I’m planning on building a new utility shed in my backyard; the original shed was old and worn out when I first bought the house 20 years ago and it absolutely has to be replaced now. In fact, if it wasn’t for the thick coating of rust that seems to be protecting it, I’m not sure that it would even be standing anymore.
I haven’t used a hammer and nails in my PCB designs before, so I ordered some plans on how to build this “Deluxe Backyard Storage Shed.” The plans include safety tips, drawings, cutting lists, step-by-step instructions, and a list of materials. But what an intimidating list of materials it is: 2 X 4’s, 2 X 6’s, shingles, hinges, a window, and the list goes on and on.
The fact is that I wouldn’t have a clue as to how to proceed with this project if it didn’t include a list of materials, and I’ll really be in trouble if the materials list has any mistakes in it. In the world of PCB manufacturing, it’s the same thing. Without a correct materials list or a Bill of Materials (BOM), we couldn’t build a circuit board either. If you’re not familiar with a PCB BOM, here’s a brief description of it and why a good and accurate BOM is so important.
A Bill of Materials, What is it?
Like the list of materials needed for building my new shed, the PCB BOM will contain every component needed to build the printed circuit board. Not only will this include the electrical components of the PCB, it will also include non-electrical items such as brackets, bolts, labels, and stickers. The majority of these items will come from the PCB CAD system where the board was designed. Additionally, other items can be added in manually as well, although as we will see, that can be a problem if the manual edits do not synchronize with the CAD database.
Each item in the PCB BOM will be listed by its part number and will contain a description of that part as well as a unique designator so that the part can be found on the board. Additional information can also be added to the BOM depending on the requirements of the board. This information can include extra fields for items such as suppliers and costs, or whether or not a specific part is to be fitted on different versions of the same board.
An example of a PCB BOM report
A Bad BOM can be Catastrophic
When a PCB bill of materials is incorrect, it can cause serious problems on the manufacturing assembly line. The wrong parts will be queued up for the automatic pick and place machines, and the correct parts may not be available. Your BOM may be incorrect for the following reasons:
Incorrect PCB library parts: If the PCB library parts are incorrect, it could result in the wrong part numbers being reported on the bill of materials.
PCB database was not updated correctly: Schematic updates during the design cycle are a normal part of PCB design. If those changes are not updated correctly though, the wrong part numbers could find their way into the BOM.
Incorrect design version used to create the BOM: Sometimes a BOM is generated from an incorrect version of the PCB database, or an older BOM is used but not updated correctly.
Incorrect manual additions or changes made to the BOM: Often non-electrical parts will get added or changed in a PCB BOM manually with a text editor. If there is a data input error here, the BOM will be incorrect.
These kinds of problems can cause lost time and money in manufacturing. At best it will take time to sort through the BOM to make the corrections and re-set the assembly line. At worst it could cause significant delays while waiting for the correct parts to be ordered, or even the expense of scrapping bad boards.
Using BOM management software like Altium’s ActiveBOM will help you to create an error-free PCB BOM
Tips for Creating a Correct and Dependable BOM
The key is to get an accurate PCB BoM into the hands of your manufacturer in order to avoid costly manufacturing errors and delays. To accomplish this, consider the following:
Follow a documented design process. By doing this you can ensure that your BOMs are created consistently. This will avoid BOMs that are created in a panic at the last moment, as well as providing a procedure for everyone to follow.
Create your manufacturing data in a unified design environment: When all of your CAD tools are working together in the same environment, you can tie the different portions of the PCB design process together. For instance, when your part data is tied together with your PCB design tools, you can work with parts that you know will be available for manufacturing.
Avoid manual changes to the BOM with text editors: Manual edits are an open invitation to introduce human error into your bill of materials. A better solution is to use BOM generation software that allows you to enter miscellaneous parts within the context of your PCB design environment. This will allow you to select the correct parts from your sources without the worry of human error.
These suggestions can help you to deliver a correct and reliable PCB BOM to your manufacturer every time. With the automation of a clean bill of materials, your engineering team can focus its efforts on design instead of documentation.
For advanced BOM creation utilities that will make your BOM tracking more efficient and less prone to error, strong PCB design software like Altium Designer will incorporate the BOM into a living, active BOM creation environment. With tools such as ActiveBOM, the entire unified design platform will tie your schematic, layout, and PCB BOM together.
If you are interested in learning how to make your BOM more reliable, find out more information by talking to an expert at Altium.
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