ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, 60 (2012), nr. 2
29
The Insulated-Gate Bipolar Transistors (IGBT)
and their Reliability
Titu-Marius I. BĂJENESCU1
Abstract
Driven by energy-efficient industrial and renewable energy applications, the demand for power semiconductors
has been increasing rapidly. The time for turn-on and turn-off of standard power modules influences the lifetime
as well. The short load cycle influences the life time of the bond wire. The junction temperature is increased to
175 °C. New IGBT modules should not use any solder layer and provide a very high reliability. Type selection of
IGBT modules to match the lifetime design taking into account the wear-out duration is very important for the
reliability of product, while the product lifetime depends on how much design margin remains in practical
system.
Keywords: Power semiconductors, power modules, IGBT, Trench-IGBT, failure rate, failure precursors, failure
mechanisms, SiC, AlSiC, AlN, sinter technology, reliability.
Introduction
The power semiconductor devices [power
diode, thyristor, power MOSFET and
insulated-gate bipolar transistor (IGBT)] are
used as switches or rectifiers in power
electronic circuits. Their popularity is the
result of the last technology advances. Only
the recently wide band gap power devices
have started to achieve an acceptable
market entry level in terms of overall
performance competitiveness in applications
with relatively lower power ratings. Silicon
Carbide (SiC) is the future material for power
semiconductors (see Figure 1).
Figure 1. Power semiconductors and their power and
frequency range in MW power electronic
applications [1]: PCT = phase controlled
thyristors; IGCT = integrated gatecommutated transistors; IGBT = insulated
gate bipolar transistor; MOSFET = metal
oxide semiconductor field effect transistor;
SiC = silicon carbide
1
Titu-Marius I. BĂJENESCU, Prof., Doctor Honoris Causa of
Military Technical Academy of Romania, and of Technical
University of Republic of Moldova; CFC, La Conversion,
Switzerland; e-mail: tmbajenesco@bluewin.ch
The problem of the failure rate during the
operational life of such devices is more and
more important and the failure analysis helps
to improve the products quality and
reliability. The power transistor is an
important element of the interface between
the command electronics and the elements
of the power electronics. The greater
currents and voltages led to new absolute
limit values for the dissipated power of these
components. Outside of the well-known
thermal power dissipation corresponding to
the operating state, there are some specific
limits for bipolar transistors: a limit for pulse
operating
and
another
for
second
breakdown. Second breakdown is still a
reliability problem in power transistors and
MOSFETs; it is controlled through device
design and the specification of safe
operating areas (in terms of safe areas of
operation for collector current and voltage
that are dependent on the biasing base
current and the switching dynamics). It was
shown that the onset of second breakdown
cannot be predicted simply in terms of
voltage and current, as had been the
practice, but that it is important to
characterize second breakdown in terms of
the energy dissipated in the transistor, and
further, that the energy threshold (or delay
time) is dependent on other factors such as
ambient temperature and the biasing base
current of the transistor [2].
Usually, it is not possible to use the full
IGBT performances of the new chip
30
ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, 60 (2012), nr. 2
technology
because
the
housing
constructions don’t permit an optimized
application. Normally, IGBT modules contain
many different materials and on top of all the
materials are IGBT-chips connected by bond
wires. All the layers inside IGBT module
(silicon, copper, ceramic) and their different
coefficients of thermal expansion (CTE) must
be considered, while – during the load and
temperature cycles – the CTE-difference will
generate deformations and eventually device
failure such a cracking of a solder layer [3]
(Figure 2). That is why the number of cycles
to failure is used in models for lifetime
estimation.
Figure 2. Solder joint cracking [4]
The new Sinter technology [5] prevent or
reduce mechanical stress between layers,
don’t use any solder layer, reduces the
thermal resistance of the IGBT module,
assures a lower junction temperature of the
IGBT chips and – compared with solder
technology – provides a higher reliability
(Figure 3).
Figure 3. The Sinter technology [5]
The Sinter technology provides 5 x higher
cycles to failure, has a better temperature
cycle capability, and reliability, thanks to the
silver layer, unbreakable joint between the
die and direct bond-copper (DBC), twice the
power cycling capability [5].
The most important requirements for
power management (PM) are: (i) optimized
IGBT-chip technology2; (ii) High reliability,
long life time, and temperature cycle
capability; (iii) low thermal impedance under
2
The IGBT chip is sintered from both side and no bond wire
is necessary.
high current density (12 A/cm2); (iv)
optimized packaging for low internal and
external stray inductance.
Failure precursors
Failure precursors indicate changes in a
measured variable that can be associated
with impending failure. By identifying
precursors to failure and by monitoring them,
system failures can be predicted and actions
can be taken to mitigate their effects. In [6]
three potential failure precursor candidates
(threshold voltage, transconductance and
collector-emitter ON voltage) are evaluated
for IGBTs. Based on the failure causes
determinated by FMMEA (failure modes,
mechanisms, and effects analysis), IGBTs
are aged using electrical-thermal stresses.
The three failure precursor candidates of
aged IGBTs are compared with new IGBTs
under a temperature range of 25-200°C. The
trends in the three electrical parameters with
changes in temperature are correlated to
device degradation, and a methodology is
presented for validating these precursors for
IGBTs prognostics using a hybrid approach.
The electrical-thermal stresses used to age
the IGBTs lead to trapped electrons in the
gate oxide resulting in an increase in the
threshold voltage of the aged parts. The
observed increase in the transconductance
(gain) and reduction in collector-emitter ON
voltage of the electrically-thermally aged
parts is possibly a result of the observed
degradation in the die-attach [6]. The three
parameters are precursors to IGBT failure as
all three parameters show changes with
increased degradation; the trend in the
evolution of parameters helps identify the
failure mechanism in operation. The
switching parameters (turn-on delay time,
turn-off fall time) are directly affected by gate
oxide damage and are also potential
precursors to IGBT failure.
Power modules failure mechanisms
Some of the identified lifetime limiting
failure mechanisms are bond wire fatigue,
terminal solder joints and large area solder
joints. The high operation temperature
influences the package lifetime. The
packaging has been improved too and offers
increased reliability and improved electrical
characteristics. Optimized module packages
have been developed to obtain maximum
performance from new generation chips [7].
ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, 60 (2012), nr. 2
The goal of module technology was
always to integrate more and more power
semiconductors. Multichip IGBT modules for
high-power applications typically include up
to 800 wedge bonds; about half of them are
bonded
onto
the
active
area
of
semiconductor devices. They are exposed to
almost the full temperature swing imposed
both by the power dissipation in the silicon
and by the ohmic self-heating of the wire
itself. The maximum DC current capability of
a bond wire is limited by melting due to
ohmic
self-heating.
During
switching
operation the current density distribution
across the section of a bond wire is strongly
inhomogeneous due to the skin effect [3].
Failure of a wire bond occurs predominantly
as a result of fatigue caused either by shear
stresses generated between the bond pad
and the wire, or by repeated flexure of the
wire.
Bond wire lift off has been observed to
affect both IGBT and freewheeling diodes.
No bond wire lift off occurs at the wire
terminations bonded onto copper lines. This
is mainly due to the fact that copper lines do
not experience large temperature swings.
The fracture mechanics at bonded interfaces
and the modelling of the crack propagation
within the welded joint with time is a quite
complex issue. There is experimental
evidence that the crack leading to the failure
is initiated at the tail of the bond wire, and
propagates within the wire material until the
bond wire completely lifts o . Bond wire
heel cracking rarely occurs in advanced
IGBT multichip modules. There are different
sources of stress, which can lead to brittle
failures. One among these is the bending
stress, which arises while mounting modules
with a bowed base plate onto a flat heat sink.
Both gross voids and extended fatigueinduced cracks can have detrimental effects
on dissipating devices. In fact, they can
significantly increase the peak junction
temperature of an IGBT or of a diode and
therefore accelerate the evolution of several
failure mechanisms including bond wire lift
off and solder fatigue. Burnout is often
associated with a short circuit condition,
where a large current flows through the
device (or through a portion of it), while it is
supporting the full line voltage [3].
The latch up is a failure mechanism
inherent to IGBT devices. This phenomenon
31
is of special relevance, because most of the
root causes mentioned above activates this
mechanism, such that it plays an important
role in determining the availability of a power
system.
By submitting a device to repeated
current load cycles, a junction temperature
swing ∆Tj is induced, creating a
corresponding temperature cycle. The
device is composed of several layers of
different materials with differing CTEs. This
causes power modules to experience shear
stress within their composition, causing
deformations and eventually leading to
module failure, such as cracking of a solder
layer [8].
Due to a closer match of CTEs between
AlSiC (CTE = 7 ppm/K) and AlN
(CTE = 4 ppm/K) no delamination is
observed in the solder layer between
ceramic substrate and AlSiC base plate even
after 20,000 thermal cycles (T=80 °C), while
clear solder failure is observed when the
base plate is copper (CTE = 17 ppm/K).
Concerning
the
performance
improvement, the IGBT semiconductor
performance at junction temperature is
above 200 °C, the increasing Tj,op, from
125 °C to 175 °C, allows the combustion
engine coolant to cool the power electronics
w/o need of a separate cooling circuit and
the
improved
reliability
in
joining
technologies. At Tj,op = 175 °C, the solder
fatigue and wire bond lift-off failures limit the
power cycling/thermal shock lifetime. As
showen in Figure 4, the material challenges
are: a lower cost water cooled integrated
heat sink and a high reliability MMC/ceramic
combination with good thermal performance
[9].
Figure 4. Improvement in power module design
32
ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, 60 (2012), nr. 2
The aluminium alloys reinforced by high
volume fractions of SiC particulates are
candidate materials for base plates of high
power electronic modules. Such AlSiC metal
matrix composites exhibit low thermal
expansion and high thermal conductivity.
The
Young
modulus,
the
thermal
conductivity and the elastic limit increase
with increasing SiC volume fraction [10].
One question can be raised in case of the
use of HEV (hybrid electric vehicles) in
countries
where
during
winter
the
temperature drops down –50 °C or less.
In [11], it has been shown, that the low
temperature improves the Trench-IGBT
performances by reducing the power losses
and the current tail influence – which have
certainly a strong influence on the device
reliability for hard-switching operations.
Automotive-use IGBT modules
If the environment and working conditions
are much more severe, the required longterm reliability is significantly different, and
the used materials will also differ (Figure 5).
Figure 5. The IGBT module layers of materials [4]
These modules are water-cooled and
their required temperature cycle tolerance is
an order of magnitude greater than that of
industrial modules [12].
If the number of temperature cycles
exceeds the industrial device capability,
cracks will appear in the solder layer bonding
together the power chips and the insulating
substrate, increasing the thermal resistance.
In a high temperature and high humidity
environment, the migration tolerance of PCB
is also an important issue.
In the power module with the latest
technologies inside, the elimination of the
base plate, the pressure technology for the
DCB to the heat sink and the sinter
technology to connect the IGBT chip to the
DCB are the measures which improve
lifetime (Figure 6) [5].
Figure 6. The power module
The sintered layer is 4.5 times thinner
than a standard soldered layer and has
4 times the thermal conductivity, resulting in
excellent thermal properties in the sintered
connection.
A homogeneous current distribution inside
the IGBT module can be influenced by
different length of bond wire and power
terminal construction. Ripple current of the
capacitor and current capability of the IGBT
module must be matching with each other
[5].
Depending on vehicle design, the inverter
may be placed in the vehicle rear,
transmission in (or near) the internal
combustion engine under the hood position,
so IGBT module must withstand severe
temperature (–40°C to +150°C) and
mechanical conditions (vibration, shock).
The hybrid vehicles driven more complex
conditions, such as the corresponding cities,
the need to frequently switch on
acceleration, deceleration, cruising the
states, so the current through the IGBT, the
voltage, with the repeated cycle of
fluctuations in traffic conditions. IGBT
modules required current, voltage, and
reliable operation during an automobile
design life of 15 years.
The nominal DC supply voltage for
electric vehicles (EVs) and hybrid electric
vehicles (HEVs) traction drives is not yet
standardised and can range from 75 V (for
micro EVs) to 400 V (for class B family
vehicles), depending on the battery type and
configuration [13]. New challenges have to
be addressed in the areas of miniaturization,
integration in electric motors, low cost, high
temperature, and standards for efficient
power electronic components. Future targets
should be advancements in the conversion
efficiency, reliability improvements for high
temperature cycling, compact and simple
cooling solutions, and cost reductions.
The future development of true ambient
intelligence systems everywhere, with
nanoelectronics penetrating all aspects of
ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, 60 (2012), nr. 2
everyday life, will create novel challenges for
the system architectures and supporting
technologies.
From Figure 7, it is clear that the
integration of Beyond CMOS technologies
into future systems as SoC/SiP becomes
urgent in order to take advantage of its high
potential in both hybridized-with-CMOS and
post-CMOS perspectives.
Reliability aspects
IGBT early failures are caused by microdefects or human errors. These defects are
originated in IGBTs and free-wheeling
diodes (FWDs), cracking in DCBs, touch of
gate and emitter wiring and so on.
Since complete removal of these
inconveniences are very difficult, screening
tests in out-going procedure are necessary
to reject such early failures.
Failure rate of random failures is relatively
stable; their duration depends on operating
conditions including environments of whole
systems where IGBT modules are installed.
a)
Figure 7. The integration of Beyond CMOS
technologies
The More Moore (MM), More than Moore
(MtM) and Beyond CMOS domains of
research are used with miniaturization and
diversification drivers for MM and MtM,
respectively.
The combination of SoC (system on chip)
and SiP (system in package) is expected to
result in highly valued systems. To reach
and support the SoC/SiP trend, the Beyond
CMOS technologies need to meet criteria of
integrate-ability and system-ability.
This can be achieved by paying more
attention to integrate-ability and systemability of Beyond CMOS and by a more
holistic and multi-disciplinary view linking
academics and industry for achieving a
critical mass and knowledge in the entire
value chain [14]. The result: Increase power
density without adversely effecting reliability.
Better utilization of IGBT module area is
an effective way of increasing power density.
Removing the barrier of maximum Tj at
150 °C allows a significant increase in power
density or simplified cooling. Increasing Tj
from 150 °C to 200 °C would increase
switching losses by approximately 10%.
Blocking
characteristics
improved
by
introducing trench and field-stop-IGBTs, will
result in lower leakage current increase,
even at Tj = 200 °C [13].
33
b)
Figure 5. a) Random failure; cosmic ray failure site.
b) Solder joint failure [16].
This means that failure rate of random
failures is equivalent to the system-specific
reliability [15].
In general, random failures are caused by
excessive stresses over maximum rating,
such as over-voltage, over-current, over-heat
and so on. The unlikely failures during wearout part are difficult to control because they
are caused by wear or fatigue of the
products. Type selection of IGBT modules to
match the lifetime design taking into account
the wear-out duration is very important for
the reliability of product, while the product
lifetime depends on how much design
margin remains in practical system.
The case temperature Tc is increased and
decreased in a relatively long-time cycle, so
that the difference between the junction
temperature Tj and Tc becomes small. When
such temperature change occurs, the
significant
stress
strain
becomes
predominant between the base and the
insulated substrate DCB.
Explanation of the failure mode of the ∆Tc
power cycle: When Tc is increased and
decreased, the largest stress strain is
caused in the soldered joint between the
insulated substrate DCB and the base due to
the difference in CTE between them.
Elimination of layers reduces the overall
thermal resistance, i.e. ceramic substrates
ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, 60 (2012), nr. 2
34
mounted directly onto heat-sink.
Summary for maintenance of reliability.
For random failures: maintain or reduce the
maximum junction temperature Tj. For wearout: maintain or reduce the ∆T.
Acknowledgment
The author wish to thank to all specialized
colleagues for the reach and repeated
informal discussions.
References
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[15]***,
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Biography
Titu I. BĂJENESCU was born in
Câmpina (Romania) on April 2,
1933; he received his engineering
training at the Polytechnic Institute
Bucharest. He served for the first 5
years in the Army Research
Institute, including tours on radio and
telecommunications maintenance,
and in the reliability, safety and maintainability office
of the Ministry of Defence (main base ground
facilities). Experience: R&D: design and manufacture
of experimental equipment for army research institute
and for air defence system. He joined Brown Boveri
(today: Asea Brown Boveri) Baden (Switzerland) in
1969, as research and development engineer.
Experience: R&D: design and manufacture of new
industrial equipment for telecommunications. In 1974
he joined Hasler Limited (today: Ascom) Berne as
Reliability Manager (recruitment by competitive
examination). Experience: Set up QRA and R&M
teams. Developed policies, procedures and training.
Managed QRA and R&M programmes. As QRA
Manager monitoring and reporting on production
quality and in-service reliability. As Switzerland
official, contributed to development of new ITU and
IEC standards. In 1981 he joined “Messtechnik und
Optoelektronik” (Neuchâtel, Switzerland, and Haar,
West Germany), a subsidiary of MesserschmittBölkow-Blohm (MBB) Munich, as Quality and
Reliability Manager (recruitment by competitive
examination). Experience: Product Assurance
Manager of “intelligent cables”. Managed applied
research on reliability (electronic components,
system analysis methods, test methods, etc.). Since
1985 he has worked as an independent consultant
and
international
expert
on
engineering
management, telecommunications, reliability, quality
and safety. Mr. Băjenescu is the author of many
technical books - published in English, French,
German and Romanian. He is university professor
and has written many papers and articles on modern
telecommunications, and on quality and reliability
engineering and management; he lectures as invited
professor, visiting lecturer or speaker at European
universities and other venues on these subjects.
Since 1991 he won many Awards and Distinctions,
presented by the Romanian Academy, Romanian
Society for Quality, Romanian Engineers Association
etc. for his contribution to reliability science and
technology.