Once the NUD Functions that a technology, product or manufacturing process must perform are identified, the Design Failure Modes & Effects Analysis (DFMEA) can be performed. Fishbone Diagrams are useful for root-cause analysis during DFMEA.

Generating a Fishbone Diagram:

Use a cross-functional team of experienced technical practitioners and subject matter experts to brainstorm a complete list of factors that can contribute to the mechanisms of failure. It is important to focus on the physics of failure and the precursor events, conditions, and dynamics that can add up to cause failure. The Fishbone Diagram documents the building blocks telling the story of how a failure can happen from the beginning to the end state of failure.

The diagram below illustrates a standard format for a Fishbone Diagram:

 

Fishbone Diagram

There are six general factors that can contribute to a functional failure. The following are considered when constructing a Fishbone Diagram:

• People
• Environment
• Materials
• Measurement / Data Acquisition Systems
• Machines / Tools / Hardware / Software / Firmware
• Methods / Processes / Common Practices

Noise Diagrams can be helpful during the construction of a Fishbone Diagram. They provide additional perspectives on sources of unwanted variation and help define the conditions of variation that are leading indicators to eventual failure as time goes by.

Generic Noise Diagram

Noise Diagrams are illustrated in Quick Guide 11.

Below is an Input-Output Constraint (I-O-C) diagram (see QG8) that includes undesirable noises that may cause unwanted changes in both the mean and standard deviation of the measured response (Y).

 

I-O-C Diagram Template

The Noise Diagram is used to identify sources of unwanted variation that affect the measured Ys. It is created by the cross-functional team in brainstorm sessions, as the Fishbone Diagram is.

 

Noise Diagram Template

The 3 Main Elements of Noise:

1. External Noise: Any form of unwanted or disruptive mass, energy or information that can get into the function from the external environment (outside of the design parameters performing the function).
2. Unit-to-Unit Noise: Any source of unwanted variation due to part-to-part, material-to-material, batch-to-batch processing or other production and packaging process variation that can negatively affect the function of the design. Tolerances are usually developed and specified to constrain these types of noise.
3. Deterioration Noise: Any form of degradation, wear-out or depletion of properties over time that can negatively affect the function of a design.

The information in the Fishbone and Noise Diagrams must be incorporated into the Design Guide (See QG4) to document the sources of variation that negatively affect the design's functions. These diagrams also set the stage for generating Robust Design Parameter Diagrams for Modeling and Simulation and Designed Experimentation (DOEs) for developing functional robustness.