The Gerber format is a file format used by printed circuit board (PCB) industry software to describe the images of a printed circuit board (copper layers, solder mask, legend, drill holes, etc.). The Gerber format is the de-facto industry standard for printed circuit board image transfer.[1][2][3]
There are two versions. RS-274X ("extended Gerber") is the most commonly used today. The previous version was a subset of EIA RS-274-D ("standard Gerber"); it is deprecated and is largely superseded by RS-274X.
The Gerber format was developed by and named after Gerber Systems Corp., a subsidiary of a company founded by Heinz Joseph “Joe” Gerber.[4] The format is now controlled and owned by Ucamco through its acquisition of Gerber Systems Corporation.[5][6]
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The RS-274X Gerber format, also known as extended Gerber or X-Gerber, is a 2D bi-level vector image description format.[7] It is a superset of RS-274-D standard Gerber, which is itself a subset of the EIA RS-274-D format for numerically controlled machines.
It is a human readable ASCII format.[8] It consists of a sequence of commands and coordinates. Its imaging primitives are line draw, flash (display) predefined shapes at a given location and outline fill. Positive and negative graphics objects can be combined.
An example of a RS-274X file:
G04 Film Name: paste_top* G04 Origin Date: Thu Sep 20 15:54:22 2007* G04 Layer: PIN/PASTEMASK_TOP* %FSLAX55Y55*MOIN*% %IR0*IPPOS*OFA0.00000B0.00000*MIA0B0*SFA1.00000B1.00000*% %ADD28R,.11X.043*% %ADD39O,.07X.022*% ... %AMMACRO19* 21,1,.0512,.0512,0.0,0.0,45.*% %ADD19MACRO19*% %LPD*% G75* G54D10* X176250Y117500D03* Y130000D03* Y163750D03* ... G54D39* X496250Y142500D03* Y137500D03* Y132500D03* Y127500D03* M02*
An RS-274X file contains the complete description of a PCB layer image without requiring any external files. It has all the imaging operators needed for a PCB image. Any aperture shape can be defined. Positive and negative objects can be combined. Planes can be specified without the need to "paint" or "vector-fill" as in RS-274-D.
RS-274X is a complete, powerful and unambiguous standard to describe a PCB layer. It can be input and processed fully automatically. This makes it well suited for fast and secure data transfer and for reliable and automated workflows.
The format specification is published.[7] A tutorial is available.[9]
Gerber files are typically produced by PCB designers using specialized Electronic Design Automation (EDA) or PCB CAD software. These files are sent to PCB fabricators where they are loaded into a CAM system to prepare data for each step of the PCB production process. In this workflow they transfer the layer information from CAD to CAM. They are also used to specify drilled hole information, which can be viewed as image layers; however, the Excellon Format is used more often to transfer drilling information.[10] Gerber files are also used to drive quality control machines, such as automated optical inspection.
A quality RS-274X file is very convenient to work with.[11] Sometimes the numerical precision is too low, causing significant rounding errors on tight-tolerance PCBs. The Gerber output resolution (grid) should always be at least a factor of 10 better than the resolution (grid) of the CAD system. Some systems still use "painting" to define copper areas instead of using outline fill, or use painted SMD pads instead of the flexible aperture definitions. Painting does not make a file invalid, but it makes it very difficult, time-consuming and error-prone for manufacturers to process. Painting should not be used.[12] Note that these issues are not due to the RS-274X format itself but due to poor implementations. Many outstanding implementations exist, producing top quality RS-274X files.
The RS-274X format does not specify which PCB layer the file represents. Extensions have been proposed to specify this.[11] In any case, it is sufficient to specify the function in the file name and to specify the format in the extension - e.g. ".GER" for Gerber. Some designers, however, use cryptic file names and document them in a free-format text file. This means that the manufacturer has to browse all the files in the dataset to find the necessary production information. In other cases the function is indicated by misusing the file extension - e.g. .BOT for Bottom Layer.[11] In this case the manufacturer has to open the file to find out the format. Contrast this with the rest of the world; nobody has to open a PDF file to know it is a PDF file.
An RS-274X file specifies a single conductor or mask layer image. The drill data is usually specified in Excellon format. The netlist, if present, is usually specified in IPC-D-356.[13] Layer names, material stack up are typically provided in informal text files or drawings. However, Ucamco itself recommends[11] the use of a subset of IPC-2581 for these. Typically, all these files are "zipped" into a single archive.
Standard Gerber, is largely superseded by extended Gerber. It was created from a subset of the Electronic Industries Association RS-274-D specification,[14] a format to drive mechanical NC machines in a wide range of industries. Be aware that the term RS-274-D is often used incorrectly (with the qualifying "Gerber" postfix omitted) to refer to the standard Gerber subset, rather than the original RS-274-D superset itself. Standard Gerber is used to drive vector photoplotters, which indeed were 2D NC machines. It is a simple ASCII format consisting of commands and X, Y coordinates.[15] An example of a Gerber RS-274-D file:
D11* X1785250Y2173980D02* X1796650Y2177730D01* X1785250Y2181480D01* X1796650Y2184580D02* D12* X3421095Y1407208D02* X3422388Y1406150D01* M02*
RS-274-D was designed in the 1960s and 1970s to drive numerical controlled machines such as vector photoplotters, machines now all replaced by raster-photoplotters. An RS-274-D file on its own is not an image description because it does not contain all information: the coordinate unit and the definitions of the apertures are not defined in the RS-274-D file. (Apertures are the basic shapes, similar to fonts in a PDF file.) The coordinate units and apertures were supposed to be set manually by the plotter operator. They were typically described in a free-format text file, called an aperture file or a wheel file (because the apertures were mounted on a wheel and rotated into the light beam), intended for human reading. There are no standards for wheel files in RS-274-D, so the designer and the plotter operator had to agree on these on a case-by-case basis.[15]
It only supports a few simple imaging operators. To work around this limitation constructions such as "stroking," also known as "painting" or "vector-fill" are needed. Standard Gerber was well-suited to drive vector plotters and was constrained by the technology then available. It was designed for a manual workflow. It is not suitable for fully automated data transfer between PCB designers and manufacturers. PCB manufacturers have to enter coordinate units and aperture definitions manually.
RS-274-D has been deprecated by Ucamco.[11]
Over the years there have been several attempts to replace Gerber by formats containing more information than just the layer image, e.g. netlist or component information.[6] None of these attempts have been widely accepted within the electronics manufacturing industry, probably because the formats are complex.[11] Gerber remains the most widely used data transfer format.[1][2][3]
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