In The Trenches
PCB board houses always want some identification of
top and bottom layers. Otherwise, things really get fouled
up. In the past, optical “targets” were added to aid in
registration between sides and to provide focusing for the
optics. More recently, this has been omitted. Round pads
are standard. However, often pin 1 of an IC is identified with
a square pad.
The wider the traces and farther apart they are, the
easier the board is to manufacture, the more reliable it will
be, and the better its performance will be. Most PCB
manufacturers can work with 0.007” traces and spaces. I
use 0.020” traces and 0.015” traces when I can. This
allows one trace between two DIP IC pins, which have a
0.050” pad. Trying to put two thin traces between 0.100”
DIP pins always seems to lead to problems.
Thin traces are more fragile and have higher resistance
than thick wires. They also handle less current. Here’s how
to estimate how much resistance it has and how much
current a trace can handle: Remember that traces are
about 0.003” thick for a standard PCB. So, you can
calculate the cross-section area by multiplying by the trace
width. A 0.020” wide trace has about a 0.060 square inch
cross section area. Look this up in a wire-table and you will
find that this is close to #32 gauge wire, which has a 0.063
square inch cross section. The table says that #32 wire has
a resistance of 0.17 Ω per foot and can carry 90 mA ( The
Radio Amateur’s Handbook, 1966).
These values are reasonably linear for different trace
cross section areas. (Twice the area is half the resistance
and twice the current, etc.) So, a 0.007” wide trace that has
a 0.021 square inch cross section area can handle 30 mA
and has 0.51 Ω/foot resistance. (Note: These current-carrying
values are very conservative, which results in negligible
heat generation. A 0.020” trace carrying about 2,000 mA
causes a 10° C or 18° F increase in temperature.)
Traces that are close together are easier to short out
and can electrically couple to each other. The traces act as
antennas and capacitors. High speed digital lines next to
high impedance analog lines are going to cause problems
every time. Long traces are just as bad as long wires. They
pick up noise and have appreciable impedance. Remember,
even though your digital design only runs at 1 MHz, the
state changes can cause transients into the GHz range.
Don’t use acute angles in traces (more than 90°). This
leaves a point that causes problems. First, it nearly acts
like a one-turn inductor. More importantly, the point is not
well supported and can easily lift. Obviously, it’s not good
to have your traces peeling off the PCB.
Using two 45° angles instead of one 90° angle will save
space. This is especially true when you have a lot of traces
running parallel to each other. The more space you can
save, the smaller and less expensive the PCB will be. Don’t
forget — you pay for the PCB by the square inch (mostly).
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