Nanosilver Paste: an Enabling Nanomaterial for Low-temperature Joining of Power Devices

Transcription

1 G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 1 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 2011 APEC Annual Meeting Fort Worth, TX March 11-13, 2011

2 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

3 Die-attaching power semiconductor chips Commonly used die-attach materials: Processing temperature Max. use temperature Electrical conductivity 10 5 (Ω-cm) -1 Thermal conductivity (W/K-cm) Die-shear Strength (MPa) Lead-free solder 260ºC < 150ºC Silver epoxy o C < 200 o C High-Pb solder 340 o C < 200 o C Drivers for new materials: Increasing demand for electronics capable of working at high temperatures in automotive electronics, solar cells, and highbrightness light-emitting diodes (LEDs); Regulatory policies by government and industrial entities to eliminate lead (Pb) from electrical and electronic products. G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 3

4 G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 4 LTJT Low Temperature Joining Technology by silver sintering 1986: Patent by H. Schwarzbauer 1991: H. Schwarzbauer and R. Kuhnert, IEEE Trans. on Industry Applications, 27 (1), pp : Initial trials and subsequent R&D effort at Semikron 1997: S. Klaka, Ph.D. Dissertation; now at ABB 2002: M. Thoben, Ph.D. Dissertation; now at Infineon 2004: C. Mertens, Ph.D. Dissertation; now at Volkswagen 2006: J. Rudzki, Ph.D. Dissertation; now at Danfoss 2007: Semikron s SKiM, 100% solder-free power modules 2009: SKiiP 4; manufacturing capacity: 350,000 5 x7 cards. SKiM SKiiP 4

5 Demonstration of improved reliability by Dr. Reinhold Bayerer of Infineon G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 5 X-ray inspection of sintered DCB bonded to a copper base plate during thermal cycling test (-40 C C, 1 hr dwell time) 73 mm 54 mm after 500 cycles after 1000 cycles after 2250 cycles

6 G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 6 Other companies in hot pursuit Volkswagen (R&D) and Danfoss (R&D) on using LTJT for both sides. From: E. Schulze, C. Mertens, and A. Lindemann, CIPS 2010 Bosch, Samsung (in development phase) GM, TI, BAE (exploring) DOE (ORNL + NREL) (exploring) + many small companies and institutions

7 G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 7 LTJT a complex manufacturing process Temperature: 240 o C 250 o C Time: 2 5 minutes Pressure: MPa or kg force per cm 2. Long process development time From: C. Gobl and J. Faltenbacher, CIPS 2010

8 A closer examination of SEMIKRON s SKiM module (solder free) G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 8 Ref: Applications: 22kW-180kW DC/AC and AC/DC invertors in electric & hybrid vehicles.

9 G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 9 X-ray CT imaging of SKiM s silver joint 5mm Pore size: 50~200μm; Concentration: about 10~20/mm µm X-ray 3D reconstructed Image of the joint layer

10 G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 10 SEM cross-sectional view of SKiM s joint SEM Work done by researchers at Oak Ridge National Lab Cu Al 2 O 3 Cu Frequently occurring porous regions in Ag sintered layer: ~ 200 µm length; Concentration about two per mm length.

11 G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 11 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.

12 CPES strategy: reduction/elimination of pressure for low-temp sintering of silver G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 12 Theoretical basis (trading chemical for mechanical force): Mackenzie-Shuttleworth Sintering Model (1960s): dρ dt γ *( + 2 r 3 1/ = P 3 applied Driving Force 1 )*(1 ρ)*(1 α *( 1) ρ Mobility 1 *ln )*1/ η 1 ρ where, γ is the surface energy; r is particle radius; P applied is external pressure/stress. Silver Particle Size 2.0 μm 100 nm 30 nm Driving Force 2.0 MPa 40 MPa 143 MPa

13 in situ nanosilver sintering in a scanning electron microscope 30 nm Ag Powder G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 13

14 Formulation and processing of nano-ag paste (nanotach ) Binder Surfactant Thinner Surfactant Organic thinner + + Ag nanopowder Uniform Dispersion Silver Paste nanotach Device Substrate Sintered joint Cool down Heat up Densification by diffusion G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 14

15 How to apply nanosilver paste for bonding small chips (<3mm x 3mm) Step I. Stencil-print to ~ 35 μm or Dispense Step II. Attach chip Ag: 275 o C, 10 min Step III. Heat in air (no pressure) Temperature ( o C ) o 50 o C 100 o C 5 o C/min Au: 300 o C, 10 min >20 o C/min 180 o C 5 o C/min RT 3 o C/min Time (minutes) G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 15

16 How to apply nanosilver paste for bonding large chips (e.g. >10 mm x 10mm) G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 16 Step I. Stencil-print to ~ 50 μm Step IV. Attach chip Step II. Dry in air Temperature RT 5 o C/min 180 o C Step V. Hot-press dry at 3 MPa and 180 o C for 5 min Time (minutes) Step III. Screen-print a fresh layer of ~ 10 μm on the dried layer Step VI. Heat in air at 270 o C (no pressure)

17 Why the need for pressure to bond largechips G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 17 Si Zero pressure gap Si 5 MPa Si 1 MPa 1 MPa

18 G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 18 Examples of chips mounted on substrates No applied pressure 2mm x 3mm Si power MOSFET on Ag-coated DBC Applied pressure < 5 MPa 2mm x 2mm SiC devices on Au-coated DBC 1.2 cm Chips joined on a copper plate 1 cm A chip joined on both sides by nanosilver sintering Chips joined on faces of an octagon substrate

19 G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 19 Integrity of nanosilver sintered joint Scanning acoustic imaging: X-ray imaging: 10 mm x 10 mm Courtesy of ABB R&D Courtesy of Bosch

20 Closer examination of the nanosilver sintered joint G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 20 Si Ag Cu Absence of large porous regions in SEM images 100µm 500µm Much fewer pores/cracks from X-ray CT images

21 Evaluation of nanosilver paste by a Fraunhofer Institute (IISB) research group From: Knoerr and Schletz, CIPS 2010 Density/Shear Strength vs. Pressure Conclusion: High bonding strength in excess of 40 MPa can be achieved with sintering at 275 o C for 5 sec and pressure as low as 2 MPa. G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 21

22 Reduction in junction-to-heatsink thermal resistance for LED packaging G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 22 From Panaccione of Luminus Devices (2010) 70 Shear Strength (MPa) mm^2 die 5.4mm^2 die 86%Ag-epoxy SN100C sintered nano-ag Rth(j-hs) deg.c/w %Ag-epoxy SN100C solder sint ered nano-ag PhlatLight mm^2 die 9.0mm^2 die

23 Z th of 20-mm 2 IGBTs attached by solder and nanosilver after temp cycling (-40 o C 125 o C) G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) NSP SN100C SAC305 Gate-Emitter Short After 500 Cycles Z th (C/W) 0.7 Soldered devices 0.65 Nano-Ag sintered device Number of Cycling (Heating time: 40 ms)

24 Cross-sectional SEM of large-area chips attached on Cu after 800 cycles (-40 o C 125 o C) G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 24

25 Die-shear testing of nanosilver joined chips on DBA during temp-cycling of -55 o C to 250 o C G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 25 Si Die-shear strength (MPa) Ag Al

26 G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 26 Conclusion LTJT by silver sintering is being implemented in manufacturing of power modules to improve reliability for higher T j 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.

27 G-Q. Lu presentation at 2011 APEC Annual Meeting (3/2011) 27 Thank you for your attention! Questions or Comments? 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