Innovative Metal System for IGBT

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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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ABB Switzerland Ltd. Innovative Metal System for IGBT Press Pack Modules

ISPSD Page 1 of 4 Cambridge, 2003

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.

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