Application of ESPI in investigating the static deformation of a lead-free joint

1. Application of ESPI in investigating the static deformation of a lead-free joint D. Karalekas1, J.Cugnoni2, J. Botsis2 1 Lab. Adv. Manufact. and Testing, University of Piraeus, Greece 2 Lab. Applied Mechanics & Reliability, EPFL, Switzerland Photomechanics 2006

2. 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

3. 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

4. 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

5. Experimental work: materials and specimens 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 Photomechanics 2006

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

7. Experimental work: setup 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 obtained phase-shifted unwrapped Photomechanics 2006

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

9. Results: elastic region Applied load: 465 N Displacement field Strain distribution Photomechanics 2006

10. Results: close to yield point Applied load: 817 N Displacement field Strain distribution Photomechanics 2006

11. Results: close to failure Applied load: 1205 N Displacement field Strain distribution Photomechanics 2006

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

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

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

15. 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

16. + 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 ESPI for strain measurements in microelectronics Photomechanics 2006

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

18. 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

19. Acknowledgments • This research work is part of the COST 531 Action “Lead-free solder materials • Dr. Karalekas contribution was made possible through COST-STSM-531-01501 • Research Center of University of Piraeus for financing the participation of Dr. Karalekas to the conference Photomechanics 2006