A bill of materials (BoM) is a list of the parts or components that are required to build a product. The BoM provides the manufacturer's part number (MPN) and the quantity needed for each component.
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At its most complex, a BoM is a multi-level document that provides build data for multiple sub-assemblies (products within products) and includes for each item: part number, approved manufacturers list (AML), mechanical characteristics and a whole range of component descriptors. It may also include attached reference files, such as part specifications, CAD files and schematics.
Originally used internally within a company, the BoM served as a way to track product changes and maintain an accurate list of required components. As manufacturing has become increasingly distributed, however, the BoM has taken on even greater importance. It serves as the primary reference file for product data when transferring product information from the original equipment manufacturer (OEM) to the electronic manufacturing services (EMS) provider and from the EMS to its vendors and suppliers.
As outsourcing expands the number of companies involved in the manufacturing process for a particular product, the need for accuracy in the BoM is critical. According to the International Electronics Manufacturing Initiative, BoM errors typically fall within three categories: completeness, consistency and correctness.
Completeness - Incomplete data is the most common BoM defect. Critical pieces of information that are often omitted include quantity, part description, reference designation and approved manufacturers list (AML). Missing AMLs reportedly cause the majority of problems.
Consistency - Information in the BoM sometimes conflicts with information provided in engineering drawings and design files. For example, quantities may not match -- there may be 10 locations for a particular component indicated on a board, but the BoM only specifies nine. Another consistency problem is format. The format of the BoM, even though it is from the same customer, can change from one transmission to the next, making it difficult to match and confirm data. Language is another stumbling block because it, too, can vary from BoM to BoM.
- Correctness - Incorrect data is a serious problem. Common errors include invalid manufacturer or supplier information, obsolete data and incorrect part numbers (i.e., the manufacturer's parts number (MPN) given does not match the description of the part, or the MPN is not recognized by the manufacturer/supplier). Again, approved manufacturer's lists (AML) seem to be the predominant problem. Additional errors can result from receipt of information in hard copy format, which requires manual re-entry of data, an error-prone and time-consuming task.
The International Electronics Manufacturing Initiative, an industry consortium of 70 electronics manufacturers, suppliers, associations, government agencies and universities, recommends that BoMs be standardized.