Abstract:
The invention is directed to a method of bonding a hermetically sealed electronics package to an electrode or a flexible circuit and the resulting electronics package that is suitable for implantation in living tissue, for a retinal or cortical electrode array to enable restoration of sight to certain non-sighted individuals. The hermetically sealed electronics package is directly bonded to the flex circuit or electrode by electroplating a biocompatible material, such as platinum or gold, effectively forming a plated rivet-shaped connection, which bonds the flex circuit to the electronics package. The resulting electronic device is biocompatible and is suitable for long-term implantation in living tissue.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 12/861,729, filed on Aug. 23, 2010, entitled Biocompatible Bonding Method and electronics Package Suitable for Implantation, which is a division of U.S. patent application Ser. No. 11/455,028, filed on Jun. 15, 2006, entitled “Biocompatible Bonding Method and Electronics Package Suitable for Implantation”, which is a divisional of U.S. patent application Ser. No. 10/174,349, filed on Jun. 17, 2002, entitled “Biocompatible Bonding Method and Electronics Package Suitable for Implantation” the disclosure of which is incorporated herein by reference, and which claims benefit of U.S. Patent application Ser. No. 60/372,062, filed on Apr. 11, 2002, entitled “Platinum Deposition for Electrodes,” the disclosure of which is incorporated herein by reference. 
     
    
     FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with government support under grant No. R24EY12893-01, awarded by the National Institutes of Health. The government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    This invention relates to an electrode array or flexible circuit, electronics package and a method of bonding a flexible circuit or electrode array to an integrated circuit or electronics package in the manufacture of a visual prosthetic. 
       BACKGROUND OF THE INVENTION 
       [0004]    Arrays of electrodes for neural stimulation are commonly used for a variety of purposes. Some examples include U.S. Pat. No. 3,699,970 to Brindley, which describes an array of cortical electrodes for visual stimulation. Each electrode is attached to a separate inductive coil for signal and power. U.S. Pat. No. 4,573,481 to Bullara describes a helical electrode to be wrapped around an individual nerve fiber. U.S. Pat. No. 4,837,049 to Byers describes spike electrodes for neural stimulation. Each spike electrode pierces neural tissue for better electrical contact. U.S. Pat. No. 5,215,088 to Norman describes an array of spike electrodes for cortical stimulation. U.S. Pat. No. 5,109,844 to de Juan describes a flat electrode array placed against the retina for visual stimulation. U.S. Pat. No. 5,935,155 to Humayun describes a retinal prosthesis for use with a flat retinal array. 
         [0005]    Packaging of a biomedical device intended for implantation in the eye, and more specifically for physical contact with the retina, presents a unique interconnection challenge. The consistency of the retina is comparable to that of wet tissue paper and the biological media inside the eye is a corrosive saline liquid environment. 
         [0006]    Thus, the device to be placed against the retina, in addition to being comprised of biocompatible, electrochemically stable materials, must appropriately conform to the curvature of the eye, being sufficiently flexible and gentle in contact with the retina to avoid tissue damage, as discussed in Andreas Schneider, Thomas Stieglitz, Werner Haberer, Hansjörg Beutel, and J.-Uwe Meyer, “Flexible Interconnects for Biomedical Microsystems Assembly, IMAPS Conference, Jan. 31, 2001. It is also desirable that this device, an electrode array, provides a maximum density of stimulation electrodes. A commonly accepted design for an electrode array is a very thin, flexible conductor cable. It is possible to fabricate a suitable electrode array using discrete wires, but with this approach, a high number of stimulation electrodes cannot be achieved without sacrificing cable flexibility (to a maximum of about 16 electrodes). 
         [0007]    A lithographically fabricated thin film flex circuit electrode array overcomes such limitations. A thin film flex circuit electrode array can be made as thin as 10 um (0.0004 inches) while accommodating about 60 electrodes in a single circuit routing layer. The flex circuit electrode array is essentially a passive conductor ribbon that is an array of electrode pads, on one end, that contact the retina and on the other end an array of bond pads that must individually mate electrically and mechanically to the electrical contacts of a hermetically sealed electronics package. These contacts may emerge on the outside of the hermetic package as an array of protruding pins or as vias flush to a package surface. A suitable interconnection method must not only serve as the interface between the two components, but must also provide electrical insulation between neighboring pathways and mechanical fastening between the two components. 
         [0008]    Many methods exist in the electronics industry for attaching an integrated circuit to a flexible circuit. Commonly used methods include wire-bonding, anisotropic-conductive films, and “flip-chip” bumping. However, none of these methods results in a biocompatible connection. Common materials used in these connections are tin-lead solder, indium and gold. Each of these materials has limitations on its use as an implant. Lead is a known neurotoxin. Indium corrodes when placed in a saline environment. Gold, although relatively inert and biocompatible, migrates in a saline solution, when electric current is passed through it, resulting in unreliable connections. 
         [0009]    In many implantable devices, the package contacts are feedthrough pins to which discrete wires are welded and subsequently encapsulated with polymer materials. Such is the case in heart pacemaker and cochlear implant devices. Flexible circuits are not commonly used, if at all, as external components of proven implant designs. The inventor is unaware of prior art describing the welding of contacts to flex circuits. 
         [0010]    Attachment by gold ball bumping has been demonstrated by the Fraunhofer group (see Hansjoerg Beutel, Thomas Stieglitz, Joerg Uwe Meyer, “Versatile ‘Microflex’-Based Interconnection Technique,” Proc. SPIE Conf on Smart Electronics and MEMS, San Diego, Cal., March 1998, vol 3328, pp 174-82) to rivet a flex circuit onto an integrated circuit. A robust bond can be achieved in this way. However, encapsulation proves difficult to effectively implement with this method. Because the gap between the chip and the flex circuit is not uniform, under fill with epoxy is not practical. Thus, electrical insulation cannot be achieved with conventional under fill technology. Further, as briefly discussed earlier, gold, while biocompatible, is not completely stable under the conditions present in an implant device since it “dissolves” by electromigration when implanted in living tissue and subject to an electric current (see M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, Houston, 1974, pp 399-405). 
         [0011]    Widespread use of flexible circuits can be found in high volume consumer electronics and automotive applications, such as stereos. These applications are not constrained by a biological environment. Component assembly onto flex circuits is commonly achieved by solder attachment. These flex circuits are also much more robust and bulkier than a typical implantable device. The standard flex circuit on the market is no less than 0.002 inches in total thickness. The trace metalization is etched copper foil, rather than thin film metal. Chip-scale package (CSP) assembly onto these flex circuits is done in ball-grid array (BGA) format, which uses solder balls attached to input-output contacts on the package base as the interconnect structures. The CSP is aligned to a corresponding metal pad array on the flex circuit and subjected to a solder reflow to create the interconnection. A metallurgical interconnect is achieved by solder wetting. The CSP assembly is then underfilled with an epoxy material to insulate the solder bumps and to provide a pre-load force from the shrinkage of the epoxy. 
         [0012]    Direct chip attach methods are referred to as chip-on-flex (COF) and chip-on-board (COB). There have been some assemblies that utilize gold wirebonding to interconnect bare, integrated circuits to flexible circuits. The flipchip process is becoming a reliable interconnect method. Flipchip technology originates from IBM&#39;s Controlled Collapse Chip Connection (C4) process, which evolved to solder reflow technique. Flipchip enables minimization of the package footprint, saving valuable space on the circuit, since it does not require a fan out of wirebonds. While there are a variety of flipchip configurations available, solder ball attach is the most common method of forming an interconnect. A less developed approach to flipchip bonding is the use of conductive adhesive, such as epoxy or polyimide, bumps to replace solder balls. These bumps are typically silver-filled epoxy or polyimide, although electrically conductive particulate of select biocompatible metal, such as platinum, iridium, titanium, platinum alloys, iridium alloys, or titanium alloys in dust, flake, or powder form, may alternatively be used. This method does not achieve a metallurgical bond, but relies on adhesion. Polymer bump flip chip also requires underfill encapsulation. Conceivably, polymer bump attachment could be used on a chip scale package as well. COB flipchip attach can also be achieved by using gold stud bumps, as an alternative to solder balls. The gold bumps of the chip are bonded to gold contacts on the hard substrate by heat and pressure. A recent development in chip-to-package attachment was introduced by Intel Corporation as Bumpless Build Up Layer (BBUL) technology. In this approach, the package is grown (built up) around the die rather than assembling the die into a pre-made package. BBUL presents numerous advantages in reliability and performance over flipchip. 
         [0013]    Known technologies for achieving a bond between a flexible circuit and a electronics package suffer from biocompatibility issues. Novel applications of a biomedical implant that utilize a flexible circuit attached to a rigid electronics package require excellent biocompatibility coupled with long term mechanical attachment stability, to assure long lived reliable electrical interconnection. 
         [0014]    Known deposition techniques for a bond, such as an electrically conductive metal bond or “rivet” are limited to thin layers. Plating is one such known method that does not result in an acceptable bond. It is not known how to plate shiny platinum in layers greater than approximately 1 to 5 microns because the dense platinum layer peels off, probably due to internal stresses. Black platinum lacks the strength to be a good mechanical attachment, and also lack good electrical conductivity. 
         [0015]    Known techniques for bonding an electronic package to a flex circuit do not result in a hermetic package that is suitable for implantation in living tissue. Therefore, it is desired to have a method of attaching a substrate to a flexible circuit that ensures that the bonded electronic package and flex circuit will function for long-term implant applications in living tissue. 
       SUMMARY OF THE INVENTION 
       [0016]    An implantable electronic device comprising a hermetic electronics control unit, that is typically mounted on a substrate, that is bonded to a flexible circuit by an electroplated platinum or gold rivet-shaped connection. The resulting electronics assembly is biocompatible and long-lived when implanted in living tissue, such as in an eye or ear. 
         [0017]    The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
       OBJECTS OF THE INVENTION 
       [0018]    It is an object of the invention to provide a hermetic, biocompatible electronics package that is attached to a flexible circuit. 
         [0019]    It is an object of the invention to attach a hermetically sealed electronics package to a flexible circuit for implantation in living tissue. 
         [0020]    It is an object of the invention to attach a hermetically sealed electronics package to a flexible circuit for implantation in living tissue to transmit electrical signals to living tissue, such as the retina. 
         [0021]    It is an object of the invention to provide a hermetic, biocompatible electronics package that is attached directly to a substrate. 
         [0022]    It is an object of the invention to provide a method of bonding a flexible circuit to a substrate with an electroplated rivet-shaped connection. 
         [0023]    It is an object of the invention to provide a method of plating platinum as a rivet-shaped connection. 
         [0024]    Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  illustrates a perspective cutaway view of an eye containing a flexible circuit electrode array. 
           [0026]      FIG. 2  is a side view of an electronics package. 
           [0027]      FIG. 3  illustrates a cutaway side view of an electronics package. 
           [0028]      FIG. 4  is a top view of a flex circuit without the electronics package. 
           [0029]      FIG. 5  presents a side view of a flex circuit with the electronics package. 
           [0030]      FIG. 6  is a side view of a flex circuit that is bonded with adhesive to a hybrid substrate. 
           [0031]      FIG. 7  is a series of illustrations of a flexible circuit being bonded using conductive metal pads to a hybrid substrate. 
           [0032]      FIG. 8  is a series of illustrations of weld staple bonding of a flexible circuit to a hybrid substrate. 
           [0033]      FIG. 9  is a sequence of steps illustrating tail-latch interconnect bonding of a flexible circuit to a hybrid substrate. 
           [0034]      FIG. 10  is a sequence of steps illustrating formation of an integrated interconnect by vapor deposition. 
           [0035]      FIG. 11  is a side view of a flexible circuit bonded to a rigid array. 
           [0036]      FIG. 12  is a side view of an electronics control unit bonded to an array. 
           [0037]      FIG. 13  is a cross-sectional view of a bonded assembly in stepwise fashion. 
           [0038]      FIG. 14  is an electroplating equipment schema. 
           [0039]      FIG. 15  is a three-electrode electroplating cell schema. 
           [0040]      FIG. 16  is a plot of showing the plating current density decrease with hole size. 
           [0041]      FIG. 17   a  is a scanning electron micrograph of a polyimide surface before plating magnified 850 times. 
           [0042]      FIG. 17   b  is a scanning electron micrograph of electrochemically deposited rivets magnified 850 times 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    The following description is the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. 
         [0044]    The present invention provides a flexible circuit electronics package and a method of bonding a flexible circuit to a hermetic integrated circuit which is useful for a number of applications, including implantation in living tissue as a neural interface, such as a retinal electrode array or an electrical sensor. The tissue paper thin flexible circuit  18 ,  FIG. 1 , transmits electrical signals to the eye  2  by means of electrodes, that are located in a stimulating electrode array  10 , that are in contact with the retina  14 . It is obvious that in addition to a stimulating electrode array or sensing electrode, the electrodes may be contacts connecting to remote electrodes.  FIG. 1  illustrates the electronics control unit  20  in a perspective cutaway view of an eye  2  containing a flexible circuit electrode array  18 . The electronics control unit  20  is hermetically sealed. The electronics control unit  20  may be a hermetic ceramic case with electronics inside, or it may be a hermetically sealed integrated circuit, or any other environmentally sealed electronics package. The stimulating electrode array  10  is implanted on the retina  14 . Flexible circuit ribbon  24  connects the stimulating electrode array  10  to the electronics control unit  20 . 
         [0045]    The flexible circuit ribbon  24  preferably passes through the sclera  16  of the eye  2  at incision  12 . Another embodiment of the invention is the flexible circuit ribbon  24  replaced by alternative means of electrical interconnection, such as fine wires or thin cable. The lens  4  of the eye  2  is located opposite the retina  14 . A coil  28 , which detects electronic signals such as of images or to charge the electronics control unit  20  power supply, located outside the eye  2 , near the lens  4 , is connected to the electronics control unit  20  by wire  30 . 
         [0046]      FIG. 2  illustrates a side view of the hermetic electronics control unit  20  and the input/output contacts  22  that are located on the bottom of the unit  20 . The input/output contacts  22  are bonded in the completed assembly to the flexible circuit  18 . Thick film pad  23  is formed by known thick film technology, such as silk screening or plating. 
         [0047]      FIG. 3  illustrates a cutaway side view of the hermetic electronics control unit  20 . The pad  23  facilitates attachment of wire  30 , and is preferably comprised of a biocompatible material such as platinum, iridium, or alloys thereof, and is preferably comprised of platinum paste. Wire  30  is preferably bonded to pad  23  by welding. The microelectronics assembly  48  is mounted on the hybrid substrate  44 . Vias  46  pass through the substrate  44  to input/output contacts  22 . Electrical signals arrive by wire  30  and exit the electronics control unit  20  by input/output contacts  22 . 
         [0048]    A top view of the flexible circuit  18  is illustrated in  FIG. 4 . Electrical signals from the electronics control unit  20  (see  FIG. 3 ) pass into bond pads  32 , which are mounted in bond pad end  33 . Flexible electrically insulating substrate  38 , is preferably comprised of polyimide. The signals pass from the bond pads  32  along traces  34 , which pass along flexible circuit ribbon  24  to the stimulating electrode array  10 . The array  10  contains the electrodes  36 , which are implanted to make electrical contact with the retina  14  of the eye  2 , illustrated in  FIG. 1 . An alternative bed of nails embodiment for the electrodes  36  is disclosed by Byers, et al. in U.S. Pat. No. 4,837,049. 
         [0049]    In  FIG. 5 , the hermetic electronics control unit  20  is illustrated mounted to flexible circuit  18 . In order to assure electrical continuity between the electronics control unit  20  and the flexible circuit  18 , the electrical control unit  20  must be intimately bonded to the flexible circuit  18  on the bond pad end  33 . A cutaway of the electronics control unit  20  ( FIG. 5 ) illustrates a bonded connection  42 . The flexible electrically insulating substrate  38  is very thin and flexible and is able to conform to the curvature of the retina  14  ( FIG. 1 ), when implanted thereon. 
         [0050]    Methods of bonding the flexible insulating substrate  18  to the hermetic electronics control unit  20  are discussed next. 
         [0051]    Platinum Conductor in Polymer Adhesive 
         [0052]    A preferred embodiment of the invention, illustrated in  FIG. 6 , shows the method of bonding the hybrid substrate  244  to the flexible circuit  218  using electrically conductive adhesive  281 , such as a polymer, which may include polystyrene, epoxy, or polyimide, which contains electrically conductive particulate of select biocompatible metal, such as platinum, iridium, titanium, platinum alloys, iridium alloys, or titanium alloys in dust, flake, or powder form. 
         [0053]    In  FIG. 6 , step a, the hybrid substrate  244 , which may alternatively be an integrated circuit or electronic array, and the input/output contacts  222  are prepared for bonding by placing conductive adhesive  281  on the input/output contacts  222 . The rigid integrated circuit  244  is preferably comprised of a ceramic, such as alumina or silicon. In step b, the flexible circuit  218  is preferably prepared for bonding to the hybrid substrate  244  by placing conductive adhesive  281  on bond pads  232 . Alternatively, the adhesive  281  may be coated with an electrically conductive biocompatible metal. The flexible circuit  218  contains the flexible electrically insulating substrate  238 , which is preferably comprised of polyimide. The bond pads  232  are preferably comprised of an electrically conductive material that is biocompatible when implanted in living tissue, and are preferably platinum or a platinum alloy, such as platinum-iridium. 
         [0054]      FIG. 6 , step c illustrates the cross-sectional view A-A of step b. The conductive adhesive  281  is shown in contact with and resting on the bond pads  232 . Step d shows the hybrid substrate  244  in position to be bonded to the flexible circuit  218 . The conductive adhesive  281  provides an electrical path between the input/output contacts  222  and the bond pads  232 . Step c illustrates the completed bonded assembly wherein the flexible circuit  218  is bonded to the hybrid substrate  144 , thereby providing a path for electrical signals to pass to the living tissue from the electronics control unit (not illustrated). The assembly has been electrically isolated and hermetically sealed with adhesive underfill  280 , which is preferably epoxy. 
         [0055]    Studbump Bonding 
         [0056]      FIG. 7  illustrates the steps of an alternative embodiment to bond the hybrid substrate  244  to flexible circuit  218  by studbumping the hybrid substrate  244  and flexible electrically insulating substrate  238  prior to bonding the two components together by a combination of heat and/or pressure, such as ultrasonic energy. In step a, the hybrid substrate  244  is prepared for bonding by forming a studbump  260  on the input/output contacts  222 . The studbump is formed by known methods and is preferably comprised of an electrically conductive material that is biocompatible when implanted in living tissue if exposed to a saline environment. It is preferably comprised of metal, preferably biocompatible metal, or gold or of gold alloys. If gold is selected, then it must be protected with a water resistant adhesive or underfill  280 . 
         [0057]    Alternatively, the studbump  260  may be comprised of an insulating material, such as an adhesive or a polymer, which is coated with an electrically conductive coating of a material that is biocompatible and stable when implanted in living tissue, while an electric current is passed through the studbump  260 . One such material coating may preferably be platinum or alloys of platinum, such as platinum-iridium, where the coating may be deposited by vapor deposition, such as by ion-beam assisted deposition, or electrochemical means. 
         [0058]      FIG. 7 , step b presents the flexible circuit  218 , which comprises the flexible electrically insulating substrate  238  and bond pads  232 . The flexible circuit  218  is prepared for bonding by the plating bond pads  232  with an electrically conductive material that is biocompatible when implanted in living tissue, such as with a coating of platinum or a platinum alloy. Studbumps  260  are then formed on the plated pad  270  by known methods. Step c illustrates cross-section A-A of step b, wherein the flexible circuit  218  is ready to be mated with the hybrid substrate  244 . 
         [0059]      FIG. 7 , step d illustrates the assembly of hybrid substrate  244  flipped and ready to be bonded to flexible circuit  218 . Prior to bonding, the studbumps  260  on either side may be flattened by known techniques such as coining. Pressure is applied to urge the mated studbumps  260  together as heat is applied to cause the studbumps to bond by a diffusion or a melting process. The bond may preferably be achieved by thermosonic or thermocompression bonding, yielding a strong, electrically conductive bonded connection  242 , as illustrated in step e. An example of a thermosonic bonding method is ultrasound. The bonded assembly is completed by placing an adhesive underfill  280  between the flexible circuit  218  and the hybrid substrate  244 , also increasing the strength of the bonded assembly and electrically isolating each bonded connection. The adhesive underfill  280  is preferably epoxy. 
         [0060]    Weld Staple Interconnect 
         [0061]      FIG. 8  illustrates the steps of a further alternative embodiment to bond the hybrid substrate  44  to flexible circuit  18  by weld staple bonding the substrate  244  and flexible electrically insulating substrate  38  together. In step a, a top view of the flexible circuit  18  is shown. Flexible circuit  18  is comprised of flexible electrically insulating substrate  38 , which is preferably polyimide, and bond pads  32  having a through hole  58  therethrough each bond pad  32  and through the top and bottom surfaces of flexible circuit  18 . The bond pads  32  are comprised of an electrically conductive and biocompatible material which is stable when implanted in living tissue, and which is preferably platinum or a platinum alloy, such as platinum-iridium. 
         [0062]      FIG. 8 , step b presents section A-A, which is shown in the illustration of step a. The through holes  58  pass completely through each bond pad  58 , preferably in the center of the bond pad  58 . They are preferably formed by plasma etching. The bond pads  58  are not covered on the top surface of flexible circuit  18  by flexible electrically insulating substrate  38 , thereby creating bond pad voids  56 . 
         [0063]      FIG. 8 , step c shows the side view of hybrid substrate  44  with input/output contacts  22  on one surface thereof. The hybrid substrate  44  is positioned, in step d, to be bonded to the flexible circuit  18  by placing the parts together such that the input/output contacts  22  are aligned with the bond pads  32 . Then wire  52 , which is preferably a wire, but may equally well be a ribbon or sheet of weldable material, that is also preferably electrically conductive and biocompatible when implanted in living tissue, is attached to input/output contact  22  and bond pad  32  to bond each aligned pair together. The wire  52  is preferably comprised of platinum, or alloys of platinum, such as platinum-iridium. The bond is preferably formed by welding using the parallel gap welder  50 , which moves up and down to force the wire  52  into the through hole  58  and into contact with input/output contact  22 . This process is repeated for each aligned set of input/output contacts  22  and bond pads  32 , as shown in step e. 
         [0064]    The weld staple interconnect bonding process is completed, as shown in step f, by cutting the wire  54 , leaving each aligned set of input/output contacts  22  and bond pads  32  electrically connected and mechanically bonded together by staple  54 . 
         [0065]    Tail-Latch Interconnect 
         [0066]      FIG. 9  illustrates yet another embodiment for attaching the hybrid substrate  244  to a flexible circuit  218  by using a tail-ball  282  component, as shown in step a. The hybrid substrate  244  is preferably comprised of a ceramic material, such as alumina or silicon. In one embodiment, a wire, preferably made of platinum or another electrically conductive, biocompatible material, is fabricated to have a ball on one end, like the preferred tail-ball  282  illustrated in step a. The tail-ball  282  has tail  284  attached thereto, as shown in the side view of step a. The tail-ball  282  is aligned with input/output contact  222  on hybrid substrate  244 , in preparation to being bonded to flexible circuit  218 , illustrated in step b. 
         [0067]    The top view of step b illustrates flexible electrically insulating substrate  238 , which is preferably comprised of polyimide, having the through hole  237  passing completely thorough the thickness and aligned with the tail  284 . The bond pads  232  are exposed on both the top and bottom surfaces of the flexible circuit  218 , by voids  234 , enabling electrical contact to be made with input/output contacts  222  of the hybrid substrate  244 . The voids are preferably formed by plasma etching. 
         [0068]    The side view of  FIG. 9 , step c, which illustrates section A-A of step b, shows the hybrid substrate  244  in position to be bonded to and aligned with flexible circuit  218 . The tails  284  are each placed in through hole  237 . Pressure is applied and the tail-balls  282  are placed in intimate contact with bond pads  232  and input/output contacts  222 . Step c illustrates that each of the tails  284  is bent to make contact with the bond pads  232 . The bonding process is completed by bonding, preferably by welding, each of the tails  284 , bond pads  232 , tail-balls  282 , and input/output contacts  222  together, thus forming a mechanical and electrical bond. Locking wire  262  is an optional addition to assure that physical contact is achieved in the bonded component. The process is completed by underfilling the gap with an electrically insulating and biocompatible material (not illustrated), such as epoxy. 
         [0069]    Integrated Interconnect by Vapor Deposition 
         [0070]      FIG. 10  illustrates a further alternative embodiment to creating a flexible circuit that is electrically and adhesively bonded to a hermetic rigid electronics package. In this approach, the flexible circuit is fabricated directly on the rigid substrate. Step a shows the hybrid substrate  44 , which is preferably a ceramic, such as alumina or silicon, having a total thickness of about 0.012 inches, with patterned vias  46  therethrough. The vias  46  are preferably comprised of frit containing platinum. 
         [0071]    In step b, the routing  35  is patterned on one side of the hybrid substrate  44  by known techniques, such as photolithography or masked deposition. It is equally possible to form routing  35  on both sides of the substrate  44 . The hybrid substrate  44  has an inside surface  45  and an outside surface  49 . The routing  35  will carry electrical signals from the integrated circuit, that is to be added, to the vias  46 , and ultimately will stimulate the retina (not illustrated). The routing  35  is patterned by know processes, such as by masking during deposition or by post-deposition photolithography. The routing  35  is comprised of a biocompatible, electrically conductive, patternable material, such at platinum. 
         [0072]    Step c illustrates formation of the release coat  47  on the outside surface  49  of the hybrid substrate  44 . The release coat  47  is deposited by known techniques, such as physical vapor deposition. The release coat  47  is removable by know processes such as etching. It is preferably comprised of an etchable material, such as aluminum. 
         [0073]    Step d illustrates the formation of the traces  34  on the outside surface  49  of the hybrid substrate  44 . The traces  34  are deposited by a known process, such as physical vapor deposition or ion-beam assisted deposition. They may be patterned by a known process, such as by masking during deposition or by post-deposition photolithography. The traces  34  are comprised of an electrically conductive, biocompatible material, such as platinum, platinum alloys, such as platinum-iridium, or titanium-platinum. The traces  34  conduct electrical signals along the flexible circuit  18  and to the stimulating electrode array  10 , which were previously discussed and are illustrated in  FIG. 4 . 
         [0074]    Step e illustrates formation of the flexible electrically insulating substrate  38  by known techniques, preferably liquid precursor spinning. The flexible electrically insulating substrate  38  is preferably comprised of polyimide. The flexible electrically insulating substrate electrically insulates the traces  34 . It is also biocompatible when implanted in living tissue. The coating is about 5 um thick. The liquid precursor is spun coated over the traces  34  and the entire outside surface  49  of the hybrid substrate  44 , thereby forming the flexible electrically insulating substrate  38 . The spun coating is cured by known techniques. 
         [0075]    Step f illustrates the formation of voids in the flexible electrically insulating substrate  38  thereby revealing the traces  34 . The flexible electrically insulating substrate is preferably patterned by known techniques, such as photolithography with etching. 
         [0076]    Step g illustrates the rivets  51  having been formed over and in intimate contact with traces  34 . The rivets  51  are formed by known processes, and are preferably formed by electrochemical deposition of a biocompatible, electrically conductive material, such as platinum or platinum alloys, such as platinum-iridium. 
         [0077]    Step h illustrates formation of the metal layer  53  over the rivets  51  in a controlled pattern, preferably by photolithographic methods, on the outside surface  49 . The rivets  51  and the metal layer  53  are in intimate electrical contact. The metal layer  53  may be deposited by known techniques, such as physical vapor deposition, over the entire surface followed by photolithographic patterning, or it may be deposited by masked deposition. The metal layer  53  is formed of an electrically conductive, biocompatible material, which in a preferred embodiment is platinum. The patterned metal layer  53  forms traces  34  and electrodes  36 , which conduct electrical signals from the electronics control unit  20  and the electrodes  36  (see  FIGS. 4 and 5 ). 
         [0078]    Step i illustrates the flexible electrically insulating substrate  38  applied over the outside surface  49  of the rigid substrate  44 , as in step e. The flexible electrically insulating substrate  38  covers the rivets  51  and the metal layer  53 . 
         [0079]    Step j illustrates the hybrid substrate  44  having been cut by known means, preferably by a laser or, in an alternative embodiment, by a diamond wheel, thereby creating cut  55 . The portion of hybrid substrate  44  that will be removed is called the carrier  60 . 
         [0080]    The flexible electrically insulating substrate  38  is patterned by known methods, such as photolithographic patterning, or it may be deposited by masked deposition, to yield voids that define the electrodes  36 . The electrodes  36  transmit electrical signals directly to the retina of the implanted eye (see  FIG. 4 ) 
         [0081]    Step k illustrates flexible circuit  18  attached to the hybrid substrate  44 . The carrier  60  is removed by utilizing release coat  47 . In a preferred embodiment, release coat  47  is etched by known means to release carrier  60 , leaving behind flexible circuit  18 . 
         [0082]    Step l illustrates the implantable electronic device of a flexible circuit  18  and an intimately bonded hermetic electronics control unit  20 . The electronics control unit  20 , which contains the microelectronics assembly  48 , is hermetically sealed with header  62  bonded to rigid circuit substrate  44 . The header  62  is comprised of a material that is biocompatible when implanted in living tissue and that is capable of being hermetically sealed to protect the integrated circuit electronics from the environment. 
         [0083]      FIG. 11  illustrates an electronics control unit  320  attached to flexible electrically insulating substrate  338 , which is preferably comprised of polyimide, by bonded connections  342 . The electronics control unit  320  is preferably a hermetically sealed integrated circuit, although in an alternative embodiment it may be a hermetically sealed hybrid assembly. Bonded connections  342  are preferably conductive adhesive, although they may alternatively be solder bumps. The bond area is underfilled with an adhesive  380 . Rigid stimulating electrode array  310  is attached to the flexible electrically insulating substrate  338  by bonded connections  342 . 
         [0084]      FIG. 12  illustrates an electronics control unit  320  attached to rigid stimulating electrode array  310  by bonded connections  342 . The bond area is then underfilled with an adhesive  380 , preferably epoxy. Bonded connections  342  are preferably conductive adhesive, although they may alternatively be solder bumps. 
         [0085]    The bonding steps are illustrated in  FIG. 13  for a flex circuit assembly that is bonded with rivets  61  that are created in situ by a deposition process, preferably by electroplating. The rivets  61  are rivet-shaped electrical connections. The substrate  68  is shown generally in  FIG. 13 . It is comprised of the hybrid substrate  44 , which is preferably a ceramic, such as alumina or silicon. The silicon would preferably be coated with a biocompatible material to achieve biocompatibility of the silicon, which is well known to slowly dissolve when implanted in living tissue. 
         [0086]    The hybrid substrate  44  preferably contains vias  46  that pass through the thickness of the hybrid substrate  44 , see  FIG. 13 , step (a). Vias  46  are not required to enable this invention, and are not present in alternative embodiments. It is preferred that the hybrid substrate  44  be rigid, although alternative embodiments utilize a non-rigid substrate. The vias  46  are integral with electrically conductive routing  35  that has been placed on the surface of the hybrid substrate  44  by known techniques. The routing is preferably comprised of a stable biocompatible material, such as platinum, a platinum alloy, or gold, most preferably platinum. 
         [0087]    A flexible electrically insulating substrate  38  is preferably comprised of two layers of an electrically insulating material, such as a polymer. Known preferred polymer materials are polyimide or Parylene. Parylene refers to polyparaxylylene, a known polymer that has excellent implant characteristics. For example, Parylene, manufactured by Specialty Coating Systems (SCS), a division of Cookson Electronic Equipment Group, located in Indianapolis, Ind., is a preferred material. Parylene is available in various forms, such as Parylene C, Parylene D, and Parylene N, each having different properties. The preferred form is Parylene C. 
         [0088]    The flexible electrically insulating substrate layers  38  are preferably of approximately equal thicknesses, as illustrated in  FIG. 13 , step (a). A trace  65  is also illustrated in  FIG. 13 , step (a), where trace  65  may be at least one, but preferably more than one, trace  65  that is electrically conductive. The traces  65  are integrally bonded to bond pads  63 . The bond pads  63  each have a bond pad hole  64  therethrough, which is in approximate alignment with first hole  57  in first electrically insulating substrate  37  and second hole  59  in the second flexible electrically insulating substrates  38 , such that there is a hole, with centers approximately aligned, through the thickness of the flexible assembly  66 . 
         [0089]    The flexible assembly  66  is placed next to the hybrid substrate in preparation for bonding,  FIG. 13 , step (b). The flexible assembly aligned holes that are formed by first substrate holes  57 , bond pad holes  64 , and second substrate holes  59  are aligned with the routing  35 . In a preferred embodiment, there is at least one via  46 , although no via  46  is required. In a preferred embodiment, an adhesive layer  39  is applied to adhesively bond the assembly together. The adhesive is preferably epoxy, silicone, or polyimide. In alternative embodiments, the assembly is not adhesively bonded. 
         [0090]    As illustrated in  FIG. 13 , step (c), a rivet  61  is formed in each flexible substrate hole to bond the assembly together. The rivets  61  are preferably formed by a deposition process, most preferably electroplating. The rivets  61  are comprised of a biocompatible, electrically conductive material, preferably platinum, although alternative embodiments may utilize platinum alloys (e.g. platinum-iridium or platinum-rhodium), iridium, gold, palladium, or palladium alloys. It is most preferred that rivet  61  be comprised of electroplated platinum, called “plated platinum” herein. 
         [0091]    Referring to  FIGS. 14 and 15 , a method to produce plated platinum according to the present invention is described comprising connecting a common electrode  402 , the anode, and a bonded assembly  70 , the cathode, to a voltage to current converter  406  with a wave form generator  430  and monitor  428 , preferably an oscilloscope. The common electrode  402 , bonded assembly  70 , a reference electrode  410 , for use as a reference in controlling the power source, which is comprised of a voltage to current converter  406  and a waveform generator  430 , and an electroplating solution are placed in a electroplating cell  400  having a means for mixing  414  the electroplating solution. Power may be supplied to the electrodes with constant voltage, constant current, pulsed voltage, scanned voltage or pulsed current to drive the electroplating process. The waveform generator  430  and voltage to current converter  406  is set such that the rate of deposition will cause the platinum to deposit as plated platinum of the present invention, the rate being greater than the deposition rate necessary to form shiny platinum and less than the deposition rate necessary to form platinum black. 
         [0092]    Because no impurities or other additives, such as lead, which is a neurotoxin and cannot be used in an implantable device, need to be introduced during the plating process to produce plated platinum of the present invention, the plated material can be pure platinum. Alternatively, other materials can be introduced during the plating process, if so desired, but these materials are not necessary to the formation of plated platinum of the present invention. 
         [0093]    Referring to  FIGS. 14 and 15 , the electroplating cell  400 , is preferably a 50 ml to 150 ml four neck glass flask or beaker, the common electrode  402 , or anode, is preferably a large surface area platinum wire or platinum sheet, the reference electrode  410  is preferably a Ag/AgCl electrode (silver, silver chloride electrode), the bonded assembly  70 , or cathode, can be any suitable material depending on the application and can be readily chosen by one skilled in the art. Preferable examples of the bonded assembly  70  include, but are not limited to, platinum, iridium, rhodium, gold, tantalum, titanium or niobium, preferably platinum. 
         [0094]    The means for mixing  414  is preferably a magnetic stirrer ( FIG. 15 ). The plating solution is preferably 3 to 30 millimoles ammonium hexachloroplatinate in 0.4 moles of disodium hydrogen phosphate, but may be derived from any chloroplatinic acid or bromoplatinic acid or other electroplating solution. The preferable plating temperature is approximately 24°-26° C. 
         [0095]    The electroplating system for pulsed current control is shown in  FIGS. 14 and 15 . While constant voltage, constant current, pulsed voltage or pulsed current can be used to control the electroplating process, pulsed current control of the plating process is preferable for plating rivets  61 , which have a height that approximates their diameter. The preferable current range to produce plated platinum of the present invention, which varies from about 50 to 2000 mA/cm 2 , is dependent on the hole dimensions,  FIG. 16 , where the response voltage ranges from about −0.45 volts to −0.85 volts. Applying power in this range with the above solution yields a plating rate in the range of about 0.05 um per minute to 1.0 um per minute, the preferred range for the plating rate of plated platinum of the present invention. The average current density may be determined by the equation y=19572x −1.46 , where y is the average current density in mA/cm 2  and x is the hole diameter in microns. Pulsed current control also allows an array of rivets to be plated simultaneously achieving uniform rivet properties. 
         [0096]    As plating conditions, including but not limited to the plating solution, surface area of the electrodes, pH, platinum concentration and the presence of additives, are changed the optimal control parameters will change according to basic electroplating principles. Plated platinum of the present invention will be formed so long as the rate of deposition of the platinum particles is slower than that for the formation of platinum gray and faster than that for the formation of shiny platinum. 
         [0097]    It has been found that because of the physical strength of plated platinum of the present invention, it is possible to plate rivets of thickness greater than 30 microns. It is very difficult to plate shiny platinum in layers greater than approximately two microns because the internal stresses of the dense platinum layer cause the plated layer to peel off. 
         [0098]    Plated platinum of the present invention can be distinguished from two other forms of electroplated platinum, specifically, platinum gray and platinum black, based on the adhesive strength of a thin film coating of these materials. Adhesive strength of thin film coatings of platinum gray and platinum black on electrically conductive articles has been measured on a Micro-Scratch Tester (CSEM Instruments, Switzerland). A 400 micron micro-scratch is formed by drawing a diamond tip, having a 10 micron spherical radius, across the coating under a load that is increased in a controlled manner from 1 millinewton to 100 millinewtons. At the “critical load”, the coating fails. Using this test, it is found that plated platinum of the present invention fails at an estimated critical load of about 70 millinewtons, platinum gray fails at a measured critical load of 61 millinewtons, while platinum black fails at a measured critical load of less than about 35 millinewtons. 
         [0099]    Likewise, the several forms of platinum are distinguished by their hardness. For example, the microhardness, as measured with a Vickers indenter, is estimated to be about 30 for plated platinum of the present invention, while it is estimated to be less than one for platinum black, it is measured to be 17 for platinum gray, and is estimated to be about 34 for shiny platinum, and it is reported to be 40 for platinum foil, and 100 for cold worked platinum (Materials Engineering, Materials Selector 1990, Penton Publishing, 1989, p 122). On a hybrid substrate  44 , a thin-layer routing  35 , preferably platinum, is sputtered and then covered with about 6 um thick flexible assembly  66 , preferably polyimide, with holes in the range from 5 um to 50 um. On each sample, preferably about 100 to 700 or more such holes are exposed for plating of rivets  61 , see  FIG. 17   a.    
         [0100]    SEM micrographs record the surface appearance before plating. The surface is chemically and electrochemically cleaned before plating. 
         [0101]    The electrodes in the test cell are arranged, so that the bonded assembly  70  (cathode) is physically parallel with the common electrode  402  (anode). The reference electrode  410  is positioned beside the bonded assembly  70 . The plating solution is added to electroplating solution level  411 . The solution is comprised of about 18 millimoles ammonium hexachloroplatinate in about 0.4 moles phosphate buffer solution. The amount of solution used depends on the number of rivets  61  to be plated. The means for mixing  414 , preferably a magnetic stirrer, is activated. 
         [0102]    A voltage waveform is generated, preferably with a 1 msec pulse width as a 500 Hz square wave, which is converted to a current signal through a voltage to current converter  406 . 
         [0103]    The pulse current is applied to the plating electrode versus anode. The electrode voltage versus Ag/AgCl reference electrode is monitored using an oscilloscope (Tektronix TDS220 Oscilloscope). The current amplitude is adjusted so that the cathodic peak voltage reaches about −0.6 v versus the Ag/AgCl reference electrode  410 . During plating, the electrode voltage tends to decrease with plating time. The current amplitude is frequently adjusted so that the electrode voltage is kept within −0.5 to −0.7 v range versus Ag/AgCl reference electrode  410 . When the specified plating time is reached, the current is eliminated. The cathode is rinsed in deionized water thoroughly. Typical plating time is in the range of about 5 to 60 minutes, preferably 15 to 25 minutes. 
         [0104]    The plated surface is examined under an optical microscope. Optical photomicrographs are taken at both low and high magnifications to record the image of the surface. The plated samples are profiled with a surface profilometer to measure the dimensions of the plated rivet. The total plated rivet has a total height of about 8 to 16 um. 
         [0105]    After plating, the pulsing current amplitudes are averaged for the total plating time and recorded. It has been demonstrated that the current density increases exponentially with sample hole decrease. The smaller the sample holes, the higher the current density required (see  FIG. 16 ). 
         [0106]    An illustrative example of a plated platinum rivet according to the present invention are micrographs produced on a Scanning Electron Microscope (SEM) at 850× taken by a JEOL JSM5910 microscope,  FIGS. 17   a  and  17   b.    
         [0107]    Furthermore, it has been found that because of the physical strength of plated platinum of the present invention, it is possible to plate rivets  61  of thickness greater than 16 um. It is very difficult to plate shiny platinum in layers greater than approximately 1 to 5 um because the internal stress of the dense platinum layer which will cause plated layer to peel off. 
         [0108]    The following example is illustrative of electroplating platinum as a rivet  61 , according to the present invention. 
       Example 
       [0109]    A flexible electrically insulating substrate comprised of a first substrate  37  and a second substrate  38  of polyimide having a total thickness of 6 um. It had 700 first substrate holes  57 , an equal number of matching bond pad holes  64 , and an equal number of matching second substrate holes  59 , all in alignment so as to create a continuous hole through flexible assembly  66  that terminates on routing  35 , arranged in 100 groups of seven on about 40 um centers,  FIG. 4   a . The hybrid substrate  44  was alumina and the routing  35  was platinum. The bond pad  63  was platinum. 
         [0110]    The assembly was cleaned by rinsing three times in 10% HCl. It was further prepared by bubbling for 10 seconds at +/−5V at 1 Hz in phosphate buffered saline. Finally, it was rinsed in deionized water. 
         [0111]    The electroplating set up according to  FIGS. 14 and 15  was comprised of an electroplating cell  400  that was a 100 ml beaker with an electroplating solution level  411  at about the 75 ml level. The solution was 18 millimoles of ammonium hexachloroplatinate in 0.4 moles phosphate buffer solution. 
         [0112]    The means for mixing  414  was a magnetic stirrer, which was activated. The voltage waveform of 1 msec pulse width as a square wave, was generated by an HP 33120A waveform generator, which is converted to current signal through a voltage to current converter  406 . The pulse current was 1 msec in pulse width at 500 Hz square wave. 
         [0113]    The pulse current was applied on the plating electrode bonded assembly  70  versus common electrode  402 . The electrode voltage versus Ag/AgCl reference electrode  410  was monitored using as a monitor  428  a Tektronix model TDS220 oscilloscope. The current amplitude was increased so that the bonded assembly  70  (cathode) peak voltage reached −0.6 v versus the Ag/AgCl reference electrode  410 . During plating, the electrode voltage decreased with plating time. 
         [0114]    The average current density was 660 mA/cm 2 , which generated response voltages of −0.5 to −0.7 volts, where the voltage was controlled by the current. A 1 msec pulse width square wave was generated by an HP 33120A Arbitrary Waveform Generator. The pulse was converted to a current signal through a voltage to current converter  406 . The pulse current was typically about 1 msec in pulse width as a 500 Hz square wave. The resulting plated platinum rivet  61  was about 32 um diameter on the button end and about 15 um tall, with about 9 um of the height extending above the polyimide substrate. The plated platinum rivet was dense, strong, and electrically conductive. 
         [0115]    Scanning Electron Microscope (SEM)/energy dispersive analysis (EDAX™) analysis were performed on the rivets  61 . SEM micrographs of the plated surface were taken showing its as-plated surface,  FIG. 17   b . Energy dispersed analysis demonstrated that the rivet  61  was pure platinum, with no detectable oxygen. 
         [0116]    The above described is the preferred embodiment of the current invention, however the platinum electrodeposition described in co-pending application “Platinum Electrode and Method for Manufacturing the Same,” Attorney Docket No. S176-USA, filed on Aug. 23, 2002 and incorporated herein by reference, is also effective for forming electrochemically deposited rivets. 
         [0117]    The rivet  61  ( FIG. 13 ) forms an electrically conductive bond with the routing  35  and with the bond pad  63 . It is obvious that the bonded assembly may be stacked with other bonded assemblies forming multiple stacked assemblies with increased stacking density. 
         [0118]    Accordingly, what has been shown is an improved flexible circuit with an electronics control unit attached thereto, which is suitable for implantation in living tissue and to transmit electrical impulses to the living tissue. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.