Flexible Thermal Ground Planes with Thicknesses Below Quarter mm

Transcription

1 Flexible Thermal Ground Planes with Thicknesses Below Quarter mm Ryan Lewis, Ph.D. Shanshan Xu, Y.C. Lee, Ronggui Yang, Li Anne Liew University of Colorado at Boulder 1

2 Future smart phones are thin Thickness (mm) Future smartphones? how thick phones used to be 2

3 3mm flexible smartphone Flexible by design! Phones crack with lbs force tests iphone 6 bendgate/index.htm 3

4 Flexible 1 mm smartphone as future microsystem OLED; 0.05 mm Glass or Polymer Cover; 0.05 mm Chips (0.075mm) PCB TGP- Thermal Ground Plane (0.20 mm) Anode/Solid State Electrolyte (0.150mm) > 4X increased storage density New Cathode/Solid State Electrolyte (0.4 mm) Aluminum (0.05 mm) Polymer Case; mm Three enabling technologies being developed at CU Boulder Flexible thermal ground planes All solid state battery Atomic layer deposition 4

5 Air Gap? Alternative Needed! chip side Back side (i.e. skin surface) 2 W/(4mm x 4mm) Polymer Air Gap Copper Polymer 66 o C 200 μm Cu 200 μm air 62 o C Air Gap Polymer Copper Polymer 45 o C Natural convection on top and bottom sides with T air = 25 o C Total Thickness = 1 mm + Air Gap T air = 25 o C Temperature distribution on back surface, i.e. skin. 43 o C For thin electronics: Copper and Graphite + Air gap not good enough! 5

6 Vacuum-Enhanced Heat Spreader: Skin Temperatures Reduced Copper Heat Spreader: Lowest temperature: 34.8 C Highest temperature: 40.2 C Temperature difference: 5.4 C Copper or Graphite vs. Vacuum-Enhanced Heat Spreader 2.5 Watt 10cm x 5cm x 250 um Vacuum-Enhanced Copper Heat Spreader Lowest temperature:35.5 C Highest temperature:36.7 C Temperature difference: 1.2 C Graphite Heat Spreader: Lowest temperature: 35.4 C Highest temperature: 38.2 C Temperature difference: 2.8 C Vacuum-Enhanced Graphite Heat Spreader Lowest temperature: 35.6 C Highest temperature: 36.4 C Temperature difference: 0.8 C Note: this 2.8 o C could be increased substantially for 100um thin graphite.

7 Vacuum-Enhanced Heat Spreader: Junction Temperatures? Max. Junction Temperature ~ 50 o C Max. Junction Temperature ~ 90 o C Vacuum-Enhanced Heat Spreader Design A Vacuum-Enhanced Heat Spreader Design B Vacuum enhanced heat spreading: a short term solution that is always better than copper/graphite + air gap. Trade off analysis for an optimum design required. Best solution: flexible thermal ground planes. 7

8 Thermal Ground Plane 10 cm x 6 cm total area Printed Circuit Board substrate Built in vacuum insulator Latent heat of phase change for heat removal 8

9 Video 9

10 TGP and Copper 10

11 TGP working physics (2-D Heat Pipe or Vapor Chamber) Evaporator Region Vapor Core Condenser Region Heat Source Liquid Returned thru a Wicking Structure Heat Sink Sintered powder Micro pillars Woven mesh Grooved Channels Y. Sungtaek Ju et al. I. J. Heat Mass Transfer, 60, 2013, C. Ding et al. JMEMS 19, 4, 2010 M. Sigurdson, et al. I.J. Heat Mass C. Ding et al. Transfer, 62, 2013, JMEMS 19, IMAPS 4, 2010 Thermal

12 TGP and Graphite Flexible TGP Graphite In plane thermal conductivity 1,000 4,000 W/m K 400 1,500 W/m K Through plane thermal conductivity Integration with PCB 3cm x 3cm < W/m K 3 15 W/m K Can be part of PCB interconnects and vias Separate piece. PCB TGP Thermal vias 9.5 cm X 5 cm X 0.1cm IMAPS Thermal ATW

13 TGP Background at Colorado 1 mm thick, 20 cm x 5 cm footprint Copper (measured) K eff > 2,000 W/mK w/o thermal resistances across Kapton; K eff > 3,000 W/mK Flexible, manufacturable, but too thick, too high power 13

14 Quarter mm TGP Results Carrying Heat (heat pipe) thermal resistance(k/w) R=ΔT/Q out R cu ref R TGP input power(w) 1600 Effective Thermal Conductivity (W/m K) k eff =(R cu /R TGP )k Cu input power(w) 8mm 8mm 5cm 2.5cm 14

15 Vapor Core Design: Critical to Thin TGP Thermal resistance R e, top R e, vapor R a, top R a, vapor R c, top R c, vapor R e, mesh R e, bottom Heat Source R a, mesh R a, bottom R c, mesh R c, bottom Heat Sink vap Thick TGP R c,bottom R e,bottom Thin TGP R c,bottom R c,mesh R e,bottom R e,mesh R c,mesh R e,mesh R vap R vap 15

16 TGP Limited by 50um Vapor Core? Vapor Core Effective Thermal Conductivity: Fourier s Law k=q/(dt/dx) 1.E+06 1.E+05 Temperature Gradient proportional to pressure gradient dt/dx~dp/dx Pressure gradient proportional to flow rate, which is proportional to heat applied dp/dx~m~q NOT A DIRECT FUNCTION OF POWER k vap (W/m K) 1.E+04 1.E+03 1.E+02 1.E+01 1.E+00 1,500 W/m K 50 μm limit Copper Ref 20 μm limit to beat Cu Vapor Core Thickness k eff 16

17 TGP Limited by 50um Vapor Core? No! As a Heat Spreader Distributed condenser Natural Convection Heat in T1 T2 T3 Measurement of uniformity of skin temperature TGP= 5x lower ΔT than Cu at 6W Temperature difference ( C) Skin temperature difference Copper VS TGP 5 0 Cu TGP Input power (W)

18 TGP s Limit? Heat Source Heat Sink Overall Thickness 100 μm? 100 μm TGP limits model Heat Load (W) Working area Condensertemperature ( o C) 18

19 Summary Copper + Air gap not enough! Short term improvement via vacuum enabled heat spreaders: copper or graphite + vacuum Thermal ground planes! PCB substrate Built in insulator layers Phase change heat transfer <0.1mm TGP with K eff > 3,000 W/m K feasible. Quarter mm, demonstrated k eff =1,400 W/m K. Prototypes are to be distributed. 19

20 Acknowledgements TGP research at CU has been supported by: University of Colorado State of Colorado AIA: award number: Defense Advanced Research Projects Agency (DARPA) Thermal Ground Plane Program N C 2006 Intelligence Community This project was supported by a grant from the Intelligence Community Postdoctoral Research Fellowship Program through funding from the Office of the Director of National Intelligence. All statements of fact, opinion, or analysis expressed are those of the author and do not reflect the official positions or views of the Intelligence Community or any other U.S. Government agency. Nothing in the contents should be construed as asserting or implying U.S. Government authentication of information or Intelligence Community endorsement of the author s views. 20