PCB BASICS: From Your
Brain to a Finished Board
“ HB OA WS I CT SO ”
by Fred Eady
You’ve seen me do it most every month in Nuts & Volts and
SERVO. I make them in two-layer form and I make them in
four-layer form. I make them as prototypes. I make them as
production quality. I identify them with silkscreen legends. I
ruggedize them with soldermask. I make them large and I make
them small. What is this thing I do that you see in Design Cycle and
SERVO every month? The answer is easy. I create printed circuit boards.
When you have finished pouring over a Nuts & Volts
Design Cycle project or one of my SERVO articles,
you have the option of having the project’s
printed circuit board (PCB) manufactured from the
ExpressPCB PCB design files that I provide via the Nuts
& Volts and SERVO websites. In most cases, that is a good
thing, as you don’t have to recreate the design work that
I have already generated for the particular project.
However, you may come up with a design idea that you
would like to put down onto a custom PCB of your own.
Making that happen is not as difficult as you might think.
To get the most from this discussion, you’ll need to be
able to run the ExpressPCB application. If you don’t
already have a copy of it, you can get it free via download
from the ExpressPCB site ( www.expresspcb.com). With a
little help from our friends at ExpressPCB, let’s cover all of
SCREENSHOT 1. My 0.050 inch grid spacing is not golden.
You may set the grid spacing to any value that you wish.
The ExpressSCH application has some very useful options
that can help you get a schematic drawn quickly. I suggest
reading the ExpressSCH quick start documentation to get
an idea of what ExpressSCH can do.
74
June 2008
the steps that are necessary to create a perfect, professional
PCB from scratch.
Do You Have a Schematic??
If not, create one before you start designing your
custom PCB. In addition to providing a road map for your
board layout to follow, capturing a schematic on paper or
electronically will force you to call out the parts you will
be using in the design.
When creating your schematic, be sure to label each
component (R1, R2, C1, C2, etc.). Odds are your design
will contain a larger number of resistors and capacitors
than integrated circuits or microcontrollers. Being able to
cross-reference a resistor in the schematic to a resistor
you have likewise labeled on the PCB will help you avoid
routing mistakes in your layout.
There is no need to spend any money on a schematic
capture program if you use ExpressPCB as your layout
tool. ExpressPCB comes bundled with an easy to use
schematic creation program called ExpressSCH that ties
into the ExpressPCB PCB layout application. ExpressSCH
includes a multitude of predefined standard components
that are ready to add to your schematic. If you have
downloaded and installed the ExpressPCB application as I
requested earlier, you already have a copy of ExpressSCH.
When the schematic is complete, the first thing you
should do before sitting down to generate your PCB
design file is to obtain all of the electronic and mechanical
components you will be mounting on your board. It is a
good practice to have the actual component in hand
when laying down a pad pattern for the part. If your
project doesn’t require any exotic electronic components,
you can save lots of time by selecting your electrical and
mechanical components based on the built-in pad layouts
found within the ExpressPCB layout application. For
example, the pad layout for the six-pin RJ- 11 female
connector that mates with the Microchip “hockey puck”
(the round MPLAB In-Circuit Debugger/Programmer
module) cable is already defined all the way down to
the Digi-Key part number in the ExpressPCB design
application. If your design calls for a part that is not in the
