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Sodium, Ammonium, Potassium Persulfates all make good copper etches. They can even have catalysts added which then allow them to be used with tin tin-lead resists. When the salts are dissolved in water the Persulfate ion is formed, which then goes on to oxidize copper to the cupric ion. Std Reaction is :-

Cu +(NH₄)₂S₂O₈ >> CuSO₄ + (NH₄)₂S₂O₄

Std makeup is 100-200g/l of Persulfate salt, 20ml/l 96% Sulphuric Acid. A mixed solution becomes exhausted in 3-5 days independent of usage.

Hydrochloric acid must not be used as toxic chlorine gas would be liberated. Once done the use of the wrong acid is never repeated.

This etch is commonly used as a micro etch (chemical sandpaper) before plating processes.