Generic Thermal Analysis for Phone and Tablet Systems
Siva P. Gurrum, Darvin R. Edwards, Thomas Marchand-Golder,
Jotaro Akiyama, Satoshi Yokoya, Jean-Francois Drouard, Franck Dahan
Texas Instruments, Inc., Dallas, Texas 75243
Abstract
Thermal management of handheld systems such as smart
phones and tablet systems is becoming increasingly
challenging due to increasing power dissipation. These
mobile systems pose a significant challenge for
implementation of traditional cooling schemes such as heat
sinks and fans due to form factor limitations. Instead, new
advanced cooling schemes have been developed. This article
presents thermal model development from an analysis of
today’s smart phone thermal management schemes and
application of these techniques to a tablet system. Application
processor temperature rise and tablet skin temperature are
reported for thermal enhancement simulations using this tablet
system. Some guiding principles are provided for efficient
thermal design of handheld systems.
Introduction
Increasing functionality of smart phones and tablet systems
is pushing the need for faster application processors with
higher computational resources, e.g., OMAP™ processors.
High performance processors sometimes dissipate higher
power, leading to higher device and skin temperatures (surface
touch temperature of phone or tablet). Thermal management
for phone and tablet systems is becoming increasingly critical
for providing functionality expected from current and future
generations of these devices.
Thermal analysis for phones has been considered by
previous researchers. Lee et al. [1] studied the impact of
volume to size ratio on chip temperature rise for handheld
devices. It is noted that surface temperature of the device
(skin temperature) reduces with increase in size of the
handheld system. Some case thermal enhancements were also
considered. Luo et al. [2] performed measurements on an
actual phone and developed a detailed numerical model as
well as a resistance network model of the phone. Thermal
enhancements through higher thermal conductivity materials
are suggested but no results are presented. The detailed
numerical model is conduction based and uses approximations
for heat transfer coefficients on the phone’s surface. Grimes
et al. [3] investigated the integration of an active fan into a
small phone. Up to 60% higher power dissipation was
possible at a fixed surface temperature constraint using the fan
with realistic flow blockages. Most of these studies are
confined only to phones and not tablets and did not include a
systematic study of the effect of heat spreaders in the system.
This article presents the results from a generic thermal
analysis of handheld systems such as smart phones and tablets.
System thermal models were constructed from a teardown
analysis of an existing popular smart phone.
Model
construction and thermal properties were extracted from
thermal measurements. These models were then extended to a
tablet form factor. Detailed thermal performance benefits are
presented for gap filler pads, tablet back-plate spreading, and
978-1-4673-1965-2/12/$31.00 ©2012 IEEE
tablet stiffener (middle-plate) spreading.
These simple
thermal enhancements using metallic spreaders within the
system are shown to reduce the temperature rise by more than
half. Skin temperature is also reported for a number of
scenarios. Recommendations for achieving optimal thermal
performance of smart phones and tablets are presented based
on the analyses. The results presented in this work will
provide valuable guidance for efficient thermal design of
handheld systems.
Display
Middle-plate (Stiffener plate)
Battery
PCB with Components
Back-plate
Figure 1: A schematic of z-direction stack-up of the phone
model developed with major components (not drawn to scale).
Smart Phone Model Development
A variety of internal constructions can be found in
different smart phones available in the market. Touch-screen
phones with a display occupying the majority of the phone
area are prominent among many manufacturers. A popular
touch-screen smart phone available in the market was used for
thermal model development in the present work. The
construction of this smart phone was determined by opening
the phone and removing different components. Figure 1
shows a simple schematic of the z-direction phone
construction used for model development in this study. Only
major components are shown in this figure. In this
construction, the battery was found adjacent to the PCB,
which enabled a thinner phone profile.
Detailed thermal model development of phones is
challenging due to the complexity of typical phone
construction. In order to develop a generic model, each major
individual component was separated from the smart phone.
Thermal properties for each of these major components were
extracted by matching simulations to temperature rise
measurements on the individual component. The major
components considered in this study included the battery,
back-plate, middle-plate (also known as stiffener plate),
display, and PCB (Printed Circuit Board).
Thermal measurements on each of the individual
components were conducted in a still air environment (natural
convection). The size of this closed box was 1 ft3. Heat
generation was provided by Minco® foil heaters (0.5 in x 0.5
in heater area) attached to each component using a simple glue
spray. The foil heater includes a metallic heating element
embedded in a Kapton® film with an aluminum backing. The
choice of the glue spray was driven by the need for a stable
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attachment with ease of removal. Grease type materials tend
to flow, whereas curable resins are difficult to remove.
Temperature rise measurements were conducted by using a
DC Power Supply to drive the current through the foil heater.
A multi-channel thermocouple reader was used for
temperature measurements using thermocouples.
All
temperature measurements were performed with 40-gauge Ttype thermocouples attached using a small piece of Kapton®
tape. Care was taken during attachment to ensure the
thermocouple bead was in good contact with the surface. In
each case, the temperature versus time history was collected
simultaneously for multiple thermocouples placed on the
component using Labview® data acquisition software.
Thermocouples were mounted on heater top, on top of the
battery adjacent to the heater, and below the battery under the
foil heater footprint. The thermocouple mounted on heater top
was used only to ensure that there was no thermal runaway in
the heater. Thermal property extraction was performed by
comparing measured temperatures with simulated values at
locations 1 and 2 shown in the figure. The dashed lines in
Figure 2 indicate simulated values with the fitted thermal
properties. In order to get good correlation between the
simulations and measurements at both locations, an
orthotropic conductivity model was necessary for the battery.
The resulting thermal conductivity values were kxy = 15
W/mK and kz = 1 W/mK, indicating the highly anisotropic
nature of the battery’s internal construction.
Component
Battery
Back-plate
Back-plate
(heater on
graphite area)
Middle-plate
Display
Battery
2
1
PCB
1 – Back of battery below
heater footprint
2 – On top of battery
adjacent to heater (~2mm
from edge)
Measured
Thermocouple Location Temperature
(oC)
Bottom of Battery
33.7
Simulated
Temperature
(oC)
34.7
Adjacent to Heater
34.9
35.8
Bottom of Backplate
46.6
48.4
Adjacent to Heater
38
39.0
Bottom of Back-plate
31
32.3
Adjacent to Heater
36.1
36.9
Bottom of Middle-plate
43.5
43.2
Adjacent to Heater
40.9
41.5
Bottom of Display
42.1
42.7
Adjacent to Heater
40.2
41.1
PCB Location 1
49.7
52.5
PCB Location 2
47.5
47.3
PCB Location 3
36.2
36.1
Table 1: Comparison of measured vs. simulated temperatures.
Simulated temperatures correspond to fit thermal properties in
Table 2.
Foil heater
Component
Figure 2: Thermal characterization and extraction of
properties for the battery.
Transient temperature rise measurements on individual
phone components described in the previous section were
used to extract thermal properties. These thermal properties
include thermal conductivity and heat capacity, which were
extracted by performing detailed Computational Fluid
Dynamics (CFD) simulations using a commercially available
tool. In each case, a steady state CFD model was first run
with different thermal conductivity values to match steady
state temperature rise at locations of interest. Subsequently,
transient simulations were performed by varying the heat
capacity to minimize least squares error between the measured
and simulated temperature response. As an example, Figure 2
shows the temperature response with the foil heater mounted
on the surface of the battery. The battery was removed from
the phone and held in the middle of the still air chamber.
Thickness
(mm)
Thermal
Conductivity
(W/mK)
Heat Capacity
(J/K.m3)
Glassy Material
1.0
1.0
1.830E+06
Backplate Metal
(effective)
0.25
10.0
1.350E+06
Graphite sheet
(effective)
0.025
kxy = 400, kz = 10
1.520E+06
Plastic Molding
--
0.2
1.484E+06
0.8
kxy = 45, kz = 1.0
1.332E+06
--
2.0
1.600E+06
Shields
0.15
15
3.640E+06
Stiffener Plate
0.275
15
3.640E+06
2.0
k xy = 1.8, kz = 0.1
1.200E+06
Battery
4.0
kxy = 15, kz = 1.0
2.193E+06
Battery Tape
0.08
0.2
1.484E+06
PCB
Package Block
Display Lumped
Table 2: Extracted thickness and thermal properties of major
materials in the smart phone evaluated in this study.
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The methodology described in the previous paragraph was
used for other components as well. The comparison between
measured and simulated temperature values at steady state are
shown in Table 1 at the fit thermal properties. The fit thermal
properties shown in Table 2 correspond to the simulated
temperature rise in Table 1. It must be noted that some of the
components were found to be composed of layers of different
materials. It is difficult to precisely identify and describe each
layer and thickness without specific information from the
smart phone vendor. The extracted construction and thermal
properties are only approximate and are intended only to
capture measured temperature rise in this work. Construction
of some of the major components was the following: (a) Backplate: stack of glassy material, metallic layer, paint, and partial
coverage with tape-based graphite film, (b) Middle-plate:
stack of metallic stiffener with tape-based graphite film, (c)
Display: stack of glassy material and a number of display
layers modeled as display lump, (d) PCB: multi-layered board
with components, shields, and gap filler pads between some
components and shields.
The PCB is modeled as a
homogeneous block with shields for major components.
more effectively. This was apparent in the temperature rise
measurements conducted with same heater on two different
locations of the back-plate: one on the metallic spreader area
and another on the tape-backed graphite layer. A schematic of
the structure and temperature measurements are shown in
Figure 3. The temperature of the heater top reduces
significantly when placed on the graphite sheet area.
(a) Schematic to show z-direction stack-up
Frontplate
Display
Middleplate
Inner Pad
Component
Shield
PCB
Shield
Component
Inner Pad
Outer Pad
Backplate
(b) Components facing the front of tablet
Top Shield enclosing
Application Processor
Heater
Backplate
Battery
Graphite
sheet
Battery
(c) Components facing the back of tablet
Heater on
metal spreader
Heater on
graphite sheet
Middle plate
Bottom shields
Figure 4: Schematic of generic tablet model, (a) represents zdirection stack-up with x-y details only representative, (b)
internal details with display facing upward, (c) internal details
with display facing downward.
Figure 3: Temperature rise measurements of the back-plate
with the heater on metal spreader area and heater on tapebased graphite sheet area.
As an example of thermal enhancement found in the
phone, the back-plate included a layer of tape-backed graphite
sheet, which is expected to spread the heat laterally much
Generic Tablet Thermal Model and Enhancements
This section describes a generic thermal model of a tablet
and discusses the impact of simple thermal enhancements.
The tablet model is derived from the thermal properties and
construction extracted from the smart phone analyzed in the
previous section. The size of the tablet is fixed at 194 x 130 x
9.5 mm, allowing for an approximately 7-inch diagonal
display. A schematic of the construction is shown in Figure 4.
Fig. 4(a) shows the z-direction stack-up within the tablet. The
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PCB has components and shields on both sides. Thermal pads
are included between the components and the shields. Fig.
4(b) describes the layout within the tablet with two large
battery areas. The size of PCB between the batteries is fixed
at 42 x 120 x 0.8 mm. The PCB is connected to the middle
plate with six screws at the periphery of the PCB. In the
baseline tablet model, the middle plate is 0.3 mm thick (0.275
mm metal and 0.025 mm graphite sheet), back-plate is 1.275
mm thick (1.0 mm glassy material, 0.25 mm metal, 0.025 mm
graphite sheet), front display region is 3 mm thick (1.0 mm
glassy material and 2 mm display layers modeled as lumped).
These are similar to the extracted construction of the smart
phone. In the case of the back-plate, the graphite sheet is
present only below the PCB footprint.
For thermal
enhancement simulations, the back-plate and middle-plate
thermal conductivities are varied over the baseline
construction. In addition, thermal pads are selectively turned
OFF and ON to show the significance of heat conduction
through solids within the tablet.
All simulations with the tablet were performed in a still air
environment at an ambient of 25oC. The tablet was placed
vertically with the longer side horizontal to earth’s gravity.
The total power was fixed at 6.0 W with power dissipated in
the application processor at 4.3 W. The remaining 1.7 W
power was distributed between the different components on
the PCB. In typical high power use cases, the application
processor runs the hottest, and is therefore of primary interest.
The temperature of the application processor is quoted as
Tj_max in the plots. The skin temperature of the tablet is
important for safe use. Typical maximum allowable skin
temperatures are quoted below 45 oC for sustained use, but
this depends considerably on the thermal properties of the
(a) All thermal pads OFF
(a)
surface and duration of exposure [4]. A still air environment
is generally more severe than when the tablet is held in hand.
Human hands with internal blood circulation tend to act as
better heat sinks than natural convection air cooling.
The first set of thermal enhancement simulations
considered the effect of back-plate thermal conductivity.
Figure 5 shows the impact of thermal pads within the tablet
system as a function of back-plate thermal conductivity.
When all the thermal pads are deactivated (OFF) in Fig. 5(a),
varying the thermal conductivity from typical plastic values
(0.2 W/mK) to that of copper (400 W/mK) changes the
junction temperature by less than 10oC. The skin temperature
is also shown for all six-sides of the tablet system. For low
thermal conductivity back-plates, the maximum back-plate
skin temperature is large due to the close proximity of high
power application processor. This temperature falls rapidly as
the back-plate thermal conductivity is increased. Providing a
better thermal path between the components and their
corresponding shields by activating the inner thermal pads
only reduces the temperature by another couple of degrees
(Fig. 5(b)). This is due to lack of good conduction (air gap)
between shields and the case of the tablet. Activating the
outer thermal pad between the bottom shields and back-plate
dramatically lowers the processor temperature, indicating the
importance of a good thermal path all the way from the heat
source to external surfaces of the tablet. The drawback due to
this is the increased skin temperature of the back-plate at
lower thermal conductivity values.
These simulations
highlight the design trade-offs between lower junction and
lower skin temperature.
(b) Inner thermal pads ON
(c) Inner and Outer thermal pads ON
(c)
(b)
Figure 5: Effect of back-plate thermal enhancements on temperature rise for the tablet system.
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(a)
not included between bottom shields and back-plate to
highlight the impact of middle-plate alone. It is also important
to note that comparing the middle-plate and back-plate
simulations for the same maximum skin temperature, middle
plate heat spreading results in a lower junction temperature.
This is due to better internal spreading within the tablet system
before the heat reaches the outer case. Such a design is
essential to result in lowest junction and case temperatures
overall.
(b)
Summary and Conclusions
A teardown analysis of a popular smart phone
accompanied by temperature rise measurements was used to
develop a thermal model of a tablet. Thermal enhancement
simulations with the tablet model were performed by varying
the thermal conductivity of back-plate and middle-plate
(stiffener) inside the tablet. The effect of gap filler pads was
also reported on the chip temperature rise and tablet skin
temperatures. Good heat conduction paths within the tablet
system are essential for lower junction temperature. As
examples, metallic spreaders and gap filler pads can serve to
create such thermal paths. It was observed that there is a tradeoff between lower junction temperature and lower skin
temperature when conducting heat from the chip directly to
the back-plate of the tablet. Significant temperature reductions
are possible with higher thermal conductivity middle-plate and
gap filler pads to transfer heat from chips to the plate (up to
40oC in the current study). For lower skin temperatures, it is
important to spread the heat effectively within the tablet
before reaching the external case. This would result in lower
junction as well as lower skin temperatures for safe use.
Temperature limitations of other components such as display
and battery should be duly considered in the overall design.
System level analyses of new phones and tablets will be
increasingly important as power dissipation levels of high
performance application processors climb. Helping to reduce
the power levels are increasingly efficient silicon processes,
dedicated processing circuits, and efficiently coded software.
Each phone requires its own specific thermal model, with the
specific construction material properties as inputs, to enable
fine tuning the thermal management solution. With the proper
materials and construction, even higher powers can be enabled
with passive cooling techniques.
(a) All inner thermal pads OFF
Outer pad between Middle plate
and Top Shield OFF
(b) All inner thermal pads ON
Outer pad between Middle
plate and Top Shield ON
Figure 6: Effect of middle plate thermal enhancements for
temperature rise in tablet.
The second set of simulations looks at the effect of middle
plate thermal conductivity at a thickness of 0.3 mm. The
results are summarized in Figure 6. Without any thermal
pads, the temperature reduces by ~15 oC by varying the
thermal conductivity from typical plastic values to that of
copper. By adding thermal pads between components and
shields, and also between application processor shield and
middle plate, dramatic reductions in temperature are possible.
The junction temperature of the application processor reduces
by as much as 40° C when changing the middle plate from
plastic to copper. In these simulations, the thermal pad was
References
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Thermal Modeling Of Heat Transfer In Handheld
Electronic Devices,” Proc. 11th ITHERM Conference,
May, 2008, pp. 604-609.
2. Luo, Z., Cho, H., Luo, X., Cho, K., “System thermal
analysis for mobile phone,” Applied Thermal Engineering,
Vol. 28, 2008, pp. 1889-1895.
3. Grimes, R., Walsh, E., and Walsh, P., “Active cooling of a
mobile phone handset,” Applied Thermal Engineering,
Vol. 30, 2010, pp. 2363-2369.
4. Roy, S. K., “An Equation for Estimating the Maximum
Allowable
Surface
Temperatures
of Electronic
Equipment,” Proc. 27th IEEE SEMI-THERM Symposium,
Santa Clara, CA, March, 2011, pp. 54.
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