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topic 0260

Need help with electroless/immersion tin plating


A discussion started in 1996 and continuing through 2018.
1996

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 much

Gordon [last name deleted for privacy by Editor]
-computers


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.

Professionals reserve the term "electroless plating" 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 thin plating on it, so the substrate metal is no longer exposed, the plating stops -- it can'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.

1996
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, finishing.com Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey


1996

Q. Ted,

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]
- computers


1996

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) 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, finishing.com
Teds signature
Ted Mooney, P.E.
finishing.com
Pine Beach, New Jersey


1999

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

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

Cor Grooff
- Netherlands

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



Alternatives to Immersion Tin plating for PCB ?

2003

Hi all,

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


2005

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



"Build your own Printed Circuit"
by Al Williams

from Abe Books
or
info on Amazon



Printed Circuits Handbook
from Abe Books

or

2006

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,

Hans Schaaper
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.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E.
finishing.com
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.

Chetan Choudhari
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.

thanks

Hanibal Najjar
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?

puvan vijayakumar
- karur, tamilnadu, india


May 18, 2016

A. 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
Jeffrey Holmes, CEF
Spartanburg, South Carolina


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,

pic of Ted Mooney
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey
Striving to live "Aloha"



Thiourea-free immersion tin plating over brass

July 3, 2018

Q. Sir,
we plate brass zip roll. It is electroless process.
We use tin chloride and thiourea to plate brass color in white.
Maybe the process known as immersion process.

Now thiourea is not available. Please guide us substitute for Thiourea which we can use.

mukand garg
- delhi, India



Tin plating pcb boards

November 13, 2018

Q. Hi everybody, I hope someone can help me. I'm an electronics hobbyist and I'm always looking for methods to upgrades my pcbs. The copper board gets rusted with time and I'm looking to cover the traces with tin.

I studied and read lots of information on the internet until I came across this site, and have many doubts.

I want to cover the copper of a pcb board with tin. The tin I have available is the solder (60/40) or soldering bars with a tin content of 50%.

I prepared an electrolyte solution I found with White vinegar and 60/40 tin. I think it should be tin acetate...

For the plating and according to Faraday's Law and taking into account a valence of +2 for the tin, I should have a current density of about 7.8 A/ft^2.

When trying to regulate the current, changing the output voltage of the power supply I see bubbling over the copper board (as a test I use a board 5 cm x 3 cm). I see the board gets plated but a black cover forms over the Surface and even precipitates. And the plating is very thin (if I scratch it slightly with sandpaper the cover is removed).

I know that the bubbling is because of electrolysis of the solution so I performed a new test but regulating the current to avoid the bubbling. According to my calculations for tinning the area that I use is 120 mA. In this new case I performed the plating with 10 mA.

I don't know if what I'm doing is correct. wWhat do I have to do to obtain a greater thickness of tin over the board? More time?

The board has to be cleaned from time to time because of the black layer deposited?

The electrolyte solution I prepared is right, or what do you recommend to use for the plating I want to perform?

I've never done this before and I'm learning.

Thanks in advance

Carlos Martin
- Argentina


November 16, 2018

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.

Brendan McNamara
- Rochester, New York, USA


November 20, 2018

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"

geoff smith
Geoff Smith
Hampshire, England



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