Deformation

1
Deformation damage of lead-free solder joints

• COST 531 Final Meeting,
• 17th-18th May 2007, Vienna
• J. Cugnoni1, J. Botsis1, V. Sivasubramaniam2, J.
  Janczak-Rusch2
• 1 Lab. Applied Mechanics Reliability, EPFL,
  Switzerland
• 2 Füge- und Grenzflächentechnologie, EMPA,
  Switzerland

2
Outline

• Overview of the project
• Global goals achievements
• Methods developments
• Experimental techniques
• Modelling
• Key results
• Elasto-plastic characterization of SAC405
• Constraining size effects
• Ductile failure effect of voids
• Future
• Bridging the length scales the disciplines

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Deformation damage of lead-free solder joints

• Size constraining effects
• Tensile / shear joints
• Effect of microstructure
• Effect of porosity content
• Failure mechanisms
• Ductile fracture
• Studied system
• SAC 405 / Cu substrates

?
4
Methods developments overview
5
Key results overview
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Thoughts about the future.

• Short term
• Time / temperature dependent properties.
• Interfacial failure cohesive elements
• Mid-Long term
• Bridging the length scales disciplines

Thermodynamics, phase diagrams
Meso
Solidification /diffusion simulation ?
Diffusion, interfaces, solidification,
microstructure
Micro
Homogenization
Continuum mechanics, damage, fracture
Macro
Need more transversal research !!
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Tensile shear specimens
Tensile specimenL120 mm, w20 mm, t1mm,
g0.25, 0.5, 0.75, 1.2, 2.4 mm Solder cross
section 20x1 mm2
Shear specimenL120mm, joint cross section2x2
mm2 Optimized for stress uniformity simple
manufacturing
thickness2mm
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Digital Image Correlation micro-level
measurements

• Why optical strain measurements??
• non-invasive measurements at a small scale
• DIC Principle
• Determine displacement for max correlation btw
  reference deformed states

Reference image
Deformed image
r(x,y)
d(x,y)
dx,dy
Advantages Versatile simple to setup
Robust in most cases

• Drawbacks
• - Resolution limited by pixel size
• - Need a random pattern

• Applications
• Local strain field measurements, small scale
  material characterization
• Finite Element validation parameter
  identification
• Fracture analysis, damage evolution

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Digital Image Correlation

• Why optical strain measurements??
• non-invasive measurements at a small scale
• DIC algorithms developments
• Tensile joints
• Small strains, small translations
• High accuracy is needed
• Spatial Correlation with cubic spline resampling
• Shear joints
• Extremely large strains, large displacement
• Need excellent robustness
• Incremental FFT-based correlation
• Advantages / Drawbacks
• Versatile simple to setup
• Robust in most cases
• - Resolution limited by pixel size
• - Need a random pattern

4 mm
10
ESPI measurements (STSM, D. Karalekas)

• Work done with Dr.Karalekas,Univ. Piraeus, Greece
  during a STSM at EPFL
• Advantages
• Sensitivity independant from magnification
  excellent for global observations
• Full field measurement
• Drawbacks
• Decorrelation
• Problems with creep tests
• Application
• Evaluate boundary conditions
• Full field displacement measurement on
  assemblies

20 mm
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DIC shear testing displacements
gt Extract the real boundary conditions for FE
analysis identification
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DIC shear testing shear strain exy
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Finite Element modelling

• Modelling? why??
• Models have the power of generalization of
  knowledge
• FE models
• Advantages
• Versatility Complex geometries,
  multi-components, multi-physics
• Ability to extrapolate knowledge gained on simple
  test cases to much more complex designs
  geometries !!
• Multi-scale modelling (homogenization)
• Drawback
• Requires an extensive reliable set of
  parameters gt huge characterization task

Combining Experiments Numerical simulation is
of prime importance
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Inverse num.-exp. identification
In-situ characterization of constitutive
parameters
Experimental
SpecimenProduction
TensileTest (DIC)
Experimental Load Displacement / Stress-Strain
response
Global / local responseof the specimen
Geometric structural effects
Identification Loop
Optimization (Least Square Fitting)
Modelling parametersConstitutive law, failure
model
Geometry BoundaryConditions
FEM
Simulated Load Displacement / Stress-Strain
response
Numerical Simulations
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• Constraining effectsTensile shear solder
  joints

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Constraints in tensile solder joints
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Parametric FE study Results
gt Constraining effects are due to the the
triaxiality (hydrostatic part) of the stress
field in the solder induced by the substrate
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Parametric FE study Results
Constraining effects are inversely proportionnal
to the gap to thickness ratio G in tensile joints
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Constraining effects experimental results
Q (sujoint - susolder) / susolder
Constraining effects Q 1/G
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Shear constraining effects
Parametric FE simulation of shear joint response
Pure shear isochoric deformation gt no
significant effects of constraints !!
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Shear Gap ultimate stress relationship

• Shear No significant effect of solder gap on
  ultimate stress

22
Constitutive law engineering response
3D FEMincludes all the geometrical effects
???
Inverse numerical identification of a 3D FEM
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• Size effects
• Tensile Shear solder joints

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Identified elasto-plastic law / size effects
Tensile joints
Mechanical properties decreasing for smaller
joints combination of scale effects porosity
Manufacturing process is also size dependant
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Identified elasto-plastic law / size effects
Shear joints
Size effect

• Tensile / shear joints
• similar elasto-plastic behaviours
• - similar size effects (manufacturing?)

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• Deformation damage mechanisms in lead-free
  solder joints

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Microstructure Fractography
Microstructure before testing
Fractography

• Pores
• created during manufacturing and grow with
  plastic deformation
• introduces large scatter in experimental data gt
  model void !!
• If porosity cannot be eliminated
• gt Include it in models as a  random  variable

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Porous metal plasticity Gurson-Tvergaard model

• Porosity content is an internal variable of the
  model f density ratio 1- void_fraction

Effect of voids
Yield surface
Hydrostatic pressure
Yield function without pores
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Shear joint response porous metal plasticity
Plastic Yielding
Void growth
Void nucleation
Ult. strain
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Ductile failure simulation
Void growth
Plastic Yielding
Void nucleation
Ult. strain

• Porous metal plasticity model can
• Predict the progressive ductile failure of metal
  up to rupture
• Simulate shear band formation localization
• Introducing  random  initial porosity gt
  statistical estimate of the failure strain in a
  given assembly