Which file format should I use? This is something I am asked frequently, and it reminds me of when I was in college and working for a PCB fabricator. I’d work my way to the sales department so I could support them with design for manufacture (DFM) analysis—a daunting task while also trying to remember all the information I was learning in my engineering classes. However, an engineer with many years of experience was a wonderful mentor to me. He not only taught me a plethora of information about PCBs, but he also taught me about the many different file formats we use for PCBs. He even shared painful legends of screen projectors and making screens.
I once asked him, “How did you get anything done with that technology?” He just smiled and shrugged. When the last light projector was unplugged, someone placed a computer in that space. The advent of CAD and CAM improved production, but also spawned new challenges. Now I want to share my own experience with Gerber, ODB++, and IPC-2581 files.
Gerber File Format
My first experience was with Gerber data. When I saw my PCB graphically represented on my computer screen, I was baffled. I remember looking through the extracted zip file and noticing how many different files there were to keep up with. Many times, the fabrication and array drawings were included in separate files as well.
At the time, I did not know Gerber files had two different formats. To be clear, Gerber data is either RS-274-D or RS-274-X. One of the drawbacks to Gerber data is that the drill files must be generated separately from the Gerber files. Many times, engineers do not know, or simply forget, to do this. This can lead to delays in manufacturing, especially when the lead time is three days or less. The time it takes for the file request to get back to the designer, and the files delivered to the factory, burns much of the time for the quick turn. From my experience, missing drill files are a common showstopper in the PCB industry.
Another drawback with Gerber data is the layer orders are not defined. This has an easy work around: Simply name the layers in order from top to bottom. In the many years that I did DFM analysis, I saw many sets of data with convoluted layer definitions. I had to contact the designer for clarification and that would hold up the board analysis. This is one part of the design we should keep simple. Finally, the netlist file must also be generated separately. This can cause further delays when the fabrication drawing requires its use, but the designer forgot to generate it.
However, Gerber files are the most widely accepted data format in the PCB industry. Because of this longevity, many programmers can produce software to import and view the data. RS-274X data includes parameters, apertures, and X and Y coordinates, along with Draw and Flash commands that RS-274D lacks. The tools used by the factories readily accept this data, producing less errors and less time spent with the board on hold. Finally, designers will have the ability to choose from more factories when getting a board quote, thus having a more sustainable PCB design. RS-274-X is a reliable format, and still the most prominent format in my daily work.
ODB++ File Format
The PCB industry is readily improving CAD and CAM data generation and handling, and the ODB++ file format does exactly that. The data is in an extensible markup language (XML) format and is a single compressed file containing sub-directories. The XML files enable CAM software to produce a more accurate representation of the PCB. I have watched this format become more widely used over the past decade, yet it’s experienced some growing pains. In its infancy, some CAM systems did not handle this data very well. When I did DFM analysis at the factory, I ran two wonderful software suites. Every board I looked at went through both systems. Occasionally, I would visually discover missing pads that neither system flagged. It was random and I had no control over it.
Through some investigation, our team determined the software used to import the ODB++ file did not import it correctly. It was occasional and random. When we looked at the accompanying Gerber data, the missing pads were “magically” generated. The CAM software I was using was the latest and the best, however, it still had some ODB++ handling bugs. I think this may be one reason why ODB++ files are not as widely accepted as Gerber files. Modern CAD, CAM, and DFM tools support this data format very well, and the lesson I learned is to always include a backup Gerber data set with the ODB++ file. So, what is the point of using the ODB++ database if I still need Gerber data?
Well, I still think the ODB++ format is an advantage for both design engineers and factory pre-production engineers (PPE). The board layers are in order upon importation and the drill files are already included. In addition, the netlist file, stackup, and bill of materials can be included. I am sure many PPEs appreciate this huge time saver. PPEs look at many boards during their work day, so any time spent going back to design engineers with questions is time that could have been spent to get another board on the manufacturing floor. This is not meant to be negative; it is just one of my observations from working in the PCB manufacturing industry.
IPC-2581 File Format
Unlike the other formats, the IPC-2581 format is an open and neutral format. The advantage here is the single file is not specific to other software and hardware. This simply makes it easier to use and integrate into new technology. This database is also in XML format. This alone makes it a winner in my book, but let’s look at some other features. First, the data is encapsulated in one single file. No more lost or forgotten drill files. All supporting drawings can be either sent to the manufacturer or eliminated from the generated data file. Sure, we can choose not to include the schematic in the data package or include it as necessary. When I worked in manufacturing, I saw schematics come with Gerber data all the time. I often wondered how many got out by accident.
My favorite aspect of this data format is that it is better at protecting intellectual property. The IPC-2581 format allows designers to export a subset of design data that is extremely difficult to re-engineer. I have no doubt these features will reduce manufacturing and assembly delays. Unfortunately, many PCB manufacturers still cannot work with this data due to factory systems. These systems are very expensive to replace and readily accept Gerber data and ODB++ databases.
I am always happy to see our industry grow. With the necessary complexity that engineers have to work with, it is nice to have a more intelligent file system. In our era, we must also protect the data we have generated. None of the three file systems are perfect. To have a more sustainable PCB we must use the most widely available options in our industry. Sometimes popular items in our society fade away, but the Gerber file system is still the most widely accepted format of the three. I do not think it is going away any time soon. As always, communicate with your PCB supplier in advance to ensure you are producing a sustainable PCB, in the format they accept.
Ryan Miller is a field applications engineer at NCAB Group.
