38 Silicon chip Australia’s electronics magazine siliconchip.com.au
available for removing copper, and it’s
now even possible to print a PCB using
conductive ink, allowing the wiring to
be ‘drawn’ directly onto a substrate.
That really is a printed circuit board!
PCB etching
You might not think that PCB etch-
ing has changed much over time; in-
deed, the basic chemical technology is
very much established and is still the
primary method of commercial PCB
manufacturers.
What has changed is the genera-
tion of the etch-resist layer, with some
clever people using novel techniques.
If you have etched your own boards,
you will have heard of ammonium per-
sulphate and ferric chloride. But many
board manufacturers use cupric chlo-
ride (green in solution) to etch their
boards instead.
When cupric chloride (CuCl 2 ) reacts
with copper, it turns into cuprous chlo-
ride (CuCl). These two compounds
both contain only copper and chlo-
rine, the difference being the ‘oxida-
tion state’ of the copper atoms.
The beauty of this method is that the
cuprous chloride (CuCl) can be turned
back into cupric chloride (CuCl 2 ) by
an oxidising agent.
This oxidising agent can be some-
thing as simple as oxygen from the air
we breathe.
Of course, the chemistry is not that
simple, and there needs to be a sup-
ply of chlorine atoms to supplement
the copper atoms that are being added,
although this can come in the form of
hydrochloric acid.
The result is an etchant that not
only doesn’t get used up; it becomes
self-generating.
There are downsides, of course.
Cupric chloride is nasty stuff, and is
worse for the environment than ferric
chloride if released, which makes it
difficult for hobbyists to use, particu-
larly if the amount of cupric chloride
keeps increasing.
That said, the actual etching works
similarly to that of ferric chloride,
with agitation and heat accelerating
the process.
Ammonium persulphate is similar,
but has the advantage that it doesn’t
stain anywhere near as much as fer-
ric chloride. It has been said that if
you walk within five metres of a fer-
ric chloride bath, it will jump the gap
and stain your clothes. A slight exag-
geration perhaps, but...
Toner transfer etch-resist
process
If you have access to a laser printer,
toner transfer is one of the best etch-
resist methods for a hobbyist.
While some toner transfer kits can
be expensive, cheaper versions are
available online. They aren’t as good,
but they can be made to work.
A PCB design is printed onto the
glossy side of the toner transfer paper
using a laser printer. It must be mir-
rored, as the transfer process mirrors
the design a second time, so it ends up
the right way around. The toner itself
becomes the resist layer.
For this to work, the copper clad
board must be spotlessly clean. Even
fingerprints can impede the etching
process.
The toner transfer paper is pressed
against the copper cladding, and heat
is applied. This can come from a
clothes iron or even a laminating ma-
chine, although it appears some lam-
inators can’t reach the temperaturesneeded to transfer the toner.
After the board cools, the toner
transfer paper is carefully peeled back,
leaving the toner attached to the cop-
per clad board, which can then be
etched.
The copper under the toner will re-
main intact, as long as it isn’t left in
the etchant too long.
You can also use this method to pro-
duce a ‘silkscreen’ layer by applying
the toner to a pre-etched board.
Incidentally, we’ve used the “ton-
er transfer” method to produce a
PCB using ordinary bond paper (ie,
from a photocopier or laser printer).
It takes quite a few attempts to get it
right and importantly, the track spac-
ing and gaps cannot be very fine. But
it does work fairly well and is a great
method for the hobbyist to try. (See
siliconchip.com.au/Article/6884).Photochemical resist processes
This involves a chemical which re-
acts to light, where the areas exposedFig.8: a PCB which was produced
from a bare copper laminate board
using a milling machine. A conical
milling bit is normally used, as
the copper and fibreglass are
fairly tough and you
want to cut a V-shaped
groove. The main
difficulty in doing a
job as good as this is
ensuring that the
PCB is perfectly flat, and perfectly
aligned with the bed of the mill.Fig.9: a screen grab of the FlatCam software which can convert Gerber files into
G-code which can then be fed to a milling machine, laser cutter or other CNC
equipment.