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PCB Glossary

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

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.

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,

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.

DXF,

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

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.