36 Silicon chip Australia’s electronics magazine siliconchip.com.au
of the manufacturers’ design rules
slightly if you are willing to accept a
higher percentage of faulty boards. Or
they may charge you extra for the more
involved processes required to manu-
facture your boards correctly.
Before we look at actual specs, let’s
get a “trap for young players” out of
the way! Track thickness and track
gaps are generally specified in “mils”.
A mil is not an abbreviation for mil-
limetre! 1mil equals one thousandth
of an inch, so a track width specified
as 12mil will be 12 thousandths of an
inch wide – about 0.3mm. Many peo-
ple have been caught over the years –
now you shouldn’t be!
Most EDA software will naturally
work in mils, although some have the
ability to work to other standards. A
tip: stick with mils, because that’s what
PCB manufacturers are expecting.
A typical rule is that copper tracks
should be no less than 6mil (six thou-
sandths of an inch or around 0.15mm)
wide and no less than 6mil apart. An-
other common rule is that the drilled
holes should be no less than 12mil in
diameter (0.3mm).
If you were to place tracks 5mil
(0.13mm) apart, they might still make
your board, but you may find that some
boards have short circuits between ad-
jacent tracks. Or they may just reject it.
You should ideally set up the design
rules before starting to lay out your
PCB, although, as a general guide, if
your board is easily hand-solderable
and you aren’t after any special board
you should be able to have your board
made by many different companies,
perhaps with some slight tweaking to
suit the stricter ones.
Most EDA programs offer automat-
ic design rule checking, so it’s worth
entering the manufacturers’ rules into
your EDA program. It will then alert
you to any violations, so you can fix
them. Some PCB manufacturers offer
downloadable design rule files that
can be imported into your EDA pro-
gram directly.
CircuitMaker’s design rules can be
accessed from the “View → Rules and
Violations” menu, which opens the
dialog shown in Fig.5. The minimum
width and clearance constraints cor-
respond to the trace width and separa-
tion noted above. Our choice of 10mil
should be achievable by most board
manufacturers (see panel!).Order process
As mentioned above, some manu-
facturers take orders via e-mail. So,
for example, if you want to order some
boards from Sydney-based LD Elec-
tronics, e-mail your zipped GerberFig.5: here is where you can enter
the manufacturers’ requirements
in CircuitMaker so that it can check
there are no violations.
For example, the Clearance rule is set to 10mil in all cases, so it will ensure
that there is a minimum of 0.01in (0.254mm) between adjacent conductors.
Generally, you only need to make a few small changes to the default rules to suit
typical manufacturers.
Fig.6: like many PCB manufacturers, PCBCart gives you an instant quote once
you have put in your PCB’s particulars. You can then log in, add the design to
your cart, upload the Gerber files, fill in your details (eg, delivery address) and
pay for the order. They’ll start manufacturing your PCBs once your order has
been submitted and will normally send you updates, and eventually a courier
tracking number, via e-mail.finishes or colours, virtually any man-
ufacturer should be able to make your
board.
Since most manufacturers have sim-
ilar rules, once you have set them up,