An old favorite, also very good at staining fingers, clothing, etc brown. Etch rate can be very high but is dependant on solution movement over the surface of the board and temperature. At 70C using Spray etching 1oz copper is removed in a little under a minute, normal etching temperature is more likely to be 45C. When etching circuits if up to 5% of HCL is added it, increases etch rate, helps to stop staining, and reduces the risk of the solution sludging. Ferric especially with extra HCL makes a very good stainless steel etchent.
When Ferric crystals are mixed with water some free HCL produced through hydrolysis.
FeCl3 + 3H2O > Fe(OH)3 + 3HCL
The basic etching reaction takes place in 3 stages. First the ferric ion oxidizes copper to cuprous chloride, which is then further oxidized to cupric chloride.
FeCl3 + Cu > FeCl2 + CuCl
FeCl3 + CuCl > FeCl2 + CuCl2
As the cupric chloride builds up at further reaction takes place,
CuCl2 + Cu > 2CuCl
The etch rate quickly falls off after about 17oz/gallon (100g/l of copper has been etched. For a typical solution containing 5.3lb/gallon (530g/l) of ferric chloride.