Note: Descriptions are shown in the official language in which they were submitted.
53
- 1 - 25307-192
TITLE OF THE INVENTIO
Cooling system used with an electronic circuit device
for cooling circuit components included therein having a thermally
conductive compound layer and method for forming the layer.
BACKGROUND OF EE IN~ENTION
The present invention relates to cooling system used
with a printed circuit board holding a number of solid electronic
circuit components, such as integrated circuit (IC) semicon-
ductor devices. More particularly, it relates to a cooliny mod-
ule, being included in the cooling system and contacting with each
of the electronic circuit components through a thermally conduct-
ive compound layer to cool these components.
There have been developed various types of cooling
structures for cooling IC semiconductor devices or large scale
IC (LSI) semiconductor devices mounted on a printed circuit board
as disclosed, for example, in U.S. Patents Ser. Numbers 3,993,123
issued to Chue et al., 4,203,129 issued to Oktay et al., 4,254,431
issued to Babuka et al., and 4,323,914 issued to Berndmaier, and
in not-examined provisionally published Japanese Patent Applica-
tions, No. 61-15353 invented by K.D. Ostergrane et al., No. 60-
160149 invented by Yamamoto et al. and No. 62-109347 invented by
Tajima. In some of these cooling structures a heat transfer ele-
ment, such as a heat transfer plate or a heat tranfer piston, is
placed in direct contact with the circuit components, being urged
to a surface of a circuit component by pressure pro~ided from a
spring, bellows, hydraulic pressure of coolant, etc., to remove
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the heat dissipa-ted Erom -the circuit components. The heat
transfer elements are exposed to a coolant direc-tly or indirectly.
The hea-t is transferred to -the coolant through the corresponding
heat transfer elements in contact with the circuit components.
In yeneral, however, the heat contact resistance of an
interface between the heat -transfer elements and the circuit
components in the above described cooling structures is rather
high and unstable, because the actual contacting area therebe-
tween is rather small and unstable due to the roughness of the
surfaces which are thermally contacting with each other. In
additlon, any change of the pressure of the spring, bellows, or
coolant pressure affects the heat contact resistance delicately
and seriously, resulting in a large loss of the heat transfer
efficiency of the cooling modules. Particularly, the use of a
spring for urging a heat transfer member to the corresponding
circuit component to be cooled, tends to cause mechanical reson-
ant vibration triggerd by an external mechanical shock, resulting
in variation of the pressure exerted.
In order to overcome the aforesaid disadvantage caused
by the prior art cooling structures, various fluid thermal con-
ductive materials such as thermally conductive inert gas, or a
li~uid metal or thermal silicon grease, or a compliant thermal
conductive material is adhesively inserted into the interface be-
tween the surfaces of a heat transfer element of a cooling module
and a circuit component. For example, thermal conductive inert
gas is introduced into the interface by Chue et al., and a low
3 - 25307-192
boiling point liquid i9 utilized to immerse a heat transfer
piston and a circuit component by Oktay et al.. However, a
complicated and costly sealiny structure for sealing the gas or
the liquid is required for both cooling modules, which is a draw-
back. In addition, the thermal conductivity of the associated
inert gas and liquid set the reduction of the relevant thermal
contact resistance within an upper limit.
While Berndmaier et al. and Babuka et al. employ liquid
metal or alloy to fill up a contact interface. Ostergrane et al
disclose the use of thermal grease in the interface of conieal
surfaces of a piston and a hat, and the use of a liquid metal
layer between the piston and a circuit element. In this struct-
ure, a rather thick layer of the thermal grease may be required to
maintain the layer on the conical surface of the piston, causing
undesirable increase in thermal resistance of the layer. In add-
ition, liquid metal of some kinds usually has a probability of
ch mical reaction with contacting material, requiring various
counter measures to prevent the reaction.
Yamamoto et al. insert a compliant sheet between a cir-
cuit component and a heat transfer plate urged toward the circuit
component by a bellows to reduce thermal contact resistance across
the interface between the component and the heat transfer plate.
A relatively thick compliant sheet, however, is required in order
to realize a desirable perfect thermal contact between the sheet
surfaces and the contacting surfaces of the relevant circuit ele-
ment and the heat transfer plate by expelling small air voids re-
57~
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maining on the contactiny surfaces. As the result, the reduction
of the whole thermal contact resistance across the interface is
adversely afEected. Tajima provides a cooling structure compris-
ing a cap having a spherical top surface, a stud haviny a concave
spherical bottom surface engageable with the spherical surface of
the cap, and a cooling hat. The cap contacts with a circuit ele-
ment with a small gap therebetween which is filled with thermal
grease, while the stud is secured to a cooling header. The cap
and the hat are in contact with each other through a layer of
thermal grease which has a considerable thickness sufficient to
protect the circuit element from being subject to a pressure.
Tajima discloses nothing about pressure to be exerted to the
thermal grease layer.
In these prior art cooling modules, much effort has
been made to reduce the heat transfer resistance across a thermal
contact interface utilizing various thermally conductive compound
material, however, the results are not suEficient to maintain a
desirably low, stable and reproducible thermal contact resistance.
SUMMARY OF THE INVENTION
The primary object of the present invention is to eli-
minate the aforementioned drawbacks of the prior art cooling
structures, and to provide a cooling system of an electronic cir-
cuit with a high performance cooling structure which can effect-
ively, steadily, and uniformly cool circuit components contained
in the circuit. The next object is to provide a stable and a re-
liable cooling structure having a stabilized and a thermally well-
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conductive layer of a liquid ma-terial in a thermal contact in-
terface between a heat transfer plate and a circuit component.
To achieve the aforesai.d objects, according to the
present invention, at first, the:re is previously conducted an
experiment to define a critical pressure Pc, namely the minimum
value of a pressure which is to be initially exerted to a layer
of thermally conductive compound located in a thermal contact
interface between a heat transfe:r plate and a circuit component.
The relationship between the heat contact resistance across the
interface and the pressure exerted thereon, represents a hyster-
esis characteristic. The heat contact resistance decreases as
the initially exerted pressure Pi increases before exceeding a
pressure, defined as a critical pressure Pc. Thereafter, the
heat transfer resistance remains unchanged as the pressure is
further increased till a maximum pressure Pm. Then the pressure
Pi is gradually decreased. Then, the heat contact resistance of
the thermal contact interface remains at the same value until the
initial pressure Pi is reduced to a small value Pa approximately
equal to zero pressure. The utilization of the above described
hysteresis characteristics between the pressure exerted on a
thermally conductive compound layer and thermal contact resist-
ance is the focus point of the present inven-tion.
In practice, an initial pressure higher than an ex-
perimentally defined critical pressure Pc, is exerted to a therm-
ally conducti~e compound layer which is already disposed in a
thermal contact interface by using a jig or a hydraulic pressure
~57~3
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of the associated coolan-t. Thus previously pressed thermally
conductive compound has a favorably low heat contact resis-tance
and is capable of maintaining the value in a stable manner under
a low pressure~ This phenomenon was found by the inventors by
conducting and repeating a number of experiments and practices.
As the result, during the operation of the relevant
electronic circuit, a pressure Pa lower than the initially exerted
pressure Pm but higher than zero, is exerted to -the thermal in-
terface sufficiently to maintain a small heat contact resis-tance
across the interface, enabling a stable and effective heat re-
moval from the relevant components. Of course, during a period
where the electronic circuit is out of operation~ the pressure Pa
is exerted to the circuit element. Hereinaf-ter, therefore, the
pressure Pa is referred to as a working pressure. With respect to
a multi-chip printed circuit board mounting a number of components
theron, pressurizing jigs of two types for exerting the initial
pressure Pi to the thermally conductive compound layers are dis-
closed in first and second embodiments. ~The use of hydraulic
pressure of the associated coolant for processing the thermally
conductive compound layer is disclosed in a third embodimen-t.
; Means for preven-ting the thermally conductive compound from
flowing off the located interface is disclosed in a fourth em-
bodiment. In all the embodiments, bellows are used as a resilient
member for elastically pressing the heat transfer plate toward the
corresponding circuit component in combination with a hydraulic
pressure of fluid coolant.
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The thermally conductive compound is selected from a
thermal grease, favourably a thermal silicone grea~e.
According to a broad aspect of the invention there is
provided a cooling system used with a printed circuit board having
at least one solid circuit component, sald cooling system
comprising: a cooling header having a coolant passage disposed
therein, a heat transfer means operatively connected to said
cooling header such that at least a part of sa.id heat tran.sfer
means is exposed to a liquid coolant flowing through said coolant
passage and such that heat may be transferred from said heat
transfer means to said liquid coolant; an elastic ~eans, connected
to said heat transfer means and said coolant header, for biasing
said heat transfer means against said solid circuit component with
a working pressure higher than zero pressure, and a thermally
conductive compound means, disposed between said heat transfer
means and said solid circuit component, establishing a thermal
contact between said heat transfer means and said solid circuit
component, wherein said thermally conductive compound means is
initially pressed with an initial pressure higher than a critical
pressure which is experimentally determined to reduce and
stabilize the thermal contact resistance between said heat
`~ transfer means and said solid circuit component under said working
pressure.
According to another broad aspect of the invention there
is provided, in a thermally heat conducting means comprising a
first member having a first surface, a second member having a
second surface, and a thermally conductive compound material
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interposed between said first surface and said second surface,
said second surface facing said first surface, said surfaces being
rough for a thermal contact, and a pressure being exerted such
that said first member and said second member are pressed to each
other, a method for processing said thermally conductive compound
means to reduce and stabilize thermal contact resistance between
said first member and said second member comprising the step~ o~:
interposing said thermally conductive compound means hetween said
heat transfer means and said solid circuit component, providing
said thermally conductive compound means with an initial pressure
higher than a critical pressure which is experimentally
determined; and reducing said initial pressure to said working
pressure higher than zero pressure.
According to another broad aspect of the inventlon there
is provided, in a cooling system used with a printed circuit board
having at least one solid circuit component, said cooling system
including a cooling header having a coolant passage disposed
therein, a hydraulic pump operatively connected to said coolant
passage for pressurizing a liquid coolant to flow through said
coolant passage, and a cooling module disposed corresponding to
said solid circuit component, said cooling module comprising a
heat transfer means operatively connected to said cooling header
such that at least a part of said heat transfer means is exposed
to the flow of said liquid coolant and such that heat may be
transferred from said heat transfer means to said liquid coolant
and an elastic means connected to said heat transfer means and
said coolant header, for biasing said heat transfer means against
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said solid circuit component with a working pressure higher than
zero pressure, and a thermally conductive compound means, disposed
between said heat transfer means and said solid cir~uit component,
establishing a thermal contact between said heat trans~er means
and said solid circuit component, a method for forminy said
thermally conductive compound means to reduce and stabilize
thermal contact reslstance between said heat transfer means and
said solid circuit component under said working pressure
comprising the steps of: interposing said thermally conductive
compound means between said heat transfer means and said solid
circuit component; providing said thermally conductive compound
means with an initial pressure higher than a critical pressure
which is experimentally determined; and reducing said initial
pressure to said working pressure which is lower than said initial
pressure.
The features and the advantages of the present invention
will be apparent upon reading the following descriptions and
claims with reference to the drawings where like reference
numerals denote like parts.
BRIEF D~SCRIPTION OF TH~ DRAWINGS
Fig. 1 is a schematic magnified cross-sectional view of
a thermal contact interface between the members containing a
thermally condu~tive compound layer therein, for explaining heat
transfer thereacross;
Fig. 2 is a schematic cross-sectional view of an experi-
mental apparatus for studying the relationship between the heat
contact resistance across the thermal contact interface of Fig. 1
~2~53
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and a pressure exerted thereon,
Fiq. 3 is a diagram representing result~ of an experi-
ment conducted by using the apparatus of Fig. 2,
Fig. 4 is a schematic cross-sectional view of a coolin~
module according to the present invention, illustrating a typical
structural configuration thereof,
Figs. 5(a) and 5(b) are schematic cross-sectional views
of a part of a cooling system according to the present invention,
illustrating the structural configuration and a method for press-
ing the thermally conductive compound layer of a first embodiment,
Figs. 6(a) and 6(b) are schematlc cross-sectional views
of a part of a cooling system according to the present invention,
illustrating the structural configuration and a method for press-
- 8 - 25307-192
ing the thermally conduc-tive compound layer of second embodiment,
Fig. 7 is a schematic cross-sectional view of a part of
cooling system according to the present invention, illustrating
its structural configuration and a method for pressing the therm-
ally conductive compound layer of a third embodiment, and
Fig. 8 is a perspective view of circuit component
mounted on a printed circuit board, illustrating the structural
configuration of a barrier means of a fifth embodiment of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
.
Generally, thermal contact resistance across any inter-
face between two surfaces of bodies is a function of contact area,
surface finishing and flatness, and applied load between the
surfaces. Actually, contact area varies dependiny on the above-
described factors e~cept the filling material. In most appli-
cations, improvement in the surface finish and the flatness is
expensive in this field and is not economical. Therefore, a
thermally conductive layer is interposed between the two surfaces,
filling small gaps in the interface area and expelling air micro-
2Q voids remaining on the surfaces, to reduce the -thermal contact
resistance thereacross.
Fig. 1 is a magnified cross-sectional view of an inter-
face between surfaces of a heat transfer plate 103 and a circuit
component 133 thermally contacting with each other through a
layer of thermally conductive compound, typically thermal grease
131. The roughness and irreguralities ranging below 1 um of the
57~3
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two surEaces are inevitable, causing a number of air ~icro--voids
100 thereon where air tends ~o be trappcd. Therma] contact re-
sistance across the in-terface can be reduced by eliminating -the
trapped air and replacing it with thermal ~rease 131, since
silicone oil has a -thermal conductivity several times higher than
that of air. Usually, the viscosit~ of a thermal grease is ad-
justed so that -the thermal grease is semi-flowable, for maintain-
ing a paste-like consistency. The thermal grease 131 con-tains
fillers consistin~ of fine particles of metal oxides or ceramics,
suspended in a carrier fluid such as silicone oil. Increase in
the mutual contact between the fine particles, or between the
fine particles and the surfaces of the interface, is considered
to reduce the thermal contact resistance of the interface filled
with the thermally conductive compound layer. ~ccording to
accumulated experience of an inventor, the heat contact resistance
across a thermal contact interface containing a thermally conduct-
ive compound therein has a hysteresis relationship with a pressure
exerted thereon as described before. The reason why this phen-
omenon occurs is not sure bu-t may be caused according to the
above-described consideration.
Fig. 2 is a schematic cross-sectional view of an ex-
perimental apparatus for studying the relationship. In the appa-
ratus, a first copper disk plate 214, a layer of thermal grease
215, a second copper disk plate 211, a heater disk 223, and a load
sensor 224 are stacked in the recited order coaxially forming a
cylinder block which is surrounded by a thick thermal insulator
~2 ~ J~
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layer 225 for thermally insulating the bottom side and the cylind-
rical side of the block. A coolan-t passage 212, namely a water-
passage is disposed in a cooliny header 220. The first copper
disk plate 214 is connected to the one end of a bellows 213 which
is water tightly secured to the cooling header 220 at the other
end, being operatively connected Io the coolant passage 212. 'rhe
coolant, water, flows through the coolant passage 212 as indicated
by an arrow 217, and then the flow direction is -turned toward the
first copper disk 214 by means of a deflector 216 disposed inside
of the water passage 212, as indicated by an arrow 218, removing
heat generated in the heater disk 223. Hereby, the cooling head-
er 220 is fixed and the cylindrical block is vertically movable
by the vertical movement o~ a table 210 on which the thermal in-
sulator 225 and the cylinder block is placed.
By the structure of the above-described configuration,
the whole heat generated in the heater disk 223 can be considered
to flow in an upward direction as indicated by an arrow 219 in
good approximation. The temperature tl of the first copper disk
plate 214 and the temperature t2 of the second copper disk plate
211 are measured by thermo-couples 221 and 222 attached to the
disk plates 214 and 211 respectively. Thus the heat contact
resistance Rcont of the thermal interface of the thermal grease
layer 215 can be easily figured out by measuring the temperatures
tl and t2, and the heating power Q inputted to the heater disk
223, following a formula:
Rcont = (t2 - tl)/Q
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The load pressure P loaded to the thermal grease layer
215 is detected by the load sensor 224, a piezoresistive device
such as a silicon pressure transducer available in the market.
Hereby, the pressure exerted to the thermal grease 215 is the sum
oE the hydraulic pressure of the coolant and the resilient press-
ure of the bellows 213. The hydraulic pressure is changeable
using a hydraulic pump and a piping line (both not shown), and
the resilient pressure of the bel:Lows 213 can be changed in some
range by adjusting the vertical position of the base table 210
upwardly or downwardly as indicated by a twin heads arrow 221.
E'ig. 3 is a diagram representing an experimental result.
The heat contact resistance Rcont is taken on the ordinate and
the load pressure P on the abscissa. At the beginning of this
experiment, the load pressure P is gradually increased from zero
to a maximum pressure Pm and kept for 3 minutes~ Thereafter, the
load pressure P is gradually reduced toward zero value. Each
experimental datum is plotted as indicated by a small circle,
initially in the leftward direction indicated by an arro~ R/ and
after reaching a point M, toward the le~tward direction indicated
by an arrow L. Apparently, the curve of Fig. 3 represents a
hysteresis character. The curve has a turning point C at a load
pressure which is referred to as a critical pressure Pc. At a
pressure higher than Pc, the heat contact resistance remains
unchanged, being maintained until the load pressure P is in-
creased to Pm, and then decreased from Pm to Pa near the zero
value. It is confirmed that the heat contact resistance at a
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load pressure of 10 grams, as indicated b~ a point A, is still
almost the same value as that at a higher load pressure. This
hysteresis character is fully utilized in the present invention.
Basically, the present invention relates to a printed
circuit board mounting a number of circuit components such as IC
chips thereon, namely a multi-chip printed circuit board. At
first, however, a single cooling module applied to a single chip
and an associated thermally conductive compound layer, are de-
scribed to explain the principle of the present invention. Fig.
4 is a schematic cross-sectional view of a cooling module, illu-
strating its structural configuration. Corresponding to the
cooling module, a circuit component 11, such as a semiconductor
IC device, is mounted on a printed circuit board 10. A thermally
conductive compound layer 15, such as a layer of thermal grease,
is interposed between the top surface of the circuit component 11
and a heat transfer plate 14 made of metal having high thermal
conductivity, such as copper. The heat transfer plate 14 is
connected to a bellows 13 in liquid tight fashion at its one end.
At the other end thereof, the bellows 13 is tightly secured to a
cooling header 20, being operatively connected to a coolant
passage 12 disposed in the cooling header 20. The coolant which
may be liquid coolant or gaseous coolant, flows through the cool-
ant passage 12, contacting the exposed surface of the heat trans-
fer plate 14. The flow of the coolant is directed toward the heat
transfer plate 14 guided by the coolant passage 12 as represented
by an arrow X such that the heat generatèd in the circuit ! I
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component ll and transferred to the heat transfer plate 14 across
the thermally conductive compound layer 15, is effectively re-
moved by the coolant. The thermally conductive compound layer 15
may be coated on the top surface of the circuit component ll or
the bottom surface, facing the circuit component 11, of the heat
transfer plate 14. As a result, both surfaces come in full con-
tact with each other even when the surfaces are uneven and rouyh.
Hereby, the cooling header 20 is fixed, and a stacked
block comprising the heat transfer plate 14, thermally conductive
compound layer 15 and the circuit component ll is vertically moved
to exert a pressure on the thermally conductive compound layer 15
by vertically moving the printed circuit board 10. With the
above-described structure of the cooling module, the thermally
conductive compound layer 15 can be pressed before operation in
order to enhance and stabilize the thermal conductivity of the
layer 15 in the manner as described before with respect to the
experiment the results of which are represented in ~ig. 3. That
is, the layer 15 is pressed by elevating the printed circuit
board 10 or by increasing the hydraulic pressure of the coolant,
or by combining both operations, to a pressure Pm higher than the
critical pressure Pc which is determined by an experiment con-
ducted beforehand. The pressure Pm is maintained for a prede-
termined time, usually for three minutes. Thereafter, the press-
ure is decreased from the pressure Pm to a working pressure Pa
which may be slightly higher than zero. However, in practice,
the working pressure Pa is selected to a higher value sufficient
~ 25307-192
to ahsorb an inevitable fluctuation of the relevant hydraulic
pressure of the coolant or the elastic pressure of -the bellows.
Thus the thermally conductive compound layer 15 establishes a
compound contact or grease contact between the heat -transfer
plate 14 and the circui-t component 11.
Figs. 5(a) and 5(b) are substantially schematic, part-
ial cross-sectional views of a cooling system of a first embodi-
ment of the present invention, illustrating a series of cooliny
modules Eor the corresponding circuit components 11 mounted on a
mul-ti-chip printed circuit board 10. The cooling modules are
connected to a fixed cooling header 20 through which a coolant
passage 12 is disposed. The coolant is pressurized by a hydraulic
pump 19 of the relevant cooling system (not wholly shown) of the
electronic circuit apparatus, running through the coolant passage
12, and removing heat generated in each circuit component 11
through the corresponding cooling module. Each cooling module
comprises a bellows 13 fixed to the cooling header 20 at one end,
being operatively connected to the coolant passage 12, and to a
hea-t transfer plate 14 at the other end. The pxinted circuit
board 10 is supported a-t its peripheral edge by a flange 22 which
is secured to the lower portion of the cooling header 20. The
axes of the cooling modules are arranged vertically to the corre-
sponding circuit components 11. Thermally conductive compound,
such as silicone thermal grease available in the market, is coated
to form a layer 15 on the top surface of the circuit component 11
or on the bottom surface of the heat transfer plate 14. Then, the
- 15 - 25307-192
printed circuit board 10 is vertically elevated usiny a movable
base plate jig 16, namely a table elevator, and is pressed against
the cooling modules. Thus, each circuit component 11 is pressed
against the corresponding heat transfer plates 14, exer-ting a
pressure P to each thermally conductive compound layer 15, as
shown in Fig. 5(a). The pressure P is increased up to Pm higher
than the critical pressure Pc which is defined by an experiment
conducted beforehand. The pressure Pm is maintained for a time,
for instance three minutes, and thereafter, the base table 16 is
lowered decreasing the pressure P until the printed circuit board
16 is held again by the flange 22 as shown in Fig. 5(b). The
printed circuit board 10 is fixed to the flange 22, In this sit-
uation, a working pressure Pa higher than zero is provided to the
thermally conductive compound layer 15. As the result, a stable
and high heat transfer from the circuit component 11 to the cool-
ant is established, providing the relevant electronic circuit
apparatus with high reliability.
Figs. 6(a) and 6(b) are substantially schematic cross-
sectional views of a cooling system of a second embodiment accord-
ing to the present invention, illustrating a series of cooling
modules engaging with the corresponding circuit components 11
mounted on a multi-chip printed circuit board 10. In the second
embodiment, the cooling header 20 is separatable into a lower
cooling header 20a and an upper cooling header 20b. The lower
cooling header 20a has an opening 20c i~ the upper portion thereof.
The relevant heat transfer plates 14 are pushed downwardly as in-
~57~;~
- 16 - 25307-192
dicated by an arrow Y with a pushing jig 17, pressing the therm-
ally conductive compound layers 15 interposed between the top
surface of the circuit components 11 and the heat transfer plates
14, against the corresponding circuit components 11. The pushing
jig 17 has a plural number of rods 21 having a ball shaped tip at
each end, and each rod 21 is insexted into each bellows 13
through the opening 20c of the lower cooling header 20b, pressing
each heat transfer plate 14 connected to the bellows 13 against
- the circuit components 11. The pushing jig 21 is driven by a
mechanical power source (not shown) such that the thermally con-
ductive compound layers 15 are subject to a pressure changing
according to the same time schedule as that of the preceding em-
bodiment. Then, the opening 20c of the lower cooling header 20a
is closed in a water-tight manner by the upper cooling header 20b,
namely a header cover, utilizing an "Q" ring 23 and screws 24 in
a conventional manner. The effect of the thus treated thermally
conductive compound layer 15 is the same as that of the first em-
bodiment.
Fig. 7 is a substantially schematic cross-sectional view
of a cooling system of a thi~d embodiment of the present invent-
ion, illustrating a series of cooling modules for the correspond-
ing circuit components 11 mounted on a multi-chip printed circuit
board 10. In the third embodiment, the structural configuration
of the cooling system is the same as that of the first embodiment,
however, instead of pressing jigs used in the preceding embodi-
ments, hydraulic pressure of coolant is used by controlling the
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- 17 - 25307-192
relevant hydraulic pump 19 connected to an opening of a cooling
head such that the thermally conductive compound layer 15 can be
subject to a pressure varying according to a similar time schedule
as employed in the Eirst and the second embodiments. ~ereby, the
other opening of the cooling header 20 is closed by a stopping
cover 26 as shown in the ~igure, or by using a stop valve (not
shown). The output pressure of the pump 19 is controlled by con-
trolling a driving motor (not shown) such that the above-described
pressurizing pattern is achieved. The effect of the cooling mod-
ules is the same as those of the preceding embodiments. Although,
the adjustment of the hydraulic pressure by controlling the re-
levant hydraulic pump is necessary, the initial pressing o~ the
thermally conductive compound layer is easily performed by the
method of the third embodiment.
Fig. 8 is a perspective view of circuit components 11
mounted on a printed circuit board 10 having square ring-shaped
barrier means 25 disposed on the top surface of the circuit com-
ponents 11, for preventing thermally conductive compound layers
15 from flowing away from the area initially disposed. The bar-
rier means 25 is formed to surround the area, and may be formed
in a single piece body with the circuit component 11 on the top
surface thereof, or may be formed separately using a material
such as ceramic, silicone rubber, etc. and adhesively dispoed on
the top surface of the circuit component 11. The height of the
barrier 25 is selected to be slightly smaller than the thickness
of the thermally conductive layer 15 in order to enable the com-
~.2~53
- 18 - 25307-192
pound layer to be pressed afterwards. The barrier means 25 may
be formed on the bottom surface of the associated heat transfer
plate 1~ (see Figs. 5, 6, and 7).
The many features and advantages of the present invent-
ion are apparent from the detailed specification and appended
claims, to cover all such features and advantages of the appara-
tus which fall in the true spirit and scope of the invention.
Further, since numerous modifications and changes will readily
occur to those skilled in the art, it is not desired to limit the
invention to the exact construction illustrated and described.
Accordingly, all suitable modifications and equivalents may be re-
stored to falling within the scope and spirit of the invention.
