Bill of Materials (BOM)
A bill of materials is the document in which every component on the board is listed with its reference code, value, manufacturer part number and quantity. Procurement, cost and the assembly programme are based on this list; a single wrong part number in the list can bring an entire production batch to a stop.
In a usable bill of materials every line carries the following: reference codes (R1, R2, R7), value and tolerance, package size, voltage or power rating, manufacturer name and manufacturer part number, quantity per board.
- The value is not enough: "100 nF" is not a procurement line; a capacitor bought without specifying the package, voltage rating and dielectric class may not give the expected behaviour.
- Alternative column: for critical parts the approved second source is kept in the same line.
- Version link: the list is numbered together with the board revision; a Rev B board is not assembled with a Rev A list.
The bulk of the cost is usually concentrated in a small number of items: power semiconductors, the controller, connectors and the enclosure. If a cost study is to be done on a project, these items are looked at first; the total of hundreds of passive components often stays below that of a single connector set.
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