Void formation by Kirkendall effect in solder joints - …

1 Void formation by Kirkendall effect in solder BiglariElfnet: reliability Athens 2006 Outline Introduction Diffusion Kirkendall effect Cases on Kirkendall void formation Concluding remarksIntroductionSoldering is a complicated processwide variety of solder alloys, substrates, process conditionswetting, solidificationLarge amount of data is available, for Lead Free solderadditional data is requiredIntroductionMechanical properties of soldered jointsMicrostructureInter-Metallic Compounds (IMC): nature and thicknessResidual stressesDefects: .. Kirkendall evolution determined byProcess conditions: soldering process parametersOperating conditions.

2 AgingMicrostructure evolutionSoldering (T, t)Dissolution of substrate in liquid solderIMC formationLiquid metal penetrationAging/Annealing (T,t)DiffusionIMC formationThermodynamics, kineticsMorphology of the microstructureDiffusionAAAdCJDdx= 201expexpexp6mmAASHQDzDRTRTRT == AAACCDtxx = AAAACJDx = BBBACJDx = ()AABXvDDx = AACCDtxx = BAABDXDXD=+ Fick IIFick IDiffusionzinc and copperLBWGTALBWK irkendall effectMarker movementMultiple Kirkendall planesstable, unstable, virtualvan Dal , Physical Review Letters, 86(15), 2001, 3352-3355 Kirkendall voids Cu3Sn ( ) Cu6Sn5( ) Case: diffusion couple Sn-CuCase: diffusion couple Sn-Cu Soldering: solid Cu dissolves in liquid Sn (supersaturised locally) formation of Cu6Sn5( ) , scallop-like uniphaseformation of Cu3Sn ( ), requires long contact times, thickness limitedt=0 Zeng et al.

3 97, 2005 SnPb solder - electrodeposited CuCase: diffusion couple Sn-CuAnnealing: Thickness of IMC increasesCu6Sn5( ), uniformCu3Sn ( ), Sn diffusion exceeds Cu diffusion at higher Tt=20days, T=150 CCase: diffusion couple Sn-CuVoids influence diffusiont=0t=20days, T=150 CCase: diffusion couple Sn-Cu Pull and sheartestingt=40days, T=125 CPad sideConcluding remarksThe Kirkendall effect manifests itself by the movement of markerplanes in a diffusion couple system. It is the result of a net mass flow, accompanied by a vacancy flow in the opposite large number of studies is dedicated to the microstructural evolution in solder -substrate systems.

4 Apart from experimental work, models are becoming available to predict the microstructures. No reference is found to predict the formation of Kirkendall voids. The voids are observed experimentally in many diffusion couple systems, but it receives not much attention. IMC layers with Kirkendall voids are an easy path for crackpropagation and determine the reliability of the solder joint.