ISSN: 1863-5598
Electronics in Motion and Conversion
ZKZ 64717
10-09
October 2009
COVER STORY
ISOPLUS-DIL™ Series
Designed for Highest Reliability
The GWM series in ISOPLUS-DIL™ package (Dual-In-Line) has been developed to
production standard. It is a transfer moulded module which combines the advantage of a
robust package (like discretes) with the functionality of a module with isolation from heat
sink and complexity in the circuit. The GMM series is an improved version and covers
market demands for SMD mountability and highest reliability and is equipped with highly
efficient fast switching TrenchMOSFETs.
By Andreas Laschek-Enders, IXYS Semiconductor GmbH, Germany
Concept of the ISOPLUS™ family
GWM /GMM package is part of the IXYS developed ISOPLUS™ family. It features isolated packages that are footprint compatible to standard housings like TO220, TO247 etc. However in the ISOPLUS™
concept the dice are soldered on a DCB instead on a copper base. A
DCB (direct copper bonded) is a “sandwich” of a ceramic with copper
layer on both sides. After soldering and wire bonding those ceramic
plates are transfer moulded similar to standard packages, see Figure
1. The Cu layer of the DCB can be structured by etching (just like a
PCB) allowing realization of complex circuitries by multi chip packaging. Buck, Boost, Phase legs, 6-packs, and 3-phase input rectifiers
are some of the configurations which are already available in ISOPLUS™ packages.
Figure 2: ISOPLUS-DIL™ package - GWM and GMM layout
bridges identical but electrically separated
(Figure 3).
Both variants are available with SMD pins
allowing the usage in PCB reflow production
process eliminating the need of selective pin
soldering. The backside of the package is
mounted on the heat sink by the use of std
inferface materials like heat transfer paste
or phase change foils.
Figure 1: Cross section of an ISOPLUS™ package
The usage of a DCB as a base carrier for the dice gives the customer the following advantages:
(A) isolation from the heat sink (up to 2.5 kV without isolating foil)
(B) higher integration
(C) multi chip solution, complex circuit topologies
(D) reduction of stray inductance
(E) low coupling capacity from chip to heat sink
(F) excellent heat transfer (low thermal impedance for isolation)
(G) very high temperature and power cycling capability
The ISOPLUS-DIL™ package is a dual in line version with max 12
pins on both sides (developed for voltages up to 150 V Products,
see Figure 8) with a package body of 37.5 x 25 mm² in size (Figure
2). The thickness of DCB’s ceramic is 0.38 mm with Cu layer 0.1 mm
thick.
GWM devices offer a 6-pack configuration with a single DC+ and DCbus bar pin connection whereas GMM parts makes use of 3 half
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Figure 3: GWM and GMM circuit diagram
GMM – a further step to higher reliability
All package constructions suffer from the difference in the expansion
coefficients of the used materials which are Si (die), solder, copper
base and wire bonds (Al). Regardless whether an external temperature change – variation of heat sink or ambient temperature – or
internally by power losses they result in mechanical stress because
of different length variations of the materials, see Figure 4.
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COVER STORY
Material
Silicon
DCB
Copper
Aluminum
Sn
Length variation 'l
Expansion
Coefficient for l=10mm & 'T=100°C
[10-6 1/K]
[ μm ]
2.5
2.5
7.4
7.4
16.5
16.5
23.0
26.7
GMM – reduction of stray inductance
In order to allow fast switching, one of the tasks is to reduce package
stray inductance. As the reduction of the current paths length is limited because of die and package dimensions another approach is to
reduce the active area in the current loop especially in those paths
with non continuous current. For a half bridge or 6-pack configuration
this is mainly an issue for the DC bus current, see Figure 6.
23.0
26.7
Figure 4: Expansion coefficients
Small variations are mainly elastic, large ones especially at the low
end of package specified temperature tend to be plastic. Temperature
cycling between min and max of package operating temperature is
an extreme stress resulting in failure modes including packaging or
die cracks, damages of solder connections either die or pins and wire
bonds lift off. Damages of the solder connection between die and
base firstly increase the thermal impedance. Under electric loads this
results in a further temperature rise and therefore to larger cracks or
voids of the solder layer. Finally this thermal run away causes the
device to fail.
ISOPLUS™ package construction principally reduces this failure
mechanism because DCB is better matched to Silicon than Copper,
see Figure 4. GWM and GMM packages withstand more than 1000
temperature cycles from – 55 °C up to +150 °C.
Power cycling induces the stress by power loss variations in the die.
Especially at low voltage designs these losses are correlated with
high current loads. For example: A 25 mm² 1200V IGBT die can handle about 30 A but a 40 V Trench MOSFET die of same size approximately 220 A. Typically the top contact of the die is a wire bond connection. The max number of bonds is limited by die size and current
density in low voltage designs is much higher than in high voltage.
Therefore bond power losses gain importance. Current density of
bonds can ramp up to more than 650 A/mm² making bond failures
more likely. For an example the power losses on a 8 mm long bond
wire for a 200 A current pulse in a TrenchMOS with 4 bonds is ~6.2
W (per wire). Usage of the max possible number and reduction of the
length of the bonds is a way to achieve high power cycling capability.
As the 3 half bridges of the GMM can be connected like a 6-pack
GWM and GMM offer the same functionality. But the split into 3 half
bridges leads to a further improvement of the internal layout with
shorter bond connections. This is shown in Figure 5.
Figure 6: Current paths in a half bridge
The switching duty cycle adds only a ripple to the output current (Figure 6: IL) but the current to the bus (Fig 6: IDC+ ,IDC-) have
“ON”/”OFF” characteristic with high ΔI/Δt at the edges. With the stray
inductance Lstray they introduce a voltage peak ΔU proportional to L
x ΔI/Δt. This may drive the device into avalanche at the switching
transient stressing the device electrically. This is valid especially
under high load currents at high supply voltage.
As it is not a simple task to calculate the stray inductance of modules
one measurement method is to apply a current pulse with known
ΔI/Δt. By measuring the voltage overshoot with a scope one can
determine Lstray according to the formula
Lstray = ΔU / ΔI/Δt.
This test has been performed on GWM and GMM on a “short circuit”
sample (Figure 7). Here the DCB has been built without die and just
the bonds only. ΔI/Δt test performed on the the DC bus current loop
clearly shows that GMM layout reduces Lstray further:
GWM: Lstray = 15 nH
GMM: Lstray = 10.5 nH
With ΔI/Δt = 1000 A/μs the voltage margin increases by 4.5 V which
is of interest when using TrenchMOSFETs with Vds of only 30 or 40
V.
The reduction of stray inductance is obvious comparing the DCB layouts of GWM and GMM, see Figure 5 and 7. The GWM DCB has a
bus bar structure inside the package with the dice placed either side.
This minimizes the internal current loop area of the bus bar but the
half bridge at the end sees the full bus bar length. Even more these
dice see also the switching activities of the other half bridges. As the
current in the bus structure is the sum of the currents of all 3 half
bridges this gives an additional overshoot when all devices are
switched at the same time.
Figure 5: DCB of GWM and GMM package
The result is an approximately 3 times higher power cycling capability
of the GMM at ΔT = 100 °C with Id = 100 A (!)
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COVER STORY
In the GMM layout the 3 phase legs are separated with their own bus connections and
the layout is optimised reducing the current
loop area further. This explains the reduction
in Lstray by about 30%. Also the internal DC
coupling between the half bridges is eliminated.
GMM – distributed power pins
The bus pins of GWM are 4 mm wide and
they are able to handle the current of the 6pack but the current entry on the board
could be problematic. Although GMM’s
power pins have a width of only 1 mm experiments have shown that under high load
conditions the PCB contact area stays cooler. In a design with a max current of ~210 A
for the 6-pack correlated to ~70A for every
phase leg of the GMM the contact area to
the PCB ran ~60°C cooler and the temperature was below the max allowed value for
the circuit board.
Another advantage of splitting up the 6-pack
into 3 identical half bridges is the optimized
Kelvin source contact. At all dies it comes
direct from the source without load current
sharing and is bonded directly to the package pin. This gives the designer the max
possible control of the die (Figure 5).
Figure 7: Current paths in GWM and GMM “short circuit” sample for measuring Lstray
Device
VDSS
V
ID25
A
R DS(ON)
typ m:
Status
GMM 3x180-004X2-SMD
40
180
1.9
engineering
GMM 3x160-0055X2-SMD
55
150
2.2
active
GMM 3x120-0075X2-SMD
75
110
4.0
active
GMM 3x100-01X1-SMD
100
90
7.5
engineering
GMM 3x60-015X1-SMD
150
60
17
engineering
www.ixys.com
Figure 8: Product table and status
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Conclusion
The GMM series is an improved ISOPLUSDIL™ package featuring very high power
and temperature cycling. The layout of 3
electrically isolated and optimized half
bridges and the use in SMD soldering production process gives the design engineer a
perfectly suited device for drives used in
robotics, automotive or battery powered
application. The GMM will be available in 40
V through 150 V. Customer special solutions
with different topologies are possible.
October 2009
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