Designing a PCB is a rewarding experience and there is a great deal of satisfaction with looking at the computer screen, reviewing the design and declaring it a completed work of engineering art. However, that feeling of satisfaction is nothing compared to the first time holding a PCB in your hands that was, for better or worse, designed entirely by you. However, there are many steps between the design on the screen and receiving the physical boards. Most people go to a manufacturer to have the boards made and it is at this point in the process that Gerber files become an incredibly important tool.
The Gerber format is an excellent common intermediary that means any of a widely disparate set of design tools can produce a format any manufacturer can read and follow. This format is not specific to PCB design, but is simply an open file format for two dimensional vector images. However, unquestionably, their most popular usage is in the PCB industry. The flip side of that is true as well. For the PCB industry, Gerber files are effectively the only software-agnostic format for transferring exactly the information needed to manufacture a design. While a Gerber file does not need any additional information to create the shape desired, the manufacturer does need other specifications, such as board thickness and finish type. If this information is not provided most manufacturers will simply use their standard setup, which, for starting designers, is typically sufficient. Recently, there have been changes to the format to include additional information besides the two-dimensional image, but the utilization of these changes by both the design programs and manufacturers is not complete.
A common point of confusion for those who have never used Gerber files before is why they need multiple files to create a single PCB. This may seem strange because the design program uses only a single file. Yet, understanding that a board is a compilation of many different layers, it becomes more obvious that each layer will require its own two-dimensional image. This also explains why the creation of Gerbers is not as simple as a single click for some PCB design tools.
Gerber generators, or the CAM processor as Eagle calls it, need to know what layers of the PCB are going to actually be created. For example, the solder paste layer does not need to go to the PCB manufacturer, but it does need to go to the assembly house. Surprisingly, there is no one standard for the file extensions and their associations with the different layers. This is sometimes frustrating, but manufacturers seem to understand the different standards, so it is fairly easy to choose a single file extension-naming standard and use it throughout the design process. Once all of the files have been created, they’re usually zipped together into a single folder. Most Gerber viewer programs can accept the entire zipped folder to display the information, instead of needing to open each file individually. It is highly recommended to review the Gerber files before sending them off to manufacturing.
Not every PCB design program is created equal and there are products that sometimes produce Gerbers with unexpected and unwanted artifacts. A good board house can catch some of these but it is risky to rely solely on them for a thorough review.
Another aspect of Gerbers that may confuse newer designers is that there are many different versions, names and, less frequently used, competing formats, making it difficult to decide which one should be used. The original Gerber format was called the RS-274-D, but this was replaced years ago with the significantly different extended Gerber format, the RS-274X. Some manufacturers still accept RS-274-D, but any PCB design software that has been updated in the last decade should be able to output RS-274X. The extended version has also seen changes over the last two decades, but most of these changes are fairly transparent to the end user. Besides Gerber files, many manufacturers require a separate format for the holes drilled in the PCB, typically in the Excellon format. This drill-specific file format includes information not typically found in Gerber files, such as how quickly and how deep a drill bit should go when drilling a hole. There are some concerns with the Excellon format as it is sometimes slightly off from the Gerber files which can lead to a slight misalignment of holes. This causes some to advocate using solely the RS-274X format for all files, including drill holes, but this is still a matter of debate. It is recommended that you read the FAQs of your PCB manufacturer or contact them directly for clarification on what their file format requirements are.
Gerber files are an unavoidable and important part of the PCB designer’s world and it is worth taking some time to become familiar with them, to standardize and streamline the Gerber creation process, and to meticulously check all generated Gerbers before sending them off to the manufacturer.