High conduction cooling module having internal fins and compliant interfaces for VLSI chip technology

The present invention dissipates the heat generated by high powered VLSI chips to a heat sink in a very efficient manner, providing a thermal resistance heretofore not possible in heat conduction module cold plate type systems. A finned internal thermal device having a flat bottom contacts the chips, while corresponding fins in a finned cooling hat mounted to a cold plate form gaps into which the fins of the finned internal thermal devices are slidably mounted. A preferred double cantilever spring between the finned internal thermal devices and fins of the finned cooling hat and a compliant thermally conductive interface such as synthetic oil between the chips and flat base of the finned internal devices provide efficient, non-rigid interfaces throughout the system, while assuring good thermal contact between the system components. The present high conduction cooling module allows for the simple incorporation of side biasing means and a grooved chip interface on the finned internal thermal devices to provide enhanced thermal performance if required.

FIELD OF THE INVENTION 
This invention relates to conduction cooling of small, flat heat generating 
devices such as integrated circuit (IC) chips, and more particularly, to a 
cooling device having a very low thermal resistance path between the heat 
generating devices and heat sink. 
As used herein, thermal resistance R is defined as R=dT/Q, where dT is the 
temperature difference and Q is the heat flow between the ends of the 
region. This relationship is a restatement of Fourier's conduction 
equation Q=kAdt/L), with R=L/kA, where L is the length of the region, k is 
the thermal conductivity of the medium and A is the cross-sectional area 
of the region. 
DESCRIPTION OF THE PRIOR ART 
The introduction of large scale integration (LSI) and very large scale 
integration (VLSI) at the chip level and very large scale integration at 
the module level by packaging multiple chips on a single multilayer 
substrate has significantly increased both circuit and power densities. 
For example, integration at the chip and module levels has resulted in 
circuit densities as high as 2.5.times.10.sup.7 circuits per cubic meter 
with the necessity of removing heat flux densities on the order of 1000 
kw/sq meter at the chip level. To remove these high heat flux densities 
various means of dissipating the heat have been investigated. A 
restriction is that the cooling fluid (e.g., water) cannot come into 
direct contact with the chips or the area wherein the chips are mounted. 
Thus, a cooling hat must be incorporated between the chips and fluid which 
may be contained in a detachable cold plate. Because the surfaces of the 
chips on a multichip module are not all coplanar due to substrate camber 
and chip and solder ball thickness variations, a single flat cooling plate 
placed in close proximity to the chips would not contact each chip evenly, 
if at all, thus leaving gaps which increase the thermal resistance to an 
unacceptable degree for state of the art VLSI applications. Filling the 
gaps with any of the known compliant thermally conductive materials would 
lower the thermal resistances, but not enough to achieve 60-120 watts per 
square cm. Providing a solid, noncomplaint heat flow path from each chip 
to the flat cooling plate would be thermally acceptable; however, such a 
rigid system would lead to solder ball fatigue, a well know phenomenon 
(e.g., see "Reliability of Controlled Collapse Interconnections" by K. C. 
Norris and A. H. Landzberg in the IBM Journal of Research and Development, 
Vol. 13 No. 3, pages 266-271, May 1979 and so is not mechanically 
acceptable. What is needed is a compliant thermal connection between the 
chips and the cooling hat. Furthermore, this connection must provide 
increased heat transfer surface area for VLSI applications and at the same 
time be capable of accommodating chip height and tilt variations. A well 
known method of achieving this is to have an internal thermal device 
contact the chip and conduct heat to the hat through a thermal interface. 
An example of a structure using such a cooling hat is the gas encapsulated 
thermal conduction module (TCM) disclosed in U.S. Pat. No. 3,993,123, 
issued to Chu et al. 
For discussing the thermal performance of a typical thermally enhanced 
module having an internal thermal device between the chips and cooling 
hat, reference is made to FIG. 1. The figure is a schematical 
cross-sectional representation of a single-chip unit of a thermally 
enhanced module having a cooling hat with separate cold plate, and the 
thermal resistance of the heat path from the chip 11 to the cold plate 16 
divided into various segments. Rext designates the external thermal 
resistance and is defined relative to the module as 
EQU Rext=dT(h-iw)/Pm 
where dT(h-iw) is the temperature difference between the hat 41 and the 
inlet water through cold plate 16 and Pm is the module power. Rext is 
divided into two parts: the interfacial resistance between the cooling hat 
41 and the mating surfaces of the cold plate 16 and the resistance between 
the cold plate surface and the circulating water inside the cold plate. 
When the cooling hat and cold plate are not two separate parts joined 
together but are instead a single entity, then the interfacial resistance 
is zero. Rint designates the internal thermal resistance, and is defined 
as 
EQU Rint=dT(c-h)/Pc 
where dT(c-h) is the temperature difference between the chip 11 and the top 
of the hat 41 and Pc is the chip power. Rint can be defined as the sum of 
five component resistances: Rc, Rc-i, Ri, Ri-h and Rh. Rc designates the 
chip resistance, Rc-i is the thermal resistance of the interface between 
the chip 11 and the internal thermal device 13, Ri is the thermal 
resistance of the internal thermal device 13, Ri-h is the thermal 
resistance between the internal thermal device 13 and the cooling hat 41 
and Rh designates the thermal resistance of the cooling hat 41. 
The thermal resistance of the interface between the chip 11 and the 
internal thermal device 13, Rc-i, is a complex function of geometric, 
physical and thermal characteristics of the contacting solids and the 
interfacial medium. Rc-i is composed of two parallel thermal resistances: 
conduction resistance through one or more solid (e.g., metallic) contact 
areas, and conduction resistance through the interfacial (e.g., fluid) 
medium used to fill the voids which may exist between chip 11 and internal 
thermal device 13. The dominant thermal conduction path is usually through 
the interfacial medium. 
The thermal resistance Ri-h between the internal thermal device 13 and the 
cooling hat 41 is a function of several parameters including the area and 
thickness of the gap between the internal thermal device 13 and the 
cooling hat 41, the cross-sectional areas of the internal thermal device 
13 and cooling hat 41, and the thermal conductivities of the internal 
thermal device 13 and cooling hat 41. 
As VLSI chips increase in circuit density, switching speed, and 
corresponding power (e.g. power densities in state of the art high-powered 
bipolar chips are presently in the 60-120 watt per square cm range), the 
thermal resistance of heat conduction systems, wherein an internal thermal 
device is placed between the chips and cooling hat, must be further 
reduced. In the TCM of U.S. Pat. No. 3,993,123 the internal thermal device 
13 is a piston which contacts the chip at one point. The TCM is very 
useful and successful in VLSI systems of the present but is not easily 
extendable to future high powered systems in all applications. 
In the present state of the art there are many structures for achieving 
enhanced heat transfer. Among these are intermeshed fin structures wherein 
the internal thermal device has fins which mate with corresponding fins in 
the cooling hat. These structures have the potential to provide improved 
thermal performance over single-surface structures such as the piston in 
the TCM because they comprise means for increasing the heat transfer area 
between the internal thermal device, which contacts the chip, and the 
cooling hat. In intermeshed fin heat sink applications, it is generally 
known to make the engagement area between the fins as large as possible. 
The gaps between mating fins must then be wide enough to facilitate 
manufacturability and assembly. However, the wide gaps that result from 
having the largest possible engagement area produce a thermal resistance 
that is unacceptable for state of the art high powered VLSI chips in the 
60-120 Watt per square cm range. Gaps which are wide enough to facilitate 
manufacturability and assembly may not be wide enough to accommodate chip 
tilt. This results in increased thermal resistance in the gap between chip 
and internal thermal device due to chip tilt. 
Finned structures which require that the finned internal thermal device be 
bonded to the chip exist in the art. However, such structures are not 
suitable for the present application because bonding makes rework 
difficult and costly. Moreover, if the internal thermal device is bonded 
to the chip, then some means for strain relieving the connection between 
this device and the hat must be found. The only methods known degrade 
thermal performance to an unacceptable level. Thus, what is needed is an 
invention which incorporates an internal thermal device which is not 
bonded to the chip and which provides a stress free thermal connection 
with the hat and so accommodates chip height and tilt variations. These 
restrictions increase the difficulty of being able to dissipate large 
amounts of heat from the chips since bonded devices and solid and rigid 
interconnections are normally better thermal conductors than the required 
compliant interfaces. 
Among the known intermeshed fin structures is the "Thermal Conductive Stud" 
taught by Dombroski et al in the IBM Technical Disclosure Bulletin, dated 
May 1977, pages 4683-4685. Dombroski et al teach that the long overlapping 
length and narrow gaps required between mating fins restrict the tilt 
range of the finned internal thermal device that contacts the chip 
surface. They teach that when the chip tilt exceeds the tilt range of the 
internal thermal device, an unacceptably large gap between said finned 
device and chip results. In an attempt to overcome this, Dombroski et al 
teach that the fins of the internal thermal device should not be integral 
but rather should be independent T-shaped studs, each of which can then 
move vertically into line contact with the tilted chip. The T-shaped studs 
are long, which increases their thermal resistance; and there are gaps 
between T-shaped studs which increase the thermal resistance between the 
studs and tilted chip. Thus, while the resistance between the fins is 
relatively low, there is added resistance between the chip and fins. 
Furthermore, the base of the finned internal thermal device is interrupted 
and so restricts the spreading of heat from chip to the fins and between 
the fins. The fact that multiple springs and T-shaped studs are required 
for each chip makes this system very costly and complex to manufacture in 
a conduction cooling module environment containing 100 or more chips. 
Hassan et al teach intermeshed finned cooling structures in the IBM 
Technical Disclosure Bulletin article entitled, "High Performance 
Chip-Cooling Technique for Cold Plate or External Air-Cooled Modules", 
dated Feb. 1984, pages 4658-4660. Hassan et al particularly disclose a 
finned internal thermal device having a low melting point solder at the 
interface of the base portion of the finned internal thermal device and 
the chip; and a centrally located coil spring plus elastomer material, 
which is used for shock absorbing purposes, or, alternatively, elastomer 
material along for both shock absorbing purposes and spring loading 
combined. The finned internal thermal device is biased towards the chip by 
the coil spring plus elastomer or elastomer alone, and chip tilt is 
specifically accommodated by large gaps between the cooling hat fins and 
the fins of the internal thermal device. Low melting point solders are 
subject to voids which can increase the thermal resistance to a level 
unacceptable for future VLSI applications. Furthermore, low melting point 
solders have not been proven to be physically stable in such interfaces; 
and a reliability concern is escape of solder from the interfaces and into 
the module circuitry, where it could cause short circuits. The coil spring 
severely impacts thermal performance by occupying area that would 
otherwise be used for heat transfer. The elastomer spring also occupies 
area critical to achieving high heat conduction, does not provide a 
significant parallel heat flow path and interferes with side-loading the 
finned internal thermal device for future applications as will be 
discussed in the Detailed Description of the present invention. 
Oguro et al in Japanese Patent Application No. J57-103337 (1982), entitled, 
"Heat Transfer and Connection Device and Method of Manufacture Thereof" 
disclose a pair of intermeshed finned structures, wherein one member of 
the pair consists of a set of fins attached to a base which is anchored to 
a chip. Oguro et al teach the use of wide gaps between intermeshed fins to 
allow assembly of the finned structure which is anchored to the chip. Wide 
gaps, however, mandate the use of tall fins to reduce the interface 
resistance between corresponding intermeshed fins. The tall fins 
contribute to a higher overall thermal resistance than corresponding 
shorter fins. In addition, the Oguro et al fins are immovable after 
assembly, and must be thin and capable of flexing so as not to break the 
chip or its delicate solder connections when the module components heat up 
and expand at different rates. The resultant structure does not provide a 
low enough thermal resistance to cool the high powered chips described 
above in a water cooled heat conduction module type system wherein a cold 
plate and cooling hat form a thermal conduction path (i.e., the minimum 
thermal resistance shown in Oguro et al is 10.degree. C./W). In addition, 
bonding a finned structure to the chip produces mechanical stress, solder 
ball fatigue, reliability and rework problems as alluded to above, 
particularly if used in conjunction with high power chips. 
Takenaka in Japanese Patent Application No. J61-67248 (1986), entitled 
"Module Cooling Structure", discloses a pair of intermeshed finned 
structures wherein one member of the pair is attached to the semiconductor 
chip. This system is mechanically too rigid for the applications covered 
by Applicants' invention for the same basic reasons as Oguro et al and 
would be subject to a multitude of mechanical stress, solder ball fatigue, 
reliability and rework problems if used in the high power applications, of 
Applicants' invention. 
Nishimura in Japanese Patent Application No. J60-126852 (1985), entitled 
"Cooling Device for Semiconductor", discloses a pair of intermeshed finned 
structures wherein one member of the pair is spring loaded against the 
chip by a centrally located coil spring as in Hassan et al. The coil 
spring has been found to impair thermal performance by occupying area that 
would otherwise be used for heat transfer, and interfering with side 
biasing for higher power applications requiring side biasing as taught by 
Applicants. Other functional disadvantages of such a spring in an 
intermeshed finned cooling system, such as non-uniform loading over the 
surfaces of the chips, will become apparent hereinafter in the Detailed 
Description. Furthermore, Nishimura teaches that the cap should be thinner 
in the regions between chip sites than in the region of each chip site. 
This would impair thermal performance for the applications of the present 
invention by increasing the thermal resistance from the fins at the sides 
of each chip site to the cap. 
Oguro in Japanese Patent Application No. J60-126853 (1985), entitled, 
"Cooling Device for Semiconductor Chip", discloses a pair of intermeshed 
finned structures wherein one member of the pair is spring loaded against 
the chip by a centrally located coil spring, thus, the thermal performance 
is impaired for reasons similar to those described above for Hassan et al 
and Nishimura. 
In view of the above requirements and the existing state of the art, it is 
apparent that there is a need in the art for a high performance heat 
conduction module that is capable of dissipating 60-120 watts per square 
cm. chips mounted in a module without requiring cooling fluid in direct 
contact with the chips or within the area wherein the chips are mounted. 
There is also a need that there be no rigid connection between the chips 
and the internal thermal device utilized in the module, and a need that 
means exist for accommodating chip tilt and chip height variations. The 
rework of chips must be easily facilitated, and a substantially uniform 
load must be applied to the chips by the internal thermal device for 
mechanical integrity advantages as well as thermal function advantages 
such as making the removal of debris at the interface of the chip and 
internal thermal device automatic so as to minimize thermal contact 
resistance. There is a further need to extend the power dissipating 
capability of the heat conduction module meeting all of these needs by 
providing means for further lowering the thermal resistance of the 
internal thermal devices. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a high performance heat 
conduction module that is capable of dissipating 60 to 120 watts per 
square cm. from heat generating devices such as semiconductor chips 
without requiring cooling fluid in direct contact with the chips or within 
the area wherein the chips are mounted. 
It is a further object that there be no rigid connection between the chips 
and the finned internal thermal device utilized. 
It is still a further object that means exist for accommodating chip tilt 
and chip height variations. 
A further object is that rework of chips be easily facilitated. 
A still further object is to provide for the automatic removal of debris at 
the interface of the chip and internal thermal device. 
Another object is that the power dissipating capability be extended as high 
as possible without adversely affecting the other objects of this 
invention. 
The above objects and other related objects are accomplished by the finned 
internal thermal device having a flat base with fins thereon and 
corresponding finned cooling hat wherein a complaint thermal medium exists 
at the chip to finned internal thermal device interface as well as at the 
interface of corresponding fins of the internal thermal device and the 
cooling hat. A unique biasing means provides a balanced force between each 
finned internal thermal device and finned cooling hat and urges the flat 
surface of each finned internal thermal device towards its corresponding 
chip through the compliant thermal medium. The biasing means typically 
occupy otherwise unused area between the cooling hat and finned internal 
thermal device, and comprise at least one arm that may or may not be 
shaped. The unique biasing means is preferably a low-profile spring with 
two or more cantilever arms preferably attached to cantilever supporting 
members, which are preferably secured to the base of the finned internal 
thermal device. Alternative configurations using various spring arms are 
possible, for example wherein the spring arms are directly attached to the 
base of the finned internal thermal device and contact corresponding areas 
of the cooling hat. The spring is preferably made from flat stock and 
having a cross member and dual cantilever supporting members secured to 
the base of the finned internal thermal device with the cantilever arms 
located in the spaces between the fins closest to the outermost sides of 
the finned internal device. For a two arm cantilever spring, the fixed 
ends of the two cantilever arms are attached to diagonally opposite sides 
of the supporting members and the free ends of the cantilever arms contact 
and push against the corresponding fins in the finned cooling hat. The low 
profile double cantilever spring provides a balanced load at the interface 
of the chip and finned internal thermal device which results in uniform 
pressure and corresponding uniform heat flow through the compliant 
interface and which provides a force which is sufficiently balanced to 
minimize the detrimental effect of debris at the chip interface. The 
low-profile double cantilever spring provides these advantages and at the 
same time allows the finned internal thermal device to move laterally and 
parallel to the chip surface though the compliant interface. The 
low-profile double cantilever spring does not occupy the region used for 
intermeshed fin heat transfer and so unlike the coil spring does not take 
away from the thermal conduction path. In fact the arms of the cantilever 
spring provide a parallel heat conduction path between the finned internal 
thermal device and the finned cap resulting in a lower thermal resistance 
than would otherwise be obtained. 
It is preferred that the compliant interface be an oil-like film having 
excellent stability and a relatively good thermal conductivity such as 
synthetic oil. The compliant film between chip and finned internal thermal 
device in combination with the unique biasing means arrangement gives a 
very low thermal resistance interface that accommodates all thermally 
induced stress in the module package, is immune to effects of 
contamination in the conductive medium, is insensitive to chip height and 
tilt variations, and allows the use of rigid intermeshed fins with very 
fine gaps. 
The cooperation of the rigid mating fins having a flat base in contact with 
the chips, double cantilever spring and compliant interface in the high 
conduction cooling module allows for an internal thermal conductance of 
approximately 1.8 W/cm.sup.2 -.degree.C. between each chip and the top of 
the cooling hat. The corresponding thermal resistance between a square cm. 
chip and the top of the cooling hat would be approximately 0.75.degree. 
C./W. 
The combination of elements also facilitates the simple incorporation of 
other features that unexpectedly have been found to lower the thermal 
resistance by an even greater amount such as the incorporation of side 
biasing means such as a springlike member formed by making a cut into an 
outer fin of one of the finned internal thermal devices. The side biasing 
means urges the finned internal thermal device into close thermal contact 
with the corresponding fins of the cooling hat on one side, but increases 
the gap on the opposite side. Nevertheless, thermal resistance has been 
found to be decreased by as much as 30%. A further enhancement comes into 
play if micro-grooves are formed in the flat face of the rigid finned 
internal thermal device.

DETAILED DESCRIPTION 
The overall mechanical assembly of the specific elements of a high 
conduction cooling module built in accordance with the present invention 
for removing heat from heat generating devices such as semiconductor chips 
is illustrated in FIG. 2. In FIG. 2 a chip 11 that must be maintained 
below a specific temperature is mounted on a substrate 10, having a c. 
ring seat 97. Preferably the chips are mounted to substrate 10 by solder 
balls, 12 i.e. See U.S. Pat. No. 3,495,133, in a face down orientation and 
substrate 10 is a multi-layered substrate. A finned internal thermal 
device 14 having substantially rigid fins on a rigid base is positioned 
over each chip and is free to move laterally in a plane parallel to the 
chip through a compliant thermal interface which is not shown in this 
view. Cooling hat 40 having rigid fins which intermesh with the rigid fins 
of the finned internal thermal device 14 is attached to or is an integral 
part of a cold plate 16. The cooling hat 40 can be fabricated from a 
single piece of material. Alternatively, the cooling hat 40 may be 
fabricated as at least two basic parts, namely, fins and a separate 
supporting frame for the fins. Preferably, the cold plate has a liquid, 
e.g. water, flowing therethrough. However, alternate means of cooling are 
available, for example, finned structures with air movement dissipating 
heat from the module. Preferably, a separator 30 provides for the 
efficient positioning of finned internal thermal devices 14 over the chips 
in the multi-layered substrate module. By using the separator 30, 
continuous channels may be formed in cooling hat 40 which facilitates ease 
of manufacture and allows for improved tolerances. A biasing means 20 
exists between the finned internal thermal devices 14 and the cooling hat 
40. The biasing means exerts a uniform biasing force between the contacted 
areas so that the flat surface of the base of the finned internal thermal 
device is urged toward the chips through the thermally compliant medium. 
The biasing means are typically springs that occupy the otherwise unused 
space between the cooling hat and the finned internal thermal devices. 
Referring to FIG. 3A, the unused space typically comprises the channels 
34, 36 which are formed between intermeshed fins and partially bound by 
the fins of the cooling hat or the fins of the finned internal thermal 
device and the space 35 between the outer surfaces of the finned internal 
thermal devices 14. The biasing means typically allow the fins of the 
finned internal thermal device and the cooling hat areas facing forward 
the chips to come as close together as possible upon compression. To 
accomplish this, the biasing means is typically a spring comprised of at 
least one arm. When the spring is fully compressed in a module having 
channels as described above, the spring does not unduly restrict the 
amount that the fins of the cooling hat and finned internal thermal device 
overlap. When the spring arms reside in the channels 34, 36 then the 
springs are preferably low profile. When the spring arms occupy at least 
the unused space 35 between finned internal thermal devices, then the arms 
may be longer so as to extend from the base of the finned internal thermal 
devices to which they are secured, to the area of the cooling hat facing 
the chips. The spring itself may be a single arm that is shaped for 
various applications as shown in FIGS. 4C, 4D and 4E. For springs which 
reside partially or completely in channels 34 or 36, it is preferred that 
the spring design permit compression of the spring to a thickness of 
approximately one to three times the thickness of a single arm of the 
spring when exposed to the forces applied by the finned internal thermal 
device and cooling hat after a module is assembled. These forces are 
preferably less than approximately 1000 grams/cm.sup.2 so as not to damage 
chip circuitry or solder balls. 
The aforementioned spring comprised of at least one arm typically fits into 
either the channels 34, 36 or the spaces 35 between the outer surfaces of 
the finned internal thermal devices, or both so that no heat transfer area 
is eliminated for the sake of accommodating the biasing means, as in the 
case of those structures in the art using large, single coil springs 
inserted into bored out areas within the intermeshed fins. When the spring 
is placed in the space 35 between the outer surfaces of the finned 
internal thermal devices, it is typically secured to each finned internal 
thermal device, most commonly, the base of each finned internal thermal 
device. The spring comprising at least one arm exerts an adequate biasing 
force to urge the base of the finned internal thermal devices toward the 
chips, while accomplishing other objects of the present invention, such as 
not applying excessive forces that may destroy chip circuitry or cause 
excess fatigue or damage to solder ball 12 connections. 
For example, discrete low-profile springs (preferably flat) may be mounted 
in the channels 34 partially bound by the fins of the cooling hat, and 
exert a biasing force that urges the finned internal thermal device toward 
the chip. Typically, this low-profile spring may comprise a material 
having spring-like properties (e.g., stainless steel wire), which is 
appropriately bent so as to be located in the channels while remaining in 
place during the life of the module. Another example of biasing means 
placement within the above described channels and spaces includes the 
securing of one end of the biasing means to an outer surface of the finned 
internal thermal device while the other end pushes off of a surface of the 
cooling hat that faces the chips. Alternatively, one end of the biasing 
means may be attached to a surface of the cooling hat, and the other end 
pushes against a surface of the finned internal thermal device facing the 
cooling hat. Given the teachings above, various springs can be placed at 
least partially in the aforementioned channels 34, 36 and spaces 35 
between finned internal thermal devices, depending on the application. In 
those applications wherein the springs occupy at least the spaces 35 
between finned internal thermal devices, the spring is usually attached to 
the finned internal device. 
It is preferred that one end of the biasing means partially occupies the 
space between the bases of the finned internal thermal devices and is 
secured to the base, while another end occupies the channel 36 partially 
bound by the fins of the finned internal thermal device and pushes against 
the surfaces of the cooling hat facing the chips. 
Preferably, the biasing means is a cantilever spring wherein the free end 
of the cantilever arms of the cantilever spring contact the bottommost 
surfaces of the fins 42, 43 of cooling hat 40 while the fixed cantilever 
end is preferably attached to the base of the finned internal thermal 
device, and at least two cantilever arms are placed in the channels 36 
between the fins of the finned internal thermal device. Alternate 
cantilever springs are operable within the confines of the present 
invention. These springs may have only a single cantilever arm, or up to 
as many cantilever arms as there are fins on the finned internal thermal 
devices 14, or cooling hat 40. The cantilever arms occupy either the 
aforementioned channels 34, 36, spaces 35 between finned internal thermal 
devices or both. The preferred cantilever spring is discussed throughout 
the remainder of the specification, even though various biasing means as 
described or anticipated above are suitable in the present invention. 
The cooperation of the basic elements of the present invention and the 
critical thermal interfaces of the high conduction cooling module are now 
described in greater detail with reference to FIG. 3A. The finned internal 
thermal devices 14 are mounted over the chips 11 and are free to move in a 
plane parallel to the top surface of the chips through a compliant thermal 
medium 29. Each finned internal thermal device 14 has a flat base 15. 
Preferably, the surface 25 of rigid base 15 adjacent to the surface of 
chips 11 is an ultra-flat surface, i.e. less than 1 micron deviation per 
centimeter. It is preferred that the outermost rigid fins 17 of the finned 
internal thermal device 14 be wider than the interior fins 18 as thermal 
performance has been found to improve under such conditions. Depending 
upon the application, all fins of the finned internal thermal device may 
be of the same size if maximum cooling potential is not required. 
It is preferred that the rigid bases of the finned internal thermal devices 
overhang each side of chips 11. The optimum amount of overhang has been 
found to be approximately one base thickness on each side of chip 11. The 
finned internal thermal devices 14 can be made from any material which is 
rigid and has a good thermal conductivity. Primary examples are aluminum, 
copper, berillya, alumina, silver, silicon carbide and combinations or 
alloys thereof. However, it is also preferred that the material be soft 
enough that debris and microscopic particulate matter which may find its 
way to the chip to rigid fin base interface are forced by the cantilever 
spring into the base of the finned internal thermal device. The thermal 
contact resistance at the chip to finned internal thermal device interface 
can increase to unacceptable levels if such debris is not compressed or 
embedded. The fins 42, 43 of cooling hat 40 are intermeshed with the fins 
17, 18 of the finned internal thermal devices 14. Cooling hat 40 and its 
associated fins 42, 43 are rigid and are also typically made of a heat 
conductive material such as aluminum, copper, berillya, alumina, silver, 
silicon carbide or combinations or alloys thereof. It is preferred that 
the distance between corresponding rigid fins 42, 43 be such that a rigid 
fin 17, 18 of the finned internal thermal device 14 can fit therein and a 
gap remain which is the minimum gap acceptable for the finned internal 
thermal device to accommodate chip tilt. In order to accommodate 
differences in chip height, it is preferred that the height of the 
channels 34 between fins 42, 43 is such that there is a vertical gap 
between the surfaces of the rigid fins 18 of the finned internal thermal 
devices facing the cooling hat and the bulk of cooling hat 40. To minimize 
the thermal resistance it is preferred that the rigid fins 43 of finned 
cooling hat 40 be made larger and longer than the rigid fins 42. Rigid 
fins 43 are those fins which overlap the outermost surface of the 
outermost fins 17 of the finned internal thermal devices 14. For both the 
finned cooling hat 40 and the finned internal thermal devices 14, the 
ratio of the length of each rigid fin to the width of each rigid fin 
should not be greater than approximately 10:1 to provide a low profile 
thermal conduction path and corresponding low thermal resistance. A 
practical upper bound on the ratio of the length of each rigid fin to the 
width of each rigid fin has been found to be less than approximately 7:1, 
based on a balance of state of the art machining practice and cost of 
fabrication. The ratio of the length of each rigid fin to the width of 
each rigid fin should not be less than approximately 3:1 when the fins are 
made from the commonly available thermally conductive materials such as 
copper and aluminum as a smaller ratio would severely limit the thermal 
performance of this invention. If maximized cooling in an environment 
where the cooling fluid cannot be brought into direct contact with the 
chip compartment is not critical or if a particular application imposes 
other restrictions, then the geometries of fins described above can vary. 
For example, the aspect ratio for cooling hat fins may be different than 
the aspect ratio of the finned internal thermal devices depending on 
material properties. 
The gaps between adjacent rigid fins of the finned cooling hat 40 and 
finned internal thermal device 14 are preferably filled with a compliant 
thermal medium 29. In some instances, the compliant thermal medium 29 will 
be the same thermal medium used between chips 11 and the base 15 of the 
finned internal thermal device, e.g. synthetic oil. The primary compliant 
thermal media utilized are heat conductive oils of low viscosity, i.e., in 
the range of approximately 350-1800 centipoise, e.g., synthetic mineral 
oils such as poly(alfaolefin). Other applicable compliant thermal media 
include thermal pastes and gels which are basically comprised of liquid or 
gel-like carriers filled with thermal filler particles, or thermally 
conducting gasses such as helium which may be used in certain 
applications. 
While the thermal conductivities of synthetic oils are approximately 8 
times less than that of thermal pastes or gels, synthetic oils are fully 
compliant and permit the incorporation of finer gaps. In addition, the use 
of synthetic oil in the gaps between intermeshed fins readily accommodates 
side-biasing of the finned internal thermal device, and this results in 
even lower thermal resistances. In an all oil system, if chip tilt is to 
be accommodated, the gaps cannot be arbitrarily small, but rather must be 
equal to or greater than the product of the corresponding chip tilt angle 
times the overlapping length between adjacent fins. In the present 
invention, there is an unexpected deviation from the usual linear thermal 
resistance versus gap relationship in the direction of improved thermal 
performance as will be described in greater detail hereinafter. The use of 
thermal pastes or gels generally requires larger gaps due to the reduced 
compliance of the paste or gel. Furthermore, the finite size of the 
particles in the paste or gel limits how close to one another the 
intermeshed fin surfaces can come. Overall, however, thermal pastes or gel 
with high thermal conductivity can be used to enhance the performance of 
finned high conduction cooling modules as disclosed herein. 
In order to provide a low profile finned internal thermal device in a 
system capable of cooling less than approximately 0.75.degree. C. per watt 
per square centimeter of chip area while accommodating height and chip 
tilt variations in a high conduction cooling module, the interface between 
the ultra flat surface of the finned internal thermal device and chip face 
is very critical. The unique incorporation of the biasing means of the 
present invention assures the integrity of this interface so that the 
finned internal thermal device 14 freely moves over the face of chip 11 
while maintaining a very low thermal resistance at the interface, and not 
causing excessive stresses that may damage chips or solder connections of 
chips to substrates. 
Referring to FIG. 4A, the preferred biasing means being a low profile 
spring 20 having at least two cantilever arms 22 is shown. The spring as 
installed is shown in FIG. 3B, wherein the fixed ends of cantilever arms 
22 are connected to supporting members 24 which are secured to the base 15 
of the finned internal thermal device 14 and attached to each other by 
cross member 26. The free end of each cantilever arm 22 preferably 
contacts the bottommost surface 23 of a rigid fin 42 of said finned 
cooling hat 40. This most preferred configuration is referred to 
throughout the specification even though other embodiments or 
configurations of the spring may exist, for example, wherein the fixed 
ends of the cantilever spring are secured to the cooling hat and wherein 
the free ends of the cantilever arms 22 contact the uppermost surface of a 
corresponding rigid fin 18, 17 of the finned internal thermal device (not 
shown). Other configurations using spring arms are also possible, for 
example, wherein the spring arms are directly attached to the base 15 of 
the finned internal thermal device 14; or wherein a single cantilever arm 
or multiple cantilever arms are used. The cantilever spring 14 is 
preferably made from flat stock having a central cross member 26 and dual 
cantilever supporting members 24 which are secured to base 15 of the 
finned internal thermal device 14. The angle between the cantilever arms 
of the cantilever spring and the base to which it is preferably secured is 
in the range of approximately 15.degree. to 60.degree., and preferably 
approximately 30.degree.. The spring also preferably contains a thermal 
conductor material such as copper, aluminum, silver or alloys thereof in 
combination with a common spring material, e.g., zirconium-copper. In such 
a system, the free ends of the cantilever arms 22 contact the bottommost 
surface 23 of fins 42 of cooling hat 40. The cantilever biasing means 
positioned as shown in combination with the fin structures and compliant 
media in the desired gaps of the present invention results in a low 
thermal resistance previously not attainable in thermal conduction module 
type systems wherein the cooling fluid cannot come into contact with the 
chip compartment. The preferred low profile double cantilever spring 
provides a balanced load at the interface of the chip and finned internal 
thermal device which results in uniform pressure and corresponding uniform 
heat flow through the compliant interface (e.g. synthetic oil) and 
provides a force which is sufficient and balanced enough to minimize the 
detrimental effects of any debris at the chip interface. The low profile 
double cantilever spring contributes to these advantages as well as 
contributing to the chip tilt and height variation accommodation as the 
ultra-flat surface of the finned internal thermal device freely moves 
laterally and parallel to the chip surface through the compliant 
interface. The low profile double cantilever spring also does not take 
away from the thermal conduction path area since it does not occupy space 
that may otherwise be allotted for finned internal thermal device area or 
for the finned area in the cooling hat, and at the same time provides a 
parallel heat flow path between the finned internal thermal device 14 and 
the cooling hat and associated fins. Biasing means having either one or 
more than two cantilever portions may also be used in high conduction 
cooling module systems made in accordance with the present invention. The 
exact number of cantilever portions used depends on the size of the chip 
and corresponding number of fins utilized. 
As shown in FIG. 4B, a plurality of low profile cantilever springs may be 
interconnected and inserted into the module as a single unit 27 to 
facilitate ease of assembly and to provide locating means for the finned 
internal thermal devices. 
Referring to FIG. 4C, double cantilever springs 52 may be attached to the 
uppermost fins of the finned internal thermal device 14 and function as 
described above. 
Referring to FIG. 4D, a spring 37 comprised of at least one serpentine arm 
32, preferably being bowed approximately at the center may be suitably 
placed in the channels. In the preferred embodiment shown in FIG. 4D, two 
serpentine arms 32 bowed at their centers are joined to fastening clip 33 
which secures the spring 37 to the base of a finned internal thermal 
device. In this preferred embodiment, securing means 31 are also 
incorporated. Variations of the serpentine arm spring are possible within 
the spirit of the present invention; for example, wherein the spring 37 is 
secured to the cooling hat, and the bowed portions of the serpentine arm 
32 contact portions of the finned internal thermal devices. A plurality of 
the springs 37 having serpentine arms 32 may be interconnected. 
Referring to FIG. 4E, yet another spring 47 is shown. In this embodiment, 
flat spring members 46 reside in the channels between finned internal 
thermal device fins, and contact the base of the finned internal thermal 
device, while spring wings 48 contact portions of the cooling hat. As in 
the case of most alternative embodiments of the present invention, this 
spring may be reversed so that flat spring members 46 reside in the 
channels between cooling hat fins, while the spring wings 48 contact 
portions of the finned internal thermal devices. 
It is generally known that the thermal performance of an intermeshed fin 
structure is dependent upon the gap between corresponding fins, and that 
thermal performance varies linearly with the change in gap. In the present 
invention, an unexpected improvement over the normally linear relationship 
between gap width and thermal performance has been found. This unexpected 
improvement occurs for gaps below approximately 0.1 mm wide in the present 
invention. Making the gaps no larger than that required for chip tilt 
accommodation results in the aforementioned unexpected enhancement in 
thermal performance. Thus, it is not possible to achieve the required 
level of thermal performance simply by increasing the intermeshed fin 
surface area. For very fine gaps, enhanced performance is possible only 
for fins having an aspect ratio greater than approximately 3 to 1, but 
less than approximately 10 to 1. State of the art machining practice sets 
this ratio at approximately 6.5 to 1. With this ratio and with gaps having 
the required degree of chip tilt accommodation, the optimum fin thickness 
is approximately 0.8 mm. Such fins are mechanically rigid and can be used 
only with compliant chip interfaces. Many previous intermeshed fin cooling 
structures accommodate chip tilt in only one direction (parallel to the 
fins), and thus could not be used for high performance applications which 
require negligible chip interface resistance under all chip tilt 
conditions. The present invention accommodates chip tilt in the direction 
parallel to the fins and also in the direction perpendicular to the fins. 
The prior intermeshed fin cooling structures do not provide a compliant 
chip interface in combination with a cantilever spring and rigid 
intermeshed fins, or a thermal resistance low enough for the high powered 
VLSI applications addressed by the present invention. This is possible 
because of the subtle but critical interplay between the gap and 
intermeshed fin characteristics described above. 
In addition to providing the advantages of low thermal resistance and the 
other advantages alluded to above, the combination of elements of the high 
conduction cooling module of the present invention allows for the 
incorporation of additional features which have unexpectedly been found to 
lower the thermal resistance by even a greater amount than that shown in 
the preferred embodiments. Due to the freely moving yet thermally 
efficient interfaces of the present invention, side biasing means may be 
incorporated between each finned internal thermal device 14 and 
corresponding portions of the finned cooling hat 40. The preferred side 
biasing means is depicted in FIG. 5A wherein a modified finned internal 
thermal device 54 is formed by a spring flap 61 being cut from the 
outermost surface of the outermost fin on the side opposite the direction 
of the desired biasing. The detailed dimensions of the spring flap 61 in 
relation to the remainder of the finned internal thermal device are 
described in the working example section to follow, along with the 
corresponding improvement in thermal resistance over the first embodiment. 
When using the side biasing means, the resultant gap between corresponding 
fins of the finned internal thermal device and finned cooling hat is 
filled with a thermally conductive compliant medium. As shown in FIGS. 5B 
and 5C a modified cantilever spring 70 having cantilever arms 72 on 
cantilever supporting members 74 on either side of cross member 76 also 
incorporates side biasing appendages 71 therein. Side biasing appendages 
71 are typically formed from the same flat metal stock as the spring 70, 
and are preferably approximately one half of the height of the fins 18 or 
less. As shown in FIG. 5C, when the modified cantilever spring 70 is 
loaded into the module, the angle between side biasing appendages 71 and 
the cantilever supporting member 74 is greater than 90.degree., preferably 
between 100.degree. and 110.degree.. 
In certain applications, a single spring 20 as shown in FIG. 4A may be used 
for both biasing the finned internal thermal device toward the chips and 
for side biasing. A means for accomplishing this with a single spring 20 
is by inclining the surfaces of the fins 17, 18 of the finned internal 
thermal devices 14 that face the cooling hat, and by having corresponding 
inclined surfaces in the cooling hat 40. 
Another alternate side biasing means is shown in FIGS. 5D and 5E. In FIG. 
5D, a resilient sheet 80, having flexible bowed portions 81 and 
interconnecting portions 82 is shown. Flexible bowed portions 81 side bias 
the finned internal thermal devices 14. The resilient sheet is preferably 
between approximately 0.05 to 0.2 mm. thick, and is mounted close to the 
base of the finned internal thermal device 14. The resilient sheet serves 
a dual purpose, as it also provides a means for locating and separating 
the finned internal thermal devices 14. FIG. 5E is a representation of the 
use of resilient sheet 80 in a cross-sectional front elevational view. The 
bowed portions 81 preferably contact and urge the finned internal thermal 
device 14 at the lower extremities thereof, e.g., towards the bottom of 
fin 17 in this instance. 
As shown in FIG. 6, another unexpected improvement over previous thermal 
conduction modules results from the modification of the face 25 of the 
finned internal thermal device in contact with the surface of chip 11 
wherein grooves 28 may be incorporated in the face. The grooves may have a 
various array of geometric patterns and are preferably in the range of 15 
to 200 microns wide and deep. The thermal resistance is improved when 
grooves are incorporated into the system described above as the working 
examples which follow will clearly demonstrate. 
Referring to FIG. 7, the fins of the finned internal thermal device and the 
corresponding fins of the cooling hat may be stepped to improve thermal 
performance by providing a shape which more closely follows the heat flux 
lines. Rigid stepped fins 55 on rigid base 51 are intermeshed with the 
stepped fins 53 of modified cooling hat 50. 
Other variations on the basic structure of this invention wherein the fins 
are tapered in a negative or positive pitch are also available. 
WORKING EXAMPLES 
The dimensions of the critical components of the present invention are 
determined by the size and power dissipation of the chips to be cooled. 
For example, in order to dissipate heat from a 30 watt chip of chip size 
6.5.times.6.5 mm. square in a system wherein poly(alfaolefin) oil is used 
as the compliant thermal medium 29 and wherein the finned internal thermal 
device 14 is made of copper, the preferred dimensions are as follows: 
Referring to FIG. 3E, there are preferably five rigid fins of the finned 
internal thermal device in the application having oil as the compliant 
thermal medium and the rigid base 15 is preferably approximately 9.1 mm. 
square and approximately 1.3 mm thick. The width of outermost rigid fins 
17 are approximately 1.3 mm., while the width of the interior fins 18 are 
approximately 0.8 mm. The vertical height of each fin from the top of the 
rigid base 15 is approximately 5.8 mm. The width of the outermost fins 43 
of cooling hat 40 are approximately 1.7 mm., while the width of the 
interior fins 42 are approximately 0.8 mm; and the length of fins 43 is 
approximately 7.1 mm, and the length of fins 42 is approximately 5.8 mm. 
The width of the oil filled gap between corresponding fins is 
approximately 0.035 mm., and the overlapping length of corresponding fins 
is approximately 4.8 mm. 
A double cantilever zirconium-copper spring having an angle of 
approximately 25 degrees between the horizontal and each cantilever was 
used. To accommodate manufacturing tolerances of the overall module, there 
is approximately a 0.8 mm. space between the top of the rigid fins 17, 18 
and the corresponding opening in cooling hat 40; and a 0.8 mm. space 
between the bottom of the fins 42 and the corresponding top of the base 
15. 
For the above described structure, the total thermal resistance is 
approximately 1.1.degree.-1.2.degree. C. per Watt. This thermal resistance 
measured between the chip and the top of the cooling hat provided for the 
required dissipation of approximately 30 watts per 6.5 mm. square chip in 
a high conduction cooling system wherein the water or other thermally 
conductive fluid cannot be directly incorporated into the chip 
compartment. Chip tilt and height variations are easily accommodated, and 
the finned internal thermal device is free to slide parallel to the 
surface of the chip and tilt with the chip in both directions. The double 
cantilever spring retained the finned internal thermal device against the 
chip surface with at least 120 grams of force, but less than approximately 
400 grams of force to prevent damage to the solder ball joints or chip 
circuitry. In addition, the temperature over the surface of the chip was 
uniform and maintained within acceptable limits. No debris was found at 
the interface, as all was embedded into the soft metal face 25 of the 
finned internal thermal device. 
In applications calling for thermal paste as the compliant thermal 
interface 29 between corresponding intermeshed fins, it is preferred that 
the fins 17, 18 of the finned internal thermal device 14 and the 
corresponding fins 42, 43 of cooling hat 40 are shortened in length by 
approximately 1.8 mm. each increased in width by approximately 0.25 mm. 
each and the number of fins be decreased from five fins to four. The 
thermal resistance from the circuits of the chip to the top of the cooling 
hat is approximately 0.7.degree.-0.75.degree. C./W for a 6.5 mm. square 
chip. 
Referring to FIG. 5A, the incorporation of the side biasing means into the 
finned internal thermal device described above results in an improvement 
of the overall thermal resistance. The incorporation of such side biasing 
means is typically applicable in applications wherein low viscosity fluids 
are used as the compliant thermal medium. The dimensions of the embodiment 
using side-biasing are comparable to those utilized in the prior example 
having thermal paste. In the side-biasing embodiment, a spring flap 61 of 
approximately 0.1 mm. is formed by cutting the outermost surface of the 
outermost rigid fin of the finned internal thermal device. An additional 
enhancement may be made to the side-biasing means by depositing a layer of 
solder approximately 0.05 mm. thick on each fin of the finned internal 
thermal device. The solder is reflowed after assembly. In this embodiment, 
the fins 42, 43 of cooling hat 40 must be non-wettable to solder. The 
thermal resistance of this system from the circuits of the chip to the top 
of the cooling hat is approximately 0.7.degree.-0.75.degree. C./W for a 
6.5 mm. square chip. 
Referring to FIG. 6, the incorporation of grooves 28 into the surface 25 of 
the base of the finned internal thermal device is shown. The grooves have 
been found to further reduce the thermal resistance by approximately 10% 
over the high conduction cooling module systems described above. The 
grooves may be between 15 and 200 microns wide and deep, but are 
preferably 15-50 microns wide and 15-20 microns deep, and spaced 
approximately 0.25 mm. apart. The grooves provide an escape path for 
excess oil or other compliant medium and particulate contamination. 
In sum, the combination of the finned internal thermal device and 
corresponding finned cooling hat with a compliant thermal medium between 
the chip surface and oppositely facing base of the finned internal thermal 
device and the unique biasing means providing a balanced load between each 
finned internal thermal device and the fins of the cooling hat that urges 
the flat base of the finned internal thermal device towards the chips 
through the compliant thermal medium provides a low thermal resistance 
capable of cooling chips of high power dissipation in a high conduction 
cooling module type system while accommodating chip tilt and height 
variations. This is accomplished without rigid interfaces that would 
damage the chip circuitry or chip solder ball connection means and without 
permanent connections that would make the rework of chips in such a high 
conduction cooling module most difficult, if not impossible. A combination 
of elements also facilitates the simple incorporation of other features 
that unexpectedly have been found to substantially lower the thermal 
resistance by an even greater amount such as side biasing means and 
grooves in the face of the base of the finned internal thermal device. 
Thus, there has been provided in accordance with the present invention an 
improved high conduction cooling module for cooling a chip or plurality of 
chips that satisfies the objects of the present invention. 
While the invention has been described in conjunction with specific 
embodiments, it is evident that many alternatives or modifications of the 
elements of the present invention will be apparent to those skilled in the 
art in light of the teachings of the present invention. 
It is therefore to be understood by those skilled in the art that the 
foregoing and other alternatives and modifications in form or detail fall 
within the true scope and spirit of the present invention.