Process Flow – Structure Tree & Block Diagram in FMEA

In Step 2: Structure Analysis of the AIAG-VDA 7-Step FMEA approach, the team must visualize the system or process using diagrams.

The three recommended tools are:

• Process Flow Diagram (PFD) – for PFMEA.
• Structure Tree – for DFMEA and PFMEA.
• Block Diagram – for DFMEA (system-level functions and interfaces).

👉 These diagrams bring clarity, alignment, and traceability before moving to Function Analysis (Step 3).

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1. Process Flow Diagram (PFD) in FMEA #

The Process Flow Diagram shows how a product moves through different process steps. It is the foundation of PFMEA.

Key Purposes

• Visualize sequence of operations.
• Define inputs and outputs at each step.
• Identify potential interfaces between operations.

Example – PFMEA Welding Process

1. Load parts in fixture
2. Align position
3. Apply electrode pressure
4. Apply current
5. Release & unload
6. Inspection

👉 Each step becomes a node in PFMEA, where risks are analyzed.

📌 Tip: Always number process steps — it helps when linking PFMEA rows to the flow diagram.

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2. Structure Tree in FMEA #

The Structure Tree shows a hierarchical breakdown of system → subsystems → components or process → process steps → work elements.

Key Purposes

• Clarifies relationships and hierarchy.
• Ensures no subsystem/component is forgotten.
• Helps identify interfaces (where many failures occur).

Example – DFMEA Electric Motor

• System: Motor Assembly
  • Subsystems: Rotor, Stator, Housing
  • Components: Bearings, Windings, Magnets, Seals

👉 Risks can then be traced top-down (system) or bottom-up (component).

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3. Block Diagram in FMEA #

The Block Diagram shows functional interactions between subsystems or components. It is mainly used in DFMEA.

Key Purposes

• Visualize signal, energy, or material flow.
• Define interfaces between subsystems.
• Ensure coverage of system-level interactions.

Example – DFMEA Electric Vehicle Powertrain

• Battery → Inverter → Motor → Transmission → Wheels

👉 Failures at interfaces (e.g., inverter ↔ motor) are often the most critical risks.

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Benefits of Using Diagrams in Structure Analysis #

1. Provides visual clarity before detailed analysis.
2. Ensures all functions, components, and steps are covered.
3. Makes cross-functional discussions easier.
4. Builds a traceable link to later FMEA steps (function, failure, risk).

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Common Mistakes to Avoid #

• Skipping diagrams and jumping directly to failure analysis.
• Making overly detailed flow diagrams (loses focus).
• Not updating diagrams when design or process changes occur.
• Ignoring interfaces, which are often high-risk zones.

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Case Study – Assembly Line PFMEA (Bolting Operation) #

• Process Flow Diagram:
  1. Operator picks bolt → 2. Positions bolt → 3. Applies torque → 4. Torque check → 5. Release.
• Structure Tree:
  • Process → Bolting operation → Torque tool → Operator handling.
• Block Diagram:
  • Operator → Torque tool → Bolt → Joint.

👉 With these three tools, the PFMEA team identified risks like under-torque, bolt omission, and tool wear.

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Key Takeaways #

• Process Flow Diagram = foundation for PFMEA.
• Structure Tree = hierarchy for DFMEA/PFMEA.
• Block Diagram = functional interactions for DFMEA.
• These tools make FMEA structured, complete, and traceable.

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Next Lesson #

👉 Continue with Lesson 3.3.3: Example – Motor → Stator → Winding