Need help with electroless/immersion tin plating of PCB
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Q. Hi, I am looking for the method of electroless tin plating of printed circuit boards. In my country I cannot simply go to a shop and buy some plating crystals, and therefore need to actually make up the stuff to do this. Electronics is my hobby, and so any help will be GREATLY appreciated.
Thanks very muchGordon [last name deleted for privacy by Editor]
TUTORIAL FOR NEWBIES:
We think Gordon is asking about depositing tin onto the copper traces on a circuit board. Although he used the term "electroless tin plating", metal finishing professionals call the process "immersion tin plating" instead, because they reserve the word "electroless" for something else ...
As high school chemistry students may have learned, if you place an iron nail into a copper sulphate solution, it becomes copper plated; or if you place a copper object into silver nitrate it becomes silver plated. The reason is that copper is more noble than iron, so dissolved copper ions will steal electrons from iron metal, turning the positively charged copper ions into neutral copper metal atoms, and dissolving the iron metal atoms into the solution as positively charged iron ions. Similarly, silver is more noble than copper, and silver will "immersion plate" onto copper. Immersion plating is sometimes alternately called "displacement plating" because one metal displaces another from solution.
The term "electroless plating" is reserved for processes which include reducing agents, such that the process is sort of "spring loaded", waiting only for the presence of a catalyst to trip the spring and cause the reducing agent to react with the dissolved metallic ions and convert them to metal. "Electroless plating" is sometimes alternately called "catalytic plating" or, if the metal that it deposits can itself serve as a catalyst, "auto catalytic plating".
One reason for drawing the distinction between "immersion plating" and "electroless plating" is that, in immersion plating, as soon as the object has a very very thin plating on it, so the substrate metal is no longer exposed, the plating stops -- it won't get any thicker. But quite thick plating is possible with "auto catalytic electroless plating" because it can continue long after the substrate is coated.
- 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 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..
Ted Mooney, P.E.
Pine Beach, New Jersey
A little while ago you assisted me by letting me know the mixture for electroless tin plating of copper on PCB. It was stannous chloride 3.8 g/l thiourea 49.5 g/l and sulphuric acid 12 ml/l. But I do not know what strength the sulphuric acid is supposed to be. Please could you assist me with this as I have now found a place to get the thiourea and would like to try the plating as soon as possible.
Thank you for you help.Gordon [returning]
A. The source doesn't say, Gordon. I must assume it's based on total weight. But you could probably use 66 degree baume (96 percent) sulphuric 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.
Ted Mooney, P.E.
Pine Beach, New Jersey
A. Anyone contemplating the use of the above recipe should first read the MSDS on thiourea:
Ed. note December 2011: Sorry, that MSDS database is no longer functional.
Alternatives to Immersion Tin plating for PCB ?(2003)
I make prototype PCB and, after etching the boards, I use Immersion tin to "plate" them and to protect from oxidation of the Copper. I buy the immersion tin solution in three components but I don't know what they are. I only know that one of them is Sulfuric acid. After reading a post by Ted Mooney I think that the three components are Stannous chloride 3.8 g/L, Thiourea 49.5 g/L, Sulfuric acid 12 ml/L since proportions are the same.
When I have to apply the solder (the protective green layer on PCBs) on the boards, before plating, I put the boards, only for a few seconds, in a Ferric Chloride solution to remove oxidation from the copper pads (the boards stay for 60 minutes at 140 °C). Even doing so, some customers (very few considering the total amount) say that it's difficult to solder. Is there another method for depositing, in the same easy way, some other metal that can be better to solder the electronic components on ? Even Gold can be taken in consideration since the effective plated area is usually very small (only the pads on a PCB with the solder).
Sorry for my English !
Thank you,Francesco Fontana
- Valenza, AL, ITALY
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
Sulphuric Acid (66 Be) 10 ml/liter
Thiourea 50 grams/liter
Use only distilled water or demineralized water.
Care must be taken for Sulphuric acid over water.Juan Oyarce
PCB-CNC - Santiago, Chile
Q. In the recipe you mention Stannous Chloride 3.8 g/L.
Is this SnCl2 [anhydrous] or SnCl2.2H2O [dihydrate] or will both do equally well?
Thanks for your reply,
hobbyist - Alkmaar, NH, The Netherlands
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 your got it in anhydrous, hydrated, or diluted form doesn't matter; rather, you need to end up with 3.8 g/L of stannous chloride.
Ted Mooney, P.E.
Pine Beach, New Jersey
July 10, 2008
Q. We are making PCB The tin plating we need should be Electroless. But how the mentioned processes are possible for the exploitation on Manufacturing Level? Which parameters are critical for maintaining?
I had tried but we got Black slurry & minor tin plating, How to reduce this problem? The Sn was released in Black Granules form I suppose. Then the process will be expensive. How to control Thickness, whatever it may be.
Owner, associate unit - Nasik, Maharashtra , INDIA
April 24, 2011
Q. Hello dear
I've tried to do electroless plating of tin on copper PCB and has worked and I've used the formula Stannous chloride 3.8 g/l, Thiourea 49.5 g/l, Sulfuric acid 12 ml/L.
But the surface wasn't bright, so how can I make the surface bright.
university teacher - Gaza
January 11, 2012
A. You can get a bright finish with Barkeepers Friend [linked by editor to product info at Amazon] - 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.Nicholas O. Lindan
- Cleveland, Ohio, USA
April 9, 2016
Q. My mixing is
Stannous chloride 3.8 g/l, Thiourea 49.5 g/l, Sulfuric acid 12 ml/L.
It is changing to silver colour, then after some time it turns to black not suitable for soldering. Can you tell me what I should do to make it good to solder?
- karur, tamilnadu, india
May 18, 2016
The plating formulations listed, with thiourea, are actually immersion plating and not electroless plating. It should be noted that thiourea is a suspected carcinogen.Lyle Kirman
Consultant - Cleveland, Ohio USA
June 6, 2016
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.
Jeffrey Holmes, CEF
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