What is a bill of materials (BOM)?

BOM: the abbreviation and what it means

BOM stands for bill of materials, in German Stückliste. Both names mean the same document: the list of every component, sub-assembly and material that goes into a product, each with its quantity. In everyday electronics work the abbreviation is used even in German-speaking engineering departments, because that is how the layout tools export it.

Which types of bill of materials exist?

By structure there are three forms. The quantity list names only which part occurs how often in the finished product, with no structure at all. The multi-level BOM maps the build across every level, from the device through the assembly down to the single resistor. The modular BOM describes one level at a time, once per assembly, and the product is composed from those. Electronics adds a second distinction that matters more in daily work: the design BOM that falls out of the layout tool, and the manufacturing BOM, which additionally carries packaging form, assembly variants and consumables such as solder paste.

eBOM and mBOM: engineering versus manufacturing bill of materials

The same assembly has two bills of materials, and confusing them regularly costs a production run. The eBOM, the engineering bill of materials, falls out of the layout tool and describes the circuit: which part sits at which position, with which value and in which package. It is ordered by reference designator, R1 through R48, and often names only the value, not the part you can buy.

The mBOM, the manufacturing bill of materials, is derived from it for purchasing and production. Three things are added: identical parts are consolidated into one line, so forty-eight individual resistors become a single line with a quantity of 48. Every line gets a manufacturer part number and the purchasing data that goes with it, because a value of 100 nF is not an order. And approved alternates are recorded so production does not stop when a part is discontinued. On top of that comes everything not visible on the board: solder paste, stencil, packaging, assembly variants.

Hand the eBOM straight to purchasing and it comes back. Transcribe it by hand and the link to engineering breaks the moment a line changes. PCBoffer performs this step on upload and keeps both sides tied together: the eBOM stays the source, the purchasable list is derived from it and keeps its own version history.

What an electronics BOM has to contain

Four entries per line decide whether the list can become a price at all: the manufacturer part number with its manufacturer, the quantity per assembly, the reference designator such as R12 or C7 that names the spot on the board, and the package. Without the package it stays open whether a 0402 part is meant or a leaded one, and the two differ considerably in assembly effort. Lines that are deliberately not populated belong in it as well, marked do not populate: without that mark somebody pays for parts that never reach the board.

BOM management: why the list is rarely correct

A bill of materials is not a fixed document. Parts go end of life, manufacturers change part numbers, engineering and purchasing work on different revisions, and spreadsheets breed spellings of the same part number that no search will ever join up again. BOM management therefore means tracing every line back to a part that really exists, being able to prove every revision, and knowing which revision was actually built. That is the difference between a list and a data set a calculation may rest on.