Best Practices in PCB Documentation
PCB manufacturers and assembly houses have the important responsibility of taking someone’s idea and turning it into a reality. CAD files, including Gerbers, are important in conveying exactly what is wanted, but it’s the personal communication that ensures what is envisioned is what is made. Different phases of the manufacturing process require different files and information, but you can expedite the overall turn-time by having the necessary information prepared prior to approaching the respective vendor. Not only will this will make it easier and faster to obtain accurate quotes, but complete and clear PCB documentation will also greatly reduce the chance of misunderstandings and errors.
Board manufacturers require, at a bare minimum, the Gerber files for your board. With this information, they can typically fabricate a board that looks like the intended design. However, the manufacturer will have to make assumptions on several things, including the laminate and finish to be used. If there are any additional requirements, such as impedance control or testing, this will not, and cannot, be done without more information. Board thickness and copper weight are also not included in Gerber files and, if a deviation from the standard thickness and weight are desired, they must specifically be requested. There is also a great deal of information that, while included in the Gerbers, can be used to verify that the manufacturer has received all of the necessary information. Number of layers, board dimensions, requested turnaround time, minimum trace width and spacing, and quantity are all items that should be included in either the Gerbers or simple order forms. However, verifying them in a readme file will help catch omissions and errors.
Assembly houses require a few of the same items as board manufacturers. A select few of the Gerber files, specifically the copper, solder paste, and silkscreen files need to be provided to give the assembly house a reference to work from, as well as show solder paste placement. In addition to the Gerber files, assembly houses need a bill of materials (BOM) and the X, Y, rotation, side (XYRS) data. The BOM needs to contain quite a bit of information to be useful. For each part there needs to be a reference designator, value, description, package type, part number (manufacturer or retailer), and even a link to the distributor webpage. If you know how parts will be packaged, such as tape and reels, cut tape, or in separate bags, including it will allow the assembly house to determine whether or not they can use a pick- and-place machine or if assembly will be done by hand.
A readme file is a common way for a designer to convey information to both PCB manufacturers and assembly houses. The readme file usually is a simple list of requested features but can also contain a description of the end product, which gives a better idea of the overall scope of the project. If a description is going to be provided, it is helpful to be clear and concise. Sometimes, instead of a readme file, designers will include text on one of the layers in the Gerber file. This is a good way of making sure that the readme information is not overlooked, however it does limit the amount of information that can be provided. It is easiest to have a standard readme file that can be updated with each new design, but this runs the risk of information not being updated.
Please note, it is not enough to simply provide the documentation, it must be in a clear and complete format. Choose naming conventions for your Gerbers that are intuitive and easy to understand. Make sure the BOM is organized with appropriate spelling. Even more importantly, make sure that the BOM is accurate with reference designators matching the appropriate parts. If possible, have a third party within your organization review the documentation to see if it is easy to understand. What seems clear and straightforward to you may not be to others.
Documentation is not a glorious part of the design process and can be easily marginalized. But with a small investment in time up front, the whole manufacturing and assembly process will be smoother and faster. If you have found that your vendors are coming back time and time again to ask clarifying questions, examine what you’re doing and what can be done to make your documentation clearer and more useful.