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Chapter V Summary and Conclusions
Chapter V Summary and Conclusions
Thermal and thermo-mechanical analysis using Finite Element Method (FEM) has
been performed to evaluate an innovative three-dimensional power module packaged by
a stacked-plate technique, namely, the MPIPPS (Metal Post Interconnected-Parallel Plate
Structure), and the results are compared with that of a wire bond module.
Thermal modeling simulates and maps the temperature and heat flow distribution in
both power modules during normal operation, taking into account effects of both size and
location of the heat source as well as the heat spreader. Thermal modeling results show
significantly lower junction temperatures in the MPIPPS module than in the wire bond
module under the same single-side cooling conditions. The hottest spot (in IGBT chip) in
the MPIPPS module has a temperature of 119
o
C and is 17
o
C lower than the hottest spot
in the wire bond module. This is due to the fact that the MPIPPS module has a more
uniform heat flow distribution. The presence of a top DBC substrate in MPIPPS module
serves as a heat conduction path so that the excessive heat generated by high-power
IGBT devices can be directed to the top DBC then flow back to lower temperature diode
devices. The maximum junction temperature is further reduced in MPIPPS module by
the implementation of double-side cooling, which can not be used in the wire bonding
technique.
The subsequent thermo-mechanical modeling has evaluated the thermally induced
responses of the power modules under two conditions. Both models are first subject to
temperature cycling, in which the temperature in the structure changes uniformly between
0 and 125
o
C. The models are also studied under power cycling condition, in which the
temperature periodically changes from 0
o
C to the power modules working temperature.
Temperature distribution obtained from thermal modeling is used in power cycling
analysis. In both cycle conditions, the aluminum wire bonds in the wire bond module
have shown plastic deformation over the regions where they attach to the devices emitter
pads. However, the results have shown a more severe stress and strain situation around
these weak locations during the temperature cycles than during the power cycling
conditions. At the other end of the bonding wires, plastic deformation only occurs in 79
temperature cycling conditions. This is because in power cycling condition, the
temperature variation around the wire-substrate interfaces is much smaller than that in the
device region. The wire bond shape distortion is also analytically predicted through FEM
analysis. At the heating stage in temperature cycling, the wire bond bows more than it
does in power cycling. In both cases the maximum displacement ranges about 10
microns at the center point of the wire bond. In the MPIPPS module, the solder joints
that link the device to the top DBC exhibit significant plastic deformation and creep
strain. Power cycling produces more plastic deformation at the solder joints between post
and device due to more local heating and thus more severe high temperature creep of
solder. Using a deformation-based thermal fatigue theory, the solder joints fatigue lives
are predicted. In temperature cycling, the fatigue life around several solder joint
locations is averaged to be about 20 cycles under the applied condition. In power
cycling, solder joints between the top DBC and the copper posts have a fatigue life of
about 200 cycles. However, the device-to-post solder joint fails within 10 cycles.
We conclude that the MPIPPS module is better in thermal management but is thermo-
mechanically less reliable than the wire bond module. 80
Appendix A:
ABAQUS Input Files
I. MPIPPS, Power Cycling,
Thermo-Mechanical Analysis
Code:
*HEADING, SPARSE
*PREPRINT, ECHO=NO
**
**UNITS M, PA, SEC, CELSIUS
**
*SUPER DELETE, ID=Z111
*SUPER, ID=Z111, RECOVERY=NO
*RETAINED DOFS
RETAINED,
*NODE,NSET=NALL
1, 7.26107E-10, 7.26107E-10, -0.000889
......
22130, 0.02208, 0.0313415, 0.001903
**
** ELEMENT DEFINITIONS
**
*ELEMENT, TYPE=C3D8, ELSET=ALN
1443, 1777, 1784, 2062, 2057, 1775,
1782,
2061, 2056
......
*ELEMENT, TYPE=C3D8, ELSET=SUBCU
463, 1779, 1786, 2064, 2059, 1777,
1784,
2062, 2057
......
** 3445, 6481, 6478, 6645, 6648, 6480,
6477,
** 6644, 6647
......
*ELEMENT, TYPE=C3D8, ELSET=A27POST
6385, 671, 674, 673, 672, 8924, 8921,
8922, 8923
......
*ELEMENT, TYPE=C3D8, ELSET=SUPER-SL
449, 842, 840, 834, 838, 674, 673,
672, 671
......
*ELEMENT, TYPE=C3D8, ELSET=DVSLDG
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2058, 2063
......
*ELEMENT, TYPE=C3D8, ELSET=SI
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** 1776, 1780
226, 1276, 1072, 1067, 1271, 1274,
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......
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6478,6481,6645,6648,6644,6647,6477,6480
*NSET,NSET=RETAINED
SBOTTOM,STOP,
**
** ELEMENT PROPERTIES, LINEAR IN
SUPERMODEL
**
** ALN
**
*SOLID SECTION, ELSET=ALN,
MATERIAL=ALN
1.,
**
** SUBCU
**
*SOLID SECTION, ELSET=SUBCU,
MATERIAL=CU
1.,
**
** 27POST
**
*SOLID SECTION, ELSET=A27POST,
MATERIAL=CU
1.,
**
** SUPER-SLDG
**
*SOLID SECTION, ELSET=SUPER-SL,
MATERIAL=SOLDER
1.,
**
** DVSLDG
**
*SOLID SECTION, ELSET=DVSLDG,
MATERIAL=SOLDER
1.,
**
** SI
**
*SOLID SECTION, ELSET=SI, MATERIAL=SI
1.,
**
** EUTECTIC SOLDER (63SN/37PB)
**
*MATERIAL,NAME=SOLDER
*ELASTIC, TYPE=ISO
2.64E+10, 0.36, 0.
1.25E+10, 0.365, 50.
6.9E+9, 0.378, 100.
**
*EXPANSION, TYPE=ISO
2.38E-5, 0.
2.94E-5, 50.
2.98E-5, 100.
**
*PLASTIC
3.64E+7, 0., 0. 81
1.25E+8, 0.5986, 0.
1.52E+7, 0., 50.
1.0382E+8, 0.598784, 50.
9.6E+6, 0., 100.
9.819E+7, 0.59861, 100.
**
** ALN
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*MATERIAL, NAME=ALN
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*ELASTIC, TYPE=ISO
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*EXPANSION, TYPE=ISO, ZERO=0.
4.5E-6,
**
** CU
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*MATERIAL, NAME=CU
**
**
*ELASTIC, TYPE=ISO
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*PLASTIC, HARDENING=KINEMATIC
1.379E+8,0.0
2.715E+8,0.098982
**
*EXPANSION, TYPE=ISO, ZERO=0.
1.7E-5,
**
** SI
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*MATERIAL, NAME=SI
**
**
*ELASTIC, TYPE=ISO
1.3E+11, 0.28
**
*EXPANSION, TYPE=ISO, ZERO=0.
4.1E-6,
**
*INITIAL CONDITIONS,TYPE=TEMPERATURE
NALL,0.0
** THE CELSIUS TEMPERATURE SCALE WAS
CHOSEN INSIDE THE SUPERELEMENT SO
** THAT WHEN THE *SLOAD CASES, HOT AND
COLD ARE CALLED OUT AT THE
** GLOBAL LEVEL, SCALE FACTORS OF 1.0
AND 0.0 (RESPECTIVELY) PROVIDE A
** DELTA-T OF 125 DEGREES. IF KELVIN WERE
USED, THE SCALE FACTORS WOULD
** BE .3141 AND 0.0.
*SLOAD CASE,ID=HOT
*TEMPERATURE
1, 32.732
2, 32.786
......
21015, 76.038
** 21017, 114.129
** 21018, 113.026
** 21019, 111.923
** 21020, 110.821
** 21022, 114.23
** 21023, 113.158
** 21024, 112.086
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** 21026, 109.941
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** 21032, 109.062
** 21033, 108.02
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** 21036, 110.205
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*