Underpinning Research
Design, Implementation and Testing
of SiC Press-Pack Power Modules
Dr Layi Alatise
Associate Professor of Power Electronics,
University of Warwick
Jose Ortiz-Gonzalez, Li Ran, Phil Mawby, Attahir Aliyu, Alberto
Castellazzi, Pushpa Rajaguru, Chris Bailey, A Junyent Ferré, S
Aldhaher and Paul Mitcheson
Contents
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Underpinning Research
Introduction to the Press-Pack Project
Reliability in Traditional Power Modules
SiC Power Device Technology
Pressure-Packaged Solutions
Warwick Press-Pack Prototype
Study of Thermal Contact Materials
Simulations on Press-Pack Devices (Greenwich)
Testing of Press-Pack Devices (Nottingham)
Gate Driving (Imperial College)
Conclusions
Cross-Cutting Research
Underpinning Research
• Cross-cutting research connects the individual themes funded by the centre
• The project is addresses a challenge that requires research cutting across
the different themes.
Advanced
Materials
& Devices
Components
& Packaging
Components and
Converters
Electric Drives and
Systems Integration
Cross-Cutting Research
Underpinning Research
Cross cutting Research Team
Underpinning Research
University Partner
Personnel
Greenwich
Prof. Chris Bailey
• Finite element modelling of
Dr Pushpa Rajaguru
electrical, mechanical and thermal
interactions in the press-pack
Imperial
Dr Paul Mitcheson
Mr. Sam Aldhaher
Dr A Junyent-Ferre
• Design of gate drive system
• Demonstration of high frequency
operation
Nottingham
Dr Alberto Castellazzi
• Power and temperature cycling
demonstration
• Test and characterisation of the
press-pack
Attahir Aliyu
Warwick
Dr O Alatise
Mr J Ortiz-Gonzalez
Prof. Li Ran
Prof. Phil Mawby
Responsibilities
• Design and assembly of the presspack
• Project lead and coordination
Project Aims and Objectives
Underpinning Research
● To develop and demonstrate SiC Power Devices in
Press-Pack
● Evaluate the Reliability Performance of SiC in Press-Pack
● Develop a design methodology using Finite Element
Analysis for SiC technology in Press-Pack
● Demonstrate Condition Monitoring techniques for
operational management of SiC Devices in Press-Pack
● Demonstrate High Frequency Switching in ultra-low
inductance press-pack technology
Contents
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Underpinning Research
Introduction to the Press-Pack Project
Reliability in Traditional Power Modules
SiC Power Device Technology
Pressure-Packaged Solutions
Warwick Press-Pack Prototype
Study of Thermal Contact Materials
Simulations on Press-Pack Devices (Greenwich)
Testing of Press-Pack Devices (Nottingham)
Gate Driving (Imperial College)
Conclusions
Electromagnetic Instability in SiC
•
SiC Schottky diodes are prone to ringing
during turn off
•
This is due to RLC resonance formed
between diode depletion capacitance
and parasitic inductance
•
Higher dI/dt in SiC aggravates the
problem
•
Lower series resistance means less
damping in SiC
SiC Schottky diode
Underpinning Research
Equivalent circuit showing diode parasitics
Si PiN diode
Power Cycling SiC Devices
Underpinning Research
Power cycling test rig and circuit schematic.
Cooling curves for SiC and Silicon Power Diodes
•
Initial power cycling tests have been done.
•
Early indications show that SiC power devices
are presently less reliable on power cycling
•
Why?
Normalised thermal resistance vs cycles
Power Cycling SiC Devices
Underpinning Research
Power cycling results reported from other researchers show SiC is less reliable
using traditional packaging techniques [1]
•
•
•
[1]
[2]
This has been reported in [1] and [2]
SiC has a Young’s Modulus 3 times higher than silicon
Also, the SiC die is thicker although the electrical drift region is thinner. These
two features cause more stresses on the SiC die attach
Ch. Herold , M. Schäfer , F. Sauerland , T. Poller , J. Lutz , O. Schilling “Power cycling capability of
Modules with SiC-Diodes” CIPS 2014
Luis A. Navarro, Xavier Perpin ˜a, ` Philippe Godignon, Josep Montserrat, Viorel Banu, Miquel
Vellvehi, and Xavier Jorda “Thermomechanical Assessment of Die-Attach Materials for Wide
Bandgap Semiconductor Devices and Harsh Environment Applications”, IEEE Transactions on
Power Electronics, vol. 29, NO. 5, May 2014.
Power Cycling of SiC Devices
SiC die
Solder
SiC thickness, t
0.1 mm
Copper
0.2 mm
Al2O3 ceramic
0.25 mm
Copper
0.2 mm
5 mm
Underpinning Research
•
SiC dies are usually
smaller and thicker
compared to Si dies.
•
The higher Youngs
modulus means that
the creep strain is
higher in SiC
compared to silicon
•
COMSOL Multiphysics
FEA package used to
study the thermomechanical stress
variation; ∆T = 50 ºC.
Traditional Packaging Systems
Underpinning Research
● Traditional power module (most common):




Semiconductor devices (die)
DCB
Wirebonds
Base Plate
Si IGBT half bridge
SKM400GB17E4
Semikron - 2015
First isolated power module
Semipack – Semikron, 1975
SiC MOSFET half bridge
CAS120M12BM2
Cree - 2014
Thermo-mechanical stress
Underpinning Research
● CTE mismatching of the different elements of the module causes
additional thermo-mechanical stress under thermal cycling conditions
● Identification of common the failure areas:
Wire bonds
Chip solder
Substrate solder
Source Metal Reconstruction
Before cycling
After cycling
Underpinning Research
•
The repetitive temperature surges that result
from power cycling causes the source metal
of chip to deform
•
This deformation is due to mechanical stress
of repetitive expansion
•
Images show source metal of low voltage
trench MOSFET before avalanche power
cycling and after 60 Million cycles of
avalanche power cycling
•
Source metal reconstruction contributes to
wirebond lift-off and device failure
Photo courtesy of Aalborg University
O. Alatise et al IEEE Transactions on Device and Materials Reliability, 2011
Contents
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Underpinning Research
Introduction to the Press-Pack Project
Reliability in Traditional Power Modules
SiC Power Device Technology
Pressure-Packaged Solutions
Warwick Press-Pack Prototype
Study of Thermal Contact Materials
Simulations on Press-Pack Devices (Greenwich)
Testing of Press-Pack Devices (Nottingham)
Gate Driving (Imperial College)
Conclusions
Material Properties of SiC
Underpinning Research
•SiC technology has a wider bandgap, higher critical field and higher thermal
conductivity compared with silicon
•These make for more efficient power devices with lower losses capable of high
temperature operation
Improved Switching Performance
Underpinning Research
Si IGBTs
SiC MOSFETs
•
Improved switching energies
measured in 1.2 kV SiC
MOSFETs and Schottky diodes
•
Significantly reduced
switching losses compared
with state of the art silicon
IGBTs and PiN diodes
Si PiN diodes
SiC Schottky
Contents
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Underpinning Research
Introduction to the Press-Pack Project
Reliability in Traditional Power Modules
SiC Power Device Technology
Pressure-Packaged Solutions
Warwick Press-Pack Prototype
Study of Thermal Contact Materials
Simulations on Press-Pack Devices (Greenwich)
Testing of Press-Pack Devices (Nottingham)
Gate Driving (Imperial College)
Conclusions
Pressure Packaging
•
One way of obviating the
problems of traditional
packaging reliability issues
is to use a pressure package
•
No wirebond or solder is
required, so solder fatigue
and wirebond lift-off from
CTE mismatch is not a
reliability concern
•
Mechanical pressure is used
to ensure that the device is
firmly
•
Press-packs have been in
use for several decades and
was traditionally designed
for wafer based thyristors in
high power applications like
grid connected converters
for HVDC, FACTS etc
ABB Mechanical clamps for press-pack
Underpinning Research
Press-Pack IGBTs: ABB vs IXYS
ABB StakPak (2002)
•
•
Individual Belleville spring loaded contact
The contact force depends on the distance to
contact according to Hooke’s law
Underpinning Research
IXYS Press-Pack (1998)
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•
•
Pressure is applied externally by clamps
Rigid collector and emitter plates are
used
Hermetic
Contents
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Underpinning Research
Introduction to the Press-Pack Project
Reliability in Traditional Power Modules
SiC Power Device Technology
Pressure-Packaged Solutions
Warwick Press-Pack Prototype
Study of Thermal Contact Materials
Simulations on Press-Pack Devices (Greenwich)
Testing of Press-Pack Devices (Nottingham)
Gate Driving (Imperial College)
Conclusions
SiC dies in Press-Pack
Underpinning Research
● Si IGBT dies (ABB) vs SiC MOSFET dies (Cree)
Cree
CPM2-1200-0025B
CPM2-1200-0040B
CPM2-1200-0080B
CPM2-1200-0160B
ABB
5SMX 76E1280
5SMX 76H1280
5SMX 76K1280
Blocking
Voltage (V)
Current
(A)
Size
(mm x mm)
1200
1200
1200
1200
60
40
20
11
4.04 x 6.44
3.10 x 5.90
3.10 x 3.36
2.39 x 2.63
1200
1200
1200
25
50
75
6.5 x 6.6
9.1 x 9.0
11.0 x 11.0
SiC MOSFET dies and Si IGBT dies properties
● Gate pad
 1.19 mm x 1.20 mm vs 0.5 mm x 0.8 mm
SiC Diodes in Press-Pack
● Diode – Single die
 Starting point
Φ 19 mm x 19.38mm
Underpinning Research
Press-Pack Diode Prototype
Heatsink
PS185
Heatsink
PS136/150
Box clamp BX42
Heatsink
PS260/150B
Diode prototype
Underpinning Research
Module assembly
200 A SiC Press-Pack Diode
Underpinning Research
•
A 200 A SiC Schottky diode
in press-pack was designed
and assembled
•
Investigations into the
impact of pressure
variation on the chips is
ongoing.
MOSFET Module Design
● Press-Pack MOSFET design
Design of the Press-Pack MOSFET module
Underpinning Research
MOSFET Module Design
Press-Pack MOSFET module
Cross sections
Underpinning Research
SiC MOSFET in Press-Pack
Underpinning Research
● Prototype is ready
● Evaluation of embedding the gate driver in the module
200 A SiC MOSFET Module Design
Underpinning Research
● Die carrier has been designed to be used with the multiple
die module
● Evaluation of the gate driving board position
Initial Design of the Press-Pack MOSFET
module – Multiple die version
Thermo-Electrical Properties
Underpinning Research
● Simplified analysis, removing the heatsink and clamp
Thermal analysis is
similar
Simplified Electrical Model of the Press-Pack diode
SiC Diode Measurements
● Using the forward voltage as a
TSEP, the thermal transient was
extracted at different forward
currents for the SiC Schottky diodes
● The junction temperature has been
measured for 10 A, 20 A and 30 A
currents at 300 N and 500 N.
Underpinning Research
Contents
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Underpinning Research
Introduction to the Press-Pack Project
Reliability in Traditional Power Modules
SiC Power Device Technology
Pressure-Packaged Solutions
Warwick Press-Pack Prototype
Study of Intermediate Contact Material
Simulations on Press-Pack Devices (Greenwich)
Testing of Press-Pack Devices (Nottingham)
Gate Driving (Imperial College)
Conclusions
ALG and Molybdenum
Underpinning Research
● Molybdenum is the traditional intermediate contact for press-pack modules.
● Aluminium Graphite (ALG2208) is an alternative contact material to
molybdenum. This is a Metal Matrix Composite comprised of Al and Graphite
ALG and Molybdenum
● Transient thermal measurements
have been performed on SiC
press-pack diodes with 2 different
intermediate thermal contacts
● The contacts are Molybdenum
and Aluminium Graphite contacts
● Measurements were done at
different currents and the junction
temperature was measured using
the forward voltage as a TSEP
Underpinning Research
ALG and Molybdenum
Underpinning Research
Aluminium Graphite
Molybdenum
Density (g/cm3)
2.3
10.22
Specific Electrical Resistance
(µΩm)
x/y=0.4 and z=0.6
0.053
Coefficient of thermal expansion
(μm/m°C)
x/y=8 and z=12
5.35
Thermal Conductivity (W/mK)
x/y=220 and z=140
138
Specific Heat Capacity (J/kgK )
850
217
Contents
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Underpinning Research
Introduction to the Press-Pack Project
Reliability in Traditional Power Modules
SiC Power Device Technology
Pressure-Packaged Solutions
Warwick Press-Pack Prototype
Study of Thermal Contact Materials
Simulations on Press-Pack Devices (Greenwich)
Testing of Press-Pack Devices (Nottingham)
Gate Driving (Imperial College)
Conclusions
Numerical Simulations
Underpinning Research
● Objective
 Develop & validate finite element models of press-pack
diode for two types of contact pad materials (Mo and AlG),
for various clamping pressure
» Junction temperature
» Current (and on resistance)
» Mechanical stress on the diode
● Modelling Methodology
 Electro-thermo-mechanical finite element modelling and
analysis
 One quarter model of the press pack single diode chip
structure by exploiting the model symmetry.
● Contact Analysis
 Surface nonlinearities exist at the interface.
 Contact analysis of the finite element code was utilised
Contact Analysis
Underpinning Research
● Electrical Contact Resistance [1]
 Analytical electrical contact resistance (REc) depends
on the contact force and the hardness of the contacting
materials
 H
REC 
2
F
  2
 1
2
Material 2
Material 1
Material 2
● Thermal Contact conductance [2]
 The thermal contact resistance (RTc) model depends on
the surface, material hardness and contact pressure at
the interface
1
m P 
 0.125k s  
RTC
 H
Material 1
0.95
[1] Paul G Slade, Electrical contacts, principles and applications, Second edition, CRC press, 2014
[2] M.M. Yovanovich, Four decades of research on thermal contact, gap and joint resistance in
microelectronics, IEEE Transactions on components and packaging technologies, Vol 28, No 2, 2005
Material Property & Boundary Condition
Underpinning Research
● Material Property
 For ‘Mo’, ‘AlG’ material properties were
extracted from manufacturer specification.
 Other material properties are extracted from
public domain[1]
 Schottky diode temperature and current
dependent resistivity is extracted from the
current/forward voltage.
● Thermal Boundary condition
 Heatsinks are removed in the FEA model to
reduce the complexity
 Appropriate heat transfer coefficient imposed
on the model
hc 
Heatsink surface area
* Natural convection
Anode / heatsink interface area
[1] J. F. Shackelford, and W. Alexander, Materials Science and Engineering Handbook, 3rd
Edition, CRC Handbook, 2001
Finite Element Modelling Results
Underpinning Research
Temperature (K) distribution on the Assembly
Electric potential (V) distribution on the Assembly
AlG pad, 400 N pressure, 20 A loading
AlG pad, 400 N pressure, 20 A loading
Temperature (K) distributions on
diode, 20 A loading, 400 N pressure
(a) Model with ‘Mo’ pad
(b) Model with ‘AlG’ pad
Diode with Mo pad model
has higher temperature
distribution
Finite Element Modelling Results
Average Temperature on Diode
Underpinning Research
The temperature reduction when using a small heatsink compared with the large heatsink for
load currents ranging from 20 A and 25A are respectively ~13°C and ~ 24°C
Average Temperature (C)
300 N Clamping Pressure
170
Mo Pad (Small Heatsink)
150
AlG Pad (Small Heatsink)
130
Mo Pad (Large Heatsink)
AlG Pad (Large Heatsink)
110
90
70
50
8
11
14
17
20
23
26
Load Current (A)
As expected, increasing the load current value increases the diode temperature distribution
for both models (ALG and Mo contact pad)
Finite Element Modelling Results
Underpinning Research
Average von Mises Stress on Diode
von Mises stress distributions of
diode
20 A loading, 400 N pressure
(a) on diode with Mo pad
(b) on diode with AlG pad
Diode with AlG pad model
has higher stress
distribution
average von Mises stress on the diode increases from large heatsink model to
small heatsink model by ≤ 3MPa due to additional thermally induced stress
Contents
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Underpinning Research
Introduction to the Press-Pack Project
Reliability in Traditional Power Modules
SiC Power Device Technology
Pressure-Packaged Solutions
Warwick Press-Pack Prototype
Study of Thermal Contact Materials
Simulations on Press-Pack Devices (Greenwich)
Testing of Press-Pack Devices (Nottingham)
Gate Driving (Imperial College)
Conclusions
Thermal Impedance Characterisation
Underpinning Research
Thermal Impedance Characterisation
Underpinning Research
ESREF 2016
Underpinning Research
Increasing clamping force
Thermal Resistance
ESREF 2016
ALG
500N Clamping Force
Underpinning Research
Mo
Power Cycling Results
Power cycling range between 20
and 90 °C
Structure function at different
number of cycles
Underpinning Research
Thermal resistance as a function of
the number of cycles
Contents
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Underpinning Research
Introduction to the Press-Pack Project
Reliability in Traditional Power Modules
SiC Power Device Technology
Pressure-Packaged Solutions
Warwick Press-Pack Prototype
Study of Thermal Contact Materials
Simulations on Press-Pack Devices (Greenwich)
Testing of Press-Pack Devices (Nottingham)
Gate Driving (Imperial College)
Conclusions
Gate Driver
● Circuit Configuration
Four individual gate drivers for each die
•
•
•
Easier to implement, less layout issues
Better control of the gate currents and the drain currents of each
die
Better synchronisation
Underpinning Research
Gate Driver
Underpinning Research
● Main Features:
- Gate drive voltage +20V to -5V
- Controlled rise and fall times
- Synchronisation to ensure all dies share the same
current
- Fully isolated interface
● Diagnostics, may include:
- measuring gate currents and waveforms of each
- measuring drain current of each die
- measuring drain voltage
- measuring temperature
Gate Driver
Underpinning Research
● Simplified gate circuit for a single die
isolation barrier
Isolated opamp
20V
Mosfet die
Gate current waveform
Isolated power supply
Rise & fall times control
-5V
Over-voltage protection
Isolated gate driver
Miller current shunt
Gate Driver
Underpinning Research
● Simplified gate circuit for a single die
Isolated opamp
Isolated power supply
Isolated gate driver
External components
Internal components
More Information
Underpinning Research
● ECCE 2016
 “An Initial Consideration of Silicon Carbide Devices in PressurePackages”
● ESREF 2016
 “Development and characterization of pressed packaging solutions
for high-temperature high-reliability SiC power modules”
Acknowledgements
Underpinning Research
● Schunk Hoffmann Carbon Technology
 ALG
● Roechling Fibracon
 PPS and PEEK machined parts
● GD Rectifiers
 Mechanical parts for the assembly (heatsinks, clamps…)
● ABTech
 Copper parts
Underpinning Research
Thank you for
your attention.
Any Questions