Mechanical characterization of lead-free solder joints

1. Mechanical characterization of lead-free solder joints J. Cugnoni*, A. Mellal*, Th. Rütti@, J. Janczak@, Pr. J. Botsis* * LMAF / EPFL ; @ EMPA Switzerland Project funded by OFES (CH) Cost 531 WG 5 & 6 Meeting, Vienna 17.01.05

2. Objectives and tasks Objectives: • identify the nature of irreversible deformation and damage; • correlate the role of micro structure on the deformation and damage mechanisms • examine the role of interface on deformation and damage of a joint; • identify appropriate constitutive equations; • characterise the role of the thermo-mechanical loading histories on the constitutive behaviour of the material and durability of various joints; • compare the results with those of the standard alloy (Sn63Pb37). Tasks • design of experiments • optical strain field measurement • observation of microstructural effects • identify constitutive laws for the lead-free alloy • construct numerical models • comparison and validation

3. Mechanical characterization • The elasto-plastic constitutive law may depend on: • strain rate and temperature • microstructure and thermal history (processing / ageing) • geometrical / mechanical constraints • characteristic size and scale effects • Characterization: • should be carried out on real solder joints • 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

4. Lead-free solder joints specimens • Specimen specifications • 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 jig • temperature cycle: heated at 40 K/min up to melting point, held 60s in liquid phase, and then rapid cooling of the jig (water).

5. Mechanical testing • Mechanical testing: • displacement control, 1mm/s up to rupture • 50 mm extensometer => average strain in the specimen • Effects of the joint thickness on mechanical properties • decreased solder gap width increases yield and tensile strengths and decreases strain (ductility) • large scatter probably mostly due to gas porosity and the averaging effect of the strain measurements

6. Ageing • Test matrix • effect of solder gap width: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 and 1.0mm • effect of room temperature ageing (TH= 0.6): 1day, 2 days, 1 week, 2 weeks, 1 month, 2 months • effect of ageing at elevated temperatures: 1 week and 2 weeks at TH =0.75 and TH = 0.9 • Effects of ageing • no visible influence of ageing at room temperature • ageing at high temperatures reduces yield and tensile strengths and increases strain (ductility)

7. A first modelling approach • The elasto-visco-plastic model (Garofalo) of classical lead solders (Shi et al., 1999 ) has been adapted to lead-free solders: • yield stress and Young's modulus adjusted for lead-free solders • hardening parameters from the classical lead solders

8. A first modelling approach • Finite element simulation of real experiments to test the "adjusted" constitutive law: • modelling of both copper and solder joint • real recorded (extensometer) displacements are applied to the FEM => simulated loads • Constitutive law shows a good agreement with experiments for thick joints (1mm) but must be improved for thin joints (0.15 mm)

9. 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"

10. 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) is glued on the surface of the specimen • the deformation of the grid is observed with a microscope (24x) and recorded through a high resolution video camera (1.3 MPixels) at 1 fps • video extensometry by motion tracking based on a Normalized Cross Correlation algorithm (NCC) • Resolution: displacement 0.2 mm, strain 0.01%

11. Mechanical characterization of constrained joints • Preliminary results: • Solder joint properties showing the constraining effects: • Yield stress, ultimate stress and ultimate strain are modified by the constraints • Properties must be determined in the most realistic conditions

12. Future work • Characterization of the solder • Compare the experimental stress-strain curve with the predictions of a FEM based on the bulk solder properties to evaluate the possibility to use directly the bulk solder stress-strain curve in real applications • Identify the elasto-visco-plastic constitutive parameters by a mixed numerical-experimental identification procedure • at a given strain rate and room temperature, with variable joint thickness (size / constraining effects) • at different strain rates and 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 • Improve the mechanical properties by inclusion of strengthening particles