Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Gamal Refai-Ahmed, Ph.D, AMD Fellow Guy Wagner, Director - Electronic Cooling Solutions William Maltz, President - Electronic Cooling Solutions
OBJECTIVE Recently, the market for tablets and hand held devices has grown at an exceptional rate. In order to successfully enable the next wave of solutions in this market segment, end user expectations are that the tablet be thin, light, quiet and has a good battery life. This presentation will outline a methodology used to analyze an existing 16 mm thick tablet with an AMD processor that is cooled with a heat pipe and an internal blower and to then demonstrate that the same technology can be packaged successfully in a thin, fanless form factor that complies with consumer market requirements. 2 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
ENGINEERING STEPS First, the architecture for a fanless design was established. Second, an existing 16 mm thick tablet was modified to show feasibility of the fanless architecture. Third, a numerical model of the physical tablet was built and validated with experimental data taken in the laboratory. Finally the validated numerical model was used to design a silent, fanless tablet less than 11 mm thick and cooled exclusively by conducting the heat to the surface of the tablet and then using natural convection and radiation to remove the heat from the product so that component reliability is ensured and the surface of the tablet will be comfortable to the user. 3 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
16 MM TABLET PHOTOS WITH BOTH COVER AND EMI SHIELD REMOVED Heat exchanger Battery EMI shield Heat pipe Blower Heat pipe detail with EMI shield removed Tablet with back cover removed SSD DC-DC converter 4 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
MODIFICATION OF 16 MM TABLET FOR FANLESS OPERATION In order to determine feasibility of a fanless design, the tablet was modified. Fujipoly thermal interface material as well as an aluminum heat spreader were used to provide a conduction path between the heat sources within the tablet and the external surface of the tablet. The next slide shows the step by step process that was followed in order to proof out the fanless design 5 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
TABLET MODIFICATION STEPS 6 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
Test Configuration Photos Showing Locations Measured with Thermocouples Experimental characterization of the tablet was needed in order to construct a CFD model that could be used to architect a fanless tablet design. Thermocouples were attached to locations on both surfaces of the tablet as well as internal locations. Locations were selected in order to have a sufficient experimental data base that could be used for the construction of the CFD model. 40 AWG type T thermocouples were used for the temperature measurements Measurement locations are shown in the next 2 slides. 7 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
EXTERNAL THERMOCOUPLE LOCATIONS 9b 9a 8 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
INTERNAL THERMOCOUPLE LOCATIONS 9 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
TEST RESULTS Test data was collected for both the unmodified tablet as well as the modified version. The data was collected under different exercise conditions and with the tablet in several orientations. The next slide shows the temperature sensitivity to orientation of the tablet for the various test conditions with both fan and without fan. Slide 12 shows the temperature rise data for several locations in the modified (fanless) tablet from start up to steady state for a specified configuration. The results suggest that a fanless design is achievable. 10 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
TEMPERATURE RISE VS ORIENTATION FAN VS FANLESS OPERATION Temperature vs Orientation 40 35 Temperature Rise (C) 30 25 20 15 10 CPU-Idle fanless Skin-Idle fanless CPU-video fanless Skin-video fanless CPU-Idle fan Skin-Idle fan CPU-video fan Skin-video fan 5 0 Horizontal 30 degrees vertical Orientation 11 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
TIME VS. TEMPERATURE RISE MODIFIED TABLET WITHOUT FAN Time vs Temperature Rise for "Despicable Me" at 30 degrees from horizontal 40.0 35.0 30.0 Temperature Rise (C) 25.0 20.0 15.0 10.0 Heat pipe 2 Heat pipe 1 TC 18 Average Skin CPU 5.0 0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 Time (minutes) 12 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
CORRELATION BETWEEN EXPERIMENTAL DATA AND SIMULATIONS FOR 16 MM TABLET 13 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
16 MM TABLET SIMULATION MODEL Top View: EMI shield hidden Isometric View: Back cover and EMI shield hidden 14 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
TABLET SIMULATION MODELS -- TOP VIEW APU Heat exchanger LCD display Original 16 mm tablet with fan Heat pipe Aluminum heat spreader 16 mm fanless tablet High conductivity TIM 15 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
THE FOLOWING TWO SLIDES SHOW THE CORRELATION BETWEEN SIMULATION AND EXPERIMENTAL DATA 16 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
Back Cover - fanless after modification 30 degrees from horizontal at 20.8 C Ambient APU 52.6 C Simulation 54.1 C Test 31.8 35.3 35.1 35.6 36.8 35.1 32.1 35.1 29.5 31.2 33.3 Actual test temperatures shown in red Simulation temperatures shown in black 17 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
Front without fan after modification 30 degrees from horizontal at 20.8 C Ambient APU 52.6 C Simulation 54.1 C Test 36.1 36.4 34.0 31.0 26.8 32.8 34.0 29.7 31.7 27.6 29.7 34.9 27.7 33.0 28.2 29.0 Actual test temperatures shown in red Simulation temperatures shown in black 18 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
THE FOLOWING TWO SLIDES SHOW THE CORRELATION BETWEEN SIMULATION AND IR IMAGES 19 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
BACK OF FANLESS TABLET IN VERTICAL POSITION RUNNING VIDEO Hot spots from the heat pipe and condenser These are not required for fanless operation 36.1 34.6 30.3 IR Camera Image, Emissivity = 0.94 Numerical Simulation 20 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
FRONT OF FANLESS TABLET IN VERTICAL POSITION RUNNING VIDEO Hot spot from heat pipe and condenser These are not required for fanless operation 35.5 29.6 33.1 31.9 IR Camera Image, Emissivity = 0.94 Numerical Simulation 21 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
Observations With good correlation between the experimental data and the simulations, the numerical model can be used to optimize the tablet design and predict temperatures for design changes. It is clear that at the current power levels, fanless operation is both possible and desirable for a tablet of this size. Since most of the heat is dissipated off of the front and rear surfaces, it should be possible to reduce the thickness substantially and still maintain the good thermal performance in fanless mode. The temperatures from the simulations and experimental data in fanless mode reach steady-state after 2 hours of continuous video operation. Even after this length of play, the tablet is still comfortable to hand hold and only feels warm to the touch. For most non-video operation, the temperature increase of the surfaces will not be noticed. 22 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
THE FOLLOWING FOUR SLIDES SHOW HOW THE HEAT SPREADER AND AIR GAP WORK TOGETHER TO PRODUCE A VIABLE FANLESS DESIGN 23 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
WHY USE A HEAT SPREADER AND AIR GAP? Most of the heat from the APU flows through a thin layer of thermal interface material (TIM) into the heat spreader. The heat spreader is composed of a high thermal conductivity material such as aluminum. PCB Substrate APU TIM Heat spreader Air gap Case - back The heat always follows the path of lowest thermal resistance. In this case, most of the heat is spread out in the heat spreader before it is conducted across the air gap to the back case of the tablet. The thickness of the air gap is important because it controls the thermal resistance from the heat spreader to the back of the case. If the gap is too thin, the thermal resistance across the air gap will be too low and the heat will cross the air gap before it has a chance to spread out sufficiently over the area or cross-section of the heat spreader. The result will be a hot spot on the back of the case. By providing the proper air gap, the highest temperature of the hot spot is reduced substantially so that the tablet remains comfortable to the touch even after several hours of playing video. 24 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
HEAT SPREADER/AIR GAP EFFECT A 145 mm x 245 mm x 1.57 mm Al heat spreader was added to the prototype for fanless operation to remove the heat from the APU and FCH and to spread it out across the back of the unit. This heat spreader was separated from the plastic back cover by an air gap. The gap allowed sufficient thermal coupling between the spreader and the back cover to remove the heat from the ICs but also prevented the hot spot on the aluminum heat spreader from reproducing itself on the back cover. As the air gap is increased, the temperature of the internal ICs increase slightly but the hot spot print-through effect is also reduced or diminished. The following slide shows simulations of the effect of increasing the thickness of the air gap. 25 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
AIR GAP EFFECT ON REAR SURFACE TEMPERATURE 21C Ambient Air Temperature Vertical Orientation 1.2 mm thick Al Heat Spreader Delta T back = 16.5C Delta T APU = 28.4C Delta T back = 15.8C Delta T APU = 29.0C 0.1 mm air gap between spreader and rear cover 0.3 mm air gap between spreader and rear cover Delta T back = 15.3C Delta T APU = 29.5C Delta T back = 14.2C Delta T APU = 30.8C 0.5 mm air gap between spreader and rear cover 1.0 mm air gap between spreader and rear cover 26 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
AIR GAP EFFECT ON REAR SURFACE 21C Ambient Air Temperature Video Mode, Vertical Orientation, 1.2 mm thick Al heat spreader Air Gap Effect on Temperatures Video Mode, Vertical Orientation 5.2 Power Dissipation vs Air Gap Thickness Temperature Rise (C) 32 30 28 26 24 Hot Spot (C) 22 APU Core (C) 20 18 16 14 12 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Air Gap Thickness between Heat Spreader and Rear Cover (mm) Power Dissipation (W) 5.1 Back Power Dissipation (W) Front Power Dissipation (W) 5 4.9 4.8 4.7 4.6 4.5 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Air Gap Thickness between Heat Spreader and Rear Cover (mm) Heat is transferred off all surfaces of the tablet by both convection and radiation. The simulation results shown above take both modes of heat transfer into account. 27 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
THIN TABLET DESIGN AND SIMULATION THE FOLLOWING SLIDES SHOW THE APPROACH TAKEN TO PRODUCE A TABLET THAT IS APPROXAMATELY 11mm THICK AND STILL MEETS THE THERMAL PERFORMANCE REQUIREMENTS 28 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
TABLET SIMULATION MODELS -- TOP VIEW APU Heat exchanger LCD display Original 16 mm tablet with fan Heat pipe Aluminum heat spreader 16 mm fanless tablet High conductivity TIM 11 mm thin fanless tablet 29 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
THIN TABLET DESIGN Using the experience gained in modifying the thick tablet, a numerical model of an 11 mm thin tablet was built in Icepak. The same power dissipation was used as in the thick tablet. However, there was a significant amount of space that could be eliminated between the display and the back cover. It was also necessary to reduce the battery thickness to 4 mm. All of the following simulations use a heat spreader plate to spread the heat from the APU and FCH ICs. The table below shows the thickness stack-up for the thin tablet with a 1.2 mm heat spreader plate. If the plate is made thinner, the tablet thickness can approach 10 mm. Thin tablet using a one-piece aluminum EMI shield heat spreader mm 1.0 Front glass 3.0 Space used for the LCD, USB connector, SD socket and SSD 1.5 Front-side PCB components and air gap between display and PCB 1.2 PCB thickness 1.5 PCB Components 1.2 Large Al heat spreader with integrated EMI shield 0.3 air gap - heat spreader to rear cover 1.0 Rear cover thickness including graphite foil 10.7 Total 30 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
COMPARISON OF TIM USAGE IN THE TWO Due DESIGNS to the elimination of the need to fill large gaps between the APU die and the heat spreader, the TIM (Thermal Interface Material) for the thin tablet can be a very inexpensive, relatively low thermal conductivity material. The simulations for the thin tablet were all run with the TIM thermal conductivity of k = 1.3 W/m-K. Volume = 4.0 mm 3 For the thick 16 mm tablet, the tests and the simulations were run with a large volume of high thermal conductivity material, k = 14.0 W/m-K. Volume = 10,500 mm 3 The result of eliminating the large gaps between the APU and the aluminum heat spreader is that the APU runs much cooler in the thin tablet because the heat can flow directly to the heat spreader through a single, thin layer of TIM. Air gap Back case Aluminum heat spreader High conductivity TIM EMI shield High conductivity TIM Heat pipe Aluminum EMI shield heat spreader Air gap Back case TIM 1 Substrate Die TIM 1 Die Cu heat spreader 16 mm Tablet 11 mm Tablet 31 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
THIN TABLES SIMULATION SUMMARY File Name AMD thin Tablet V12 AMD thin Tablet V17 AMD thin Tablet V18 Description Fanless, smooth aluminum back Fanless, smooth aluminum back, 1.25x power Fanless, smooth aluminum back, 1.5x power APU delta T (C) 30.4 37.6 44.8 FCH delta T (C) 23.4 28.8 34.2 DRAM delta T (C) 22.4 27.6 32.8 Rear Average delta T (C) 11.6 14.1 16.6 Rear Hot Spot delta T (C) 14.3 17.4 20.6 Front Average delta T (C) 9.0 10.8 12.7 Front Hot Spot delta T (C) 12.4 15.1 17.8 Power (W) 9.7 12.1 14.6 Air Gap (mm) 0.3 0.3 0.3 Heat Spreader 0.3mm Al 0.3mm Al 0.3mm Al Orientation Vertical Vertical Vertical Notes 0.3mm aluminum heat spreader 0.3mm aluminum heat spreader 0.3mm aluminum heat spreader 32 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
AMD Thin Tablet V12 With Smooth Aluminum Back Vertical Position running Video - 21C Ambient, 0.3 mm Air Gap Back 14.3 C above ambient Front 1 mm thick Aluminum Back 1.0 mm smooth Al back case 0.3 mm air gap Substrate APU Die 0.3 mm Aluminum heat spreader TIM 1 33 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
AMD Thin Tablet V12, V17 and V18 With Smooth Aluminum Back Vertical Position running Video 21 C Ambient, 0.3 mm Air Gap, 0.3 mm Al heat spreader 100 Temperature Rise as a function of System Power y = 1.257x 1.3438 Temperature Rise (C) 10 y = 1.8499x 0.8994 y = 1.6485x 0.8617 y = 1.8723x 0.7106 APU Rear Hot Spot Average Rear Case Average Front Case Power (APU) Power (Rear Hot Spot ) Power (Average Rear Case) Power (Average Front Case) 1 9 10 11 12 13 14 15 Total System Power (W) 34 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
AMD Thin Tablet V12, V17 and V18 With Smooth Aluminum Back Vertical Position running Video 21 C Ambient, 0.3 mm Air Gap, 0.3 mm Al heat spreader 4.5 Power Dissipation from Back as a function of System Power 4 Power (W) 3.5 3 Radiated Power Convected Power 2.5 2 9 10 11 12 13 14 15 Total System Power (W) 35 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
THIN FANLESS TABLET OBSERVATIONS The 11 mm thin tablet exhibits much lower APU temperatures due to the reduction in thickness of the thermal interface material. It is possible to adjust the rear hot spot temperature on the back case by adding extra air space between the heat spreader and the case. This can be done at the expense of a slight increase in tablet thickness. The IC temperatures have good margin even with a 1 mm air gap during video playback. Aside from providing extra rigidity for the thin tablet, making the back case from stamped aluminum significantly decreases the hot spot temperature as well as the temperatures on the display side of the tablet. A graphite foil heat spreader applied to the inside surface of the polycarbonate back case can be just as effective at reducing the back case hot spots as an aluminum case. However, the front surface of the tablet be warmer with the graphite foil. The temperatures from the simulations and experimental data in fanless mode take about 2 hours of continuous video operation to reach steady state. Even after this length of play, the tablet is still comfortable to hand hold and only feels warm to the touch. For most non-video operation, the temperature increase of the surfaces will be barely detectable. 36 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required
Trademark Attribution AMD, the AMD Arrow logo and combinations thereof are trademarks of Advanced Micro Devices, Inc. in the United States and/or other jurisdictions. Other names used in this presentation are for identification purposes only and may be trademarks of their respective owners. 2011 Advanced Micro Devices, Inc. All rights reserved. 37 Best Engineering Practice to Extend the Free Air-Cooling Limit in Tablet Hand Held Devices AMD TFE 2011 Confidential NDA Required