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Copper plate + tin plate without rinse
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I run a small printed circuit board shop producing prototypes and one offs. Recently I have been questioning the possibility of removing the rinse stage between the copper plating and tin plating stages. Both these baths contain sulfuric acid and divalent metal. I am curious of how tolerant the tin bath would be to copper sulfate drag in, since copper sulfate is the only foreign species that would be introduced. As I understand copper ions become reduced when in contact with tin metal and will therefore precipitate out of solution. A few quick tests showed that if the panel is removed directly from copper into tin plating baths the tin deposits produce dark patches for first 10-20 seconds and then disappear completely appearing as normal finish. The main problem I am unsure of is possibility of copper and tin codepositing. This may harm the tin plate resistance in the copper etching stage. I'd be interested in knowing if there has been any attempt to purposely skip rinses between two plating stages and rely on the second bath to react with drag in from the first bath ?
Adam Seychell- Melbourne, Australia
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Don't do it! The copper will contaminate the bath. You will either end up dummying it out, or building a new bath.
Phil Pace- Tulsa, Oklahoma
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I'm going to assume that the Sn is being used as an etch resist in this case. Initially, you won't have a big problem, but as the Cu concentration rises (and the relative porportion of Cu to Sn in the deposit), it will interfere with the etch. Etchants are designed to not dissolve Cu. Also, you might wind up with a copper/tin intermetallic on the surface which is not solderable.
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James Totter, CEF - Tallahassee, Florida |