Printed circuit board

Not to be confused withPrinted electronics.

For other uses, seePCB (disambiguation).

"PC board" redirects here. For the mainboard of personal computers, seeMotherboard.

"Panelization" redirects here. For the page layout strategy, seeN-up.

Aprinted circuit board(PCB), also calledprinted wiring board(PWB), is a medium used to connect or"wire"componentsto one another in acircuit.It takes the form of alaminatedsandwich structure ofconductiveandinsulatinglayers: each of the conductive layers is designed with a pattern of traces, planes and other features (similar to wires on a flat surface)etchedfrom one or more sheet layers of copperlaminatedonto or between sheet layers of a non-conductive substrate.Electrical components may be fixed to conductive pads on the outer layers, generally by means ofsoldering,which both electrically connects and mechanically fastens the components to the board. Another manufacturing process addsvias,drilled holes that allow electrical interconnections between conductive layers.

Printed circuit board of a DVD player

Part of a 1984 SinclairZX Spectrumcomputer board, a printed circuit board, showing the conductive traces, thethrough-hole pathsto 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 includewire wrapandpoint-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 automationsoftware 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.

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 ofsurface 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 2014and is estimated to reach $79 billion by 2024.

History

Predecessors

Before the development of printed circuit boards, electrical and electronic circuits werewired point-to-pointon 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 withjumper wiresbysoldering,or sometimes usingcrimpconnectors, 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 Edisonexperimented with chemical methods of plating conductors onto linen paper in 1904. Arthur Berry in 1913 patented a print-and-etchmethod in the UK, and in the United StatesMax Schoopobtained a patentto flame-spray metal onto a board through a patterned mask. Charles Ducas in 1925 patented a method of electroplating circuit patterns.

Predating the printed circuit invention, and similar in spirit, wasJohn Sargrove's 1936–1947 Electronic Circuit Making Equipment (ECME) that sprayed metal onto aBakeliteplastic board. The ECME could produce three radio boards per minute.

Early PCBs

Proximity fuze Mark 53 production line 1944

The Austrian engineerPaul Eislerinvented 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 Germanmagnetic influence naval mines.

Around 1943 the United States began to use the technology on a large scale to makeproximity fuzesfor use in World War II.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 bescreenprintedwith metallic paint for conductors and carbon material forresistors,with ceramic disc capacitors and subminiature vacuum tubes soldered in place.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 theInstitute of Electrical and Electronics Engineers(IEEE) awarded Harry W. Rubinstein itsCledo Brunetti Awardfor 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, theUniversity of Wisconsin-Madison,for his innovations in the technology of printed electronic circuits and the fabrication of capacitors.This invention also represents a step in the development ofintegrated circuittechnology, as not only wiring but also passive components were fabricated on the ceramic substrate.

Post-war developments

In 1948, the US released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after theAuto-Semblyprocess was developed by the United States Army. At around the same time in the UK work along similar lines was carried out byGeoffrey Dummer,then at theRRDE.

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.Motorola soon began using its trademarked term for the process, PLAcir, in its consumer radio advertisements.Hallicrafters released its first "foto-etch" printed circuit product, a clock-radio, on November 1, 1952.

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 wireleads,and a PCB had holes drilled for each wire of each component. The component leads were then inserted through the holes andsolderedto the copper PCB traces. This method of assembly is calledthrough-holeconstruction.In 1949, Moe Abramson and Stanislaus F. Danko of the United States Army Signal Corps developed theAuto-Semblyprocess in which component leads were inserted into a copper foil interconnection pattern anddip soldered.The patent they obtained in 1956 was assigned to the U.S. Army.With the development of boardlaminationandetchingtechniques, 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 awave-solderingmachine. 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 usingmicroviatechnology to produce High-Density Interconnect (HDI) PCBs.

Recent advances

Recent advances in3D printinghave 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 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 offlip chippackages.Some PCBs have optical waveguides, similar to optical fibers built on the PCB.

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 copperfoil,laminated to the substrate. Chemical etching divides thecopperinto separate conducting lines called tracks orcircuit traces,pads for connections, vias to pass connections between layers of copper, and features such as solid conductive areas forelectromagnetic shieldingor 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 fromcorrosionand reduces the chances of soldershortsbetween traces or undesired electrical contact with stray bare wires. For its function in helping to prevent solder shorts, the coating is called solder resist orsolder mask.

The pattern to be etched into each copper layer of a PCB is called the "artwork". The etching is usually done usingphotoresistwhich 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 wayphotographscan be mass-duplicated fromfilm negativesusing aphotographic printer.

FR-4glass epoxyis the most common insulating substrate. Another substrate material iscotton paperimpregnated withphenolic resin,often tan or brown.

When a PCB has no components installed, it is less ambiguously called aprinted wiring board(PWB) oretched wiring board.However, the term "printed wiring board" has fallen into disuse. A PCB populated with electronic components is called aprinted circuit assembly(PCA),printed circuit board assemblyorPCB assembly(PCBA). In informal usage, the term "printed circuit board" most commonly means "printed circuit assembly" (with components). TheIPCpreferred term for an assembled board iscircuit card assembly(CCA),and for an assembledbackplaneit isbackplane 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 printingwas 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 asground planeor 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-1980sCommodore 64home computer

A box ofdrill bitsused 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,transistorsand an integrated circuit

A PCB in acomputer 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 fortransistors,diodes,IC chips,resistors,and capacitors. Through-hole mounting may be used for some large components such aselectrolytic capacitorsand connectors.

The first PCBs usedthrough-hole technology,mounting electronic components by lead 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 besolderedeither manually or by awave solderingmachine.

Surface-mount technologyemerged 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 semiconductorsurface 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 as1N4148small-signal switch diodes, are actually significantly cheaper than SMD equivalents.

Electrical properties

Each trace consists of a flat, narrow part of thecopperfoil that remains after etching. Itsresistance,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 thansignal traces.In a multi-layer board one entire layer may be mostly solid copper to act as aground planefor shielding and power return. Formicrowavecircuits,transmission linescan be laid out ina planar formsuch asstriplineormicrostripwith carefully controlled dimensions to assure a consistentimpedance.In radio-frequency and fast switching circuits theinductanceandcapacitanceof 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 indistributed-element filters,antennae,andfuses,obviating the need for additional discrete components. High density interconnects (HDI) PCBs have tracks or vias with a width or diameter of under 152 micrometers.

Materials

Laminates

Laminates are manufactured by curing layers of cloth or paper withthermosetresin under pressure and heat to form an integral final piece of uniform thickness. They 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, andresinpercentage are used to achieve the desired final thickness anddielectriccharacteristics. Available standard laminate thickness are listed in ANSI/IPC-D-275.

The cloth or fiber material used,resinmaterial, 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 isfire retardant,thedielectric constant(e_r), theloss tangent(tan δ), thetensile strength,theshear strength,theglass transition temperature(T_g), and the Z-axisexpansion 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 arepolytetrafluoroethylene(Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well known pre-preg materials used in the PCB industry areFR-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 withball 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 stock with unetched copper on it is called "copper-clad laminate".

With decreasing size of board features and increasing frequencies, small nonhomogeneities like uneven distribution of fiberglass or other filler, thickness variations, and bubbles in the resin matrix, and the associated local variations in the dielectric constant, are gaining importance.

Key substrate parameters

The circuit-board substrates are usually dielectric composite materials. The composites contain a matrix (usually anepoxy resin) and a reinforcement (usually a woven, sometimes nonwoven, glass fibers, sometimes even paper), and in some cases a filler is added to the resin (e.g. ceramics; titanate ceramics can be used to increase the dielectric constant).

The reinforcement type defines two major classes of materials: woven and non-woven. Woven reinforcements are cheaper, but the high dielectric constant of glass may not be favorable for many higher-frequency applications. The spatially nonhomogeneous structure also introduces local variations in electrical parameters, due to different resin/glass ratio at different areas of the weave pattern. Nonwoven reinforcements, or materials with low or no reinforcement, are more expensive but more suitable for some RF/analog applications.

The substrates are characterized by several key parameters, chiefly thermomechanical (glass transition temperature,tensile strength,shear strength,thermal expansion), electrical (dielectric constant,loss tangent,dielectric breakdown voltage,leakage current,tracking resistance...), and others (e.g.moisture absorption).

At theglass transition temperaturethe resin in the composite softens and significantly increases thermal expansion; exceeding T_gthen exerts mechanical overload on the board components - e.g. the joints and the vias. Below T_gthe thermal expansion of the resin roughly matches copper and glass, above it gets significantly higher. As the reinforcement and copper confine the board along the plane, virtually all volume expansion projects to the thickness and stresses the plated-through holes. Repeated soldering or other exposition to higher temperatures can cause failure of the plating, especially with thicker boards; thick boards therefore require a matrix with a high T_g.

The materials used determine the substrate'sdielectric constant.This constant is also dependent on frequency, usually decreasing with frequency. As this constant determines thesignal propagation speed,frequency dependence introduces phase distortion in wideband applications; as flat a dielectric constant vs frequency characteristics as is achievable is important here. The impedance of transmission lines decreases with frequency, therefore faster edges of signals reflect more than slower ones.

Dielectric breakdown voltage determines the maximum voltage gradient the material can be subjected to before suffering a breakdown (conduction, or arcing, through the dielectric).

Tracking resistance determines how the material resists high voltage electrical discharges creeping over the board surface.

Loss tangent determines how much of the electromagnetic energy from the signals in the conductors is absorbed in the board material. This factor is important for high frequencies. Low-loss materials are more expensive. Choosing unnecessarily low-loss material is a common engineering error in high-frequency digital design; it increases the cost of the boards without a corresponding benefit. Signal degradation by loss tangent and dielectric constant can be easily assessed by aneye pattern.

Moisture absorption occurs when the material is exposed to high humidity or water. Both the resin and the reinforcement may absorb water; water also may be soaked by capillary forces through voids in the materials and along the reinforcement. Epoxies of the FR-4 materials are not too susceptible, with absorption of only 0.15%.Teflonhas very low absorption of 0.01%.Polyimidesand cyanate esters, on the other side, suffer from high water absorption. Absorbed water can lead to significant degradation of key parameters; it impairs tracking resistance, breakdown voltage, and dielectric parameters. Relative dielectric constant of water is about 73, compared to about 4 for common circuit board materials. Absorbed moisture can also vaporize on heating, as duringsoldering,and cause cracking anddelamination,the same effect responsible for "popcorning" damage on wet packaging of electronic parts. Careful baking of the substrates may be required to dry them prior to soldering.

Common substrates

Often encountered materials:

• FR-2,phenolic paperor phenolic cotton paper, paper impregnated with aphenol formaldehyde resin.Common in consumer electronics with single-sided boards. Electrical properties inferior to FR-4. Poor arc resistance. Generally rated to 105 °C.
• FR-4,a wovenfiberglasscloth impregnated with anepoxy resin.Low water absorption (up to about 0.15%), good insulation properties, good arc resistance. Very common. Several grades with somewhat different properties are available. Typically rated to 130 °C.
• Aluminum,ormetal core boardorinsulated metal substrate(IMS), clad with thermally conductive thin dielectric - used for parts requiring significant cooling - power switches, LEDs. Consists of usually single, sometimes double layer thin circuit board based on e.g. FR-4, laminated on aluminum sheet metal, commonly 0.8, 1, 1.5, 2 or 3 mm thick. The thicker laminates sometimes also come with thicker copper metalization.
• Flexible substrates- can be a standalone copper-clad foil or can be laminated to a thin stiffener, e.g. 50-130 μm
  • KaptonorUPILEX,apolyimidefoil. Used forflexible printed circuits,in this form common in small form-factor consumer electronics or for flexible interconnects. Resistant to high temperatures.
  • Pyralux,a polyimide-fluoropolymer composite foil.Copper layer can delaminate during soldering.

Less-often encountered materials:

• FR-1, like FR-2, typically specified to 105 °C, some grades rated to 130 °C. Room-temperature punchable. Similar to cardboard. Poor moisture resistance. Low arc resistance.
• FR-3, cotton paper impregnated with epoxy. Typically rated to 105 °C.
• FR-5, woven fiberglass and epoxy, high strength at higher temperatures, typically specified to 170 °C.
• FR-6, matte glass and polyester
• G-10,woven glass and epoxy - high insulation resistance, low moisture absorption, very high bond strength. Typically rated to 130 °C.
• G-11, woven glass and epoxy - high resistance to solvents, high flexural strength retention at high temperatures.Typically rated to 170 °C.
• 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
• PTFE,( "Teflon" ) - expensive, low dielectric loss, for high frequency applications, very low moisture absorption (0.01%), mechanically soft. Difficult to laminate, rarely used in multilayer applications.
• PTFE, ceramic filled - expensive, low dielectric loss, for high frequency applications. Varying ceramics/PTFE ratio allows adjusting dielectric constant and thermal expansion.
• RF-35, fiberglass-reinforced ceramics-filled PTFE. Relatively less expensive, good mechanical properties, good high-frequency properties.
• Alumina,a ceramic. Hard, brittle, very expensive, very high performance, good thermal conductivity.
• Polyimide,a high-temperature polymer. Expensive, high-performance. Higher water absorption (0.4%). Can be used from cryogenic temperatures to over 260 °C.

Copper thickness

Copper thickness of PCBs can be specified directly or as the weight of copper per area (in ounce per square foot) which is easier to measure. Oneouncepersquare footis 1.344 mils or 34 micrometers thickness.Heavy copperis a layer exceeding three ounces of copper per ft²,or approximately 0.0042 inches (4.2 mils, 105 μm) thick. Heavy copper layers are used for high current or to help dissipate heat.

On the common FR-4 substrates, 1ozcopper per ft²(35 μm) is the most common thickness; 2 oz (70 μm) and 0.5 oz (17.5 μm) thickness is often an option. Less common are 12 and 105 μm, 9 μm is sometimes available on some substrates. Flexible substrates typically have thinner metalization. Metal-core boards for high power devices commonly use thicker copper; 35 μm is usual but also 140 and 400 μm can be encountered.

In the US, copper foil thickness is specified in units ofouncesper square foot (oz/ft²), commonly referred to simply asounce.Common thicknesses are 1/2 oz/ft²(150 g/m²), 1 oz/ft²(300 g/m²), 2 oz/ft²(600 g/m²), and 3 oz/ft²(900 g/m²). These work out to thicknesses of 17.05 μm (0.67thou), 34.1 μm (1.34thou), 68.2 μm (2.68 thou), and 102.3 μm (4.02 thou), respectively.

oz/ft2	g/m2	μm	thou
1/2 oz/ft2	150 g/m2	17.05 μm	0.67 thou
1 oz/ft2	300 g/m2	34.1 μm	1.34 thou
2 oz/ft2	600 g/m2	68.2 μm	2.68 thou
3 oz/ft2	900 g/m2	102.3 μm	4.02 thou

1/2 oz/ft²foil is not widely used as a finished copper weight, but is used for outer layers when plating for through holes will increase the finished copper weight Some PCB manufacturers refer to 1 oz/ft²copper foil as having a thickness of 35 μm (may also be referred to as 35 μ, 35micron,or 35 mic).

• 1/0 – denotes 1 oz/ft²copper one side, with no copper on the other side.
• 1/1 – denotes 1 oz/ft²copper on both sides.
• H/0 or H/H – denotes 0.5 oz/ft²copper on one or both sides, respectively.
• 2/0 or 2/2 – denotes 2 oz/ft²copper on one or both sides, respectively.

Construction

Design

A board designed in 1967; the sweeping curves in the traces are evidence of freehand design using adhesive tape

Manufacturing starts from the fabrication data generated bycomputer aided design,and component information. The fabrication data is read into the CAM (Computer Aided Manufacturing) software. CAM performs the following functions:

1. Input of the fabrication data.
2. Verification of the data
3. Compensation for deviations in the manufacturing processes (e.g. scaling to compensate for distortions during lamination)
4. Panelization
5. Output of the digital tools (copper patterns, drill files, inspection, and others)

Initially PCBs were designed manually by creating aphotomaskon a clearmylarsheet, usually at two or four times the true size. Starting from the schematic diagram the component pin pads were laid out on the mylar and then traces were routed to connect the pads. Rub-ondry transfersof common component footprints increased efficiency. Traces were made with self-adhesive tape. Pre-printed non-reproducing grids on the mylar assisted in layout. The finished photomask wasphotolithographicallyreproduced onto a photoresist coating on the blank copper-clad boards.

A PCB as a design on a computer (left) and realized as a board assembly populated with components (right). The board is double sided, with through-hole plating, green solder resist and a white legend. Both surface mount and through-hole components have been used.

Modern PCBs are designed with dedicated layout software, generally in the following steps:

1. Schematic capturethrough anelectronic design automation(EDA) tool.
2. Card dimensions and template are decided based on required circuitry and enclosure of the PCB.
3. The positions of the components andheat sinksare determined.
4. Layer stack of the PCB is decided, with one to tens of layers depending on complexity.Groundandpower planesare decided. A power plane is the counterpart to a ground plane and behaves as anACsignal ground while providing DC power to the circuits mounted on the PCB. Signal interconnections are traced on signal planes. Signal planes can be on the outer as well as inner layers. For optimalEMIperformance high frequency signals are routed in internal layers between power or ground planes.
5. Line impedanceis determined using dielectric layer thickness, routing copper thickness and trace-width. Trace separation is also taken into account in case of differential signals.Microstrip,striplineor dual stripline can be used to route signals.
6. Components are placed. Thermal considerations and geometry are taken into account.Viasand lands are marked.
7. Signal tracesarerouted.Electronic design automation tools usually create clearances and connections in power and ground planes automatically.
8. Fabrication data consists of a set ofGerber files,a drill file, and a pick-and-place file.

Panelization

Several small printed circuit boards can be grouped together for processing as a panel. A panel consisting of a design duplicatedn-times is also called ann-panel, whereas amulti-panelcombines several different designs onto a single panel. The outer tooling strip often includestooling holes,a set ofpanel fiducials,atest coupon,and may includehatched copper pouror similar patterns for even copper distribution over the whole panel in order to avoid bending. The assemblers often mount components on panels rather than single PCBs because this is efficient. Panelization may also be necessary for boards with components placed near an edge of the board because otherwise the board could not be mounted during assembly. Most assembly shops require a free area of at least 10 mm around the board.

Depaneling

Main article:depaneling

The panel is eventually broken into individual PCBs along perforations or grooves in the panelthrough milling or cutting. For milled panels a common distance between the individual boards is 2–3 mm. Today depaneling is often done by lasers which cut the board with no contact. Laser depaneling reduces stress on the fragile circuits, improving the yield of defect-free units.

Copper patterning

The first step is to replicate the pattern in the fabricator's CAM system on a protective mask on the copper foil PCB layers. Subsequent etching removes the unwanted copper unprotected by the mask. (Alternatively, a conductive ink can be ink-jetted on a blank (non-conductive) board. This technique is also used in the manufacture ofhybrid circuits.)

1. Silk screen printinguses etch-resistant inks to create the protective mask.
2. Photoengravinguses a photomask and developer to selectively remove a UV-sensitive photoresist coating and thus create a photoresist mask that will protect the copper below it. Direct imaging techniques are sometimes used for high-resolution requirements. Experiments have been made with thermal resist.A laser may be used instead of a photomask. This is known asmaskless lithographyor direct imaging.
3. PCB millinguses 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 aplotter,receiving commands from the host software that control the position of the milling head in the x, y, and (if relevant) z axis.
4. Laser resist ablationinvolves spraying black paint onto copper clad laminate, then placing the board intoCNClaser plotter. The laser raster-scans the PCB and ablates (vaporizes) the paint where no resist is wanted. (Note: laser copper ablation is rarely used and is considered experimental.)
5. Laser etching,in which the copper may be removed directly by a CNC laser. Like PCB milling above, this is used mainly for prototyping.
6. EDMetchinguses anelectrical dischargeto remove a metal from a substrate submerged into adielectric fluid.

The method chosen depends on the number of boards to be produced and the required resolution.

Large volume

• Silk screen printing – Used for PCBs with bigger features
• Photoengraving – Used when finer features are required

Small volume

• Print onto transparent film and use as photo mask along with photo-sensitized boards, then etch. (Alternatively, use a film photoplotter.)
• Laser resist ablation
• PCB milling
• Laser etching

Hobbyist

• Laser-printed resist: Laser-print onto toner transfer paper, heat-transfer with an iron or modified laminator onto bare laminate, soak in water bath, touch up with a marker, then etch.
• Vinyl filmand resist, non-washable marker, some other methods. Labor-intensive, only suitable for single boards.

Etching

PCB copper electroplating line in the process of pattern plating copper

PCBs in process of having copper pattern plated (note the blue dry film resist)

The two processing methods used to produce a double-sided PWB with plated-through holes

The process by which copper traces are applied to the surface is known asetchingafter the subtractive method of the process, though there are also additive and semi-additive methods.

Subtractive methods remove copper from an entirely copper-coated board to leave only the desired copper pattern. The simplest method, used for small-scale production and often by hobbyists, is immersion etching, in which the board is submerged in etching solution such asferric chloride.Compared with methods used for mass production, the etching time is long. Heat and agitation can be applied to the bath to speed the etching rate. In bubble etching, air is passed through the etchant bath to agitate the solution and speed up etching. Splash etching uses a motor-driven paddle to splash boards with etchant; the process has become commercially obsolete since it is not as fast as spray etching. In spray etching, the etchant solution is distributed over the boards by nozzles, and recirculated by pumps. Adjustment of the nozzle pattern, flow rate, temperature, and etchant composition gives predictable control of etching rates and high production rates.As more copper is consumed from the boards, the etchant becomes saturated and less effective; different etchants have different capacities for copper, with some as high as 150 grams of copper per liter of solution. In commercial use, etchants can be regenerated to restore their activity, and the dissolved copper recovered and sold. Small-scale etching requires attention to disposal of used etchant, which is corrosive and toxic due to its metal content.The etchant removes copper on all surfaces not protected by the resist. "Undercut" occurs when etchant attacks the thin edge of copper under the resist; this can reduce conductor widths and cause open-circuits. Careful control of etch time is required to prevent undercut. Where metallic plating is used as a resist, it can "overhang" which can cause short circuits between adjacent traces when closely spaced. Overhang can be removed by wire-brushing the board after etching.

In additive methods the pattern iselectroplatedonto a bare substrate using a complex process. The advantage of the additive method is that less material is needed and less waste is produced. In the full additive process the bare laminate is covered with a photosensitive film which is imaged (exposed to light through a mask and then developed which removes the unexposed film). The exposed areas are sensitized in a chemical bath, usually containing palladium and similar to that used for through hole plating which makes the exposed area capable of bonding metal ions. The laminate is then plated with copper in the sensitized areas. When the mask is stripped, the PCB is finished.

Semi-additive is the most common process: 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 bare original copper laminate from the board, isolating the individual traces. Some single-sided boards which have plated-through holes are made in this way.General Electricmade consumer radio sets in the late 1960s using additive boards. The (semi-)additive process is commonly used for multi-layer boards as it facilitates theplating-through of the holes to produce conductiveviasin the circuit board.

Industrial etchingis usually done withammonium persulfateorferric chloride.For PTH (plated-through holes), additional steps ofelectroless depositionare done after the holes are drilled, then copper is electroplated to build up the thickness, the boards are screened, and plated with tin/lead. The tin/lead becomes the resist leaving the bare copper to be etched away.

Lamination

Cut through a SDRAM-module, a multi-layer PCB (BGAmounted). Note thevia,visible as a bright copper-colored band running between the top and bottom layers of the board.

Multi-layer printed circuit boards have trace layers inside the board. This is achieved by laminating a stack of materials in a press by applying pressure and heat for a period of time. This results in an inseparable one piece product. For example, a four-layer PCB can be fabricated by starting from a two-sided copper-clad laminate, etch the circuitry on both sides, then laminate to the top and bottom pre-preg and copper foil. It is then drilled, plated, and etched again to get traces on top and bottom layers.

The inner layers are given a complete machine inspection before lamination because mistakes cannot be corrected afterwards. Automatic optical inspection (AOI) machines compare an image of the board with the digital image generated from the original design data. Automated Optical Shaping (AOS) machines can then add missing copper or remove excess copper using a laser, reducing the number of PCBs that have to be discarded.PCB tracks can have a width of just 10 micrometers.

Drilling

Eyelets (hollow)

Holes through a PCB are typically drilled withdrill bitscoated withtungsten carbide.Coated tungsten carbide is used because board materials are abrasive. High-speed-steel bits would dull quickly, tearing the copper and ruining the board. Drilling is done by computer-controlled drilling machines, using adrill fileorExcellon filethat describes the location and size of each drilled hole.

Vias

Holes may be made conductive, by electroplating or inserting hollow metal eyelets, to connect board layers. Some conductive holes are intended for the insertion of through-hole-component leads. Others used to connect board layers, are calledvias.

Micro vias

When vias with a diameter smaller than 76.2 micrometers are required, drilling with mechanical bits is impossible because of high rates of wear and breakage. In this case, the vias may belaser drilled—evaporated bylasers.Laser-drilled vias typically have an inferior surface finish inside the hole. These holes are calledmicro viasand can have diameters as small as 10 micrometers.

Blind and buried vias

It is also possible withcontrolled-depthdrilling, 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 calledblind viaswhen they connect an internal copper layer to an outer layer, orburied viaswhen they connect two or more internal copper layers and no outer layers. Laser drilling machines can drill thousands of holes per second and can use either UV or CO₂lasers.

The hole walls for boards with two or more layers can be made conductive and then electroplated with copper to formplated-through holes.These holes electrically connect the conducting layers of the PCB.

Smear

For multi-layer boards, those with three layers or more, drilling typically produces asmearof the high temperature decomposition products of bonding agent in the laminate system. Before the holes can be plated through, this smear must be removed by a chemicalde-smearprocess, or byPlasma etching.The de-smear process ensures that a good connection is made to the copper layers when the hole is plated through. On high reliability boards a process called etch-back is performed chemically with a potassium permanganate based etchant or plasma etching. The etch-back removes resin and the glass fibers so that the copper layers extend into the hole and as the hole is plated become integral with the deposited copper.

Plating and coating

Proper plating or surface finish selection can be critical to process yield, the amount of rework, field failure rate, and reliability.

PCBs may be plated with solder, tin, or gold over nickel.

After PCBs are etched and then rinsed with water, thesolder maskis applied, and then any exposed copper is coated with solder, nickel/gold, or some other anti-corrosion coating.

It is important to use solder compatible with both the PCB and the parts used. An example isball grid array(BGA) using tin-lead solder balls for connections losing their balls on bare copper traces or using lead-free solder paste.

Other platings used areorganic solderability preservative(OSP),immersion silver(IAg),immersion tin(ISn),electroless nickel immersion gold(ENIG) coating,electroless nickel electroless palladium immersion gold(ENEPIG), and directgold plating(over nickel).Edge connectors,placed along one edge of some boards, are often nickel-plated thengold-platedusing ENIG. Another coating consideration is rapid diffusion of coating metal into tin solder. Tin forms intermetallics such as Cu₆Sn₅and Ag₃Cu that dissolve into the Tin liquidus or solidus (at 50 °C), stripping surface coating or leaving voids.

Electrochemical migration(ECM) is the growth of conductive metal filaments on or in a printed circuit board (PCB) under the influence of a DC voltage bias.Silver, zinc, and aluminum are known to growwhiskersunder the influence of an electric field. Silver also grows conducting surface paths in the presence of halide and other ions, making it a poor choice for electronics use. Tin will grow "whiskers" due to tension in the plated surface. Tin-lead or solder plating also grows whiskers, only reduced by reducing the percentage of tin. Reflow to melt solder or tin plate to relieve surface stress lowers whisker incidence. Another coating issue istin pest,the transformation of tin to a powdery allotrope at low temperature.

Solder resist application

A PCB with red solder mask and white silkscreen

A PCB with green solder mask and yellow silkscreen

Areas that should not be soldered may be covered with solder resist (solder mask). The solder mask is what gives PCBs their characteristic green color, although it is also available in several other colors, such as red, blue, purple, yellow, black and white. One of the most common solder resists used today is called "LPI" (liquid photoimageable solder mask).A photo-sensitive coating is applied to the surface of the PWB, then exposed to light through the solder mask image film, and finally developed where the unexposed areas are washed away. Dry film solder mask is similar to the dry film used to image the PWB for plating or etching. After being laminated to the PWB surface it is imaged and developed as LPI. Once but no longer commonly used, because of its low accuracy and resolution, is to screen print epoxy ink. In addition to repelling solder, solder resist also provides protection from the environment to the copper that would otherwise be exposed.

Legend / silkscreen

A legend (also known assilkorsilkscreen) is often printed on one or both sides of the PCB. It contains thecomponent designators,switch settings, test points and other indications helpful in assembling, testing, servicing, and sometimes using the circuit board.

There are three methods to print the legend:

1. Silkscreen printingepoxy ink was the established method, resulting in the alternative name.
2. Liquid photo imaging is a more accurate method than screen printing.
3. Inkjet printingis increasingly used. Inkjet printers can print variable data, unique to each PCB unit, such as text, aserial number,or abar code.

Bare-board test

Boards with no components installed are usuallybare-board testedfor "shorts" and "opens". This is calledelectrical testorPCB e-test.A short is a connection between two points that should not be connected. An open is a missing connection between points that should be connected.For high-volume testing, arigid needle adaptermakes contact with copper lands on the board.The fixture or adapter is a significant fixed cost and this method is only economical for high-volume or high-value production. For small or medium volume productionflying probetesters are used where test probes are moved over the board by an XY drive to make contact with the copper lands. There is no need for a fixture and hence the fixed costs are much lower. The CAM systeminstructsthe electrical tester to apply a voltage to each contact point as required and to check that this voltage appears on the appropriate contact points and only on these.

Assembly

PCB withtest points

In assembly the bare board is populated (or "stuffed" ) with electronic components to form a functionalprinted circuit assembly(PCA), sometimes called a "printed circuit board assembly" (PCBA).Inthrough-hole technology,the component leads are inserted in holes surrounded by conductivepads;the holes keep the components in place. Insurface-mount technology(SMT), the component is placed on the PCB so that the pins line up with the conductivepadsorlandson the surfaces of the PCB; solder paste, which was previously applied to the pads, holds the components in place temporarily; if surface-mount components are applied to both sides of the board, the bottom-side components are glued to the board. In both through hole and surface mount, the components are thensoldered;once cooled and solidified, the solder holds the components in place permanently and electrically connects them to the board.

There are a variety ofsolderingtechniques used to attach components to a PCB. High volume production is usually done with apick-and-place machineand bulk wave soldering for through-hole parts orreflow ovensfor SMT components or through-hole parts, but skilled technicians are able to hand-solder very tiny parts (for instance 0201 packages which are 0.02 in. by 0.01 in.)under amicroscope,using tweezers and a fine-tipsoldering iron,for small volume prototypes.Selective solderingmay be used for delicate parts. Some SMT parts cannot be soldered by hand, such asball grid array(BGA) packages. All through-hole components can be hand soldered, making them favored for prototyping where size, weight, and the use of the exact components that would be used in high volume production are not concerns.

Often, through-hole and surface-mount construction must be combined in a single assembly because some required components are available only in surface-mount packages, while others are available only in through-hole packages. Or, even if all components are available in through-hole packages, it might be desired to take advantage of the size, weight, and cost reductions obtainable by using some available surface-mount devices. Another reason to use both methods is that through-hole mounting can provide needed strength for components likely to endure physical stress (such as connectors that are frequently mated and demated or that connect to cables expected to impart substantial stress to the PCB-and-connector interface), while components that are expected to go untouched will take up less space using surface-mount techniques.For further comparison, see theSMT page.

After the board has been populated it may be tested in a variety of ways:

• While the power is off,visual inspection,automated optical inspection.JEDECguidelines for PCB component placement, soldering, and inspection are commonly used to maintainquality controlin this stage of PCB manufacturing.
• While the power is off,analog signature analysis,power-off testing.
• While the power is on,in-circuit test,where physical measurements (for example, voltage) can be done.
• While the power is on,functional test,just checking if the PCB does what it had been designed to do.

To facilitate these tests, 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 exerciseboundary scantest features of some components. In-circuit test systems may also be used to programnonvolatile memorycomponents 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. TheJTAGtest architecture provides a means to test interconnects between integrated circuits on a board without using physical test probes, by using circuitry in the ICs to employ the IC pins themselves as test probes.JTAGtool vendors provide various types of stimuli and sophisticated algorithms, not only to detect the failing nets, but also to isolate the faults to specific nets, devices, and pins.

When boards fail the test, technicians maydesolderand replace failed components, a task known asrework.

Protection and packaging

PCBs intended for extreme environments often have aconformal coating,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 werewax;modern conformal coats are usually dips of dilute solutions of silicone rubber, polyurethane, acrylic, or epoxy. Another technique for applying a conformal coating is for plastic to besputteredonto the PCB in a vacuum chamber. The chief disadvantage of conformal coatings is that servicing of the board is rendered extremely difficult.

Many assembled PCBs arestaticsensitive, and therefore they must be placed inantistatic bagsduring transport. When handling these boards, the user must begrounded (earthed).Improper handling techniques might transmit an accumulated static charge through the board, damaging or destroying components. The damage might not immediately affect function but might lead to early failure later on, cause intermittent operating faults, or cause a narrowing of the range of environmental and electrical conditions under which the board functions properly.

Cordwood construction

A cordwood module

Cordwood construction was used inproximity fuzes.

Cordwood construction can save significant space and was often used withwire-ended componentsin applications where space was at a premium (such asfuzes,missile guidance, and telemetry systems) and in high-speedcomputers,where short traces were important. In cordwood construction, axial-leaded components were mounted between two parallel planes. The name comes from the way axial-lead components (capacitors, resistors, coils, and diodes) are stacked in parallel rows and columns, like a stack of firewood. 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 specialnickel-leaded components had to be used to allow reliable 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 mechanical 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 at the cost of being difficult to remove the boards or replace any component that is not at the edge.

Before the advent ofintegrated circuits,this method allowed the highest possible component packing density; because of this, it was used by a number of computer vendors includingControl Data Corporation.

Types

Further information:Monoboard

Breakout boards

A breakout board can allow interconnection between two incompatible connectors.

This breakout board allows anSD card's pins to be accessed easily while still allowing the card to be hot-swapped.

A minimal PCB for a single component, used forprototyping,is called abreakout board.The purpose of a breakout board is to "break out" the leads of a component on separate terminals so that manual connections to them can be made easily. Breakout boards are especially used for surface-mount components or any components with fine lead pitch.

Advanced PCBs may contain components embedded in the substrate, such as capacitors and integrated circuits, to reduce the amount of space taken up by components on the surface of the PCB while improving electrical characteristics.

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. As of 2010,Multiwire is still available through Hitachi.

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 a 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 reducedcrosstalk(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.

Corrections can be made to a Multiwire board layout more easily than to a PCB layout.

Uses

Printed circuit boards have been used as an alternative to their typical use for electronic andbiomedical engineeringthanks to the versatility of their layers, especially the copper layer. PCB layers have been used to fabricate sensors, such as capacitive pressure sensors and accelerometers, actuators such as microvalves and microheaters, as well as platforms of sensors and actuators forLab-on-a-chip(LoC), for example to performpolymerase chain reaction(PCR), and fuel cells, to name a few.

Repair

Manufacturers may not support component-level repair of printed circuit boards because of the relatively low cost to replace compared with the time and cost of troubleshooting to a component level. In board-level repair, the technician identifies the board (PCA) on which the fault resides and replaces it. This shift is economically efficient from a manufacturer's point of view but is also materially wasteful, as a circuit board with hundreds of functional components may be discarded and replaced due to the failure of one minor and inexpensive part, such as a resistor or capacitor. This practice is a significant contributor to the problem ofe-waste.

Legislation

Main articles:Restriction of Hazardous Substances DirectiveandChina RoHS

In many countries (including allEuropean Single Marketparticipants,theUnited Kingdom,Turkey,andChina), legislation restricts the use oflead,cadmiumandmercuryin electrical equipment. PCBs sold in such countries must therefore use lead-free manufacturing processes and lead-free solder, and attached components must themselves be compliant.

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, maximumoperating temperature,electrical tracking, heat deflection, and direct support of live electrical parts.

See also

• Breadboard
• BT-Epoxy- resin used in PCBs
• Certified interconnect designer- qualification for PCB designers
• Occam process- solder-free circuit board manufacture method

Further reading

• Tavernier, Karel (September 2015)."PCB Fabrication Data - Design to Fabrication Data Transfer"(PDF).V6.Archived(PDF)from the original on March 8, 2022.RetrievedMay 9,2022.
• Colotti, James (2022)."Analog, RF and EMC Considerations in Printed Wiring Board (PWB) Design"(PDF).Revision 5.IEEE,Long Island Section.Archived(PDF)from the original on May 8, 2022.RetrievedMay 9,2022.

• Media related toPrinted circuit boardsat Wikimedia Commons