Step 1 | Review the design
Use a blueprint or schematic of the design/product to identify each component and interface.
Reasons for the review:
- Help assure all team members are familiar with the product and its design.
- Identify each of the main components of the design and determine the function or functions of those components and interfaces between them.
- Make sure you are studying all components defined in the scope of the DFMEA.
Use a print or schematic for the review.
- Add Reference Numbers to each component and interface.
Try out a prototype or sample.
- Invite a subject matter expert to answer questions.
- Document the function(s) of each component and interface.
Step 2 | Brainstorm potential failure modes
Review existing documentation and data for clues.
Consider potential failure modes for each component and interface.
- A potential failure mode represents any manner in which the product component could fail to perform its intended function or functions.
- Remember that many components will have more than one failure mode. Document each one. Do not leave out a potential failure mode because it rarely happens. Don’t take shortcuts here; this is the time to be thorough.
Prepare for the brainstorming activity.
- Before you begin the brainstorming session, review documentation for clues about potential failure modes.
- Use customer complaints, warranty reports, and reports that identify things that have gone wrong, such as hold tag reports, scrap, damage, and rework, as inputs for the brainstorming activity.
- Additionally, consider what may happen to the product under difficult usage conditions and how the product might fail when it interacts with other products.
Step 3 | List potential effects of failure
There may be more than one effect for each failure.
The effect is related directly to the ability of that specific component to perform its intended function.
- An effect is the impact a failure could make should it occur.
- Some failures will have an effect on customers; others on the environment, the process the product will be made on, and even the product itself.
The effect should be stated in terms meaningful to product performance. If the effects are defined in general terms, it will be difficult to identify (and reduce) true potential risks.
Step 4 | Assign Severity rankings
The Severity ranking is based on the severity of the consequences of failure.
Assign a severity ranking to each effect that has been identified.
- The severity ranking is an estimate of how serious an effect would be should it occur.
- To determine the severity, consider the impact the effect would have on the customer, on downstream operations, or on the employees operating the process.
The severity ranking is based on a relative scale ranging from 1 to 10.
- A “10” means the effect has a dangerously high severity leading to a hazard without warning.
- Conversely, a severity ranking of “1” means the severity is extremely low.
The severity ranking is based on the severity of the consequences of failure.
Step 5 | Assign Occurrence rankings
The Occurrence ranking is based on how frequently the cause of the failure is likely to occur.
We need to know the potential cause to determine the occurrence ranking because, just like the severity ranking is driven by the effect, the occurrence ranking is a function of the cause.
- The occurrence ranking is based on the likelihood, or frequency, that the cause (or mechanism of failure) will occur.
- If we know the cause, we can better identify how frequently a specific mode of failure will occur.
The occurrence ranking scale, like the severity ranking, is on a relative scale from 1 to 10.
- An occurrence ranking of “10” means the failure mode occurrence is very high; it happens all of the time. Conversely, a “1” means the probability of occurrence is remote.
- See FMEA Checklists and Forms for an example DFMEA Occurrence Ranking Scale.
Your organization may need to customize the occurrence ranking scale to apply to different levels or complexities of design. It is difficult to use the same scale for a modular design, a complex design, and a custom design.
- Some organizations develop three different occurrence ranking options (time-based, event-based, and piece-based) and select the option that applies to the design or product.
- See FMEA Checklists and Forms for an examples of Custom DFMEA Ranking Scales. (Examples of custom scales for severity, occurrence, and detection rankings are included in this Resource Center.)
Step 6 | Assign Detection rankings
The Detection ranking is based on the chances the failure will be detected prior to the customer finding it.
To assign detection rankings, consider the design or product-related controls already in place for each failure mode and then assign a detection ranking to each control.
- Think of the detection ranking as an evaluation of the ability of the design controls to prevent or detect the mechanism of failure.
- A detection ranking of “1” means the chance of detecting a failure is almost certain. Conversely, a “10” means the detection of a failure or mechanism of failure is absolutely uncertain.
Prevention controls are always preferred over detection controls.
- Prevention controls prevent the cause or mechanism of failure or the failure mode itself from occurring; they generally impact the frequency of occurrence. Prevention controls come in different forms and levels of effectiveness.
- Detection controls detect the cause, the mechanism of failure, or the failure mode itself after the failure has occurred BUT before the product is released from the design stage.
To provide DFMEA teams with meaningful examples of Design Controls, consider adding examples tied to the Detection Ranking scale for design related topics such as:
- Design Rules
- DFA/DFM (design for assembly and design for manufacturability) Issues
- Simulation and Verification Testing
Step 7 | Calculate the RPN
RPN = Severity x Occurrence x Detection.
- The RPN is the Risk Priority Number. The RPN gives us a relative risk ranking. The higher the RPN, the higher the potential risk.
- The RPN is calculated by multiplying the three rankings together. Multiply the Severity Ranking times the Occurrence Ranking times the Detection Ranking. Calculate the RPN for each failure mode and effect.
- Editorial Note: The current FMEA Manual from AIAG suggests only calculating the RPN for the highest effect ranking for each failure mode. We do not agree with this suggestion; we believe that if this suggestion is followed, it will be too easy to miss the need for further improvement on a specific failure mode.
Since each of the three relative ranking scales ranges from 1 to 10, the RPN will always be between 1 and 1000. The higher the RPN, the higher the relative risk. The RPN gives us an excellent tool to prioritize focused improvement efforts.
Step 8 | Develop the action plan
Define who will do what by when.
Taking action means reducing the RPN.
- The RPN can be reduced by lowering any of the three rankings (severity, occurrence, or detection) individually or in combination with one another.
- A reduction in the Severity Ranking for a DFMEA is often the most difficult to attain. It usually requires a design change.
- Reduction in the Occurrence Ranking is accomplished by removing or controlling the potential causes or mechanisms of failure.
- A reduction in the Detection Ranking is accomplished by adding or improving prevention or detection controls.
What is considered an acceptable RPN?
- The answer to that question depends on the organization.
- For example, an organization may decide any RPN above a maximum target of 200 presents an unacceptable risk and must be reduced. If so, then an action plan identifying who will do what by when is needed.
There are many tools to aid the DFMEA team in reducing the relative risk of those failure modes requiring action.
The following recaps some of the most powerful action tools for DFMEAs.
Design of Experiments (DOE)
- A family of powerful statistical improvement techniques that can identify the most critical variables in a design and the optimal settings for those variables.
Mistake-Proofing (Poka Yoke)
- Techniques that can make it impossible for a mistake to occur, reducing the Occurrence ranking to 1.
- Especially important when the Severity ranking is 10.
Design for Assembly and Design for Manufacturability (DFA/DFM)
- Techniques that help simplify assembly and manufacturing by modularizing product sub-assemblies, reducing components, and standardizing components.
- Simulation approaches include pre-production prototypes, computer models, accelerated life tests, and value-engineering analyses.
Step 9 | Take action
Implement the improvements identified by your DFMEA team.
- The Action Plan outlines what steps are needed to implement the solution, who will do them, and when they will be completed.
- A simple solution will only need a Simple Action Plan while a complex solution needs more thorough planning and documentation.
- Most Action Plans identified during a DFMEA will be of the simple “who, what, & when” category. Responsibilities and target completion dates for specific actions to be taken are identified.
- Sometimes, the Action Plans can trigger a fairly large-scale project. If that happens, conventional project management tools such as PERT Charts and Gantt Charts will be needed to keep the Action Plan on track.
Step 10 | Calculate the resulting RPN
Re-evaluate each of the potential failures once improvements have been made and determine their impact on the RPNs.
- This step in a DFMEA confirms the action plan had the desired results by calculating the resulting RPN.
- To recalculate the RPN, reassess the severity, occurrence, and detection rankings for the failure modes after the action plan has been completed.