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Deformation and damage of lead free materials and joints

Deformation and damage of lead free materials and joints. J. Cugnoni*, A. Mellal*, Th. Rütti @ , J. Janczak @ , J. Botsis* * LMAF / EPFL ; @ EMPA Switzerland Project funded by OFES (CH) Cost 531 Mid-Term Meeting, Lausanne, 25.02.2005. Nature of Irreversible Deformations. Interface.

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Deformation and damage of lead free materials and joints

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  1. Deformation and damage of lead free materials and joints J. Cugnoni*, A. Mellal*, Th. Rütti@, J. Janczak@, J. Botsis* * LMAF / EPFL ; @ EMPA Switzerland Project funded by OFES (CH) Cost 531 Mid-Term Meeting, Lausanne, 25.02.2005

  2. Nature of Irreversible Deformations Interface Micro Structure Thermo-mechanical History Size / Constraining Effects Manufacturing Objectives and tasks Objectives: ConstitutiveEquations

  3. Designof Experiments Validation and Comparison with SnPb Complete Characterization of SnAgCu Experimental OpticalStrainMeasurement Micro Structure Analysis Constitutive LawType Finite Element Model Modelling Effects of Constraints Investigations on Size Effects Mixed Num. / Exp.Identification Objectives and tasks

  4. Mechanical characterization • Elasto-plastic constitutive law : • Characterization: • should be carried out on real solder joints (size and constraining effects) • temperature, strain rate and joint thickness are independent parameters and must be changed • a correlation between thermal history, microstructure and constitutive behaviour must be found

  5. Lead-free solders specimens • Bulk solder specimen: • Solder bar from manufacturer glued in special fixtures, 25 x 6 mm cylinder • Idealized joint specimen: • Dimension: 120 x 20 x 1 mm, joint thickness from 0.1 to 1 mm • Solder: ECOREL Sn-4.0Ag-0.5Cu • Production: • joint cast in a special fixture, temperature cycle: heated at 40 K/min up to melting point, held 60s in liquid phase, and then rapid cooling of the jig (water).

  6. Mechanical characterization of constrained joints • Objectives • characterize the stress - strain law of lead-free solders in a real joint (constrained) • optical strain measurement technique to measure the real strains of the solder only (not the average strains of the joint) • Optical measurement technique • a grid of fine dots (pitch = 0.2 mm) glued on the surface or the natural pattern of the material is used • the deformation of the surface pattern is observed through a microscope (24x) and recorded by a high resolution video camera (1.3 MPixels) at 1 frame per second • custom made video extensometry (Matlab) by motion tracking based on a Normalized Cross Correlation (NCC) or Digital Image Correlation (DIC) algorithm • Resolution: displacement 0.2 mm, strain 0.01%

  7. Bulk solder properties • Preliminary results: • specimens of pure solder produced in several ways • important effects of thermal history and processing • properties must be characterized "in-situ"

  8. Mixed num. / exp. identification of the constitutive parameters Initial guess x = x0 Numerical SolutionSnum(x) Parameter updatingx( minimization ofd) Experimental dataSexp Error normd(Snum,Sexp) d > dmin d < dmin Identified parametersx Mixed num./exp. identification Identify constitutive properties even with very complex stress / strain field => use a finite element model instead of a simple analytical solution Bulk solder data Parametric FEM (Matlab - Abaqus) DIC measurement (Matlab) : load - displacement curve Non linear least squares optimization (Matlab Optim Toolbox)

  9. Initial guess / identified solution Initial guess Identified solution We can determine the real constitutive parameters of the solder inside a constrained joint Execution time ~ 1h, ~ 50 FE solutions

  10. Future work • Characterization of the solder • Identify the elasto-visco-plastic constitutive parameters with our mixed numerical-experimental identification procedure and an optical strain measurement • at a given strain rate and room temperature, with variable joint thickness (size / constraining effects) • at different strain rates and temperatures • investigate the constraining and size effects • Comparison with bulk solder properties at different strain rates / temperatures • Microstructure evolution (in collaboration with EMPA, Switzerland) • Correlate the mechanical properties with the microstructure of the solder • Evaluate the evolution of micro structure and mechanical properties in function of the thermal history

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