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

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.

Back-plate

PCB with Components

Middle-plate (Stiffener plate)

Display

Battery

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

978-1-4673-1965-2/12/$31.00 ©2012 IEEE 1488

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 T-

type 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.

1

2

Battery

Foil heater

1 – Back of battery below

heater footprint

2 – On top of battery

adjacent to heater (~2mm

from edge)

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.

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 Thermocouple Location

Measured

Temperature

(oC)

Simulated

Temperature

(oC)

BatteryBottom of Battery 33.7 34.7

Adjacent to Heater 34.9 35.8

Back-plateBottom of Backplate 46.6 48.4

Adjacent to Heater 38 39.0

Back-plate

(heater on

graphite area)

Bottom of Back-plate 31 32.3

Adjacent to Heater 36.1 36.9

Middle-plateBottom of Middle-plate 43.5 43.2

Adjacent to Heater 40.9 41.5

DisplayBottom of Display 42.1 42.7

Adjacent to Heater 40.2 41.1

PCB

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.

ComponentThickness

(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

PCB 0.8 kxy = 45, kz = 1.0 1.332E+06

Package Block -- 2.0 1.600E+06

Shields 0.15 15 3.640E+06

Stiffener Plate 0.275 15 3.640E+06

Display Lumped 2.0 kxy = 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

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) Back-

plate: 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.

Heater

Graphite

sheet

Heater on

metal spreader

Heater on

graphite sheet

Figure 3: Temperature rise measurements of the back-plate

with the heater on metal spreader area and heater on tape-

based 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

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.

Battery

Top Shield enclosing

Application Processor

Bottom shields

Middle plate

(b) Components facing the front of tablet

(c) Components facing the back of tablet

(a) Schematic to show z-direction stack-up

Backplate

Outer Pad

Shield

PCB

Shield

Middleplate

Display

Frontplate

ComponentInner Pad

ComponentInner Pad

Backplate

Battery

Figure 4: Schematic of generic tablet model, (a) represents z-

direction stack-up with x-y details only representative, (b)

internal details with display facing upward, (c) internal details

with display facing downward.

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

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.

(a) All thermal pads OFF (b) Inner thermal pads ON (c) Inner and Outer thermal pads ON

(a) (b) (c)

Figure 5: Effect of back-plate thermal enhancements on temperature rise for the tablet system.

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(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

(a) (b)

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

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.

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 trade-

off 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.

References

1. Lee, L., Gerlach, D. W., and Joshi, Y. K., “Parametric

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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