Analysis of Solder Joint Reliability of High Power LEDs by Transient Thermal Testing and Transient Finite Element Simulations

An innovativesensitivetestmethodisdevelopedtodetectsolderjointcrackingforhighpowerLED packages.Themethodisbasedontransientthermalanalysisandcanfullyreplacethestilldominating light-on test.Forexperimentalapplicationofthemodel,testgroupsofLEDpackagesweresolderedwith two differentleadfreesolders(SnAgCu305andInnolotFL-640)onAluminumInsulatedMetalSubstrate (Al-IMS) andexposedtotemperaturecycles.Transientthermalmeasurementswereperformeddirectly afterassemblyandafterspecific cyclenumbers.Afterdataprocessingtheincreaseoftherelativethermal resistance betweentheinitialsignalat \u201c0\u201d cyclesand \u201cn\u201d cyclesisobtainedandcorrelatedwithcracksin the solderjointbycrosssections.BasedontheCADandmaterialdataoftheLEDpackagea finite element (FE) modelissetup.Thetime-resolvedtemperaturecurvesareproperlyreproducedbytransientthermal simulation. Themeasured \u201c0\u201d cyclecurvesare fitted usingtheFEmodelbyadjustingafewmaterial parameterswithintheirallowedtolerancerange.Aparametersensitivityanalysisisperformed.The impact ofacrackinthesolderjointbetweenpackageandprintedcircuitboard(PCB)onthetime resolvedtemperaturecurveissimulated.Thesimulatedcrackpropagatesfromthecornerofthepackage to itscenter.Theexperimentalmeasuredcurvesarereproduced.Basedonthesimulationafailurecriteria is defined, representingacracklengthbetween20%and30%ofthesolderjointarea,andWeibullcurves are calculated.AhighercreepresistanceforthetestgroupsolderedwithInnolotFL-640comparedtothe test groupsolderedwithSAC305isobserved