Much of the electronics industry’s PCB design, assembly, and quality control follows standards published by the IPC
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
Through-hole PCB technology almost completely replaced earlier electronics assembly techniques such as point-to-point
construction. From the second generation of computers in the 1950s until surface-mount technology became popular in the late
1980s, every component on a typical PCB was a through-hole component.
Through-hole manufacture adds to board cost by requiring many holes to be drilled accurately, and limits the available routing
area for signal traces on layers immediately below the top layer on multilayer 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.
Main article: Surface-mount technology
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. 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;
prices of semiconductor surface mount devices (SMDs) are determined more by the chip itself than the package, with little
price advantage over larger packages. Some wire-ended components, such as 1N4148 small-signal switch diodes, are actually
significantly cheaper than SMD equivalents.
Circuit properties of the PCB
Each trace consists of a flat, narrow part of the copper foil that remains after etching. The resistance, determined by width and
thickness, of the traces 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 the form of stripline and 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; but they can be
used as a deliberate part of the circuit design, obviating the need for additional discrete components.
Excluding exotic products using special materials or processes all printed circuit boards manufactured today can be built using
the following four materials:
Resin impregnated B-stage cloth (Pre-preg)
Laminates are manufactured by curing under pressure and temperature layers of cloth or paper with thermoset resin to form
an integral final piece of uniform thickness. The size can be up to 4 by 8 feet (1.2 by 2.4 m) in width and length. Varying cloth
weaves (threads per inch or cm), cloth thickness, and resin percentage are used to achieve the desired final thickness and
dielectric characteristics. Available standard laminate thickness are listed in Table 1:
Although this specification has been superseded and the new specification does not list standard sizes, these are still the
most common sizes stocked and ordered for manufacturer.
The cloth or fiber material used, resin material, and the cloth to resin ratio determine the laminate’s type designation (FR-4,
CEM-1, G-10, etc.) and therefore the characteristics of the laminate produced. Important characteristics are the level to which
the laminate is fire retardant, the dielectric constant (er), the loss factor (tδ), the tensile strength, the shear strength, the glass
transition temperature (Tg), and the Z-axis expansion coefficient (how much the thickness changes with temperature).
There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the
requirements of the circuit. Some of these dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well
known prepreg materials used in the PCB industry are FR-2 (phenolic cotton paper), FR-3 (cotton paper and epoxy), FR-4
(woven glass and epoxy), FR-5 (woven glass and epoxy), FR-6 (matte glass and polyester), G-10 (woven glass and epoxy),
CEM-1 (cotton paper and epoxy), CEM-2 (cotton paper and epoxy), CEM-3 (non-woven glass and epoxy), CEM-4 (woven
glass and epoxy), CEM-5 (woven glass and polyester). Thermal expansion is an important consideration especially with ball
grid array (BGA) and naked die technologies, and glass fiber offers the best dimensional stability.
FR-4 is by far the most common material used today. The board with copper on it is called “copper-clad laminate”.
Copper thickness of PCBs can be specified as units of length (in micrometers or mils) but is often specified as weight of copper
per area (in ounce per square foot) which is easier to measure. One ounce per square foot is 1.344 mils or 34 micrometres
The printed circuit board industry defines heavy copper as layers exceeding 3 ounces of copper, or approximately 0.0042
inches (4.2 mils, 105 μm) thick. PCB designers and fabricators often use heavy copper when design and manufacturing circuit
boards in order to increase current-carrying capacity as well as resistance to thermal strains. Heavy copper plated vias transfer
heat to external heat sinks. IPC 2152 is a standard for determining current-carrying capacity of printed circuit board traces.
Safety certification (US)
Safety Standard UL 796 covers component safety requirements for printed wiring boards for use as components in devices or
appliances. Testing analyzes characteristics such as flammability, maximum operating temperature, electrical tracking, heat
deflection, and direct support of live electrical parts.
Multiwire is a patented technique of interconnection which uses machine-routed insulated wires embedded in a non-conducting
matrix (often plastic resin). It was used during the 1980s and 1990s. (Kollmorgen Technologies Corp, U.S. Patent 4,175,816
filed 1978) Multiwire is still available in 2010 through Hitachi. There are other competitive discrete wiring technologies that have
been developed (Jumatech, layered sheets).
Since it was quite easy to stack interconnections (wires) inside the embedding matrix, the approach allowed designers to forget
completely about the routing of wires (usually a time-consuming operation of PCB design): Anywhere the designer needs a
connection, the machine will draw a wire in straight line from one location/pin to another. This led to very short design times (no
complex algorithms to use even for high density designs) as well as reduced crosstalk (which is worse when wires run parallel
to each other—which almost never happens in Multiwire), though the cost is too high to compete with cheaper PCB
technologies when large quantities are needed.
Cordwood construction can save significant space and was often used with wire-ended components in applications where
space was at a premium (such as missile guidance and telemetry systems) and in high-speed computers, where short traces
were important. In “cordwood” construction, axial-leaded components were mounted between two parallel planes. The
components were either soldered together with jumper wire, or they were connected to other components by thin nickel ribbon
welded at right angles onto the component leads. To avoid shorting together different interconnection layers, thin insulating
cards were placed between them. Perforations or holes in the cards allowed component leads to project through to the next
interconnection layer. One disadvantage of this system was that special nickel-leaded components had to be used to allow the
interconnecting welds to be made. Differential thermal expansion of the component could put pressure on the leads of the
components and the PCB traces and cause physical damage (as was seen in several modules on the Apollo program).
Additionally, components located in the interior are difficult to replace. Some versions of cordwood construction used soldered
single-sided PCBs as the interconnection method (as pictured), allowing the use of normal-leaded components.
Before the advent of integrated circuits, this method allowed the highest possible component packing density; because of this,
it was used by a number of computer vendors including Control Data Corporation. The cordwood method of construction was
used only rarely once semiconductor electronics and PCBs became widespread.