as the reducing agent.
The chemistry is Mg (0) + Sn (+2) > Mg (+2) + Sn (0) - i.e., tin metal.
The chalk is in the process to act as an abrasive. It is a poor choice, since it has a high pH and an inert abrasive would work better. The ammonium sulphate is in the formula to keep the pH low; at higher pH's the process won't work. I do question whether the ammonium sulphate would keep the process low enough to make the reaction occur. Sodium bisulphate would probably work better. This formula - and other similar formulas - would have to be made fresh to prevent the reaction of the reducing metal with the stannous salt. An alternative reducing metal would be zinc dust ⇦this on eBay or Amazon [affil links] .

(I have had almost 50 years of experience with electroless tin plating using active metals as reducing agents.)

A. Hello, Gordon,
The Metal Finishing Guidebook lists three formulations for immersion deposits of tin onto copper, only one of which is cyanide-free:
Stannous chloride 3.8 g/L,
Thiourea ⇦this on eBay or Amazon [affil links] 49.5 g/L,
Sulfuric acid 12 ml/L,
temperature 80-120 °F.

However, immersion deposits of tin may not be a fully acceptable substitute for tin-lead electroplating. Good luck.

A. The source doesn't say, Gordon. I must assume it's based on total weight. But you could probably use 66 degree baumé (96 percent) sulfuric acid without being far off. There are several other formulas for generic immersion tin solutions; you might want to consult some of the standard reference books on this. Good luck.

A. Anyone contemplating the use of the above recipe should first read the MSDS on thiourea:

http://physchem.ox.ac.uk/MSDS/T/thiourea

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Ed. note December 2011: Sorry, that MSDS database is no longer functional.

A. You might get this publication: "Implementing Cleaner Printed Wiring Board Technologies: Surface Finishes" (EPA 744-R-00-002); it can be ordered from the Pollution Prevention Information Clearinghouse online through the EPA Website or download from www.epa.gov/dfe. The Design for the Environment EPA project did a good job in their "Making Holes Conductive" project.

A. Sir,

Immersion Tin is primarily a three-step process: Cleaning - Microetching - Immersion Tin. But Some suppliers give an option of conditioning the copper surface before Immersion Tin. Please could someone clarify whether I should go for conditioning step before Immersion Tin?

A. Try an overlay with less adhesive or do not rub it as hard at the edge.

A. Hi There,

So you found that dreadful formula too ... was that the one with Sodium Metaphosphate as well?

Okay, I gave up on trying to chemically get a bright finish and then buffed the P.C.B's with a fine cutting compound it will just take off that annoying dull/oxide layer and little tin, leaving the sought after highly solderable bright tin.

Have Fun.

Mr.Fyfe,

Thanks for your response to my query. I am glad to note that my persistence resulted in the much sought after bright finish. Nothing was changed in the Stannous Chloride, Thiourea and HCl formulation.

No,I did not use Sodium Metaphosphate. Only the method of application was modified. Instead of dunking the PCB in the solution, I briefly swabbed the copper conductors. As soon as the PCB was evenly covered with the plated tin, it was rinsed in water. I immediately got the shiny tin finish. Light buffing with a soft cloth made the plating even more shiny -- though, this is not necessary.

The secret is in not immersing the PCB in the solution for prolonged period but quickly swabbing it with soaked cotton. Immersing the work piece for any length of time -- except very very briefly -- results in the dark gray tin deposit which you claimed to have removed with the polishing compound. I too tried to do just that but by the time the gray deposit is entirely removed, the tin starts to come off too. Perhaps I was not using the right polishing compound. As a matter of fact, I also tried Stannous Chloride, Thiourea, citric acid formulation and got a mirror finish on an inch-square copper piece. But in this case you just dip and pull out the PCB a couple of times to get an even coat of tin, or the swabbing has to be real even, otherwise you get dark gray stains in places. Of course, for both the methods to succeed the copper surface has to be meticulously clean. Hand rubbing with a fine polishing compound -- I used a pre-wax auto rubbing compound ⇦this on eBay or Amazon [affil links] -- and acetone ⇦this on eBay or Amazon [affil links] Warning! highly Flammable! wipe will accomplish this. No touching with fingers after that! Regards

Azhar

A. I have used Stannous Chloride for years to prepare Electroless Tin Plating for PCB. You can try the following recipe:

• Stannous Chloride 5 grams/liter
• sulfuric Acid (66 Be) 10 ml/liter
• Thiourea ⇦this on eBay or Amazon [affil links] 50 grams/liter
• Use only distilled water or demineralized water.

Care must be taken for sulfuric acid over water.

A. Hi Hans. That sourcebook did not say. However, the usual meaning of concentrations like this is on a final weight basis -- i.e., you need 3.8 g/L of the chemical stannous chloride. Whether you got it from anhydrous or hydrated doesn't matter; you need to end up with 3.8 g/L of SnCl₂, stannous chloride.

Regards,

A. You can get a bright finish with Barkeepers Friend ⇦this on eBay or Amazon [affil links] - it probably goes by another name in Gaza... It is a cleaner used by restaurants for polishing brass, copper and such. The active ingredient is 5% oxalic acid - which shouldn't be any problem - mixed with a mild abrasive and a bit of detergent.

A. The plating formulations listed, with thiourea ⇦this on eBay or Amazon [affil links] , are actually immersion plating and not electroless plating. It should be noted that thiourea is a suspected carcinogen.

A. Hi, Puvan.

The immersion tin coating is very thin.

If it turns black in a few hours or days, then it was not carefully rinsed and dried.

If it turns dark after weeks or months, then it is due to alloying of copper with the thin tin layer.

So ... either rinse and dry it better, or use it sooner.

A. ... or plate it thicker.

Hi, Puvan. Some suppliers claim that a proper conditioning step before the immersion plating process will increase the thickness to 1 µ (.65 µm of pure tin) which they say will remain solderable for a year.

Regards,

A. I am not a tin plating expert, but I doubt you are going to have much luck getting quality plating with a solution of vinegar and tin solder.

I'd recommend buying a proper tin plating solution from a supplier as a first step; if you really can't do that, at the very least find a better source of tin.

A. Hi Carlos.
There are many reasons why your idea is unlikely to work.
Unmodified acetate solution will not produce an acceptable deposit and with both tin and lead in solution you are trying to deposit an alloy; even more unlikely.
Printed circuits are commonly plated with both tin and tin/lead but using far more complex solutions.
It is common practice to plate the circuit pattern through a resist and use the deposit as the etch resist. Once the pattern is formed you have a major problem achieving electrical continuity.
There are two simple ways of coating the conductors.
Boards are dipped in a molten solder bath - Quick and effective but needs a large heated pot of solder.
Or, for you the easiest way is to use an immersion tin bath (sometimes wrongly called electroless tin.)
You can make your own (see above discussion) but the chemicals are available cheaply and ready mixed on the internet.
Just search "Immersion Tin"

A. Hi Carlos. As Geoff Smith noted, it's better to call the process 'immersion' or 'displacement' plating because to most platers 'electroless' implies an autocatalytic process, as in the electroless copper and electroless nickel baths which can be used on plastic & other non-conductive substrates.

This page already lists formulations at least 4 times -- one by me, one by Juan_Oyarce, one by puvan_vijayakumar, one by Hanibal_Najjar (plus notes about where to find many others), and you say you've tried many times. This leaves an obvious question:

Are you sure the substrate isn't the problem, rather than your many formulation attempts? What substrate are you trying to immersion plate onto?

Luck & Regards,

A. Hi Carlos. I've never prepared it myself, so I can't advise exactly what is going wrong. However, tin is well known for its stannous (white precipitate) form, so I suspect that something went wrong with the pH or the lack of thiourea ⇦this on eBay or Amazon [affil links] . Are you using stainless steel instead of glassware?

Are you sure you can't just buy an immersion tin process as Geoff Smith suggests? -- thiourea is carcinogenic, so somewhat random experiments with it are probably not a good idea.

Luck & Regards,

A. Hi. The best solution, when possible, is simply to not wait. Solderability always decreases with age :-)
But Jeffrey Holmes tells us below that when failure comes soon it's due to poor rinsing and drying. And Robert Probert tells us on thread 24114 that plain water rinsing probably won't do it; you need a dip in an alkaline cleaner to clear the tin residues.

Luck & Regards,