The world over, a majority of designers and fabricators follow the Gerber RS-274X as the de facto standard when designing and fabricating their PCBs. The evidence of its popularity notwithstanding, Gerber has a number of practical limitations. Often, these limitations lead to a variety of problems when fabricating PCBs.
Brief History of the Gerber File Format
Ucamco developed the Gerber file format in the 1960s, when it was the Gerber Systems Corporation, and a leading provider of early photo-plotter systems using numerical controls. Their first format, RS-274D, was a subset of EIA RS-274-D, supporting their vector-based photo-plotters. Widely adopted, RS-247D remained the standard format for vector-based photo-plotters until the 1980s.
Raster scan plotters began replacing vector-based photo-plotters in the 1980s. These newer plotters were bitmap-based, requiring a completely different data format. Consequently, in 1998, Barco ETS, who had acquired Gerber Systems, released a single standard image format, and named it the Extended Gerber or GerberX. This was later renamed as the RS-274X format and is still in use today.
The latest Gerber RS-274X presents a complete image description format. Therefore, the Extended Gerber file holds the complete description of a layer of the PCB, and provides the operator with everything necessary to generate a PCB image, including the definition of any aperture shape. Requiring no external aperture files, painting, or vector-fill, the RS-274X standard specifies all pads and planes clearly and simply. Its simplicity has made it the de facto standard followed by nearly 90% of the world’s PCB designers and fabricators.
Problems with the Gerber File Format
Despite its wide acceptance and use, the RS-274X Gerber file format has its own shortcomings. The trouble is the standard does not address all aspects of fabrication and assembly, as required by the PCB fabricator.
Although Gerber RS-274X is extremely accurate and reliable when rendering images of copper shapes precisely on signal and plane layers, it does not transfer the layer stackup order accurately. Moreover, data sets and information regarding materials, drill data, netlist, pick-and-place data, bill of materials, test point reports, and more need to be generated by separate processes by different utilities. This means, the Gerber RS-274X format is incapable of transferring the complete information from the design domain to the manufacturing domain.
In the absence of a defined layer order being transferred to the manufacturer, fabrication has no way of deciding the order of copper layers and may miss a layer or two altogether. With the layer order missing, the drill data may generate holes relative to an incorrect layout. This mismatch can happen with the entire assembly data, and at all aspects of the fabrication process. Usually, with Gerber RS-274X, there is no defined way a fabricator can know about missing output data, wrong source file version, and these can render boards useless.
Designers usually get over the above shortcomings using a well-maintained design methodology and best practices shared with the fabricators. In general, they utilize Gerber RS-274X with minimum fabrication issues. However, maintaining ideal conditions all the time is difficult, things can slip up, causing problems to the fabricator and assembly houses, and now they have to face the brunt of the responsibility and sort through the issue. This also leads to fabricators and assembly houses being forced to spend a great deal of time and resource in inspecting and verifying the entire data for all incoming jobs, simply to minimize manufacturing issues.
Solutions and Alternatives to the Gerber File Format
Eminent PCB manufacturers such as Rush PCB Inc., eliminate the problems by adopting design transfer standards that addresses all aspects of the fabrication and assembly process. Two new open standards are available, and these enable efficient and accurate data exchange from the PCB designer to the manufacturing fabricators and assemblers. Ucamco administers one of these standards, the Gerber X2, while the IPC Consortium administers the other, the IPC-2581. Both are open standards, free from any proprietary restrictions.
The Gerber X2 File Format
The Gerber X2 is an expanded version of the GerberX format. In addition to the layout image data, Gerber X2 now includes design data as well. The X2 fabrication files now include the board layer order and stackup information that so far, fabricators had to interpret and verify manually. In the same way, a set of drill files is also included within the X2 fabrication files, detailing the location, drill size, plated/non-plated information, and the layer span.
The X2 attribute system qualifies objects with specifications such as file function, part, pad function, and more that add intelligence to the traditional image data improving the automation process. For instance, the file function specifies a file as top copper layer, top solder mask, while part specifies whether the PCB is a single or a panelized array, and pad function defines the pad as belonging to a via, through-hole, SMT, or fiducial. The Gerber X2 format directs all outputs to one single folder.
As the Gerber X2 is both forward and backward compatible with the RS-274X standard, it helps any X2 reader to interpret Gerber RS-274X files correctly. Therefore, fabricators using the Gerber X2 process will have no trouble interpreting legacy fabrication files created in the Gerber RS-274X format and vice versa.
The IPC-2581 File Format
Contributors from a wide range of PCB industry segment initiated, developed, and drove the IPC-2581 standard. These industries included MES, CAD/CAM and PLM vendors, PCB fabricators, contract manufacturers, as well as OEMs. The IPC-2581 is a single data format and within a single file, contains all aspects of the PCB design, such as layer stackup, materials, assembly, and test details.
With the IPC-2581 standard, the designer can include details of layer stack and information on materials to ensure proper layer order. The standard is suitable for stackups of complex board design such as related to rigid-flex boards, and is capable of handling special materials. It can also include drill and mill data for blind, buried, and filled via types. It also supports information on back drilling, V-grooves, slots, and cavities. For bare board testing, designers can include the net-list as well.
In addition to a complete set of fabrication data, the IPC-2581 can also hold assembly data. Therefore, it can contain not only the pick-and-place information, but also the information on polarity and rotation of a component, enabling support for both stacked and embedded components.
In addition to assembly drawings, the IPC-2581 standard has the capability to generate the documentation for bill of materials and purchasing. Therefore, the standard can tie up with PLM/ERP system data to create links between design and supply chain facilities. The greatest advantage of the IPC-2581 is one single file containing the entire data related to fabrication and assembly.
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