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Journal ArticleDOI

A Method for Thermal Performance Characterization of Ultrathin Vapor Chambers Cooled by Natural Convection

01 Mar 2016-Journal of Electronic Packaging (American Society of Mechanical Engineers)-Vol. 138, Iss: 1, pp 010903

AbstractVapor chamber technologies offer an attractive approach for passive cooling in portable electronic devices. Due to the market trends in device power consumption and thickness, vapor chamber effectiveness must be compared with alternative heat spreading materials at ultrathin form factors and low heat dissipation rates. A test facility is developed to experimentally characterize performance and analyze the behavior of ultrathin vapor chambers that must reject heat to the ambient via natural convection. The evaporator-side and ambient temperatures are measured directly; the condenser-side surface temperature distribution, which has critical ergonomics implications, is measured using an infrared (IR) camera calibrated pixel-by-pixel over the field of view and operating temperature range. The high thermal resistance imposed by natural convection in the vapor chamber heat dissipation pathway requires accurate prediction of the parasitic heat losses from the test facility using a combined experimental and numerical calibration procedure. Solid metal heat spreaders of known thermal conductivity are first tested, and the temperature distribution is reproduced using a numerical model for conduction in the heat spreader and thermal insulation by iteratively adjusting the external boundary conditions. A regression expression for the heat loss is developed as a function of measured operating conditions using the numerical model. A sample vapor chamber is tested for heat inputs below 2.5 W. Performance metrics are developed to characterize heat spreader performance in terms of the effective thermal resistance and the condenser-side temperature uniformity. The study offers a rigorous approach for testing and analysis of new vapor chamber designs, with accurate characterization of their performance relative to other heat spreaders.

Topics: Heat spreader (69%), Thermal resistance (66%), Heat sink (65%), Heat transfer (64%), Heat pipe (64%)

Summary (2 min read)

Introduction

  • A heat pipe or vapor chamber can passively transport heat from a localized generation source to a diffuse heat rejection surface at a low temperature gradient.
  • The sealed vapor chamber contains a working fluid, and vapor is generated at the evaporator section located over the hot spot.
  • Heat was rejected on the condenser side using a finned heat sink cooled by forced air convection.
  • The thermal performance of the vapor chamber was assessed based on its thermal resistance and condenser-side temperature uniformity [5].
  • This product sector necessitates a paradigm shift in thermal management, where the external surface temperature threshold is dictated by user considerations, rather than by device operating temperature limits.

Experimental Facility

  • An experimental facility is developed to evaluate the performance of ultrathin vapor chambers at low heat loads.
  • The intrinsic challenge in vapor chamber characterization under such conditions is estimation of the percentage of heat input rejected through the vapor chamber versus parasitic losses through other pathways.
  • From the recorded images, a pixel-by-pixel calibration of the surface temperature versus sensor output was performed.
  • Finally, a thermocouple is inserted at the center of the copper heater block to measure the junction temperature.
  • Active data processing is performed in a LABVIEW interface to determine when steady-state conditions have been reached, defined as when the standard deviation of the junction temperature for the last 150 data points is less than 0.02 K.

Calibration of the Test-Section Heat Loss

  • A calibration procedure is implemented that predicts heat loss from the test section.
  • The test section temperatures were recorded for heat loads in the nominal range of 0.15–4 W. Key characteristics of the metal heat spreaders used for the calibration process are listed in Table 1.
  • The lateral cell lengths increase in the outward direction from 0.25 mm to 2.25 mm.
  • With a sufficient match to the experimental data, the heat transported through the heat spreaders and the heat loss through the insulation block can be easily extracted from the numerical data.
  • By evaluating the thermal resistance of both the copper and aluminum heat spreaders, as shown in Fig. 6(b), the influence of the junction-to-ambient temperature on the overall heat loss can be incorporated into the regression.

Results and Performance Metrics

  • The copper vapor chamber has 0.2-mm-thick copper walls, uses water as the working fluid, and is lined with a single layer of copper mesh (pore sizes of approximately 50–100 lm).
  • The data obtained from the tests were used to assess the behavior of the vapor chamber relative to the solid copper heat spreader of the same dimensions.
  • The large thermal resistance contributed by the condenser-side natural convection (in addition to the comparatively smaller thermal resistances of the copper block and conductive epoxy layer) should be omitted from the device thermal resistance assessment for the current configuration, since its inclusion would mask any variations in performance of the actual device under test.
  • 1=RMS Ts Ts;m Q VC 1=RMS Ts Ts;m Q Cu (4) For an ideal heat spreader, the temperature profile would be a uniform temperature on the condenser surface at Ts,m if the convective boundary condition on the condenser is uniform.

Conclusions

  • A novel approach was developed for characterization of vapor chambers of ultrathin form factor.
  • Given their intended application in portable electronics platforms, the experimental facilities are designed to evaluate performance at low heat input powers with heat rejection to the ambient by natural convection.
  • The condenser surface temperature distribution was monitored because of ergonomics implications that govern the thermal management requirements for these applications.
  • The high thermal resistance due to natural convection in the heat dissipation pathway necessitates careful calibration of the parasitic heat losses from the system.
  • The testing methodology developed is an important tool for the development of vapor chambers and heat spreaders intended for use in portable electronics platforms.

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Purdue University
Purdue e-Pubs
CTRC Research Publications Cooling Technologies Research Center
2016
A Method for %ermal Performance
Characterization of Ultra-%in Vapor Chambers
Cooled by Natural Convection
G. Patankar
S. Mancin
J. A. Weibel
Purdue University, jaweibel@purdue.edu
M. A. McDonald
S V. Garimella
Purdue University, sureshg@purdue.edu
Follow this and additional works at: h=p://docs.lib.purdue.edu/coolingpubs
<is document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for
additional information.
Patankar, G.; Mancin, S.; Weibel, J. A.; McDonald, M. A.; and Garimella, S V., "A Method for <ermal Performance Characterization of
Ultra-<in Vapor Chambers Cooled by Natural Convection" (2016). CTRC Research Publications. Paper 301.
h=p://dx.doi.org/10.1115/1.4032345

Gaurav Patankar
Cooling Technologies Research Center,
an NSF I/UCRC,
School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907
e-mail: gpatank@purdue.edu
Simone Mancin
Cooling Technologies Research Center,
an NSF I/UCRC,
School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907
e-mail: simone.mancin@unipd.it
Justin A. Weibel
Cooling Technologies Research Center,
an NSF I/UCRC,
School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907
e-mail: jaweibel@purdue.edu
Suresh V. Garimella
1
Cooling Technologies Research Center,
an NSF I/UCRC,
School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907
e-mail: garimell@purdue.edu
Mark A. MacDonald
Intel Corporation,
5200 Elam Young Pkwy,
Hillsboro, OR 97124
e-mail: mark.macdonald@intel.com
A Method for Thermal
Performance Characterization
of Ultrathin Vapor Chambers
Cooled by Natural Convection
Vapor chamber technologies offer an attractive approach for passive cooling in portable
electronic devices. Due to the market trends in device power consumption and thickness,
vapor chamber effectiveness must be compared with alternative heat spreading materials
at ultrathin form factors and low heat dissipation rates. A test facility is developed to
experimentally characterize performance and analyze the behavior of ultrathin vapor
chambers that must reject heat to the ambient via natural convection. The evaporator-
side and ambient temperatures are measured directly; the condenser-side surface temper-
ature distribution, which has critical ergonomics implications, is measured using an
infrared (IR) camera calibrated pixel-by-pixel over the field of view and operating tem-
perature range. The high thermal resistance imposed by natural convection in the vapor
chamber heat dissipation pathway requires accurate prediction of the parasitic heat
losses from the test facility using a combined experimental and numerical calibration
procedure. Solid metal heat spreaders of known thermal conductivity are first tested, and
the temperature distribution is reproduced using a numerical model for conduction in the
heat spreader and thermal insulation by iteratively adjusting the external boundary con-
ditions. A regression expression for the heat loss is developed as a function of measured
operating conditions using the numerical model. A sample vapor chamber is tested for
heat inputs below 2.5 W. Performance metrics are developed to characterize heat
spreader performance in terms of the effective thermal resistance and the condenser-side
temperature uniformity. The study offers a rigorous approach for testing and analysis of
new vapor chamber designs, with accurate characterization of their performance relative
to other heat spreaders. [DOI: 10.1115/1.4032345]
Introduction
A heat pipe or vapor chamber can passively transport heat from
a localized generation source to a diffuse heat rejection surface at
a low temperature gradient. There is an extensive body of research
on the investigation of heat pipes and vapor chambers for the ther-
mal management of electronics [1]. A vapor chamber is used to
mitigate the temperature rise of sensitive components by spread-
ing heat away from local hot spots. The sealed vapor chamber
contains a working fluid, and vapor is generated at the evaporator
section located over the hot spot. The vapor is driven outward and
condenses on the inner surface of the opposing wall. A porous
wick passively pumps the condensed liquid back to the evaporator
(Fig. 1). Portable electronic device platforms such as smartphones
and tablets are trending toward thinner, compact designs with
more embedded functionality (and thereby more waste heat gener-
ation from active components). Due to power consumption and
size constraints, it is not practical to use active air cooling meth-
ods, or heat rejection surfaces with large area enhancement, to dis-
sipate heat. In such instances, ultrathin vapor chambers may offer
a viable solution.
Prior studies have experimentally assessed the performance of
heat pipes and vapor chambers using a standard testing approach
in which the heat is spread from a heat source to a cold plate or
air-cooled heat sink, and the heater-to-ambient thermal resistance
is determined [2]. The transport behavior of internal components
of a vapor chamber, such as the effective thermal resistance across
Fig. 1 Schematic diagram of vapor chamber operation
1
Corresponding author.
Contributed by the Electronic and Photonic Packaging Division of ASME for
publication in the J
OURNAL OF ELECTRONIC PACKAGING. Manuscript received
September 12, 2015; final manuscript received November 16, 2015; published online
March 10, 2016. Assoc. Editor: Ashish Gupta.
Journal of Electronic Packaging MARCH 2016, Vol. 138 / 010903-1Copyright
V
C
2016 by ASME
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the evaporator wick [3,4], can also be assessed. In a testing config-
uration closely related to the current study, Wang et al. [5] investi-
gated copper vapor chambers that contained interlaced grooves
and channels as the wick structure. The hot spot heat input was
supplied through a copper platen with an embedded rake of ther-
mocouples for heat flux measurement. Heat was rejected on the
condenser side using a finned heat sink cooled by forced air con-
vection. Thermal grease was used to reduce contact resistance
between the vapor chamber and the heat sink; thermocouples
were embedded in this grease layer between the heat sink and the
vapor chamber to measure surface temperatures. The thermal per-
formance of the vapor chamber was assessed based on its thermal
resistance and condenser-side temperature uniformity [5]. This
vapor chamber testing configuration has commonly been used
[69]. Other variants use a liquid-cooled cold plate on the con-
denser side [10]. This testing configuration is tailored for high-
power or high-density cooling applications for which the heat flux
induces a large temperature gradient in the heater platen that can
be accurately measured, and where the condenser-side heat rejec-
tion method mimics the intended application. For example,
Mochizuki et al. [9] tested input heat fluxes from 20 to 100 W
over 1 cm
2
, Wong et al. [6] from 300 to 400 W over 1–4 cm
2
,
and Chen et al. [8] from 20 to 80 W over 2cm
2
. The lowest
reported heat fluxes investigated under this vapor chamber testing
configuration include measurements by Koito et al. [11] from 16
to 32 W/cm
2
and Wang et al. [5] from 4 to 10 W/cm
2
.
In portable electronic platforms, heat is rejected to the ambient
via natural convection directly from the device surface (typically
a smooth, flat surface that does not incorporate a finned heat sink
due to lack of space and the low operating power). Hence, it is
necessary to develop good characterization techniques for vapor
chambers cooled by natural convection. To the authors’ knowl-
edge, experimental investigation of vapor chambers operating at a
low power density where the condenser-side boundary condition
is one of natural convection has not been considered in the litera-
ture. Also, due to the proximity of the vapor chamber condenser
surface to the device skin in thin form factor platforms, an assess-
ment of the condenser-side surface temperature distribution is
extremely important. This product sector necessitates a paradigm
shift in thermal management, where the external surface tempera-
ture threshold is dictated by user considerations, rather than by
device operating temperature limits. According to Moritz and
Henriques [12], roughly 5 mins of contact with temperatures of
50
C can cause skin tissue damage; to avoid this condition, per-
formance throttling would be dictated by user comfort standards.
Berhe [13] defined ergonomic temperature limits on handheld
devices of 41
C for aluminum surfaces and 43
C for plastic
surfaces. It is clear from this review that existing metrology
approaches stress a characterization of the total thermal resistance
of the vapor chamber, while few studies analyze the condenser-
side temperature distribution. A rigorous mapping of the surface
temperature distribution is necessary to characterize vapor cham-
ber performance for portable electronics applications.
This paper presents an approach for characterizing the perform-
ance of ultrathin vapor chambers for portable electronics plat-
forms operating at low power. An experimental test facility is
developed that subjects the vapor chamber to a hot spot on the
evaporator side and rejects heat from the condenser side by natu-
ral convection. Precise evaluation of performance at very low
power densities (1 W/cm
2
) is enabled by a combined experimen-
tal and numerical approach for calibration of the heat transport
through the vapor chamber. Keeping the heat source at the mini-
mum possible temperature and mitigating hotspots on the con-
denser surface are key functional requirements. Hence, in addition
to the conventional thermal resistance metric, IR measurement of
the condenser-side surface temperature allows characterization of
the vapor chamber performance in terms of temperature distribu-
tion. The assessment is based on the performance of an ultrathin
vapor chamber relative to a solid heat spreader with identical
outer dimensions.
Experimental Facility
An experimental facility is developed to evaluate the perform-
ance of ultrathin vapor chambers at low heat loads. The intrinsic
challenge in vapor chamber characterization under such condi-
tions is estimation of the percentage of heat input rejected through
the vapor chamber versus parasitic losses through other pathways.
To measure extremely low heat loads, a test section is typically
designed to eliminate heat losses (an isolated system, e.g.,
Ref. [2]). To evaluate performance of a vapor chamber rejecting
heat to the ambient via natural convection, which inserts a large
associated thermal resistance in the primary heat rejection path-
way, it is difficult to create a sufficiently isolated system. An alter-
native approach is to control the heat losses in a manner that
allows for accurate estimation and calibration, as implemented in
the current study.
Test Section Design and Instrumentation. A schematic dia-
gram of the test section configuration is shown in Fig. 2. The test
section is comprised of the heat spreader sample, with insulation
and a centered heater block on the underside; the top side of the
heat spreader is exposed to ambient air. The test section insulation
is made of PEEK (k ¼ 0.25 W m
1
K
1
) with outer dimensions of
150 mm 115 mm 25.4 mm. A 92 mm 52 mm 0.7 mm deep
recess milled into the top surface of the insulation seats the heat
spreader sample. In the center, a 10 mm 10 mm square pocket
was machined to insert the heater block assembly. As shown in the
inset of Fig. 2, the hot spot heat input is simulated using a
10 mm 10 mm thin-film polyimide heater attached using thermally
conductive paste to the base of a 10 mm copper heater block that
ensures uniform distribution of the heat load imposed on the
spreader. A uniform, thin layer of high-conductivity epoxy was
applied onto the bottom surface of the heat spreader to cover the area
overlapping the copper heater block. This allowed a consistent joint
to be formed between the heat spreader and heater block across all
samples to yield consistent calibration. The top surface of the vapor
chamber is cooled by natural convection to the surrounding air.
A photograph of the experimental facility is shown in Fig. 3.A
sample is shown inserted into the test section, and the auxiliary
components for temperature and power measurements are visible.
The spatial temperature distribution on the top surface of the
heat spreader is measured by a mid-wave IR camera (Indigo
Fig. 2 Schematic diagram of the test section (top inset shows
the heater block assembly)
010903-2 / Vol. 138, MARCH 2016 Transactions of the ASME
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Merlin MID, FLIR) positioned above the test section. Visualization
of the surface temperature via an IR camera allows for the develop-
ment of performance metrics based on the surface temperature distri-
bution. Calibration of the IR camera measurement was carried out
using a reference black body (Blackbody Source Model 2004, SBIR)
with a known emissivity (0.985 6 0.015). The temperature of the
black body was incremented in steps of 5
C from 20
Cto100
C.
From the recorded images, a pixel-by-pixel calibration of the surface
temperature versus sensor output was performed. A sixth-o rder poly-
nomial curve was fit to the data to obtain a correlation between sensor
output and temperature. Subsequent evaluation of the calibration at
selected blackbody reference temperatures in the 20
Cto100
C
range showed a maximum mean error averaged over the field of view
of 0.2
C. The top surface of the heat spreader sample is painted black
(#1602, Krylon) to impart a known emissivity of 0.96 [14]; the ratio
of the cal ibration black body emissivity to the surface emissivity is
used to correct the IR temperature measurement.
As shown in Fig. 2, a total of 30 thermocouples are embedded
throughout the insulation block to monitor the temperature. In par-
ticular, the thermocouple locations are classified into groupings of
those embedded under the top, side, and bottom surfaces (four
each) and in the middle of the insulation block (nine). Eight ther-
mocouples are placed in grooves along the surface in contact with
the bottom of the heat spreader. Finally, a thermocouple is inserted
at the center of the copper heater block to measure the junction
temperature. This deployment of thermocouples is essential to the
calibration procedure used for estimation of the heat loss from the
insulation block, as described in the following section. Each ther-
mocouple was individually wired to a reference junction that is
placed in a dry-block ice point reference (TRCIII, Omega). The
thermocouples were individually calibrated using a thermostatic
oil bath (7103 Micro-Bath, Fluke) and two factory-calibrated
resistance temperature detectors (RTD, 6 0.1 K), one each for the
ice point and the oil bath. Following calibration, the thermocouple
temperature measurements have an absolute uncertainty of
6 0.3 K. The ambient temperature is measured using an RTD.
The electrical power supplied to the film heater is determined
by measuring the voltage drop across the resistance heating ele-
ment and across a shunt resistance placed in series with the film
heater. The electrical input power has a measured uncertainty of
0.2% (governed by the shunt resistance uncertainty).
Test Procedure. A strict experimental procedure is followed
for all tests to ensure repeatability of the measurements. The IR
camera is switched on at least 1 hr prior to starting the test to
ensure that the sensor cools down to a steady temperature for
reduced noise in the images. Boards are placed around the test
section so as to prevent air flow disturbances in the surrounding
ambient. To acquire each data point, the electrical power input to
the heater is set at the desired value; all the monitored data are
recorded every 4 s using an NI cDAQ 9178 data acquisition chas-
sis with NI 9124 thermocouple, NI 9217 RTD, and NI 9205 volt-
age input modules. Active data processing is performed in a
LABVIEW interface to determine when steady-state conditions have
been reached, defined as when the standard deviation of the junc-
tion temperature for the last 150 data points is less than 0.02 K.
The time usually taken to reach steady-state conditions is approxi-
mately 3 hrs. After steady-state conditions are reached, the per-
formance is monitored for an extended period ( 30 min) to
obtain a large steady-state data set; IR images are acquired at
5-min intervals during this period. Due to the small fluctuations in
ambient temperature that affect the test section temperatures at
steady state, a set of 150 data points is selected from the steady-
state data set which has the lowest standard deviation in junction
temperature. An average over these data is used for subsequent
analysis, and associated with the specific steady-state IR image
taken during this interval. This procedure for acquiring a single
data point is repeated for each heat input power.
Calibration of the Test-Section Heat Loss
A calibration procedure is implemented that predicts heat loss
from the test section. The experimental step of the calibration pro-
cedure evaluates heat spreading in two thin metal plates of known
thermal conductivity, viz., copper and aluminum. The test section
temperatures were recorded for heat loads in the nominal range of
0.15–4 W. Key characteristics of the metal heat spreaders used for
the calibration process are listed in Table 1.
A numerical model of the test section is generated to simulate
conduction in the heater block assembly, insulation block, and
heat spreader. As shown in Fig. 4, the model boundary conditions
have a constant heat flux applied at the base of the heater block, a
thermal resistance at the interface between the insulation and heat
Table 1 Heat-loss calibration data set
Copper Aluminum
Thermal conductivity (W m
1
K
1
) 387.6 202.4
Outer dimensions (mm) 90 55 0.7 90 51 0.635
Electrical heat input (W) 0.17–4.16 0.16–3.88
# of data points 8 10
Fig. 3 Photograph of the experimental facility
Fig. 4 Exploded view of the numerical conduction model do-
main and boundary conditions
Journal of Electronic Packaging MARCH 2016, Vol. 138 / 010903-3
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spreader, and convection coefficients on each external surface. A
grid-independent rectangular mesh is used to discretize the geom-
etry using a total of 325,000 cells. The peripheral regions of the
insulation block have a uniform coarse mesh; the solution is
insensitive to further refinements because of the low temperature
gradients in these regions. The copper heater block assembly has
a finer mesh. In the heat spreader, a gradient-based mesh is used
for refinement near the hotspot. The lateral cell lengths increase in
the outward direction from 0.25 mm to 2.25 mm. There are 20
cells across the thickness of the heat spreader near the hotspot.
The properties of the heat spreader are specified for the sample
being tested according to Table 1. The governing energy equation
is solved using the finite-volume method implemented in the com-
mercial software
ANSYS FLUENT [15].
The primary objective of the numerical model is to predict the
boundary conditions and overall heat losses that cannot be deter-
mined directly from the available experimental data. A formal
procedure is implemented in order to iterate on the boundary con-
ditions in the model in order to produce good agreement between
the experimental and numerical values of temperature at the loca-
tions in the test section measured by thermocouples. For each cali-
bration data point, the free variables in the numerical simulation
are the heat transfer coefficients on the top, side, and bottom
surfaces of the insulation block and the heat spreader top surface.
Tuning of the thermal resistance at the interface between the insu-
lation and heat spreader to a fixed value of 0.02 m
2
KW
1
across
all test cases yielded the best agreement with experimental data
(equivalent to an air gap thickness of 0.5 mm).
For the initial guess value, a prediction of the natural convec-
tion heat transfer coefficient at each surface with a different orien-
tation obtained from standard correlations was imposed, and then
was subsequently iterated to generate a match with the thermocou-
ple data. Priority was given to first match the junction temperature
closest to the heat source, and then finer adjustments to the bound-
ary heat transfer coefficients (increments of 0.5 W m
2
K
1
)were
made to minimize the overall average deviation from the experi-
mental temperature data. Simple rules were applied that ensure the
heat transfer coefficients increased from the downward to upward
facing surfaces according to the physical behavior expected. With a
sufficient match to the experimental data, the heat transported
through the heat spreaders and the heat loss through the insulation
block can be easily extracted from the numerical data.
The values of the external heat transfer coefficients were found
to be in the range from 4 to 16 W m
2
K
1
. In the current study,
where low heat loads are applied and the overall heat loss is a sig-
nificant percentage of the overall heat input, a single value for the
heat transfer coefficient on all exposed surfaces did not yield suffi-
cient accuracy in the match between experimental and numerical
temperatures. When these values were allowed to independently
vary, the temperature mismatch between the measured and com-
puted values was significantly decreased for all test cases.
The temperature mismatch between the test and the simulation,
averaged over all the cases, is 0.34 K, with a standard deviation of
0.56 K. Figure 5 shows a comparison between the simulated tem-
peratures after iterating on the boundary conditions compared
with the measured values for a selected copper spreader test case.
The thermocouple groups (as discussed under “Test Section Design
and Instrumentation section) are on the bottom, side, and top
surfaces of the insulation block, inside the insulation block
(internal), embedded below the heat spreader, and at the junction.
For the selected case, the junction temperature is matched most
closely (difference of 0.02 K); the maximum difference is observed
for the heat spreader group of thermocouples (difference of 0.71 K).
Using this calibration procedure, the uncertainty in the eval-
uated heat loss from the test section is roughly estimated based on
both the resolution of the heat transfer coefficient increments used
during the iteration process and the ultimate temperature mis-
match at the surface-embedded thermocouple locations. Using
these component uncertainty values for each case, and expressing
the heat loss as a single equation of the form
Q
loss
¼ h A ðT T
amb
Þ (1)
a standard propagation of errors can be used estimate the uncer-
tainty in the predicted heat loss; this uncertainty varies from 3%
to 14% of the calculated heat loss for the test cases described in
Table 1.
The heat loss values extracted from the calibration of the cop-
per and aluminum heat spreaders are plotted in Fig. 6(a). A gener-
alized regression is developed for the heat loss value as a function
of the electrical input power and the junction-to-ambient tempera-
ture difference, as given by
Q
loss
¼ a
T
j
T
amb
Q
ele

b
"#
Q
ele
(2)
This form of the equation assumes that the heat loss value is pro-
portional to the electrical input power (and that there is no heat
loss at zero input power). This relationship can be clearly observed
in Fig. 6(a) (dashed lines indicate best linear fit to the data points).
The proportionality constant would then depend on the thermal re-
sistance of the sample being tested. The ratio of junction-to-
ambient temperature difference and the electrical input reflects this
thermal resistance. By evaluating the thermal resistance of both the
copper and aluminum heat spreaders, as shown in Fig. 6(b),the
influence of the junction-to-ambient temperature on the overall
heat loss can be incorporated into the regression. The result of the
calibration yields the constants a ¼ 0.14 and b ¼ 0.57, which can
subsequently be used to calculate the heat losses through the insula-
tion block when evaluating heat spreading devices that have an
unknown thermal resistance and heat spreading behavior. The val-
ues of these constants are specific to the current test section design;
a similar calibration procedure would need to be employed with
any change in the experimental setup.
Results and Performance Metrics
A representative vapor chamber device with outer dimensions
of 90 mm 50 mm 0.8 mm, obtained from a commercial ven-
dor, is characterized to demonstrate the testing approach devel-
oped. The copper vapor chamber has 0.2-mm-thick copper walls,
uses water as the working fluid, and is lined with a single layer of
copper mesh (pore sizes of approximately 50–100 lm). The heat
spreading behavior of the vapor chamber is evaluated for 12
device power levels (electrical heat input minus losses) ranging
from 0.4 to 2.2 W, and resulting in vapor chamber area-weighted
mean condenser-side surface temperatures from 24.2
Cto
50.3
C, and maximum condenser-side surface temperatures in the
range of 32.8
C to 55.9
C. During testing, the ambient air
Fig. 5 Comparison of thermocouple temperatures obtained
from experiments against those from the simulations at an elec-
trical heat input of 1 W and ambient temperature of 298.2 K.
Each bar is an average temperature from each grouping of
thermocouples.
010903-4 / Vol. 138, MARCH 2016 Transactions of the ASME
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Journal ArticleDOI
TL;DR: This paper identifies drivers for progress and immediate and future challenges based on discussions at the 3rd Workshop on Thermal Management in Telecommunication Systems and Data Centers held in Redwood City, CA, USA, on November 4–5, 2015.
Abstract: This paper reviews thermal management challenges encountered in a wide range of electronics cooling applications from large-scale (data center and telecommunication) to small-scale systems (personal, portable/wearable, and automotive). This paper identifies drivers for progress and immediate and future challenges based on discussions at the 3rd Workshop on Thermal Management in Telecommunication Systems and Data Centers held in Redwood City, CA, USA, on November 4–5, 2015. Participants in this workshop represented industry and academia, with backgrounds ranging from data center thermal management and energy efficiency to high-performance computing and liquid cooling, thermal management in wearable and mobile devices, and acoustic noise management. By considering a wide range of electronics cooling applications with different lengths and time scales, this paper identifies both common themes and diverging views in the thermal management community.

80 citations


Cites background from "A Method for Thermal Performance Ch..."

  • ...of heat rejection to ambient air via natural convection [75]....

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Journal ArticleDOI
Abstract: Vapor chamber technologies offer an attractive approach for passive heat spreading in mobile electronic devices, in which meeting the demand for increased functionality and performance is hampered by a reliance on conventional conductive heat spreaders. However, market trends in device thickness mandate that vapor chambers be designed to operate effectively at ultra-thin (sub-millimeter) thicknesses. At these form factors, the lateral thermal resistance of vapor chambers is governed by the saturation temperature/pressure gradient in the confined vapor core. In addition, thermal management requirements of mobile electronic devices are increasingly governed by user comfort; heat spreading technologies must be designed specifically to mitigate hot spots on the device skin. The current work considers these unique transport limitations and thermal requirements encountered in mobile applications, and develops a methodology for the design of vapor chambers to yield improved condenser-side temperature uniformity at ultra-thin form factors. Unlike previous approaches that have focused on designing evaporator-side wicks for reduced thermal resistance and delayed dryout at higher operating powers, the current work focuses on manipulating the condenser-side wick to improve lateral heat spreading. The proposed condenser-side wick designs are evaluated using a 3D numerical vapor chamber transport model that accurately captures conjugate heat transport, phase change at the liquid–vapor interface, and pressurization of the vapor core due to evaporation. A biporous condenser-side wick design is proposed that facilitates a thicker vapor core, and thereby reduces the condenser surface peak-to-mean temperature difference by 37% relative to a monolithic wick structure.

47 citations


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Journal ArticleDOI
Abstract: The behavior of a vapor chamber is strongly coupled to the thermophysical properties of the working fluid within. It is well known that these properties limit the maximum power (heat load) at which a vapor chamber can operate, due to incidence of the capillary limit. At this limit, the available capillary pressure generated within the wick structure balances the total pressure drop incurred along the path of fluid flow within the wick. A common figure of merit prioritizes working fluids that maximize this capillary-limited operating power. The current work explores working fluid selection for ultra-thin vapor chambers based on a thermal performance objective, rather than for maximized power dissipation capability. A working fluid is sought in this case that provides the minimal thermal resistance while ensuring a capillary limit is not reached at the target operating power. A resistance-network-based model is used to develop a simple analytical relationship for the vapor chamber thermal resistance as a function of the working fluid properties, operating power, and geometry. At small thicknesses, the thermal resistance of vapor chambers becomes governed by the saturation temperature gradient in the vapor core, which is dependent on the thermophysical properties of the working fluid. To satisfy the performance objective, it is shown that the choice of working fluid cannot be based on a single figure of merit containing only fluid properties. Instead, the functional relationship for thermal resistance must be analyzed taking into account all operating and geometric parameters, in addition to the thermophysical fluid properties. Such an approach for choosing the working fluid is developed and demonstrated.

28 citations


Journal ArticleDOI
Abstract: For flat plate heat pipes, under the premise of holding high thermal spreading performance meanwhile being as thin as possible for cooling smartphones, the flow resistance should be minimized and the capillary force be maximized. Firstly, the mechanism of superhydrophilicity in a specially treated wick was explored. Secondly, a visual study on a microscale flat plate heat pipe which can regulate two-phase flow was conducted. The typical two-phase flow patterns were identified in the heating and cooling zones under different heat loads and working orientations. Under the joint action of the above functions, a 500 µm thick flat plate heat pipe was developed and tested under natural air convection and compared with graphite and copper flakes. The results demonstrate that the microscale flat plate heat pipe has a maximum equivalent thermal conductivity up to 2.88 × 104 W/(m⋅K), more than 80 times the value of copper and 36 times of graphite , which is superior to any reported thin film heat spreader so far. The proposed microscale flat plate heat pipe is an ideal solution to cool high-end smartphone chips.

24 citations


Journal ArticleDOI
Abstract: Vapor chambers can offer a passive heat spreading solution for thermal management in electronics applications ranging from mobile devices to high-power servers. The steady-state operation and performance of vapor chambers has been extensively explored. However, most electronic devices have inherently transient operational modes. For such applications, it is critical to understand the transient thermal response of vapor chamber heat spreaders and to benchmark their transient performance relative to the known behavior of metal heat spreaders. This study uses a low-cost, 3D, transient semi-analytical transport model to explore the transient thermal behavior of thin vapor chambers. We identify the three key mechanisms that govern the transient thermal response: (1) the total thermal capacity of the vapor chamber governs the rate of increase of the volume-averaged mean temperature; (2) the effective in-plane diffusivity governs the time required for the spatial temperature profile to initially develop; and (3) the effective in-plane conductance of the vapor core governs the range of the spatial temperature variation, and by extension, the steady-state performance. An experiment is conducted using a commercial vapor chamber sample to confirm the governing mechanisms revealed by the transport model; the model accurately predicts the experimental measurements. Lastly, the transient performance of a vapor chamber relative to a copper heat spreader of the same external dimensions is explored as a function of the heat spreader thickness and input power. The mechanisms governing the transient behavior of vapor chambers are used to explain the appearance of key performance thresholds beyond which performance is superior to the copper heat spreader. This work provides a foundation for understanding the benefits and limitations of vapor chambers relative to metal heat spreaders in transient operation and may inform the design of vapor chambers for improved transient performance.

16 citations


Additional excerpts

  • ...3K after this calibration procedure [36]....

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References
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Journal Article
TL;DR: Although Leach, Peters, and Rossiter made a careful study of the pathological characteristics of different kinds of burns of guinea-pig skin, the extent to which these chges are representative of those that occur in cutaneous burning in man was not disclosed.
Abstract: A. R. Moxr, M.D., and F. C. HENRIQuES, JR., Ph.D. (From the Departmen of Legal Medcixe, Harvrd Medical School, Boston, Mass.) Although it is common knowledge that there is an inverse relationship between the intensity of a thermal exposure and the amount of time required to produce a burn, there is remarkably little available information as to the rate at which burning of huiman skin occurs at any given surface temperature or as to the pathogenesis and pathological characteristics of burns in which the duration and degree of rise in intracutaneous temperature was known or could be calculated with any degree of accuracy. Considerable information regarding the time-temperature thresholds at which cutaneous burning occurs in animals is provided by the experiments of Hudack and McMaster1 and of Leach, Peters, and Rossiter.2 In the former, water at temperatures ranging between 420 and 670C. either was applied directly or was passed through a thinwalled glass chamber, the base of which was brought in contact with the skin of mice. In the experiments performed by Leach, Peters, and Rossiter water was pumped through a metal chamber at temperatures ranging between 450 and 8o0C. and the base of the chamber was held in contact with the skin of guinvea-pigs for varying periods of time. Both groups of investigators observed that the time required to produce injury diminished rapidly as the temperature of the water was raised. The former reported that a source temperature of 440C. was critical for the causation of hyperthermic edema. The latter reported that the critical temperature for causing permanent and irreversible injury of guinea-pig skin lies between 500 and SS°C. Neither of the above-cited investigations provided data from which the timetemperature requirements for the production of burns of huiman skin could be estimated. Although Leach, Peters, and Rossiter 2 made a careful study of the pathological characteristics of different kinds of burns of guinea-pig skin, the extent to which these chges are representative of those that occur in cutaneous burning in man was not disclosed.

1,255 citations


"A Method for Thermal Performance Ch..." refers background in this paper

  • ...Visualization of the surface temperature via an IR camera allows for the development of performance metrics based on the surface temperature distribution....

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Book
12 Oct 2006
Abstract: Heat Pipes 6th Edition, is an essential guide for practicing engineers and an ideal text for postgraduate students. This book takes a highly practical approach to the design and selection of heat pipes. This new edition has been updated with new information on the underlying theory of heat pipes and heat transfer, fully updated applications, new data sections, updated chapters on design and on electronics cooling applications. Reay's book is a useful reference as well as an accessible introduction for those approaching the topic for the first time. It contains all information required to design and manufacture a heat pipe. It is suitable for use as a professional reference and graduate text. It is revised with greater coverage of key electronic cooling applications.

491 citations


"A Method for Thermal Performance Ch..." refers background in this paper

  • ...A test facility is developed to experimentally characterize performance and analyze the behavior of ultrathin vapor chambers that must reject heat to the ambient via natural convection....

    [...]


Journal ArticleDOI
Abstract: The thermal resistance to heat transfer into the evaporator section of heat pipes and vapor chambers plays a dominant role in governing their overall performance. It is therefore critical to quantify this resistance for commonly used sintered copper powder wick surfaces, both under evaporation and boiling conditions. The objective of the current study is to measure the dependence of thermal resistance on the thickness and particle size of such surfaces. A novel test facility is developed which feeds the test fluid, water, to the wick by capillary action. This simulates the feeding mechanism within an actual heat pipe, referred to as wicked evaporation or boiling. Experiments with multiple samples, with thicknesses ranging from 600 to 1200 μm and particle sizes from 45 to 355 μm, demonstrate that for a given wick thickness, an optimum particle size exists which maximizes the boiling heat transfer coefficient. The tests also show that monoporous sintered wicks are able to support local heat fluxes of greater than 500 W cm −2 without the occurrence of dryout. Additionally, in situ visualization of the wick surfaces during evaporation and boiling allows the thermal performance to be correlated with the observed regimes. It is seen that nucleate boiling from the wick substrate leads to substantially increased performance as compared to evaporation from the liquid free surface at the top of the wick layer. The sharp reduction in overall thermal resistance upon transition to a boiling regime is primarily attributable to the conductive resistance through the saturated wick material being bypassed.

213 citations


"A Method for Thermal Performance Ch..." refers background in this paper

  • ...An experimental facility is developed to evaluate the performance of ultrathin vapor chambers at low heat loads....

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Journal ArticleDOI
Abstract: Flat miniature heat pipes (FMHP's) are shown to be very promising in the cooling of electronic component systems. This investigation presents a detailed experimental and theoretical analysis on maximum heat transfer capabilities of two copper-water FMHP's with diagonal trapezoidal micro capillary grooves and one copper-water FMHP with axial rectangular micro capillary grooves. Maximum heat flux on the evaporator wall of the 120-mm long axial grooved heat pipe, with a vapor channel cross-sectional area of approximately 1.5 x 12 mm 2 and rectangular grooves of dimensions 0.20 mm wide by 0.42 mm deep, exceeded 90 W/cm 2 in the horizontal orientation and 150 W/cm 2 in the vertical orientation. Theoretical prediction of the capillary limitation in the horizontal orientation agreed reasonably well with the experimental data.

205 citations


"A Method for Thermal Performance Ch..." refers methods in this paper

  • ...A numerical model of the test section is generated to simulate conduction in the heater block assembly, insulation block, and heat spreader....

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  • ...A sample vapor chamber is tested for heat inputs below 2.5 W. Performance metrics are developed to characterize heat spreader performance in terms of the effective thermal resistance and the condenser-side temperature uniformity....

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Journal ArticleDOI
Abstract: A numerical analysis is carried out on a flat-plate type heat pipe called “vapor chamber”. The vapor chamber is an advanced cooling heat spreader for high-performance microchips, such as new generation CPUs in personal computers and workstations. The mathematical model of the vapor chamber formulated in this study is a two-phase closed disk-shaped chamber and is placed between a small heat source and a large heat sink. Wick sheets and a wick column are provided inside the vapor chamber to circulate the working fluid. By solving the equations of continuity, momentum and energy numerically, the velocity, pressure and temperature distributions inside the vapor chamber are obtained. From the numerical results, the capillary pressure head necessary to circulate the working fluid is estimated and the temperature drop inside the vapor chamber is determined. These numerical results are useful for the design and improvement of the vapor chamber. Experimental investigation is also carried out, and fairly good agreement is obtained with the numerical results, which confirms the validity of the present numerical analysis.

100 citations


"A Method for Thermal Performance Ch..." refers methods in this paper

  • ...2, the hot spot heat input is simulated using a 10 mm 10 mm thin-film polyimide heater attached using thermally conductive paste to the base of a 10 mm copper heater block that ensures uniform distribution of the heat load imposed on the spreader....

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