Process Capability Stduy

1ProcessCapabilityStudy2PurposeTounderstandcapabilityconceptandhowtostudyprocesscapability;Tounderstandrelationbetweenprocesscapabilitystudyandgagestudy,SPC,DOE.3Whatiscapabilityofaprocess?TerminologyWhytodoprocesscapabilitystudy?GagestudyandcapabilitystudySPCandcapabilitystudyDOEandcapabilitystudyHowtodoprocesscapabilitystudy?Content4Whatiscapabilityofaprocess?Capabilityistheextenttowhichastableprocessisabletomeetspecifications.Aprocessissaidtobestableifitonlyexhibitscommoncausevariation.(Controlchartshownopointsoutofcontrol)5Terminology(1)DOE-DesignOfExperiment,synonymouswithdesignedexperiments,orstatisticallydesignedexperiments,itisamethodicalprocessthateffectivelyusesresourcestostudytheimpactofvariousfactorsonaprocess;VariableData-Quantitativedatathatcanbemeasuredandcanvaryfrompiecetopiece;AttributeData-Qualitativeorcategoricaldatacanbecounted.Attributedataischaracteristiccounts.6Terminology(2)Mean-Ameasureofthecentraltendency;StandardDeviation-Ameasureofspread(variability).USL-UpperSpecificationLimit.Thenumericalvalue,abovewhichdefectsoccur.LSL-LowerSpecificationLimit.Thenumericalvalue,belowwhichdefectsoccur.7Terminology(3)SpecialCause-Atypeofvariationthatisshownbyanoutofcontrolsituationfromacontrolchart.Itsuggestssomethingspecialhashappenedtocreateachangeintheprocess.CommonCause-Anaturaltypeofvariationthatcomesfromthenormaloperatingconditionsofaprocess.GageStudy-Astudytoquantifythedifferenttypesofmeasurementerrorassociatedwithagageormeasurementsystem.8Whytodocapabilitystudy?Capabilitystudiesgiveanestimateoftheexpecteddefectrateforaprocessthatisincontrol;Itshouldbeperformedtoevaluatenewequipment,comparedifferentmachines;Itcanalsobeperformedonanongoingbasis.9GagestudyandcapabilitystudyAllmeasurementdevicesusedtocollectdataforacapabilitystudyshouldhaveacceptablegageerror;Gagestudyincludingbias,linearity,stability,repeatabilityandreproducibility;Otherwise,thestudyisinvalid.10SPCandcapabilitystudyAcapabilitystudycanonlybeperformedonastableprocessthatisincontrol;Suitablecontrolchartshouldbeusedtomonitorstabilityofprocessanalyzed.11DOEandcapabilitystudyCapabilitystudycanbeconductedtoprioritizewhereexperimentationshouldbeperformed,tobestutilizeresources;Capabilitystudiesperformedbothbeforeandafterexperimentationcanbeusedtoquantifyanyimprovementmadeintheprocess.12Howtodocapabilitystudy?(1)Capabilitystudywithvariabledata.Capabilitystudywithattributedata;Capabilitystudycanbecategorizedbydatatype.13CapabilityStudyWithVaraibles(Cp,Cpk,Pp,Ppk)14Capabilitystudywithvariables(1)Anormaldistributionorclosetoitbeingassumedwithparametersstudied;Priortocapabilitystudy,anormalitystudyshouldbeconducted;Avariablecontrolchartshouldbeusedduringcapabilitystudy.15Capabilitystudywithvariables(2)Cp-Acommoncapabilityindexforvariabledata,whichbeingusedtoevaluateprocess’spotentialcapability.Cp=(USL-LSL)/6S16Capabilitystudywithvariables(3)Cpk-Themostcommoncapabilityindexforvariabledata,whichbeingusedtoevaluateprocess’spotentialcapability.Cpk=minimum((USL-X)/3S,(X-LSL)/3S)17Capabilitystudywithvariables(4)TheCpisequaltotheCpkwhenandonlywhentheprocessiscenteredbetweenitsspecificationlimits.18Capabilitystudywithvariables(5)Pp-Acommoncapabilityindexforvariabledata,whichbeingusedtoevaluateprocess’sperformance;ThecalculationformulaissameasCpexceptdifferentderivingofS.19Capabilitystudywithvariables(6)Ppk-Anothercommoncapabilityindexforvariabledata,whichbeingusedtoevaluateprocess’sperformance.ThecalculationissameasCpkexceptdifferentderivingofS.20Capabilitystudywithvariables(7)CpandCpkonlyincludescommoncausevariation;PpandPpkincludesbothcommoncausevariationandspecialcausevariation,sostableprocessisn’tapplicable.21Capabilitystudywithvariables(8)Cp1.67Overhighcapability1.67Cp1.33Sufficientcapability1.33Cp1.0Passablecapability1.0Cp0.67Insufficientcapability0.67CpPoorcapabilityCriteriaOfCapabilityIndex:Cp22Capabilitystudywithvariables(9)0.330.671.001.331.672.000.3368.268%84.000%84.134%84.134%84.13447%84.13447%0.67-95.450%97.722%97.725%97.72499%97.72499%1.00--99.730%99.865%99.86501%99.86501%1.33---99.994%99.99683%99.99683%1.67----100.000%99.99997%2.00-----99.9999998%CpkCpFirstPassRateInaccordancewithCpandCpk23CapabilityStudyWithAttributes(P,DPMO)24Capabilitystudywithattribute(1)FirstpassrateandDPMOisthemetricusedtoassesstheprocesscapability;FirstPassRate-Passratewithoutanyrework;DPMO-DefectsPerMillionOpportunities.Aprocesscapabilityindexthatquantifiesthedefectrateofaprocess.Anopportunitycanbeasolderjoint,component,etc.25Capabilitystudywithattribute(2)Unlikevariabledataindices,theyaredirectmeasureofprocessquality;Theytypicallyrequiresmoredatathanotherindices,butprovidesamoreaccurateestimateofthedefectrate.26Capabilitystudywithattribute(3)Duringcapabilitystudy,defectcontrolchartshouldbeusedtoensurestableprocess;Usedtosetprocessyieldtarget.27ReviewWhathaveweleaned?Andanyquestion?