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

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. Middle-plate (Stiffener plate) PCB with s 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 ft 3. 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. 2 1 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 k xy = 15 W/mK and k z = 1 W/mK, indicating the highly anisotropic nature of the battery s internal construction. (heater on graphite area) Middle-plate PCB Thermocouple Location Measured Temperature ( o C) Simulated Temperature ( o C) Bottom of 33.7 34.7 Adjacent to Heater 34.9 35.8 Bottom of Backplate 46.6 48.4 Adjacent to Heater 38 39.0 Bottom of 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 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. Thickness (mm) Thermal Conductivity (W/mK) Heat Capacity (J/K.m 3 ) Glassy Material 1.0 1.0 1.830E+06 Backplate Metal (effective) Graphite sheet (effective) 0.25 10.0 1.350E+06 0.025 k xy = 400, k z = 10 1.520E+06 Plastic Molding -- 0.2 1.484E+06 PCB 0.8 k xy = 45, k z = 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 Lumped 2.0 k xy = 1.8, k z = 0.1 1.200E+06 4.0 k xy = 15, k z = 1.0 2.193E+06 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. 1489

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) : 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 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 Middleplate Inner Pad Shield PCB Shield Inner Pad Outer Pad Backplate (b) s facing the front of tablet Top Shield enclosing Application Processor Backplate Graphite sheet (c) s facing the back of tablet Heater on metal spreader Heater on graphite sheet Middle plate Bottom shields 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 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 1490

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 25 o C. 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 o C 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 10 o C. 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. 1491

(a) (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 (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 o C 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 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 40 o C 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. 11 th 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. 27 th IEEE SEMI-THERM Symposium, Santa Clara, CA, March, 2011, pp. 54. 1492