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Gerber format

Standard file format used for designing printed circuit boards

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Gerber is used in PCB fabrication data. PCBs are designed on a specialized electronic design automation (EDA) or a computer-aided design (CAD) system. The CAD systems output PCB fabrication data to allow fabrication of the board. This data typically contains a Gerber file for each image layer (copper layers, solder mask, legend or silk...). Gerber is also the standard image input format for all bare board fabrication equipment needing image data, such as photoplotters, legend printers, direct imagers or automated optical inspection (AOI) machines and for viewing reference images in different departments. For assembly the fabrication data contains the solder paste layers and the central locations of components to create the stencil and place and bond the components.

There are two major generations of Gerber format:

  • Extended Gerber, or RS-274X. This is the current Gerber format. In 2014, the graphics format was extended with the option to add meta-information to the graphics objects. Files with attributes are called X2 files; those without attributes are X1 files.
  • Standard Gerber, or RS-274-D. This obsolete format was revoked.

The official website contains the specification, test files, notes and the Reference Gerber Viewer to support users and especially developers of Gerber software.

PCB fabrication data

PCBs are designed on a specialized electronic design automation (EDA) or a computer-aided design (CAD) system. The CAD systems then outputs PCB fabrication data to allow fabrication of the board. Fabrication data contains a Gerber file for each image layer and drill span (copper layers, solder mask, legend or silk...) (For historic reasons drill data is also transferred in NC formats although Gerber files are often of better quality.) Typically, all these files are "zipped" into a single archive that is sent to the PCB bare board fabrication shop. The fabricator loads them into a computer-aided manufacturing (CAM) system to prepare data for each step of the PCB production process.

The .FileFunction attribute is the standardized method to link each layer in the PCB with its corresponding Gerber file in the fabrication data.

If attributes are not supported, then only informal methods are available. A simple informal method is to express the file function clearly in the file name. Sometimes the file extension is abused to indicate the file function - e.g. .BOT for the bottom layer rather than the standard extension .GBR. In industry this is considered poor practice and engineers should use the appropriate X2 attribute instead.

PCB Fabrication Data must comply with a number of rules: all layers must be aligned, a profile layer must be included, etc.

The CAD netlist can be embedded in the Gerber files. However, for historic reasons, netlists often are described in a separate file in IPC-D-356A, an electrical test format.

The material stack up, components and finishes are typically provided in informal text files or drawings. In 2018 Ucamco has published a specification for an extension of the Gerber format to cover this fabrication documentation.

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