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.

Thermal Performance and Reliability of Bonded 

Interfaces for Power Electronics Packaging 

Applications

ASTR 2011 Workshop

Douglas DeVoto

Douglas.DeVoto@nrel.gov

Phone: 303-275-4256 

Friday, September 30, 2011 

NREL/PR-5400-52468
 NATIONAL RENEWABLE ENERGY LABORATORY

Outline 

• Program Overview

• Background and Objectives

• Experimental Approach and Results

• Modeling Approach and Results

• Summary and Future Direction
 NATIONAL RENEWABLE ENERGY LABORATORY

Advanced Power Electronics and Electric Motors (APEEM) 

Research Targets, Challenges, and Research Areas

Power Electronics

($/kW) (kW/kg) (kW/l)

7.9 10.8 8.7

5 12 12

3.3 14.1 13.4

Motors

($/kW) (kW/kg) (kW/l)

11.1 1.2 3.7

7 1.3 5 

4.7 1.6 5.7

Traction Drive System

($/kW) (kW/kg) (kW/l) Efficiency

19 1.06 2.6 >90%

12 1.2 3.5 >93%

8 1.4 4 >94%

Research Areas

Power 

Electronics

Traction Drive 

System

PEEM Thermal 

Management

• Innovative topologies

• Temperature-tolerant devices

• Packaging

• Capacitors

• Vehicle charging

Electric 

Machines

• Permanent magnet (PM) motors

• Magnetic materials

• High-performance non-PM motors

• New materials

• Thermal system integration

• Heat transfer technologies

• Thermal stress and reliability

• Benchmarking technologies

• Innovative system designs

Challenges

Year

2010

2015

2020

Reduce Dependence on Oil

Via Electrification of Vehicle Drives

Requirements: 55 kW peak for 18 sec; 30 kW continuous; 15-year life

Technical Targets
 NATIONAL RENEWABLE ENERGY LABORATORY

NREL APEEM Research Focus Areas

Achieve technology 

integration at a lower 

system cost

Enable increased power 

density at lower cost

Improve reliability of 

new technologies

Credit: Mark Mihalic, NREL Credit: Ken Kelly, NREL
 NATIONAL RENEWABLE ENERGY LABORATORY

Background 

BIM 2

Silicon die

Direct-bond-copper (DBC) or 

Direct-bond-aluminum (DBA)

Copper or aluminum 

baseplate/coldplate

BIM 1

100 

110 

120 

130 

140 

150 

160 

0 2 4 6 8 10 12 

Distance across the package (mm)

Temperature ( 0C)

100 mm2K/W 

20 mm2K/W 

10 mm2K/W 

5 mm2K/W 

• Power electronics are essential in an electric 

vehicle drive train for controlling the output of 

the electric motor

• Excessive temperature (>150°C for silicon [Si] 

devices) can degrade the performance, life, and 

reliability of power electronics components

• Interfaces in the package can pose a major 

bottleneck to heat removal

• Conventional thermal interface materials (TIMs) 

do not meet thermal performance and reliability 

targets—the industry trend is towards bonded 

interface materials (BIMs)

• Bonded interfaces, such as solder, degrade at 

higher temperatures and are prone to 

thermomechanical failure under large 

temperature cycling.

Credit: Mark Mihalic, NREL
 NATIONAL RENEWABLE ENERGY LABORATORY

Objectives

• Overall Objective

– Investigate thermal performance and reliability of novel bonded interface 

materials (such as sintered silver and thermoplastics with embedded carbon 

fibers) for power electronics applications to meet the following specific 

objectives: 

o Thermal resistance of 5 mm2K/W 

o Thermal resistance maintenance of 5 mm2K/W after 2,000 thermal cycles 

from -40°C to 150°C. 

• Benefits

– High-performance, reliable, low-cost bonded interfaces enable: 

o Compact, light-weight, low-cost packaging

o High-temperature coolant and/or air cooling.
 NATIONAL RENEWABLE ENERGY LABORATORY

Failure Mechanisms

Voids Cohesive Fracture Adhesive Fracture

Bonded Interface Materials

Direct-Bond-Copper (DBC) 

Aluminum Nitride (AlN) Cracks Copper (Cu) to AlN Delamination

Credit: G.Q. Lu, Virginia Tech

Credit: Doug DeVoto, NREL (all remaining photos)
 NATIONAL RENEWABLE ENERGY LABORATORY

NREL Reliability Capabilities

Non-Destructive Imaging

(C-mode scanning acoustic 

microscope [C-SAM])

0 Cycles

40 Cycles

Ceramic Electrical 

Failure (High-potential 

[hi-pot] tester) 

Thermal Cycling

Thermal Resistance

(ASTM stand) 

Sample Deformation

(Laser profilometer) 

Finite Element Analysis 

(FEA) Modeling

Credit: Doug DeVoto, NREL (all photos)

Sample Synthesis

(Hot press)
 NATIONAL RENEWABLE ENERGY LABORATORY

Approach 

Sample Synthesis

BIM 

Mechanical 

Characterization

Reliability 

Calculation

Thermal Testing/

Characterization

Synthesis of samples using 

stencil printer and hot press

Cycling of 

samples in a 

thermal shock 

chamber

Characterization of samples via 

steady-state thermal resistance 

tester, hipot tester, C-SAM, and 

X-ray imaging

Shear tests to extract

mechanical characteristics 

of BIMs

Number of 

cycles to crack 

initiation/

delamination

Fatigue life 

prediction

Strain energy 

density per cycle

Extraction of Anand model 

parameters

Experimental Approach Numerical Approach - FEA/Calculations
 NATIONAL RENEWABLE ENERGY LABORATORY

Bond 

Material

Type

Name Comments

Solder Sn63Pb37 Baseline (lead-based solder)

Solder SAC305 Lead-free solder

Sintered silver Heraeus C1075A Based on micron-size silver 

particles

Sintered silver nanoTach® Based on nanoscale silver 

particles

Adhesive Btech HM-2 Thermoplastic (polyamide) film 

with embedded carbon fibers

• Five samples of each BIM (between DBC/copper baseplate) were 

synthesized for testing and included:

- Silver coating on the DBC and baseplate

- DBC base on an AlN substrate

- An interface between 50.8 mm x 50.8 mm footprint

• Samples followed manufacturer-specified reflow profiles, and bonds 

were inspected for quality.

DBC/Base Plate Assembly

Credit: Doug DeVoto, NREL

Sample Assembly
 NATIONAL RENEWABLE ENERGY LABORATORY

Thermal Resistance Measurements

30

40

50

60

70

80

PbSn SAC Ag Sinter

(μm)

Ag sinter

(nm)*

Btech

HM-2

Test Resistance (mm

2K/W) 

Test Resistance before Cycling

#1

#2

#3

#4

#5

*DBC of samples 2 and 5 delaminated from baseplate after ASTM test.

• Each sample was tested for thermal resistance before the temperature cycling 

was initiated

• Resistance values included package resistance and two layers of 25μm Dow TC 

5022 interfacing between a sample and the test stand blocks. 

Credit: Doug DeVoto, NREL
 NATIONAL RENEWABLE ENERGY LABORATORY

• Cycle Profile

- Thermal extremes from -40°C to 150°C 

- Ramp rate of 10°C/minute, with a

dwell/soak time of 10 minutes

- Adherence to JEDEC Standard 22-A104D 

for temperature cycling.

Shock Chamber

Credit: Doug DeVoto, NREL

-50

0

50

100

150

200

0:00 0:30 1:00 1:30 2:00 2:30 3:00

Temperature (°C)

Time (h:mm)

Shock Chamber Testing

Sample Profile

Temperature Profile

Thermal Cycling 
 NATIONAL RENEWABLE ENERGY LABORATORY

Bonded Interface Materials Testing

13

• Failure Mechanisms: 

– BIM, including voids and adhesive/cohesive fractures

– DBC, including Cu-to-AlN delamination and AlN cracks.

Material 0 cycles 20 cycles 40 cycles 100 cycles 200 cycles

PbSn 2%–10% voiding - - DBC delamination Additional DBC 

delamination

Tin-silvercopper (SnAg-Cu or 

SAC) 

19%–33% voiding Minor DBC 

delamination - Additional DBC 

delamination

Silver (Ag) 

Sinter (μm)

Poor adhesion, 

AlN cracking

Minor DBC 

delamination

Additional DBC 

delamination Testing ended

Ag Sinter 

(nm)

BIM cohesive 

fracturing in two

of five samples

BIM fracturing in 

three remaining

samples 

Testing ended

Btech HM-2 Successful bond Bond quality 

maintained - Excessive DBC 

delamination Testing ended
 NATIONAL RENEWABLE ENERGY LABORATORY

PbSn

• Kester Sn63/Pb37 256HA

– Printing 

o A 50.8 μm (2-mil) stainless-steel stencil was raised to 101.6 μm (4 mil) 

with Kapton tape and placed over the baseplate. The stencil pattern has 9 

mm x 9 mm square openings with 1 mm separations. The solder material 

was applied to the top of the stencil and a squeegee was used to ensure 

uniform application.

– Reflow 

o The assembled sample passed through a Sikama reflow oven following 

the recommended profile

– Initial Results

o Low percentage of voiding (2%–10%), solder thickness of 50 to 75 μm

– Cycling Results 

o Two samples show large liftoff of the top Cu layer, with the remaining 

three samples revealing minor liftoff.

0 Cycles

100 Cycles

200 Cycles The top Cu layer delaminating from AlN is shown 

after 100 cycles (left) and 200 cycles (right). Time (s)

0 50 100 150 200 250 300

Temperature (oC)

0

50

100

150

200

250

Reflow Profile for EP256HA Pb-Sn solder

T on Cu plate

Recommended

Actual

Credit: Doug DeVoto, NREL (all photos)
 NATIONAL RENEWABLE ENERGY LABORATORY

Ag Sinter (μm)

• Heraeus C1075A (silver conductor paste)

– Printing 

o Two layers of paste were applied with a 50.8 μm stencil, and each layer 

was dried after application at 80°C for 30 minutes

– Sintering 

o Binder was burned off for 2 hours at 250°C, and then the assembly was 

heated to 280°C and underwent 30 MPa of pressure for 30 minutes

– Initial Results

o High-bonding temperatures caused cracking within the AlN layer and poor 

adhesion at the edges was revealed in the C-SAM images

– Cycling Results 

o Together, top Cu and AlN warp and delaminate from the lower Cu layer. 

Cycling of samples will not continue.

0 Cycles

20 Cycles

40 Cycles

Credit: Doug DeVoto, NREL (all photos)
 NATIONAL RENEWABLE ENERGY LABORATORY

Thermally Conductive Adhesive Film

• Btech HM-2 (Carbon Fibers within Polymer Matrix)

– Bonding 

o HM-2 was cut to the baseplate dimensions. The sample assembly was placed 

in the hot press and raised to 190°C, and then 0.689 MPa (100 psi) of pressure 

was applied.

– Initial Results

o C-SAM images showed less contrast with thermoplastics, but uniform bonds 

were obtained. 

– Cycling Results 

o After 100 cycles, excessive DBC delaminations were present. Delaminations 

occurred both between the AlN and lower Cu as well as the AlN and top Cu. 

0 Cycles

20 Cycles

Screw 

Jack

Cold 

Plates

Hot 

Plates

Credit: Doug DeVoto, NREL (all photos)

100 Cycles
 NATIONAL RENEWABLE ENERGY LABORATORY

BIM Mechanical Characterization

• Shear testing fixture was designed for 

BIM specimens. The fixture will allow for 

sample testing at various strain rates 

and temperatures.

• Script developed to derive Anand 

viscoplastic parameters from strain rate 

test data. This will allow the behavior of 

new solder and sinter materials to be 

modeled in FEA simulations.

• Template established for performing 

FEA simulations of the sample 

geometry to determine the strain energy 

density per thermal cycle. This will be 

used as a fatigue-life indicator.

Example Stress-Strain Test Data

Courtesy: Andreas Vlahinos, 

Atmospheric Environment Service (AES) 
 NATIONAL RENEWABLE ENERGY LABORATORY

0 50 100 150 200 250 300

-40

-20

0

20

40

60

80

100

120

140

 Time (min)

 Temperature (C)

 Temperature Cycling

FEA Modeling

Parameters

– Reflow temperature = 125°C 

– Room temperature = 22°C 

– Ramp rate = 10°C/minute

Anand’s viscoplastic material model 

for the solder/sintered material

Temperature-dependent material 

properties for Cu and AlN

Courtesy: Andreas Vlahinos, AES (all graphics)

DBC 

(AlN substrate)

Copper 

baseplate

Sintered/solder 

layer
 NATIONAL RENEWABLE ENERGY LABORATORY

Stress Distribution

• Equivalent stress distribution in the 

upper/top and middle layer of the bonded 

joint

• Stresses are higher in the layer adjoining 

the DBC and higher in the corner regions.

Courtesy: Andreas Vlahinos, AES (all graphics)

Upper/top layer of bonded joint Middle layer of bonded joint
 NATIONAL RENEWABLE ENERGY LABORATORY

Plastic Work Density Distribution

• Plastic work density distribution in the bonded joint 

 region 

• Work density higher in the corner regions—failures 

likely to originate here

• Plastic work density/strain energy density versus 

 cycles-to-failure correlation is the new contribution of 

this effort.

Courtesy: Andreas Vlahinos, AES (all graphics)

Quarter Symmetry Model
 NATIONAL RENEWABLE ENERGY LABORATORY

Collaborations 

Collaborator Nature of Interaction/Collaboration

Electrical and Electronics 

Technical Team, GM, 

Delphi

• Technical guidance

Oak Ridge National 

Laboratory 

• Packaging

National Institute of 

Standards and 

Technology

• Electrothermal modeling of power modules 

Virginia Tech • Synthesis of soldered and sintered joints

• X-ray imaging

• Provider of nanosilver paste

Btech • Provider of HM-2 thermoplastic (polyamide) 

with embedded carbon fibers

Atmospheric 

Environment Service

• Finite element modeling support
 NATIONAL RENEWABLE ENERGY LABORATORY

Summary 

• Approach

– Synthesis of various joints between DBA/DBC and baseplate (Cu/Al), thermal shock/temperature cycling, 

thermal resistance measurements, high-potential test and joint inspection (X-ray, C-SAM), and strain 

energy density versus cycles-to-failure models

• Current Work

– Synthesized a number of bonded interfaces between the DBC/Cu base plate based on different BIM 

technologies, including lead-based and lead-free solder, sintered silver (micron-size and nanosilver), and 

thermoplastic

– Initiated FEA for bonded interface geometries

– Initiated thermal resistance measurement and quality characterization for the different bonded interfaces 

prior to thermal shock/cycling.

• Future Work

– For bonded interface between the DBC/Cu baseplate:

o Refine the bond synthesis process

o Continue thermal cycling of the bonded samples

o Characterize thermal resistance and bond quality

o Characterize BIM to obtain stress-versus-strain mechanical constitutive relationships

o Perform FEA modeling to infer strain energy density, defect initiation, and propagation

o Correlate strain energy density to cycles-to-failure.

– Investigate the thermal performance and reliability of the bonded interfaces between DBA/aluminum as 

well as alternatives to AlN, such as Si3N4.
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.

For more information, contact:

Principal Investigator

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov

Phone: 303-275-4062

APEEM Task Leader

F. Patrick McCluskey

Patrick.McCluskey@nrel.gov

Phone: 303-275-3653

Acknowledgements: 

Susan Rogers, U.S. Department of 

Energy

Team Members:

Doug DeVoto

Mark Mihalic

Tim Popp