mechanical characterization of lead-free solder joints
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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
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