Mechanical characterization of lead-Mechanical characterization of lead-free solder jointsfree solder joints

J. Cugnoni*, A. Mellal*, Th. RüttiJ. Cugnoni*, A. Mellal*, Th. Rütti@@, J. Janczak, J. Janczak@@, Pr. J. Botsis*, Pr. J. Botsis*

* LMAF / EPFL ; * LMAF / EPFL ; @@ EMPA EMPA

SwitzerlandSwitzerland

Project funded by OFES (CH)Project funded by OFES (CH)

Cost 531 WG 5 & 6 Meeting, Vienna 17.01.05Cost 531 WG 5 & 6 Meeting, Vienna 17.01.05

LMAF / EPFLLMAF / EPFLObjectives and tasksObjectives and tasks

Objectives:Objectives: identify the nature of irreversible deformation and damage;identify the nature of irreversible deformation and damage; correlate the role of micro structure on the deformation and damage correlate the role of micro structure on the deformation and damage

mechanismsmechanisms examine the role of interface on deformation and damage of a joint;examine the role of interface on deformation and damage of a joint; identify appropriate constitutive equations;identify appropriate constitutive equations; characterise the role of the thermo-mechanical loading histories on the characterise the role of the thermo-mechanical loading histories on the

constitutive behaviour of the material and durability of various joints; constitutive behaviour of the material and durability of various joints; compare the results with those of the standard alloy (Sn63Pb37). compare the results with those of the standard alloy (Sn63Pb37).

TasksTasks design of experiments design of experiments optical strain field measurement optical strain field measurement observation of microstructural effectsobservation of microstructural effects identify constitutive laws for the lead-free alloyidentify constitutive laws for the lead-free alloy construct numerical models construct numerical models comparison and validationcomparison and validation

LMAF / EPFLLMAF / EPFLMechanical characterizationMechanical characterization

The elasto-plastic constitutive law The elasto-plastic constitutive law may depend on:may depend on:

strain rate and temperature strain rate and temperature microstructure and thermal history microstructure and thermal history

(processing / ageing)(processing / ageing) geometrical / mechanical constraintsgeometrical / mechanical constraints characteristic size and scale effectscharacteristic size and scale effects

Characterization:Characterization: should be carried out on real solder should be carried out on real solder

jointsjoints temperature, strain rate and joint temperature, strain rate and joint

thickness are independent thickness are independent parameters and must be changedparameters and must be changed

a correlation between thermal history, a correlation between thermal history, microstructure and constitutive microstructure and constitutive behaviour must be foundbehaviour must be found

LMAF / EPFLLMAF / EPFLLead-free solder joints specimensLead-free solder joints specimens

Specimen specificationsSpecimen specifications Dimension: 120 x 20 x 1 mm, joint thickness from 0.1 to 1 mmDimension: 120 x 20 x 1 mm, joint thickness from 0.1 to 1 mm Solder: ECOREL Sn-4.0Ag-0.5CuSolder: ECOREL Sn-4.0Ag-0.5Cu

Production: Production: joint cast in a special jigjoint cast in a special jig temperature cycle: heated at 40 K/min up to melting point, held 60s in temperature cycle: heated at 40 K/min up to melting point, held 60s in

liquid phase, and then rapid cooling of the jig (water).liquid phase, and then rapid cooling of the jig (water).

LMAF / EPFLLMAF / EPFLMechanical testingMechanical testing

Mechanical testing:Mechanical testing: displacement control, 1displacement control, 1m/s up to rupturem/s up to rupture 50 mm extensometer => average strain in the specimen50 mm extensometer => average strain in the specimen

Effects of the joint thickness on mechanical propertiesEffects of the joint thickness on mechanical properties decreased solder gap width increases yield and tensile strengths and decreased solder gap width increases yield and tensile strengths and

decreases strain (ductility)decreases strain (ductility) large scatter probably mostly due to gas porosity and the averaging effect large scatter probably mostly due to gas porosity and the averaging effect

of the strain measurementsof the strain measurements

Solder gap width

Yield strength

Tensile strength

Young’s modulus

Strain at fracture

[µm] [MPa] [MPa] [GPa] [%]

181 41.8 ± 0.1 42.9 ± 3.3 114.4 ± 13.1 0.042% ± 0.007%

204 40.9 ± 3.0 44.5 ± 2.7 100.6 ± 4.5 0.050% ± 0.001%

395 36.3 ± 5.6 42.4 ± 6.1 107.6 ± 9.1 0.048% ± 0.004%

526 34.7 ± 4.2 41.0 ± 3.7 101.8 ± 6.3 0.056% ± 0.004%

611 29.4 ± 1.2 42.5 ± 0.5 95.3 ± 7.6 0.078% ± 0.008%

795 39.6 ± 5.6 46.9 ± 5.4 92.7 ± 4.7 0.072% ± 0.015%

935 39.3 ± 1.1 49.5 ± 1.5 105.7 ± 9.0 0.077% ± 0.018%

1107 35.8 ± 1.3 46.4 ± 1.3 90.8 ± 19.3 0.076% ± 0.005%

LMAF / EPFLLMAF / EPFLAgeingAgeing

Test matrixTest matrix effect of solder gap width: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8 and 1.0mmeffect 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 (Teffect of room temperature ageing (THH= 0.6): 1day, 2 days, 1 week, 2 weeks, 1 month, 2 = 0.6): 1day, 2 days, 1 week, 2 weeks, 1 month, 2

monthsmonths effect of ageing at elevated temperatures: 1 week and 2 weeks at Teffect of ageing at elevated temperatures: 1 week and 2 weeks at THH =0.75 and T =0.75 and THH = 0.9 = 0.9

Effects of ageingEffects of ageing no visible influence of ageing at room temperatureno visible influence of ageing at room temperature ageing at high temperatures reduces yield and tensile strengths and increases strain ageing at high temperatures reduces yield and tensile strengths and increases strain

(ductility)(ductility)

TH = 0.90

TH = 0.75

TH = 0.60

TH = 0.60

Ageing temperature

0.081 ± 0.01398.4 ± 2.833.4 ± 2.023.7 ± 1.31.0

0.072 ± 0.017104.4 ± 2.937.5 ± 0.127.3 ± 0.31.0

0.068 ± 0.009100.8 ± 8.341.1 ± 2.629.7 ± 3.11.0

0.051 ± 0.006109.8 ± 3.150.5 ± 5.747.8 ± 2.00.1

[%][GPa][MPa][MPa][mm]

Strain at fracture

Young’s modulus

Tensile strength

Yield strength

solder gap width

TH = 0.90

TH = 0.75

TH = 0.60

TH = 0.60

Ageing temperature

0.081 ± 0.01398.4 ± 2.833.4 ± 2.023.7 ± 1.31.0

0.072 ± 0.017104.4 ± 2.937.5 ± 0.127.3 ± 0.31.0

0.068 ± 0.009100.8 ± 8.341.1 ± 2.629.7 ± 3.11.0

0.051 ± 0.006109.8 ± 3.150.5 ± 5.747.8 ± 2.00.1

[%][GPa][MPa][MPa][mm]

Strain at fracture

Young’s modulus

Tensile strength

Yield strength

solder gap width

LMAF / EPFLLMAF / EPFLA first modelling approachA first modelling approach

The elasto-visco-plastic model (Garofalo) of classical lead The elasto-visco-plastic model (Garofalo) of classical lead solders (Shi et al., 1999 ) has been adapted to lead-free solders (Shi et al., 1999 ) has been adapted to lead-free solders:solders:

yield stress and Young's modulus adjusted for lead-free soldersyield stress and Young's modulus adjusted for lead-free solders hardening parameters from the classical lead solders hardening parameters from the classical lead solders

TR

QBA ncr exp)(sinh

0

10

20

30

40

50

60

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Plastic strain

Str

ess

(MP

a)

Sn-Pb (Shi et al.) Sn-Ag-Cu

Young modulus(GPa)

Poisson’s ratio

Elastic behavior

56 0.35

Plasticity

Yield stress = 32.5 (MPa)Linear hardening up to rupture:

Ultimate stress = 33 (MPa)Ultimate strain = 0.02 (-)

Creep behavior

A = 96200 (sec-1)B = 0.087 (MPa-1)

n = 3.3Q = 67437 (J mol-1)

R=8.314 (J mol-1 K-1)

LMAF / EPFLLMAF / EPFLA first modelling approachA first modelling approach

Finite element simulation of real experiments to test the Finite element simulation of real experiments to test the "adjusted" constitutive law:"adjusted" constitutive law:

modelling of both copper and solder jointmodelling of both copper and solder joint real recorded (extensometer) displacements are applied to the real recorded (extensometer) displacements are applied to the

FEM => simulated loadsFEM => simulated loads Constitutive law shows a good agreement with experiments for Constitutive law shows a good agreement with experiments for

thick joints (1mm) but must be improved for thin joints (0.15 mm)thick joints (1mm) but must be improved for thin joints (0.15 mm)Lead-free joint

1mm gap

0

100

200

300

400

500

600

700

800

900

0.000 0.005 0.010 0.015 0.020 0.025 0.030Displacement (mm)

Fo

rce

(N)

Experiment Simulation

Lead-free joint0.15mm gap

0

200

400

600

800

1000

1200

0.000 0.005 0.010 0.015 0.020 0.025 0.030

Displacement (mm)

Forc

e (N

)

Experiment Simulation

LMAF / EPFLLMAF / EPFLBulk solder propertiesBulk solder properties

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

Bulk solder stress-strain curves

0.00E+00

1.00E+07

2.00E+07

3.00E+07

4.00E+07

5.00E+07

6.00E+07

7.00E+07

0.00% 0.20% 0.40% 0.60% 0.80% 1.00% 1.20% 1.40% 1.60% 1.80% 2.00%

Strain

Str

ess

(Pa)

PureSolder SnAg Pure Solder SnAgCu (casted, slow cooling) Pure Solder SnAgCu (bulk)

LMAF / EPFLLMAF / EPFL

Mechanical characterization of Mechanical characterization of constrained jointsconstrained joints

ObjectivesObjectives characterize the stress - strain law of characterize the stress - strain law of

lead-free solders in a real joint lead-free solders in a real joint (constrained)(constrained)

optical strain measurement technique to optical strain measurement technique to measure the real strains of the solder measure the real strains of the solder only (not the average strains of the joint)only (not the average strains of the joint)

Optical measurement techniqueOptical measurement technique a grid of fine dots (pitch = 0.2 mm) is a grid of fine dots (pitch = 0.2 mm) is

glued on the surface of the specimenglued on the surface of the specimen the deformation of the grid is observed the deformation of the grid is observed

with a microscope (24x) and recorded with a microscope (24x) and recorded through a high resolution video camera through a high resolution video camera (1.3 MPixels) at 1 fps(1.3 MPixels) at 1 fps

video extensometry by motion tracking video extensometry by motion tracking based on a Normalized Cross Correlation based on a Normalized Cross Correlation algorithm (NCC)algorithm (NCC)

Resolution: displacement 0.2 Resolution: displacement 0.2 m, strain m, strain 0.01%0.01%

LMAF / EPFLLMAF / EPFL

Mechanical characterization of Mechanical characterization of constrained jointsconstrained joints

Preliminary results:Preliminary results: Solder joint properties showing the constraining effects: Solder joint properties showing the constraining effects:

Yield stress, ultimate stress and ultimate strain are modified by the constraintsYield stress, ultimate stress and ultimate strain are modified by the constraints

Properties must be determined in the most realistic conditionsProperties must be determined in the most realistic conditionsSolder joint properties

0.0E+00

5.0E+06

1.0E+07

1.5E+07

2.0E+07

2.5E+07

3.0E+07

3.5E+07

4.0E+07

4.5E+07

0.00% 0.20% 0.40% 0.60% 0.80% 1.00% 1.20% 1.40% 1.60% 1.80% 2.00%

Strain

Str

ess

(Pa)

Solder only (VideoExt) Average strain (over 15mm) Bulk Solder SnAgCu

LMAF / EPFLLMAF / EPFLFuture workFuture work

Characterization of the solderCharacterization of the solder Compare the experimental stress-strain curve with the predictions of a Compare the experimental stress-strain curve with the predictions of a

FEM based on the bulk solder properties to evaluate the possibility to FEM based on the bulk solder properties to evaluate the possibility to use directly the bulk solder stress-strain curve in real applicationsuse directly the bulk solder stress-strain curve in real applications

Identify the elasto-visco-plastic constitutive parameters by a mixed Identify the elasto-visco-plastic constitutive parameters by a mixed numerical-experimental identification procedurenumerical-experimental identification procedure

at a given strain rate and room temperature, with variable joint thickness at a given strain rate and room temperature, with variable joint thickness (size / constraining effects)(size / constraining effects)

at different strain rates and temperatures at different strain rates and temperatures

Microstructure evolution Microstructure evolution (in collaboration with EMPA, Switzerland)(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 Evaluate the evolution of micro structure and mechanical properties in

function of the thermal historyfunction of the thermal history Improve the mechanical properties by inclusion of strengthening Improve the mechanical properties by inclusion of strengthening

particlesparticles