New developments of direct bonding on non-precious metal ... · Pressure silver sintering by far...

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 SchmittHeraeus Electronics | Heraeus Deutschland GmbH & Co. KG

OUTLINE

IMAPS-UK | Die Attach Workshop | New Material Developments in Sintering | Heraeus Electronics | Ly May Chew22.11.20182

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

1 2 New Material Development for Al

and Ni Surfaces

1. Background

2. Experimental− Sintering process flow


− Die shear test

− Scanning electron microscopy (SEM) coupled with

energy dispersive X-ray spectroscopy (EDX)

3. Results

4. Summary and outlook

1 | BACKGROUND


Pressure silver sintering by far offerssuperior 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-to-use micro Ag sinter pastefor pressure sintering on bare Cu surface and precious metal surfaces

High melting temperature (961°C)

High thermal and electrical conductivity

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 63rd Electronic

Components and Technology Conference, May. 2013, pp. 1746-17522. W. Schmitt, L.M. Chew, "Silver Sinter Paste for SiC Bonding with Improved Mechanical Propertise", IEEE 67th Electronic Components and Technology Conference, May. 2017, pp. 1560-1565

1 | PRESSURE SINTERING PROCESS FLOW


Paste Printing

Silver sinter paste:ASP 338-28

Substrates:Si3N4 AMB (Ag & Au metallization, bare Cu)

Stencil thickness:75 µm

Pre Drying ofPrinted Paste

Convection Oven

Temperature:120 °C

Drying time:20 min

Atmosphere:N2 (50 ppm O2)

Hot DiePlacement

Dies:

Ag metallized Si dies

(4 x 4 mm, 10 x 10 mm)

Placement temperature:

130 °CPlacement pressure:

400 g

Placement time:

2 s

Pressuresintering

Protective FoilPFA Foil

Pressure: 10 MPa

Temperature: 230 °C

Time: 3 min

Sinter atmosphere: Air

1 | CHARACTERIZATION


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 acoustic microscopy (SAM) identify void, drying channels and delamination in sintered joint

(Initial, 1000 and 2000 cycles)

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

Evaluation of reliability of sintered joint

Scanning electron microscopy (SEM) coupled withenergy dispersive X-ray spectroscopy (EDX)

elemental analysis of joint interface between silver sinteredlayer and bare Cu substrate

(after HTS)


Die shear testEvaluation of bonding strength of sintered joint

Die shearfailure mode:

High bondingstrength

Low bonding strength

1 | DIE SHEAR STRENGTH BEFORE AND AFTER TCT (-40°C/+150°C)


The average initial die shear strength for Agmetallized 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 resultingin higher bonding strength

After 2000 cycles, the average die shear strength ofbare Cu substrate is fairly similar to that of Agmetallized substrate

24 dies were sheared to generate an individual boxplot

1 | DIE SHEAR FAILURE MODE – BEFORE AND AFTER TCT


4 mm x 4 mm Ag metallized die

Initial 2000 cyclesAdhesive / cohesive break Cohesive break in the sintered layer

adhesive / cohesive break cohesive break

Ag

NiAu

bare Cu

Ag sinter layer(die)

Ag sinterlayer

(substrate)

Ag sinterlayer

(substate)




Ag sinterlayer

(substrate)

adhesive break on the substrate cohesive break



cohesive break adhesive break on the substrate

Ag sinterlayer

(substrate)

Bare Cusubstrate

Au metallizedsubstrate

1 | DIE SHEAR STRENGTH BEFORE AND AFTER HTS AT 250 °C


The average die shear strength increasedafter HTS

Ag, Au, Cu continued to diffuse duringHTS increasing Ag layer densificationresulting 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

24 dies were sheared to generate an individual boxplot

1 | DIE SHEAR FAILURE MODE BEFORE AND AFTER HTS


4 mm x 4 mm Ag metallized die Initial 1000 h storage

Ag

adhesive / cohesive break cohesive break in the Cu layer

NiAu

bare Cu

Ag sinter layer

Ag sinter layer

(die)

Ag sinter

layer(substrate)

Ag sinter layer

(substrate)

Ag sinter layer

(die)Ag sinter layer

(die)

adhesive break on the substrate cohesive break in the sintered layer

adhesive break on the substrate cohesive break in the Cu layer

Cu layerAg sinter layer

(die)

Adhesive / cohesive break Cohesive break in the sintered layer

Ag sinter layer

(substrate)

Cu layer

Cu layer

cohesive break in the Cu layer

cohesive break in the sintered layer

cohesive break in the Cu layer

250 h storage

sinter layer

substrate

Ag

Cu

Au metallizedsubstrate

Bare Cusubstrate

NiAu

sinter layer

substrate

Ag

Cu

1 | ELEMENTAL ANALYSIS BY SEM-EDX


Ag content decreases and concurrently theamount of Cu and O increases whenscanning 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 substratewhich show that the layer is silver


Bending testEvaluation of bonding strength of sintered joint (quick method)

1 | BENDING TEST – BEFORE AND AFTER TCT (-40 °C/+150 °C)


Initial 2000 cycles10 mm x 10 mm Ag metallized die

Ag

NiAu

bare Cu

Die attached stronglyon substrate

High bonding strength


Scanning acoustic microscopyIdentification of void, drying channels & delamination in sintered joint

1 | SAM – BEFORE AND AFTER TCT (-40 °C / +150 °C)


Void, drying channel and delaminationwere not detected in the sintered layer

for all the samples before and after 2000 thermal cycles

Homogeneous sintered layer was obtained

homogeneous pressure distributionwas applied during sintering process

High reliable sintered joint on bare Cu surface was achieved by pressure sintering under airatmosphere using ASP 338-28 sinter paste

Sintering process continued to occur during TCT and HTS and is completed after certain time ofstorage 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 toprevent interdiffusion of Ag/Cu

SAM and bending test results demonstrate that high bonding strength of sintered joint with nodelamination 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 68th Electronic Components and Technology Conference, May. 2018, pp. 323-330

SUMMARY


OUTLINE


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


3. Results

4. Summary

1 2 New Material Development for Al

and Ni Surfaces

1. Background

2. Experimental− Sintering process flow


− Die shear test

− Scanning electron microscopy (SEM) coupled with

energy dispersive X-ray spectroscopy (EDX)

3. Results

4. Summary and outlook

2 | BACKGROUND


Die

Substrate

Base Plate

Surfaces Sinterable

Ag, Au, Cu √

Al, Ni X

Ag, Au,

Cu, Ni

Ag, Au

Cu, Al,

AlSiC

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 63rd Electronic

Components and Technology Conference, May. 2013, pp. 1746-17522. W. Schmitt, L.M. Chew, "Silver Sinter Paste for SiC Bonding with Improved Mechanical Propertise", IEEE 67th Electronic Components and Technology Conference, May. 2017, pp. 1560-1565

3. 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 68th Electronic

Components and Technology Conference, May. 2018, pp. 323-330

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)

Solder Ag Sinter

Solder Ag Sinter

High Power Packages Development Trend

Higher current capacity >>> Higher operating temperature >>> Higher reliability

2 | PRESSURE SINTERING PROCESS FLOW


Paste Printing

Silver sinter paste:

ASP 338-28

LTS 342-54

Substrates:

Al2O3 DCB (Ni

plated, bare Cu)

Al plate (99.5 % Al)

Stencil thickness:

75 µm

Pre Drying ofPrinted Paste

Convection Oven

Temperature:120 °C

Drying time:

10 min

Atmosphere:

N2 (50 ppm O2) / Air

Hot DiePlacement

Dies:Ag metallized Si dies

(2 x 2 mm, 4 x 4 mm)

Placement temperature:

130 °C

Placement pressure:

400 g

Placement time:

3 s

Pressuresintering

Protective Foil

PFA Foil

Pressure: 10 MPa

Temperature: 250 °C

Time: 3 min

Sinter atmosphere:

Air

Ni DCB Cu DCBAl plate

2 | CHARACTERIZATION


Die shear test measure bonding strength of sintered joint

(Initial)

Scanning acoustic microscopy (SAM) identify void, drying channels and delamination in sintered joint

Evaluation of bonding strength of sintered joint

Scanning electron microscopy (SEM) coupledwith energy dispersive X-ray spectroscopy (EDX)

elemental analysis of joint interface between silver

sintered layer and substrate(after die shear test)

2 | HERAEUS COMMERCIAL SINTER PASTE (ASP 338-28)


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

IMAPS-UK | Die Attach Workshop | New Material Developments in Sintering | Heraeus Electronics | Ly May Chew

Scanning acoustic microscopyIdentification of void, drying channels & delamination in sintered joint

22.11.201822

2 | SAM – newly developed sinter paste (LTS 342-54)


Ni DCB Cu DCBAl plate

No void and drying channel in the sintered layer

IMAPS-UK | Die Attach Workshop | New Material Developments in Sintering | Heraeus Electronics | Ly May Chew

Die shear testEvaluation of bonding strength of sintered joint

22.11.201824

Die shearfailure mode:

High bondingstrength

Low bonding strength

2 | DIE SHEAR TEST – newly developed sinter paste (LTS 342-54)


Ni DCB

Cu DCB

Al plate



2 | SEM – AFTER DIE SHEAR (Ni DCB SAMPLE)


newly developed sinter paste (LTS 342-54)

Ag

Ni

CuP

Ni DCB

Ag sintered layer

Ag

NiCu

P

Interdiffusion

between Ag and Ni

2 | SEM – AFTER DIE SHEAR (Al PLATE SAMPLE)



Ag

Al plate

Ag sintered layer

Al

O

Ag

Al

O

Interdiffusion

between Ag and Al

2 | SEM – AFTER DIE SHEAR (Cu DCB SAMPLE)



Ag

Cu

O

Cu DCB

Ag sintered layer

Ag

Cu

Interdiffusion

between Ag and Cu

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 342-54 sinter paste

Average die shear strength above 15 N/mm² was obtained for all samples (Ni DCB, Al plate and CuDCB) 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 thatsintered joint created on Ni, Al and Cu surfaces and an interdiffusion of Ag/Ni, Ag/Al and Ag/Cuoccurred at the interface between sintered layer and substrate

SUMMARY


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New developments of direct bonding on non-precious metal ... · Pressure silver sintering by far...

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