Nanosilver Paste: an Enabling Nanomaterial for Low ... · Nanosilver Paste: an Enabling Nanomaterial for Low-temperature Joining of Power Devices. Guo-Quan (GQ) ... Silver Paste

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


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


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:


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


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


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


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


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.


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.


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


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®.


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


30 nm Ag Powder

in situ nanosilver sintering in a scanning electron microscope


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®)


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)


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)


Why the need for pressure to bond large- chips

Zero pressure

1 MPa

5 MPa

1 MPa

gap

Si Si

Si


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


Integrity of nanosilver sintered joint

Scanning acoustic imaging:

10 mm x 10 mm

X-ray imaging:

Courtesy of ABB R&D Courtesy of Bosch


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


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.


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


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


Cross-sectional SEM of large-area chips attached on Cu after 800 cycles (-40oC –

125oC)


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


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.


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

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Nanosilver Paste: an Enabling Nanomaterial for Low ... · Nanosilver Paste: an Enabling Nanomaterial for Low-temperature Joining of Power Devices. Guo-Quan (GQ) ... Silver Paste