There are a lot of important aspects of building a low-volume product, and managing costs—both in terms of time and money—is often pretty high on the list. When it comes to low-volume PCB assemblies, the focus is more often on the functionality of the PCB instead of making the best parts and materials choices. Without proper management, however, the bill of materials (BOM) can quickly get out of control, costing you time and money.
The good news is that with some added care on your part, and by working with a contract manufacturer who specializes in providing high-quality, low-volume builds, you can have the best of both worlds—functional circuit boards and an accurate BOM that helps streamline your project costs. The key is to design your board and manage your components correctly so that your contract manufacturer can build the boards you need without any problems. Here, then, are some of the best practices that you can put into place for your PCB assembly BOMs.
Best Practices When Creating Your PCB Assembly BOM
Generating a bill of materials report of your printed circuit board is a fairly simple task with most types of PCB design CAD tools. Usually, it is just a matter of selecting the report type and format you want from a menu of different reporting options.
The critical part of PCB BOM creation is in designing the schematic and choosing the correct parts that BOM report generator will use to pull its information from. For an accurate BOM, you will want to make sure the components you are using are functionally correct, are available, and are at the best price.
There are also some other details you should consider for your BOM as well. Some of these may not be included in the information that is listed in the schematic components, and it may take some additional work from you to include. Here are some of those details to keep in mind:
- Component manufacturer: Your CM should have many different component manufacturers they work with, and you can often source parts at a better price and lead time than what you are aware of. If you need a specific component manufacturer, however, due to the electrical performance for any of your parts, it is a good practice to note that on your BOM so your CM will understand immediately what the needs are for that part.
- NRND, EOL, or OBS components: It is helpful to alert your CM if you are purposely using parts that are not recommended for new designs (NRND), have gone end-of-life (EOL), or are obsolete (OBS). Usually, your CM will recommend substitutions for readily available parts, however, for your low-volume builds, this may not be acceptable.
- Verify the part descriptions with the part numbers: Sometimes parts can be updated in the design without their part numbers being updated. Also, it is not unheard of for the CAD library part to have incorrect information in it that is passed on to the BOM. It is a good practice to confirm your parts in the final BOM to make sure your CM orders that parts that you are expecting.
- Inspect the BOM for format: CAD tools have a lot of options when it comes to generating a BOM report, and it isn’t unusual to see certain classes of parts omitted, or reference designators left off, or the BOM sorting off the wrong data field. Always check to make sure that you are sending out a BOM that is formatted the way the CM needs it to be.
- Validate quantities & reference designators to package types: Make sure that all of your parts are represented accurately. Sometimes, a designer will inadvertently renumber a reference designator to the wrong prefix, like using an “R” for a capacitor, or a socket gets replaced with an IC, but the socket reference isn’t updated accordingly.
Paying attention to BOM details like this can save you costly problems in assembly if they are dealt with before the board goes to manufacturing.
Knowing what your CM needs on your PCB assembly BOM for low-volume builds helps prevent problems and delays.
Assembly Difficulties Due to BOM Problems
When the above details are not accounted for, there can be a host of problems with the bill of materials for the printed circuit board. These problems can cause assembly delays or affect the performance of the assembled PCB. Some of the problems that might happen include:
- Schedule delays due to waiting for components from a specific manufacturer when any equivalent would have worked.
- Functional problems because a certain manufacturer’s part was not specified, and the substitute part did not have the same performance characteristics as was used during prototyping.
- Performance problems due to questionable or outdated NRND, EOL, or OBS components that had to be purchased from non-approved sources.
- Incorrect part numbers or descriptions can result in the wrong part being installed during assembly.
- The component does not assemble correctly to the PCB land pattern because the incorrect footprint shape for the component was specified in the design.
Problems like these can be traced back to issues with the BOM, and they can cost you time and money. The best thing you can do is to make sure that your design and BOM information is correct before you commit to manufacturing. The good news is that you have a partner in this process that has a vested interest in helping you to create a good bill of materials information.
How to be Assured of a Complete and Accurate BOM
Having accurate bill of materials information is just as important to your contract manufacturer as it is to you. They want their assembly lines to run smoothly, and they want your board build to be successful. Because of this, your CM will usually work with you to make sure that the BOM for your PCB is the best that it can be.
At VSE, we do a full BOM analysis and “scrub” of all BOMs on incoming projects with the following goals in mind:
- Identify long lead-times and inventory issues.
- Identify NRND, EOL, and OBS issues.
- Identify lower-cost alternatives for expensive components.
- Validate part numbers with their descriptions.
If any of these problems are discovered in our analysis, we will make recommendations to you for corrective actions. We will also continue to monitor the status of your project’s components during the life of the build, so we have advanced warning of any potential changes to their availability. We can do this because of the relationships that we have built up over the years with reputable component brokers. Once completed, we will generate a final “clean” BOM for your project so that your documentation is completely up to date and in sync with your design data.
