Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HEAT PIPE THERMAL MANAGEMENT APPARATUS
This application claims the benefit of U.S. Provisional Application No.
60/065,418 filed November
13, 1997.
This invention relates to a heat pipe apparatus for thermal management, and
more particularly to
improved thermal management devices comprising a heat pipe and molded heat
sink components.
Still more particularly, the invention relates to a thermal management
apparatus comprising a heat
pipe component in thermal communication with a heat sink component wherein the
heat sink
component comprises a moldable, thermally conductive, filled polymer and
preferably an injection
moldable, thermally conductive, filled liquid crystal polymer. Preferably, the
thermal management
apparatus will be formed as a unitary structure by an insert molding
operation.
The market segment for electrical devices such as for windings of motors,
transformers and
solenoids is increasingly moving to miniaturization of such devices. This in
turn leads to a rise in
internal equipment operating temperature resulting in not only a need for
higher temperature ratings
on insulation materials used for these applications, but also a need for
improved methods for
removal and dissipation of heat. Heat generation is also a problem in a great
variety of electronic
devices comprised of a semiconductor component, such as, for example laser
diodes, light-emitting
diodes, thyristors, microwave electron transfer devices and the like.
Semiconductor or multichip
modules that consist, for example, of monocrystalline silicon having millions
of transistors
positioned on or near a single surface of the chip produce considerable
amounts of heat energy in
operation. Removal of byproduct heat is important to the lifetime of
electronic components and the
art has continually sought improved thermal management systems and methods for
managing the
high levels of heat output associated with such devices.
Thermal management has long been the subject of extensive study and research.
Early practice
relied on the use of heat sinks, including carriers and housings, constructed
of metals and alloys
selected for their high thermal conductivity, and such devices continue to
find wide use. More
recent innovations and modifications in materials have included, for example,
combinations of metal
housings with metal-coated diamond chips or wafers, intended to take advantage
of the fact that
diamonds have the highest thermal conductivity known. The obvious shortcomings
of devices based
on diamond, particularly including the practical considerations imposed by the
limited size of
diamond components, as well as high cost, fed to the development of metal
matrix composites
containing diamond particles as a filler to increase thermal conductivity.
Other solutions for thermal
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management problems designed particularly for use with high power density
devices include liquid-
cooled heat sink structures and chip module housings that rely on liquid
nitrogen as the coolant.
A disadvantage of the prior art structures lies in the complexity associated
with their fabrication.
Generally, thermal management devices have been constructed of metal,
primarily because of the
requirement for excellent heat transfer characteristics in combination with
good mechanical
properties. There are substantial coefficient of thermal expansion (CTE)
differences between most
metals used in thermal management devices and the electronic components that
will be cooled, as
well as between these components and the plastic case and components housing
the devices. These
differences may impose substantial mechanical stress on mating components,
providing
opportunities for failure during use. Matching CTE properties of heat sink
materials with those of
semiconductors requires the use of dense alloys that are difficult to machine
and adds significantly
to the weight of the device. Compensating for large differences in CTE is also
practiced, but this
requires complex designs that are difficult to fabricate. Further, effective
dissipation of the heat by
convection is a function of surface area. As thermal loads increase it becomes
necessary to employ
convective heat exchange components with still larger surface areas, again
adding weight and
impacting design flexibility.
Lower density materials have been suggested as metal replacements in thermal
management.
Particularly attractive are structures comprising carbon or crystalline
graphite; both materials are
highly thermally conductive, have substantially lower densities than the
metals they replace and may
be made into structures with a low and even negative CTE. Although light
weight graphite
structures and carbon-carbon composites are known and accepted for use in heat
sink and other
thermal management applications, fabricating complex structures from these
materials is generally
difficult and thus such components may be more costly than those constructed
from metal.
Thermoplastic resins with good molding properties are readily available as are
castable and
moldable thermoset resins. However, resins generally have a high thermal
expansion coefficient and
are poor conductors of heat. Few are capable of withstanding thermal cycling
over a wide range of
temperatures without undergoing failure through creep or warping or, in the
case of rigid thermoset
resins, cracking or similar failure due to thermomechanical stress. Adding
fillers to resins as a
method for reducing CTE and thereby improving dimensional stability is well
known and widely
used in the resin formulating arts and may also be found useful for improving
thermal conductivity.
Particulate materials including conductive carbon or graphite fillers,
spherical particles of various
metals, glass or carbon black, non-spherical metal or ceramic particles,
stainless steel filaments,
aluminum fibers and the like are disclosed in the art and characterized as
particularly useful where
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improved thermal conductivity is desired. However, where resins. containing
these fillers have been
employed for thermal management purposes only modest improvement has been
realized.
Commonly, filled thermoset resins used commercially in the electronics
industry have thermal
conductivities on the order of 2 to 4 W/mK, while injection moldable filled
thermoplastic
formulations are disclosed with thermal conductivities in the 4 to 9 W/mK
range. The thermal
conductivity of even the most conductive of the filled resin formulations
disclosed in the art falls
below 10 W/mK, while most commercially-available filled resins comprising such
highly
conductive fillers as metallic filaments and the like are still much lower in
thermal conductivity,
generally as low as 2 to 3 W/mK. Filled resins, and particularly filled
thermoplastic resins, thus
have found limited acceptance and are generally better suited for use where
thermal loads are low
and where minimizing the size of the heat exchange device is not an important
design factor.
More recently, computer manufacturers have turned to heat pipes as means for
e~ciently and
rapidly transferring heat away from a heat source such as a microprocessor
semiconductor
component for further dissipation. Typically a heat pipe will be a hollow
metal tube partially filled
with a fluid, although alternative forms include heat pipes comprising a solid
heat conducting
material may also be employed in these structures. In use, the evaporator or
heat input zone of the
heat pipe will be thermally coupled either directly to the semiconductor
structure being cooled or,
more commonly, to an interposed heat sink in thermal communication with the
device. Heat
removed to the condenser or heat dissipation zone of the heat pipe will be
dissipated into the
surroundings by means of thermally coupled cooling fins or a second heat sink
element such as a
thermal plate or the like.
Good thermal transfer between the components is important for effective and
efficient operation and
the metal fins and heat sink elements are therefore generally swaged, soldered
or brazed to the heat
pipe. Alternatively, the components may be held in mechanical contact by
fasteners, clamping
devices or the like, and thermally conductive adhesives have also been
employed for these purposes.
Where the heat pipe element of an assembly is intended to be displaced, for
example, rotably or
slidably relative to a heat sink element while in use and therefore cannot be
permanently attached
thermal grease has been employed to fill airgaps and provide continuous
contact region between the
contacting surfaces of the parts. See U.S. 5,598,320. Hinged computing devices
have also been
disclosed wherein a heat pipe serves as the pintle or hinge pin to transfer
heat to the display housing
through the gudgeon receiving the pintle. See U.S. 5,621,613.
It is also known in the art to fabricate heat pipe panels having internal
micro heat pipes by forming
channels within a substrate followed by enclosing the channels. For example,
heat panels of vapor
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deposited tungsten or tungsten-rhenium alloy having internal tubular
passageways foaming micro
heat pipes have been disclosed. See U.S. 5,598,632.
Although the use of heat pipes may greatly improve the efficiency of heat
removal, it will be
understood that the removed heat will then be dissipated, normally into the
surroundings, generally
requiring the use of heat sinks, such as thermal panels or the like. In
practice these latter
components most often have taken the form of a rigid metal structure such as a
thermal plate placed
in thermal communication with the environment, for example, as an external
feature or structural
component of the case of portable electronic devices. Requirements imposed by
the thermal
management device are thus seen to continue to impact and restrict design
flexibility, as well as to
increase the overall weight of the device.
The art continues to seek more flexible solutions for thermal management
problems. Even when
further improvements in heat removal have been achieved, heat loads have
continued to increase
with the demand for ever smaller electronic devices. Heat sinks necessarily
must increase in size to
provide adequate dissipation of the removed heat to the surroundings, in turn
impeding the trend
toward miniaturization. A thermal management apparatus comprising heat
dissipating means
formed of thermally conductive, lower density, readily molded structural
materials suitable for use
as the case component of an electronic device would be a useful advance in the
thermal management
art.
SUMMARY OF THE INVENTION
The improved thermal management apparatus of this invention comprises a heat
pipe in thermal
communication with a molded, thermally conductive heat sink comprising a
filled, thermally-
conductive resin. The resin can be thermoplastic or thermoset. Examples of
thermoplastic resins
which are suitable for this invention include liquid crystal polymers (LCPs),
aliphatic polyamides,
polyphthalamides, acrylonitrile butadiene styrene resins (ABS), and polyaryl
ether resins such as
PPO and PPS resins. Several thermoset resins, including epoxy resins, cyanate
resins, thermoset
polyesters and phenolic resins are also suitable for use in the present
invention. Thermoset resins
are particularly useful when transfer molding is used to maufacture the molded
heat sink. Other
molding techniques such as compression and injection molding can also be used.
In one preferred embodiment the heat sink component is injection molded from a
thermally-
conductive, filled liquid crystal polymer. The heat pipe will be positioned in
the molded heat sink to
place selected portions of said heat pipe in thermal communication with the
heat sink, preferably by
insert molding to afford an integral unitary construction having excellent
thermal transfer
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characteristics and without the need for thermal grease or the like. In
another embodiment the heat
sink component is compression molded from a thermally-conductive, filled epoxy
resin.
The good thermal properties and dimensional stability and the excellent
mechanical strength of
filled LCP resins together with low mold shrinkage and ease of fabrication by
injection molding into
parts having close tolerances and very thin sections make these thermally
conductive molding
compounds particularly well-suited for constructing components having complex
designs that are
highly desirable and useful in thermal management, particularly for electrical
and electronic devices.
..
DETAILED DESCRIPTION
In the. preferred embodiment, the thermal management devices of this invention
wii3 comprise at least
one. heat pipe, together with one or more molded thermoplastic or thermoset
heat sink components.
For the purposes of this invention, a heat sink includes any structure to
which heat is transferred.
The thermoplastic and thermoset formulations suitable for these purposes will
be thermally conductive,
preferably having a thermal conductivity greater than I ~ W/mK and as great as
600 W/mK or
more, readily moldable at temperatures that wilt not cause damage to the heat
pipe, and with high melt
IS flow at the molding temperature. Still more preferred will be thermoplastic
formulations which may
be described as having high tensile modulus values, generally greater than 7 x
106 psi, approaching the
stiffness and rigidity of lighter metals including magnesium and aluminum.
A preferred thermoset formulation suitable for use in this invention will
comprise a thermoset resin,
such as an epoxy resin, filled with discontinuous pitch-based carbon fiber.
Thermoset resins are well
known and described in the art and are characterized by having low viscosity
in the uncured state
which facilitates wet out of the carbon fibers.
A preferred thermoplastic formulation suitable for use in the practice of this
invention will comprise a
liquid crystal polymer (LCP) resin filled with discontinuous pitch-based
carbon fiber.
LCP resins are well known and described in the art. Those further
characterized as thermotropic liquid
crystal polymers (LCP) exhibit optical anisotropy when molten, together with a
remarkably low melt
viscosity at melt fabrication temperatures. When further compounded with high
levels of filler, even
to levels as great as 7~ wt% based on weight of resin and filler, such LCP
resins maintain good melt
processing character and moIdability.
The preferred LCP resins are aromatic polyesters derived from monomers
selected from one or more
aromatic dicarboxylic acids and one or more aromatic diols, together with one
or more aromatic
hydroxycarboxylic acids.
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Representative of aromatic dicarboxylic acids useful in forming the LCP resins
useful in the practice of
this invention are aromatic dicarboxylic acids such as terephthalic acid, 4,4'-
diphenyldicarboxylic acid,
4,4'-triphenyldicarboxylic acid, 2,6-naphthalenedicarboxylic acid,
diphenylether-4,4'-dicarboxylic
acid, diphenoxyethane-4,4'-dicarboxylic acid, diphenoxybutane-4,4'-
dicarboxylic acid, diphenylethane-
4,4'-dicarboxylic acid, isophthalic acid, diphenyl ether-3,3'-dicarboxylic
acid, diphenoxyethane-3,3'
dicarboxylic acid, diphenylethane-3,3'-dicarboxylic acid and naphthalene-1,6-
dicarboxylic acid, and
derivatives of the aforementioned aromatic dicarboxylic acids substituted with
alkyls, alkoxyls or
halogens such as chloroterephthalic acid, dichloroterephthalic acid,
bromoterephthalic acid,
methyiterephthalic acid, dimethylterephthalic acid, ethylterephthalic acid,
methoxyterephthalic acid
and ethoxyterephthalic acid.
Aromatic diols which may be found useful in forming the LCP resins include
hydroquinone,
resorcinol, 4,4'-dihydroxydiphenyl, 4,4'-dihydroxytriphenyl, 2,6-naphthalene
diol, 4,4'-
dihydroxydiphenyl ether, bis(4-hydroxyphenoxy)ethane, 3,3'-dihydroxydiphenyl,
3,3'-
dihydroxydiphenyl ether, 1,6-naphthalenediol, 2,2-bis(4-hydroxyphenyl)propane,
2,2-bis(4-
hydroxyphenyl)methane, etc., and alkyl, alkoxyl or halogen derivatives of the
aforementioned aromatic
diols, such as chlorohydroquinone, methylhydroquinone, 1-butylhydroquinone,
phenylhydroquinone,
methoxyhydroquinone, phenoxyhydroquinone, 4-chlororesorcinol, 4-
methylresorcinol, etc.
Aromatic hydroxycarboxylic acids which may be found useful in Forming the LCP
resins include 4-
hydroxybenzoic acid, 3-hydroxybenzoic acid, 6-hydroxy-2-naphthoic acid and 6-
hydroxy-1-naphthoic
acid, etc., and alkyl, alkoxyl or halogen derivatives of the aromatic
hydroxycarboxylic acids such as 3-
methyl-4-hydroxybenzoic acid, 3,5-dimethyl-4-hydroxybenzoic acid, 2,6-dimethyl-
4-hydroxybenzoic
acid, 3-methoxy-4-hydroxybenzoic acid, 3,5-dimethoxy-4-hydroxybenzoic acid, 6-
hydroxy-5-methyl-
2-naphthoic acid, 6-hydroxy-5-methoxy-2-naphthoic acid, 3-chloro-4-
hydroxybenzoic acid, 2-chloro-
4-hydroxybenzoic acid, 2,3-dichloro-4-hydroxybenzoic acid, 3,5-dichloro-4-
hydroxybenzoic acid, 2,5-
dichloro-4-hydroxybenzoic acid, 3-bromo-4-hydroxybenzoic acid, 6-hydroxy-5-
chloro-2-naphthoic
acid, 6-hydroxy-7-chloro-2-naphthoic acid and 6-hydroxy-5,7-dichloro-2-
naphthoic acid, etc.
LCP resins comprising thio-containing analogs of these monomers, e.g. aromatic
thiol-carboxylic
acids, dithiols and aromatic thiol phenols, as well as the amide analogs
derived from hydroxylamines
and aromatic diamines, are also known in the art, and these resins may also be
found useful in the
practice of this invention.
As stated, the polymers useful in the practice of this invention will be those
LCP resins that are
anisotropic in the melt. Those skilled in the art will understand that whether
the polymer will be
anisotropic in the melt will be determined by the particular components
selected, the composition
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ratios in the polymer and the sequence distribution and will thus.select
monomers and composition
parameters according to experience and following knowledge and practice common
in the LCP resin
art.
The LCP resins especially preferred for use in the practice of this invention
will contain at least
10 mol%, preferably from 10 toy 90 mol,% of repetitive units containing a
naphthalene
moiety . , such , as, for example, a 6-hydroxy-2-naphthoyl, 2,6-
dioxynaphthalene or 2,6-
dicarboxynaphthalene moiety or the Iike. Particularly useful are polyesters
containing from'~o~,10-
90 mol%, preferably ~be~l 6~-85 mol%, more preferably 70-80 mol% of such
naphthalene units
together with, .90-10 mol%, preferably 20-30 mol% hydroxybenzoic acid-derived
units.
Polyesters containing from 30 to 70 mol%, preferably 40 to 60 mol% of
hydroxybenzoic acid units, from20 to~a~bat~ 30 mol% of 2,6-naphthalene diol-
derived units and
from 20 toy 30 mol% terephthafic acid-derived units may also be found useful.
Further representative.of the LCP resins that may be found useful are
polyesters containing from t
to ~het~ 40 mol%, preferably 20-30 mol% of 6-hydroxy-2-naphthalic acid-derived
units, from 10
15 mot% to SO.mol%, preferably 25-40 mol% hydroxybenzoic acid-derived units,
from 5
mol% to ~e~ 30 mol%, preferabiydabad~ 15-25 mol% hydroquinone-derived units
and from ~ mol%
to 30 mol%, preferably~s~i 15-25 moI% terephthalic acid-derived units;
polyesters containing
from 10 - 90 mol% of 6-hydroxy-2-naphthalic acid-derived units., from 5 to
~out145 mot%
terephthalic acid-derived units and 5 to 45 mol% of hydroquinone-derived
units; polyesters
20 containing 10-40 mol% 6-hydroxy-2-naphthalic acid-derived units,. together
with 10-40
mol% each of terephthalic acid-derived. units and hydroquinone-derived units:
and polyesters
containing 60-80 mol% 6-hydroxy-2-naphthalic acid-derived units, together with
~bou~ 10-20
mol% each of terephthalic acid-derived units and hydroquinone-derived units.
The molecular weight of the LCP resins employed in the practice of this
invention, as represented by
the resin intrinsic viscosity (LV.), will be at least ~pg~a~ 0.1 dl/g,
preferably will lie in the range of
from0.1 to ~be~ 10.0 dl/g when dissolved at 60°C. in pentafluorophenol
at a concentration of
0.1 wt%.
LCP resins and methods for their preparation are well known and widely
described in the art, and a
number of suitable LCP resins are readily available from commercial sources.
Particularly suitable
are the LCP resins sold by Amoco Polymers, Inc. as Xydar~ LCP resins.
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Thermally conductive fillers suitable for use in the practice of -this
invention include aluminum
nitride, boron nitride, alumina, graphite, pyrolytic graphite, aluminum,
copper and other metallic
particles, diamond, silicon carbide, and preferably, carbon fibers.
Carbon fibers suitable for use in the practice of this invention include
highly-graphitized carbon
fiber having a high thermal conductivity and a low or negative coefficient of
thermal expansion
produced from pitch. As used herein, the term "carbon fibers" is intended to
include graphitized,
partially graphitized and ungraphitized carbon reinforcing fibers or a mixture
thereof. The preferred
carbon fibers will be pitch-based carbon fiber having a thermal conductivity
greater than 1~e~1600
W/mK, preferably greater than'øbe~1900 W/mK, and still more preferably greater
than 1000 W/mK.
t0 Fiber with even greater thermal conductivities, as high as 1300 'W/mK up to
the thermal
conductivity of single crystal graphite, 1800 W/mK and higher will also be
suitable. Thermally-
conductive formulations may also be obtained using pitch-based carbon fiber
having a thermal
conductivity as low as 300 W/mK.
Pitch-based carbon fiber with thermal conductivities falling in the range of
from 600 W/mK greater
than 1100 W/mK, a density of from 2.16 to above 2.2 g/cc and a very high
tensile modulus, from
110x106 psi to greater than 120x106 psi, is readily obtainable from commercial
sources.
Commercial carbon fiber is ordinarily supplied in the form of continuous
carbon fiber tow or yarn
comprising a plurality, usually from 1000 to 20,000 or more, of carbon
filaments 5 to 20 microns in
diameter with the axially-aligned filaments providing strength in the fiber
direction of the tow.
Generally, where discontinuous fiber is employed the fiber may either be
chopped tow, generally
greater than 1 /4" in length, ordinarily from 1 /4" to 3/4" in length, or a
carbon particulate
with a length of from feet 25 to 1000 microns, preferably from 50 to 200
microns
obtained by milling or granulating carbon fiber.
The thermoplastic resins, including LCP resin, and carbon fiber may be readily
combined and
compounded generally by following processes and procedures commonly employed
in the resin
compounding art. For example, discontinuous carbon fiber may be dry mixed or
similarly combined
with the dry resin in any convenient form using any suitable, conventional
mixing means and then fed
to a compounding extruder, thereby producing a filled extrudate which may be
chopped for use in
further fabrication steps. Alternatively, thermoplastic resins together with
the requisite quantity of
carbon fiber in the form of continuous carbon fiber tow may be fed to a single
screw extruder and
extruded as a strand or pultruded, chopped to form pellets and collected.
Thennoset resins and carbon
fiber may be readily combined and compounded by dry blending or compounded by
way of mixers,
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extruders, stirrers, roll-mills, and impregnation devices such as prepreg
machines. In addition,
dipping, spraying or other coating processes may also be used.
As is generally known in the art, further processing of resin formulations
containing glass fiber, carbon
fiber or similar brittle fiber fillers using shearing means such as extruders,
injection molding machines
or the like generally will cause fiber breakage and further reduce the size of
the fibrous particulates.
The average size of the particulates in filled and molded articles produced by
high shear processing
means then will generally lie in the range of from 35 to 200 microns, ir-
espective of whether the fiber
was initially supplied in the fonm of continuous tow or as chopped or milled
fiber, and the aspect ratio
filler will be greater than ~be~4 and range on average up to 10. ~ When
processed using low-
shear conditions, for example, through compression molding or transfer molding
of when used in a
( flowable formulation as found in potting resin applications, the fiber
damage and breakage will be
minimized and the articles will then contain substantially greater length
fibers, ranging from 100
microns to as great as 1/4", i.e., the original length of the chopped tow
fiber, with a correspondingly
higher aspect ratio, generally above 10.
Generally, the filled resin formulations useful in the practice of the
invention will comprise from ~be~(
to 80 wt%, preferably from t45 to lebe~8 80 wt% and still more preferably from
f~beu~ 60
to 75 wt% carbon fiber and, correspondingly; from- 80 to ~e~t 20 wt%, more
preferably
from ~be~# 55 to f20 wt% and most preferred from ~e~g 40 to beat 25 wt%resin.
The
formulations may further include such plasticizers and processing aids, as
well as thermal stabilizers,
20 oxidation inhibitors, flame retardants, additional fillers including
reinforcing fillers and fiber, dyes,
pigments and the like as are conventionally employed in the compounding arts
for use with such
molding resins. It will be readily recognized that the utility of the filled
molding compounds of this
invention lies in the substantial thermal conductivity exhibited by the
material. These additional
components, as well as the amounts employed, will thus be selected to avoid or
at least minimize any
reduction in the thermal conductivity of the formulation.
The filled resin will preferably be injection molded with the heat pipe using
an insert molding
operation to provide a unitary structure. Insert molding processes typically
include providing an insert
within the mold and injecting the plastic material about the insert or desired
portions of the insert to
complete the component. In the practice of this invention a heat pipe is
inserted within the mold and
the filled LCP resin is then injected to surround the desired portion of the
heat pipe, filling the mold to
form the heat sink. The filled LCP resin, upon cooling, forms a molded heat
sink component having a
near interference fit at all points of contact with the surface of the heat
pipe thereby affording
excellent heat transfer between the components.
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As will be understood, the heat pipe is subject to being damaged when
subjected to high temperatures
or pressures. Heat pipes are intended to operate in particular environments
and within a particular
range of temperatures depending in part upon the working fluid, the materials
of construction and the
design. To prevent bursting of the heat pipe by overpressuring, the end seal
of heat pipe may be
designed to rupture when subjected to temperatures significantly above the
design upper limit.
Further, subjecting a heat pipe to high external pressure or other severe
mechanical stress may cause
the pipe to bend or distort and become inoperable. Highly filled polymers are
generally difficult to
injection mold and form a viscous melt that flows with difficulty, requiring
high injection pressures
together with stock temperatures well above the polymer melt temperature in
order to fill the mold
cavity. Non-uniform flow of the highly viscous melt within the mold cavity
will subject the heat pipe
insert to severe mechanical stress, causing the pipe to distort and even
become bent or folded. Filled
LCP resins such as those employed in the practice of this invention generally
will have a low melt
viscosity, and wilt permit molding using relatively low melt temperatures in
the range 400-700° F
(200-370° C), welt within the design limit for many heat pipes.
Moreover, excessive injection
pressures are not required to fill the mold thereby avoiding damage to the
heat pipe through
mechanical stress.
Inasmuch as these formulations are thermoformable, any of a variety of
conventional molding
equipment and processes adaptable for use in insert molding operations may be
employed to mold the
filled LCP resin with the heat pipe to form unitary thermal management devices
according to the
invention.
A wide variety of extrudable and injection moldable thermoplastic resins other
than LCP resins are
generally known in the art, and such resins, when filled with high modulus
carbon fiber to provide
thermally conductive resins that are injection moldable, may also be found
suitable for the purposes of
this invention. For example, aliphatic polyamides including those widely
available commercially such
as nylon 6, nylon 6,6, nylon 4,6, nylon 11 and the like; polyphthalamides,
including the commercially
available polymers of one or more aliphatic diamines such as hexamethylene
diamine, 2-
methylpentamethylene diamine and the like with terephthalic acid compounds as
well as copolymers
thereof with additional dicarboxylic acid compounds such as isophthalic acid,
adipic acid, naphthalene
dicarboxylic acid and the like; polyarylate resins including polyethylene
terephthalate (PET) resins,
polybutylene terephthalate (PBT) resins and the like; arylene polycarbonate
resins including
poly(bisphenol A carbonate); the well known polyaryl ether resins such as PPO
resins, including the
thioether analogs thereof such as PPS resins and the like and the
corresponding sulfone- and ketone-
linked polyaryl ethers such as polyether sulfones, polyphenylether sulfones,
polyether ketones,
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polyphenyl ether ketones and the like, are well known and filled and unfilled
resin formulations
containing such thermoplastics are readily available from a variety of
commercial sources.
Such thermoplastics, when filled with thermally conductive fillers
particularly including carbon fiber
as described herein above, may be suitably thermally-conductive for many
thermal management
applications. However, to obtain the high level of thermal conductivity needed
for use in heat sinks
for heat dissipation without requiring a significant increase in surface area,
and in other thermal
management applications where high thermal loads are anticipated, it is
necessary to employ a high
level of carbon fiber loading, generally at least as great as 45 wt%,
preferably from 50 to 80 wt%.
When filled to these levels, most such thermoplastic materials may become
quite difficult to mold,
requiring pressures and elevated temperatures not as well suited for insert
molding operations using
heat and pressure sensitive inserts such as heat pipes or the like.
Thermally conductive, filled thermoset resins are also known and many are used
commercially as
thermally conductive potting, encapsulating, adhesive and coating materials as
well as sheet molding
compounds, bulk molding compounds or the like, particularly for thermal
management in electronic
applications. Conventional thermoset resins including epoxy resins, cyanate
resins, novolacs, resoles
and similar thermosetting phenolic resins, thermoset polyesters, and the like
may usefully be
combined with chopped carbon fiber tow or milled or granulated carbon fiber as
described herein
above to provide thermoset molding resins and materials with thermal
conductivity suitable for use in
thermal management devices. Such formulations may be formed and fabricated by
conventional
means, ordinarily by use of compression molding or transfer molding processes,
or by use of a B-
staged resin composition in a thermoforming step or the like. When
appropriately formed and then
thermoset or cured, filled resin articles may be produced with high thermal
conductivities, from 2-5
W/mK to as high as 80-100 W/mK or more depending upon the level of filler
employed, together with
the dimensional stability at elevated temperatures that generally is
recognized to be characteristic of
most thermoset materials. Filled elastomers that may be thermoformed and cured
to provide tough,
flexible parts are also known in the art, and these also may be made thermally
conductive through use
of suitable fillers.
EXAMPLES
Materials employed in the following examples include:
K-1100X Carbon fiber, obtained as Thornel~ UHM carbon fiber K1100X from Amoco
Performance Products with published specifications including a tensile modulus
of about 130x106 psi,
a density of 2.21 g/cc, and a thermal conductivity of I 100 WImK.
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P-120 Carbon fiber, obtained as Thornel~ carbon fiber P-120 from Amoco
Polymers, Inc.
with published specifications including a tensile modulus of 120x106 psi, a
density of 2.17 glcc, and a
thermal conductivity of 900 W/mK.
E600X Carbon fiber, obtained as Thornel~ carbon fiber E-600X from Amoco
Polymers, Inc.
with published specifications including a tensile modulus of 120x106 psi, a
density of about 2.14 g/cc,
and a thermal conductivity of 600 W/mK.
Radel A Polyether sulfone resin, obtained as Radel~ A3800 polyaryl ether
sulfone from
Amoco Polymers, Inc.
PPA-1 Polyphthalamide resin, obtained as Amodel~ polyphthalamide resin from
Amoco
Polymers, Inc.
Nylon 6,6 Polyhexamethylene adipamide
LCP Liquid crystal polymer, obtained as Xydar~ SRT 900 resin from Amoco
Polymers,
Inc.
Mechanical properties were determined according to ASTM standardized testing
methods unless
otherwise noted.
Thermal conductivities were obtained from measurement of power/heat input and
temperature
differentials along multiple paths and determination of cross-sectional heat
flow under steady-state
conditions. Calibration of the device was made using aluminum or copper panels
of known thermal
conductivity.
Thermal conductivity is calculated using the Fourier Conduction Law
q=KA OT ,
0X
where q = power input; A = cross-sectional area; K = therma! conductivity; 0 T
= temperature
differential between resistive thermal devices in the direction of thermal
flow; and 0 X = distance
between temperature measurement devices. The data are reported in W/mK.
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Filled Resin Formulations -
Example 1. Chopped K-1100 carbon fiber tow with a nominal length of 1/4" was
dry mixed with
Xydar LCP resin pellets and extrusion compounded and chopped to provide
pellets of filled resin
containing 45 wt% target levels of carbon fiber. Test specimens (6"x6"x1/16")
were prepared by
injection molding the dried resin pellets using a HPM 75 ton injection molding
machine. Properties
are summarized in Table I.
Examples 2-5. The procedures of Example 1 were substantially followed in
providing a series of
filled Xydar resins at levels of 10, 45 and 60 wt% carbon fiber. The pellets
were injection molded as
in Example 1 to provide flat panels and 4"x4"x1/8" test specimens. Thermal
conductivities are
summarized in the following Table I.
Table I. Xydar LCP Resins Filled With Chopped K1100 Carbon Fiber.
Ex. No. 1 2 3 4 S
Fiber contentl (wt%) 45 10 45 45 60
Mold Temp. °F - 220° I 50 - 100° 100°
220°
Thermal conductivity
flow dir. (w/m 23.9 7.5 21.1 20.7 21.4
K)
transverse (w/m 13.2 8.1 20.4 21.2 21.3
K)
Spec. Gr. (g/cc) 1.66 1.44 1.66 1.66 1.72
Tensile Str. (Kpsi)15.4 - - - -
Tensile Mod. (Mpsi)4.7 - - - -
E (%) I.5 - _ - -
Flex Str. (lcpsi)22.2 20.6 25.6 25.0 24.8
Flex Mod. (Mpsi) 3.8 1.8 4.9 4.6 5.0
Vol. Resist (ohm-cm)1.5 - - - -
HDT, 264 psi ( - 251 275 274 276
F)
Izod Impact
unnotched (ft-lbs)6.4 13.1 10.9 11.7 1.4
notched (ft-lbs/in.)3.2 - - - -
Notes: 1. Fiber Content = nominal wt% fiber.
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It will be apparent that formulations comprising less than about 20 wt% carbon
fiber are lacking in
thermal conductivity. Although there is some variation, the filled materials
may be molded to be
anisotropic with respect to thermal properties, or substantially isotropic in
the plane of the molded
article. It appears that for most systems, there is some fiber alignment along
the fiber plane, with the
greater degree of alignment occurring most often in the flow direction.
However, the properties
normal to the flow plane indicate that little three-dimensional fiber
orientation occurs for most
injection moldings.
Compositions according to the invention may also be manufactured using other
thermoplastic resins
to provide thermally-conductive parts.
Examples 6 and 7 Continuous K-1100X carbon fiber was compounded with Radel
polyether
sulfone by feeding continuous fiber together with the polysulfone resin to a
Killion 1.5" single screw
extruder, extruding the compounded resin into strand, and chopping the strand
to form filled
polysulfone pellets. The feed rates were controlled to provide strand with 18
wt% and with 33 wt%
target levels of carbon fiber. The pellets were then dried and compression
molded to provide 4" by
4" by 1/8" coupons for use as test specimens. Properties are summarized in
Table II.
Examele 8 Pultruded rod was prepared from nylon 6,6 resin by feeding the resin
and continuous
K1100x carbon fiber to the extruder at a feed rate controlled to provide rod
having 60 wt% target
level of carbon fiber. The pultruded rod was chopped to give 1/2" pellets.
Test plaques were
injection molded from dried pellets using an HPM 75 ton injection molding
machine. Properties are
summarized in Table II.
Examales 9 10 and 11 Chopped E-600X carbon fiber tow with a nominal length of
1/4" was dry
mixed with PPA-1 resin pellets and extrusion compounded and chopped to provide
pellets of filled
resin containing 10, 50 and 70 wt% target levels of carbon fiber. Test
specimens were prepared by
injection molding the dried resin pellets as previously described. Properties
are summarized in
Table II.
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Table II
Ex. No. 6 7 8 9 10 11
Resin typel Radel Radel nylon PPA-1 PPA-1 PPA-1
6,6
Fiber type2 K1100 K1100 P-120 E 600X E 600X E 600X
Fiber content2 18 33 (60) 10 (9.6)50 (44)70
{wt%)
Fiber length2 50 50 80 80 80 -
(w)
Thermal conductivity
flow dir. (W/mK) 13 18 37 - 15.2 33.8
transverse (W/mK)- - 27 - 19.7 21.0
thickness (W/mK) - - - - 1.7 5.8
CTE
flow dir. (ppm) - - 4.5 - 10.3 6.4
transverse (ppm) - - 19 - 6.0 16.1
Notes: 1. For resin identification, see text. 2. For fiber identification, see
text; Fiber (:ontent =
nominal wt% fiber; values in ( ) are actual, determined by gravimetric
analysis; Fiber Length =
average fiber length in molded sample, determined microscopically.
Compositions according to the invention may also be compression molded to
provide thermally-
conductive parts.
Examele 12 A composition consisting of 50 wt% Radel polyether sulfone and 50
wt% chopped
K-1100 carbon fiber tow with a nominal length of 1/4" was prepared by solution
impregnation. The
mixture was compression molded to provide thermal property test specimens. The
in-plane (or x
and y direction) thermal conductivities were 90.4 W/mK and 102.9 W/mK. The
coefficient of
thermal expansion was 4.0 ppm/°F.
Example 13 A dry blend consisting of 50 wt% Radel polyether sulfone powder and
50 wt%
chopped K-1100 carbon fiber tow with a nominal length of 1" was compression
molded to provide
thermal property test specimens. The in-plane (or x direction) thermal
conductivity was 67.8
W/mK. Some wetting-out difficulties were observed.
Exam le 14 A dry blend consisting of 50 wt% Radel polyether sulfone powder and
50 wt% P-
120 carbon fiber milled or granulated to give 200 micron particles was
compression molded to
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provide thermal property test specimens. The in-plane (or x direction) thermal
conductivity was
37.0 W/mK. Some wetting-out difficulties were observed.
Thermoset resins such as the common epoxy potting resins may also be filled
with carbon fiber,
molded and cured to give thermally-conductive parts.
Examine 15 A filled epoxy composition consisting of 70 wt% epoxy resin and 30
wt% P-120
carbon fiber milled or granulated to give 200 micron particles was prepared by
combining the liquid
resin and the particulate and hand mixing, then pouring into a mold and
allowing the plaque to cure,
providing a therman property test specimen. The in-plane (or x direction)
thermal conductivity was
W/mK.
10 Insert Molded Thermal Devices
E~ple 16 A thermal device comprising a 3 mm x 160 mm heat pipe and an
injection molded
heat sink was constructed, using the 60 wt% carbon fiber-filled LCP resin
formulation of Example S.
The mold cavity, measuring l2mm x 12 mm x 96 mm, was fixtured to center and
support the heat
pipe inserted into the mold cavity. The closed mold was then injected with
filled LCP resin
15 formulation, using an HPM 75 ton injection molding machine as previously
described. After
ejecting the cooled molding, the centering fixture was removed to provide a
heat pipe embedded at
the condenser portion to a length of S 1 mm in the injection-molded resin. The
block was then milled
to form a plurality of fins lmm in thickness and 4-5 mm in height, centered on
and disposed
normally to the axis of the heat pipe and spaced 1 mm apart along the embedded
length. The overall
weight of the heat sink was 12 grams. The source end of the heat pipe was
imbedded to a length of
14 mm in a heat transfer block. The heat transfer block was electrically
heated at a constant power
input of 5.5 watts. Temperature of the source and the ambient temperature were
measured by
thermocouples, the rise in the temperature for constant power input being
inverse to the ability of the
structure to dissipate heat.
The die/source temperature was 71° C, while the ambient temperature was
23° C, giving a thermal
resistance of 8.6° C/watt.
Comparative Example A 3 x 160 mm heat pipe was attached at the condenser end
and with an
embedded length of 95 mm to a commercial cast magnesium finned heat sink
measuring 13 mm x
20 mm x 140 mm in length, having a weight of 26 grams. Thermal adhesive was
applied to the
contacting surfaces between the heat sink and the heat pipe. The source end
was embedded to a
length of 14 mm in a heat transfer block, and heated as in Example 16. The
die/source temperature
was 71 ° C, and the ambient temperature was 25° C, giving a
thermal resistance value of 8.3 °C/watt.
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It will thus be seen that the injection molded heat sink provides
substantially the same degree of heat
dissipation as the larger and considerably heavier cast magnesium heat sink of
the prior art.
It will be apparent that the invented molded thermal management devices
represent a substantial
advance in the art and provide significantly improved materials for use in
thermal management
applications. The thermally conductive resin formulations used in forming the
devices of this
invention are readily fabricated using conventional processing means, and are
generally tough
materials having excellent mechanical properties and good dimensional
stability. These improved
thermally conductive resin molding compounds may find wide application for use
in fabricating
thermal management components.
It will thus be seen that the thermal management devices of the invention may
be described as
comprising a heat pipe together with a molded heat sink, wherein the heat sink
component is
preferably insert molded with the heat pipe to form a thermal management
device having an integral
unitary construction. The thermally conductive heat sink component may be
further characterized as
molded from filled thermoplastic or thermoset molding compounds having,
depending upon the
amount of conductive filler in the formulation, a thermal conductivity greater
than about 5 WImK,
preferably greater than about 10 W/mK, and more preferably from about 80 to as
great as 600
W/mK and still more preferably from about 100 to about 450 W/mK, together with
an unusually
low coefficient of thermal expansion, again depending upon the type and level
of conductive filler,
of generally less than 10 ppm/°C. The filled thermoplastic injection
molding compounds employed
in the more preferred embodiments of the invention may be further described as
comprising from
about 80 to about 20 wt%, preferably about 50 to about 20 wt% of a
thermoplastic LCP resin and
about 20 to about 80 wt%, preferably about 50 to about 80 wt% carbon fiber,
said carbon fiber
having a thermal conductivity greater than about 600 W/mK, preferably greater
than about 750
W/mK, more preferably greater than about 900 W/mK, and still more preferably
greater than 1000
W/mK.
It will be understood that the filled resin formulations may further comprise
such plasticizers,
processing aids, stabilizers and the like as are conventionally used in the
resin compounding and
molding resin arts. The formulations are particularly suited for use producing
thermal management
devices for electrical and electronic use, where the art has lacked suitable,
readily-fabricated
materials with high thermal conductivity.
Further modifications and variations will be readily apparent to those skilled
in the resin arts. For
example, it will be readily understood by those skilled in the art that a wide
variety of thermal
management devices may be designed employing a plurality of heat pipe
components embedded in
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heat sink, including heat spreader or thermal plane, components irr an insert
molding operation. The
use of further or post-molding operations are also contemplated, including
overmolding the thermal
management device, optionally including attached electrical or electronic
components, for example
to provide an attached, hermetically sealed housing. Such further embodiments
and modifications
will be seen to fall well within the skill of those engaged in the art and
thus will be understood to lie
within the scope the invention as defined by the appended claims.
