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

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Outline

Introduction General remarks Goal of research work

Experimental Work Materials and Specimens Experimental Procedure

Results Stress-strain curve Displacement fields Failure initiation

Conclusions

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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

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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

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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

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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 (-)

Stre

ss (P

a) J7C-AJ7C-BJ7C-CJ8C-BESPI SP1ESPI 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)

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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

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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