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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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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
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1998InierSocietyConference on Thermal Phenomena