PFMEA PCB Assembly (SMT) Example

The scope is a double-sided SMT assembly with mixed-technology components (BGAs, QFNs, fine-pitch ICs, 0402 passives, two through-hole connectors hand-soldered downstream). The line is paste print → SPI → placement → pre-reflow AOI → reflow → post-reflow AOI → ICT → conformal coating → final inspection. Bare board specification, schematic and BOM are frozen — those belong to the DFMEA. This PFMEA covers everything that happens after the board enters the SMT line.

1. Bare PCB receiving and MSL-controlled storage
2. Solder paste printing (Type 4 SAC305, stencil 100 µm)
3. Solder paste inspection (SPI) — 100 % volume / area
4. Component placement (chip shooter + fine-pitch placer)
5. Pre-reflow AOI
6. Reflow soldering (10-zone convection oven, nitrogen)
7. Post-reflow AOI + X-ray sampling on BGAs
8. In-circuit test (ICT) and functional test
9. Selective conformal coating
10. Final inspection and packaging

• Open joints and insufficient solder — almost always traceable to paste deposit (SPI) rather than reflow.
• Solder bridges — fine-pitch placement combined with excess paste or pad design.
• Tombstoning — chip resistors and capacitors lifted on one end, driven by paste asymmetry and uneven heating ramp.
• BGA cold joints and voids — the hardest failures to detect; X-ray sampling is the only realistic check.
• Wrong component / wrong orientation — feeder loading errors, reel orientation, MSL handling.
• Conformal coating defects — capillary creep, pinholes, coating under tall parts trapping flux.
• ICT escapes — false pass driven by probe wear; the single highest-impact escape mode on most lines.

Ten representative rows. Use the ranking tables for calibrated S/O/D scales.

SMT lines run with dense automated inspection, but every inspection has a known false-pass rate. The PFMEA must reflect that, not assume 100 % capture.

• Prevention: MSL dry storage, feeder verification, stencil cleaning schedule, reflow profile validation, ICT probe PM, BGA stencil design review.
• Detection: 100 % SPI volume measurement, pre- and post-reflow AOI, X-ray sampling on BGAs (typically 1 per shift per type), ICT, functional test, UV inspection on conformal coat.

• Implement automated under-stencil wipe based on SPI volume trend, not a fixed print count.
• Add a daily reflow oven thermal profile verification with a KIC profiler and trigger an alarm on time-above-liquidus deviation.
• Move ICT probe replacement from cycle-count to a correlation-study trigger — replace any probe whose Cpk drops below 1.33 on the golden-board check.
• Validate any new BGA introduction with a paste-print study before release to production.

The High-AP rows are dominated by latent-defect failure modes: BGA cold joints, BGA voiding, ICT escapes. These share a structural problem — Detection is fundamentally limited because the failure is not visible to optical inspection. The realistic Detection floor is 4 or higher even with X-ray sampling. The AP guide rules promote any High Severity (≥ 7) with Medium Occurrence and weak Detection straight to AP High, which is the correct engineering conclusion for these failures.

• Treating AOI as Detection 2. Modern AOI is genuinely Detection 3–4 on joint quality; it is not a guarantee against cold joints.
• Forgetting MSL handling. Moisture absorption on bare boards and on moisture-sensitive ICs is a common audit finding.
• Single row for "soldering" instead of separating paste, placement and reflow. Each has different causes and different controls.
• Ignoring probe wear in ICT. False-pass is the dominant escape mode on mature lines.
• Conformal coating treated as a downstream finishing step rather than a process step with its own failure modes.

IPC and customer EMS auditors look for: paste deposit SPC linked to SPI data, reflow profile control (KIC or equivalent profiling on a defined cadence), ICT correlation study evidence, and BGA X-ray sampling plan with a defensible sample size. The PFMEA must reference these by control name, not generically. Misalignment between the PFMEA, the SMT process Control Plan and the work instructions is a common observation. Use the FMEA audit checklist as a pre-submission review.

• FMEA Example — base PFMEA walkthrough.
• What is FMEA? — methodology primer.
• AIAG-VDA 7-Step Process — the methodology this example follows.
• Action Priority Guide — how H/M/L is derived from S, O, D.
• Severity Rating Guide, Occurrence Guide, Detection Guide.
• Common Mistakes and Audit Checklist.
• FMEA → Control Plan for the downstream handoff.
• Process FMEA Software — build this PFMEA in Qhubio with deterministic AP.