Performance and Reliability of Bonded 

Interfaces for High-Temperature 

Packaging

PI: Paul Paret

National Renewable Energy Laboratory

June 19, 2018

DOE Vehicle Technologies Office

2018 Annual Merit Review and Peer Evaluation Meeting

This presentation does not contain any proprietary, confidential, or otherwise restricted information.

Project ID: ELT080
NREL | 2

Overview

• Project start date: FY17

• Project end date: FY19

• Percent complete: 60%

• Cost

• Size and Weight

• Performance and Lifetime

Timeline Barriers

• Interactions/collaborations

– Virginia Polytechnic Institute and 

State University (Prof. G. Q. Lu)

– Oak Ridge National Laboratory (ORNL)

• Project lead

– National Renewable Energy Laboratory 

(NREL)

Partners

• Total project funding

– DOE share: $767K

• Funding for FY 2017: $492K

• Funding for FY 2018: $275K

Budget
NREL | 3

Relevance

Metalized Substrate

Sintered-Silver

Base Plate

• Sintered silver synthesized using high pressure (30-40 MPa) exhibits 

excellent reliability; survived for more than 2,000 cycles

• High bonding pressure results in manufacturing complexities and 

catastrophic damage to devices and substrates

Pressure-less 

sintered silver

• Mechanical characterization

• Thermal cycling reliability

• Thermomechanical modeling

Crack Propagation in Pressure-Assisted Sintered Silver

Credit: Paul Paret
NREL | 4

Overall Approach

• Conduct a mechanical characterization study (stress 

versus strain relationship) of sintered silver

• Evaluate reliability of sintered silver under thermal 

cyclic loading conditions

• Develop finite element analysis models that can 

capture the fatigue behavior of sintered silver
NREL | 5

Approach – Mechanical Characterization

Double Lap Sample Shear Testing

• Shear testing was conducted at multiple 

displacement rates and temperatures

• Peak shear stress values were recorded

• Both nano-silver and hybrid/nano-micron 

silver were included in the study

• Impact of variation in sintering pressure was 

investigated on hybrid-sintered silver

Shear Test Output

Credit: Douglas DeVoto Credit: Douglas DeVoto
NREL | 6

Approach – Reliability Evaluation

CTE 1

CTE 2

CTE 1

CTE 2

CTE 1

CTE 2

Side Views

Top View

• Main output from accelerated testing would be cycles-to-failure

Copper – Invar Coupons

Φ 25.4 mm

Credit: Douglas DeVoto

CTE – co-efficient of thermal expansion

-100

0

100

200

0 5 10 15 20 25

Temperature (°C)

Time (min)

Thermal load
NREL | 7

Approach – Thermomechanical Modeling

Φ 25.4-mm Sample

Strain Energy Density 

Contour Plot

J-Integral Plot

Continuum Mechanics

Fracture Mechanics

Crack extension (mm/cycle)

J-Integral (J/m2)

Lifetime Prediction 

Model

Lifetime Prediction 

Model

Strain Energy Density

Cycles to Failure
NREL | 8

Milestones

Month / Year Description of Milestone or Go/No-Go Decision Status

December 2017 Conduct thermomechanical modeling simulations to obtain 

strain energy density and J-integral values Completed

May 2018

Complete mechanical characterization tests on double-lap 

samples

Select a constitutive model for sintered silver from the 

resulting stress-strain curves (Go/No-Go)

Ongoing

August 2018 Conduct accelerated thermal cycling tests on round coupons to 

obtain cycles-to-failure (Go/No-Go) Ongoing

September 2018

Formulate a lifetime predictive model correlating modeling 

outputs and experimental results Ongoing

January 2019

Develop a preliminary crack propagation model of sintered 

silver material with the appropriate constitutive model as 

input To be started

August 2019

Conduct additional accelerated tests to validate the lifetime 

prediction model; use these results to improve the accuracy of 

the lifetime prediction model

To be started
NREL | 9

Technical Accomplishments and Progress

Synthesis of double-lap samples

0

50

100

150

200

250

300

0 100 200 300 400

Temperature (°C)

Time (mins)

140°C 0.43°C/min 

3.23°C/min 

60 mins 

Sintering profiles of nano-silver (left) and hybrid-silver (right)

C-mode scanning acoustic microscope 

(C-SAM) images of sintered silver bond 

regions in double-lap samples
NREL | 10

Technical Accomplishments and Progress

Observation of Failure Mechanism – Nano-silver

• A few double-lap samples were cross-sectioned and subjected to shear test

• A digital microscope took images of the sintered silver interface 

• Cohesive fracture mechanism was observed to be the failure mechanism

Cross-section view Interface before failure Cohesive fracture

Credit: Paul Paret Credit: Joshua Major Credit: Joshua Major
NREL | 11

Technical Accomplishments and Progress

Shear Stress Results – Nano-silver

0

4

8

12

16

20

24

0.01 0.0001

Shear Stress (MPa)

Displacement Rate (mm/s)

Impact of displacement rate and ambient 

temperature

RT 200°C

0

4

8

12

16

20

24

0.01 0.0001

Shear Stress (MPa)

Displacement Rate (mm/s)

Impact of sintering temperature

270°C 250°C

• Shear stress values drop with rise in ambient temperature, possibly due to creep strain

• Lower displacement rates allow more time, causing the defects to grow thereby weakening the 

sintered silver microstructure

• A rise in sintering temperature from 250°C to 270°C resulted in slight improvement in shear stress 

values

RT – room temperature
NREL | 12

Technical Accomplishments and Progress

Shear Stress Results – Hybrid-silver

0

4

8

12

16

20

24

28

32

36

0.01 0.0001

Shear Stress (MPa)

Displacement Rate (mm/s)

Impact of displacement rate and ambient 

temperature

-40°C RT 200°C

• No significant difference in the shear stress results between -40°C and room 

temperature

• Rate dependence is strong at elevated temperatures (200°C)

• Pressure-assisted sintering results in a denser bond with low porosity; results in high 

shear strength

• Between 3 MPa and 10 MPa of sintering pressure, shear stress variation was relatively 

insignificant

0

10

20

30

40

50

60

70

80

0.01 0.0001

Shear Stress (MPa)

Displacement Rate (mm/s)

Impact of sintering pressure

0 MPa 3 MPa 10 MPa
NREL | 13

Technical Accomplishments and Progress

Reliability Evaluation – Hybrid-silver

3 MPa sintering pressure

22 mm 16 mm 10 mm

10 MPa sintering pressure

22 mm 16 mm 10 mm

C-SAM images of pressure-assisted hybrid-sintered silver samples at zero cycles
NREL | 14

Technical Accomplishments and Progress

Strain Energy Density Results

Interface

Outer 

Coupon

1

2

3

4

5

8 12 16 20 24

Strain Energy Density/Cycle 

(MPa)

Diameter (mm)

Cu-Invar

0

1

2

3

4

5

Strain Energy Density (MPa)

0 2 4 6 8 10 12

Distance (mm)

Strain energy density variation – 22 mm 

diameter

Ramp up High dwell Ramp down Low dwell
NREL | 15

Responses to Previous Year Reviewers’ Comments

The reviewer commented that Anand viscoplastic 

and other material models were used in the study 

to simulate sintered silver layer

Simulation results to date are based on the Anand 

viscoplastic model only. Mechanical characterization 

results will be used to develop other material model 

parameters

The reviewer pointed to correlating the reliability 

aspects of the sintered silver joint with its porosity

Sintered silver samples manufactured using 3 MPa 

and 10 MPa of sintering pressure will be subjected 

to thermal cycling. Investigating the reliability of 

samples made using different sintering pressures 

will provide insights into the effects of porosity on 

thermomechanical fatigue 
NREL | 16

Collaboration and Coordination with 

Other Institutions

• Virginia Tech: technical partner on the synthesis of sinteredsilver samples

• ORNL : technical guidance

• Private industries in power electronics
NREL | 17

Remaining Challenges and Barriers

• Existence of a correlation between shear strength and 

reliability of sintered silver may be uncertain

• Developing void-free round large-area bonded coupons for 

reliability evaluation with pressure-less sintering process is 

not trivial

• The suitability of modeling parameters such as strain energy 

density and J-integral for accurately predicting the lifetime of 

sintered silver joints needs to be validated
NREL | 18

Proposed Future Research – FY18

• Complete accelerated thermal cycling test 

on sintered silver until failure

• Develop a lifetime prediction model 

correlating modeling outputs with 

experimental results

• Validate the thermomechanical model 

with experimental results; make changes 

to the model to improve its accuracy

“Any proposed future work is subject to change based on 

funding levels”

Crack extension (mm/cycle)

J-Integral (J/m2)

Lifetime Prediction 

Model
NREL | 19

Proposed Future Research – FY19

• Submit a journal paper capturing the various findings of the 

project, including both modeling results and experimental data

• Investigate the reliability of other high-temperature bonded 

interface materials

– Cu – Al , Cu – Sn alloys 

• Initiate a multiscale modeling approach to predict failure due 

to fatigue in high-temperature bonded interface materials

“Any proposed future work is subject to change based on funding levels”
NREL | 20

Summary

• DOE Mission Support

– Sintered silver is a promising material for enabling low-cost, lightweight, and reliable 

power electronics packages that can operate at high temperatures

• Approach

– Synthesis of sintered silver bonds, mechanical characterization, reliability evaluation, 

thermomechanical modeling, and lifetime prediction models

• Accomplishments

– Conducted mechanical characterization tests and studied the impact of microstructure, 

ambient temperature, displacement rate, sintering temperature, and sintering pressure 

on the shear strength of sintered silver

– Conducted thermomechanical simulations, and obtained strain energy density/cycle and 

J-integral/cycle results

– Synthesized round CTE-mismatched coupons for reliability evaluation of sintered silver

• Collaborations

– Virginia Tech

– ORNL

– Power electronics industry partners
NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency 

and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

www.nrel.gov

NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency 

and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for 

Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. 

Funding provided by U.S. Department of Energy Office of Energy Efficiency and Vehicle Technologies 

Office. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. 

Government. The U.S. Government retains and the publisher, by accepting the article for publication, 

acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to 

publish or reproduce the published form of this work, or allow others to do so, for U.S. Government 

purposes.

Thank You

Acknowledgments

Susan Rogers, U.S. Department of 

Energy

EDT Task Leader

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov 

Phone: (303)-275-4062

Team Members

Douglas DeVoto (NREL)

Joshua Major (NREL)

For more information, contact

Principal Investigator

Paul Paret

Paul.Paret@nrel.gov

Phone: (303)-275-4376

NREL/PR-5400-71205