Hardware Autopsy
short-circuit two or more traces. But if the
traces are too far apart, the resulting board
might be too large to fi t its purpose or too
expensive to mass-produce. Traces also
exhibit impedance and can have undesired
traits—traces of certain lengths will even
pick up radio signals—so they can’t be too
long or too wide. Their width is typically
determined by the amount of current they’re
expected to carry, so power traces are usu-
ally much wider than signal traces.
Laying out electronic traffi c patterns
is only half the equation—mounting the
actual electronic components is the next
step. There are two main ways to accom-
plish the task: through-hole construction
and surface mount. In some cases, both
techniques are used.
In through-hole construction, the contact
wires (leads) on the component are passed
through holes drilled in the PCB and then
soldered to the traces. In surface-mount
construction, the part is soldered directly
to a solder pad on the board’s surface.
Surface-mount technology was developed in
the 1960s and was in wide use by the early
1990s. It’s the most common manufacturing
technique in use today because it allows for
much greater circuit density than through-
hole construction and because it is very well
suited to automated assembly.
Small surface-mount devices (SMDs) are
delivered to the assembly line on reels (long,
tapelike rolls of components), while larger
components are stored in plastic tubes or
trays. As the PCB moves down the line, a
paste consisting of solder particles and fl ux
is applied to the solder pads. Robotic devic-
es known as pick-and-place machines select
a component and set it in the appropriate
location. Once all the components have
been placed on the PCB, it moves down the
line to a soldering oven.
The temperature of the PCB and the
components is gradually raised until the
solder particles melt, bonding the leads to
the board. Maximum temperature—which is
determined by the least heat-tolerant com-
ponent on the PCB—is achieved in what is
known as the refl ow zone and is typically
maintained for less than 60 seconds. The
board is then cooled, allowing the solder to
resolidify. In many cases, an SMD is held to
the board only by its leads, which are sol-
dered to the PCB.
The last step in PCB manufacturing is to
apply a conformal coat to prevent oxidation,
corrosion, and short circuits caused by con-
densation. Manufacturers used wax in the
early days, but polyurethane, acrylic varnish,
and epoxy are more common now.r&dBREAKING DOWN TECH —PRESENT AND FUTURE
62 MAXIMUMPC JUNE 2007 JUNE 2007 MAXIMUMPC 63
White Paper: Printed Circuit Boards
LENS SHELL
This isn’t the
actual lens; it’s
just a protec-
tive shell with
an integrated
focus ring.USB Webcam
It wasn’t all that long ago that we were marveling at the wonder of 1MP digital cameras;
now, USB webcams offer that and much more.Any requests? What
hardware—new
or old—would
you like to see go
under Maximum
PC’s autopsy knife?
Email your sug-
gestions to input@
maximumpc.com.LENS SHELL
This isn’t the
actual lens; it’s
just a protec-
tive shell with
an integratedAny requests? What
hardware—new
or old—would
you like to see go
Maximum
autopsy knife?
Email your sug-
input@
maximumpc.com.CMOS IMAGE SENSOR
The lens focuses the
image on the image sen-
sor (this one has 1.3 million
pixels). The image sensor
converts light into elec-
trons and passes them to
a processor on the back
of the PCB. This camera
uses a Sonix Technology
SN9C2028AF, an all-in-
one design with interfaces
to the CMOS image sen-
sor, SDRAM, and USB;
a compression engine;
an LCD driver; and an
embedded 16-bit DSP.MULTIFUNCTION
BASE Like most
webcams, this one
has a base that
allows you to set it
atop a desk or CRT
monitor or clip it to
the edge of an LCD.MICROPHONE
A mic is an essential
component in a web-
cam—unless you enjoy
pantomime. This one
is monophonic, as you
might have guessed;
it interfaces with your
PC’s sound system
via USB.LENS Here’s the
webcam’s actual opti-
cal lens. This one is
a wide-angle model,
but its zoom capabil-
ity is strictly digital.