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Abstract of devices, making thermal management a key

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Abstract

The heat density in three-dimensional (3D) power module
packages is significantly increased because of the stacked structure and the
small footprints of devices, making thermal management a key issue to maintain
the performance of these power modules. This paper proposes a low temperature
co-fired ceramic (LTCC) heat spreading interposer as a novel cooling solution
for 3D packages, and investigates the effect of various high thermally
conductive materials on the thermal performance of the interposer, including
graphene and pyrolytic graphite sheet (PGS). A two-chip face-to-face stacked
configuration was used as the model for analysis and thermal simulations.
Steady state temperature tests were performed on the LTCC interposers of
different structures and integrated with different thermal spreading materials,
and their thermal performance were compared.

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Keywords

Three-dimensional (3D) package, Low temperature co-fired
ceramic (LTCC) interposer, thermal management

1.   
Introduction

The power density of current mobile electrified systems is
expected to increase significantly in order to achieve economic and
environmental benefits. As a result, new power electronic module packaging architecture,
such as the three-dimensional (3D) package with interposer, has gained more and
more attention for its enhanced system performance. In a 3D package, multiple
chips are stacked vertically with an interposer between them to achieve higher
electrical interconnect density and short interconnect length 1. Many
researches have been carried out to demonstrate the low inductance advantage of
wire bondless 3D packaging of semiconductor devices, which can solve the
problems of ringing, overshoots and EMI emissions 2. On the other hand, the
heat density in the 3D packages is much higher than the conventional 2D
packages due to the higher circuit density and closer distances between chips.
To guarantee the electrical performance and improve the thermo-mechanical
reliability of the 3D modules, it becomes a key issue to dissipate heat near
devices more efficiently to avoid hot spots.

Conventional cooling methods for a 3D package utilize direct
bond copper (DBC) substrates at both top and bottom of the power module as
double-sided cooling. Heat is removed from top and bottom DBCs. However,
additional cooling solutions are needed when the double-side cooling is not
sufficient for the high heat density situations. Other than introducing liquid
cooling integrated within the 3D package, in which situation mechanical
components such as pumps and valves are needed, it is essential to utilize high
thermally conductive material to conduct heat near the power devices
horizontally to the outer ambient 3.

Silicon and glass interposers have been researched because of their
capability to achieve high I/O density, and the possibility to improve their
thermal characteristics by through-package vias (TPVs) 4. However, silicon
interposer has the disadvantage of high electrical loss, while the thermal conductivity
of glass interposer is as low as 0.8-1.0 W/(m*K). Besides, only vertical TPVs
can be built and utilized within the silicon and glass interposers, which
limits the design of interconnect distribution and the usage of long horizontal
edge of interposers 5.

In this paper, we propose the low temperature co-fired ceramic
(LTCC) as interposer used in 3D module packages. First, compared to silicon and
glass whose coefficient of thermal expansion (CTE) is 2.6 ppm/? and 54 ppm/?,
respectively, LTCC tape has a CTE of 4.4 ppm/? 6, which is close to the value
of silicon carbide (SiC), 4.0 ppm/?. Matching CTE values improve the
reliability of module by reducing the risk of warpage or delamination between
layers 7. Second, LTCC substrate can accommodate complex interconnect and
cavity structures, which allows coordinated integration of both electrical and
thermal power routes for heterogeneous 3D power module structures, hence the
LTCC interposer can offer cooling ability without degrading the electrical performance
of 3D module. And its low loss characteristics make it suitable for high
frequency applications up to 100GHz. Last but not least, LTCC can be used as
substrate to integrate materials such as graphene and pyrolytic graphite sheet
(PGS), which have high thermal conductivity and small volume, making them
potential solid-state thermal conduits within the LTCC tape to aide spreading
of heat away from hot spots within the module.

As various materials are proposed, it is important to evaluate
their effects for thermal management. The study herein performs the thermal
analysis of LTCC interposer test coupons with different candidate materials
using simulation and temperature measurement tests, their thermal performance
is compared and discussed.

2.   
Method

To study and measure the thermal performance of LTCC
interposers with different thermally conductive materials, a two-chip
face-to-face stacked configuration was used for analysis as shown in Figure
1.  In this 3D module, two chips are
placed on two opposite sides of LTCC interposer and connected by through via
arrays in the LTCC interposer, DBC substrates are attached on the other side of each
chip to dissipate the generated heat by chips as double-sided cooling. The two power chips of size 3.10 mm
x 3.36 mm dissipating a total 20W power are considered in this structured.   

When LTCC heat spreading interposer is
integrated, the maximum junction temperature of devices decreases as shown in
Figure 2. As the thermal conductivity of a LTCC interposer increases to 100
W/(m*K), the maximum device temperature reduces from 115?, when no interposer
is presented, to 87?.  Hence, in order to
reduce the chip temperature, higher thermal conductivity of interposer is
desired. To achieve that, high thermally conductive materials are integrated
within the LTCC interposer.

2.1.    
Thermal spreading material preparations

Graphene nano-powder and pyrolytic graphite sheet (PGS) were
chosen to form thermal conduits within the LTCC interposer considering their
high thermal conductivities.

The graphene nano-powder of 5-30 nm average
flake thickness was made into dispersion in ethyl acetate by a mixer at 2500
RPM for 30 minutes, followed by an ultrasonic bath for 10 minutes to maximize
the dispersion. Then the graphene dispersion was mixed with silver (Ag) paste matrix
to improve the thermal conductivity as its in-plane thermal conductivity is
much higher than the cross-plane thermal conductivity, so that the heat can
flow to sides of the power module along the longest thermal path as compared to
thermal paths to the top and bottom.

PGS from Panasonic is a graphite polymer film with an in-plane
thermal conductivity ranging from 700 to 1950 W/(m*K), which can diffuse the
heat generated by heat sources horizontally through it in the power package.
Two different types of PGS, A-M and A-DM, were used. Both sheets have same 10
µm thickness and same in-plane thermal conductivity of 1950 W/(m*K). What’s
different is that the A-M type has only 10 µm adhesion tape on one side, while
the A-DM type has one more 10 µm polyester insulation tape on the other side.
The cross-section view of the two PGSs is shown in Figure 3 8.

2.2.    
LTCC interposer substrate fabrication

Considering the material properties, two different LTCC
interposer structures were designed, one is with surface channels where
material under test is filled in, and the other one is with material attached
on interposer surfaces.

In the case of graphene nano-powder and sliver paste composite,
both one-side and double-sided surface channels with two different lengths (30
mm and 77.5 mm) are designed and fabricated within LTCC substrates, as shown in
Figure 4 and Figure 5. Cavity was punched on some LTCC green tapes, and then
they were stacked with blank LTCC tapes according to designs shown in Figure 4.
For the one-side channel, LTCC co-fired silver paste is filled into the top two
layers of LTCC tape. As for the double-sided channels, besides LTCC interposer
sample with channels filled with only co-fired silver paste was prepared,
sample with LTCC co-fired silver paste embedded in the 2nd and 5th layers, and
then graphene and sliver composite filled in the top 1st and bottom 6th layers
after LTCC tape firing, was also fabricated. Graphene-sliver mixture was filled
in the post-fire process, since graphene decomposes at around 400?, while LTCC firing
can go as high as 850? in order to transfer the soft
green tapes to solid ceramic.

As for the case of PGS, The PGSs were attached on both top and
bottom surfaces of LTCC interposers of 77.5 mm long using its adhesion tape.
The fabricated sample is shown in Figure 6.

2.3.    
Steady state temperature tests

To test the thermal performance of different
LTCC interposer samples, temperature measurement method as shown in Figure 7 is
used. Tests were conducted with the LTCC interposer samples placed horizontally
and exposed to ambient air, a 3~5cm wide distance was kept between test coupons
if more than one was under test at the same time. Two power film resistors were
placed face to face on both top and bottom surfaces of the LTCC interposer, acting
as power device heat sources dissipating same and constant power during test. T-type
thermocouples connected to a data logger were placed between each resistor and
interposer to measure and record the device temperatures. Equilibrium was
assumed when the temperature variation was within ±0.1°C/min, and the device
temperatures at both top and bottom were recorded when stable. The experimental
setup are shown in Figure 8.

3.   
Results and discussion

Table 1 shows the test results for the LTCC interposers in which
thermal conduit channels of different lengths and with different materials filled
into, including silver (Ag) paste only and the mixture of graphene and silver
paste. Maximum temperatures at both power film resistors, which acted as the
heat sources, attached on top and bottom surfaces of LTCC interposer after
steady state were recorded.

 

Test Sample

Ttop (?)

Tbottom (?)

30mm

No
channel

Blank tape

97.5

91.5

One-side
channel

Ag

82.7

86.6

Double-sided
channels

Ag

81.5

79.3

mixture

76.8

71.8

77.5mm

No
channel

Blank tape

94.2

83.2

One-side
channel

Ag

82.8

85.9

Double-sided
channels

Ag

80.9

72.2

mixture

68.5

64.3

Table 1: Device temperature measurement
results for LTCC interposer with surface channels.

As can be seen from Table 1, for same
lengths, when the same silver paste is used, the interposer with double-sided
thermal conduit channels shows a better thermal performance with lower device temperatures.
And the addition of graphene nano-powder helps further improve the thermal performance
of the LTCC interposer. The maximum temperature drop compared to LTCC
interposer without thermal conductive channels happens when graphene and silver
paste mixture is filled in the double-sided channels, and the values are 20.7?
and 25.7?, respectively for 30mm and 77.5mm channels.

Table 2 shows the test results when PGS was attached onto LTCC
interposer surfaces. It demonstrates that the PGS enhances the thermal
performance of LTCC substrate and reduces the device temperature by 8.3~13.8°C,
similar to the thermal simulation results shown in Figure 9, where the addition
of PGS to LTCC interposer helps reduce the maximum junction temperature in the
power module from 95.7°C to 83.8°C, a 11.9°C temperature drop.  Notice that the bottom device temperature is
lower than that of top device because ceramic substrate underneath test coupons
helped release the heat from the bottom during experiment, as shown in Figure
8(b). While in the thermal simulation, only air convection is considered.

Between the two types of PGS, the A-M type without polyester
insulation tape performs better than A-DM type with a further 4.5~5.4°C reduced
device temperatures. On the other hand, when layer up the A-M type PGS, the
thermal conductivity of LTCC interposer is reduced and the device temperatures
increase. This could result from the adhesion tape on the top layer of PGS
acting as an insulation and blocks the heat transfer.

4.   
Conclusions

This paper proposes LTCC interposer integrated with high
thermally conductive materials as thermal management for 3D power electronic
module packages. LTCC is chosen as substrate for interposer because it’s able
to provide a high density, high reliability, high performance and low-cost
interconnect package. The thermal performance of LTCC interposers with graphene
nano-powder and pyrolytic graphite sheets is investigated by the steady state
temperature test. It’s found that both materials help reduce device temperature
in a 3D package when they were applied to LTCC interposer. Specifically, the
mixture of graphene nano-powder and sliver paste results in bigger improvement
in thermal performance than PGS, but it has high requirement of dispersion
uniformity since the nanoparticles are prone to agglomeration. The PGS is easy
to use, stable and affordable, making it a potential heat spreader on LTCC
interposer. More different types of PGS with different thickness and different
thermal conductivity are worth investigating, and the known parameters of them
make simulation possible before application and experiments to find potential
candidate and to compare.

Acknowledgments

This work was supported by the National Science Foundation
Engineering Research Center for Power Optimization of Electro Thermal Systems
(POETS) with cooperative agreement EEC-1449548.

References

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5.  Si Huang,
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6.   http://www.dupont.com/content/dam/dupont/products-and-services/electronic-and-electrical-materials/documents/prodlib/GreenTape_Design_Layout_Guidelines.pdf

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