BOM best practices: MPNs, designators, DNP parts, quantities and attrition

31 October 2026 · Sonani Electronics Team

A bill of materials is a purchase instruction, not a memory aid. If a stranger can buy the correct reel and place it on the correct pads, the BOM is finished. If they have to ask what "C12" means, or whether "DNP" is a part number, it is not. The notes below are the checks we wish every file had before it reached a quote.

We source and deliver the parts on the lines you give us. Where a line is a bare board or assembly, that work is done through our manufacturing partners. The quote is the price, on a single GST invoice, with no hidden charges. You can send the sheet through upload a BOM.

One manufacturer part number per line

The manufacturer part number is the orderable number, including the suffix that selects the real goods. A family name is not enough. "100nF" is a value. "10k 0402" is still missing dielectric, voltage and tolerance.

Put the number in its own column, with no extra words, so a search box can take it as-is. A description column can say "100nF X7R 16V 0402". An internal stores code goes in a different column. Never make it the only identifier.

If two numbers are both acceptable, put the preferred one in the part-number column and the second in "approved alternate". Say whether it is equal or only a fallback. Do not overwrite the original, and do not write "or equivalent". How to check package, pinout, voltage and dielectric is in finding alternatives.

Designators have to match the quantity

Every placed part has a designator: R1, C14, U3, J2. The list of designators and the quantity are the same fact written twice, and they must agree.

Quantity 4 with C1 C2 C3 C4 is clear. Quantity 4 with C1-C8 is a contradiction. A quantity with no designators can only be bought as loose parts. Say so if that is what you want.

Group designators only when they share one manufacturer part number. 100nF and 1µF are two lines even if both are 0402. A missing designator is fine when the schematic deleted it, and not fine when the part is still on the board. Through-hole parts stay on the BOM. A screw or standoff can be listed if you mark it "not placed on the PCB".

DNP means do not buy it and do not place it

DNP, DNI and "no-fit" mean the footprint is on the PCB and the part must not be fitted. Keep the line so the next revision still shows it, and set a column to DNP. Purchase quantity for that line is zero.

A DNP note buried in a comment, with a normal quantity, and with the designator still in the centroid file, will be placed. Take DNP designators out of the pick-and-place file. Those file rules are in preparing Gerber, BOM and pick-and-place files. If the part is only off on this build, write "DNP on this build".

Quantities: per board, number of boards, purchase quantity

These are three different numbers. Mixing them up orders the wrong count even when every part number is right.

Number	Meaning
Quantity per board	How many of this part are fitted on one PCB.
Number of boards	How many PCBs you are building. Write it once, in the email or in a cell at the top, not only inside a formula.
Purchase quantity	What you want bought. Usually per board times number of boards, plus any extra.

Write "Build 25 boards" in the message. If the sheet and the message disagree, we ask. We will not guess. "Parts only, 25 sets" and "25 PCBA" are different orders. See PCB vs PCBA vs SMT stencil.

Attrition: extra parts, written as extra parts

Small parts get lost. A 0402 resistor sticks to a tweezer, a reel ends early, a placement machine dumps a few at setup. Expensive ICs are the opposite problem: buying two "just in case" can matter more than the empty pocket on the reel. There is no single extra percentage that is right for every line, and we will not invent one.

Write the extra in its own column, separate from the per-board quantity. A few loose passives, no extra on an expensive IC, or a spare connector are decisions you make per line. None of them is a rule we will invent for you.

If the extra column is blank, we buy the build quantity and we do not add a surplus. If an assembly partner needs more to run the job, we ask before buying it. Do not hide attrition inside the per-board quantity. "1.1" is not a designator count.

Columns worth having, and columns that cause trouble

A sheet we can quote without a meeting usually looks like this:

Designator
Manufacturer part number
Manufacturer, if you know it and the number alone is easy to confuse
Description, value, dielectric, voltage, tolerance
Package or footprint name
Quantity per board
Extra to buy
Fitted or DNP
Approved alternate, if any
Note, for "customer consigns this" or "place by hand"

Do not jam the part number, the DNP flag and the alternate into one remarks cell. Colour alone disappears in a CSV. Two assembly variants need two files, or a variant column, not a font only you can see.

Parts you already hold, and parts we must import

Mark consigned lines "customer will supply" and give the quantity you will hand over, the package, and whether the parts are on a reel or loose. Loose unmarked parts are a placement risk. We would rather ask for the reel label than guess a marking code.

For lines we buy, India and import can be mixed on one quote. Imported parts typically reach Surat in about 12-15 working days, India-sourced parts about 2-4 working days, depending on stock. The quote will say which lines are which. You do not file the customs entry. The purchase is a single GST invoice with no hidden charges. The longer explanation is in importing components to India.

You can check a number on parts search before you freeze the BOM. A search hit is a candidate, not an automatic approval to change the schematic.

A pass over the sheet before you send it

Every line that is fitted has a manufacturer part number and a package.
Designator counts equal the per-board quantities.
DNP lines are flagged in a column and removed from the pick-and-place file.
The number of boards is written in the message.
Any extra quantity sits in its own column.
Alternates are in their own column and have been checked.
Consigned parts are marked, with the quantity you will provide.
The file name includes the project and the revision.

Then upload the BOM or send it with the Gerber. If you also want assembly, send the centroid file from the same revision. If you only want parts, say so in the first line of the message so the quote is not built as a PCBA.

Common questions

Can I send a PDF?

A spreadsheet or CSV is better. A PDF is acceptable when the columns and part numbers are complete. A photo of a screen is a last resort.

Should quantity already include attrition?

No. Keep per-board quantity equal to the designators, and put attrition in a separate column. If you only have an "order qty" column, add the per-board figure as well.

One internal code covers ten resistor values. Is that a problem?

Yes, if the code does not change with value, tolerance or package. Split the lines by manufacturer part number. Keep the internal code as an extra column.

Do DNP parts appear on the GST invoice?

Not as goods, when the purchase quantity is zero. They can stay on the BOM you sent. The invoice covers what you purchased, on a single GST invoice, with no hidden charges.