MECHANICAL CHARACTERIZATION OF SOLDER

AT THE MICRO-SCALE THROUGH SCANNING

ELECTRON MICROSCOPY AND DIGITAL IMAGE

CORRELATION

Santaneel Ghosh, Konstantin Yamnitski, Ibrahim Guven and Erdogan Madenci

Department of Aerospace and Mechanical Engineering The University of Arizona

TECHCON 2005October 24, 2005

2

Objective

Mechanical Testing of Lead-free Solder

Specimen Preparation

SEM-DIC

ODIC

Results

Summary

OUTLINE

Validation and application of a non-contact measurement technique utilizing Scanning Electron Microscope and Optical Setup/Digital Image Correlation to Characterize Solder Material at Micro-Scale

OBJECTIVE

Specimen Preparation• V-groove etching • Encapsulation of copper wire• Polishing• Solder reflowing

Testing• Mechanical testing using SEM-DIC• Mechanical testing using ODIC Results• Tensile test results• Comparison to the available bulk and small-scale lead-free

solder properties

Mechanical Testing of Lead-free Solder

Silicon wafer with V-groove

Silicon Wafer

V Groove

Copperwires

Solder balls

Copper wires and solder balls are placed in the V-groove

Copper wire and solder ball diameter = 300 m

Specimen Preparation

Solder balls

Cu wires

Encapsulated copper wire End polished copper wire

Copper wire

Specimen Preparation

Wax

Cu wire section

Place glass to:(a) keep components in place(b) assist flow of flux

Apply flux

Keep at 250 °C 3 to 4 min.

Heat up to 250 °C

Specimen Preparation

Reflow in progress

Solder column is formed

Turn off heaterRemove glassRemove from hot surface at 85 °CWait until room temperatureRemove specimen from V groove

Solder column is formed

Specimen Preparation

Specimen Preparation

• Cleaning: All residue & debris is removed by ultrasonic cleaning

• Sputtering: Gold sputter specimen to allow SEM imaging of non-conductive regions

Solder column after reflow

m m

DIC identifies features along the surface of the solder joint

Distance between points 1 and 2 is (L) in the undeformed configuration

This configuration serves as the reference image

Mechanical Testing – Application of DIC

Initial (unloaded) configuration

Final (loaded) configuration

DIC identifies the same features by correlating the undeformed and deformed images

Distance between points 1 and 2 is (L in the deformed configuration

This configuration serves as the object image

Mechanical Testing – Application of DIC

• Displacement of each point in the reference image is calculated by a DIC software, ADASIM from Fraunhofer Institut.

• Strains are calculated

• Stresses are calculated using

L

P

A

Mechanical Testing – Application of DIC

Reference image Object image

Evaluation of stress–strain relationship

SEM image of a test specimen after solder reflow

Mechanical Testing inside SEM

Mechanical Testing inside SEM

In-house measurement apparatus

Specimen placement in the loading fixture

Mechanical Testing inside SEM

Solder

Cu wire

Cu wire

Groove

Point 1 Point 1 Point 2 Point 2

Unloaded state Under a load of 3.83 N

Mechanical Testing inside SEM

Optical digital image of test specimen after solder reflow

Solder

Cu

Cu

Mechanical Testing using Optical System

Unloaded state Under a load of 4.67 N

Region of strain calculations

Mechanical Testing using Optical System

SEM-DIC Measurements

Load (kg-f)

Load (N)

Stress (MPa)

Displacement (pixel)

Displacement (m)

Strain (%)

0.00 0.00 0.00 0.00 0.00 0.00

0.15 1.47 17.42 0.21 0.08 0.03

0.23 2.26 26.70 1.56 0.56 0.23

0.30 2.94 34.83 4.18 1.50 0.61

0.38 3.73 44.12 6.01 2.16 0.87

0.44 4.32 51.09 7.18 2.58 1.04

Sample specimen test results (SEM-DIC)

Results

Optical DIC (ODIC) Measurements

Load (kg-f)

Load (N)

Stress (MPa)

Strain (%)

0.00 0.00 0.00 0.00

0.09 0.88 12.49 0.06

0.19 1.86 26.37 0.13

0.30 2.94 41.64 0.22

0.39 3.83 54.13 0.45

0.47 4.61 65.23 0.69

Sample specimen test results (ODIC)

Results

Load-displacement response (SEM-DIC)

0

1

2

3

4

5

0 1 2 3 4 5Displacement (Micron)

Lo

ad

(N

)

specimen 5

specimen 6

specimen 7

specimen 8

specimen 9

Results

Stress-strain response from SEM-DIC and ODIC

Results

Stress - Strain Diagram for SEM and OPTICAL DIC

0

10

20

30

40

50

60

0 0.5 1 1.5 2

Strain (%)

Str

ess (M

Pa) specimen 7 SEM-DIC

specimen 8 SEM-DIC

specimen 9 SEM-DIC

specimen 5 SEM-DIC

specimen 6 SEM-DIC

specimen 3 ODIC

Results

0

10

20

30

40

50

60

0.0 0.5 1.0 1.5 2.0

Strain (%)

Str

ess

(MP

a)

specimen 5

specimen 6

specimen 7

specimen 8

specimen 9

Stress-strain response from SEM-DIC with error bars

Young’s modulus from SEM-DIC and ODIC

Results

SEM-DICAvg: 30.5 GPaSt Dev. : 3.6 GPa

0

5

10

15

20

25

30

35

40

Specimen 9(SEM-DIC)

Specimen 8(SEM-DIC)

Specimen 7(SEM-DIC)

Specimen 6(SEM-DIC)

Specimen 5(SEM-DIC)

Specimen 3(ODIC)

Yo

un

g's

Mo

du

lus

(GP

A)

0

10

20

30

40

50

60

Sn 96

.5 A

g 3.

5

Sn 96

.5 A

g 3.

5

Sn 96

.5 A

g 3.

5

Sn 95

.5 A

g 3.

8 Cu0

.7

Sn 95

.8 A

g 3.

5 Cu0

.7

Sn 95

.5 A

g 3.

8 Cu0

.7

Composition

Yo

un

g's

Mo

du

lus

(G

Pa

)

Young’s modulus for bulk lead-free solder from literature

Results

Avg.: 41.5 GPaSt. Dev.: 10.4 GPa

0

10

20

30

40

50

60

70

Sn 96

.3 A

g 3.

7

Sn 95

.5 A

g 4

Cu0.5

Sn 95

.5 A

g 4

Cu0.5

Sn 96

.5 A

g 3.

5

Sn 99

.3 C

u0.7

Sn 96

.5 A

g 3

Cu0.5

Sn 95

.5 A

g 4

Cu0.5

Composition

Yo

un

g's

Mo

du

lus

(G

Pa

)

Young’s modulus for joint scale lead-free solder from literature

Results

Avg.: 48.4 GPaSt. Dev.: 9.9 GPa

Comparison of Young’s modulus measurement against literature

Results

0

10

20

30

40

50

60

70

Experiments

Yo

un

g's

Mo

du

lus (

GP

A)

Avg.: 30.5 GPaSt. Dev.: 3.6 GPa

Summary

• Joint scale lead-free solder specimen preparation technique implemented successfully

• Performed tensile tests within the SEM and using the Optical Setup

• Both SEM-DIC and ODIC measurements for Young’s modulus are lower than those reported in literature