1. 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. LMAF / EPFL 2 Objectives and tasks Objectives: Manufacturing Size / Constraining Effects Thermo- mechanical History Micro Structure Interface Nature of Irreversible Deformations Constitutive Equations

3. LMAF / EPFL 3 Validation and Comparison with SnPb Complete Characterization of SnAgCu Investigations on Size Effects Effects of Constraints Objectives and tasks Modelling Experimental Finite Element Model Constitutive Law Type Mixed Num. / Exp. Identification Micro Structure Analysis Optical Strain Measurement Design of Experiments

4. LMAF / EPFL 4 Mechanical characterization Elasto-plastic constitutive law : Characterization: should be carried out on real solder joints (size and constraining effects) 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 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 a correlation between thermal history, microstructure and constitutive behaviour must be found

5. LMAF / EPFL 5 Lead-free solders specimens Bulk solder specimen: Solder bar from manufacturer glued in special fixtures, 25 x 6 mm cylinder 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 Dimension: 120 x 20 x 1 mm, joint thickness from 0.1 to 1 mm Solder: ECOREL Sn-4.0Ag-0.5Cu Solder: ECOREL Sn-4.0Ag-0.5CuProduction: 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). 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. LMAF / EPFL 6 Mechanical characterization of constrained joints Objectives characterize the stress - strain law of lead- free solders in a real joint (constrained) 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 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 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 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 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  m, strain 0.01% Resolution: displacement 0.2  m, strain 0.01%

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

8. LMAF / EPFL 8 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 Mixed num. / exp. identification of the constitutive parameters Initial guess x = x 0 Numerical Solution S num (x) Identified parameters x Experimental data S exp Error norm  (S num,S exp ) Parameter updating x ( minimization of  )  >  min  <  min DIC measurement (Matlab) : load - displacement curve Non linear least squares optimization (Matlab Optim Toolbox) Parametric FEM (Matlab - Abaqus) Bulk solder data

9. LMAF / EPFL 9 Initial guess / identified solution Initial guessIdentified solutionParam  0 (MPa) Q inf (MPa) b (-) Initial321750 Final24.911.5344 Execution time ~ 1h, ~ 50 FE solutions We can determine the real constitutive parameters of the solder inside a constrained joint

10. LMAF / EPFL 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 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 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 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 Evaluate the evolution of micro structure and mechanical properties in function of the thermal history