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PCB Glossary
Circuit Board Components

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Acid Trap

A feature of copper tracking which captures excess etching chemistry during processing resulting in under or over etching.

Analogue circuit

This is a type of circuit where the variation of the voltage is the signal quantity.These circuits have short signal paths with large areas of screening.

Annular Ring

The width of the copper pad area that remains after a hole is drilled through a pad.

Aperture

This is the shape and size of the tool which draws or flashes pads or tracks respectively. The early vector plotters created tracks and pads by shining a light through selected apertures in a wheel, which was indexed over photographic film. Each aperture is referred to as a "D" code, the term arising from the adapted original machine NC language where it meant "drill"tool.Photographic films are now plotted using laser raster plotters, where all the shapes and sizes are produced in software, and there is no limitation of a mechanical wheel.

Aperture List

The list of the shapes and sizes of the apertures used to create pads and tracks.

Artwork

The photographic tool used to manufacture a printed circuit board. Usually this is film work, but could be hand laid with tape, or could refer to the Gerber files themselves.

ATE

Automatic (Automated) Test Equipment

Au/Ni

This is a type of PCB surface finish. It consists of 0.1 microns of immersion gold over 5 microns of electroless nickel. It is commonly used as a PCB surface finish for mounting SMD components and for low use PCB edge connector contacts.

Auto-routing

The process by which a CAD layout package automatically places copper tracking and components in accordance with the connectivity defined by the net-list.

Bare Board

This refers to a bare board to distinguish it from a populated pcb.

Base Material

This is the material which forms the structural and insulating layer between the copper layers.

Bed of nails electrical tester

A machine which by way of an adaptor jig ("the test jig"), connects every test point of a board under test to an input channel on a computer. the computer scans the test points and compares the connectivity to either a net list file or to the connectivity of a learnt "golden" board.

Bill of Materials, (BOM).

A document that sets out quantity required, manufacturer's part numbers, device descriptions, value, type or size, and reference designators.

Blind Via

This is a copper plated hole which passes from the surface of a PCB and terminates at a copper feature on an inner layer. It does not pass all the way through the PCB.

Breakout

This is a PCB fault where part of a drilled hole cuts through its associated copper pad.

Bridging

A PCB fault where excess solder or copper causes a short between 2 adjacent copper features.

Buried Via

A buried via is a copper plated hole which interconnects between 2 internal layers of a multi-layer PCB. They are not visible externally.

CEM-1

Epoxy paper laminate with glass fabric coating

CEM-3

Epoxy felt laminate with glass fabric coating

Chamfer

Usually applied to an edge connector area of a PCB to relieve the sharp edges where the PCB is inserted into an edge connector.

Circuit

The copper connections which allow electrical currents to pass between components, and create a functioning electrical or electronic device.

Circuit Boards

See Printed circuit board

Circuit Layer

One of the layers of a PCB containing conductors, including tracking, ground and voltage planes.

Component Hole

The holes through which leaded components are fitted.

Conductor Spacing

The gap between adjacent copper tracks

Controlled impedance

This is defined in ohms, commonly 70 ohm or 50 ohm. It is a characteristic of a copper track which is governed by the dielectric constant of the base laminate, the width of the track, the thickness of the dielectric, and the thickness of the copper track, and the configuration of the adjacent copper features.

Copper foils

Often used in multilayer printed circuit board construction, copper foils are commonly available in 18 micron, 35 micron, and 70 micron thicknesses.

Copper weights (FR4 laminate)

1/2 oz. per square foot of copper = copper 17 microns thick = copper weighing 152.5 grams per square metre = copper 0.7 thousandths of an inch thick
1 oz. per square foot of copper = copper 35 microns thick = copper weighing 305 grams per square metre = copper 1.4 thousandths of an inch thick
2 oz. per square foot of copper = copper 70 microns thick = copper weighing 610 grams per square metre = copper 2.8 thousandths of an inch thick
3 oz. per square foot of copper = copper 105 microns thick = copper weighing 915 grams per square metre = copper 4.2 thousandths of an inch thick
Copper density = 8.96 grams per cubic centimetre

Coverlay

This is a material applied to the outside layers of a flexible circuit to insulate the copper conductor. Coverlays are normally produced with a drilling process. Limitations of the drilling process only allow round or oval holes. Square openings or dense features are not practical.

Design Rules Check

Normally checked by a computer program, features on the circuit which breach design rules are identified. Checks undertaken include: track to track gaps, track to pad gaps, annular ring sizes, track to board edge gaps, acid trap detection, unterminated track checks, legend to pad clearance, solder resist clearance, etc.

Desmear

When multilayer printed circuit boards are drilled, the drill cuts through lands of copper on the inner and outer layers. Resin from the base laminate is smeared over the exposed copper where plated copper must eventually form a connection. This resin must be removed to ensure a sound electrical connection will be made by the plated copper. This is the desmear process.

DFM

Design For Manufacture.

DXF

Document Exchange Format or .dxf format is files format by which many mechanical drawing programmes can share data. It is commonly the means by which PCB designs drafted on Autocad can be transferred to PCB manufacturers. Newbury Electronics can read DXF files into their PCB manufacturing software, and convert it into Gerber for PCB manufacture.

Digital Circuits

These are characterised by a signal which is either on or off, and is typically either 0 or 5 volts or 0 and 3 volts. High speed digital circuitry is largely replacing analogue circuits.

Dielectric Constant, Dk

That property of a dielectric that determines the electrostatic energy per unit volume for unit potential grade. The transmission speed of an electromagnetic wave in a dielectric medium is the speed of light divided by the square root of Dk. Air has a Dk of 1.0. FR4 has a Dk of 4.4.

Dielectric loss (tan d)

The energy absorbed by the dielectric media is measured by tan d. Attenuation is proportional to tan d and signal frequency. For standard FR4 multilayer printed circuit boards materials, tan d is 0.02 which translates into serious losses at at frequencies above 1 Ghz. For circuits operating at 1 Ghz or higher, a material such as PTFE with a tan d of 0.001 is preferred.

Double-sided board

PCB with a copper tracking on both sides, but no inner layers.

Drill Table

A list of drill sizes used in making the PCB.

DPF

Disc Plotter Format or .dpf files is a proprietary file format used by Barco for plotting PCB files. Circuit board features are defined by "contour lines" which are "filled" with copper. It is generally considered to be superior to Gerber, but it is not widely used. Newbury Electronics can read DPF files into their PCB manufacturing software, and convert it into Gerber for PCB manufacture.

Edge Connector

Usually this will consist of gold plated tags at the edge of the PCB which mate with an appropriate connector.

Electroless copper

This is the copper deposited on the inside of drilled holes to a thickness of about 0.5 microns. The process is based on the catalytic deposition of copper on to all the surfaces of the substrate. Subsequently, the thickness of copper will increased by electrolytic plated copper of about 20 to 25 microns.

Electroless Nickel - Immersion Gold

Normally this will be 0.1 microns of gold over 5 microns of nickel plated over just the copper pads. This finish provides a flat and oxidation free surface for mounting surface mount components.

Etch Factor

The ratio of etch depth to the amount the resist is undercut during etching.

Etching

This is the process of removing copper from copper clad laminate to create insulation between the copper tracking.

Fiducial marker

Usually a copper pad, this marker provides a common datum for registering other PCB layers or machines (eg pick & place) to the component placement points (copper pads) on the PCB.

Flash

Vector plotters would "flash" an image of the pads required on PCB. Tracking would be "drawn".

Flexi-Rigid

see Flexible Printed Circuit.

Flexi

see Flexible Printed Circuit.

Flexi circuit

see Flexible Printed Circuit.

Flexible Circuit

see Flexible Printed Circuit.

Flexible circuits

Single sided flexible circuits consist of a single copper conductor layer on a flexible dielectric film. Single sided circuits can be fabricated with or without coverlayers. Similarly, double sided circuits consist of two copper layers insulated with a dielectric normally connected with plated through holes.

Flexible Printed Circuit

A printed circuit board (pcb) manufactured on non-rigid material, commonly polyimide sheet.

Flying probe electrical tester

A machine which electrically tests printed circuit boards by moving test probes to the nets under test. Modern machines will test at over 2000 points a minute.

FR-2

Phenolic paper laminate

FR-3

Epoxy paper laminate

FR-4

This is the standard glass epoxy substrate with a glass transition temperature of 135 deg C. Its mechanical strength and in particular flexural strength and impact resistance are greatly superior to those of phenolic and epoxy paper laminates. FR stands for flame resistant.

FR-5

This is an enhanced glass epoxy substrate with a glass transition temperature of 165 deg C

Glass transition temperature (Tg)

The Tg of a resin system is the temperature at which the material transforms from a relatively rigid or glassy state to a more deformable or softened state.

Gerber Data

This is data that encapsulates a PCB design. It is configured in a vector format and arose from an adaptation of a simple NC drill programming language widely used in the 1960's. Gerber data is unique to the PCB industry and is still the universal data format for transferring data from CAD packages to PCB manufacturers despite attempts to popularise alternatives. Gerber data is officially designated as RS-274-D (without embedded aperture codes) and RS-274-X (with embedded aperture codes).

Ground Plane

A large area of solid or hatched copper on a printed circuit board used as an electrical ground or shield.

HASL

Hot Air Solder Leveled. This is a process whereby a printed circuit board is immersed for a few seconds in a vat of molten solder and then rapidly withdrawn, passing between two air knives which blast away surplus solder from the surface and the through holes. The thickness of solder is between 10 and 25 microns. The solderability and shelf life of HASL is excellent.

HPGL

Hewlett Packard Graphics Language is commonly used for driving pen plotters. It is vector driven language similar to Gerber. Newbury Electronics can read HPGL into their PCB manufacturing software, and convert it into Gerber for PCB manufacture.

Immersion Silver

A type of solderable finish applied to bare copper. It consists of a layer of organic anti-tarnish material applied over metallic silver which is applied over copper.

Import net-list

The term used to describe the process of transferring a net list from a schematic layout software to PCB design CAD software.

Inner Layer

The layers of a circuit board that are sandwiched in between the outer copper layers. These may be tracking layers or ground plane layers.

Land

That part of a pad surrounding a component hole or a via hole.

Lead-free solder

These solders are based on 95% tin metal with the addition of small amounts (around 1%) of variously silver, copper, bismuth and other metals.
Standard tin/lead solder is 60% tin, 40% lead. Most lead-free solders have a melting point some 30/35 degrees C above that of 60/40 tin/lead solder. Therefore the soldering process has to be performed at temperatures of around 255 degrees C rather than the more normal 235 degrees C. The maximum temperature rating of most components is 260 degrees C, thus restricting the use of an optimum temperature.

Legend

Silk screened component identification layer. Sometimes called an "ident".

Micro sectioning

A micro section is a polished sample of a cross-section of a pcb, usually through a PTH hole. It is normally mounted in clear resin. Under magnification, a micro-section is used to verify the integrity of plating down the hole.

Mil

One thousands of an inch

Misregistration

The mis-alignment of features, e.g. solder resist to pad misregistration.

Mixed Technology

Describes the assembly process used when pin through hole, surface mount, and other mounting technologies are placed on the same printed circuit board.

MLPCB

see Multilayer Printed Circuit Board

ML4

see Multilayer Printed Circuit Board

ML6

see Multilayer Printed Circuit Board

ML8

see Multilayer Printed Circuit Board

MLPWB

see Multilayer Printed Circuit Board

ML

see Multilayer Printed Circuit Board

Moisture absorption

Moisture is the enemy of a printed circuit board. Absorbed moisture raises Dk, expands the board, and causes thermal defects such as substrate blisters and barrel cracking during soldering. If storage time are short, or humidity is low, then moisture will not be a problem. However if the pcbs are stored in high humidity for several months or more, then they should be separated, racked and baked at 120 degrees C for 1 hour before assembly.

Multilayer Board

A PCB with more that 2 layers.

Multilayer Printed Circuit Board

A printed circuit board with more than 2 layers, commonly 4 layers , 6 layers or 8 layers. The internal layers of a multilayer printed circuit board may be either ground planes or tracking or a mixture of both. Connection between the layers are made by copper plated through holes.

Net

A net consists of all the points that are connected together by copper.

Net End

A net end is a pad or plated through hole at the end of a branch of an electrical circuit.

Net list

A list of net numbers and their associated X and Y coordinates, and the surface upon which they can be found. This is the basic information from which a test program for an electrical test machine can be constructed since it physically identifies the locations of the nets to be tested. Another type of net list is a CAD software design package net list which identifies connectivity between components, but does not identify their physical location.

Nick

A cut or notch in a track or pad.

Open

An absence of expected circuit continuity.

Organic coatings

Organic solderability preservative (OSP) coatings are organic compounds that selectively bond with copper to provide a protective layer over the copper. Benzimidazoles, benzotriazoles, imidazoles or substituted benzimidazoles are often employed to restrict copper oxidation. The organic material reacts with copper to form a thin coating which prevents moisture and air from coming into contact with the copper surface, thus preserving solderability. OSP's are cheap, but are not mechanically robust, and will not withstand multiple heat cycles.

Pad

The portion of the conductive pattern on printed circuits designated for the mounting or attachment of components.

Panel

A piece of laminate which contains multiple images of a circuit.

PCB

Printed Circuit Board.

PCB prototype

An initial low volume production run of printed circuit boards (pcbs).

Photomech

The department of a printed circuit board factory which uses photo-sensitive materials to print images on printed circuit boards.

Photoplotter

A machine for creating a photographic image of a printed circuit board from electronic data.

Photo resist or plating resist

A light sensitive film, which when exposed to UV light under a photographic mask, polymerises and becomes hardened. The soft unexposed areas may be developed away (washed off) with the use of alkaline chemistry often in a conveyorised processor.

Plated Through Hole

A hole in which electrical connection is made between internal or external conductive patterns, or both, by the plating of metal on the wall of the hole.

Polyimide laminate

A high temperature laminate system with a Tg of around 250 degrees C. Suitable for applications with working temperatures of over 200 degrees C.

Printed Wiring Board

See printed circuit board

Printed Circuit Board

A sheet of glass reinforced plastic upon which is etched a copper pattern of conductive tracks to provide an electrical inerconnect between electronic components which are either soldered to copper pads on the printed circuit board (pcb), or mounted through holes drilled in the printed circuit board (pcb).

PWB

See printed circuit board

Resist

A coating material that is used to mask or protect selected areas of a circuit board from etching or plating processes.

Router

A machine that profiles laminate to the shape and size of the required printed circuit board.

RS-274-X

A variant of Gerber data which contains aperture shape information in addition to the usual tool selection and movement commands.

Scoring

The process of part cutting through laminate with twin rotary cutters which leaves a thin web of material. This web may be broken by "snapping" after assembly.

Screen Printing

The transfer of a pattern onto a surface by forcing a suitable material through a screen with a squeegee.

Short circuit

Erroneous low resistance connections between two or more networks or isolated points.

Silver finish

see immersion silver

Single Sided Board

A printed circuit board that contains tracks and pads on one side of the board and no plating down the holes.

SMOBC

Solder Mask Over Bare Copper. This is to distinguish this more modern process from "solder mask over tin/lead"

SMT

Surface Mount Technology. Components are mounted directly on to printed circuit boards rather than by wires passing through holes. Most modern printed circuit boards will use SMD technology.

Solder Mask, Solder Resist

A solder mask ensures that solder is only placed on the printed circuit board in those areas where it is required. i.e. on component land areas.

Solder Side

On printed circuit boards with components on only one side, the side of the PCB that is opposite to the component side.

Solderability

This is the ability of solder to wet a component quickly. It is influenced by thermal demand of the components, wettability and resistance to soldering heat of the components.

Step & Repeat

The number of duplicate images placed on a panel in both the X axis and Y axis.

Surface Mount Technology, (SMT).

Components are mounted directly on to printed circuit boards rather than by wires passing through holes. Most modern printed circuit boards will use SMD technology.

Teardrop

A teardrop refers to the shape of a pad which tapers evenly into its associated track.

Tester

A machine that checks a PCB for the connectivity of its circuits from the design net list.

Thermal Relief

Through holes are connected to ground planes by way of thermal relief pads, otherwise the heat sink effect of the ground plane would render the temperature in the through hole during soldering insufficient to form a good connection.

Track

An electrical connection between two or more points on a PCB.

Underwriters Laboratories (UL)

The U.S. based safety standard certification organization. US insurance companies will not issue product liability insurance without UL approval of products. In respect of electrical goods, the UL is largely interested in fire hazards and electrical insulation. FR4 laminate is approved to the 94-V0 rating.

Via

A plated through hole which makes a connection from one side of a printed circuit board to the other without having a component lead inserted.