System, Subsystem, and Component Breakdown in FMEA

A successful FMEA begins with a clear structure analysis. The AIAG-VDA 7-Step FMEA Handbook recommends breaking down the subject into System, Subsystem, and Component levels.

👉 This hierarchical approach helps the FMEA team:

• Define relationships between elements.
• Identify interfaces where failures often occur.
• Ensure no critical part or process is overlooked.

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What is System, Subsystem, and Component Breakdown? #

1. System – The highest-level product or process under analysis.
2. Subsystem – Major building blocks that make up the system.
3. Component – The smallest elements or parts within a subsystem.

📌 For PFMEA, this hierarchy applies as Process → Process Step → Work Element.

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Why Breakdown is Important in FMEA #

• Provides clarity on what exactly is being analyzed.
• Helps avoid confusion and duplication during Function & Failure Analysis.
• Simplifies complex designs or processes into manageable levels.
• Enables traceability from high-level functions down to specific parts.

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Example 1 – DFMEA: Electric Motor #

• System: Electric Motor Assembly
• Subsystems:
  • Rotor
  • Stator
  • Housing
  • Bearings
• Components:
  • Rotor: Shaft, Laminations, Permanent Magnets
  • Stator: Windings, Core, Insulation
  • Housing: Covers, Seals
  • Bearings: Inner/outer rings, Lubrication

👉 This breakdown helps the DFMEA team analyze risks at each level (e.g., winding insulation failure, bearing seizure).

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Example 2 – PFMEA: Welding Process #

• Process (System): Spot Welding Operation
• Process Steps (Subsystems):
  1. Load components
  2. Position components
  3. Apply electrode force
  4. Apply current
  5. Release and unload
• Work Elements (Components):
  • Operator handling
  • Fixture alignment
  • Electrode condition
  • Current supply

👉 This breakdown helps the PFMEA team identify risks at the operator level, equipment level, and environment level.

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Example 3 – FMEA-MSR: Braking System Monitoring #

• System: ABS (Anti-lock Braking System)
• Subsystems:
  • ECU
  • Wheel Speed Sensors
  • Hydraulic Modulator
• Components:
  • ECU: Microcontroller, Software, Power Supply
  • Sensor: Magnet, Wiring, Connector
  • Modulator: Valve, Pump

👉 The breakdown ensures the team considers hardware + software + sensor integration risks.

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Best Practices for Breakdown #

1. Use structure trees for system-level FMEAs.
2. For processes, always prepare a Process Flow Diagram before PFMEA.
3. Keep the breakdown consistent with FMEA scope defined in Step 1.
4. Capture interfaces (System ↔ Subsystem, Subsystem ↔ Component).
5. Don’t go into unnecessary micro-detail (avoid listing screws, washers, etc., unless safety-critical).

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Common Mistakes to Avoid #

• Jumping straight to failures without structure breakdown.
• Over-detailing → analysis becomes unmanageable.
• Ignoring interfaces (e.g., ECU ↔ Sensor connections).
• Not aligning breakdown with APQP/DFMEA/PFMEA templates.

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Case Study – Suspension Assembly PFMEA #

• System: Front Suspension Assembly
• Subsystems: Shock Absorber, Control Arm, Spring, Bushings
• Components:
  • Shock Absorber: Tube, Piston, Oil Seal
  • Control Arm: Arm Body, Ball Joint
  • Spring: Coil Material, Heat Treatment
• Findings: Team identified that oil seal leakage could cause shock absorber failure, which was missed in earlier generic process reviews.

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Key Takeaways #

• System = Entire product/process
• Subsystem = Major functional blocks
• Component = Detailed parts/work elements
• Breakdown ensures clarity, completeness, and traceability.
• A structured breakdown is the foundation for Function and Failure Analysis (Steps 3 & 4).

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Next Lesson #

👉 Continue with Lesson 3.3.2: Process Flow → Structure Tree & Block Diagram