MTBF-公式介绍

MTBFIntroduction利用加速寿命试验的手法及增加取样数量的方式，来缩短测试产品出现失效的时间，以便在短期内达到符合产品的平均失效间隔时间MTBF(MeanTimeBetweenFailure)，进而保证产品在可用期的可靠度。其实施步骤可参考「平均失效间隔时间验证作业程序」(TG-SX00-Q066)ItisusingacceleratedlifetestsandincreasingsamplesizestoshortenthefailuretimeofproductssothatwecanreachtheMTBF(MeanTimeBetweenFailure)forproductsinashortperiod.Astoitsprocedures,wecanreferto“MTBFprediction/DemonstrationStandard”(TG-SX00-Q066).(1).根据客户要求之测试条件及标准，计算出在正常环境下累积MTBF所需的时间(T)。Calculatingtherequiredtime(T)beingaccumulatedasMTBFinnormalenvironmentaccordingtothedemandedtestconditionsandcriteriaofcustomers’.(a).MTBF之累积一般均采用单阶时间检剔数据法(即预先设定试验截止的时间点)，在试验中若发现不良品，就必须参考固定时间试验GEM(GeneralizedExponentialModel)表，计算出累积MTBF所需的时间。由GEM表之试验比，可以看出不良数越多，则累积MTBF所需的时间(T)越长。TimescreeningmethodisgenerallyusedfortheaccumulationofMTBF.Ifthereisanydefectedsamplefoundduringtesting,itisamusttocalculate,referringtofixedtimetestsGEM(GeneralizedExponentialModel),thetimeneededtoaccumulateMTBF.Itcanbeseenthatthemorefailurequantityis,thelongertime(T)isneededtoaccumulateMTBF.(b).使用GEM表须按照客户指定的信赖水平来查表，若客户无特别要求，则建议选择90%信赖水平为基础，此为目前业界常用的水平。WemustfollowtheconfidencelevelassignedbycustomerstouseGEMtable.Confidencelevelof90%,whichisthemostcommonusedinthefield,累积MTBF所需的时间TimeneededtoaccumulateMTBF(T)试验比(M)=平均失效间隔时间Meantimeintervalforfailure(MTBF)=T=MMTBFissuggestedtobeadopted.(c).在试验中若发现不良品，则将不良品修复，并登记一次不良。修复之不良品，可继续加入试验累积MTBF，同一件产品可重复修复，直到无法修复为止。IfthereisanyNGsamplefound,itshouldberepairedandthislogneedstoberecorded.RepairedNGsamplescancontinuebeingtestedtoaccumulateMTBF,andoneproductcanberepairedrepeatedlyuntilitcannotberepairedanymore.(d).假设试验时每天开机的时数为H，待测产品数量为N，则可算出累积MTBF所需的时间(T)需要几天(D)：AssumethatthesamplequantityundertestisNandthedailypowerontimeisH,andthenwecancalculatehowmanydays(D)thetime(T)neededtoaccumulateMTBF.HND=T=D=T/(HN)(2).利用加速寿命试验的手法，缩短累积MTBF所需的时间。ShortenthetimeneededtoaccumulateMTBFbyusingacceleratedlifetest.(a).若加速寿命试验所考虑的环境应力为温度，且失效模式符合指数分布，则可采用Arrhenius模式作为加速寿命试验之模式，其加速因子A可表示为：WecanuseArrheniusmodeasacceleratedlifetestmodeiftheconsideredenvironmentstressistemperatureandthefailuremodemeetsexponentialdistribution.Itsacceleratedfactorcanbeexpressedas:A=e(E/K)()式中K为Boltzmann常数(=8.62310-5eV/K)，E为活化能，Tn为正常环境之温度，Ta为加速寿命试验之温度。上式系假设试件在两种不同温度下之活化能相同所获得的结果，故应用时两种试验温度差距不宜过大，否则所预估之加速因子可信度将降低。一般电子产品在正常有用期失效之活化能趋近于0.6~0.8eV。WhereKisBoltzmannconstant(=8.623x10-5eV/K),Eisactivationenergy,Tnisthenormaltemperature,andTaisthetemperatureforacceleratedlifetests.TheformulaaboveisderivedfromassumingthatDUT’shavethesameactivationenergyareundertwodifferenttemperature,Tn1Ta1thereforethetesttemperaturedifferenceshadbetterbelimitedotherwisethecredibilityofpredictedacceleratedfactorswilllower.Theactivationenergyofcommonelectricalproductsduringnormalusefulperiodapproaches0.6~0.8eV.(b).在加速寿命试验中，一般皆采用提高环境温度来达到加速劣化的目的。假设在应力环境中老化的加速因子为A，则可算出在应力环境中累积MTBF所需的天数(Da)为：Raisingtemperatureisusuallyadoptedtoreachthepurposeofacceleratedaging.AssumethatagingfactorinstressenvironmentisA,andthenwecancalculatethedays(Da)neededtoaccumulateMTBFinstressenvironment:HNA(Da)=T=Da=T/(HNA)(c).假设一般使用者每天开机时数为U小时，则由Da之天数可预估使用者在正常环境下使用P天之状况：Assumethatthepower-on-hourperdayofcommonusersisU,andthenwecanpredicttheusagestatusundernormalenvironmentofusersafterPdays:HADa=UP=P=(HADa)/U上式中若P=30，就是预估使用者开机30天(I+30)的状况；若P=90，就是预估使用者开机90天(I+90)的状况。IntheaboveifP=30,itistopredictthestatusofusersfor30days(I+30):ifP=90,itistopredictthestatusofusersfor30days(I+90)(3).在试验初期若发现重大之遐疵或大量之不良品出现，则可考虑停止试验，将对策导入所有的待测产品，并重新开始累积MTBF。如此可避免过多的不良品，造成试验时间之延长。IfthereisanymajordefectoragreatquantityofNGproductsturnsup,stoppingtestingcanbeconsidered.AfterphasingincountermeasuretoallDUT’s,restarttoaccumulateMTBF.ThusithelpstoavoidtoomanyNGsamplesortheextensionofthetesttime.(4).执行MTBF之环境条件及累积时数，须符合客户之要求。TheenvironmentalconditionsandaccumulatedtimeforperformingMTBFmustmeetcustomers’requirements.(5).MTBF累积，可使用不同时间点所产出之成品分批累积，但必须具备相同设计及相同制程之产品才可累积。ItisallowedtouseinbatchesthefinishedproductsatdifferenttimepointstoaccumulateMTBF;however,theproductsmusthavethesamedesignandprocess.(6).待测物必须是经过早夭期筛选(例如：Run-in)之良品。Devicesundertestmustbegoodsamplespassingthescreenofearlylifeperiod(Ex:Run-in).(7).加速寿命试验中所选用的温度条件(环境应力)，必须有适当的考虑，其可施加应力之大小应受「所造成失效模式与实际作用时发生者相同」的约束。加速因子与应力之大小不一定有成正比的关系，若无限制的提高应力大小，则可能会有异常的失效模式出现。Thetemperatureconditions(environmentstress)selectedinacceleratedlifetestmustbeconsideredproperly.Thestrengthofappliedstressmustbelimitedby“thefailuremodecausedisthesameasthathappensinpracticaloperation”.Itisnonecessarythattherelationshipofdirectproportionexistsbetweentheacceleratedfactorsandthestrengthofstresses.Abnormalfailuremodesmayoccurifthereisnorestrictiononraisingthestrengthofstresses.(8).若考虑待测物作完试验后，仍然维持可以出货之状态，则必须设定待测物试验时间的上限值，以避免因过度劣化而影响出货的质量。MaximumtimelimitfortestingmustbesettoavoidoveragingifwestillhopethattheDUT’scankeepingoodconditionaftertestingandbesold.