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Acid tin plating is rough

I am setting up an acid tin sulphate bath for the first time and having problems with dull ('burned') and rough deposits. The bath composition is; 11% by volume (200 g/l) sulfuric acid and 12 g/l (1.6 oz/gal) tin as stannous sulfate. The only additive used is 2 g/l of gelatin which I found necessary to minimize 'treeing' or crystal growth. The bath is not agitated nor am I using anodes bags. For my application only a matte finish of about 7 µm thickness is necessary. A small amount of roughness is acceptable also. The problem I'm having is hydrogen evolution when cathode current density gets above 1 A/dm^2 (about 0.5 V). The deposits are gray and quite rough after first few microns. If current density is reduced < 0.7 A/dm² (about 0.1 V) then no H₂ develops and deposits are smoother but do not initially cover the entire copper substrate. I suspect this is because at such low current densities the voltage is below the deposition potential of tin on copper. If I manually agitate! (even very slightly) the resistance decreases to about half and then returns back in about 15 seconds. When not agitating the current density is almost constant between 0.1 V and 0.4 V. Above 0.4 V hydrogen begins and current increases.

Do these symptoms sound normal for a bath like this ? Are additives essential for smooth finishes in acid tin sulphate bath ?

My other concern is solution becoming yellow a few days after fresh test bath makeup while sitting in open air. After several weeks it becomes almost white and cloudy with precipitate. I've read many texts on how stannous tin (Sn₂⁺) oxidizes to stannate tin (Sn₄⁺) with any free oxygen dissolved in the bath. I figure the only way to avoid this is to minimize air exposure. I'm interested to know if all acid tin sulphate baths need regular replenishment of stannous sulfate to correct these losses ?

Adam

Adam Seychell
- Australia (Melbourne)
2000

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We ran a acid tin bath , small for several years.

I can not imagine trying to run a tin bath without proprietary additives. It can be done, but you will do a lot of reading of old literature, old patents and endless hours of Hull Cell Testing.

We used both mechanical agitation and pump agitation thru a filter.

The yellow is a fact of life. There is at least one proprietary solution to fix it, but it cost as much as a new tank of solution and generated over half as much sludge.So, we replaced the tank when we could not make it work or we had a critical job.

Pardon my being ugly, but there is no excuse for not bagging your anodes. They will slough and you will get rough plate.

With better additives, your voltage will not be quite as critical.

James Watts
- Navarre, Florida
2000

This is one letter that graphically shows the importance of buying proprietary chemistry for a plating bath. Any of the large manufacturers/suppliers can help on this. If Gelatin alone worked on acid tin baths to secure a bright deposit, everyone would be using it.

The major manufacturers have far better systems available for this. You may check on Finishing.com for advertisers; or other periodicals. The better Tin Proprietary systems will give beautiful mirror bright deposits, if that is what you desire. Matte formulations are available on request from most suppliers.

Suggest consulting with your local supplier for the best advice. If you are still not happy, come on back to Finishing.com

Ed Budman [dec]
- Pennsylvania
With deep sadness we advise that our good friend Ed passed away Nov. 24, 2018
2000

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Adam, Mr Watts and Mr Budman are total correct .I would like to also add a little to this conversation.

One is with a Dull bath they say you do not need cooling but if you want any kind of consistency in your product you still need it, We ran a filters and cooling (in tank filters) with 50 or 100 micron just to keep solution agitated, You must like most plating solutions keep solution fresh at point of deposit. you don't want to add air agitation as it will build ?tin sulphate(white precipitate)?some is a necessary evil. Be care full if these items are to be soldered deposit must be thin. Proprietary Brightener and or components is a must and make sure they are use for your application

Chris Snyder
plater - Charlotte, North Carolina
2000

Hi Adam ,

Welcome to the wonderful world of electroplating , all acid tin solutions begin to discolour and go cloudy they eventually look like soup , Gelatin is a very poor additive in Acid Tin solutions , it needs to be added every day and sometimes twice a day , forget about it , go to one of the local suppliers (Enthone -OMI, Macdermid, Atotech, R O Hull & Co , Elite )and get the proper system , usually a 2 component system , you will need constant filtration thru a 50 micron filter and use the return as solution agitation together with cathode rod agitation , you will need to make additions of Stannous sulphate on a regular basis, and you will also need to bag your anodes.

Regards

John Tenison - Woods
- Victoria Australia
2000