printed circuit board not to be confused with printed electronics printed circuit redirects here for the defunct company see printed circuit corporation part of a 1984 sinclair zx spectrum computer board a pcb showing the conductive traces vias the throughhole paths to the other surface and some electronic components mounted using throughhole mounting a printed circuit board pcb mechanically supports and electrically connects electronic components or electrical components using conductive tracks pads and other features etched from one or more sheet layers of copper laminated onto andor between sheet layers of a nonconductive substrate components are generally soldered onto the pcb to both electrically connect and mechanically fasten them to it printed circuit boards are used in all but the simplest electronic products they are also used in some electrical products such as passive switch boxes alternatives to pcbs include wire wrap and pointtopoint 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 specialized cad software is available to do much of the work of layout massproducing 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 only has to be done once pcbs can also be made manually in small quantities with reduced benefits pcbs can be singlesided one copper layer doublesided two copper layers on both sides of one substrate layer or multilayer outer and inner layers of copper alternating with layers of substrate multilayer 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 602 billion in 2014 history before the development of printed circuit boards electrical and electronic circuits were wired pointtopoint 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 laborintensive 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 printandetch method in the uk and in the united states max schoop obtained a patent 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 multilayer 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 fuses 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 mid1950s after the autosembly 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 even as circuit boards became available the pointtopoint chassis construction method remained in common use in industry such as tv and hifi 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 an example of handdrawn etched traces on a pcb predating the printed circuit invention and similar in spirit was john sargroves 19361947 electronic circuit making equipment ecme which sprayed metal onto a bakelite plastic board the ecme could produce three radio boards per minute during world war ii the development of the antiaircraft proximity fuse 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 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 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 throughhole plating green solder resist and a white legend both surface mount and throughhole components have been used 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 throughhole construction in 1949 moe abramson and stanislaus f danko of the united states army signal corps developed the autosembly 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 us army 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 wavesoldering 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 throughhole 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 highdensity interconnect hdi pcbs hdi technology allows for a denser design on the pcb and significantly smaller components as a result components can be closer and the paths between them shorter hdis use blindburied vias or a combination that includes microvias with multilayer hdi pcbs the interconnection of stacked vias is even 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 4layer hdi microvia pcb cost is equivalent in quality to an 8layer throughhole pcb however the cost is much lower 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 manufacturers may not support componentlevel 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 boardlevel repair the technician identifies the board pca on which the fault resides and replaces it this shift is economically efficient from a manufacturers point of view but is also materially wasteful as a circuit board with hundreds of good 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 of ewaste overview 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 em 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 a printed circuit board can have multiple copper layers a twolayer board has copper on both sides multi layer boards sandwich additional copper layers between layers of insulating material conductors on different layers are connected with vias which are copperplated holes that function as electrical tunnels through the insulating substrate throughhole component leads sometimes also effectively function as vias after twolayer pcbs the next step up is usually fourlayer often two layers are dedicated as power supply and ground planes and the other two are used for signal wiring between components 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 throughhole mounted components are now uncommon surface mounting is used for transistors diodes ic chips resistors and capacitors throughhole mounting may be used for some large components such as electrolytic capacitors and connectors 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 massreproduced in a way similar to the way photographs can be massduplicated from film negatives using a photographic printer in multilayer 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 platedthrough before the layers are laminated together only the outer layers need be coated the inner copper layers are protected by the adjacent substrate layers fr4 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 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 assembled boards is circuit card assembly cca and for assembled backplanes it is backplane assemblies card is another widely used informal term for a printed circuit assembly a pcb may be silkscreen printed with a legend identifying the components test points or identifying text originally an actual silkscreen printing process was used for this purpose but today other finer quality printing methods are usually used instead normally the screen printing is not significant to the function of the pcba a minimal pcb for a single component used for prototyping is called a breakout 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 surfacemount components or any components with fine lead pitch advanced pcbs may contain components embedded in the substrate characteristics throughhole technology throughhole technology throughhole leaded resistors the first pcbs used throughhole 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 singlesided with an unplated component side or more compact doublesided boards with components soldered on both sides horizontal installation of throughhole 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 parts mechanical strength soldering the leads and trimming off the ends leads may be soldered either manually or by a wave soldering machine throughhole 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 multilayer boards since the holes must pass through all layers to the opposite side once surfacemounting came into use smallsized smd components were used where possible with throughhole mounting only of components unsuitably large for surfacemounting due to power requirements or mechanical limitations or subject to mechanical stress which might damage the pcb eg by lifting the copper off the board surface throughhole devices mounted on the circuit board of a mid1980s commodore 64 home computer a box of drill bits used for making holes in printed circuit boards while tungstencarbide bits are very hard they eventually wear out or break drilling is a considerable part of the cost of a throughhole printed circuit board surfacemount technology surfacemount technology surface mount components including resistors transistors and an integrated circuit surfacemount technology emerged in the 1960s gained momentum in the early 1980s and became widely used by the mid1990s 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 throughhole 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 onequarter to onetenth of the size and weight of throughhole 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 wireended components such as 1n4148 smallsignal switch diodes are actually significantly cheaper than smd equivalents a pcb in a computer mouse the component side left and the printed side right circuit properties of the pcb 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 multilayer 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 radiofrequency 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 obviating the need for additional discrete components materials rohs compliant pcb the european union bans the use of lead among other heavy metals in consumer items a piece of legislature called the rohs for restriction of hazardous substances directive pcbs to be sold in the eu must be rohscompliant meaning that all manufacturing processes must not involve the use of lead all solder used must be leadfree and all components mounted on the board must be free of lead mercury cadmium and other heavy metals laminates 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 12 by 24m 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 ansiipcd275 the cloth or fiber material used resin material and the cloth to resin ratio determine the laminates type designation fr4 cem1 g10 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 t the tensile strength the shear strength the glass transition temperature tg and the zaxis 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 fr4 fr1 cem1 or cem3 well known prepreg materials used in the pcb industry are fr2 phenolic cotton paper fr3 cotton paper and epoxy fr4 woven glass and epoxy fr5 woven glass and epoxy fr6 matte glass and polyester g10 woven glass and epoxy cem1 cotton paper and epoxy cem2 cotton paper and epoxy cem3 nonwoven glass and epoxy cem4 woven glass and epoxy cem5 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 fr4 is by far the most common material used today the board stock with unetched copper on it is called copperclad 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 circuitboard substrates are usually dielectric composite materials the composites contain a matrix usually an epoxy 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 eg ceramics titanate ceramics can be used to increase the dielectric constant the reinforcement type defines two major classes of materials woven and nonwoven woven reinforcements are cheaper but the high dielectric constant of glass may not be favorable for many higherfrequency applications the spatially nonhomogeneous structure also introduces local variations in electrical parameters due to different resinglass 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 rfanalog 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 eg moisture absorption at the glass transition temperature the resin in the composite softens and significantly increases thermal expansion exceeding tg then exerts mechanical overload on the board components eg the joints and the vias below tg the 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 platedthrough 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 tg the materials used determine the substrates dielectric constant this constant is also dependent on frequency usually decreasing with frequency as this constant determines the signal 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 lowloss materials are more expensive choosing unnecessarily lowloss material is a common engineering error in highfrequency 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 an eye 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 fr4 materials arent too susceptible with absorption of only 015 teflon has very low absorption of 001 polyimides and 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 during soldering and cause cracking and delamination common substrates often encountered materials fr2 phenolic paper or phenolic cotton paper paper impregnated with a phenol formaldehyde resin common in consumer electronics with singlesided boards electrical properties inferior to fr4 poor arc resistance generally rated to 105c fr4 a woven fiberglass cloth impregnated with an epoxy resin low water absorption up to about 015 good insulation properties good arc resistance very common several grades with somewhat different properties are available typically rated to 130c aluminum or metal core board or insulated 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 eg fr4 laminated on aluminum sheet metal commonly 08 1 15 2 or 3mm thick the thicker laminates sometimes also come with thicker copper metalization flexible substrates can be a standalone copperclad foil or can be laminated to a thin stiffener eg 50130m kapton or upilex a polyimide foil used for flexible printed circuits in this form common in small formfactor consumer electronics or for flexible interconnects resistant to high temperatures pyralux a polyimidefluoropolymer composite foil copper layer can delaminate during soldering lessoften encountered materials fr1 like fr2 typically specified to 105c some grades rated to 130c roomtemperature punchable similar to cardboard poor moisture resistance low arc resistance fr3 cotton paper impregnated with epoxy typically rated to 105c fr5 woven fiberglass and epoxy high strength at higher temperatures typically specified to 170c fr6 matte glass and polyester g10 woven glass and epoxy high insulation resistance low moisture absorption very high bond strength typically rated to 130c g11 woven glass and epoxy high resistance to solvents high flexural strength retention at high temperatures typically rated to 170c cem1 cotton paper and epoxy cem2 cotton paper and epoxy cem3 nonwoven glass and epoxy cem4 woven glass and epoxy cem5 woven glass and polyester ptfe teflon expensive low dielectric loss for high frequency applications very low moisture absorption 001 mechanically soft difficult to laminate rarely used in multilayer applications ptfe ceramic filled expensive low dielectric loss for high frequency applications varying ceramicsptfe ratio allows adjusting dielectric constant and thermal expansion rf35 fiberglassreinforced ceramicsfilled ptfe relatively less expensive good mechanical properties good highfrequency properties alumina a ceramic hard brittle very expensive very high performance good thermal conductivity polyimide a hightemperature polymer expensive highperformance higher water absorption 04 can be used from cryogenic temperatures to over 260c 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 one ounce per square foot is 1344 mils or 34 micrometers thickness heavy copper is a layer exceeding three ounces of copper per ft2 or approximately 00042inches 42mils 105m thick heavy copper layers are used for high current or to help dissipate heat on the common fr4 substrates 1 oz copper per ft2 35m is the most common thickness 2 oz 70m and 05 oz 18m thickness is often an option less common are 12 and 105m 9m is sometimes available on some substrates flexible substrates typically have thinner metalization metalcore boards for high power devices commonly use thicker copper 35m is usual but also 140 and 400m can be encountered 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 design a board designed in 1967 the sweeping curves in the traces are evidence of freehand design using adhesive tape initially pcbs were designed manually by creating a photomask on a clear mylar sheet 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 rubon dry transfers of common component footprints increased efficiency traces were made with selfadhesive tape preprinted nonreproducing grids on the mylar assisted in layout the finished photomask was photolithographically reproduced onto a photoresist coating on the blank copperclad boards modern pcbs are designed with dedicated layout software generally in the following steps schematic capture through an electronic design automation eda tool card dimensions and template are decided based on required circuitry and case of the pcb the positions of the components and heat sinks are determined layer stack of the pcb is decided with one to tens of layers depending on complexity ground and power planes are decided a power plane is the counterpart to a ground plane and behaves as an ac signal 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 optimal emi performance high frequency signals are routed in internal layers between power or ground planes line impedance is determined using dielectric layer thickness routing copper thickness and tracewidth trace separation is also taken into account in case of differential signals microstrip stripline or dual stripline can be used to route signals components are placed thermal considerations and geometry are taken into account vias and lands are marked signal traces are routed electronic design automation tools usually create clearances and connections in power and ground planes automatically gerber files are generated for manufacturing manufacturing pcb manufacturing consists of many steps pcb cam manufacturing starts from the fabrication data generated by computer aided design and component information the fabrication data is read into the cam computer aided manufacturing software cam performs the following functions input of the fabrication data verification of the data compensation for deviations in the manufacturing processes eg scaling to compensate for distortions during lamination panelization output of the digital tools copper patterns drill files inspection and others panelization several small printed circuit boards can be grouped together for processing as a panel a panel consisting of a design duplicated ntimes is also called an npanel whereas a multipanel combines several different design onto a single panel the outer tooling strip often includes tooling holes a set of panel fiducials a test coupon and may include hatched copper pour or 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 the panel is eventually broken into individual pcbs along perforations or grooves in the panel 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 defectfree units copper patterning the first step is to replicate the pattern in the fabricators cam system on a protective mask on the copper foil pcb layers subsequent etching removes the unwanted copper alternatively a conductive ink can be inkjetted on a blank nonconductive board this technique is also used in the manufacture of hybrid circuits silk screen printing uses etchresistant inks to create the protective mask photoengraving uses a photomask and developer to selectively remove a uvsensitive photoresist coating and thus create a photoresist mask direct imaging techniques are sometimes used for highresolution requirements experiments were made with thermal resist pcb milling uses a two or threeaxis 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 laser resist ablation spray black paint onto copper clad laminate place into cnc laser plotter the laser rasterscans the pcb and ablates vaporizes the paint where no resist is wanted note laser copper ablation is rarely used and is considered experimental laser etching the copper may be removed directly by a cnc laser like pcb milling above this is used mainly for prototyping 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 photosensitized boards then etch alternatively use a film photoplotter laser resist ablation pcb milling laser etching hobbyist laserprinted resist laserprint onto toner transfer paper heattransfer with an iron or modified laminator onto bare laminate soak in water bath touch up with a marker then etch vinyl film and resist nonwashable marker some other methods laborintensive only suitable for single boards subtractive additive and semiadditive processes the two processing methods used to produce a doublesided pwb with platedthrough holes subtractive methods remove copper from an entirely coppercoated board to leave only the desired copper pattern in additive methods the pattern is electroplated onto 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 semiadditive 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 tinlead or other surface platings are then applied the mask is stripped away and a brief etching step removes the nowexposed bare original copper laminate from the board isolating the individual traces some singlesided boards which have platedthrough holes are made in this way general electric made consumer radio sets in the late 1960s using additive boards the semiadditive process is commonly used for multilayer boards as it facilitates the platingthrough of the holes to produce conductive vias in the circuit board media all and pcb copper electroplating line in the process of pattern plating copperpcbs in process of having copper pattern plated note the blue dry film resist chemical etching chemical etching is usually done with ammonium persulfate or ferric chloride for pth platedthrough holes additional steps of electroless deposition are done after the holes are drilled then copper is electroplated to build up the thickness the boards are screened and plated with tinlead the tinlead becomes the resist leaving the bare copper to be etched away the simplest method used for smallscale production and often by hobbyists is immersion etching in which the board is submerged in etching solution such as ferric 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 motordriven 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 litre of solution in commercial use etchants can be regenerated to restore their activity and the dissolved copper recovered and sold smallscale 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 exposed by the resist undercut occurs when etchant attacks the thin edge of copper under the resist this can reduce conductor widths and cause opencircuits careful control of etch time is required to prevent undercut where metallic plating is used as a resist it can overhang which can cause shortcircuits between adjacent traces when closely spaced overhang can be removed by wirebrushing the board after etching lamination cut through a sdrammodule a multilayer pcb note the via visible as a bright coppercolored band running between the top and bottom layers of the board multilayer 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 fourlayer pcb can be fabricated by starting from a twosided copperclad laminate etch the circuitry on both sides then laminate to the top and bottom prepreg 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 afterwards mistakes cannot be corrected the automatic optical inspection system compares an image of the board with the digital image generated from the original design data drilling eyelets hollow holes through a pcb are typically drilled with drill bits made of solid coated tungsten carbide coated tungsten carbide is used because board materials are abrasive highspeedsteel bits would dull quickly tearing the copper and ruining the board drilling is done by computercontrolled drilling machines using a drill file or excellon file that describes the location and size of each drilled hole 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 throughholecomponent leads others used to connect board layers are called vias 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 laser drilledevaporated by lasers laserdrilled vias typically have an inferior surface finish inside the hole these holes are called micro vias it is also possible with controlleddepth drilling laser drilling or by predrilling 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 hole walls for boards with two or more layers can be made conductive and then electroplated with copper to form platedthrough holes these holes electrically connect the conducting layers of the pcb for multilayer boards those with three layers or more drilling typically produces a smear of 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 chemical desmear process or by plasmaetch the desmear 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 etchback is performed chemically with a potassium permanganate based etchant or plasma the etchback 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 are plated with solder tin or gold over nickel and a resist for etching away the unneeded underlying copper after pcbs are etched and then rinsed with water the solder mask is applied and then any exposed copper is coated with solder nickelgold or some other anticorrosion coating matte solder is usually fused to provide a better bonding surface for bare copper treatments such as benzimidazolethiol prevent surface oxidation of bare copper the places to which components will be mounted are typically plated because untreated bare copper oxidizes quickly and therefore is not readily solderable traditionally any exposed copper was coated with solder by hot air solder levelling hasl the hasl finish prevents oxidation from the underlying copper thereby guaranteeing a solderable surface this solder was a tinlead alloy however new solder compounds are now used to achieve compliance with the rohs directive in the eu which restricts the use of lead one of these leadfree compounds is sn100cl made up of 993 tin 07 copper 005 nickel and a nominal of 60ppm germanium it is important to use solder compatible with both the pcb and the parts used an example is ball grid array bga using tinlead solder balls for connections losing their balls on bare copper traces or using leadfree solder paste other platings used are osp organic surface protectant immersion silver iag immersion tin electroless nickel with immersion gold coating enig electroless nickel electroless palladium immersion gold enepig and direct gold plating over nickel edge connectors placed along one edge of some boards are often nickelplated then goldplated another coating consideration is rapid diffusion of coating metal into tin solder tin forms intermetallics such as cu6sn5 and ag3cu that dissolve into the tin liquidus or solidus50c 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 solder resist application areas that should not be soldered may be covered with solder resist solder mask one of the most common solder resists used today is called lpi liquid photoimageable solder mask a photosensitive 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 printing a legend is often printed on one or both sides of the pcb it contains the component 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 silk screen printing epoxy ink was the established method it was so common that legend is often misnamed silk or silkscreen liquid photo imaging is a more accurate method than screen printing ink jet printing is new but increasingly used ink jet can print variable data unique to each pwb unit such as text or a bar code with a serial number bareboard test boards with no components installed are usually bareboard tested for shorts and opens 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 highvolume production a fixture or a rigid needle adapter makes contact with copper lands on the board the fixture or adapter is a significant fixed cost and this method is only economical for highvolume or highvalue production for small or medium volume production flying probe testers 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 system instructs the 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 with test connection pads in assembly the bare board is populated or stuffed with electronic components to form a functional printed circuit assembly pca sometimes called a printed circuit board assembly pcba in throughhole technology the component leads are inserted in holes surrounded by conductive pads the holes keep the components in place in surfacemount technology smt the component is placed on the pcb so that the pins line up with the conductive pads or lands on the surfaces of the pcb solder paste which was previously applied to the pads holds the components in place temporarily if surfacemount components are applied to both sides of the board the bottomside components are glued to the board in both through hole and surface mount the components are then soldered once cooled and solidified the solder holds the components in place permanently and electrically connects them to the board there are a variety of soldering techniques used to attach components to a pcb high volume production is usually done with a pick and place machine or smt placement machine and bulk wave soldering or reflow ovens but skilled technicians are able to handsolder very tiny parts for instance 0201 packages which are 002 in by 001 in under a microscope using tweezers and a finetip soldering iron for small volume prototypes some smt parts cannot be soldered by hand such as bga packages all throughhole 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 throughhole and surfacemount construction must be combined in a single assembly because some required components are available only in surfacemount packages while others are available only in throughhole packages or even if all components are available in throughhole packages it might be desired to take advantage of the size weight and cost reductions obtainable by using some available surfacemount devices another reason to use both methods is that throughhole 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 pcbandconnector interface while components that are expected to go untouched will take up less space using surfacemount techniques for further comparison see the smt 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 jedec guidelines for pcb component placement soldering and inspection are commonly used to maintain quality control in this stage of pcb manufacturing while the power is off analog signature analysis poweroff testing while the power is on incircuit 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 incircuit test may also exercise boundary scan test features of some components incircuit 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 the jtag test 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 jtag tool 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 may desolder and replace failed components a task known as rework protection and packaging pcbs intended for extreme environments often have a conformal 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 were wax 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 be sputtered onto 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 are static sensitive and therefore they must be placed in antistatic bags during transport when handling these boards the user must be grounded 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 even bare boards are sometimes static sensitive traces have become so fine that its quite possible to blow an etch off the board or change its characteristics with a static charge this is especially true on nontraditional pcbs such as mcms and microwave pcbs cordwood construction this section needs additional citations for verification please help improve this article by adding citations to reliable sources unsourced material may be challenged and removed december 2016 learn how and when to remove this template message a cordwood module cordwood construction was used in proximity fuzes cordwood construction can save significant space and was often used with wireended components in applications where space was at a premium such as fuzes missile guidance and telemetry systems and in highspeed computers where short traces were important in cordwood construction axialleaded 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 nickelleaded 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 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 singlesided pcbs as the interconnection method as pictured allowing the use of normalleaded 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 multiwire boards multiwire is a patented technique of interconnection which uses machinerouted insulated wires embedded in a nonconducting matrix often plastic resin it was used during the 1980s and 1990s kollmorgen technologies corp us patent 4175816 filed 1978 as of 2010 multiwire was 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 timeconsuming operation of pcb design anywhere the designer needs a connection the machine will draw a wire in a straight line from one locationpin 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 otherwhich 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 more easily than to a pcb there are other competitive discrete wiring technologies that have been developed electronics portal breadboard cid design for manufacturability pcb electronic packaging electronic waste multichip module occam process another process for the manufacturing of pcbs pointtopoint construction printed electronics creation of components by printing printed circuit board milling printed electronic circuit similar name different part stripboard veroboard wire wrap pcb materials conductive ink laminate materials btepoxy composite epoxy material cem15 cyanate ester fr2 fr4 the most common pcb material polyimide ptfe polytetrafluoroethylene teflon pcb layout software list of eda companies comparison of eda software the wikibook practical electronics has a page on the topic of pcb layout the wikibook practical electronics has a page on the topic of hole sizes for throughhole parts wikimedia commons has media related to printed circuit boards pcb fabrication data a guide the gerber format specification authority control gnd 40196276 