Flexible Thermal Ground Planes with Thicknesses Below Quarter …kelvinthermal.com/content/uploads/TGP_IMAPS_2014_Oct... · 2015-03-02 · Graphite Heat Spreader: Lowest temperature:

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 [email protected]

1IMAPS ‐ Thermal 2014

Future smart phones are thin

http://pocketnow.com/2012/05/16/remember‐how‐thick‐phones‐used‐to‐be

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Thickness (mm)

Future smartphones?


3mm flexible smartphone

Phones crack with 90‐150 lbs force

http://www.consumerreports.org/cro/news/2014/09/consumer‐reports‐tests‐iphone‐6‐bendgate/index.htm

Flexible by design!


Flexible 1‐mm smartphone as future microsystem

Chips (0.075mm)OLED; 0.05 mm

Glass or Polymer Cover; 0.05 mm

Anode/Solid State Electrolyte (0.150mm)

New Cathode/Solid State Electrolyte (0.4 mm)

Aluminum (0.05 mm)Polymer Case; 0.025 mm

PCB TGP- Thermal Ground Plane (0.20 mm)

> 4X increased storage density

Three enabling technologies being developed at CU‐Boulder• Flexible thermal ground planes• All solid state battery• Atomic layer deposition


Air Gap? Alternative Needed!

66 oC

45 oCNatural convectionon top and bottom sideswith Tair = 25oC

Air Gap

2 W/(4mm x 4mm)Polymer

Copper

Polymer

Total Thickness = 1 mm + Air Gap

For thin electronics:•Copper and Graphite + Air‐gap not good enough!

Temperature distributionon back surface, i.e. skin.

62 oC

43 oCTair = 25oC

Air Gap Polymer

Copper

Polymer

chip‐side Back‐side (i.e. skin surface)

200 μm Cu200 μm air


Copper Heat Spreader:Lowest temperature: 34.8°CHighest temperature: 40.2°CTemperature difference: 5.4°C

Vacuum-Enhanced Heat Spreader: Skin Temperatures Reduced

10cm x 5cm x 250 umCopper or Graphite vs. Vacuum-Enhanced Heat Spreader

2.5 Watt

Graphite Heat Spreader:Lowest temperature: 35.4 °CHighest temperature: 38.2 °CTemperature difference: 2.8 °C

Note: this 2.8oC could be increased substantially for 100um thin graphite.

Vacuum-Enhanced Copper Heat SpreaderLowest temperature:35.5 °CHighest temperature:36.7 °CTemperature difference: 1.2°C

Vacuum-Enhanced Graphite Heat SpreaderLowest temperature: 35.6 °C Highest temperature: 36.4 °CTemperature difference: 0.8 °C

Vacuum-Enhanced Heat Spreader – Design A

Vacuum-Enhanced Heat Spreader: Junction Temperatures?

Vacuum-Enhanced Heat Spreader – Design B

Max. Junction Temperature ~ 90 oCMax. Junction Temperature ~ 50 oC

•Vacuum‐enhanced heat spreading: a short term solution that is alwaysbetter than copper/graphite + air gap.•Trade‐off analysis for an optimum design required.•Best solution: flexible thermal ground planes.


Thermal Ground Plane

• Printed Circuit Board substrate

• Built‐in “vacuum” insulator

• Latent heat of phase‐change for heat removal

10 cm x 6 cm total area


Video


TGP and Copper


TGP working physics(2-D Heat Pipe or Vapor Chamber)

Vapor Core

Heat Source Heat SinkLiquid Returned thrua Wicking Structure

Evaporator Region Condenser Region

Sintered powder Micro‐pillars Woven mesh Grooved Channels

Y. Sungtaek Ju et al. I. J. Heat Mass Transfer, 60, 2013, 163‐169

M. Sigurdson, et al. I.J. Heat Mass Transfer, 62, 2013, 178‐183

C. Ding et al. JMEMS 19, 4, 2010

C. Ding et al. JMEMS 19, 4, 2010 11IMAPS ‐ Thermal 2014

TGP and GraphiteFlexible TGP Graphite

In‐plane thermal conductivity 1,000 – 4,000W/m‐K 400 ‐ 1,500 W/m‐K

Through‐plane thermal conductivity

<0.03‐100 W/m‐K 3 – 15 W/m‐K

Integration with PCB Can be part of PCB interconnects and vias

Separate piece.

12IMAPS – Thermal ATW 2014

Thermal vias

PCB‐TGP

3cm x 3cm

9.5 cm X 5 cm X 0.1cm

TGP Background at Colorado

Copper (measured)

w/o thermal resistances across Kapton; Keff > 3,000 W/mK

Keff > 2,000 W/mK

1‐mm thick, 20 cm x 5 cm footprint

•Flexible, manufacturable, but too thick, too high power 13IMAPS ‐ Thermal 2014

Quarter‐mm TGP Results• Carrying Heat (heat‐pipe)

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Effective Thermal

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uctiv

ity (W

/m‐K)

input power(W)

keff=(Rcu/RTGP)kCu

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thermal re

sistance(K/W)

input power(W)

Rcu ref

RTGPR=ΔT/Qout

8 mm 8mm

2.5cm

5cm


Vapor Core Design: Critical to Thin TGP

• Thermal resistance

Heat Source Heat Sink

Ra, vapor

Ra, meshRa, bottom

Ra, top

Rc, meshRc, bottom

Rc, vaporRc, top

Re, meshRe, bottom

Re, vaporRe, top

Rvap

Re,mesh

Re,bottomRc,bottomRc,mesh

Rvap

Re,mesh

Re,bottom

Rc,mesh

Rc,bottom

Thick TGP Thin TGP

vap


TGP Limited by 50um Vapor Core?• Vapor Core Effective Thermal

Conductivity:Fourier’s Law k=Q/(dT/dx)

‐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

keff

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1 10 100 1000

k vap(W

/m‐K)

Vapor Core Thickness

Copper Ref 20 μm limit to “beat” Cu

1,500 W/m‐K 50 μm limit


TGP Limited by 50um Vapor Core? No!

• As a Heat Spreader– Distributed condenser

• Measurement of uniformity of skin temperature

• TGP= 5x lower ΔT than Cu at 6W

Heat in

Natural Convection

T1

T2

T3

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rature differen

ce (°C)

Input power (W)

Skin temperature difference – Copper VS TGP

TGP

Cu


TGP’s Limit?

Heat Source Heat Sink

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Heat Loa

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Condensertemperature (oC)

Working area

Overall Thickness100 μm?

100 μm TGP limits model


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 Keff > 3,000 W/m‐K feasible.

• Quarter‐mm, demonstrated keff=1,400 W/m‐K.Prototypes are to be distributed.


Acknowledgements

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.

• TGP research at CU has been supported by:– University of Colorado– State of Colorado AIA:

award number: 2015‐3212– Defense Advanced Research Projects Agency

(DARPA)Thermal Ground Plane Program N6601‐08‐C‐2006

– Intelligence Community

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Documents

Flexible Thermal Ground Planes with Thicknesses Below Quarter …kelvinthermal.com/content/uploads/TGP_IMAPS_2014_Oct... · 2015-03-02 · Graphite Heat Spreader: Lowest temperature: