the bill of materials (BOM)
/ BOM rhymes with mom /
The bill of materials is the complete shopping-and-assembly list for your board: every component you need, how many of each, and exactly which part to buy. If the Gerbers tell the factory how to make the bare board, the BOM tells it (and you) what to populate the board with. It is the unglamorous spreadsheet that turns a design into something purchasable and assemblable — and a board is only as buildable as its BOM is complete and correct.
A good BOM has one row per distinct part, listing the reference designators that use it (R1, R5, R9), the quantity, the value or description (10 kohm, 1 percent, 0603), and crucially the manufacturer part number and a distributor order number. That exact part number is what removes all ambiguity: ten kilohm is not enough information, because the right part also has a specific package, tolerance, power rating, and voltage rating. The EDA tool generates the BOM straight from the schematic, so every placed component is accounted for and nothing is forgotten.
Why it matters: the BOM is where a design meets the real, messy supply chain. A part that is out of stock, discontinued, or has a months-long lead time can stall an otherwise finished board, so designers check availability while choosing parts and often list a second-source alternative. Cost adds up here too — the BOM is where you see what the board really costs to build. The honest reality: many a perfect design has been delayed not by a circuit error but by one tiny BOM line that could not be bought. Keep it accurate, complete, and sourced.
One BOM line reads: reference R1, R5, R9; quantity 3; description 10 kohm 1% 0.1 W 0603; manufacturer part Yageo RC0603FR-0710KL; distributor stock number and price each. With that, anyone can buy and stuff exactly the right resistors.
Exact part numbers, quantities, and sources — what turns a design into a buildable board.
A value alone (10 kohm) is not a buyable BOM line. Without the package, tolerance, ratings, and a real manufacturer part number, the assembler must guess — and the cheapest way to delay a board is a BOM full of parts that turn out to be unavailable.