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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, Greece2 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
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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
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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
Results: vertical displacement field
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Observation region: 22x16 mm, displ. range: ~10 microns
Results: elastic region
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Displacement field Strain distribution
Applied load: 465 N
Results: close to yield point
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Displacement field Strain distribution
Applied load: 817 N
Results: close to failure
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Displacement field Strain distribution
Applied load: 1205 N
Results: displacement variation at interfaces of
the solder joint gap
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Applied load: 1205 N
Results: local measurements
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Displacement fieldDisplacements at interfaces
Increased magnificationObservation region: ~ 10 x 8 mm
Results: average stress-strain curves
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Stress-strain curve, 0.2 mm gap
0,00E+00
1,00E+07
2,00E+07
3,00E+07
4,00E+07
5,00E+07
6,00E+07
7,00E+07
8,00E+07
0 0,002 0,004 0,006 0,008 0,01 0,012 0,014 0,016 0,018 0,02
Strain (-)
Str
ess
(Pa) J7C-A
J7C-B
J7C-C
J8C-B
ESPI SP1
ESPI SP2
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
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Full field ESPI and FEA example: validation of boundary conditions
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FEM Hypothesis:homogeneous displacement field
far from the joint
Cu Solder
FEMCu
Boundary conditionsfrom hypothesis
Stress / strain field
ESPI:measure actual displacement field
RealisticBoundary conditions
More RealisticFE 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-531-01501
Research Center of University of Piraeus for financing the participation of Dr. Karalekas to the conference
Photomechanics 2006