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3.3 Failure Mode and Effects Analysis (FMEA)

FMEA is not paperwork; it is the enginemathematical prediction of riskthe reduction.future. It assignsforces engineers to stare into the abyss of "What could go wrong?" and build a quantitativebridge over it before the first prototype is scrapped. If you treat FMEA as a checkbox exercise to satisfy an auditor, you will find the failure modes in the field, where they cost 100x more to fix.

The Mechanics of Risk (The RPN Engine)

We quantify risk using the Risk Priority Number (RPN) to potentialstrip failuresaway toemotion prioritizeand engineeringfocus resources.on

The RPN Calculation

data.

Formula: RPN = Severity (S) \timesx Occurrence (O) \timesx Detection (D)

The Variables:

  • Severity (1-10)S): HowThe badimpact ison the failure?end user.
    • Constraint: You cannot "inspect" Severity down. You can only reduce it by changing the Design (e.g., adding a fuse).
    • 10 = Hazard / Safety hazardRisk without warning).warning.
    • 1 = No discernible effect.
  • Occurrence (1-10)O): HowThe oftenprobability willof itthe happen?cause happening.
    • Driver: Reduced by Process Capability (10Cpk) =and Inevitable).robust design margins.
  • Detection (1-10)D): WillThe probability that we findcatch it before it ships?escapes.
    • Driver: (Improved by testing and inspection.
    • 10 = CannotAbsolute beuncertainty detected,(No check).
    • 1 = Auto-detect/prevent)Error Proofing (Physics prevents the defect).

Pro-Tip: Never average RPN scores. A single line item with Severity 10 and RPN 90 is infinitely more critical than an item with Severity 3 and RPN 200. Safety trumps statistics.

Execution Strategy: DFMEA vs. PFMEA

Do not confuse the intent. One protects the design; the other protects the build.

DFMEA (Design FMEA):

  • Focus: Component physics, circuit topology, geometry, material properties.
  • Example Failure: "Capacitor derating insufficient for voltage spike."
  • Mitigation: Select higher voltage rating component.

ThePFMEA Action(Process ThresholdFMEA):

  • High RPN:Focus: AnyThe itemmachine, withthe operator, the environment, the method.
  • Example Failure: "Operator installs capacitor backwards."
  • Mitigation: Add polarity marking to PCB silkscreen + AOI check.

Scoring Calibration:

  • IF Severity = 9 or 10 (Safety/Regulatory) -> THEN Action is Mandatory regardless of RPN.
  • IF RPN > 100 -> THEN Mitigation plan is required.
  • IF Detection = 10 (orVisual Severityinspection 9/10)by requireshuman) mandatory-> mitigationTHEN Reject as a primary control for critical features. Humans are only 80% effective.

The AIAG-VDA Harmonization (The New Standard)

Modern Quality Engineering is moving away from pure RPN towards Action Priority (AP) levels (High, Medium, Low) to prevent "gaming the numbers" (e.g., poka-yokeartificially tooling,lowering designDetection change)to get RPN < 100).

Logic Flow:

  • High Priority: Severity 9-10 with any meaningful Occurrence. -> Action: Review at Management Level.
  • Medium Priority: Severity 7-8 with Moderate Occurrence. -> Action: Review at Engineering Level.

Final Checklist

FMEAControl FactorPoint

DefinitionCritical Requirement

MitigationNon-Negotiable StrategyRule

Severity Scoring

Impact9/10 on= UserSafety/Regulatory.

DesignNever Changelower OnlySeverity based on "good testing."

OccurrenceMitigation

FrequencyPrefer ofPrevention Cause(Poka-Yoke) over Detection.

Process"Retrain ControlOperator" /is CapabilityNOT a valid long-term fix.

DetectionLoop Closure

AbilityRe-score toRPN Findafter mitigation.

TestRPN Coveragemust /drop, Poka-Yokeor the action was useless.

RPNLiving LimitDocument

ThresholdUpdate forwith Actionevery RMA/NC.

MitigationIf Actiona Planfailure occurs in the field, the FMEA failed. Update it.

S/O/D Anchor

Use a standard scoring table.

Do not guess; use the defined criteria.


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