Hi, The ASM International Handbook Vol. 9, 9th Edition on Metallography and Microstructures lists 21 different etchants for coppers, but states that the following etchant is by far the most widely used etchant:
20mL NH4OH (amonium hydroxide), 0-20mL H2O (water), 8-20mL 3% H202 (hydrogen peroxide) (Immersion or swabbing 1 min. H2O2 content varies with copper content of alloy to be etched. Use fresh H2O2 for best results.