I have put together a lot of kit furniture over the years, with various levels of success. I will begrudgingly take responsibility for the errors that I made by over-tightening screws, putting the pieces in backward, and scratching shiny surfaces. The thing that really drives me nuts, however, is when I find problems with the assembly documentation. Assembly steps are sometimes missing or illustrations may be confusing. The worst, however, is when the included parts don’t match the parts list.
As annoying as it is for your new bookshelf to be shelved due to a parts list problem, however, it’s far worse when your circuit board can’t be built because the bill of materials (BOM) is incorrect. Having problems with your BOM will slow down the production of your board because your contract manufacturer must spend time cleaning it up. This will not only add time to the schedule, but it could also add cost. It could even potentially cause a bad board build or stop the project altogether. Let’s look at some ways that you can make sure that your bill of materials for electronic components is clean and ready for production before you send out your board to the CM.
A Bill of Materials for Electronic Components Starts with Correct Data
A bill of materials (BOM) that is not accurate can cause a lot of problems when it comes time for your contract manufacturer to build your circuit boards. Before you compile the BOM data in preparation for sending it out, you should take some time to verify its accuracy. Focus on three main areas of concern:
- Verify component availability: If circuitry has been copied from a previous design, or the CAD library hasn’t been updated, the PCB BOM may end up specifying components that are no longer available. This could be because a part may be in the process of being phased out and is no longer recommended for new designs (NRND), or it has completely gone end-of-life (EOL), or it is even obsolete (OBS). It could also mean that a specific part specified is too expensive, or that the lead times will delay the build. Any of these reasons can cause problems by delaying the manufacturing of your board or even forcing a redesign.
- Confirm the accuracy of the CAD data: Usually, the CAD parts that are available from vendors and third-party CAD library services are built correctly, but for parts that have been altered or built by hand, that may not be the case. Your design process should have component checking built into it for new parts introduced to your library, and you should confirm that the parts you are using are correct as well. To do this, make sure that the schematic symbol and component information match the part datasheets, and that the correct PCB layout footprint is being used.
- Make sure the design is fully updated: Most modern PCB design CAD systems have redundant checks in them, but you should still confirm that everything in your design has been updated before you complete it. This will verify that the BOM data matches your design intent. Another critical part of this process is to make sure that you create a final BOM report after you have finalized the last design updates and synchronization between the schematic and the layout. Too many BOM inaccuracies occur because someone made a last-minute change to the design that altered a reference designator and then forgot to create a new BOM report.
These are the main problem areas to watch out for in generating accurate BOM data. The next step is equally important, however, and that is organizing the data so that it is in a usable format.
A Clean Bill of Materials Needs Optimization and Organization
Now that you are working from exact data, the next step is to organize that data into a workable BOM report. At a minimum, the following information needs to be included in your BOM:
- Part numbers. In situations in which a company is using both vendor and internal numbers, both should be included.
- Component values and tolerances.
- Part descriptions.
- Quantities of each part number.
- Reference designators.
Additionally, contract manufacturers also like to see information on non-standard components. For instance, if you are planning on using a part that is known to be NRND or EOL, your CM should be notified. You should also notify your CM which parts can be sourced from any manufacturer (i.e. a generic part), and which parts should come from a specified manufacturer due to specific properties or parametric characteristics that must be maintained.
With all the relevant data now prepared, the final step is formatting the BOM report for usage. Here is where it is a good idea to work with your CM to find out what format will best serve their needs. In some cases, a BOM will be listed with each part on a separate line item, while others will group reference designators from like part numbers together.
Whichever way you are requested to provide this information to your CM, you will find that most PCB design tools have formatting utilities built into their report generators. This will make your job a lot easier by eliminating the tedious and error-prone manual editing and formatting process of cleaning up a BOM report.
The Benefits of Working with Your CM on Your Bill of Materials
In addition to finding out what BOM formatting requirements that your CM has by working ahead with them, you will also find a lot of help there, too. Contract manufacturers typically have deep relationships with component vendors that will help get the best prices and the lowest lead times of the parts you need. Another extremely helpful resource you will find at a quality CM is the engineering expertise to help you with part selection based on how the parts will work in the circuitry that you are designing.
At VSE, you will find the component and design engineering expertise that will do a full BOM analysis on your design before we run it through manufacturing. In addition to fully scrubbing your BOM to generate a clean BOM document to be returned to you, we also:
- Identify long lead time and other inventory issues.
- Identify NRND, OBS, and EOL parts.
- Identify lower-cost alternatives for expensive components.
- Validate part descriptions versus part numbers.
For all of these potential issues, we will suggest to you component alternatives that will minimize the impact of any of these conditions. We can do this due to the long history we have of working together with reputable component vendors, suppliers, and brokers to quickly get you the best parts for your project
