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.

Term	Definition
Device	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.

Case	Description
1	PredictionsarebasedontheBlackBoxoptionwithunit/systemburn-in<=1hour.Thereisnodeviceburn-in.Alldevicesareassumedtobeoperatingat40degreesCelsiusandat50percentratedelectricalstress.
2	PredictionsarebasedontheBlackBoxoptionwithunit/systemburn-in>1hour.Thereisnodeviceburn-in.Alldevicesareassumedtobeoperatingat40degreesCelsiusandat50percentratedelectricalstress.
3	ThisisthegeneralcaseforMethodIreliabilitypredictions.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.

Case	Description
L1	Devicesarelaboratorytestedandhavenoburn-in.
L2	Unitsarelaboratorytestedandhavenounit/deviceburn-in.
L3	Devicesarelaboratorytestedandhavehadburn-in.
L4	Unitsarelaboratorytestedandhavehadunit/deviceburn-in.

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

Case	Description
III(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.