U.S. patent application number 10/414884 was filed with the patent office on 2003-11-06 for conformal thermal interface material for electronic components.
Invention is credited to Bunyan, Michael H., Sorgo, Miksa de.
Application Number | 20030207064 10/414884 |
Document ID | / |
Family ID | 21777389 |
Filed Date | 2003-11-06 |
United States Patent
Application |
20030207064 |
Kind Code |
A1 |
Bunyan, Michael H. ; et
al. |
November 6, 2003 |
Conformal thermal interface material for electronic components
Abstract
A thermally-conductive interface for conductively cooling a
heat-generating electronic component having an associated thermal
dissipation member such as a heat sink. The interface is formed as
a self-supporting layer of a thermally-conductive material which is
form-stable at normal room temperature in a first phase and
substantially conformable in a second phase to the interface
surfaces of the electronic component and thermal dissipation
member. The material has a transition temperature from the first
phase to the second phase which is within the operating temperature
range of the electronic component.
Inventors: |
Bunyan, Michael H.;
(Chelmsford, MA) ; Sorgo, Miksa de; (Windham,
NH) |
Correspondence
Address: |
JOHN A MOLNAR JR
PARKER-HANNIFIN CORPORATION
6035 PARKLAND BOULEVARD
CLEVELAND
OH
44124-4141
US
|
Family ID: |
21777389 |
Appl. No.: |
10/414884 |
Filed: |
April 16, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10414884 |
Apr 16, 2003 |
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09714680 |
Nov 16, 2000 |
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09714680 |
Nov 16, 2000 |
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08801047 |
Feb 14, 1997 |
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6054198 |
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60016488 |
Apr 29, 1996 |
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Current U.S.
Class: |
428/40.5 ;
156/247; 257/E23.107; 428/41.8; 524/399 |
Current CPC
Class: |
Y10T 428/1419 20150115;
H01L 23/3737 20130101; Y10T 428/31909 20150401; Y10T 428/1452
20150115; H01L 2224/32245 20130101; C09K 5/06 20130101; H01L
2924/3011 20130101; Y10T 428/1476 20150115; Y10T 428/2822 20150115;
Y10T 428/2826 20150115 |
Class at
Publication: |
428/40.5 ;
428/41.8; 524/399; 156/247 |
International
Class: |
C09J 001/00 |
Claims
What is claimed is:
1. A thermal interface material which undergoes a phase change at
microprocessor operating temperatures to transfer heat generated by
a heat source to a heat sink, the material comprising: a phase
change substance which softens at about the operating temperature
of the heat source, the phase change substance including: a polymer
component, and a melting point component mixed with the polymer
component, which modifies the temperature at which the phase change
substance softens, the melting point component melting at around
the microprocessor operating temperatures and dissolving the
polymer component in the melting point component; and a thermally
conductive filler dispersed within the phase change substance.
2. The thermal interface material of claim 1, wherein the phase
change substance has a viscosity of from 1 to 100 poise at the
operating temperature of the heat source.
3. The thermal interface material of claim 1, wherein the phase
change substance has a viscosity of from 5 to 50 poise in the
temperature range of 60 to 120.degree. C.
4. The thermal interface material of claim 1, wherein the phase
change substance has a melting point of 30-120.degree. C.
5. The thermal interface material of claim 1, wherein the polymer
component includes an elastomer selected from the group consisting
of silicone, acrylic polymers, natural rubber, synthetic rubber,
and combinations thereof.
6. The thermal interface material of claim 1, wherein the polymer
component has a Mooney viscosity of up to 40 ML4.
7. A thermal interface material which undergoes a phase change at
microprocessor operating temperatures to transfer heat generated by
a heat source to a heat sink, the material comprising: a phase
change substance which softens at about the operating temperature
of the heat source, the phase change substance including: a polymer
component, and a melting point component mixed with the polymer
component, which modifies the temperature at which the phase change
substance softens, the melting point component being selected from
the group consisting of C.sub.12-C.sub.16 alcohols, acids, esters,
petroleum waxes, wax-like compounds, low molecular weight styrenes,
methyl triphenyl silane materials, and combinations thereof; and a
thermally conductive filler dispersed within the phase change
substance.
8. The thermal interface material of claim 7, wherein the melting
point component is a C.sub.12-C.sub.16 alcohol or acid selected
from the group consisting of myristyl alcohol, cetyl alcohol,
stearyl alcohol, myristyl acid, stearic acid, and combinations
thereof.
9. The thermal interface material of claim 7, wherein the melting
point component is a wax or a waxlike compound selected from the
group consisting of microcrystalline wax, paraffin waxes,
cyclopentane, heceicosyl, 2-heptadecanone, pentacosaneyl, silicic
acid, tetraphenyl ester, octadecanoic acid,
2-[2-[2-(2hydroxyethoxy) ethoxy]ethoxy]ethyl ester, cyclohexane
docosyl, polystyrene, polyamide resins, disiloxane 1,1,1,
trimethyl-3,3, triphenyl silane, and combinations thereof.
10. The thermal interface material of claim 1, wherein the polymer
component has a solubility parameter which is within +1 and -1 of
the solubility parameter of the melting point component.
11. The thermal interface material of claim 1, wherein: the polymer
component is at a concentration of from 10-80% by weight; the
filler is at a concentration of from 10-80% by weight; and the
melting point component is at a concentration of from 10-80% by
weight.
12. The thermal interface material of claim 11, wherein: the
polymer component is at a concentration of from 10-70% by weight;
the filler is at a concentration of from 10-70% by weight; and the
melting point component is at a concentration of from 15-70% by
weight.
13. The thermal interface material of claim 1, wherein the
thermally conductive filler has a bulk thermal conductivity of
between about 0.5 and 1000 watts meter per degree Kelvin.
14. The thermal interface material of claim 1, wherein the thermal
interface material has a thermal conductivity of at least 0.8 watts
meter per degree Kelvin.
15. The thermal interface material of claim 1, wherein the
thermally conductive filler is selected from the group consisting
of boron nitride, aluminum oxide, nickel powder, copper flakes,
graphite powder, powdered diamond, and combinations thereof.
16. The thermal interface material of claim 1, wherein the
thermally conductive filler has an average particle size of from
about 2 to 100 microns.
17. A thermal interface material comprising: a polymer component at
a concentration of from 10-80% by weight of the material, the
polymer component including an elastomer; a melting point component
at a concentration of from 10-80% by weight of the material, the
melting point component including a wax or a waxlike compound, the
polymer component having a solubility parameter which is within +1
and -1 of the solubility parameter of the melting point component;
and a thermally conductive filler at a concentration of from 10-80%
by weight of the material.
18. The thermal interface material of claim 17, wherein the
elastomer is selected from the group consisting of silicone,
acrylic polymers, natural rubber, synthetic rubber, and
combinations thereof.
19. The thermal interface material of claim 17, wherein the melting
point component includes a compound selected from the group
consisting of microcrystalline wax, paraffin waxes, cyclopentane,
heceicosyl, 2-heptadecanone, pentacosaneyl, silicic acid,
tetraphenyl ester, octadecanoic acid, 2-[2-[2-(2hydroxyethoxy)
ethoxy]ethoxy]ethyl ester, cyclohexane docosyl, polystyrene,
polyamide resins, disiloxane 1,1,1, trimethyl-3,3, triphenyl
silane, C.sub.12-C.sub.16 alcohols, C.sub.12-C.sub.16 acids, and
combinations thereof.
20. The thermal interface material of claim 1, wherein the melting
point component includes a compound selected from the group
consisting of microcrystalline wax, paraffin waxes, cyclopentane,
heceicosyl, 2-heptadecanone, pentacosaneyl, silicic acid,
tetraphenyl ester, octadecanoic acid, 2-[2-[2-(2hydroxyethoxy)
ethoxy]ethoxy]ethyl ester, cyclohexane docosyl, polystyrene,
polyamide resins, disiloxane 1,1,1, trimethyl-3,3, triphenyl
silane, C.sub.12-C.sub.16 alcohols, C.sub.12-C.sub.16 acids, and
combinations thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 09/714,680, filed Nov. 16, 2000; which is an
application for reissue of U.S. patent application Ser. No.
08/801,047, filed Feb. 14, 1997, now U.S. Pat. No. 6,054,198,
granted Apr. 25, 2000, the disclosure of each of which is expressly
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates broadly to a heat transfer
material which is interposable between the thermal interfaces of a
heat-generating, electronic component and a thermal dissipation
member, such as a heat sink or circuit board, for the conductive
cooling of the electronic component. More particularly, the
invention relates to a self-supporting, form-stable film which
melts or softens at a temperature or range within the operating
temperature range of the electronic component to better conform to
the thermal interfaces for improved heat transfer from the
electronic component to the thermal dissipation member.
[0003] Circuit designs for modern electronic devices such as
televisions, radios, computers, medical instruments, business
machines, communications equipment, and the like have become
increasingly complex. For example, integrated circuits have been
manufactured for these and other devices which contain the
equivalent of hundreds of thousands of transistors. Although the
complexity of the designs has increased, the size of the devices
has continued to shrink with improvements in the ability to
manufacture smaller electronic components and to pack more of these
components in an ever smaller area.
[0004] As electronic components have become smaller and more
densely packed on integrated boards and chips, designers and
manufacturers now are faced with the challenge of how to dissipate
the heat which is ohmicly or otherwise generated by these
components. Indeed, it is well known that many electronic
components, and especially semiconductor components such as
transistors and microprocessors, are more prone to failure or
malfunction at high temperatures. Thus, the ability to dissipate
heat often is a limiting factor on the performance of the
component.
[0005] Electronic components within integrated circuit
traditionally have been cooled via forced or convective circulation
of air within the housing of the device. In this regard, cooling
fins have been provided as an integral part of the component
package or as separately attached thereto for increasing the
surface area of the package exposed to convectively-developed air
currents. Electric fans additionally have been employed to increase
the volume of air which is circulated within the housing. For high
power circuits and the smaller but more densely packed circuits
typical of current electronic designs, however, simple air
circulation often has been found to be insufficient to adequately
cool the circuit components.
[0006] Heat dissipation beyond that which is attainable by simple
air circulation may be effected by the direct mounting of the
electronic component to a thermal dissipation member such as a
"cold plate" or other heat sink. The heat sink may be a dedicated,
thermally-conductive metal plate, or simply the chassis of the
device. However, and as is described in U.S. Pat. No. 4,869,954,
the faying thermal interface surfaces of the component and heat
sink typically are irregular, either on a gross or a microscopic
scale. When the interfaces surfaces are mated, pockets or void
spaces are developed therebetween in which air may become
entrapped. These pockets reduce the overall surface area contact
within the interface which, in turn, reduces the efficiency of the
heat transfer therethrough. Moreover, as it is well known that air
is a relatively poor thermal conductor, the presence of air pockets
within the interface reduces the rate of thermal transfer through
the interface.
[0007] To improve the efficiency of the heat transfer through the
interface, a layer of a thermally-conductive material typically is
interposed between the heat sink and electronic component to fill
in any surface irregularities and eliminate air pockets. Initially
employed for this purpose were materials such as silicone grease or
wax filled with a thermally-conductive filler such as aluminum
oxide. Such materials usually are semi-liquid or sold at normal
room temperature, but may liquefy or soften at elevated
temperatures to flow and better conform to the irregularities of
the interface surfaces.
[0008] For example, U.S. Pat. No. 4,299,715 discloses a wax-like,
heat-conducting material which is combined with another
heat-conducting material, such as a beryllium, zinc, or aluminum
oxide powder, to form a mixture for completing a
thermally-conductive path from a heated element to a heat sink. A
preferred wax-like material is a mixture of ordinary petroleum
jelly and a natural or synthetic wax, such as beeswax, palm wax, or
mineral wax, which mixture melts or becomes plastic at a
temperature above normal room temperature. The material can be
excoriated or ablated by marking or rubbing, and adheres to the
surface on which it was rubbed. In this regard, the material may be
shaped into a rod, bar, or other extensible form which may be
carried in a pencil-like dispenser for application.
[0009] U.S. Pat. No. 4,466,483 discloses a thermally-conductive,
electrically-insulating gasket. The gasket includes a web or tape
which is formed of a material which can be impregnated or loaded
with an electrically-insulating, heat conducting material. The tape
or web functions as a vehicle for holding the meltable material and
heat conducting ingredient, if any, in a gasket-like form. For
example, a central layer of a solid plastic material may be
provided, both sides of which are coated with a meltable mixture of
wax, zinc oxide, and a fire retardant.
[0010] U.S. Pat. No. 4,473,113 discloses a thermally-conductive,
electrically-insulating sheet for application to the surface of an
electronic apparatus. The sheet is provided as having a coating on
each side thereof a material which changes state from a solid to a
liquid within the operating temperature range of the electronic
apparatus. The material may be formulated as a meltable mixture of
wax and zinc oxide.
[0011] U.S. Pat. No. 4,764,845 discloses a thermally-cooled
electronic assembly which includes a housing containing electronic
components. A heat sink material fills the housing in direct
contact with the electronic components for conducting heat
therefrom. The heat sink material comprises a paste-like mixture of
particulate microcrystalline material such as diamond, boron
nitride, or sapphire, and a filler material such as a fluorocarbon
or paraffin.
[0012] The greases and waxes of the aforementioned types heretofore
known in the art, however, generally are not self-supporting or
otherwise form stable at room temperature and are considered to be
messy to apply to the interface surface of the heat sink or
electronic component. To provide these materials in the form of a
film which often is preferred for ease of handling, a substrate,
web, or other carrier must be provided which introduces another
interface layer in or between which additional air pockets may be
formed. Moreover, use of such materials typically involves hand
application or lay-up by the electronics assembler which increases
manufacturing costs.
[0013] Alternatively, another approach is to substitute a cured,
sheet-like material for the silicone grease or wax material. Such
materials may be compounded as containing one or more
thermally-conductive particulate fillers dispersed within a
polymeric binder, and may be provided in the form of cured sheets,
tapes, pads, or films. Typical binder materials include silicones,
urethanes, thermoplastic rubbers, and other elastomers, with
typical fillers including aluminum oxide, magnesium oxide, zinc
oxide, boron nitride, and aluminum nitride.
[0014] Exemplary of the aforesaid interface materials is an alumina
or boron nitride-filled silicone or urethane elastomer which is
marketed under the name CHO-THERM.RTM. by the Chomerics Division of
Parker-Hannifin Corp., Woburn, Mass. Additionally, U.S. Pat. No.
4,869,954 discloses a cured, form-stable, sheet-like,
thermally-conductive material for transferring thermal energy. The
material is formed of a urethane binder, a curing agent, and one or
more thermally conductive fillers. The fillers may include aluminum
oxide, aluminum nitride, boron nitride, magnesium oxide, or zinc
oxide.
[0015] U.S. Pat. No. 4,782,893 discloses a thermally-conductive,
electrically-insulative pad for placement between an electronic
component and its support frame. The pad is formed of a high
dielectric strength material in which is dispersed diamond powder.
In this regard, the diamond powder and a liquid phase of the high
dielectric strength material may be mixed and then formed into a
film and cured. After the film is formed, a thin layer thereof is
removed by chemical etching or the like to expose the tips of the
diamond particles. A thin boundary layer of copper or other metal
then is bonded to the top and bottom surfaces of the film such that
the exposed diamond tips extend into the surfaces to provide pure
diamond heat transfer paths across the film. The pad may be joined
to the electronic component and the frame with solder or an
adhesive.
[0016] U.S. Pat. No. 4,965,699 discloses a printed circuit device
which includes a memory chip mounted on a printed circuit card. The
card is separated from an associated cold plate by a layer of a
silicone elastomer which is applied to the surface of the cold
plate.
[0017] U.S. Pat. No. 4,974,119 discloses a heat sink assembly which
includes an electronic component supported on a printed circuit
board in a spaced-apart relationship from a heat dispersive member.
A thermally-conductive, elastomeric layer is interposed between the
board and the electronic component. The elastomeric member may be
formed of silicone and preferably includes a filler such as
aluminum oxide or boron nitride.
[0018] U.S. Pat. No. 4,979,074 discloses a printed circuit board
device which includes a circuit board which is separated from a
thermally-conductive plate by a pre-molded sheet of silicone
rubber. The sheet may be loaded with a filler such as alumina or
boron nitride.
[0019] U.S. Pat. No. 5,137,959 discloses a thermally-conductive,
electrically insulating interface material comprising a
thermoplastic or cross linked elastomer filled with hexagonal boron
nitride or alumina. The material may be formed as a mixture of the
elastomer and filler, which mixture then may be cast or molded into
a sheet or other form.
[0020] U.S. Pat. No. 5,194,480 discloses another
thermally-conductive, electrically-insulating filled elastomer. A
preferred filler is hexagonal boron nitride. The filled elastomer
may be formed into blocks, sheets, or films using conventional
methods.
[0021] U.S. Pat. Nos. 5,213,868 and 5,298,791 disclose a
thermally-conductive interface material formed of a polymeric
binder and one or more thermally-conductive fillers. The fillers
may be particulate solids, such as aluminum oxide, aluminum
nitride, boron nitride, magnesium oxide, or zinc oxide. The
material may be formed by casting or molding, and preferably is
provided as a laminated acrylic pressure sensitive adhesive (PSA)
tape. At least one surface of the tape is provided as having
channels or through-holes formed therein for the removal of air
from between that surface and the surface of a substrate such as a
heat sink or an electronic component.
[0022] U.S. Pat. No. 5,321,582 discloses an electronic component
heat sink assembly which includes a thermally-conductive laminate
formed of polyamide which underlays a layer of a boron
nitride-filled silicone. The laminate is interposed between the
electronic component and the housing of the assembly.
[0023] Sheet-like materials of the above-described types have
garnered general acceptance for use as interface materials in
conductively-cooled electronic component assemblies. For some
applications, however, heavy fastening elements such as springs,
clamps, and the like are required to apply enough force to conform
these materials to the interface surfaces to attain enough surface
for efficient thermal transfer. Indeed, for certain applications,
materials such as greases and waxes which liquefy, melt, or soften
at elevated temperature sometimes are preferred as better
conforming to the interface surfaces. It therefore will be
appreciated that further improvements in these types of interface
materials and methods of applying the same would be well-received
by the electronics industry. Especially desired would be a thermal
interface material which is self-supporting and form-stable at room
temperature, but which is softenable or meltable at temperatures
within the operating temperature range of the electronic component
to better conform to the interface surfaces.
BROAD STATEMENT OF THE INVENTION
[0024] The present invention is directed to a heat transfer
material which is interposable between the thermal interfaces of a
heat-generating, electronic component and a thermal dissipation
member. The material is of the type which melts or softens at a
temperature or range within the operating temperature range of the
electronic component to better conform to the thermal interfaces
for improved heat transfer from the electronic component to the
thermal dissipation member. Unlike the greases or waxes of such
type heretofore known in the art, however, the interface material
of the present invention is form-stable and self-supporting at room
temperature. Accordingly, the material may be formed into a film or
tape which may be applied using automated equipment to, for
example, the interface surface of a thermal dissipation member such
as a heat sink. In being self-supporting, no web or substrate need
be provided which would introduce another layer into the interface
between which additional air pockets could be formed.
[0025] It therefore is a feature of the present invention to
provide for the conductive cooling a heat-generating electronic
component. The component has an operating temperature range above
normal room temperature and a first heat transfer surface
disposable in thermal adjacency with a second heat transfer surface
of an associated thermal dissipation member to define an interface
therebetween. A thermally-conductive material is provided which is
form-stable at normal room temperature in a first phase and
conformable in a second phase to substantially fill the interface.
The material, which has a transition temperature from the first
phase to the second phase within the operating temperature range of
the electronic component, is formed into a self-supporting layer.
The layer is applied to one of the heat transfer surfaces, which
surfaces then are disposed in thermal adjacency to define the
interface. The energization of the electronic component is
effective to heat the layer to a temperature which is above the
phase transition temperature.
[0026] It is a further feature of the invention to provide a
thermally-conductive interface for conductively cooling a
heat-generating electronic component having an associated thermal
dissipation member such as a heat sink. The interface is formed as
a self-supporting layer of a thermally-conductive material which is
form-stable at normal room temperature in a first phase and
substantially conformable in a second phase to the interface
surfaces of the electronic component and thermal dissipation
member. The material has a transition temperature from the first
phase to the second phase which is within the operating temperature
range of the electronic component.
[0027] Advantages of the present invention include a thermal
interface material which melts or softens to better conform to the
interfaces surfaces, but which is self-supporting and form-stable
at room temperature for ease of handling and application. Further
advantages include an interface material which may be formed into a
film or tape without a web or other supporting substrate, and which
may be applied using automated methods to, for example, the
interface surface of a thermal dissipation member. Such member then
may be shipped to a manufacturer for direct installation into a
circuit board to thereby obviate the need for hand lay-up of the
interface material. Still further advantages include a thermal
interface formulation which may be tailored to provide controlled
thermal and viscometric properties. These and other advantages will
be readily apparent to those skilled in the art based upon the
disclosure contained herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] For a fuller understanding of the nature and objects of the
invention, reference should be had to the following detailed
description taken in connection with the accompanying drawings
wherein:
[0029] FIG. 1 is a fragmentary, cross-sectional view of an
electrical assembly wherein a heat-generating electronic component
thereof is conductively cooled in accordance with the present
invention via the provision of an interlayer of a
thermally-conductive material within the thermal interface between
the heat transfer surfaces of the component and an associated
thermal dissipation member;
[0030] FIG. 2 is a view of a portion of the thermal interface of
FIG. 1 which is enlarged to detail the morphology thereof;
[0031] FIG. 3 is a cross-sectional end view which shows the
thermally-conductive material of FIG. 1 as coated as a film layer
onto a surface of a release sheet, which sheet is rolled to
facilitate the dispensing of the film; and
[0032] FIG. 4 is a view of a portion of the film and release sheet
roll of FIG. 3 which is enlarged to detail the structure
thereof.
[0033] The drawings will be described further in connection with
the following Detailed Description of the Invention.
DETAILED DESCRIPTION OF THE INVENTION
[0034] Referring to the drawings wherein corresponding reference
characters indicate corresponding elements throughout the figures,
shown generally at 10 in FIG. 1 is an electrical assembly which
includes a heat-generating digital or analog electronic component,
12, supported on an associated printed circuit board (PCB) or other
substrate, 14. Electrical component 12 may be an integrated
microchip, microprocessor, transistor, or other semiconductor, or
an ohmic or other heat-generating subassembly such as a diode,
relay, resistor, transformer, amplifier, diac, or capacitor.
Typically, component 12 will have an operating temperature range of
from about 60-80.degree. C. For the electrical connection of
component 12 to board 14, a pair of leads or pins, 16a and 16b, are
provided as extending from either end of component 12 into a
soldered or other connection with board 14. Leads 16 additionally
may support component 12 above board 14 to define a gap,
represented at 17, of about 3 mils (75 microns) therebetween.
Alternatively, component 12 may be received directly on board
14.
[0035] As supported on board 14, electronic component 12 presents a
first heat transfer surface, 18, which is disposable in a thermal,
spaced-apart adjacency with a corresponding second heat transfer
surface, 22, of an associated thermal dissipation member, 20.
Dissipation member 20 is constructed of a metal material or the
like having a heat capacity relative to that of component 12 to be
effective is dissipating thermal energy conducted or otherwise
transferred therefrom. For purposes of the present illustration,
thermal dissipation member 20 is shown as a heat sink having a
generally planar base portion, 24, from which extends a plurality
of cooling fins, one of which is referenced at 26. With assembly 10
configured as shown, fins 26 assist in the convective cooling of
component 12, but alternatively may be received within an
associated cold plate or the like, not shown, for further
conductive dissipation of the thermal energy transferred from
component 12.
[0036] The disposition of first heat transfer surface 18 of
electronic component 12 in thermal adjacency with second heat
transfer surface 22 of dissipation member 20 defines a thermal
interface, represented at 28, therebetween. A thermally-conductive
interlayer, 30, is interposed within interface 28 between heat
transfer surfaces 18 and 22 for providing a conductive path
therethrough for the transfer of thermal energy from component 12
to dissipation member 20. Such path may be employed without or in
conjunction with convective air circulation for effecting the
cooling of component 12 and ensuring that the operating temperature
thereof is maintained below specified limits.
[0037] Although thermal dissipation member 20 is shown to be a
separate heat sink member, board 14 itself may be used for such
purpose by alternatively interposing interlayer 30 between surface
32 thereof and corresponding surface 34 of electronic component 12.
In either arrangement, a clip, spring, or clamp or the like (not
shown) additionally may be provided for applying an external force,
represented at 32, of from about 1-2 lbs.sub.f for improving the
interface area contact between interlayer 30 and surfaces 18 and 22
or 32 and 34.
[0038] In accordance with the precepts of the present invention,
interlayer 30 is formed of a self-supporting film, sheet, or other
layer of a thermally-conductive material. By "self-supporting" it
is meant that interlayer 30 is free-standing without the support of
a web or substrate which would introduce another layer into the
thermal interface between air pockets could be formed. Typically,
the film or sheet of interlayer 30 will have a thickness of from
about 1-10 mils (25-250 microns) depending upon the particular
geometry of assembly 10.
[0039] The thermally-conductive material forming interlayer 30 is
formulated to be form-stable at normal room temperature, i.e.,
about 25.degree. C., in a first phase, which is solid, semi-solid,
glassy, or crystalline, but to be substantially conformable in a
second phase, which is a liquid, semi-liquid, or otherwise viscous
melt, to interface surfaces 18 and 22 of, respectively, electronic
component 12 and thermal dissipation member 20. The transition
temperature of the material, which may be its melting or glass
transition temperature, is preferably from about 60 or 70.degree.
C. to about 80.degree. C., and is tailored to fall within the
operating temperature of electronic component 12.
[0040] Further in this regard, reference may be had to FIG. 2
wherein an enlarged view of a portion of interface 28 is
illustrated to detail the internal morphology thereof during the
energization of electronic component 12 effective to heat
interlayer 30 to a temperature which is above its phase transition
temperature. Interlayer 30 accordingly is shown to have been melted
or otherwise softened from a form-stable solid or semi-solid phase
into a flowable or otherwise conformable liquid or semi-liquid
viscous phase which may exhibit relative intermolecular chain
movement. Such viscous phase provides increased surface area
contact with interface surfaces 18 and 22, and substantially
completely fills interface 28 via the exclusion of air pockets or
other voids therefrom to thereby improve both the efficiency and
the rate of heat transfer through interface. Moreover, as depending
on, for example, the melt flow index or viscosity of interlayer 30
and the magnitude of any applied external pressure 36 (FIG. 1), the
interface gap between surfaces 18 and 22 may be narrowed to further
improve the efficiency of the thermal transfer therebetween. Any
latent heat associated with the phase change of the material
forming interlayer 30 additionally contributes to the cooling of
component 12.
[0041] Unlike the greases or waxes of such type heretofore known in
the art, however, interlayer of the present invention
advantageously is form-stable and self-supporting at room
temperature. Accordingly, and as is shown generally at 40 in FIG.
3, interlayer 30 advantageously may be provided in a rolled, tape
form to facilitate its application to the substrate by an automated
process. As may be better appreciated with additional reference to
FIG. 4 wherein a portion, 42, of tape 40 is shown in enhanced
detail, tape 40 may be formed by applying a film of interlayer 30
to a length of face stock, liner, or other release sheet, 44.
Interlayer 30 may be applied to a surface, 46, of release sheet 44
in a conventional manner, for example, by a direct process such as
spraying, knife coating, roller coating, casting, drum coating,
dipping, or like, or an indirect transfer process utilizing a
silicon release sheet. A solvent, diluent, or other vehicle may be
provided to lower the viscosity of the material forming interlayer
30. After the material has been applied, the release sheet may be
dried to flash the solvent and leave an adherent, tack-free film,
coating, or other residue of the material thereon.
[0042] As is common in the adhesive art, release sheet 44 may be
provided as a strip of a waxed, siliconized, or other coated paper
or plastic sheet or the like having a relatively low surface energy
so as to be removable without appreciable lifting of interlayer 30
from the substrate to which it is ultimately applied.
Representative release sheets include face stocks or other films of
plasticized polyvinyl chloride, polyesters, cellulosics, metal
foils, composites, and the like.
[0043] In the preferred embodiment illustrated, tape 40 may be
sectioned to length, and the exposed surface, 48, of interlayer 30
may be applied to interface surface 22 of dissipation member 20
(FIG. 1) prior to its installation in assembly 10. In this regard,
interlayer exposed surface 48 may be provided as coated with a thin
film of a pressure sensitive adhesive or the like for adhering
interlayer 30 to dissipation member 20. Alternatively, interface
surface 22 of dissipation member 20 may be heated to melt a
boundary layer of interlayer surface 48 for its attachment via a
"hot-melt" mechanism.
[0044] With tape 40 so applied and with release sheet 44 protecting
the unexposed surface, 50, of interlayer 30, dissipation member 20
(FIG. 1) may be packaged and shipped as an integrated unit to an
electronics manufacturer, assembler, or other user. The user then
simply may remove release sheet 44 to expose surface 50 of
interlayer 30, position surface 50 on heat transfer surface 18 of
electronic component 12, and lastly apply a clip or other another
means of external pressure to dispose interlayer surface 50 in an
abutting, heat transfer contact or other thermal adjacency with
electronic component surface 18.
[0045] In one preferred embodiment, interlayer 30 is formulated as
a form-stable blend of: (a) from about 25 to 50% by weight of a
pressure sensitive adhesive (PSA) component having a melting
temperature of from about 90-100.degree. C.; (b) from about 50 to
75% by weight of an .alpha.-olefinic, thermoplastic component
having a melting temperature of from about 50-60.degree. C.; and
(c) from about 20 to 80% by weight of one or more
thermally-conductive fillers. "Melting temperature" is used herein
in its broadest sense to include a temperature or temperature range
evidencing a transition from a form-stable solid, semi-solid,
crystalline, or glassy phase to a flowable liquid, semi-liquid, or
otherwise viscous phase or melt which may be characterized as
exhibiting intermolecular chain rotation.
[0046] The PSA component generally may be of an acrylic-based,
hot-melt variety such as a homopolymer, copolymer, terpolymer,
interpenetrating network, or blend of an acrylic or (meth)acrylic
acid, an acrylate such as butyl acrylate, and/or an amide such as
acrylamide. The term "PSA" is used herein in its conventional sense
to mean that the component is formulated has having a glass
transition temperature, surface energy, and other properties such
that it exhibits some degree of tack at normal room temperature.
Acrylic hot-melt PSAs of such type are marketed commercially by
Heartland Adhesives, Germantown, Wis., under the trade designations
"H600" and "H251".
[0047] The .alpha.-olefinic thermoplastic component preferably is a
polyolefin which may be characterized as a "low melt" composition.
A representative material of the preferred type is an amorphous
polymer of a C.sub.10 or higher alkene which is marketed
commercially by Petrolite Corporation, Tulsa, Okla., under the
trade designation "Vybar.RTM. 260." Such material may be further
characterized as is set forth in Table 1.
1TABLE 1 Physical Properties of Representative Olefinic Polymer
Component (Vybar .RTM. 260) Molecular Weight 2600 g/mol Melting
Point (ASTM D 36) 130.degree. F. (54.degree. C.) Viscosity (ASTM D
3236) @ 210.degree. F. (99.degree. C.) 357.5 cP Penetration (ASTM D
1321) @ 77.degree. F. (25.degree. C.) 12 mm Density (ASTM D 1168)
0.90 g/cm.sup.3 @ 75.degree. F. (24.degree. C.) @ 200.degree. F.
(93.degree. C.) 0.79 g/cm.sup.3 Iodine Number (ASTM D 1959) 15
[0048] By varying the ratio within the specified limits of the PSA
to the thermoplastic component, the thermal and viscometric
properties of the interlayer formulation may be tailored to provide
controlled thermal and viscometric properties. In particular, the
phase transition temperature and melt flow index or viscosity of
the formulation may be selected for optimum thermal performance
with respect to such variables as the operating temperature of the
heat generating electronic component, the magnitude of any applied
external pressure, and the configuration of the interface.
[0049] In an alternative embodiment, a paraffinic wax or other
natural or synthetic ester of a long-chain (C.sub.16 or greater)
carboxylic acid and alcohol having a melting temperature of from
about 60-70.degree. C. may be substituted for the thermoplastic and
PSA components to comprise about 20-80% by weight of the
formulation. A preferred wax is marketed commercially by Bareco
Products of Rock Hill, S.C. under the trade designation
"Ultraflex.RTM. Amber," and is compounded as a blend of
clay-treated microcrystalline and amorphous constituents. Such wax
is additionally characterized in Table 2 which follows.
2TABLE 2 Physical Properties of Representative Paraffinic Wax
Component (Ultraflex .RTM. Amber) Melting Point (ASTM D 127)
156.degree. F. (69.degree. C.) Viscosity (ASTM D 3236) @
210.degree. F. (99.degree. C.) 13 cP Penetration (ASTM D 1321) @
77.degree. F. (25.degree. C.) 29 mm @ 110.degree. F. (43.degree.
C.) 190 mm Density (ASTM D 1168) @ 77.degree. F. (25.degree. C.)
0.92 g/cm.sup.3 @ 210.degree. F. (99.degree. C.) 0.79
g/cm.sup.3
[0050] In either of the described embodiments, the resin or wax
components form a binder into which the thermally-conductive filler
is dispersed. The filler is included within the binder in a
proportion sufficient to provide the thermal conductivity desired
for the intended application. The size and shape of the filler is
not critical for the purposes of the present invention. In this
regard, the filler may be of any general shape including spherical,
flake, platelet, irregular, or fibrous, such as chopped or milled
fibers, but preferably will be a powder or other particulate to
assure uniform dispersal and homogeneous mechanical and thermal
properties. The particle size or distribution of the filler
typically will range from between about 0.25-250 microns (0.01-10
mils), but may further vary depending upon the thickness of
interface 28 and/or interlayer 30.
[0051] It additionally is preferred that the filler is selected as
being electrically-nonconductive such that interlayer 30 may
provide an electrically-insulating but thermally-conductive barrier
between electronic component 12 and thermal dissipation member 20.
Suitable thermally-conductive, electrically insulating fillers
include boron nitride, alumina, aluminum oxide, aluminum nitride,
magnesium oxide, zinc oxide, silicon carbide, beryllium oxide, and
mixtures thereof. Such fillers characteristically exhibit a thermal
conductivity of about 25-50 W/m-K.
[0052] Additional fillers and additives may be included in
interlayer 30 to the extent that the thermal conductivity and other
physical properties thereof are not overly compromised. As
aforementioned, a solvent or other diluent may be employed during
compounding to lower the viscosity of the material for improved
mixing and delivery. Conventional wetting opacifying, or
anti-foaming agents, pigments, flame retardants, and antioxidants
also may be added to the formulation depending upon the
requirements of the particular application envisioned. The
formulation may be compounded in a conventional mixing
apparatus.
[0053] Although not required, a carrier or reinforcement member
(not shown) optionally may be incorporated within interlayer 30 as
a separate internal layer. Conventionally, such member may be
provided as a film formed of a thermoplastic material such as a
polyimide, or as a layer of a woven fiberglass fabric or an
expanded aluminum mesh. The reinforcement further supports the
interlayer to facilitate its handling at higher ambient
temperatures and its die cutting into a variety of geometries.
[0054] The Example to follow, wherein all percentages and
proportions are by weight unless otherwise expressly indicated, is
illustrative of the practicing of the invention herein involved,
but should not be construed in any limiting sense.
EXAMPLE
[0055] Master batches representative of the interlayer formulations
of the present invention were compounded for characterization
according to the following schedule:
3TABLE 3 Representative Interlayer Formulations Ultraflex .RTM.
Sample Vybar .RTM. 260.sup.1 H600.sup.2 Amber.sup.3 Filler (wt. %)
No. (wt. %) (wt. %) (wt. %) BN.sup.4 ZnO.sub.2.sup.5 Al.sup.6 3-1
45 22 33 3-2 47 17 36 3-3 47 17 6 30 3-6 40 60 3-7 40 19 41 3-8 50
25 25 3-10 34 16 50 5-1 67 33 .sup.1.alpha.-olefinic thermoplastic,
Petrolite Corp., Tulsa, OK .sup.2acrylic PSA, Heartland Adhesives,
Germantown, WI .sup.3paraffinic wax, Bareco Products Corp. Rock
Hill, SC .sup.4Boron nitride, HCP particle grade, Advanced
Ceramics, Cleveland, OH .sup.5Zinc oxide, Midwest Zinc, Chicago,
II; Wittaker, Clark & Daniels, Inc., S. Plainfield, NJ
.sup.6Alumina, R1298, Alcan Aluminum, Union, NJ
[0056] The Samples were thinned to about 30-70% total solids with
toluene or xylene, cast, and then dried to a film thickness of from
about 2.5 to 6 mils. When heated to a temperature of between about
55-65.degree. C., the Samples were observed to exhibit a
conformable grease or paste-like consistency. The following thermal
properties were measured and compared with conventional silicone
grease (Dow 340, Dow Corning, Midland, Mich.) and metal
foil-supported wax (Crayotherm.RTM., Crayotherm Corp., Anaheim,
Calif.) formulations:
4TABLE 4 Thermal Properties of Representative and Comparative
Interlaver Formulations Thermal Thermal Sample Impedance.sup.5
Conductivity.sup.5 No. Formulation Filler (wt. %) Thickness (mils)
(.degree. C.-in/w) (w/m-K) 3-1 blend.sup.1 62% Al 6 0.14 1.7 3-2
blend 62% Al 4 0.12 1.3 3-3 blend 62% Al/BN 4 0.09 1.7 3-6
wax.sup.2 60% Al 2.5 0.04 2.3 5-1 wax 50% BN 4 0.10 1.5 3-7 blend
62% ZnO.sub.2 4 0.14 1.1 3-8 blend 30% BN 2.5 0.07 1.5 3-10 blend
70% ZnO.sub.2 3 0.12 0.95 Crayotherm wax/foil.sup.3 ZnO.sub.2 2.5
0.11 0.93 3-2 blend 62% Al 5 0.26 0.74 3-6 wax 60% Al 5 0.30 0.65
5-1 wax 50% BN 5 0.12 1.64 Dow 340 grease.sup.4 ZnO.sub.2 5
(true.sup.6) 0.36 0.54 .sup.1of Vybar .RTM. and H600
.sup.2Ultraflex .RTM. Amber .sup.3metal foil-supported wax
.sup.4silicone grease .sup.5measured using from about 10-300 psi
applied external pressure .sup.6spacers used to control
thickness
[0057] The foregoing results confirm that the interlayer
formulations of the present invention retain the preferred
conformal and thermal properties of the greases and waxes
heretofore known in the art. However, such formulations
additionally are form-stable and self-supporting at room
temperature, thus affording easier handling and application and
obviating the necessity for a supporting substrate, web, or other
carrier.
[0058] As it is anticipated that certain changes may be made in the
present invention without departing from the precepts herein
involved, it is intended that all matter contained in the foregoing
description shall be interpreted as illustrative and not in a
limiting sense. All references cited herein are expressly
incorporated by reference.
* * * * *