Design FMEA (DFMEA) Tutorial
Design FMEA (DFMEA) Tutorial
Design FMEA (DFMEA) Tutorial
These steps of the FMEA will be discussed and explained later in detail, but it is helpful
to see the overall flow, at this overview level, for now. Try to match the additional
following examples to this flow to give you a feel for how Design FMEAs can be used.
For this abbreviated example, there are 4 basic functions of the car door. One
way (failure mode) the door could potentially fail to perform the last 2 basic
functions listed is by the interior lower door panels becoming corroded.
The failure mode could potentially lead to deteriorated door panel life (a failure
mode effect of door panel corrosion), resulting in poor appearance and impaired
function of the interior door hardware.Note: it is acceptable to break down the
effect in a chain of events as shown if it better describes the problem being
worked on. The effect is described from the perspective of the customer,
although the description need not be in the customers language. The severity
of the deteriorated door life effect is rated as a 6 here.
There are 4 potential design-based root causes of the door panel corrosion (the
failure mode here) listed. Note that while these causes may show up in
fabrication or assembly, they are design-based, that is, the designers specify
their values or settings. Each cause receives its own probability of occurrence
rating score, based on the likelihood of the cause happening and its resulting in
the failure mode.
The only design verifications (DVs) presently in place are the Design groups
phase design reviews. The ability of the design reviews to prevent the designbased cause (or detect the potential failure mode) from happening is assessed
for each of the causes listed.
Then the RPN is calculated for each cause so that their corrective/preventive
actions can be prioritized. The highest scores become the highest priorities. The
fact that none of the scores is close to the maximum possible score (1000) does
not mean anything. Just the relative scores are meaningful.
The information developed from the DFMEA will provide excellent input for the
earlier phases of the Concurrent Engineering or Integrated Product
Development processes, and vice versa. Having some timing overlap
(concurrency) between the DFMEA and the PFMEA will further reduce the Time
to Market.
If an existing design, on which there is already a DFMEA, is applied in a
different environment or usage, then the FMEA should be focused on the impact
of the new environment or application.
The DFMEA process will be explained and demonstrated using the numbering
scheme shown on the preceding blank DFMEA form. You may want to
periodically refer back to this blank form as each step and column of the
process is described. There will also be a flowchart that progressively develops
in sequence with the numbered steps.
The function of the part is the task that it must perform to meet its design intent.
it should be described in a
way that is concise, exact and clear to all participants and users
Use a verb-noun format
Examples:
Isolate
electrical
data
signals
Provide
mating
surface
to
part
B
Provide
dielectric
material
for
impedance
Provide
engaging
tab
for
cam
Last
for
40,000
cycles
Operate
within
specified
environmental
conditions
Be manufactured at specified productivity and quality rates Right about now
in the FMEA process would be a good time to perform the Fault Tree Analysis
FTA. Then you would transfer the information to the FMEA form as you go
through the steps. Note that you will likely adding to that information, but it is an
excellent way to make the FMEA more thorough.
Although the DFMEA assumes the part will be manufactured to the design intent, it
should still take into account realistic limitations of the manufacturing and assembly
processes, e.g., clearances for tooling, achievable tolerance capabilities, realistic
parts/hour rates, etc.
Design FMEA Use this blank form to develop your own example.
How
could
this
part
fail?, or
Could it break, deform, wear, corrode, bind, leak, short, open, etc.?
Assume
that the failure could occur for now. Later we will deal with the likelihood of its
occurrence. Refer back to the list of functions when asking the What If
questions. Also refer back to the FTA to help ensure completeness here.
List each potential failure mode for the part. Note that for these examples, the
failure modes are shown following the parts basic functions (optional).
Other possible failure modes could be a part oxidizing, fracturing, sticking,
loosening, not transferring a force, not assembling readily with a mating part,
drift, disengaging during operation, etc.
cannot
assemble
mating
part
*
Worn
mating
surface
mating
part
does
not
engage
* Loss of signal transmission integrity incorrect data transmission
* Tab edge does not engage cam auxiliary bolt sticks
* Cannot assemble lost time and wasted costs in Final Assembly
* Corrosion part loses yield strength, with a lower MTBF
The loss of dielectric is an example of a failure mode resulting in a low
withstanding voltage, which in turn results in the assembly sorting out (the
effect). Using a chain of events is a good technique if it helps you better explain
the failure modes and their effects. Look for the outcomes or consequences of
the failure mode on the part, assembly, other parts, end-user, etc. You should
also
include
safety
or
regulatory
non-compliance
outcomes.
Other examples include unpleasant odor, unstable, regulatory non-compliance,
intermittent
operation,
or
poor
appearance.
Assuming that the failures have occurred, give specific descriptions of the ways
in which customers could observe each failure. Use the perspective of the
external or internal customer, but it isnt necessary to use the customers
terminology
here.
Note that there could be more than one effect for a given failure mode, or, an
effect could be the result of several failure modes. Dont forget to refer back to
your FTA. Note: MTBF stands for mean time between failures.
The severity or estimated consequence of the effect on a 1 to 10 scale, where a
1 is a minor nuisance and a 10 indicates a severe total failure of the system.
When weighing the consequence (effect) of the failure, ask How serious would
the effect of this failure be to the customer, assuming it has appended?
To reduce the severity of the effect of a product failure mode, a part design
action is usually required.
The part design action to reduce a high rating may include design additions or
revisions that mitigate the resultant severity of the failure, e.g., seat belts in a
car. Severity is the first of the 3 Risk Priority Numbers (RPN) rating
criteria. Think of the estimated consequence or seriousness of the effect,
assuming it does exist*, on the external and/or internal customer. If the effect is
critical, the severity is probably high. *Do not consider the likelihood that the
defect will occur in scoring this criterion; this will be evaluated in the second
criterion (Occurrence).
A sample of a Severity ranking scale is shown here:
Severity Ranking
Ranking
Description
The effect of the failure is of such a minor nature as to be undetectable by the customer. For exam
out of
specification on a non-key quality characteristic but not
have any noticeable effect on the system.
2-3
The failures effect is of a minor nature, although it is detectable by the customer, it causes only sl
noticeable
degradation in the system performance.
4-6
The failures effect causes some customer dissatisfaction and some system degradation.
7-9
The failures effect causes major customer dissatisfaction and major system degradation. serious
10
Sudden, catastrophic failure without any prior warning. very serious legal implications.
The company should adopt these sample ratings to what is appropriate to their
industry. This table of a Severity ranking scale provides guidance for assigning criteria
values to the failure mode effects on a consistent basis. It is only a sample and should
not be used without adaptation to the industry and company. The project team should
evaluate each effect and reach a consensus on their ranking values. The idea is to find
comparative differences among the effects so that their associated causes can later be
prioritized. Failure modes with a severity rank of 1 probably do not need to be analyzed
further.
It is an indication of a design weakness, the consequence of which is the failure mode.
Consider the possible design mechanisms and/or causes of each failure
mode. Analyze what conditions can help bring about the failure modes.
Make sure the list of causes is thorough. This helps point the way toward
preventive/corrective
actions
for
all pertinent causes. Consider the difference between effects, contributing
causes, and root causes.
Examples of causes:
* Wrong polymer specified moisture absorption loss of dielectric
* Excess annealing malleable base material alloy bent tab
* Maximum material condition stack-up worn mating surface
* Insufficient gold plating specified loss of signal transmission integrity
* Least material condition stack-up gap Tab edge does not engage cam
Here you are looking for the potential design-based root causes of the potential
failure modes. This includes causes that occur in production but are due to the
design. Do not include causes that are due strictly to errors or flaws in the
production process save these for the PFMEA. Again, refer to the completed
FTA to help ensure the list of causes is as thorough as possible, and that the
root causes are identified. Other possible causes could include improper
material specified for process or end-use operating environment, incorrect
algorithm, incorrect software specification, insufficient re-lubrication capability,
incorrect cam path, etc. Other possible failure mechanisms could include creep,
fatigue, wear, galvanic action, EMI (electromagnetic interference), etc.
* The probability that the cause will happen, and that it will result in the failure
mode, on a 1 to 10 scale. A 1 reflects a remote possibility of it happening and a
10 indicates a high probability of the cause and its failure occurring in major
proportions.
* Ask How likely to happen is the cause? and then How likely is it to result in
the
potential
failure
mode?
* Generally, to reduce the probability of the failure mode happening, a design
change will be required.
Occurrence is the second of the 3 Risk Priority Numbers (RPN) rating
criteria. Think of the relative likelihood or probability of the cause actually
happening during the design life of the part and then resulting in the failure
mode.
One possible source of reference data is the reject and service history for
similar components. A sample of a Probability of Occurrence ranking scale is
shown below:
Description
2-3
Low failure rate with similar parts having similar functions in previous designs or processes.
4-6
Moderate failure rate with similar parts having similar functions in previous designs or processes.
occurred occasionally in the past but not in major proportions
7-9
Frequent failure rate with similar parts having similar functions in previous designs or processes.
10
The company should adopt these sampling ratings to their industry and include
specific probability values. This table of a Probability of Occurrence ranking
scale provides guidance for assigning criteria values to the causes and their
failure modes on a consistent basis. It is only a sample and should not be used
without adaptation to the industry and company.
The project team should evaluate the likelihood of each cause and its associated failure
mode and reach a consensus on their ranking values. The idea is to find comparative
differences among the causes so they can later be prioritized (with the total RPN
scores).
* The design verification steps that have been or are being used with the same or
similar designs.
* List all current design controls which are intended to prevent the specific
design-based
causes(s)
of
the
potential
failure
modes from occurring, or are intended to detect the design-based causes(s) of
the potential failure or the resultant failure mode.
Detectability is the third of the 3 Risk Priority Number (RPN) rating criteria. It is
an assessment of the ability of the DV program to identify a potential design
weakness before release to production. It is a relative ranking (within the FMEA)
of the ability of the DV to assure the design adequacy for the failure mode
and/or cause or failure mechanism under study.
A high score means a low probability of detection, and a low score indicates a
high probability of detection. Because of this possible confusion, some users
call this criterion the Probability of Non-Detection.
A sample of a Probability of Detection ranking scale is shown on the next page.
Ranking
Description
1-2
There is a very high probability that the design weakness, when it exists, will be detected before th
the DV program. A very low probability of non-detection.
3-4
High probability that the design-based cause of the failure will be detected before the parts releas
probability of non-detection.
5-6
Moderate probability that the design-based cause of the failure will be detected before the parts r
moderate probability of non-detection
7-8
Low probability that the design-based cause of the failure will be detected before the parts releas
non-detection.
9-10
Very low probability that the design-based failure will be detected before the parts release to prod
non-detection.
The company should adopt these sample rankings to their industry and include specific
probability values. A low score means that there is a low probability that the failure
mode or defect based on a design weakness, when the failure mode does exist*, will
escape the DV controls before the design is released to Production and the defect has
a chance to reach the customer. *Do not consider the likelihood that the defect will
occur in scoring this criterion; this was evaluated in the second criterion (Occurrence).
A high score for Detectability means that there is a high probability that the
failure mode or defect, when it exists, will escape the DV controls before the
design is released to Production. If there is more than one design control that
could prevent or detect a given failure mode or its cause under study, consider
in the scoring the one that would provide the best prevention or detection.
This summary of alternate ranking criteria can also be used as a guideline for
developing your own standardized set of values. In the table, P refers to the
Probability of an event. For example, for a rank of 6 in Occurrence, the
probability of the cause and the failure mode happening is approximately 1 in
80.
At a level of 6 in Severity (of the failure modes effect), there would be some customer
dissatisfaction, starting to approach a high level with some interoperability of the part.
For a rank of 6 in Detectability, the probability of missing the failure mode would be
somewhere around 1 in 100. These probability values might be appropriate for some
industries but not for others.
To reduce a severity ranking, a design revision will be required. A design revision will
also be required to reduce an occurrence rating, by removing or controlling one or more
of the failure modes causes. Increasing DV actions only is not as desirable since it
does not address the severity or occurrence of the failure mode.
The solutions should either be based on data, or they should call for a trial or
investigation to generate the necessary data for determining the best solution(s)
to solve the root causes. One of the risks in the FMEA process is for the team to
generate Corrective/Preventive Actions that are not proven with observed
data. The Actions should end up calling for revised tolerances, specifications,
etc.
The responsible party should usually be someone on the project team, even
though the person(s) actually doing the tasks may be someone else.
DFMEA Quiz
1. In the DFMEA process, the potential or known failure modes are identified,
then the causes are identified, and then the effects are identified.
Answer:Incorrect. The effects are identified before the causes in the DFMEA
process.
2. A DFMEA would normally contain the causes that occur in production but are
due to the design and causes that are due to errors or flaws in a
production
process
that
was
called
out
by
the
designer.
Answer:Incorrect. A DFMEA should not include causes that are due to
execution
errors
or
flaws in a production process (unless the product cannot be
economically/reasonably manufactured by the process called out by the
designer).
3. It is a good idea to start the PFMEA before the DFMEA is finished.
Answer:
correct
4. The following would be good examples of a part function for a light bulb no
out-of-box
failures
and
provide
bright
light.
Answer:Incorrect. Better examples would be to provide 99.7% defect-free turnon
reliability,
and to provide 75 watts of light for 450 hours. These are more exact, and
clearer
to
the
DFMEA
team.
5. Asking What does the customer experience as a result of the failure
mode of or Will the component or assembly be inoperative,intermittently
operative,
noisy,
not
durable,
etc.?
are
examples
of
questions asked
Answer: Correct
in
the
step
in
which
the
effects
are
determined.
6. Asking what could happen to cause a loss of function, such as How could this part
fail?, or Could it break, deform, wear, corrode, bind, leak, short, open, etc.? are
examples of questions asked in the step in
which the causes are determined.
Answer: Incorrect. These questions should be asked in the step in which failure modes
are identified.
7. When describing the effects, it is important to use the customers terminology.
Answer: Incorrect. Use the perspective of the external or internal customer, but it isnt
necessary to use the customers terminology here.
8. Do not consider the likelihood that the defect will occur in scoring the severity rating
criterion.
Answer: Correct.
9. In the DFMEA process, the potential or known failure modes are identified, then the
causes are identified, and then the effects are identified.
Answer: Incorrect. The effects are identified before the causes in the DFMEA process.
10. Generally, to reduce the probability of the failure mode happening, better design
verifications/controls will be required.
Answer: Incorrect. Generally, to reduce the probability of the failure mode happening, a
design change will be required.
11. For the Occurrence rating, a 1 reflects a remote possibility of it happening and a 10
indicates a high probability of the cause and its failure occurring in major proportions.
Answer: Correct
12. If none of failure mode causes are rated high, e.g., none have RPN scores over 500
or 600, then there is a good chance you missed some key failure modes or causes.
Answer: Incorrect. The absolute RPN scores do not mean anything. The relative
ranking provides prioritization of the causes preventive/corrective action.