Between the mid-1980s and late 1990s, Spy magazine published a monthly “Separated at Birth” column/pictorial that would pair two selected photos of unrelated celebrities, cartoon characters, or movie characters. The authors would then point out similar physical characteristics from each photo. Mentally picture a space alien from a popular movie paired with a sultry singer to gain some appreciation for their humor.
Separated at Birth? Well… Kinda… Sorta
Looking closely, we can see the similarities—minus the humor—between an engineering bill of materials (EBOM) and a manufacturing bill of materials (MBOM). In our work with PCB design, we usually use an EBOM and always refer simply to a Bill of Materials (BOM). An EBOM associates with a CAD or EDA tool and uses engineer preferences and processes as an organizing guide. The scope of an EBOM remains rather narrow and refers only to the physical products subject to the engineering design process. An EBOM also excludes items or components from other disciplines but shows all the components needed for a specific area like PCB design.
As an example, the PCB design software that we use generates the PCB bill-of-materials. The list includes all components needed to build a specific board. Parts information from the BOM also feeds into the CAD system and becomes stored as part of a specific CAD library. Every PCB design BOM includes a comment that provides unique identification for the component, the manufacturer, a description of the component, a reference designator, package type, and a footprint. The BOM may also include supporting documentation along with tolerances and values that assist with selecting and placing the part.
When you think of a manufacturing bill of materials, picture the brother or sister who always made it to the dinner table before anyone else and managed to—somehow—get the best food. As time went along, this person became stronger, bigger, and more assertive as you wondered what happened.
An MBOM serves as an essential part of the project and shows all the mechanical and electrical parts, assemblies and subassemblies, tools and equipment, packaging, and documentation needed to produce a finished, ready-to-ship product. The MBOM also includes components, off-the-shelf (OTS) parts, and made-to-specification (MTS) parts as well as software components such as firmware. The list includes all parts scheduled for purchase, all parts used for the design process, and all parts in the inventory. An MBOM individually lists each screw for each sub-assembly, all the packing materials, instructional materials, and all the materials needed to complete the final assembly.
As a result, an MBOM has a different structure and takes a deeper look at the needed materials from the idea stage through the production stage. Every MBOM organizes by sub-assemblies in the order needed for the manufacturing process. An MBOM also uses detailed descriptions for each stage of the assembly process. Manufacturing work instructions (MWI) provide the detailed information for producing a sub-assembly, including separate part numbers and assembly time.
(Alt text: Manual PCB assembly)
Everybody Needs Somebody
Manufacturing teams rely on a very structured process and need to have all the process steps listed in a specific order. For example, using an FPGA would show separate entries for the blank device, the firmware, and the programmed device. The teams need this because of prioritization and decision demands. Some steps of a manufacturing process may occur in-house. Others may flow to a valued vendor. When outsourcing occurs, costs, quality, and product usability and flexibility may impact processes and the MBOM. Teams can use the MBOM to identify approved distributors with its data. All of this allows the product manufacturer to use something like a new handheld device to efficiently and cost-effectively deliver the product to the customer.
How does this occur? Since the MBOM lists parts scheduled for purchase and available inventory, a manufacturer can make decisions about production runs and determine how those decisions impact the introduction of the new product. In effect, the MBOM assists resource planning at the enterprise level, the materials level, and at the manufacturing level. For all those reasons, an MBOM must provide a very precise picture and thorough control of the parts needed for the product. Any inaccuracies result in the delivery of the wrong parts, the purchase of additional parts, or other circumstances that can result in lost time and additional costs.
(Alt text: Electronic components)
A Few Important Notes
BOM management tools can assist with your design and the collaboration that must occur between departments. When setting up your MBOM, a thorough approach pays dividends and sometimes answers questions before anyone can ask them. Adhere to language that everyone can understand is also important. A thorough, easy-to-understand approach and language builds project communication and helps prevent disasters like large quantities of the wrong part delivered to the manufacturer or a third-party location. As your project grows and migrates from concept to full-production, don’t neglect to track all changes and versions of your MBOM and maintain a change log. Your changelog should document why changes have occurred.
With versatile PCB design software like Altium Designer, you can access advanced tools for bill of materials management. For example, you can use ActiveBOM for unprecedented control over your design and provides a straightforward way to manage your components and project costs.
For more information on how to use and optimize BOM management tools for your PCB manufacturing needs, talk to an Altium expert.
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