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Contract No. DE-AC36-08GO28308
Thermal Performance and
Reliability Characterization of
Bonded Interface Materials
(BIMs)
Preprint
D
. DeVoto, P. Paret, M. Mihalic,
and S
. Narumanchi
National Renewable Energy
Laboratory
A
. Bar-Cohen and K. Matin
Defense Advanced Research Projects Agency
P
resented at ITherm 2014
Orlando, Florida
May 27
-30, 2014
Conference Paper
NREL/CP-5400-61107
August 2014
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1
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Thermal Performance and Reliability Characterization of Bonded Interface Materials (BIMs)
Douglas DeVoto, Paul Paret, Mark Mihalic, and Sreekant Narumanchi
National Renewable Energy Laboratory
15013 Denver West Parkway
Golden, CO 80401
Email: sreekant.narumanchi@nrel.gov
Avram Bar-Cohen and Kaiser Matin
Defense Advanced Research Projects Agency
675 N. Randolph Street, Arlington, VA 22203
ABSTRACT
Thermal interface materials (TIMs) are an important
enabler for low thermal resistance and reliable electronics
packaging for a wide array of applications. There is a trend
towards bonded interface materials (BIMs) because of their
potential for low thermal resistance (<1 mm
2
-K/W). However,
due to coefficient of thermal expansion mismatches between
various layers of a package, thermomechanical stresses are
induced in BIMs and the package can be prone to failures and
integrity risks. Deteriorated interfaces can result in high
thermal resistance in the package and degradation and/or
failure of the electronics. The Defense Advanced Research
Projects Agency’s (DARPA) Thermal Management
Technologies (TMT) Program has addressed this challenge,
supporting the development of mechanically compliant, low
resistivity nano-thermal interface (NTI) materials. Prior
development of these materials resulted in samples that met
DARPA’s initial thermal performance and synthesis metrics.
In this present work, we describe the testing procedure and
report the results of thermal performance and reliability
characterization of an initial sample set of three different NTI-
BIMs tested at the National Renewable Energy Laboratory.
KEY WORDS: thermal interface, bonded interface material,
accelerated testing, temperature cycling, aging, thermal
resistance, transient technique, steady-state technique
NOMENCLATURE
A metering block cross-sectional area, m
2
BIM bonded interface material
c
p
specific heat, J/g-K
DARPA Defense Advanced Research Projects Agency
k thermal conductivity of metering block, W/m-K
l thickness of the test sample, cm
NREL National Renewable Energy Laboratory
NTI nano-thermal interface
R thermal resistance, mm
2
-K/W
t time, s
T temperature, °C
TIM thermal interface material
TMT Thermal Management Technologies
x bondline thickness, mm
Δx
1
distance between T
1
and T
2
or T
3
and T
4
, m
Δx
2
distance between T
2
or T
3
and sample interface,
m
Greek
a thermal diffusivity, cm
2
/s
λ thermal conductivity, W/m-K
ρ bulk density, g/cm
3
Subscripts
avg average
s top top of the interface material
s bot bottom of the interface material
top top metering block
bot bottom metering block
1,2,3,4 locations of temperature measurement in the
metering blocks
INTRODUCTION
Modern electronic packages continue to increase in
processing power by shrinking transistors in each subsequent
generation of the silicon devices. With these advances,
thermal management of the package becomes more
challenging. A successful cooling solution must address the
needs of a chip that operates at higher power levels and higher
heat fluxes. Hot spots in the chip can result in localized heat
fluxes that are several times greater than the chip’s average
heat flux. Package and die stacking designs will limit access to
the back side of chips for cooling, thus creating higher heat
density packages. Innovative thermal management solutions
are needed to ensure that future chip package architectures can
operate at their maximum performance potential.
DARPA’s T
MT Program aims to address these thermal
design concerns through several focus areas: novel air-cooled
heat sinks, two-phase heat spreaders, TIMs/BIMs, and
thermoelectric coolers. In an electronic package, the TIM/BIM
is a critical component that fills up the air gaps between
various layers thereby providing a path for heat dissipation.
Historically, polymeric interface materials such as greases,
gels and phase change materials [1, 2] have been used in
various packaging applications. However, the trend towards
miniaturization and increasing power densities has resulted in
the need for development of low-resistance packaging
configurations with TIM/BIM layer resistances on the order of
1 mm
2
-K/W or less. An additional and important requirement
is also that the TIM/BIM should be reliable. For
environmental reasons, non-lead-based solutions are being
developed and include lead-free solders [3-7], high-pressure
sintered silver [8, 9], low-pressure/temperature sintered silver
[10, 11], as well as thermoplastic adhesives [12]. None of
these materials though have yielded thermal resistances below
1 mm
2
-K/W for large-area attachments (> 50 mm X 50 mm
cross-sectional area footprint) while demonstrating reliability
under harsh accelerated testing conditions. The goal of the
DARPA NTI Program is to develop materials with thermal
resistance on the order of 1 mm
2
-K/W while demonstrating
robustness and reliability. In this work, we report the
2
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characterization of thermal performance and reliability of
BIMs synthesized by several performers in DARPA’s NTI
Program. Standardized test samples were bonded by the
performers for accelerated thermal testing. Their initial
thermal performance was characterized by the xenon flash
transient measurement technique. Xenon flash measurements
were taken at periodic intervals during and after the
completion of accelerated testing. Rigorous finite element
analysis was used to revise a steady-state experimental setup
for measuring the thermal resistance of the samples between
standardized copper test blocks.
DESCRIPTION OF TEST SAMPLES
A variety of interface materials were evaluated to meet
DARPA’s thermal and reliability requirements. They include
aligned carbon nanotubes, laminated graphite and solder, and
copper nanosprings. GE has established an assembly
technique that forms metal nanosprings by the glancing angle
deposition process [13]. The number of springs, diameter of
spring wire, radius of winding, number of windings, and
overall spring length can be controlled by the glancing angle
deposition process. This allows the process to engineer the
desired shear and compressive compliance within the interface
while also optimizing for minimal thermal resistance.
Teledyne developed a bonding process that vertically aligns
graphite platelets within the contact area between two surfaces
[14]. The platelets are first aligned and compressed into thin
layers before a solder binds the graphite layers to each other
and to the surfaces. The Georgia Institute of Technology has
led an effort to develop a low temperature process that grows
and aligns carbon nanotubes as a thin interface material [15].
The thermal performance and reliability of the performers’
interface materials were characterized by utilizing 10-mm X
10-mm cross-sectional footprint samples of silicon bonded to
copper via the BIMs, shown in Fig. 1. The silicon diodes used
for creating the bonded samples were 350 µm thick and were
provided with a backside metallization of aluminum/titanium/
nickel/silver. The copper coupons were 1 mm thick and were
not provided with any metallization. Surface preparation and
additional metallization processing were allowed for the teams
to optimize the bond strength with their interface materials.
After bonding, bondline thicknesses varied amongst the
performers’ samples from 70 to 325 µm.
Fig. 1 Silicon and copper coupons
INITIAL THERMAL RESISTANCE MEASUREMENTS
Bonded samples were evaluated for thermal performance
using a Netzsch LFA 447 Nanoflash instrument. The
Nanoflash operates following the ASTM E-1461-13 test
standard [16]. A xenon flash pulse directs energy towards the
underside of a test sample. An infrared detector with a 7.8 mm
aperture records the sample’s top-side rise in temperature as a
function of time. This technique is demonstrated in Fig. 2.
Fig. 2 Xenon flash measurement technique
Under adiabatic conditions, this allows for the thermal
diffusivity of the sample to be calculated by the following
equation:
= 0.1388
(1)
where:
a = thermal diffusivity
l = thickness of the test sample
t
50
= the time at which 50% of the temperature rise has
occurred
Previous knowledge of a sample’s bulk density and specific
heat also allows calculation of its thermal conductivity, as
shown in the following equation:
(
)
=
(
)
(
)
(
)
(2)
where:
T = temperature
λ = thermal conductivity
ρ = bulk density
c
p
= specific heat
With knowledge of the sample’s bondline thickness, the
thermal resistance of the interface layer can be calculated:
=
(3)
where:
R = thermal resistance
x = bondline thickness
λ = thermal conductivity
Prior to testing, all samples were sprayed with DGF-123 Dry
Graphite Film Spray. This uniform graphite coating allows for
consistent absorptivity of the xenon flash pulse and emissivity
to the infrared detector between test samples.
3
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Initial thermal resistance measurements are summarized in
Table 1. The performers will be designated as Performer A, B,
or C for all discussions of results.
Table 1. Initial sample thermal resistance (mm
2
-K/W)
Sample
Number
Performer
A
Performer
B
Performer
C
1 28.8 3.3 8.4
2 16.7 3.4 3.4
3 13.3 2.7 2.2
4 4.6 2.6 2.1
5 13.4 2.9 1.2
6 9.8 84.1 0.9
7 3.6 19.1 2.4
8 4.3 78.4 1.5
9 4.9 18.6 1.4
10 46.8 21.5 1.2
11 15.6 13.3 1.3
12 4.9 11.9 0.8
13 3.9 3.6 2
14 4.7 4.8 --
15 4.4 3.7 --
Fig. 3 Initial sample thermal resistance (mm
2
-K/W)
Performer A synthesized a sample with a bondline thermal
resistance of 3.6 mm
2
-K/W, and approximately half of the
samples were measured to have thermal resistances lower than
5 mm
2
-K/W. However, a significant number of samples were
measured with high resistances. This indicates the potential for
a low thermal resistance material but also that synthesis
variations are present in the production process. Performer B
samples followed a similar pattern with one sample measuring
2.6 mm
2
-K/W, and half of the samples measuring below 5
mm
2
-K/W. Again, the remaining samples yielded high thermal
resistances. Performer C produced samples that measured less
than 1 mm
2
-K/W and was the only team that produced
samples with little deviation in thermal resistance
measurements.
In addition to transient thermal measurements with the
Nanoflash apparatus, acoustic microscopy images were taken
of the bondlines for qualitative evaluations of the interfaces.
Fig. 4 Acoustic images of samples from Performer A (top
left), Performer B (top right), Performer C (bottom
left), and lead solder as a reference (bottom right)
In general, darker areas indicate a strong bond between the
silicon and copper coupons while lighter areas denote the
likely presence of voiding or delamination. Performer A and B
samples typically had large areas of discontinuity while
Performer C samples consistently showed minimal variation in
bond quality (Fig. 4). The presence of lighter, poorer bond
areas in samples correlated with higher thermal resistance
measurements. For reference, a sample bonded with lead-
solder is shown with a high percentage of voiding.
RELIABILITY TESTING AND CHARACTERIZATION
The bonded samples were subjected to accelerated tests in
the form of temperature cycling as well as thermal aging at an
elevated temperature. In thermal aging tests, the samples were
exposed to a temperature of 130°C for 300 hours. In thermal
cycling tests, the samples were subjected to temperatures from
-40°C to 80°C at low (3°C/minute) and high (25°C/minute)
ramp rates. Transient thermal measurements with the
Nanoflash apparatus were performed to characterize the
thermal performance of all samples prior to, during, and after
accelerated testing. Acoustic microscopy was used to monitor
the condition of the interfaces during the same analysis
intervals.
Samples thermally aged at 130°C were inspected every 100
hours. After 300 hours, Performer A samples all showed an
increase in thermal resistance, as shown in Table 2 and Fig. 5.
In several cases, the thermal resistance of the bondline within
samples approached 100 mm
2
-K/W, indicating that a failure of
the interface would occur shortly if thermal aging continued.
0.1
1
10
100
1 3 5 7 9 11 13 15
Thermal Resistance (mm
2
-K/W)
Sample Number
Performer A
Performer B
Performer C
