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A bill of materials or product structure (sometimes bill of material, BOM or associated list) is a list of the raw materials, sub-assemblies, intermediate assemblies, sub-components, parts, and the quantities of each needed to manufacture an end product. A BOM may be used for communication between manufacturing partners or confined to a single manufacturing plant. A bill of materials is often tied to a production order whose issuance may generate reservations for components in the bill of materials that are in stock and requisitions for components that are not in stock. There are two types of bill materials.
A BOM can define products as they are designed (engineering bill of materials), as they are ordered (sales bill of materials), as they are built (manufacturing bill of materials), or as they are maintained (service bill of materials). The different types depend on the business need and use for which they are intended. In process industries, the BOM is also known as the formula, recipe, or ingredients list. The phrase "bill of material" (or "BOM") is frequently used by engineers attributively to refer not to the literal bill, but to the current production configuration of a product, to distinguish it from modified or improved versions under study or in test.
Sometimes the term "pseudo-bill of materials" or "pseudo-BOM" is used to refer to a more flexible or simplified version. Often a place-holder part number is used to represent a group of related (usually standard) parts that have common attributes and are interchangeable in the context of this BOM.In electronics, the BOM represents the list of components used on the printed wiring board or printed circuit board. Once the design of the circuit is completed, the BOM list is passed on to the PCB layout engineer as well as the component engineer who will procure the components required for the design.
BOMs are of hierarchical nature, with the top level representing the finished product which may be a sub-assembly or a completed item. BOMs that describe the sub-assemblies are referred to as modular BOMs. An example of this is the NAAMS BOM that is used in the automotive industry to list all the components in an assembly line. The structure of the NAAMS BOM is System, Line, Tool, Unit and Detail.
The first hierarchical databases were developed for automating bills of materials for manufacturing organizations in the early 1960s. At present, this BOM is used as a data base to identify the many parts and their codes in automobile manufacturing companies.
A bill of materials "implosion" links component pieces to a major assembly, while a bill of materials "explosion" breaks apart each assembly or sub-assembly into its component parts.
A modular BOM (or variant parts list) can be displayed in the following formats:

A single-level BOM (or unit list) that displays the assembly or sub-assembly with only one level of children. Thus it displays the components directly needed to make the assembly or sub-assembly.
An indented BOM (or structural parts list) that displays the highest-level item closest to the left margin and the components used in that item indented more to the right.
Modular (planning) BOMA single-level BOM resolved to list the effectively needed quantities of components to produce a product (rather than to list each individual part by its logical name) is also called quantity synopsis parts list.A BOM can also be visually represented by a product structure tree, although they are rarely used in the workplace. For example, one of them is Time-Phased Product Structure where this diagram illustrates the time needed to build or acquire the needed components to assemble the final product. For each product, the time-phased product structure shows the sequence and duration of each operation.

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