US 6915566 B2
A method for the fabrication of a double-sided electrical interconnection flexible circuit (200) particularly useful as a substrate for an area array integrated circuit package. A copper matrix with studs (203) is pressed through a dielectric film (201) having a copper layer on the opposite surface, thereby forming an intermediate structure for a flex circuit with self-aligned solid copper vias in a one step process. The contacts are reinforced by plating both surfaces with a layer of copper, and conventional processes are used to complete the circuit patterning.
1. A method of manufacturing a flex circuit on a flexible base polymer film including the steps of:
a) superimposing on said film an embossing tool having raised areas comprising a pattern of conductors and vias corresponding to a circuit design, wherein, said raised areas are coated with a thin layer of metal, comprising copper,
b) applying heat and pressure to simultaneously emboss the film and to transfer said thin metal layer from the embossing tool to the polymer film,
c) removing the embossing tool,
d) embossing a pattern corresponding to that of the second surface of a flex circuit, and simultaneously transferring a thin layer of metal into the embossed pattern,
e) physically removing the embossing tool,
f) plating a layer of copper to fill the vias and conductor patterns on both sides of the film,
g) plating a layer of nickel and gold onto the exposed copper patterns, and
h) applying a solder mask on the surface of the film surrounding the solder ball contact pads.
This application claims priority under 35 USC § 119 based upon Provisional Patent Application No. 60/122,297, filed Mar. 1, 1999.
The present invention relates generally to flexible circuits and more particularly to a method of fabricating flexible circuits for integrated circuit package interconnections.
As the demand for cheaper, faster and lower power consuming integrated circuits increases, so must the packing density at the circuit board level. Not only have techniques continually evolved to meet the demand for minimizing dimensions of the transistors and of the electrical interconnections which integrate semiconductor devices, but also the packaging technology has advanced resulting in smaller integrated circuit packages with improved electrical and thermal performance.
Ball Grid Array (BGA) and many Chip Scale Packages (CSP) are integrated circuit packages which are assembled to an external circuit board using an array of solder balls confined within the area of the package. The solder balls are electrically connected to an external circuit board, and the external connections to the chip are made through conductive vias and conductor traces in the package substrate. An example of an area array package is shown in
Integrated circuit chips 104 are electrically connected to the interconnect circuitry on the substrate either by wire bonding or by flip chip 103 connections. Typical substrates are of rigid printed wiring board construction, or for more advanced small and closely spaced circuits, an unsupported flex circuit provides interconnections. Electrical connection between the printed circuitry 105 on the chip side of the substrate and the external contact solder balls is typically realized by conductive vias 106 through the substrate.
Printed circuits are typically fabricated by preparing by an enlarged-scale artwork master of a circuit pattern and conductor paths, and then the enlarged-scale artwork master is photographically reduced to the desired size. Screens and masks are fabricated according to the reduced circuit pattern for the application of photoresist materials. Processes including etching, screening, plating, laminating, vacuum deposition, via hole formation, and protective coating application are used to fabricated both supported printed circuits and unsupported flexible circuits.
Substrates for area array packages on flex circuits are most commonly fabricated on a dielectric polymer base film in reel or sheet format. A copper metal is applied to both surfaces of the dielectric film either by bonding a thin foil to the base film, or by vapor depositing the metal and subsequently plating to the necessary conductor thickness. Interconnect traces and contact pads are patterned and etched in the metal. Several techniques are known for electrically interconnecting the conductors and contacts on either side of the substrate. Vias are typically formed by mechanical punching or by laser ablation, and the vias are filled with a conductor by plating, by metal deposition during the film metallization, or by filling with conductive pastes. Completing the fabrication of a circuit includes plating a layer of nickel and a thin film of gold over the conductors for environmental protection, and to support solder contacts. A solder mask coating is applied to the circuitry on the surface to be soldered in order to control the solder run out.
All of these fabrication techniques require multiple plating and etching steps. In the case of laser ablation where one via is formed at a time, the process is repetitive and time consuming. Further, the existing techniques for making conductive vias suffer from difficulties in making the through holes consistently conductive, and in aligning the top and bottom circuits with the vias. Via conductivity failures occur either as electrical opens or as intermittent contacts resulting from separations in the thin conductor walls during thermal excursions due to expansion mismatch between the conductor and the substrate, from incompletely coating by vapor deposition, from air pockets entrapped in the vias during plating from both sides of the circuit, from fatigue failures of the thin conductors, from marginal conductivity of the filling material, and/or from marginal contact due to misalignment of the circuit and vias.
It is accordingly desirable to provide a reliable flexible circuit substrate for integrated circuit area array packages, and a method of manufacture that permits high volume production without the reliability issues identified for current processes, and which eliminates the need for costly vapor deposition or multiple laser drilling.
The principal object of the present invention is to provide a reliable, double sided electrical interconnection flexible circuit which enables interconnecting integrated circuit chips to an external circuit, and a method of manufacturing the flex circuit. The preferred embodiment of the invention provides a flexible circuit substrate for area array packages including conductor patterns on the first surface of a flexible dielectric film which permit interconnection between the chip terminals and from the chip terminals to conductive vias through the dielectric film. It further includes, on the second surface a plurality of contact pads for attachment of solder balls for external contact to a second level of interconnection, and the conductors on the two surfaces are interconnected by reliable, solid metal conductive vias. The unique conductive vias comprise metal studs etched from a metal matrix, preferably comprising copper, and the studs when pressed through the dielectric film contact the conductive material on the first surface of the dielectric. The copper matrix with raised studs is attached to the second surface of the dielectric film, and the unraised portion of the matrix provides the pre-patterned solder ball contact pads.
The flex circuit of the present invention, therefore, includes a plurality of studs etched from a metal matrix with a which serve as both the tool to punch apertures for the vias, and to simultaneously fill the vias. The unraised film portion of the metal matrix provides the base metal which will subsequently be patterned to form the solder ball contacts, and thus the film and the studs become a part of the finished flex circuit.
It is further an object of the invention to provide a predecessor embossing tool for use of and inclusion into the flexible circuit, and a method of manufacturing the tool. The embossing tool comprises a copper matrix etched to form raised studs corresponding to a pattern of conductive vias in a flexible circuit. The etched studs of the copper embossing tool when adhered to the base dielectric film, and a hydraulic force applied, punch a plurality of cylindrical apertures in the film, and simultaneously fill the apertures. When pushed through the dielectric, the studs contact conductors on the opposite surface of to solid conductive vias through the dielectric.
To complete the fabrication of the intermediate base structure for a flex film, additional copper is electroplated onto the surfaces to both increase the conductor thickness, and to seal the connection between the solid Cu via and the copper film on the first surface, thereby insuring robust electrical contact. Subsequently, the copper on both surfaces is photopatterned and etched to provide interconnections and bump solder pads. The patterns on either side are aligned to the vias, thereby eliminating the problems associated with aligning patterns on opposite sides of a film.
It is further an object of the invention to provide a method for fabricating a flex circuit substrate which has high yield, low cost, and is amenable to mass production. In addition to the etched metal matrix, a die set precisely matched to the etched studs facilitates punching apertures for vias in the film. Reel to reel processing and transport are the preferred techniques for fabrication of substrates which may then be assembled into packages using the same format. However, the processes are not limited to this technique, nor is the specific flex circuit application.
In an alternate embodiment, a metal embossing tool is coated in the selective raised areas with a thin film of copper which under temperature and pressure adheres to the dielectric film simultaneously with the embossing process. The embossing tool is removed, leaving depressed interconnect patterns, contact pads and/or vias coated with copper. The exposed copper is subsequently plated with copper. The process eliminates patterning and etching in the manufacture of a flex circuit, as well as the need for a solder mask on solder contact surface.
In accordance with the preferred embodiment of the current invention, a double sided flexible interconnection circuit 200 for use as an integrated package substrate is shown in cross-sectional view in
A typical pattern of solder ball contact pads 204 for a flex circuit package substrate is shown in
A plurality of solid metal conductive vias 203 electrically connect the patterned conductors on each surface of the dielectric film. The conductive vias of the preferred embodiment of the current invention comprise metallic studs etched from a metal matrix. The studs are attached to an unraised portion of the metal matrix on the second surface 201 a of the flex circuit. The metal film matrix adhered to the dielectric flex film provides the base metal of the solder ball contact pad 204. The metal matrix with etched studs comprises copper.
An etched metal stud 203 contacts the interconnection pattern 202 on the first surface 201 b of the dielectric film, and is intimately attached to the base copper metal of the solder ball contact pad 204 on the second surface 201 a of the dielectric film 210, thereby forming a the solid metal conductor via 203. Reliability of the copper stud contact to the conductor pattern on both surfaces of the dielectric film is insured by a layer of plated copper 206 to fill any voids at the interfaces.
The device of this invention has been described wherein the metal matrix with raised studs is a portion of the solder ball pads on the second surface of the dielectric film. In an alternate embodiment, the surfaces are reversed and the metal matrix with studs forms the base metal layer of the interconnect pattern or first surface of the dielectric film, and the copper studs engage with the solder ball contact pads on the second surface.
Interconnection circuitry on the flex circuit substrate further includes a thin film of nickel and gold over the exposed copper patterns to provide environmental protection and a bondable surface. A solder mask surrounds the solder ball pads on the second surface to complete the patterned area of the flex circuit.
The preferred embodiment has been described as a flexible circuit which constitutes the major component and interconnection for an integrated circuit substrate, however, it is not limited to this application, but is intended to cover flexible circuits having interconnections on two sides.
Turning now to a method of fabricating the flex circuit of the current invention having a plurality of conductive vias comprising the etched studs of a metal matrix which serve to electrically interconnect the circuitry on the two surfaces of the flex film substrate. It is necessary to fabricate a metal matrix having raised studs corresponding to a pattern of vias for a specified flex circuit. The matrix with raised studs constitutes the tool used to punch the apertures for vias, to fill the vias with the studs, and to provide the base metal for the solder ball contact pads on the second surface of the dielectric film.
A reverse image of the photomask for the embossing tool is used to fabricate a die plate precisely matched to the vias. The die plate provides an opening through which residue from the punching operation will be extruded; this will be discussed later under the subject of mechanization.
The apertures are filled by the copper studs 315 simultaneously with the punching to forming a plurality of solid copper vias in the dielectric film 402. Within the same process, the copper matrix film 311 contacts the adhesive material 416 on the second surface 406 of the dielectric film. In
The vertical surfaces of the copper studs 315 of the studs opposite the film matrix 311 terminate in contact with the copper layer 403 on the first surface of the dielectric. The copper stud 315 and the copper layer 403 are press fit forming an intact solid copper layer 408 on the second surface of the flex film.
The process steps illustrated in
The photoresist is removed by conventional stripping methods and in
Processes and material required for completing the metallization overlying the patterned copper circuits are typical of those used for flex circuit formation, except that alignment of the patterns is specifically to filled vias, as opposed to some existing technology wherein the surfaces are patterned and subsequently vias formed which must align to and connect the patterns on both surfaces.
Mechanization for fabricating a flexible circuit for use as an integrated circuit area array package substrate is well adapted to reel to reel film transport. The appropriate film width will be selected for single or multiple packages, as required by the end user transport equipment. Continuous processing to form an intermediate structure for a flex circuit includes the following steps: alignment of a metal matrix having etched studs to a dielectric film, and to a die plate having apertures specifically matched to the vias, compressing the matrix and film, heating to adhere the matrix to a heat sensitive adhesive and plating the copper coated structure. Further, continuous processing to pattern the circuit includes the steps of photolithography, copper etching, and electroless plating the patterns.
The film with assembled metal matrix is subsequently transported to a position in the press having a solid plate wherein the film assemblage is heated and compressed, which enables the copper studs to be press fit against the copper film on the second surface and against the walls of the dielectric film, as shown in
An alternate technique for fabricating the conductor patterns and vias of a flex circuit package substrate using an embossing tool includes the following steps. Securing a metal matrix with embossed studs and/or conductor patterns fabricated from known technology such as electroforming or etching, coating the raised areas with a loosely held thin film of copper, transferring the copper film to a dielectric film using heat and pressure, physically removing the matrix, and subsequently plating the appropriate thickness of copper over the thin film of copper. The process is repeated on the second surface having a pattern of solder ball contact pads aligned to the preformed vias. The embossed-film is plated with copper from both sides, using the transferred metal as the seed layer.
A schematic drawing of the tool for side one is given in FIG. 7. In
The copper film is selectively applied to raised portions 701, 702, and 703 of the tool from a copper powder in suspension adhering to thermoplastic adhesive on the tool, or by use of an embossing tool comprising iron, permanently masking the area where no copper is desired and plating a film of copper on the raised portions.
Similar mechanization facilitates the embossing process for this transfer method as the preferred embodiment described earlier; i.e. reel to reel processing with heated press to effect the embossing procedure.
Advantages of the emboss and transfer method are that no etching or photolithography are required in the flex circuit process itself and that the conductor and contact pads are recessed into the dielectric film.
While the invention has been described in connection with preferred embodiments, it is not intended to limit the scope to a particular form set forth, but on the contrary, it is intended to cover alternatives, modifications and equivalents as may be included within the spirit of the invention and the scope of the invention as defined by the appended claims.