34 Silicon chip Australia’s electronics magazine siliconchip.com.au
be lines inside the board itself, which
indicate the presence of slots (for ex-
ample, for isolation) and other cutouts.
These lines are traced by a rout-
ing machine to give the board its fi-
nal shape.
As this is done as the final step, any
slots defined here will not be cop-
per plated. If you need copper plated
slots, eg, to solder flat component pins
into, they are defined in the drill lay-
er, using something known as a “G85”
command. These are made before the
through-plating process is applied.
It may seem odd that the drill file
has a .TXT file extension, as if it is
a text file, but Gerber and Excellon
files are text-based; you can open any
of these files in a text editor program
like Notepad.
You will see a series of coordinates
and commands, which will look fa-
miliar if you are used to working with
CNC machinery.
The above is only a brief overview,
but should give you an idea of what
to expect when creating PCBs for your
own use. We won’t go into any detail
about creating Gerber files; if you are
using CircuitMaker, we explained how
to generate Gerber files in the January
2019 article. Other EDA programs will
have their own instructions on how to
export Gerber files.
Just make sure that you provide all
the required layers. In many cases,
exporting the drill (Excellon) file is a
separate step to producing the other
Gerber files, so don’t forget to do it!
And it’s always a good idea to check
the Gerber files before sending them
off for manufacture, as it’s quite com-
mon for some elements to be missing
or extra elements to be present. We’ll
explain how to do that shortly.Exporting PCBs as PDFs
You might have noticed that SILICON
CHIP has historically published our
PCBs as EPS or PDF files, a tradition
that we continue to this day, although
we now also offer commercially pro-
duced boards for virtually all of our
published designs.
The main reason for doing this is
that it’s easy to print such files at home
to produce a negative or positive maskthat can be transferred to a resist mask
for home etching. Both formats store
and preserve dimensions, which is
critical.
Some commercial manufacturers
may be able to make a PCB from such
files, but since they only describe the
copper layers, you need Gerber files to
have a proper board made with a sol-
der mask, holes drilled to the correct
sizes and so on.
These days, the PDFs we supply are
mainly useful so that you clearly can
see where tracks run on the board. Un-
less you really want to make boards
yourself, the commercial boards are
quite inexpensive considering the
high qualityHow to view Gerber files
We use the free open source pro-
gram “gerbv” to check and validate
our Gerber files.
It’s available for Windows and
Linux. The latest Windows version can
be downloaded from https://source-
forge.net/projects/gerbv/files/ and it
is available as a system-installed soft-
ware package for many Linux distri-
butions (eg, “sudo apt-get install ger-
bv” in Debian-based distributions like
Ubuntu).
As well as displaying Gerber files
and allowing you to view and manip-
ulate the layers, it also has the option
of exporting to PDF, which is handy
if you want to make PCBs using some
of the more hobbyist oriented tech-
niques.
But note that most versions of gerbv
do a poor job of exporting to PDF when
the PCB contains copper pours (large
areas of copper which are not to be
removed); these tend to get pixelated.
An up-and-coming version claims
to solve this.Fig.2: opening the files shown
in Fig.1 in ‘gerbv’ produces this
display. Colours are assigned
randomly to each layer, for example,
bottom layer copper is purple
and top layer copper is cyan.
Transparent rendering allows you
to see all the layers in full, even
where they overlap. The actual PCB
produced by this file is shown at
right.
Fig.3: the code on the white ‘silkscreen’ overlay of this board
(ringed in red), was added by the PCB manufacturer. It allows them to figure out
to whom to send this PCB after it has been cut out of the large panel that was
manufactured (known as “depanelisation”). Note how clean the tracks and pads
are, and how accurately the holes have been drilled on this low-cost board.