Abstract:
The present invention features additions of nano-structures to interconnect conductor fine particles (spheres) to: (1) reduce thermal interface resistance by using thermal interposers that have high thermal conductivity nano-structures at their surfaces; (2) improve the anisotropic conductive adhesive interconnection conductivity with microcircuit contact pads; and (3) allow lower compression forces to be applied during the microcircuit fabrication processes which then results in reduced deflection or circuit damage. When pressure is applied during fabrication to spread and compress anisotropic conductive adhesive and the matrix of interconnect particles and circuit conductors, the nano-structures mesh and compress into a more uniform connection than current technology provides, thereby eliminating voids, moisture and other contaminants, increasing the contact surfaces for better electrical and thermal conduction.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a Division of U.S. patent application Ser. No. 10/402,293, filed Mar. 31, 2003, now U.S. Pat. No. 7,645,512. 
    
    
     FIELD OF THE INVENTION 
     The invention pertains to electrical circuit interconnections and, more particularly, to the addition of nano-structures that facilitate thermal dissipation and electrical conductivity in microcircuits that are fabricated using conductive adhesives and anisotropic conductive adhesives. 
     BACKGROUND OF THE INVENTION 
     Discussion of the Related Art 
     Clearly, the continuing development of microcircuits includes, among others, the objectives of: application of flexible printed circuits, increased capacity (more switching functions in smaller devices), and a host of robustness issues, such as moisture control, improved shock-resistance, and use in higher temperature applications. These issues become more crucial when printed circuits are used in environments in which they are shocked or vibrated, as in machinery or fighter planes, or when high temperature, moisture, or contamination is experienced, as in industrial corrosive and high-humidity environments and in the engine compartment vehicle. To achieve these objectives, improvements in the connections between microcircuit components and the circuit chip or printed circuit board must be made. 
     Conductive adhesives and anisotropic conductive adhesives (ACA) have been used regularly in microcircuit fabrication, and their composition has been well described. Isotropic conductive adhesives (ICA) are hereinafter referred to as conductive adhesives. 
     These adhesive compositions consist primarily of an insulating adhesive resin carrier in which a matrix of interconnect fine particles is suspended. For the purpose of this description, such fine particles and the device and circuit board connections, including metal, metallized polymer, carbon or carbonaceous, micron or sub-micron sized shapes, including spheres, rods, tubes, conductors for heat transfer or electrical connection, printed circuit substrates and lands, and/or other regular and irregularly shaped particles and connectors, upon which nano-structures are attached or grown, are referred to as spheres. The spheres in the adhesive compositions, when squeezed under pressure during microcircuit fabrication, interconnect the components and layers of the microcircuit chip or circuit board. It is to be particularly emphasized that the nano-structures are grown on the flat surfaces of the conductor pads, printed circuit substrates and connectors and are not limited to the surface of particles in dispersion in an adhesive matrix. In other words, any body on which surface these nano-structures are grown is referred to as a sphere regardless of its shape. 
     For the purpose of description, the nano-structures are drawn as columns in the figures, but they may be spikes, cylinders, tubes, hemispheres, fibers, or any other regular or irregular shape; they are referred to as nano-structures. 
     The adhesive compositions have several purposes including, but not limited to: providing the carrier medium for the matrix of interconnect spheres to be distributed between the microcircuit devices and conductor pads; providing the thermal path for heat that is generated by the switching functions; the cured adhesive supports and electrically insulates between interconnection particles and conductors on the microcircuits, and it prevents moisture or other contaminants from getting into or being entrapped within the interconnections. 
     Several problems arise from the use of conductive adhesives and anisotropic conductive adhesive that affect the capacity of the microcircuit, specifically, thermal dissipation and electrical interconnection. Those effects, in turn, can limit the number of circuit switches on, or logic operations performed by, a microcircuit. One of these problems is a need to apply high pressure to the microcircuit during fabrication that can damage or misalign parts of the circuit. Also, entrapped air in voids has lower thermal conductivity and can limit heat dissipation from the microcircuit. Third, increased resistance in the interconnect can result from insufficient interconnect particle to contact surface connection. 
     It would be advantageous to provide conductive adhesives and anisotropic conductive adhesive interconnects in which thermal and electrical interconnection resistance and distortion or damage of circuit boards are reduced or eliminated. With existing interconnects, regardless of specific metal or metallized polymer or carbonaceous material used for interconnects, or whether their surfaces are smooth or irregular, the thermal and electrical conductivity and board distortion or damage problems described above exist to varying degrees. 
     SUMMARY OF THE INVENTION 
     The present invention adds nano-structures to interconnect conductor spheres to: reduce thermal interface resistance by using thermal interposers that have high thermal conductivity nano-structures at their surfaces; improve the conductive adhesives and anisotropic conductive adhesive interconnection conductivity with microcircuit contact pads; and, allow lower compression forces to be applied during the microcircuit fabrication processes which then results in reduced deflection or circuit damage. 
     Accordingly, the present invention provides an innovative improvement in conductive adhesives and anisotropic conductive adhesive interconnection technology by growing or attaching nano-structures to the interconnect particles and the microcircuit connection pads which address the problems listed above. When pressure is applied during fabrication to spread and compress conductive adhesives and anisotropic conductive adhesive and the matrix of interconnect particles and circuit conductors, the nano-structures mesh and compress into a more uniform connection than current technology provides, thereby eliminating voids, moisture, and other contaminants, increasing the contact surfaces for better electrical and thermal conduction. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       A complete understanding of the present invention may be obtained by reference to the accompanying drawings, when considered in conjunction with the subsequent detailed description, in which: 
         FIGS. 1   a ,  1   b , and  1   c , taken together, show a schematic diagram depicting, in general, the prior art process of anisotropic conductive adhesive with a matrix of interconnect spheres electrically and thermally connecting contacts and conductors of a microcircuit or circuit board; 
         FIG. 2  is a schematic diagram of one example of prior art liquid crystal display (LCD) technology showing the compressed metallized polymer conductive sphere within the anisotropic conductive adhesive interconnecting the circuit conductor to the ITO metallization layer on the LCD glass; 
         FIG. 3  is a schematic diagram depicting a prior art thermal interposer wherein a single smooth-walled particle contacts similarly smooth surfaces. 
         FIGS. 4   a ,  4   b ,  4   c  and  4   d , taken together, show a schematic diagram depicting, in general, the nano-structures of the present invention attached to or grown from an interconnect sphere and a thermally conductive tube; 
         FIG. 5  is a schematic diagram depicting an assembly of a chip to a heat sink using a thermal plane as an interface with nano-structures attached; and 
         FIGS. 6   a ,  6   b , and  6   c , taken together, show a schematic diagram depicting the nano-structures meshing and compressing into a more uniform connection than current technology provides, thereby eliminating voids, moisture, and other contaminants, increasing the contact surfaces for better electrical and thermal conduction. 
         FIGS. 6   d  and  6   e , taken together, show a schematic diagram depicting the nano-structures grown on filler material particles and heat sink to improve their contact within the anisotropic adhesive system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Generally speaking, the invention pertains to electrical circuit interconnections. More specifically, the invention features the addition of nano-structures that facilitate thermal dissipation and electrical conductivity in microcircuits, and reduce circuit board deflection when fabricated using anisotropic conductive adhesives. 
     An anisotropic conductive adhesive system for fabricating microcircuits consists primarily of an insulating adhesive resin carrier in which a matrix of interconnect spheres is suspended. The spheres in the adhesive compositions, when squeezed under pressure during microcircuit fabrication, interconnect the components and layers of the microcircuit chip or circuit board. 
     Referring to  FIGS. 1   a ,  1   b , and  1   c , there are shown schematic drawings of typical microcircuit fabrication of the prior art. Assemblies of circuit fabrication are schematically represented in  FIG. 1   a  by upper and lower circuit boards  10  and  12 , respectively, which may be conductors, components, substrates, circuit boards, chips, or devices. Boards  10  and  12  have device connectors or printed circuits or thermal conductors  14  as shown on the lower board  12 . 
     An anisotropic conductive adhesive  16  ( FIG. 1   b ) is applied between the upper and lower boards  10 ,  12 . The anisotropic conductive adhesive  16  consists of a polymer carrier  18  with a matrix of interconnect spheres  20 . 
     Pressure (arrows  24 ) is applied to the upper and lower boards  10 ,  12  ( FIG. 1   c ) forcing the anisotropic conductive adhesive  16  throughout the spaces on and between the boards  10 ,  12 , and compressing the interconnect spheres  20  to make the interconnections between the boards  10 ,  12  and device connectors or printed circuits or thermal conductors  14 . 
     Referring now to  FIG. 2 , there is shown a schematic diagram of a liquid crystal device (LCD), which is one specific type of microelectronic circuit using anisotropic conductive adhesive fabrication. The anisotropic conductive adhesive  16  with the polymer carrier  18  and spheres  20  has been pressed so that conductor  25 , an example of the upper assembly  10  ( FIG. 1   a ), is interconnected to ITO metallized layer  26  on glass substrate  28 , an example of the lower board  12  ( FIG. 1   a ). 
     Referring now to  FIG. 3 , which is an enlarged view of  FIG. 2  showing the contact between a single particle  20  ( FIG. 20 ), the device surface  10  and the board  28 . Voids or pockets  22 ′ of adhesive carrier  16  or contaminants in those pockets  22 ′, such as moisture, keep the heat transfer and electrical conductivity low between the device surface  10 , device connectors or printed circuits or thermal conductors, not shown, and the interconnect spheres  20 . 
     Referring now to  FIGS. 4   a ,  4   b ,  4   c  and  4   d , there are shown two of many shapes onto which nano-structures  50  are attached or grown.  FIG. 4   b  shows an interconnect sphere  20  with nano-structures  50 , enlarged in  FIG. 4   a .  FIG. 4   d  shows a thermally conductive tube  52  with nano-structures  50 , enlarged in  FIG. 4   c.    
     It should be understood that interconnects and thermal conductors may be made in shapes other than spheres and tubes. Also, it should be understood that shapes other than the flat surfaces shown in diagrams for conductors and circuit boards can be used. And, further, shapes other than the columns shown may be used for the nano-structures. 
     Typically, the size range of fine particle interconnects and thermal conductors, represented here by a sphere  20  and a tube  52 , are 1 to 20 microns (1.times.10.sup.−6 meter) in diameter. 
     The nano-structures  50  attached to or grown from the surfaces of spheres  20  and thermal conductor tubes  52  are 1 to 200 nano-meters (1.times.10.sup.−9 meter) in size. The materials the nano-structures can be made from include: carbon, metal, polymers, metallized polymers, electrical and thermal conducting materials and the like; the shapes of these nano-structures include columns, spikes, cylinders, tubes, hemispheres, fibers, regular, and irregular shapes. 
     Referring now to  FIG. 5 , there is shown a schematic diagram of a fabricated circuit with the inventive nano-structures  50  attached to or grown from interconnect spheres  20  or thermal conductor tubes  52 , and inventive nano-structures  50  attached to or grown from boards  10 ,  12 . Anisotropic conductive adhesive  16  is disposed throughout the spaces between the boards  10 ,  12 . The invention improves the prior art by adding nano-structures  50  to the interconnect spheres  20 , and the surfaces of boards  10 ,  12 , which mesh and compress into a more uniform connection  22 ″, thereby eliminating voids, moisture and other contaminants  22 ′″, increasing the contact surfaces  22 ″ for better electrical and thermal conduction. 
     As the number of nano-structures  50  attached to or grown on the surfaces of boards  10 ,  12 , spheres  20 , and thermal conductor tubes  52  are increased by making them uniform and consistently spaced, the thermal conduction and electrical connection are improved. 
     Additionally, because the interconnect contact surface  22 ″ is increased with the meshing of the nano-structures, improved contact can be achieved with lower pressure (arrows  24 ,  FIG. 1   c ) applied to the circuit components and circuit boards  10 ,  12 , and connections  14  ( FIG. 1   a ) than is required by conventional techniques. Lower pressure results in reduced distortion and less likelihood of damage of the circuit boards  10 ,  12  and connections  14 . 
     Referring now to  FIGS. 6   a ,  6   b , and  6   c , there are shown schematic diagrams depicting one specific type of fabricated circuit, assembly of a chip  62  to a heat sink  64 , with a thermal plane  60 , shown for the purpose of example. Nano-structures  50  are attached to or grown from a thermal plane interface  60  and circuit chip  62  and heat sink  64 . 
     Thermal planes  60  may be made of rigid or flexible metal, metallized soft substrate, or any other high thermal conductivity material. Thermal plane shapes may be corrugated waves as shown or any other regular or irregular shape that fits the needs of the circuit fabrication. 
     Assemblies of circuit fabrication are schematically represented in  FIG. 6   b  by thermal plane interface  60  and circuit chip  62  and heat sink  64 . 
     In  FIG. 6   c , pressure (arrows  24 ) is applied to the circuit chip  62  and heat sink  64  to make the interconnections between the thermal plane interface  60  and circuit chip  62  and heat sink  64 . 
     In operation, the nano-structures  50  may be attached to or grown from the surfaces of the spheres  20 , electrical and thermal conductors, device connectors  14 , and other surfaces of circuit assemblies by sputtering, dissolving in highly volatile solution and spray coat, sol-gel, fluidized bed, epitaxial growth, chemical vapor deposition (CVD), precipitations, or any other process that befits the needs of the circuit fabrication. 
     Referring now to  FIGS. 6   d  and  6   e , there are shown schematic diagrams depicting an assembly of a heat sink  64  and a circuit element  62  nano-structures  50  grown on filler material particles  80  or heat sink  64  to improve their contact within the anisotropic adhesive system. In contrast to the arrangement of  FIGS. 5 ,  6   a ,  6   b , and  6   c  in this arrangement, the filler particles are much smaller than the gap  82  between the package elements  62  and  64 , and therefore, they will cluster together in the wall layer  84  and core layer  86 , and a thermal path requires several particles  80  to bridge the gap  82 . 
     In  FIG. 6   d , nano-structures  50  are attached to or grown from heat sink  64 , and in  FIG. 6   e , nano-structures  50  are attached to or grown from filler material particles  82 . 
     Alternate embodiments of the present invention may be implemented with nano-structures appended to or grown from the surface of any contact surface, such as a flexible card with bowed circuit lands, where the meshing of the nano-structures maintains better contact between the interconnect spheres, thermal tubes, circuit conductors, and components. 
     Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure, and covers changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.