Flip chip mounting technique

Abstract

The invention provides processes for bonding a flip chip to a substrate in a manner that maximizes reliability of the bonding operation. Electrically conductive polymer bumps are formed on bond pads of a flip chip and the flip chip polymer bumps are at least partially hardened. Electrically conductive polymer bumps are formed on bond pads of a substrate, and a layer of electrically insulating adhesive paste is then applied on the substrate, covering the substrate polymer bumps with the adhesive. The bond pads of the flip chip are then aligned with the bond pads of the substrate and the at least partially hardened flip chip polymer bumps are then pushed through the substrate adhesive and at least partially into the substrate polymer bumps. In a further method, electrically conductive polymer bumps are formed on bond pads of a flip chip and the flip chip polymer bumps are at least partially hardened. A layer of electrically insulating adhesive paste is formed on a substrate having bond pads, covering the bond pads with the adhesive. The bond pads of the flip chip are aligned with the bond pads of the substrate, and then the at least partially hardened flip chip polymer bumps are pushed through the substrate adhesive with pressure sufficient for the flip chip polymer bumps to directly contact and deform the substrate bond pads.

Description

BACKGROUND OF THE INVENTION

This invention relates to methods for electrically connecting a flip chip to a substrate.

Flip chip mounting is an increasingly popular technique for directly electrically connecting an integrated circuit chip to a substrate such as a circuit board. In this configuration, the active face of the chip is mounted face down, or “flipped” on the substrate. The electrical bond pads on the flip chip are aligned with corresponding electrical bond pads on the substrate, with the chip and substrate bond pads electrically connected by way of an electrically conductive material. The flip chip mounting technique eliminates the use of bond wires between a chip or chip package and the substrate, resulting in increased reliability of the chip-to-substrate bond.

A wide range of electrically conducting compositions have been proposed for making the interconnection between flip chip and substrate bond pads.

Solder balls, gold bumps, gold stud bumps, and other conventional metal bump configurations have been used extensively. Aside from metallic compositions, electrically conducting polymer compositions are gaining wide acceptance as flip chip interconnection bump materials. In a flip chip mounting technique employing polymer interconnections, electrically conductive polymer bumps are formed on the bond pads, typically of the flip chip, and are polymerized or dried to effect bonding to the substrate bond pads, whereby both an electrical and a mechanical adhesive bond between the flip chip and the substrate bond pads is produced. Electrically conductive polymer materials are particularly well-suited for flip chip mounting techniques because of their ease of application, because they eliminate many of the unwanted characteristics of metallic interconnections, e.g., solder flux, and because for some polymer materials reworkability of faulty flip chips is enabled by simple heating of the material.

Conventionally, once a flip chip is bonded to a substrate, whether by metallic or by polymer bump interconnections between the chip and substrate bond pads, an underfill material is dispensed between the chip and the substrate. The underfill material is typically provided as a liquid adhesive resin that can be dried or polymerized. The underfill material provides enhanced mechanical adhesion and mechanical and thermal stability between the flip chip and the substrate, and inhibits environmental attack of chip and substrate surfaces.

SUMMARY OF THE INVENTION

The invention provides processes that exploit the superior bonding capabilities of electrically conductive polymer materials for bonding a flip chip to a substrate in a manner that maximizes reliability of the bonding operation. In a first mounting process provided by the invention, electrically conductive polymer bumps are formed on bond pads of a flip chip and the flip chip polymer bumps are at least partially hardened. Electrically conductive polymer bumps are formed on bond pads of a substrate, and a layer of electrically insulating adhesive paste is then applied on the substrate, covering the substrate polymer bumps with the adhesive.

The bond pads of the flip chip are then aligned with the bond pads of the substrate and the at least partially hardened flip chip polymer bumps are then pushed through the substrate adhesive and at least partially into the substrate polymer bumps. Preferably, the flip chip polymer bumps are pushed through the substrate adhesive and into the substrate polymer bumps to a depth sufficient to produce electrical connections between the flip chip polymer bumps and the substrate bond pads. For some applications, it can be preferred to push the flip chip polymer bumps through the adhesive and through the substrate polymer bumps to directly contact the flip chip polymer bumps with the substrate bond pads.

This process results in electrical and mechanical bonding of the polymer bumps between the chip and substrate bond pads, either directly or via the substrate polymer bumps, even though the adhesive layer was applied on the substrate in a manner that covered the substrate polymer bumps. The flip chip polymer bumps displace the adhesive and penetrate the substrate polymer bumps by the mounting technique of the invention. As a result, the area around the polymer bumps between the chip and the substrate is filled with the adhesive, in the manner of an underfill. A separate, post-bond underfill process is therefore not required.

In addition, the substrate polymer bumps can operate as an intermediary, compensating for non-coplanar flip chip bumps. If some flip chip bumps are shorter than others, it can be assured that most, if not all, flip chip bumps are electrically connected to substrate bond pads, if not directly, then by way of the substrate polymer bumps. The overall reliability of the bonding process is thereby significantly enhanced by the inclusion of substrate polymer bumps.

In embodiments provided by the invention, the adhesive paste applied to the substrate can be at least partially dried or at least partially cured, as appropriate for the selected paste material, before the step of pushing the polymer bumps through the adhesive on the substrate. Similarly, the step of at least partially hardening the polymer bumps can be carried out by at least partially drying or by fully polymerizing the polymer bumps, as appropriate for the selected bump material.

In further embodiments provided by the invention, heat is applied to the flip chip as the bumps are pushed through the adhesive on the substrate. Heat can also be applied to the flip chip after the polymer bumps contact the bond pads of the substrate. Pressure is preferably applied to the flip chip during the bonding process for a selected duration, based on material characteristics of the adhesive and of the polymer bumps.

The height of the substrate polymer bumps, as-formed, preferably is between about 30% and about 150% of the height of the flip chip polymer bumps as-formed. The diameter of the substrate polymer bumps, as-formed, preferably is between about 10% and about 70% greater than the diameter of the flip chip polymer bumps as-formed. Preferably, the flip chip polymer bumps have a bump height as-formed that is greater than the adhesive paste thickness as-applied on the substrate, more preferably having a bump height that is at least about 25% greater than the adhesive paste thickness; the as-applied adhesive thickness is preferably at least as great as the substrate polymer bump height as-formed.

The flip chip polymer bumps, the substrate polymer bumps, and the adhesive paste can each be distinctly formed of, e.g., a thermoplastic material, a thermoset material, or a B-stage thermoset material. The polymer bumps can include hard particles, preferably that have jagged edges or sharp points which protrude from the bump. Such particles are preferably electrically conductive.

Both the flip chip and substrate polymer bump formation and the adhesive application to the substrate can be carried out by, e.g., a stenciling process or a screen printing process. The adhesive can further be applied by, e.g., a dispensing process. Preferably, the adhesive paste is stenciled with a stencil that includes openings at stencil locations corresponding to substrate polymer bump locations.

The invention provides a further method for mounting a flip chip on a substrate. In this method, electrically conductive polymer bumps are formed on bond pads of a flip chip and the flip chip polymer bumps are at least partially hardened. A layer of electrically insulating adhesive paste is formed on a substrate having bond pads, covering the bond pads with the adhesive. The bond pads of the flip chip are aligned with the bond pads of the substrate, and then the at least partially hardened flip chip polymer bumps are pushed through the substrate adhesive with pressure sufficient for the flip chip polymer bumps to directly contact and to deform the bond pads of the substrate.

It is recognized that such bond pad deformation can significantly enhance the electrical connection between a polymer bump and a bond pad. This is understood to be enabled by a range of mechanisms including, among other things, enhancement of mechanical contact area between the polymer bump and bond pad. It is further recognized that the structural integrity and reliability of the bond between a polymer bump and a bond pad is significantly enhanced by such bond pad deformation. The invention provides a discovery that flip chip bumps formed of a polymer material have sufficient mechanical integrity to deform substrate bond pads.

Preferably, heat and pressure are applied to the flip chip as the flip chip polymer bumps are pushed through the substrate adhesive. The temperature of the chip heating and the degree of applied chip pressure are preferably selected based on the thickness of the substrate bond pads to enable deformation of the substrate bond pads. Bond pad deformation of less than about 50% of flip chip polymer bump height as-formed is preferred.

In accordance with the invention, the flip chip polymer bumps can be pushed through the substrate adhesive with a pressure sufficient for the flip chip polymer bumps to be burnished by the substrate bond pads. It can be preferred that the pressure be sufficient for the flip chip polymer bumps to be vertically compressed between the flip chip and the substrate to a compressed height that is less than the as-formed flip chip polymer bump height.

In embodiments provided by the invention, the substrate can be mechanically flexible, whereby the flip chip polymer bumps deform the substrate as well as the substrate bond pads. The substrate bond pads can be provided as copper, optionally including nickel and gold layers, preferably with each layer being sufficiently thin to accommodate bond pad deformation.

The flip chip mounting techniques of the invention are widely applicable to a range of substrate materials and flip chip mounting configurations. The flexibility in adhesive application and polymer bump formation methods allow for versatility in material formulation for application-specific considerations. In general, the flip chip mounting technique can be employed as a superior alternative for most conventional flip chip mounting processes that employ solder or other metallic bumps and conventional post-bond underfill processes, resulting in enhanced mounting quality and improved process efficiency.

Other features and advantages of the flip chip mounting methods of the invention will be apparent from the following description and accompanying drawing, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a flip chip being mounted to a substrate by a method in accordance with the invention;

FIG. 2 is a schematic cross-sectional view of a flip chip held above a substrate, identifying the height, H, of a polymer bump on a flip chip bond pad and identifying the thickness, t, of a layer of adhesive on the substrate;

FIG. 3 is a schematic cross-sectional view of a flip chip bonded to a substrate by a method in accordance with the invention in which polymer bumps are provided on the flip chip;

FIG. 4 is a schematic cross-sectional view of a flip chip held above a substrate in which polymer bumps are provided on both the flip chip and the substrate and an adhesive layer is provided over the polymer bumps of the substrate;

FIG. 5 is a schematic plan view of a stencil design provided in accordance with the invention for applying an adhesive material to a substrate which includes polymer bumps on the substrate bond pads;

FIG. 6 is a schematic cross-sectional view of a flip chip bonded to a substrate by a method in accordance with the invention in which polymer bumps are provided on the flip chip and on the substrate;

FIG. 7 is a schematic cross-sectional view of a flip chip bonded to a substrate by a method in accordance with the invention in which polymer bumps of the flip chip deform the substrate bond pads; and

FIG. 8 is a schematic cross-sectional view of a flip chip bonded to a substrate by a method in accordance with the invention in which polymer bumps of the flip chip deform the substrate bond pads and the substrate.

DETAILED DESCRIPTION OF THE INVENTION

As indicated in FIG. 1 , in accordance with the invention, polymer bumps 2 of an IC chip 1 can be directly bonded to the electrodes, i.e., bond pads, 4 of a substrate 3 , such as a circuit board, with an electrically insulating adhesive 5 located between the IC chip 1 and the circuit board. Referring also to FIG. 4 , further in accordance with the invention, on the substrate bond pads 4 there can be provided polymer bumps 8 .

The IC chip is a flip chip, in that the active surface of the chip is bonded face down on the substrate. The flip chip polymer bumps 2 are therefore applied to electrode pads of the flip chip on the active surface of the chip. Where no substrate polymer bumps are included, direct bonding of the flip chip polymer bumps to the substrate electrodes is accomplished in accordance with the invention by pushing the polymer bumps of the chip through the adhesive on the substrate to displace the adhesive in the area of the substrate electrodes such that the flip chip polymer bumps are directly bonded to the substrate electrodes. The adhesive around the bonded polymer bumps underfills the volume between the chip and the substrate, and thereby eliminates the need for a post-bond underfill dispensing step. If polymer bumps are provided on the substrate bond pads, the chip polymer bumps are pushed to penetrate the substrate polymer bumps, preferably to directly contact the substrate electrodes, but at a minimum, to be at least partially encased by the substrate polymer bumps.

The chip polymer bumps and if included, the substrate polymer bumps, are formed by, e.g., stenciling electrically-conductive paste on the electrode pads 6 , i.e., bond pads, of the IC chip 1 , and the bond pads 4 of the substrate 3 . A metal mask is satisfactory as a plate material for this stenciling of the polymer bumps on the bond pads.

The invention contemplates additional techniques for producing polymer bumps on the bond pads of the flip chip and the substrate. Screen printing, dispensing, transfer printing, laser jetting, roller coating, vacuum suction through a stencil, photolithographic techniques, and other processes can be employed, in the known conventional manner, and as described below with regard to adhesive application to the substrate, to produce polymer bumps on the bond pads of the flip chip and optionally on the bond pads of the substrate. For many applications, a stenciling process is particularly well-suited due to its ease of control and precision. It is preferred that the polymer bumps be formed on flip chips in wafer form, prior to dicing of the wafer, but such is not required. It is to be recognized, however, that for many applications, conventional manufacturing processes do not enable formation of polymer bumps on single flip chips in a manner that provides adequate bump integrity and uniformity.

Whatever process is selected to form the polymer bumps on the chip bond pads and optionally the substrate bond pads, it is preferred that any oxidizing layer present on the bond pads first be removed, in the conventional manner, and that a layer of a good electrically conducting material be applied to the bond pads. For example, a layer of nickel, nickel-gold, palladium, or other conductor is preferably applied to the bond pads by, e.g., a sputtering, electroplating, immersion plating, or other suitable technique, as is conventional.

Heat-curable resin paste, e.g., epoxy resin paste or other such resin paste, containing electrically-conductive particles, e.g., silver particles, or other such material, can be used as the electrically-conductive paste. Specifically, the polymer bumps can be formed of any of a thermoset, B-stage thermoset, thermoplastic, or other suitable polymer, in each case provided as an electrically conducting polymer by, e.g., the addition of particles, flakes, or other form of an electrically conducting material, e.g., Ag, Ag—Pd, Au, Cu, Ni, or other suitable conducting material to the polymer. It is preferred that the selected flip chip polymer bump material be characterized by a relatively high glass transition temperature, such that the flip chip bumps remain mechanically robust in the process of directly connecting the flip chip polymer bumps to the substrate electrodes by displacing the adhesive and optionally, polymer bumps, on the substrate, in the manner described in detail below.

Well-suited materials for producing polymer bumps include EPO-TEK® E4110-PFC; EPO-TEK® E2101; EPO-TEK® E3084PFC and EPO-TEK® H20E-175PFC-1, both of which have glass transition temperatures above about 175° C.; EPO-TEK® E2101, EPO-TEK® H20E-PFC, and EPO-TEK® E3114PFC, all of which have glass transition temperatures between about 110° C. and about 130° C.; EPO-TEK® K5022-115BE and EPO-TEK® K5022-115BG, both of which are thermoplastic materials having a melting point that is less than about 170° C.; and EPO-TEK® EE149-6, which is a B-stage thermoset having a glass transition temperature of between about 130° C. and about 150° C.

Preferably, the condition of the flip chip polymer bump material is that when formed as a bump, the material has sufficient hardness and sufficient morphological features to adequately push through polymer bumps, if included on substrate bond pads, and to displace the insulating adhesive on a substrate to which the chip is being mounted, to preferably enable direct connection of the bump to the substrate electrode. Specifically, the flip chip bumps should not be coined, i.e., flattened, by the bump forming process. Instead, the flip chip bumps preferably are characterized as being substantially half-hemispherical, or cone-shaped or otherwise pointed. Such enables ease of adhesive displacement and substrate bump penetration as the flip chip bumps are pushed through the adhesive and substrate bumps. This is further enabled if the flip chip polymer bumps are characterized by a relatively high Shore-D hardness, e.g., between about 70 and about 90.

The mechanical robustness of the flip chip bumps is preferably enhanced, in accordance with the invention, once the bumps are formed on the chip, by fully curing, or less preferably, by partially curing the flip chip bumps, if formed of a curable, i.e., polymerizable, material; similarly, mechanical robustness is enhanced in accordance with the invention by fully or partially drying the flip chip bumps, if of a thermoplastic material. Correspondingly, reference hereinafter to a process of at least partially “hardening” flip chip polymer bumps is meant to refer to any of the processes of partially or fully curing or drying the bumps, depending on the bump composition. For example, a thermoset material can be partially cured by subjecting it to a temperature of about 150° C. for about 15 minutes; a B-stage thermoset material can be at least partially dried by subjecting it to a temperature of about 75° C. for between about 30 minutes to about 40 minutes; and the solvents in a thermoplastic material can be driven off to at least partially dry the material by subjecting it to a temperature of about 100° C. for about 1 hour. These example flip chip bump hardening process conditions produce a bump material of sufficient mechanical robustness to successfully push through and displace adhesive on a substrate. Whatever polymerization or drying process conditions are employed, it is preferable that they provide an adequate degree of drying or partial polymerization that renders the polymer, in bump form, sufficiently mechanically robust to push through the adhesive on a substrate. It is to be recognized, therefore, that for many applications, partial polymerization may not be preferable. If a surface skin is formed on the flip chip bumps by partial polymerization of the bumps, the bumps could burst during the bonding process step of piercing through the substrate adhesive. A flip chip bump surface skin can also diminish the wetting of and adhesion to the substrate bond pads by the flip chip bumps. It is thus more preferable to fully cure flip chip polymer bumps if such are formed of a polymerizable material.

It is also preferred that the robustness of the substrate polymer bump material be at least somewhat enhanced by partial curing or drying once formed on the substrate bond pads. This mechanical enhancement is preferred to enable the substrate polymer bumps to withstand the deposition of the adhesive layer on the substrate after the polymer bumps are formed on the substrate bond pads, as explained in detail below. Skin curing of the substrate polymer bump surface is all that is generally required to produce the desired robustness.

If the selected flip chip and/or substrate polymer bump material is curable, is not necessarily required to be heat-curable, i.e., polymerizable by heat. Polymer bump material that is polymerized at room temperature or with, e.g., microwave energy, can be employed. Polymer bump material that is photocurable by exposure to, e.g., visible light, an E-beam, or ultraviolet light, can also be employed. If reworkability of a mounted flip chip is important for a given application, then a polymer bump material that is not at all cured, e.g., a thermoplastic, is preferred.

The flip chip bump hardness and morphology can be further enhanced by including relatively hard, irregularly-shaped, sharp-edged, or pointed abrasive particles in the bump material. When substantially homogeneously mixed in the material, as-formed in a bump, some of the particles' edges or points can be expected to protrude from the bump surface, forming jagged bump edges or pointed projections that enhance the ability of the flip chip bump to push through and displace the insulating adhesive and penetrate polymer bumps if included on the substrate bond pads, as well as oxides that may be present on the surface of the substrate bond pads. Such an additive can be electrically conducting or insulating. Preferably the added hard particles are less than about 20 μm in diameter, and more preferably the diameter distribution of the particles is between about 3 μm and about 13 μm.

Well-suited additives include particles, such as flakes, of, e.g., diamond, either natural or synthetic, silicon carbide, boron nitride, aluminum nitride, aluminum oxide, quartz, nickel, silica, and mica, as well as gold flake, palladium flake, silver flake, and powdered electrically conductive or insulating epoxy resin that has been cured and ground, and other suitable particle, powder, or flake materials that are characterized by relatively high degree of hardness. Requirements of a given application must be considered in selecting a suitable additive. For example, if electrical conductivity is of highest consideration, then powdered electrically conductive epoxy resin can be the preferable additive. In general, the harder an additive material, the less must be incorporated in the polymer bump to enhance the mechanical properties of the bump. As a result, nonconducting additives such as diamond and the less costly aluminum oxide can be preferred due to the very small amounts required to be added to achieve enhanced mechanical properties while minimally impacting on the electrical properties of the bump.

For some applications, electrically conductive material added to the bump polymer material to render it electrically conducting will itself provide irregularly-shaped, sharp-edged or pointed, abrasive particles. For example, silver flake, although not as hard as, e.g., diamond or silicon carbide, when included in a polymer bump to produce electrical conductivity of the bump will produce surface jaggedness and irregularities that enhance the ability of the flip chip bump to pierce through and displace the substrate adhesive and substrate polymer bumps, if included. It is contemplated in accordance with the invention that additional hard particles beyond the particles included for electrical conductivity of the polymer are to be added to enhance the mechanical properties of the flip chip polymer bumps, if such enhancement is desired, such as improving the ability of the bumps to pierce through surface oxides on the substrate pads.

While the hardness of the included particles is of importance, the jagged irregularities of the particles extending from the flip chip bump are found to particularly aid in piercing and displacing adhesive and substrate polymer bumps as the flip chip bump is pushed through the adhesive and substrate polymer bumps, if included. Plating of the flip chip polymer bump surface with a hard material, rather than inclusion of hard particles in the flip chip bump, is not contemplated by the invention, as this alternative is found to not be effective. A hard but generally smooth plating surface does not aid in piercing and displacing adhesive and substrate polymer bumps, if included, through which the flip chip bump is pushed. In addition, given that the flip chip bump surface is substantially coated with the plating material, the plating can compromise the electrical conductivity of the bump.

Furthermore, a flip chip bump plating surface can limit the lateral expansion of the flip chip bump as it is pressed against a substrate electrode. As explained in detail below, the flip chip polymer bump is a particularly effective interconnection in that the bump laterally expands as it is pressed between a chip and a substrate, whereby the flip chip polymer bump covers more of the area of a bond pad than would, say, a conventional metal bump. Limitation of the flip chip bump lateral expansion by a plating surface is to be avoided as the reduced bump surface area on the bond pad would result in lower adhesion to the substrate bond pad as well as a higher electrical contact resistance through the flip chip bump.

It is preferred in accordance with the invention that substrate polymer bumps, if included, be formed of a diameter that is, e.g., between about 5% and about 60% larger than the diameter of the flip chip bumps. It is further preferred that the substrate polymer bumps be formed of a height that is between about 30% and about 150% of the height of the flip chip bumps. These geometric conditions enable a substrate bump to at least partially encase a flip chip bump as the flip chip bump is pushed into the substrate bump, as described in detail below.

The chemistry of the substrate bumps is preferably compatible with that of the flip chip bumps such that robust adhesion between the flip chip and substrate bumps is maintained after the bonding process. If reworkability of the chip-substrate bond is desired for a given application, it is preferable that both the substrate and flip chip bumps be formed of a thermoplastic material. If the bumps of one of the substrate or flip chip are thermoplastic, but the bumps of the other are a thermoset or B-stage thermoset, it can not be guaranteed that reworkability will be possible, and more importantly, the bump-to-bump adhesion may be inadequate, due to differences in polarity. Therefore, if thermoplastic bumps are to be employed on the flip chip or substrate, it is preferred that thermoplastic bumps be employed on both the flip chip or substrate. Combinations of flip chip and substrate bumps that are thermoset or B-stage thermoset are understood to be compatible. For example, polymer bumps formed of EPO-TEK® E2101 are compatible with polymer bumps formed of EPO-TEK® H20E-PFC, both materials from Epoxy Technology of Billerica, Mass. Whatever material is selected for the substrate bumps, it is further preferred that the substrate bumps not be dried or polymerized, such that the ability of the flip chip polymer bumps to penetrate the substrate polymer bumps is maximized.

Turning now to the electrically insulating adhesive, the adhesive applied to the substrate can be, e.g., a heat-curable material, such as a thermoset or a B-stage thermoset, a thermoplastic material, or a mixture of the two. The preferred alternative is that of a heat-curable electrically insulating adhesive film. The form of this film is, e.g., a single sheet, but laminated films, or other films, can be used.

It is preferred in accordance with the invention, in general, that the insulating adhesive be substantially completely electrically nonconductive, and be substantially nonvoiding, i.e., not characterized by a tendency to form voids. Because the insulating adhesive eliminates the need for application of a conventional underfill material between the chip and substrate after the chip is bonded to the substrate, it is contemplated by the invention that the adhesive be characterized by the material properties that are desirable for an underfill material, such as relatively high modulus, low coefficient of thermal expansion, and high glass transition temperature. Preferably the adhesive can be snap cured, i.e., polymerized, if curing is desired, but such is not required. Also, if the adhesive is provided as a thermoset or B-stage thermoset, then the adhesive is preferably characterized by a tendency to shrink during polymerization in the chip bonding process, such that the flip chip polymer bumps and the chip are held in compression against the substrate. This condition of compression aids in the lateral expansion of the flip chip polymer bumps as described above, and maximizes the electrical, mechanical, and dimensional integrity of the direct bond between the flip chip polymer bumps and the substrate bond pads, as well as between the chip and the substrate.

If the adhesive is provided as a thermoplastic, it preferably is characterized by a tendency to be easily softened by a heated flip chip polymer bump being pushed through the material, whereby the bump can be easily compressed and laterally expanded by pressure applied to the chip on which the bump is connected. Such can be achieved, e.g., where the melting temperature of the adhesive is less than that of the flip chip polymer bumps. This results in maximization of the electrical, mechanical, and dimensional bond integrity in the manner enabled by thermoset and B-stage thermoset materials.

The substrate adhesive can contain a thermally-conductive filler, e.g., aluminum oxide, and/or an insulating filler, such as silica particles, but such is not required. The adhesive further can contain a spacer material that is of a dimensional regime corresponding to a desired chip-to-substrate distance when the chip is bonded to the substrate. Example spacer materials include glass beads, polystyrene particles, or other materials mixed in with the adhesive to set a minimum adhesive thickness once applied to a substrate.

Whatever filler condition is selected, preferably the resulting adhesive is characterized by a relatively high glass transition temperature or low melting point, e.g., between about 120° C. and about 175° C.; a relatively low thermal expansion coefficient, e.g., between about 10 ppm/° C. and about 40 ppm/° C.; a relatively high temperature degradation temperature, e.g., greater than about 350° C.; and by low outgassing, e.g., less than about 1.0% at about 300° C.

The invention contemplates the provision of the adhesive material in forms other than the single sheet or laminated sheet films mentioned above, and in a range of compositions. Whatever composition is provided, in accordance with the invention the composition is characterized as a paste when it is applied to the substrate. As herein specified, a paste is of sufficient viscosity and thixotropy that it does not readily flow, in the nature of a conventional liquid, once applied to a substrate. But the thixotropy of the paste can be broken by application forces, e.g., shear forces, during the application process, such that the paste becomes fluid enough to be applied, e.g., through a stencil or mesh screen, in the manner described below. A paste therefore enables application in a somewhat liquid state, but once applied tends to hold its as-applied shape.

A first general class of adhesive compositions provided by the invention is that of an electrically nonconductive film material, i.e., a material that has been rendered as a dry, non-tacky film after application to a substrate as a paste. The nonconductive film material can include a solvent as-applied to the substrate to aid in application to the substrate, which can be carried out by any of a range of techniques as described below. Examples of suitable nonconductive film materials are thermoplastics, B-stage thermosets, mixtures of the two, and other like compositions. One particularly well-suited thermoplastic material is EPO-TEK® K5022-115BT2, available from Epoxy Technology, of Billerica, Mass. Several particularly well-suited B-stage thermoset materials are EPO-TEK® TE154-8, EPO-TEK® TE154-9, EPO-TEK® TE154-10, EPO-TEK® TE154-15, EPO-TEK® B9021-1, and EPO-TEK® B9021-6, all available from Epoxy Technology, of Billerica, Mass.

Once a selected B-stage thermoset or thermoplastic material is applied to the substrate, solvent in the material, if present, is removed from the material to form a solvent-free, partially- or fully-dried adhesive film. A B-stage thermoset material can be dried by subjecting it to, e.g., a temperature of about 75° C. for between about 30 minutes to about 40 minutes. The solvent can be driven from a thermoplastic material by subjecting it to, e.g., a temperature of about 100° C. for about 1 hour. These example processes are intended only as general guidelines and it is to be recognized that the particular conditions of a given material must be considered in selecting process parameters.

A second general class of adhesive compositions provided by the invention is that of an electrically nonconductive paste material, i.e., a material that is maintained as a paste after it has been applied to a substrate. Example paste materials include thermosets and hot melt thermoplastics. Preferably the selected paste material does not include a solvent, which could likely result in voiding of the material after application to a substrate. The selected nonconductive paste material is applied to a substrate, in the manner described below, without a drying step; the material therefore remains as an adhesive paste, rather than a dry adhesive film.

Thus the distinction between nonconductive pastes of the invention and nonconductive films of the invention is that the nonconductive films are pastes that have been rendered dry or solvent-free, and therefore are in a non-tacky state in which they can be handled, whereas the undried pastes are maintained as such. Given these distinctions, the adhesive material to be employed for a given application is therefore preferably selected based on the particular needs of the application. For example, a bonding process employing a dried nonconductive film requires an added drying step, but enables the pre-production of substrates with the film for introduction into a production line that cannot accommodate an insulating adhesive application step. A bonding process employing a nonconductive paste does not accommodate handling of the substrates, but requires fewer process steps and enables a polymer bump bonding process that requires less chip pressure than for a dried, nonconductive film adhesive process.

Several thermosets that are particularly well-suited as nonconductive paste materials include EPO-TEK® TE179-1, EPO-TEKS® TE179-2, EPO-TEK® TE179-3, EPO-TEK® T6116M, EPO-TEK® B9126-20, EPO-TEK® 353NDT, and EPO-TEK® 115SMT, all available from Epoxy Technology of Billerica, Mass. In general, it is preferred that whatever thermoset is selected, it is characterized by a relatively high modulus, e.g., greater than about 10 GPa.

In general, if the selected substrate is of a flexible material, e.g., polyester, then a thermoplastic adhesive is preferably employed to maintain the flexible nature of the substrate. If a rigid substrate, e.g., a circuit board, is selected, then a thermoset or B-stage thermoset adhesive can also be preferable for various applications. For any substrate material, if reworkability of flip chips is required, then a thermoplastic adhesive is preferred.

The selected adhesive material can be applied to a substrate in any of a range of techniques provided by the invention. All techniques meet the common requirement that they form a layer of adhesive that substantially or entirely covers the substrate area at which a chip is to be bonded, including the bond pads of the substrate, if no polymer bumps are provided on the substrate bond pads; in this scenario, the substrate bond pads are not exposed; they are covered by the adhesive. Alternatively, if polymer bumps are provided on the substrate bond pads, it is preferred that such bumps be formed on the bond pads prior to adhesive application to maximize the electrical and mechanical contact of the polymer bumps to the substrate bond pads. It is not required to form the adhesive on the substrate in an exact pattern correlation with the expected chip area. It is preferable, however, that the applied adhesive area on the substrate be at least slightly larger than the area of the chip to be attached or that the adhesive material flow out from under the chip during placement of the chip sufficient to form a fillet of adhesive around the edges of the chip.

In one example technique, already mentioned above, and generally limited to the class of B-stage thermoset and thermoplastic nonconductive films described above, the film is applied to a stand-alone carrier substrate and then partially or fully dried or cured. After drying or curing, the film is cut to a selected size and shape and then transferred to the intended working substrate destination. Care must be exercised to ensure that air gaps are not formed between the film and the substrate, particularly in the areas of the substrate bond pads and polymer bumps, if included on the bond pads.

For many applications, it can be more preferable to employ a screen print or stencil operation that enables application of an adhesive material directly to an intended working substrate. Such application techniques substantially eliminate the production of voids, or air gaps, between the adhesive and the substrate at the location of the substrate bond pads and polymer bumps, if included on the bond pads. Both screen printing and stenciling operations are particularly well-suited for use with adhesive materials that include a solvent as-applied to the substrate; the solvent lowers the viscosity of the material, thereby aiding in effective printing or stenciling of the material. As discussed in detail below, substrate adhesive stencil application can be preferable to substrate adhesive screen printing application when polymer bumps are included on the substrate bond pads.

In an example screen printing process provided by the invention, the selected adhesive material, e.g., any of the thermoset, B-stage thermoset, or thermoplastic materials given above, is squeegeed through a wire mesh screen using, e.g., a metal or polymer-based squeegee, onto the intended substrate. The screen can be formed in the conventional manner, of, e.g., stainless steel mesh or other metal, or a polyester-type plastic weave.

In the conventional manner, the diameter of the wire mesh is selected based on the selected adhesive thickness. For many applications, it is desirable to provide an emulsion pattern on the screen to define the adhesive application area on the substrate. Such an emulsion pattern can be provided, in the conventional manner, on the bottom side of the screen. The emulsion thickness, like the wire mesh diameter, is preferably selected, in the conventional manner, based on the selected adhesive thickness. If the wire mesh is too thick for a desired adhesive thickness, and/or the emulsion is too thin for a desired adhesive thickness, gaps in the adhesive can be formed on the substrate at the location of cross-over of the mesh wires. The general rheology of a selected adhesive material must also be considered with regard to the screen wire diameter and the screen emulsion thickness. With proper selection of wire mesh diameter and emulsion thickness for a selected adhesive thickness and thixotropic characteristic, complete coverage of a substrate by a screen printing process can be achieved.

In a stencil operation provided by the invention for applying adhesive material of e.g., thermoset, B-stage thermoset, or thermoplastic directly to a substrate, the adhesive material is pushed by a squeegee through open holes provided in a metal stencil onto the intended working substrate. The metal stencil thickness is selected, in the conventional manner, based on the selected adhesive thickness. The squeegee can be formed of, e.g., metal or polymer. The openings in the stencil can be formed by, e.g., a subtractive chemical etch process, a subtractive laser etch process, an additive electroforming process, or other suitable stencil patterning forming technique.

For many applications, screen printing can be preferable to stenciling when no substrate polymer bumps are included, given the relatively large substrate area over which the adhesive material is to be applied. Conversely, for applications in which substrate polymer bumps are included, and for applications employing a relatively thin layer of adhesive, stenciling can be preferable to eliminate the formation of voids in the adhesive at the cross-over locations of a screen printing wire mesh. Stenciling operations also can be expected to be more reliable over the life of a production line in that unlike a mesh screen, a metal stencil general does not clog with material being stenciled and exhibits a longer operational time to fatigue.

Referring to FIG. 5 , for applications where substrate polymer bumps are included, it can be preferable to carry out a stenciling operation employing a stencil that is designed particularly for adhesive application to the substrate in a manner that does not mechanically distort the substrate polymer bumps. The stencil 10 can include, e.g., an array of openings 12 located across the substrate area corresponding to the chip placement. The stencil preferably includes polymer bump openings 14 located in correspondence with the polymer bumps 8 on the substrate bond pads. With this arrangement of openings, the substrate polymer bumps are not located under a solid region of the stencil. As a result, when adhesive is stenciled onto the substrate through the mask holes, the mask does not press against and deform the substrate polymer bumps and the adhesive is applied with minimum shearing force, such that the geometry of the bumps is substantially maintained. In addition, during the stenciling process, the adhesive applied to the substrate through the polymer bump openings of the stencil can cover and surround the substrate polymer bumps, thereby maximizing its underfill capabilities.

One can recognize that the advantages provided by this stencil design cannot be had with a mesh screen, and thus screen printing is not preferred when substrate polymer bumps are included. Specifically, because screen printing requires that the screen be pressed down to the substrate surface, deformation of the substrate bond pads is not easily avoided during adhesive application to the substrate. If screen printing is to be employed, it is desired that the mesh spacing be large enough to accommodate the substrate polymer bumps.

Alternative to screen printing and stenciling operations, the invention further provides a dispensing operation for applying the adhesive material as, e.g., a thermoset, B-stage thermoset, or thermoplastic, directly to an intended working substrate. Here the selected adhesive material is dispensed from a dispenser onto the substrate, after formation on the bond pads of substrate polymer bumps, if included. The dispenser can be provided as, e.g., a single needle, or a showerhead needle of multiple parallel dispensing points, or other suitable needle configuration. Dispenser pumps can be provided as conventional air piston or positive displacement pump configurations. Whether configured as a single needle, showerhead needle, or other configuration, the dispenser is preferably controlled to produce a selected dispensed drop pattern on the substrate at each chip attachment location on the substrate. Suitable patterns include concentric circles and squares of dispensed drops, centered at the locations of chip attachment to the substrate; but it is to be recognized that lines of dispensed drops and complex patterns of dispensed drops can be preferable for some applications. The needle diameter and the volume of adhesive pushed out of the needle during one dispensing operation determine the dispensed drop dimensions. In general, a single needle dispenser is preferred over a showerhead needle for its ability to produce a dispensed drop that reliably expands to provide continuous coverage of a selected substrate area.

In a further example adhesive material application process provided by the invention, a polymer such as a thermoset, B-stage thermoset, or thermoplastic adhesive material is applied to an intended working substrate by a transfer print, pad print, stamp print, or roller print process. In general, in all of these operations, a transfer element is employed to pick up, or collect, the selected adhesive from an adhesive reservoir and then to deposit the collected adhesive on the intended working substrate. The adhesive is released from the transfer element to the substrate typically as a result of differing surface tensions between the adhesive and the substrate and the transfer element and the adhesive. Such adhesive application techniques are not preferred in accordance with the invention when substrate polymer bumps are included.

The transfer element can be provided as, e.g., a patterned pad of suitable material such as metal or rubber, as a patterned roller, as an array of pins fastened to a support plate, or other configuration in which can be provided a differentiated pattern. The pattern preferably corresponds to the chip attachment locations on the substrate. If a patterned roller coater is employed, it is preferably designed such that adhesive is collected on a patterned portion of the roller when the roller is rolled through an adhesive reservoir, the patterned portion then applying the adhesive to the substrate when the substrate is passed under the roller or the roller is passed over the substrate.

In a further adhesive material application technique, a selected adhesive material, here specifically a thermoset polymer, is spin-coated on the substrate. A solvent-based adhesive is here preferably employed to enable spread of the material and to maximize planarity across the substrate. Conventional spin-coating process parameters can here be employed. For some applications it can be preferable to apply multiple layers of spin-coated material to the substrate to obtain a desired adhesive layer thickness. Spin coating is not a preferable adhesive application technique when substrate polymer bumps are included.

It is to be recognized that the details of the adhesive material application techniques just described are applicable to the corresponding polymer bump formation techniques earlier described.

The invention is not limited to the example adhesive material application operations described above. All that is required is a technique that enables the formation of a selected adhesive on an intended working substrate at locations of the substrate corresponding to chip attachment areas, with the adhesive coating, i.e., covering, the bond pads of the substrate so that the bond pads are not exposed through the adhesive, if substrate polymer bumps are not included. If substrate polymer bumps are included, it is preferable that the technique enable formation of the selected adhesive on the substrate and over and around the polymer bumps on the substrate bond pads.

It is preferable that the viscosity and the specific gravity of a selected adhesive material be tailored for a selected adhesive application technique. For screen printing and stenciling operations, the adhesive preferably is characterized by relatively high viscosity and thixotropy. For dispensing operations, the adhesive preferably is characterized by relatively low to medium viscosity and by relatively high thixotropy. For transfer printing and for spin coating operations, the adhesive preferably is characterized by relatively low viscosity and by relatively medium thixotropy.

It is preferred in accordance with the invention that the adhesive material be applied to the substrate rather than the chip. Where no substrate polymer bumps are included, adhesive material application to the substrate prior to attachment of any components to the substrate enables the production of a uniform and planar layer of adhesive across the extent of the substrate, resulting in repeatable, reliable mechanical and electrical characteristics of the chip-to-substrate bond. Furthermore, the dicing of a wafer would likely damage adhesive applied to the wafer and dispensing on single chips is not practical; dispensed paste cannot be reliably maintained on the chip. Even if adhesive application could be reliably carried out on a bumped chip or wafer, such would negate the ability to verify the functionality of the chip just prior to its attachment to a substrate. It is therefore contemplated in accordance with the invention that the adhesive material be applied to the intended working substrate, not the chip.

Referring to FIG. 2 , in accordance with the invention, the thickness of the applied layer of adhesive on the substrate is to be selected in tandem with the height of the polymer bumps formed on the flip chip and the polymer bumps formed on the substrate if such are included. It is to be recognized that the bump diameter and the Shore-D of the bump must be considered in selecting the bump height.

In general, the height, H, of a flip chip polymer bump is preferably greater than the thickness, t, of the layer of substrate adhesive when no substrate polymer bumps are included. In FIG. 2 , the heights of the chip bond pads 6 and the substrate electrodes 4 are greatly exaggerated; under typical conditions, the flip chip bump height and the adhesive layer can be specified to meet this condition while neglecting the electrode and bond pad heights. If a higher degree of precision is desired, then the height, H, should include the height of the flip chip bump as well as the height of the bond pad supporting the bump on the chip. It is further to be recognized that for any flip chip bump height, the adhesive layer must cover the substrate bond pads where no substrate polymer bumps are included, and thus must be at least as thick as the height of the substrate bond pads.

For applications where no substrate polymer bumps are included, if flip chip bump height is greater than the adhesive thickness, it is preferable that when the chip is pressed against the adhesive layer on the substrate, the flip chip bumps do not act as stand-offs holding the chip above the adhesive layer. Contact of the face of the chip with the adhesive layer is substantially required for the adhesive layer to fully underfill the distance between the chip face and the substrate. Therefore, it is preferable for flip chip bumps to be vertically compressed between the chip and the substrate when the face of the chip is pushed until it is in contact with the top of the adhesive layer. This results in the desirable lateral expansion of the flip chip bump on the bond pad, a preferable condition as explained above.

A flip chip bump height greater than adhesive layer thickness therefore can ensure maximization of the mechanical and electrical integrity of both the chip and bump bond if during the bonding process, the flip chip bumps are compressed to an extent sufficient for the face of the chip to come into contact with the adhesive layer. Such cannot be reliably expected if the flip chip bump is too large or hard to be sufficiently compressed or too low small and short to reach the substrate bond pad, a condition that would occur if the flip chip bump height is less than or the same as the adhesive layer thickness. The optimal bump height for a given adhesive layer thickness is preferably selected based on considerations for a given application, but in general, a larger flip chip bump height is better than a smaller bump height, and a flip chip bump height compression of between about 20% and about 50%, i.e., a compressed flip chip bump height of between about 50% and about 80% of pre-bonded bump height, is desirable. For many applications the flip chip bump height is optimally about twice the thickness of the adhesive layer. In one example, a flip chip bump height of about 50 μm is employed and an adhesive thickness of about 25 μm is employed.

Referring again to FIG. 4 , where polymer bumps are included on the substrate bond pads, the thickness of the applied layer of adhesive on the substrate is preferably large enough to ensure that the chip face opposite the substrate is fully in contact with the adhesive layer when the flip chip polymer bumps are in contact with the substrate bond pads or at least partially encased by the substrate polymer bumps. To meet this requirement, it can be preferred that the adhesive layer be thicker than the height of the substrate bumps. This results in covering of the substrate bumps by the adhesive layer. It can also be preferred that the adhesive layer thickness enable the formation of a fillet at the edges of the flip chip when the flip chip is seated on the substrate. However, the adhesive layer preferably is not so thick as to cover the top surface of the flip chip once the chip is seated on the substrate.

It is preferred that the substrate 3 , if a circuit board, be a resin, i.e., polymeric, board, but other types of substrates can be used. For example, the substrate can be formed of polyimide, paper, epoxy glass, plastic, ceramic, acetylbutylstyrene(ABS), polyester (PET), polyvinylchloride (PVC), and other suitable substrate materials.

Referring back to FIG. 1 , it is preferred that the substrate be held via suction, i.e., vacuum, on a suction stage 7 to prevent the substrate from shifting as pressure is applied to the chip against the substrate during the bonding process. Such suction is particularly important for bonding chips to relatively flexible substrates such as polyimide. In one particularly well-suited configuration, the suction stage is provided as a metal plate having an array of holes through the thickness of the plate, through which suction can be drawn against the substrate. An array of suction holes enables a uniform degree of suction across the substrate, in turn reducing the possibility of bending or wrinkling thin and flexible substrates. It is also found to be preferable to apply a reduced level of suction through many holes rather than to apply an increased level of suction through only a few or one hole. In the latter instance, a condition of bending or wrinkling of a thin substrate is difficult to avoid. For many applications, it is also preferable that the suction holes not be located directly underneath the chip placement site, such that planarity of the substrate is maintained at such sites.

For an application in which a nonconducting film is first formed on a carrier substrate and then applied to an intended working substrate, when the substrate 3 is held via suction on a suction stage 7 , it is preferable for the insulating adhesive film 5 to first be set on the substrate 3 , with heat thereafter applied while pressing the film against the substrate 3 , for an application in which a nonconductive film is first formed and then applied to the substrate. Bonding of the film to the substrate can then be accomplished after bringing the film into close contact with the electrodes 4 of the circuit board 3 and the selective application of pressure and heat. Applying heat, e.g., a heating temperature of about 80° C., while pressing the insulating adhesive film 5 against the substrate 3 can be accomplished using a reserve heating tool, and then bonding of the flip chip to the substrate can be accomplished using a bonding tool 8 . This procedure is not required for the other adhesive application techniques described above.

The bonding tool 8 is preferably mounted to enable movement of the tool in all three axis directions, X, Y, and Z, as well as rotation to a specified angle. Furthermore, the bonding tool preferably has a built-in heater and is set to enable vacuum suction holding of the IC chip 1 to the tool 8 . Likewise, it is preferable that suction stage 7 have a built-in heater to allow pre-warming and/or supplementary heating of the substrate during the bonding process. Optionally, it can be preferable that the suction stage 7 be rendered optically transparent and incorporate a UV light source for enabling photo-initiated curing of the adhesive layer 5 . Because a wide range of applicable substrates 3 consist of thin and transparent or semi-transparent films, e.g., PET, PEN, or polyimide, and the substrate bond pads 4 and other connecting circuitry are generally sufficiently spaced apart to allow transmission of light from beneath the substrate, a light source may be advantageously located or directed from underneath the substrate to expose the adhesive during the bonding process.

Once the adhesive layer has been applied to the substrate in the desired manner, an individual flip chip, having polymer bumps formed on the chip bond pads in the manner described above, is provided on the bonding tool 8 , typically, e.g., a collet, and heated to the desired bonding temperature. Preheating of the chip is not required by the invention, but can be preferred for many bonding applications, because if the chip is heated when it comes into contact with the adhesive, heat transfer from the chip into the adhesive aids in lowering of the adhesive viscosity. This results in a correspondingly increased ability for the chip bumps to penetrate the substrate adhesive. If preheating is to be carried out, such is preferably maintained until the heat is transferred completely through the chip and the polymer bumps on the chip.

Thereafter, heat-pressure attachment, i.e., bonding, is achieved by precisely positioning the IC chip 1 on the underlying circuit board. This requires the lateral alignment of the chip bond pads with the substrate bond pads. In one example configuration, this can be achieved by employing fiducials on the chip and the substrate, in the conventional manner, in an alignment process employing an upper microscope and a lower microscope, with a camera, e.g., a CCD camera, provided for controlling the alignment process.

Once the respective bond pads of the chip and substrate are aligned, bonding can be satisfactorily achieved by piercing through the insulating film 5 with the polymer bumps 2 of the IC chip 1 . The heating temperature and the pressure for the bonding are preferably controlled to specified values. For applications in which no substrate polymer bumps are included, as each flip chip polymer bump is pushed through the adhesive 5 the heated bump lowers the viscosity of the adhesive, displacing the adhesive in the region of its path such that the flip chip polymer bump can make direct contact with a corresponding substrate electrode. Application of pressure to the chip is preferably maintained at least until the chip face is in contact with the top surface of the adhesive on the substrate and the polymer bumps of the chip are preferably in contact with the substrate electrodes.

It is to be recognized that for applications where substrate polymer bumps are not included, complete displacement of the substrate adhesive from the substrate bond pads by the flip chip polymer bumps is not in general required. Some amount of residual adhesive may remain on the substrate bond pads. In this condition, electrical conduction between the flip chip polymer bumps and the substrate bond pads can be at least partially established by point contacts formed of the sharp-edge or pointed projections incorporated in the bumps as previously mentioned.

Referring also to FIG. 6 , if polymer bumps are included on the substrate bond pads, the flip chip polymer bumps are pushed through the adhesive to displace the adhesive overlying the substrate polymer bumps. The flip chip polymer bumps are then pushed into the substrate polymer bumps, penetrating the substrate polymer bumps. Optimally, the flip chip polymer bumps are pushed through the substrate polymer bumps to make contact with the substrate bond pad, in the manner of bump 16 in FIG. 6 and as noted above.

If such is not achievable or practical, then at a minimum it is preferred that the flip chip bumps penetrate the substrate bumps to a depth sufficient to enable electrical connection between the flip chip bumps and the substrate bond pads via the substrate polymer bumps. For many applications, it can be preferred that a flip chip polymer bump penetrate a substrate polymer bump to a depth whereby at least about 50% of the flip chip polymer bump is covered by the substrate polymer bump. The substrate bumps thereby can operate as an intermediary between the flip chip bumps and the substrate bond pads, as shown for flip chip bumps 18 , 20 in FIG. 6 . As a result, the substrate bumps can compensate for non-coplanar bumps on the flip chip; even if some flip chip bumps are shorter than others, the majority, if not all, of the flip chip bumps can still can be electrically connected to the substrate bond pads, via the substrate bumps.

With chip heating maintained during the bonding, the adhesive wets the chip surface upon contact with the layer. If the adhesive material is a thermoset or B-stage thermoset, conduction of heat from the flip chip and flip chip bumps to the adhesive layer cures the adhesive, causing the adhesive layer to shrink and pull the flip chip bumps and flip chip into compression against the substrate. Compression of the chip against the adhesive layer is desirable, as explained above, to ensure that no gap space exists between the chip and the substrate, i.e., to ensure that the adhesive layer is completely underfilling the distance between the chip and the substrate. Compression of the chip further ensures that the flip chip polymer bumps are compressed and laterally expanded on substrate bond pads, whereby lateral gaps between the bumps and the adhesive are eliminated.

If the adhesive material is a thermoplastic, it is softened by the heated flip chip bumps as they push through it. As a result, pressure applied to the chip compresses the bumps and laterally expands the bumps in the manner described above, resulting in an equally effective bond. In accordance with the invention, a heating schedule can be selected in which no preheating of the chip and bumps is carried out. Alternatively, the polymer bumps can be fully pushed through the adhesive and contacted to the substrate electrodes prior to heating of the completed assembly for curing the adhesive.

The following guidelines are examples of parameter values that can be generally applied to the bonding process, with the caveat that the chemistry of particular polymer bump and adhesive materials employed in the bonding process must be considered:

Parameter                        Range of Values
Adhesive Film Thickness          25 μm-75 μm
Polymer Bump Height              50 μm-150 μm
Polymer Bump Hardness            Shore-D 70-90
Bond Temperature                 150° C.-350° C.
Bond Pressure                    10 grams-1000 grams
Pressure Dwell Time              0.3 seconds-10 seconds
Bump Compression                 20%-50%
Contact Coverage of Bump on Electrode >50%
                                 (less with point contact)

It is to be recognized that consideration must also be made for the number of polymer bumps to be bonded between a given chip and a substrate and the bond pressure required for each bump. Because the required bond pressure of each bump is cumulative to the chip, a relatively high total pressure can be required on the chip to enable bonding of all chip bumps. This can result in damage to the chip where very large numbers of bumps are provided. It is therefore preferable to determine the total pressure necessary for bonding the total number of polymer bumps of a chip and then to verify that the required total pressure will not damage the chip. If damage appears to be possible, a reduced pressure application is warranted, with compensating adjustment of materials properties as required. A particularly advantageous compensation technique is the provision of unpolymerized, non-dried polymer bumps on the substrate bond pads. As described above and shown in FIG. 6 , such enable electrical connection of flip chip bumps to substrate bond pads via the substrate polymer bumps.

Referring to FIGS. 7 and 8 , for many applications, it can be preferred to select bonding pressure and temperature characteristics that enable deformation of substrate bond pads by the flip chip polymer bumps. Specifically, the bonding temperature and pressure can be selected based on the substrate bond pad material composition and thickness to enable the polymer bumps of the flip chip to deform the substrate bond pads. The invention provides a discovery that flip chip bumps formed of a polymer material have sufficient mechanical integrity to deform substrate bond pads.

It is recognized that such bond pad deformation can significantly enhance the electrical connection between a polymer bump and a bond pad. This is understood to be enabled by a range of mechanisms including, among other things, enhancement of mechanical contact area between the polymer bump and bond pad. It is further recognized that the structural integrity and reliability of the bond between a polymer bump and a bond pad is significantly enhanced by bond pad deformation.

Substrate bond pad deformation preferably is of an extent to produce some degree of curvature of the bond pads, as shown in FIG. 7 , and more preferably to produce a significant bond pad curvature, as shown in FIG. 8 . The vertical extent of features in FIGS. 7 and 8 are shown out of scale for clarity. Whatever degree of bond pad curvature occurs, it preferably accommodates the maintenance of some amount of adhesive between the flip chip and the substrate, to inhibit electrical shorting between bond pads and to inhibit contact between the chip and the substrate.

FIG. 7 represents an example in which the bond pads of the substrate are deformable but the substrate itself is not deformable. FIG. 8 represents an alternative example in which both the bond pads and the substrate material itself are deformable, as in the case of, e.g., a Mylar substrate. The use of a mechanically flexible substrate can be preferable to enable this degree of bond pad deformation. It can also be preferred to select bond pad materials and thicknesses that accommodate deformation. For example, copper bond pad material for a wide range of thicknesses can be deformed by the flip chip polymer bumps. If non-oxidizing layers are included over a bond pad, such layers preferably are thin enough to accommodate deformation while sufficiently preventing oxidation.

It is recognized that deformation of substrate bond pads by flip chip polymer bumps can result in burnishing of the flip chip polymer bumps to some extent. This enables a measure of flip chip bump penetration into substrate bond pad oxide, if present, and can be effectively employed for diagnostic analysis of the extent of bond pad deformation by a given flip chip bump. For many applications, it is recognized that some combination of substrate bond pad deformation and flip chip bump compression and lateral expansion can be preferred.

Referring to FIGS. 3 and 6 , once the bonding process is complete, the polymer bumps 2 , 16 , 18 , 20 are connected between the bond pads 6 of the flip chip and the bond pads 4 of the substrate, either directly or via substrate polymer bumps. Preferably, there is little or no electrically insulating adhesive between the bond pads and electrodes—only the polymer bump material from flip chip bumps and from substrate bumps if such were included. Preferably, the entire region surrounding the polymer bumps between the chip and the substrate is filled by the adhesive; as a result, little or no gaps exist between the chip and the substrate, although some voids are occasionally unavoidable. The adhesive layer thereby functions as an underfill material between the chip and the substrate. No additional material need be applied between the chip and the substrate to provide the functionality of an underfill.

Thus, in accordance with the invention, as shown in FIGS. 3 and 6 , the IC chip 1 bumps 2 , 16 , 18 or 20 are composed of polymer bumps 2 formed from electrically conductive paste. Compared with other bumps, like solder bumps formed with a ball bonding method, or Au bumps formed with a plating method, the polymer bumps 2 provide unexpectedly superior performance. Specifically, the polymer bumps successfully enable piercing and displacement of the insulating adhesive film and substrate polymer bumps to a degree comparable with that of the metal bumps. This is enabled, in part, by heating of the polymer bumps as they are pressed against the adhesive, whereby the insulating adhesive is effectively displaced due to its lower viscosity. But in addition, the polymer bumps differ greatly from e.g., solder bumps or Au bumps in that during the bonding process the polymer bumps can be compressed to expand across the contact area of the electrodes 4 of the substrate 3 . This substantially enhances the quality of the mechanical and electrical bond over that made with a conventional metal bump. This also enables a condition in which the chip is compressed against the adhesive layer on the substrate, thereby ensuring that the adhesive layer fills the volume between the chip and the substrate around the polymer bumps.

In addition, the polymer bumps have the characteristic that in the state where the bump surface is suitable for bonding, the material added to the polymer to render it electrically conducting, e.g., silver particles, provided as is conventional substantially homogeneously through the polymer bump, are deposited, i.e., are inherently found to exist, on the surface of the bump. The particles form minute concave and convex surface regions that aid in the piercing through and displacement of the substrate adhesive layer as the polymer bumps are pushed through the layer. As explained above, additional particles or flakes can be added to the polymer bump material to enhance its material properties for pushing through and displacing the substrate adhesive. Such is not generally available for conventional metal bump technology. There is also a notable difference between the polymer bumps and conventional metal bumps in that, during bonding, the bump resin paste plays the role of an adhesive, where conventional metal bumps generally do not provide an adhesive mechanism.

Therefore, even though minute through-holes are not provided in the insulating adhesive film 5 for inserting the polymer bumps 2 of the IC chip 1 , direct bonding of the bumps to the substrate electrodes can be achieved by piercing the film and substrate bumps, if included, with the flip chip bumps. The method thus makes the preparation of the insulating adhesive film 5 easier, thereby greatly improving the general applicability of the technique. Mechanically strong bonding of the chip to the substrate can be achieved with superior reliability.

It is therefore found that as described above, it is possible to bond flip chip polymer bumps to circuit board bond pads, optionally via substrate polymer bumps, using an insulating film with no through-holes. This condition facilitates preparation of the insulating adhesive film, and markedly improves the general applicability of the technique. It also enables strong bonding in a state with adequate reliability.

Based on the discussion above, it is found that the flip chip mounting method of the invention accomplishes polymer bump-to-substrate bonding and chip-to-substrate underfilling simultaneously. The process therefore eliminates the need for an additional underfill step, eliminating the capital cost of equipment for the additional underfill step and resulting in increased production throughput and lower process cost. Because the electrically nonconducting adhesive layer can be formed and applied by any of a wide range of techniques, the mounting method of the invention enables production of flip chip assemblies on virtually any substrate material, including low-temperature plastics such as polyvinylchloride, as well as paper and other exotic substrate materials. The flexibility of the adhesive application also enables the mounting method to be automated for a selected process, e.g., for reel-to-reel processing where the adhesive layer is applied as a film that is first formed on a carrier substrate. The flexibility of the adhesive application also allows versatility in material formulation for application-specific considerations, e.g., with the inclusion of thermally-conductive fillers.

The operation of the flip chip mounting method of the invention in vertically compressing and laterally expanding polymer bumps as they are bonded to substrate electrodes substantially eliminates the production of voids between the bumps and the adhesive located around the bumps, due, e.g., to entrapped air, solvents, or other volatiles. The flip chip bump compression is a result of the pressing of the chip against the adhesive layer on the substrate, of the shrinking of the adhesive layer as it is fully polymerized, in the case of thermoset and B-stage thermoset adhesives, and of ease of lateral bump expansion against a softened adhesive, in the case of thermoplastic adhesives. As a result, the flip chip mounting method of the invention produces an underfill between the chip and the substrate that is more robust than conventional underfill materials that are dispensed between the chip and the substrate after the chip is bonded to the substrate. Virtually complete coverage of adhesive material between the flip chip and the substrate is enabled by the mounting method of the invention.

It is recognized, of course, that those skilled in the art may make various modifications and additions to the flip chip bonding techniques described above without departing from the spirit and scope of the present contribution to the art. Accordingly, it is to be understood that the protection sought to be afforded hereby should be deemed to extend to the subject matter of the claims and all equivalents thereof fairly within the scope of the invention.

Claims

1. A method for mounting a flip chip on a substrate, comprising the steps of:forming electrically conductive polymer bumps on bond pads of a flip chip; at least partially hardening the flip chip polymer bumps; forming electrically conductive polymer bumps on bond pads of a substrate; applying a layer of electrically insulating adhesive paste on the substrate, covering the substrate polymer bumps with the adhesive; aligning the bond pads of the flip chip with the bond pads of the substrate; pushing the at least partially hardened flip chip polymer bumps through the substrate adhesive and at least partially into the substrate polymer bumps.

2. The method of claim 1 wherein the step of pushing the at least partially hardened flip chip polymer bumps comprises pushing the flip chip polymer bumps through the substrate adhesive and through the substrate polymer bumps to directly contact the flip chip polymer bumps with the substrate bond pads.

3. The method of claim 1 wherein the step of pushing the at least partially hardened flip chip polymer bumps comprises pushing the flip chip polymer bumps through the substrate adhesive and into the substrate polymer bumps to a depth sufficient to produce electrical connections between the flip chip polymer bumps and the substrate bond pads.

4. The method of claim 1 wherein the step of at least partially hardening the flip chip polymer bumps comprises at least partially drying the flip chip polymer bumps.

5. The method of claim 1 wherein the step of at least partially hardening the flip chip polymer bumps comprises fully polymerizing the flip chip polymer bumps.

6. The method of claim 1 wherein the step of pushing the at least partially hardened flip chip polymer bumps comprises applying heat to the flip chip as the flip chip polymer bumps are pushed.

7. The method of claim 1 wherein the step of pushing the at least partially hardened flip chip polymer bumps comprises applying pressure to the flip chip.

8. The method of claim 2 further comprising a step of applying heat to the flip chip after the flip chip polymer bumps contact the bond pads of the substrate.

9. The method of claim 1 wherein as-formed height of the substrate polymer bumps is between about 30% and about 150% of as-formed height of the flip chip polymer bumps.

10. The method of claim 1 wherein as-formed diameter of the substrate polymer bumps is between about 10% and about 70% greater than as-formed diameter of the flip chip polymer bumps.

11. The method of claim 1 wherein the step of forming flip chip polymer bumps comprises forming flip chip polymer bumps having a bump height as-formed that is greater than adhesive paste thickness as-applied on the substrate.

12. The method of claim 11 wherein the flip chip polymer bump height as-formed is at least about 25% greater than adhesive paste thickness as-applied.

13. The method of claim 1 wherein as-applied thickness of the substrate adhesive is at least as great as substrate polymer bump height as-formed.

14. The method of claim 1 wherein the flip chip and substrate polymer bumps are formed of a thermoplastic material.

15. The method of claim 1 wherein the substrate polymer bumps are formed of a thermoset material.

16. The method of claim 1 wherein the flip chip polymer bumps are formed of a thermoset material.

17. The method of claim 1 wherein the flip chip polymer bumps are formed of a B-stage thermoset material.

18. The method of claim 1 wherein the substrate polymer bumps are formed of a B-stage thermoset material.

19. The method of claim 1 wherein the step of forming flip chip polymer bumps comprises stenciling flip chip polymer bumps; and wherein the step of forming substrate polymer bumps comprises stenciling substrate polymer bumps.

20. The method of claim 1 wherein the applied layer of insulating adhesive paste comprises a thermoplastic.

21. The method of claim 1 wherein the applied layer of insulating adhesive paste comprises a thermoset.

22. The method of claim 1 wherein the applied layer of insulating adhesive paste comprises a B-stage thermoset.

23. The method of claim 1 wherein the step of applying the adhesive paste to the substrate comprises stenciling the adhesive paste on the substrate.

24. The method of claim 23 wherein the step of stenciling the adhesive paste on the substrate comprises stenciling the adhesive paste with a stencil that includes openings at stencil locations corresponding to substrate polymer bump locations.

25. The method of claim 1 wherein the step of applying the adhesive paste to the substrate comprises screen printing the adhesive paste on the substrate.

26. The method of claim 1 wherein the step of applying the adhesive paste to the substrate comprises dispensing the adhesive paste on the substrate.

27. The method of claim 1 further comprising a step of at least partially drying the substrate adhesive paste before the step of pushing the flip chip polymer bumps through the substrate adhesive.

28. The method of claim 1 wherein the flip chip polymer bumps include hard particles.

29. The method of claim 26 wherein the particles included in the flip chip polymer bumps are electrically conductive.

30. A method for mounting a flip chip on a substrate, comprising the steps of:forming electrically conductive polymer bumps on bond pads of a flip chip; at least partially hardening the flip chip polymer bumps; applying a layer of electrically insulating adhesive paste on a substrate having bond pads, covering the bond pads with the adhesive; aligning the bond pads of the flip chip with the bond pads of the substrate; pushing the at least partially hardened flip chip polymer bumps through the substrate adhesive with pressure sufficient for the flip chip polymer bumps to directly contact and deform the bond pads of the substrate.

31. The method of claim 30 wherein the step of pushing the at least partially hardened flip chip polymer bumps comprises applying pressure to the flip chip.

32. The method of claim 31 wherein the step of pushing the at least partially hardened flip chip polymer bumps comprises applying heat to the flip chip as the flip chip polymer bumps are pushed through the substrate adhesive.

33. The method of claim 32 wherein the flip chip heating temperature and pressure applied as the flip chip polymer bumps are pushed through the substrate adhesive are selected based on thickness of the substrate bond pads to enable deformation of the substrate bond pads.

34. The method of claim 30 wherein the step of pushing the at least partially hardened flip chip polymer bumps comprises pushing the flip chip polymer bumps to produce substrate bond pad deformation that is less than about 50% of flip chip polymer bump height as-formed.

35. The method of claim 30 wherein the step of pushing the at least partially hardened flip chip polymer bumps comprises pushing the flip chip polymer bumps with a pressure sufficient for the flip chip polymer bumps to be burnished by the substrate bond pads.

36. The method of claim 30 wherein the step of pushing the at least partially hardened flip chip polymer bumps comprises pushing the flip chip polymer bumps with a pressure sufficient for the flip chip polymer bumps to be vertically compressed between the flip chip and the substrate bond pads to a compressed height that is less than flip chip polymer bump height as-formed.

37. The method of claim 30 wherein the substrate is characterized as being mechanically flexible, and wherein the step of pushing the at least partially hardened flip chip polymer bumps comprises pushing the flip chip polymer bumps to deform the substrate as well as the substrate bond pads.

38. The method of claim 30 wherein the substrate bond pads comprise copper.

39. The method of claim 30 wherein the substrate bond pads comprise a gold layer and a nickel layer, each layer being sufficiently thin to accommodate bond pad deformation.