Glossary of Terms

Aperture On a mechanical photoplotter, light is shone through an aperture and focused onto the film, exposing it and leaving the shape of the aperture in the finished design. For component pads, apertures are "flashed" by turning the light on and off again without moving the film. To draw tracks and silkscreen outlines, the film is moved while the light is on, leaving a trail exposed behind it. Modern photoplotters emulate this behaviour using a laser beam which can be precisely focused to a tiny spot on the film.

CNC Computer Numerically Controlled -- a class of machine tools which take a series of instructions from a computer to produce any number of copies of a tangible product from a prepared "blank".

D-Code A code which specifies an aperture on the photoplotter. Historically only D10-D29 and D70-D73 were available. Modern laser photoplotters are not so restricted. However, in case anyone is still using an ancient photoplotter, BCP supports aperture substitution. This means that the D-codes for "rotated" pads can be "borrowed" for vias, if needed. For instance if D10 is a round or square pad, D20 can be substituted with D10. This leaves D20 available for another pad (the substitution is only performed once, so the D20 value will not be immediately re-substituted with D10!) for e.g. D80.

Footprint The copper pads, silkscreen outline and bounding box for a component. Components housed in the same package may share the same footprint.

Excellon File An output file for manufacturing. It contains instructions to drill all the holes in the design on a CNC drilling machine.

Gerber File An output file for manufacturing. It contains instructions to draw one layer of the design on a photoplotter, including the specifications of apertures used in a machine-readable format for modern photoplotters. BCP can add comments to its Gerber files.

Photoplotter A machine tool which produces an image on photographic film, which once developed and fixed can be transferred to a circuit board by a further photographic process.

Postscript File An output file suitable for laser printing.

Silkscreen A printed layer on the board with the outlines of components.

Solder Mask A coating (traditionally, usually green, but nowadays black, blue, white and red are common) on the board with a lesser affinity for solder than the bare board material. Gaps in the solder mask correspond to the component pads.

SPICE Simulation Program with Integrated Circuit Emphasis -- a famous program, whose input file format is well known and has been extended to create the input files for BCP.

Via A connection from one side of a double sided board to the other, consisting of a plated-through hole with a pad on each side. The pad will be a solid circle whose diameter is equal to the track width plus its associated via hole size. Where tracks are of different widths on each side of the board, the via will be determined by the wider of the two.

Viewport The viewport is an imaginary window which can be moved about over the design.

Gerber and Postscript output files can be compared to a fairground-organ book and a MIDI file of the same song: all the same notes, instrument sounds and timings are there, but represented in a different form; either palpably as slots in cardboard, or invisibly as zeros and ones in a computer's memory. (And some organs have more pipes, so are capable of generating more notes, than others.) BCP's design file can be thought of as the full concert score for all the instruments in the orchestra: all the information required to produce any individual player's piece (corresponding to one of the output files), in yet another different form (sheet music; or a representation that BCP finds easiest to work with).