Used to be the preferred etchent for tin tin-lead plated boards but has been replaced by Ammoniacal and Sulfuric Peroxide etchents. Chromic-Sulphuric etchent has only a slow etch rate and low copper holding capacity. The solution is also a serious pollution concern and very expensive to dispose off. Normal copper etching reaction is:-

3Cu + 2HCrO₄^- + 14H⁺ >> 3Cu²⁺ 2Cr³⁺ + 8H₂O

The hydrogen ions are normally provided by Sulphuric acid, beware Hydrochloric acid must not be used as toxic chlorine gas would be liberated.

Std makeup with the addition of sodium sulfate which increases the etch rate is, 240g/l  Chromic Acid, 40g/l Sodium Sulfate, 180g/l 96% Sulphuric Acid. Oddly enough air agitation reduces etch rate.