Nanosilver Paste: an Enabling Nanomaterial for Low ... · Nanosilver Paste: an Enabling...

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

G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 2

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

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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

G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 7

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)

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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