Design FMEA (DFMEA) TutorialIn this tutorial, you will
learn:Completing the Design FMEA Form: Identifying the Product
Function Identifying Potential Failure Modes Identifying Potential
Failure Effects Determining the Severity of the Effect Identifying
Potential Cause(s) of the Failure Mode Determining the Probability
of Occurrence of the Failure Mode Identifying Design Verifications
for the Causes Determining the Probability of Non-Detection of the
Failure ModeUsing the Completed DFMEA Form: calculating the Risk
Priority Number (RPN) determining Corrective and Preventive Actions
prioritizing Actions Based on the RPNWhat IsADesignFMEA?* FMEA is a
method for identifying potential or known failure modes and
providing corrective and preventive actions.* The Design FMEA is a
disciplined analysis of the part design with the intent to correct
or prevent the design-based failure modes prior to the first
production run.The FMEA Process:* Identify potential or known
failure modes* Identify the effects and causes of each failure
mode* Prioritize the identified failure modes based on the
frequency of occurrence, the severity of the failure mode, and the
likelihood of detection* Provide for follow-up corrective and
preventive actions* Follow upDesignFMEAProcess Flow
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 asa 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
design-based 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 Design FMEA (DFMEA) needs to be done early in the product
design cycle, after the design concept has been selected since it
needs detailed part functions; it should be continually updated as
the program develops. FMEA can be applied as a supporting method as
part of the Concurrent Engineering development process. Try for
some concurrency between the Design and Process FMEAs.
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.Function of the Part The function of the part is the task
that it must perform to meet its design intent. it should be
described in away that is concise, exact andclear to all
participants and users Use a verb-noun formatExamples: 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
theFMEA 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.
FMEA Process Flow Failure Mode and Effect of Failure
ModePotential Failure Mode of the Function* The ways in which the
part could fail to perform its intended design function.. it should
be described in a negative way.* Stress how the part could fail,
not whether it will fail, to conform to the design specification*
Try asking what could happen to cause a loss of function: How could
this part fail?,or Could it break, deform, wear, corrode, bind,
leak, short, open, etc.?
*Examples of failure modes: Isolate electrical data signals loss
of dielectric Provide mating surface to part B worn mating surface
Provide dielectric material for impedance loss of signal
transmission integrity Provide engaging tab for cam bent tab
Corrosion Cannot assemble Over-deflectionAssumethat the failure
could occurfor 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 forthe 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.FMEA Process Flow Effect
of FailureMode
PotentialEffect(s) of the Potential Failure Mode* The outcome of
the occurrence of the failure mode on the part or assembly
(system).* Describe the effect of the failure specifically, and in
terms of customer reaction:What does the customer (external or
internal) experience as a result of the failure mode ofor Will the
component or assembly be inoperative, intermittently operative,
noisy, not durable, etc.?* Consider whether the effect can also
impact other functionsor parts.Examples of effects:* Loss of
dielectric low withstanding voltage assembly shorts out* Bent tab
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) ofthe 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 orseriousness 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 RankingRankingDescription
1The effect of the failure is of such a minor nature as to be
undetectable by the customer.For example, the part may be out
ofspecification on a non-key quality characteristic but not have
any noticeable effect on the system.
2-3The failures effect is of a minor nature, although it is
detectable by the customer, it causes only slight annoyance without
any noticeabledegradation in the system performance.
4-6The failures effect causes some customer dissatisfaction and
some system degradation.
7-9The failures effect causes major customer dissatisfaction and
major system degradation. serious safety/legal implications.
10Sudden, catastrophic failure without any prior warning. very
serious legal implications.
Thecompany should adopt these sample ratings towhat 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
1probably 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 causesof 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 camHere 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.
Otherpossible 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.* AskHow 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.Occurrenceis the second
of the 3 Risk Priority Numbers (RPN) rating criteria.Think of the
relative likelihood or probability of the cause actuallyhappening
during the design life of the part andthen 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:Probability OfOccurrence
RankingRankingDescription
1Remote possibility of happening.Where similar parts have been
used for similar functions in previous design or processes,
failures have been non-existent.
2-3Low failure rate with similar parts having similar functions
in previousdesigns or processes.
4-6Moderate failure rate with similar parts having similar
functions in previous designs or processes.It is generally a
failure that had occurred occasionally in the past but not in major
proportions
7-9Frequent failure rate with similar parts having similar
functions in previous designs or processes.
10High probability of failure.
The company should adopt these sampling ratings totheir 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.
Examples: design checklists, peer reviews, reliability analysis,
variation stack-up analysis, FEA, rapid prototyping, handoff
checklists, environmental stress analysis, etc. Design controls are
intended to assure the design adequacy for the failure mode and/or
cause under consideration.Asuggested technique when listing the
Design Controls in their column is to add the prefix p to DVs which
are intended to prevent the failure mode cause or the prefix d to
DVs which are intended to detect the failure mode or its cause. It
is not uncommon for a company just beginning to use FMEA to find
that it does not have many DVs.Detectability or Probability of
Detection of the Potential Failure Mode* The probability that the
failure mode caused by the design would be detected by the DV
before the part is released to production on a 1 to 10 scale, where
a 1 indicates a high probability and a 10 indicates a low
probability.* Ask If there is a design weakness, how likely is it
to be detected by our current DVs before the part is released to
production?
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.RankingDescription
1-2There is a very high probability that the design weakness,
when it exists, will be detected before the part is released to
production, usually bythe DV program.A very low probability of
non-detection.
3-4High probability that the design-based cause of the failure
will be detected before the parts release to production, usually by
the DV program.A low probability of non-detection.
5-6Moderate probability that the design-based cause of the
failure will be detected before the parts release to production,
usually by the DV program.A moderate probability of
non-detection
7-8Low probability that the design-based cause of the failure
will be detected before the parts release to production.High
probability ofnon-detection.
9-10Very low probability that the design-based failure will be
detected before the parts releaseto production.Very high
probability of non-detection.
The company should adopt these sample rankings to their
industryand 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 detecta given failure mode or its cause under study,
consider in the scoring the one that would provide the best
prevention or detection.Quantifying FMEA: Risk Assessment
This is an alternate set of rating guidelines.Customized
guidelines should be developed for your industry and company.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 someinteroperability 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.FMEA Risk
Priority Number* The Risk Priority Number (RPN) is calculated by
multiplying together the values of the severity, occurrence, and
detection risk assessmentcriteria.* The RPN provides an indicator
of the relative seriousness and priority of each failure mode.The
higher the RPN, the more relatively serious is the failure
mode.FMEA Risk Priority Number FormulaRPN=Severity x Occurrence
xDetectionAlthoughsome FMEA instruction manuals say the RPN values
are used to rank and compare the seriousness of failure modes, the
scoring is actually done for each cause.The Corrective/Preventive
Actions will be generated for the higher priority causes. The
purpose of generating the RPN scores is to prioritize for which
causes there should be Corrective and/or Preventive Actions
developed and completed.Note: the absolute values of the RPN do not
mean anything.As long as you are consistent in applying the
criteria, then their RPN scores can be usedfor relative ranking and
prioritizing of the causes and their actions within the project.It
is notadvised to compare/rank actions between projects from
separate FMEAs unless the teams followed the same ranking criteria
scales, and did so very consistently.
Recommended Corrective/Preventive Action* The Recommended
Corrective/Preventive Actions for the root causes of the
prioritized failure modes should be described specifically
andbriefly.* The intent of any recommended action is to reduce the
severity, occurrence, and then detection rankings.* It should be
stated in a positive way, e.g., Increase spring wire diameter by
.001 in. or Conduct trials to identify the optimum annealing method
or Develop a more robust engagement tab design.
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.Responsibility Information for the
Corrective/Preventive Action The Recommended Corrective/Preventive
Actions should eachshow the person or department responsible for
resolution, and the EstimatedCompletion Date. The action items
should be reported on and monitored. The actual actions taken later
are described.The severity, occurrence, and detection criteria are
re-evaluated after the actions are taken, then a new RPN is then
calculated.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 Quiz1.In the DFMEA process, the potential
or known failure modes areidentified, 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 aproduction 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: correct4. 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 turn-on 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 oforWill the component or assembly be
inoperative,intermittently operative, noisy, not durable, etc.? are
examples of questions asked in the step in which the effects are
determined.Answer: Correct
6. Asking what could happen to cause a loss of function, such
asHow could this part fail?, orCould 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:
Correct12. 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.
