International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 1- Feb 2014
Loss Density Distribution and Power Module
Failure Modes of IGBT
Amit Thakur#1, Y S Thakur#2, Dr. D.K. Sakravdia#3
1
Ujjain Engineering College Ujjain, India
Ujjain Engineering College Ujjain, India
3
Ujjain Engineering College Ujjain, India
2
Abstract- The major cause of IGBT devices failure
is heat. There is a need of higher power densities and
decreased packaging solutions in the industries
commercial applications. In order to meet the
through-life reliability targets for power modules, it
is critical to understand the response of typical wearout mechanisms, for example wire-bond lifting and
solder degradation So the concentration is on the
tools that need to take into account detailed device
structures and different physical phenomena /
parameters that take place inside the semiconductor
structure during operation and also to present the
effect of power cycling frequency, load current and
mean temperature on temperature variations within
the power module structure and its impact on the life
consumption for two common wear-out mechanisms
(the bond wire and the substrate-solder). It is shown
that bond wire degradation is the dominant failure
mechanism for all power cycling conditions whereas
substrate solder failure dominates for externally
applied (ambient or passive) thermal cycling.
Keywords: IGBT, Lumped parameter, Power cycling,
Thermal Modeling, Thermal stress.
I.
Introduction
IGBT devices are now being operated much nearer to
their overload limits to decrease product costs and
improve the efficiency, because of which the risk of
adverse thermal conditions increases. Lumped
parameter thermal models and numerical techniques
such as finite difference and finite element, utilizing
the results from this analysis have been developed.
So for understanding of the loss mechanism, loss
distribution, and thermal coefficients/ characteristics
variation with temperature in power electronic
ISSN: 2231-5381
devices is, therefore, a requisite for the development
of the design tools which enable accurate prediction
of temperature rise and better smart chip design for
self-protection. Power module failures are frequently
triggered by the thermo-mechanical driven wear-out
processes affecting the device packaging, for
example, bond wire lift-off or solder cracking.
Thermal stress is generated at various locations
within the power module by thermal deformation
because of the difference of the coefficients of
thermal expansion (CTE) of power modules'
component materials. Failures may occur at different
rates for different module designs and applications
[1, 2] where the thermal cycling can be as a result of
changing environmental or operational (load)
conditions.
II. Determination of loss density
distribution
The main aim of thermal modeling is to predict the
temperature rise rather than the loss distribution. This
method can be extended to the investigation of the
distribution of losses within a device using the
temperature distribution as the main indicator of
losses. Since temperature distribution measurements
are impossible to take place inside an IGBT device
due to its small dimensions, a previously developed
electrical model based on Hefner's model [2], [3] is
used to estimate the power losses inside the structure.
The power losses can then be taken from the
electrical model and be distributed across the areas
where these losses occur, Figure 1.
Thermal modeling is used to predict the
temperature rise in the IGBT device. However, the
accurate prediction of these temperatures depends
strongly on the accurate loss distribution and accurate
determination of thermal coefficient variation with
temperature. The distribution of losses can be used to
calculate the temperature rise within the device. A
portion of the energy delivered to the IGBT terminals
is dissipated as heat and the remainder is stored in the
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International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 1- Feb 2014
internal capacitances. Using the internal current and
voltage components it is possible to calculate the
instantaneous power that is dissipated as heat within
the IGBT.
In the same way the holes flowing through the basecollector depletion layer generate heat by the
dissipated power given by
Pbc = Vbc (IT-IMOS)
(2)
Where IT is the total current
Figure 1: Loss density distribution for thermal
analysis
The losses in this model can be calculated
analytically, and these are related to the schematic
representation of the IGBT equations shown in
Figure 2. The dashed-circled components represent
the elements of current contributing to the power loss
in the device. Figure 3 shows turn-on anode current,
anode voltage, internal dissipated power and terminal
power waveforms for a clamped inductive load [4]. A
comparison between the carrier dissipated power and
the terminal power is shown in with the dissipated
power being larger than the terminal power at turn
on. During turn-off or power-up of anode voltage,
energy is stored in the drain-source and gate- drain
capacitances that discharge through the MOSFET
and that energy is dissipated as heat within the device
[5]. After turn-on the dissipated power becomes less
than the terminal power as the diffusion capacitance
is charged. This dissipated power will be used in the
calculation of temperatures in different regions in the
IGBT. The corresponding dissipated power is given
by
PMOS = Vbc. IMOS,
(1)
Where Vbc is the base collector voltage and IMOS is the
current passing through the channel.
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Figure 2: Analogue representation of IGBT model
equations
The equivalent power dissipated in the base
resistance and in the emitter-base depletion layer (due
to carrier crystal structure collisions) is given by
PP = Veb. IT + Rb.IT2
(3)
Where Veb is the emitter-base diffusion depletion
voltage and Rb is the conductivity-modulated base
resistance.
The evaluation of the dissipated power by
the electrical model is sufficient to predict the
temperature in the different regions inside the IGBT
[6]. So, in the following study, thermal and electrical
models are coupled to give an adequate model of the
IGBT allowing an accurate description of the electrothermal behavior.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 1- Feb 2014
III. Effects of power cycling frequency on
power module failure mechanism
In power cycling applications, thermal cycling for
both the bond wire and the substrate-solder is
affected differently by the converter modulation
frequency. To show the effect of the modulation
frequency on temperature variations, real time
temperature estimates for the junction and substratesolder layers of a typical half bridge model were
obtained for a converter modulated with a range of
power cycling sinusoidal frequencies between 1mHz
and 100Hz with a constant current amplitude of
300A. The PWM switching frequency was fixed at 5
kHz and the ambient (coolant) temperature was 40ºC.
Comparison between the junction and the substratesolder temperature estimates is shown in Fig. 3. At
frequencies below 0.01 Hz the temperature variations
are independent of frequency and reflect the thermal
resistance of the heat transfer path. For higher
frequencies, the effect of heat capacity attenuates the
variations with the substrate solder seeing a
proportionally smaller variation compared to the
bond wires.
Figure 4: Comparison between bond wire and
substrate-solder life time model mechanisms over a
range of power cycling frequency
IV. Effect of high temperature on power
module failure mechanisms
The study the effect of high temperature on life
consumption of power modules is to be implemented
over a range of mean temperatures. In order to
achieve a variable set of mean temperature for each
test, the ambient temperature in the real time model
was changed. As a result, the life time of the
corresponding component could be studied over a
range of mean temperature values. The lifetime
consumption of the substrate-solder layer is known to
be very dependent on the layer's mean temperature.
Fig. 5 shows the results of the life expectancy of the
substrate-solder layer when the test was repeated over
a range of mean temperatures.
Figure 3: Comparison between the junction and the
substrate-solder temperature estimates over a range
of modulation frequencies
The number of cycles to fail for both the bond wire
and the substrate-solder were determined for the
power cycling conditions described above. Fig. 4
illustrates a comparison (in number of cycles) as a
function of the applied sinusoidal modulation
frequency. The test results in this figure clearly show
that the substrate-solder layer always fails first.
Figure 5: Comparison of Life consumption for
substrate-solder layer over a range of mean
Temperatures
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International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 1- Feb 2014
These tests show that although the thermal cycling
has a big effect in causing failure in substrate-solders,
it is clear that at high temperatures the substratesolder failure is faster. As a result, high mean
temperatures can significantly reduce the substratesolder life. Fig. 6 illustrates the experimentally
observed effects of both mean temperature and
thermal cycling range on wire-bond wear out,
illustrating that exposure to high temperature can
reduce the degradation rate. Fig. 7 compares the life
consumption of both the bond wire and the substratesolder as a function of power cycling frequency when
the layer's mean temperature was fixed at 80°C. It is
clear from these results that high mean temperature
[7] has its direct impact on increasing the life
consumption rate in solders meanwhile wire bond
failure is not affected by the layer's temperature. On
the contrary with high temperatures the bond wire
failure is slower.
V. Load current effect on failure
mechanisms
Life time tests were taken over different values of
load current for both the bond wire and the substratesolder. For each load current the tests covered a range
of power cycling frequencies allowing to compare the
effect of load current over a variant range of thermal
cycling on both the bond wire and solder-substrate
life consumption [8]. Typical results are shown in
Fig. (8) and (9) respectively. Comparing the results in
Fig. 8 shows that with higher load current the bond
wire wears out faster. At the same time, the load
current has the same impact on the substrate-solder
layer failure as illustrated in Fig. 9.
Figure 8: Effect of load current on bond wire life
time consumption
Figure 6: Bond wire wear-out rate for different ranges
of thermal cycling
Figure 9: Effect of load current on substrate-solder
life time consumption
VI. Conclusion
Figure 7: Comparison of Life consumption for bond
wire and substrate-solder layer at high temperatures
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The equations derived the loss distribution inside the
IGBT structure is sufficient to predict the temperature
in the different regions inside the module. In addition
the model can be used for device modeling. The
effect of power cycling frequency on temperature
variations and its impact on the life consumption for
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International Journal of Engineering Trends and Technology (IJETT) – Volume 8 Number 1- Feb 2014
the two common power module wear-out
mechanisms (the bond wire and the substrate-solder)
under a range of prospective in-service conditions
was presented. The results showed the effect of
power cycling frequency, load current and the layer's
temperature on both failure mechanisms. It was
shown that the solder failure is very dependent on the
layer's mean temperature while the bond wire
degradation is more sensitive to temperature
variations (∆T). Under the majority of power cycling
conditions the bond-wire is identified as the dominant
failure mechanism because it is subject to the greatest
temperature variation. Conversely, under passive or
ambient thermal cycling the substrate solder can be
expected to fail first since in this case both bond-wire
and solder see similar temperature variations.
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