r&dBREAKING DOWN TECH —PRESENT AND FUTURE
62 MAXIMUMPC JUNE 2007
A
ustrian inventor Paul Eisler invented the
printed circuit while living in the United
Kingdom in 1943, but there was almost
no demand for the technology—which
was more expensive but less reliable than
the existing alternative—until the U.S.
Army adopted it for use in proximity fuses
in the antiaircraft shells used to counter
Germany’s V1 rockets.
Prior to Eisler’s invention, electronic cir-
cuits were wired point-to-point, meaning that
each component was mounted to a heat-
resistant material—such as Bakelite—and
connected with copper wires and solder (a
process that is nearly impossible to automate).
Although educated as an engineer, one of
Eisler’s fi rst jobs was as a printer. Combining
his knowledge of these two disciplines, he
came up with the idea of using an etch-resis-
tant ink to print a circuit pattern onto copper
foil bonded to an insulating substrate. When
the substrate is immersed in an etching chem-
ical, the excess copper is removed and only
the desired circuit pattern remains.
The problem with Eisler’s idea was that
foil manufacturers were largely uninterested
in producing copper foil that was suffi cient-
ly thin for the job. And when that obstacle
was fi nally overcome, it took several more
years to develop the proper adhesives for
bonding the foil to the insulating base.
Although the United States issued a 1948
dictate that all airborne equipment must
use printed circuits, the technology didn’t
begin to offer signifi cant technical and price
advantages over the hand-wired competi-tion until the late 1950s.
The breakthrough came with Moe
Abramson’s and Stanislaus F. Danko’s
development of the Auto-Sembly process,
which they created while working for the
U.S. Army Signal Corps; they fi led for their
patent in 1950, and it was granted six years
later. The Auto-Sembly process inserted
component leads directly into the cop-
per foil interconnection pattern and then
dip-soldered the entire board, eliminating
the need to drill holes in the PCB for each
component’s wire leads. The dip-soldering
process immerses the PCB in molten solder,
but the solder adheres only to surfaces that
are not protected by a solder mask (typically,
a green-tinted liquid epoxy that lends PCBs
their characteristic color).SAME AS IT EVER WAS
The PCB-manufacturing process has
changed relatively little since then,
although three “subtractive” processes
have evolved to remove the copper to
reveal the fi nal circuit layout. In silk-screen
printing, a layer of copper is glued to a
fi brous substrate, a circuit mask is drawn
on the copper using an etch-resistant ink,
and the excess copper is removed in a
chemical bath (ferric chloride and ammo-
nium persulfate being two examples). The
second alternative, photoengraving, uses
a computer and a laser printer to produce
an etch-resistant photomask that is glued
to the copper foil prior to etching. The fi nalsubtractive method uses a mechanical
milling machine to cut the excess copper
foil from the substrate, much like a plotter
would draw a schematic.
Complex PCBs are created by laminat-
ing thin, separately etched, conductive
boards. Such multilayer boards have cir-
cuit traces inside the PCB, not just on the
top or bottom of the board. This enables
electrons to travel on and through multiple
layers within the PCB, vastly increasing the
board’s capacity for electrical traffi c without
increasing its surface area at all—much like
an urban subway increases a city’s capacity
for moving people around without clogging
its streets with traffi c. In some designs, an
entire layer is used to distribute power while
another acts as a ground plane.
Drilled holes, known as vias, connect
the copper tracks from one layer of the
board to another. A complex multilayer
PCB might have both blind vias (which
are visible on only one side of the fi nished
board) and buried vias (which aren’t visible
at all when the board is assembled). A via
is typically either electroplated or has a
small rivet inserted into it.PCB LAYOUT AND
POPULATING
When designing a PCB, great attention is
paid to the pathways or signal traces that
the electrons will follow as they move from
one component to the next. If the traces are
placed too close together, excess solder canWhite Paper: Printed Circuit Boards
Some printed circuit boards have components mounted on both sides, while multilayer boards have traces running
through the middle of the board. In these cases, a via is drilled to establish an electrical connection between the layers.HOW IT WORKS
BURIED VIAIt’s easy to take printed Traces on a multilayer PCB
circuit boards for granted—
we see them everywhere.
But these seemingly simple
devices are quite remark-
able works of engineering.
BY MICHAEL BROWNBLIND VIAPLATED THROUGH-HOLE BUILDUP LAYER