1 / 30

Constraining and size effects in lead-free solder joints

Constraining and size effects in lead-free solder joints. J. Cugnoni 1 , J. Botsis 1 , V. Sivasubramaniam 2 , J. Janczak-Rusch 2 1 Lab. Applied Mechanics & Reliability, EPFL, Switzerland 2 Füge- und Grenzflächentechnologie, EMPA, Switzerland. Nature of Irreversible Deformations. Objectives.

akamu
Download Presentation

Constraining and size effects in lead-free solder joints

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Constraining and size effects in lead-free solder joints J. Cugnoni1, J. Botsis1, V. Sivasubramaniam2, J. Janczak-Rusch2 1 Lab. Applied Mechanics & Reliability, EPFL, Switzerland 2 Füge- und Grenzflächentechnologie, EMPA, Switzerland J.Cugnoni, joel.cugnoni@epfl.ch

  2. Nature of Irreversible Deformations Objectives Global Project Interface Micro Structure ConstitutiveEquations Thermo-mechanical History Size / ConstrainingEffects Manufacturing Deformation & damage of lead-free solder joints • Plastic constitutive law of Sn-4.0Ag-0.5Cu solder • Variable solder gap width • Effects of constraints • Effects of size ? J.Cugnoni, joel.cugnoni@epfl.ch

  3. Rigid substrates: - impose lateral stresses at the interfaces - additionnal 3D stresses => apparent hardening => constraining effects Plastic deformation of solder: - constant volume - shrinks in lateral directions Solder joint in tension: - stiff elastic substrates - plastic solder (n~=0.5) Constraints in solder joints J.Cugnoni, joel.cugnoni@epfl.ch

  4. w g L t Parametric FE study • Goal: • study the constraining effects as a function of geometry • Method: • parametric FE simulation of 30 joint geometries with the same materials • parameters: • gap to thickness ratio G = g / t • width to thickness ratio W = w / t • indicators: • constraining effect ratio Q = (sujoint - susolder) / susolder • triaxiality ratio R = p / sm J.Cugnoni, joel.cugnoni@epfl.ch

  5. FEM Cu Cu Solder Stress field in constrained solder s11 s22 Front surface view 47 MPa 76 MPa 37 MPa 70 MPa Mid-plane view J.Cugnoni, joel.cugnoni@epfl.ch

  6. FEM Cu Cu Solder Stress field in constrained solder Von Mises eq. stress Hydrostatic pressure Front surface view 54 MPa -47 MPa 58 MPa -37 MPa Mid-plane view J.Cugnoni, joel.cugnoni@epfl.ch

  7. Parametric FE study: Results => Constraining effects are due to the the triaxiality of the stress field in the solder induced by the substrate J.Cugnoni, joel.cugnoni@epfl.ch

  8. Parametric FE study: Results => Constraining effects are inversely proportionnal to the gap to thickness ratio G (asymptotic effect in the form of 1/G) J.Cugnoni, joel.cugnoni@epfl.ch

  9. Parametric FE study: Results • Constraining effects are: • strongly dependent on the gap to thickness ratio G for G<0.5 • slightly affected by the width to thickness ratio W for W<2. J.Cugnoni, joel.cugnoni@epfl.ch

  10. Apparent stress - strain curve of the solder in a joint Constitutive law of the solder is needed for FE simulations is what we usually measure independent of geometry depends on geometry Constitutive law & constraints 3D FEM:includes all the geometrical effects ??? Inverse numerical identification of a 3D FEM J.Cugnoni, joel.cugnoni@epfl.ch

  11. In situ characterization method In-situ characterization of constitutive parameters Experimental SpecimenProduction TensileTest (DIC) Experimental Load - Displacement Curve Apparent engineering stress-strain response of the joint Constraining Effects Identification Loop Optimization (Least Square Fitting) Constitutive stress-strain lawof the solder Geometry FEM Simulated Load - Displacement Curve Numerical Simulations J.Cugnoni, joel.cugnoni@epfl.ch

  12. Experimental setup Tensile tests: • Sn-4.0Ag-0.5Cu solder • production: 1-2 min at 234°C (heating rate 3-4°C/min) and rapid cooling in water • 0.25 to 2.4 mm gap width • Displacement ramp 0.5 mm/s Digital Image Correlation: • is used to determine the displacement "boundary condition" near the solder layer • gauge length =~ 1.5 x solder gap • Displacement res. up to 0.1 mm J.Cugnoni, joel.cugnoni@epfl.ch

  13. micro - Digital Image Correlation micro-DIC measurements: • Requirements: • DIC needs medium & high frequency details in each sub images => random pattern • micro-measurements: spacial & displacement resolution limited mainly by the pattern • no change in magnification & no loss of focus => difficult with optical microscopy • Pattern created by: • rough polishing (contrast in reflexion, uniform light field) • spray paint (best results for global measurements) • Inkjet printing (in progress) 2 - 4 mm J.Cugnoni, joel.cugnoni@epfl.ch

  14. Digital Image Correlation algorithm DIC algorithm: • Features: • Custom developed in Matlab & C • Based on linear / cubic sub-pixel interpolation • Displacement and derivatives (optional) • Optimization: • original "brute" search • simplex or gradient based optimizer • hybrid "pyramidal" search & gradient optimizer • hybrid FFT-based DSC & gradient fine search • Performance: • up to 0.02 pixel displacement resolution (ideal pattern) 4 mm J.Cugnoni, joel.cugnoni@epfl.ch

  15. Similar results for G > 0.5 Constrained stress-strain curves Clear hardening for G < 0.5 Constraining & scale effects => can't compare these curves Identify constitutive properties J.Cugnoni, joel.cugnoni@epfl.ch

  16. Imposed displacement from testing Cu Simulated load-displacement curve Sn-Ag-Cu Elongation of solder Finite Element Modelling • 3D FEM of 1/8th of the specimen • Copper: • Elastic behaviour: ECu = 112 GPa, n = 0.3 • Solder: • Elasto-plastic with isotropic exponential & linear hardening • Chosen to fit bulk solder plastic response • 5 unknown parameters: J.Cugnoni, joel.cugnoni@epfl.ch

  17. Load - displacement curves Blue: initial load-displ. curve Red: identified load-displ. curveBlack: measured load-displ. curve Inverse identification procedure • Identification parameters: • Objective function e(a): • difference of measured and simulated load-displacement curves • non-linear least square optimization algorithm to solve: • Solution time: • 50 FE solutions required to identify the material properties (~2h) • Accuracy: • max error +/-4% on load – displacement curve J.Cugnoni, joel.cugnoni@epfl.ch

  18. Identified constitutive parameters Mechanical properties decreasing for smaller joints: combination of scale effects & porosity !! Manufacturing process is also size dependant !! Removed constraining effects => can compare with bulk specimen Bulk specimen appears much softer !! In-situ characterization !! J.Cugnoni, joel.cugnoni@epfl.ch

  19. + 15 % Constraining effects 2.4 mm J.Cugnoni, joel.cugnoni@epfl.ch

  20. + 22 % Constraining effects 1.2 mm J.Cugnoni, joel.cugnoni@epfl.ch

  21. + 30 % Constraining effects 0.7 mm J.Cugnoni, joel.cugnoni@epfl.ch

  22. + 37 % Constraining effects 0.5 mm J.Cugnoni, joel.cugnoni@epfl.ch

  23. + 78 % Constraining effects 0.25 mm J.Cugnoni, joel.cugnoni@epfl.ch

  24. decrease of yield & ultimate stress ~10 MPa Size effects constraining effects ~ 35 MPa J.Cugnoni, joel.cugnoni@epfl.ch

  25. 2.4mm 0.7mm 0.5mm (vacuum) Microstructure & Fractography Microstructure before testing Fractography • Pores: • created during manufacturing and grows with plastic deformation • introduces large scatter in experimental data => modelling? • interacts with the interfaces => critical defect!! • size of pores ~ constant for all gap but more influence in thinner joints J.Cugnoni, joel.cugnoni@epfl.ch

  26. FE model DIC measurements plastic damage & void growth in center => crack Damage mechanisms Thick Joint G>1 = small triaxiality Fractography J.Cugnoni, joel.cugnoni@epfl.ch

  27. Fractography DIC measurements Damage mechanisms FE model Thin joint G<0.5 = High triaxiality void growth & crack at interface J.Cugnoni, joel.cugnoni@epfl.ch

  28. Conclusions • Constraining effects: • Proportionnal to triaxility of the stress field in the solder • Inversely proportionnal to the gap to thickness ratio G • Can completely modify the solder joint response: • in an ideal case, ultimate stress increased by a factor of 6 compared to the ult. stress of the solder material itself • Must be taken into account in Characterization & Design • In-situ characterization method: • A versatile & powerful technique for characterization of small size & thin layer materials produced with realistic processing and geometry conditions • Can determine actual constitutive properties from constrained materials J.Cugnoni, joel.cugnoni@epfl.ch

  29. Conclusions • Size & scale effects in lead-free solders • Actual constitutive properties are size dependant: • In the present case, ult. stress decreases by 20% from 2.4mm to 0.2mm joints due to effects of porosity. • material scale effects & the "scaling" of the production methods have a combined influence. • Constraining effects: • Constraining effects are size dependant ~(1/G) with G=g/t • Up to 80% of additionnal hardening due to plastic constraints • solder joint response & constitutive properties are NOT equivalent • stress-strain response solder joint curves are geometry dependant => should not be compared for diff. geometries J.Cugnoni, joel.cugnoni@epfl.ch

  30. Realistic Experiment (DIC) Mixed num-expidentification: realistic properties Design / processvalidation FE Analysis & optimization Future developments • In-situ characterization: • Apply to shear tests • Extend to identification of visco-elasto-plasticity with damage • Reduced object size • Industrial aspects: • Apply the in-situ characterization method to an industrial electronic package (for example BGA) • Determination of the mechanical properties of a solder joint under realistic loading conditions (power-cycles) J.Cugnoni, joel.cugnoni@epfl.ch

More Related