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Short answer: Every BOM change should go through an ECO. The ECO records what changed (added, removed or modified lines), why it changed, what it affects (where-used, stock, work in progress, service), and whether the change is interchangeable. Interchangeability decides whether the item gets a new revision or a new part number. The change is released in PDM or PLM only after review, and the before and after BOMs are kept together with the ECO.
Steps
1. Raise the ECO with a reason and the affected items.
2. Export the before BOM from the released revision.
3. Make the change in CAD or PDM on a work-in-progress revision.
4. Export the after BOM, compare it with the before BOM, and attach the change list.
5. Run where-used on every changed part to find other assemblies that use it.
6. Decide on interchangeability (form, fit, function). If the change is interchangeable, raise the revision. If it isn't, assign a new part number.
7. Set the disposition for existing stock and WIP: use as is, rework or scrap.
8. Review and approve with design, manufacturing and purchasing.
9. Release and notify. Update the drawings and their revision tables.
Worked example (generic)
A filter in a gas stick is replaced by a different model with the same connection type but a longer body. The longer body means a tube segment changes too. The before/after compare shows one line changed (the filter part number) and one line changed in quantity (tube length). Where-used finds two other gas stick assemblies that use the old filter, so the ECO decides whether those change too. Because the new filter isn't dimensionally interchangeable, it gets a new part number, not a revision.
Common mistakes
- Editing a released BOM in place with no ECO.
- Skipping where-used, which leaves other assemblies inconsistent.
- Raising a revision for a change that isn't interchangeable.
- Not keeping the before and after BOMs with the ECO.
What a good ECO record contains
| Field | Why it matters |
|---|---|
| ECO number and title | Traceability across documents |
| Reason (problem or improvement) | Reviewers judge whether the change solves it |
| Affected items with old and new revision | Exact scope |
| Before and after BOM, with the change list | Proof of what changed |
| Where-used results | Other assemblies affected |
| Interchangeability decision | Revision vs new part number |
| Disposition of stock, WIP and field units | Purchasing, production and service actions |
| Effectivity (date, serial number or lot) | When the change takes effect |
| Approvals | Who accepted it |
Edge cases
- Changes that only affect quantity. A quantity change is still a BOM change and still needs an ECO. It often matters to purchasing more than a description change does.
- Supplier part number changes with no physical change. If the supplier renames a part but it is physically identical, many companies update the approved-vendor data without changing their own part number. Agree the rule.
- Changes inside a purchased subassembly. If the vendor changes an internal part, decide whether that affects your form, fit and function. If it does, you may need a new part number for the subassembly.
- Effectivity for machines in build. Machines already in production may get the old BOM, the new one, or a retrofit. State it in the ECO, not in an email.
- Multiple ECOs on the same item. Sequence them. Two parallel ECOs editing the same BOM is a common source of lost changes.
- Deviations. A one-time use of a different part for one build is a deviation, not a permanent ECO. Keep the two processes separate.
A second worked example (generic)
Purchasing reports that a fastener supplier has discontinued a screw. Engineering selects an equivalent screw from a different supplier that meets the same standard and property class and has the same dimensions.
- Interchangeability: form, fit and function are the same, so the company keeps its own part number and updates the approved vendor list.
- The BOM doesn't change, so the compare shows no differences, which is the expected result.
- The ECO still records the vendor change, the reason and the effectivity, so the next audit can trace it.
Had the replacement screw been a different length, the BOM would change, the screw would get a new part number, and where-used would show every assembly that needs updating.
FAQ
Do drawing-only changes need an ECO?
In most systems, yes. Any change to a released document is controlled.
Who decides interchangeability?
Engineering, using form, fit and function, with input from service.
How do I spot unintended BOM changes?
Compare the exported before and after BOMs line by line, or with a BOM compare tool.
What is effectivity?
The point from which a change applies: a date, a serial number or a lot.
Do I need a new part number for a material change?
Usually yes, if the material affects function or interchangeability. Follow your company's form, fit and function rule.
Can ECOs be handled in PDM?
Yes. Most PDM and PLM systems support change workflows with approvals and attached documents.