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).
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
- Load parts in fixture
- Align position
- Apply electrode pressure
- Apply current
- Release & unload
- 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.
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).
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.
Benefits of Using Diagrams in Structure Analysis #
- Provides visual clarity before detailed analysis.
- Ensures all functions, components, and steps are covered.
- Makes cross-functional discussions easier.
- Builds a traceable link to later FMEA steps (function, failure, risk).
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.
Case Study – Assembly Line PFMEA (Bolting Operation) #
- Process Flow Diagram:
- 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.
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.
Next Lesson #
👉 Continue with Lesson 3.3.3: Example – Motor → Stator → Winding