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Fine solids formation in tin plating bath

2003

I work as a process eng. in a plant where we have an electrolytic acid copper and tin line for PCB based components. My question is addressed to know if the situation we are having in our tin bath is normal or not, even though we are not facing real problems with quality of plating. This is a stannous sulphate-acid bath and with the use we have found white-yellowish fine-solid residues formation on the tank. Although analysis show what expected from our supplier in means of Sn and acid Concentration and we do the corresponding additions to keep it, the appearance has changed from the original amber color of the make up. If we agitate the solution becomes "chocolate shake" like and after settling it turns back to its actual gray-brown color. Originally we had Sn nuggets on Ti baskets as our anodes, but we found out the hard way that Ti should not be used here (baskets were literally consumed). Now, I'm about to place tin bars to replace this anodes but before this I would like to know if the solid formation is normal (if so what this could be) and if not if I need to remove it before putting the new anodes or not. Then what are main characteristics to look into the bath, besides analysis and Hull cell that could help us to ensure that it is in normal conditions. Your comments will be appreciated. Regards,

David A [last name deleted for privacy by Editor]
electronics - San José,Costa Rica

Hi, David. I can't quite answer your question, and don't exactly understand your description of the bath being like a chocolate shake when agitated and gray-brown when settled. However, other people have reported strange color to their tin baths on other letters here, and they may either give your assurance that nothing is seriously wrong or get you on track to understanding it.

Please see letter 20394 where a "dark coffee with cream" color is described, and the situation sounds very similar to yours. Good luck!

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 31, 2008