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 Management and 

Reliability of Power Electronics 

and Electric Machines

Sreekant Narumanchi

Organization: National Renewable Energy Laboratory 

Email: sreekant.narumanchi@nrel.gov

Phone: 303-275-4062

NREL Team Members: Kevin Albrecht, Kevin Bennion, Emily Cousineau, 

Doug DeVoto, Xuhui Feng, Charlie King, Joshua Major, Gilbert Moreno, 

Paul Paret, Meghan Walters

3D PEIM Symposium

Raleigh, NC

June 13, 2016

NREL/PR-5400-66554 This presentation does not contain any 

proprietary or confidential information.
2

Importance of Thermal Management and Reliability

• Excessive temperature degrades the performance, life, and 

reliability of power electronics and electric machines. 

• Advanced thermal management technologies enable 

o Keeping temperature within limits

o Higher power densities

o Lower cost materials, configurations and system.

• Improve lifetime/reliability and develop new predictive lifetime 

models.
3

DOE Vehicle Technologies Office Electric Drive Technologies 

(EDT) Program Targets

(on-road status)

• Discrete Components

• Silicon Semiconductors

• Rare-Earth Motor Magnets

• Fully Integrated Components

• Wide-Bandgap (WBG) Semiconductors

• Non Rare-Earth Motors

3

From DOE EV Everywhere Grand Challenge Blueprint, 

http://energy.gov/sites/prod/files/2016/05/f31/eveverywhere_blueprint.pdf 
4

Advanced 

Packaging 

Reliability

Electric Machines 

Thermal 

Management

Power Electronics 

Thermal 

Management

NREL EDT Research Focus Areas

Research Focus Areas Will Reduce Cost, Improve Performance and Reliability

Enabling Materials

Photo Credit: Jana Jeffers, NREL

Photo Credits: Doug DeVoto, NREL Photo Credits: Doug DeVoto and Gilbert Moreno, NREL
5

Power Electronics Thermal Management
6

Power Electronics Thermal Management Strategy 

• Packages based on WBG devices 

require advanced materials, 

interfaces, and interconnects 

o Higher temperature capability

o Higher effective thermal conductivity 

• Low-cost techniques to increase heat 

transfer rates

o Coolants – water-ethylene glycol 

(WEG), air, transmission coolant, 

refrigerants

o Enhanced surfaces

o Flow configurations

• System-level thermal management 

(capacitor and other passives)

WBG Module Packaging Design with Integrated Cooling

Device

Metalized Substrate

Substrate

Attach

Integrated Base/Cold Plate

Die 

Attach

Interconnect Encapsulant

Enclosure

Terminal

Coolant 

Inlet

Coolant 

Outlet
7

The Challenge with Interfaces/Interface Materials

• Interfaces can pose a major bottleneck to heat removal.

• Bond materials, such as solder, degrade at higher temperatures and are prone to 

thermomechanical failure.

• Problem can become more challenging for configurations employing WBG devices.

2

2.5

3

3.5

4

4.5

100 110 120 130 140 150 160

Distance across the Package (mm)

Temperature (°C)

New Degraded

Degraded Substrate Attach

132.5°C 156.7°C

Module Packaging Design with Integrated Cooling

Device

Metalized Substrate

Substrate

Attach

Integrated Base/Cold Plate

Die Attach

Interconnect Encapsulant

Enclosure

Terminal

Coolant 

Inlet

Coolant 

Outlet
8

Pump Laser

Function Generator

Lock-in Amplifier

Multi-mode 

optical fiber

Collimation lenses

Lens tube

Modulated 

pump laser

Optical 

chopper

Probe laser

Reflecting 

mirrors

Photodiode

Modulated 

pump laser

Heat transfer

Probe laser

Probed 

temperature 

variation

Sample

Thermal Resistance of Thermoplastics with Embedded 

Carbon Fibers

• Thermoplastics with 

embedded carbon fibers 

show very good thermal 

performance

Frequency-domain transient thermoreflectance experiment configuration Thermoplastic 

film 

HM-2 

Bondline

thickness (μm)

60

Bulk thermal 

conductivity 

(W/m·K)

37.5 ± 6.8

Contact 

resistance 

(mm2·K/W)

3.1 ± 1.1

Total thermal 

resistance 

(mm2·K/W)

7.5 ± 1.9

Photo: Courtesy of BtechCorp
9

• Bonded interface resistance in the range of 0.4 to 2 mm2K/W 

is possible.

 Materials developed in the DARPA programs are in this range.

o Copper nanowires

o Boron-nitride nanosheets (0.4 mm2K/W for 30- to 50-μm bondline thickness)

o Copper nanosprings (1 mm2K/W for 50-μm bondline thickness with very good reliability)

o Graphite solder

o Nanotube-based

Other Bonded Interface Materials
10

Integrated Module Heat Exchanger

• Up to 100% increase in power per die 

area

• Up to 34% increase in coefficient of 

performance (efficiency)

NREL integrated module heat exchanger 

Patent No.: US 8,541,875 B2

Photo Credits: Kevin Bennion, NREL
11

Liquid Jet-Based Plastic Heat Exchanger

Metalized 

substrate

Base plate

Plastic manifold 

Device

Bonded interface

material (BIM)

Wire/ribbon 

bonds

WEG jets

Enhanced

surface

• Up to 12% increase in power 

density

• Up to 36% increase in specific 

power

Photo Credit: Doug DeVoto, NREL

Photo Credit: Gilbert Moreno, NREL
12

Two-Phase Cooling for Power Electronics

Fundamental 

Research

Module-Level 

Research

Inverter-Scale 

Demonstration

Vapor Liquid

Evaporator

Characterized performance of 

HFO-1234yf and HFC-245fa

Achieved heat transfer rates of up 

to ~200,000 W/m2-K

Reduced thermal resistance by 

over 60% using immersion twophase cooling of a power module

Quantified refrigerant volume 

requirements

Dissipated 3.5 kW of heat with only 

250 mL of refrigerant

Predicted 58%–65% reduction in 

thermal resistance via indirect and 

passive two-phase cooling

Power electronics 

modules

Evaporator

Air-cooled 

condenser Photo Credit: Bobby To, NREL

Photo Credit: Gilbert Moreno, NREL

Photo Credit: Gilbert Moreno, NREL
13

WBG Power Electronics Thermal Management 

Create thermal models of 

an automotive inverter

Simulate WBG operation 

using the inverter model

Explore advanced cooling 

strategies

Quantify the inverter 

component temperatures 

under elevated device 

temperatures

Validate the thermal models

Evaluate different module 

topologies

Experimentally validate some 

key thermal management 

concepts 

cold plate

copper

copper-molybdenum direct-bond copper

cold plate

Develop thermal 

management concepts to 

enable WBG power 

electronics

Identify the primary thermal 

paths through which heat is 

conducted from the devices to 

the other components

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14 16

Specific thermal resistance: R"th ,j-l (mm2-K/W)

Flow rate (liters per minute)

 Experiment

 Model: maximum

 Model: average

Photo Credit: Scot Waye, NREL
14

Advanced Packaging Reliability

14
15

Bonded Interface Material Reliability

• Thermoplastics 

yield very good 

reliability

• Reliability of 

sintered silver is 

better than 

solder

Photo Credits: Doug DeVoto, NREL
16

Bonded Interface Material Reliability

• Thermoplastics 

yield very good 

reliability

• Reliability of 

sintered silver is 

better than 

solder
17

Impact of Geometric Variations on Reliability

Profile Joint Thickness

(μm) ΔW (MPa) Predicted experimental 

cycles to failure – Nf

Thermal cycle

50 2.65 465

100 1.36 1,400

150 0.93 2,600

• Joint Thickness

• Substrate Variation

Profile Substrate

Coefficient of 

Thermal Expansion 

(x 10-6/°C)

ΔW (MPa)

Predicted 

experimental 

cycles to failure –

Nf

Thermal cycle

Si3N4 2.8 1.36 1,400

AlN 4.5 1.08 2,000

Al2O3 8.1 0.44 9,000

Predictive Lifetime Model, Nf = 2312.5 (ΔW)-1.645
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Electric Machines Thermal Management 
DOE APEEM FY14 Kickoff Meeting 19

Stator 

Laminations

Slot-Winding

Rotor 

Laminations Rotor 

Hub

Case

Air Gap

Stator-Case 

Contact

Nozzle/Orifice

ATF 

Impingement

Shaft

ATF Flow

ATF Flow

Stator Cooling Jacket

End-Winding

Motor Axis

Electric Machines Thermal Management Strategy

Problem

• Multiple factors impacting heat 

transfer are not well quantified or 

understood.

Contributing Factors

1. Direction-dependent thermal 

conductivity of lamination stacks

2. Direction-dependent thermal 

conductivity of slot windings and 

end windings

3. Thermal contact resistances (statorcase contact, slot-winding interfaces)

4. Convective heat transfer coefficients 

for ATF cooling 

5. Cooling jacket performance

4

1

1

2

3

5

Motor Cooling Section View

ATF: Automatic Transmission Fluid
20

Transmission Oil Jet Heat Transfer Characterization

Heat transfer coefficients of all target 

surfaces at 50°C inlet temperature

• Surface features increase heat 

transfer

18 AWG surface target

Note: Heat transfer coefficient calculated from the base projected area (not wetted area)

Top View

Side View

50°C Inlet Temperature

0

2000

4000

6000

8000

10000

12000

0 2 4 6 8 10 12

Heat Transfer Coefficient [W/m

2K]

Velocity [m/s]

18 AWG

22 AWG

26 AWG

Baseline

Photo Credit: Gilbert Moreno, NREL
21

Lamination Stack Effective Thermal Conductivity

Measured Stack Thermal 

Resistance

Lamination-to-Lamination 

Thermal Contact Resistance

Effective Through-Stack 

Thermal Conductivity

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

0 200 400 600

Asymptotic Stack Thermal Conductivity 

[W/m-K]

Pressure [kPa]

M19 26 Gauge

M19 29 Gauge

HF10

Arnon7

Error bars represent 95% confidence level
22

Transverse Winding Thermal Conductivity

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

3 4 5 6 7

Thermal Conductivity [W/m-K]

Sample Thickness [mm]

22 AWG Transverse

data

FEA

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

3 4 5 6 7

Thermal Conductivity [W/m-K]

Sample Thickness [mm]

19 AWG Transverse

data

FEA

• Error bars represent 95% measurement 

uncertainty (U95) 

• Finite element analysis (FEA) model results 

based on measured sample copper fill factor 

• FEA assumes hexagonal or closed-pack wire 

pattern

Photo Credit: Emily Cousineau, NREL
23

• Industry-led inverter development with VTO and AMO funding

– Delphi Inverter (VTO)

– GM Inverter (VTO)

– Wolfspeed Wide-Bandgap Inverter (VTO)

– John Deere Wide-Bandgap Inverter (AMO)

• UQM Technologies motor development (VTO funding)

Participation in Industry-Led Projects

VTO: Vehicle Technologies Office

AMO: Advanced Manufacturing Office
24

Summary

• Low-cost, high-performance thermal management technologies are helping meet 

aggressive power density, specific power, cost, and reliability targets for power 

electronics and electric machines.

• NREL is working closely with numerous industry and research partners to help 

influence development of components that meet aggressive performance and 

cost targets

o Through development and characterization of cooling technologies

o Thermal characterization and improvements of passive stack materials and interfaces.

• Thermomechanical reliability and lifetime estimation models are important 

enablers for industry in cost- and time-effective design.
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:

NREL APEEM Section Supervisor

Sreekant Narumanchi

sreekant.narumanchi@nrel.gov

Phone: (303) 275-4062

Acknowledgments:

Susan Rogers and Steven Boyd 

EDT Program

Vehicle Technologies Office

Advanced Manufacturing Office

U.S. Department of Energy

Industry and Research Partners

Ford, GM, FCA, John Deere, Tesla, Toyota, Oak Ridge National Laboratory, 

DARPA, Virginia Tech, University of Colorado Boulder, University of Wisconsin, 

Carnegie Mellon University, Texas A&M University, North Carolina State University, 

3M, NBETech, Curamik, DuPont, GE Global Research, Semikron, Kyocera, Sapa, 

Delphi, Btechcorp, Remy/BorgWarner, Heraeus, Henkel, Wolverine Tube Inc., 

Wolfspeed, Kulicke & Soffa, UQM Technologies Inc., nGimat LLC