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Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
1
Particle reinforced lead-free solders
A Comparative study on reinforcing Sn-4Ag-0.5Cu solder with
nano and micron sized Cu particles
V.Sivasubramaniam1, 2, J.Janczak-Rusch1, J.Botsis2, J.Cugnoni2
1-Laboratory of Joining and Interface Technology, EMPA Material Science and Technology, Dübendorf, Switzerland
2-Laboratory of Applied Mechanics and Reliability, EPFL, Lausanne, Switzerland
COST 531 Final Meeting, Vienna,17-05-2007
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
2
Overview
Motivation - Composite solders
Materials and processing method
Microstructural analysis of particle reinforced solders
Mechanical properties of solder joints
Failure analysis- Fractography
Conclusion and Future Work
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
3
Motivation-Composite approach
Controlling the growth and formation of intermetallics at the interface (eg. Cu/solder) – during higher service temperature
Source:Szu-Tsung Kao et. al , J.Electron. Materials,Vol.35,No.3 (2006)
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
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Materials used-Current work
Matrix alloy Sn-4Ag-0.5Cu (supplied by Alpha metals)
Base metal - Standard Cu
Reinforcement particles
Cu particles 3-20μm Cu2O nano particle 150nm
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
5
Synthesis and Characterization of nano Cu2O particles
Cu(NO3)2
[232.59] [143.08]
160°C; ArN O
[111.14]
495HO
OH
Cu2O6 + 2.5 x H2O2
Chemical reduction route (literature source…)
Cu2O
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
6
Specimen preparation
Manual Mixing of particles with SAC405 paste using mortar
Initially (melt) samples were made in circular (AlN) crucible for microstructural analysis
Mortar
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
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Microstructural analysis- SAC405+2 wt% micro Cu
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32
Distance in microns
wt%
Sn L
Cu K
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
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Microstructural analysis- SAC405+2 wt% nano Cu2O
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12
Distance in microns
wt.
%
SnL
CuK
All of the Cu2O has been converted in to CuSn IMC!
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
9
Quality control of joints-X-ray Radiography
Step gage from SAC405 cast
width of tensile specimen joint
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
10
Mechanical Characterization-Reproducibility
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
11
Mechanical Characterization-Results
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
12
Fractography results of tested composite solder joints
micro Cu nano Cu
cup and cone
ductile fracture!
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
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Mechanical characterization-Aged composite solder joint
Dotted curves –Aged joints isothermally for 24hrs @ 130°C
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
14
Interfacial IMC of composite solder joint
SAC405+2wt% micron-Cu
before ageing after ageing
20µ
20µ20µ
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
15
Microstructural analysis of composite solder joint ‘after’ ageing
SAC405+2wt% Nano Cu SAC405+2wt% micron Cu
No appreciable interfacial layer growth for solders with Nano Cu
Cu core was evident even after ageing for samples reinforced with
micron Cu
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
16
Conslusions
Composite solder prepared with nanometer scale Cu particles are expected to
have better creep properties (higher strain rates are achievable)
Potential solution for microelectronics interconnects where good tensile and
plastic properties are required
Growth control of interfacial layer for joints made with nanoscale Cu is evident
Mechanism of increase in strain rate for joints with nanoscale Cu
reinforcement must still explained
Future Work
Creep tests for Composite solders
Ageing for longer duration
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
19
Composite solder joints-Tensile test specimen preparation
Tensile test specimen-0.5mm gap width
0.5mm gap width, 2wt% reinforcement with microCu and ultrafineCu2O
4 specimens per batch
Test conditions:
Tensile test-Instron Microtester 5848-cross head speed of 0.5µ/sec
Digital image Correlation (DIC) technique developed by Dr. J.Cugnoni
DIC setup for localized strain field measurement
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
20
Mechanical Characterization-Microhardness (load:200gms,30sec)
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0
Vic
kers
har
dn
ess
(HV
)
SAC 4052wt% ultra fine Cu2O
2wt% micro Cu
2wt% micro Ni
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
21
Stress-strain curve- 0.5mm- 2wt% ultra fine Cu2O
0
10000000
20000000
30000000
40000000
50000000
60000000
0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50%
strain
stre
ss(P
a)
(RT)
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
22
Influence of particle reinforcement on the mechanical behaviour
0
10
20
30
40
50
60
70
80
0 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004Strain (-) [gage length=50 mm]
Str
ess
(MP
a)
"Sn-4Ag-0.5Cu @ 234degC,without vacuum"
"Sn-4Ag-0.5Cu,Ni part. 5%wt. @ 234degC,with vacuum"
"Sn-4Ag-0.5Cu,Cu part. 5%wt. @ 234degC,with vacuum"
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
23
Results from previous tests (old temp. profile)
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
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• The driving force of the moving of the Cu from substrate is due to the concentration difference in solders and substrate
• Ni and Cu have similar chemical characteristics,thus their diffusion in Sn is expected to be similar but due to the simultaneus presence of Cu,Ni in Ni reinforced Sn3.5Ag which apparently accelerates formation of (Cu-Ni-Sn) IMC
• However since there is not much growth of interfacial layer in Ni reinforced Sn3.5Ag below 100C its quite suitable for applications such as computers where the service conditions are below 100C
• Cu6Sn5 particles inhibits the growth of interfacial IMC by increasing the activation energy from 0.8eV to 1.23eV in SnPb
• It appears that creep resistance FeSn2 > Ni3Sn4 > Cu6Sn5 > Eutectic SnAg
Literature review - Consolidated inferences
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
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Microstructural analysis of composite solder joint ‘before’ ageing
SAC405+2wt% ultra fine Cu2O
SAC405+2wt% micron-Cu
10microns
10microns
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
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Microstructural analysis of composite solder joint ‘after’ ageing
SAC405+2wt% nano Cu SAC405+2wt% micron-Cu
Venkatesh Sivasubramaniam - COST 531 Final Meeting, Vienna,17-05-2007
27
nano Cu2O particles
Disperse in SAC405 paste
Strong flux(OM338)
Reflowed at 240°C
SAC405 reinforced with CuSn(IMC)
Cu2O Cu