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FloTHERM

FloTHERM

FloTHERM uses advanced CFD techniques to predict airflow, temperature, and heat transfer in components, boards, and complete systems, including racks and data centers. It's also the industry's best solution for integration with MCAD and EDA software. Learn more about FloTHERM

Engineering Edge

A Tablet for Everything

FloTHERM XT™, FloTHERM®, & T3Ster® Cure Thermal Headaches in Tablet Computers

By Guy Wagner and William Maltz, Electronic Cooling Solutions

It is human nature to follow the path of least resistance. To that end, it has never been easier or quicker to get what you want, when you want it. The demand for this instant gratification has not only resulted in an explosion of online shopping and drive-thru conveniences such as coffee shops and fast food chains, but has also dramatically changed how we work, play, and learn. No longer do we visit the library to get a book, buy newspaper or wait for the evening news to find out what is happening in the world. We read the news, finish an urgent work report, complete the next level of Angry Birds and find out when the next train is due and all from the same little device: The Tablet

The growth of the tablet market is reflected in the continued decline of the PC market, with consumers generally choosing to replace their laptops with tablets rather than purchasing both.[1] Cheaper prices and a wide range of form factors and sizes has resulted in manufacturers designing devices that are slimmer, sleeker and more compact than ever before.

The thermal design of next generation handheld devices must address both comfortable surface touch temperatures and maximum temperature limitations of internal critical components while also meeting aggressive industrial design requirements. This article discusses the challenges in meeting these requirements in tablet designs.

Thermal models of tablets were created using FloTHERM XT to help understand the maximum allowable power dissipation under various operating conditions. The models were also used to conduct parametric studies to determine the best way to move heat from the internal components out to the case of the tablet where it can be dissipated.

Maximum Power Dissipation

Handheld devices are increasingly capable of running applications that used to require laptop and desktop computers. The requirement that these devices provide better performance with a smaller form factor presents significant challenges, especially when one considers that passive cooling is also a requirement. Several studies have focused on the cooling challenges of hand-held devices; Brown et al, Lee et al, Mongia et al, Huh et al, and Gurrum et al. [2-6]

Figure 1. Total power dissipation removed by passive cooling of a tablet

The maximum possible power dissipation by natural convection and radiation has been calculated for this study and is shown in Figure 1 (overleaf). With a 25°C ambient condition at sea level, the maximum total power dissipation was calculated with a requirement that the surface temperature not exceed a touch temperature of 41°C. This is the maximum aluminum enclosure comfort touch temperature as presented by Berhe.[7]

It can be seen that the theoretical maximum total power dissipation is 13.9 watts when the device is suspended vertically in midair with conduction and radiation occurring from all surfaces as shown in Figure 2. When the device is horizontal, the maximum dissipation falls to 13.1 watts. When the device is placed on a horizontal adiabatic surface, heat transfer occurs from the sides and front surface only and the maximum power dissipation is reduced to 7.9 watts as shown in Figure 3. This condition occurs when the user places the tablet on a blanket or pillow effectively blocking heat transfer from the back surface. These values establish bounds for the maximum amount of heat that can be dissipated by the tablet for different orientations in still air.

In order to calculate the total power dissipation, the following assumptions were made: a typical tablet size of 180 mm by 240 mm and an ideal condition of uniform surface temperature. With a conservative assumption of a surface emissivity of 0.8, the radiant heat transfer accounts for more than half of the total power dissipation at an ambient temperature of 25°C.

In order to achieve a 41°C touch temperature, design parameters need to be considered carefully. It is important to design the tablet to be as isothermal as possible to maximize the amount of heat transfer to the surroundings. The reason for this is so that all surface areas of the tablet are as far above ambient temperature as possible to maximize heat transfer without exceeding the maximum allowable touch temperature. When a surface is no longer available for heat transfer, such as when the tablet is placed on a blanket, the amount of power that can be dissipated while staying under the maximum touch temperature drops significantly since heat transfer is effectively blocked from the back surface.

The Value of Numerical Models

In order to analyze the impact of different thermal management techniques, a detailed computational fluid dynamics (CFD) thermal model is constructed using FloTHERM. Since the actual thermal characterization data of the main processor in the tablet may not be known, the thermal characteristics of the processor can be measured with a high degree of accuracy using Mentor Graphics' T3Ster® to determine the thermal resistance from the processor IC to the case and the PCB. This allows accurate capture of heat flow from the top and bottom of the processor. The thermal model of the processor that was generated using T3Ster can be directly dropped into the tablet thermal model in FloTHERM. The results of a detailed thermal model of the interior of a tablet are shown in Figure 4. Notice how the automatic adaptive meshing in FloTHERM XT follows the small surface details of the components to accurately capture convective and radiant heat transfer from these surfaces.

Hot-Spot Temperature Reduction

Since the goal is to keep the touch temperature at or below 41°C, determining the effect of high conductivity heat spreaders will have a major impact on the design. Some parametric studies were run to determine the effect of making the back side of the case with materials of varying thermal conductivity. The results are shown in Figure 5 and summarized graphically in Figure 6. This study assumes the same power dissipation for each simulation. The only parameter that is being changed is the thermal conductivity of the case. As a reference point, typical thermal conductivity of most plastics is in the range of 0.2 W/mK while aluminum approaches 200 W/mK. This makes aluminum about 1000 times as thermally conductive as plastic. Hot-spot temperature reduction can be achieved by either providing a high conductivity heat spreader inside the case of the tablet or by making the case itself out of high conductivity material. One must keep in mind that the maximum touch temperature is also a function of the conductivity of the case. As the conductivity goes down the maximum comfortable touch temperature goes up.

Figure 2. 41°C Isothermal tablet in vertical position

Figure 3. 41°C Isothermal tablet in horizontal position on an adiabatic surface

Figure 4. Detailed model of the PCB created in FloTHERM® XT

As an example, if the case is made of plastic with a thermal conductivity in the range of 0.2 W/mK, the case temperature that the user senses appears to be cooler than that of an alumunum case since the low thermal conductivity of the plastic results in less heat being conducted between the case and the skin. Since the surface area of the case is large in relation to the thickness of the plastic, heat transfer to the air is not reduced significantly over that of an aluminum case. This of course assumes that the heat is spread on the inside of the plastic case using a high-conductivity aluminum plate or a graphite sheet.

Deriving a Thermal Model of a Processor

When building a thermal model of a tablet, the thermal characteristics of the processor are not always known with a high degree of accuracy. It is also true that data sheets from the suppliers of thermal interface materials may not accurately reflect the thermal resistance of the interface material and the wetting properties of the material between the processor chip or lid and the heat spreader. To overcome this limitation and get an accurate thermal model of the processor, T3Ster was used to determine the thermal resistance from the processor IC to the lid or heat spreader and the PCB. T3Ster is able to do a dynamic thermal characterization of the thermal resistance paths of a packaged semiconductor device.

The transient temperature response of the die is recorded as a function of a step input in power to the die and a structure function is derived from the transient temperature response that characterizes the thermal resistance of all the materials in the thermal path. Figure 7 shows the structure function that was derived for a processor using T3Ster. Note that the thermal resistance from junction to case is measured at 0.23 K/W using this technique. This thermal model of the processor package is then put back in to the CFD simulation and numerical experiments can be run to determine the change in processor junction temperature as other elements in the thermal path such as heat spreader materials and dimensions, air gap thickness between the heat spreader and back case and case materials are changed.

Figure 5. Back-side hot spot temperature change as a function of case thermal conductivity

Figure 6. Effect of thermal conductivity of the case on temperatures

Summary

The maximum power dissipation of the internal components is not only governed by the size of the tablet but is a strong function of how well that heat is spread internally to reduce hot-spot temperatures. Few engineers realize the importance played by radiation in dissipating the heat from the exposed surfaces of a tablet. It is not until precise calculations are made that the importance of radiation is realized in the thermal design of the tablet. If the emissivities of the various surfaces are high, over half of the heat transfer to the surroundings is due to radiation. Overall heat transfer is maximized by reducing hot spot temperatures and spreading the heat so that all surfaces are effectively providing maximum heat transfer through convection and radiation.

Figure 7. The cumulative structure function measured for the processor and lid using T3Ster®

In summary, building an accurate thermal model of the tablet allows the designer to rapidly test the effect of design and material changes without incurring the high cost and schedule delays of testing prototypes. A thermal model allows the thermal design engineer to investigate far more alternatives than building prototypes. This results in a highly engineered tablet design that better meets the expectations of the user while providing an edge over the competition. High quality thermal models speed time to market and lower development costs. With the accuracy of the latest simulation software, the intermediate step of building and testing thermal prototypes can be reduced or eliminated. The only need is final thermal verification of production prototype samples.

References

  1. Preliminary data from the International Data Corporation (IDC) Worldwide Quarterly Tablet Tracker
  2. Brown, L., Seshadri, H., Cool Hand Linux®
  3. -Handheld Thermal Extensions, Proceedings of the Linux Symposium, Vol. 1, pp 75 –80, 2007
  4. Gurrum, S.P., Edwards, D.R., MarchandGolder, T., Akiyama, J., Yokoya, S., Drouard, J.F., Dahan, F., Generic Thermal Analysis for Phone and Tablet Systems, Proceedings of IEEE Electronic Components and Technology Conference, 2012
  5. [Huh, Y., Future Direction of Power Management in Mobile Devices, IEEE Asian Solid-State Circuits Conference, 2011
  6. Lee, J., Gerlach, D.W., Joshi, Y.K., Parametric Thermal Modeling of Heat Transfer in Handheld Electronic Devices, Proceedings of the 11th IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, I-THERM, pp 604609, 2008
  7. Mongia, R., Bhattacharya, A., Pokharna, H., Skin Cooling and Other Challenges in Future Mobile Form Factor Computing Devices, Microelectronics Journal, Vol. 39, pp 992 –1000, 2008
  8. Berhe, M.K., Ergonomic Temperature Limits for Handheld Electronic Devices, Proceedings of ASME InterPACK'07, Paper No. IPACK2007-33873
 
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