Taking Dormant States into Account in Reliability Predictions
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=+
λP=[(λOBπGπDCOπTO)+(λTCBπGπCRπDT)+(λEOSπG)+(λSJ)]+[(λEBπGπDCNπRHT)]
Aswithothermodelingmethods,theassemblyfailureratesarethencomputedbyaddingupalloftheindividualpartfailurerates,andtheoverallsystemfailurerateiscomputedbyrollingupalloftheindividualassemblyfailurerates.
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