Guide
Alternate and substitute parts: recording them without breaking the count
Record an alternate so that only one populated line owns each designator. Put it on a line with no designators and no quantity to buy, in a note, or in a separate approved-manufacturer table keyed by internal part number. Repeating the designators on the alternate doubles the count and leaves the assembler guessing which part to fit.
Steps
Recording an alternate
Qualify it first
An alternate is a part you have checked against the original for fit, ratings and footprint. Record who approved it and when.
Give it one home
Keep alternates against the internal part number in a mapping table, not scattered across BOMs.
Keep one populated line per part
The preferred part owns the designators and the quantity, so the count check stays meaningful.
Show the alternate without designators
List it on its own line with the quantity blank and a role of Alternate, or in the notes as alt: maker and MPN.
State the policy
Say whether the assembler may substitute without asking: any listed alternate, only with approval, or never.
Re-run the count
Every populated line must still have a quantity equal to its designator count, and no designator may sit on two populated lines.
Terms
Alternate, second source, substitute
The words vary between companies. In many shops an alternate is a part approved in advance as interchangeable, a second source is an equivalent part from a second manufacturer, and a substitute is a replacement chosen when the original cannot be had. Whichever words you use, define them in the BOM notes and treat the approved list as a controlled record. Mapping approved parts to one internal number is covered in MPN vs internal part number.
Lifecycle
When the alternate becomes the main part
If the preferred part reaches end of life, or the alternate becomes the one you actually buy, promote it: swap the roles, release a new BOM revision and log why, as described in BOM revision control. Do not overwrite the MPN and lose the history of what was approved. An alternate is not a duplicate either; see BOM consolidation and duplicate lines. Whether a substitute meets compliance, safety or customer-approval requirements is an engineering and regulatory decision, and this guide gives no compliance advice.
Comparison
Five ways to record an alternate
| Method | How it looks | Effect on the count | Verdict |
|---|---|---|---|
| Extra row repeating the designators | Two lines both list R1 to R4 | Count looks doubled; both lines look fitted | Avoid |
| Extra row with no designators and no quantity | Role set to Alternate, quantity blank | None; the count still ties | Good |
| Note on the preferred line | alt: Yageo RC0402FR-0710KL | None | Good for one or two alternates |
| Separate approved-manufacturer table | Mapping keyed by internal PN | None; the BOM keeps one line per part | Best when there are many |
| Extra row marked DNP on the same designators | Fitted line plus a DNP line on R1 to R4 | Fitted count ties; the DNP line shares by design | Stuffing options only, not sourcing |
Whichever you choose, apply it to the whole BOM.
Pitfalls
What goes wrong
- Doubled designators. The alternate repeats them, so the count reads twice over and a sum of quantities buys both parts.
- Silent substitution. The assembler swaps a part and nobody records it, so the build no longer matches the BOM.
- Unqualified alternates. A part with the same value but a different voltage rating, height or footprint is not an alternate.
- DNP standing in for an alternate. A DNP line marks a part left off the board. It does not approve a replacement; see DNP in a BOM.
- Alternates in free text only. A notes cell is easy to lose in an export. Check that it survived.
- Mixed sources in one lot. Some builds need a single manufacturer per lot or per board, so say whether mixing is allowed.
- Stale approvals. An alternate approved for an earlier board revision may not suit the current one. Re-check after a change.
The tool
How BOMParser helps
When a PDF table has an item column, BOMParser reads a row with no item number and no quantity that holds only a manufacturer and a part number as an alternate source. It adds the row to the notes of the line above as alt: followed by the maker and part number, and lists that in the parser notes, so the count stays on one line. It flags a designator on more than one populated line, and lets a DNP line share a designator with a fitted one. It does not judge whether a part is a valid alternate. Convert a file on the BOM to CSV page; pricing lists the plans. Free: 1 file per day, up to 10 pages, no account. Files over 10 pages are converted up to page 10 on the free allowance; a page code converts the whole file, up to 150 pages.
FAQ
Alternate part questions
What is the difference between an alternate and a substitute?
Usage varies. Often an alternate is a part approved in advance as interchangeable, and a substitute is a replacement chosen when the original cannot be had. Define your terms in the BOM notes.
How do I list an alternate without double-counting?
Give it a line with no designators and no quantity to buy, put it in the notes of the preferred line, or keep it in a separate approved-manufacturer table. Only the preferred line carries the designators.
Should an alternate repeat the reference designators?
No. Two populated lines on the same designators make the count look doubled and leave the assembler to guess which part to fit.
Can the assembler substitute parts without asking?
Only if you say so. State the policy in the BOM or the order: any listed alternate, a listed alternate with notice, or nothing without written approval.
Where should approved alternates be kept?
In one mapping table keyed by internal part number, with manufacturer, MPN, role and approval date. BOMs then refer to that table instead of each carrying its own list.
How do I show a part that is fitted in some builds only?
That is an assembly variant, not an alternate. Use a DNP flag per variant or one BOM per variant, and name the variant in the file.
Keep reading
Related guides
MPN vs internal part number
Why a BOM needs both, and how to keep a mapping table.
DNP in a BOM
How to mark do-not-populate lines, and how they get read.
BOM consolidation and duplicate lines
Merging same-MPN lines without losing designators.