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Research Papers

Design of Thermal Ground Planes for Cooling of Foldable Smartphones

[+] Author and Article Information
Ali Nematollahisarvestani, Ryan J. Lewis, Yung-Cheng Lee

Department of Mechanical Engineering,
University of Colorado at Boulder,
427 UCB, 1111 Engineering Dr.,
Boulder, CO 80309

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received July 9, 2018; final manuscript received January 1, 2019; published online March 1, 2019. Assoc. Editor: Xiaobing Luo.

J. Electron. Packag 141(2), 021004 (Mar 01, 2019) (11 pages) Paper No: EP-18-1056; doi: 10.1115/1.4042472 History: Received July 09, 2018; Revised January 01, 2019

Foldable smartphones are expected to be widely commercialized in the near future. Thermal ground plane (TGP), known as vapor chamber or two-dimensional flat heat pipe, is a promising solution for the thermal management of foldable smartphones. There are two approaches to designing a TGP for foldable smartphones. One approach uses two TGPs connected by a graphite bridge and the other approach uses a single, large, and foldable TGP. In this study, different thermal management solutions are simulated for a representative foldable smartphone with screen dimensions of 144 × 138.3 mm2 (twice the screen of iPhone 6 s with a 10 mm gap). In addition, the simulation includes two heat sources representing a main processor with dimensions of 14.45 × 14.41 mm2 and power of 3.3 W (A9 processor in iPhone 6S) and a broadband processor with dimensions of 8.26 × 9.02 mm2 and power of 2.5 W (Qualcomm broadband processor). For the simulation, a finite element method (FEM) model is calibrated and verified by steady-state experiments of two different TGPs. The calibrated model is then used to study three different cases: a graphite heat spreader, two TGPs with a graphite hinge, and a single, large, and foldable TGP. In the fully unfolded configuration, using a graphite heat spreader, the temperature difference across the spreader's surface is about 17 °C. For the design using two TGPs connected by a graphite bridge, the temperature difference is about 7.2 °C. Finally, for the design using a single large TGP with a joint region, the temperature difference is only 1–2 °C. These results suggest that a single foldable TGP or a configuration with two TGPs outperform the graphite sheet solution for the thermal management of foldable smartphones.

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Figures

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Fig. 1

Graphical representation of a foldable smartphone. TGP or other cooling systems can be placed on the backside of the phone and on the top of the processors where the heat is generated.

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Fig. 2

Side view of a TGP showing its internal structure. Black arrows show the circulation path of water. The actual vapor core thickness is 160 μm while the effective vapor core thickness (dv) is smaller.

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Fig. 3

Plot for the function f(T) versus temperature

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Fig. 4

Top view of one of the two TGPs used for model calibration and validation. The active region is a 5 × 10 cm2 rectangle.

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Fig. 5

Sketch of the experimental setup including the TGP, thermocouples, thermal interface material, and the associated dimensions for different components

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Fig. 6

Parametric study with two different TGPs labeled as TGP1 and TGP2. The plots show a reasonable agreement between the experimental data and simulation results.

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Fig. 7

Internal components of a representative foldable phone in fully unfolded, partially unfolded and fully folded states. The hot spot is close to the main processor referred to as “main processor”. The coldest side of display is the right edge of the display referred to as “OLED”. The distance between the major heat sources (processors) and the coldest side of the display is very long when the phone is fully unfolded. Therefore, this study focuses on the thermal management solutions in the fully unfolded state.

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Fig. 8

The case study on the thermal management of a potential foldable smartphone using 400 μm thick graphite sheet

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Fig. 9

Simulation results for two TGPs connected by a graphite bridge used for the thermal management of a foldable smartphone. Note that in this figure, the bridge covers each TGP. The covering length is shown as Lc = 5 mm.

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Fig. 10

Schematic diagram of the side view for two TGPs connected by a graphite bridge used for cooling of a foldable smartphone

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Fig. 11

The thermal resistance network diagram for a two-TGP configuration for thermal management of a foldable phone

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Fig. 12

Comparison between the resistance network model results and FEM simulation results

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Fig. 13

Temperature distribution across the surface of the two-TGP configuration with a graphite bridge. Each TGP has a width of w = 138.3 mm and length of Lv of 6.7 cm.

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Fig. 14

Simulation results for a large foldable TGP with a joint region used for thermal management of a foldable smartphone. Each bridge is 1 cm wide and the spacing between the bridges is 1 cm.

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Fig. 15

Simulation results for a large foldable TGP with a joint region used for thermal management of a foldable smartphone. Each bridge is 2 cm wide and the spacing between them is 2 cm.

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Fig. 16

Simulation results for a large foldable TGP with a joint region used for thermal management of a foldable smartphone. Each of the cavities in the torsional joint has a thickness of 3 mm and length of 15 mm.

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Fig. 17

Simulation results for a large foldable TGP with a joint region used for thermal management of a foldable smartphone. The torsional joints are inclined for enhanced mechanical performance.

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