5.7 Defect Taxonomy & Disposition Rules (Linked to MRB)

Ambiguity in defect classification paralyzes production lines. If an operator cannot instantly distinguish between a "process indicator" and a "functional failure," they will either scrap good parts (wasting money) or pass bad parts (risking liability). This chapter establishes a binary, physics-based taxonomy for non-conformities and defines the rigid logic path for their disposition. We remove emotion from the decision process: the physics of the defect determines its fate, not the schedule pressure.

Defect Severity Classifications

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

Critical Defect (Severity 1)

• Definition: The deviation renders the product unsafe, illegal, or hazardous.
• Physics/Risk: Creepage distance violation, exposed live parts, toxic material (RoHS failure), or lithium battery damage.
• Action:
  • If Critical Defect is found → Then Stop Ship / Stop Build immediately.
  • If root cause is unknown → Then Quarantine the entire lot and upstream inventory.

Major Defect (Severity 2)

• Definition: The deviation results in functional failure or prevents proper assembly (Form/Fit/Function).
• Physics/Risk: Open circuits, short circuits, dimensional interference, or significant reduction in anticipated life-cycle.
• Action:
  • If defect affects function → Then Reject unit.
  • If defect rate > AQL limit → Then Trigger SCAR (Supplier Corrective Action Request).

Minor Defect (Severity 3)

• Definition: A deviation from standards that does not affect Function or Safety, often cosmetic or a slight process drift.
• Physics/Risk: Visual imperfections not visible to the end-user, or dimensional deviations within tolerance stacking limits.
• Action:
  • If defect is invisible to customer → Then Log for process control (yield tracking) but continue processing.
  • If accumulation of minors exceeds visual limit → Then Reject.

Pro-Tip: Do not confuse "Minor" with "Acceptable." A Minor defect is still a non-conformance. If your process produces frequent Minor defects, your CpK is degrading, and a Major defect is inevitable.

Disposition Logic Gates

Once classified, the defect must be processed. Use this logic to determine the physical path of the hardware.

RTV (Return to Vendor)

• Trigger: Incoming material fails AQL inspection.
• Logic:
  • If component cost < shipping/inspection cost → Then Scrap locally and credit note (with vendor agreement).
  • If stock is zero and line is down → Then 100% Sort/Screen (charge labor back to vendor).

Scrap

• Trigger: Non-recoverable failure where rework cost > 50% of unit value or physics prohibits repair.
• Logic:
  • If PCB substrate is cracked, burned, or delaminated → Then Scrap. (Structural integrity is gone; do not attempt repair).
  • If Intermetallic Compound (IMC) is compromised by excessive heat cycles → Then Scrap.

Rework (Standard)

• Trigger: Deviation can be corrected to spec using a pre-validated, documented process (e.g., IPC-7711).
• Logic:
  • If a Standard Operating Procedure (SOP) exists → Then Execute Rework.
  • If no SOP exists or "creative" soldering is required → Then Stop. Escalate to MRB.

UAI (Use As Is)

• Trigger: Component is out of spec, but Engineering validates zero risk.
• Logic:
  • If deviation is purely cosmetic on an internal surface → Then UAI.
  • If deviation reduces margin (e.g., 5% less capacitance) but circuit remains stable → Then UAI (Requires signed deviation).

The Material Review Board (MRB)

The MRB is the "Supreme Court" of quality. It is not a daily meeting to approve sloppy work; it is a risk assessment body for complex dispositions.

When to Trigger MRB:

1. Structural Modification: Disposition requires cutting traces, drilling holes, or adding jumper wires (changing the "Form").
2. Reliability Risk: The defect might degrade long-term performance (e.g., corrosion, slight moisture exposure).
3. High Value: The scrap cost exceeds the threshold (e.g., > $2,000).
4. Customer Impact: The deviation requires a permanent change to the drawing or a customer waiver.

MRB Output:

• Technical Justification: A physics-based reason why the deviation is acceptable.
• Containment: A plan to prevent the defect from recurring.
• Expiry: The decision applies only to the specific serial numbers listed. It is not a permanent rule change.

Final Checklist

Control Point	Critical Requirement	Risk Avoided
Critical Defect	Stop Build authority delegated to Quality Techs.	Safety / Regulatory Liability
Rework Limits	Max 2 attempts per joint/location.	PCBA Delamination / Pad Lift
MRB Threshold	Cost > $X,XXX or Safety/Reliability Risk.	Financial Loss / Field Failure
UAI Authority	Requires Engineering & Quality Manager Sign-off.	Unauthorized Design Change
Scrap Logic	Structural damage (Cracks/Burns) = Mandatory Scrap.	Latent Field Failure
Traceability	All MRB/Rework actions recorded by Serial Number.	Recall Management