Micro channel cooling

8/13/2019 Micro channel cooling

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ABSTRACT

Micro channel cooling is proposed for cooling of the high heat flux electronics. Generally, for

high heat flux components liquid cooling is preferable cooling method. In these cases, the

dominant thermal resistance is in the fluid. This is due to low thermal conductivity of fluids.

Silicon has been used extensively as a heat sin material because of its compatibility to

silicon integrated circuits, its high thermal conductivity, and the ease with which it can be used

in fabricating!high aspect!ratio channels. Silicon heat collector has a dense array of

microchannels etched into the surface that wor to transfer heat energy to a fluid which is

circulated through the entire pacage. That heat energy is then pumped to a nearby radiator

where it can be dispersed into the surrounding atmosphere with the aid of a fan, or throughpassive convection cooling.

Micro!channel heat!sins represent the most compact and efficient method of

transferring heat from a power source to a fluid. "y minimi#ing the si#e of the slow!moving

fluid boundary layer and increasing the area of contact between the heat sin fins and the fluid,

the microchannel heat sin removes heat $% times more efficiently than conventional methods

lie liquid!cooled cold plates. &arge heat flux from the component base can be removed with a

much smaller '()$%* surface temperature rise.

+eat transfer coefficients in a microchannel can be very high due to the thinner

boundary layer. +owever, the high!pressure drop associated with microchannel flow prevents

it to be employed in a micro!fluid loop with micro!pump due to the pumping power limit.

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1.INTRODUCTION:

Micro!channel cooling is an effective method to enhance cooling for electronic devices.

The problem of boundary layer development as a liquid coolant travels downstream persists in

convection micro!channel heat sin.

ig (.% - ooling system

1.1 Invention- The use of micro!channel as a visible cooling system was proposed by

T/012 M34 5 613S1 'G12M34 14GI4112S* . They designed cooled hat sin by

etching micro channel heat sin with $%um wide and 7%%um height on a silicon substrate.

luid flow inside channel is at the heart of many natural and man!made systems. +eat

and mass transfer is accomplished across the channel walls in biological systems, such as

brain, lungs, idneys, intestines, blood vessels etc.. as well as in man!made systems, such as

heat exchanges, nuclear reactions, air separation units, desalination etc8.

In general, the transport process occur across the channels walls where as bul flowtaes place through the channel of cross!sectional area. The channel cross section thus serves

as a conduct to transport fluid to and away from channel walls.

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2.HEAT SINK-

In electronic systems, a heat sinkis a passive heat exchanger component that cools a

device by dissipating heat into the surrounding air. In computers, heat sins are used to cool

central processing units or graphic processors. +eat sins are used with high!powersemiconductor devices such as power transistors and optoelectronic devices such as lasers and

light emitting diodes '&19s*, wherever the heat dissipation ability of the basic device pacage

is insufficient to control its temperature.

3 heat sin is designed to increase the surface area in contact with the cooling medium

surrounding it, such as the air. 3pproach air velocity, choice of material, fin 'or other

protrusion* design and surface treatment are some of the factors which affect the thermal

performance of a heat sin. +eat sin attachment methods and thermal interface materials also

affect the eventual die temperature of the integrated circuit. Thermal adhesive or thermal

grease fills the air gap between the heat sin and device to improve its thermal performance.

Theoretical, experimental and numerical methods can be used to determine a heat sin:s

thermal performance.

ig ;.% - 3 finned heatsin and fan clipped onto a microprocessor, with a smaller heatsin

without fan in the bacground.

2.1 OBJECTIVES:

3 channel serves to accomplish two ob


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ii* "ring fluid to the wall and remove fluid away from the walls as the transport

process is accomplished.

The rate of transport process depends on surface area, which varies with the diameter 9 for a

circular tube, where as flow rate depend on cross sectional area. In human body, the head and

mass transfer occurs inside lung and idney=s with flow channels approaching capillary

dimensions of around >um.

Mini channel 7mm?@ 9?;%% um

Micro channel ;%% um ?@ 9 ? (% um

Transitional micro channel (% um?@ 9 ? ( um

+eat transfer and fluid flow in mini channel and micro channel

a* Single!phase gas flow in micro channels

b* Single!phase liquid flow in micro channels

c* Single!phase electro!inetic flow in micro channels

d* low boiling in micro!channels

e* ondensation in micro!channels

f* "iomedical applications of micro!channel flows.

2.2 T!es o" #ICRO$CHANNE% &oo'in( sste)*s-

a+ Sin('e !hase 'i,-i "'o/ in )i&0o$&hanne' -

i* Micro!pumps, micro!valves, micro!sensors and analy#er of biological materials.

ii* +eat removal system, cooling of mirrors in high power laser system.

iii* 3dvances in biometric and genetic engineering require controlled fluid transport

and control in passages of several micrometers.

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Aater is treated as continuous media.

eo)et0 o!ti)iation- +eat dissipation rate

low rate

6ressure drop

luid temperature rise

luid in to surface temperature difference

3+ Sin('e !hase E'e&t0o$kineti& "'o/ in )i&0o$&hanne's :

1lectro!inetic flow system is used in lab!on!a chip devices. The lab!on!a!chip devicesare miniature bio!medical or chemistry laboratory on a small glass or plastic chip.

&ab!on!a!chip has a networ of micro!channels, electrodes sensors and electrical

circuits. 1lectrodes are placed at strategic locations on the chip. 3pplying electric field along

micro!channel control the liquid flows and other operations are done on a chip.

The ey microfluidic functions required in various lab!on!chip devices include pumping,

mining, thermal cycling, dispensing and separating most of these processes are electromagnetic

processes.

&+ 4'o/ 3oi'in( in )i&0o$&hanne' :

6rocess taes places in compact evaporator applications. 3utomative air conditioning

evaporators use small passage with flute function heat exchanges. 1xtruded channel with

passage diameters smaller than (nm being applied in compact condensor application.

low boiling is pursued in heat removal from high heat flex devices 'computer chip*,

laser diodes and components.

onsiderations -!

i* +igh heat transfer coefficient during flow boiling.

ii* +igher heat removal capability for given mass flow rate of coolant.

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Avanta(es- ! 3bility of fluid to carry large amount of thermal energy through latent heat of

vapouri#ation.

+ Sin('e !hase (as "'o/:

In this type of micro!channel cooling system , the gas flow is of viscous type and the gas was

compressed. 2eynolds number and mach numbers were used in this type of flow system. The

wall effects and gas flow regimes were used.

e+ 4'o/ 3oi'in( in )i&0o$&hanne's:

This process taes place incompact evaporator applications. 3utomotive air conditioning

evaporators use small passage with plate fin heat exchanger. 1xtruded channels with passage

diameter smaller than (mm being applied in compact condenser application.

low boiling is pursued in heat removal from high heat flux devices 'computer chips, laser

diodes and components*.

2.5 Consie0ations:

(* high heat transfer coefficient during flow boiling.

;* +igher heat removal capability for given mass flow rate of coolant.

Avanta(es:

3bility of fluid to carry large amount of thermal energy latent heat of vapori#ation.

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2.6Bio$)ei&a' a!!'i&ation:

i*Transport and manipulation of living cells and biological macromolecules place increasinglycritical demand on maintaining system conditions with acceptable ranges.

ii*Micro!channel geometry used in changing the temperature of small liquid volumes in 943

chains.

iii*oncentration of solutes, nutrients, gases metabolic products are maintained within

specified tolerances to ensure cell proliferation with bio!reactors of micro!channel cooling

system.

onsequently, convective heat transfer performance of a heat sin deteriorates in the

direction resulting in elevated maximum temperature and significient temperature gradient

across heat sin.

Heat sink- 3 passive heat exchange component that cools a device by dissipating heat into

surrounding air.

+eat sin is used with high power semi conductors devices 'power transistors,optoelectronic devices'lasers, light emitting devices'#ed*.

2.7 T!es o" "'o/s in )i&0o$&hanne' &oo'in( sste):

(* %a)ina0 "'o/ th0o-(h )i&0o$&hanne's-

The fluid flows in the channel in parallel layers of channels with no disruptions between layersand channels. The liquid tends to flow without mixing in the channels.

/se - micro!scale cooling system

a* or circular micro!pipes and convection ducts.

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ig ;.$'a* - &aminar flow through micro!channels

;* Inte(0ate )i&o$&hanne' &oo'in( "o0 5D e'e&t0oni& &i0&-it a0&hite&t-0e

In this type of micro!channels the fluid flows in all layers of the chip. These devices

are in two or more than two layers.. The di!electric liquid flows in all direction covering

all channel=s of the sin.

;.$ 'b* - Integrated micro!channel cooling

5+ #i&0o$Channe' Coo'in( "o0 hi(h !o/e0 se)i &on-&to0 evi&es:

In this type of channel cooling system used in semi!conductor devices lie transistors, bi!polar

chips. This system is used in linear operations of electronic equipments.

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ig ;.$'c* - Micro!channel cooling for high power semiconductor devices

Micro!scale pumping technologies for micro!channel cooling systems

(%%@ h '$%* 'T(!T;*

T!e o" )eta's -se: si'i&on8 a''-)ini-)

5.Heat Re)ova' /ith #i&0o$&hanne' Heat Sinks:

Aith component heat dissipation levels reaching $%% A)cm;and beyond, conventional

air cooling systems are inadequate for removing excess heat. 2esearch has intensified toward

developing more innovative chip cooling techniques. The ultimate goal is to reduce thermal

resistance from the chip


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&iquid!cooled micro!channel heat sins and coolers have been shown to be a very effective

way to remove high heat load. 3 large heat transfer coefficient can be achieved by reducing the

channel hydraulic diameter. In a confined geometry the small flow rate within micro!channels

produces laminar 'smooth* flow, which results in a heat transfer coefficient inversely

proportional to the hydraulic diameter. In other words, the narrower the channels in the heat

sin, the higher the heat transfer coefficient.

5.1 A 90a&ti&a' I)!'e)entation O" Si'i&on #i&0o$&hanne' Coo'e0s:

More than twenty!five years ago, Tucerman and 6ease first described the use of silicon

micro!channel cooling for very high power densities . +owever, the coolers could not be

fabricated easily and pressure drops were very high. 3s chip power densities are now

increasing beyond air cooling limits, a variety of liquid cooling methods are being investigated.

9ue to the high heat transfer coefficient associated with it, micro!channel cooling is an

attractive approach, but several practical issues need to be addressed. 2eviews of micro!

channel cooling are available, though only a few recent publications discuss integration of

micro!channel coolers with pacaged silicon chips. we addressed some of the practical issuesfor implementing silicon micro!channel cooling in a single chip module 'SM*.

7.; Si'i&on #i&0o$&hanne' Desi(n

2ecent progress in high!rate, deep reactive ion etching '92I1* of Si has greatly

simplified the fabrication of silicon micro!channel coolers. 3lso, methods for reducing the

pressure drop have been reported including subdividing the flow into multiple heat exchanger

#ones with shorter channel lengths and manifold designs with large cross!sectional area 'i.e.

equal, or larger than, the channel cross!sectional area*.

3 7!9 rendering of part of an assembled micro!channel cooler is shown in igure (, where the

manifold chip is on top and is shown as semi!transparent green. In operation, alternate

#ig#agged rows of fluid vias are used as inlets and outlets, so the ;% x ;% mm micro!channel

cooler is divided into six parallel!fed heat exchanger #ones and the flow length between the

inlets and outlets is about 7 mm. The fluid vias in the manifold chip were formed as #ig#agged

arrays of circular openings instead of elongated slots to reduce the lielihood of the manifold

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wafers breaing during fabrication and assembly. The micro!channel coolers had a %.B mm seal

region around the perimeter, so the actively cooled area was (C.D x (C.D mm in si#e.

ig 7.;'a*- Silicon plate

igure 7.;'b* - 9 rendering of a portion of an assembled micro!channel cooler having six heat

exchanger #ones.

The manifold chip also contained distribution channels etched about ;$% microns deep on the

side facing the micro!channels to help redistribute the flow to, or from, the fluid vias. 3lso, the

fin segments were removed from the regions under the fluid vias on the channel chip to further

aid in the redistribution of the flow at the fluid vias. Staggered fins were used on the channel

chip, which allow easy flow redistribution, but appropriate filtration of the coolant is still

required. The micro!channel coolers were fabricated using photolithography and deep Si 92I1

on ($% mm wafers at a M1Ms foundry. The manifold and channel wafers were fusion bonded

together and then diced to produce the completed Si micro!channel coolers. The channel depth

was about ;$% microns.

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5.5 EO#ETR O9TI#I;ATION O4 #ICRO$CHANNE% COO%IN

SSTE#:

(* +eat dissipation rate

;* low rate

7* 6ressure drop

>* luid temperature rise

$* luid inlet to surface temperature difference.

5.6 Avanta(es o" )i&0o &hanne' &oo'in( sste)s:

(* ools integrated circuits.

;* +eat is absorbed by walls and heat is moved to the radiator.

7* ools integrated chips $% times faster than convection cooling.

>* 1asy to manufacture

$* Eery effective for small areas 'FBcentimeter square*

6.#ICRO$CHANNE% HEAT SINK DESIN:

The micro channel heat sin was designed in such a way that it taes the heat from the

surrounding of the chip and it collects to the plate 'channels*. 3nd this collected heat, after

moving to the channels was detected by the sensors at floe passages and walls of the channel

plates and gets cooled by the fans, di!electric liquid. In the micro!channel design, the plates

were designed in such a way that the gap was maintained between plate to plate as minimum

width as $%um and height 7%%um and length as $%%um. The spacing is provided between each

plate for the air passage.

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Microchannel heat sink

Structure constaints

Substrate thickness t 100 m (Silicon wafer thickness)

Aspect ratio H/Ww 30 (D!")

&

A

+

t

AwAc

&

A

+

t

AwAc

Tm

Tb

Tw

Tm

Tb

Tw

Heat input q

Tm

Tb

Tw

Tm

Tb

Tw

Heat input q

ig >.% - Micro!channel heat sin

To design the heat sin, we use si'i&on !'atesas the channels for the heat sin.

6.1 #ICRO$CHANNE% COO%IN ARRANE#ENT:

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ig >.('a* - Micro!channel cooling arrangement

ig >.('b* - Thermal management solution for a typical destop or server application

In the above figures it is explained that the cooling system was placed on the chip. 3nd on the

chip a socet was placed and on the socet a small substrate was placed, this substrate transfer

heat to the under!fill. This under fill has alternate sins, this underfill transfer heat to the

pacage die'chip plate* and the heat is transferred to interface material and then moved to

integrated heat spreader of first and second stage and finally they were moved to the heat sin

micro!channel walls and plates..

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ig >.('c* - ross section of Micro!channel cooling

6.2 4EATURES O4 THIS SSTE#:

1+ This system was related to 6eltier effect 'property of T1 material*

2+ &ow efficiency 'input power larger than dissipated heat*

5+ &ow heat flux dissipation rate '7A)cm;*

6+ &imited operation temperature range

6.5 #i&0o&hanne' &oo'in(:

1ither single phase or two!phase, can dissipate heat of B%% A)cm;'ost effective, compact,

low acoustic noise*

Cha''en(es:

a+ 9a&ka(in(:

+ermetic seal, interface with microelectronics

!6rocess and materials compatibility-

I yields 'if monolithic*, T1 matching

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3+ oolant and coolant clogging-

oolant selection 'temperature range, thermal properties*, filtering

&+ +ot spots

Thermal stress!!!performance of I=s

6.6 #ONO%ITHIC HEAT SINK:

ig >.> - Monolithic heat sin

6.6.1 TECHINICA% ISSUES:

(. ptimum design

;. 6rocess and material capability of I=S

7. +ermetic sealing

>. ooling

$. 6umping

D. Temperature gradient

B. ost.

6.7 #ICRO$CHANNE%S 4OR %I


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(* Straight parallel fins

;* &abyrinth

7* ffset strip fins

>* Split flow

6.= #ICRO$CHANNE% 4OR AIRCOO%IN-

(* 2adial

;* 6ins 'micro heat sins*

6.> CHANNE% SECTION SHA9ES

(* Square ) 2ectangular H +igh ) &ow aspect ratios

;* Triangular

7* Trape#oidal

>* ircular channels

6.?#OTIVATION:

Micro!channels are amongst most effective of high heat!flux heat transfer technologies! can

provide temperature control, uniformity and stability, and are also suitable for spot cooling.

(* Ahy the increased interest in micro!channels

;* 4ew manufacturing techniques 'etching, vapour deposition, diffusion bonding8.

1xtruded aluminum multi!channel tubes*

7* 4ew marets for miniature heat!exchangers ' electronic cooling 5 automotive*.

4ew needs 'military and commercial* for more aggressive cooling techniques.

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6.@ #AJOR ISSUES O4 #ICRO$CHANNE% COO%IN:

(* +igh efficiency in heat dissipation

;* &ower the temperature of chips, boards and systems

7* &ower the temperature gradient of chip and board 'hot spots*

>* Minimal increase in weight and volume

$* 4o detrimental effect on I. yield and performance 'monolithic approach*

D* 2eliable

B* 1nvironmental friendly

C* ost effective

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7.#ICRO E%ECTRONICS COO%IN:

1+ hip level cooling

2+ "oard level cooling

7.2 #ETHOD O4 CHI9 %EVE% COO%IN:

(* +igh thermal conductivity material 'F7A)cm;*

;* Gas impingement 'F7w)cm;*

7* Single phase liquid cooling

B%%A)cm;J 2equire pump to move coolant

>* Two phase cooling

+igh thermal conductivity

6ump may not be required 'heat pipe*

$* Thermal electric cooling

&ow efficiency and operation temperature

7.5 #ICRO$9U#9IN SSTE#:

The micro!pump is an electronic equipment, it pumps the di!electric liquid from tan to the

channel cooling system. This pump is very small in si#e and it is very much useful for

transferring the cooled liquid to the system.

There different types of pumps used in micro channel cooling systems. They are -

(* mechanical pumps

;* 1lectrical pumps

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7* 0inetic pumps

>* Magnetic pumps.

urrently all electronic equipments are using electrical pumps for more efficiency.

ig $.7 -ross!section of a liquid dosage system with micropump and flow sensor

7.6 E%ECRTO$OS#ATIC 9U#9 IN #ICRO$CHANNE% COO%IN :

The electro osmotic pump is based on electroosmosis through an ultra!fine porous glass

filter. The silanol groups deprotonate on the glass surface while in contact with an

electrolyte.

3s an external electric field is applied through the structure, ions in the bul fluid

move, and simple ion drag creates a net motion of the bul liquid. 1lectro!osmotic

pumping of the liquid with desired flow rates and pressures is achieved.

The pump developed at Stanford, consists of an ultra!fine porous glass filter dis with a

diameter of diameter of 7% mm and a thicness of ; mm, platinum electrodes, a

catalytic gas re!combiner, and plexiglas machined parts. The woring fluid is (mM

buffered de!ioni#ed water '( mM 4a;">B(% +; dissolved into ( liter de!ioni#ed

water*. The pumps provide flow rates of more than ;% ml)min and pressure drops of

higher than ; bar with about ($% E applied voltages. The electro!osmotic pump has no

moving mechanical parts, generates little noise, and is very compact with minimal

volume.

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ig $.>'a*- 1lecro!osmatic pump

1lectroosmoticmicrochannel cooling-

ompact, high!tech cooling devices for microprocessors-

The technology combines two!phase convection in microchannels silicon heat sins

with a novel electroosmotic pump to achieve minimal heat sin volume at the chip

bacside.

The microchannel heat sin is approximately the si#e of the chip and allows remote

heat re


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ig $.> 'b* - ross section of 1lectro osmotic micro channel cooling

ig $.>'c* - Thermal resistance for various cooling fluids

7.7 9HASE CHANE #ATERIA%S IN #ICRO CHANNE% COO%IN:

ig $.$ - 6hase change materials in micro!channel cooling

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7.= #i&0o &oo'in( te&hni,-e:

L 6assive cooling

H +eat pipe, no##le array, heat exchanger

L 3ctive cooling micro cooler

H Micro heat exchanger 'MM2*

H Micro compressor

L 1lectrostatic diaphragm

L entrifugal 'MIT*

L 1lectro!inetic 'Stanford*

Sorption compressor 'Twenty*

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=.CONC%USION

Micro!channel cooling system is very useful for the electronic devices for the immediate

cooling of high heat flux devices. It cools the electric chips, boards $% times faster than

convection cooling.

urrently this system is developing in defense sector of high power electronics. 3nd this

system should be applied in all electronic equipment:s 'power plants, power grids, electronic

based automobiles..etc*

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Re"e0en&es:

(. Tucerman, 9. and 6ease, 2., +igh!6erformance +eat Sining for E&SI, I111

1lectron 9evice &etters, May (C(.

;. Nu, A. and Mudawar, I., Measurement and orrelation of ritical +eat lux in Two!

6hase Microchannel +eat Sins, Int. Oournal +eat and Mass Transfer, ;%%>.

7. Marston, 0., Gaynes, M., "e#ama, 2. and olgan, 1., 3 6ractical Implementation of

Silicon Microchannel oolers, 1lectronics ooling Maga#ine, 4ovember ;%%B.

Micro channel cooling

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