Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation-in-part of, and claims priority from, U.S. patent application Ser. No. 11/248,720 filed Oct. 11, 2005, and still pending. 
     
    
     TECHNICAL FIELD 
       [0002]    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 
       [0003]    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 and ultimately failure. 
         [0004]    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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    Greases have been developed with thermal conductivities significantly better than the corresponding conductivities of filled adhesives. Typical problems with greases include to 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. 
         [0008]    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. 
         [0009]    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. 
         [0010]    The ability to contain an electrically conductive liquid within an electronic package presents significant challenges. The liquid must be reliably retained in the thermal interface throughout the life of the package if shorting is to be avoided and effective resistance is to be minimized. To solve the problems of liquid metal migration, various seal and gasket mechanisms have been disclosed. 
         [0011]    Although, these various mechanisms mitigate liquid metal migration, some disclosures include elastomeric or polymeric components in the thermal path which is thermally undesirable. Other disclosures include various seals which increase the bondline thickness (BLT) of the liquid metal, thereby, increasing the bulk thermal resistance of the interface. These elastomeric components are not hermetic and therefore do not prevent air or moisture from entering the thermal joint. 
         [0012]    In addition, corrosion will propagate through the thermal interface should any air gaps be present. Surface asperities of the heat source and heat exchanger increase the potential for voids. This is further exacerbated when the metal changes between the liquid and the solid state within the temperature range of the package. 
         [0013]    U.S. Pat. No. 4,413,766, granted to Webster on Nov. 8, 1983 discloses a void-free design to bond a metallic sheet to a ceramic substrate wherein grooves are added to the metallic sheet to facilitate gas escape. 
         [0014]    U.S. Pat. No. 4,650,107, granted to Keser on Mar. 17, 1987 discloses a design to promote bubble-free soldering of two components wherein V-shaped perforations or mesh facilitate deaerating of the solder joint while using high clamping pressure. 
         [0015]    U.S. Pat. No. 4,915,167, 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 exposed interface material. 
         [0016]    U.S. Pat. Nos. 5,323,294 and 5,572,404, granted to Layton, et al. on Jun. 21, 1994 and Nov. 5, 1996, respectively, and U.S. Pat. No. 5,561,590, granted to Norell, et al. on Oct. 1, 1996 disclose a heat transfer module in which a compliant, absorbent body containing liquid metal is surrounded by a seal, said body is spaced apart from the seal area. As the module is clamped between a heat source and heat exchanger, liquid metal is squeezed out of the porous structure to fully fill the space within the seal area. 
         [0017]    U.S. Pat. No. 5,909,056, granted to Mertol on Jun. 1, 1999 discloses a thermal interface structure in which a phase change thermal interface material is contained within a protrusion on a heat spreader and a dam ring, which is attached to the backside of a semiconductor chip. 
         [0018]    U.S. Pat. No. 6,016,006, granted to Kolman, et al. on Jan. 18, 2000 discloses a method for applying thermal interface grease between an integrated circuit device and a cover plate in which a seal encloses the region of the device. Thermal grease is injected into the cavity region via an inlet port in the cover plate thereby filling the interface between device and plate. 
         [0019]    U.S. Pat. No. 6,037,658, granted to Brodsky, et al. on Mar. 14, 2000 discloses a heat transfer surface in which a thermally conductive fluid is contained by both an absorbent medium and a seal to inhibit migration. 
         [0020]    U.S. Pat. No. 6,097,602, granted to Witchger on Aug. 1, 2000 discloses a thermal interface structure in which a phase change interface material is surrounded by a fabric carrier dike structure. The dike is adhesively attached to both the electronic circuit package and heat sink, thereby preventing interface material from migrating from the joint. 
         [0021]    U.S. Pat. Nos. 6,281,573 and 6,656,770, granted to Atwood, et al. on Aug. 28, 2001 and Dec. 2, 2003, respectively, disclose 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 liquid metal interface material and thereby limit oxidation and migration. 
         [0022]    U.S. Pat. No. 6,292,362, granted to O&#39;Neal, et al. on Sep. 18, 2001 discloses a thermal interface material module in which a flowable interface material is deposited in the center opening of a picture-frame carrier and a gasket is mounted to the carrier. With the application of heat, the reservoir area between the interface material and gasket is filled. 
         [0023]    U.S. Pat. No. 6,665,186, granted to Calmidi, et al. on Dec. 16, 2003 discloses a liquid metal interface material held in place by a flexible seal, such as an O-ring, which also accommodates expansion and contraction of the liquid. The seal also allows for air venting and filling of liquid metal. 
         [0024]    U.S. Pat. No. 6,732,905, granted to Humpston, et al. on May 11, 2004 discloses a method for void-free component attachment wherein a thru-hole vent is formed in the backside face of one component. 
         [0025]    U.S. Pat. Nos. 6,761,928, 6,617,517, 6,372,997, granted to Hill, et al. on Jul. 13, 2004, Sep. 9, 2003, and Apr. 16, 2002, respectively, and U.S. Pat. No. 6,940,721, granted to Hill on Sep. 6, 2005 disclose 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. The low melt alloy on the foil carrier, placed between a heat source and heat exchanger, will become molten during the operational temperatures of the heat source. 
         [0026]    U.S. Pat. No. 6,849,941, granted to Hill, et al. on Feb. 1, 2005 discloses a liquid metal interface material in which the material is bonded (in solid form) to a solid base member and includes a sealing material set into a annular groove (within the base member) surrounding the periphery of the bonded interface. 
         [0027]    U.S. Pat. No. 6,891,259, granted to Im, et al. on May 10, 2005 and U.S. Pat. Application No. 20030085475, filed by Im, et al. on Oct. 10, 2002 disclose a semiconductor package in which a dam substantially surrounds the thermal interface material. The package lid includes injection holes for the dispensation of the dam and interface material. 
         [0028]    U.S. Pat. No. 7,030,485 and U.S. Pat. Application No. 20060138644, both by Houle, et al., granted on Apr. 18, 2006 and filed on Feb. 23, 2006, respectively, disclose the use of a plastically deformable material (indium, tin, etc.) to create channels or guides between a IC chip and heat spreader. A liquid metal, such as Gallium, fills the spaces/channels and may even be circulated. 
         [0029]    U.S. Pat. No. 7,169,650 and U.S. Pat. Application No. 20030173051, both by Rinella, et al. granted on Jan. 30, 2007 and filed on Mar. 12, 2002, respectively, disclose a method of forming a thermal interface in which a semi-solid metal, injected through an inlet on a heat spreader plate, fills the gap between a die and the cavity formed in the heat spreader plate. 
         [0030]    U.S. Pat. Application No. 20030183909, filed by Chiu on Mar. 27, 2002 discloses a method of forming a thermal interface in which a thermal interface material is dispensed through and inlet in a heat spreader in order to fill the interface between the spreader and chip. 
         [0031]    U.S. Pat. Application No. 20040217467, filed by Rumer, et al. on May 28, 2004 discloses a heat spreader comprised of a convex or concave surface which contacts a thermal interface material (between chip and heat spreader) in order to mitigate stress from thermal expansion and contraction. 
         [0032]    U.S. Pat. Application No. 20040261980, filed by Dani, et al. on Jun. 30, 2003 discloses a heat dissipating device, such as a heat spreader, comprised of surface features (channels, grooves, serrations) to facilitate adhesion of a thermal interface material and arrest interface cracking or delamination. 
         [0033]    U.S. Pat. Application No. 20040262766, filed by Houle on Jun. 27, 2003 discloses a liquid metal interface contained within a cold-formed o-ring barrier positioned directly on the chip. Once the barrier is established between the heat spreader and chip, liquid metal is introduced into the interface via a channel in the spreader. 
         [0034]    U.S. Pat. Application No. 20050073816, filed by Hill on Jan. 7, 2004 discloses a liquid metal interface assembly in which an o-ring or shim sealing member surrounds the liquid metal interface material to shield the interface from the atmosphere. 
         [0035]    U.S. Pat. Application No. 20060131738, filed by Furman, et al. on Sep. 6, 2005 discloses a liquid metal layer with diffusion barrier layers on both interface surfaces and a wetting layer over each barrier layer. The liquid metal may also include metallic or inert particles for viscosity modification. 
         [0036]      FIGS. 1A through 3  (Prior Art) show various methods of forming a void-free, high thermal performance thermal interface within electronic assemblies  100 .  FIG. 1A  illustrates an electronic assembly  100  comprised of a thermal interface structure  102  positioned between a heat spreader lid  104  and electronic component  106 , which is comprised of an IC chip  108 , package substrate  110  and electrical interconnection vias  112 . The interface structure  102  is comprised of a metallic core  120  encapsulated by a metallic interface composition  122 . An adhesive layer  114  bonds the heat spreader lid  104  to the electronic component package substrate  110 . It can be seen in  FIG. 1B  that the lid  104  has now been mounted to the package substrate  110  with an adhesive layer  114  located on the lid flange  116 . During operation of the electronic component  106 , the resultant heat will cause excess metallic interface composition  122  to flow out of the thermal interface, thereby creating a fillet outside the IC chip perimeter. Unfortunately, oxidation, present on the surface of the metallic interface composition  122  prior to heating and flowing, creates a “skin” and prohibits filling of the surface asperities present on the lid  104  and IC chip  108 .  FIG. 1   c , a magnified sectional view of  FIGS. 1   a  and  1   b , illustrates the resultant air gaps  123  due to the layer of oxidation  125  inhibiting flow of interface material. The non-hermetic interface allows oxygen and moisture to penetrate into these air gaps  123  and continue oxidation/corrosion of the metallic interface composition  122  within the interface between chip  108  and lid  104 . 
         [0037]    Within  FIG. 2  (Prior Art), it can be seen that a metallic thermal interface composition is injected (by a dispenser  124 ) through a hole  126  in the heat spreader lid  104  to yield a filled thermal interface joint  128 . Without a barrier or seal, interface material would have the tendency to migrate out of the joint. The use of a seal will promote full filling of the thermal joint as well. Additionally, the hole  126 , filled with the interface composition would certainly possess lower thermal conductivity than the typical materials (copper, aluminum) comprising heat spreader lids. 
         [0038]      FIG. 3  (Prior Art), similar to  FIG. 2 , illustrates an electronic assembly  100  comprised of a thermal interface structure  130  sandwiched between an IC chip  108  and heat spreader lid  104 . The lid  104  includes at least one gas permeable plug  132  located within holes  134  in the lid  104 . A barrier or seal  136  is placed near the perimeter of the IC chip  108 , thereby creating a seal and space between the lid  104  and IC chip  108 . Liquid interface material  138  is injected into the holes  134  in the lid  104 , thereby filling the space comprising the thermal interface joint. Should the barrier be of polymeric composition, heat transfer would be reduced near the perimeter of the chip. A metallic barrier would require a bonding and hermetic seal in order for the gas permeable plugs to be effective. Barrier bonding may induce unwanted stresses between the IC chip  108  and the lid  104 . Additionally, the holes  134  in the lid would also created undesirable thermal impedance between the chip  108  and lid  104 . 
       SUMMARY OF THE INVENTION 
       [0039]    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. 
         [0040]    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. 
         [0041]    Yet, another feature of the present invention is to provide a corrosion resistant interface system in which the metallic interface composition flows and fills the surface asperities on both the electronic component and heat exchanger thereby sealing the interface from moisture and oxygen. 
         [0042]    One additional feature of the present invention is to provide an improved metal thermal interface system in which the metallic interface composition directionally flows to mitigate any entrapped air voids within the thermal interface. 
         [0043]    A further feature of the present invention is to provide an improved metal thermal interface system which includes structures to vent entrapped air between the electronic component and heat exchanger, thereby increasing heat transfer and environmental reliability. 
         [0044]    Still another feature of the present invention is to provide an improved metal thermal interface system which includes structures to accommodate excess metallic interface material during deployment, thereby ensuring a minimum interface thickness for maximum thermal performance. 
         [0045]    One additional feature of the present invention is to provide a metallic interface composition including oxygen gettering elements to promote wetting to oxide layers present on the surface of the electronic component chip and heat exchanger. 
         [0046]    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 integrated circuit (IC) packages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0047]      FIGS. 1   a  through  1   c , sectional views, illustrate an electronic assembly including a foil-based thermal interface structure deployed between an IC chip and heat spreader lid, as known in the art. 
           [0048]      FIG. 2 , a sectional view, illustrates an electronic assembly in which thermal interface material is injected through a hole in the heat spreader lid, as known in the art. 
           [0049]      FIG. 3 , a sectional view, illustrates an electronic assembly in which thermal interface material is added though a vent to fill the space inside of a barrier formed between the IC chip and lid, as known in the art. 
           [0050]      FIGS. 4   a  through  4   d , sectional views, illustrate the sequence of flowing, filling, and sealing of metallic interface material within a thermal interface joint of the present invention. 
           [0051]      FIGS. 5   a  and  5   b , sectional views, illustrate the initial deployment of one metallic thermal interface embodiment of the present invention. 
           [0052]      FIGS. 6   a  and  6   b , sectional views, illustrate the flowing and filling of metallic interface material (between an electronic component and heat exchanger) of the present invention. 
           [0053]      FIGS. 7   a  and  7   b , sectional views, illustrate final deployed state of one metallic thermal interface embodiment seen in  FIGS. 5   a ,  5   b ,  6   a  and  6   b.    
           [0054]      FIGS. 8   a  through  8   c , partial sectional views, illustrate the change of interface structure thickness to lateral dimensional change as the metallic interface structure (of the present invention) is fully deployed. 
           [0055]      FIG. 9 , an isometric view, illustrates a vent structure (which is gas permeable, yet liquid impermeable) to expel entrapped air during deployment of the present invention. 
           [0056]      FIG. 10 , an isometric view, illustrates another embodiment of the present invention in which a diaphragm structure is added to accommodate excess metallic interface material during deployment. 
           [0057]      FIGS. 11   a  and  11   b , partial sectional views, illustrate how the diaphragm layer acts as a reservoir for excess metallic interface material. 
           [0058]      FIG. 12 , an isometric view, illustrates yet another embodiment of the present invention in which additional features are added to structurally support the vent diaphragm layers during deployment. 
           [0059]      FIG. 13 , an isometric view, illustrates still another embodiment of the present invention in which the diaphragm and vent layers are positioned serially along channel features to facilitate venting of entrapped air and contain excess metallic interface material. 
           [0060]      FIGS. 14 through 16 , isometric views, illustrate various metallic interface material shapes in the present invention. 
           [0061]      FIG. 17 , a sectional view, illustrates one metallic interface material structure embodiment of the present invention between an IC chip and heat sink. 
           [0062]      FIG. 18 , a sectional view, illustrates another metallic interface material structure embodiment of the present invention positioned between an IC chip and heat spreader lid and between the lid and heat sink. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0063]      FIGS. 4   a  through  4   d  illustrate a sequence of the present invention in which the thermal interface structure  140  flows and fills the space between the electronic component and heat exchanger to yield a highly conductive and hermetic thermal interface joint. An electronic assembly  100  includes a heat exchanger  104  (depicted as a heat spreader lid), a thermal interface structure  140  positioned between the lid  104  and an electronic component  106 . The component  106  is comprised of an IC chip  108 , package substrate  110  and electrical interconnection vias  112  (on the chip  108  and substrate  110 ). 
         [0064]    Within  FIG. 4   a , a thermal interface structure  140  includes a metallic seal member  142  (comprised of an inner and outer perimeter) which is positioned near the perimeter of the IC chip  108  and is comprised of a metallic interface composition. It can be seen that the metallic seal member  142  does not extend beyond the periphery of the IC chip  108 . A coating layer  144  encapsulates the metallic seal member on all faces with the exception of the inner perimeter of the member  142 . The coating layer  144  may be of metallic or polymeric composition. 
         [0065]    The interface structure  140 , when disposed between the lid  104  and IC chip  108 , creates an interface space  146  between the electronic component and heat spreader and a seal to each of their respective surfaces. With the application of heat (from the electronic component  106  or external source), the metallic seal member  142  will flow (flow arrows  148 ) into the space  146  and fill all the surface asperities of both heat spreader lid and IC chip. 
         [0066]      FIG. 4   b  illustrates the melting and flowing of the liquefied metallic interface composition comprising the metallic seal member  142 . Pressure applied external to the lid  104  or the weight of the heat exchanger  104  also promotes the flowing of the liquefied metallic seal member  142  and filling of the interface space  146 . As the seal member  142  continues to melt, the space  146  between the lid  104  and IC chip  108  is reduced in volume. 
         [0067]    As seen in  FIG. 4   c , the interface space  146  between the lid  104  and IC chip  108  has been fully filled with the metallic interface composition, comprising the metallic seal member  142 . The coating layer  144  assists in containing the flowing interface composition within the perimeter of the metallic seal member  142 . Due to the collapse of the metallic seal member  142  during melting, the adhesive layer  114 , applied to the heat spreader lid  104  at the outer lid flange  116  and package substrate  110 , is now bonded to the electronic component package substrate  110 . Seal materials include silicones, polysulphides, polyurethanes, polyimides, polyesters, epoxides, cyanate esters, olefins and sealing glasses. A continuous seal may be applied between the heat spreader lid flange  116  and package substrate  110 , thereby reducing the amount of moisture ingression within the lid cavity. 
         [0068]      FIG. 4   d , a magnified view of  FIGS. 4   a  through  4   c , illustrates the filling of surface asperities  152  present on the IC chip  108  and heat spreader lid  104 . To enhance heat transfer, an asperity filling material, such as a grease or phase change polymeric compound, may be disposed on the coating layer where it contacts either the IC chip or heat exchanger. 
         [0069]    The metallic interface composition (comprising the metallic seal member  142 ) may be comprised of the metallic elements of bismuth, gallium, indium and tin and their alloys. 
         [0070]    It is desirable for the composition to be liquid over the operating temperature of the electronic component (&lt;100° C.). This allows the metal to adequately flow into all surface asperities of the heat spreader lid  104  and IC chip  108  and accommodate thermomechanical stresses from temperature cycling. 
         [0071]    In another embodiment of the present invention, “reactive” elements or intrinsic oxygen gettering elements are added to the metallic interface composition to further facilitate wetting to the lid  104  and IC chip  108 . The resulting composition has a higher affinity for surface oxides and promotes oxide to oxide bonding, thereby reducing the thermal impedance at the lid  104  and chip  108  contact interfaces. Oxygen getter elements include alkali metals (Li, Na, and K), alkaline-earth metals (Mg and Ca), zinc, refractory metals (Ti, Zr, Hf, Ta, V, and Nb), rare earth metals (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Yb), and mixtures and alloys thereof. 
         [0072]      FIGS. 5   a  and  5   b  through  7   a  and  7   b  illustrate a sequence of interface deployment and formation of a metallic interface structure  202  within an electronic assembly  200  of the present invention. Unlike the deployment illustrated in  FIGS. 4   a  through  4   d , the thermal interface structure  202  is deployed between the heat spreader lid  104  and heat exchanger or heat sink  168 . Therefore, the electronic component  198  is comprised of an IC chip  108 , package substrate  110 , electrical interconnection vias  112  (on the chip  108  and substrate  110 ), heat spreader lid  104  and a thermal interface material  155  between the IC chip  108  and lid  104 . 
         [0073]    Within  FIGS. 5   a  and  5   b  (a sectional structure  220  of  FIG. 5   a  on lines  5   b - 5   b ), it can be seen that a thermal interface structure  202  (of the present invention) is sandwiched between an electronic component  198  and heat exchanger  168 , thereby forming an electronic assembly. A metallic interface composition  204 , positioned near the periphery of the electronic component  198 , includes at least one inboard face  205  (facing toward the center of the electronic component  198 ) and at least one outboard face  207  (facing toward the perimeter of the electronic component  198 ). 
         [0074]    An encapsulating structure (comprised of cover layers  206 , a duct layer  208  and coating layers  210 ) and metallic interface composition  204 , create a seal (and resultant space  212 ) between the electronic component  198  and heat exchanger  168 . The seal includes an inner perimeter (as illustrated by the space  212  formed between the electronic component  198  and heat exchanger  168 ) and outer perimeter, illustrated by the region extending laterally from the surfaces which the coating layers  210  contact the electronic component  198  and heat exchanger  168 . The coating layers  210  may be adhesive to further facilitate sealing. 
         [0075]    It can be seen that the metallic interface composition  204  is encapsulated (by cover layers  206 , duct layer  208  and coating layers  210 ) with the exception of the inboard face  205  of the metallic interface composition  204 . The sandwiching of the duct layer  208  (between the cover layers  206 ) and encapsulation will facilitate the flow of liquefied metallic interface material into the space  212 . Vents comprised of ducts  214 , vent apertures  216  and an optional vent screen structure  218  connect the space  212  (defined by the seal inner perimeter) to the region beyond the outer perimeter of the seal. An air permeable, liquid impermeable vent screen structure  218  may be added to seal the vent aperture  216 . 
         [0076]      FIGS. 5   a  and  5   b  also illustrate a metallic interface composition  204  which is applied around the perimeter of the electronic component in a non-uniform manner. Additionally, the asymmetry illustrates a disproportionately large quantity of interface composition  204  positioned (on the electronic component) opposite the vents (comprised of ducts  214  and vent apertures  216 ). In the present embodiment of the invention, this asymmetry of the interface composition  204  (and resultant interface structure  202 ) creates a space between the heat exchanger  168  and electronic component  198  (prior to liquefaction of the metallic interface composition  204 ) which is in the form of a trapezoid having its narrow most dimension at the vent side of the structure  202 . Combined with the lack of encapsulation on at least one inboard face  205  of the interface composition  204 , the asymmetric (disproportionately large quantity of) metallic interface composition  204  facilitates a directional flow (when liquefied) toward the vents (comprised of apertures  216 ), resulting in the mitigation of entrapped air during deployment. 
         [0077]    Now, within  FIGS. 6   a  and  6   b  (a sectional structure  220  of  FIG. 6   a  on lines  6   b - 6   b ), a compressive force  222  and thermal energy are applied to the metallic interface structure  202 , resulting in the liquefaction and subsequent dimensional change of the metallic interface composition  204 . With continued liquefaction (and collapse of the structure  202 ), the composition  204  will flow directionally  224  toward the ducts  214  and vent apertures  216 , thereby forcing air  226 , originally entrapped within the space  212 , to be expelled through the vent screen  218  via the vent apertures  216 . The flowing composition  204  flows into the space  212  and fills the surface asperities of both electronic component  198  and heat exchanger  168 . As the composition continues to liquefy, the space  212  between component  198  and exchanger  169  is reduced in volume. 
         [0078]    As seen in  FIGS. 7   a  and  7   b  (a sectional structure  220  of  FIG. 7   a  on lines  7   b - 7   b ), the interfacial space  212  (inner perimeter region of the seal) has been fully filled with the metallic interface composition  204 . The ducts  214  and vent apertures  216  may also be filled with the composition  204 ; however, the vent screen structure  218  prohibits the passage of liquefied composition  204  to the outer perimeter region of the seal. With the complete liquefaction and flow of the interface composition  204 , it can also be seen that the heat exchanger  168  is now relatively planar with the electronic component  198 . 
         [0079]      FIGS. 8   a  through  8   c , magnified sectional views of the interface structure  202  (of  FIGS. 5 through 7 ) between an electronic component  198  and heat exchanger  168 , illustrate the dimensional changes (on the structure  202 ) which result from liquefaction of the interface composition and compressive loading. 
         [0080]      FIG. 8   a  illustrates cover layers  206  formed over the metallic interface composition  204  (in solid form) with a duct layer  208  and coating layers  210  comprising the seal. 
         [0081]    Within  FIG. 8   b , it can be seen that with the liquefaction of the metallic interface composition  204  (and compressive force  222 ), a dimensional change results in a flow  224  of the composition  204  and reduction of the interfacial space thickness (between the component  198  and exchanger  168 ). As the interface composition  204  liquefies, flows and thins, the formed cover layers  206  (a portion of the encapsulating structure) will change shape by extending laterally  228 . 
         [0082]    Now,  FIG. 8   c , illustrates the completion of interface composition  204  flow and filling of the interfacial space. By fully extended laterally  228 , the cover layers  206  (a portion of the encapsulating structure) enable the interfacial structure  202  to fully collapse without wrinkles or creases, thereby reducing thermal impedance through the interface composition thickness. 
         [0083]      FIG. 9  (an isometric view) illustrates the assembly of individual layers to yield a complete thermal interface structure  202  embodiment of the present invention as illustrated in  FIGS. 5 through 8 . In this embodiment, the coating layers  210  may be adhesive which facilitates the bonding of the vent screen layer  218  and attachment of the structure  202  to the component  198  and/or heat exchanger  168 . The duct layer  208  may be a solid transfer adhesive to facilitate bonding to the cover layers  206 . It can also be seen that the metallic interface composition  204  has at least one tapered dimension. 
         [0084]    As seen in  FIG. 10  (an isometric view) and  FIGS. 11   a  and  11   b  (partial sectional views), the embodiment illustrated in  FIG. 9  includes a diaphragm layer  230  (which may be adhesively attached to the coating layer) which acts as an adjustable reservoir to accommodate excess metallic interface composition  204  during liquefaction and deployment. It is desirable to minimize the interface composition thickness (and thereby reduce the thermal impedance), accommodate differences of surface planarity and roughness between various components and heat exchangers, and eliminate migration of the interface composition  204  while liquid. The elastic diaphragm layer  230  adjusts to changing interface composition volumes while providing necessary back pressure to keep the interface composition  204  within the interfacial space (between the electronic component  198  and heat exchanger  168 ). 
         [0085]    As the liquefied interface composition  204  continues to flow through the ducts  214  and into the vent apertures  216 , the diaphragm layer  230  will stretch immediately adjacent each filled aperture  216 , thereby creating an adjustable reservoir for excess interface composition  204 . 
         [0086]      FIG. 12  (an isometric view) illustrates structural supporting layers (added to the embodiment seen in  FIG. 10 ) to mitigate any delaminating forces on the vent screen layer  218  and diaphragm layer  230  as the interface composition  204  is liquefied and forced into the vent apertures  216 . The additional support may include cap layers  232  adhesively bonded (via an adhesive layer  234 ) to both the vent screen layer  218  and diaphragm layer  230 . The cap layers  232  act as stiffeners to mitigate any peel or delaminating forces between the vent screen layer  218 , diaphragm layer  230  and their respective coating layers  210 . 
         [0087]      FIG. 13  (an isometric view) illustrates yet another embodiment of the present invention wherein the diaphragm and vent layers (both immediately adjacent vent apertures  216 ) are positioned serially along the ducts  214 . As metallic interface composition  204  is liquefied and flowed, any entrapped air may be expelled via the liquid impermeable vent screen layer  218 . Any excess interface composition  204  may be accommodated within the flexible volume created by the diaphragm material expanding immediately adjacent the vent apertures  216 . 
         [0088]      FIGS. 14 through 16  (isometric views) illustrate various metallic interface composition  204  shapes which facilitate the directional flow (upon liquefaction of the composition) toward the ducts and vent apertures (seen in  FIGS. 5 through 13 ). As seen in  FIG. 14 , a minimum interface composition shape  204  may be comprised of at least one leading edge segment  217 . 
         [0089]      FIG. 15  illustrates one leading edge segment  217  combined with leg segments  215  to yield a complete metallic interface composition shape  204 . It can also be seen that the leg segments&#39;  215  thickness is reduced from the leading edge segment connection region  221  down to the leg tips  223 . For ease of manufacture, the leg faces  219  may be uniform in width from the segment connection region  221  to the leg tips  223 . 
         [0090]    Within  FIG. 16  it can be seen that a metallic interface composition shape  204  includes tapered leg faces  219  in addition to a taper of each leg segments&#39;  215  thickness (both (from the segment connection region  221  down to the leg tips  223 ). The taper of the leg segments&#39;  215  thickness and width may be accomplished by soldering metallic interface composition (in liquid form) onto a tapered width pattern on a substrate (such as one of the cover layers seen in  FIGS. 5-13 ). The liquid interface composition will deposit a volume of the composition in proportion to the exposed surface on the substrate, yielding a taper in thickness as the substrate&#39;s pattern width tapers. 
         [0091]      FIG. 17  illustrates a “bare die” electronic assembly  240  including an electronic component  106  (comprised of an IC chip  108 , package substrate  110 , and electrical interconnection vias  112 ), heat exchanger (heat sink)  168 , and thermal interface structure  202  (of the present invention). The thermal interface structure  202  facilitates a high performance thermal path between the IC chip  108  and heat sink  168 . 
         [0092]    Within  FIG. 18 , it can be seen that a “lidded” electronic assembly  250  includes a heat spreader lid  104  (with the thermal interface structure  202  of the present invention between the lid  104  and IC chip  108 ) and a heat sink  168 , also with a thermal interface structure  202  of the present invention. The resultant electronic assembly, providing a metallic-based heat path from the IC chip  108  to the heat sink  168 , would possess high thermal performance with a high degree of reliability and ease of deployment. 
         [0093]    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.

Technology Category: 5