In The Trenches
Finally, not all printers have precise
paper-handling mechanisms. That is,
the scale may not be exactly 1:1. A
2.5% scale error, which is not noticeable
for printing, will cause a quarter inch
size error in a 10 inch PCB. This is
enough to foul up PCB mounting, cause
problems with large PCB connectors,
and create large headaches. (I know
this from experience.)
There is also a technique that
prints the pattern directly onto thin,
flexible, copper sheets that feed into
your printer. Then, you glue this to a
base board and etch it. This eliminates
the ironing step. (I haven’t tried it.)
The technique that I prefer,
unfortunately, is no longer generally
available. It uses a pen-plotter to draw
a special resist-ink directly onto the
bare copper PCB. Then you etch it.
It’s accurate to the precision of the
plotter (0.001” resolution for my
plotter). Both the plotters and the ink
are now very hard to get.
The last method is mentioned
only for completeness; it can’t be used
by hobbyists. It’s called “Multiwire”
from Advanced Interconnection
Technology ( www.ait-atlanta.com).
They don’t use copper PC boards at
all. They glue/embed thin, insulated
wires into the base material. Because
the wires are insulated, they can cross
over each other without shorting. To
make an electrical connection, a hole
is drilled that cuts through the wire
and exposes the copper end. Then,
the hole is plated, which electrically
connects the end of the wire to the
metalized hole and the rest of the circuit.
This method is very reliable and is
used in some military applications.
Basic PCB Layout
For me, the first step in the physical
layout of a PCB is a good schematic.
With the exception of glue logic
(discrete logic gates) and multiple
opamp packages, I arrange the IC
pins in physical order, as viewed from
the top. I usually start with the IC that
has the most pins and then place the
other parts to create a visually simple
schematic (if practical). This means
only a few crossed lines. It’s always
easier to lay out a PCB with fewer
crossed traces. This also creates a
physical relationship between the
schematic and the actual board,
which is useful in troubleshooting.
I always place components and
pads on a 0.100” grid, if possible.
This makes calculations between the
parts and case easy to figure.
Basically, it’s convenient. I also
immediately identify the top and
bottom layers with text “component
side” and “solder side.” The “solder
side” text is mirrored (reversed left to
right) because I see that layer from
the top with my layout software. All
software packages I know edit the
bottom layer, as viewed from the top.
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