Taking Dormant States into Account in Reliability Predictions

cliffcrag

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.