fab Short for fabrication. Used both for the manufacture of printed circuit boards and semiconductors.

(board) fabrication The bare-board manufacturing process, which begins after design but before assembly. Individual processes include layer lamination, metal addition/subtraction, drilling, plating, routing and cleaning.

fabrication drawing A drawing used to aid the construction of a printed board. It shows all of the locations of the holes to be drilled, their sizes and tolerances, dimensions of the board edges, and notes on the materials and methods to be used. Called ‘fab drawing’ for short. It relates the board edge to at least one hole location as a reference point, so that the NC drill file can be properly lined up.

false triggering An incorrect change of state of a digital device due to a spurious signal received by that device.

fatigue failure Mechanical failure of a material caused by application of repeated cycles of stress (force) and strain (movement) over a period of time. These forces may be due to vibration or caused by changes in temperature, and in PCAs may result in cracked plated through-holes, open solder joints, or board delamination.

fatigue resistance The ability of a solder joint to resist vibration.

fault isolation A test procedure for locating the area of a circuit that is causing a performance anomaly or failure.

FCC = Federal Communications Commission An independent United States government agency that is charged with regulating interstate and international communications by radio, television, wire, satellite, and cable.

feed-through A conductor that connects patterns on opposite sides of a printed circuit board, e.g. an eyelet, plated through hole or clinched jumper wire.

feed-through via A plated-through hole in a PWB used to route a trace vertically in the board, that is, from one layer to another.

FEM = finite-element modelling A method of using a software program to simulate the response of a PCA to various mechanical or thermal conditions. A mathematical model of an assembly is constructed, exposed to mechanical or thermal stimulation, and analysed for its response to those inputs.

female connector Consists of an insulated moulding fitted with socket contacts that allows male and female connectors to be plugged together. (In the USA female connectors can be called receptacles).

FET A field-effect transistor is a unipolar device, which functions as a voltage amplifier.

fibre exposure A condition in which reinforcing fibres within the base material are exposed in machined, abraded or chemically attacked areas.

fiducial (mark) A geometric feature incorporated into the artwork of a printed wiring board or into a stencil.

At fabrication, fiducials are etched on the outside surface of a circuit board or a panel at the same time as the conductive pattern is formed, and provide an optically measurable reference point for subsequent manufacturing and assembly processes, to confirm proper artwork registration and orientation.

At assembly, fiducials are used as an optical target by the machine vision system to identify its exact location and orientation. These reference points serve to align and position boards and stencils both for printing and SMT component placement. Fiducial marks are particularly necessary for the accurate placement of fine pitch components.

Field-Programmable Gate Array (FPGA) A device containing a large number of logic gates that can be interconnected internally by the user of the device to form an application-specific circuit.

fill area A large conductive area such as a ground or power plane.

fillet A general term used to describe the normally concave surface of the solder at the intersection of the metal surfaces of the solder connection that is formed with a component lead or termination and a PWB land pattern. In particular, it relates to its shape and strengthening function. Visual inspection criteria are largely based on the size and the shape of the fillet.

[Derived from the Latin filus = a thread used as a head-band; became a strip of any material suitable for binding; then in carpentry a strip of wood used to strengthen an angle]

find number An item number that cross-references a part callout on an assembly drawing to its entry in a parts list or bill of materials.

fine-line design Printed circuit design permitting two (rarely three) traces between adjacent DIP pins. It entails the use of a either dry film solder mask or liquid photoimageable solder mask, both of which are more accurate than wet solder mask.

fine-pitch A term which always refers to the distance between lead centres of device packages, but whose value depends on the date of the definition. Previously lead pitches below 1.27mm (0.050in) were regarded as fine pitch; in 2002, given improvements in printing and paste technology, the frontier probably lies at 0.63mm (0.025in).

finger See gold finger.

flash A non-volatile programmable semiconductor memory product. Flash devices retain the contents of their memory when the power is turned off.

flatpack Common designator for a two or four-sided integrated circuit package with gull wing or flat leads. The component leads extend from the sides and have their termination plane on the same linear plane as the base of the package, with standard spacing between leads. Commonly, the lead pitches are at 1.27mm (0.05in) centres, but lower pitches also may be used.

flex circuit Printed circuitry that utilizes flexible rather than rigid laminate material.

flexible buffer material If devices packaged in a brittle material (such as glass diodes or ceramic ICs) are coated with rigid conformal coating material, temperature cycling may cause the packages to crack due to the difference in the CTEs of the two materials. This may be prevented by applying a resilient material (such as silicone rubber) to the part as a buffer between it and the conformal coating.

flip chip (device) A leadless structure that is electrically and mechanically connected to the substrate via contact lands or solder bumps.

flip-chip (technology) A ‘Chip-On-Board’ technology in which the silicon chip is inverted (‘flipped’) and interconnected mechanically and electrically to a conductor pattern on the substrate by conductive bumps on the chip. The bumps are formed on the active surface of the chip, which is turned over for attachment.

Flip-chip bumps are usually of solder, deposited on the bonding pads in vacuum, then reflowed. This process needs complex metallisation on the bonding pads. Originally developed by IBM, the process is also known as C4 (controlled collapse chip connection).

flood bar A device on a stencil printing system that drags solder paste back to the starting point after the squeegee has made a printing stroke.

flux A chemically active agent that speeds the wetting process of metals with molten solder: a short-form way of describing a complex of rosin, activators and solvents. When heated, fluxes remove minor surface oxidation, minimise oxidation of the base metal, and promote the formation of an intermetallic layer between solder and base metal. Flux will not remove oils, dirt, or fingerprints – only a solvent can remove these.

flux residue A by-product of the soldering operation which may or may not need to be removed from the board, depending on the nature of the residue. Generally, highly active fluxes are corrosive and conduct electricity, so must be removed completely. Even with so-called ‘no-clean’ fluxes there will usually be some residue which has an impact on the cosmetic performance but is otherwise not deleterious to circuit reliability

flying probe tester Computer-controlled In-Circuit Test system in which contacts are directed to specific nodes on a board to complete an electrical test.

foil See conductive foil.

footprint 1) The pattern and space on a board taken up by a component. 2) The hole, pad and conductor pattern associated with a specific electronic component package configuration. A non-preferred term for land pattern. See also decal

form, fit, and function Interchangeability classifications that determine the equivalence of products when design changes are implemented. If the physical form of an assembly, its ability to fit in the same place as the previous design, or its functional operation changes, it should be considered a new product, and given a new number.

fractured joint A joint where the component lead has separated from the solder fillet, usually occurring during lead clipping after soldering.

functional test An electrical test of a component, sub-assembly, or entire assembly that simulates part of the intended function of the product, verifying that the product is likely to comply with its overall function specification. Note that, for reasons of practicality and test time, it is unusual for this functional test to cover the entire range of function of the product.

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