Printed circuit board
A printed circuit board (PCB), also called printed wiring board (PWB), is a medium used to connect or "wire"
components to one another in a circuit. It takes the form of a laminated sandwich structure of conductive and insulating
layers: each of the conductive layers is designed with a pattern of traces, planes and other features (similar to wires on a flat
surface) etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive
substrate.[1] Electrical components may be fixed to conductive pads on the outer layers in the shape designed to accept the
component's terminals, generally by means of soldering, to both electrically connect and mechanically fasten them to it.
Another manufacturing process adds vias, plated-through holes that allow interconnections between layers.
Printed circuit board of a DVD player
Part of a 1984 Sinclair ZX Spectrum computer
board, a printed circuit board, showing the
conductive traces, the through-hole paths to the
other surface, and some electronic components
mounted using through-hole mounting
Printed circuit boards are used in nearly all electronic products. Alternatives to PCBs include wire wrap and point-to-point
construction, both once popular but now rarely used. PCBs require additional design effort to lay out the circuit, but
manufacturing and assembly can be automated. Electronic design automation software is available to do much of the work of
layout. Mass-producing circuits with PCBs is cheaper and faster than with other wiring methods, as components are
mounted and wired in one operation. Large numbers of PCBs can be fabricated at the same time, and the layout has to be
done only once. PCBs can also be made manually in small quantities, with reduced benefits.[2]
PCBs can be single-sided (one copper layer), double-sided (two copper layers on both sides of one substrate layer), or multi-
layer (outer and inner layers of copper, alternating with layers of substrate). Multi-layer PCBs allow for much higher
component density, because circuit traces on the inner layers would otherwise take up surface space between components. The
rise in popularity of multilayer PCBs with more than two, and especially with more than four, copper planes was concurrent
with the adoption of surface mount technology. However, multilayer PCBs make repair, analysis, and field modification of
circuits much more difficult and usually impractical.
The world market for bare PCBs exceeded $60.2 billion in 2014[3] and is estimated to reach $79 billion by 2024.[4][5]
History
Predecessors
Before the development of printed circuit boards, electrical and electronic circuits were wired point-to-point on a chassis.
Typically, the chassis was a sheet metal frame or pan, sometimes with a wooden bottom. Components were attached to the
chassis, usually by insulators when the connecting point on the chassis was metal, and then their leads were connected directly
or with jumper wires by soldering, or sometimes using crimp connectors, wire connector lugs on screw terminals, or other
methods. Circuits were large, bulky, heavy, and relatively fragile (even discounting the breakable glass envelopes of the
vacuum tubes that were often included in the circuits), and production was labor-intensive, so the products were expensive.
Development of the methods used in modern printed circuit boards started early in the 20th century. In 1903, a German
inventor, Albert Hanson, described flat foil conductors laminated to an insulating board, in multiple layers. Thomas Edison
experimented with chemical methods of plating conductors onto linen paper in 1904. Arthur Berry in 1913 patented a print-
and-etch method in the UK, and in the United States Max Schoop obtained a patent[6] to flame-spray metal onto a board
through a patterned mask. Charles Ducas in 1925 patented a method of electroplating circuit patterns.[7]
Predating the printed circuit invention, and similar in spirit, was John Sargrove's 1936–1947 Electronic Circuit Making
Equipment (ECME) that sprayed metal onto a Bakelite plastic board. The ECME could produce three radio boards per
minute.
Early PCBs
Proximity fuze Mark 53 production line 1944
The Austrian engineer Paul Eisler invented the printed circuit as part of a radio set while working in the UK around 1936.
In 1941 a multi-layer printed circuit was used in German magnetic influence naval mines.
Around 1943 the USA began to use the technology on a large scale to make proximity fuzes for use in World War II.[7] Such
fuzes required an electronic circuit that could withstand being fired from a gun, and could be produced in quantity. The
Centralab Division of Globe Union submitted a proposal which met the requirements: a ceramic plate would be screenprinted
with metallic paint for conductors and carbon material for resistors, with ceramic disc capacitors and subminiature vacuum
tubes soldered in place.[8] The technique proved viable, and the resulting patent on the process, which was classified by the
U.S. Army, was assigned to Globe Union. It was not until 1984 that the Institute of Electrical and Electronics Engineers
(IEEE) awarded Harry W. Rubinstein the Cledo Brunetti Award for early key contributions to the development of
printed components and conductors on a common insulating substrate. Rubinstein was honored in 1984 by his alma mater, the
University of Wisconsin-Madison, for his innovations in the technology of printed electronic circuits and the fabrication of
capacitors.[9][10] This invention also represents a step in the development of integrated circuit technology, as not only wiring
but also passive components were fabricated on the ceramic substrate.
Post-war developments
In 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer
electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army. At around the
same time in the UK work along similar lines was carried out by Geoffrey Dummer, then at the RRDE.
Motorola was an early leader in bringing the process into consumer electronics, announcing in August 1952 the adoption of
"plated circuits" in home radios after six years of research and a $1M investment.[11] Motorola soon began using its
trademarked term for the process, PLAcir, in its consumer radio advertisements.[12] Hallicrafters released its first "foto-
etch" printed circuit product, a clock-radio, on 1 November 1952.[13]
Even as circuit boards became available, the point-to-point chassis construction method remained in common use in industry
(such as TV and hi-fi sets) into at least the late 1960s. Printed circuit boards were introduced to reduce the size, weight, and
cost of parts of the circuitry. In 1960, a small consumer radio receiver might be built with all its circuitry on one circuit board,
but a TV set would probably contain one or more circuit boards.
Originally, every electronic component had wire leads, and a PCB had holes drilled for each wire of each component. The
component leads were then inserted through the holes and soldered to the copper PCB traces. This method of assembly is
called through-hole construction. In 1949, Moe Abramson and Stanislaus F. Danko of the United States Army Signal
Corps developed the Auto-Sembly process in which component leads were inserted into a copper foil interconnection pattern
and dip soldered. The patent they obtained in 1956 was assigned to the U.S. Army.[14] With the development of board
lamination and etching techniques, this concept evolved into the standard printed circuit board fabrication process in use today.
Soldering 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 inefficient since drilling holes is expensive and consumes drill bits and the
protruding wires are cut off and discarded.
From the 1980s onward, small surface mount parts have been used increasingly instead of through-hole components; this has
led to smaller boards for a given functionality and lower production costs, but with some additional difficulty in servicing faulty
boards.
In the 1990s the use of multilayer surface boards became more frequent. As a result, size was further minimized and both
flexible and rigid PCBs were incorporated in different devices. In 1995 PCB manufacturers began using microvia
technology to produce High-Density Interconnect (HDI) PCBs.[15]
Recent advances
Recent advances in 3D printing have meant that there are several new techniques in PCB creation. 3D printed electronics
(PEs) can be utilized to print items layer by layer and subsequently the item can be printed with a liquid ink that contains
electronic functionalities.
HDI (High Density Interconnect) technology allows for a denser design on the PCB and thus potentially smaller PCBs
with more traces and/or components in a given area. As a result, the paths between components can be shorter. HDIs use
blind/buried vias, or a combination that includes microvias. With multi-layer HDI PCBs the interconnection of several vias
stacked on top of each other (stacked vías, instead of one deep buried via) can be made stronger, thus enhancing reliability in
all conditions. The most common applications for HDI technology are computer and mobile phone components as well as
medical equipment and military communication equipment. A 4-layer HDI microvia PCB is equivalent in quality to an 8-
layer through-hole PCB, so HDI technology can reduce costs. HDI PCBs are often made using build-up film such as
ajinomoto build-up film, which is also used in the production of flip chip packages.[16][17] Some PCBs have optical
waveguides, similar to optical fibers built on the PCB.[18]
Composition
An example of hand-drawn etched traces on a
PCB
A basic PCB consists of a flat sheet of insulating material and a layer of copper foil, laminated to the substrate. Chemical
etching divides the copper into separate conducting lines called tracks or circuit traces, pads for connections, vias to pass
connections between layers of copper, and features such as solid conductive areas for electromagnetic shielding or other
purposes. The tracks function as wires fixed in place, and are insulated from each other by air and the board substrate
material. The surface of a PCB may have a coating that protects the copper from corrosion and reduces the chances of
solder shorts between traces or undesired electrical contact with stray bare wires. For its function in helping to prevent solder
shorts, the coating is called solder resist or solder mask.
The pattern to be etched into each copper layer of a PCB is called the "artwork". The etching is usually done using
photoresist which is coated onto the PCB, then exposed to light projected in the pattern of the artwork. The resist material
protects the copper from dissolution into the etching solution. The etched board is then cleaned. A PCB design can be mass-
reproduced in a way similar to the way photographs can be mass-duplicated from film negatives using a photographic printer.
FR-4 glass epoxy is the most common insulating substrate. Another substrate material is cotton paper impregnated with
phenolic resin, often tan or brown.
When a PCB has no components installed, it is less ambiguously called a printed wiring board (PWB) or etched wiring
board.[19] However, the term "printed wiring board" has fallen into disuse. A PCB populated with electronic components is
called a printed circuit assembly (PCA), printed circuit board assembly or PCB assembly (PCBA). In informal usage,
the term "printed circuit board" most commonly means "printed circuit assembly" (with components). The IPC preferred
term for an assembled board is circuit card assembly (CCA),[20] and for an assembled backplane it is backplane assembly.
"Card" is another widely used informal term for a "printed circuit assembly". For example, expansion card.
A PCB may be printed with a legend identifying the components, test points, or identifying text. Originally, silkscreen
printing was used for this purpose, but today other, finer quality printing methods are usually used. Normally the legend does
not affect the function of a PCBA.
Layers
A printed circuit board can have multiple layers of copper which almost always are arranged in pairs. The number of layers
and the interconnection designed between them (vias, PTHs) provide a general estimate of the board complexity. Using more
layers allow for more routing options and better control of signal integrity, but are also time-consuming and costly to
manufacture. Likewise, selection of the vias for the board also allow fine tuning of the board size, escaping of signals off
complex ICs, routing, and long term reliability, but are tightly coupled with production complexity and cost.
One of the simplest boards to produce is the two-layer board. It has copper on both sides that are referred to as external
layers; multi layer boards sandwich additional internal layers of copper and insulation. After two-layer PCBs, the next step
up is the four-layer. The four layer board adds significantly more routing options in the internal layers as compared to the two
layer board, and often some portion of the internal layers is used as ground plane or power plane, to achieve better signal
integrity, higher signaling frequencies, lower EMI, and better power supply decoupling.
In multi-layer boards, the layers of material are laminated together in an alternating sandwich: copper, substrate, copper,
substrate, copper, etc.; each plane of copper is etched, and any internal vias (that will not extend to both outer surfaces of the
finished multilayer board) are plated-through, before the layers are laminated together. Only the outer layers need be coated;
the inner copper layers are protected by the adjacent substrate layers.
Component mounting
Through-hole (leaded) resistors
Through-hole devices mounted on the circuit
board of a mid-1980s Commodore 64 home
computer
A box of drill bits used for making holes in
printed circuit boards. While tungsten-carbide
bits are very hard, they eventually wear out or
break. Drilling is a considerable part of the cost
of a through-hole printed circuit board.
Surface mount components, including resistors,
transistors and an integrated circuit
A PCB in a computer mouse: the component
side (left) and the printed side (right)
"Through hole" components are mounted by their wire leads passing through the board and soldered to traces on the other
side. "Surface mount" components are attached by their leads to copper traces on the same side of the board. A board may
use both methods for mounting components. PCBs with only through-hole mounted components are now uncommon. Surface
mounting is used for transistors, diodes, IC chips, resistors, and capacitors. Through-hole mounting may be used for some
large components such as electrolytic capacitors and connectors.
The first PCBs used through-hole technology, mounting electronic components by leads inserted through holes on one side of
the board and soldered onto copper traces on the other side. Boards may be single-sided, with an unplated component side, or
more compact double-sided boards, with components soldered on both sides. Horizontal installation of through-hole parts with
two axial leads (such as resistors, capacitors, and diodes) is done by bending the leads 90 degrees in the same direction,
inserting the part in the board (often bending leads located on the back of the board in opposite directions to improve the part's
mechanical strength), soldering the leads, and trimming off the ends. Leads may be soldered either manually or by a wave
soldering machine.[21] Through-hole manufacture adds to board cost by requiring many holes to be drilled accurately, and it
limits the available routing area for signal traces on layers immediately below the top layer on multi-layer boards, since the
holes must pass through all layers to the opposite side. Once surface-mounting came into use, small-sized SMD components
were used where possible, with through-hole mounting only of components unsuitably large for surface-mounting due to power
requirements or mechanical limitations, or subject to mechanical stress which might damage the PCB (e.g. by lifting the
copper off the board surface).
Surface-mount technology emerged in the 1960s, gained momentum in the early 1980s, and became widely used by the mid-
1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto the
PCB surface, instead of wire leads to pass through holes. Components became much smaller and component placement on
both sides of the board became more common than with through-hole mounting, allowing much smaller PCB assemblies with
much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labor costs and
greatly increasing production rates compared with through-hole circuit boards. Components can be supplied mounted on
carrier tapes. Surface mount components can be about one-quarter to one-tenth of the size and weight of through-hole
components, and passive components much cheaper. However, prices of semiconductor surface mount devices (SMDs) are
determined more by the chip itself than the package, with little price advantage over larger packages, and some wire-ended
components, such as 1N4148 small-signal switch diodes, are actually significantly cheaper than SMD equivalents.
Electrical properties
Each trace consists of a flat, narrow part of the copper foil that remains after etching. Its resistance, determined by its
width, thickness, and length, must be sufficiently low for the current the conductor will carry. Power and ground traces may
need to be wider than signal traces. In a multi-layer board one entire layer may be mostly solid copper to act as a ground plane
for shielding and power return. For microwave circuits, transmission lines can be laid out in a planar form such as stripline or
microstrip with carefully controlled dimensions to assure a consistent impedance. In radio-frequency and fast switching
circuits the inductance and capacitance of the printed circuit board conductors become significant circuit elements, usually
undesired; conversely, they can be used as a deliberate part of the circuit design, as in distributed-element filters, antennae,
and fuses, obviating the need for additional discrete components. High density interconnects (HDI) PCBs have tracks
and/or vias with a width or diameter of under 152 micrometers.[22]
