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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.
Performance and Reliability of Interface Materials for Automotive Power Electronics
Sreekant Narumanchi National Renewable Energy Laboratory [email protected] (303) 275-4062 NREL Team members: Doug DeVoto, Mark Mihalic, Paul Paret Industry Session Applied Power Electronics Conference Long Beach, CA March 19, 2013 NREL/PR-5400-57964
This presentation does not contain any proprietary or confidential information
2
Outline • Background • State-of-the-art of interface materials/interfaces • Non-bonded thermal interface materials (TIMs)
• Thermal resistance
• Bonded interface materials (BIMs) • Thermal resistance
• Reliability of bonded interfaces (accelerated testing)
• Modeling of BIMs
• Summary
3
Importance of Thermal Management and Reliability • Excessive temperature degrades
the performance, life, and reliability of power electronics and electric motors.
• Advanced thermal management technologies enable
– keeping temperature within limits
– higher power densities – lower cost materials,
configurations and system.
• Improve lifetime/reliability as well as develop new predictive lifetime models.
Reduce cost, improve reliability
Courtesy: Oak Ridge National Laboratory (ORNL)
4
State-of-the-Art of Interfaces/Interface Materials • Interfaces (especially polymeric interface materials) can pose a major bottleneck to heat
removal. • BIMs, such as solder, degrade at higher temperatures and are prone to thermomechanical
failure. • Need for improved reliability as well as predictive lifetime design tools for lowering cost. • Important for configurations employing wide bandgap devices.
Metallized substrate
Base plate/heat exchanger
Device
Bonded interfaces/ materials
Wire/ribbon bonds
Delamination Voiding Cracking Bonded Interface
2
2.5
3
3.5
4
4.5
100 110 120 130 140 150 160Dist
ance
acro
ss th
e Pa
ckag
e (m
m)
Temperature (°C)
New Degraded
Degraded Substrate Attach
132.5°C 156.7°C
5
Thermal resistance of various non-bonded TIMs
0
50
100
150
200
250
300
350
400
0 0.05 0.1 0.15 0.2Thickness (mm)
Ther
mal
resi
stan
ce (m
m2 K
/W)
Arctic Silver 5
Thermalcote-251G
Wacker Silicone P12
Keratherm KP77
Dow Corning TC5022
Shinetsu-X23-7783D-S
Chomerics XTS8010
Chomerics XTS8030
3M AHS1055M
Honeywell PCM45
172 kPa, ~ 75 C sample temperature TIM thickness in all cases is 100 μm
0102030405060708090
100
100 200 300 400 500 600Pressure (kPa)
Resi
stan
ce (m
m2 K/
W)
Graftech graphite - 125 microns (initial)Indium -150 microns (initial)Shinetsu X23-7783D-S (no thickness control)Honeywell PCM45G (no thickness control)Single-sided CNTSingle-sided CNT+Paraffin
• Red dashed line in the two figures above is the target thermal resistance (3 to 5 mm2K/W).
• Most non-bonded TIMs do not come close to meeting thermal specification of 3 to 5 mm2K/W at approximately 100-μm bond line thickness.
0 25 50 75
100 125 150 175 200 225 250 275
Ther
mal
cote
25
1G
Wac
ker
Sili
cone
P12
Arc
tic S
ilver
5
Ker
athe
rm
KP
77
Cho
mer
ics
XTS
8010
Cho
mer
ics
XTS
8030
Hon
eyw
ell
PC
M45
Dow
Cor
ning
TC
-502
2
Shi
nets
u X-
23-
7783
-D
3M A
HS
105
5M
Res
ista
nce
(mm
2 K/W
)
Bulk resistance Contact resistance
6
Thermal Resistance of Sintered Silver and Solder
Samples Thickness (µm)
Resistance (mm2K/W)
Silvered Cu-Cu sintered interface
20 5.8
27 8.0
64 5.4
Cu-Cu soldered interface (SN100C)
80 1.0
150 4.8
200 3.7
• The thermal resistance tests were performed using the NREL ASTM TIM apparatus – Average sample temperature ~ 65°C,
pressure is 276 kPa (40 psi). • The silvered Cu-Cu sintered interface
showed promising thermal performance.
• Results hinted at some problems with the bonding of the silvered Al-Al interface.
• The initial thermal results for a lead-free solder (SN100C) interface were promising.
• Bonded interface resistance in the range of 1 to 5 mm2K/W is possible.
ASTM test fixture
7
Thermal Resistance of Thermoplastics with Embedded Fibers
Sample
Thermoplastics with embedded carbon fibers
• Thermoplastic films (provided by Btech) bonded between 31.8-mm-diameter copper disks.
• Promising thermal results (8 mm2K/W for 100-µm bondline thickness).
• Continuing work at NREL to further decrease contact resistance to approach target thermal performance, as well as characterize reliability.
Sequence of bonding steps
8
Reliability of Bonded Interfaces
• Investigate the reliability of emerging BIMs to meet the thermal performance target of 3 to 5 mm2K/W.
• Identify failure modes in emerging BIMs. • Experimentally characterize their life under known conditions. • Develop lifetime estimation models.
Sample Assembly
Traditional Power Electronics Package
Silicon die
Metalized substrate
Base plate
BIM
Wire/ribbon bonds
9
Approach
BIM Mechanical Characterization
Sample Synthesis
Reliability Calculation
Thermal Testing/ Characterization
Synthesis of samples using vacuum solder reflow station
and hot press
Cycling of samples in a
thermal shock chamber
Characterization of samples via hipot tester, acoustic microscope,
and laser profilometer
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 viscoplastic parameters
Experimental Approach Modeling Approach – Finite Element Analysis/Calculations
10
Approach – Sample Assembly • Five samples of each BIM were synthesized for testing and
included: – Silver plating on the substrate and base plate. – Substrate based on a Si3N4 active metal bonding process;
base plate material is copper. – An interface between substrate and base plate with
50.8-mm x 50.8-mm footprint. • Samples followed manufacturer-specified reflow profiles, and
bonds were inspected for quality.
Sample Assembly
Bond Material Type Name Comments
Solder Kester Sn63Pb37 Baseline (lead-based solder)
Sintered Silver Semikron Based on Semikron synthesis process
Adhesive Btech HM-2 Thermoplastic (polyamide) film with embedded carbon fibers
11
Approach – Temperature Cycling • Cycle Profile
– -40°C to 150°C – 5°C/minute ramp rate – 10 minute dwell/soak time
• Failure Mechanisms – BIM: voids and cohesive or
adhesive/interfacial fractures – Substrate: Cu-to-Si3N4 delamination and
Si3N4 cracking
Cohesive Fracture Voids Substrate Delamination and Cracking Adhesive/Interfacial Fracture
Time
Failu
re P
aram
eter
Time
Failure Limit s1 s2
s3 s1<s2<s3
s3 s2
s1
t(s3) t(s2) t(s1)
12
Reliability of Thermoplastic-based BIM • Btech HM-2 (Carbon fibers within polyamide
matrix) – Bonding
o HM-2 was cut to the base plate dimensions o The sample assembly was placed in the hot press and raised to
195°C o 1 MPa (150 psi) of pressure was applied
– Reliability Results o After 2,000 cycles, the bonded interface remained defect-free
Cold Plate
Sample Hot Plates
Cold Plate
Screw Jack
Hot Press Sample Assembly
0 Cycles
1,000 Cycles
2,000 Cycles
13
Reliability of Sintered Silver-based BIM • Semikron Sintered Silver
– Bonding o Si3N4 edges were ground off to match the metallization layer. o The sample assembly was placed in a hot press and raised to its
processing temperature, then pressure was applied. o Compression testing of substrates at ORNL showed cracking of
substrates required between 30 MPa to 50 MPa of pressure. – Reliability Results
o Uniform bonds were obtained. o Adhesive fracture has initiated at the bonding perimeter.
Sample Assembly
0 Cycles
1,000 Cycles
2,000 Cycles
14
• After 2,000 cycles, perimeter fracturing has reached 11% to 21%.
2,000 Cycles
0%
5%
10%
15%
20%
25%
0 500 1,000 1,500 2,000De
lam
inat
ion
Number of Cycles
Sample 1
Sample 2
Sample 3
Sample 4
Reliability of Sintered Silver-based BIM
Credit: Paul Paret
15
BIM Finite Element Modeling (FEM)
-80
-40
0
40
80
120
160
0 2000 4000 6000 8000 10000 12000 14000
Tem
pera
ture
(°C)
Time (s)
Temperature Cycling Profile • Temperature cycling parameters: – -40°C to 150°C – 5°C/minute ramp rate – 10 minute dwell/soak time
• Viscoplastic material model applied to solder layer
• Temperature-dependent elastic material properties incorporated for base plate and substrate
Base plate
Solder layer
Substrate
Quarter Symmetry Model
16
BIM FEM
• Strain energy density in the bonded joint region shown to decrease as fillet radius increases (11.3 MPa and 11.2 MPa for 1.5 mm and 2 mm, respectively).
• Strain energy density versus cycles-to-failure correlation to be obtained for lead-based and lead-free solders.
Solder Layer Top Surface (1.5-mm radius) Solder Layer Top Surface (2-mm radius)
17
Summary • TIMs/BIMs are a key enabling technology for compact, light-weight, low-cost,
reliable packaging and for high-temperature coolant and air-cooling technical pathways.
• Characterization of thermal performance of TIMs/BIMs
– 3 to 5 mm2K/W resistance at 100 μm is a difficult target for non-bonded TIMs
– BIMs can meet this thermal target immediately after bonding – main question is reliability
• Characterization of reliability of BIMs
– Synthesis of various joints between substrates and base plate, thermal shock/temperature cycling, high-potential test and joint inspection (C-SAM), and strain energy density versus cycles-to-failure models
– Thermoplastic BIM is very reliable after 2,000 cycles, sintered silver BIM showing some significant edge delamination
• Initiated FEM for solder-bonded interface geometries – ultimate goal is to develop predictive lifetime model for BIM.
18
Summary
• Current/Future Work
– Complete 2,000 thermal cycles on all selected materials using Si3N4-based substrates
– Report on reliability of each BIM under specified accelerated test conditions
– Derive viscoplastic parameters for lead-based and lead-free solders from double-lap shear test experiments
– Develop strain energy density versus cycles-to-failure predictive lifetime model for lead-based solder
– Expand strain energy density versus cycles-to-failure predictive lifetime model to lead-free solders
– Improve process for large-area sintered silver-based interface, and eventually develop predictive lifetime model
For more information contact: Principal Investigator Douglas DeVoto [email protected] Phone: (303) 275-4256 APEEM Task Leader Sreekant Narumanchi [email protected] Phone: (303) 275-4062
Acknowledgments: Susan Rogers and Steven Boyd U.S. Department of Energy Team Members: Mark Mihalic Paul Paret
All photos in this presentation except for one on slide 14, credit: Douglas DeVoto, NREL
Collaborations: Semikron Btech Curamik Delphi GM Virginia Tech ORNL Heraeus Kyocera Shin-etsu Dow Corning 3M Parker Chomerics