Abstract:
A metal thermal interface structure for dissipating heat from electronic components comprised a heat spreader lid, metal alloy that is liquid over the operating temperature range of the electronic component, and design features to promote long-term reliability and high thermal performance.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to the field of heat transfer structures between electronic components and their associated heat exchangers and, more particularly, to a thermal interface system which utilizes a metal alloy interface, materials and design features to stabilize the alloy while exposed to various environmental conditions.  
       BACKGROUND OF THE INVENTION  
       [0002]     Today&#39;s electronic components generate significant amounts of heat which must be removed to maintain the component&#39;s junction temperature within safe operating limits. Failure to effectively conduct away heat leaves these devices at high operating temperatures, ultimately resulting in decreased performance and reliability.  
         [0003]     The heat removal process involves heat conduction between the electronic component and heat exchanger, or heat sink, via a thermal interface material. Small irregularities and surface asperities on both the component and heat sink surfaces create air gaps and therefore increase the resistance to the flow of heat. The thermal resistance of the interface between these two surfaces can be reduced by providing an interface material which fills the air gaps and voids in the surfaces.  
         [0004]     An ideal medium for transferring heat from one surface to another should have low interfacial or contact thermal resistance, high bulk thermal conductivity and the ability to achieve a minimum bond-line thickness. Additional desirable qualities include product stability, ease of deployment, product reworkability, low cost and non-toxicity.  
         [0005]     Liquids have low interfacial resistance because they wet a surface forming a continuous contact with a large area. Most liquids do not, however, have very high conductivity. Solids, and in particular metals, have very high conductivity but high interfacial resistance. Most common heat transfer materials combine highly conductive particles with a liquid or plastic in order to exploit both characteristics. Examples of the former are greases and gels while the latter include filled epoxies, silicones and acrylics.  
         [0006]     Greases have been developed with thermal conductivities significantly better than the corresponding conductivities of filled adhesives. Typical problems with greases include pumping and dry out, both of which can cause the conducting medium to lose contact with one or both of the heat transfer surfaces. Pumping occurs when the structure is deformed, due to differential thermal expansion or mechanical loads, and the grease is extruded. The oils contained in a grease can be depleted by evaporation or by separation and capillary flow.  
         [0007]     Liquid metal alloys (liquid at the operating temperature of the electronic component), such as alloys of bismuth, gallium and indium, potentially offer both low interfacial resistance and high conductivity. Several alloys of gallium with very low melting points have also been identified as potential liquid metal interface materials. Thermal performance of such an interface would be more than one order of magnitude greater than many adhesives typically in use.  
         [0008]     Although liquid metal alloys offer both low interfacial resistance and high conductivity, they have historically suffered from various reliability issues including corrosion/oxidation, intermetallic formation, drip-out, dewetting, and migration. Unless mitigated, these mechanisms will continue to degrade the interface, resulting in a thermally related catastrophic failure of the actual electronic component.  
         [0009]     The ability to contain an electrically conductive liquid within an electronic package presents significant challenges. The liquid must be reliably retained in its enclosure throughout the life of the package if shorting is to be avoided. In addition, air must be excluded from the space between the heat transfer surfaces if the effective resistance is to be minimized. This is difficult due to the volume expansion of the liquid and is exacerbated if the metal changes between the liquid and the solid state within the temperature range of the package.  
         [0010]     U.S. Pat. No. 4,092,697, granted to Spaight on May 30, 1978 discloses a conductive or non-conductive film (plastic or metallic) whose perimeter is attached to a heat sink surface thereby creating a pouch. Grease, powdered metal or low melt alloy is inserted within the pouch while the film interfaces the chip or source to be cooled. This design would prevent the interface material from migrating.  
         [0011]     U.S. Pat. No. 4,233,645, granted to Balderes, et al. on Nov. 11, 1980 discloses a deformable heat transfer member (between a heat source and heat exchanger) comprised of a porous block of material and thermally conductive liquid retained within the block by surface tension. This design would also prevent the liquid interface material from migrating out of the thermal joint.  
         [0012]     U.S. Pat. No. 4,323,914, granted to Berndlmaier, et al. on Apr. 6, 1982 discloses methods of protecting both a chip and heat exchanger from gallium-indium or mercury based alloys by coating the interface surfaces with parylene and chromium metal.  
         [0013]     U.S. Pat. No. 4,915,177, granted to Altoz, et al. on Apr. 10, 1990 discloses a low melting point thermal interface material which is contained between the heat source and heat exchanger by applying a sealant to completely encapsulate the interface material.  
         [0014]     U.S. Pat. No. 5,198,189, granted to Booth, et al. on Mar. 30, 1993 discloses a gallium-indium alloy with non-reactive particles which are added in order to increase the viscosity of the alloy and mitigate migration of material from the thermal joint.  
         [0015]     U.S. Pat. No. 6,343,647, granted to Kim, et al. on Feb. 5, 2002 discloses a liquid metal interface material in which the alloy&#39;s operating temperature falls between its liquidus and solidus point, thereby reducing the amount of oxidation and increasing the alloy&#39;s viscosity.  
         [0016]     U.S. Pat. No. 6,372,997, granted to Hill, et al. on Apr. 16, 2002 discloses a low melting point alloy coating both sides of a surface enhanced metallic foil, thereby providing a carrier to support and contain the liquid metal alloy.  
         [0017]     U.S. Pat. No. 6,656,770, granted to Atwood, et al. on Dec. 2, 2003 discloses both a solder-based seal (between the ceramic cap/heat exchanger and package substrate) and an elastomeric gasket (between the ceramic cap/heat exchanger and chip) to near hermetically seal the cavity containing a Gallium alloy interface material and thereby limit ingress of oxygen, oxidation and migration.  
         [0018]     U.S. Pat. No. 6,665,186, granted to Calmidi, et al. on Dec. 16, 2003 discloses a gallium based interface material held in place by a flexible seal, such as an O-ring, which also accommodates expansion and contraction of the liquid.  
       SUMMARY OF THE INVENTION  
       [0019]     Accordingly, it is the overall feature of the present invention to provide an improved thermal interface system in order to more effectively transfer thermal energy from an electronic component to a heat exchange structure.  
         [0020]     One feature of the present invention is to provide an improved thermal interface system comprised of a metal interface which exhibits a high degree of surface wetting and possessing relatively high bulk thermal conductivity.  
         [0021]     An additional feature of the present invention is to provide an improved metal thermal interface system which is liquid over the operating temperature of the electronic component, thereby minimizing the stresses placed on the electronic component by the heat exchange structure.  
         [0022]     Yet, another feature of the present invention is to provide an improved metal thermal interface system which utilizes diffusion barrier layers to promote chemical compatibility between the metal interface and heat exchange components.  
         [0023]     A further feature of the present invention is to provide an improved metal thermal interface system which includes materials and design features, such as moisture seals, encapsulants, desiccants and corrosion inhibitors, to promote long-term stability and reliability by mitigating corrosion.  
         [0024]     Still another feature of the present invention is to provide an improved metal thermal interface system which includes barrier structures to preclude metal interface migration and preserve high heat transfer.  
         [0025]     Lastly, it is a feature of the present invention to combine all of these unique design aspects and individual fabrication techniques into effective and manufacturable thermal interface system for electronic components, including Flip Chip and Cavity-Down IC packages. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  illustrates an electronic component package including a heat spreader lid, corrosion inhibitor and metal interface material comprising the thermal interface structure embodiment of the present invention.  
         [0027]      FIGS. 2   a  and  2   b  illustrate the use of a metal interface barrier structure within the present invention.  
         [0028]      FIG. 3  illustrates another heat spreader lid embodiment of the present invention.  
         [0029]      FIG. 4  illustrates another electronic component package embodiment comprising an IC die and thermal interface structure. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     Described below are several embodiments of the present invention which illustrate various ways the present invention can be implemented. In the descriptions that follow, like numerals represent like numerals in all figures. For example, where the numeral  14  is used to refer to a particular element in one figure, the numeral  14  appearing in any other figure refers to the same element.  
         [0031]     As seen in  FIG. 1 , an electronic component package  10 , comprised of a thermal interface structure  12 , electronic component  14 , and package substrate  16 , is illustrated. The electronic component  14  may be an IC chip (die) or other discrete device fabricated from silicon or a compound semiconductor material. The illustrated component  14  is a Flip Chip die which includes one face for electrical connection to the package substrate  16  (via flip chip solder balls  18 ) and an opposite face  20  to which a thermal interface structure  12  may be attached for removing generated heat.  
         [0032]     The thermal interface structure includes a heat spreader lid  22  which may be metallic, composite or ceramic in composition. The lid is formed to include an underside cavity  24  and an outer flange  26 . Within the lid cavity  24 , a metal interface  28  is applied directly to the cavity surface  29  (by mechanical scrubbing or ultrasonic agitation) or to a diffusion barrier layer  30  which may be deposited on surface of the cavity  29 . A metal (such as alloys of bismuth, gallium, indium and tin) which is liquid over the operating temperature of the electronic component is needed to allow the metal to adequately flow into all surface asperities of the lid cavity surface  29  and die  14 . Suitable diffusion barrier layer materials include chromium, iron, molybdenum, nickel, niobium, tantalum or tungsten.  
         [0033]     A corrosion inhibiting material  32 , such as a moisture desiccant, vapor phase or liquid phase corrosion inhibitor, is disposed within the lid cavity  24 . These materials, in powder or granular form, may be first applied to an absorbent or adhesive substrate/medium to facilitate deployment within the lid cavity  24 .  
         [0034]     Moisture desiccants can adsorb significant amounts of water even at low humidity levels. The reduction of humidity within the lid cavity  24  results in greatly reduced corrosion rates on the metal interface  28 .  
         [0035]     The moisture desiccant may be selected from the group consisting of silica gel; molecular sieve zeolites; activated clays, such as a montmorillonite clay; activated alumina; anhydrous calcium sulfate; anhydrous calcium chloride; anhydrous calcium bromide; anhydrous lithium chloride; anhydrous zinc chloride; anhydrous barium oxide; anhydrous calcium oxide and combinations thereof.  
         [0036]     Vapor phase corrosion inhibitors are compounds transported in a closed environment to the site of corrosion by volatilization from a source. The vapors protect metallic surfaces through the deposition or condensation of a protective film or coating. Upon contact with the metal interface  28 , the vapor of these salts condenses and is hydrolyzed by any moisture to liberate protective ions, thus mitigating any corrosion.  
         [0037]     The vapor phase corrosion inhibitor may be selected from the group consisting of nitrites, benzoates, sulfonates, primary amines, secondary amines, tertiary amines, diamines, aliphatic polyamines, ethers, salts of quaternary ammonium compounds, amine salts, aromatic amines, nonaromatic heterocyclic amines, heterocyclic amines, alkanolamines, substituted alkanolamines, thiols, thioethers, sulfoxides, thiourea, substituted thioureas, substituted thiocarbonyl esters, phosphonium salts, arsonium salts, phosphates, sulfonates, molybdates, corresponding salts and combinations thereof.  
         [0038]     The vapor phase corrosion inhibitor may additionally be selected from the group consisting of sodium nitrite, dicyclohexylamine, sodium benzoate, hexadecylpyridinium iodide, dodecylbenzyl quinolinium bromide, propargyl quinolinium bromide, cyclohexylammonium benzoate, ammonium benzoate, dicyclohexylammonium nitrite and dicyclohexylamine chromate, benzotriazole, mercaptobenzothiazole, sodium dinonylnaphthalene sulfonate, triethanolamine dinonylnaphthalene sulfonate, calcium dinonylnaphthalene sulfonate, magnesium dinonylnaphthalene sulfonate, barium dinonylnaphthalene sulfonate, zinc dinonylnaphthalene sulfonate, lithium dinonylnaphthalene sulfonate, ammonium dinonylnaphthalene sulfonate, ethylenediamine dinonylnaphthalene sulfonate, diethylenetriamine dinonylnaphthalene sulfonate, 2-methylpentanediamine dinonylnaphthalene sulfonate, sodium molybdate, corresponding salts and combinations thereof.  
         [0039]     Liquid phase corrosion inhibitors blend with the liquid moisture present to protect surfaces through various mechanisms including the creation of passivation layers, raising the pH of the moisture, or reducing the electrical conductivity of the moisture layer. Liquid phase corrosion inhibitor candidates include sodium metaborate, sodium nitrite, sodium chromate and sodium silicate.  
         [0040]     The lid  22  is attached to the electronic component package substrate  16  via the outer flange  26  and a continuous seal  34 . Seal materials include silicones, polysulphides, polyurethanes, polyimides, polyesters, epoxides, cyanate esters, olefins and sealing glasses. A continuous seal between the heat spreader lid flange  26  and package substrate  16  greatly reduces the amount of moisture ingression within the lid cavity  24 , resulting in reduced film formation and corrosion on the metal interface  28 .  
         [0041]     Reference is now made to  FIGS. 2   a  and  2   b  wherein a containment band  36 , which forms a physical barrier to metal interface migration, is illustrated within the electronic component package  10 .  
         [0042]     As shown in  FIG. 2   a , the containment band  36 , affixed to the lid cavity surface  29 , may be composed of a polymeric or fabric material which is coated (Teflon, for example) to mitigate any adhesion by the metal interface  28 . The illustrated embodiment includes a diffusion barrier layer  30  which extends to the outer periphery of the metal layer  28  and the inner diameter of the containment band  36 .  
         [0043]      FIG. 2   b  (sectional view of  FIG. 2   a  on lines  2   b - 2   b ) depicts the containment band  36  positioned around the periphery of the metal layer  28  along with corrosion inhibiting material  32  disposed within the heat spreader lid cavity  24  (on the lid cavity surface  29 ).  
         [0044]      FIG. 3 , similar to  FIG. 2 , illustrates an alternative heat spreader lid embodiment wherein the lid  22  may be joined to at least one additional ring or stiffener  38  (via an adhesive layer  34 ) thereby creating a heat spreader core cavity  24 .  
         [0045]     Reference is now made to  FIG. 4  wherein another embodiment of the present invention is illustrated. The Cavity-Down style electronic component package  40  is comprised of a heat spreader core  22  formed with a cavity  24  and outer flange  26  on the core&#39;s underside. The core  22 , which may be metallic, composite or ceramic in composition, provides structural integrity for a circuitry layer  42  which is attached to the outer flange  26 . The IC die  14  includes a backside  20  which is attached to the cavity surface  29  within the core  22  via a metal interface  28 . A diffusion barrier layer  30  may be deposited on the cavity surface  29  to mitigate the possibility of intermetallic formation between the core  22  and metal interface  28 .  
         [0046]     A plurality of metallic bond wires  44  provide electrical continuity between the die  14  and circuitry layer  42 . An encapsulating material  46 , disposed within the heat spreader core cavity  24 , is applied over the IC die  14 , the exposed portion of the metal interface  28  and bond wires  44 , thereby protecting the metallic and semiconducting surfaces from moisture, contamination and physical damage. To maintain a precise position on the core  22 , the IC die  14  may be partially fastened (at the corners or edges) to the cavity surface  29  by an adhesive prior to wire bonding or encapsulation.  
         [0047]     Several embodiments of the present invention have been described. A person skilled in the art, however, will recognize that many other embodiments are possible within the scope of the claimed invention. For this reason, the scope of the invention is not to be determined from the description of the embodiments, but must instead be determined solely from the claims that follow.