Using Telcordia Calculation Methods to Adjust Failure Rates
ImprovetheAccuracyofYourReliabilityPredictions!ThegeneralpurposeofthecalculationmethodsoutlinedintheTelcordia(Bellcore)document,PredictionProcedureforElectronicEquipment,istotakeintoaccountadditionalinformationaboutthedevices,units,andsystemsunderanalysis.Thesecalculationmethodsconsidervariousburn-in,field,andlaboratorytestdata.Theyalsoprovideforcalculatingtheinfantmortalityrate,orfirst-yeardropout,forthesystemsunderanalysis.However,thisparticulartopicisbeyondthescopeofthisbrief.
BeforereviewingthethreeTelcordiacalculationmethods,itisimportantthatyouunderstandthebasicterminologythatisusedtodescribethem.ThefollowingtabledefinestermsusedwithintheTelcordia(Bellcore)document.
TermDefinitionDevice
Anelectricalpartwithwell-definedelectricalcharacteristics.Devicesincludeintegratedcircuits,diodes,resistors,andmore.
Unit
Anassemblyofdevicestypicallyatthelowestreplaceablelevel.Unitsincludecircuitpacks,modules,powersupplies,plug-indevices,andmore.
System
Acompleteassemblythatperformsanoperationalfunction.
Steady-StateFailureRate
Theconstantfailurerateafteroneyearofoperation,providinginformationaboutlong-termproductperformance.
Burn-in
Theoperationofadeviceunderacceleratedtemperatureorotherstressconditionstostabilizeitsperformance.
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.
CaseDescription1PredictionsarebasedontheBlackBoxoptionwithunit/systemburn-in<=1hour.Thereisnodeviceburn-in.Alldevicesareassumedtobeoperatingat40degreesCelsiusandat50percentratedelectricalstress.
2PredictionsarebasedontheBlackBoxoptionwithunit/systemburn-in>1hour.Thereisnodeviceburn-in.Alldevicesareassumedtobeoperatingat40degreesCelsiusandat50percentratedelectricalstress.
3ThisisthegeneralcaseforMethodIreliabilitypredictions.Itcantakeintoaccountdeviceburn-inandvaryingtemperatureandstressdata.
MethodII(BlackBoxIntegratedwithLaboratoryData)
ThepurposeofMethodIIistoadjustthepredictedMTBF(MeanTimeBetweenFailures)ofaunitordevicebasedonavailablelaboratoryortestdata.UsingMethodII,thesteady-statefailurerateiscalculatedasaweightedaverageofthemeasuredlaboratoryfailurerateandtheMethodIgenericfailurerate,withtheweightsdeterminedbythelaboratorydata.
Whenlaboratorytestsareveryinformative,theMethodIIbasefailurerateisheavilyinfluencedbythelaboratorydata.Whenlaboratorytestsarelessinformative,theMethodIIbasefailurerateisheavilyinfluencedbytheMethodIgenericfailurerate.Thefactorstakenintoconsiderationintheweightingoflaboratorydataincludethenumberofdevicefailuresduringlaboratorytest,thenumberofdevicestested,theactualtimedevicesweretested,andthetemperatureaccelerationduringtest.
Whenlaboratorydataisincluded,thecalculationsforpredictingsteady-statefailureratesaredependentuponwhetherdevicesorunitshavehadpreviousburn-in.InthepreviousBellcorestandards,fourdifferentcasesofMethodIIaredefined.Thetablebelowdescribeseachofthesecases.
CaseDescriptionL1Devicesarelaboratorytestedandhavenoburn-in.
L2Unitsarelaboratorytestedandhavenounit/deviceburn-in.
L3Devicesarelaboratorytestedandhavehadburn-in.
L4Unitsarelaboratorytestedandhavehadunit/deviceburn-in.
MethodIII(BlackBoxIntegratedwithFieldData)
ThepurposeofMethodIIIistoadjustthepredictedMTBFofaunitordevicebasedonfielddata.MethodIIIiscalculatedasaweightedaverageoftheobservedfieldfailurerateandtheMethodIgenericfailurerate.Thenumberoftotaloperatinghoursduringfieldtestingdeterminestheweights.
Whenthenumberoftotaloperatinghoursislarge,theMethodIIIbasefailurerateisheavilyinfluencedbythefielddata.Whenthenumberoftotaloperatinghoursissmall,theMethodIIIbasefailurerateisheavilyinfluencedbytheMethodIprediction.
InpreviousBellcorestandards,threedifferentcasesofMethodIIIaredefined.Thefollowingtabledescribeseachofthesecases.
CaseDescriptionIII(a)Providesfailureratepredictionsfordevices,units,orsubsystemsbasedonactualin-serviceperformance.
III(b)Providesfailureratepredictionsfordevices,units,orsubsystemsbasedonin-serviceperformanceaspartofanothersystem.Adjustmentsaremadetotheseestimatestotakeintoaccountdifferencesinoperatingconditionsandenvironments.
III(c)Providesfailureratepredictionsfordevices,units,orsubsystemsbasedonthein-serviceperformanceofsimilarequipmentfromthesamemanufacturer.Adjustmentsaremadetotheseestimatestotakeintoaccountdifferencesbetweentheoperatingconditionsandenvironmentsofthetwosystems.
TheTelcordiacalculationmethodsdescribedinthisdocumentaresupportedintheRelexReliabilityPredictionmodule.Inadditiontosupportingawiderangeofpredictionmodels,includingMIL-HDBK-217,Telcordia,RDF,etc.,RelexReliabilityPredictionallowstheTelcordiacalculationmethodstobeappliedtoanylinear-basedmodel.ThismeansthatyoucanadjustthefailureratesforvirtuallyanyreliabilitypredictionmodeltoaccountforlaboratoryandfielddatabyusingtheseTelcordiacalculationmethods.
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