innovative metal system for igbt
TRANSCRIPT
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Innovative Metal System for IGBTPress Pack Modules
S. Gunturi, J. Assal, D. Schneider, S. Eicher
ISPSD, April 2003, Cambridge, England
Copyright [2003] IEEE. Reprinted from the International Symposium onPower Semiconductor Devices and ICs.
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Innovative Metal System for IGBT
Press Pack Modules
S. Gunturi *, J. Assal #, D. Schneider#, S. Eicher#*
ABB Switzerland Ltd, Corporate Research, 5405 Dttwil, [email protected]# ABB Switzerland Ltd, Semiconductors, Fabrikstrasse 3, 5600 Lenzburg, Switzerland
Abstract. Two important design aspects
encountered in IGBT press pack modules used for
HVDC applications are short circuit failure mode
(SCFM) and intermittent operating life (IOL)
capabilities. The requirement that press-pack
IGBT (PPI) fail safely into a short causes a design
conflict with the modules desired capability to
survive a high number of power cycles in normal
operation. An innovative materials design to
optimise this trade-off is described. The failure
mechanism that leads to an open circuit after the
PPI has operated extensively in SCFM was found
to be liquid metal corrosion of the baseplate
followed by the formation of intermetallics with
poor conductivity and silicone gel degradation.
The beneficial effects of dry interface plating
materials to avoid thermomechanical fatigue
under IOL conditions are described.
INTRODUCTION
IGBT press pack modules with high blocking
voltages up to 10 kV are offering new possibilities in
power systems applications, e.g. HVDC transmission
and power quality management, as well as in driveand traction applications. High flexibility and easy
handling are obtained by a non-hermetic, modular
design, in which each silicon (Si) chip is pressed by
an individual contact spring [1], see also figure 1.
These modules are currently used successfully in
HVDC transmission systems with a power ranging upto 300 MW. This translates into operating junction
temperatures of up to 125C and temperature cycles
of up to 100C. Therefore, the capability in
intermittent operating life (IOL) i.e. power cycling is
crucial for the reliable operation of the modules.
In HVDC applications, dozens of modules are
connected in series to block dc-link voltages of up to
100 kV. To prevent shut down of the system due to a
defect arising in a module, redundant modules are
included in the system, such that the surviving
modules share the voltage and the failed module is
still able to carry the load current. Accordingly, a
stable short circuit condition through the failed
module must be formed and guaranteed until the
system is serviced. This so-called short circuit failure
mode (SCFM) has an important consequence on the
press pack design. A single failed chip and its contact
system, which is illustrated in figure 2a, take up thewhole module current of up to 1500 A (phase-rms).
To reduce the resistance of the failure path through
the chip, a metal platelet is used in contact with the
silicon chip [2]. When the chip fails it dissipates, for avery short duration, a sufficiently high energy to melt
the platelet and forms a stable alloy with silicon.
Metals like silver and aluminium are preferred as they
form low melting eutectic alloys with silicon.
Figure 1. IGBT press pack modulecomposed of four submodules.
(a)
soldermaterial
pin
silicone gel
platelet
Si chip
base plate
(b)
IGBTs emitter
surface
gate pad1000 N
thermal
expansion
thermal
expansion
platelet
Si chip
current flow
Figure 2. (a) Design of the contacted Si-chip.(b) Emitter interface of the Si-chip.
Unfortunately Ag and Al have high coefficients
of thermal expansion (CTE = 19 and 23 ppm/K,
respectively) compared to Si (3 ppm/K) causing a
trade-off between IOL and SCFM performance. As
illustrated in figure 2b the changes in temperature,due to power cycling, cause relative lateral
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movements of more than 10 m at the chip to platelet
interface. These cyclic motions combined with high
current densities and high operating temperatures can
damage the chip surface, thereby generating early
electrical failures.
EXPERIMENTAL
Short circuit failure mode. The IGBT modules for
testing in SCFM were initially destroyed by applying
an over-voltage to form the short circuit in one of the
Si chips. Thereafter they were subjected to high
currents in the range of 1000-1500 A and load cycling
to accelerate the degradation of the contact system.
The voltage drop across the module was recorded as a
function of time. Specially built modules were also
tested to monitor the voltage drop across the various
materials interfaces. Tests were interrupted at various
stages of the degradation process. In order to identify
the aging mechanisms, the samples were sectionedand prepared for metallurgical analysis by grinding
and polishing to reveal the alloying zone as shown in
figure 3. Samples were analysed using optical,
scanning acoustic microscopy, scanning electron
microscopy aided by EDX for chemical compositionanalysis and micro hardness testing techniques to
determine the mechanisms leading to the failure by
opening up of the short circuit.
Intermittent Operating Lifetime. In order to
simulate the operating conditions, power cycling wastested on the modules as shown in figure 1. An
applied DC current between 1500 A and 1700 A wasswitched on and off every 30 through the stacks
composed alternatively of modules and coolers. The
contact resistances of the complete module and of the
interface between the metallization of the emitter sideof the chip and the contact platelet were previously
measured, to ensure a controlled T (typically of40C and 80C) between the switching on and off
cycle periods. During the test the collector-emitter
voltage of each module was measured regularly and,
in case of a failure, by-passed. Every 10000 cycles,
the test was stopped, the stacks were dismantled andevery device was electrically tested (gate-emitter and
collector-emitter leakage currents, and gate-emitterthreshold voltage). Failed modules were replaced by
new ones, stacks were remounted and the test was
started again. The interfaces at the emitter surface of
the chips and the platelets, as well as the nature of thefailures were analyzed using optical microscopy and
scanning electron microscopy (SEM) with chemical
composition analysis capability (EDX).
RESULTS & DISCUSSION
Short circuit failure mode. Statistical modelling
activities [3] and the available results from the field
confirm that the IGBT modules meet the requirednumber of operating hours. However, experiments
under accelerated conditions were performed to study
the possible failure mechanisms after long term
operation. Two modes which result in the undesired
opening up of the short circuit were identified, firstly
a failure in the alloying zone and secondly a
degradation of the dry interfaces due to the the
silicone gel creeping into them. These results arediscussed below.
Al-plate
Si - chip
Pb-solder
Mo-baseplate
Figure 3. Alloying zone.
The microstructure of a typical cross section
through the alloying zone during the early stages of
operation in SCFM is shown in figure 3. It revealed
predominantly, the formation of a hemispherical Al-
Si alloy above the Mo baseplate and in the Al-Si
interface. Compositional analysis by EDX in the SEMconfirmed that this alloy had the composition ranging
from 8 to 25 wt. % Si (either side of the eutectic
composition of 12.7 wt. % Si in Al) in various regions
of the alloying zone. A low Si content in the alloywould be preferable as otherwise the resistivity and
hence power dissipated in the alloy increase with theSi content resulting in rapid degradation of the
materials. Spherical chunks of Pb from the solder
alloy (joining the chip to the base plate) were found
embedded in this alloy, as Pb is insoluble in Al.
Further, away from the Al-Si interface, platelets ofprimary Si, that did not melt were embedded in a
matrix of Al. Although platelets of Si were present
they do not lead to the immediate destruction of the
current conducting path, but only contribute to the
higher voltages observed during the early stages of
the test and dissolve during subsequent melting andspreading of the alloy with time. Small volume
fractions of Ni-Al intermetallic needles and platelets
were also observed in the Al-Si alloy. They are
formed from the interaction of the Ni plating on the
Al platelet and Mo baseplate used to facilitate
adequate solderability of the Si chip. Ni-Alintermetallics in general possess poor conductivities,
but were not formed in significant volume fractions to
affect the conductivity of the alloy. During the early
stages of operation the top surface of the Mo
baseplate displayed regions in which the Al-Si alloy
penetrated into the Mo baseplate. However at that
early stage, there was still a good contact between thebaseplate and the Al-Si alloy.
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At a later stage in testing, examination of the Si-
Mo interface in samples that were interrupted after
longer operation but prior to failure, in the SEM
revealed a number of long cracks branching in all
directions in the Mo plate (figure 4). These cracks
were formed due to the liquid Al (from the Al-Si
alloy) corroding the Mo grain boundaries. Thecorroded grains were then drawn into the Al-Si alloy.
Over long periods of operation, diffusion of Si and Al
occurs into the Mo particles forming various
intermetallics (eg. Mo(Si, Al)2, Al4Mo etc), figure 5,
some of which are predicted by the phase diagrams.
Their volume fraction increased with time and a
majority of the alloy was observed to be composed of
the intermetallics (supported by the microhardness
measurement profiles) close to failure. The inherently
high resistivity of intermetallics (MoSi2 has a
resistivity that is one order higher than pure metals)
along with the cracking in the baseplate increase the
resistance to current flow and increase the ohmic heatthat is dissipated in the Si-Mo interface. Such increase
in heat causes further deterioration of the baseplate
and finally failure by oxidation of the alloy. Liquid
metal corrosion of Mo [4] and formation of various
intermetallics [5] were reported earlier.
Figure 4. Molten Al penetrates along Mo grainboundaries.
Figure 5. Intermetallics in the alloying zone.
A second mechanism which results in an open
circuit is due to the silicone gel creeping in to a
majority of the dry interfacial contact area after the
gel potting operation during the production ofmodules. The presence of silicone gel in the interface
during SCFM (when high temperatures are generated
in the dry contacts) (figure 6) resulted in the
embrittlement of the gel and the formation of hard
silica (SiO2) due to oxidation of the methyl groups in
polydimethylsiloxane, which starts in air at
temperatures >180C. Aging experiments on the gel
in the temperature range of 200-275C confirmed theformation of SiO2. Formation of hard layers of silica
from the soft silicone gel that creeps into the
interfaces prevents further electrical contact points
from being established after the initial contact points
have deteriorated by aging/oxidation leading to higher
power dissipation and failure of the contact by creep
in the press-pin and oxidation.
Ag-plating
Ag-plating
pin foot
gel
Figure 6. Silicone gel (dark) in a dry
interface between two plated parts (bright).
Intermittent Operating Lifetime. As mentioned
above, aluminium is needed to ensure long lifetime in
SCFM. Unfortunately aluminium has a high CTE
(23 ppm/K) compared to that of silicon (3 ppm/K).
This generates mechanical fatigue and can cause early
failures under IOL. Therefore, the coating material ofthe platelet must be chosen carefully to meet the
following requirements: (1) high electrical
conductivity, (2) low coefficient of friction, (3) no
oxide formation, and (4) chemically inert below
150C with the IGBT top metal or the silicone gel
under the influence of humidity. In addition, thecoating process must be compatible with Al bulk
material and cost effective.
Figure 7. Failed IGBT (left) and platelet (right)
after 6000 cycles in IOL T= 80C.The location of the failure is marked.
Palladium coatings (Pd) fulfil all these
requirements reasonably well for most applications,
including HVDC. IOL experiments reveal lifetimes(expressed as 10% failure probabilities) of more than
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100000 cycles and 10000 cycles forT= 40C and
80C, respectively. The typical failures exhibit
transfer and sticking of Ag from the chip
metallization on to the Pd coating of the platelet by
interdiffusion of Ag and Pd. Such sticking damages
the emitter surface of the IGBTs, and finally causes
failure of the electronic devices (figures 7 and 8).
Figure 8. SEM of a Pd-coated Al-platelet
(2000 cycles, T= 80C, no electrical failure).Ag particles are situated at the border of thecircular marking. The structure of the IGBT
emitter surface is visible.
In several applications like traction, SCFM
capability is not required as in HVDC. In those cases,
IOL can be dramatically improved by using Mo as
bulk material for the platelet. Mo, with its CTE of
5 ppm/K, is a traditional contact material for
semiconductor devices. To protect it against
oxidation, rhodium (Rh) is used as plating. Figure 9shows the excellent IOL results of using a Mo
platelet on an IGBT after 5000 cycles in IOL at T=80C (the test was interrupted before failure).
Figure 9. IGBT after 5000 cycles in IOL
T= 80C. The emitter surface is notdamaged and the markings left by the Mo
platelet are barely visible.
Using contact platelets with this combination,
IOL tests with T of 80C, exhibit an improvedlifetime close to 100000 cycles. In our first tests we
have seen a shift in the collector-emitter leakage
current from the nA to the A range after
approximately 100 k cycles, but no catastrophic IGBT
failure occurred (chips maintained their switching
capability). The root cause of this observation remainsto be investigated. Mo clearly improves the IOL
capability but experiments show that SCFM lifetime
is reduced by a factor of 10 in comparison to Al
platelets. Therefore, a Mo platelet in direct contact
with the chip enables excellent IOL performance for
modules which are not intended for use in
applications where extended SCFM life is required,
e.g. traction or industrial applications. It should,
however, be noted that even the construction with theMo platelet fails safely into a short. However, such a
short will not remain stable over extremely long
periods as with Al or Ag platelets.
CONCLUSIONS
We present here a press pack IGBT module
construction that fails safe into a short and is able to
maintain this short for a long time. A relatively stable
Al-Si alloy formed under SCFM conditions is able to
carry the load current of the module stack, whose
lifetime is limited by liquid Al from the Al-Si alloy
corroding the Mo baseplate, thus forming cracks andvarious intermetallics with poor conductivity over
long-term operation. When the volume fraction of the
intermetallics increases to a critical level, the power
dissipation and hence heat dissipated increases in the
alloy leading to failure (open circuit) by oxidation.Silicone gel creeping into the dry interfaces also leads
to failure by forming hard SiO2 in the contact
interfaces.It has been shown that high IOL capability and
good SCFM life are conflicting requirements. For
HVDC stations where SCFM is necessary, Al coatedwith Pd is used for the contact platelet and, thus,
power cycling lifetime above 100 k cycles are reachedfor T= 40C. In that case, failures are due to the
high mismatch of the CTEs between Al and Si, and
the interdiffusion process between the Ag top metal of
the electronic device and the Pd coating. For
applications where high IOL capability is critical andlong SCFM life is not required (e.g. traction), a Mo
platelet coated with Rh, is adopted. This construction
exhibits superior IOL lifetime of about 1 mio cycles
forT= 40C.
REFERENCES
[1] S. Kaufmann et al. Innovative Press PackModules for High Power IGBTs. Proc. 13
th
ISPSD 2001, Osaka, Japan (2001). pp. 59.
[2] T. Lang, H. Zeller, Short-circuit resistant IGBT
module, Patent number: US 6426561 B1
[3] R. Schlegel et al. Reliability of non-Hermetic
Pressure Contact IGBT Modules Micro-
electronics Reliability, 41 (2001). pp. 1689.
[4] Metals Handbook Properties and Selection: Non
Ferrous Alloys and Special Purpose Materials,
10th
Edn., Vol.2, ASM Intl.
[5] N. Tunca, R.W. Smith Intermetallic Compound
Layer Growth at the Interface of Solid Refractory
Metals Molybdenum and Niobium with MoltenAluminium. Met. Trans. 20A (1989). pp. 825.