In The Trenches
Ground and Power
Proper grounding is critical. It’s a
topic that books have been written
on. Obviously, I can only touch on the
subject here. A ground and power
plane are ideal, but often it’s simply
too expensive (it doubles the cost of
I usually start with a wide ring
(0.100” to 0.250”) around the PCB
that is my ground. This allows me to
easily put large ground areas around
the PCB mounting holes (which are
typically near the edge of the PCB).
That means that a star washer and a
metal screw to the chassis make a
good chassis-ground connection. It
also means that ground traces only
have to go to any board edge. This
simplifies the layout. If there is any
concern about ground loops (more
than one ground current flowing in
one conductor), I simply cut the ring.
In this way, I can control where
and how the current flows in the
trace. If the design is fairly simple, I’ll
spend extra time and try to make it
single-sided and use the second side
as a ground plane. There are always
more connections to ground than to
any other circuit node. Properly
understanding how and where
ground current flows in your layout
cannot be overemphasized.
I use a similar ring technique for
power, but on the other side. I carefully
avoid the PCB mounting holes so that
power/ground shorts will not occur.
Running the traces back-to-back creates
a small capacitor. This helps to reduce
noise on the power line.
When I’ve finished the layout, I
increase the width of the power and
ground traces as much as I can. If
there are large areas without traces,
I’ll fill them with solid copper connected
to power or ground. More copper
often helps and very rarely hurts PCB
Surface Mount technology (SMT)
has added more facets to PCB layout.
The parts are very small and the lead
pitch (spacing between the centers of
adjacent leads) can be very small. It’s
usually not possible to run any traces
between the leads. This makes
routing more difficult and increases
the number of vias.
It’s virtually impossible to use a
single-side layout with high pin count
SMT ICs. The pad sizes and shapes
are different for different parts. You
will need to refer to the manufacturer
for these specifications. Testing and
repair need to be considered more
with SMT designs because it’s hard to
probe tiny, closely-spaced pins. Think
about how can you lay out the board
to make servicing easy.
Routing the PCB
The proper placement of traces is
something that is learned. The only
way to do this is to actually route PCBs.
A common and effective technique is to
run horizontal traces on one layer and
vertical traces on the other layer.
(Obviously, this doesn’t work for
single-layer boards.) This tends to
create boards with a lot of vias.
Don’t run parallel traces closer
than necessary. Segregate analog
and digital areas. Put ground traces
next to sensitive analog lines to act as
shields. Do the same for high-speed
clock lines to reduce EMI
(ElectroMagnetic Interference) and
crosstalk. Pay close attention to parts
placement. It’s amazing how this can
Do not use an auto-router, unless
it’s really smart (and expensive). The
typical, low-cost routers only connect
points together. They don’t consider
the length of the trace, which traces
are analog and which are digital,
which lines are sensitive, ground
loops, etc. As we’ve seen, subtle
points can make a significant
difference in how the PCB performs.
NUTS & VOLTS