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    Using Telcordia Calculation Methods to Adjust Failure Rates

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    ImprovetheAccuracyofYourReliabilityPredictions!

    ThegeneralpurposeofthecalculationmethodsoutlinedintheTelcordia(Bellcore)document,PredictionProcedureforElectronicEquipment,istotakeintoaccountadditionalinformationaboutthedevices,units,andsystemsunderanalysis.Thesecalculationmethodsconsidervariousburn-in,field,andlaboratorytestdata.Theyalsoprovideforcalculatingtheinfantmortalityrate,orfirst-yeardropout,forthesystemsunderanalysis.However,thisparticulartopicisbeyondthescopeofthisbrief.
    BeforereviewingthethreeTelcordiacalculationmethods,itisimportantthatyouunderstandthebasicterminologythatisusedtodescribethem.ThefollowingtabledefinestermsusedwithintheTelcordia(Bellcore)document.

    [table=98%][tr][td=1,1,140]Term[/td][td=1,1,660]Definition[/td][/tr][tr][td=1,1,140]Device
    [/td][td=1,1,660]Anelectricalpartwithwell-definedelectricalcharacteristics.Devicesincludeintegratedcircuits,diodes,resistors,andmore.
    [/td][/tr][tr][td=1,1,140]Unit
    [/td][td=1,1,660]Anassemblyofdevicestypicallyatthelowestreplaceablelevel.Unitsincludecircuitpacks,modules,powersupplies,plug-indevices,andmore.
    [/td][/tr][tr][td=1,1,140]System
    [/td][td=1,1,660]Acompleteassemblythatperformsanoperationalfunction.
    [/td][/tr][tr][td=1,1,140]Steady-StateFailureRate
    [/td][td=1,1,660]Theconstantfailurerateafteroneyearofoperation,providinginformationaboutlong-termproductperformance.
    [/td][/tr][tr][td=1,1,140]Burn-in
    [/td][td=1,1,660]Theoperationofadeviceunderacceleratedtemperatureorotherstressconditionstostabilizeitsperformance.
    [/td][/tr][/table]

    YourchoiceofaparticularTelcordiacalculationmethoddependsuponyouranalysisrequirementsandtheamountofdataavailable.Generaldescriptionsofeachmethodanditsdifferentcasesareprovidedintheremainingpagesofthisbrief.Formoredetailedinformation,pleaserefertotheTelcordia(Bellcore)documentitself.
    Note:TheterminologyfrompreviousBellcorestandards—MethodI,MethodII,andMethodIII—isreplacedinTelcordiaIssue1withBlackBoxTechnique,BlackBoxTechniqueIntegratedwithLaboratoryData,andBlackBoxTechniqueIntegratedwithFieldData,respectively.Theunderlyingcalculationproceduresarethesame;onlytheterminologyhasbeenupdatedtopromotebetterunderstanding.Althoughthesectionsthatfollownotebothsetsofterminologyintheheading,theMethodI,II,andIIIterminologyisusedwithinthesetopicsforsuccinctness.

    MethodI(BlackBox)
    MethodIisgenerallyreferredtoasaPartsCountmethodbecausethesteady-statefailurerateforaunitisassumedtobethesumofthesteady-statefailureratesforitsdevices.BecauseMethod1isbasedongenericfailuredataforvariousdevicetypes,itisusedwhenspecificpartdataisunavailable.
    InthepreviousBellcorestandards,threedifferentcasesofMethodIaredefined.Case1andCase2,whichareverygeneral,bothassumethatoperatingtemperatureis40degreesCelsiusandratedstressis50percent.Case1assumesaburn-inoflessthanorequalto1hour,whileCase2assumesaburn-inofgreaterthan1hour.Case3providesfortheuseofvariabletemperatureandstressvalues.Case1issimilartotheMIL-HDBK-217PartsCountmethodology,andCase3issimilartotheMIL-HDBK-217PartsStressmethodology.
    [table=98%][tr][td=1,1,70]Case[/td][td=1,1,630]Description[/td][/tr][tr][td=1,1,70]1[/td][td=1,1,630]PredictionsarebasedontheBlackBoxoptionwithunit/systemburn-in<=1hour.Thereisnodeviceburn-in.Alldevicesareassumedtobeoperatingat40degreesCelsiusandat50percentratedelectricalstress. [/td][/tr][tr][td=1,1,70]2[/td][td=1,1,630]PredictionsarebasedontheBlackBoxoptionwithunit/systemburn-in>1hour.Thereisnodeviceburn-in.Alldevicesareassumedtobeoperatingat40degreesCelsiusandat50percentratedelectricalstress.
    [/td][/tr][tr][td=1,1,70]3[/td][td=1,1,630]ThisisthegeneralcaseforMethodIreliabilitypredictions.Itcantakeintoaccountdeviceburn-inandvaryingtemperatureandstressdata.
    [/td][/tr][/table]

    MethodII(BlackBoxIntegratedwithLaboratoryData)
    ThepurposeofMethodIIistoadjustthepredictedMTBF(MeanTimeBetweenFailures)ofaunitordevicebasedonavailablelaboratoryortestdata.UsingMethodII,thesteady-statefailurerateiscalculatedasaweightedaverageofthemeasuredlaboratoryfailurerateandtheMethodIgenericfailurerate,withtheweightsdeterminedbythelaboratorydata.
    Whenlaboratorytestsareveryinformative,theMethodIIbasefailurerateisheavilyinfluencedbythelaboratorydata.Whenlaboratorytestsarelessinformative,theMethodIIbasefailurerateisheavilyinfluencedbytheMethodIgenericfailurerate.Thefactorstakenintoconsiderationintheweightingoflaboratorydataincludethenumberofdevicefailuresduringlaboratorytest,thenumberofdevicestested,theactualtimedevicesweretested,andthetemperatureaccelerationduringtest.
    Whenlaboratorydataisincluded,thecalculationsforpredictingsteady-statefailureratesaredependentuponwhetherdevicesorunitshavehadpreviousburn-in.InthepreviousBellcorestandards,fourdifferentcasesofMethodIIaredefined.Thetablebelowdescribeseachofthesecases.
    [table=98%][tr][td=1,1,70]Case[/td][td=1,1,630]Description[/td][/tr][tr][td=1,1,70]L1[/td][td=1,1,630]Devicesarelaboratorytestedandhavenoburn-in.
    [/td][/tr][tr][td=1,1,70]L2[/td][td=1,1,630]Unitsarelaboratorytestedandhavenounit/deviceburn-in.
    [/td][/tr][tr][td=1,1,70]L3[/td][td=1,1,630]Devicesarelaboratorytestedandhavehadburn-in.
    [/td][/tr][tr][td=1,1,70]L4[/td][td=1,1,630]Unitsarelaboratorytestedandhavehadunit/deviceburn-in.
    [/td][/tr][/table]

    MethodIII(BlackBoxIntegratedwithFieldData)
    ThepurposeofMethodIIIistoadjustthepredictedMTBFofaunitordevicebasedonfielddata.MethodIIIiscalculatedasaweightedaverageoftheobservedfieldfailurerateandtheMethodIgenericfailurerate.Thenumberoftotaloperatinghoursduringfieldtestingdeterminestheweights.
    Whenthenumberoftotaloperatinghoursislarge,theMethodIIIbasefailurerateisheavilyinfluencedbythefielddata.Whenthenumberoftotaloperatinghoursissmall,theMethodIIIbasefailurerateisheavilyinfluencedbytheMethodIprediction.
    InpreviousBellcorestandards,threedifferentcasesofMethodIIIaredefined.Thefollowingtabledescribeseachofthesecases.

    [table=98%][tr][td=1,1,70]Case[/td][td=1,1,630]Description[/td][/tr][tr][td=1,1,70]III(a)[/td][td=1,1,630]Providesfailureratepredictionsfordevices,units,orsubsystemsbasedonactualin-serviceperformance.
    [/td][/tr][tr][td=1,1,70]III(b)[/td][td=1,1,630]Providesfailureratepredictionsfordevices,units,orsubsystemsbasedonin-serviceperformanceaspartofanothersystem.Adjustmentsaremadetotheseestimatestotakeintoaccountdifferencesinoperatingconditionsandenvironments.
    [/td][/tr][tr][td=1,1,70]III(c)[/td][td=1,1,630]Providesfailureratepredictionsfordevices,units,orsubsystemsbasedonthein-serviceperformanceofsimilarequipmentfromthesamemanufacturer.Adjustmentsaremadetotheseestimatestotakeintoaccountdifferencesbetweentheoperatingconditionsandenvironmentsofthetwosystems.
    [/td][/tr][/table]

    TheTelcordiacalculationmethodsdescribedinthisdocumentaresupportedintheRelexReliabilityPredictionmodule.Inadditiontosupportingawiderangeofpredictionmodels,includingMIL-HDBK-217,Telcordia,RDF,etc.,RelexReliabilityPredictionallowstheTelcordiacalculationmethodstobeappliedtoanylinear-basedmodel.ThismeansthatyoucanadjustthefailureratesforvirtuallyanyreliabilitypredictionmodeltoaccountforlaboratoryandfielddatabyusingtheseTelcordiacalculationmethods.

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