BOM & components
Prototype BOM vs Production BOM: What Changes Before Manufacturing
Why loose prototype BOMs are usually correct, what has to be nailed down before production, and the handover that quietly goes wrong.
What changes between a prototype BOM and a production BOM?
The components mostly stay the same. What changes is how precisely each line is specified, and how much is written down about why.
A prototype BOM is a shopping list for five boards you are going to debug. A production BOM is a contract with an assembler who has never met you and will build four hundred units from it without asking questions.
| Prototype BOM | Production BOM | |
|---|---|---|
| Part identification | often generic — "10k 0603 1%" | exact MPN and manufacturer |
| Sourcing | whatever the distributor had | approved sources |
| Alternatives | not stated | stated for the lines that matter |
| DNP lines | informal, often in your head | explicit and marked |
| Lifecycle checked | rarely | yes, for the parts that matter |
| Revision identity | file name | in the data, tied to a board revision |
| Who reads it | you | purchasing, the assembler, you in three years |
Why generic parts are fine for prototypes
There is a tendency to treat loose prototype BOMs as sloppiness. Mostly they are not — they are appropriate.
When you order five boards, you buy from whoever has stock today. Specifying one exact manufacturer for a 10k resistor costs time and achieves nothing: the circuit does not care, and if the specified part is out of stock you lose a week waiting for a resistor.
The place to be precise early is the small number of lines where the part actually determines behaviour: the regulator whose feedback network you tuned, the crystal, the connector that has to mate with something, the op-amp whose offset matters. Those deserve exact parts from day one, because changing them later invalidates your measurements.
Be loose where the circuit is insensitive and precise where it is not. That is not a shortcut, it is correct engineering.
What has to be nailed down before production
1. Every line gets a manufacturer and an MPN
"10k 0603 1%" cannot be bought by someone who is not you. An assembler receiving that line will either guess, substitute, or stop and ask. Two of those three are bad.
2. Approved alternatives for the lines that can take them
The moment you commit to a single source on a passive, you have created a single point of failure in a supply chain you do not control. Listing one approved alternative for the generic parts removes most of that risk.
3. DNP lines become explicit
On prototypes you know R7 is not fitted because you decided it last Tuesday. An assembler does not. Every unfitted position has to be marked, kept on the list, and shown on the assembly drawing.
4. Lifecycle status gets checked once
Not for all 142 lines — for the ones that would hurt. Processors, FPGAs, regulators, connectors, anything custom or single-sourced. A part that is already NRND before you start manufacturing is a problem you have bought on purpose.
5. The revision gets a real identity
bom_final_v3_OK.xlsx stops being acceptable. The list has to say which board revision it belongs to, and that has to live in the data rather than in a file name.
The handover that goes wrong
The most common failure is not technical. It is that the prototype BOM gets promoted to a production BOM by renaming the file.
What happens next is predictable. The assembler receives a list with twelve lines that say "100 nF 0402" with no manufacturer. They do what any reasonable supplier does: they substitute from stock. Nine of those substitutions are fine. One is a Y5V where the design needed X7R, and the board works at room temperature and fails at 60 °C.
Nobody did anything wrong. The BOM did not carry the information.
A practical maturity checklist
Before a BOM is allowed to leave engineering:
- [ ] Every line has a manufacturer and an MPN.
- [ ] No line says "or equivalent" without saying which equivalents.
- [ ] Critical lines are marked as critical and have no unapproved alternatives.
- [ ] Non-critical lines have at least one approved alternative where possible.
- [ ] Every DNP position is listed, marked and explained.
- [ ] Lifecycle checked for processors, regulators, connectors and anything
single-sourced.
- [ ] The BOM states its board revision.
- [ ] Quantities match the reference designator counts.
- [ ] Someone who did not write it has read it.
Worked example: one line, three stages
A single line from the fictional ORION CONTROL UNIT Main Control Board.
Prototype (rev A):
| Ref | Description | MPN |
|---|---|---|
| C12–C18 | 100 nF 0402 | — |
Bought from whatever was in the drawer. Entirely reasonable.
Pre-production (rev B):
| Ref | Description | Manufacturer | MPN |
|---|---|---|---|
| C12–C18 | 100 nF 16 V X7R 0402 | Murata | GRM188R71H104KA93D |
The dielectric is now specified, because testing showed the rail was sensitive to it. This is the step most teams skip.
Production (rev C):
| Ref | Qty | Description | Manufacturer | MPN | Alternative | Status |
|---|---|---|---|---|---|---|
| C12–C18 | 7 | 100 nF 50 V X7R 0402 | Murata | GRM188R71H104KA93D | CL05B104KO5NNNC (Samsung), paper-qualified | Active |
Sample data. Voltage rating raised after a transient was found, one approved alternative added, lifecycle checked.
Does a prototype BOM need revision control?
Lightly, yes — and the reason is not process hygiene.
If you build five boards from one list, change two parts, and build five more, you now have two populations of hardware that look identical and behave differently. When one of them fails you will want to know which list it was built from.
For a prototype, this does not need a process. It needs the list to be saved alongside the build, identified by something more durable than "the version I sent on Tuesday".
Taking a BOM from prototype to production is mostly about writing down decisions you already made. The part that breaks later is keeping those decisions attached to the board revision they belong to — so that the list, the Gerbers and the assembly drawing still describe the same hardware when somebody reorders in two years.
In XKEMA the bill of materials belongs to the board, and both carry their own revisions.
BOM management How to create an electronics BOM * Approved alternatives in an electronics BOM