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Printed circuit board
In electronics, printed circuit boards, or
PCBs, are used to mechanically support and electrically connect
electronic components using conductive pathways, or traces, etched
from copper sheets laminated onto a non-conductive substrate.
Alternative names are printed wiring board (PWB), and etched wiring
board. Populating the board with electronic components forms a
printed circuit assembly (PCA), also known as a printed circuit
board assembly (PCBA).
PCBs are rugged, inexpensive, and can be highly
reliable. They require much more layout effort and higher initial
cost than either wire-wrapped or point-to-point constructed
circuits, but are much cheaper, faster, and consistent in high
volume production. Much of the electronics industry's PCB design,
assembly, and quality control needs are set by standards that are
published by the IPC organization.
History
Physical Composition
Design
Patterning (etching)
Lamination
Drilling
Exposed conductor plating and coating
Solder resist
Screen Printing
Test
Populating
Protection & Packing
Safety Certification (US)
"Cordwood" construction
Multiwire boards
Surface-mount technology
History
The
inventor of the printed circuit was the Austrian engineer Paul
Eisler (1907–1995) who, while working in England, made one circa
1936 as part of a radio set. Around 1943 the USA began to use the
technology on a large scale to make rugged radios for use in World
War II. After the war, 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. Before printed
circuits (and for a while after their invention), 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'
leads were then passed through the holes and soldered to the PCB
trace. 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. 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 wasteful since drilling holes is expensive and
the protruding wires are merely cut off.
In recent years, the use of surface mount parts has gained
popularity as the demand for smaller electronics packaging and
greater functionality has grown.
Physical
Composition
Most PCBs are composed of between one and
twenty-four conductive layers separated and supported by layers of
insulating material (substrates) laminated (glued with heat,
pressure & sometimes vacuum) together.
The most common substrate for single-sided
PCBs is FR-2 (synthetic resin bonded paper) which is cheap and easy
to drill but is prone to cracking and is not good for making plated
through hole boards. Double sided and multilayer boards tend to use
FR-4 (a fiberglass type material). Other substrates include power
electronic substrate and Kapton (used to make flexible electronics).
The conductive layers are almost invariably made of copper, which
sometimes is gold-coated.
Layers may be connected together through
drilled holes called vias. To form an electrical connection, the
holes are either electroplated or small rivets are inserted. Even
though they may not form electrical connections to all layers, these
holes are typically drilled completely through the PC board for
simplicity of design and manufacture. The exception are high-density
PCBs, which may have blind vias (which are visible only on one
surface), or buried vias (which are visible on neither).
Design
The patterns on a PCB are usually drawn using
electronic design automation software. That PCB design software
generates vector graphics output drawings, most often in Gerber file
format.
Patterning
(etching)
The vast majority of printed circuit boards are
made by adhering a layer of copper over the entire substrate,
sometimes on both sides, (creating a "blank PCB") then removing
unwanted copper after applying a temporary mask (eg. by etching),
leaving only the desired copper traces. A few PCBs are made by
adding traces to the bare substrate (or a substrate with a very thin
layer of copper) usually by a complex process of multiple
electroplating steps.
There are three common "subtractive" methods
(methods that remove copper) used for the production of printed
circuit boards:
Silk screen printing uses etch-resistant inks
to protect the copper foil. Subsequent etching removes the unwanted
copper. Alternatively, the ink may be conductive, printed on a blank
(non-conductive) board. The latter technique is also used in the
manufacture of hybrid circuits. Photoengraving uses a photomask and
chemical etching to remove the copper foil from the substrate. The
photomask is usually prepared with a photoplotter from data produced
by a technician using CAM,or computer-aided manufacturing software.
Laser-printed transparencies are typically employed for phototools;
however, direct laser imaging techniques are being employed to
replace phototools for high-resolution requirements.
PCB milling uses a two or three-axis mechanical
milling system to mill away the copper foil from the substrate. A
PCB milling machine (referred to as a 'PCB Prototyper') operates in
a similar way to a plotter, receiving commands from the host
software that control the position of the milling head in the x, y,
and (if relevant) z axis. Data to drive the Prototyper is extracted
from files generated in PCB design software and stored in HPGL or
Gerber file format.
"Additive" processes also exist. The most
common is the "semi-additive" process. In this version, the
unpatterned board has a thin layer of copper already on it. A
reverse mask is then applied. (Unlike a subtractive process mask,
this mask exposes those parts of the substrate that will eventually
become the traces.) Additional copper is then plated onto the board
in the unmasked areas; copper may be plated to any desired weight.
Tin-lead or other surface platings are then applied. The mask is
stripped away and a brief etching step removes the now-exposed
original copper laminate from the board, isolating the individual
traces.
The additive process is commonly used for
multi-layer boards as it facilitates the plating-through of the
holes (vias) in the circuit board.
Lamination
Some PCBs have trace layers inside the PCB and
are called multi-layer PCBs. These are formed by bonding together
separately etched thin boards.
Drilling
Holes, or vias, through a PCB are typically
drilled with tiny drill bits made of solid tungsten carbide. The
drilling is performed by automated drilling machines with placement
controlled by a drill tape or drill file. These computer-generated
files are also called numerically controlled drill (NCD) files or "Excellon
files". The drill file describes the location and size of each
drilled hole.
When very small vias are required, drilling
with mechanical bits is costly because of high rates of wear and
breakage. In this case, the vias may be evaporated by lasers.
Laser-drilled vias typically have an inferior surface finish inside
the hole. These holes are called micro vias.
It is also possible with controlled-depth
drilling, laser drilling, or by pre-drilling the individual sheets
of the PCB before lamination, to produce holes that connect only
some of the copper layers, rather than passing through the entire
board. These holes are called blind vias when they connect an
internal copper layer to an outer layer, or buried vias when they
connect two or more internal copper layers and no outer layers.
The walls of the holes, for boards with 2 or
more layers, are plated with copper to form plated-through holes
that electrically connect the conducting layers of the PCB. For
multilayer boards, those with 4 layers or more, drilling typically
produces a smear comprised of the bonding agent in the laminate
system. Before the holes can be plated through, this smear must be
removed by a chemical de-smear process, or by plasma-etch.
Exposed
conductor plating and coating
The pads and lands to which components will be
mounted are typically plated, because bare copper oxidizes quickly,
and therefore is not readily solderable. Traditionally, any exposed
copper was plated with solder. This solder was a tin-lead alloy,
however new solder compounds are now used to achieve compliance with
the RoHS directive in the EU, which restricts the use of lead. Other
platings used are OSP (organic surface protectant), immersion
silver, electroless nickel with immersion gold coating (ENIG), and
direct gold. Edge connectors, placed along one edge of some boards,
are often gold plated.
Solder
resist
Areas that should not be soldered to may be
covered with a polymer solder resist (solder mask) coating. The
solder resist prevents solder from bridging between conductors and
thereby creating short circuits. Solder resist also provides some
protection from the environment.
Screen
printing
Line art and text may be printed onto the outer
surfaces of a PCB by screen printing. When space permits, the screen
print text can indicate component designators[1], switch setting
requirements, test points, and other features helpful in assembling,
testing, and servicing the circuit board.
Screen print is also known as the silk screen,
or, in one sided PCBs, the red print.
Lately some digital printing solutions have
been developed to substitute the traditional screen printing
process. This technology allows printing variable data onto the PCB,
including serialization and barcode information for traceability
purposes.
Test
Unpopulated boards may be subjected to a
bare-board test where each circuit connection (as defined in a
netlist) is verified as correct on the finished board. For
high-volume production, a Bed of nails tester ,a fixture or a Rigid
needle adapter is used to make contact with copper lands or holes on
one or both sides of the board to facilitate testing. A computer
will instruct the electrical test unit to send a small amount of
current through each contact point on the bed-of-nails as required,
and verify that such current can be seen on the other appropriate
contact points. For small- or medium-volume boards, flying-probe
testers use moving test heads to make contact with the copper lands
or holes to verify the electrical connectivity of the board under
test.
Populating
After the PCB is completed, electronic
components must be attached to form a functional printed circuit
assembly, or PCA. In through-hole construction, component leads may
be inserted in holes and electrically and mechanically fixed to the
board with a molten metal solder, while in surface-mount
construction, the components are simply soldered to pads or lands on
the outer surfaces of the PCB.
Often, through-hole and surface-mount
construction must be combined in a single PCA because some required
components are available only in surface-mount packages, while
others are available only in through-hole packages.
JEDEC guidelines for PCB component placement,
soldering, and inspection are commonly used to maintain quality
control in this stage of PCB manufacturing.
After the board is populated, the populated
board may be tested with an in-circuit test system. To facilitate
this test, PCBs may be designed with extra pads to make temporary
connections. Sometimes these pads must be isolated with resistors.
The in-circuit test may also exercise boundary scan test features of
some components. In-circuit test systems may also be used to program
nonvolatile memory components on the board.
In boundary scan testing, test circuits
integrated into various ICs on the board form temporary connections
between the pcb traces to test that the ICs are mounted correctly.
Boundary scan testing requires that all the ICs to be tested use a
standard test configuration procedure, the most common one being the
Joint Test Action Group (JTAG) standard.
Protection
and packaging
PCBs intended for extreme environments often
have a conformal coat, which is applied by dipping or spraying after
the components have been soldered. The coat prevents corrosion and
leakage currents or shorting due to condensation. The earliest
conformal coats were wax. Modern conformal coats are usually dips of
dilute solutions of silicone rubber, polyurethane, acrylic, or
epoxy. Some are engineering plastics sputtered onto the PCB in a
vacuum chamber.
Many assembled PCBs are static sensitive, and
therefore must be placed in antistatic bags during transport. When
handling these boards, the user must be earthed; failure to do this
might transmit an accumulated static charge through the board,
damaging or destroying it. Even bare boards are sometimes static
sensitive. Traces have gotten so fine that it's quite possible to
blow an etch off the board (or change its characteristics) with a
static charge. This is especially true on non-traditional PCBs such
as MCMs and microwave PCBs.
Safety
Certification (US)
Safety Standard UL 796 covers component safety
requirements for printed wiring boards 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.
The boards may use organic or inorganic base
materials in a single or multilayer, rigid or flexible form.
Circuitry construction may include etched, die stamped, precut,
flush press, additive, and plated conductor techniques.
Printed-component parts may be used.
The suitability of the pattern parameters,
temperature and maximum solder limits shall be determined in
accordance with the applicable end-product construction and
requirements.
"Cordwood"
construction
A cordwood module.Cordwood construction can
give large space-saving advantages and was often used with
wire-ended components in applications where space was at a premium
(such as missile guidance and telemetry systems). In 'cordwood'
construction, two leaded components are mounted axially between two
parallel planes. Instead of soldering the components, they were
connected to other components by thin nickel tapes welded at right
angles onto the component leads. To avoid shorting together of
different interconnection layers, thin insulating cards were placed
between them. Perforations or holes in the cards would allow
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. Some versions of cordwood construction used single sided
PCBs as the interconnection method (as pictured). This meant that
normal leaded components could be used.
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 now appears to have fallen into disuse, probably
because high packing densities can be more easily achieved using
surface mount techniques and integrated circuits.
Multiwire
boards
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. (Augat Inc., U.S. Patent 4,648,180)
Since it was quite easy to stack
interconnections (wires) inside the embedding matrix, the approach
allowed 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), reduced
cross talk (an electrical phenomenon appearing where a current in
one wire generates another current in another conductor, that is
highly amplified when wires are parallel - this nearly never happens
in Multiwire), but costs too high to compete with cheaper PCB
technologies when large quantities are needed.
Surface-mount
technology
Main article: surface-mount technology
Surface mount components, including resistors,
transistors and an integrated circuit.Surface-mount technology was
developed in the 1960s, gained momentum in Japan in the early 1980s
and became widely used globally by the mid 1990s. Components were
mechanically redesigned to have small metal tabs or end caps that
could be directly soldered to the surface of the PCB. Components
became much smaller and component placement on both sides of the
board became far more common with surface-mounting than through-hole
mounting, allowing much higher circuit densities. Surface mounting
lends itself well to a high degree of automation, reducing labour
cost and greatly increasing production and quality rates. SMDs can
be one-quarter to one-tenth the size and weight, and passive
components can be one-half to one-quarter the cost of through-hole
parts. Integrated circuits (where the chip itself is the most
expensive part) are often priced the same regardless of package type
however. As of 2006, some wire-ended components, such as small
signal switch diodes (philips 1N4148 for instance), are actually
significantly cheaper than corresponding SMD versions.
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