Applied Reliability Symposium 2009 M Turner

2009 ARS, North America, San Diego2009 ARS, North America, San DiegoTrack 2, Session 2Track 2, Session 2

Begins at 10.30AM, 9 JuneBegins at 10.30AM, 9 June

Current Time:5:22 PM

Improving Organization Improving Organization Competitiveness.Competitiveness.

How Quantitative Accelerated Life How Quantitative Accelerated Life Testing Can Present a Value Testing Can Present a Value

Proposition. Proposition.

Mark Turner, C.R.P.Flextronics International

Mark Turner, C.R.P.Flextronics International

Mark Turner, Flextronics Medical Systems Slide Number: 2Session 2Track 2

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IntroductionIntroduction

Reliability impacts the organization’s bottom line. Accelerated life tests are common, but are

they always successful?

Reliability impacts the organization’s bottom line. Accelerated life tests are common, but are

they always successful?

Reliability engineers have a responsibility to deliver accurate and meaningful data.

o Our results must deliver real value. - Achieving this helps our organizations to become more competitive, and can drive up margins.

- This is vital in a challenging economic climate.

Reliability engineers have a responsibility to deliver accurate and meaningful data.

o Our results must deliver real value. - Achieving this helps our organizations to become more competitive, and can drive up margins.

- This is vital in a challenging economic climate.


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Offer a refined accelerated life test methodology that: Is technically sound. Delivers accurate, meaningful results. Significantly reduces reliability

demonstration test overheads. Reduces product time to market. Drives up margins. Enables our organizations to become

more competitive. Drives warranty costs down and profits

up!

Offer a refined accelerated life test methodology that: Is technically sound. Delivers accurate, meaningful results. Significantly reduces reliability

demonstration test overheads. Reduces product time to market. Drives up margins. Enables our organizations to become

more competitive. Drives warranty costs down and profits

up!

Presentation aimsPresentation aims

$$$


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Two distinct approaches: Accelerated Life Testing (ALT).

o Identifies a product’s life characteristics

Reliability Demonstration Testing (RDT).o Demonstrates whether a minimum reliability target has

been achieved.o Assumes an exponential distribution.o A Parametric Binomial Test approach is often

adopted.

How do we select between each approach

Two distinct approaches: Accelerated Life Testing (ALT).

o Identifies a product’s life characteristics

Reliability Demonstration Testing (RDT).o Demonstrates whether a minimum reliability target has

been achieved.o Assumes an exponential distribution.o A Parametric Binomial Test approach is often

adopted.

How do we select between each approach

Obtaining Reliability MetricsObtaining Reliability Metrics


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RDT MethodologyRDT Methodology

Runtime=[c+(0.667x#Failures)]xMTBF

Weibull++ Design of Reliability Tests utility can also be used.


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A product has a target MTBF of 250,000 hours. We need to conduct a reliability demonstration

test to a confidence level of 70%.o Runtime = [1.2+(0.667x0)]x250,000 = 300,000 hrs.o Using 200 test units, test duration = 1,500 hrs.

During the test a unit fails.o Runtime=[1.2+(0.667x1)]x250,000 = 466,750 hrs.o Revised test duration = 2,334 hrs.

A product has a target MTBF of 250,000 hours. We need to conduct a reliability demonstration

test to a confidence level of 70%.o Runtime = [1.2+(0.667x0)]x250,000 = 300,000 hrs.o Using 200 test units, test duration = 1,500 hrs.

During the test a unit fails.o Runtime=[1.2+(0.667x1)]x250,000 = 466,750 hrs.o Revised test duration = 2,334 hrs.

An RDT ExampleAn RDT Example


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Capacity. Lab space or walk-in chambers.

Cost. Test Samples. Manpower. Data acquisition equipment. Power demands.

But customers often demand a reliability test. Reliability has a direct impact on customer satisfaction,

retention, and ultimately our long term success.

Capacity. Lab space or walk-in chambers.

Cost. Test Samples. Manpower. Data acquisition equipment. Power demands.

But customers often demand a reliability test. Reliability has a direct impact on customer satisfaction,

retention, and ultimately our long term success.

Common RDT ProblemsCommon RDT Problems


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Custom designs. Modular designs.Platform and derivative approach.

Custom designs. Modular designs.Platform and derivative approach.

Product Design StrategiesProduct Design Strategies


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Accelerated Life TestsAccelerated Life Tests

Activation Energy = 0.7eV. Activation energy of each failure mechanism will

differ! Not all components within a system will

experience an equal failure rate acceleration for a given set of conditions.

o The system may be subject to a constant acceleration.o The activation energy of each component failure

mechanism may differ significantly.

Activation Energy = 0.7eV. Activation energy of each failure mechanism will

differ! Not all components within a system will

experience an equal failure rate acceleration for a given set of conditions.

o The system may be subject to a constant acceleration.o The activation energy of each component failure

mechanism may differ significantly.


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An MTBF Estimate will help to identify the most unreliable components. FIT rates may not be totally accurate.

o But identifies the “weak links”

o Provides an opportunity for reliability improvement.

o Indicates that reliability vs. stress may be non- linear.

Failure Rates


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Acceleration ModelingAcceleration Modeling

Elevated stress levels introduce life acceleration.

The ALT aim:o Experience failures at high stress conditions.o Identify acceleration factor.o Extrapolate life at high stress to expected life under

end-usage conditions. Model accuracy

o Dependent on number of stress levels within the test.– 2 levels = linear extrapolation.– >2 levels can identify non-linear life-stress

relationships.

Elevated stress levels introduce life acceleration.

The ALT aim:o Experience failures at high stress conditions.o Identify acceleration factor.o Extrapolate life at high stress to expected life under

end-usage conditions. Model accuracy

o Dependent on number of stress levels within the test.– 2 levels = linear extrapolation.– >2 levels can identify non-linear life-stress

relationships.


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Relies on having an understanding of the product design and the physics of failure. Is not based any life or activation energy assumptions! A better question to ask is:

o What are the most likely components or factors that could cause the product to fail?

Relies on having an understanding of the product design and the physics of failure. Is not based any life or activation energy assumptions! A better question to ask is:

o What are the most likely components or factors that could cause the product to fail?

A Proposed ALT MethodologyA Proposed ALT Methodology

What activation energy

should I use?


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A Proposed QALT MethodologyA Proposed QALT Methodology

Provides flexibility in industrial applications. This methodology will be reviewed in detail, during which an example will be presented…

N

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Y


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Running Example (1)Running Example (1)

Power supply platform design. Temperature identified as main stressor. ALT aim:

o Establish the design’s mean life and failure distribution.

o Design RDT for future derivative products.

ALT limitations: o Only two thermal chambers are available.

Power supply platform design. Temperature identified as main stressor. ALT aim:

o Establish the design’s mean life and failure distribution.

o Design RDT for future derivative products.

ALT limitations: o Only two thermal chambers are available.


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The QALT Methodology Flow (1)The QALT Methodology Flow (1)

Identify limited life components. Estimate their expected life.

• Manufacturers data.• Stress levels.• Standards based reliability

prediction.• Thermal analysis.• Computer simulation. N

N

Y

Conduct Physics of

Failure Review


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Identify stressors, performance limits & design margin. Determine degradation potential – avoid phase change!

Increasing stress

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Y

N

Determine Stress Limits

Conduct Failure Analysis


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PoF Review: Several limited life components: electrolytic

capacitors and some semiconductors.

Preliminary thermal review. Conducted on 5 units.

o All component temperatures monitored.

PoF Review: Several limited life components: electrolytic

capacitors and some semiconductors.

Preliminary thermal review. Conducted on 5 units.

o All component temperatures monitored.

110100908070605040

0 10 20 30Time (hours)

Te

mp

era

ture

(ºC

)

Thermal Profile


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Failure points: 99°C, 100°C, 102°C, 103°C and 105°C. Mean = 101.8°C, Standard deviation = 2.39°C.

Power analysis. Nominal usage temperature (23°C). Output power increased in 1W increments. Failure points:

o 50W, 51W, 52W, 52W and 53W.o Mean = 51.6W, standard deviation = 1.14W.

Test repeated at midpoint of 40°C & 102°C= 71°C, and at 102°C.

Failure points: 99°C, 100°C, 102°C, 103°C and 105°C. Mean = 101.8°C, Standard deviation = 2.39°C.

Power analysis. Nominal usage temperature (23°C). Output power increased in 1W increments. Failure points:

o 50W, 51W, 52W, 52W and 53W.o Mean = 51.6W, standard deviation = 1.14W.

Test repeated at midpoint of 40°C & 102°C= 71°C, and at 102°C.


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The performance variation can then be plotted.

The performance variation can then be plotted.

Temperature (°C)Temperature (°C)

Po

we

r (W

)P

ow

er

(W)

Failure distribution Failure distribution

Maximum stress limit. Remaining below this ensures that any product failures are the result of life acceleration and not due to excessively high stress levels.

Destruct Region

Design Margin

Specification Limits


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Multiple stressors may be used to accelerate a product’s life.

Multiple stressors may be used to accelerate a product’s life. If equipment constraints exist,

identify the most suitable stressors.

o Establish impact of each factor.

o Estimate acceleration factor using PoF.

o Avoid acceleration factors >10-15.

o In design environments Taguchi arrays are ideal.

N

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ConductDOE


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During the DOE each limited life item should be monitored. What impact each stress has on

component life.

o Rule of thumb:

Component life halves for every

10-15°C increase above 40°C.

o Rule of thumb:

Component life halves for every

10-15°C increase above 40°C.

– Determine exact relationship.

o Also consider indirect stress effects.

ConductDOE

Define Operational Limits & estimate

Acceleration Factor


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Design of Experiment. Additional stressors: input power cycling, input

voltage variation and output power. 23 DOE, focusing on how these stressors

affected product wear-out.o How each limited life component performs during

each run.

o Which operating conditions are significant.

o What interactions exist between stressors.

o Input voltage & output power selected from ALT.– This is combined with temperature.

Design of Experiment. Additional stressors: input power cycling, input

voltage variation and output power. 23 DOE, focusing on how these stressors

affected product wear-out.o How each limited life component performs during

each run.

o Which operating conditions are significant.

o What interactions exist between stressors.

o Input voltage & output power selected from ALT.– This is combined with temperature.


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Nominal output power, constant

input voltage

Increased output power, power

cycled

Increased output power, constant

input voltage


…#1 #2 #3 #4 #n

Chamber 1A

Chamber 1B

High Stress 1

High Stress 2

Nominal output power, power

cycled

…#1 #2 #3 #4 #n

...#1 #2 #3 #4 #nChamber 2A

Chamber 2B

…#1 #2 #3 #4 #n

ALT setup Chamber conditions inverse. Acceleration factor based on most critical component –

the weak link!

ALT setup Chamber conditions inverse. Acceleration factor based on most critical component –

the weak link!

Temperature (°C)908070605040

Acc

ele

ratio

n F

act

or

05

101520253035

1A 1B 2A 2B


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N

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– First pass approximation that can be refined later.

Define Sample Size

& QALT duration

It is reasonable to assume a Weibull shape parameter Beta of 1.0. Product population is assumed to

reside in bathtub curve constant failure rate region. Calculate sample quantity using Parametric binomial approach.

o Divide time duration by acceleration factor estimate.

It is reasonable to assume a Weibull shape parameter Beta of 1.0. Product population is assumed to

reside in bathtub curve constant failure rate region. Calculate sample quantity using Parametric binomial approach.

o Divide time duration by acceleration factor estimate.


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Sample size. Dependent on thermal chamber capacity (80 units).

Sample allocation. Inverse ratio of acceleration factor.

o Chamber 1A : 15 units.o Chamber 1B : 12 units.o Chamber 2A : 23 units.o Chamber 2B : 30 units.

Reliability demonstration (70% confidence level):o Chamber 1A : 332 hours.o Chamber 1B : 252 hours.o Chamber 2A : 503 hours.o Chamber 2B : 663 hours.

Sample size. Dependent on thermal chamber capacity (80 units).

Sample allocation. Inverse ratio of acceleration factor.

o Chamber 1A : 15 units.o Chamber 1B : 12 units.o Chamber 2A : 23 units.o Chamber 2B : 30 units.

Reliability demonstration (70% confidence level):o Chamber 1A : 332 hours.o Chamber 1B : 252 hours.o Chamber 2A : 503 hours.o Chamber 2B : 663 hours.


The accelerated life test should be continued to validate the conclusions…

Parametric Binomial test duration without

acceleration required a duration of 3,750 hours


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Derived Product QALT DesignDerived Product QALT Design

Field data from the derived product yields a Weibull shape parameter beta of 1.3. Technology was unchanged, but some minor

changes were made. Accelerated RDT is conducted on further derived

products.o Same thermal chambers and sample quantity.o Acceleration factors will remain unchanged.

– Chamber 1A : 15 units, RDT duration 949 hours.– Chamber 1B : 12 units, RDT duration 719 hours.– Chamber 2A : 23 units, RDT duration 1439 hours.– Chamber 2B : 30 units, RDT duration 1896 hours.

Field data from the derived product yields a Weibull shape parameter beta of 1.3. Technology was unchanged, but some minor

changes were made. Accelerated RDT is conducted on further derived

products.o Same thermal chambers and sample quantity.o Acceleration factors will remain unchanged.

– Chamber 1A : 15 units, RDT duration 949 hours.– Chamber 1B : 12 units, RDT duration 719 hours.– Chamber 2A : 23 units, RDT duration 1439 hours.– Chamber 2B : 30 units, RDT duration 1896 hours.


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Derived Product Accelerated RDTDerived Product Accelerated RDT

Product derived from initial platform design. Life data analysis identified that acceleration

factor for chamber 2B (constant input voltage, nominal output power) was 6.2, not 5.66.

Derived products will be subjected to 70% confidence RDT, rather than repeat QALT.

o 250,000 hour MTBF demonstration would require 130 units for a duration of 16 weeks, if no failures occur.

o This can be reduced using the acceleration factor of 6.2, and running the RDT at 65°C.

Product derived from initial platform design. Life data analysis identified that acceleration

factor for chamber 2B (constant input voltage, nominal output power) was 6.2, not 5.66.

Derived products will be subjected to 70% confidence RDT, rather than repeat QALT.

o 250,000 hour MTBF demonstration would require 130 units for a duration of 16 weeks, if no failures occur.

o This can be reduced using the acceleration factor of 6.2, and running the RDT at 65°C.


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Reliability demonstration test options: Assume maximum RDT capacity is 500 units.

Reliability demonstration test options: Assume maximum RDT capacity is 500 units.

Derived Product Accelerated RDTDerived Product Accelerated RDT

One product to market every 6 weeks.

OR

Five products to market every

4 weeks!


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Can be used to reduce ALT duration.

Higher stress levels increase acceleration factor! To extrapolate between stress levels and usage

condition, a degradation mechanism has to exist.

Can be used to reduce ALT duration.

Higher stress levels increase acceleration factor! To extrapolate between stress levels and usage

condition, a degradation mechanism has to exist.

Step Stress TestsStep Stress Tests

t1 t2 t3

Stress 3

Stress 2

Stress 1f1,d1

f2,d2

f3,d3

F1-3: Probability of failure at stress levels 1-3.

d1-3: Probability of degradation at stress levels 1-3.


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Analysis ModelsAnalysis Models

Constant & step stress tests: Cumulative damage model.

o Enables the generation of contour plots.– Indicates likelihood of all failure modes being identical.

– Requires Alta and Weibull++.

– Note that this results in the Beta from each individual batch being reported in Weibull++, not the overall Beta.

Cyclic stress tests: General Log-Linear model.

Constant & step stress tests: Cumulative damage model.

o Enables the generation of contour plots.– Indicates likelihood of all failure modes being identical.

– Requires Alta and Weibull++.

– Note that this results in the Beta from each individual batch being reported in Weibull++, not the overall Beta.

Cyclic stress tests: General Log-Linear model.

Eta

Beta


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SummarySummary

Reliability demonstration tests. Can take a long time to conduct. Can be expensive due to sample demand.

QALT is ideal for platform designs. Enables tests to be based on PoF, rather than

activation energy assumption. Identified acceleration factor can be used in

derived product reliability demonstrations. Reduces test duration and/or sample needs. Enables organizations to be more competitive.

Reliability demonstration tests. Can take a long time to conduct. Can be expensive due to sample demand.

QALT is ideal for platform designs. Enables tests to be based on PoF, rather than

activation energy assumption. Identified acceleration factor can be used in

derived product reliability demonstrations. Reduces test duration and/or sample needs. Enables organizations to be more competitive.


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Where to Get More InformationWhere to Get More Information

M Turner, “A practical application of quantitative accelerated life testing in power systems engineering,” Soon to be published in the IEEE Transactions on Reliability.

Reliasoft RS 521 Quantitative Accelerated Life Testing.

M Turner, “A practical application of quantitative accelerated life testing in power systems engineering,” Soon to be published in the IEEE Transactions on Reliability.

Reliasoft RS 521 Quantitative Accelerated Life Testing.


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Mark TurnerMark Turner

Senior Staff Reliability Engineer,

Flextronics Medical Systems.

(469) 229-2530

[email protected]

Creating Value That Increases Customer Competitiveness

Senior Staff Reliability Engineer,

Flextronics Medical Systems.

(469) 229-2530

[email protected]

Creating Value That Increases Customer Competitiveness


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QuestionsQuestions

Thank you for your attention.

Q & A?

Thank you for your attention.

Q & A?

Documents

Applied Reliability Symposium 2009 M Turner