History of circuit boards, PCB assembly, PCB boards and PCB printing
Find useful information about circuit boards and PCB printing, the history of PCB boards. Electronic Interconnect is your 1-stop shop for PCB assembly and manufacturing of printed circuit boards.
A printed circuit board
, or PCB, is a self-contained module
of interconnected electronic components found in devices ranging from common
beepers, or pagers, and radios to sophisticated radar and computer systems. The
circuits are formed by a thin layer of conducting material deposited, or
"printed," on the surface of an insulating board known as the substrate.
Individual electronic components are placed on the surface of the substrate and
soldered to the interconnecting circuits. Contact fingers along one or more
edges of the substrate act as connectors to other PCBs or to external
electrical devices such as on-off switches. A printed circuit board may have
circuits that perform a single function, such as a signal amplifier, or
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There are three major types of printed circuit board
construction: single-sided, double-sided, and multi-layered. Single-sided
boards have the components on one side of the substrate. When the number of
components becomes too much for a single-sided board, a double-sided board may
be used. Electrical connections between the circuits on each side are made by
drilling holes through the substrate in appropriate locations and plating the
inside of the holes with a conducting material. The third type, a multi-layered
board, has a substrate made up of layers of printed circuits separated by
layers of insulation. The components on the surface connect through plated
holes drilled down to the appropriate circuit layer. This greatly simplifies
the circuit pattern.
Components on a printed circuit board are electrically
connected to the circuits by two different methods: the older "through hole
technology" and the newer "surface mount technology." With through hole
technology, each component has thin wires, or leads, which are pushed through
small holes in the substrate and soldered to connection pads in the circuits on
the opposite side. Gravity and friction between the leads and the sides of the
holes keeps the components in place until they are soldered. With surface mount
technology, stubby J-shaped or L-shaped legs on each component contact the
printed circuits directly. A solder paste consisting of glue, flux, and solder
are applied at the point of contact to hold the components in place until the
solder is melted, or "reflowed," in an oven to make the final connection.
Although surface mount technology requires greater care in the placement of the
components, it eliminates the time-consuming drilling process and the
space-consuming connection pads inherent with through hole technology. Both
technologies are used today.
Two other types of circuit assemblies are related to the
printed circuit board. An integrated circuit, sometimes called an IC or
microchip, performs similar functions to a printed circuit board except the IC
contains many more circuits and components that are electrochemically "grown"
in place on the surface of a very small chip of silicon. A hybrid circuit, as
the name implies, looks like a printed circuit board, but contains some
components that are grown onto the surface of the substrate rather than being
placed on the surface and soldered.
Printed circuit boards evolved from electrical connection
systems that were developed in the 1850s. Metal strips or rods were originally
used to connect large electric components mounted on wooden bases. In time the
metal strips were replaced by wires connected to screw terminals, and wooden
bases were replaced by metal chassis. But smaller and more compact designs were
needed due to the increased operating needs of the products that used circuit
boards. In 1925, Charles Ducas of the United States submitted a patent
application for a method of creating an electrical path directly on an
insulated surface by printing through a stencil with electrically conductive
inks. This method gave birth to the name "printed wiring" or "printed circuit."
In the 1943, Paul Eisler of the United Kingdom patented a
method of etching the conductive pattern, or circuits, on a layer of copper
foil bonded to a glass-reinforced, non-conductive base. Widespread use of
Eisler's technique did not come until the 1950s when the transistor was
introduced for commercial use. Up to that point, the size of vacuum tubes and
other components were so large that the traditional mounting and wiring methods
were all that was needed. With the advent of transistors, however, the
components became very small, and manufacturers turned to printed circuit
boards to reduce the overall size of the electronic package.
Through hole technology and its use in multi-layer PCBs was
patented by the U.S. firm Hazeltyne in 1961. The resulting increase in
component density and closely spaced electrical paths started a new era in PCB
design. Integrated circuit chips were introduced in the 1970s, and these
components were quickly incorporated into printed circuit board design and
There is no such thing as a standard printed circuit board.
Each board has a unique function for a particular product and must be designed
to perform that function in the space allotted. Board designers use
computer-aided design systems with special software to layout the circuit
pattern on the board. The spaces between electrical conducting paths are often
0.04 inches (1.0 mm) or smaller. The location of the holes for component leads
or contact points are also laid out, and this information is translated into
instructions for a computer numerical controlled drilling machine or for the
automatic solder paster used in the manufacturing process.
Once the circuit pattern is laid out, a negative image, or
mask, is printed out at exact size on a clear plastic sheet. With a negative
image, the areas that are not part of the circuit pattern are shown in black
and the circuit pattern is shown as clear.
The substrate most commonly used in printed circuit boards
is a glass fiber reinforced (fiberglass) epoxy resin with a copper foil bonded
on to one or both sides. PCBs made from paper reinforced phenolic resin with a
bonded copper foil are less expensive and are often used in household
The printed circuits are made of copper, which is either
plated or etched away on the surface of the substrate to leave the pattern
desired. (See "additive" and "subtractive" processes described in step 3 under
The Manufacturing Process). The copper circuits are coated with a layer of
tin-lead to prevent oxidation. Contact fingers are plated with tin-lead, then
nickel, and finally gold for excellent conductivity.
Purchased components include resistors, capacitors,
transistors, diodes, integrated circuit chips, and others.