Data Preparation: Data Formats
The first step in the manufacturing process of a printed circuit board is commonly referred to as data preparation. This is an operation in which information provided by the customer or designer is converted for use by the manufacturer to build the pcb. There are standard data formats required by the bare board manufacturer. The commonly accepted formats are as follows…
1) RS-274-X is the Gerber extended photo plotter language. Each layer of artwork is provided in its own Gerber file. Each file specifies the location, shape and size of all features on the artwork layer in one data file. Common file extensions include .PHO, .LGX and .GBX. This is the second generation of the Gerber format. The original version of the artwork format provided the data locations in one Gerber file and the shape and size in a second file called an aperture list.
2) Excellon 2 CNC drill format. Each file contains the through hole locations used by a pcb design. The file specifies location and size of drilled holes. Drill locations and size for blind and buried vias are provided in their own unique drill files. Common file extensions are .DRL, .NCD and .TXT.
3) IPC-356 net list format. The Gerber and Drill formats are non-intelligent data formats. They only specify location, shape and size. CAM software reads in the Gerber and drill information and generate a generic net list based on what is interpreted by the software as being connected based on applied rules. The IPC-356 net list file contains the connectivity information from the designer’s CAD software. The CAM software imports the IPC-356 data format and compares it to the generic net list. Differences between the two net lists are then identified. Doing so minimizes the possibility of interpretation errors. Common file formats are .356 and .NET.
Most printed circuit board manufacturers rely on these three formats to build the design. They are common and widely accepted by all manufacturers.
There is another less common format available that combines all three pieces of information. The format is called ODB++. This format contains all of the artwork, drill and CAD intelligence in one database file. The file is typically provided with a .TGZ file extension which is in a UNIX compressed TAR file. Supplying the data in the ODB++ format has not gained wide acceptance as of yet. Although acceptance is slowly gaining. Specialty software is required is required to export and import this data format in some cases.
ODB++ is not the first all inclusive data format to be released to the pcb industry. There were at least two other all inclusive database formats prior to ODB++. These are IPC-350 and Gencam. These two database formats were an attempt to simplify the transfer of printed circuit design data errors between the designer and the manufacturer. Each format is controlled by a design specification and were driven by the IPC. Unfortunately, there were dozens of software providers writing code to allow their software to take advantage of these formats. Interpretation errors of the specifications were a concern to some and unfortunately some errors were made that resulted in non-conformances. The same problem happened when the extended Gerber format was first being incorporated into pcb software. After the bugs were worked out of the extended Gerber format it was stable and reliable. New formats introduce the potential of mistakes from human error. IPC-350 and Gencam never gained wide acceptance. Why risk scrapping out a design on a new format when the extended Gerber format already works?
ODB++ on the other hand is the working environment of the Frontline Genesis CAM software platform. This is the driving force behind the ODB++ format gaining acceptance by the industry. The extended Gerber format became accepted because it was a photo-plotter format used by many printed circuit board manufacturers. The format gained acceptance by designers in part because it was already being used by the manufacturing industry. Unlike IPC-350 and Gencam, the ODB++ format is slowly gaining acceptance. This is in part due to the fact that it (like extended Gerber) is being used by the industry by companies that use Fronline Genesis as their front end CAM platform. There are still some concerns over data interpretation by programmers. However, as time progresses I believe the ODB++ format shall ultimately become the data transfer format of the pcb industry.