A bill of materials (BOM) serves as a complete list of all the materials and parts—virtually every item—that a manufacturer needs to create a certain product. To be effective, the BOM needs to include not only the raw materials but also any subassemblies, subcomponents, and parts—and the precise quantities of each.
The exact format for a BOM will vary depending on the nature of the product being manufactured, but it is typical for two distinctly different types of BOM to be associated with each product—one used for the engineering phase when a product is first being developed, and another type of BOM used when the product rolls out to mass production for shipping to customers.
A good BOM always includes certain essential elements:
- BOM level: Each part or assembly in the BOM must receive a number or ranking that explains where it fits into the BOM hierarchy. This makes it easier for anyone to understand the BOM.
- Part number: The BOM should assign a part number to each item, which allows anyone involved in the manufacturing cycle to reference and identify parts instantly. To avoid confusion, each part must receive only one part number.
- Part name: Each part, material, or assembly should also include a detailed, unique name that allows anyone to identify the part easily without having to reference other sources.
- Phase: Make sure to record the lifecycle stage of each part in the BOM. For example, for parts that are the process of being completed, a term like "In Production" can be used. Other terms, such as "Unreleased" or "In Design" can be used for parts that have not yet been approved. Such terms are especially helpful during new product introductions since they allow progress to be tracked easily.
- Description: A comprehensive, informative description of each material or part must be included. This description helps you and others identify parts and distinguish between similar parts and materials.
- Quantity: The number of each part used in each assembly must be specified in order for the BOM to serve as an accurate purchasing tool.
- Unit of measure: The BOM must specify what unit of measure is being used to quantify the part or material. Terms like "each," "inches," "feet," "ounces," and similar identifiers of quantity can be used. This information helps ensure that correct quantities are purchased and delivered to assembly lines.
- Procurement type: Each part should be identified as something that is purchased off the shelf or manufactured according to project specifications.
- Reference designators: When a product includes printed circuit board assemblies (PCBAs), the BOM should have reference designators that explain in detail how the part fits on the circuit boards.
- BOM notes: Make sure to include any additional information that is necessary for those who will use the BOM.
Key Questions to Answer When Creating a BOM
You'll need to have certain information to create a BOM. Among the questions you should be asking are as follows:
- Are There Consumables That Need to Be Itemized? Manufacturers sometimes fail to include mass-quantity consumable items—such as glue, wires, fasteners, labels, and boxes—in the BOM. It is important that all necessary materials are included in your BOM in order for it to serve as an accurate procurement and production document.
- Are There Files That Must Be Attached to Your BOM? Many BOMs will need to be appended with supporting documentation, such as computer-aided design (CAD) drawings, part datasheets, and subassembly instructions. Make sure any attached files are correctly matched to specific BOM-level items.
- Who Will Use the BOM? Consider everyone who might need to reference the BOM, and make sure it includes all the information that each of these people will need throughout the lifecycle of the product's manufacture and packaging.
- How Will You Track BOM Revisions? Your BOM may go through several versions during the design phase, so you must have a way to identify different BOM versions and distinguish between them. This will ensure that when production begins, everyone is referencing the correct version.
The engineering bill of materials (EBOM) defines the finished product as it was originally designed. It lists the items, parts, components, subassemblies, and assemblies in the product as engineering designed it. The EBOM is often created by the product engineers based on a CAD drawing. For a finished product, more than one EBOM may be created.
A precise and accurate EBOM is essential, especially for a new product, since this is the document that ensures the correct materials and parts—in the correct quantities—are available when the item is being manufactured. To ensure that the parts are available when required, the purchasing department needs information on what vendors to purchase items from and how much lead time is required for each ordered part. The purchasing department will negotiate to obtain the best price for each part in efforts to reduce the overall cost of the finished product.
The ramifications of errors in the EBOM are serious. Incorrect quantities can cause production to be stopped. Any delay can lead to financial loss as the manufacturer attempts to find missing parts or postpones manufacturing to start a different production order.
The manufacturing bill of materials (MBOM) contains information on all the parts and assemblies required to build a complete and shippable product. This includes all the packaging materials required to ship the finished product to the customer. The MBOM includes not only all the information required for manufacturing but also any processes that must be performed on the item before it is completed. When a materials resource planning (MRP) analysis is run, the details of the MBOM are used to calculate when materials need to be purchased and when the manufacturing order needs to start, based on the suggested delivery date to the customer.
Several elements are involved in creating an MBOM. For some companies, the MBOM must have a validity date range. For example, when new products are being tested, the manufacturer may want to restrict the MBOM's use to one or two months. If, after the test period, the product requires some modification to either key elements or to the packaging, the MBOM can be changed and a new validity date can be established. Alternatively, an entirely new MBOM can be created.
Configurable Bill of Materials
A configurable BOM is for a finished product that meets an individual customer's specifications. The configurable bill of material contains all the components that are required to manufacture the material to that customer's detailed requirements. Frequently this involves labeling and packaging with slight variations for an individual product being manufactured for several different customers.