I am not a wizard in the kitchen like many members of my family, so I am always amazed at the wonderful and tasty dishes that appear almost, as if by magic, at our family events. I’m not entirely sure what sorcery is involved in making such delicious food. I do know that, in addition to the years of practice that have gone into developing these culinary skills, there is also a spellbook: a handed down history of generational family recipes used to produce these feasts.
After you’ve designed a printed circuit board, your contract manufacturer will create reliable PCBs from your design. Like our great family feasts, however, your CM needs more than just the raw materials to produce your boards, they also need a good recipe. That recipe is in the manufacturing documentation that you create. Therefore, your documentation must be complete and accurate.
One of the most important documents that you will create for this process is the Gerber file, which is used to fabricate the raw circuit board. To help you to be better prepared, here’s a look at how Gerber files are explained and how they’re used in the manufacturing of your circuit board.
Gerber Files Explained: What Is a Gerber File?
Gerber files are open ASCII vector format files that contain information on each physical board layer of your PCB design. Circuit board objects, like copper traces, vias, pads, solder mask and silkscreen images, are all represented by a flash or draw code, and defined by a series of vector coordinates. These files are used by PCB manufacturers to translate the details of your design into the physical properties of the PCB.
The Gerber files are typically generated by the PCB design software that you’re using, although the process of doing so will vary with each CAD tool. Most of today’s Gerber files are generated according to the RS-274X Gerber format, which supersedes obsolete formats such as the RS-274-D Gerber standard. As a text file, Gerber data does not have to have a specific identifying file name but is often given a common extension such as .gb, or .gbr.
How Gerber Files Are Used In PCB Manufacturing
The PCB manufacturing technology used today has come a long way over the past decades. Previously, vector photoplotters were used to create the tooling film used in the PCB manufacturing process. A focused light conducted through an aperture was used to expose the film to create the flashes and draws for each individual pad and trace.
There was only a minimal set of apertures available and designers had to be creative in restricting their flashes and lines to only the apertures that were available to them. These older vector machines have now been replaced by a newer breed that uses a raster laser process to expose the film. For larger pieces of film that had a lot of line drawing on it, a vector photoplotter could take many hours. This time has been cut down to a matter of minutes with a laser plotter.
Gerber file history can be traced back to the needs of the original vector photoplotters. In order to give the plotter its instructions, a Gerber file contained minimal plotter configuration information, and X/Y coordinates followed by a flash or drawing command and which aperture position to use. The Gerber data has increased in functionality over the year; it now includes additional configuration information as well as macro and aperture definitions.
The laser plotters used today still use the same Gerber information, but the aperture restrictions of the older vector plotters no longer apply. The laser plotters convert the Gerber coordinates into a raster file, and that information instructs the laser plotter on how, where, and what is to be created on the films. For example, the aperture definitions convey the thickness and sizes of the traces and pads while the drawing commands define whether lines, polygon fills or flashes are to be created. Then the laser sweeps across the film exposing the image as it goes.
Time does not sit still though, and the world of Gerber files continues to change. Circuit board manufacturers have now begun using direct laser imaging to create PCB images directly onto the copper, and bypassing the need for film. There are also new database formats that are being used to create PCB images that contain much more intelligent board design data such as net connectivity data. But it’s safe to say that Gerber files will continue to remain in its current form for some time to come, and you need to be aware of the part they play in PCB manufacturing.
How To Generate Gerber Files in a PCB Design
After your design is complete and you’ve done the final check, the next step will be to generate the Gerber files for your PCB manufacturer. The process of doing so varies according to the PCB design software that you’re using. Some older tools may take many steps to set up and generate files, but today, most CAD packages have simplified the process of creating these files.
Typically the Gerber files you will need to produce will be an individual file for each physical layer of the board. If your PCB design is a six-layer board with four signal layers and two planes for power and ground, then you will need to output those layers into six Gerber files. Additionally, you will need to generate a separate Gerber file for the top and bottom solder mask layers, the top and bottom silkscreen layers (if required), and the top and bottom solder paste layers (if required). The amount of silkscreen and paste layers will depend on if your design requires silkscreen and paste on both the top and the bottom of the board. Some manufacturers may ask for additional Gerber layers for board outlines or fabrication and assembly information, but those requests are usually for unique design configurations.
You will also need to configure the Gerber file generator before you create the files so that the units, formats, and standards are set up in a way that is consistent with your design standards and the standards of your manufacturer. Although not a Gerber file, an NC drill file is usually created along with the Gerber files. This file instructs the drilling machines used by the board fabricator where to drill the holes in the board. This file is very similar to a Gerber formatted file in that it also contains drill size information and vector data for the different drill locations.
Documentation After Gerber Files
Lastly, make sure to document your Gerber, NC Drill, and other manufacturing files well so that your contract manufacturer can build the board according to your intent. You will need to create fabrication and assembly drawings, pick and place data, test data, a schematic netlist, along with a readme file to finalize the process. The best thing that you can do is to work ahead with your CM to know exactly what it is that they need from you for the best results.