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    Taking Dormant States into Account in Reliability Predictions

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    ReviewtheTwoDifferentMethodsDevisedbytheReliabilityAnalysisCenter(RAC)

    Althoughthedormantornon-operatingfailurerateofasystemisoftenassumedtobe0,thisisgenerallynotthecase.Totakedormantstatesintoaccountinreliabilitypredictions,theReliabilityAnalysisCenter(RAC)hasdevisedtwodifferenttechniques:
    • Dormantfailureratescanbeestimatedthroughtheuseofdormantenvironmentsandconversionfactorsasdescribedin”TopicA16:DormantAnalysis”oftheRomeLaboratoryReliabilityEngineer’sToolkit(CommercialPracticesEdition).
    • DormantfailureratescanbecalculatedbyPRISMRACRatesmodels,whichrecognizeandaccountforthefundamentallydifferentfailuremechanismsinvolvedindormantstorageasdescribedintheRACPRISM®User’sManual.
    ThisdocumentsummarizeshowtheRACconversionfactormethodandPRISMRACRatesmodelstakedormantstatesintoaccountwhencalculatingfailurerates.Fordetailedinformationaboutthesemethods,pleaserefertotheabove-referencedRACpublications.
    ConversionFactorMethod
    Estimationsofdormantfailureratesaredeterminedbasedonoperatingfailureratesandtakingintoaccountthevariationbetweentheoperatinganddormantenvironments.Todeterminethedormantfailurerate,theactivefailurerateofeachpartisfirstcomputed.Then,theactivefailurerateismultipliedbytheappropriatevaluefromatableofactive-to-passiveconversionfactors.Thisconversionalsotakesintoaccounttheparttype.
    Forexample,whenanintegratedcircuitisinagroundenvironmentwhenactiveandalsowhendormant,thefailurerateofthispartinthedormantenvironmentisestimatedtobe0.08timesitsactivefailurerate.Ifthesamepartisplacedinanactiveenvironmentofspaceandadormantenvironmentofground,ithasanestimateddormantfailurerateof0.30timesitsactivefailurerate.
    Onceboththeactiveanddormantfailureratesforapartarecomputed,thedutycycleisusedtocomputethetotalfailurerateofthatpart:
    λpart=λactive*DutyCycle+λpassive*(100−DutyCycle)
    Assemblyfailureratesarethencomputedbyaddingupalloftheindividualpartfailurerates.Finally,theoverallsystemfailurerateiscomputedbyrollingupalloftheindividualassemblyfailurerates.
    RACRatesModelMethod
    PRISMRACRatesmodelsrecognizeandaccountforthefactthatthefailuremechanismsinvolvedinthedormantstatearefundamentallydifferentfromthoseinnormaloperation.Thismethodisvastlydifferentfromtheconversionfactormethod,whichassumesthatthefailuremechanismsinvolvedinthedormantstatearethesameasthoseinnormaloperationbutoccuratalesserrate.
    Toprovideaccuratefailureratepredictionsthattakeintoaccountbothoperatingandnon-operatingconditions,RACRatesmodelsconsiderfiveseparatecontributionstothetotalpartfailurerate:
    • Operatingbasefailurerate(λOBπGπDCOπTO)
    • Non-operatingor”environmental”basefailurerate(λEBπGπDCNπRHT)
    • Temperaturecyclingbasefailurerate(λTCBπGπCRπDT)
    • Failurerateduetoelectricaloverstress(λEOSπG)
    • Failurerateduetosolderjoints(λSJ)
    RACRatesmodelsforcomputingfailureratesareavailableforthefollowingparttypes:capacitors,diodes,integratedcircuits,resistors,transistors,andthyristors.WhilethereisalsoaRACRatesmodelforpredictingthefailurerateofsoftware,itisstructuredverydifferentlyfromthoseforpredictingpartfailureratesandisthereforenotaddressedinthisarticle.
    TheRACRatesmodelforpredictingthefailurerateofanintegratedcircuitfollows:
    λP=(λOBπGπDCOπTO)+(λEBπGπDCNπRHT)+(λTCBπGπCRπDT)+(λEOSπG)+(λSJ)
    Where:
    λP = Predictedfailurerate(infailurespermillioncalendarhours)
    λOB = Operatingbasefailurerate
    πG = Reliabilitygrowthfactor
    πDCO = Operatingdutycyclefactor
    πTO = Operatingtemperatureaccelerationfactor
    λEB = Non-operatingor”environmental”basefailurerate
    πDCN = Non-operatingdutycyclefactor(proportionaltotimeinthenon-operatingstate)
    πRHT = Non-operatingtemperatureandrelativehumidityaccelerationfactor
    λTCB = Temperaturecyclingbasefailurerate
    πCR = Temperaturecyclingrateaccelerationfactor
    πDT = TemperaturecyclingΔTaccelerationfactor
    λSJ = Failurerateduetosolderjoints
    λEOS = Failurerateduetoelectricaloverstress

    IntheRACRatesmodel,theoperatingbasefailureratecontribution(λOBπGπDCOπTO),temperaturecyclingbasefailureratecontribution(λTCBπGπCRπDT),failurerateduetoelectricaloverstress(λEOSπG)contribution,andfailurerateduetosolderjoints(λSJ)contributionallaffectthecalculationoftheoperatingfailurerateforthepart.Thenon-operatingbasefailurerate(λEBπGπDCNπRHT)contributionaffectsthecalculationofthenon-operatingfailurerateforthepart.
    Onceboththeoperatingandnon-operatingportionsoftheRACRatesmodelarecomputed,theyareaddedtogethertocomputethetotalfailurerateforthatpart:
    TotalFailureRateforIntegratedCircuit=[OperatingPortions]+[Non-OperatingPortions]
    λP=[(λOBπGπDCOπTO)+(λTCBπGπCRπDT)+(λEOSπG)+(λSJ)]+[(λEBπGπDCNπRHT)]
    Aswithothermodelingmethods,theassemblyfailureratesarethencomputedbyaddingupalloftheindividualpartfailurerates,andtheoverallsystemfailurerateiscomputedbyrollingupalloftheindividualassemblyfailurerates.

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