The Science and Application of Nanosilver Chipbonding Material Guo-Quan (GQ) Lu, Professor Dept. of MSE and ECE, Virginia Tech, USA 2014 APEC Annual Meeting Fort Worth, TX March 19th, 2014 G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 1 Outline I. Conventional LTJT by silver sintering versus nanosilver-enabled LTJT II. Sintering behavior of nanosilver paste III. Drying behavior of large-area nanosilver bond-line IV. Application of nanosilver for making doubleside cooled power modules V. Summary G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 2 High-temperature packaging research at Virginia Tech’s Center for Power Electronics Systems (CPES) Conventional Power Modules One-side cooling; Solder: fatigue; low-melting temp; and low thermal conductivity Th(Homologus Temperature) = Toperating/Tmelting 1. Planar device assembly 2. Encapsulant 3. Die-attach material (Nanoscale Ag paste) Power Device 4. Substrate Source: Knoerr, Kraft, and Schletz, Fraunhofer Institute for ISDT G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 3 Sintered silver joints improve reliability Dr. Reinhold Bayerer of Infineon (-40°C to 150°C, 1 hr dwell) Danfoss Double-side Silver Sintered Modules ~ 100 x higher Rudzki et al., 2012 PCIM. G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 4 Conventional 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 G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 5 The science of replacing mechanical force by thermodynamic driving force Theoretical basis (trading chemical for mechanical force): Mackenzie-Shuttleworth Sintering Model (1960s): 1 1 dρ 3 γ 1/ 3 ) *1 / η = * ( + Papplied ) * (1 − ρ ) * (1 − α * ( − 1) * ln ρ 1− ρ dt 2 r Driving Force 30 nm Ag Powder Mobility 100 nm Ag Powder G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 6 Outline I. Conventional LTJT by silver sintering versus nanosilver-enabled LTJT II. Sintering behavior of nanosilver paste III. Drying behavior of large-area nanosilver bond-line IV. Application of nanosilver for making doubleside cooled power modules V. Summary G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 7 Formulation of nanosilver paste Surfactant Binder Surfactant + Organic Ag nanothinner + powder Thinner Uniform Dispersion Nanosilver paste nanoTach® Use of organics to prevent: a) nano-particles from agglomeration and cracking; b) surface diffusion at low temperature so that rapid densification can take place at high temperature. G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 8 Removal of organics is necessary for sintering Exothermic peak from polymer combustion 1 2 3 1 2 4 3 4 G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 9 Oxygen is necessary for sintering 400 100 Paste: N-080528-IIB 96 300 Weight/% 10oC/min 92 250 200 88 12.4min 144.7oC 12.1min 141.6oC 18.7min 84 o 207.3 C 80 -5 0 5 10 15 20 27.9min 298.1oC 150 100 31.1min 321.0oC 50 22.6min 245.6oC 25 Temperature/oC 350 Air NSP-10-Air Nitrogen NSP-10-N2 0 30 35 40 Time/min G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 10 ~ 50% shrinkage of bond-line thickness due to drying and sintering 100 2 Weight loss curve 0 -2 95 90 -4 85 -6 -8 -10 Weight /% Change of Thickness /um 4 80 Paste: N-080528-IIB Thickness shrinkage curve -12 75 0 20 40 60 80 100 120 Time/min G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 11 Outline I. Conventional LTJT by silver sintering versus nanosilver-enabled LTJT II. Sintering behavior of nanosilver paste; III. Drying behavior of large-area nanosilver bond-line; IV. Application of nanosilver for making doubleside cooled power modules V. Summary G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 12 Understanding the kinetics of paste drying for pressure-less bonding of large IGBT chips Bonding large chips Glass “chip” nanoAg paste Gaps or debonding Cracks 1 cm and oo o o ooC 127 o 122 140 25 180 100 C 39 C 167 53 186 CC 75 184 113 180 186 154 C C 1 cm G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 13 A diffusion-viscous analysis of bond-line drying Kinetic processes: h chip paste 1. Solvent evaporation at the chip edges (liquid to gas transition); 2. Solvent molecular diffusion within the bond-line; 3. Shrinkage of the bond-line; 4. Stress development within the bondline cracks and delamination σz σy σx Unit cell of paste Solvent evaporation and diffusion due to thermodynamics and kinetics Intention to shrink due to surface tension Shrinkage constrained due to bonding at substrate and chip Unit cell failure due to high internal stress G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 14 Modeling result of stresses in the bond-line 1 cm Nano-Ag paste Temperature (°C) Glass chip 200 150 100 50 Drying 0 0 10 min 20 16 14 45 min 22 0 min 4 25 55 35 1 cm σx cracking 22 min 45 25min 35 min 55 20 min 16 0 4 min 10 min 14 min 10 20 30 40 Time (min) 50 60 σz debonding 22 min 0 20 25 16 55 4 45min min 10 min 35 14 G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 15 How to eliminate bond-line defects? cracks & debonding Zero pressure bonding Chip Substrate Silver paste Max internal stress causing debonding: 2.7 MPa Max internal stress causing cracking: 11.0 MPa Die-shear strength <10 MPa B. Chemical Route A. Mechanical Route 10 mm x 10 mm chip Pressure-free Void contentbonding < 2% Drying at 3 MPa 3 MPa Press chip Silver paste substrate Max internal stress causing debonding: 1.3 MPa Max internal stress causing cracking: 2.8 MPa Die-shear strength >25 MPa Chip Substrate Addition of easy-flow component into the paste to allow silver particles 2 mm slide over one another reducing internal stresses Die-shear strength > 25 MPa G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 16 Comparison to soldering large-area chips Acoustic Imaging X-ray Imaging Void content > 15% > $150 K SST Vacuum Reflow Soldering system PINK Formic Acid Soldering system Void < 1% G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 17 Outline I. Conventional LTJT by silver sintering versus nanosilver-enabled LTJT II. Sintering behavior of nanosilver paste III. Drying behavior of large-area nanosilver bond-line IV. Application of nanosilver for making doubleside cooled power modules V. Summary G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 18 Motivation Extra cooling loop required for power electronics 105oC coolant Cooled to 65oC Engine Power Electronics Source: NREL Radiator cooling High-temp power electronics capable of Tj ~ 200oC would eliminate the extra cooling loop lower cost. G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 19 Nanosilver enabled double-side cooled, planar power modules (Version I: half-bridge) Three-phase Inverter Current stateof-the-art IGBT Module Planar, double-side cooled module G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 20 Version II of nanosilver sintered planar power modules (quarter-bridge) Top DBC- Negative Bus Gate/Emitter output Bottom DBC- Positive Bus Two Q-bridge modules connected to form a half-bridge module 54mm 25.4mm 4mm thin G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 21 Three-phase testing of half- and quarter-bridge nanosilver sintered planar power modules Vcc A+ B+ C+ Phase A Phase B Phase C A- A+ DC Link Sense A- C- B- B+ B- C+ C- APS Gate Driver Board Input Signal Bus Bar Current Output Current (Phase A) DC-Link Voltage Vge of High-Side IGBT (Phase A) 3-phase testing results at 750V 75 Amps G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 22 Most recent developments – bonding to copper in controlled atmosphere Pressure-free heating in pure nitrogen or forming gas (4%H2-N2) G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 23 High die-shear strength and reliable sintered silver-to-copper joints Temperature cycling (Temp range from -40oC to 125oC) Power cycling (∆T = 135oC) G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 24 Most recent developments – nanosilver preform Step I: Preform transfer to chip or substrate Step II: Pressure-sintering G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 25 Summary The LTJT by silver sintering is emerging as a competitive lead-free, die-attach solution for manufacturing power devices/modules because sintered silver joints are significantly more reliable. By studying the drying and sintering properties of a nanosilver paste, the paste formulations have been optimized for pressure-free bonding of large-area (13 mm x 13 mm) IGBT chips. Using nanosilver paste can significantly lower the cost of implementing LTJT because it requires simpler tooling and offers higher throughput and yield. G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 26 Thank you for your attention! Questions or Comments? Acknowledgements: • US Office of Naval Research • US Army Research Laboratory • US Department of Energy • NBE Technologies, LLC • US National Science Foundation & Chinese NSF • Prof. K. Ngo, Dr. G. Lei, Dr. J.N. Calata, Dr. J. Mei, Dr. K. Xiao, H. Zheng, Dr. T. Wang, Li Jiang, D. Berry, Y. Yao, X. Cao, Prof. X. Chen, Dr. J. Bai, Dr. Z. Zhang, and Dr. S. Luo G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 27 G-Q. Lu presentation at 2014 APEC Annual Meeting (3/2014) 28
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