红墨水试验

1DescriptionoftheMethodDevelopedforDyePenetrantAnalysisofCrackedSolderJointsBackgroundTheextensionofcracksinsolderjointsafterfatiguetestingisusuallyevaluatedusingcross-sectioningofthesolderjoints.Theprintedboardassembly,orapartofit,ispottedinepoxyandisthencutclosetothesolderjointsthatwillbeexaminedandthesampleisgroundintothesolderjoints.AsshowninFigure1,theformationofcracksinsolderjointscanbeveryirregularandvaryconsiderablyeveninjointslocatedclosetoeachother.Figure1PhotographshowingtheextentanirregularityofcrackinginsolderjointstoaBGAafterthermalcycling.Thecrackinghasbeenanalysedusingadyepenetrantandtheareascolouredredindicatehowfarthecrackshadpropagatedduringthethermaltest.Thus,toobtainathree-dimensionalviewoftheextensionofcrackinginasolderjoint,polishingmustbedoneinstepswhiletheextensionofcrackingissuccessivelydocumented.Justmakingonecutthroughasolderjointmaygiveacompletingmisleadingresult.Forexample,ifacutismadefromAtoBinFigure1,itwouldindicateacompletelycrackedjointwhereasacutfromCtoDwouldindicatethatalmostnocrackinghasoccurred.Furthermore,anumberofsolderjointsneedtobeexaminedinordertogetstatisticallyvalidresults.Allsolderjointsinarowcanbeexaminedsimultaneously.IfallsolderjointstoaBGAshallbeexamined,thismustberepeatedforeveryrowofjoints.Thisisaverytimeconsumingandcostlyprocess.Furthermore,itisdifficulttodeterminetheexactpositionofacut.AsalreadyshowninFigure1,dyepenetrantanalysisoffersathree-dimensionalviewofcracksinsolderjointstoacomponent.ThetechniquewasoriginallydevelopedbyMotorola’sLandMobileProductsSector’sAdvancedManufacturingTechnologygroup[1].Thecomponentassembledontheprintedcircuitboardwasfloodedwithadyepenetrantliquidtodefinethecrackedarea.Afterhavingappliedthedye,thecomponentwasremovedbybendingtheboardnumeroustimes.Thefracturedsolderjointswerethenvisuallyinspectedandphotographsweremadetorecordthepositionthatdyehadpenetratedintothejoint.Thismethodworkswellifthecracksinthesolderjointsarelarge.Iftheyarenotlargeenough,thepadsontheboardwillripofffromthelaminate.2Inordertofacilitatetheremovalofcomponentswithsmallcracksinthesolderjoints,IVFhasimprovedthetechniquetoremovethecomponentsinaprojectfundedbyESA[2].ApplicationofDyeInordertofacilitatetheapplicationofthedye,adamiscreatedaroundthecomponentusingamodellingwax.Ifthereareviaholesbeneaththecomponent,atapeisappliedtotheviaholesontheoppositesideoftheboardtopreventthedyefromescapingthroughthem.Ifthatisnotpossible,thesampleislaidinaglassbeaker.Thedamisthenfilledwiththedye(SteelRedfromDYKEM)oralternativelythedyeispouredintotheglassbeakeruntilthesampleiscompletelysubmergedinthedye.Theboardortheglassbeakerisplacedinavacuumchamberandtwoevacuationsaremadedownto100mbarinordertoremoveanyairentrappedinthecracks.Sincesolventsinthedyeevaporateduringthisprocess,itisdoneasfastaspossibletopreventtheviscosityofthedyegettingtoohigh.Thesurplusdyeisthenpouredoutandthesampleisdriedat100°Cfor15minutes.RemovalofComponentsTheuppersideofthecomponentpackageisroughenedusingagrindingpaperandisthendriedwithaclothwettedwithacetone.Asteelcylinderwithathreadedholeisgluedtothecomponentusingatwo-partepoxyglue,PlasticPaddingSuperSteelfromLoctite(Fig.2).Thesteelcylinderisshotblastedinordertoimprovetheadhesionoftheglue.Theboardisthenfixedbyscrewstoanaluminiumplatewithathicknessof1cm.Ahookisscrewedtothesteelcylinderandthesampleisarrangedsothatapullingforcecanbeappliedthroughthehook(Fig.3).ForBGAcomponentshavingonlyperipheraljointsitmaybebettertodrillaholethroughthecomponentandapplyapullingforceasshowninFigure4.Thealuminiumplateisplacedonaheatingplatewiththeheatcontrolledbyathermocoupleattachedtotheuppersideoftheboard.Thetemperaturesofthesolderjointsareregisteredusinganotherthermocouple.Apullingforceofabout50gramsperjointisappliedtothepackage.Thisforcecausesthesoldertocreepandthepackagecanberemovedwithoutbendingtheboard.Ifthesolderjointsareseverelycracked,itmaybepossibletoremovethepackagewithinafewhoursatroomtemperature.However,inmostcasesitisnecessarytoheatthesamplestobeabletoremovethepackageswithinareasonabletime.Thehighertemperature,thefasterisitpossibletoremovethecomponent.Byheatingthesolderjointsto140°C,evenpackageswithonlysmallcracksincornerjointscanberemovedwithinafewhours.However,attemperaturesabove120°,thecolourofthedyefadesslightly.Also,agoodtemperaturecontrolsystemisrequiredtoensurethatthetemperatureoftheboardwillnotriseabovethemeltingpointofthesolderwhenthepackagebecomesloose.Ifthemaximumtemperatureislimitedto120°C,itmaytake10-40hourstoremoveaBGAcomponenthavingabout600I/Osifthecracksareverysmallinthesolderjoints.3Figure2.AcomponentwithasteelcylindergluedtoitFigure3.Arrangementforapplyingapullingforcetoapackage4Figure1ArrangementforapplyingapullingforcetoaBGAbydrillingaholethroughthecomponent.CaseStudyofDBGAThisexampletakenfromReference2showshowdyepenetrantanalysishasbeenusedtoevaluatetheextentofcrackinginsolderjointstotwothermallycycledDimpleBGA(DBGA)packages.TestVehicleTheDBGApackageshadabodysizeof17x17mmand228I/Oswithapitchof1.0mm.Bothpackageshaddaisychaininterconnections.Th