Laboratory of Applied Mechanics and Reliability: Research Activities

1. Laboratory of Applied Mechanics and Reliability: Research Activities • Experimental mechanics • Static, cyclic & fatigue testing, uniaxial / biaxial test, climatic chamber (-10°C to 250°C). • Vibration testing for reliability analysis and characterization • Bio-material testing: bone & ligament tensile testing (hydrostatic pressure, controlled environment). J.Cugnoni, joel.cugnoni@epfl.ch

2. Finite element simulation & design optimization • Non-linear static, temperature dependent elasto-visco-plasticity & contact mechanics (Abaqus Standard / TACT) • Non-linear transient analysis (Abaqus Explicit): impacts, drop tests • Vibration and modal dynamics: identification & optimization (MAFE) • Simulation of composite structures (static / dynamic) and design optimization (failure stress, residual stresses) • Bio materials: characterization of constitutive relations (bone, ligament, muscle), non-linear simulation & optimization (prosthetics) Stresses in a composite insulator subjected to bending (plasticity, contacts, composites, SEFAG AG) Deformation of a race catamaran (carbon-nomex sandwich, Decision D35) J.Cugnoni, joel.cugnoni@epfl.ch

3. 0.5 mm Mixed num. / exp. identification Initial parameters FE Solution Optimization Experimental data Error norm d > dmin d < dmin Identified parameters Innovative measurement / computing technologies Deformation of a cracked epoxy plate(ESPI) • embedded Bragg fiber sensors (OLCR) for measuring strain/stress distributions inside polymeric materials • in-plane and out-of-plane electronic speckle pattern interferometry (ESPI, resolution< 0.3mm) • versatile 2D digital image correlation (DIC, resolution up to 0.1mm) • mixed numerical – experimental identification for in-situ characterization of materials • multi-parametric finite element analysis & automated design optimization • Custom finite element softwares and tools for advanced simulations topics (adhesion-contact models, damping of thick composite shells,…) Deformation in a lead-free solder joint (DIC) Mixed num/exp identificationfor in-situ characterization of material properties J.Cugnoni, joel.cugnoni@epfl.ch

4. On-going projects • Deformation and damage of lead-free solder joints (in collaboration with EMPA, part of COST Action 531 Lead-free solders) • Ceramic-metal joints produced by novell brazing fillers (in collaboration with EMPA) • Collaboration with EMPA on the development of particle reinforced lead-free solder materials • Characterization of the elastic and damping properties in composite shells • Coupling adhesion and friction for modeling the interfaces in composite materials • Crack bridging and crack fiber interaction in composites • Experimental stress analysis using optical fiber sensors • Experimental and numerical studies in dental biomechanics • Durability of composite-metal joints: optimization of composite insulators (KTI / SEFAG AG) • Design and optimization of ultralight sandwich-core snowboards (KTI / Nidecker SA) Sandwich panel FEA Composite brazing material for ceramic-metal joints Lead-free solder joints: simulation & testing J.Cugnoni, joel.cugnoni@epfl.ch

5. Objectives Nature of Irreversible Deformations Designof Experiments Methodology Interface Micro Structure ConstitutiveEquations Experimental OpticalStrainMeasurement Micro Structure Analysis Constitutive LawType Finite Element Model Thermo-mechanical History Size / Constraining Effects Modelling Mixed Num. / Exp.Identification Manufacturing Project: deformation and damage of lead-free solder joints J.Cugnoni, joel.cugnoni@epfl.ch

6. Problem description • Mechanical properties of lead-free solder joints are highly process dependant • In-situ characterization of the mechanical properties in real solder joints can provide accurate data for modelling and optimizing the reliability of solder joints • Stress / strain fields inside the solder joints are very heterogeneous and classical characterization techniques are not suitable (no analytical solution) • By combining the advantages of finite element modelling and digital image correlation, a novell mixed numerical – experimental identification procedure can be used to extract accurate constitutive properties of the solder material from a real solder joint J.Cugnoni, joel.cugnoni@epfl.ch

7. Real experiment Tensile test on a real joint, DIC strain measurement (optical microscopy) Numerical experiment Finite element model with initial constitutive parameters (elasto plasticity) Optimization method Fit the constitutive parameters of FEM on experimental data (iterative non-linear least-square minimization) 200mm Measured & simulated load – displacement curves In-situ characterization of solder properties J.Cugnoni, joel.cugnoni@epfl.ch

8. Identification results Load - displacement curves Red: identified load-displ. curveBlack: measured load-displ. curve Relative errors • Identified parameters: Young’s modulus, yield stress, ultimate stress, exponential hardening rate • Solution time: ~1h / 40 FE solutions required to identify the material properties • Accuracy: max error +/-4% on load – displacement curve J.Cugnoni, joel.cugnoni@epfl.ch

9. Evolution of plastic deformations FEA Exp J.Cugnoni, joel.cugnoni@epfl.ch

10. 1 2 Effects of constraints & processing • Effects of the constraints imposed by the substrates (geometry dependant):comparison of the joint avg.stress / avg.strain curve and the actual solder stress-strain curve • Effects of the processing parameters: comparison of bulk & actual solder properties J.Cugnoni, joel.cugnoni@epfl.ch

11. FEA of plastic deformations in a solder joint Average Strain: 2%Max Strain: 10% 1 Outside Inside • Two plastic deformation regions: • At the interface on the outside surface • In the center of the joint 2 J.Cugnoni, joel.cugnoni@epfl.ch

12. Plasticity - Damage evolution: remarks • Need to distinguish two phenomena: • Elasto-plastic deformation: global behavior of the solder joint • Failure : local behavior, depends on the interfacial properties • Evolution during testing: • Plasticity is “homogeneous” up to 90% of max load => most of the stress-strain curve can be used for characterization • Failure in the last 15 seconds of the test: • concentration of deformations in the corner & on the interface • crack propagation: 1 crack on each interface (A), 1 single crack (B) A B J.Cugnoni, joel.cugnoni@epfl.ch

13. Next steps in the present project • Sn-4.0Ag-0.5Cu: • Evaluate the possible size effects • Identify the dependency of the mechanical properties wrt strain rate (viscous effects) & temperature • Particle-reinforced lead-free solder (with EMPA): • Sn-4.0Ag-0.5Cu + 5% Cu or Ni particles • Improvement of the processing parameters / material composition, study of the microstructure (EMPA) • Testing, in-situ characterization of the mechanical properties, modelling of the composite solder material, FE studies (EPFL) J.Cugnoni, joel.cugnoni@epfl.ch

14. Realistic Experiment (DIC) Mixed num-expidentification: realistic properties Design / processvalidation FE Analysis & optimization Future developments • Industrial aspects: • Apply the in-situ characterization method (DIC / mixed num./exp. Identification) to a real industrial package • Determination of the mechanical properties of a solder joint unter realistic loading conditions (power-cycles) • FE simulation, parametric study & optimization of a real package (geometry / material / processing) under realistic loading conditions (thermo-cycling, vibrations, …) J.Cugnoni, joel.cugnoni@epfl.ch