I once attended a party where the hostess was running out of beverage cups, and she asked me to run to the store and buy more. When I asked what she wanted, she replied with, “Oh you know, the red plastic ones.” I bought what she needed, but just to be funny, I went ahead and also got a container of the mini shot-sized red plastic cups as well and presented those to her first. Don’t worry, I was smart enough to give her the real cups before she took a swing at me.
Handing her the wrong-sized cups made for an amusing moment, but it’s not nearly as funny when incorrect or insufficient parts are purchased for your printed circuit board assembly. Too often, a simple parts list is submitted to the PCB contract assembler instead of a finalized bill of materials. Without complete and correct information, it can cause confusion as to what is actually needed to build the board.
To prevent this from happening on your project, here’s a look at some of the possible differences between a PCB bill of materials and parts list, and what you should know about them as you prepare your PCB design for assembly.
The Differences Between a PCB Bill of Materials and Parts List
Although both a parts list and a bill of materials contains information on the components used in a printed circuit board design, there are some important differences to be aware of.
A parts list is a compilation of all of the components used in creating the PCB design. This list may come from handwritten notes or be computer-generated, and it is usually a working document while the design is being developed of what the engineer intends. Because of this, a parts list may be incomplete or even missing important information, such as reference designators or descriptions. The parts themselves may be listed in several ways, including by their internal company part number, a manufacturer’s part number, a suppliers catalog part number, or even by an abbreviated name or description.
A bill of materials, or BOM, on the other hand, is a complete list of the components and materials needed to build a PCB. It will carry full information on each part used on the board, including the manufacturer’s part number, an internal company part number (if used), values and tolerances, descriptions, quantities, and individual reference designators. BOMs are usually computer-generated by the PCB design CAD tools and will be formatted to present their data either by individual components (reference designators) or by grouping the components by their part numbers.
Knowing which of these documents to send to your PCB contract manufacturer is important because sending the parts list instead of the BOM can lead to some unintended consequences.
Problems When You Submit a Parts List Instead of a Full BOM
Although you may be very familiar with the design you are sending out for manufacturing, your contract manufacturer may be looking at it for the first time. With only a parts list to work from instead of a full BOM, there may be some confusion when the CM is preparing the components to use on the board. For instance, a parts list may not contain complete or up-to-date information on a component. Without a proper manufacturer’s part number, description, and reference designator, the CM will have to get clarification from you before they can proceed. A parts list may also not include some items that are essential for manufacturing such as IC sockets, mounting hardware, labels, or other materials.
If the CM ends up working from a parts list instead of a BOM generated from the CAD tools, it could slow down or stop manufacturing for the following reasons:
- Components may be specified that end up being unavailable due to higher prices or long lead times, causing manufacturing delays.
- Components may also end up having an end-of-life (EOL), not recommended for new designs (NRND), or obsolete (OBS) status, forcing the CM to use more expensive parts or redesign the board.
- There may be delays waiting for a specified component that could have been replaced with a less expensive and easier-to-obtain equivalent part.
- The wrong components could be loaded into the automated assembly machines.
- The footprint on the PCB may not match the corresponding component specified in the parts list.
- The component specified may fit on the board, but it may not be the correct functional part.
The best thing you can do is to ensure your CM has a complete, CAD-generated BOM to work with. After design completion, you should thoroughly check and review it, make sure the schematic and the layout are synchronized, and then generate the final BOM. This way, you can be certain you will have the most up-to-date BOM possible to send out for manufacturing. Once your CM gets this BOM, they can then help you with additional services.
How Your PCBA Contract Manufacturer Can Help
Having a clean BOM to work with is a high priority for your CM because it will help to ensure that the manufacturing process goes smoothly and yield a higher-quality run of circuit boards. Therefore, the CM will spend time working with you and your BOM to ensure it has complete information and is ready for manufacturing.
At VSE, we’ve perfected our BOM scrubbing process to provide you with the following:
- Analysis: We will thoroughly analyze your BOM to identify lower-cost alternatives for expensive components.
- Examine: We will examine your BOM for components with long lead times that could result in inventory issues.
- Identify: We will identify components that have an EOL, NRND, or OBS status to consider advance purchases or potential replacements.
- Validate: We will validate part numbers with part descriptions to ensure that your boards are built with the parts that you intended.
- Monitor: For ongoing assemblies, we will continue to monitor the status of the components to get advanced warning of parts that may be going EOL, NRND, or OBS.
- Generate: Finally, we will generate a completely scrubbed BOM to be returned to you for your internal documentation package.
