Higher Junction Temperature in Power Modules – a demand from
hybrid cars, a potential for the next step increase in power density
for various Variable Speed Drives
Dr. Reinhold Bayerer, Infineon Technologies AG, Max-Planck-Str. 5, Warstein, Germany
Abstract
Removing the barrier of maximum junction temperature at 150°C will allow a significant increase in
power density or simplified cooling. The barrier is set by solder fatigue and wire bond lift off at intermittent operation. New Packaging technologies can eliminate the reliability issue. Future IGBT generations will take advantage of the progress in packaging and will show major improvements based on
Silicon. For the application of higher temperatures in power electronics, special concern is on PCB’s
and passives. When spatial temperature profiles of heat sinks are taken into account, those components can stay within their limits.
1
Introduction
Data sheets of power devices show maximum
ratings in junction temperature (TJ) of 150°C or
175°C (Fig. 1). The maximum TJ for switching
conditions, i.e. inverter operation is usually 25°C
less. This lower temperature TJ,op is also based
on power cycling and other reliability requirements. It is the intermittent operation of power
electronics, which links TJ,op to power cycling reliability.
FS50R12KT3 (example of old generation)
FS50R12KT4 (example of new generation)
Fig. 1 Extraction of IGBT modules’ data sheets.
Maximum junction temperature ratings are split
into DC and operational.
Modules with a resulting maximum TJ,op of 125°C
belong to the former module generations, i.e. up
to IGBT3/1200V, for example. More recently,
power modules, which allow an operation up to
150°C, became available. These modules with a
TJ,max of 175°C belong to the new module genera-
tion IGBT4/1200V, for example. A major reliability
improvement was required to ensure this increase in operating temperature [9], [6].
Driven by automotive requirements and hybrid
car application further increase of junction temperatures is on the roadmap. It would simplify
power electronics within a car, because the coolant of the combustion engine could cool the
power electronics, as well. Otherwise a separate
cooling circuit is required. The separate cooling
means more space weight and cost for power
electronics in a hybrid car. For other drives applications like industrial applications the question
arises, if they could also benefit from the related
increase of power density.
Industrial drives represent an example of power
electronics, which went through several steps in
power density. We may look back to mercury vapour rectifiers and the break through when power
semiconductors – thyristors and diodes – became available. In the 80’s the Darlington transistors had been introduced. Passive components
to control switching slopes of the thyristor became obsolete. But three stage Darlington transistors wasted approximately 30% of chip area
for the first two stages. Therefore the introduction
of IGBT meant another increase in power density.
Furthermore, it was easier to control and more
robust and has today significantly fewer losses.
In the mean time, several development steps followed increasing the power density in IGBT (Fig.
16). It led to more and more current and power
density in modules (Fig. 2). In turn, heat sinks
had to dissipate more and more power. In case of
air-cooled heat sinks, the thermal impedance of
the heat sink to ambient is dominating the whole
chain from chip to ambient. Therefore, the capability of power dissipation through air-cooled Al
heat sinks limits power density at today’s levels.
The limit could move by higher heat sink temperatures.
short circuit test results, for example, justify this
conclusion. The semiconductor does not set the
limit in junction temperature (up to 200°C), but
lifetime of solder joints and interconnects rather
sets the limit.
Eoff-FS100R12KE3
30
25
2 x Inom
20
Eoff [mJ]
1.5 x Inom
15
Inom
10
0.5 x Inom
2 x Inom
5
0
0
50
100
Tvj [°C]
150
200
Fig. 3 1200V Trench-Field-stop-IGBT, turn-off
losses up to 200°C
2
Temperature limits for
Power Semiconductors
High temperature characteristics of Silicon devices, i.e. emitter controlled free wheeling diodes
and IGBT with trench and field-stop, had been
studied [1], [2], [3], [4] (Fig. 3, Fig. 4). Going from
TJ=150°C to TJ=200°C switching losses would
increase by 10%, approximately. Blocking characteristics improved by introducing trench and
field-stop-IGBTs, which results in less growth of
leakage current with TJ. Therefore 600V- and
1200V-Chips losses caused by leakage current
stay below critical limits, even at TJ=200°C. The
short circuit condition represents another question mark at elevated junction temperature, because it deposits a large amount of energy within
the chip. A steep rise of TJ is the consequence.
As it adds to the starting temperature a higher
Tj,op is assumed to be critical. The work in [4] focuses on the feasibility of maintaining the short
circuit capability and dispels doubts.
The overall conclusion with respect to chip characteristics is that 600V and 1200V IGBT and diodes can operate at up to 200°C junction, in principle. Blocking characteristics, switching and
1.E-03
1.E-04
cut-off current (A)
Fig. 2 Evolution of IGBT module. Shrink of
150A-1200V Half-bridges. First generation IGBTmodules required 62mm-package (left). Today
(IGBT4) the function fits in EasyBRIDGETM2B.
The footprint shrunk by a factor of 2.5.
1.E-05
1.E-06
1.E-07
IGBT3
IGBT2
1.E-08
1.E-09
0
50
100
150
200
T (°C)
Fig. 4 Leakage (cut-off) current representing
blocking capability of two 600V-IGBT generations. IGBT2/IGBT3-characteristics are from devices with a current rating of 20A.
3
Reliability Limit for Junction Temperatures
One important characteristic with respect to reliability is the power-cycling lifetime. Former module generations rated at TJ,op =125°C had been
represented by the dashed power cycling curve
of Fig. 5. For the new generation, which can operate up to TJ,op =150°C the solid line applies. If
material optimization (Fig. 1). The root cause
here was solder fatigue and wires had not been
lifted the number of power cycles can increase
further as soon as solder is replaced or improved.
1.E+07
1.E+06
N f / Cycles
the former generation’s life is represented by a
special point at ∆TJ=80°C (cycling from TJ=45°C
to ∆TJ=125°C) the new generation withstands
similar number of cycles at 25°C higher ∆TJ and
TJ indicated by the arrow. Progress in wire bonding technology improved the reliability. As the
number of power-cycles to failure (Nf) increased
by a large factor, solder fatigue became a factor
competing with wire bond lift-off as failure
mechanism [1]. Wire bonds and solder joints
reached their limits. For further increase of power
cycling capability or further increase of TJ accompanied by higher ∆TJ, even lead rich solders
with higher melting point would not be an alternative in terms of reliability [5].
1.E+05
7
1 .10
1.E+04
Nf / cycles
6
1 .10
60
80
90 100 110 120 130 140 150 160
∆ TJ / K
Fig. 6 Improvement of cycles achieved in power
cycling by wire material optimization. Arrow indicates delta between standard wires (lower group
of data) and optimized wires (upper 2 data
points). Root cause of failure for upper points is
solder fatigue.
5
1 .10
4
1 .10
3
1 .10
40
60
80
100
120
∆TJ / K
140
160
Fig. 5 Power cycling lifetime, i.e. Nf-∆TJ relation
(min. values) [6]; modules for industrial application with max. TJ,op=125°C, lower, doted trace;
typical Nf for new module generation with
IGBT4/1200V at max. TJ,op of 150°C, solid line.
Arrow indicates increased ∆TJ of 25°C.
4
70
Required Changes of
Power Modules
Assembly and Interconnection technologies have
to change for enabling high temperature operation in the range of 175°C to 200°C, especially if
∆TJ follows in increase. One example is the predominately used Al heavy wire bonding technology, which has to change. The analysis of end of
life failures reveals cracks running through the
wire but not along the interface to the chips. Consequently, there is a potential to improve, e.g. by
modification of the wire material. Examples of
Power cycling tests indicate such a progress by
NTV - also known as low temperature joining
technique – can replace solder joints. It is known
since its invention in 1986 [7] and is in production
for high power thyristors and diodes at Infineon
Technologies since then. Power cycling results
prove the elimination of fatigue [8]. As the NTV
process is not that attractive for power modules,
from the process point of view, other assembly
technologies, which can be considered to meet
the target of eliminating the solder fatigue, are
investigated, too. Power cycling results of NTV
and such alternatives look also promising (Fig. 7).
In such tests, the chip to substrate interface was
stable. The devices failed by wire bond lift-off or
third failure reasons [8]. When the previously
mentioned data point at 80°C (refer to dashed
curve of Fig. 5) is transferred to a TJ of 175°C
and a related ∆TJ of 130°C (see arrow in Fig. 7)
results show significant potential. Integrating wire
bond improvements and new assembly technology makes TJ,op of 175°C feasible. Further optimization of assembly and interconnection technologies should even allow 200°C.
Large area joints of DCB (direct copper bonded
substrate) to base plate can benefit from the new
joining techniques, as well. Examples by NTV
and their passive thermal cycling tests prove the
required reliability. Fig. 8, shows DCB substrates
sintered to a 3 mm Cu base plate of the 62mmmodule footprint. During the thermal cycling test,
the NTV joint delaminates after 500 cycles over a
small distance at the corners. The crack propagates very slowly up to 2250 cycles; the original
thermal contact to the base plate is almost maintained. It represents a tremendous improvement
against soldered layers.
technology is also implemented in new modules
for automotive application (Fig. 10).
1.E+07
Nf / Cycles
1.E+06
TM
Fig. 9 PrimePACK , direct copper bonded terminals for high temperature application
1.E+05
1.E+04
1.E+03
40 50 60 70 80 90 100 110 120 130 140 150 160
∆TJ / K
Fig. 7 Power Cycling results from modules with
new die attach technologies (data above
∆TJ=120°C). Root cause of failure is bond wire
lift-off. Other data are copied from Fig. 5. Arrow
points to target for a TJ,op of 175°C.
Fig. 8 USM inspection of DCB-substrate
bonded to a Copper base plate; bonded area
54mm x 73mm; delaminating of NTV-bond during
thermal shock test (-40°C … 150°C, 1 h dwell
time at each temperature); from left to right USM picture before test, after 500 cycles, after
1000 cycles, after 2250 cycles.
5
Terminals
A further important area for improvement is solder joints of heavy power terminals. A direct copper bonding technology is introduced to replace
these solder joints. New modules like PrimeTM
PACK take already advantage of this solder
free interconnection (Fig. 9). The interconnection
TM
Fig. 10 New IGBT Module HYBRIDPACK 2 for
hybrid car application. Direct copper bonded terminals and integrated heat sink.
6
Impact of new die attach
technology on transient
thermal impedance
The new die attach methods not only improve the
reliability and power cycling but also improve the
thermal contact between chip and substrate. It is
the consequence of the replacement of solder
material with low thermal conductivity. As the total
chain of thermal impedance from junction to ambient has other dominating elements it has almost no impact on total thermal impedance but it
affects the transient thermal impedance in the
early phase. Short circuit pulses have been used
to study this transient thermal impedance with
high
Measuring pulse
Heating pulse
∆t
resolution in time (Fig. 11). Longer first short circuit pulse deposits energy in the chip and a 2nd
pulse serves as sense of junction temperature.
The relation between short circuit current level
and junction temperature leads to the extraction
of temperature rise after the first pulse. Comparison of transient thermal impedance of soldered
and chips attached by the new method shows a
faster transfer of heat from the chip into the substrate. This benefit allows for further increase of
current density and loss reduction in the IGBT.
7
The automotive demand
for hybrid cars
In terms of required number of power cycles at
certain ∆TJ the hybrid application may look very
well covered by the modules, which fulfil the requirements of industrial applications. The difference makes the temperature profiles and their
impact especially on solder. Underlying slower
passive cycles in power cycling conditions may
stress solder layers additionally and reduce the
lifetime, when the coolant of the combustion engine is used.
top side contact
sheet of metal, eg. Cu
NTV
Fig. 11 Short circuit test of IGBT for transient
thermal impedance evaluation. Collector Current:
Yellow trace. Gate voltage: Green trace. Accumulated energy by short circuit current: Brown trace.
nd
First pulse is used to heat the chip and 2 pulse
is used to sense the temperature after a delay ∆t.
Time scale is 2µs/div.
160
Temperature (°C)
140
Test data of soldered chip
Simulation (solder)
120
Test dataof new die attach
Simulation (new die attach)
100
80
60
40
20
0
200
400
time(µs)
600
800
1000
Fig. 12 Transient temperature within chip after
short circuit pulse of 5µs. New die attach method
leads to faster transfer of heat from chip into substrate. Simulation is based on a RC-network,
which is equivalent to the thermal chain.
Si chip
Cu- ceramic - Cu
MMC plate
Al heat sink
channel to distribute the coolant
structure brazed to act as cooling fins
envelope
channel to distribute the coolant
Fig. 13 Example of module setup based on NTV.
Metal layer on top of Chip interfaces the wire
bond and chip. The integrated heat sink is based
on a low cost MMC plate equipped with a fin
structure by Al brazing. Thermal expansion of
MMC plate is less than CTE of Copper.
A module set up, which combines new assembly
and interconnection technologies with an integrated heat sink (Fig. 13), can supply further functionality. As the heat sink is required to consist of
Aluminum, a special metal matrix material with
low content of SiC or other ceramic fillers may
result in a cost effective and reliable solution with
excellent overall thermal impedance.
8
Industrial application
As the capability of power dissipation through aircooled heat sinks limits power density, more
power dissipation and consequently more power
density requires higher heat sink temperature
(Fig. 14). Higher heat sink temperature is enabled by an increase of TJ,op. Therefore, higher TJ
will help with further increase in power density,
especially in air-cooled systems. Allowance of
higher TJ,op and Tsink may also allow to use simpler heat sinks, in some case.
the extended reliability gained by the new packaging technologies, described above.
FLIR Systems
174.4 °C
Li2
Sp1
150
Li3
100
50
Sp2
Li1
25.2
°C
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
Fig. 14 Air-cooled heat sink for IGBT module;
growing power density is illustrate by a shrinking
red square on the heat sink - from left to right.
Power injected into the heat sink is kept constant
for the different sizes of squares. Upper boxes
show temperature profiles on the heat sink. Temperature on the heat sink is growing from left to
right due to higher power density.
With the consideration to increase heat sink temperatures concerns, come up. Higher heat sink
temperature may thermally overstress PCBs and
passives, which are assembled on heat sinks, as
well. To judge this risk, temperature profiles on
the heat sink have to be investigated. Already
from (Fig. 14) one can imagine that higher power
density causes the maximum temperatures to be
more concentrated at the heat source i. e. below
the power module. Infrared measurements of
such temperature profiles confirm the temperature to drop within a short distance from the
module. Components like PCBs and passives do
not seem to be that much affected, if placed outside the module mounting area. Considering
such profiles during the design of a system,
higher TJ looks feasible, also from the passives
point of view. Furthermore, passives may not stay
at their limits of today.
High voltage drives applications, i.e. traction applications may not approach the junction temperature as high as foreseen for the 1200V devices but such application may take advantage of
Bezeichnung
Li1
Li2
Li3
Cursor
-
Min
44.3
41.6
79.1
Max
104.9
97.5
174.0
TM
Fig. 15 EconoPACK 3 on a state-of-the-art heat
sink. IR-measured temperature profiles TJ (Li3),
Tsink with some distance from the module (Li1,
Li2). Junction temperature is at increased value
of 175°C.
9
Conclusion
Power modules can approach the 175°C point as
operating temperature and 200°C in a further
step. This development will further extend the
possibilities of silicon based power devices like
IGBT [Fig. 16]. It will show up in future IGBT
Generations.
New high temperature packaging technologies
are on their way to replace solder and standard
wire bonds. Utilization of the high temperature
capabilities in variable speed drives will lower the
effort of cooling and system cost. Challenges
arising from the temperature budget on surrounding passive components have to be solved by optimized system design and/or improved thermally
robust components The increase of power density is also a contribution to extended use of controlled drives and thus to energy saving.
VCEsat(125°C) [V] @ 75A
4
2nd Gen
3rd Gen
4th Gen
3.5
6th Gen
5th Gen
3 1st Gen
1200 V / 75 A IGBT
2.5
2
1.5
1
1988
1992
1996
2000
2004
2008
2012
Fig. 16 IGBT progress for 75A/1200V chip. Latest Generation, IGBT4 operates at 150°C. Further generation will allow higher TJ.
10
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