BACKGROUND OF THE INVENTION
The present invention relates broadly to a thermal management material which is interposable as an interface between, for example, the heat transfer surfaces of a heat-generating, electronic component, such as an integrated circuit (IC) chip, 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 double-side, pressure sensitive adhesive tape adapted for attaching a metal heat sink to a plastic-encapsulated or “packaged,” heat-generating electronic component.
Circuit designs for modem 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 power 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 circuits 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 or circuit board of the device. However, beyond the normal temperature gradients between the electronic component and the heat sink, an appreciable temperature gradient is developed as a thermal interfacial impedance or contact resistance at the interface between the bodies.
That is, 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 heat transfer area and the overall efficiency of the heat transfer through the interface. 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 heat transfer efficiency through the interface, a layer of a thermally-conductive, electrically-insulating 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 solid 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 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.
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. 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 or pad 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® by the Chomerics Division of Parker-Hannifin Corp., 77 Dragon Court, Woburn, Mass. 01888. 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, which may include aluminum oxide, aluminum nitride, boron nitride, magnesium oxide, or zinc oxide, range in particle size from about 1-50 microns (0.05-2 mils).
U.S. Pat. No. 4,654,754 discloses a “thermal link” for providing a thermal pathway between a heat source and a heat sink. In one embodiment, a thermally conductive elastomeric material, such as a silicone filled with silver-copper particles, is formed into a mat having a plurality of raised sections. The raised sections deform under low pressure to conform to the space between the heat source and the heat sink.
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. 4,842,911 discloses a composite interfacing for the withdrawal and dissipation of heat from an electronic, solid-state device by an associated heat sink. The interfacing consists of dual layers of a compliant silicone rubber carried on either side of a porous glass cloth. The layers are filled with finely-divided heat-conducting particles which may be formed of alumina or another metal oxide, or an electrically-conductive material such as nickel or graphite. One of the silicone layers is pre-vulcanized, with the other being cured and bonded in place once the interfacing has been applied to the heat sink surface for abutment with the electronic device.
Commonly-assigned U.S. Pat. No. 4,869,954 discloses a form-stable material for use in transferring thermal energy from an electronic component to a heat sink. The material is formulated as the reaction product of a urethane resin and a curing agent, and is filled with one or more thermally conductive fillers such as zinc oxide, aluminum oxide, magnesium oxide, aluminum nitride, or boron nitride. The material may be formed as including a support layer of a glass cloth, plastic mesh or film, or a metal mesh or foil.
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.
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 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. 5,060,114 discloses a conformable, gel-like pad having a thermally-conductive filler for conducting heat away from a packaged electronic power device. The pad is formed of a cured silicone resin which is filled with a thermally-conductive material such as aluminum powder, nickel, aluminum oxide, iron oxide, beryllium oxide, or silver. A thin sheet of a thermally-conductive metal such as aluminum is positioned in contact with the surface of the conformable pad for increased thermal transfer.
Commonly-assigned 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. 5,151,777 discloses an interface device of thermally coupling an integrated circuit to a heat sink. The device includes a first material, such as copper, having a high thermal conductivity, which is provided to completely surround a plurality of inner core regions. The inner core regions contain a material such as an iron-nickel alloy having a low coefficient of thermal expansion.
Commonly-assigned 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.
Commonly-assigned 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. Such a tape is marketed commercially by the Chomerics Division of Parker-Hannifin Corp., Woburn, Mass., under the tradename THERMATTACH®.
U.S. Pat. No. 5,309,320 discloses a “conduction converter” for a printed circuit board having electronic components. The converter includes a body of a thermally conductive dielectric material, such as an alumina-filled RTV silicone, which is molded to the exact configuration of the electronic components. The converter may be clamped intermediate a cold plate and the circuit board to conductively remove heat from the electronic components.
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.
Commonly-assigned U.S. Pat. No. 5,510,174 discloses a thermally-conductive, titanium diboride (TiB2) filler providing improved thermal conductivity at low application pressures. The filler may be incorporated into elastomers, films, and tapes.
U.S. Pat. No. 5,545,473 discloses a thermally conductive interface for electronic components. The interface is formed of an open structure fluoropolymer material such as an expanded polytetrafluoroethylene. Thermally conductive particles, which may be formed of a metal or metal oxide, or another material such as boron nitride, aluminum nitride, diamond powder, or silicone carbide, are attached to portions of the fluoropolymer material.
U.S. Pat. Nos. 5,533,256 and 5,471,027 disclose a method of joining a multi-layered ceramic (MLC) electronic package. The method involves the use of a double-sided, pressure-sensitive, thermally-conductive adhesive tape to directly bond the heat sink to an upper, exposed surface of the chip as mounted on a circuit board.
International Publication No. WO 96/37915 discloses an electronic assembly including an active circuit having surface mount components, an insulating layer, and an aluminum heat sink. The insulating layer comprises an unfilled thermoplastic sheet having adhesive layers on opposite sides thereof. The adhesive layers preferably are selected as a thermoplastic or thermosetting adhesive or pressure sensitive adhesive formulation containing a thermally-conductive and, optionally, electrically-conductive filler material which may be a metallic, inorganic, or ceramic particulate. The unfilled sheet preferably is a thin film of an engineering thermoplastic such as a polyester, polyetherimides, polyimide, or the like. A preferred adhesive is a solvent-borne, water-based, or hot melt thermoplastic adhesive.
U.S. Pat. No. 4,606,962 discloses an electrically and thermally conductive adhesive transfer tape for attaching individual semiconductor dies or chips to conductive substrates. The transfer tape comprises a flexible, low-adhesion carrier web to which is lightly adhered a layer of an adhesive containing electrically and thermally conductive particles. The particle containing adhesive layer is removed from the carrier web and compressed between the die and the substrate for attaching the die to the substrate.
A double coated film tape is marketed commercially by 3M, St. Paul, Minn., under the tradename “9731.” A firm silicone PSA system is coated on the inside of a 0.055-inch (0.14 mm) thick polyester film carrier, with a high performance acrylic adhesive being coated on the outside of the carrier. Such tape is stated to feature the strong holding power of a silicone adhesive to various silicone surfaces, along with the high adhesion of an acrylic adhesive to a variety of surfaces.
Sheets, pads, and tapes of the above-described types have garnered general acceptance for use as interface materials in the conductive cooling of electronic component assemblies such as the semiconductor chips, i.e., dies described in U.S. Pat. No. 5,359,768. It will be appreciated, however, that further improvements in these types of interface materials are called for in response to developments made by the electronics industry. Specifically regarding the fabrication of semiconductor dies, heretofore such dies typically were packaged by encapsulation in a ceramic chip carrier. External connections provided on the chip carrier allow for the chip to be mounted onto a printed circuit board (PCB) by wire bonding electrical leads on the carrier through a common mounting surface on the board, or by surface mounting the carrier directly to the mounting surface of the board.
Recently, the industry trend has been away from ceramic chip carrier packages and toward plastic packages. Usually molded of an engineering thermoplastic material such as polyethylene terephthalate (PETP), polyphenylene sulfide (PPS), polyetherimide (PEI), polyetherether ketone (PEEK), polyetherketone (PEK), or polyimide (PI), or a thermosetting material such as an epoxy or an epoxy-phenolic composite, these plastic chip packages typically are less expensive than their ceramic counterparts. However, the thermal interface materials previously known in the art were designed for use with ceramic substrates, and not for use with the low surface energy, plastic substrates now common in the commercial electronics market. Especially desired, therefore, would be a thermal interface material which is particularly adapt to bond or otherwise couple a plastic packaged die or other heat generating electronic component to a metal thermal dissipation member, such as a plate or pin fin heat sink, or to another metal surface such as that of a cold plate or chassis. The preferred material would provide an effective thermal interface between the electronic component and the heat sink, while exhibiting improved adhesion to low surface energy substrates.
BROAD STATEMENT OF THE INVENTION
The present invention is directed to a thermal interface, and particularly to a double-sided, thermally-conductive, pressure sensitive adhesive (PSA) tape or other laminate adapted for bonding or otherwise coupling a plastic packaged electronic die or other component in a conductive heat transfer relationship with a metal thermal dissipation member such as a metal heat sink or chassis. In this regard, the interface includes a PSA surface or side formed of a layer of a thermally-conductive, first PSA composition having an affinity to the low energy surface of the die material, and an opposing second PSA surface or side formed of a layer of a second PSA composition which is different from the first composition and which has an affinity to the higher energy surface of the heat sink or chassis material.
In a preferred embodiment, the second PSA composition is formulated as an acrylic-based PSA which is rendered thermally conductive via its loading with a thermally-conductive particulate filler such as an aluminum oxide. The first PSA composition, in turn, is preferably is formulated as a silicone-based PSA which, optionally, is rendered thermally conductive via its loading with a thermally-conductive. particulate filler such as an aluminum oxide. For ease of handling and application, the adhesive layers may be coated on respective sides of a carrier interlayer, such as a polymeric film or a metal foil, to form a tape which may be faced on one or both sides with a release liner and then wound on a roll. In use, individual interface elements may be machine or manually cut from the roll as configured to conform to the margins of the associated heat transfer surfaces of, for example, the electronic component and heat sink. Then, with the release liner or liners removed, each side of the tape may be adhered under pressure to the corresponding heat transfer surface of the electronic component or heat sink. Optionally, the surfaces of the adhesives layers may be embossed with a cross-hatched pattern, as is shown in the commonly-assigned U.S. Pat. Nos. 5,213,868 and 5,298,791, for additional conformably to the heat transfer surfaces with minimal air pockets.
A feature of a preferred embodiment of the present invention therefore is to provide an assembly which includes a heat-generating source, a thermal dissipation member, and a thermally conductive interface disposed intermediate the heat-generating source and the thermal dissipation member to provide a thermally conductive pathway therebetween. The heat-generating source has a first heat transfer surface formed of a first material, such as a plastic, having a low surface energy, with the thermal dissipation member having a second heat transfer surface which is formed of a second material, such as a metal, having a surface energy substantially higher than the surface energy of the first material. The second heat transfer surface of the thermal dissipation member is disposed opposite the first heat transfer surface of the heat-generating source in a spaced-apart, heat transfer adjacency therewith. The interface includes a first pressure sensitive adhesive (PSA) surface bonded to at least a portion of the first heat transfer surface of the heat-generating source and an opposing second pressure sensitive adhesive (PSA) surface bonded to at least a portion of the second heat transfer surface of the heat-generating source. The first PSA surface is presented from a layer of a thermally conductive first pressure sensitive adhesive composition, preferably silicone-based, having an affinity to the first heat transfer surface of the heat generating source. The second PSA surface is presented from a layer of a second pressure sensitive adhesive composition, preferably acrylic-based, different from the first composition and having an affinity to the second heat transfer surface of the thermal dissipation member.
Another feature of a preferred embodiment of the present invention is to provide a thermally conductive interface disposable intermediate a heat-generating source having a first heat transfer surface formed of a first material, such as a plastic, having a low surface energy, and a thermal dissipation member having a second heat transfer surface which is formed of a second material, such as a metal, having a surface energy substantially higher than the surface energy of the first material, and which is disposable opposite the first heat transfer surface of the heat-generating source in a spaced-apart, heat transfer adjacency therewith. The interface includes a first pressure sensitive adhesive (PSA) surface which is bondable under pressure to at least a portion of the first heat transfer surface of the heat-generating source, and an opposing second pressure sensitive adhesive (PSA) surface bondable under pressure to at least a portion of the second heat transfer surface of the heat-generating source. The first PSA surface is presented from a layer of a thermally conductive, first pressure sensitive adhesive composition, preferably silicone-based, having an affinity to the first heat transfer surface of the heat generating source. In turn, the second PSA surface is presented from a layer of a second pressure sensitive adhesive composition, preferably acrylic-based, different from the first composition and having an affinity to the second heat transfer surface of the thermal dissipation member.
Another feature of a preferred embodiment of the present invention is to provide a method of attaching a heat-generating source having a first heat transfer surface formed of a first material having a low surface energy to a thermal dissipation member having a second heat transfer surface formed of a second material having a surface energy substantially higher than the surface energy of the first material. The method involves providing a thermally conductive tape including opposing first and second pressure sensitive adhesive (PSA) surfaces. The first PSA surface is presented from a layer of a thermally conductive, first pressure sensitive adhesive composition having an affinity to the first heat transfer surface of the heat generating source, with the second pressure sensitive adhesive (PSA) surface being presented from a layer of a second pressure sensitive adhesive composition which is different from the first composition and has an affinity to the second heat transfer surface of the thermal dissipation member. In no particular order, at least a portion of the first and second heat transfer surfaces of the heat-generating source and thermal dissipation member are bonded under pressure to the corresponding first or second PSA surface of the tape such that the first heat transfer surface is disposed opposite the second heat transfer surface of the heat generating source in a spaced-apart, conductive heat transfer adjacency therewith.
Advantages of the present invention include an interface material that may be provided in the form of a double-sided, pressure sensitive adhesive tape, and that is particularly adapted for bonding a low surface energy substrate, such as a plastic packaged electronic component, without the use of a primer to a metal heat sink or chassis wall. Additional advantages include a double-sided, thermally-conductive adhesive tape which may be provided in a roll for automated application, and which can be consistently applied using either automated or manual processes for precise thermal and adhesive properties. Still further advantages include a thermal interface offering an easy “peel and stick” installation without the use of mechanical fasteners such as clips or screws. Yet further advantages include a thermal interface that can be removed after application for repair or rework. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.