5.7 Defect taxonomy & disposition rules: MRB linkage

Ambiguity in defect classification can quickly bring a production line to a standstill. When an operator or Quality Inspector cannot immediately tell the difference between a minor process indicator and a critical functional failure, they face a difficult choice. They might scrap perfectly good parts, wasting valuable resources, or they might accidentally pass defective parts, which risks field failures and damages the brand. This chapter establishes a clear, physics-based system for classifying all non-conformities and defines a consistent, logical process for deciding their final outcome. The goal is to remove subjective debate from the decision-making process. The disposition of the hardware is determined by the fundamental nature of the defect itself, not by external pressures like the production schedule.

Defect severity classifications

We classify physical defects based solely on their direct impact on Safety, Function, and Assembly Form/Fit. The financial cost of the component itself is not a factor in this classification.

Critical defect (severity 1)

• Definition: A physical deviation that makes the finished product unsafe to use, legally non-compliant, or hazardous to the user.
• Physics/Risk Examples: A violation of high-voltage PCB creepage distance, exposed live mains parts through the chassis, use of banned materials (a RoHS failure), or severe structural damage to a high-density lithium battery cell.
• Action:
  • When a Critical Defect is found anywhere on the line, you must immediately initiate a Stop Ship / Stop Build order.
  • If the true Root Cause is currently unknown, you must quarantine the entire production lot and immediately lock all related upstream inventory in the ERP system.

Major defect (severity 2)

• Definition: A deviation that causes an immediate functional failure out of the box, or that physically prevents proper assembly downstream (a hard Form/Fit/Function failure).
• Physics/Risk Examples: Hard open circuits, dead short circuits, severe dimensional interference (where a plastic housing won’t snap closed), or a defect that guarantees a significant reduction in the product’s expected lifespan (like severe solder voiding).
• Action:
  • If the defect directly affects the required function, reject the unit immediately.
  • If the defect rate for a lot exceeds the calculated AQL limit, you must immediately trigger a formal SCAR (Supplier Corrective Action Request) against the vendor or an internal 8D analysis for the SMT line.

Minor defect (severity 3)

• Definition: A visual or dimensional deviation from preferred standards that does not affect Product Function or Safety. These are often purely cosmetic or serve as early warnings of slight machine process drift.
• Physics/Risk Examples: Surface visual imperfections that are completely invisible to the end-user, or minor dimensional deviations that still fall safely within the calculated mechanical tolerance limits.
• Action:
  • If the defect is structurally sound and invisible to the customer, log it for process control tracking (for yield and Cₚₖ monitoring) but continue processing the unit.
  • If the accumulation of multiple minor cosmetic defects on a single unit exceeds the defined visual limit, reject the unit.

Critical Factor

Engineering must consistently train operators not to confuse “Minor” with “Acceptable.” A Minor defect is still a formal non-conformance. If the manufacturing process produces frequent Minor defects, it indicates the Cₚₖ is degrading. If the process is not recentered, a severe Major defect becomes statistically likely over time.

Disposition logic gates

Once Quality has correctly classified a defect, the physical hardware must be processed off the line. Use the following logic to determine the correct path for the non-conforming hardware.

”RTV” (Return to Vendor)

• Trigger: Incoming raw materials or sub-assemblies definitively fail IQC/AQL inspection before they are used on the production floor.
• Decision Logic:
  • If the cost to return and inspect the component is greater than the component’s unit cost, scrap the parts locally and formally request a credit note (with prior written agreement from the vendor).
  • If local safety stock is depleted and production is at risk of stopping, perform a 100% screening operation on the remaining suspect inventory. The labor and overhead costs for this operation should be charged back to the vendor.

Scrap

• Trigger: A non-recoverable physical failure where the cost of rework exceeds 50% of the unit’s value, or where the fundamental physics of the failure prohibit a reliable, long-lasting repair.
• Decision Logic:
  • Scrap the board if the bare FR4 PCB substrate is cracked, deeply burned by an iron, or internally delaminated. The core structural integrity is compromised; do not attempt a repair on a cracked board.
  • Scrap the board if the Intermetallic Compound (IMC) of a specific joint is metallurgically compromised by excessive, repeated heat cycles from failed rework attempts.

Rework (standard)

• Trigger: The specific defect can be safely corrected back to the original print specification using a pre-validated, formally documented process (e.g., IPC-7711).
• Decision Logic:
  • If a formal Standard Operating Procedure (SOP) exists for this specific defect, execute the rework.
  • If no SOP exists, or if the technician would need to use unproven “creative” soldering techniques, stop immediately and escalate the unit to the MRB.

UAI (use as is)

• Trigger: A component is technically out of specification on paper, but Engineering has validated that there is zero operational risk to the customer in the field.
• Decision Logic:
  • Officially tag the unit as UAI if the deviation is purely cosmetic and on an internal, hidden surface.
  • Tag the unit as UAI if the deviation slightly reduces the design margin (e.g., a batch of capacitors is at the lower end of the specified tolerance) but the circuit remains completely stable under worst-case testing. This requires a formally signed deviation from Engineering.

The material review board (MRB)

The MRB is the final authority for product quality decisions in the factory. It is not a routine meeting to approve substandard work to meet shipping goals. It is a structured risk assessment body reserved for complex, non-standard dispositions.

When to Trigger the MRB:

1. Structural Modification: The proposed disposition requires permanently changing the product’s form, such as cutting internal copper traces, drilling non-plated holes, or adding “white” jumper wires.
2. Reliability Risk: The initial defect, or the proposed rework, might unknowingly degrade long-term field performance. Examples include minor corrosion observed or slight prior moisture exposure of MSL-rated parts.
3. High Value: The cost to scrap the assembly exceeds a 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 that clearly documents why the deviation is considered acceptable. “We need to ship it today” is not a valid engineering reason.
• Containment: A physical or procedural plan to be deployed immediately to prevent the identical defect from occurring on the next shift.
• Expiry: The MRB decision applies only to the specific physical serial numbers listed in the document. It is never a permanent rule change.

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Recap: Defect Severity Classification and Disposition Rules

Severity	Classification Criteria	Required Action	MRB Trigger / Notes
Critical (1)	Defect creates safety hazard, legal non-compliance, or user danger.	Initiate Stop Ship/Build. Quarantine lot and lock upstream inventory.	Mandatory for any Critical defect.
Major (2)	Defect causes immediate functional failure or prevents assembly (hard FFF failure).	Reject unit. Trigger SCAR/8D if lot defect rate exceeds AQL.	Required if defect rate exceeds AQL.
Minor (3)	Defect is cosmetic/visual or a process indicator; does not affect safety or function.	Log for process control (Cₚₖ). Reject if visual limit on unit is exceeded.	Not typically required for single defect.
Scrap	Rework cost >50% of unit value, or defect is irreparable (e.g., cracked PCB, compromised IMC).	Scrap the unit.	Required if scrap cost > $2,000 per assembly.
Rework	Defect can be corrected to original spec using a pre-validated, documented process (e.g., IPC-7711).	Execute per existing SOP.	Trigger MRB if no validated SOP exists and creation of one is required.
UAI (Use As Is)	Deviation poses zero operational risk; is cosmetic/internal or has validated engineering margin.	Tag unit formally. Requires signed engineering deviation.	Required for any UAI disposition.
RTV (Return to Vendor)	Non-conformance found at IQC before production use.	Return for credit or scrap locally if cost-effective. Perform 100% sort at vendor’s cost if line is down.	Decision based on component cost vs. return logistics.