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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. |
