G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 1
2011 APEC Annual MeetingFort Worth, TX
March 11-13, 2011
Nanosilver Paste: an Enabling Nanomaterial for Low-temperature Joining of Power Devices
Guo-Quan (GQ) Lu, Professor
Dept. of MSE and ECE, Virginia Tech, USA
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Outline
I. LTJT – not soldering, not epoxy curing
II. Nanosilver paste for LTJT
III. How to apply nanosilver paste for joining
IV. Mechanical, thermal, and reliability results of sintered nanosilver joints
V. Summary
G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 3
Die-attaching power semiconductor chips
Processing temperature
Max. use temperature
Electrical conductivity105 (Ω-cm)-1
Thermal conductivity(W/K-cm)
Die-shear Strength (MPa)
Lead-free solder 260ºC < 150ºC 0.75 0.70 35
Silver epoxy 100 – 200oC < 200oC 0.1 0.1 – 0.5 10 – 40
High-Pb solder 340oC < 200oC 0.45 0.23 15
Increasing demand for electronics capable of working at high temperatures in automotive electronics, solar cells, and high-brightness light-emitting diodes (LEDs);Regulatory policies by government and industrial entities to eliminate lead (Pb) from electrical and electronic products.
Commonly used die-attach materials:
Drivers for new materials:
G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 4
1986: Patent by H. Schwarzbauer1991: H. Schwarzbauer and R. Kuhnert, IEEE Trans. on Industry Applications, 27 (1), pp.93 – 95.1994: Initial trials and subsequent R&D effort at Semikron1997: S. Klaka, Ph.D. Dissertation; now at ABB2002: M. Thoben, Ph.D. Dissertation; now at Infineon2004: C. Mertens, Ph.D. Dissertation; now at Volkswagen2006: J. Rudzki, Ph.D. Dissertation; now at Danfoss2007: Semikron’s SKiM®, 100% solder-free power modules2009: SKiiP®4; manufacturing capacity: 350,000 5”x7” cards.
LTJT –
Low Temperature Joining Technology by silver sintering
SKiM® SKiiP®4
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Demonstration of improved reliability by Dr. Reinhold Bayerer of Infineon
X-ray inspection of sintered DCB bonded to a copper base plate during thermal cycling test (-40°C -
150°C, 1 hr dwell time)
after 500 cycles after 1000 cycles after 2250 cycles54 mm
73 mm
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Other companies in hot pursuit• Volkswagen (R&D) and Danfoss (R&D) on using LTJT
for both sides.
• Bosch, Samsung (in development phase)
• GM, TI, BAE (exploring)
• DOE (ORNL + NREL) (exploring)
• + many small companies and institutions
From: E. Schulze, C. Mertens, and A. Lindemann, CIPS’2010
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LTJT –
a complex manufacturing process
Temperature: 240oC –
250oC
Time: 2 –
5 minutes
Pressure: 20 –
40 MPa or 200 –
400 kg force per cm2.
Long process development time
From: C. Gobl and J. Faltenbacher, CIPS’2010
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A closer examination of SEMIKRON’s SKiM module (solder free)
Ref: http://www.semikron.com
Applications: 22kW-180kW DC/AC and AC/DC invertors in electric & hybrid vehicles.
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X-ray CT imaging of SKiM’s silver joint
500µm
5mm
X-ray 3D reconstructed Image of the joint layer
Pore size: 50~200μm;
Concentration: about 10~20/mm2.
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SEM cross-sectional view of SKiM’s joint
Cu
Cu
Al2 O3
Frequently occurring porous regions in Ag sintered layer: ~ 200 µm length;Concentration about two per mm length.
SEM Work done by researchers at Oak Ridge National Lab
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LTJT work at CPES
2001: Initiated LTJT project;
2003: Started development of nanosilver paste for pressure-less sintering;
2004: Filed patent on nanosilver paste;
2005: Z. Zhang, Ph.D. dissertation on pressure-assisted LTJT ;
2005: G.F. Bai, Ph.D. dissertation on nanosilver-LTJT;
2005: Completion of technology transfer to start-up, NBE Tech;
2007: 2007 R&D 100 award to nanosilver paste as nanoTach®;
Today: 40+ companies are evaluating nanoTach®.
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where, γ is the surface energy; r is particle radius;Papplied is external pressure/stress.
Silver Particle Size 2.0 μm 100 nm 30 nm
Driving Force 2.0 MPa 40 MPa 143 MPa
Mackenzie-Shuttleworth Sintering Model (1960s):
Theoretical basis (trading chemical for mechanical force):
Mobility
ηρρ
αργρ /1*)1
1ln*)11(*1(*)1(*)(*23 3/1
−−−−+= appliedP
rdtd
Driving Force
CPES strategy: reduction/elimination of pressure for low-temp sintering of silver
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30 nm Ag Powder
in situ nanosilver sintering in a scanning electron microscope
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SubstrateDevice
Densificationby diffusion
Heat up
Cool down
Sintered joint
Surfactant Ag nano-powder
Organicthinner
Uniform Dispersion
Silver PastenanoTach®
+ +BinderThinner
Surfactant
Formulation and processing of nano-Ag paste (nanoTach®)
G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 15
How to apply nanosilver paste for bonding small chips (<3mm x 3mm)
Stencil-print to ~ 35 μm Dispense
Attach chip
orStep I.
Step II.
Time (minutes)
Tem
pera
ture
(o
C)
RT
10 30 4020 50 60 70 80 90
180oC
oC
)
Ag: 275oC, 10 min
10 30 4020 50 60 70 80 90
Au: 300oC, 10 min
>20oC/min
100oC
50oC
5oC/min
5oC/min
3oC/min
Step III. Heat in air(no pressure)
G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 16
How to apply nanosilver paste for bonding large chips (e.g. >10 mm x 10mm)
Step I. Stencil-print to ~ 50 μm
Step II. Dry in air
Time (minutes)
Tem
pera
ture
RT
30
180oC
5oC/min
35
Step III. Screen-print a fresh layer of ~ 10 μm on the dried layer
Step IV. Attach chip
Step V. Hot-press dry at 3 MPa and 180oC for 5 min
Step VI. Heat in air at 270oC (no pressure)
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Why the need for pressure to bond large- chips
Zero pressure
1 MPa
5 MPa
1 MPa
gap
Si Si
Si
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Examples of chips mounted on substrates
2mm x 2mm SiC devices onAu-coated DBC
2mm x 3mm Si power MOSFETon Ag-coated DBC
No applied pressure
1 cm1.2 cm
Applied pressure < 5 MPa
Chips joined on faces of an octagon substrate
A chip joined on both sides by nanosilver sintering
Chips joined on a copper plate
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Integrity of nanosilver sintered joint
Scanning acoustic imaging:
10 mm x 10 mm
X-ray imaging:
Courtesy of ABB R&D Courtesy of Bosch
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Closer examination of the nanosilver sintered joint
500µm
Si
Cu
Ag
Absence of large porous regions in SEM images
Much fewer pores/cracks from X-ray CT images
100µm
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Density/Shear Strength vs. Pressure
Evaluation of nanosilver paste by a Fraunhofer Institute (IISB) research group
From: Knoerr and Schletz, CIPS’2010
Conclusion: High bonding strength in excess of 40 MPacan be achieved with sintering at 275oC for5 sec and pressure as low as 2 MPa.
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Reduction in junction-to-heatsink thermal resistance for LED packaging
From Panaccione of Luminus Devices (2010)
0
0.5
1
1.5
2
2.5
3
3.9mm^2 die 9.0mm^2 die
Rth
(j-hs
) deg
.C/W
86%Ag-epoxy
SN100C solder
sint ered nano-Ag
0
10
20
30
40
50
60
70
3.9mm^2 die 5.4mm^2 die
Shea
r Str
engt
h (M
Pa)
86%Ag-epoxy
SN100C
sintered nano-Ag
PhlatLight
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0 100 200 300 400 500 600
0.65
0.7
0.75
0.8
Number of Cycling
Z th(C
/W)
(Heating time: 40 ms)(Heating time: 40 ms)
Gate-Emitter Short After 500 Cycles
NSP
SN100C
SAC305
Zth of 20-mm2
IGBTs attached by solder and
nanosilver after temp cycling (-40oC –
125oC)
Nano-Ag sintered device
Soldered devices
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Cross-sectional SEM of large-area chips attached on Cu after 800 cycles (-40oC –
125oC)
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Die-shear testing of nanosilver joined chips on DBA during temp-cycling of -55oC to 250oC
Die
-she
ar s
tren
gth
(MPa
)
Si
Ag
Al
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Conclusion
LTJT by silver sintering is being implemented in manufacturing of power modules to improve reliability for higher Tj applications;
Use of nanosilver paste can significantly lower the pressure required in LTJT within the same short processing time for die-attaching large chips, and eliminates pressure for bonding small chips, such as LED.
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Questions or Comments?
Thank you for your attention!
Acknowledgements:• US Office of Naval Research
• US Army Research Laboratory
• US National Science Foundation & Chinese NSF
• G. Lei, J.N. Calata, K. Xiao, H. Zheng, T. Wang, Y. Mei, X. Cao, X. Chen, K. Ngo, and S. Luo