A bill of materials (BOM) is the shopping list for your board. The person who reads it has to turn each line into a purchase order for an exact part, and has no way to ask the part's designer what was meant. A BOM that a buyer can read without guessing is the single biggest schedule saver in a PCBA order.
What each line must say
| Column | Why it matters |
|---|---|
| Reference designators (R1, R2, C7) | Link the line to the placement file and the silkscreen |
| Quantity | Must equal the number of designators on the line |
| Value | 10 kΩ, 100 nF, and the unit, so nothing depends on the reader |
| Footprint or package | 0402, SOT-23, SOIC-8, so the part fits the pads |
| Manufacturer | Part numbers are only unique per manufacturer |
| Manufacturer part number (MPN) | The exact item to buy |
| Description | Tolerance, voltage, dielectric, power: whatever the MPN does not make obvious |
| Notes | Do-not-place, alternates, special handling |
The manufacturer part number is the line that matters. A distributor's own order code is useful as a hint, but it names a stock item that may not exist in the factory's supply chain.
The reference designator is the key that ties the files together.
- A BOM row says what to buy: value, footprint, manufacturer and part number.
- The placement row with the same reference designator says where it goes: X, Y, rotation and side.
- The position is measured from an origin, which must be the one the Gerber files use.
- The footprint on the board must fit the package of the part in the BOM row.
- The reference designator is the link. If it is in one file and not in the other, a part is bought and not placed, or placed and not bought.
A good row and a bad row
Bad:
| Designators | Qty | Value | Part |
|---|---|---|---|
| C1, C2 | 2 | 100n | capacitor |
Which tolerance? Which voltage? Which dielectric? Which size? A buyer will pick one, and it may not be the one you meant. A 100 nF capacitor in a decoupling position, a timing circuit and a filter are three different requirements.
Good:
| Designators | Qty | Value | Footprint | Manufacturer | MPN | Description |
|---|---|---|---|---|---|---|
| C1, C2 | 2 | 100 nF | 0402 | (named) | (exact number) | MLCC, X7R, 16 V, 10 % |
The good row says what the part is, in two independent ways: by number and by description. If the number is out of stock, the description shows what would be an acceptable substitute.
Alternates
Say which alternates are allowed. There are three honest answers per line:
- No alternate. The part is critical (a precision reference, a radio module, a regulated power part) and only this part may be used.
- Equivalent allowed. Anything that meets the description is fine. This works well for passives.
- Listed alternates. Name the second and third MPN. Do this for any part that is scarce.
Without this, a buyer who finds the part out of stock must stop and ask you, which costs days, or must decide for you, which is a risk you did not choose.
Do not place, and parts you add by hand
Some parts are on the schematic but should not be assembled: an optional resistor, a connector you fit later, a debug header. Mark them "do not place" (DNP, or "do not fit") in the BOM and remove them from the placement file, or at least mark them in both. A line that is in the BOM but not in the placement file is the most common source of a question.
If you want something to be left out of reflow and added by hand, say so too.
Parts that you supply (consigned parts)
You may send some or all of the parts. Do this when a part is hard to buy, when you hold a stock, or when you must use a specific lot. Rules that make it work:
- Mark the line "consigned" and say who supplies it.
- Send a few more than the board count. Machines lose parts while loading and during set-up, and a small passive on a cut tape is easier to lose than a reel.
- Send the parts in their original packaging, labelled with the MPN, on tape or reel for machine placement. Loose parts cannot be machine-placed efficiently, and may mean hand placement.
- Make sure moisture-sensitive parts (many ICs) are in sealed bags with the humidity indicator card intact.
Life-cycle and availability
Before you send the BOM, check each line for:
- Not recommended for new designs (NRND) or end of life (EOL). A part may still be sold from stock, but the stock will not last.
- Long lead time. Some parts, such as microcontrollers, power parts and certain connectors, can take many weeks to arrive and are the real cause of a long schedule. See Lead times and how to plan them.
- Single source. Only one maker, and no alternates.
- Counterfeit risk. Parts bought from an unfamiliar broker, rather than the maker or an authorised distributor, can be relabelled or recycled.
Minimum order quantities and attrition
Parts are sold in packs: a full reel of 5,000 resistors, a cut tape of 100, a tray of 90 chips. The price per piece for your quantity is rarely the price per piece in the catalogue, and there is a minimum you pay for even if you need less. Two ideas help:
- Minimum order quantity (MOQ). The smallest number the seller will sell. You pay for it whether you use all of it or not.
- Attrition (overage). The extra parts bought to cover losses in assembly. Small passives are lost more often than large ICs, so factories usually add a percentage for them, and nothing for expensive parts. Ask what rule is used and whether the leftover parts come back to you.
For a prototype run of five boards, the part cost is often dominated by MOQs, not by the quantity on the boards. If you plan a repeat order, leftover parts are a stock you can reuse.
Check the BOM before you send it
- Sum the quantity column: it should equal the number of placed parts in the placement file.
- Every reference designator appears once, in exactly one line.
- Every footprint matches the package of the MPN. Open the datasheet for any part you have not used before.
- Polarised parts (diodes, electrolytic capacitors, ICs) have a clear pin-1 or polarity mark in the silkscreen and the assembly drawing.
- Every line has an MPN or is marked DNP.
Rotation and polarity are where the BOM, the placement file and the footprint disagree most often. The drawing shows one part three ways.
- The centre of the part is the point the placement file gives as X and Y.
- Pin 1 has a dot. The rotation in the placement file is measured against how the footprint is drawn in the library, not against the part on the reel.
- A quarter turn: the same part rotated. If the library and the placement file disagree about zero, the part lands turned.
- The bottom side is mirrored, so pin 1 is on the other side. The file must say which side the part is on.
How this shows up when you order here
The 25 checks at upload look at the board files. They do not decide whether a part is the right one for your circuit, and they do not check stock. An engineer reads the BOM before parts are bought and asks you about missing alternates, scarce parts and lines that cannot be bought. The price follows your options and is shown in US dollars. See what drives the price, and upload at the upload page.