In electronics assembly, most defects that reach a customer passed through several detection steps: SPI, AOI, X-ray, ICT or flying probe, functional test, and often a visual check. That makes the escape question in an 8D more specific than in many other industries. A customer quality engineer reading a PCBA 8D will look for two separate root causes, each with evidence. This page explains the difference, gives a station-by-station way to analyse the escape, and shows illustrative examples. Your customer’s format and rules come first.
Occurrence vs escape: two different questions
- Occurrence root cause: why the process made the defect. Examples: reflow profile, stencil or paste, pad design, placement, wrong reel, moisture-sensitive parts handled outside limits.
- Escape root cause: why detection did not stop the defective board before it shipped. The answer sits in test coverage, inspection programs, thresholds, sampling, and review decisions.
A D4 that explains only the occurrence leaves the customer asking what else could slip through the same gap. A D4 that explains only the escape (“added X-ray”) leaves the defect being made. The general method is on the 8D escape point page; this page applies it to PCBA lines.
Walk the detection chain station by station
For the escape cause, go through each station the board passed and answer one question per station. Use records, not memory: the inspection program, the test coverage report, the X-ray sampling plan, and the review-station log for the affected serial numbers.
SPI (solder paste inspection)
Was the paste deposit inside limits when printed, and were the limits tight enough to flag this deposit?
AOI (automated optical inspection)
Could the camera see the joint at all? Was the component in the inspection program, and was the call overridden at the review station?
X-ray (AXI or sample X-ray)
Were hidden joints such as BGA, QFN, or bottom-terminated parts covered, and at what sample rate?
ICT or flying probe
Did the test coverage report include this net or part? An untested net cannot catch a fault on it.
Functional test (FCT)
Did the test conditions trigger the failure mode? Intermittent or temperature-dependent faults may pass at room temperature.
Visual inspection and rework
Was the board reworked, and was the rework re-inspected with the same method as first-pass boards?
The escape point is the first station that could reasonably have caught the defect and did not. Name that station and the reason, for example “BGA joints outside the X-ray sample” or “net not in ICT coverage”. “AOI missed it” names a station, not a cause.
Want this on paper for the next investigation? The free printable 8D / D4 checklist (PDF) has separate D4 checks for occurrence, escape, and verification. The free 8D checklist page explains each check.
PCBA examples: occurrence and escape side by side
The examples below are illustrative and fictional, not from real cases.
BGA head-in-pillow (illustrative)
Occurrence: Reflow profile soak too short for a thick board after a stencil change; reproduced on test boards with the old and new profile.
Escape: AOI cannot see under a BGA, and X-ray was sampled at one panel per lot, so affected panels were not imaged. ICT passed because the joint made contact at room temperature.
Tombstoned 0402 resistor (illustrative)
Occurrence: Uneven pad design plus paste volume imbalance; SPI data showed the imbalance on the affected pads.
Escape: AOI flagged the part, but the call was accepted as a false call at the review station. Review rules allowed acceptance without a second look.
Wrong-value capacitor (illustrative)
Occurrence: Feeder loaded with the wrong reel after a changeover; the reel verification scan was skipped.
Escape: The capacitor sat on a net not covered by ICT, and the functional test did not exercise the circuit at the affected tolerance.
Writing a PCBA 8D? Get a free Accept / Conditional / Reject advisory on your 8D text, with missing occurrence or escape evidence named, before it goes to the customer.
8D Gate is a free pre-send readiness advisory. It doesn't replace the customer's decision. We don't store your 8D body in our database; you can delete anytime. Anthropic processes the text for the review.
Evidence that supports each cause
- Occurrence: reproduction on test boards, SPI or reflow profile data, cross-sections, component lot records, machine logs for the affected time window.
- Escape: the inspection program version, a coverage report showing the net or part was not tested, the sampling plan, review-station decisions for the affected serial numbers.
- Both: the link between affected serial numbers or date codes and the cause. Without traceability the D3 scope and the D4 claim are hard to check.
If the D4 ends at “operator accepted a false call”, ask why the review rules allowed it. See why operator error is not a root cause and weak D4 root cause examples.
D5–D6: one action for each cause
Pair each root cause with its own corrective action and its own verification. For occurrence, show the process change and results from boards built after it. For escape, show that the changed detection step now catches the defect, for example by running known-bad boards or seeded defects through it, and record the results. Extra inspection added as containment in D3 is not the permanent escape fix unless you state it as such and verify it. See D6 verification of effectiveness and how to write a D4 root cause.
FAQ
- Is “AOI missed it” an escape root cause?
- Not on its own. It names the station, not the reason. State why the station could not or did not detect the defect: not visible, not programmed, threshold too loose, or call overridden.
- Do I need two corrective actions?
- Usually yes: one that stops the defect being made and one that would catch it if it happened again. Adding a test without fixing the process only moves the defect to scrap.
- What does 8D Gate do with a PCBA 8D?
- It reviews the 8D report text as a pre-send readiness check. You get an Accept / Conditional / Reject advisory with missing evidence and questions, including whether occurrence and escape are both covered. It isn’t a customer decision. The review tool is v0.1: it reviews English 8D reports, and the interface and report are in English. Templates are also available in German, Czech and Polish.
Free pre-send readiness check
Before the PCBA 8D goes out, run the text through the free supplier 8D readiness check.
Paste text or upload PDF, DOCX, or Excel. You get an Accept / Conditional / Reject advisory, a score, section ratings, missing evidence, and questions to close gaps, with a PDF download. It is a pre-send readiness check for your team, not a customer decision.
8D Gate is a free pre-send readiness advisory. It doesn't replace the customer's decision. We don't store your 8D body in our database; you can delete anytime. Anthropic processes the text for the review.
Get the free 8D / D4 checklist
Keep the one-page version ready for the next board failure. It covers D1–D8 and goes deeper on D4 occurrence, escape, and verification.
Get the free 8D / D4 checklist