NEW DEVELOPMENTS OF DIRECT BONDING ON NON-PRECIOUS METAL SURFACES BY PRESSURE SILVER SINTERING

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1 NEW DEVELOPMENTS OF DIRECT BONDING ON NON-PRECIOUS METAL SURFACES BY PRESSURE SILVER SINTERING IMAPS-UK Die Attach Workshop 22 November 2018 MTC, Coventry Ly May Chew, Wolfgang Schmitt Heraeus Electronics Heraeus Deutschland GmbH & Co. KG

2 OUTLINE 1 2 New Material Development for Cu Surface 1. Background 2. Experimental Sintering process flow Temperature cycling test (TCT) with -40 C/+150 C High temperature storage at 250 C Die shear test Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) Bending test Scanning acoustic microscopy (SAM) 3. Results 4. Summary New Material Development for Al and Ni Surfaces 1. Background 2. Experimental Sintering process flow Scanning acoustic microscopy (SAM) Die shear test Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) 3. Results 4. Summary and outlook 2

3 1 BACKGROUND High melting temperature (961 C) High thermal and electrical conductivity Pressure silver sintering by far offers superior thermal and electrical conductivity Eliminating precious metal finishing on substrate Significant compatibility to present supply chain Lower the entry barrier to adopt silver sinter solution Development of a safe-touse micro Ag sinter paste for pressure sintering on bare Cu surface and precious metal surfaces 1. T. Krebs, S. Duch, W. Schmitt, S. Kötter, P. Prenosil, S. Thomas, "A Breakthrough in Power Electronics Reliability New Die Attach and Wire Bonding Materials", IEEE 63 rd Electronic Components and Technology Conference, May. 2013, pp W. Schmitt, L.M. Chew, "Silver Sinter Paste for SiC Bonding with Improved Mechanical Propertise", IEEE 67 th Electronic Components and Technology Conference, May. 2017, pp

4 1 PRESSURE SINTERING PROCESS FLOW Paste Printing Pre Drying of Printed Paste Hot Die Placement Pressure sintering Silver sinter paste: ASP Substrates: Si 3 N 4 AMB (Ag & Au metallization, bare Cu) Stencil thickness: 75 µm Convection Oven Temperature: 120 C Drying time: 20 min Atmosphere: N 2 (50 ppm O 2 ) Dies: Ag metallized Si dies (4 x 4 mm, 10 x 10 mm) Placement temperature: 130 C Placement pressure: 400 g Placement time: 2 s Protective Foil PFA Foil Pressure: 10 MPa Temperature: 230 C Time: 3 min Sinter atmosphere: Air 4

5 1 CHARACTERIZATION Evaluation of reliability of sintered joint Temperature cycling test (TCT) Test conditions: Testing equipment: Vötsch VT 7012 Conditions: -40 C/+150 C (air to air) High temperature storage (HTS) Test conditions: Testing equipment: Convection oven Conditions: 250 C in air atmosphere Die shear test measure bonding strength of sintered joint (Initial, 1000 and 2000 cycles, 250 h and 1000 h) Bending test quick method to measure bonding strength of sintered joint (Initial, 2000 cycles) Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) elemental analysis of joint interface between silver sintered layer and bare Cu substrate (after HTS) Scanning acoustic microscopy (SAM) identify void, drying channels and delamination in sintered joint (Initial, 1000 and 2000 cycles) 5

6 Die shear failure mode: Low bonding strength High bonding strength Die shear test Evaluation of bonding strength of sintered joint 6

7 1 DIE SHEAR STRENGTH BEFORE AND AFTER TCT (-40 C/+150 C) 24 dies were sheared to generate an individual boxplot The average initial die shear strength for Ag metallized substrate is higher than that for Au metallized and bare Cu substrate self-diffusion of Ag to Ag is faster than interdiffusion of Ag/Au and Ag/Cu The average die shear strength increased after TCT sintering process is not yet completed under the mild sintering conditions (230 C, 10 MPa, 3 min) used in this study Ag, Au, Cu continued to diffuse during TCT increasing Ag layer densification resulting in higher bonding strength After 2000 cycles, the average die shear strength of bare Cu substrate is fairly similar to that of Ag metallized substrate 7

8 1 DIE SHEAR FAILURE MODE BEFORE AND AFTER TCT 4 mm x 4 mm Ag metallized die Initial Adhesive / cohesive break 2000 cycles Cohesive break in the sintered layer Ag Ag sinter layer (die) Ag sinter layer (substrate) Ag sinter layer (die) Ag sinter layer (substate) adhesive / cohesive break cohesive break NiAu Ag sinter layer (die) Au metallized substrate adhesive break on the substrate Ag sinter layer (die) Ag sinter layer (substrate) cohesive break bare Cu Ag sinter layer (die) Bare Cu substrate adhesive break on the substrate Ag sinter layer (substrate) Ag sinter layer (die) cohesive break 8

9 1 DIE SHEAR STRENGTH BEFORE AND AFTER HTS AT 250 C 24 dies were sheared to generate an individual boxplot The average die shear strength increased after HTS Ag, Au, Cu continued to diffuse during HTS increasing Ag layer densification resulting in higher bonding strength No significant difference in the average die shear strength between 250 h and 1000 h storage sintering process continued to occur during HTS and is completed after a certain time of storage 9

10 1 DIE SHEAR FAILURE MODE BEFORE AND AFTER HTS 4 mm x 4 mm Ag metallized die Initial 250 h storage 1000 h storage Adhesive / cohesive break Cohesive break in the sintered layer Ag Ag sinter layer (die) Ag sinter layer (substrate) adhesive / cohesive break Cu Cu layer cohesive break in the Cu layer sinter layer substrate Ag Ag sinter layer cohesive break in the Cu layer NiAu Ag sinter layer (die) Au metallized substrate adhesive break on the substrate Ag sinter layer (substrate) cohesive break in the sintered layer sinter layer Au Ni substrate Cu Ag Ag sinter layer (die) Ag sinter layer (substrate) cohesive break in the sintered layer 10 bare Cu Ag sinter layer (die) Bare Cu substrate adhesive break on the substrate Cu layer cohesive break in the Cu layer Cu layer cohesive break in the Cu layer

11 1 ELEMENTAL ANALYSIS BY SEM-EDX Ag content decreases and concurrently the amount of Cu and O increases when scanning move towards the Cu substrate a sudden increase in Ag content and a reduction in Cu content at the layer between the copper oxide layer and the Cu substrate which show that the layer is silver 11

12 Bending test Evaluation of bonding strength of sintered joint (quick method) 12

13 1 BENDING TEST BEFORE AND AFTER TCT (-40 C/+150 C) 10 mm x 10 mm Ag metallized die Ag Initial 2000 cycles NiAu Die attached strongly on substrate High bonding strength bare Cu 13

14 Scanning acoustic microscopy Identification of void, drying channels & delamination in sintered joint 14

15 1 SAM BEFORE AND AFTER TCT (-40 C / +150 C) Void, drying channel and delamination were not detected in the sintered layer for all the samples before and after 2000 thermal cycles Homogeneous sintered layer was obtained homogeneous pressure distribution was applied during sintering process 15

16 SUMMARY High reliable sintered joint on bare Cu surface was achieved by pressure sintering under air atmosphere using ASP sinter paste Sintering process continued to occur during TCT and HTS and is completed after certain time of storage at 250 C (sintering conditions used in this study: 230 C, 10 MPa, 3 min) After 250 h storage at 250 C, Cohesive break in the Cu layer was observed for Ag metallized and bare Cu substrates SEM-EDX results demonstrate that interdiffusion between Ag and Cu occurred and generated a strong joint Cohesive break in the silver sintered layer was observed for NiAu metallized substrates Ni layer acts as a barrier to prevent interdiffusion of Ag/Cu SAM and bending test results demonstrate that high bonding strength of sintered joint with no delamination was obtained for all samples even after 2000 thermal cycles L.M. Chew, W. Schmitt, C. Schwarzer, J. Nachreiner, "Micro-silver sinter paste developed for pressure sintering on bare Cu surfaces under air or inert atmosphere" IEEE 68 th Electronic Components and Technology Conference, May. 2018, pp

17 OUTLINE 1 2 New Material Development for Cu Surface 1. Background 2. Experimental Sintering process flow Temperature cycling test (TCT) with -40 C/+150 C High temperature storage at 250 C Die shear test Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) Bending test Scanning acoustic microscopy (SAM) 3. Results 4. Summary New Material Development for Al and Ni Surfaces 1. Background 2. Experimental Sintering process flow Scanning acoustic microscopy (SAM) Die shear test Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) 3. Results 4. Summary and outlook 17

18 2 BACKGROUND High Power Packages Development Trend Higher current capacity >>> Higher operating temperature >>> Higher reliability Ag, Au, Cu, Ni Cu, Al, AlSiC Ag, Au Die Substrate Base Plate Solder Solder Ag Sinter Ag Sinter Surfaces Ag, Au, Cu Al, Ni Sinterable X Eliminating precious metal finishing on substrate Significant compatibility to present supply chain Lower the entry barrier to adopt silver sinter solution Development of a safe-to-use micro Ag sinter paste for pressure sintering on non-precious metal surfaces (Al, Ni) 1. T. Krebs, S. Duch, W. Schmitt, S. Kötter, P. Prenosil, S. Thomas, "A Breakthrough in Power Electronics Reliability New Die Attach and Wire Bonding Materials", IEEE 63 rd Electronic Components and Technology Conference, May. 2013, pp W. Schmitt, L.M. Chew, "Silver Sinter Paste for SiC Bonding with Improved Mechanical Propertise", IEEE 67 th Electronic Components and Technology Conference, May. 2017, pp L.M. Chew, W. Schmitt, C. Schwarzer, J. Nachreiner, "Micro-silver sinter paste developed for pressure sintering on bare Cu surfaces under air or inert atmosphere" IEEE 68 th Electronic Components and Technology Conference, May. 2018, pp

19 2 PRESSURE SINTERING PROCESS FLOW Paste Printing Pre Drying of Printed Paste Hot Die Placement Pressure sintering Silver sinter paste: ASP LTS Substrates: Al 2 O 3 DCB (Ni plated, bare Cu) Al plate (99.5 % Al) Convection Oven Temperature: 120 C Drying time: 10 min Dies: Ag metallized Si dies (2 x 2 mm, 4 x 4 mm) Placement temperature: 130 C Placement pressure: 400 g Protective Foil PFA Foil Pressure: 10 MPa Temperature: 250 C Time: 3 min Stencil thickness: 75 µm Atmosphere: N 2 (50 ppm O 2 ) / Air Placement time: 3 s Sinter atmosphere: Air Ni DCB Al plate Cu DCB 19

20 2 CHARACTERIZATION Evaluation of bonding strength of sintered joint Scanning acoustic microscopy (SAM) identify void, drying channels and delamination in sintered joint Die shear test measure bonding strength of sintered joint (Initial) Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) elemental analysis of joint interface between silver sintered layer and substrate (after die shear test) 20

21 2 HERAEUS COMMERCIAL SINTER PASTE (ASP ) Al plate Ni DCB Die back side with Ag sintered layer Die back side with Ag sintered layer Sinter joint was not formed on Al surface dies with Ag sintered layer were detached from Al plate and Ni DCB right after pressure sintering Sinter paste was printed on Al substrate but after sintering sintered layer was only found on the backside of the die which is Ag metallization 21

22 Scanning acoustic microscopy Identification of void, drying channels & delamination in sintered joint 22

23 2 SAM newly developed sinter paste (LTS ) Ni DCB Al plate No void and drying channel in the sintered layer 23 Cu DCB

24 Die shear failure mode: Low bonding strength High bonding strength Die shear test Evaluation of bonding strength of sintered joint 24

25 2 DIE SHEAR TEST newly developed sinter paste (LTS ) Al plate Ni DCB cohesive break in the sintered layer cohesive break in the sintered layer Cu DCB 25

26 2 SEM AFTER DIE SHEAR (Ni DCB SAMPLE) newly developed sinter paste (LTS ) Ag Ag sintered layer Ag Ni Cu P Cu Ni Interdiffusion between Ag and Ni P Ni DCB 26

27 2 SEM AFTER DIE SHEAR (Al PLATE SAMPLE) newly developed sinter paste (LTS ) Al Ag sintered layer Ag Ag Interdiffusion between Ag and Al O Al plate Al O 27

28 2 SEM AFTER DIE SHEAR (Cu DCB SAMPLE) newly developed sinter paste (LTS ) Ag Ag Ag sintered layer Cu Cu O Cu DCB Interdiffusion between Ag and Cu 28

29 SUMMARY It is feasible to create sinter joint on Ni, Al and Cu surfaces by pressure sintering at 250 C, 10 MPa, 3 min using LTS sinter paste Average die shear strength above 15 N/mm² was obtained for all samples (Ni DCB, Al plate and Cu DCB) after pressure sintering. Cohesive break in the sintered layer was observed for all samples SEM-EDX results of all samples after pressure sintering and after die shear tests demonstrate that sintered joint created on Ni, Al and Cu surfaces and an interdiffusion of Ag/Ni, Ag/Al and Ag/Cu occurred at the interface between sintered layer and substrate 29

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