40 Silicon chip Australia’s electronics magazine siliconchip.com.au
to light change in chemical composi-
tion, allowing the unwanted parts of
the layer to be chemically removed,
leaving just the areas required to pro-
tect the copper underneath during the
etching process.
You usually print the copper pattern
as a mask on transparency film, then
place that mask on top of the photo-
chemical layer, which is attached to
the copper laminate. You then expose
it to UV light, either using a light box
or by exposing it to sunlight.
The resist layer is then treated in a
developing solution to remove the un-
desired parts of the resist mask, after
which the board is etched as it would
be for other resist types.
This is fairly close to the method
used in factories for PCB manufactur-
ing. It is vital to ensure that the resist
layer is not exposed to light unneces-
sarily, as this lessens the effectiveness
of the process.
Options for using a photochemi-
cal resist include both pre-sensitised
boards, films that can be laminated to
copper and even liquid photo-resist
that can be painted onto copper-clad
fibreglass. There are also options for
negative and positive resists.
A negative resist is one that hard-
ens where exposed to light, so the re-
maining etch resist layer corresponds
to clear spaces in the transparency;
the final PCB result is the negative of
what is printed onto the film.
With a positive resist, the areas
which are exposed to light are the ar-
eas which are then removed, and the
areas which were not exposed remain
to resist the etchant. Both options are
shown in Fig.7.
Again, there are variations on this
theme where a pattern printed onto
plain (bond) paper is used to expose
the PCB photoresist. It is important
that the PCB pattern is in contact with
the resist (ie, it is printed “wrong read-
ing”) so light scatter within the paper
is minimised.
Fairly obviously, exposure times are
rather significantly longer than when
using transparency film.Etch resist pens
Etch resist pens are typically used
to touch up or repair the resist layer
already applied to a board, where it
has not transferred or printed cor-
rectly. They are also sometimes used
to quickly sketch a very small PCB de-
sign by hand.
But they can also be used as part
a CAM (computer aided manufac-
ture) process. This involves the use
of FlatCam (http://flatcam.org/) and a
3D printer.
Rather than using FlatCam to mill
an isolation path, it can also be used
to trace a resist path using a pen. The
etch resist pen is attached to the head
of the 3D printer, and it is command-
ed to lay down a resist path by the G-
code that FlatCam generates.
It’s a marvellously simple method,
as it doesn’t require any permanent
changes to your 3D printer; the pen canbe held in place with a rubber band.
The difficulty is in converting the
Gerber files to an appropriate set of
commands to drive the 3D printer. The
best option we found is to use gerbv to
convert the Gerber file to .png graph-
ics, followed by using the http://svg-
2stl.com/ website to convert these to
an .stl file.
The .stl file can then be converted
by any 3D ‘slicer’ program to files that
can be printed on a 3D printer. You
need a custom ‘slicing’ profile for the
pen, so it can be lifted when moving
between points; many programs offer
a ‘lift between extrudes’ option, which
is suitable.
By the way, most etch-resist pens
work much better if the board is
‘baked’ before etching, to cure the re-
sist layer. This is also true of many
other methods, especially photo resist.Filament extrusion
We’ve also seen a similar method
but without even needing the pen;
a 3D printer can be used to extrude
plastic filament onto a blank copper
PCB, with the filament forming the
etch-resist layer.
Flexible filament appears to be the
best choice. This helps to prevent the
plastic from lifting off the PCB during
the etching process.Printing conductive material
If you have access to a 3D printer,
you can also consider directly print-
ing wiring using a conductive filament.
But note that conductive filaments are
not as good conductors as copper, so
this method is mainly for low-power
applications.
It’s also pretty much impossible to
tin the conductive filament; you need
to melt the component leads into the
filament. We’re not sure how perma-
nent the result is!
A typical 3D printer nozzle width
of 0.4mm corresponds to a minimum
track width of 16mil, so this method
isn’t capable of producing the fine de-
tails of other methods, and small SMD
footprints will be impossible.
But it appears that having a 3D
printer can still be a useful tool for
making PCBs.Voltera V-One PCB Printer
An extreme example of this is the
Voltera V-One PCB Printer, which can
not only produce double-sided PCBs
up to 127 x 104mm using proprietaryFig.10: the Voltera V-One can “print” a double-sided PCB up to 127 x 104mm.
It’s an expensive way to produce a board but when time is money...