11

Deformation and damage of lead free Deformation and damage of lead free materials and jointsmaterials and joints

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

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

SwitzerlandSwitzerland

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

Cost 531 Mid-Term Meeting, Lausanne, 25.02.2005Cost 531 Mid-Term Meeting, Lausanne, 25.02.2005

22

LMAF / EPFLLMAF / EPFLObjectives and tasksObjectives and tasks

Objectives:Objectives:

ManufacturingS

ize

/ Con

stra

inin

g

Eff

ects

Thermo-

mechanical H

istory

Micro S

tructure

Inte

rfac

e

Nature of Irreversible Deformations

ConstitutiveEquations

33

LMAF / EPFLLMAF / EPFL

Validation and Comparison

with SnPb

Compl

ete

Chara

cter

izat

ion

of

SnAgC

u

Inve

stig

atio

ns o

n Siz

e Effe

cts

Effects of Constraints

Objectives and tasksObjectives and tasks

Modelling

Experimental

Finite Element Model

Constitutive LawType

Mixed Num. / Exp.Identification

Micro StructureAnalysis

OpticalStrain

Measurement

Designof

Experiments

44

LMAF / EPFLLMAF / EPFLMechanical characterizationMechanical characterization

Elasto-plastic constitutive law :Elasto-plastic constitutive law :

...,,,, LTpvmy

...,,, LTEE cst

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

joints (size and constraining effects)joints (size and constraining effects) 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 a correlation between thermal history, microstructure and history, microstructure and constitutive behaviour must be foundconstitutive behaviour must be found

55

LMAF / EPFLLMAF / EPFLLead-free solders specimensLead-free solders specimens

Bulk solder specimen:Bulk solder specimen: Solder bar from manufacturer glued in special fixtures, 25 x 6 mm cylinder Solder bar from manufacturer glued in special fixtures, 25 x 6 mm cylinder

Idealized joint specimen:Idealized joint specimen: 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 fixture, temperature cycle: heated at 40 K/min up to melting joint cast in a special fixture, temperature cycle: heated at 40 K/min up to melting

point, held 60s in liquid phase, and then rapid cooling of the jig (water).point, held 60s in liquid phase, and then rapid cooling of the jig (water).

66

LMAF / EPFLLMAF / EPFL

Mechanical characterization of Mechanical characterization of constrained jointsconstrained joints

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

free solders in a real joint (constrained)free solders in a real joint (constrained) optical strain measurement technique to optical strain measurement technique to

measure the real strains of the solder only measure the real strains of the solder only (not the average strains of the joint)(not the average strains of the joint)

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

the surface or the natural pattern of the the surface or the natural pattern of the material is usedmaterial is used

the deformation of the surface pattern is the deformation of the surface pattern is observed through a microscope (24x) and observed through a microscope (24x) and recorded by a high resolution video camera recorded by a high resolution video camera (1.3 MPixels) at 1 frame per second(1.3 MPixels) at 1 frame per second

custom made video extensometry (Matlab) by custom made video extensometry (Matlab) by motion tracking based on a Normalized Cross motion tracking based on a Normalized Cross Correlation (NCC) or Digital Image Correlation (NCC) or Digital Image Correlation (DIC) algorithm Correlation (DIC) algorithm

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

77

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

0.0% 0.5% 1.0% 1.5% 2.0% 2.5% 3.0% 3.5% 4.0%

Strain

Str

ess (

Pa)

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

88

LMAF / EPFLLMAF / EPFLMixed num./exp. identificationMixed num./exp. identification

Identify constitutive properties even with very complex stress / strain field Identify constitutive properties even with very complex stress / strain field

=> use a finite element model instead of a simple analytical solution=> use a finite element model instead of a simple analytical solution

Mixed num. / exp. identification of the constitutive parameters

Initial guess x = x0

Numerical Solution Snum(x)

Identified parameters x

Experimental data Sexp

Error norm (Snum,Sexp)

Parameter updating x( minimization of )

> min

< min

N

i

iinum

F

uFuFnormError

1

2

exp

exp )()(

2

1 ],,[ 0 bQParameters y x

DIC measurement(Matlab) :

load - displacement curve

Non linear least squares

optimization (Matlab Optim Toolbox)

Parametric FEM (Matlab - Abaqus)

Bulk solder data

99

LMAF / EPFLLMAF / EPFLInitial guess / identified solutionInitial guess / identified solution

Initial guess Identified solution

ParamParam 0 0

(MPa)(MPa)QQinf inf

(MPa)(MPa)b b (-)(-)

InitialInitial 3232 1717 5050

FinalFinal 24.924.9 11.511.5 344344

Execution time ~ 1h, ~ 50 FE solutions

We can determine the real constitutive parameters of the

solder inside a constrained joint

1010

LMAF / EPFLLMAF / EPFLFuture workFuture work

Characterization of the solderCharacterization of the solder Identify the elasto-visco-plastic constitutive parameters with our mixed Identify the elasto-visco-plastic constitutive parameters with our mixed

numerical-experimental identification procedure and an optical strain numerical-experimental identification procedure and an optical strain measurementmeasurement

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

at different at different strain ratesstrain rates and and temperaturestemperatures

investigate the constraining and size effectsinvestigate the constraining and size effects Comparison with bulk solder properties at different strain rates / Comparison with bulk solder properties at different strain rates /

temperaturestemperatures

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