- Email: [email protected]
Micro cooling systems for high density packaging
Rev. G&I. 0 Elsevier,
Therm. Paris
(1998)
37, 781-787
Micro cooling systems for high density packaging* Bernd Gromoll Siemens
AC, Corporate
Technology,
Erlangen
Abstract-Future 3D electronics packaging systems will require micro cooling systems that can be integrated and permit the continued use of air as a coolant. To achieve this, new types of silicon micro heat exchangers were made using an anisotropic etching process. Various heat exchanger configurations and sizes were made using sandwich and stacking techniques. They can be used either as a heat exchanger for direct cooling with compressed air or as a heat pipe and thermosyphon for indirect cooling with fan-blown air. The performance characteristics of the various cooling systems are stated. The micro-heat-pipe can be used for power loss densities of up to 3 Wcm-‘, the direct air cooling up to 15 Wcm-’ and the thermosyphon up to 25 Wcm-‘. Cooling performances are achieved that are otherwise only possible with liquid cooling. The practical application of the micro cooling system is demonstrated using the example of the Pentium processor. With a power loss of 15 W. the high-performance micro cooling system is able to limit the increase in operating temperature to 15 K. The volume of the micro heat exchanger is 2.5 cm3 and therefore considerably smaller than that of standard heat sinks. 0 Elsevier, Paris turbulence/ turbulence/
mass mass
transfer transfer
/ gas-liquid model
interface
/ numerical
simulation
/ large
eddy
simulation
/ Navier-Stokes
equation
/ sheared
Resume - Les futurs systemes de packaging Clectroniques 3D exigeront des micro-systemes de refroidissement aptes a I’integration et permettant de continuer d’utiliser l’air comme agent de refroidissement. Pour satisfaire a ce besoin, on a fabrique de nouveaux types de micro-Cchangeurs de chaleur en silicone, selon un processus de gravure anisotropique. Differentes configurations et tailles d’echangeurs de chaleur ont et6 developpees, qui utilisent les techniques sandwich et d’empilage. 11s peuvent Ctre utiiises, soit comme echangeurs de chaleur pour le refroidissement direct par air cornprime, soit comme caloducs et thermosiphons pour le refroidissement indirect par air souffle. Les caracteristiques des performances des differents systemes de refroidissement sont presentees. Le micro-caloduc peut etre utilise pour des densites de dissipation allant jusqu’a 3 Wcm-‘, le refroidissement direct par air jusqu’a 15 Wcm-’ et le thermosiphon jusqu’a 25 Wcm-2. De telles performances de refroidissement ne peuvent Ctre obtenues autrement qu’avec un refroidissement par liquide. Une application pratique du microsysdme de refroidissement est present&e par I’exemple du processeur Pentium. Avec une dissipation de 15 W, le micro-systeme de refroidissement haute performance est capable de limiter I’echauffement a 15 K. Le volume du micro-Cchangeur de chaleur est de 2,s cm3, et est done considerablement plus petit que pour les radiateurs standard destines a cet usage. @ Elsevier, Paris masse/ cisaillee
interface / modirle
gaz-liquide de transfert
/ simulation de masse
numerique
/ simulation
1. INTRODUCTION In electronics the power loss to be dissipated is constantly on the increase [l, 21. For example, it has increased from 4 W for 486type processors to 16 W for 586-type processors, quadrupling within a single development generation. Moreover, the trend in packaging is moving from 2D to 3D architecture. To be able to dissipate power loss from 3D systems without hindering * Portions reprinted with the permission of Proceedings of Semitherm 94, Catalog number 94 CH 3413-2 ISSN 1065-2221 (pp. 53-58). 0 1997 IEEE.
des
grandes
Cchelles
/ equation
de Navier-Stokes
/ turbulence
the development of electronics towards greater packaging densities, cooling systems are required that can be integrated, that can dissipate high powers per volume. Not only the size of the cooling system but also its contact with the heat source and the coolant used are of great importance. Liquid cooling generally gives cooling performances per volume that are one or two orders of magnitude higher than for air cooling. Nevertheless, air has been and still is the preferred coolant in electronics. Air cooling is generally cheap, easy to implement and reliable. To be able to continue to use air as the coolant for the packaging densities that are already emerging, new approaches are required. With the previously used ‘open’ cooling technology using fan
781
B. Cromoll
growth of a Si Nitride Rim by chemical vapor deposition (SI,N,). photolithographic procession (restst spin coating) - exposed to uv-light developed In aqueous alkali hydroxides
wafer RIE
etching of the Silicon Nitride by reactive Ion-etching (RIE), glow etching of the Silicon Nitride by glow discharge plasma treatment with SF, - stripping of the resist. anisotroplc etching by KOH and water (300 urn). plasma treatment (SF,) of the Si Nltrlde (stripping)
&---
anodic
bonding
microcooler
(1 kV. 3OO’C)
separation
-_J
L
Figure
1. Si-microcooler:
planartechnical
manufacturing
process.
+ fin heat exchangers, large-volume heat exchangers and very high air speeds, i.e. large surfaces and large fans are required. The high heat flux at the component level must first be distributed by heat conduction, in order to enter the performance range of air cooling. A further increase in integration density is only possible to a limited extent. If air is to be retained as the coolant, micro cooling systems must be developed that achieve a high heat transfer coefficient at the power loss source itself, despite their small dimensions, and still permit either direct or indirect cooling with air.
2. STRUCTURE OF THE MICRO AIR COOLING SYSTEM In the Siemens laboratories a micro cooler for very high heat fluxes was developed as the basis of a future air-cooling system. The micro cooler is made in silicon etching technology. a standard process of silicon technology. The planar manufacturing method involves a photolithographic masking process and subsequent anisotropic etching (along 100 crystal orientation) (figure 1). This technology provides numerous variations and is suitable for mass production. Unlike the individual channels first presented by Tuckerman and Pease [3] and Philips [4], the form chosen here has a pyramid structure for its heat exchanging surface (figure 2). The pyramid was chosen in order to bring about as turbulent an air flow as possible. The constant change in direction of the fluid as it flows through forms a thin boundary layer on the front of the pyramid with a high heat transfer coefficent. This structure also significantly increases the surface area for heat exchange. With an active heat exchange surface of 1 cm2 an edge of 1 mm and intake and outlet for the coolant with a length
782
of 5 mm. the total area of the basic form of the heat exchanger is 1.6 cm2. With a 3” wafer it is possible to etch 15 heat exchangers in one production process (figure 3). A glass plate is bonded to the etched wafer to cover it and the heat exchanger is separated. Ceramic connection pieces are bonded to the input and the output to which the flexible tubes for the coolant can be attached. If the intermediate edges are etched out, other heat exchanger configurations can be made from the wafer with the same mask. The basic forms are comiected in series or parallel on the wafer. In this way larger forms can be made. The heat exchangers can thus be adapted to the size of the components to be cooled (figure 4).
Figure
2. Structure
of the micro
heat-exchanger
surface
Micro
cooling
systems
for
high
density
packaging
!%I)bonded glass plate i-
iitii ---2mw
“...“..^.^..^..^-^“^-” ::::w
iE v
5b) 2-layer sandwich-desian
siivar bond X) elayar stack-design -b --w Figure
3. Etched
wafer
with
separated
micro
heat-exchanger.
-b 10mm
4a)
-a (or %paratad cross tlow: air/vapor, liquid) Figure
50 mm 4b)
5. Various
heat
exchanger
designs.
ic be achieved by stacking these sandwich forms (figure 5). They are bonded together using conductive silver. The micro coolers were manufactured by our silicon process technology department.
3. PERFORMANCE CHARACTERISTICS OF VARIOUS MICRO COOLING SYSTEMS
Figure 4. Various separated
from
the
types wafer.
of
heat
exchanger
configurations
The maximum size is determined by the size of the wafer used. The wafer can be covered by a second etched wafer instead of the glass plate. The surfaces then are bonded by conductive silver, which has good heat conducting properties. This sandwich technique is shown in figure 5. The cross-section available for air flow is doubled, the flow resistance of the coolant is reduced and the heat exchanging surface enlarged. Still larger cross-sections and heat-exchanging surfaces can
3.1. Forced
convection
with
air
The heat to be dissipated is conducted directly from the heat source through the heat exchanger to the cooling air. The heat exchanger is enclosed with flexible connections for the input and output of compressed air. In the experiment, the compressed air was generated by an oilfree electrically driven compressor. In practical use, say, in an electronics cubicle, a central air compressor might be used to supply the various micro heat exchangers mounted on the critical components via a compressed air accumulator and flexible tubing.
783
B. Gromoll
The heat to be dissipated was simulated by an electrically heated copper block in the experiment (figure 6). The copper block was completely insulated so that heat exchange between the heat source and the heat exchanger could only take place via the rectangular contact surface. The thermal contact was improved by the use of heat conducting paste. The volume Aow and the input and output temperature of the cooling air were measured with Pt 100. The temperature in the copper block was detected by a Pt 100 thermometer that was fitted a few millimetres under the contact surface in a drill-hole. The temperature was offset by the temperature difference required to transfer the heat by conduction from the measuring point to the contact point. This temperature drop was calculated and taken into account for the calculation of the htc. The measurements were made using heat exchangers of types 5a: 5b and 5c shown in figure 5. The air volume tlow was varied. The results for various heat fluxes are shown in figure 6. All types of heat exchangers demonstrated a strong dependency on volume flow. Within the range tested, the temperature difference for a constant heat flux decreases almost linearly as the volume flow increases. However, the pressure drop increases almost linearly with increasing volume flow rate. The highest heat transfer coefficients achievable by forced convection are around 100 W.m-‘.K-l, but with the micro heat exchanger, values of over 1 000 W,rn-‘.K-’ can be achieved as the measured values in figure G show (5b: 10 W @40 K, htc = 2 500 W.nl-2.Kp1). The performance of liquid cooling is therefore already
Y a.c
:6a
;ww m-fpnr~~60pfW ) ltw vobne (!a)
:
32 0.13 180
0.03 180
t0.13
0.70 %zJk 0.41
0.34
j
a .k L 9 i 60. ; .: 5:
__. ~.” “.A-._*,.,.--.w...---t,J 0
2
4
6
3
10
heat flux 4 in W/cm’ Figure
784
6. Microcooler-forced
convection
air.
12
14
attained. The cause of this is the structure of the heat exchange surface and the extremely high air speed. For a volume flow of 500 L.h-l, for example, the air For comparison, figure 6 also gives speed is 200 rn+-’ results from Azar [5] and Goldberg [6] measured on special heat sinks with close fin spacing. As far as heat exchange goes, these heat exchangers yield comparable values. Their volume of 5 and 2 cm3.cmm2 is however much larger than that of the micro-heat exchangers with 0.1 cm3.cme2.
3.2. Microthermosyphon air convection
circuit
+ forced
With higher heat fluxes, direct air cooling does not suffice, even with micro heat exchangers. To avoid the pumps required for a pure liquid cooling system. the evaporation principle is a suitable solution. This principle permits higher heat flux at the heat source than convection with air and can be used in a closed circuit in the form of a thermosyphon without the use of components with moving parts. Air cooling is thus indirect because the thermosyphon circuit forms the heat transfer element bet,ween the heat source and the cooling air (figure 7). The cooling circuit consists of two micro heat exchangers, an evaporator and a condenser connected via flexible tubes (3 mm in diameter). The evaporator is thermally linked to the heat source. In the experiment the heat source was simulated by an electrically heated copper block (see Section 3.1). The heat is transferred by evaporating and condensing the coolant. The coolant consisted of water or FC 72. The coolant circulates because of the density difference between the vapour and the liquid! so that no moving parts such as pumps and valves are required. The condenser must be located above the evaporator. A type 5c heat exchanger is used as condenser with separated cross flow of coolant and compressed air. The circuit regulates itself by evaporation of the coolant when heat is absorbed from the heat source. Despite the small dimensions of the heat exchangers and the flexible tubing, there was no impairment of evaporation. condensation and circulation of the coolant. For the type 4b/5a evaporator, a relatively steep rise in temperature with the heat flux, an almost typical heat transport by conduction was recorded (figure 7). The cause of this is that, because of the very small volume of this type, an increasingly large part of the evaporator surface was covered by vapour which displaced the liquid coolant. With FC 72, which has a higher vapour density than water, a somewhat lower temperature difference was obtained. By using type 4b/5b, which has twice the volume, the typical evaporation process was obtained. With a heat flux density of, for example, 25 W.cmm2, the temperature difference was 21 K. This corresponds to a heat transfer coefficient of 12 000 W.m-‘.K-‘. a value
Micro
condenser
Figure 7. Thermosyphon and condenser.
type4cf!k
cycle
with
cooling
systems
croWlow
microcooler
as evaporator
that is otherwise only achieved with liquid cooling. The temperature curve agrees with measurements by Jaeger et al. [S] on etched silicon surfaces. The microthermosyphon under examination here can be operated with heat fluxes of 30 Wcm-‘. At higher temperatures film boiling begins and the temperature increases sharply. In practice, it is possible to imagine a common central condenser to which the various evaporators are connected via flexible tubing so that a concentrated dissipation of heat into the cooling air only occurs at one place.
3.3. Micro heat convection
pipe + forced
air
For smaller powers the thermosyphon principle can be applied in the form of a heat pipe, in order to remove the heat effectively from the heat source and dissipate into the air. A two-layered extended heat
for
high
density
packaging
exchanger type 4bj5b in sandwich technology is used for the heat pipe. The heat exchanger has a length of 60 mm, a width of 10 mm and a depth of 1 mm. Before being filled, the heat pipe is carefully cleaned and then it is filled with completely degassed coolant, here FC 72, via an intermediate container and under vacuum. The lower part of the heat exchanger serves as evaporator, the upper part as condensers. The coolant is transported by gravity supported by the capillary action of the triangular central structure formed by the pyramids. The optimum filling quantity was determined from the change in vapour pressure depending on the power entered. In the experiment, the power loss was simulated by a foil heater that was bonded to the evaporator part of the heat pipe. The evaporator and the heating foil were thermally insulated in order to be able to determine the performance. The heat was dissipated through microfins on the condenser over which compressed air was blown (figure 8). The cooling performance of the micro heat pipe is shown in figure 8. When power is applied the temperature rises almost linearly. This behaviour would be typical for heat conduction. The temperature change with an initially slow increase is to be expected, as measured by Sato [9] however for a larger copper heat pipe with intermediate insulation (figure 8). The cause of the deviating temperature change is the high proportion of heat transferred by conduction through the silicon housing, itself compared to the proportion transferred by the evaporating coolant. In figure 8, the measured heat transfer through the heat pipe without a coolant is shown for comparison. Much higher temperatures are obtained. The effective heat conductivity of the heat pipe is increased by 40 % due to the evaporation/condensation process. With a heat flux of 2 Wcm-’ a temperature difference of 24 K was measured. This is equivalent to a heat transfer coefficient of 833 W.m-‘.K-‘. Similar values and a similar behaviour were found by Peterson [lo] but for 39 parallel micro heat pipes etched in a 1” silicon wafer. The micro heat pipe shown here can be used for heat fluxes up to 3 Wcm-‘. At higher heat fluxes, ‘dry out’ begins and heat is mainly transferred by conduction (figure 8). For relatively small heat fluxes, the heat pipe can be used for the selective cooling of ‘hot spots’, e.g. in the centre of the ceramic housing of microprocessors.
4. PRACTICAL APPLICATION ON A PENTIUM-TYPE MICROPROCESSOR The Intel example for a maximum of 66 MHz)
Pentium processor was chosen as a typical the use of micro cooling (figure 9). With power loss of 16 W (at a clock frequency and a housing surface of 30 cm’, the result 785
B. Gromoll
Figure
9. Cooling
of pentium
wilhout heat sink: v=1m/s h=33Whn’ K \,,,+
,,J)
dummy
with
microcooler.
0.35’ pin fin heat sink: v=3mis v=tmls h=66Whn’K h=117W/m’K O--J
thermallimit
~
microcwler V =36WVh Dp = 0.2 bar h = 333 Wlm’K
0
1
2 heath
3
2
4
8. Micro
heat
pipe-forced
convection
8
10 12 14
16 18 20
10. Heat
sink
performance
22 24 26 28
30 32
in W with
Pentium-dummy.
air.
is a mean heat flux of 0.55 Wcm-‘. This is three times higher than on its predecessor, the 486. With a permissible allowed operating temperature of 85 “C and an ambient temperature of e.g. 35 “C, a heat transfer coefficient of at least 110 W.rn-‘.K-’ is required to remove the power loss. With air this is only possible with a large heat exchanger if the heat flux density is first distributed and if the air speed is sufficiently high. The limits of standard air cooling are therefore reached with the Pentium processor. To demonstrate the power of micro cooling technology in comparison to standard technology, a micro heat exchanger of the sandwich structure as shown in figures 4c and 5b was used to cool the Pentium processor. The microcooler was adapted to the housing dimensions of the processor (4.8 cm x 5.5 cm: for lanes). The cooling air was led in via three connections; figure 4~. The cooler has a volume
786
6
heat input Figure
Figure
4
Q In W/cd
of 2.5 cm’ and a height of 1 mm. For the experiments, a thermal dummy of the processor was first used. That is sufficient for a comparison of the various cooling systems. The structure comprised a ceramic plate with the dimensions of the processor housing. The heat to be dissipated was provided by a foil heater that was bonded in the centre of the lower side of the plate. The temperature on the upper and lower sides of the housing were measured by temperature sensors (Pt 100) bonded in recesses. To provide a basis for comparison. the ‘processori was first measured without a heat sink in a wind tunnel. Air was blown over the housing parallel to the surface. After that, a 0.35” pin fin heat sink was attached and the different temperatures depending on the power and the air-flow speed measured. Heat conducting paste was used as the contact medium between the housing and the
Micro
cooling
systems
heat sink. The results are shown in figure 10. Without a heat sink, the housing temperature rises rapidly. If the allowed temperature rise is set at 50 K, it is reached after only 5 W. With the pin fin heat sink, higher powers are possible. But here too, the nominal power of 16 W for a clock frequency of 66 MHz only falls within the thermally permissible range at flow rates greater than 3 rn.s-‘. The micro cooler yields significantly better values. At 16 W, the temperature increase is 15 K for V = 3 500 L.h-’ and 5 K for V = 6 000 L.h-‘, that is only one third of the value obtained with the pin fin heat sink. With 2.5 cm3, the volume of the micro heat exchanger is smaller by a factor of 10. The freedom to use the processor is much enhanced with the micro cooling system. This applies to the operating temperature and therefore also rise in clock frequency and freedom of installation options.
5. SUMMARY
AND OUTLOOK
It has been demonstrated that many forms of heat exchangers can be manufactured in silicon etching technology. These can be used as a basic component to construct micro cooling systems that use air as a coolant either directly or indirectly. Cooling performances per unit of area of the power loss source surface are achieved that are comparable with those of liquid cooling systems. The volume of the cooling system is significantly smaller than that of standard systems. Prom the thermal point of view, this allows a further increase in the integration density of electronic components while retaining air as the coolant. The practical application on the Pentiumtype processor shows that the use of micro cooling systems for this performance range is already worthwhile because operating temperatures can be clearly reduced. Although micro cooling technology is still in its infancy, it shows a lot of promise. However, before it can be technically applied to mass-produced products,
for
high
density
packaging
there are various questions to be clarified. These concern the tightness, resistance against mechanical and thermal loads, reliability and the cost of the system. REFERENCES [l] density 1992, 310.
Gromoll B., Trends in cooling packaging, IEEE-Proceedings The Hague, Netherlands, May
[2] Gromoll microcooling ings Eurotherm 14-16, 1993. [3] sinking
technology for of the Comp. 4-8, 1992, pp.
highEuro 304-
B., Application and performance of Sisystems for electronic devices, in: ProceedSeminar 29, Delft, The Netherlands, June
Tuckerman for VLSI,
D.B., Pease R.F., High performance IEEE Electr. Dev. Lett. EDL-2, 126,
heat 1981.
[4] Phillips R.J., Microchannel heat sinks, Lincoln Lab. J., vol. 1, 1988, pp. 31-48. [5] Azar K., McLeod R.S., Carou R.E., Narrow channel heat sink for cooling of high powered electronic components, Semitherm Proceed.1 992 Austin TX, February 3-5, 1992, pp. 12-20. [6] Goldberg N., Narrow channel forced air heat IEEE Trans. on Comp. Hybrids and Manuf.Techn., 2CHMT-7, No 1, March 1984. [7] Hilbert C., High performance therm Proceedings pp. 108-l 14.
Sommerfeldt S., Cupta O., microchannel air cooling, 1990, Phoenix, AZ February
sink, V.C.
Herrell D., IEEE Semi6-8, 1990,
[8] Jaeger C.R. et al., Re-entrant cavity heat sink formed by anisotropic etching and Si direct wafer bonding, IEEE Semitherm Proceed. 1992, Austin TX, February 3-5, 1992, pp. 25-30. [9]
Sato 1992,
Aug. [lo] heat 115
pipes (1993)
K. et al., Micro pp. 15-20.
heat
Peterson G.P., Experimental fabricated in Si-wafers, 751-756.
pipe,
Furukawa
Review
investigation J. Heat Trans.-T.
10,
of micro ASME
[l 11 Cromoll B., Advanced micro air-cooling systems for high density packaging, Proceedings of 10th Semitherm Symposium, San Jose, CA, USA, February 1-3, 1994.
787
Therm. Paris
(1998)
37, 781-787
Micro cooling systems for high density packaging* Bernd Gromoll Siemens
AC, Corporate
Technology,
Erlangen
Abstract-Future 3D electronics packaging systems will require micro cooling systems that can be integrated and permit the continued use of air as a coolant. To achieve this, new types of silicon micro heat exchangers were made using an anisotropic etching process. Various heat exchanger configurations and sizes were made using sandwich and stacking techniques. They can be used either as a heat exchanger for direct cooling with compressed air or as a heat pipe and thermosyphon for indirect cooling with fan-blown air. The performance characteristics of the various cooling systems are stated. The micro-heat-pipe can be used for power loss densities of up to 3 Wcm-‘, the direct air cooling up to 15 Wcm-’ and the thermosyphon up to 25 Wcm-‘. Cooling performances are achieved that are otherwise only possible with liquid cooling. The practical application of the micro cooling system is demonstrated using the example of the Pentium processor. With a power loss of 15 W. the high-performance micro cooling system is able to limit the increase in operating temperature to 15 K. The volume of the micro heat exchanger is 2.5 cm3 and therefore considerably smaller than that of standard heat sinks. 0 Elsevier, Paris turbulence/ turbulence/
mass mass
transfer transfer
/ gas-liquid model
interface
/ numerical
simulation
/ large
eddy
simulation
/ Navier-Stokes
equation
/ sheared
Resume - Les futurs systemes de packaging Clectroniques 3D exigeront des micro-systemes de refroidissement aptes a I’integration et permettant de continuer d’utiliser l’air comme agent de refroidissement. Pour satisfaire a ce besoin, on a fabrique de nouveaux types de micro-Cchangeurs de chaleur en silicone, selon un processus de gravure anisotropique. Differentes configurations et tailles d’echangeurs de chaleur ont et6 developpees, qui utilisent les techniques sandwich et d’empilage. 11s peuvent Ctre utiiises, soit comme echangeurs de chaleur pour le refroidissement direct par air cornprime, soit comme caloducs et thermosiphons pour le refroidissement indirect par air souffle. Les caracteristiques des performances des differents systemes de refroidissement sont presentees. Le micro-caloduc peut etre utilise pour des densites de dissipation allant jusqu’a 3 Wcm-‘, le refroidissement direct par air jusqu’a 15 Wcm-’ et le thermosiphon jusqu’a 25 Wcm-2. De telles performances de refroidissement ne peuvent Ctre obtenues autrement qu’avec un refroidissement par liquide. Une application pratique du microsysdme de refroidissement est present&e par I’exemple du processeur Pentium. Avec une dissipation de 15 W, le micro-systeme de refroidissement haute performance est capable de limiter I’echauffement a 15 K. Le volume du micro-Cchangeur de chaleur est de 2,s cm3, et est done considerablement plus petit que pour les radiateurs standard destines a cet usage. @ Elsevier, Paris masse/ cisaillee
interface / modirle
gaz-liquide de transfert
/ simulation de masse
numerique
/ simulation
1. INTRODUCTION In electronics the power loss to be dissipated is constantly on the increase [l, 21. For example, it has increased from 4 W for 486type processors to 16 W for 586-type processors, quadrupling within a single development generation. Moreover, the trend in packaging is moving from 2D to 3D architecture. To be able to dissipate power loss from 3D systems without hindering * Portions reprinted with the permission of Proceedings of Semitherm 94, Catalog number 94 CH 3413-2 ISSN 1065-2221 (pp. 53-58). 0 1997 IEEE.
des
grandes
Cchelles
/ equation
de Navier-Stokes
/ turbulence
the development of electronics towards greater packaging densities, cooling systems are required that can be integrated, that can dissipate high powers per volume. Not only the size of the cooling system but also its contact with the heat source and the coolant used are of great importance. Liquid cooling generally gives cooling performances per volume that are one or two orders of magnitude higher than for air cooling. Nevertheless, air has been and still is the preferred coolant in electronics. Air cooling is generally cheap, easy to implement and reliable. To be able to continue to use air as the coolant for the packaging densities that are already emerging, new approaches are required. With the previously used ‘open’ cooling technology using fan
781
B. Cromoll
growth of a Si Nitride Rim by chemical vapor deposition (SI,N,). photolithographic procession (restst spin coating) - exposed to uv-light developed In aqueous alkali hydroxides
wafer RIE
etching of the Silicon Nitride by reactive Ion-etching (RIE), glow etching of the Silicon Nitride by glow discharge plasma treatment with SF, - stripping of the resist. anisotroplc etching by KOH and water (300 urn). plasma treatment (SF,) of the Si Nltrlde (stripping)
&---
anodic
bonding
microcooler
(1 kV. 3OO’C)
separation
-_J
L
Figure
1. Si-microcooler:
planartechnical
manufacturing
process.
+ fin heat exchangers, large-volume heat exchangers and very high air speeds, i.e. large surfaces and large fans are required. The high heat flux at the component level must first be distributed by heat conduction, in order to enter the performance range of air cooling. A further increase in integration density is only possible to a limited extent. If air is to be retained as the coolant, micro cooling systems must be developed that achieve a high heat transfer coefficient at the power loss source itself, despite their small dimensions, and still permit either direct or indirect cooling with air.
2. STRUCTURE OF THE MICRO AIR COOLING SYSTEM In the Siemens laboratories a micro cooler for very high heat fluxes was developed as the basis of a future air-cooling system. The micro cooler is made in silicon etching technology. a standard process of silicon technology. The planar manufacturing method involves a photolithographic masking process and subsequent anisotropic etching (along 100 crystal orientation) (figure 1). This technology provides numerous variations and is suitable for mass production. Unlike the individual channels first presented by Tuckerman and Pease [3] and Philips [4], the form chosen here has a pyramid structure for its heat exchanging surface (figure 2). The pyramid was chosen in order to bring about as turbulent an air flow as possible. The constant change in direction of the fluid as it flows through forms a thin boundary layer on the front of the pyramid with a high heat transfer coefficent. This structure also significantly increases the surface area for heat exchange. With an active heat exchange surface of 1 cm2 an edge of 1 mm and intake and outlet for the coolant with a length
782
of 5 mm. the total area of the basic form of the heat exchanger is 1.6 cm2. With a 3” wafer it is possible to etch 15 heat exchangers in one production process (figure 3). A glass plate is bonded to the etched wafer to cover it and the heat exchanger is separated. Ceramic connection pieces are bonded to the input and the output to which the flexible tubes for the coolant can be attached. If the intermediate edges are etched out, other heat exchanger configurations can be made from the wafer with the same mask. The basic forms are comiected in series or parallel on the wafer. In this way larger forms can be made. The heat exchangers can thus be adapted to the size of the components to be cooled (figure 4).
Figure
2. Structure
of the micro
heat-exchanger
surface
Micro
cooling
systems
for
high
density
packaging
!%I)bonded glass plate i-
iitii ---2mw
“...“..^.^..^..^-^“^-” ::::w
iE v
5b) 2-layer sandwich-desian
siivar bond X) elayar stack-design -b --w Figure
3. Etched
wafer
with
separated
micro
heat-exchanger.
-b 10mm
4a)
-a (or %paratad cross tlow: air/vapor, liquid) Figure
50 mm 4b)
5. Various
heat
exchanger
designs.
ic be achieved by stacking these sandwich forms (figure 5). They are bonded together using conductive silver. The micro coolers were manufactured by our silicon process technology department.
3. PERFORMANCE CHARACTERISTICS OF VARIOUS MICRO COOLING SYSTEMS
Figure 4. Various separated
from
the
types wafer.
of
heat
exchanger
configurations
The maximum size is determined by the size of the wafer used. The wafer can be covered by a second etched wafer instead of the glass plate. The surfaces then are bonded by conductive silver, which has good heat conducting properties. This sandwich technique is shown in figure 5. The cross-section available for air flow is doubled, the flow resistance of the coolant is reduced and the heat exchanging surface enlarged. Still larger cross-sections and heat-exchanging surfaces can
3.1. Forced
convection
with
air
The heat to be dissipated is conducted directly from the heat source through the heat exchanger to the cooling air. The heat exchanger is enclosed with flexible connections for the input and output of compressed air. In the experiment, the compressed air was generated by an oilfree electrically driven compressor. In practical use, say, in an electronics cubicle, a central air compressor might be used to supply the various micro heat exchangers mounted on the critical components via a compressed air accumulator and flexible tubing.
783
B. Gromoll
The heat to be dissipated was simulated by an electrically heated copper block in the experiment (figure 6). The copper block was completely insulated so that heat exchange between the heat source and the heat exchanger could only take place via the rectangular contact surface. The thermal contact was improved by the use of heat conducting paste. The volume Aow and the input and output temperature of the cooling air were measured with Pt 100. The temperature in the copper block was detected by a Pt 100 thermometer that was fitted a few millimetres under the contact surface in a drill-hole. The temperature was offset by the temperature difference required to transfer the heat by conduction from the measuring point to the contact point. This temperature drop was calculated and taken into account for the calculation of the htc. The measurements were made using heat exchangers of types 5a: 5b and 5c shown in figure 5. The air volume tlow was varied. The results for various heat fluxes are shown in figure 6. All types of heat exchangers demonstrated a strong dependency on volume flow. Within the range tested, the temperature difference for a constant heat flux decreases almost linearly as the volume flow increases. However, the pressure drop increases almost linearly with increasing volume flow rate. The highest heat transfer coefficients achievable by forced convection are around 100 W.m-‘.K-l, but with the micro heat exchanger, values of over 1 000 W,rn-‘.K-’ can be achieved as the measured values in figure G show (5b: 10 W @40 K, htc = 2 500 W.nl-2.Kp1). The performance of liquid cooling is therefore already
Y a.c
:6a
;ww m-fpnr~~60pfW ) ltw vobne (!a)
:
32 0.13 180
0.03 180
t0.13
0.70 %zJk 0.41
0.34
j
a .k L 9 i 60. ; .: 5:
__. ~.” “.A-._*,.,.--.w...---t,J 0
2
4
6
3
10
heat flux 4 in W/cm’ Figure
784
6. Microcooler-forced
convection
air.
12
14
attained. The cause of this is the structure of the heat exchange surface and the extremely high air speed. For a volume flow of 500 L.h-l, for example, the air For comparison, figure 6 also gives speed is 200 rn+-’ results from Azar [5] and Goldberg [6] measured on special heat sinks with close fin spacing. As far as heat exchange goes, these heat exchangers yield comparable values. Their volume of 5 and 2 cm3.cmm2 is however much larger than that of the micro-heat exchangers with 0.1 cm3.cme2.
3.2. Microthermosyphon air convection
circuit
+ forced
With higher heat fluxes, direct air cooling does not suffice, even with micro heat exchangers. To avoid the pumps required for a pure liquid cooling system. the evaporation principle is a suitable solution. This principle permits higher heat flux at the heat source than convection with air and can be used in a closed circuit in the form of a thermosyphon without the use of components with moving parts. Air cooling is thus indirect because the thermosyphon circuit forms the heat transfer element bet,ween the heat source and the cooling air (figure 7). The cooling circuit consists of two micro heat exchangers, an evaporator and a condenser connected via flexible tubes (3 mm in diameter). The evaporator is thermally linked to the heat source. In the experiment the heat source was simulated by an electrically heated copper block (see Section 3.1). The heat is transferred by evaporating and condensing the coolant. The coolant consisted of water or FC 72. The coolant circulates because of the density difference between the vapour and the liquid! so that no moving parts such as pumps and valves are required. The condenser must be located above the evaporator. A type 5c heat exchanger is used as condenser with separated cross flow of coolant and compressed air. The circuit regulates itself by evaporation of the coolant when heat is absorbed from the heat source. Despite the small dimensions of the heat exchangers and the flexible tubing, there was no impairment of evaporation. condensation and circulation of the coolant. For the type 4b/5a evaporator, a relatively steep rise in temperature with the heat flux, an almost typical heat transport by conduction was recorded (figure 7). The cause of this is that, because of the very small volume of this type, an increasingly large part of the evaporator surface was covered by vapour which displaced the liquid coolant. With FC 72, which has a higher vapour density than water, a somewhat lower temperature difference was obtained. By using type 4b/5b, which has twice the volume, the typical evaporation process was obtained. With a heat flux density of, for example, 25 W.cmm2, the temperature difference was 21 K. This corresponds to a heat transfer coefficient of 12 000 W.m-‘.K-‘. a value
Micro
condenser
Figure 7. Thermosyphon and condenser.
type4cf!k
cycle
with
cooling
systems
croWlow
microcooler
as evaporator
that is otherwise only achieved with liquid cooling. The temperature curve agrees with measurements by Jaeger et al. [S] on etched silicon surfaces. The microthermosyphon under examination here can be operated with heat fluxes of 30 Wcm-‘. At higher temperatures film boiling begins and the temperature increases sharply. In practice, it is possible to imagine a common central condenser to which the various evaporators are connected via flexible tubing so that a concentrated dissipation of heat into the cooling air only occurs at one place.
3.3. Micro heat convection
pipe + forced
air
For smaller powers the thermosyphon principle can be applied in the form of a heat pipe, in order to remove the heat effectively from the heat source and dissipate into the air. A two-layered extended heat
for
high
density
packaging
exchanger type 4bj5b in sandwich technology is used for the heat pipe. The heat exchanger has a length of 60 mm, a width of 10 mm and a depth of 1 mm. Before being filled, the heat pipe is carefully cleaned and then it is filled with completely degassed coolant, here FC 72, via an intermediate container and under vacuum. The lower part of the heat exchanger serves as evaporator, the upper part as condensers. The coolant is transported by gravity supported by the capillary action of the triangular central structure formed by the pyramids. The optimum filling quantity was determined from the change in vapour pressure depending on the power entered. In the experiment, the power loss was simulated by a foil heater that was bonded to the evaporator part of the heat pipe. The evaporator and the heating foil were thermally insulated in order to be able to determine the performance. The heat was dissipated through microfins on the condenser over which compressed air was blown (figure 8). The cooling performance of the micro heat pipe is shown in figure 8. When power is applied the temperature rises almost linearly. This behaviour would be typical for heat conduction. The temperature change with an initially slow increase is to be expected, as measured by Sato [9] however for a larger copper heat pipe with intermediate insulation (figure 8). The cause of the deviating temperature change is the high proportion of heat transferred by conduction through the silicon housing, itself compared to the proportion transferred by the evaporating coolant. In figure 8, the measured heat transfer through the heat pipe without a coolant is shown for comparison. Much higher temperatures are obtained. The effective heat conductivity of the heat pipe is increased by 40 % due to the evaporation/condensation process. With a heat flux of 2 Wcm-’ a temperature difference of 24 K was measured. This is equivalent to a heat transfer coefficient of 833 W.m-‘.K-‘. Similar values and a similar behaviour were found by Peterson [lo] but for 39 parallel micro heat pipes etched in a 1” silicon wafer. The micro heat pipe shown here can be used for heat fluxes up to 3 Wcm-‘. At higher heat fluxes, ‘dry out’ begins and heat is mainly transferred by conduction (figure 8). For relatively small heat fluxes, the heat pipe can be used for the selective cooling of ‘hot spots’, e.g. in the centre of the ceramic housing of microprocessors.
4. PRACTICAL APPLICATION ON A PENTIUM-TYPE MICROPROCESSOR The Intel example for a maximum of 66 MHz)
Pentium processor was chosen as a typical the use of micro cooling (figure 9). With power loss of 16 W (at a clock frequency and a housing surface of 30 cm’, the result 785
B. Gromoll
Figure
9. Cooling
of pentium
wilhout heat sink: v=1m/s h=33Whn’ K \,,,+
,,J)
dummy
with
microcooler.
0.35’ pin fin heat sink: v=3mis v=tmls h=66Whn’K h=117W/m’K O--J
thermallimit
~
microcwler V =36WVh Dp = 0.2 bar h = 333 Wlm’K
0
1
2 heath
3
2
4
8. Micro
heat
pipe-forced
convection
8
10 12 14
16 18 20
10. Heat
sink
performance
22 24 26 28
30 32
in W with
Pentium-dummy.
air.
is a mean heat flux of 0.55 Wcm-‘. This is three times higher than on its predecessor, the 486. With a permissible allowed operating temperature of 85 “C and an ambient temperature of e.g. 35 “C, a heat transfer coefficient of at least 110 W.rn-‘.K-’ is required to remove the power loss. With air this is only possible with a large heat exchanger if the heat flux density is first distributed and if the air speed is sufficiently high. The limits of standard air cooling are therefore reached with the Pentium processor. To demonstrate the power of micro cooling technology in comparison to standard technology, a micro heat exchanger of the sandwich structure as shown in figures 4c and 5b was used to cool the Pentium processor. The microcooler was adapted to the housing dimensions of the processor (4.8 cm x 5.5 cm: for lanes). The cooling air was led in via three connections; figure 4~. The cooler has a volume
786
6
heat input Figure
Figure
4
Q In W/cd
of 2.5 cm’ and a height of 1 mm. For the experiments, a thermal dummy of the processor was first used. That is sufficient for a comparison of the various cooling systems. The structure comprised a ceramic plate with the dimensions of the processor housing. The heat to be dissipated was provided by a foil heater that was bonded in the centre of the lower side of the plate. The temperature on the upper and lower sides of the housing were measured by temperature sensors (Pt 100) bonded in recesses. To provide a basis for comparison. the ‘processori was first measured without a heat sink in a wind tunnel. Air was blown over the housing parallel to the surface. After that, a 0.35” pin fin heat sink was attached and the different temperatures depending on the power and the air-flow speed measured. Heat conducting paste was used as the contact medium between the housing and the
Micro
cooling
systems
heat sink. The results are shown in figure 10. Without a heat sink, the housing temperature rises rapidly. If the allowed temperature rise is set at 50 K, it is reached after only 5 W. With the pin fin heat sink, higher powers are possible. But here too, the nominal power of 16 W for a clock frequency of 66 MHz only falls within the thermally permissible range at flow rates greater than 3 rn.s-‘. The micro cooler yields significantly better values. At 16 W, the temperature increase is 15 K for V = 3 500 L.h-’ and 5 K for V = 6 000 L.h-‘, that is only one third of the value obtained with the pin fin heat sink. With 2.5 cm3, the volume of the micro heat exchanger is smaller by a factor of 10. The freedom to use the processor is much enhanced with the micro cooling system. This applies to the operating temperature and therefore also rise in clock frequency and freedom of installation options.
5. SUMMARY
AND OUTLOOK
It has been demonstrated that many forms of heat exchangers can be manufactured in silicon etching technology. These can be used as a basic component to construct micro cooling systems that use air as a coolant either directly or indirectly. Cooling performances per unit of area of the power loss source surface are achieved that are comparable with those of liquid cooling systems. The volume of the cooling system is significantly smaller than that of standard systems. Prom the thermal point of view, this allows a further increase in the integration density of electronic components while retaining air as the coolant. The practical application on the Pentiumtype processor shows that the use of micro cooling systems for this performance range is already worthwhile because operating temperatures can be clearly reduced. Although micro cooling technology is still in its infancy, it shows a lot of promise. However, before it can be technically applied to mass-produced products,
for
high
density
packaging
there are various questions to be clarified. These concern the tightness, resistance against mechanical and thermal loads, reliability and the cost of the system. REFERENCES [l] density 1992, 310.
Gromoll B., Trends in cooling packaging, IEEE-Proceedings The Hague, Netherlands, May
[2] Gromoll microcooling ings Eurotherm 14-16, 1993. [3] sinking
technology for of the Comp. 4-8, 1992, pp.
highEuro 304-
B., Application and performance of Sisystems for electronic devices, in: ProceedSeminar 29, Delft, The Netherlands, June
Tuckerman for VLSI,
D.B., Pease R.F., High performance IEEE Electr. Dev. Lett. EDL-2, 126,
heat 1981.
[4] Phillips R.J., Microchannel heat sinks, Lincoln Lab. J., vol. 1, 1988, pp. 31-48. [5] Azar K., McLeod R.S., Carou R.E., Narrow channel heat sink for cooling of high powered electronic components, Semitherm Proceed.1 992 Austin TX, February 3-5, 1992, pp. 12-20. [6] Goldberg N., Narrow channel forced air heat IEEE Trans. on Comp. Hybrids and Manuf.Techn., 2CHMT-7, No 1, March 1984. [7] Hilbert C., High performance therm Proceedings pp. 108-l 14.
Sommerfeldt S., Cupta O., microchannel air cooling, 1990, Phoenix, AZ February
sink, V.C.
Herrell D., IEEE Semi6-8, 1990,
[8] Jaeger C.R. et al., Re-entrant cavity heat sink formed by anisotropic etching and Si direct wafer bonding, IEEE Semitherm Proceed. 1992, Austin TX, February 3-5, 1992, pp. 25-30. [9]
Sato 1992,
Aug. [lo] heat 115
pipes (1993)
K. et al., Micro pp. 15-20.
heat
Peterson G.P., Experimental fabricated in Si-wafers, 751-756.
pipe,
Furukawa
Review
investigation J. Heat Trans.-T.
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of micro ASME
[l 11 Cromoll B., Advanced micro air-cooling systems for high density packaging, Proceedings of 10th Semitherm Symposium, San Jose, CA, USA, February 1-3, 1994.
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