Physical and Mechanical Characteristics

of Pb-free Solder - Copper Joints

Project funded by OFES (Switzerland)

Within the framework of :Cost Action 531:Within the framework of :Cost Action 531:

J. Janczak-Rusch* , T. Rütti*,

A. Mellal@ and John Botsis@

*EMPA; @EPFL

New generation particle reinforced Pb-free solders: initial tests

• Experimental setup– tensile test specimen dimensions: 20mm x 1mm cross section– solder alloy: Sn-3.5Ag-0.5 Cu– soldering cycle: 6 min ( 30 s) to melt, 1 min ( 10 s) in liquid phase, fast

cooling (solder jig placed into water); peak temperature: 230°C– tensile test conditions: Instron 5848 MicroTester in displacement control

mode at 1µm/s; strain measured with a 50mm clip gauge

• 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, 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

Thermal cycling

Extensometer's displacement

0

0.005

0.01

0.015

0.02

0.025

0.03

0 50 100 150 200 250 300

Time (s)

Dis

pla

cem

ent

(mm

)

Recorded load

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100200

300400

500600

700800

900

0 50 100 150 200 250 300

Time (s)

Lo

ad (

N)

h

b

e

t

h = 50 mm

b = 20 mm

e = 1 mm

t = variable

Mechanical testing

Preliminary results• Results

– a notable number of gas pores form in the solder joint under the given soldering conditions

– yield stress, tensile stress, Young’s modulus and strain at fracture are not significantly influence by ageing at room temperature

– ageing at high temperatures reduces yield and tensile strengths and increases strain (ductility)

– decreased solder gap width increases yield and tensile strengths and decreases strain (ductility)

Ageing temperature

solder gap width

Yield strength

Tensile strength

Young’s modulus

Strain at fracture

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

TH = 0.60 0.1 47.8 ± 2.0 50.5 ± 5.7 109.8 ± 3.1 0.051 ± 0.006

TH = 0.60 1.0 29.7 ± 3.1 41.1 ± 2.6 100.8 ± 8.3 0.068 ± 0.009

TH = 0.75 1.0 27.3 ± 0.3 37.5 ± 0.1 104.4 ± 2.9 0.072 ± 0.017

TH = 0.90 1.0 23.7 ± 1.3 33.4 ± 2.0 98.4 ± 2.8 0.081 ± 0.013

Typical microstructures

cross-section (etched)

Sn-4.0Ag-0.5Cu solder joint

typical fracture surface

Purpose

Numerical simulation of solder joints mechanical behavior

Model description: Geometry (h = 50 mm, extensometer’s length) Boundary conditions: same as experiment, T=25°C FE Mesh (Plane stress, 2 axial symmetries) Constitutive law

•Copper: Elastoplastic (E=112 GPa, =0.33, y=69 MPa)

•Solder: elasto-viscoplastic (E=56 GPa, =0.35, y=32.5 MPa)

creep: Garofalo’s type power law

Finite Element Analysis

h

b

x

y

sin expncr Q

A BRT

Lead-free joint1mm gap

0

200

400

600

800

1000

0,000 0,005 0,010 0,015 0,020 0,025 0,030

Displacement (mm)

Fo

rce

(N)

Experiment Simulation

Calibration of constitutive model:

Finite Element Analysis

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

Simulation of different thickness specimen:

Finite Element Analysis

New generation particle reinforced Pb-free solders

• Next steps– soldering under vacuum/ reduced pressure to

eliminate gas pores– testing of different base solder alloys: Sn-Ag and

Sn-Cu compared to Sn-Ag-Cu– incorporation of strengthening particles into solder

alloy