Metal finishing Q&As since 1989
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SnPb plating solution gets turbid within a week
Our SnPb plating solution is getting turbid very fast (within a week). This is the 2nd time after we washed and cleaned all of the pipings and tanks. During circulation we were using 50% sodium hydroxide, 10% v/v and we have thorough cleaning and rinsing to remove the sodium hydroxide but the problem back again after a week. Any experience or idea about this problem?
Plating Engineer - Batam, Riau, Indonesia
2004
Is the turbidity causing any plating problem? Does analysis reveal any contaminants? In other words, do you have an actual problem or just an expectation that the solution should not get turbid? The reason I ask is that I have seen excellent deposits plated from cream-colored and coffee-with-cream-colored tin plating tanks.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004
The turbidity is probably SN+4. A part of the SN+2 in the solution turns into SN+4. Ted is right, this turbidity has no influence what so ever on the deposit's properties.
Sara Michaeli
Tel-Aviv-Yafo, Israel
2004
One possible cause for the turbidity is the oxidation of Sn(II) to Sn(IV), as is said above. The only time I encountered this was when I had a small air leak in the filters and when the solution was being passed through the pump it dragged in air through a poor seal. In my case, a 1,000 liter tank went from clear to milky overnight. Although having some stannic ions in the bath will not affect the deposit, if its concentration gets too high it will affect the plating efficiency as you are using twice the number of electrons to reduce stannic as you need for stannous.
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2004
Your filter pump may be causing part of this by literally beating the solution to death forming gas (air?)(foaming) inside the pump cavity. We used the solution until it got to a dark creme, almost yellow and at that point the plating efficiency was bad enough to replace the tank. Enthone used to sell a product to treat the tank, but it was slow and cost more than the cost of replacing the tank. Waste treatment bumped the cost of replacement up to a little over treating. Replacement of the tank was a lot quicker, less work and the more efficient way of doing it on our small tank.
James Watts- Navarre, Florida
2004
Trevor, you are wrong. Once the SN+2 turns into SN+4 in an acidic media, the solution becomes turbid because the SN+4 is not desolved any more in the solution. It means that the stannic does not participate in the plating reaction. It means that you do not need twice as many electrons, it is just out of the game.
Sara Michaeli
Tel-Aviv-Yafo, Israel
2004
My guess would be you are dragging water in with your parts that has enough halides in it to form insoluble precipitants.
If you can make up your pre-dip with DI water and change it often enough, the problem should disappear if this is the cause.
John Nelson- Leesburg, VA, US
2004
2004
Sara, I agree with you to a certain extent. Once Sn(II) has been oxidised and precipitated out as Sn(IV)in an acidic solution it will cause turbidity and will no longer take part in any electrochemical reaction. However, there will be a concentration equilibrium between the soluble and insoluble Sn(IV); this equilibrium will be dependent on the type of anions present. I think it would be a safe bet to assume the anion is fluoroborate, but speculation is a dangerous thing! Since there is an equilibrium, there will be a certain amount of Sn(IV) in solution and capable of being reduced to Sn(0) by four electrons. Furthermore, if the oxidation is caused by the unwanted ingress of oxygen, the Sn(II) will continue to be depleted as the Sn(IV) is produced and precipitated out. This will obviously cause a shortfall in the tin level in the electrolyte and the wrong alloy composiiton will start to be deposited. Since the bath will contain a fine suspension of (relatively) insoluble Sn(IV), it is quite possible that it could be co-deposited and therefore cause a non-homogenous deposit.
I would therefore suggest that the best way to ensure the white turbidity does not cause any problems is to detect its cause and eliminate it - then there will be no problem!
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
Dear Trevor,
Sn(IV) could be co-deposited and cause an in-homogonous deposit ? Do you have any data or chemical equation for me.
Thank you!
Jerry- Taiwan
2004
The Sn(IV) could be deposited as a composite coating in the form of insoluble Sn(OH)4 or SnO2, depending on the pH. My guess would be it is SnO2, as the pH will be quite low, even at the cathode. The Sn(IV) material, being insoluble, can be considered like any other insoluble material used in composites, such as PFTE, diamond, SiC etc. You will, of course, have to have the correct plating conditions to achieve this!
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2004
2006
This one happened in our process too. We listed all the possible cause and simulated it in our mini plating tank.
We found 3 things affects our turbidity problem, one is the filtration problem,solution is not circulating well.
2. Anode cleanliness, this one will exhibit Sn+4 deposit if the anode surface is coated with sludge(sn+4) and the last one is High Current supply, once high current is subjected to the solution H2 gas liberated as we all know, this one resulted to turbidity of the solution.
LF - Philippines
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