This guide covers why incoming inspection is necessary even when manufacturers inspect at shipment, five inspection categories with specific check items for each, how to set acceptance standards using IPC-A-600, AQL-based sampling for large lots, and how to build an inspection record system that improves supplier quality over time.
PCB manufacturers conduct outgoing inspection before shipment — so why inspect again on receipt? Two reasons make incoming inspection non-negotiable.
- Acceptance standard misalignment: A manufacturer's internal acceptance criteria and your own quality requirements are not always the same. With overseas suppliers especially, the interpretation of terms like "allowable scratch depth" or "acceptable solder mask void" can differ significantly. Incoming inspection is where you verify against your own standard, not the manufacturer's.
- Transit damage: Warping, surface scratches, packaging failures, and moisture absorption can all occur between the manufacturer's shipping dock and your receiving dock. None of these will be caught by the manufacturer's outgoing inspection. Incoming inspection is the last quality gate before these boards enter your assembly process.
The cost equation: Defects found at incoming inspection cost a fraction of what they cost when discovered after PCBA assembly — and a tiny fraction of what they cost in the field. A structured incoming inspection process is one of the highest-return quality investments available.
PCB incoming inspection covers five distinct categories. Each requires different equipment and serves a different quality objective.
🔍
Visual Inspection
SCOPE: Full lot or AQL sample · Tool: 10× loupe
- Surface scratches, contamination, foreign material
- Solder mask delamination or pinholes
- Silkscreen print offset or missing marks
- Edge burrs or chipping
- Board warpage (place flat — check corner lift)
📐
Dimensional Inspection
SCOPE: Sample (first + last of lot) · Tool: Calipers, micrometer
- Finished board thickness vs. specification
- Outer dimensions (length, width) within tolerance
- Hole diameter — random sample of key holes
- Hole position accuracy relative to pad centers
⚡
Electrical Testing
SCOPE: Verify manufacturer report + spot-check · Tool: Multimeter / TDR
- Request the manufacturer's electrical test report — confirm method (flying probe or dedicated fixture) and coverage rate
- Perform spot-check open/short continuity test on a sample to cross-validate
- For impedance-controlled boards: request coupon measurement report
🔥
Solderability Testing
SCOPE: Sample boards · Tool: Reflow / flow solder test
- Run a sample board through actual reflow or flow solder process
- Inspect solder wettability on pads — look for dewetting or non-wetting
- OSP finish is especially storage-sensitive; verify before committing full lot to assembly
Additional: Impedance Verification
For boards where controlled impedance was specified in the purchase order, request a measurement report from the manufacturer showing coupon (test coupon on the production panel) TDR results. If your facility has a TDR, perform spot-check verification on a sample of received boards. Impedance variation is invisible to visual and continuity inspection — it must be actively measured.
⚠ OSP surface finish has a limited shelf life. OSP (Organic Solderability Preservative) finishes degrade with time and exposure to humidity. Always check manufacturing date and storage conditions on delivery. If boards have been in transit or storage for an extended period, prioritize solderability testing before releasing the lot to assembly.
IPC-A-600: the baseline acceptance standard
IPC-A-600 (Acceptability of Printed Boards) is the primary international standard for PCB visual inspection. It defines three product classes with progressively stricter acceptance criteria:
1
Class 1
General Electronics
Consumer products and applications where cosmetic imperfections are acceptable as long as the board functions.
Least strict
2
Class 2
Dedicated Service
Industrial, communications, and commercial equipment requiring high reliability and extended service life.
Standard — recommended
3
Class 3
High Reliability
Medical devices, aerospace, military, and applications where failure is not an option.
Most strict
Specify the class in your purchase order. Don't assume the manufacturer will inspect to your required class — state it explicitly. The same physical defect may be acceptable under Class 1 and unacceptable under Class 2. Align on this before ordering, not at incoming inspection.
AQL-based sampling for large lots
Full inspection of every board in a large lot is not always practical or necessary. AQL (Acceptable Quality Level) based sampling provides a statistically sound approach: inspect a defined sample size, and accept or reject the lot based on the number of defects found in the sample.
- Common AQL levels for PCBs: AQL 1.0 is typical for critical defects (electrical shorts/opens), AQL 2.5 for major defects (dimensional, surface finish), AQL 4.0 for minor defects (cosmetic)
- Agree the sampling plan with your quality team before the first delivery — don't make sampling decisions ad-hoc at receiving
- Reference IPC-A-600 acceptance criteria for each defect type when judging whether a board passes or fails the visual inspection
- If a lot fails AQL: apply a pre-agreed escalation procedure — typically 100% sort of the failed lot, followed by a corrective action request to the manufacturer
Inspection results are only useful if they're recorded consistently. Document the following for every incoming inspection:
Incoming Inspection Record — Required Fields
Inspection Date
YYYY-MM-DD
Lot Number
Manufacturer lot / PO reference
Ordered Qty
Quantity per purchase order
Received Qty
Quantity actually delivered
Inspection Method
100% / AQL sample — sample size
Defects Found
Type and count per category
Pass / Fail
Accept / Reject / Conditional accept
Inspector
Name and sign-off
Accumulate these records by manufacturer. Recurring defect patterns become visible over time and provide documented evidence for corrective action requests.
Records accumulated across deliveries allow you to track quality trends by manufacturer. If the same defect type appears repeatedly from one supplier — say, consistent warpage, or recurring solder mask pinholes — the documented record gives you both the evidence and the leverage to request a formal corrective action and improvement plan.
Records also protect you commercially. In a dispute over responsibility for defects discovered downstream in assembly or field, your incoming inspection records establish what condition the boards were in when you received them — and whether you accepted them conditionally or without reservation.
Summary
Effective PCB incoming inspection is built on four pillars: inspecting the right things (visual, dimensional, electrical, solderability), using agreed standards (IPC-A-600 class aligned with your product), applying proportionate sampling (AQL for large lots), and maintaining consistent records. The goal is not to catch every defect — it's to detect systemic problems before they reach assembly, and to build the quality data that drives supplier improvement over time.