thermal management of a laptop computer with synthetic air micrqjets

8/10/2019 Thermal Management of a Laptop Computer With Synthetic Air Micrqjets

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Thermal Managementof a Laptop Computer with

Synthetic Air Micrqjets

J. Stephen Camp bell, Jr.

Boeing North American

Aircraft and Missile Systems Division

1800

Satellite Blvd.,

DL23

Duluth,

CA

30097-4099

W.

Z. Black, A. Glezer, J. G. Hartley

Woodruff School of Mech anical Engineering

Georgia Institute of Technology

Abstract

This paper discusses experiments conducted to

determine the effectiveness of synthetic air microjets in

cooling packaged thermal test chips and a laptop computer

processor. The details of the experiments may be found in

Reference

1.

A small electromagnetic actuator was used

to

create the jets. When AC voltage was applied

to

the assembly,

or microjet, a pulsating jet of air was forced out through an

orifice in one face of the actuator. Design variables included

the number and diameter of the orifices. Tlhe magnitude and

frcquency

of

the input signal were held constant.

Initially, a microjct was used to cool a heated,

packaged thermal test chip. Th e setup was such that the jet(s)

of air impinged directly on the package, and the distance

between the microjet and the heated surface was varied. For

the remaining experiments, the microjet was used to cool the

processor of a laptop compu ter. In these tests, the air jet(s )

impinged on the plate covering the processor. Various orifice

plate designs and methods of baffling and sealing were

employed

to

increase the cooling efficiency. Occasionally, the

microjet was used in conjunction with

a

small

5-V

fan to

determine the effect

of

global cooling on the microjet

effectiveness.

Synthetic air microjets were shown

to

be effective in

cooling both the test chips and the laptop processor. For the

former tcsts, the microjet produced an average

26

percent

reduction in c hip temperature rise compared to the tcmperature

rise that existed under natural convection conditions.

For

the

latter tests, the processor temperature rise was decreased by

22

percent compared to the temperature rise without microjet

cooling.

Introduction

In the almost

40

years since the introduction of the

integrated circuit, the microelectronics industry has seen

tremendous changes. With the advent

of LSI

and

VLSI

tcchnology, the number of circuits pe r chip has increased on

a

very rapid pace, as hav e

the

power requirements

of

the

chips.

While chip sizes have also grown, these increases have

occurred at a slower rate than the increases in chip power [2].

Therefore, higher heat fluxes have resulted.

As

such, thermal

0-7803-4475-8/98/ l0.000

998

IEEE

43

management has taken on an increasingly important

rolc

in the

microelectronics industry.

In recent years, the introduction of portable electronic

devices such as cellular phones and laptop computers has

presented additional challenges for thermal management

engineers. Th e small size of these devices often restricts the

use of standard cooling devices such as fans and heat sinks. A

1996 study

by

Xie et al, wherein several methods of cooling

the processor of a laptop computer were examined,

represented a general survey of laptop computer cooling

techniques [3]. As devices such as these increase in power

(and popularity), the need

for

thermal management research in

this area has become clear.

The use of forced air convection cooling for

electronic devices is common.

Numerous studies have dealt

with the subject

of

heat removal from a surface via a steady,

impinging jet of air.

A 1977

survey by Martin

[4 ]

discussed

several aspects of jet cooling, including the influences

of

orifice-to-surface spacin g and orifice diameter. Jambunathan

gave a similar study

of

heat transfer resulting from a single,

round jet impinging on a heated surface [ 5 ] The use of an

array of air jets in cooling a simulated electronic package was

examined

by

Hamad ah [6]. Wh ile studies dealing with steady

jets

are plentiful, the use

of

pulsating jets in heat transfer

is

relatively unexplored.

A

1960 study by Nevins and Ball

discussed the use of unsteady jets created by a compressor,

nozzle system and pneumatic controller [7].

In this paper, the results of an experimental study on

the heat removal capabilities of synthetic air microjets are

presented. Th e microjets were first used to cool packaged

thermal test chips. Thermal resistance data was calculated for

the packages with and without microjet cooling.

A more

rigorous test of the microjets cooling performance was

provided in experiments involving the cooling of a laptop

computer processor.

The microjet laptop cooling data was

compared with data collected when the laptop processor was

energized without microjet cooling to determine the

effectiveness of the microjet as a thermal management device.

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Micro ets

the design of each p ackag e and gives the nomenclature used to

identify each p acka ge in the remainder of this paper.

A novel technique

of

producing an air jet was used i n

this research and was tested

to

determine its effectiveness

in

Table

1 .

Package design table.

cooling a heated chip. Instead of using a compressor and

nozzle , the jet was created by a small electromagnetic actuator.

When AC voltage was applied to the actuator wires, a

pulsating jet of air was forced out through an orifice plate

which was attached one face of the actuator. The amplitude

and frequency of the jet were dctcrmined by the input voltage.

Th e diameter of the hole in the orifice plate was either 1.6 or

0.8 nun.

For some tests, an array

of

holes was used to create

multiple jets. Henceforth, the actuator assembly will be

referred to

as

a microjet, while the term air je t will refer to the

actual stre am of air that leaves the orifice.

A test oscillator was used to generate a sinusoidal

wave to power the microjet. T he input voltage

to

the microjet

was limited to 15

V

for all tests to avoid damaging the

actuator.

Since the test oscillator was unable

to

produce a

signal of this magnitude, a

6-V

wave was produced by the test

oscillator and fed into a signal amplifier. An oscilloscope was

used to monitor the signal before and after amplification. The

optimal signal frequency was determined by varying the

frequency on the test oscillator and observing the change in jet

strength. A signal frequency of 100

Hz

was determined to

produce the strongest jet and this frequency was used in all

the jet cooling tests. This frequency was chosen also because

it virtually eliminated the noise associated with generating the

air

jet.

Jet velocities were measured with a miniature pitot

probe mounted on a computer-controlled traversing

mechanism

[8].

For a microjet with a single

1.6-mm

diameter

orifice, the jet centerline velocity at

a

distance of two

diameters from the orifice plate was approximately 14 d s

Thermal Test Chir, Cooling ExDeriments

Thermal T est Packages and Chips

The chip carriers used in this research were multi-

layer ceramic pin grid array PGA) carriers manufactured by

Kyocera, Inc. A total of four different carriers of varying size

and cavity orientation were analyzed. Tw o of the four carriers

were 68 pin, while the other two were 144 pin. Thre e of the

carriers had a cavity-up design, where the chip cavity was on

the opposite side of the carrier from the pins; the remaining

carrier had a cavity-down design, where the cavity was on the

same side a s the pins.

Package assembly and wirebonding were performed

in the cleanroom of the Georgia Tech Microelectronics

Resea rch Center. The thermal test chips used in this researc h

were supplied by Delco Electronics. Onc e chips were bonded

inside the carriers, the wirebonding was performed such that

corresponding pins on the different packages carried the sam e

electrical value; in this fashion, a single socket and test board

could be used for all the tests. Tw o different methods

of

protecting the chips were employed. On e of the assembled

packages had an encapsulating material which filled the chip

cavity and completely covered the chip. The other packages

had a ceramic lid that covered the chip. Table

1

summarizes

44

No. Name I Pins I Cavity Cover

1

Lid

6 8 D L 68

Down

Lid

3 1 4 4 U L

68

up

Lid

144UE

68

up Encap.

A

144-pin plastic PGA socket and thermal lest board

formed the packa ge mounting assembly, as shown in Figure I.

The socket pins which carried electrical signals between the

packages and the test board were soldered

to

the board. In

addition, the socket corner pins were soldered

to

the board to

ensure a mechanical connection between the t wo components.

One end

of

the test board was inserted into an edge connector,

allowing electrical communication between the package and

test equipment through the boards metallic traces.

Sub s t r a t e

\

C h i p

So c k e t

\ Pins

Figure

1:

Chi p cooling experiment system geometry.

After package assembly and testing were complete,

each chip was calibrated separately. Each chip had

a

diode

bridge, the voltage drop across which varied linearly with the

chip temperature. The calibration process involved inserting a

package into the socket, heating the assembly to a known

temperature in an oven, and mcasuring the diode forward

voltage dro p at that temperature. This procedure was repeated

at selected temperatures until a curve of diode voltage drop as

a function of temperature was generated.

ExDerimental Setup

Once the calibration curves for the various packages

were produced, the packages were individually subjected to

variable heating loads to determine their temperature rise as a

function of heat input. Each chip had a buried resistive

element which provided heating to the chip surface when

connected to

DC

power. The general procedure for chip

heating involved mounting the device on the test board and

mounting the assembly horizontally onto a test stand. Then

heating current was provided to the device and the diode

voltage drop was recorded. Tests were performed without and

without microjet cooling. In addition, experiments were

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performed wherein the package was cooled by a pin fin heat

sink in natural convection.

For the chip cooling tests, the package/test board

assembly was mounted on the test stand. Th e microjet

was

mounted above the heated package such that the jet would

impinge onto the center of the top surface of the package,

as

shown in Figure

2.

For

the microjet mounting assembly,

mounting holes were drilled at the four corners of the microjet

orifice plate,

so

that it could be attached to

a

small Plexiglas

plate. This assembly was supported ov er the heated package

by threaded rods inserted through holes in tlhe Plexiglas plate

and thermal test board. This arrangem ent permitted the height

of the m icrojet over the chip to be adjusted.

lexiglas

plate

\

icrojeL

Figure

2:

Microjet mounted o ver test chip.

A

36-gauge T-type thermocouple was used to

measure the ambient temperature in the chip cooling

experiments. Once the diode voltage drop at

a

given

power level was known, the chip temperature was calculated

using the chip calibration curve. With these values known, the

package junction-to-ambient thermal resistance

e,,

was

calculated from

where Tj, T_and P were the junction

or

chip temperature, the

ambient temperature and the chip power dissipation,

respectively.

Package junction-to-case thermal resistance

Ojc)

was

calculated with Tj in the above equation replaced by T he

case temperature. For all packages, the case thermocouple was

located directly above the chip. Packag e e values were an

average

of

16

OC/W

for

packages

3

and 25, and an average of

2

C W for packages

9

and 26.

Results

of

Cooling Test Packages with Microiets

The first test involved studying microjet cooling

effectiveness

as a

function

of

microjet height above the

package using the setup described in the previous section.

All

tests carried out with

a

single orifice microjet had

a

1.6-mm

orifice, while

all

tests carried out with

a

multi-orifice microjet

had an array

of

nine

0.8 mm

orifices. In each

test,

the microjet

was operated at

15 V

peak-to-peak with a

frequency of 100

Hz.

Figure

3

shows the change in package

e,, as

a

function

of

microjet height above the top surface of the

package for package 4 for both the single-orifice and multi-

orifice microjets. For these Lests, the package was en ergi zed t o

1

W. he results show that the cooling effectiveness increased

with increasing microjet height until

an

optimum height was

reached. Further increases

in

microjet height beyond this p oint

caused the cooling effectiveness to drop. Whe n a single-

orifice microjet was used to cool the packages, the optimum

microjet height w as between 38 and

44 mm.

When a multi-

orifice microjet was used, the optimum height was between

35

and 40 mm.

Figure

3

also shows that the single-orifice microjet

provided better cooling for package

4

than

did

the multi-

orifice microjet. This phenomenon was observ ed

for

other

package designs as well. The superior performance of the

single-orifice microjet is due to fact that the air leaving the

multiple, smaller orifices is weaker than the air leaving the

single, large orifice. Consequently the air velocity at the point

of impingement on the package top surface was less for the

multi-orifice configuration than for the single-orifice

configuration. Furthermore, jet spreading hindered the

performance of the multi-orifice microjet, particularly when it

was used

to

cool the smaller packages, because the spreading

caused some

of

the air to

m i s s

the package entirely.

30

2 9

. 28

U,

a

2 27

.-

26 t

25 I

2.5

3 3.5 4

4.5 5 5.5

Microjet

height cm)

Figure 3: Effect of microjet height on

e

for package 4 a t

1W.

Table 2 shows the percent decrease in package e

produced when several packages were cooled with

a

single-

or i f ice

microjet. Th e power level for each chip was

2

W.

Each test was carried out

at

the optimum microjet cooling

height for each package. In terms of comparing the thermal

performance

of

single-orifice microjet cooling with natural

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Also shown schematically in Figure 6 are the heat

sink and fan. The heat sink had conical-shaped, staggere d fins

with rounded tips and was mounted over

the

cold pla te. When

the laptop was completely assembled, the tips of the fins were

i n contact with the underside of the keyboard. Sin ce the width

of the sink was greater than that of the cold plate, the sink

acted as

a

third heat spreader. The heat sink bas e was roughly

6.4x4.4

cm, with a 2.5x1.2 cm

cu tou t

in one corner.

A

thin,

rubbery material reduced the thermal interface resistance

between the cold plate and sink.

The fan operated at

5

VDC, drawing

0.07 A,

and was

used to draw air in through ports in the laptop external case

and over the sink fins. Power for the fan cam e from the

motherboard. Th e manufacturers rated volumetric air flow

for the fan was 0.92 cfm (0.026 m3/min)

[9].

A thermal

feedback

loop

between the motherboard and fan was assumed

to exist such that the fan would be activated if the processor

temperature exceeded a predetermined level. Sin ce the fan

never came

on,

that limiting temperature was evidently not

surpassed.

For

some of the experiments, the goal was

to

determine the cooling capabilities

of

the fan. In these cases,

the fan was disconnected from the motherboard and

wired

directly to a DC power supply so that independent operation of

the fan could b e assured.

The experimental setup involved mounting the

microjet directly over the laptop processor.

To

position the

microjet in the sp ace between the cold plate and k eyboar d, the

heat sink had to be removed. Using the threaded ro ds, the

microjet was mounted such that the jet impinged directly onto

the cold plate surface. Th e microjets orifice plate h ad eith er a

single, central hole which was either 1.6 or 0.8 mm in

diameter,

or

an array of

0.8-mm

holes. In addition , the shape

of the orifice plate itself was varied. Th e square, single-orifice

plate and the cut, multi-hole orifice plate designs

are

shown in

Figures 7a and 7b , respectively. The four outer holes in both

the square and machined o rifice plates are the mo unti ng holes.

(a) Square , single-orifice plate

(b) Cut, multi-orifice plate

Figure 7: M icrojet orifice plate designs for la ptop cooling

(not to scale).

Baffling and Sealin g

The flow of air from a microjet is created by a closed

electromagnetic actuator. Since the microjet is

a

closed

system, it draws in air and expels air alternately, with a

frequency based on the microjet input signal. W hen th e gap

between the microjet orifice and heated surface

is

sufficiently

large, as in the test chip experiments, the air circulation pattern

47

is as shown in Figure

8a.

In this case, during the microjets

out stroke,

the

expelled air impinges on the heated surface

and flows radially outward, moving nearly parallel to the

surface

[lo].

On the microjets intake stroke, the air flows

i n

approxim ately parallel to the orifice plate. Conver sely, if the

microjet orifice is very close to the heated surface, a

recirculation flow pattern is created (Figure 8b) wherein the

microjet draws

in

the air that has impinged on the heated

surface. Therefore, the separation between the orifice plate

and the heated surface will hav e

a

large impact on the cooling

capabilities of the microjet.

A

large separation distance

ensures that the heated surface ex periences the impingcment of

coo1 air that has not prev iously passed over the heated surface.

On the other hand, if the spac ing is small, the air heated after

impingement will be drawn back into the microjet; thus, the

microjet recirculates hot air an d minimal cooling will occur.

Inflow

(a) Large separation between heated surface and microjet.

(b) Small separation between heated surface and microjet.

Figure

8: Air

je t circulation patterns.

To avoid the recirculation phenomenon, baffling and

sealing were employed to create air ducts that channcled thc

air

to

and from the microjet. On e technique used to direct air

flow was to place

a

thin horizontal baffle between the microjet

orifice and heated surface (see Figure

9).

This baffle had

a

hole that was slightly larger than the orifice of the m icrojet to

compensate

for

jet spreading. In some cases, the fan was

added to dra w air between either the m icrojet orifice plate and

baffle plate, or between the baffle plate and heated surface.

The former setup (shown in the figure) supplied the microjet

with cool intake

air;

the latter removed the hot air after

impingement.

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A d h e s i v e

t ape

a n

On f i c c plat e

\

Air f low

---

Figure 9: Laptop jet cooling setup.

When the fan was used (either with or without the

horizontal baffle), an airflow channel was created by adding

vertical baffles on both sides of the gap between the microjet

and orifice plate. With this setup , the air drawn by the fan

entered the laptop through the external casc ports, then flowed

between the microjet orifice plate, the cold plate, and the

channel walls. To ensure that the fan dre w in cool air through

this path, other internal gaps and natural openings (such as the

gap between the motherboard and laptop external case) were

covered with sealing material.

Experimental Runs

A program designed to exercise the laptop processor

and cause chip heating was installed in the laptop hard drive

193. The software was an

MS DOS 6.2

batch file which

opened a

DOS

Edit window. When this window was open and

a

menu was pulled down, the power drawn

by

the processor

was at a maximum. The power stayed relatively constant

while the software was running.

In all laptop tests the components were

at

ambient

temperature at the beginning of the test. The processor self-

heating program was started at time zero, and the processor

tem per atur e began to rise immediately thereafter. A 36-gauge

type

T

thermocouple, located on the bottom side of

the

processor encapsulant, was used

to

measure the processor

temperature. The thermocouple bead was surrounded with

high-conductivity thermal grease and then taped down.

Table 3

is

a

summary

table which lists each laptop

cooling experiment and describes the setup used in each

experiment.

As

the table indicates, the baffling design and

quality of the sealing improved throughout the course

of

the

study; hence the descriptions poor sealing, medium

sealing, and good sealing. T hes e improvem ents included t h e

use of better materials for the flow channel and the use of more

sealing materials

to

cover more of the openings and gaps.

Table

3

also lists two control cases which are

indicated by Runs 1 and

2

Run 1 involved a heat sink but no

fan, and Run 2 involved the 5-V fan but had no heat sink. In

the oth er experiments where the fan

was

used, its input voltage

was varied. In all the jet cooling runs, the microjet was run at

15

V peak-to-peak and at a frequency of 100Hz.

Table

3.

Laptop cooling experimental runs.

Laptop Jet Cooling Results

The results of the laptop cooling experiments are

given in Figures

10-13.

In Figure 10, the processor

temperature as a function of time is plotted

for

Runs 6 and 7.

Referring to Table

3,

the horizontal baffle plate was used in

Run

6

but not

in

Run

7.

Clearly, the removal of the baffle

plate had a beneficial impact on the processor temperature,

causing the temperature rise to decrease from 64.9C to 61.6C

(averaged over the last ten minutes of heating).

A

possible

explanation for this behavior in Run

6

could be that the

microjet was forced

to

draw intake air from the thin gap

between these

two

plates. Conversely,

in

Run 7, the microjet

could draw air through a larger opening.

Also

the hole

in

the

48

baffle plate used in Run 6 might not have been exactly

oriented with the centerline of the microjet orifice, thus

causing interference with the air stream.

The effect of improved sealing is shown in Figure

1

1

which co mpares the processor temperature rise for R uns 3 and

4. As

indicated in Table 3, the sealing design was improved

between thc two runs. While the precise difference between

poor and medium sealing is difficult to quantify, basically

it involv es more liberal use

of

sealing material to cover more

of

the possible air leakage points. Th e effect of this

improvement in sealing is evident in the results shown i n the

figure.

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In Figure 12, the eflects of improved sealing, the

number

of

microjet orifices and the orifice plate design are

compared. From the dat a, it can be seen that when the 3

V

fan

is used without a microjet (Run 5 ) , it does not provide

particularly effective coo ling . However, the addition of

microjet cooling

to

the coo ling provided by the fan, along with

improved baffling (Run

7),

leads to approximately a

7C

drop

in processor temperaturc.

The next comparison shown in the figure

is

between

Runs

7

and

8,

where the latter test used the multi-orifice

machined plate. Th e cha nge in microjet design caused

a

further 8C drop in processo r temperature rise. Obviously, the

presence of more

air

jets, which are able

to

cover more

of the

cold plate area with cooling air,

is

quite beneficial. In

addition, the machined orifice plate allows t he microjet

to

take

in

cooler

air from the reg ions around and above the microjet,

rather than just below it.

As

such, this orifice plate design

represents an improvement over the single-hole orifice plate

design. This result is in con tras t to the results of the test chip

cooling experiments. Therefo re, a single air jet has more

cooling capability in situations where the distance between the

orifice plate and heated su rfac e is large. How cvcr, when the

orifice plate-heated surface spacing is small, more efficient

cooling is provided by an array of air jets.

Finally, the performance of the microjet setup that

produced the greatest drop in the laptop CPU temperature

(Run 8) was compared with the baseline test when the

5-V

fan

was operated alone (Run

2)

and the temperature produced

when the finned heat sink was used alone (Run 1 . These

comparison cases are show n in Figure 13. While the microjet

system described in Run 8 cools the processor

to

a lower

temperature than when the fan is used as the sole cooling

device, it lags slightly behind the cooling provided by the heat

sink. This result is mainly due to the fact that the heat sink

base is larger than the area of the processor, so it provides

significant heat spreading.

70 I 1

506oi mc=n=;

30

20

0 20

40

60

80

100 120

Time min)

Figure 10:Effect of horizontal baffling on laptop CPU

temperature.

-m-

Run

3

-+-

Run 4

10

O

4 I

I

I I I

0 20 40 60

80

100

120

Time

(min)

Figure

11:

Effect of sealing on laptop

CPU

temperature.

70

6

50

40

5

=:

30

2

10

-

0

0 15 30 45 60 75 90 105 120

Time

min)

Figure 12: Effect of baffling, sealing and o rifice plate design

on laptop

CPU

temperature.

t i

-

0 1

0

15

30 45 6

75

90

105 120

Time min)

Figure 13: Laptop jet cooling compared with baseline data.

49

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Phenomena


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Conclusions

The use of synthetic air jets produced by an

electromagnetic actuator was shown to be effective in

electronic cooling applications, both on packaged thermal test

chips and on the CPU

of

a laptop computer. Microjet design

variables included the number and diameter of the jet orifices

and the height of the microjet above the heated surf ace. When

used to cool the thermal test chips, the microjet produced an

average 26 percent drop in chip temperature rise when

compared

to

the temperature rise that exists under natural

convection conditions. This value was approximately

comparable to that provided when a heat sink is attached to the

package and the fins are cooled by natural convection. For

the

application of jet cooling in the laptop computer, design

variables such as baffling and sealing were studied. Using the

optimum combination

of

the various parameters (baffling,

sealing, orifice size and number), the microjet was able to

lower the processor operating temperature rise by 22 percent

when compared to the laptop operating without the m icrojet.

At this point in the research, the design

of

the

microjet (i.e. material, actuating device, etc.) has not been

optimized. I n addition, further improvements in baffling and

sealing are still possible. Therefore, even though the microjet

has been shown

to

be rclativcly effective in electronics

cooling, optimization of the microjet design and improvements

in

the baffling and sealing could lead to improved je t cooling

capabilities.

References

1.

J.

S.

Campbell, Jr., Establishment

of

an Analytical and

Experimental Test Facility

for

the Evaluation

of

Thermal

Managem ent in Microelectronic Packages. Masters

Thesis, Geo rgia Institute

of

Technology, 1997.

so

2 .

3.

4.

5

6 .

7 .

8.

9.

R. Simons, Microelectronics Cooling and SEMITHERM:

A Look Back,

lOlh

IEEE SEMI-THERM Symposium,

1-

16. 1994.

H.

Xie,

et

al, Thermal Solutions to Penlium Processors in

TCP in Notebooks and Sub-Notebooks, IEEE Trans. on

Comp., Hybrids, and Man. Tech., Part

A

19(1),

54-64,

March, 1996.

H. Martin, Heat and Mass Transfer between Impinging

Gas Jets and Solid Surfaces, Advances in H eat Transfer,

1-60, 1977.

K. Jambunathan, et al, A Review of Heat Transfer Data

for Single Circular Jet Impingement, Inti. J. of Heat and

Fluid

Flow,

13(2), 106-115, June, 1992.

T.

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