Probably the main etchent used within the circuit board industry. Due to it's very smelly nature it is only used in spray etching applications.  Normal copper etching reaction is:-

Initial reaction, which results in a build up of cuprous ions,

Cu²⁺  + Cu^O  >>  2Cu⁺

Secondary reaction, oxygen is absorbed from the air which re-oxidizes the cuprous ions back to cupric ions,

2Cu⁺ + 1/2O₂ + H₂O >>  2Cu²⁺ + 2OH

The basic etching reaction is the same as cupric chloride etching but to work the copper (II) ions require complexing with ammonium chloride and ammonia.

To work free ammonia is present, which always finds it's way past any extraction equipment resulting in strong odors. Ammonia loss results in sludging, as do excess water or excess copper. Its an almost trouble free process when correctly run but can result in hours of machine cleaning if not.