COST Action 531: -

baconossifiedMechanics

Oct 29, 2013 (3 years and 9 months ago)

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Lambrinou Konstantina



imec restricted 2007

1

COST Action 531:

“Lead
-
Free Solder Materials”

Final Meeting

17
-
18 May 2007

Vienna, Austria

Bulk Embrittlement of Sn
-
Based
Pb
-
Free Solder Alloys

Konstantina Lambrinou

IPSI/REMO Group

IMEC, Leuven, Belgium

Lambrinou Konstantina



imec restricted 2007

3

Outline


General Introduction


Brittle vs. Ductile Failure


Factors Affecting the Fracture Resistance



Impact Testing of Sn
-
Based Pb
-
Free Solders


Impact Testing of Bulk Solder Specimens


Impact Testing of Solder Joints



Conclusions



References & Acknowledgments

Lambrinou Konstantina



imec restricted 2007

4

[1]

Brittle vs. Ductile Fracture



Brittle

fracture
:

occurs

with

little

or

no

plastic

deformation

prior

to

failure,

and

at

high

speeds

(
e
.
g
.

2000

m/s

in

steels)



Ductile

fracture
:

characterised

by

appreciable

plastic

deformation

prior

to

failure
,

and

high

energy

consumption



Toughness
:

ability of a material to
resist fracture



Ductility
:

ability of a material to
deform plastically without fracturing

High toughness:

combination of
strength and
ductility

[2]

Lambrinou Konstantina



imec restricted 2007

5

Factors Affecting the Fracture Resistance

Material

Composition

Crystal structure

Microstructure


Conditions

Processing Conditions

Service Conditions


Properties

Strength,
E,
Fracture Behaviour/Resistance
,

Ductile
-
to
-
Brittle Transition
, etc.


‘Intrinsic’ Factors

‘Extrinsic’ Factors

Lambrinou Konstantina



imec restricted 2007

6

Factors Affecting the Fracture Resistance


Composition

(alloy

selection,

addition

of

elements

that

increase

toughness

or

removal

of

those

that

degrade

it)



Microstructure

(grain

size,

size

and

spatial

distribution

of

second
-
phase

particles,

orientation

of

flaws)



Crystal

structure
,

nature

of

electron

bond
,

atomic

order



Presence

of

notches

(internal,

external)



Service

conditions

(temperature,

strain

rate,

constraint)

Lambrinou Konstantina



imec restricted 2007

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Dislocations and Plastic Deformation


Dislocations

enable

the

plastic

deformation

of

metallic

materials

by

means

of

a

process

known

as

slip


Slip

is

the

process

of

dislocation

motion

that

results

in

the

plastic

deformation

of

crystalline

materials

[3]

[3]

Lambrinou Konstantina



imec restricted 2007

8

Crystal Structure and Fracture Resistance


Peierls
-
Nabarro

stress

(P
-
N

stress
)
:

stress

required

to

move

dislocations

through

a

crystal

lattice


Close
-
packed

materials,

like

fcc

and

hcp

metals
,

exhibit

lower

P
-
N

stress

than

bcc

and

bct

metals


P
-
N

stress

is

temperature
-
dependent
:

it

increases

as

the

temperature

decreases!


Yield

strength

and

P
-
N

stress

are

interrelated,

and

so

are

their

temperature

dependences


Al alloy

(fcc)

[3]

Stress


Temperature



u


0

Small flaw

Limit for large
flaws

NDT


With flaw

NDT


Flaw free

5,000 psi

A

B

C

D

E

F

L

H

J

K

Steel

(bcc)

[4]

bct
(Sn)

bcc

Lambrinou Konstantina



imec restricted 2007

9

Crystal Structure and Fracture Resistance


Relative

change

of

yield

to

tensile

strength

in

bcc

(and

bct
)

metals

leads

to

low
-
temperature

embrittlement
!

[2]

Ductile
-
to
-
Brittle
Transition Temperature
(DBTT)

Lambrinou Konstantina



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10

Notches and Fracture Resistance


Notches

create a
triaxial stress state

in the material


Notch

toughness
:

the

ability

of

a

material

to

absorb

energy

in

the

presence

of

a

sharp

notch



IMCs
:
internal ‘notches’
!

[3]

50

m

TC:

0

-

100
o
C

SAC 405

10

m

Ag
3
Sn IMC

Lambrinou Konstantina



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Strain Rate, Temperature and Fracture Resistance


Slow loading rate
:

max load in


10 s; d

/
dt


10
-
5

s
-
1


Intermediate
loading rate
:

max load in 1 s; d

/
dt


10
-
3

s
-
1


Dynamic
loading rate
:

max load in


0.001 s; d

/
dt


10 s
-
1


Fracture

toughness

of

bcc/bct

metals
:

increases

with

increasing

temperature

and

decreasing

loading

rate

Low yield strength steel

[1]

[1]

Lambrinou Konstantina



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12

Material Constraint and Fracture Resistance


Constraint
:

refers

mainly

to

the

transition

from

plane
-
stress

to

plane
-
strain

condition

in

the

material


Plane
-
stress
:

stress

is

zero

in

the

thickness

direction


Plane
-
strain
:

strain

is

zero

in

direction

normal

to

both

axis

of

applied

stress

and

direction

of

crack

growth


Plane
-
Stress

Plane
-
Strain

Min Constraint

Max Constraint

[2]

[2]

Lambrinou Konstantina



imec restricted 2007

13

Material Constraint and Fracture Resistance


Change

in

the

sample

thickness

changes

the

degree

of

constraint
:

plane
-
stress

to

plane
-
strain

condition


Change

in

the

sample

thickness

may

shift

the

DBTT
!

[2]

A283 steel

Lambrinou Konstantina



imec restricted 2007

14

Outline


General Introduction


Brittle vs. Ductile Failure


Factors Affecting the Fracture Resistance



Impact Testing of Sn
-
Based Pb
-
Free Solders


Impact Testing of Bulk Solder Specimens


Impact Testing of Solder Joints



Conclusions



References & Acknowledgments

Lambrinou Konstantina



imec restricted 2007

15

Charpy V
-
Notch (CVN) Impact Testing


CVN Impact Testing
:

reproduces very strenuous service
conditions (
high strain rates
,
triaxial stress state

due to
the presence of sharp notches, and
low temperatures
)

[]

ASTM E 23
-
06

[5]


‘Mini
-
Charpy’ setup
:

real solder joint sizes!

IMEC, Belgium

[3]

[3]

Lambrinou Konstantina



imec restricted 2007

16

CVN Impact Tests of Bulk Sn
-
Based Solders


Tested

solder

alloys
:

SAC

305
,

SAC

405
,

99
.
99
%
Sn
,

Sn
-
5
%
Ag
,

Sn
-
0
.
7
%
Cu
,

Sn
-
0
.
7
%
Cu
-
0
.
1
%
Ni
,

Sn
-
37
%
Pb


Test

temperature
:

-
195
o
C

to

+
100
o
C

Notch
: 2.5 mm

10

10

55 mm
3

[6, 7]

5

5

55 mm
3

[6, 7]

Notch
: 1.3 mm

Notch
: 2.5 mm

5 μm



Behaviour of Sn37Pb
:
compromise between
Pb
-
rich phase (
fcc
)

and
Sn
-
rich phase (
bct
)

Lambrinou Konstantina



imec restricted 2007

17

Results from CVN Impact Tests on Bulk Samples:

SAC 405

vs.
99.99%Sn

Intergranular fracture

Test at 20

C

Test at
-
75

C

SAC 405

Test at 20

C

Test at
-
190

C

99.99% Sn

Lambrinou Konstantina



imec restricted 2007

18

Results from Mini
-
Charpy Impact Tests on

SAC 405

Solder Joints

Test at 23

C

Test at
-
88

C

Test at 23

C

Test at
-
41

C

Test at
-
88

C

Test at
-
78

C

Lambrinou Konstantina



imec restricted 2007

19

Results from Mini
-
Charpy Impact Tests on

SAC 305

Solder Joints

Test at 24

C

Test at
-
104

C

Test at 23

C

Test at
-
51

C

Test at
-
104

C

Test at
-
85

C

Lambrinou Konstantina



imec restricted 2007

20

Outline


General Introduction


Brittle vs. Ductile Failure


Factors Affecting the Fracture Resistance



Impact Testing of Sn
-
Based Pb
-
Free Solders


Impact Testing of Bulk Solder Specimens


Impact Testing of Solder Joints



Conclusions



References & Acknowledgments

Lambrinou Konstantina



imec restricted 2007

21

Conclusions


The

fracture

behaviour

of

Sn
-
based

Pb
-
free

solders

is

very

similar

to

that

of

bcc

metals,

due

to

the

similarity

of

the

bcc

and

bct

(Sn)

crystal

structures


The

fracture

behaviour

of

Sn
-
based

solder

alloys

is

affected

by
:


the

service

conditions

(
temperature
,

strain

rate
,

and

degree

of

material

constraint
)


the

size

distribution
,

spacing
,

and

acuity

of

IMCs


At

low

temperatures,

embrittlement

of

Sn

is

a

fact!


When

testing

a

Sn
-
based

solder

alloy

with

a

certain

composition

in

impact
,

it

is

important

to

realise

that

the

sample

size

affects

the

exact

DBTT

value!

Lambrinou Konstantina



imec restricted 2007

22

Outline


General Introduction


Brittle vs. Ductile Failure


Factors Affecting the Fracture Resistance



Impact Testing of Sn
-
Based Pb
-
Free Solders


Impact Testing of Bulk Solder Specimens


Impact Testing of Solder Joints



Conclusions



References & Acknowledgments

Lambrinou Konstantina



imec restricted 2007

23

References
(1)

[
1
]

J
.
M
.

Barsom,

S
.
T
.

Rolfe,

“Fracture

and

Fatigue

Control

in

Structures
:

Applications

of

Fracture

Mechanics”,

ASTM

Manual

Series
:

MNL
41
,

West

Conshohocken,

PA,

USA,

1999

[
2
]

R
.
W
.

Hertzberg,

“Deformation

and

Fracture

Mechanics

of

Engineering

Materials”,

John

Wiley

&

Sons,

Inc
.
,

New

York,

USA,

1996

[
3
]

D
.
R
.

Askeland,

P
.
P
.

Phulé,

“The

Science

and

Engineering

of

Materials”,

Thomson,

Toronto,

Canada,

2006

[
4
]

http
:
//www
.
key
-
to
-
steel
.
com

[
5
]

ASTM

E

23
-
06
:

“Standard

Test

Methods

for

Notched

Bar

Impact

Testing

of

Metallic

Materials”,

ASTM

International,

2006

[
6
]

P
.

Ratchev,

T
.

Loccufier,

B
.

Vandevelde,

B
.

Verlinden,

S
.

Teliszewski,

D
.

Werkhoven,

B
.

Allaert,

“A

Study

of

Brittle

to

Ductile

Fracture

Transition

Temperatures

in

Bulk

Pb
-
Free

Solders”,

Proceedings

of

EMPC

2005

(IMAPS
-
Europe)
,

June

12
-
15
,

2005
,

Brugge,

Belgium,

pp
.

248
-
252

Lambrinou Konstantina



imec restricted 2007

24

References
(2)

[
7
]

P
.

Ratchev,

B
.

Vandevelde,

B
.

Verlinden,

“Brittle

to

Ductile

Fracture

Transition

in

Bulk

Pb
-
Free

Solders”,

in

press

for

IEEE
-
Transactions

on

Components

and

Packaging

Technologies

[
8
]

P
.

Ratchev,

B
.

Vandevelde,

B
.

Verlinden,

“Effect

of

the

Intermetallics

Particle

Size

on

the

Brittle

to

Ductile

Fracture

Transition

in

a

Bulk

Sn
-
4
wt
%
Ag
-
0
.
5
wt
%
Cu

Solder”,

CD
-
ROM

Proceedings

of

IPC/JEDEC

10
th

International

Conference

on

Lead
-
Free

Electronic

Components

and

Assemblies
,

October

17
-
19
,

2005
,

Brussels,

Belgium

Lambrinou Konstantina



imec restricted 2007

25

Acknowledgments



IMEC
:


Dr. Bart Vandevelde



Paresh Limaye



Frederic Duflos




K.

U.

Leuven
:

Prof. Bert Verlinden





Wout Maurissen



Financial

support

by

IWT

(Flemish

Government)

in

the

framework

of

the

ALSHIRA

(
A
spects

of

L
ead
-
Free

S
oldering

for

Hi
gh
-
R
eliability

A
pplications)

Project

Lambrinou Konstantina



imec restricted 2007

26

Thank you!

Lambrinou Konstantina



imec restricted 2007

27


Stress
-
intensity

factor,

K
I
:

describes

the

stress

field

ahead

of

a

sharp

crack

(in

MPa∙m
1
/
2
)


K
I

is

affected

by

the

specimen

geometry,

the

applied

load,

the

shape

and

size

of

flaws

in

the

material


Elements of Fracture Mechanics



Fracture

toughness,

K
c
:

critical

K
I

value

at

failure
;

it


represents

the

material

resistance

to

crack

propagation


K
c

is

a

material

property
;

it

is

affected

by

temperature,


loading/strain

rate,

and

material

constraint

Mode I

Mode II

Mode III

[2]

Edge crack

Through
-
thickness crack

[1]

Lambrinou Konstantina



imec restricted 2007

28

Composition and Fracture Resistance


Phase transformation leading to
embrittlement of Sn
:


-
Sn
(‘white’ Sn)






-
Sn
(‘grey’
Sn

or

‘tin pest’
)


-
Sn
(bct structure)





-
Sn
(diamond cubic
structure)


Sluggish:

18 months incubation period

13.2ºC


V



+
26%



Suppression by
adding
retardants
:

Sb
(0.5%),
Bi
(0.3%),
(Pb


5%
)

[5, 6]

Sn
-
0.5%Cu
; aged at
-
18ºC

Lambrinou Konstantina



imec restricted 2007

29

Second
-
Phase Particles and Fracture Resistance


Brittle

second
-
phase

particles,

like

IMCs
,

show

a

very

high

probability

of

acting

as

sites

of

crack

nucleation


Crack

nucleation

occurs

by

dislocation

coalescence
,

since

second
-
phase

particles

tend

to

‘pin’

dislocations


Dislocation

‘pinning’

limits

the

material’s

ability

for

plastic

deformation,

and

is

often

accompanied

by

strengthening

(known

as

‘precipitation

hardening’
)


Dislocation

‘pinning’
:

for

specific

size

and

spatial

distribution

of

second
-
phase

particles



Size distribution
and spacing of IMCs:

influence the fracture
behaviour
of solders!

6061
-
T4 Al alloy

[2]

Lambrinou Konstantina



imec restricted 2007

30

CVN Impact Tests of Bulk Sn
-
Based Solders


Tested solder alloy
:

as
-
cast

and

annealed
SAC 405


Test temperature
:

-
195
o
C to +100
o
C


Sample size
: 10

10

55 mm
3

5

m



m



m



m

As
-
cast

150

C, 100 h

150

C, 1000 h

175

C, 1000 h

[8]

Material Condition

DBTT (

C
)

As
-
cast

-
28


6

100 h at 150

C

-
42


5

1000 h at 150

C

-
40


5

1000 h at 175

C

-
48


5



Size distribution, spacing,
and sharpness of IMCs:

affect solder embrittlement!

Lambrinou Konstantina



imec restricted 2007

31

Mini
-
Charpy Results from
SAC 305

(Test at Room Temperature)

Lambrinou Konstantina



imec restricted 2007

32

Mini
-
Charpy Results from
SAC 305

(Test close to
-
100

C)

Sn

Lambrinou Konstantina



imec restricted 2007

33

Mini
-
Charpy Results from
SAC 405

(Test at Room Temperature)

Lambrinou Konstantina



imec restricted 2007

34

Mini
-
Charpy Results from
SAC 405

(Test close to
-
100

C)

Cu
6
Sn
5

IMC

Sn

Sn

Bond Pad

Lambrinou Konstantina



imec restricted 2007

35

Mini
-
Charpy Results from
Sn
-
37%Pb

(Test at Room Temperature)