The process begins with your standard CAD-layout and drilling files in Gerber,
HP-GL, Excellon, or one of several other
formats. You import these files to your
Windows-based PC, which runs the prototyping
machine, and use vendor-supplied
software, which has two roles. First,
it converts the CAD file to a milling file,
which guides the spindle head’s X-, Y-,
and Z-axis motion. Second, it lets you
specify some factors that the initial plot
does not. After all, a physical layout is
more than just a simple interpretation of
the connected points of a schematic diagram.
Nearly every physical-board layout
needs some intervention in which you
include rules for insulation spacing,
ground and isolation paths, and the
amount of copper that you must remove
from certain areas (Figure 1). Additionally,
many RF and wireless designs now
use the pc board’s copper as an antenna,
a filter, or a stripline element, which requires
complex shapes or carefully sized
and shaped traces and spaces.
Once you adjust the board’s layout and
set some parameters, you’re almost ready
to go. You set your blank board on the
machine’s work surface and register it in
place via pins that project from the surface.
These pins not only define the
board’s location for the top-surface pass,
but also ensure either front-to-back
alignment when you turn the board over
for a two-sided board or layer-to-layer
alignment for a multilayer board.
The high-speed spindle mills away undesired
copper. Unfortunately, a single
milling, drilling, or routing tool in most cases cannot handle every aspect of your
board (Figure 2). The machine signals
that you need to change tool bits, which
takes a few seconds; the good news is that
the software that drives the machine optimizes
its routine so that you usually
need to change each bit only once per
board. Some of the more expensive machine
models even eliminate this human
intervention by interfacing with optional
multistation tool holders and changers
so that you can walk away during the entire
operation.
Typical double-sided boards take
about one to two hours to complete.
These machines are precise and repeatable
and can produce pc layouts compatible
with today’s requirements: 0.1- to
0.2-mm (4- to 8-mil) minimum track width and track spacing and holes as
small as 0.2 to 0.3 mm (8 to 12 mil), depending
on the model. Don’t worry that
you can fabricate only small boards, either.
Depending on the model, you can
load a blank board as large as 42338 cm
(16.5315 in.) in the LPKF unit and
40361 cm (16324 in.) in the larger of
the T-Tech units, and you can mill to
within a few centimeters of the edge.
When you finish the circuit milling
and drilling, the spindle head and appropriate
tool finishes the job by routing
any internal cutouts you have and then
routing the board outline so you can remove
your finished board. (This outline
does not have to be rectangular; it can
have curves and card edges.) As a nice
touch, if your pc board is small, you can specify—via the software—that you
want several boards laid out and fabricated
side by side, so you have more than
one bare board. You can then load one
board with components and use the unloaded
one for signal tracing or as a debugging
guide.
Most pc boards have plated throughholes
to conduct signals, power, or heat
either from the top layer to other layers
or between inner layers.With these pcprototyping
systems, you can achieve
through-hole conductivity in several
ways, depending on the number of holes
you have to plate through. You can press
small eyelets into the holes (the open eyelet
can also accept a component lead); insert
“via” pins that completely fill the
hole; automatically dispense a conductive,
solderable paste that bonds to the
layers; or use a completely closed chemical
plating system that vendors offer,
which emulates conventional plating but
with minimal chemical hazard. |
