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Etching problems with PCB tin resist

Q. Was anyone able to use the phosphoric Acid to prevent this blacking of the Tin? My own personal testing has shown that in sodium persulphate the ability to prevent blackening of the Tin is inconsistent and also has random areas where copper won't etch due to ablation of the Tin onto the area during the etch (speculation only on that point).

Chris Thornton
- plymouth [Florida]
July 15, 2022

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I am interested in building PCBs using pattern plating method. I would like to know if it's possible to use pure tin plate as etch resist for doing pattern plated boards. In the PCB industry it seems common to use lead/tin alloy for the plating resist. In a test I found that pure tin plate will dissolve in an ammonium persulphate etch bath. Tin/lead alloy (40/60 solder) did not dissolve but rather formed a thin black coating. In the "Printed Circuit Handbook" by Clyde F. Coombs, Jr. it comments on how additives are needed to prevent solder plate blackening, thus agreeing with my test. The closest information I've found is the addition of phosphoric acid to ammonium persulphate. Before I go out and buy drum of H3PO4 (minimum order qty) I would like to first get opinions that it works.

Adam Seychell
- Melbourne, Australia
2000

A. Tin is commonly used as an etch resist in commercial PWB manufacturing. The use of 60/40 solder was common when the etch resist was reflowed to preserve solderability, but with the advent of HASL and other final board finishes, its use has diminished.

As far as buying a drum of H3PO3 is concerned, try a lab supply house for smaller quantities instead of an industrial chemical firm. You'll pay a higher price/unit volume, but you won't have as much money tied up in feedstock.

James Totter, CEF
- Tallahassee, Florida
2000