Printed circuit board

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Not to be confused with printed electronics.
"Printed circuit" redirects here. For the defunct company, see Printed Circuit Corporation.
Part of a 1983 Sinclair ZX Spectrum computer board; a populated PCB, showing the conductive traces, vias (the through-hole paths to the other surface), and some mounted electronic components




The world market for bare PCBs reached nearly $60 billion in 2012.[2]


A board designed in 1967; the sweeping curves in the traces are evidence of freehand design using self-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. Rub-on dry transfers of common component footprints increased efficiency. Traces were made with self-adhesive tape. Pre-printed non-reproducing grids on the mylar assisted in layout. To fabricate the board, the finished photomask was photolithographically reproduced onto a photoresist coated on the blank copper-clad boards.

Nowadays PCBs are designed with dedicated layout software, generally in the following steps: [3]

  1. Schematic capture through an electronic design automation (EDA) tool.
  3. The positions of the components and heat sinks are determined.
  4. 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.[4]
  5. Line impedance is determined using dielectric layer thickness, routing copper thickness and trace-width. Trace separation is also taken into account in case of differential signals. Microstrip, stripline or dual stripline can be used to route signals.
  6. Components are placed. Thermal considerations and geometry are taken into account. Vias and lands are marked.
  7. Signal traces are routed. Electronic design automation tools usually create clearances and connections in power and ground planes automatically.
  8. Gerber files are generated for manufacturing.[5][6][7][8]


PCB manufacturing consists of many steps.

PCB CAM[edit]

Manufacturing starts from the PCB fabrication data generated by CAD.[5] The Gerber or Excellon files in the fabrication data are never used directly on the manufacturing equipment but always read into the CAM (Computer Aided Manufacturing) software. CAM performs the following functions:[9]

  1. Input of the Gerber data[5][8]
  2. Verification of the data; optionally DFM
  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, solder resist image, legend image, drill files, automated optical inspection data, electrical test files,...)[5]


Panelization is a procedure whereby a number of PCBs are grouped for manufacturing onto a larger board - the panel. Usually a panel consists of a single design but sometimes multiple designs are mixed on a single panel. There are two types of panels: assembly panels - often called arrays - and bare board manufacturing panels. The assembler often mount components on panels rather than single PCBs because this is efficient.[10] The bare board manufactures always uses panels, not only for efficiency, but because of the requirements the plating process. Thus a manufacturing panel can consist of a grouping of individual PCBs or of arrays, depending on what must be delivered. [5]

The panel is eventually broken apart into individual PCBs; this is called depaneling. Separating the individual PCBs is frequently aided by drilling or routing perforations along the boundaries of the individual circuits, much like a sheet of postage stamps. Another method, which takes less space, is to cut V-shaped grooves across the full dimension of the panel. The individual PCBs can then be broken apart along this line of weakness.[11]Today depaneling is often done by lasers which cut the board with no contact. Laser panelization reduces stress on the fragile circuits.

Copper patterning[edit]

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. (Alternatively, a conductive ink can be ink-jetted on a blank (non-conductive) board. This technique is also used in the manufacture of hybrid circuits.) The patterning method depend volume and resolution requirements.

  1. Silk screen printing uses etch-resistant inks to create the protective mask.
  2. Photoengraving uses a photomask and developer to selectively remove a UV-sensitive photoresist coating and thus create a photoresist mask. Direct imaging techniques are sometimes used for high-resolution requirements. Experiments were made with thermal resist.[12]
  4. Laser resist ablation Spray black paint onto copper clad laminate, place into CNC laser 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.[clarification needed])

Patterning method by volume[edit]

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

Large volume[edit]
  • Silk screen printing– used for PCBs with bigger features
  • Photoengraving–used when finer features are required.
Small volume[edit]
  • Print onto transparent film and use as photo mask along with photo-sensitized boards. (i.e., pre-sensitized boards), then etch. (Alternatively, use a film photoplotter)
  • Laser resist ablation.
  • PCB milling.

Subtractive, additive and semi-additive processes[edit]


Subtractive methods remove copper from an entirely copper-coated 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.


The (semi-)additive process is commonly used for multi-layer boards as it facilitates the plating-through of the holes to produce conductive vias in the circuit board.

PCB copper electroplating line in the process of pattern plating copper.

Chemical etching[edit]





Inner layer automated optical inspection (AOI)[edit]

The inner layers are given a complete machine inspection before lamination because afterwards mistakes cannot be corrected. The automatic optical inspection system scans the board and compares it with the digital image generated from the original design data. [14]


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 prepreg and copper foil. It is then drilled, plated, and etched again to get traces on top and bottom layers.


Eyelets (hollow).


Holes may be made conductive, by electroplating or inserting metal eyelets (hollow), to electrically and thermally connect board layers. Some conductive holes are intended for the insertion of through-hole-component leads. Others, typically smaller and 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 drilled—evaporated by lasers. Laser-drilled vias typically have an inferior surface finish inside the hole. These holes are called micro vias.[15][16]


The hole walls for boards with two or more layers can be made conductive and then electroplated with copper to form plated-through holes.[17]  These holes electrically connect the conducting layers of the PCB. For multi-layer 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 de-smear process, or by plasma-etch. 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.[18] 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[edit]

PCBs[19] are plated with solder, tin, or gold over nickel as a resist for etching away the unneeded underlying copper.[20]

After PCBs are etched and then rinsed with water, the solder mask is applied, and then any exposed copper is coated with solder, nickel/gold, or some other anti-corrosion coating.[21][22]


It is important to use solder compatible with both the PCB and the parts used. An example is ball 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 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 nickel plated then gold plated. Another coating consideration is rapid diffusion of coating metal into Tin solder. Tin forms intermetallics such as Cu5Sn6 and Ag3Cu that dissolve into the Tin liquidus or solidus(@50C), 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.[24][25] Silver, zinc, and aluminum are known to grow whiskers under 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 the percentage Tin replaced. Reflow to melt solder or tin plate to relieve surface stress lowers whisker incidence. Another coating issue is tin pest, the transformation of tin to a powdery allotrope at low temperature.[26]

Solder resist application[edit]

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).[27]  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 develop as LPI. Once common but no longer commonly used because of its low accuracy and resolution is to screen print epoxy ink. Solder resist also provides protection from the environment.

Legend printing[edit]

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 and servicing the circuit board.[28][29]

There are three methods to print the legend.

  1. Silk screen printing epoxy ink was the established method. It was so common that legend is often misnamed silk or silkscreen.
  2. Liquid photo imaging is a more accurate method than screen printing.
  3. Ink jet printing is new but increasingly used. Ink jet can print variable data such as a text or bar code with a serial number.

Bare-board test[edit]

Unpopulated boards are usually bare-board 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 high-volume production a fixture or a rigid needle adapter is used to make contact with copper lands on the board. Building the adapter is a significant fixed cost and is only economical for high-volume or high-value 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.[30] 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.


PCB with test connection pads

After the printed circuit board (PCB) is completed, electronic components must be attached to form a functional printed circuit assembly,[31][32] or PCA (sometimes called a "printed circuit board assembly" PCBA). In through-hole construction, component leads are inserted in holes. In surface-mount (SMT - surface mount technology) construction, the components are placed on pads or lands on the outer surfaces of the PCB. In both kinds of construction, component leads are electrically and mechanically fixed to the board with a molten metal solder.







Protection and packaging[edit]



PCB characteristics[edit]

Much of the electronics industry's PCB design, assembly, and quality control follows standards published by the IPC organization.

Through-hole technology[edit]

Through-hole (leaded) resistors




Surface-mount technology[edit]

Surface mount components, including resistors, transistors and an integrated circuit

Surface-mount technology emerged in the 1960s, gained momentum in the early 1980s and became widely used by the mid-1990s. Components were mechanically redesigned to have small metal tabs or end caps that could be soldered directly onto the PCB surface, instead of wire leads to pass through holes. Components became much smaller and component placement on both sides of the board became more common than with through-hole mounting, allowing much smaller PCB assemblies with much higher circuit densities. Surface mounting lends itself well to a high degree of automation, reducing labor costs and greatly increasing production rates. Components can be supplied mounted on carrier tapes. Surface mount components can be about one-quarter to one-tenth of the size and weight of through-hole components, and passive components much cheaper; prices of semiconductor surface mount devices (SMDs) are determined more by the chip itself than the package, with little price advantage over larger packages. Some wire-ended components, such as 1N4148 small-signal switch diodes, are actually significantly cheaper than SMD equivalents.

Circuit properties of the PCB[edit]



Excluding exotic products using special materials or processes all printed circuit boards manufactured today can be built using the following four materials:

  1. Laminates
  2. Copper-clad laminates
  4. Copper foil


Laminates are manufactured by curing under pressure and temperature layers of cloth or paper with thermoset resin to form an integral final piece of uniform thickness. The size can be up to 4 by 8 feet (1.2 by 2.4 m) in width and length. Varying cloth weaves (threads per inch or cm), cloth thickness, and resin percentage are used to achieve the desired final thickness and dielectric characteristics. Available standard laminate thickness are listed in Table 1:

Table 1
Standard laminate thickness per ANSI/IPC-D-275[37][Note 1]
IPC Laminate
in inches
in millimeters
  IPC Laminate
in inches
in millimeters
L1 0.002 0.05   L9 0.028 0.70
L2 0.004 0.10   L10 0.035 0.90
L3 0.006 0.15   L11 0.043 1.10
L4 0.008 0.20   L12 0.055 1.40
L5 0.010 0.25   L13 0.059 1.50
L6 0.012 0.30   L14 0.075 1.90
L7 0.016 0.40   L15 0.090 2.30
L8 0.020 0.50   L16 0.122 3.10


  1. ^ Although this specification has been superseded and the new specification does not list standard sizes,[38] these are still the most common sizes stocked and ordered for manufacturer.


There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the requirements of the circuit. Some of these dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well known prepreg materials used in the PCB industry are FR-2 (phenolic cotton paper), FR-3 (cotton paper and epoxy), FR-4 (woven glass and epoxy), FR-5 (woven glass and epoxy), FR-6 (matte glass and polyester), G-10 (woven glass and epoxy), CEM-1 (cotton paper and epoxy), CEM-2 (cotton paper and epoxy), CEM-3 (non-woven glass and epoxy), CEM-4 (woven glass and epoxy), CEM-5 (woven glass and polyester). Thermal expansion is an important consideration especially with ball grid array (BGA) and naked die technologies, and glass fiber offers the best dimensional stability.


Copper thickness[edit]

Copper thickness of PCBs can be specified as units of length (in micrometers or mils) but is often specified as weight of copper per area (in ounce per square foot) which is easier to measure. One ounce per square foot is 1.344 mils or 34 micrometres thickness.


Safety certification (US)[edit]


Multiwire boards[edit]



Corrections can be made to a Multiwire board more easily than to a PCB.[39]

Cordwood construction[edit]

A cordwood module

Cordwood construction can save significant space and was often used with wire-ended components in applications where space was at a premium (such as missile guidance and telemetry systems) and in high-speed computers, where short traces were important. In "cordwood" construction, axial-leaded components were mounted between two parallel planes. The components were either soldered together with jumper wire, or they were connected to other components by thin nickel ribbon welded at right angles onto the component leads. To avoid shorting together different interconnection layers, thin insulating cards were placed between them. Perforations or holes in the cards allowed component leads to project through to the next interconnection layer. One disadvantage of this system was that special nickel-leaded components had to be used to allow the interconnecting welds to be made. Differential thermal expansion of the component could put pressure on the leads of the components and the PCB traces and cause physical damage (as was seen in several modules on the Apollo program). Additionally, components located in the interior are difficult to replace. Some versions of cordwood construction used soldered single-sided PCBs as the interconnection method (as pictured), allowing the use of normal-leaded components.




The Austrian engineer Paul Eisler invented the printed circuit as part of a radio set while working in England around 1936. Around 1943 the USA began to use the technology on a large scale to make proximity fuses for use in World War II.[41] After the war, in 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army. At around the same time in Britain work along similar lines was carried out by Geoffrey Dummer, then at the RRDE.

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.
A PCB in a computer mouse. The component side (left) and the printed side (right).
The component side of a PCB in a computer mouse; some examples for common components and their reference designations in the legend.





Historically many measurements related to PCB design were specified in multiples of a thousandth of an inch, often called "mils". For example, DIP and most other through-hole components have pins located on a grid spacing of 100 mils, in order to be breadboard-friendly. Surface-mount SOIC components have a pin pitch of 50 mils. SOP components have a pin pitch of 25 mils. Level B technology recommends a minimum trace width of 8 mils, which allows "double-track" – two traces between DIP pins.[46][47]

See also[edit]

Schematic Capture (KiCad)
PCB layout (KiCad)
3D View (KiCad)

PCB materials

PCB layout software


  1. ^ IPC-14.38
  2. ^ IPC World PCB Production Report 2013
  3. ^ Printed Circuit Board Design Flow Methodology
  4. ^ See appendix D of IPC-2251
  5. ^ a b c d e Tavernier, Karel. "PCB Fabrication Data - A Guide". Ucamco. Retrieved 8 January 2015. 
  6. ^ Vermeire, Filip. "PCB Fabrication Data Example 1". Ucamco. Ucamco. Retrieved 7 January 2015. 
  7. ^ Vermeire, Filip. "PCB Fabrication Data Example 2". Ucamco. Ucamco. Retrieved 7 January 2015. 
  8. ^ a b "The Gerber File Format Specification". Ucamco. Retrieved 8 January 2015. 
  9. ^ "Front-end tool data preparation". Eurocircuits. Retrieved 2 Sep 2013. 
  10. ^ "Making a PCB - Educational movies". Eurocircuits. Eurocircuits. Retrieved 20 January 2015. 
  11. ^ Kraig Mitzner, Complete PCB Design Using OrCad Capture and Layout, pages 443–446, Newnes, 2011 ISBN 0080549209.
  12. ^ Itshak Taff, Hai Benron. "Liquid Photoresists for Thermal Direct Imaging". The Board Authority, October 1999. 
  13. ^ a b R. S. Khandpur,Printed circuit boards: design, fabrication, assembly and testing, Tata-McGraw Hill, 2005 ISBN 0-07-058814-7, pages 373–378
  14. ^ "Inner layer inspection". Eurocircuits. Retrieved 31 Aug 2013. 
  15. ^
  16. ^ Microvia Fabrication: When to drill, When to Blast
  17. ^ "Making Holes Conductive". Electronic Chemicals. Retrieved 5 Sep 2012. 
  18. ^ "Electro-Brite E-Prep Desmear/Etchback". OM Group, Inc. Retrieved 5 Sep 2012. 
  19. ^ Appendix F Sample Fabrication Sequence for a Standard Printed Circuit Board, Linkages: Manufacturing Trends in Electronics Interconnection Technology, National Academy of Sciences
  20. ^ Production Methods and Materials 3.1 General Printed Wiring Board Project Report – Table of Contents, Design for the Environment (DfE), US EPA
  21. ^ George Milad and Don Gudeczauskas. "Solder Joint Reliability of Gold Surface Finishes (ENIG, ENEPIG and DIG) for PWB Assembled with Lead Free SAC Alloy." [1]
  22. ^ "Nickel/Gold tab plating line"
  23. ^ Soldering 101 – A Basic Overview
  24. ^ IPC Publication IPC-TR-476A, "Electrochemical Migration: Electrically Induced Failures in Printed Wiring Assemblies," Northbrook, IL, May 1997.
  25. ^ S.Zhan, M. H. Azarian and M. Pecht, "Reliability Issues of No-Clean Flux Technology with Lead-free Solder Alloy for High Density Printed Circuit Boards", 38th International Symposium on Microelectronics, pp. 367–375, Philadelphia, PA, September 25–29, 2005.
  26. ^ Clyde F. Coombs Printed Circuits Handbook McGraw–Hill Professional, 2007 ISBN 0-07-146734-3, pages 45–19
  27. ^ "liquid photoimageable solder masks" (PDF). Coates Circuit Products. Retrieved 2 Sep 2012. 
  28. ^ "Silk-screen and cure". Eurocircuits. Retrieved 31 Aug 2013. 
  29. ^ "Towards a more rational silkscreen". Optimum Design Associates. Retrieved 31 Aug 2013. 
  30. ^ "Electrical test". Eurocircuits. Retrieved 13 Apr 2015. 
  31. ^ Ayob M. and Kendall G. (2008) A Survey of Surface Mount Device Placement Machine Optimisation: Machine Classification. European Journal of Operational Research, 186(3), pp 893–914 (
  32. ^ Ayob M. and Kendall G. (2005) A Triple Objective Function with a Chebychev Dynamic Pick-and-place Point Specification Approach to Optimise the Surface Mount Placement Machine. European Journal of Operational Research, 164(3), pp 609–626 (
  33. ^ Borkes, Tom. "SMTA TechScan Compendium: 0201 Design, Assembly and Process" (PDF). Surface Mount Technology Association. Retrieved 2010-01-11. 
  34. ^ JTAG Tutorial (
  35. ^ Shibu. Intro To Embedded Systems 1E. Tata McGraw-Hill. p. 293. ISBN 978-0-07-014589-4. 
  36. ^ Electronic Packaging:Solder Mounting Technologies in K.H. Buschow et al (ed), Encyclopedia of Materials:Science and Technology, Elsevier, 2001 ISBN 0-08-043152-6, pages 2708–2709
  37. ^ "Design Standard for Rigid Printed Boards and Rigid Printed Board Assemblies". IPC. September 1991. IPC-4101. 
  38. ^ "Specification for Base Materials for Rigid and Multilayer Printed Boards" (IPC-4101). ANSI/IPC. December 1997. ANSI/IPC-D-275. 
  39. ^ David E. Weisberg. "Chapter 14: Intergraph". 2008. p. 14-8.
  40. ^ US 1256599 
  41. ^ a b Charles A. Harper, Electronic materials and processes handbook, McGraw-Hill,2003 ISBN 0-07-140214-4, pages 7.3 and 7.4
  42. ^ Brunetti, Cledo (22 November 1948). New Advances in Printed Circuits. Washington DC: National Bureau of Standards. 
  43. ^ IEEE Cledo Brunetti Award,
  44. ^ Engineers' Day, 1984 Award Recipients, College of Engineering, University of Wisconsin-Madison,
  45. ^ US 2756485  assigned to US Army. July 31, 1956.
  46. ^ Kraig Mitzner. "Complete PCB Design Using OrCad Capture and Layout". 2011.
  47. ^ "TINA PCB DesignManual".

External links[edit]