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Gerber format

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A Gerber File is a file format used by printed circuit board (PCB) manufacturing machines to lay out electrical connections such as traces, vias, and pads (the component "footprints" on the PCB). A Gerber file can also contain information for drilling and milling the circuit board.

These files are generated by PCB CAD software, and are sent to manufacturers where they are loaded into a CAM system to prepare data for each step of the PCB production process.

History

  • The Gerber Format was originally a subset of EIA RS-274-D.
  • On 27-Aug-1980 the first edition of the GERBER FORMAT, Plot Data Format Reference Book was published by Gerber Systems Corporation as a specification to drive the photoplotter of this company. At this time the photoplotter had a limited set of fixed apertures of different shapes (typically round and rectangular and a few others) in different sizes. An aperture could be exposed at a specific coordinate (flash) or the head was moved with open shutter from one coordinate to another to generate line or circular arc segments.
  • Mechanical Gerber plotters used apertures made of photographic film, where the transparent portion could be any shape. Flash apertures in custom shapes were used for surface mount pads or edge connector pads or targets to align the layers. If a circle was used to draw a line, the middle of the line would be exposed to more light than the edges, so a Draw aperture was typically a doughnut shape with the center dark for uniform coverage. This is no longer necessary with raster plotters.
  • There was no uniform position for the film apertures, so an aperture list (plain text) was used to inform the operator where to install the apertures before running the plot. The machine had a certain number of available aperture position numbers that were not consecutive 1..max, and usually not all were used for any given board plot. The person generating the plot file had responsibility to ensure that the aperture list used positions that were actually available and matched the plot file.
  • In 1986 the Gerber Format was extended to support apertures with variable sizes to produce rectangles of arbitrary sizes within a given range and tapered lines. This functionality is not in practical use any more.
  • Around the year 1990 the Gerber Format was adopted by several other photoplotter vendors and also computer-aided manufacturing systems for PCB manufacturing. It became the de-facto standard.
  • On 26-Apr-1991 with the availability of raster-scan capability the Gerber Format was extended for polygon areas and Extended Mass Parameter, allowing to dynamically define apertures of different shape and sized and more
  • The last edition of the GERBER FORMAT, Plot Data Format Reference Book was published on 31-Jan-1993 by Gerber Systems Corporation.
  • Gerber Systems Corporation was taken over by Barco Graphics, Gent, Belgium in 1997. On 1998-09-21 the RS-274X Format User’s Guide was published by the Barco Graphics - Gerber Systems Corporation. Barco Graphics' PCB dvision is now called Ucamco (Former Barco ETS).
  • The RS-274X Format User's Guide can be downloaded from the Ucamco website, at www.ucamco.com.


Example Gerber layers, showing the top overlay (silkscreening), top solder resist (protective film), top layer copper traces, and bottom layer copper traces of a printed circuit board.

Usage of the Gerber Format

These files are produced by PCB designers using specialized Electronic Design Automation (EDA) software, such as OrCAD Layout, GEDA, Agilent Advanced Design System, Altium Designer, Mentor Expedition, Cadence Allegro, CadSoft EAGLE, DipTrace, Ultiboard TARGET 3001!, Sunstone PCB123, Number One System Easy-PC, PADS, KiCad, Zuken Cadstar, or Zuken CR5000.

The EIA RS-274D Gerber format always assumed a set of command files - one for each PCB layer - and one aperture file. The command files consists of short commands and X, Y coordinates and are usually called "Gerber files". The "aperture files" describe the shape of the apertures (image elements) used. The aperture files are a free text files, not intended to be processed by a computer, but to be read by a human operator. (Sometimes it is even put in a PDF file.) Every EDA software had its own aperture file format. The RS-274D is not truly an image description format. It is an NC format, designed to drive mechanical Gerber photoplotters, where an operator would read the aperture file, and set up the photoplotter accordingly. It was very well designed for that purpose, and, hence very successful. It was not designed for automatic processing, for driving raster-scan photoplotters, or to transfer data to a CAM system. It is often (ab)used for the latter purpose and than the aperture information has usually to be re-entered manually.

The newer RS-274X Gerber format - also known as X-Gerber or Extended Gerber - was designed to overcome the limitations of RS-274D. The apertures are unambiguously specified in the file, there is no need for an external file. Any aperture shape can be defined. Positive and negative objects can be defined. Large areas can be clearly specified as filled polygons, without the need to "paint" or "vector-fill" as on mechanical Gerber plotters, and, hence, on RS-274D. RS-274X is a complete, powerful and unequivocal description of a single PCB layer. There is no need to manually link a free format external file. It is perfectly suitable for automatic processing.

Unfortunately, the RS-274X output of a some systems is poorly implemented. Even invalid RS-274X files circulate, and manufacturers are expected to process them. Sometimes the numerical accuracy is poor. Several systems still use "painting" to fill polygon areas instead of using the powerful polygon fill feature, or use painted SMD pads instead of using the flexible aperture definitions in RS-274X. Painting does not make the file invalid, but it makes it very difficult and time consuming for manufacturers to work with the file. It is not clear why painting is still used. A good quality RS-274X file is very easy to work with. The issue is with poor usage, not with the format.

RS-274X is an excellent bi-level image description format. RS-274D is not an image description format, it is an NC format used to drive mechanical photoplotters. Mechanical photoplotters are not manufactured anymore since the mid 80's, and very few, if any, are still in use. They are as outdated as the mechanical typewriter. It is incomprehensible that RS-274D is still used so often.

The RS-274X format does not specify wich PCB layer the file represents. This can be specified in the file name, the file extension, in a separate file. No standard exist, and usually this information has to be re-entered manually. The most practical way is to give unequivocal file names, or extensions. Unfortunately, the file names are often of no use, and the manufacturer has to browse all files in the data set to hunt for the information what is meant by a given file.

A typical set of RS-274X file names is:

  • .GBL - Gerber Bottom Layer
  • .GTL - Gerber Top Layer
  • .GBS - Gerber Bottom Solder Resist
  • .GTS - Gerber Top Solder Resist
  • .GBO - Gerber Bottom Overlay
  • .GTO - Gerber Top Overlay
  • .GBP - Gerber Bottom Paste
  • .GTP - Gerber Top Paste
  • .GKO - Gerber Keep-Out Layer
  • .GM1 - Gerber Mechanical 1
  • .GM2 - Gerber Mechanical 2
  • .GPT - Gerber Top Pad Master
  • .GPB - Gerber Bottom Pad Master

Alternatives

Over the years there have been several attempts to replace Gerber by formats containing more information than just the layer image, e.g. the function of the layer, plating, etc. None of these have been able to replace the dominance of Gerber.

  • IPC-D-350 C Printed Board Description in Digital Format', 1989. This specification was standardized as IEC 61128 in 1992 and withdrawn in 2001
  • DPF Format, now at v7, from Ucamco
  • The Electronic Design Interchange Format, EDIF
  • ODB++ from Valor Computerized Systems Ltd, Israel
  • GenCAM: IPC-2511A Generic Requirements for Implementation of Product Manufacturing Description Data and Transfer Methodology, 2000
  • GenCAM: IPC-2511B Generic Requirements for Implementation of Product Manufacturing Description Data and Transfer XML Schema Methodology, 2002
  • Offspring: IPC-2581 Generic Requirements for Printed Board Assembly Products Manufacturing Description Data and Transfer Methodology, 2004
  • STEP AP210: ISO 10303-210, Electronic assembly interconnect and packaging design, first edition 2001, second edition 2008 (to be published)

See also

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