U.S. patent number RE41,576 [Application Number 09/714,680] was granted by the patent office on 2010-08-24 for conformal thermal interface material for electronic components.
This patent grant is currently assigned to Parker-Hannifin Corporation. Invention is credited to Michael H. Bunyan, Miksa deSorgo.
United States Patent |
RE41,576 |
Bunyan , et al. |
August 24, 2010 |
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), deSorgo; Miksa (Lago Vista, TX) |
Assignee: |
Parker-Hannifin Corporation
(Cleveland, OH)
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Family
ID: |
21777389 |
Appl.
No.: |
09/714,680 |
Filed: |
November 16, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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60016488 |
Apr 29, 1996 |
|
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Reissue of: |
08801047 |
Feb 14, 1997 |
06054198 |
Apr 25, 2000 |
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Current U.S.
Class: |
428/515; 428/349;
428/220; 428/40.5; 428/348; 428/41.3; 165/185; 428/41.8;
156/324.4 |
Current CPC
Class: |
C09K
5/06 (20130101); H01L 23/3737 (20130101); Y10T
428/1476 (20150115); Y10T 428/1419 (20150115); Y10T
428/2826 (20150115); H01L 2224/32245 (20130101); Y10T
428/2822 (20150115); Y10T 428/1452 (20150115); H01L
2924/3011 (20130101); Y10T 428/31909 (20150401) |
Current International
Class: |
B32B
9/00 (20060101); B32B 27/08 (20060101); C09J
5/02 (20060101); B32B 27/32 (20060101); B32B
33/00 (20060101); F28F 7/00 (20060101) |
Field of
Search: |
;165/185,DIG.44
;428/40.5,41.3,41.8,220,348,349,515 ;156/247,306.6,324.4 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
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dated Jul. 1991. cited by other .
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1994. cited by other .
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2000. cited by other .
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Zero-stress Film Adhesive for Substrate Attach, published in Hybrid
Circuits No. 18, Jan. 1989. cited by other .
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Primary Examiner: Ahmed; Sheeba
Attorney, Agent or Firm: Molnar, Jr.; John A.
Parent Case Text
This application claims the benefit of U.S. Provisional Application
No.: 60/016,488 filing date Apr. 29, 1996.
Claims
What is claimed:
.[.1. A method of conductively cooling a heat-generating electronic
component having 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 a thermal
dissipation member to define an interface therebetween, said method
comprising the steps of: (a) providing a thermally-conductive
material which is form-stable at normal room temperature in a first
phase and conformable in a second phase to substantially fill said
interface, said material having a transition temperature from said
first phase to said second phase within the operating temperature
range of said electronic component, and said material consisting
essentially of at least one resin or wax component blended with at
least one thermally-conductive filler; (b) forming said material
into a self-supporting and free-standing film layer, said layer
consisting essentially of said material and having a thickness of
from about 1-10 mils; (c) applying said layer to one of said heat
transfer surfaces; (d) disposing said heat transfer surfaces in
thermal adjacency to define said interface; and (e) energizing said
electronic component effective to heat said layer to a temperature
which is above said phase transition temperature..].
.[.2. The method of claim 1 further comprising an additional step
between steps (d) and (e) of applying an external force to at least
one of said heat transfers defining said interface..].
.[.3. The method of claim 1 wherein said thermal dissipation member
is a heat sink or a circuit board..].
.[.4. The method of claim 1 wherein said layer is applied in step
(c) to the heat transfer surface of said electronic
component..].
.[.5. The method of claim 1 wherein said self-supporting layer is
formed in step (b) by coating a film of said material onto a
surface of a release sheet, and wherein said layer is applied in
step (c) by adhering said film to one of said heat transfer and
removing said release sheet to expose said film..].
.[.6. The method of claim 1 wherein said material is provided in
step (a) as consisting essentially of a blend of: (i) from about 20
to 80% by weight of a paraffinic wax component having a melting
temperature of from about 60-70.degree. C.; and (ii) from about 20
to 80% by weight of one or more thermally-conductive
fillers..].
.[.7. The method of claim 6 wherein said material has a phase
transition temperature of from about 60-80.degree. C..].
.[.8. The method of claim 6 wherein said one or more
thermally-conductive fillers is selected from the group consisting
of boron nitride, alumina, aluminum oxide, aluminum nitride,
magnesium oxide, zinc oxide, silicon carbide, beryllium oxide, and
mixtures thereof..].
.[.9. A thermally-conductive interface for interposition between a
heat-generating electronic component having 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 a thermal dissipation member, said interface
comprising a self-supporting and free-standing film layer having a
thickness of from about 1-10 mils and consisting essentially 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 said interface surfaces, said material having a
transition temperature from said first phase to said second phase
within the operating temperature range of said electronic
component, and said material consisting essentially of at least one
resin or wax component blended with at least one
thermally-conductive filler..].
.[.10. The interface of claim 9 which is coated as a film onto a
surface of a release sheet..].
.[.11. The interface of claim 9 wherein said material consisting
essentially of a blend of: (a) from about 20 to 80% by weight of a
paraffinic wax component having a melting temperature of from about
60-70.degree. C.; and (b) from about 20 to 80% by weight of one or
more thermally-conductive fillers..].
.[.12. The interface of claim 11 wherein said material has a phase
transition temperature of from about 60-80.degree. C..].
.[.13. The interface of claim 11 wherein said one or more
thermally-conductive fillers is selected from the group consisting
of boron nitride, alumina, aluminum oxide, aluminum nitride,
magnesium oxide, zinc oxide, silicon carbide, beryllium oxide, and
mixtures thereof..].
14. A method of conductively cooling a heat-generating electronic
component having 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 a thermal
dissipation member to define an interface therebetween, said method
comprising the steps of: (a) providing a thermally-conductive
material which is form-stable at normal room temperature in a first
phase and conformable in a second phase to substantially fill said
interface, said material having a transition temperature from said
first phase to said second phase within the operating temperature
range of said electronic component and comprising a blend of: (i)
from about 25 to 50% by weight of an acrylic pressure sensitive
adhesive component having a melting temperature of from about
90-100.degree. C.; (ii) 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 (iii) from about 20
to 80% by weight of one or more thermally-conductive fillers; (b)
forming said material into a self-supporting layer; (c) applying
said layer to one of said heat transfer surfaces; (d) disposing
said heat transfer surfaces in thermal adjacency to define said
interface; and (e) energizing said electronic component effective
to heat said layer to a temperature which is above said phase
transition temperature.
15. The method of claim 14 wherein said material has a phase
transition temperature of from about 70-80.degree. C.
16. The method of claim 14 wherein said one or more
thermally-conductive fillers is selected from the group consisting
of boron nitride, alumina, aluminum oxide, aluminum nitride,
magnesium oxide, zinc oxide, silicon carbide, beryllium oxide, and
mixtures thereof.
17. A thermally-conductive interface for interposition between a
heat-generating electronic component having an operating
temperature range above normal room temperature and a first heat
transfer surface disposal in thermal adjacency with a second heat
transfer surface of a thermal dissipation member, said interface
comprising 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 said
interface surfaces, said material having a transition temperature
from said first phase to said second phase within the operating
temperature range of said electronic component, and comprising a
blend of: (a) from about 25 to 50% by weight of an acrylic pressure
sensitive adhesive 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.
18. The interface of claim 17 wherein said material has a phase
transition temperature of from about 70-80.degree. C.
19. The interface of claim 17 wherein said one or more
thermally-conductive fillers is selected from the group consisting
of boron nitride, alumina, aluminum oxide, aluminum nitride,
magnesium oxide, zinc oxide, silicon carbide, beryllium oxide, and
mixtures thereof.
Description
BACKGROUND OF THE INVENTION
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.
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.
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.
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.
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.
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.
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.
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 electronically-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.
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.
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.
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.
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.
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.
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.
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 silicon
elastomer which is applied to the surface of the cold plate.
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.
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.
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.
U.S. Pat. No. 5,194,480 discloses another thermally-conductive,
electricallyinsulating filled elastomer. A preferred filler is
hexagonal boron nitride. The filled elastomer may be formed into
blocks, sheets, or films using conventional methods.
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.
U.S. Pat. No. 5,321,582 discloses an electronic component heat sink
assembly which includes a thermally-conductive laminate formed of
polyamide which underlies a layer of a boron nitride-filled
silicone. The laminate is interposed between the electronic
component and the housing of the assembly.
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 as 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
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.
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.
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 mono-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.
Advantages of the present invention include a thermal interface
material which melts of 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
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:
FIG. 1 is a fragmentary, cross-sectional view of an electrical
assembly wherein a heatgenerating 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 interace between the heat transfer surfaces of
the component and an associated thermal dissipation member;
FIG. 2 is a view of a portion of the thermal interface of FIG. 1
which is enlarged to detail the morphology thereof,
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
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
The drawings will be described further in connection with the
following Detailed Description of the Invention.
DETAILED DESCRIPTION OF THE INVENTION
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.
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.
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.
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.
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.
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.
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 enerigization 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.
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.
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.
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.
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.
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.
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."
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.
TABLE-US-00001 TABLE 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) 357.5 cP @ 210.degree. F. (99.degree. C.)
Penetration (ASTM D 1321) 12 mm @ 77.degree. F. (25.degree. C.)
Density (ASTM D 1168) @ 75.degree. F. (24.degree. C.) 0.90
g/cm.sup.3 @ 200.degree. F. (93.degree. C.) 0.79 g/cm.sup.3 Iodine
Number (ASTM D 1959) 15
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.
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.
TABLE-US-00002 TABLE 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)
13 cP @ 210.degree. F. (99.degree. C.) Penetration (ASTM D 1321) @
77.degree. F. (25.degree. C.) 29 mm @ 110.degree. F. (43.degree.
C.) 190 mm Density (ASTM D 1168) @ 75.degree. F. (25.degree. C.)
0.92 g/cm.sup.3 @ 210.degree. F. (99.degree. C.) 0.79
g/cm.sup.3
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.
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-.degree.K.
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.
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.
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
Master batches representative of the interlayer formulations of the
present invention were compounded for characterization according to
the following schedule:
TABLE-US-00003 TABLE 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, IL; Wittaker, Clark & Daniels, Inc., S.
Plainfield, NJ .sup.6Alumina, R1298, Alcan Aluminum, Union, NJ
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.TM., Crayotherm Corp., Anaheim,
Calif.) formulations:
TABLE-US-00004 TABLE 4 Thermal Properties of Representative and
Comparative Interlayer Formulations Thermal Thermal Sample Formu-
Filler Thickness Impedance.sup.5 Conductivity.sup.5 No. lation (wt.
%) (mills) (.degree. C.-in/w) (w/m-.degree. K.) 3-1 blend 62% Al 6
0.14 1.7 3-2 blend 62% Al 4 0.12 1.3 3-3 blend 62% 4 0.09 1.7 Al/BN
3-6 wax.sup.2 60% Al 2.5 0.04 2.3 5-1 wax 50% 4 0.10 1.5 BN 3-7
blend 62% 4 0.14 1.1 ZnO.sub.2 3-8 blend 30% 2.5 0.07 1.5 BN 3-10
blend 70% 3 0.12 0.95 ZnO.sub.2 Crayo- wax/foil.sup.3 ZnO.sub.2 2.5
0.11 0.93 therm 3-2 blend 62% Al 5 0.26 0.74 3-6 wax 60% Al 5 0.30
0.65 5-1 wax 50% 5 0.12 1.64 BN Dow grease.sup.4 ZnO.sub.2 5
(true.sup.6) 0.36 0.54 340 .sup.1blend of 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
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.
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.
* * * * *