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Thermal Management of Flexible Electronic

Systems

(what are the unique thermal/mechanical design features

of a flexible electronic system)

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Modes of Heat Transfer in Electronic Systems

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•Conduction (relatively simple analysis, will introduce today)

•Convection (mostly empirical and numerical)•Radiation (often negligible

at low operating temperatures)•Multimode (combinations of the above modes)

Modes of heat transfer

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Classification of levels of packaging…why ???

Thermal

Mechanical

Electrical

Reliability

Functional

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Thermal aspects

First level….conduction heat transfer

Second level….convection and conduction

System level….thermodynamic considerations,

overall system balance, application

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•The design of a system should not be a set of individual requirements such as thermal, mechanical, electrical and functional. There should be one design that is optimal and meets all of the requirements.

•Case in point thermal and mechanical requirements.

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•Thermal and mechanical requirements are often conflicting.

•For example a large Cu heat sink is likely to adversely impact interconnect reliability (fatigue, shock, vibration)

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•Interconnects are vulnerable failure points in all systems

•Chip level interconnects are the most vulnerable (size and materials)

•This has a direct impact on thermal management and the mechanical design of the system

•Flexible electronics offer an advantage

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•Conduction (relatively simple analysis, will introduce today)

•Convection (mostly empirical and numerical)•Radiation (often negligible

at low operating temperatures)•Multimode (combinations of the above modes)

Modes of heat transfer

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Conduction

Heat transfer mode in solids and stationary fluids. Heat is transferred via random molecular interactions.

In gases: random molecular collisions (translational + vibration + rotational components)

In solids: combination of free electron transport and latticevibrations

In liquids: similar to gases but closer bonding between molecules

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ConductionRate of heat transfer: Fourier’s “Law” (isotropic material):

dxdTkqTkq −=∇−= "" ;~~ 1-D conduction

Temperature predictions: Heat diffusion equation(Fourier Law + energy balance)

tT

kc

kq

zT

yT

xT

∂∂

=+∂∂

+∂∂

+∂∂ ρ&

2

2

2

2

2

2 Rectangular coordinates;k is constant

• Need 6 boundary conditions + 1 initial condition

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Concept of Thermal Resistance

0=q&

02

2

=dx

Td

( ) 112 TLxTTT +−=

LkTTq )( 21

" −=

TLkAAqq ∆⎟

⎠⎞

⎜⎝⎛== "

Voltage (V)

Analogous to V= IR

or

or

L

•

•T1

T2q ′′

x

• Steadystate:

Current (I)

kALqTRth /=

∆=

Slab:

⎟⎟⎠

⎞⎜⎜⎝

⎛=

1

2ln2

1rr

LkRth π

⎟⎟⎠

⎞⎜⎜⎝

⎛−=

21

1141

rrkRth π

Cylindrical shell:

Spherical shell:

Assuming:• 1-D

• No internal heatgeneration:

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Concept of Thermal Resistance

In electronic packaging, a single chip package contains manymaterials. An overall resistance is defined:

qTT

R cjjc

−= : Internal thermal resistance

Junction temperature Case temperature

Typical values: 80 K/W: plastic package, no spreader

12 -20 K/W: plastic package with spreader

5 - 10 K/W: ceramic package

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•Conduction (relatively simple analysis, will introduce today)

•Convection (mostly empirical and numerical)•Radiation (often negligible

at low operating temperatures)•Multimode (combinations of the above modes)

Modes of heat transfer

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Thermal management in a second level packageGoverning Equations for fluids in motion:

Assuming the flow to be steady with constant properties except for the thermal conductivity in general and the density in the buoyancy term specifically, and neglecting viscous dissipation, the governing equations are,:

Continuity (mass conservation):

∇. V = 0

Momentum conservation:

ρ (V.∇)V= - ∇p + µ∇2V+ ρg β(T-T∞)i

Energy conservation:

ρcp (V. ∇)T = ∇.(k∇T)

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•Navier Stokes equations fairly complex to solve

•Most practical applications require a numerical solution

•Several commercial codes are available, provide good design tools

•It is possible to solve conjugate conduction/convection./radiation problems

Thermal management in a second level package…cont.

FLOTHERM (TM)

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diffuser ducts

computer equipment racks

return vents for hot exhaust air(on parallel walls)

cold aisles(chilled air supply)

13.42 m long

6.05 m wide

diffuser ducts

computer equipment racks

return vents for hot exhaust air(on parallel walls)

cold aisles(chilled air supply)

13.42 m long

6.05 m wide

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z=0.8m

z=6.5m

z=12m

z=16m

y

x

z

z=20m

Most Racks ~19 kW

z=0.8m

z=6.5m

z=12m

z=16m

y

x

z

y

x

z

z=20m

Most Racks ~19 kW

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On the other hand there are very interesting small scale problems at the micro level; Example of micro-channel

cooling

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Channels in Perpendicular DirectionTemperature Distribution

Fluid flow direction

Fluid f

low di

rectio

n

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Temperature at various cross-sections along the length

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Variation of Maximum Temperature with Velocity(Single Channel with Adiabatic BCs)

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 3.6 4.00

20

40

60

80

100

120

140

160

Chi^2/DoF = 2.77172R^2 = 0.99924 y0 31.89508 ±1.50026A1 224.53191 ±16.14905t1 0.03871 ±0.00338A2 41.0406 ±3.37751t2 0.62888 ±0.12918

y0 + A1e^(-x/t1) + A2e^(-x/t2)

Simulated Data Second order exponential decay

Tem

pera

ture

(o C)

Velocity (m/s)

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Variation of Pressure Drop with Velocity(Single Channel with Adiabatic BCs)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0

4000

8000

12000

16000

20000

24000

28000P

ress

ure

(Pa)

Velocity (m/s)

B Polynomial Fit of Data2_B

Y = A + B1*X + B2*X^2Parameter Value Error------------------------------------------------------------A -30.44094 18.76507B1 4372.04107 37.73392B2 630.1282 9.51187

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•Conduction (relatively simple analysis, will introduce today)

•Convection (mostly empirical and numerical)•Radiation (often negligible

at low operating temperatures)•Multimode (combinations of the above modes)

Modes of heat transfer

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Example using thermal grease as the interface material

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Thermal grease in a ceramic single chip package

Without grease

Rint = 10 C/WWith grease

Rint = 3.5 C/W

Why not use adhesive ?

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AirAir

Air

Rcap-air

Rcard-air1

Rcard-air2

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Resistance Equation Value oC/W

Spreading in Si Rj-c = ln(ro-ri)/2 π kt 0.6

Through a solder joint Rc4 = l/kA (Assume all solder joints conduct in parallel)

1.1

Spreading in the substrate Rsubstrate= ln(ro-ri)/2 π kt 6.4

Through a copper pin Rpin = l/kA 0.0

Spreading in the card Rcard= ln(ro-ri)/2π kt 1.8

Convection to air Rcap-air = 1/hA 5.0

Convection to air Rcard-air1= 1/hA 30.0

without grease

Rint = 10.7; Rext = 4.8

Rtotal = 15

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AirAir

Air

Rcap-air

Rcard-air1

Rcard-air2

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Resistance Equation Value oC/W

Thermal grease resistance Rgrease = l/kA 2 to 4

With grease

Rint = 2.5; Rext = 5.2

Rtotal = 7.7

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Example using an adhesive as a thermal interface material

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CHIP

ENCAPSULANT

HEAT SINKTHERMALADHESIVE

STIFFENER

BGA C4s

PRINTED CIRCUIT BOARD

ORGANIC SUBSTRATEPower planes

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Heat sink resistance (top) Rhs

Adhesive resistance Radh

Chip(heat source) Rchip

C4 resistance Rc4Substrate resistance Rsubst.

BGA resistance RBGA

Card spreading resistance Rspr

Heat sink resistance (legs) Rhsl

Convective resistance or external resistance REXT

Air

Air

Air

Rint = 0.5 to 1.5 C/W

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Heat sink resistance (top) Rhs

Chip(heat source) Rchip

C4 resistance Rc4Substrate resistance Rsubst.BGA resistance RBGA

Card spreading resistance Rspr

Heat sink resistance (legs) Rhsl

Convective resistance or external resistance REXT

Air

Air

Air

Rint = 3 to 9 C/W

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Thermal Resistances of Electronic Packages

• A number of resistances are used, often poorly defined.

Junction-to-case:

Case-to-ambient:

Junction-to-ambient:

Problems: Measurement location, substrate conduction, ambient fluid properties, flow regime, accounting of systemboundaries.

RT T

qjcj c=−( )

R T Tqca

c a= −( )

RT T

qjaj a=−( )

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Other resistances that need to be added:• Interfaces (interfacial voids and defects)

• Spreading (bottlenecks)

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Contact Resistance

T1

T2

∆T

xct q

TR ∆=,

(K/W)

xct q

TR′′

∆=′′, (m2K/W)

ctR ,′′ : available from tables

Depends upon: contact surface materials,interface pressure, interfacial material

xq

Line ofcontact

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Conduction Spreading Resistance

Near the chip, the conduction is 2 or 3-D. Need to solve heat diffusion equation exactly to get Rth. For many cases this can be done in closed form (if chip approximated as uniform heat flux circular source)

W

rab r r

z z z(a) (b) (c)

akHRth π

= ⎟⎠⎞

⎜⎝⎛

bw

baH , Available from charts

adiabatic boundary

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Thermal and Mechanical designs are intertwined

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• During every thermal cycle interconnections must ‘absorb’ the CTE mismatch

• They must be designed to survive (at an acceptable failure rate) through end of life of the product

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Underfill or encapsulation

Converts some of the shear loading to bending

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• Chip not mechanically coupled(strongly) to chip carrier

+

• Long slender wire bonds = excellent fatigue life

Soft interface

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Thermal grease provides a flexible heat transfer path, (phase change, low modulus materials, etc…)

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0

20

40

60

80

Inva

r

Cu

Inva

rC

u

Kov

ar Fe Al

Fr-4

InvarSiliconCu Invar CuAl203KovarQuartzFeCuAlsolderFr-4Adhesives

CTE value for different packaging materials, PPM/Degree K.

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At room temperature

At elevated

temperature

At room temperature

Fundamentals of Electronics Packaging; B. Sammakia

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Additional examples from actual systems

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Chapter 12 Figure 3b

11.85.33.7Module heat flux (W/cm2)

643319Chip heat flux (W/cm2)

1.87.711.6Rint (oC/W)

2000600300Max module power (W)

2774Max chip power (W)

199019851980Year

ES9000ES/30903081IBM system

Table 1. Showing the relative incremental improvements made to the TCM, from Simons (1995).

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Nozzle

Incoming air

Impingement

heat sink

Ceramic cap

Ceramicsubstrate

Thermalgrease

Chapter 12 Figure 2 Schematic diagram of the IBM 4381

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fluorocarbon liquid, FC-77, under single phase forced convection conditions. The flow velocity is about 1 m/s over the single chip packages.

Chapter 12 Figure 8 Cray 2 system

fluorocarbon liquid, FC-77

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Bellows provides mechanical spring

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Flex provides mechanical spring

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•In all of the designs shown here there is a flexible part of thesystem designed to protect the interconnects at the chip level.

•Examples: pistons, bellows and springs (weakly coupled to the chip)

•Grease: allows the chip and heat-sink to expand independently

•Flexible chip carrier: the chip carrier deforms and reduces the stresses on the interconnect

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Gas gap/Spring/Thermal grease/Oil

Wire bond/TAB/Solder Bumps with Under fill

High modulus adhesive

Metal heat sink

Metal heat sink

Substrate

Solder interconnections

(no underfill)

IBM TCM (308X,309X,ES9000)

IBM 4381

Hitachi M680

Mitsubishi high thermal conduction module

Fujitsu bellows system

VAX 9000

NECSX3

Substrate

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Uses Bellows and water impingement cooling0.56 total

304600Fujitsu VP 2000

Uses direct liquid immersion cooling. 600-700Cray 2

Chips backbonded to heat spreader using diamond paste. TAB bonding reduced mechanical stresses.

30 at chip level

VAX 9000

Liquid cooling utilizing micro channels in substrate

0.52.88.44.215.1377NTT

Water cooling, copper pistons touching chips.272000IBM ES 9000

Water cooling, aluminum pistons touching chips.1.57.20.42.27500IBM 3090

Air cooling, impingement heat sink. Ceramic cap and thermal grease reduce stress

890.52.23.890IBM 4381

Water cooling, studs touching chips.Total5

0.31.6>5.4250NEC SX LCM

Water cooled.Total60

0.20.9>0.560Honeywell SLIC

Air cooling, CTE matching by using SiC heat spreader

24.610.10.50.816Hitachi RAM

Thin air gap between cap and heat spreader on the chip. This reduces the stresses.

4.330.40.83436Mitsubishi HTCM

RextRintQ”’Q”QP

Thermal and Mechanical design featuresThermal Characteristics Technology

Where P = total module power, W Q = Max chip power, W Q” = heat flux, W/cm2 Q”’ = Heat density, W/cm3

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Future flexible electronic systems that are completely integrated can be

‘wrapped’ around heat-sinks and cold plates for maximum thermal

performance and reliability

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References:

•Allen, E., Donohue, B. and Pei, H. (1987), “Cooling the VAX 8000 Family of Computers - An Overview,” Proc. of the Technical Conf., Seventh Annual Intl. Electronic Packaging Society Conf., v. 2, held in Boston, MA, pp. 712-717.•Bar-Cohen, A. (1987), “Thermal Management of Air- and Liquid Cooled Multi-Chip Modules,” IEEE Trans. Components, Hybrids and Manufacturing Technology, v. CHMT-10, no. 2, pp. 159-175. •Bar-Cohen, A. (1988), “An Addendum and Correction to Thermal Management of Air- and Liquid-Cooled Multi-Chip Modules,” IEEE Trans. Components, Hybrids and Manufacturing Technology, v. CHMT-11, no. 3, pp. 333-334.•Biskeborn, R.G., Horvath, J.L. and Hultmark, E.B. (1984), “Integral Cap Heat Sink Assembly for the IBM 4381 Processor,” Proc. of the Technical Conf., Fourth Annual Intl. Electronic Packaging Society Conf., held in Baltimore, MD (October 29-31, 1984), pp. 468-474.•Carlson, D.M., (1994), “Cray J90 Computer System Air-Cooled Supercomputer Packaging,” Proc. of the Technical Program, 1994 Intl. Electronics Packaging Conf., Atlanta, GA (September 25-28, 1999), pp. 708-721.•Chu, R.C., Hwang, U.P. and Simons, R.E. (1982), “Conduction Cooling for an LSI Package - A One-Dimensional Approach,” IBM J. of Research and Development, v. 26, no. 1, pp. 45-54.•Cray, S. R. (1986), “Immersion Cooled High Density Electronic Assembly,” U. S. Patent 4,590,538 (May, 1986).•Danielson, R.D., Krajewski, N. and Brost, J. (1986), “Cooling a Superfast Computer,” Electronic Packaging and Production, (July, 1986), pp. 44-45.•Emoto, Y, Tsuchiya, M., Ogiwara, S., Sasaki, T., Kobayashi, F.and Otsuka, K. (1986), “A High Performance Package Approach on 1st Level Packaging For Main Frame,” Proc. of the 36th Electronic Components Conf., Organized by the IEEE Components, Hybrids, and Manufacturing Technology, held in Seattle, WA (May 5-7, 1986), pp. 564-570.•.‘National Technology Roadmap for Semiconductors: Technology needs’, 1999, http://notes.sematech.org/PublNTRS.nsf/home.htm .•.”The Electronics Industry Report”, Prismark partners annual report, 1998-99.

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•Fitch, J.S. (1990), “A One-Dimensional Thermal Model for the VAX 9000 Multi-Chip Units,” in Thermal Modeling and Design of Electronic Systems and Devices, R. A. Wirtz and G. L. Lehman, (editors), presented at the ASME Winter Annual Meeting, Dallas, TX (November 25-30, 1990), American Society of Mechanical Engineers, New York, NY, HTD-Vol. 153, pp. 59-64.•Gani, L., Graf, M.C., Rizzolo, R.F. and Washburn, W.F. (1991), “IBM Enterprise System/9000 Type 9121 Model 320 Air-Cooled Processor Technology,” IBM J. of Research and Development, v. 35, no. 3, pp. 342-351.•Hopper, S., Edwards, D.L. and Young, S.P. (1992), “The Thermal Management of IBM's ES/9000 Advanced Thermal Conduction Module,” Proc. of the 42nd Electronic Components and Technogy Conf., Organized by IEEE Soc. on Components, Hybrids and Manufacturing Technology, held in San Diego, CA (May 18-20, 1992), pp. 997-1001.•Kaneko, K. Seyama, K. and Suzuki, M. (1990), “LSI Packaging and Cooling Technologies for Fujitsu VP2000 Series,”, Fujitsu Scientific & Technology J., v. 41, no. 1, pp. 12-19.•Kalita, B. and English, W. (1985), “Cooling the VAX 8600 Processor,” Digital Technical Journal, no. 1, (August 1985).•Kaneko, K., Kuwabara, K., Kikuchi, S. and Kano, T. (1991), “Hardware Technology for Fujitsu VP2000 Series,” Fujitsu Scientific & Technology J., v. 37, no. 2, pp. 158-168.•Knickerbocker, J.U., Leung, G.B., Miller, W.R., Young, S.P., Sands, S.A. and Indyk, R.F. (1991), “IBM System/390 Air-Cooled Alumina Thermal Conduction Module,” IBM J. of Research and Development, v. 35, no. 3, pp. 330-341.•Kobayashi, F., Anzai, A., Yamada, M., Takahashi, A., Yamazaki, S. and Toda, G.,(1986), “Packaging Technologies for the Ultrahigh-Speed Processor Hitachi M-680 H/M-682 H,” Proc. of the 36th Electronic Components Conf., Organized by the IEEE Components, Hybrids, and Manufacturing Technology, held in Seattle, WA (May 5-7, 1986), pp. 571-577.•Kohara, M., Nakao, S, Tsutsumi, K., Shibata, H., and Nakata, H. (1983), “High thermal conduction package technology for for flip chip devices,” IEEE Trans. Components, Hybrids, Manuf. Technology, Vol. CHMT-6, pp. 267-271.•Lyman, (1982), “Special Report - Supercomputers Demand Innovation in Packaging and Cooling,” Electronics, (September 22, 1982), pp. 136-142.•McElroy, J. (1984), “Packaging of a High Performance VAX,” Proc. of the Intl. Electronic Packaging Soc. Conf.,•McElroy, J. (1985), “Packaging the VAX8600 Processor,” Digital Technical Journal, no. 1, (August 1985).•McPhee, M., O'Toole, T.S. and Yedvabny, M. (1990), “Cooling The VAX 9000,” Electro/90, Conference Record, pp. 288-292, Boston, MA (May 9-11, 1990).•Mizuno, T., Okano,M., Matsuo, Y. and Watari, T. (1987), “Cooling Technology for the NEC SX Supercomputers,” Proc. of the Intl. Symposium on Cooling Technology for Electronic Equipment, Hawaii, pp. 110-125.

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•Okutani, K., Otsuka, K., Sahara, K. and Satoh, K. (1984), “Packaging Design of a SiC Ceramic Multi-Chip RAM Module,”Proc. of the Technical Conf., Fourth Annual Intl. Electronic Packaging Society Conf., held in Baltimore, MD (October 29-31, 1984), pp. 299-304.•Pei, J., Heng, S., Charlantini, R., and Gildea, P. (1990), “Cooling Components Used in VAX 9000 Family of Computers,”Proc. of the Technical Conf. 1990 Intl. Electronics Packaging Conf., (Marlborough, MA, September 10-12, 1990), pp. 587-601.•Sechler, R. F. (1993), “RISC System/6000 Central Processing Unit,” Proc. of the Intl. Conf. & Exhibition on MultichipModules, (Denver, CO 14-16 April, 1993), SPIE vol. 1986, pp. 22-27.•Simons, Robert, E. (1995), “The evolution of IBM high performance cooling technology,” IEEE Transactions on Components, Packaging and Manufacturing Technology-Part A, Vol. 18, No. 4. •Watari, T. and Murano, H. (1985), “Packaging Technology for NEC SX Supercomputer,” Proc. of the 1985 IEEE Electronic Component Conference, organized by IEEE Components, Hybrids, and Manufacturing Technology, pp. 192-198.•Werbizky, G.G. and Haining, F.W. (1985), “Circuit packaging for large scale integration”, in Proc. IEEE Electronic Component Conf., pp. 187-191.•Yamamoto, Udagawa, Y. and Okada, T. (1986), “Cooling and Packaging for FACOM M-780,” Fujitsu, v. 37, no. 2, pp. 124-134.•Yamamoto, Udagawa, Y. and Suzuki, M. (1987), “Cooling System for FACOM M-780 Large-Scale Computer,” Proc. of the Intl. Symposium on Cooling Technology for Electronic Equipment, Hawaii, pp. 110-125.•.T.M.Niu, Bahgat Sammakia and Sanjeev Sathe “Void effect modeling of flip chip encapsulation on ceramic substrates”, Transactions of the ASME, IMECE 1997, Seattle WA•.P. Totta, S. Khadpe, N. Koopman, T. Reilly, M. Sheaffer, “Chip to Package Interconnections,” Chap. 8 of Microelectronics Packaging Handbook, Part II, ed. Rao Tummala, Eugene J. Rymaszewski, Alan G. Klopfenstein, 2nd ed., Chapman & Hall 1997.•.D. Suryanarayana, R. Hsiao, T.P. Gall, J.M. McCreary, “Flip-Chip Solder Bump Fatigue Enhanced by Polymer Encapsulation,” IEEE Trans. of Components and Hybrid Manufacturing Technology, Vol. 14, No. 1, 1991, p. 218.•.G. Moore, “Cramming More Components onto Integrated Circuits,” Electronics Magazine, Vol. 38, No. 8, April 19, 1965, p. 114.•.Schaller, R.R. “Moore’s Law: Past, Present and Future,” IEEE Spectrum, Vol. 34, No. 6, June 1997, p. 52.•.Chu, R. C. (1986), "Heat Transfer in Electronic Systems," Heat Transfer - 1986, Proc. of the 8th Intl. Heat Transfer Conf., held in San Francisco, Hemisphere Publishing Co., Washington, DC, v. I, pp. 293-305.•.Bar-Cohen, (1987), "Thermal Management of Air- and Liquid Cooled Multi-Chip Modules," IEEE Trans. Components, Hybrids and Manufacturing Technology, v. CHMT-10, no. 2, pp. 159-175.

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•.Chu, R. C. and R. E. Simons, (1990), "Review of Thermal Design for Multi-Chip Modules," Proc. of the Technical Program, pp. 1633-1642, NEPCON-WEST, National Electronic Packaging and Production Conf., Anaheim, CA (February 26 - March 1, 1990).•Pei, J., Heng, S., Charlantini, R., and Gildea, P., (1990), “Cooling Components Used in VAX 9000 Family of Computers,” Proc. of the Technical Conf. 1990 Intl. Electronics Packaging Conf., (Marlborough, MA, September 10-12, 1990), pp. 587-601.•Oktay, S. and Kammerer, H.C. (1982), “A Conduction Cooled Module for High Performance LSI Devices,” IBM J. of Research and Development, v. 26, no. 1, pp. 55-66.•Oktay, S., Dessauer, B. and Horvath, J.L. (1983), “New Internal and External Cooling Enhancements for the Air-Cooled IBM 4381 Module,” ICCD '83, Proc. of the IEEE Intl. Conf. on Computer Design: VLSI in Computers, held in Port Chester, NJ (November 1, 1983). •Han and Guo,"Thermal Deformation Analysis of Various Electronic Packaging Products by Moire and Microscopic MoireInterferometry," Journal of Electronic packaging, Transaction of the ASME,Vol. 117, pp. 185-191, 1995. •Guo, Lim, Chen and Woychik,"Solder Ball Connect (SBC) Assemblies under Thermal Loading: I. Deformation Measurement via Moire Interferometry, and Its Interpretation," IBM Journal of Research and Development, Vol. 37, No. 5, pp. 635-648, 1993. Choi, Guo, LaFontaine, and Lim, "Solder Ball Connect (SBC) Assemblies under Thermal Loading: II. Strain Analysis via Image,Processing, and Reliability Considerations," IBM Journal of Research and Development, Vol. 37, No. 5,pp. 649-659, 1993. •Y. Joshi, “Thermal management in electronic packages”, professional course presented at ITherm 2002, San Diego.