Color of tin plating bath; problem with acid tin brightener

A. Enthone, for one company, sells a material to treat their tin solution. But our cost analysis showed it was cheaper to generate a new tank solution than treat the old one; less down time and pain also.

A. Hoba-

Just because the bath changed color does not mean you have a problem. Acid tin baths do discolor as you use them. Is the tin deposit still giving you good results? What is being told to you is a clarification chemical? A bath three months old should not require this treatment. Do you filter the bath constantly? You should have a chiller to maintain temperature no more than 65-70 °F.

You should also be using sulfuric acid at 10% as your acid after the cleaning step. Chlorides can kill acid tin. If you are using muriatic acid, that's not good.

A. I disagree in part with Mr. Hemp. The color change your bath is experiencing is normal due to the oxidation of the tin in the solution from stannous to stannic. Generally, if you wish to use color as a guide to when a bath should be discarded, or cut or "clarified" (floc'd) the color to look for is a dark "coffee with cream". At this point you will find that you are having difficulty maintaining brightness despite additions of additives. This would be different from chloride contamination which regardless of color will result in a dull or etched finish despite treatments or addition agents.

As to temperature, some brightener systems enable operation at temperatures up to 95 °F. Ideally, the trade off between conductivity, chilling cost and brightener cost is about 80 to 85 °F with modern brightener systems.

A. Dear Sir,

The problem of your bath becoming yellow is in fact an oxidation to metastanic salts - It should not be a problem to plate from an electrolyte in that stage providing you are depositing from Sn+2. The presence of such by-product in large amounts causes premature yellow deposits and sometimes problems with solderability.

You may reduce the presence of the metastanic salts in your bath by filtering from time to time with "granular" activated carbon "not powder", the same type you use in water softening equipment or deionizers. It depends on what kind of brightener system you are using, the bath will be able to work at around 40 °C !

The use of Clarifiers or polyelectrolytes designed specifically for that purpose by vendors of processes is also an alternative; however, you should bear in mind that the amount of sludge produced by this treatment is tremendous and carries along a major portion of the good brightener as well as some Sn+2 from your bath.

A. Hi, David. Sorry, I can't quite answer your question. However, other people have reported colors with their tin baths which may either give your assurance that nothing is seriously wrong or get you on track to understanding it.

So we appended your inquiry to a thread where a "dark coffee with cream" color is described as the point where the bath must be discarded or fixed, and that description sounds pretty similar to your "gray-brown ... chocolate milk shake". Good luck!

Regards,