A component shortage should trigger an approval request, not an automatic BOM change. For turnkey PCB assembly, permission to purchase components should be separate from permission to substitute them.
Before procurement starts, define which alternatives are acceptable, who can approve changes and which checks are required. This prevents a purchasing decision from becoming an unreviewed product change.
Set Substitution Rules on the BOM
Use exact manufacturer part numbers, including relevant suffixes. A description such as “10 kΩ resistor, 0603” does not fully specify tolerance, power rating, temperature coefficient or other application requirements.
| Approval Status | What Procurement May Do |
|---|---|
| No substitution | Purchase only the specified manufacturer part number. Report shortages before ordering an alternative. |
| Approved alternatives listed | Purchase an exact part number from the approved list, subject to any stated restrictions. |
| Approval required | Submit a proposed alternative for written approval before purchasing or placing it. |
If a BOM line has no substitution status, specify that written approval is required. Do not leave the purchasing team to interpret silence as permission.
Require a Specific Replacement Proposal
A useful approval request identifies the affected part, the proposed replacement and the differences that matter. “Equivalent component available” is not enough to make a decision.
- Affected item: BOM revision, line number, reference designators and original manufacturer part number.
- Proposed replacement: manufacturer, complete part number and datasheet.
- Reason: shortage, discontinuation, delivery constraint or cost reduction.
- Technical differences: electrical ratings, package, pinout, operating conditions and any required design changes.
- Purchasing impact: source, available quantity, lead time and price difference.
- Approval scope: the order, quantity and product revision covered by the request.
Check More Than Footprint and Nominal Value
A replacement can fit the pads and still change circuit performance. The review should follow the component’s role in the actual circuit.
| Component Group | Checks Before Approval |
|---|---|
| Resistors and capacitors | Tolerance, voltage and power ratings, temperature behavior and package dimensions. For capacitors, also review dielectric, DC-bias behavior and ESR where relevant. |
| MOSFETs, diodes and power devices | Voltage and current ratings, losses, switching characteristics, thermal performance and pin assignment. |
| Regulators and analog ICs | Pin compatibility, operating range, accuracy, stability requirements, startup behavior and surrounding circuit compatibility. |
| MCUs and memory | Pinout, supply requirements, memory configuration, firmware compatibility and programming method. |
| Connectors | Mating compatibility, keying, contact rating, plating, mounting dimensions and mechanical retention. |
| Crystals and oscillators | Frequency tolerance, stability, load requirements, output type and startup conditions. |
For safety-related, RF or otherwise qualified designs, determine whether the change affects product approvals or requires additional validation. Package compatibility alone does not establish interchangeability.
Separate Part Approval from Source Approval
Approving a manufacturer part number does not automatically approve every seller offering it. State any required purchasing channels and traceability records. If the proposed source falls outside those requirements, treat it as a separate approval request.
For mixed sourcing, identify which components you will supply and which the assembly supplier will purchase. The same substitution rules should apply to both; responsibility for buying a part does not grant authority to change it.
Define Whether Approval Is Temporary or Permanent
A replacement approved for one urgent order should not silently become the default for future production. Record whether approval applies to a specific purchase order, a limited quantity or all future builds under a revised BOM.
Keep purchasing approval and production approval separate when validation is still pending. If parts must be bought to protect availability, state that they cannot be assembled or shipped in finished products until the required checks are accepted.
Use This Note in Your PCBA RFQ
Purchase components to the supplied BOM revision and approved alternative list. Any unlisted manufacturer part number requires written approval from our designated engineering contact before purchase or assembly. Submit the proposed part, datasheet, differences, source, price and delivery impact. Approval applies only to the stated order and quantity unless incorporated into a revised BOM. Identify approved substitutions in the delivered build records.
Order & Project Support: When requesting a PCBA quotation from Benchuang, include your approved alternatives, restricted components and substitution approval contact with the BOM. If any parts are unavailable, ask for those lines to be identified separately so you can resolve them before committing the production schedule.
