Bill of materials

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.

An example of a BOM for a mechanical assembly (in German)

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 BOM can also be visually represented by a product structure tree, although they are rarely used in the workplace.[1] For example, one of them is Time-Phased Product Structure[2] 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.

Structure

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.

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.

Usage

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.

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.

Types

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.

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.[3]

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.[1]
Modular (planning) BOM

A 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.[4]

Configurable BOM

A configurable bill of materials (CBOM) is a form of BOM used by industries that have multiple options and highly configurable products (e.g. telecom systems, data-center hardware (SANS, servers, etc.), PCs, cars).[5]

The CBOM is used to dynamically create "end-items" that a company sells. The benefit of using CBOM structure is that it reduces the work-effort needed to maintain product structures. The configurable BOM is most frequently driven by "configurator" software, however it can be enabled manually (manual maintenance is infrequent because it is unwieldy to manage the number of permutations and combinations of possible configurations). The development of the CBOM is dependent on having a modular BOM structure in place. The modular BOM structure provides the assemblies/sub-systems that can be selected to "configure" an end-item.

While most configurators utilize top-down hierarchical rules syntax to find appropriate modular BOMs, maintenance of very similar BOMs (i.e., only one component is different for various voltages) becomes highly excessive. A newer approach, (bottom-up/rules-based structuring) utilizing a proprietary search engine scheme transversing through selectable componentry at high speeds eliminates the planning modular BOM duplications. The search engine is also used for all combinatorial feature constraints and GUI representations to support specification selections.

To decide which assembly variant of the parts or components is to be chosen, they are attributed by the product options which are the characteristic features of the product. If the options of the product build an ideal Boolean algebra,[6] it is possible to describe the connection between parts and product variants with a Boolean expression, which refers to a subset of the set of products.[7][8][9]

Parts which will not be assembled at all in one or more variants are typically marked as "DNP" (for "do not populate" or "do not place") in the affected variants. Other less frequently used designators for this include "NP" ("no placement", "not placed"), "NF" ("no fit", "not fitting"), "DNM" ("do not mount"), "NM" ("not mounted"), "NU" ("not used"), "DNI" ("do not install", "do not insert"), "DNE" ("do not equip"), "DNA" ("do not assemble"), "DNS" ("do not stuff"), "NOFIT" etc.[nb 1]

Multi-level BOM

A multi-level bill of materials (BOM), referred to as an indented BOM, is a bill of materials that lists the assemblies, components, and parts required to make a product in a parent-child, top-down method. It provides a display of all items that are in parent-children relationships. When an item is a sub-component, of a (parent) component, it can in-turn have its own child components, and so on. The resulting top-level BOM (item number) would include children; a mix of finished sub-assemblies, various parts and raw materials. A multi-level structure can be illustrated by a tree with several levels. In contrast, a single-level structure only consists of one level of children in components, assemblies and material.

Notes

The abbreviation "NB" or "n.b." (for nicht bestücken) is commonly used in German-speaking countries.

References

Reid, R. Dan; Sanders, Nada R. (2002). Operations Management. John Wiley & Sons. pp. 457–458. ISBN 0-471-32011-0.
Malakooti, Behnam (2013). Operations and Production Systems with Multiple Objectives. John Wiley & Sons. ISBN 978-1-118-58537-5.
"pseudo bill of material". BusinessDictionary.com. Archived from the original on 2020-08-05. Retrieved 2018-04-17.
Hochheimer, Norbert (2011). Das kleine QM-Lexikon: Begriffe des Qualitätsmanagements aus GLP, GCP, GMP und EN ISO 9000 (in German). John Wiley & Sons. p. 277. ISBN 978-3-52733076-8. Retrieved 2022-04-04. (334 pages)
"Super BOM". SAP ERP 6.0. SAP. 2012. Archived from the original on 2013-02-17. Retrieved 2012-07-17.
Herlyn, Wilmjakob Johannes (2012). PPS im Automobilbau – Produktionsprogrammplanung und -steuerung von Fahrzeugen und Aggregaten [PPS in automobile engineering – Production program planning and control of vehicles and assemblies] (in German). Munich, Germany: Carl Hanser Verlag. ISBN 978-3-446-41370-2.
Herlyn, Wilmjakob Johannes (January 1990). Zur Problematik der Abbildung variantenreicher Erzeugnisse in der Automobilindustrie [On the problems of mapping high-variation products in the automobile industry]. Fortschritt-Berichte VDI, Reihe 16: Technik und Wirtschaft (in German). Vol. 52. Düsseldorf, Germany: VDI Verlag. ISBN 3-18-145216-5. ISBN 978-3-18-145216-5. Retrieved 2021-01-29. (127 pages)
Paul, Matthias R. (2012-01-20) [2005]. "Eine durch den Nutzer erweiterbare Klammer-Präfix-Notation zur flexiblen Definition und Verwaltung von Bestückungsvarianten durch Auflösung logischer Bedingungen". BOM-MPL (Program documentation). 1.06 (also 1.09 as of 2013-03-04) (in German). Retrieved 2012-02-05. (NB. A ULP-based BOM management tool for the EDA tool EAGLE 5 to 7 with support for part databases and EAGLE's assembly variants and attributes in addition to a more flexible system based on parentheses notation stored as prefixes to part values defining multi-level logical conditions and dependencies of assembly groups and possible alternative part values. Originally devised by the author as an easy-to-understand symbolic notation assisting a user manually processing variant BOMs before CadSoft added support for variants and attributes to EAGLE, it was later integrated into the author's BOM tool for automatic variant management.)
Frischen, Christian; Marbach, Anastasia; Tichla, Florian; Mantwill, Frank (2019-09-18). Written at Hamburg, Germany. Krause, Dieter; Paetzold, Kristin; Wartzack, Sandro (eds.). "Durchgängige Variantensteuerung mit Hilfe der regelbasierten Komplexstückliste". DS 98: Proceedings of the 30th Symposium Design for X (DFX 2019). DfX (in German and English). Jesteburg, Germany: Mechanical Engineering & Computer-assisted Product Development, Helmut-Schmidt-Universität: 13–24. doi:10.35199/dfx2019.2. Archived from the original on 2021-03-27. Retrieved 2021-03-28. (12 pages)