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sulfuric Acid Content Keeps Going Up

We do electroplating of tin on copper wire, using stannous sulfate and sulfuric acid.It is a continuous process and I get a very good coating and the coating is not flaky. on analysis of the bath the concentration of sulfuric acid increase to double and the Tin content in the bath reduces.I cannot understand why does this happen.Also there is foaming in the tank.

Asit Kumar
- Delhi, India
2000

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Hi Asit: Generally increasing acid and decreasing metal is indicative of low anode reaction efficiency, so instead of melting away the Tin anodes as Stannous(Sn2+) the anodes are using a higher than normal amount of the current generating acid through hydrolysis of water. This usually happens when using inert anodes like platinized Ti, or some other inert material emitting current.

But with Tin, since Tin has both a +2 (plateable Tin) valence state and a +4 (un-plateable Tin) valence state, an improper anode/cathode surface area combined with improper anode solution flow may generate this symptom. Stagnant solution flow around tin anodes can cause emitted tin+2 to further oxidize to tin+4. Too much current on the anode can cause the same plus generate acid.

I've always designed tin & solder baths with a mesh material separating anode and cathode chambers and left the anodes free in the bath without bags. Then pump from the anode chamber thru a bank of filters back into the cathode chamber. Also never use air/N2 agitation. Always use patterned spargers. Introduction of O2 in any form will precipitate the Tin. Feel free to contact me for add'l info. Dave SUNNYvale CA

Dave Kinghorn
Chemical Engineer
SUNNYvale, California
2000

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Could you explain a bit more detail how to increase Sn+2 Content in Sulfonic bath. Is it easy to chemically, electrolytically or mechanically add Sn+2 in the bath.

regards,

MOHD MISRI
plating - NS, Malaysia
2006

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