Your native CAD files are the blueprints of your PCB design. They show the component locations, component footprints, and electrical schematic of your PCB. The type of design file you use depends on what software you’re using to create your PCB, as well as its specific functionality. In this article, I’ll introduce you to the most common PCB design file formats and how each file format is used.
PCB Manufacturing Files
PCB Fabrication Files
- Gerber files - are perhaps the most commonly cited file type used in PCB design, but they are not actually design files per se. This 2D ASCII file format is used in CAM applications because it is a simple vector file format that can technically be used to reconstruct a PCB layout. Bugs are rare and can be easily removed manually. Gerber files contain the imagery used to build stencils and tooling used in the PCB fabrication process. Information like the location of drill hits, solder paste, mask openings, etched copper, and component pad placement is contained in these files. The two commonly used Gerber formats are RS-274X and Gerber X2.
- NC Drill File - The drilling and routing data of PCB is defined by NC (numerically controlled) drill files. The NC drill file is a secondary file sent to the manufacturer in addition to the Gerber file. The size, position and number of holes to be drilled into the PCB are referred to by drill files. The number of drill files depends upon the complexity of circuit board wiring. The IPC standard relevant to drilling and routing files is IPC-NC-349.
- IPC-NC-349 - IPC-NC-349 is the machine-readable format of NC drill files; it is utilized by computer-controlled routing tools for drilling. The information about drilling and routing can be transferred between PCB manufacturers, designers, and consumers with IPC-NC-349 file types.
- XNC Format - A strict subset of IPC-NC-349 is the exchange NC (XNC) format which is highly compatible with existing software used for NC drill files.
- Excellon drill file - Excellon format is used as an input to drilling machines and it contains essential instructions to operate CNC drilling machines. There are two versions of the Excellon drill file format (Versions 1 and 2).
- ODB++ - ODB++ is a newer data format than Gerber and was created to help prevent errors and data miscommunication in PCB fabrication. This file format enables exchange of data between design and manufacturing, as well as between design tools from different CAD and CAM companies. When ODB++ is in use, its data is kept in a hierarchical order of files and folders in a single ZIP file.
- IPC-2581 - The CAM data transfer standard defined in IPC-2581 is based on XML data structures and was created from the ODB++ format. The IPC-2581 standard has the backing of a group of manufacturers and suppliers, but it hasn't seen the same level of usage as Gerbers and ODB++. This file format contains all the necessary fabrication information including layer build-up data, drill data, copper details for each layer etc. Furthermore, a complete list of electronic components along with their placement coordinates and rotational information is included.
- DXF/DWG - DXF/DWG files are sometimes found in deliverable packages as they will include mechanical specifications. DWG files are an exclusive file format used in CAD systems like AutoCAD. It is possible to convert between Gerbers, DXFs, and DWGs.
- IPC-D-365 netlist - A IPC-D-365 netlist is generated from your design data and is used by a fabrication house to define automated testing specifications, as well as to check for unexpected opens/shorts in your Gerber files using CAM software. It is a human-readable ASCII file that has instructions for PCB systems consisting of pins and XY coordinates (start and end points) for each network node.
PCB Assembly Files
- Pick-and-place (centroid) - A pick-and-place file, sometimes called a centroid file, is used to program a pick-and-place machine used in PCB assembly. The information about the orientation and position of all parts mounted on the board surface, along with each reference designator, is contained in a pick-and-place file.
- Bill of materials (BOM) - A bill of materials (BOM) contains the list of all components that will appear in a PCB assembly. This list comprises part quantity, footprints, do not install (DNI) parts, manufacturing part numbers, and vendor part numbers. It also contains a list of some other crucial components such as wiring, sub-assemblies, and raw materials that need to be assembled on the circuit board. BOMs are usually in XLSX format.
Native CAD Files
A specific software program is used to build the CAD files that contain all the information in your PCB design. All the necessary information required to generate circuit board fabrication files is contained in your native PCB CAD files. The CAD files also contain all the information showing component placement, part numbers, copper elements, and application-specific information that is used within your CAD application.
Schematic sheets define the electrical specifications for your PCB design. They are used in front-end engineering to define functional blocks in a board, to define pin-to-pin connections between components, to specify components by part number, and to build simulations in CAD programs with SPICE simulators. The electrical connections between various components in a circuit board are depicted graphically in schematics and the CAD program will check that these connections are synchronized with the PCB layout.
PCB Layout File
All physical specifications for a PCB are defined in a PCB layout file. The PCB layout includes all important information that will be used to eventually generate Gerber/drill files used to fabricate the board. There is no "universal" PCB layout file format; all of these files are application-specific with proprietary file formats, although CAD applications contain converter tools used to migrate between different file formats.
- Fab drawings - Fabrication drawings will graphically show drill details, board dimensions, stack-up details, impedance details, slot details, layer details, and via details (blind/buried, through-hole, layer pairs, etc.). The main Gerber layers shown in a fabrication drawing include the copper layers (signal and plane layers), paste layers, and mask layers. The fabrication drawings will also include a set of fabrication notes, which will outline many of the IPC standards conformance requirements that the fabricated PCB must meet in order to be considered acceptable.
- Assembly drawings - Assembly drawings show a graphical outline of component placement, including outlines and designators for some of the major components in the PCBA. The idea is to show a high-level orientation of components on the board so that a machine operator can check the rotation of each assembled part. Typically the "major components" will include certain connectors, main processor, and some of the physically larger components. Additional performance, testing, and inspection requirements can be specified in this drawing in a set of assembly notes.
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