5.7 Defect taxonomy & disposition rules: MRB linkage

Ambiguity in defect classification rapidly paralyzes production lines. If an operator or Quality Inspector cannot instantly distinguish between a minor “process indicator” and a critical “functional failure,” they will inevitably either scrap perfectly good parts (wasting precious capital) or inadvertently pass functionally bad parts (risking severe field liability and brand damage). This chapter establishes a binary, physics-based taxonomy for all non-conformities and defines the rigid, non-negotiable logic path for their final disposition. Human emotion, ego, and debate are removed from the decision process: the fundamental physics of the defect determines the hardware’s fate, Not the current production schedule pressure.

Defect severity classifications

Physical defects are classified based purely on their direct engineering impact on Safety, Function, and Assembly Form/Fit, never on the financial dollar cost of the component itself.

Critical defect (severity 1)

• Definition: The physical deviation renders the finished product actively unsafe to use, legally non-compliant, or hazardous to the user.
• Physics/Risk Examples: High-Voltage PCB creepage distance violation, exposed live mains parts through the chassis, use of toxic/banned material (RoHS failure), or severe structural damage to a high-density lithium battery cell.
• Action:
  • IF a Critical Defect is found anywhere on the line -> THEN explicitly Stop Ship / Stop Build immediately.
  • IF the true Root Cause is currently unknown -> THEN Quarantine the entire production lot and immediately lock all active upstream inventory in the ERP.

Major defect (severity 2)

• Definition: The deviation practically results in an immediate functional failure out of the box, or structurally prevents proper downstream assembly (a hard Form/Fit/Function failure).
• Physics/Risk Examples: Hard open circuits, dead short circuits, severe dimensional interference (the plastic housing won’t snap closed), or a physics-based defect that guarantees a significant reduction in the anticipated product life-cycle (like severe solder voiding).
• Action:
  • IF the defect directly affects required function -> THEN Reject the unit immediately.
  • IF the defect rate historically exceeds the calculated AQL limit for the lot -> THEN immediately trigger a formal SCAR (Supplier Corrective Action Request) against the vendor or an internal 8D against the SMT line.

Minor defect (severity 3)

• Definition: A visual or dimensional deviation from preferred standards that conclusively does not affect Product Function or Safety; it is often purely cosmetic or serves as an early warning of slight machine process drift.
• Physics/Risk Examples: Surface visual imperfections completely invisible to the end-user, or minor dimensional deviations that stack safely within calculated mechanical tolerance limits.
• Action:
  • IF the defect is structurally sound and invisible to the customer -> THEN Log the defect for process control tracking (yield tracking and Cₚₖ limits) but actively continue processing the unit.
  • IF the physical accumulation of multiple minor cosmetic defects exceeds the defined visual limit -> THEN Reject the unit.

Pro-Tip: Engineering must relentlessly train operators to never confuse “Minor” with “Acceptable.” A Minor defect is still a formal non-conformance flag. If the manufacturing process produces frequent Minor defects, the Cₚₖ is degrading over time, and a severe Major defect is statistically inevitable if the machine is not recentered.

Disposition logic gates

Once a defect is correctly classified by Quality, the physical hardware must be processed off the line. This rigid logic gate must be used to flawlessly determine the physical path of the non-conforming hardware.

”RTV” (Return to Vendor)

• Trigger: Incoming raw material or sub-assemblies definitively fail IQC/AQL inspection before hitting the floor.
• Decision Logic:
  • IF the component unit cost is less than the required return shipping and administrative inspection cost -> THEN Scrap the parts locally and formally request a credit note (with prior written vendor agreement).
  • IF local safety stock is zero and the production line is hard down -> THEN perform a 100% Sort/Screen operation (and explicitly charge the labor and overhead costs back to the offending vendor).

Scrap

• Trigger: A non-recoverable physical failure where the calculated rework labor cost > 50% of the unit’s value, or fundamental physics prohibits a reliable, long-lasting repair.
• Decision Logic:
  • IF the bare FR4 PCB substrate is cracked, deeply burned by an iron, or internally delaminated -> THEN Scrap it. (The core structural integrity is gone; never attempt a hero repair on a cracked board).
  • IF the Intermetallic Compound (IMC) of a specific joint is metallurgically compromised by excessive, repeated heat cycles from failed rework -> THEN Scrap the board.

Rework (standard)

• Trigger: The specific defect deviation can be safely corrected back to the original print spec using a pre-validated, formally documented process (e.g. IPC-7711).
• Decision Logic:
  • IF a formal Standard Operating Procedure (SOP) uniquely exists for this defect -> THEN Execute the Rework.
  • IF no SOP exists or “creative, unproven” soldering techniques are required by the technician -> THEN Stop immediately. Escalate the unit to the MRB.

UAI (use as is)

• Trigger: The component is technically out of spec on paper, but Engineering validates zero operational risk to the customer in the field.
• Decision Logic:
  • IF the deviation is purely cosmetic on an internal, hidden surface -> THEN officially tag as UAI.
  • IF the deviation slightly reduces engineering margin (e.g. a batch of capacitors has 5% less capacitance than the design target) but the active circuit remains completely stable under worst-case testing -> THEN tag as UAI (This requires a formally signed deviation from Engineering).

The material review board (MRB)

The MRB is the absolute “Supreme Court” of product quality in the factory. It is definitely not a daily stand-up meeting designed to cheerfully approve sloppy production work to hit shipping goals; it is a highly structured risk assessment body reserved for complex, non-standard dispositions.

When to Trigger the MRB:

1. Structural Modification: The proposed disposition actively requires cutting internal copper traces, drilling non-plated holes, or adding “white” jumper wires (permanently changing the “Form”).
2. Reliability Risk: The initial defect (or the proposed rework) might unknowingly degrade long-term field performance (e.g. minor corrosion observed, or slight prior moisture exposure of MSL parts).
3. High Value: The calculated scrap assembly cost significantly exceeds the predefined threshold (e.g. > $2,000 per assembly).
4. Customer Impact: The proposed deviation requires a permanent engineering change to the approved drawing or a formal, signed customer waiver to ship.

MRB Output Requirements:

• Technical Justification: A rigorous, physics-based engineering reason clearly documenting exactly why the deviation is considered acceptable. “We need to ship it today” is not an engineering reason.
• Containment: A physical or procedural plan rapidly deployed to prevent the identical defect from occurring on the very next shift.
• Expiry: The MRB decision applies only to the specific physical serial numbers explicitly listed in the document. It is never a permanent rule change.

Final Checkout: Defect taxonomy & disposition rules (linked to MRB)

Control Point	Guiding Principle
Critical Defect	”Stop Build” authority should be explicitly delegated to floor Quality Techs.
Rework Limits	Enforce a Strict Maximum of 2 thermal attempts per individual joint/pad location to prevent PCBA delamination.
MRB Threshold	Triggered immediately when the physical Scrap Cost > $X,XXX or there is any Safety/Reliability Risk.
UAI Authority	Formally requires explicitly signed approval by both the Engineering & Quality Managers.
Scrap Logic	Structural base material damage (Cracks/Burns) = Automatic, Mandatory Scrap.
Traceability	All MRB/Rework physical actions must be permanently logged by individual Serial Number to track field failures.