Nano-structure enhancements for anisotropic conductive material and thermal interposers

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-Part of U.S. Ser. No. 10/402,293, filed Mar. 31, 2003, the entirety of which is expressly incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. 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 materials and anisotropic conductive adhesives.

2. 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) (or merely “conductive adhesives”) provide uniform conductivity along all axes, and thus do not isolate adjacent conductors which are each embedded in the matrix. The distance across a gap between conductors intended to be electrically connected, and the distance between adjacent conductors intended to be electrically isolated may be of similar scale. Thus, insulating barriers are required for isolation, making bulk dispensing problematic.

These isotropic 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, metalized 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 may be grown on flat surfaces of the conductor pads, the active device (chip), printed circuit substrates and connectors and are not limited to the surface of particles in dispersion within an adhesive matrix. In other words, any body on whose 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 isotropic 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 isotropic 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 provides a matrix, such as an electrically insulating adhesive or stable non-adhesive material, which may be a polymer, grease, gel, or the like, filled with conductive interconnect particles, the particles (and optionally adjacent surfaces) having nano-structures, adapted to: reduce thermal interface resistance; improve the conductivity between with microcircuit contact pads joined by the matrix; 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 compositions and anisotropic conductive composition interconnection technology by growing or attaching nano-structures to the interconnect particles and optionally the microcircuit connection pads, adapted to address one or more of the problems listed above. When pressure is applied during fabrication to spread and compress conductive matrix of interconnect particles and circuit conductors, the nano-structures mesh and compress along the compression axis 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.

The matrix may be curable or stable, and is preferably electrically insulating. The matrix preferably has a high thermal conductivity, which may be an isotropic phenomena, but is preferably not loaded with non-conductive particles in addition to the conductive ones, and more preferably is loaded only with particles having nanostructures formed thereon.

The loading ratio of particles in an anisotropic conductive matrix is preferably such that, after compressing and curing (for a curable matrix), the particles form a continuous electrically conductive path between two electrical connection pads spaced by the matrix, while electrical connection pads offset with respect to the compression axis are electrically isolated relative to the electrical connection pads directly in line with the compression axis, and the isolation is greater than reflected solely by the distance between the respective electrical connection pads that would be implied by the distance and geometry alone.

The particles may be shaped as platelets, ovoids or disks, which naturally tend to align with respect to a compression axis, with relatively large expanses of parallel surface between adjacent particles.

It is therefore an object to provide an anisotropic conductive composition, comprising: an electrically insulating carrier composition, which prior to any curing preferably has a high viscosity; a plurality of electrically and thermally conductive interconnect particles suspended in the electrically insulating carrier composition, each having an outer surface; and a plurality of electrically conducting elongated nanostructures affixed to and selectively extending away from the outer surface along the elongated axis, wherein the plurality of conductive interconnect particles are provided within a range of concentration sufficient to provide a substantially anisotropic electrical conduction pattern aligned with the compression axis and a high probability of interacting with each other, under compression of a film of the anisotropic conductive composition, and wherein the conducting elongated nanostructures on a respective conductive interconnect particle are adapted to engage and interlock with the conducting elongated nanostructures on another respective conductive interconnect particle when mutually compressed, to form an efficient thermal and electrical conduction path.

It is another object to provide a method of forming an anisotropic conductive path, comprising: providing an anisotropic conductive composition, comprising a plurality of electrically and thermally conductive interconnect particles suspended in an electrically insulating carrier composition, each having an outer surface, with a plurality of electrically conducting elongated nanostructures affixed to and selectively extending away from the outer surface along the elongated axis, wherein the plurality of conductive interconnect particles are provided within a range of concentration sufficient to provide a substantially anisotropic electrical conduction pattern aligned with the compression axis and a high probability of interacting with each other, under compression of a film of the anisotropic conductive composition; and dispensing the anisotropic conductive composition between at least adjacent two pairs of opposed contact surfaces; and compressing the anisotropic conductive composition to selectively form electrical conduction paths across the opposed contact surfaces maintaining electrical isolation between non-opposed contact surfaces, wherein the conducting elongated nanostructures on a respective conductive interconnect particle are adapted to engage and interlock with the conducting elongated nanostructures on another respective conductive interconnect particle when mutually compressed, to form an efficient thermal and electrical conduction path.

It is a still further object of provide a module, comprising: at least adjacent two pairs of opposed contact surfaces, having an intracontact gap between the contact surfaces of a respective pair and an intercontact gap between respective pairs; a continuous portion of an anisotropic conductive composition provided in the intracontact gap and the intercontact gap, comprising a plurality of electrically and thermally conductive interconnect particles suspended in an electrically insulating carrier composition, each having an outer surface, with a plurality of electrically conducting elongated nanostructures affixed to and selectively extending away from the outer surface along the elongated axis, wherein the plurality of conductive interconnect particles are provided within a range of concentration sufficient to provide a substantially anisotropic electrical conduction pattern aligned with the compression axis and a high probability of interacting with each other, under compression of a film of the anisotropic conductive composition; a selectively formed electrically conductive path across the intracontact gaps comprising commonly occurring engaged and interlocked conducting elongated nanostructures, and a selectively formed electrically isolating space in the intercontact gap in which engaged and rarely occurring interlocked conducting elongated nanostructures bridging the intercontact gap.

The insulating carrier composition may comprise an adhesive polymer resin or a thermal grease.

The conductive interconnect particles may have a metallic shell comprising the conducting elongated nanostructures. The conductive interconnect particles may have a polymeric or carbonaceous core.

The conductive interconnect particles may be, for example, spherical, ovoid, disk-like or tubular, for example.

The nanostructures may be are selected from the group consisting of one or more of columns, spikes, cylinders, tubes and fibers.

At least two pairs of opposed contact surfaces may be immersed in said anisotropic composition, wherein after compression, an electrical current selectively flows through the anisotropic conductive composition between opposed contact surfaces and does not selectively flow between non-opposed contact surfaces. At least one contact surface may be coated with a plurality of electrically conducting elongated nanostructures affixed to and selectively extending away from the contact surface along the elongated axis, wherein the conducting elongated nanostructures on the contact surface are adapted to engage and interlock with the conducting elongated nanostructures on an adjacent conductive interconnect particle when mutually compressed, to form an efficient thermal and electrical conduction path.

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. 1a, 1b, and 1c, 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. 4a, 4b, 4c and 4d, 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. 6a, 6b, and 6c, 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. 6d and 6e, 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 on the surfaces of conductive particles in an insulating interface matrix, and optionally on the surfaces being interconnected, that facilitate thermal dissipation and electrical conductivity in microcircuits, and reduce circuit board deflection when fabricated using anisotropic conductive adhesives.

An anisotropic conductive interface system for fabricating microcircuits consists primarily of an insulating carrier matrix in which conductive interconnect particles are suspended. The particles in the matrix, which is, for example, an organic or inorganic oil, forming a grease, or an electrically insulating curable polymer resin adhesive composition, when squeezed under pressure during microcircuit fabrication, interconnect the components and layers of the microcircuit chip or circuit board.

Referring to FIGS. 1a, 1b, and 1c, there are shown schematic drawings of typical microcircuit fabrication of the prior art. Assemblies of circuit fabrication are schematically represented in FIG. 1a 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 composition 16 (FIG. 1b) is applied between the upper and lower boards 10, 12. The known anisotropic conductive composition 16 consists of a curable polymer resin carrier 18 filled with sufficient quantity of conductive interconnect particles 20, which are typically spherical.

Pressure (arrows 24) is applied to the upper and lower boards 10, 12 (FIG. 1c) forcing the anisotropic conductive composition 16 throughout the spaces on and between the boards 10, 12, and compressing the conductive interconnect particles 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 composition 16 with the carrier 18 and conductive interconnect particles 20 has been pressed so that conductor 25, an example of the upper assembly 10 (FIG. 1a), is interconnected to an indium-tin-oxide (ITO) metalized layer 26 on glass substrate 28, an example of the lower board 12 (FIG. 1a).

Referring now to FIG. 3, which is an enlarged view of FIG. 2 showing the contact between a single conductive interconnect particle 20, the device surface 10 and the board 28. Voids or pockets 22′ of anisotropic conductive composition 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. 4a, 4b, 4c and 4d, there are shown two of many shapes onto which nano-structures 50 are attached or grown. FIG. 4a shows an interconnect particle 20 with nanostructures 50, enlarged in FIG. 4a. FIG. 4d shows a thermally conductive tube 52 with nano-structures 50, enlarged in FIG. 4c.

It should be understood that interconnects and thermal conductors may be made in shapes other than spheres shown in FIG. 4b and tubes shown in FIG. 4d. 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. The nano-structures are preferably formed in such way that under mild compression, the nano-structures engage with each other and may interlock. For example, an elongated nano-structure may extend from a particle or surface, and on compression, engage the nano-structures of an adjacent particle or surface, and perhaps pierce its shell. The nano-structures preferably have an elongated axis which extends normal to the surface from which it extends, to fill less than 50% of the space in which they protrude, especially at the outermost reach of the nano-structures, to facilitate interaction of the nano-structure of adjacent particles or surfaces. Alternatively stated, the nano-structures preferably do not present a dense mat of structures. The nano-structures may have a higher hardness than the core of the conductive interconnect particles. The core, on the other hand, may be deformable, e.g., plastically deformable, when subject to pressure and/or piercing from 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 in diameter.

The nano-structures 50 attached to or grown from the surfaces of conductive interconnect particles 20 and thermal conductor tubes 52, for example, are 1 to 200 nanometers in length. 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-structure include columns, spikes, cylinders, tubes, hemispheres, fibers, regular, and irregular or inhomogeneous 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 conductive interconnect particles 20 or thermal conductor tubes 52, and inventive nano-structures 50 attached to or grown from boards 10, 12. Anisotropic conductive composition 16, which in this case can be an adhesive, curable matrix, liquid or gel, is disposed throughout the spaces between the boards 10, 12. The invention improves the prior art by adding nano-structures 50 to the conductive interconnect particles 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, conductive interconnect particles 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. 1c) applied to the circuit components and circuit boards 10, 12, and connections 14 (FIG. 1a) than is required by conventional techniques. Lower pressure results in reduced distortion and likelihood of damage of the circuit boards 10, 12 and connections 14.

Referring now to FIGS. 6a, 6b, and 6c, 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. 6b by thermal plane interface 60 and circuit chip 62 and heat sink 64.

In FIG. 6c, 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. 6d and 6e, 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, 6a, 6b, and 6c, 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.

Therefore, in this embodiment, the loading of particles is intentionally maintained below a level which would lead isotropic conductivity, but sufficient to ensure conduction under compression. It is noted that the matrix is present in a liquid or gel form during compression, and the nano-structures preferably engage each other to reduce slippage and transport of particles after contact while pressure is applied, while the matrix material is more free to flow between chains of particles. Therefore, under compression of the uncured or liquid matrix, there is preferably a relative selective concentration effect on of the conductive interconnect particles between the narrower gaps where conductive pads are present, and a selective dilution effect of the conductive interconnect particles in the intercontact spaces.

In the embodiments of FIGS. 1-6c, the anisotropic conductive composition is sought to be squeezed at high pressure to a layer thickness corresponding to a particle. The thermal conductivity is perhaps enhanced by the thin target interface layer, but the loading density of conductive particles (which provide substantial thermal conductivity) is limited in these applications by the need for suitable viscosity to permit forming a thin uniform layer without voids, and the need to avoid clumping of particles and loss of anisotropicity.

In FIG. 6d, nano-structures 50 are attached to or grown from heat sink 64, and in FIG. 6e, 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.