Printed circuit board
A printed circuit board or PCB interconnects electronic components without
discrete wires. Alternative names are printed wiring board or PWB.
The simplest PCB is a layer of copper foil glued to a sheet of plastic,
often an epoxy glue reinforced with fibreglass. The excess foil is removed,
usually by chemical etching, and components are attached to the remaining
foil, usually by soldering.
Printed circuit technology normally uses photolithography to make the
conductors. Sometimes it uses mechanical deposition to attach conductive
layers to an insulating substrate.
The photomask is usually prepared with a photoplotter from data produced by
a technician using computer-aided PCB design software. Some persons claim
that they can produce low-resolution photoplots by printing a design to a
laser printer, printing on the sheets used to make transparent presentations.
PCBs are rugged, inexpensive, and can be highly reliable. They are harder to
repair than wire wrap boards. They require much more design than either
wire-wrapped or point-to-point constructed equipment.
History
Printed circuits were first used in World War II for rugged radios, but
entered commercial use in the 1950s.
Before printed circuits, point-to-point construction was used. For
prototypes, or small production runs, wire wrap can be more efficient.
Originally, every electronic component had wire leads, and the PCB had holes
drilled for each wire of each component. The components were then soldered
to the PCB. This method is called through-hole construction. This could be
done automatically by passing the board over a ripple, or wave, of molten
solder in a wave-soldering machine.
However, the wires and holes are wasteful. It costs money to drill the
holes, and the wires are merely cut off.
In the 1960s, a technique called surface mount was invented. It became
widely used in the late 1980s. Components were mechanically redesigned to
have small metal tabs or pads that could be directly soldered to the surface
of the PCB.
Often an automated machine removes the parts from reels, and sticks them to
the PCB. A silk-screened application of solder paste, a mixture of solder
and flux, holds the parts in place.
The board is pre-heated, passed through an oven containing infrared lamps
whose heat melts the solder, then slowly cooled. The pre-heating and
controlled cooling prevent the parts from cracking when one edge of the part
is cold and another edge is hot from the solder.
The parts and pads of the PCB are designed so that the surface tension of
the molten solder centers the parts on their copper pads.
The result was components that were one quarter to one tenth of the size and
weight, and half to a quarter of the cost of wire-mounted parts.
Design
Usually an electrical engineer designs the circuit, and a technician designs
the PCB. PCB design is a specialized skill. There are numerous tricks and
standards used to design a PCB that is easy to manufacture and yet small and
inexpensive. (see PCB layout guidelines).
Some PCBs for high-frequency RF work use plastics with special
characteristics in order to avoid detuning the radio. PCBs in vacuum or
spacecraft often have solid copper or aluminum cores to carry away
components' heat.
The width and spacing of conductors on a PCB is very important. If
conductors are too close, solder can short adjacent connectors, and the PCB
will be difficult to repair. If too far apart, the PCB may be too large and expensive.
Removing large areas of copper wastes etchant and increases pollution. Also,
a PCB etches more consistently if all regions have the same average ratio of
copper to bare plastic. Therefore, designs may widen connectors, leave
unconnected copper in place, or cover large areas of bare plastic with
arrays of small, electrically isolated copper diamonds or squares.
Most PCBs have between one and sixteen conductive layers laminated (glued)
together. In more complex PCBs, two or more of the layers are dedicated to
providing ground and power. These ground planes and power planes detune
accidental antennas, and provide efficient distribution of power.
Multi-layer boards are needed for complex digital circuits such as computers.
Ground and power planes are rectangular sheets of conductor that occupy
entire layers. They distribute electrical power and heat better than simple
conductors. Specialized conduction-cooled designs rely on the PCB to conduct
away all the waste heat, unlike the air-cooling method more commonly used.
Multilayer PCBs have alignment marks and holes to align layers and permit
the PCB to be mounted in equipment that automatically places and solders
components. Some designs place alignment and etch test-patterns on break-off
tabs that can be removed before installation.
Layers may be connected together through drilled holes called vias. Either
the holes are electroplated or small rivets are inserted. Vias cost $0.02 to
$0.50 depending on the design. High-density PCBs may have blind vias, which
are visible only on one surface, or buried vias, which are visible on
neither, but these are expensive to build and difficult or impossible to
inspect after manufacture.
Good designers minimize the number of vias to reduce the cost of drilling.
On older, two-layer PCBs, it was common to solder a wire through the hole.
Holes are drilled with tiny carbide drill-bits or by lasers. The drilling is
performed by drilling machines with computerized placement using a "drill
tape" or "drill file."
Component leads are inserted in holes or mounted on the surface "pads" and
electrically and mechanically fixed to the board with a molten metal solder.
A solder mask is a plastic layer that resists wetting by solder (the solder
is said to "bead up"), and keeps islands of solder from running together. It
also protects the outside conductors layers from abrasion and corrosion.
A silkscreen legend on the top or bottom surface of the board provides
readable information about component part numbers and placement that aids in
manufacturing and repair.
PCBs intended for extreme environments often have a conformal coat, which is
applied by dipping or spraying. The coat prevents corrosion and electrical
shorting from condensation. The earliest conformal coats were wax. Modern
conformal coats are usually dips of dilute solutions of silicone rubber or
epoxy. Some are engineering plastics sputtered onto the PCB in a vacuum chamber.
Mass-production PCBs have small pads for automated test equipment to make
temporary connections. Sometimes the pads must be isolated with resistors.
PCB designers often design power supply circuits, including placement of
bypass capacitors. These store power for high-speed integrated circuits, and
are usually mounted near the integrated circuit.
PCB designers must often renumber components.
To aid manual repair, diodes, capacitors and integrated circuits should be
oriented in the same way.
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