Feasible Study On Characterization And Comparison Of Accelerated Life Tests And The Compatible Software For Automotive Component.
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CHAPTERI
INTRODUCTION
Thischaptercontainstheprojectbackgr
oundoftheresearchstudywhichentitled“
Feasiblestudy
on characterizationandcomparisonofacceleratedlife testsandthecompatiblesoftwareforautomotive
component”. This chapter is including the background of the proj
ect whereas a little bit of project
summary will be include, objectives of the project’
s research, scope, and problem statement of the
project.
1.1BackgroundofProject
Traditional life data analysis involves
analyzing times-to- failure data (of a product, system or
component)obtainedundernormaloperatingconditio
product, system or component. In many situations, a
nsinordertoquantifythelifecharacteristicsof
the
nd for many reasons, such life data (or times-to-
failuredata)isverydifficult,ifnotimpossible,
toobtain.Thereasonsforthisdifficultycanincl
udethe
longlifetimesoftoday'sproducts,thesmalltime
periodbetweendesignandreleaseandthechallenge
of
testingproductsthatareusedcontinuouslyundern
ormalconditions.
Giventhisdifficulty,andtheneedtoobservefailu
resofproductstobetterunderstandtheirfailure
modes and their life characteristics, reliability p ractitioners have attempted to devise methods to fo
rce
these products to fail more quickly than they would
under normal use conditions. In otherwords, they
haveattemptedtoacceleratetheirfailures.Overth
eyears,thetermacceleratedlifetestinghasbeen
used
todescribeallsuchpractices.
Avarietyofmethodsthatservedifferen
thetermisusedinthisreference,acceleratedlif
purposeofquantificationofthelifecharacteristi
tpurposeshavebeentermed“acceleratedlifetesti
etestinginvolvesaccelerationoffailureswiththe
csoftheproductatnormaluseconditions.
ng.” As
single
Morespecifically,acceleratedlifetestingcanbe
dividedintothreeareas:qualitativeaccelerated
testing, Environmental Stress Screening and quantita tive accelerated life testing. In qualitative
accelerated testing, the engineer is mostly interes
attemptingtomakeanypredictionsastotheproduc
ted in identifying failures and failure modes withou
t’slifeundernormaluseconditions.Inquantitative
t
accelerated life testing, the engineer is intereste
d in predicting the life of the product at normal u
conditions, from data obtained in an accelerated li
fe test. Each type of Accelerated Life Tests (ALT)
analyseswillbeindetailexplanationincludingits
example.
se
1.2ProblemStatement
In the past, life test analysis is used to predict
the life span of components. Normally, it takes
longer to estimate the life span. This can cause pr
oblems to production line whereas it will reduce the
production rate and takes longer to complete a prod
uct. With using accelerated life test analysis, the
usefulreliabilityinformationcouldbeobtainedin
amatterofweeksinsteadofmonths.
Reliability is a very important measure of modern e
lectronic products and its growth and
improvementarevitaltosafeguardingprogressinm
anufacturingindustries.Reliabilityatfirstseems to
be just a number without any meaning to many who do
not have a deep understanding of its proper
application. However, it is the only possible means b
andtestingproceduresmaybemeasured.
y which the robustness of modern manufacturing
An integrated reliability program includes all relia
bility testing programs such as Design
ValidationTesting,AcceleratedLifeTesting,HighlyAcce leratedLifetesting,ReliabilityDemonstration
Testing,andOn-Going ReliabilityTests.Thisshouldbeimplementedinsuch
awaythatallthereliability
needsfortheproductarecoveredandthereisnoe
xtraeffort.Thedesignofanoptimalreliabilityte
sting
program will also help reduce the overall life cycl
e costs of the product while improving the products
reliability.
1.3Objective
Theseanalysisobjectivesare
toanalyzethecharacteristicoftheacceleratedli
tocomparewithAcceleratedLifeTest(ALT)softwares.
otherfactors)onproductlife.
Designacceleratedteststhatwillbethemosteffec
reducethetesttimerequiredtoobtainreliability
fetestanalysisand
Understandandquantifytheeffectsofstress(or
tivetoachievedesiredobjectives.Significantly
metricsforaproduct,whichcanresultinfaster
market,lowerproductdevelopmentcostsandimprove
time-to-
ddesigns.
1.4Scope
Thescopeofthisanalysisis
toobserveandstudythemethodology.Anothermajor
scopeofthis
projectistomakecomparisonsoftheAcceleratedLi
feTests(ALT).An AcceleratedLifeTests(ALT)
software must been chosen to find the compatible ALT software th
at can be used on automotive
component.
CHAPTERII
LITERATUREREVIEW
Inthischapter,thereisbrieflyexplanationregar
dingtothepurposeofthisprojectwhichistofind
thecompatibleAcceleratedLifeTests (ALT)softwareforautomotivecomponent.EachofALTwill
brieflyexplainedincludingitsexample.Thefunctio nsandcomparisonofallALTsoftwarewillbestated.
be
2.1Overview
In typical life data analysis, the practitioner ana
undernormalconditionsinordertoquantifytheli
lyzes life data of a product's sample operating
fecharacteristicsoftheproductandmakepredicti
ons
about all of the products in the population. In lif
e data analysis, the practitioner attempts to make
predictionsaboutthelifeofallproductsinthep
opulationby"fitting"astatisticaldistributiont olifedata
fromarepresentativesampleofunits.Theparameter
izeddistributionforthedatasetcanthenbeused
estimate important life characteristics of the prod uct such as reliability or probability of failure a
specifictime,themeanlifefortheproductandfa
ilurerate.
to
ta
Lifedataanalysisrequiresthepractitionerto:
l
l
l
l
Gatherlifedatafortheproduct.
Selectalifetimedistributionthatwillfittheda
Estimatetheparametersthatwillfitthedistributi
Generateplotsandresultsthatestimatethelifech
product.
taandmodelthelifeoftheproduct.
ontothedata.
aracteristics,likereliabilityormeanlife,ofth
e
2.2AcceleratedLifeTest(ALT)
Eachtypeoftestthathasbeencalledanaccelerate
dtestprovidesdifferentinformationaboutthe
product and its failure mechanisms. Generally, accel erated tests can be divided into three types:
Qualitative Tests (Torture Tests or Shake and Bake Tests
), ESS and Burn-in and finally Quantitative
AcceleratedLifeTests.
2.2.1QualitativeTests
QualitativeTestsareteststhatyieldfailureinform
referredtobymanynamesincluding:
ation(orfailuremodes)only.Theyhavebeen
• ElephantTests
• TortureTests
• HALT(HighlyAcceleratedLifeTesting)
• Shake&BakeTests
Qualitativetestsareperformedonsmallsampleswith
thespecimenssubjectedtoasinglesevere
levelofstress,toanumberofstresses,ortoat
ime-varyingstress(i.e.,stresscycling,coldtohot,
etc.).If
the specimen survives, it passes the test. Otherwise,
appropriate actions will be taken to improve the
product’sdesigninordertoeliminatethecause(s)offail
ure.Qualitativetestsareusedprimarilytoreveal
probable failure modes. However, if not designed prop
erly, they may cause the product to fail due to
modesthatwouldhaveneverbeenencounteredinreal
life.
Agoodqualitativetestisonethatquicklyreveals
thosefailuremodesthatwilloccurduringthe
lifeoftheproductundernormaluseconditions.In
general,qualitativetestsarenotdesignedto
yieldlife
data that can be used in subsequent analysis or for
“Accelerated Life Test Analysis.”
In general,
qualitative tests do not quantify the life (or reli
ability) characteristics of the product under norma
conditions.HighlyAcceleratedLifeTesting(HALT)ischos
enunderthistopicforfurtherdiscussion.
2.2.1.1HighlyAcceleratedLifeTesting(HALT)
l use
Inrecentyears,thetesttechniquesknownasHALT(High
lyAcceleratedLifeTesting)andHASS
(HighlyAcceleratedStressScreening)havebeengain ingadvocatesandpractitioners.Thesetestmethods,
quite different from standard life testing, design
verification testing and end-of- production testing, are
becoming recognized as powerful tools for improving
increasingcustomersatisfaction.
AHighlyAcceleratedLifeTest(HALT)isastr
esstestingmethodology foracceleratingproduct
Itiscommonlyappliedtoelectronicequipmentand
is
weaknesses in newly- developed equipment. Thus it
reliabilityduringtheengineeringdesignprocess.
performed to identify and thus help resolve design
greatlyreducestheprobabilityofin-servicefailu
Progressively more severe environmental
product reliability, reducing warranty costs and
resi.e.itincreasestheproduct'sreliability.
stresses are applied building to a level significantly
beyondwhattheequipmentwillseein-service.
Bythismethodweaknessescanbeidentifiedusinga
smallnumberofsamples(sometimesoneor
twobutpreferablyatleastfive)intheshortestpo
ssibletimeandatleastexpense.Asecondfunction
of
HALT testing is that it characterizes the equipment un
operatinglimitsanddesignmargins.Individualcom
electronicsystemscanbesubjectedtoHALTtesting.
der test, and identifies the equipment's safe
ponents,populatedprintedcircuitboards,andwhole
The size of the test sample is governed by many fact
available, cost, type of stresses applied, and phys
ors including the number of samples
ical size. For example, component manufacturers can
typically test thousands of individual components a t one time whereas often it is not economically
feasibletowriteoffmorethanafewitemsofvery
expensiveequipmentbecauseproductionquantitieso
r
theapplicationdoesnotjustifythecost.
AgeneralprincipalisthatwhilstHALTtestcanandshou
ldbeconductedatunitlevel,itisvery
desirable to conduct it at sub-assembly and piecepart level as well. Temperature cycling and random
vibration, power margining and power cycling are the
most common form of failure acceleration for
electronic equipment. HALT does not measure or determi
ne equipment reliability but it does serve to
improvethereliabilityofaproduct.Itisanempi
ricalmethodusedacrossindustrytoidentifythel
imiting
failuremodesofaproductandthestressesatwhich
thesefailuresoccur.
On military design and development programs HALT is cond
ucted before qualification testing.
Bysodoing,significantcostsavingscanaccrue,b
ecausetheformalqualificationoftheequipmentan
subsequentcustomeracceptancewillproceedmorerap idlyandatlowercost,andtheneedformultiple
redesignsandrepeattesting(regressiontests)will begreatlyreducedoreliminated.
d
ThereisnoonerecipeforaHALTtest,butdifferentst
ressesareappliedwithdifferentfailures
occurringduringeach.Thetypesofstresstypically
employedare:
1. ColdStep
2. HotStep
3. RapidTemperatureCycling
4. SteppedVibration(random)
5. Combined Environment stress (temperature cycling and
random vibration plus power switching
andpowermargining)
InHALTthesestressesareappliedinacontrolled,in
continuously monitored for failures. Once the weaknes
actions taken, the limits of the product are clearl
extendedasfaraspossible.Amuchmorematurepro
higherdegreeofreliability.
crementalfashionwhiletheunitundertestis
ses of the product are uncovered and corrective
y understood and the operating margins have been
ductcanbeintroducedmuchmorequicklywitha
When HALT testing is applied during the design process
, it can produce a very robust product
withoutunduecost,becauseimprovementsaretargete donlywheretheyareneeded.Asfailuremodesare
discoveredandunderstoodtheproductlifecanincr
easesignificantly.Thismakestheproductmorerobu
st
andriskoffailurereducesdrastically.
Individualcomponentssuchasresistors
,capacitors,anddiodes,printedcircuitboards
,andwhole
electronicproductssuchascellphones,PDAs,andTV
s,eventuallyfailatdifferentratesunderdifferen
end-user stress levels. For example, a typical cons
umer-owned television will not likely be operated at
t
temperatures outside the range of normal living acc
ommodations, or subjected to mechanical stress by
being repeatedly dropped. A cell phone, on the other
hand, may be dropped from 3 or 4 feet off the
groundfairlyoften,andsubjectedtoavaryingran
geofvibrations.
Acommercialtelephoneswitchmayberequiredtooper
ateinremoteinstallationsrangingfrom
Barrow, Alaska to Phoenix, Arizona, at an ambient tem
perature range of from minus 50 to plus 120
degrees Fahrenheit. Components used in military and
aerospace applications may be subjected to even
more severe operating temperature requirements as we
ll as high G- forces and ionizing radiation,
sometimessimultaneously,tomeetspecificMIL-SPEC standards.
Therefore,failureratedataused to select anydev
iceinaproductmust correlatewith thestress
levelsintheproductorapplication.Toaccomplish
this,therequiredlifetimeandoperatingcondition sfor
theproductintowhichthecomponentsaredesignedm
ustfirstbedetermined.Forinstance,intheabove
examples, the television set may only be required t
telephoneswitchmayberequiredtooperatewithoutb
o operate through its warranty period, whereas the
eingservicedfortenyearsormore.
Componentsusedinamissilemayonlyberequiredt
ooperateforafewhoursoftestingandafew
minutesofactualuse,buttheirfailureratewillb
eexpectedtobezeroduringthatperiod.Devicesu
sedin
satellitesorspacevehicleswherereplacementisn
otpossibleareexpectedtohaveazerofailurerat
efor
the lifetime of the vehicle. Knowing the required fai
lure rate as determined by the application,
components can be selected based on the failure rat
above.
e data supplied by the manufacturer as described
AtestchamberdesignedforHALTtestingisreallyrequ
iredforsatisfactoryandsuccessfulHALT
testing.Atemperatureramprateofatleast45degr
eesC/minuteisrequired,andsomeHALTchambers
canachievecloserto60degreesC/minute.Toachie
evaporationofliquidnitrogenisrequired.Chamber
vethesehighramp- rates,coolingbymeansofthe
sequippedwithoneortwocompressorsforcooling,
suchasthosetypicallyemployedduringqualificati ontesting,arereallynotadequate.Suchchambers
are
typically only capable of 15- 18 degrees C/minute, and are subject to frequent br
eakdowns if always
pushedtotheirlimit.
Asuitablechamberalsohastobecapableofapplyin
grandomvibrationwithasuitablespectral
densityprofileinrelationtofrequency.Ithasto
becapableofdoingthisatthesametimeasitap
plies
temperaturecyclingsoacombinedchamberisessent
ialratherthanseparatechambersforvibrationand
temperaturecycling.Whereaschambersusedforqual ificationtestingoftenuseelectro- dynamicvibrators
(like a giant moving- coil loudspeaker). Chambers designed for HALT testing
use pneumatic hammers
arrangedtostrikethebaseplateuponwhichtheequ
ipmentismounted.Thehammersproducesixdegree
of freedom random vibration: they produce simultane
ous vibration in each principle axis and rotations
abouteachaxis.Thiscompareswiththesingleaxiso
fvibrationprovidedbyelectro-dynamicvibrators.
2.2.2EnvironmentalStressScreening(ESS)andBurn
The second type of accelerated test cons
-In
ists of ESS and Burn-
In testing. ESS is a process
involving the application of environmental stimuli
to products (usually electronic or electromechanica l
products) on an accelerated basis. The surviving population, upon completion of screeni ng, can be
assumedtohaveahigherreliabilitythanasimilar
unscreenedpopulation.ThegoalofESSistoexpose,
identifyandeliminatelatentdefectswhichcannot
bedetectedbyvisualinspectionorelectricaltest
ingbut
which will cause failures in the field. ESS is perform
ed on the entire population and does not involve
sampling.
Developedtohelpelectronicsmanufacturersdetect
widelyusedinmilitaryandaerospaceapplications,
elaborate,forexample,switchinganelectronicore
productdefectsandproductionflaws,ESSis
lesssoforcommercialproducts.Thetestsneednot
lectricalsystemonandoffafewtimesmaybeenoug
to catch some simple defects that would otherwise be
productwasfirstused.
be
h
encountered by the end user very soon after the
Teststypicallyincludethefollowing:
·
Temperaturevariations
·
Vibrationtests
Pressure
·
·
Flexibilitytests
ESS can be performed as part of the manufacturing pr
qualificationtesting.AnESSsystemusuallyconsists
andwiring,andafunctionaltester.Thesesystemsca
ocess or it can be used in new product
ofatestchamber,controller,fixturing,intercon
nect
nbepurchasedfromavarietyofcompaniesinthe
environmentaltestindustry.
Thestressscreeningfromthisprocesswillhelpfind
infantmortalityintheproduct.Findingthese
failures before the product reaches the customer yi
elds better quality and lower warranty expenses.
Associated military terminology includes an operatio nal requirements document (ORD) and on- going
reliabilitytesting(ORT).
To conduct an effective screen, the product must be
capable of surviving the high stimulation
levelsneededtoacceleratethefailuremechanismo
fassemblyrelateddefects.Theparticipationofdes
ign
andreliabilityengineeringistodeterminethelim
itsofenvironmentalstimulationwhichtheproductc
an
endurebeforeitsperformanceispermanentlydegrad ed.Amechanically“weak” designmaybechanged
toimproveitsmarginswithrespecttoaspecificf
ormofenvironmentalstimulus.
TheformofESSchosenbythefactoryisdependenton
thefailuremechanismsfortherelevant
fieldfailures.AnESSprogramisfaultywhenitdoes
notexposethelocusoffaultsseenbythecustomer
TheESSprocessisadynamicprocesswhichmustchange
reason, it is not appropriate to “spec”
productlife.
asproductfailurebehaviorchanges.Forthis
an ESS regimen and leave the regimen unchanged throu
ghout
Again,thepurposeofanyenvironmentalstressscree
ning(ESS)regimenistoexposethehidden
defects that wereintroduced during themanufacturin g process. Moresuccinctly, manufacturing defects
areprecipitatedfromlatenttopatent.
.
ESS, however, is not designed to find deficiencies in
product design although in many cases it
does expose design deficiencies. Rather than design
an ESS program to find design weaknesses, an
importantingredientofproductqualificationmust
betheundertakingofenvironmentaltestingtoensu
that the design is robust enough to meet its design
componenttechnologyandassemblytechniqueshavec
re
goals. As the electronics industry has matured,
hangedprofoundly.
Thefirstproducttobesubjectedtoenv
ironmentalstressscreeningintheformof“burn-in
” was
productsdesignedwithvacuumtubetechnology.Burn- incanberegardedasaspecialcaseofESS.For
these products, high temperature burn- in was the best stimuli for precipitating latent def
ects. Early
transistorandintegratedcircuittechnologydefect
swerealsoefficientlystimulatedtofailureusing
temperatureburn- in.Duringthistime,mostproductdefectswerecompo
seeamuchdifferentfailurebehaviorforelectroni
mostproductdefectsarerelatedtotheassemblypr
high
nentrelateddefects.Todaywe
cproducts.Componentshavebecomesoreliablethat
ocess.Astheproductfaultspectrumhaschanged,th
screening stimuli for rapid defect precipitation mu
efficientlystimulatedtoanobservablefailure.
e
st also change to ensure that latent defects are
According to MIL-STD-883C, Burnin is a test performed for the purpose of screening
eliminatingmarginaldevices.Marginaldevicesare
thosewithinherentdefectsordefectsresultingfro
or
m
manufacturing aberrations which cause time and stre
ss-dependent failures. As with ESS, Burn- in is
performedontheentirepopulation.Theintentionis
todetectthoseparticularcomponentsthatwouldfa
il
as a result of the initial, high-failure rate porti
on of the bathtub curve of component reliability
. If the
burn-in period is made sufficiently long (and, perhaps,
artificially stressful), the system can then be
trustedtobemostlyfreeoffurtherearlyfailures
oncetheburn-inprocessiscomplete.
Apreconditionforasuccessfulburn-in
isabathtub- likefailurerate,thatis,therearenoticeable
early failures with a decreasing failure rate follow
ing that period. By stressing all devices for acer
tain
burn-in time the devices with the highest failure rate fa
il first and can be taken out of the cohort. The
devices that survive the stress have a later positi
ongoingfailurerate).
Thusbyapplyingaburn-in,earlyinofareducedyieldcausedbytheburn-
usesystemfailurescanbeavoidedattheexpense(
inprocess.Whentheequivalentlifetimeofthestr
into the increasing partof thebathtub-likefailur
productlifetime.Inamatureproductionitisnot
rate. Todeterminethefailuretimedistributionfor
havetodestroyaverylargenumberofdevices.
When possible, it is better to eliminat
on in the bathtub curve (with an appropriately lower
tradeoff)
essisextended
e-rate curve, the effect oftheburn- in is areduction of
easytodeterminewhetherthereisadecreasingfail
ure
avery low percentageoftheproduction, onewould
e the root cause of early failures than doing a bur
n-in.
Becauseofthis,aprocessthatinitiallyusesburn
forfailuresareidentifiedandeliminated.Forele
-inmayeventuallyphaseitoutasthevariousroot
causes
ctroniccomponents,burn- inisfrequentlyconductedat
elevated temperature and perhaps elevated voltage.
componentsmaybeundercontinuoustestorsimplyt
This process may also be called heat soaking
estedattheendoftheburn-inperiod.
. The
Thereisanotheruseofthetermbysome
audiophiles ,wholeavenewaudioequipmentturnedon
for multiple days or weeks, to get the components to
achieve optimal performance. However, many
debatesariseaboutthebeneficialeffectsofthis
practice.
2.2.3QuantitativeAcceleratedLifeTests
QuantitativeAcceleratedLifeTesting,unlikethequali
tativetestingmethods(i.e.,TortureTests,
Burn-in, etc.) described previously, consists of quantit ative tests designed to quantify the life
characteristics of the product, component or system
under normal use conditions, and thereby provide
“Reliability Information.” Reliability information can include the determinati on of the probability of
failure of the product under use conditions, mean l
ife under use conditions, and projected returns and
warrantycosts.Itcanalsobeusedtoassistinthe
performanceofriskassessments,designcomparison
etc.AcceleratedLifeTestAnalysis(ALTA)softwarehasbeen
s,
chosenforQALT.
2.2.3.1AcceleratedLifetestAnalysis(ALTA)
Accelerated Life Testing can take the form
of “Usage Rate Acceleration” or “
Overstress
Acceleration.” Both Accelerated Life Test methods are d
escribed next. Because “ Usage Rate
Acceleration” test data can be analyzed with typical life data an
alysis methods, the Overstress
Acceleration method is the testing method. For all l
productisrequiredsincethefailureoftheproduc
ife tests, some time-to- failure information for the
tistheeventwewanttounderstand.
In other words, if we wish to understand, measure, and
predict any event. Most products,
components or systems are expected to perform their
functions successfully for long periods of time,
competitive,thetimerequiredtoobtaintimes-to- failuredatamustbeconsiderablylessthantheexp
ected
lifeoftheproduct.
Two methods of acceleration, “Usage Rate
beendevisedtoobtaintimesto-failuredataatan
Acceleration” and “Overstress Acceleration,”
have
acceleratedpace.
For products that do not operate continuously, one
can accelerate the time it takes to induce
failures by continuously testing theseproducts. Thi s is called “Usage Rate Acceleration.”
For products
forwhich“UsageRateAcceleration” isimpractical,onecanapplystressatlevelsthat
exceedthelevels
that aproduct willencounterundernormalusecondi
tionsandusethetimes- to failuredataobtainedin
thismannertoextrapolatetouseconditions.Thisi
scalled“OverstressAcceleration.”
i)UsageRateAcceleration
For products that do not operate continuously under
normal conditions, if the test units are
operatedcontinuously,failuresareencounteredear lierthaniftheunitsweretestedatnormalusage.
For
example, amicrowaveoven operates forsmall periods
oftimeevery day. One can accelerateatest on
microwaveovensbyoperatingthemmorefrequentlyun
weassumeanaveragewasheruseof6hoursaweek,on
tilfailure.Thesamecouldbesaidofwashers.If
ecouldconceivablyreducethetestingtime28-
fold by testing these washers continuously. Data obta
ined through usage acceleration can be analyzed
with the same methods used to analyze regular timesto-failure data. The limitation of “ Usage Rate
Acceleration” arises when products, such as computer servers and p
eripherals, maintain a very high or
even continuous usage. In such cases, usage acceler
ation, even though desirable, is not a feasible
alternative. In these cases the practitioner must s
timulate the product to fail, usually through the
applicationofstress.Thismethodofacceleratedli
fetestingiscalled“OverstressAcceleration”.
ii)OverstressAcceleration
For products with very high or continuo
stimulatetheproducttofailinalifetest.
us usage, the accelerated life- testing practitioner must
Thisisaccomplishedbyapplyingstressthatexceeds
thestressthataproductwillencounterunder
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CHAPTERI
INTRODUCTION
Thischaptercontainstheprojectbackgr
oundoftheresearchstudywhichentitled“
Feasiblestudy
on characterizationandcomparisonofacceleratedlife testsandthecompatiblesoftwareforautomotive
component”. This chapter is including the background of the proj
ect whereas a little bit of project
summary will be include, objectives of the project’
s research, scope, and problem statement of the
project.
1.1BackgroundofProject
Traditional life data analysis involves
analyzing times-to- failure data (of a product, system or
component)obtainedundernormaloperatingconditio
product, system or component. In many situations, a
nsinordertoquantifythelifecharacteristicsof
the
nd for many reasons, such life data (or times-to-
failuredata)isverydifficult,ifnotimpossible,
toobtain.Thereasonsforthisdifficultycanincl
udethe
longlifetimesoftoday'sproducts,thesmalltime
periodbetweendesignandreleaseandthechallenge
of
testingproductsthatareusedcontinuouslyundern
ormalconditions.
Giventhisdifficulty,andtheneedtoobservefailu
resofproductstobetterunderstandtheirfailure
modes and their life characteristics, reliability p ractitioners have attempted to devise methods to fo
rce
these products to fail more quickly than they would
under normal use conditions. In otherwords, they
haveattemptedtoacceleratetheirfailures.Overth
eyears,thetermacceleratedlifetestinghasbeen
used
todescribeallsuchpractices.
Avarietyofmethodsthatservedifferen
thetermisusedinthisreference,acceleratedlif
purposeofquantificationofthelifecharacteristi
tpurposeshavebeentermed“acceleratedlifetesti
etestinginvolvesaccelerationoffailureswiththe
csoftheproductatnormaluseconditions.
ng.” As
single
Morespecifically,acceleratedlifetestingcanbe
dividedintothreeareas:qualitativeaccelerated
testing, Environmental Stress Screening and quantita tive accelerated life testing. In qualitative
accelerated testing, the engineer is mostly interes
attemptingtomakeanypredictionsastotheproduc
ted in identifying failures and failure modes withou
t’slifeundernormaluseconditions.Inquantitative
t
accelerated life testing, the engineer is intereste
d in predicting the life of the product at normal u
conditions, from data obtained in an accelerated li
fe test. Each type of Accelerated Life Tests (ALT)
analyseswillbeindetailexplanationincludingits
example.
se
1.2ProblemStatement
In the past, life test analysis is used to predict
the life span of components. Normally, it takes
longer to estimate the life span. This can cause pr
oblems to production line whereas it will reduce the
production rate and takes longer to complete a prod
uct. With using accelerated life test analysis, the
usefulreliabilityinformationcouldbeobtainedin
amatterofweeksinsteadofmonths.
Reliability is a very important measure of modern e
lectronic products and its growth and
improvementarevitaltosafeguardingprogressinm
anufacturingindustries.Reliabilityatfirstseems to
be just a number without any meaning to many who do
not have a deep understanding of its proper
application. However, it is the only possible means b
andtestingproceduresmaybemeasured.
y which the robustness of modern manufacturing
An integrated reliability program includes all relia
bility testing programs such as Design
ValidationTesting,AcceleratedLifeTesting,HighlyAcce leratedLifetesting,ReliabilityDemonstration
Testing,andOn-Going ReliabilityTests.Thisshouldbeimplementedinsuch
awaythatallthereliability
needsfortheproductarecoveredandthereisnoe
xtraeffort.Thedesignofanoptimalreliabilityte
sting
program will also help reduce the overall life cycl
e costs of the product while improving the products
reliability.
1.3Objective
Theseanalysisobjectivesare
toanalyzethecharacteristicoftheacceleratedli
tocomparewithAcceleratedLifeTest(ALT)softwares.
otherfactors)onproductlife.
Designacceleratedteststhatwillbethemosteffec
reducethetesttimerequiredtoobtainreliability
fetestanalysisand
Understandandquantifytheeffectsofstress(or
tivetoachievedesiredobjectives.Significantly
metricsforaproduct,whichcanresultinfaster
market,lowerproductdevelopmentcostsandimprove
time-to-
ddesigns.
1.4Scope
Thescopeofthisanalysisis
toobserveandstudythemethodology.Anothermajor
scopeofthis
projectistomakecomparisonsoftheAcceleratedLi
feTests(ALT).An AcceleratedLifeTests(ALT)
software must been chosen to find the compatible ALT software th
at can be used on automotive
component.
CHAPTERII
LITERATUREREVIEW
Inthischapter,thereisbrieflyexplanationregar
dingtothepurposeofthisprojectwhichistofind
thecompatibleAcceleratedLifeTests (ALT)softwareforautomotivecomponent.EachofALTwill
brieflyexplainedincludingitsexample.Thefunctio nsandcomparisonofallALTsoftwarewillbestated.
be
2.1Overview
In typical life data analysis, the practitioner ana
undernormalconditionsinordertoquantifytheli
lyzes life data of a product's sample operating
fecharacteristicsoftheproductandmakepredicti
ons
about all of the products in the population. In lif
e data analysis, the practitioner attempts to make
predictionsaboutthelifeofallproductsinthep
opulationby"fitting"astatisticaldistributiont olifedata
fromarepresentativesampleofunits.Theparameter
izeddistributionforthedatasetcanthenbeused
estimate important life characteristics of the prod uct such as reliability or probability of failure a
specifictime,themeanlifefortheproductandfa
ilurerate.
to
ta
Lifedataanalysisrequiresthepractitionerto:
l
l
l
l
Gatherlifedatafortheproduct.
Selectalifetimedistributionthatwillfittheda
Estimatetheparametersthatwillfitthedistributi
Generateplotsandresultsthatestimatethelifech
product.
taandmodelthelifeoftheproduct.
ontothedata.
aracteristics,likereliabilityormeanlife,ofth
e
2.2AcceleratedLifeTest(ALT)
Eachtypeoftestthathasbeencalledanaccelerate
dtestprovidesdifferentinformationaboutthe
product and its failure mechanisms. Generally, accel erated tests can be divided into three types:
Qualitative Tests (Torture Tests or Shake and Bake Tests
), ESS and Burn-in and finally Quantitative
AcceleratedLifeTests.
2.2.1QualitativeTests
QualitativeTestsareteststhatyieldfailureinform
referredtobymanynamesincluding:
ation(orfailuremodes)only.Theyhavebeen
• ElephantTests
• TortureTests
• HALT(HighlyAcceleratedLifeTesting)
• Shake&BakeTests
Qualitativetestsareperformedonsmallsampleswith
thespecimenssubjectedtoasinglesevere
levelofstress,toanumberofstresses,ortoat
ime-varyingstress(i.e.,stresscycling,coldtohot,
etc.).If
the specimen survives, it passes the test. Otherwise,
appropriate actions will be taken to improve the
product’sdesigninordertoeliminatethecause(s)offail
ure.Qualitativetestsareusedprimarilytoreveal
probable failure modes. However, if not designed prop
erly, they may cause the product to fail due to
modesthatwouldhaveneverbeenencounteredinreal
life.
Agoodqualitativetestisonethatquicklyreveals
thosefailuremodesthatwilloccurduringthe
lifeoftheproductundernormaluseconditions.In
general,qualitativetestsarenotdesignedto
yieldlife
data that can be used in subsequent analysis or for
“Accelerated Life Test Analysis.”
In general,
qualitative tests do not quantify the life (or reli
ability) characteristics of the product under norma
conditions.HighlyAcceleratedLifeTesting(HALT)ischos
enunderthistopicforfurtherdiscussion.
2.2.1.1HighlyAcceleratedLifeTesting(HALT)
l use
Inrecentyears,thetesttechniquesknownasHALT(High
lyAcceleratedLifeTesting)andHASS
(HighlyAcceleratedStressScreening)havebeengain ingadvocatesandpractitioners.Thesetestmethods,
quite different from standard life testing, design
verification testing and end-of- production testing, are
becoming recognized as powerful tools for improving
increasingcustomersatisfaction.
AHighlyAcceleratedLifeTest(HALT)isastr
esstestingmethodology foracceleratingproduct
Itiscommonlyappliedtoelectronicequipmentand
is
weaknesses in newly- developed equipment. Thus it
reliabilityduringtheengineeringdesignprocess.
performed to identify and thus help resolve design
greatlyreducestheprobabilityofin-servicefailu
Progressively more severe environmental
product reliability, reducing warranty costs and
resi.e.itincreasestheproduct'sreliability.
stresses are applied building to a level significantly
beyondwhattheequipmentwillseein-service.
Bythismethodweaknessescanbeidentifiedusinga
smallnumberofsamples(sometimesoneor
twobutpreferablyatleastfive)intheshortestpo
ssibletimeandatleastexpense.Asecondfunction
of
HALT testing is that it characterizes the equipment un
operatinglimitsanddesignmargins.Individualcom
electronicsystemscanbesubjectedtoHALTtesting.
der test, and identifies the equipment's safe
ponents,populatedprintedcircuitboards,andwhole
The size of the test sample is governed by many fact
available, cost, type of stresses applied, and phys
ors including the number of samples
ical size. For example, component manufacturers can
typically test thousands of individual components a t one time whereas often it is not economically
feasibletowriteoffmorethanafewitemsofvery
expensiveequipmentbecauseproductionquantitieso
r
theapplicationdoesnotjustifythecost.
AgeneralprincipalisthatwhilstHALTtestcanandshou
ldbeconductedatunitlevel,itisvery
desirable to conduct it at sub-assembly and piecepart level as well. Temperature cycling and random
vibration, power margining and power cycling are the
most common form of failure acceleration for
electronic equipment. HALT does not measure or determi
ne equipment reliability but it does serve to
improvethereliabilityofaproduct.Itisanempi
ricalmethodusedacrossindustrytoidentifythel
imiting
failuremodesofaproductandthestressesatwhich
thesefailuresoccur.
On military design and development programs HALT is cond
ucted before qualification testing.
Bysodoing,significantcostsavingscanaccrue,b
ecausetheformalqualificationoftheequipmentan
subsequentcustomeracceptancewillproceedmorerap idlyandatlowercost,andtheneedformultiple
redesignsandrepeattesting(regressiontests)will begreatlyreducedoreliminated.
d
ThereisnoonerecipeforaHALTtest,butdifferentst
ressesareappliedwithdifferentfailures
occurringduringeach.Thetypesofstresstypically
employedare:
1. ColdStep
2. HotStep
3. RapidTemperatureCycling
4. SteppedVibration(random)
5. Combined Environment stress (temperature cycling and
random vibration plus power switching
andpowermargining)
InHALTthesestressesareappliedinacontrolled,in
continuously monitored for failures. Once the weaknes
actions taken, the limits of the product are clearl
extendedasfaraspossible.Amuchmorematurepro
higherdegreeofreliability.
crementalfashionwhiletheunitundertestis
ses of the product are uncovered and corrective
y understood and the operating margins have been
ductcanbeintroducedmuchmorequicklywitha
When HALT testing is applied during the design process
, it can produce a very robust product
withoutunduecost,becauseimprovementsaretargete donlywheretheyareneeded.Asfailuremodesare
discoveredandunderstoodtheproductlifecanincr
easesignificantly.Thismakestheproductmorerobu
st
andriskoffailurereducesdrastically.
Individualcomponentssuchasresistors
,capacitors,anddiodes,printedcircuitboards
,andwhole
electronicproductssuchascellphones,PDAs,andTV
s,eventuallyfailatdifferentratesunderdifferen
end-user stress levels. For example, a typical cons
umer-owned television will not likely be operated at
t
temperatures outside the range of normal living acc
ommodations, or subjected to mechanical stress by
being repeatedly dropped. A cell phone, on the other
hand, may be dropped from 3 or 4 feet off the
groundfairlyoften,andsubjectedtoavaryingran
geofvibrations.
Acommercialtelephoneswitchmayberequiredtooper
ateinremoteinstallationsrangingfrom
Barrow, Alaska to Phoenix, Arizona, at an ambient tem
perature range of from minus 50 to plus 120
degrees Fahrenheit. Components used in military and
aerospace applications may be subjected to even
more severe operating temperature requirements as we
ll as high G- forces and ionizing radiation,
sometimessimultaneously,tomeetspecificMIL-SPEC standards.
Therefore,failureratedataused to select anydev
iceinaproductmust correlatewith thestress
levelsintheproductorapplication.Toaccomplish
this,therequiredlifetimeandoperatingcondition sfor
theproductintowhichthecomponentsaredesignedm
ustfirstbedetermined.Forinstance,intheabove
examples, the television set may only be required t
telephoneswitchmayberequiredtooperatewithoutb
o operate through its warranty period, whereas the
eingservicedfortenyearsormore.
Componentsusedinamissilemayonlyberequiredt
ooperateforafewhoursoftestingandafew
minutesofactualuse,buttheirfailureratewillb
eexpectedtobezeroduringthatperiod.Devicesu
sedin
satellitesorspacevehicleswherereplacementisn
otpossibleareexpectedtohaveazerofailurerat
efor
the lifetime of the vehicle. Knowing the required fai
lure rate as determined by the application,
components can be selected based on the failure rat
above.
e data supplied by the manufacturer as described
AtestchamberdesignedforHALTtestingisreallyrequ
iredforsatisfactoryandsuccessfulHALT
testing.Atemperatureramprateofatleast45degr
eesC/minuteisrequired,andsomeHALTchambers
canachievecloserto60degreesC/minute.Toachie
evaporationofliquidnitrogenisrequired.Chamber
vethesehighramp- rates,coolingbymeansofthe
sequippedwithoneortwocompressorsforcooling,
suchasthosetypicallyemployedduringqualificati ontesting,arereallynotadequate.Suchchambers
are
typically only capable of 15- 18 degrees C/minute, and are subject to frequent br
eakdowns if always
pushedtotheirlimit.
Asuitablechamberalsohastobecapableofapplyin
grandomvibrationwithasuitablespectral
densityprofileinrelationtofrequency.Ithasto
becapableofdoingthisatthesametimeasitap
plies
temperaturecyclingsoacombinedchamberisessent
ialratherthanseparatechambersforvibrationand
temperaturecycling.Whereaschambersusedforqual ificationtestingoftenuseelectro- dynamicvibrators
(like a giant moving- coil loudspeaker). Chambers designed for HALT testing
use pneumatic hammers
arrangedtostrikethebaseplateuponwhichtheequ
ipmentismounted.Thehammersproducesixdegree
of freedom random vibration: they produce simultane
ous vibration in each principle axis and rotations
abouteachaxis.Thiscompareswiththesingleaxiso
fvibrationprovidedbyelectro-dynamicvibrators.
2.2.2EnvironmentalStressScreening(ESS)andBurn
The second type of accelerated test cons
-In
ists of ESS and Burn-
In testing. ESS is a process
involving the application of environmental stimuli
to products (usually electronic or electromechanica l
products) on an accelerated basis. The surviving population, upon completion of screeni ng, can be
assumedtohaveahigherreliabilitythanasimilar
unscreenedpopulation.ThegoalofESSistoexpose,
identifyandeliminatelatentdefectswhichcannot
bedetectedbyvisualinspectionorelectricaltest
ingbut
which will cause failures in the field. ESS is perform
ed on the entire population and does not involve
sampling.
Developedtohelpelectronicsmanufacturersdetect
widelyusedinmilitaryandaerospaceapplications,
elaborate,forexample,switchinganelectronicore
productdefectsandproductionflaws,ESSis
lesssoforcommercialproducts.Thetestsneednot
lectricalsystemonandoffafewtimesmaybeenoug
to catch some simple defects that would otherwise be
productwasfirstused.
be
h
encountered by the end user very soon after the
Teststypicallyincludethefollowing:
·
Temperaturevariations
·
Vibrationtests
Pressure
·
·
Flexibilitytests
ESS can be performed as part of the manufacturing pr
qualificationtesting.AnESSsystemusuallyconsists
andwiring,andafunctionaltester.Thesesystemsca
ocess or it can be used in new product
ofatestchamber,controller,fixturing,intercon
nect
nbepurchasedfromavarietyofcompaniesinthe
environmentaltestindustry.
Thestressscreeningfromthisprocesswillhelpfind
infantmortalityintheproduct.Findingthese
failures before the product reaches the customer yi
elds better quality and lower warranty expenses.
Associated military terminology includes an operatio nal requirements document (ORD) and on- going
reliabilitytesting(ORT).
To conduct an effective screen, the product must be
capable of surviving the high stimulation
levelsneededtoacceleratethefailuremechanismo
fassemblyrelateddefects.Theparticipationofdes
ign
andreliabilityengineeringistodeterminethelim
itsofenvironmentalstimulationwhichtheproductc
an
endurebeforeitsperformanceispermanentlydegrad ed.Amechanically“weak” designmaybechanged
toimproveitsmarginswithrespecttoaspecificf
ormofenvironmentalstimulus.
TheformofESSchosenbythefactoryisdependenton
thefailuremechanismsfortherelevant
fieldfailures.AnESSprogramisfaultywhenitdoes
notexposethelocusoffaultsseenbythecustomer
TheESSprocessisadynamicprocesswhichmustchange
reason, it is not appropriate to “spec”
productlife.
asproductfailurebehaviorchanges.Forthis
an ESS regimen and leave the regimen unchanged throu
ghout
Again,thepurposeofanyenvironmentalstressscree
ning(ESS)regimenistoexposethehidden
defects that wereintroduced during themanufacturin g process. Moresuccinctly, manufacturing defects
areprecipitatedfromlatenttopatent.
.
ESS, however, is not designed to find deficiencies in
product design although in many cases it
does expose design deficiencies. Rather than design
an ESS program to find design weaknesses, an
importantingredientofproductqualificationmust
betheundertakingofenvironmentaltestingtoensu
that the design is robust enough to meet its design
componenttechnologyandassemblytechniqueshavec
re
goals. As the electronics industry has matured,
hangedprofoundly.
Thefirstproducttobesubjectedtoenv
ironmentalstressscreeningintheformof“burn-in
” was
productsdesignedwithvacuumtubetechnology.Burn- incanberegardedasaspecialcaseofESS.For
these products, high temperature burn- in was the best stimuli for precipitating latent def
ects. Early
transistorandintegratedcircuittechnologydefect
swerealsoefficientlystimulatedtofailureusing
temperatureburn- in.Duringthistime,mostproductdefectswerecompo
seeamuchdifferentfailurebehaviorforelectroni
mostproductdefectsarerelatedtotheassemblypr
high
nentrelateddefects.Todaywe
cproducts.Componentshavebecomesoreliablethat
ocess.Astheproductfaultspectrumhaschanged,th
screening stimuli for rapid defect precipitation mu
efficientlystimulatedtoanobservablefailure.
e
st also change to ensure that latent defects are
According to MIL-STD-883C, Burnin is a test performed for the purpose of screening
eliminatingmarginaldevices.Marginaldevicesare
thosewithinherentdefectsordefectsresultingfro
or
m
manufacturing aberrations which cause time and stre
ss-dependent failures. As with ESS, Burn- in is
performedontheentirepopulation.Theintentionis
todetectthoseparticularcomponentsthatwouldfa
il
as a result of the initial, high-failure rate porti
on of the bathtub curve of component reliability
. If the
burn-in period is made sufficiently long (and, perhaps,
artificially stressful), the system can then be
trustedtobemostlyfreeoffurtherearlyfailures
oncetheburn-inprocessiscomplete.
Apreconditionforasuccessfulburn-in
isabathtub- likefailurerate,thatis,therearenoticeable
early failures with a decreasing failure rate follow
ing that period. By stressing all devices for acer
tain
burn-in time the devices with the highest failure rate fa
il first and can be taken out of the cohort. The
devices that survive the stress have a later positi
ongoingfailurerate).
Thusbyapplyingaburn-in,earlyinofareducedyieldcausedbytheburn-
usesystemfailurescanbeavoidedattheexpense(
inprocess.Whentheequivalentlifetimeofthestr
into the increasing partof thebathtub-likefailur
productlifetime.Inamatureproductionitisnot
rate. Todeterminethefailuretimedistributionfor
havetodestroyaverylargenumberofdevices.
When possible, it is better to eliminat
on in the bathtub curve (with an appropriately lower
tradeoff)
essisextended
e-rate curve, the effect oftheburn- in is areduction of
easytodeterminewhetherthereisadecreasingfail
ure
avery low percentageoftheproduction, onewould
e the root cause of early failures than doing a bur
n-in.
Becauseofthis,aprocessthatinitiallyusesburn
forfailuresareidentifiedandeliminated.Forele
-inmayeventuallyphaseitoutasthevariousroot
causes
ctroniccomponents,burn- inisfrequentlyconductedat
elevated temperature and perhaps elevated voltage.
componentsmaybeundercontinuoustestorsimplyt
This process may also be called heat soaking
estedattheendoftheburn-inperiod.
. The
Thereisanotheruseofthetermbysome
audiophiles ,wholeavenewaudioequipmentturnedon
for multiple days or weeks, to get the components to
achieve optimal performance. However, many
debatesariseaboutthebeneficialeffectsofthis
practice.
2.2.3QuantitativeAcceleratedLifeTests
QuantitativeAcceleratedLifeTesting,unlikethequali
tativetestingmethods(i.e.,TortureTests,
Burn-in, etc.) described previously, consists of quantit ative tests designed to quantify the life
characteristics of the product, component or system
under normal use conditions, and thereby provide
“Reliability Information.” Reliability information can include the determinati on of the probability of
failure of the product under use conditions, mean l
ife under use conditions, and projected returns and
warrantycosts.Itcanalsobeusedtoassistinthe
performanceofriskassessments,designcomparison
etc.AcceleratedLifeTestAnalysis(ALTA)softwarehasbeen
s,
chosenforQALT.
2.2.3.1AcceleratedLifetestAnalysis(ALTA)
Accelerated Life Testing can take the form
of “Usage Rate Acceleration” or “
Overstress
Acceleration.” Both Accelerated Life Test methods are d
escribed next. Because “ Usage Rate
Acceleration” test data can be analyzed with typical life data an
alysis methods, the Overstress
Acceleration method is the testing method. For all l
productisrequiredsincethefailureoftheproduc
ife tests, some time-to- failure information for the
tistheeventwewanttounderstand.
In other words, if we wish to understand, measure, and
predict any event. Most products,
components or systems are expected to perform their
functions successfully for long periods of time,
competitive,thetimerequiredtoobtaintimes-to- failuredatamustbeconsiderablylessthantheexp
ected
lifeoftheproduct.
Two methods of acceleration, “Usage Rate
beendevisedtoobtaintimesto-failuredataatan
Acceleration” and “Overstress Acceleration,”
have
acceleratedpace.
For products that do not operate continuously, one
can accelerate the time it takes to induce
failures by continuously testing theseproducts. Thi s is called “Usage Rate Acceleration.”
For products
forwhich“UsageRateAcceleration” isimpractical,onecanapplystressatlevelsthat
exceedthelevels
that aproduct willencounterundernormalusecondi
tionsandusethetimes- to failuredataobtainedin
thismannertoextrapolatetouseconditions.Thisi
scalled“OverstressAcceleration.”
i)UsageRateAcceleration
For products that do not operate continuously under
normal conditions, if the test units are
operatedcontinuously,failuresareencounteredear lierthaniftheunitsweretestedatnormalusage.
For
example, amicrowaveoven operates forsmall periods
oftimeevery day. One can accelerateatest on
microwaveovensbyoperatingthemmorefrequentlyun
weassumeanaveragewasheruseof6hoursaweek,on
tilfailure.Thesamecouldbesaidofwashers.If
ecouldconceivablyreducethetestingtime28-
fold by testing these washers continuously. Data obta
ined through usage acceleration can be analyzed
with the same methods used to analyze regular timesto-failure data. The limitation of “ Usage Rate
Acceleration” arises when products, such as computer servers and p
eripherals, maintain a very high or
even continuous usage. In such cases, usage acceler
ation, even though desirable, is not a feasible
alternative. In these cases the practitioner must s
timulate the product to fail, usually through the
applicationofstress.Thismethodofacceleratedli
fetestingiscalled“OverstressAcceleration”.
ii)OverstressAcceleration
For products with very high or continuo
stimulatetheproducttofailinalifetest.
us usage, the accelerated life- testing practitioner must
Thisisaccomplishedbyapplyingstressthatexceeds
thestressthataproductwillencounterunder