siliconchip.com.au Australia’s electronics magazine July 2019 33
Many PCB manufacturers can also
create PCBs with aluminium cores,
rather than fibreglass, which is used
in high-dissipation devices, like radar
systems and LED arrays. That’s because
aluminium conducts heat away from
parts much better than fibreglass.
Anatomy of a PCB
We covered the anatomy of a two-
layer PCB in our CircuitMaker article,
but it’s also possible to get four-layer
(or more) PCBs made at a reasonable
price. Here, we’ll explain a bit more
about how commercial operators make
PCBs, and how this changes with the
numbers of layers.
Whether the design has two or more
layers, the early stages are not too dis-
similar to the home etching process
you might have tried. It starts with a
sheet of fibreglass (the most common
type is called FR4) clad on both sides
with copper. A resist layer is applied to
match the desired copper pattern, and
the board is ‘etched’ by removing the
exposed copper with a chemical that
dissolves copper not covered by resist.
The board is then drilled (and any
slots to be plated are routed), but this
is about where the similarity ends.
A process for plating copper into the
holes is used to create vias (which con-
nect to the copper on both sides) and
other plated-through holes. Then, an
insulating solder mask layer is printed
onto both sides of the board, followed
by the silkscreen layer, which may
be on one or both sides. The exposed
copper is then coated with a protec-
tive layer of solder, or possibly silver
or gold plating.
Finally, the boards are ‘depanelised’
(ie, cut apart). Typically, several differ-
ent designs (or copies of the same de-
sign) are processed at the same time on
a large panel for efficiency (24in x 24in
[610mm x 610m] is a typical panel size),
so they need to be separated.
This is usually done by a CNC rout-
ing machine, which can also rout slots
and other shapes within the individual
boards too.
For a four-layer PCB, the inner layers
are etched as for a two-layer board, us-
ing a thinner core than the final prod-
uct. The outer layers of copper are then
laminated to the core using ‘pre-preg’,
which is actually uncured fibreglass
laminated with copper foil.
The outer layers of the PCB are then
etched. The later steps proceed as for
a two-layer board.
The four-layer technique can be ex-
tended to more layers as necessary, and
there are variations where two or more
two-layer boards can be sandwiched to
give a similar result.
In any case, to make a board, espe-
cially one with many layers, we need
information about what each layer will
look like.
For a typical two-layer board, this
amounts to six layers worth of infor-
mation: two copper layers, two solder
mask layers and two silkscreen layers
(each pair is for the top and bottom).
There also needs to be information
about the final board shape and the size
and location of the drill holes and slots,
making for a total of eight files. All this
information is typically exported from
your EDA program of choice.
On top of this will be information
such as how thick the finished board
will be and what thickness of copper is
used. Other features such as silkscreen
and solder mask colour can often be
specified too. These specifications are
usually made in a separate step, though.File formats
Practically all PCB manufacturers
will accept so-called ‘Gerber’ files for
the manufacture of PCB designs. It is
also called RS-274X.A single PCB design results in not
one, but rather multiple Gerber files,
usually packaged in a .zip archive.
We’ve emphasised the importance of
the layers because, in the standard
Gerber format, each layer is described
by a separate file. The file extension of
each file dictates what role it has. Fig.1
shows a typical set of Gerber files de-
scribing a single PCB.
From top to bottom, the layer names
refer to the bottom copper, bottom
overlay (silkscreen), bottom solder
mask, mechanical (board outline) lay-
er, top copper layer, top overlay, top
solder mask and drill file. The .zip file
describing the board simply contains
these eight files.
The drill file is in a slightly differ-
ent format to the other files, generally
known as “Excellon” format; it is simi-
lar to Gerber but not identical. This is
because the drill file was traditionally
used to control a CNC drilling/routing
machine, while the Gerber files were
originally intended to be used with
optical plotters that ‘exposed’ a light-
sensitive resist layer.
The Excellon file instructs the ma-
chine to select a particular bit, then use
that bit to drill at a series of locations,
while the other files contain an assort-
ment of shapes, such as rectangles and
circles, which are combined to create
the board pattern. These shapes are
called ‘apertures’.
They literally were used as aper-
tures for the optical plotters, but these
days, the resist is applied differently
and the Gerber files have simply be-
come a standard way to describe the
required patterns.
The Gerber files are now rendered
by a computer, but the photochemical
resist process survives, with the aper-
tures replaced by a single computer-
printed transparency.
The overlay and copper layers are
rendered positively. That means that
the Gerber file indicates where there
should be copper or “silkscreen” ink.
The solder mask is rendered nega-
tively, meaning the file dictates where
there are holes in the solder mask.
In other words, an empty copper file
would result in no copper on the
board, while an empty solder mask
file would result in the board being
covered in the solder mask.
The board outline layer is treated
differently again. It consists of a se-
ries of lines or arcs which dictate the
outline of the board. There may alsoFig.1: the eight Gerber files typically
required to manufacture a double-
sided PCB. In order, they are:
bottom layer copper, bottom overlay
(silkscreen), bottom solder mask
apertures, board outline/routing, top
layer copper, top overlay (silkscreen),
top solder mask apertures, drilled
holes and slots, and the zip package
which contains the above.