Application of ESPI in investigating the static deformation of a lead-free joint D. Karalekas 1, J.Cugnoni 2, J. Botsis 2 1 Lab. Adv. Manufact. and Testing, University of Piraeus, Greece 2 Lab. Applied Mechanics & Reliability, EPFL, Switzerland Photomechanics 2006

Outline Introduction  General remarks  Goal of research work Experimental Work  Materials and Specimens  Experimental Procedure Results  Stress-strain curve  Displacement fields  Failure initiation Conclusions Photomechanics 2006

Introduction Lead (Pb)-containing solders have been used extensively in microelectronics applications Recent trends of worldwide environmental legislation for toxic materials and consumer demand for “green” products are accelerating the transition from Pb-containing to Pb-free solders in the electronic industry Reliability assessment of risk factors associated with Pb-free solders requires:  Better understanding of of Pb-free solders’ metallurgical and mechanical properties  Predictions of package/assembly field performance, where FEM based projections require new valid constitutive equations and new fatigue damage criteria based on the thermomechanical loading history for the new materials Photomechanics 2006

Introduction Non-contact optical methods are desirable in measuring “real- time” thermomechanical deformation of such materials ESPI is a versatile method since it requires little or no special specimen preparation, being able to measure in-plane and out-of-plane deformation with high sensitivity The goal of the research work was to measure strain field evolution in Pb-free joints (global strain field & local strain field near the stress concentrations) at different load levels:  In the elastic domain, near yield stress  In the early plastic domain  Near rupture, to observe failure initiation and development Photomechanics 2006

Experimental work: materials and specimens Photomechanics 2006 Specimen configuration: 60 mm long copper plates Sn-4.0Ag-0.5Cu solder alloy 0.2, 0.5 & 1mm solder gap width Customized specimen mounting devices Pure tension condition 2kN load cell Displacement controlled

Experimental work: setup Photomechanics 2006 ESPI arrangement for in-plane measurements

Experimental work: setup Photomechanics 2006 ESPI configuration: 633 nm wavelength He-Ne laser 768 x 572 pixels CCD camera Standard phase-shifting Differential measurements in successive steps of 3 μm as obtainedphase-shiftedunwrapped

Results: vertical displacement field Photomechanics 2006 Observation region: 22x16 mm, displ. range: ~10 microns

Results: elastic region Photomechanics 2006 Displacement fieldStrain distribution Applied load: 465 N

Results: close to yield point Photomechanics 2006 Displacement fieldStrain distribution Applied load: 817 N

Results: close to failure Photomechanics 2006 Displacement fieldStrain distribution Applied load: 1205 N

Results: displacement variation at interfaces of the solder joint gap Photomechanics 2006 Applied load: 1205 N

Results: local measurements Photomechanics 2006 Displacement field Displacements at interfaces Increased magnification Observation region: ~ 10 x 8 mm

Results: average stress-strain curves Photomechanics 2006 Digital Imaging Correlation ESPI

Results: discussion ESPI proved to be very usefull to:  evaluate the overall displacement / strain field.  understand the evolution of plastic deformations in a thin solder joint under tension.  check the "boundary conditions" imposed to the specimen during the test.  In the present case, reveals the effects of slight misalignement of fixtures on the inhomogeneous evolution of the plastic field (plastic instability near rupture) Photomechanics 2006

ESPI for strain measurements in microelectronics + Sensitivity independant from magnification: excellent for global observations Full field measurement Good spatial resolution Monitoring of the damage evolution - Decorrelation when increasing magnification: not suitable for local measurements Very sensitive to out of plane displacements & rotations Incremental loading not suitable with creep Photomechanics 2006

Full field ESPI and FEA example: validation of boundary conditions Photomechanics 2006 FEM Hypothesis: homogeneous displacement field far from the joint Cu Solder FEM Cu Boundary conditions from hypothesis Stress / strain field ESPI: measure actual displacement field Realistic Boundary conditions More Realistic FE Results

Conclusions  Comparison of ESPI and DIC results was satisfactory  Capture and calculation of strain field evolution was possible  ESPI experimental data clearly determined area of crack initiation at the solder-Cu interface  Local studies were not possible due to decorrelation of speckles  ESPI is an attractive technique for global characterization of solder joints Photomechanics 2006

Acknowledgments  This research work is part of the COST 531 Action “Lead- free solder materials  Dr. Karalekas contribution was made possible through COST-STSM  Research Center of University of Piraeus for financing the participation of Dr. Karalekas to the conference Photomechanics 2006