6.7 Defect Taxonomy & Disposition Rules (Linked to MRB)

Ambiguity in defect classification paralyzes production lines and allows latent failures to escape. A subjective debate over whether a scratch is "Minor" or "Major" wastes engineering hours and degrades the baseline for acceptable quality. This chapter establishes a rigid, physics-based taxonomy for non-conformities and defines the exact logic path for their disposition, removing emotion from the decision process.

Defect Severity Taxonomy

Classify defects based on their impact on Safety, Function, and Assembly, not on the cost of the component.

Critical Defect (Severity 1)

Definition: The deviation renders the product unsafe or legally non-compliant.

• Impact: Risk of injury, fire, shock, or regulatory violation (e.g., creepage distance violation, exposed live part, toxic material presence).
• Action:
  • If Critical Defect found -> Then Quarantine entire lot immediately.
  • If Root Cause is not identified -> Then Stop Ship / Stop Build.

Major Defect (Severity 2)

Definition: The deviation results in functional failure or significant reduction in usability/reliability.

• Impact: Product does not work, life-cycle is compromised, or unit cannot be assembled (Fit/Form/Function failure).
• Action:
  • If functional/reliability risk -> Then Reject unit.
  • If systemic (rate > AQL limit) -> Then Trigger SCAR (Supplier Corrective Action Request).

Minor Defect (Severity 3)

Definition: The deviation is cosmetic or a slight departure from specifications that does not affect Function or Safety.

• Impact: Visual imperfection not visible to the end-user, or minor dimensional deviation within tolerance stacking limits.
• Action:
  • If defect does not affect customer perception -> Then Count against yield but continue processing.
  • If accumulation of minor defects exceeds visual standard -> Then Reject.

Pro-Tip: Do not confuse "Minor" with "Acceptable." A Minor defect is still a non-conformance. If the process produces frequent Minor defects, the process capability (Cpk) is degrading.

Disposition Logic

Once a defect is classified, execute the disposition based on the following logic gates. Do not invent new categories.

1. RTV (Return to Vendor)

Trigger: Incoming material fails AQL inspection or functional testing.

Logic:

• If part is cheaper than inspection cost -> Then Scrap at vendor expense.
• If stock is critical for continuity -> Then 100% Sort (charge back to vendor).

2. Scrap

Trigger: Non-recoverable non-conformance where rework is impossible or costs > 50% of unit value.

Logic:

• If structural integrity is compromised (e.g., cracked PCB, bent chassis) -> Then Scrap. DO NOT attempt mechanical straightening.

3. Rework (Standard)

Trigger: Deviation can be corrected to meet original specifications using a validated process.

Logic:

• If rework process exists and is documented -> Then Execute Rework.
• If rework requires unproven methods (e.g., jumper wires on a final board) -> Then Escalate to MRB.

4. UAI (Use As Is)

Trigger: Deviation is outside spec but poses zero functional or reliability risk.

Logic:

• If dimension is out of tolerance but fits mating part -> Then UAI (requires Engineering Approval).
• If visual blemish is on internal surface -> Then UAI.

Material Review Board (MRB) Triggers

The MRB is not a committee for approving bad quality; it is a risk assessment body for complex dispositions. Only escalate to MRB when the standard disposition logic fails or high-value risks are involved.

Escalate to MRB IF:

1. Structural Modification: The proposed disposition changes the Fit, Form, or Function (e.g., drilling new holes, cutting traces).
2. Reliability Risk: The defect might degrade long-term performance (e.g., oxidized contacts, moisture exposure).
3. High Value: The cost of scrap exceeds the pre-defined threshold (e.g., > $5,000).
4. Customer Impact: The deviation might be noticeable to the customer, requiring a deviation waiver.

MRB Output Requirements:

• Technical Justification: Physics-based reason why the deviation is acceptable.
• Containment: Plan to prevent further production of this defect.
• Expiry: The decision applies only to the specific serial numbers or lot identified. It does not set a precedent for future production.

Final Checklist

Parameter	Rule / Threshold
Critical Defect Action	Immediate Stop Build; Quarantine Lot.
Major Defect Criteria	Fails Function, Fit, or Safety.
Rework Constraint	Allowed only with validated, documented instruction.
MRB Trigger (Cost)	Scrap value > [Insert Threshold, e.g., $1,000].
MRB Trigger (Risk)	Any risk to Reliability or Safety.
UAI Documentation	Must include Engineering signature and expiration (Lot/Qty).
Disposition Time Limit	Non-conforming material must be dispositioned within 48 hours.