Ten Steps In Performing A DFMEA

Step 1

Review the design—Use a blueprint or schematic of the design/product to identify each component and interface.

Step 2

Brainstorm potential failure modes—Review existing documentation and data for clues.

Step 3

List potential effects of failure—There may be more than one for each failure mode.

Step 4

Assign Severity rankings—Based on the severity of the consequences of failure.

Step 5

Assign Occurrence rankings—Based on how frequently the cause of the failure is likely to occur.

Step 6

Assign Detection rankings—Based on the chances the failure will be detected prior to the customer finding it.

Step 7

Calculate the RPN—Severity X Occurrence X Detection = RPN.

Step 8

Develop the action plan—Define who will do what by when.

Step 9

Take action—Implement the improvements identified by your DFMEA team.

Step 10

Re-calculate the resulting RPN—Re-evaluate each of the potential failures once improvements
have been made and determine the impact of the improvements.

Step 1: Review the Design

There are several reasons for reviewing the process

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.

The function of the component is the value-adding role that component performs or provides. In
other words it is the task it performs.

Many components have more than one function.

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Step 2: Brainstorm Potential Failure Modes

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.

Using the list of components and related functions generated in Step 1, as a team, brainstorm the potential failure modes for each function.

Don’t take shortcuts here; this is the time to be thorough.

Prepare for the brainstorming session.

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.

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Step 3: List Potential Effects of Failure

Determine the effects associated with each failure mode. 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 if it occurred.

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.

As with failure modes, use descriptive and detailed terms to define effects.

The effect should be stated in terms meaningful to product or system performance.

If the effects are defined in general terms, it will be difficult to identify (and reduce) true potential risks.

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Step 4: Assign Severity Rankings

Assign a severity ranking to each effect that has been identified.

The ranking scales are mission critical for the success of a DFMEA because they establish the basis for determining risk of one failure mode and effect relative to another.

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 same ranking scales for DFMEAs should be used consistently throughout an organization. This will make it possible to compare the RPNs from different FMEAs to one another.

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.

See FMEA Checklists and Forms for an example DFMEA Severity Ranking Scale..

The best way to customize a ranking scale is to start with a standard, generic scale and then modify it to be more meaningful to your organization.

By adding organization-specific examples to the ranking definitions, DFMEA teams will have an easier time using the scales. The use of examples saves teams time and improves the consistency of rankings from team to team.

As you add examples specific to your organization, consider adding several columns with each column focused on a topic. One topic could provide descriptions of severity levels for customer satisfaction failures and another for environmental, health, and safety issues. However, remember that each row should reflect the same relative impact, or severity, on the organization or customer.

One topic could provide descriptions of severity levels for operational failures, another column for customer satisfaction failures, and a third for environmental, health, and safety issues.

See FMEA Checklists and Forms for an example DFMEA Severity Ranking Scale.

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Step 5: Assign Occurrence Rankings

Next, consider the potential cause or failure mechanism for each failure mode; then assign an occurrence ranking to each of those causes or failure mechanisms.

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.

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, and happens all of the time. Conversely, a “1” means the probability of occurrence is remote.

See FMEA Checklists and Forms for an example DFMEA Severity 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 example DFMEA Severity Ranking Scale.

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Step 6: Assign Detection Rankings

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.

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.

The Detection ranking scale, like the Severity and Occurrence scales, is on a relative scale from 1 to 10.

A Detection ranking of “1” means the chance of detecting a failure is certain.

Conversely, a “10” means there is absolute certainty of non-detection. This basically means that there are no controls in place to prevent or detect.

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

See FMEA Checklists and Forms for an example DFMEA Severity Ranking Scale.

Taking a lead from AIAG, consider three different forms of Custom Detection Ranking options. Custom examples for Mistake-Proofing, Gauging, and Manual Inspection controls can be helpful to PFMEA teams.

See FMEA Checklists and Forms for an example DFMEA Severity Ranking Scale.

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Step 7: Calculate the RPN

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.

Trainer's 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.

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Step 8: Develop the Action Plan

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. It usually requires a A change to the design of the product.

Reduction in the Occurrence ranking is accomplished by removing or controlling the potential causes.

Mistake-proofing tools are often used to reduce the frequency of occurrence.

A reduction in the Detection ranking can be accomplished by adding or improving prevention or detection controls.

Adding process fail-safe shut-downs, alarm signals (sensors or SPC), and validation practices including work instructions, set-up procedures, calibration programs, and preventative maintenance are all detection ranking improvement approaches.

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 100 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 failure modes requiring action. Here are some of the powerful tools that should be considered:

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.

Design of Experiments (DOE)

A family of powerful statistical improvement techniques that can identify the most critical variables in a process and the optimal settings for these variables.

Simulations

Simulation approaches include pre-production prototypes, computer models, accelerated life tests, and value-engineering analyses.

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Step 9: Take Action

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 PFMEA 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. A good project management software program can be indispensible.

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.

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Step 10: Recalculate the Resulting RPN

This step in a DFMEA confirms the action plan had the desired results by recalculating 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.

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