Patent Publication Number: US-6660116-B2

Title: Capacitive filtered feedthrough array for an implantable medical device

Description:
This application is a divisional of application Ser. No. 09/515,385, filed Mar. 1, 2000, now U.S. Pat. No. 6,414,835. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to electrical feedthroughs of improved design and to their method of fabrication, particularly for use with implantable medical devices. 
     BACKGROUND OF THE INVENTION 
     Electrical feedthroughs serve the purpose of providing an electrical circuit path extending from the interior of a hermetically sealed case or housing to an external point outside the case. Implantable medical devices (IMDs) such as implantable pulse generators (IPGs) for cardiac pacemakers, implantable cardioverter/defibrillators (ICDs), nerve, brain, organ and muscle stimulators and implantable monitors, or the like, employ such electrical feedthroughs through their case to make electrical connections with leads, electrodes and sensors located outside the case. 
     Such feedthroughs typically include a ferrule adapted to fit within an opening in the case, one or more conductor and a non-conductive hermetic glass or ceramic seal which supports and electrically isolates each such conductor from the other conductors passing through it and from the ferrule. The IMD case is typically formed of a biocompatible metal, e.g., titanium, although non-conductive ceramics materials have been proposed for forming the case. The ferrule is typically of a metal that can be welded or otherwise adhered to the case in a hermetically sealed manner. 
     Typically, single pin feedthroughs supported by glass, sapphire and ceramic were used with the first hermetically sealed IMD cases for IPGs. As time has passed, the IPG case size has dramatically reduced and the number of external leads, electrodes and sensors that are to be coupled with the circuitry of the IPG has increased. Consequently, use of the relatively large single pin feedthroughs is no longer feasible, and numerous multiple conductor feedthroughs have been used or proposed for use that fit within the smaller sized case opening and provide two, three, four or more conductors. 
     Many different insulator structures and conductor structures are known in the art of multiple conductor feedthroughs wherein the insulator structure also provides a hermetic seal to prevent entry of body fluids through the feedthrough and into the housing of the medical device. The conductors typically comprise electrical wires or pins that extend through a glass and/or ceramic layer within a metal ferrule opening as shown, for example, in commonly assigned U.S. Pat. Nos. 4,991,582, 5,782,891, and 5,866,851 or through a ceramic case as shown in the commonly assigned &#39;891 patent and in U.S. Pat. No. 5,470,345. It has also been proposed to use co-fired ceramic layer substrates that are provided with conductive paths formed of traces and vias as disclosed, for example, in U.S. Pat. Nos. 4,420,652, 5,434,358, 5,782,891, 5,620,476, 5,683,435, 5,750,926, and 5,973,906. 
     Such multi-conductor feedthroughs have an internally disposed portion configured to be disposed inside the case for connection with electrical circuitry and an externally disposed portion configured to be disposed outside the case that is typically coupled electrically with connector elements for making connection with the leads, electrodes or sensors. The elongated lead conductors extending from the connector elements effectively act as antennae that tend to collect stray electromagnetic interference (EMI) signals that may interfere with normal IMD operations. At certain frequencies, for example, EMI can be mistaken for telemetry signals and cause an IPG to change operating mode. 
     This problem has been addressed in certain of the above-referenced patents by incorporating a capacitor structure upon the internally facing portion of the feedthrough ferrule coupled between each feedthrough conductor and a common ground, the ferrule, to filter out any high frequency EMI transmitted from the external lead conductor through the feedthrough conductor. The feedthrough capacitors originally were discrete capacitors but presently can take the form of chip capacitors that are mounted as shown in the above-referenced &#39;891, &#39;435, &#39;476, and &#39;906 patents and in further U.S. Pat. Nos. 5,650,759, 5,896,267 and 5,959,829, for example. Or the feedthrough capacitors can take the form of discrete discoidal capacitive filters or discoidal capacitive filter arrays as shown in commonly assigned U.S. Pat. Nos. 5,735,884, 5,759,197, 5,836,992, 5,867,361, and 5,870,272 and further U.S. Pat. Nos. 5,287,076, 5,333,095, 5,905,627 and 5,999,398. 
     These patents disclose use of discoidal filters and filter arrays in association with conductive pins which are of relatively large scale and difficult to miniaturize without complicating manufacture. It is desirable to further miniaturize and simplify the fabrication of the multi-conductor feedthrough assembly 
     Although feedthrough filter capacitor assemblies of the type described above have performed in a generally satisfactory manner, the manufacture and installation of such filter capacitor assemblies has been relatively time consuming and therefore costly. For example, installation of the discoidal capacitor into the small annular space between the terminal pin and ferrule as shown in a number of these patents can be a difficult and complex multi-step procedure to ensure formation of reliable, high quality electrical connections. 
     Other problems have arisen when chip capacitors have been coupled to conductive trace and via pathways of co-fired multi-layer metal-ceramic substrates disclosed in the referenced &#39;652, &#39;358, &#39;891, &#39;476, &#39;435, &#39;926, and &#39;906 patents. The conductive paths of the feedthrough arrays and attached capacitors suffer from high inductance which has the effect of failing to attenuate EMI and other unwanted signals, characterized as “poor insertion loss”. 
     A high integrity hermetic seal for medical implant applications is very critical to prevent the ingress of body fluids into the IMD. Even a small leak rate of such body fluid penetration can, over a period of many years, build up and damage sensitive internal electronic components. This can cause catastrophic failure of the implanted device. The hermetic seal for medical implant (as well as space and military) applications is typically constructed of highly stable alumina ceramic or glass materials with very low bulk permeability. The above-described feedthroughs formed using metal-ceramic co-fired substrates, however, have not been hermetic because the metal component of the substrate corrodes in body fluids, and the substrates have cracked from stresses that developed from brazing and welding processes. 
     Withstanding the high temperature and thermal stresses associated with the welding of a hermetically sealed terminal with a premounted ceramic feedthrough capacitor is very difficult to achieve with the &#39;551, &#39;095 and other prior art designs. The electrical/mechanical connection to the outside perimeter or outside diameter of the feedthrough capacitor has a very high thermal conductivity as compared to air. The welding operation typically employed in the medical implant industry to install the filtered hermetic terminal into the IMD case opening can involve a welding operation in very close proximity to this electrical/mechanical connection area. Accordingly, in the prior art, the ceramic feedthrough capacitors are subjected to a dramatic temperature rise. This temperature rise produces mechanical stress in the capacitor due to the mismatch in thermal coefficients of expansion of the surrounding materials. 
     In addition, in the prior art, the capacitor lead connections must be of very high temperature materials to withstand the high peak temperatures reached during the welding operation (as much as 500 C. °). A similar, but less severe, situation is applicable in military, space and commercial applications where similar prior art devices are soldered instead of welded by the user into a bulkhead or substrate. Many of these prior art devices employ a soldered connection to the outside perimeter or outside diameter of the feedthrough capacitor. Excessive and unevenly applied soldering heat has been known to damage such prior art devices. Accordingly, there is a need for a filter capacitor and feedthrough array in a single assembly that addresses the drawbacks noted above in connection with the prior art. 
     In particular, a capacitive filtered feedthrough array is needed that is subjected to far less temperature rise during the manufacture thereof. Moreover, such an improvement would make the assembly relatively immune to the aforementioned stressful installation techniques. 
     Moreover, a capacitive filtered feedthrough array is needed which is of simplified construction, utilizing a straightforward and uncomplicated assembly, that can result in manufacturing cost reductions. Of course the new design must be capable of effectively filtering out undesirable EMI. The present invention fulfills these needs and provides other related advantages. 
     SUMMARY OF THE INVENTION 
     A capacitive filtered feedthrough assembly is formed in accordance with the present invention in a solid state manner to employ highly miniaturized conductive paths each filtered by a discoid capacitive filter embedded in a capacitive filter array. A non-conductive, co-fired metal-ceramic substrate is formed from multiple layers that supports one or a plurality of substrate conductive paths and it is brazed to a conductive ferrule, adapted to be welded to a case, using a conductive, corrosion resistant braze material. The metal-ceramic substrate is attached to an internally disposed capacitive filter array that encloses one or a plurality of capacitive filter capacitor active electrodes each coupled to a filter array conductive path and at least one capacitor ground electrode. Each capacitive filter array conductive path is joined with a metal-ceramic conductive path to form a feedthrough conductive path. Bonding pads are attached to the internally disposed ends of each feedthrough conductive path, and corrosion resistant, conductive buttons are attached to and seal the externally disposed ends of each feedthrough conductive path. Each capacitor ground electrode is electrically coupled with the ferrule. 
     Preferably, a plurality of such feedthrough conductive paths are formed, and each capacitive filter comprises a plurality of capacitor active and ground electrodes, wherein the capacitor ground electrodes are electrically connected in common. 
     Moreover, preferably, a plurality of conductive, substrate ground paths are formed extending through the co-fired metal-ceramic substrate between internally and externally facing layer surfaces thereof and electrically isolated from the substrate conductive paths. The capacitor ground electrodes are coupled electrically to the plurality of conductive, substrate ground paths and to the ferrule. 
     In addition, preferably, the capacitive filter array conductive paths are formed by solder filling holes extending through the filter array substrate between internally and externally facing array surfaces thereof. The application of the solder also joins the externally facing array surface with the internally facing metal-ceramic substrate layer surface and electrically joins the capacitive filter array conductive paths with the metal-ceramic conductive paths to form the feedthrough conductive paths. 
     Utilization of an internally grounded, metal-ceramic substrate providing a plurality of conductive substrate paths in stacked, aligned, relation to a capacitive filter array as disclosed herein provides a number of advantages: 
     A hermetic seal is achieved by brazing a co-fired metal-ceramic substrate with low permeability to a metallic ferrule. The inventive ferrule-substrate braze joint design minimizes the tensile stresses in the co-fired substrate, thus preventing cracking of the co-fired substrate during brazing and welding. In addition, the ferrule has a thin flange which minimizes stress applied to the co-fired substrate during welding. Corrosion of the co-fired metal phase of the substrate is prevented by protecting the exposed metal vias and pads with corrosion resistant metallizations and braze materials. 
     Because the capacitive filter array is displaced from the ferrule and supported by the metal-ceramic substrate, the heat imparted to the ferrule flange during welding causes minimal temperature elevation of the capacitive filter array, and does not cause damage to it. 
     The attachment of the conductive paths of the outward facing capacitive filter surface to the metallized layers of the inward facing surface of the metal-ceramic substrate using reflow soldering provides secure attachment and low resistance electrical connection and simplifies manufacturing. The use of conductive epoxy compounds for adhesion is thereby avoided. Conductive epoxy adhesion layers can bridge the non-conductive ceramic between adjacent conductive paths and cause electrical shorts. And voids can occur in bridging the conductive paths of the metal-ceramic substrate and the capacitive filter elements. 
     The reflow soldering attachment of the of the conductive paths of the outward facing capacitive filter surface to the metallized layers of the inward facing surface of the metal-ceramic substrate also is advantageous in that the solder flow takes place in an oven under uniformly applied temperature to the entire assembly, thereby avoiding damage that can be caused in hand soldering such parts together. 
     The capacitor ground electrodes of the discoidal capacitors of the capacitive filter array are electrically coupled together and through the plurality of substrate ground paths of the metal-ceramic substrate and then through the braze to the ferrule. The plurality of substrate ground paths are selected in total cross-section area to provide a total ground via cross-section area that minimizes the inductance of the filtered feedthrough assembly, resulting in favorable insertion loss of EMI and unwanted signals. 
     Size of the feedthrough is decreased by eliminating the pins, the pin braze joints, and the welds between the pins. The pin-to-pin spacing of two single pin or unipolar feedthroughs is typically on the order of 0.125 inches. The above-described capacitive filtered feedthrough array provides a spacing of 0.050 inches between adjacent conductive paths. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other advantages and features of the present invention will be appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiment of the invention when considered in connection with the accompanying drawings, in which like numbered reference numbers designate like parts throughout the figures thereof, and wherein: 
     FIG. 1 is a perspective view of the filtered feedthrough assembly of the present invention adapted to be fitted into an opening of a case of a hermetically sealed electronic device showing the externally disposed portion configured to be disposed outside and face outwardly from the case; 
     FIG. 2 is a perspective view of the filtered feedthrough assembly of the present invention adapted to be fitted into an opening of a case of a hermetically sealed electronic device showing the internally disposed portion configured to be disposed inside the case and face inward; 
     FIG. 3 is a plan view looking toward the internally disposed portion of the filtered feedthrough assembly of the present invention; 
     FIG. 4 is a cross-section side view of the filtered feedthrough assembly taken along lines  4 — 4  of FIG. 3; 
     FIG. 5 is an expanded end portion of the cross-section view of FIG. 4 
     FIG. 6 is a cross-section end view of the filtered feedthrough assembly taken along lines  6 — 6  of FIG. 3; 
     FIG. 7 is an exploded view of the components of the filtered feedthrough assembly of FIGS. 1-6; 
     FIG. 8 is a perspective view of the filtered feedthrough assembly of the present invention fitted into an opening of a half portion of the case of a hermetically sealed electronic device showing the externally disposed portion outside the case; 
     FIG. 9 is a perspective view of the filtered feedthrough assembly of the present invention fitted into the opening of the case half portion of FIG. 7 showing the internally disposed portion inside the case and electrically connected to an electrical component; 
     FIG. 10 is a flow chart illustrating the steps of fabricating the multi-layer, co-fired metal-ceramic substrate adapted to be brazed with the capacitive filter array formed in the steps of FIG. 11, the ferrule, and other components in the steps of FIG. 12; 
     FIG. 11 is a flow chart illustrating the steps of fabricating the capacitive filter array adapted to be brazed with the co-fired metal-ceramic substrate formed in the steps of FIG. 10, the ferrule, and other components in the steps of FIG. 12; and 
     FIG. 12 is a flow chart illustrating the steps of fabricating the filtered feedthrough assembly from the capacitive filter array formed in the steps of FIG. 11, the co-fired metal-ceramic substrate formed in the steps of FIG. 10, the ferrule, and other components. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     FIGS. 1-7 depict the filtered feedthrough assembly  10  adapted to be fitted into an opening  204  of a case  202  of a hermetically sealed electronic device  200  as shown in FIGS. 8 and 9 and manufactured in accordance with the flow chart steps of FIGS. 10-12. The feedthrough assembly  10  has an internally disposed portion  12  configured to be disposed inside the case  202  and an externally disposed portion  14  configured to be disposed outside the case  202 . 
     The filtered feedthrough assembly  10  shown in FIGS. 1-9 comprises a electrically conductive ferrule  20  having a ferrule wall  22  with an inner wall surface  24  defining a centrally disposed ferrule opening  30  and extending between opposed internal and external sides  26  and  28 . When ferrule  20  is fitted into the case opening  204 , the internally facing side  26  is adapted face toward the inside of the case  202 , and the externally facing side  28  is adapted face toward the exterior of the case  202 . The electrically conductive ferrule  20  further comprises a relatively thin welding flange  32  extending outwardly of the ferrule wall  22  away from the ferrule opening  30  for a predetermined distance defining a flange width FW. The flange  32  is formed to have a relatively thin flange thickness FT for absorbing stress caused by thermal welding energy applied to the ferrule  20  in the process of welding the flange  32  to the case  202  around the case opening  204  as shown in FIGS. 8 and 9. 
     The ferrule  20  is preferably formed of a conductive material selected from the group consisting of niobium, titanium, titanium alloys such as titanium-6Al-4V or titanium-vanadium, platinum, molybdenum, zirconium, tantalum, vanadium, tungsten, iridium, rhodium, rhenium, osmium, ruthenium, palladium, silver, and alloys, mixtures and combinations thereof. Niobium is the optimal material for forming the ferrule  20  because it has a coefficient of thermal expansion (CTE) that is compatible with the CTE of the substrate  40  so that heat-induced during brazing of the metal-ceramic substrate edge to the ferrule inner wall surface  24  does not damage the substrate  40 . 
     The multi-layer, co-fired metal-ceramic substrate  40  shown in detail in FIGS. 5 and 7 has an internally facing major surface or side  42  and an externally facing major surface or side  44  that are joined by a common substrate edge  46 . The substrate  40  is dimensioned and shaped to fit within the ferrule opening  30  with the common substrate edge  46  in close relation to the ferrule inner wall surface  24 . The common substrate edge  46  is brazed to the ferrule inner wall surface  24  using a substrate-ferrule braze joint  48 . 
     The metal-ceramic substrate  40  is formed of a plurality of planar ceramic layers  52 ,  54 ,  56 ,  58  and  60 . Each ceramic layer is shaped in a green state to have a layer thickness and a plurality of via holes extending therethrough between an internally facing layer surface and an externally facing layer surface. The co-fired metal-ceramic substrate ceramic material comprises one of the group consisting essentially of alumina, aluminum nitride, beryllium oxide, silicon dioxide, and glass-ceramic materials that has a CTE compatible with the CTE of the material of the ferrule. 
     A plurality (nine in the depicted example) of conductive paths, e.g. path  50  shown in FIGS. 5 and 6, extend through the layers  52 - 60  of co-fired metal-ceramic substrate  40  and are electrically isolated from one another by the ceramic material. Conductive path  50  (and all the other conductive paths) comprises a plurality of electrically conductive vias  62 ,  64 ,  66 ,  68  extending through the plurality of layer thicknesses and a like plurality of electrically conductive traces  72 ,  74  and  76  formed on certain of the internally or externally facing layer surfaces such that the conductive trace  72  joins the conductive vias  62  and  64 , the conductive trace  74  joins the conductive vias  64  and  66 , and the conductive trace  76  joins the vias  66  and  68  to form the conductive path  50 . The layer holes and vias  62 - 68  filling them are staggered in the elongated direction of the feedthrough assembly  10  as shown in FIGS. 4 and 5 but are aligned in the narrow direction as shown in FIG.  6 . The conductive vias and traces are formed of a refractory metal, e.g., tungsten, as described further in reference to FIG.  10 . 
     A further plurality (twenty in the depicted example) of ground paths  118  each comprising substrate ground paths  118  extending through all layers  52 - 60  spaced apart around the periphery of the co-fired metal-ceramic substrate  40 . The ground paths  118  also comprise one or more ground trace, e.g. ground plane traces or layers  132 ,  134  and  136  shown in FIG. 5, extending peripherally along the substrate layer surfaces from the substrate ground paths  118  to the substrate edge  46 . The ground trace  132  assists in making electrical contact with the ground solder joint  130  and with the substrate-ferrule braze joint  48 . The ground traces  134  and  136  extend to a metallization layer  140  formed over the substrate edge  46 . The number of substrate ground paths  118  substrate ground paths  118  formed in this manner is selected to provide a total ground via cross-section area that minimizes the inductance of the filtered feedthrough assembly  10  resulting in favorable insertion loss of EMI and unwanted signals. 
     Each such conductive path  50  extends all the way through the substrate  40  between the internally facing side  42  and the externally facing side  44 . On the externally facing side  44 , ceramic layer  60  is formed in the green state with a plurality of button cavities  80  each aligned with a via  68  of layer  58 . A substrate conductor pad or button  70  is fitted within each button cavity  80  of the layer  60  and adhered to via  68  by a button braze joint  78  formed of gold or a nickel-gold alloy. The pads or bonding buttons  70  are preferably formed of a conductive material selected from the group consisting of niobium, platinum or a platinum-iridium alloy, titanium, titanium alloys such as titanium-6Al-4V or titanium-vanadium, molybdenum, zirconium, tantalum, vanadium, tungsten, iridium, rhodium, rhenium, osmium, ruthenium, palladium, silver, and alloys, mixtures and combinations thereof. In this way, a plurality of externally disposed bonding buttons  70  are supported along the externally disposed feedthrough portion  14 , and each externally disposed bonding button  70  is electrically conducted with an electrically conductive path  50  of the metal-ceramic substrate  40 . 
     A substrate conductor plated pad  82  is formed of gold or a gold alloy on the internally facing surface of layer  52  and electrically coupled to via  62 . A solder layer  128  adheres to plated pad  82  during assembly of the feedthrough assembly  10  described below with reference to FIG.  12 . The plurality of ceramic layers are shaped punched with holes, and printed with the traces and vias and assembled together in the ceramic green state, and the assembly is then co-fired from the green state to form the substrate  40  as further described below in reference to FIG.  10 . 
     The discoidal capacitive filter array  90  is formed of a ceramic capacitive filter array substrate  92  having an internally facing filter substrate side  94  and an externally facing filter substrate side  96  joined by a common filter substrate edge  98 . The discoidal capacitor filter array  90  is formed with a plurality of discoidal capacitive filters, e.g., capacitive filter  100 , that are electrically connected with a respective one of the substrate conductive paths, e.g., conductive path  50 , and provide a filtered electrically conductive path between the internally disposed bonding pad  120  and the externally disposed button  70 . The number of conductive paths so formed can vary from the nine that are depicted in FIGS. 1-9. 
     The capacitive filter array substrate  92  is preferably formed of layers of barium titanate and precious metal traces in the manner described below with reference to FIG.  11 . The plurality of capacitive filters  100  and the filter array conductive paths  110  associated therewith are formed in and electrically isolated from one another by the ceramic material and extend between the internally facing filter substrate side  94  and the externally facing filter substrate side  96 . Each filter array conductive path  110  is formed of melted solder pre-forms as described below that fill a respective capacitor filter hole  108  extending between the internally facing filter substrate side  94  and an externally facing filter substrate side  96 . 
     Each discoidal capacitive filter  100  comprises at least one capacitor electrode formed within the filter substrate and extending outward from a filter array conductive path  110  in overlapping spaced relation to at least one common ground plate. The number and dielectric thickness spacing of the capacitor electrode sets varies in accordance with the capacitance value and the voltage rating of the discoidal capacitor. The capacitor active and ground electrodes are formed of silver thick films, silver-palladium alloy thick films, or silver-platinum alloy thick films disposed on inner capacitive filter layer surfaces during the fabrication of the capacitive filter array  90 . The number of capacitor active and ground electrodes, the sizes of each and the spacing and overlapping relation can be varied for each discoidal capacitive filter  100  within the capacitive filter array  90  and between differing models of such capacitive filter arrays  90  to tailor the filter characteristics to the circuitry of the particular IMD. In the depicted example, the capacitive filters  100  either have three capacitor active electrodes  112 ,  114 ,  116  or two capacitor active electrodes  122 ,  124  that are spaced from three ground electrodes  102 ,  104 ,  106  that extend inward from the filter substrate edge  98 . In operation, the discoidal capacitor permits passage of relatively low frequency electrical signals along the conductive path it is coupled with, while shielding and decoupling/attenuating undesired interference signals of typically high frequency. 
     The ground solder joint  130 , preferably formed of solder or a conductive epoxy, adheres against a metallized layer  111  formed on the filter substrate edge  98  as described below in reference to FIG. 12 that electrically connects the three capacitor ground electrodes  102 ,  104 ,  106  together. The ground solder joint  130  also electrically connects the three capacitor ground electrodes  102 ,  104 ,  106  to the ferrule  20  through the conductive ground trace  132 , the plurality of substrate ground paths  118 , and the substrate-ferrule braze joint  48 . The ground solder joint  130  can be formed of ABLEBOND.RTM.8700 electrically conductive silver-filled epoxy adhesive provided by ABLESTIK LABORATORIES of Rancho Dominguez, Calif. Other suitable electrically conductive glue or epoxy-based adhesives and other suitable materials may also be employed in the present invention to form the ground solder joint  130 . Such materials include gold or copper-filled epoxies, carbon or graphite-filled epoxies or even electrically conductive plastics acting effectively as adhesive joints after their application and upon cooling, such as at least some of the electrically conductive plastics or polymers disclosed in U.S. Pat. No. 5,685,632. The ground solder joint  130  and the solder layers  128  mechanically join the externally facing filter substrate side to the internally facing substrate side. The solder layers  128  electrically join each filter array conductive path  110  to a substrate conductor pad  82  of each substrate conductive path  50 . 
     The substrate-ferrule braze joint  48  is preferably formed of 99.9% or purer gold or a nickel-gold alloy that adheres to the metallization layer  140  on substrate edge  46  and to the ferrule wall  22  and provides a hermetic seal of the ferrule  20  with the metal-ceramic substrate  40 . The substrate-ferrule braze joint  48  may also be formed of: (a) gold alloys comprising gold and at least one of titanium, niobium, vanadium, nickel, molybdenum, platinum, palladium, ruthenium, silver, rhodium, osmium, indium, and alloys, mixtures and thereof; (b) copper-silver alloys, including copper-silver eutectic alloys, comprising copper and silver and optionally at least one of indium, titanium, tin, gallium, palladium, platinum, and alloys, mixtures and combinations thereof; and (c) silver-palladium-gallium alloys. 
     The filtered feedthrough assembly  10  thus provides a plurality of miniaturized, electrically isolated, and capacitively filtered, electrical conductors formed of conductive path  50  and  110  extending between a respective internal bonding pad  120  of the internally disposed portion  12  and bonding button  70  of the externally disposed portion  14  when the feedthrough assembly  10  is affixed into an opening  204  in the case  202  of the electronic device, e.g., the IMD  200  of FIGS. 8 and 9. The case  202  for an IMD is preferably fabricated of titanium, and the ferrule flange  32  is welded thereto. The ferrule  20  is preferably formed of niobium because niobium has a comparable CTE to the CTE of AlO 2  which is a preferred substrate ceramic material. However, the ferrule may be formed of titanium, titanium alloys such as titanium-6Al-4V or titanium-vanadium, platinum, molybdenum, zirconium, tantalum, vanadium, tungsten, iridium, rhodium, rhenium, osmium, ruthenium, palladium, silver, and alloys, mixtures and combinations thereof. 
     FIG. 10 is a flow chart illustrating the steps of fabricating the multi-layer, co-fired metal-ceramic substrate  40  adapted to be brazed with the capacitive filter array  90  formed in the steps of FIG. 11, the ferrule  20 , and other components in the steps of FIG.  12 . In step S 100  the ceramic layers  52 - 60  are preferably tape cast from conventional ceramic or low temperature co-fired ceramic, such as alumina, aluminum nitride, beryllium oxide, silicon dioxide, etc., that has a CTE compatible with the CTE of the material of the ferrule  20 . Preferably, 88%-96% pure alumina (AlO 2 ) is tape cast using conventional “green sheet” techniques on glass-ceramic or MYLAR support materials. In general, such techniques start with a ceramic slurry formed by mixing a ceramic particulate, a thermoplastic polymer and solvents. This slurry is spread into ceramic sheets of predetermined thickness, typically about 0.006-0.010 inches thick, from which the solvents are volitized, leaving self-supporting flexible green sheets. 
     In step S 102 , the holes that will be filled with conductive material to form the vias  62 - 68  of each conductive path  50  and the aligned ground vias, as well as the button cavities  80  of the layer  60  are made, using any conventional technique, such as drilling, punching, laser cutting, etc., through each of the green sheets from which the ceramic layers  52 - 60  are formed. The vias  42  may have a size appropriate for the path spacing, with about a 0.004 inch diameter hole being appropriate for 0.020 inch center to center path spacing. 
     In step S 104 , the via holes are filled with a paste of refractory metal, e.g., tungsten, molybdenum, or tantalum paste, preferably using screen printing. In step S 104 , the conductive traces, e.g. traces  72 ,  74 ,  76 , are also applied to particular surface areas of the ceramic layers  52 - 60  over the vias. The traces may comprise an electrical conductor, such as copper, aluminum, or a refractory metal paste, that may be deposited on the green sheets using conventional techniques. The traces may be deposited, sprayed, screened, dipped, plated, etc. onto the green sheets. The traces may have a center to center spacing as small as about 0.020 inch (smaller spacing may be achievable as trace forming technology advances) so that a conductive path density of associated vias and traces of up to 50 or more paths per inch may be achieved. 
     In these ways, the via holes are filled and the conductive traces are applied to the green sheets before they are stacked and laminated in step S 106  using a mechanical or hydraulic press for firing. The stacked and laminated ceramic layers are trimmed to the external edge dimensions sufficient to fit within the ferrule opening, taking into account any shrinkage that may occur from co-firing of the stacked layers. In step S 110 , the assembly of the stacked, laminated and trimmed green sheets is co-fired to drive off the resin and sinter the particulate together into a multi-layer metal-ceramic substrate  40  of higher density than the green sheets forming the layers  52 - 60 . The green sheets shrink in thickness when fired such that a 0.006 inch thick green sheet typically shrinks to a layer thickness of about 0.005 inch. The green sheets may be fired using conventional techniques, with low temperature co-fired ceramic techniques being recommended when copper or aluminum are used. 
     In step S 112 , the outer edge  46  and the inward and outward facing substrate surfaces  42  and  44  are machined and polished to size and finish specifications. Then, in step S 114 , the various regions of the outward facing surface  44  are metallized to form the button braze joints  78  for each conductive path  50  and the band-shaped, ground plane layer  136  electrically connecting all of the substrate ground paths  118  together at the outward facing ends thereof. The substrate edge  46  is also metallized with metallization layer  140 . These metallization layers are preferably sputtered films of niobium, titanium, tungsten, molybdenum or alloys thereof. The machining and polishing of the outer edge  46  which is then metallized improves the dimensional tolerances of the co-fired substrate  40  which in turn enables the reliable use of the substrate-ferrule braze joint  48  that is formed in step S 300  of FIG.  12 . 
     In a preferred embodiment of the present invention, where pure gold is employed to form the substrate-ferrule braze joint  48 , a 25,000 Angstrom thick layer of niobium is preferably sputtered onto substrate edge  46  and on edge bands of the inward facing surface  44  to form the band-shaped, ground plane or trace layers  132  and  136  by vacuum deposition using a Model No. 2400 PERKINELMER.RTM. sputtering system. The niobium layer is most preferably between about 15,000 and about 32,000 Angstroms thick. These metallization layers may not be required if metals such as: (i) gold alloys comprising gold and at least one of titanium, niobium, vanadium, nickel, molybdenum, platinum, palladium, ruthenium, silver, rhodium, osmium, iridium., and alloys, mixtures and thereof; (ii) copper-silver alloys, including copper-silver eutectic alloys, comprising copper and silver and optionally at least one of indium, titanium, tin, gallium, palladium, platinum; or (iii) alloys, mixtures or combinations of (i) or (ii) are employed for the substrate-ferrule braze joint  48 . 
     FIG. 11 is a flow chart illustrating the steps of fabricating the capacitive filter array  90  adapted to be brazed with the co-fired metal-ceramic substrate  40  formed in the steps of FIG. 10, the ferrule  20 , and other components in the steps of FIG.  12 . The capacitive filter array  90  is also formed of layers of ceramic material, preferably barium titanate, and screen printed, conductive, capacitor active and ground electrodes that are co-fired to form a monolithic structure. 
     In step S 200 , the barium titanate ceramic layers are tape cast, and the capacitor active and ground electrodes are screen printed on the surfaces thereof in step S 202 . The capacitor electrodes are formed of silver thick films, silver-palladium alloy thick films, or silver-platinum alloy, thick films. The layers are stacked and laminated using a mechanical or hydraulic press in step S 204 , and the stacked and laminated layers are machined and drilled to form the capacitor conductive path receiving, capacitive filter holes  108  in steps S 206  and S 208 . 
     The partly completed capacitor filter array  90  is fired in step S 210  to form the monolithic structure. Then, in steps S 212  and S 214 , the edges of the active capacitive filter electrodes  112 ,  114 ,  116  or  122 ,  124  exposed by the capacitor holes  108  and the capacitive filter ground electrodes  102 ,  104  and  106  are coupled together electrically in common or “terminated”. A conductive metal frit that contains one of silver, palladium, platinum, gold and nickel alloys thereof, is placed in the capacitor holes  118  and along the array side  98  and melted to form the termination layers  109  and  111  shown in FIGS. 5 and 6. Most commonly, the conductive frit comprises one of silver, silver-palladium alloy or nickel-gold alloy. Alternatively, the capacitor holes  118  and the array side  98  may be electroplated with layers of nickel and gold. In step S 212 , the capacitor filter array  90  is fired again to densify the termination layer materials. 
     FIG. 12 is a flow chart illustrating the steps of fabricating the filtered feedthrough assembly  10  from the capacitive filter array  90  formed in the steps of FIG. 11, the co-fired metal-ceramic substrate  40  formed in the steps of FIG. 10, the ferrule  20 , and other components. First, the metal-ceramic substrate  40  is fitted into the ferrule opening and hermetically sealed thereto using the gold or gold alloy substrate-ferrule braze joint  48  and the externally disposed contact buttons  70  are sealed into the button cavities  80 . Then, the capacitor filter array  90  is attached to the interior facing surface of the metal-ceramic substrate  40 , the capacitor filter conductive paths  110  fill the filter holes  108  using reflow solder techniques, and the internally disposed ground solder joint  130  and the plurality of interior contact pads  120  are attached. 
     In step S 300 , the ferrule  20 , braze preforms that melt to form the substrate-ferrule braze joint  48 , the metal-ceramic substrate  40 , and the externally disposed contact buttons  70  in the button cavities  80  are stacked into a braze fixture. Advantageously, these components that are assembled together in step S 300  self center and support one another in the braze fixture. This improves the ease of manufacturing and increases manufacturing batch yields. The stacked assembly is subjected to brazing temperatures in a vacuum or inert gas furnace in step S 302 , whereby the braze preforms melt to form the substrate-ferrule braze joint  48  and the buttons  70  fill the button cavities  80  and adhere to the braze joints  78 . As the assembly cools, the ferrule contracts more than the co-fired substrate, which puts the co-fired substrate in a state of compression. 
     In step S 304 , the conductive plated pads  82  and the band-shaped, ground plane or trace layer  132  electrically connecting all of the substrate ground paths  118  together at each inward facing end thereof are adhered onto the surface  42  as metallization layers. Each metallization layer preferably comprises sputtered films, first of titanium, then of nickel, and finally of gold, so that a three film metallization layer is formed in each case. 
     In step S 306 , the discoid capacitive filter array  90 , reflow solder, and the interior contact pads  120  are assembled onto the inward facing surfaces of the sub-assembly formed in step S 304 , and these components are heated in step S 308 . The heating causes the solder to flow into and fill the capacitive filter conductive path holes  108  to complete the formation of the capacitive filter conductive paths  110  and the solder pads  128  shown in FIG.  7  and to adhere the internally disposed bonding pad  120 . The solder may be an indium-lead or tin-lead alloy, and the internally disposed bonding pads  120  may be formed of Kovar alloy plated with successive layers of nickel and gold. The final layer that is exposed to air and that lead wires are bonded or welded to as shown in FIG. 9 preferably is gold. 
     In step S 310 , the ground solder joint  130  is molded around and against the filter substrate edge  98  and the band-shaped, ground plane or trace layer  132 . The ground solder joint  130  electrically connects the three ground electrodes  102 ,  104 ,  106  together and to the ferrule  20  through the plurality of substrate ground paths  118  and the substrate-ferrule braze joint  48 . The ground solder joint  130  also mechanically bonds the discoid capacitive filter array  90  with the multi-layer metal-ceramic substrate  40 . Since the ground solder joint  130  does not need to provide a hermetic seal, it may be formed of a number of materials as described above. 
     In the sputtering steps of the present invention, a DC magnetron sputtering technique is preferred, but RF sputtering techniques may less preferably be employed. A DC magnetron machine that may find application in the present invention is an Model 2011 DC magnetron sputtering device manufactured by ADVANCED ENERGY of Fort Collins, Colo. 
     The pin-to-pin spacing of two single pin or unipolar feedthroughs is typically on the order of 0.125 inches. The above-described capacitive filtered feedthrough array provides a spacing of 0.050 inches between adjacent conductive paths. The feedthrough assembly  10  can be formed providing the nine capacitively filter array conductive paths within a ferrule  20  that is 0.563 inches long and 0.158 inches wide. 
     While the present invention has been illustrated and described with particularity in terms of a preferred embodiment, it should be understood that no limitation of the scope of the invention is intended thereby. The scope of the invention is defined only by the claims appended hereto. It should also be understood that variations of the particular embodiment described herein incorporating the principles of the present invention will occur to those of ordinary skill in the art and yet be within the scope of the appended claims.