Abstract:
A capacitor structure comprises a shallow drawn case having a first major side and a peripheral wall extending therefrom, the first major side having a first interior surface and the wall having a peripheral interior surface. A lid is sealingly coupled to the case along adjacent edges of the lid and the wall, the lid and said case forming an encasement of the capacitor structure, the lid comprising a second interior surface. A cathode material is disposed proximate the first and second interior surfaces, and an anode is positioned intermediate the cathode material and has a peripheral portion positioned proximate the adjacent edges. A protective layer on the peripheral portion protects the anode during the sealing process. A first insulative separator is positioned between the anode and the cathode material.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to capacitors, and more particularly to a thin electrolytic capacitor suitable for use in an implantable medical device such as an implantable cardiac defibrillator (ICD). 
     BACKGROUND OF THE INVENTION 
     ICDs are devices that are typically implanted in a patient&#39;s chest to treat very fast, and potentially lethal, cardiac arthythmias. These devices continuously monitor the heart&#39;s electrical signals and sense if, for example, the heart is beating dangerously fast. If this condition is detected, the ICD can deliver one or more electric shocks, within about five to ten seconds, to return the heart to a normal heart rhythm. These defibrillation electric shocks may range from a few micro-joules to very powerful shocks of approximately twenty-five joules to forty joules. 
     Early generations of ICDs utilized high-voltage, cylindrical capacitors to generate and deliver defibrillation shocks. For example, standard wet slug tantalum capacitors generally have a cylindrically shaped conductive casing serving as the terminal for the cathode and a tantalum anode connected to a terminal lead electrically insulated from the casing. The opposite end of the casing is also typically provided with an insulator structure. 
     One such capacitor is shown and described in U.S. Pat. No. 5,369,547 issued on Nov. 29, 1994 and entitled “Capacitor”. This patent disclosed an electrolytic capacitor that includes a metal container that functions as a cathode. A porous coating, including an oxide of a metal selected from the group consisting of ruthenium, iridium, nickel, rhodium, platinum, palladium, and osmium, is disposed proximate an inside surface of the container and is in electrical communication therewith. A central anode selected from the group consisting of tantalum, aluminum, niobium, zirconium, and titanium is spaced from the porous coating, and an electrolyte within the container contacts the porous coating and the anode. 
     U.S. Pat. No. 5,737,181 issued on Apr. 7, 1998 and entitled “Capacitor” describes a capacitor that includes a cathode material of the type described in the above cited patent disposed on each of two opposed conducting plates. A metal anode (also of the type described in the above cited patent) is disposed between the cathode material coating and the conducting plates. 
     U.S. Pat. No. 5,982,609 issued Nov. 9, 1999 and entitled “Capacitor” describes a capacitor that includes a cathode having a porous coating including an amorphous metal oxide of at least one metal selected from the group consisting of ruthenium, iridium, nickel, rhodium, rhenium, cobalt, tungsten, manganese, tantalum, molybdenum, lead, titanium, platinum, palladium, and osmium. An anode includes a metal selected from the group consisting of tantalum, aluminum, niobium, zirconium, and titanium. 
     While the performance of these capacitors was acceptable for defibrillator applications, efforts to optimize the mechanical characteristics of the device have been limited by the constraints imposed by the cylindrical design. In an effort to overcome this, flat electrolytic capacitors were developed. U.S. Pat. No., 5,926, 362 issued on Jul. 20, 1999 and entitled “Hermetically Sealed Capacitor” describes a deep-drawn sealed capacitor having a generally flat, planar geometry. The capacitor includes at least one electrode provided by a metallic substrate in contact with a capacitive material. The coated substrate may be deposited on a casing side-wall or connected to a side-wall. The capacitor has a flat planar shape and utilizes a deep-drawn casing comprised of spaced apart side-walls joined at their periphery by a surrounding intermediate wall. Cathode material is typically deposited on an interior side-wall of the conductive encasement which serves as one of the capacitor terminals; e.g. the cathode. The other capacitor terminal (the anode) is isolated from the encasement by an insulator/feedthrough structure comprised of, for example, a glass-to-metal seal. It is also known to deposit cathode material on a separate substrate that is placed in electrical communication with the case. In another embodiment, the cathode substrate is insulated from the case using insulators and a separate cathode feedthrough. 
     A valve metal anode made from metal powder is pressed and sintered to form a porous structure, and a wire (e.g. tantalum) is imbedded into the anode during pressing to provide a terminal for joining to the feedthrough. A separator (e.g. polyolefin, a fluoropolymer, a laminated film, non-woven glass, glass fiber, porous ceramic, etc.) is provided between the anode and the cathode to prevent short circuits between the electrodes. Separator sheets are sealed either to a polymer ring that extends around the perimeter of the anode or to themselves. 
     A separate weld ring and polymer insulator may be utilized for thermal beam protection as well as anode immobilization. Prior to encasement welding, a separator encased anode is joined to the feedthrough wire by, for example, laser welding. This joint is internal to the capacitor. The outer metal encasement structure is comprised essentially of two symmetrical half shells that overlap and are welded at their perimeter seam to form a hermetic seal. After welding, the capacitor is filled with electrolyte through a port in the encasement. 
     The above described techniques, present concerns relating to both device size and manufacturing complexity. The use of overlapping half-shields results in a doubling of the encasement thickness around the perimeter of the capacitor thus reducing the available interior space for the capacitor&#39;s anode. This results in larger capacitors. Space for the anode material is further reduced by the presence of the weld ring and space insulator. In addition, manufacturing processes become more complex and therefore more costly, especially in the case of a deep-drawn encasement. 
     A further disadvantage of the known design involves the complexity of the anode terminal-to-feedthrough terminal weld joint. As was described, a tantalum anode lead is imbedded into the anode and is joined via laser welding to a terminal lead of the feedthrough. This is typically accomplished by forming a “J” or “U” shape with one or more of the leads, pressing the terminal end of these leads together, and laser welding the interface. In order to accomplish this, one must either perform this step prior to welding the feedthrough ferrule into the encasement or sufficient space must be provided in the capacitor anode structure to facilitate clamping and welding while the anode is in the case. This results in additional manufacturing complexity while the latter negatively impacts device size. 
     As stated previously, a separator material is provided on the anode and may be sealed to itself to form an envelope. The anode is typically on the order of 0.1 inch thick. As a result, the sealing operation is complex, and significant separator material typically overhangs the anode. This overhang must be accommodated in the design and typically either reduces the size of the anode or increases the size of the capacitor. Furthermore, due to the proximity of thermally sensitive separator material to the encasement, the separator is in direct contact with the cathode/encasement structure. Weld parameters must therefore be carefully selected to prevent thermal damage of the separator material. When cathode material is deposited on a separate substrate, as described above, substrate thickness further reduces the space available for anode material or increases the size of the capacitor. 
     Thus, while the development of flat electrolytic capacitors significantly reduces size and thickness, defibrillation capacitors are still the largest components in current ICDs making further downsizing a primary objective. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an aspect of the invention, there is provided a capacitor structure, comprising a shallow drawn case having a first major side and a peripheral wall extending therefrom, the first major side having a first interior surface and the wall having a peripheral interior surface. A lid is sealingly coupled to the case along adjacent edges of the lid and the wall, the lid and the case forming an encasement of the capacitor structure, the lid comprising a second interior surface. A cathode material is disposed proximate the first and second interior surfaces, and an anode is positioned intermediate the cathode material and has a peripheral portion positioned proximate the adjacent edges. A protective layer on the peripheral portion protects the anode during the encasement sealing process. A first insulative separator is positioned between the anode and the cathode material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
     FIG. 1 is a cross-sectional view of an electrolytic capacitor in accordance with the teachings of the prior art; 
     FIGS. 2,  3 , and  4  are front, side, and top cross-sectional views of a flat electrolytic capacitor in accordance with the teachings of the prior art; 
     FIGS. 5,  6 , and  7  are front cross-sectional, side cross-sectional, and scaled cross-sectional views of an electrolytic capacitor in accordance with a first embodiment of the present invention and suitable for use in an implantable medical device; 
     FIGS. 8, and  9  are side cross-sectional and scaled cross-sectional views of an electrolytic capacitor in accordance with a further embodiment of the present invention; 
     FIG. 10 is a cross-sectional view of a capacitor/anode encasement structure in accordance with the teachings of the prior art; 
     FIG. 11 is a cross-sectional view of a novel capacitor/anode encasement assembly; 
     FIG. 12 is a cross-sectional view of an alternative capacitor/anode encasement assembly; 
     FIG. 13 illustrates a first novel technique for electrically coupling an anode lead wire through a ferrule in an electrolytic capacitor; and 
     FIGS. 14-18 illustrate alternate techniques for electrically coupling an anode lead wire through a ferrule in an electrolytic capacitor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing exemplary embodiments of the invention. Various changes to the described embodiments may be made in the function and arrangement of the elements described herein without departing from the scope of the invention. 
     FIG. 1 is a cross-sectional view of an electrolytic capacitor in accordance with the teachings of the prior art. It comprises a cylindrical metal container  20  made of, for example tantalum. Typically, container  20  comprises the cathode of the electrolytic capacitor and includes a lead  22  that is welded to the container. An end scat of cap  24  includes a second lead  26  that is electrically insulated from the remainder of cap  24  by means of a feed-through assembly  28 . Cap  24  is bonded to container  20  by, for example, welding. Feed-through  28  of lead  26  may include a glass-to-metal seal through which lead  26  passes. An anode  30  (e.g., porous sintered tantalum) is electrically connected to lead  26  and is disposed within container  20 . Direct contact between container  20  and anode  30  is prevented by means of electrically insulating spacers  32  and  34  within container  20  that receive opposite ends of anode  30 . A porous coating  36  is formed directly on the inner surface of container  20 . Porous coating  36  may include an oxide of ruthenium, iridium, nickel, rhodium, platinum, palladium, or osmium. As stated previously, anode  30  may be made of a sintered porous tantalum. However, anode  30  may be aluminum, niobium, zirconium, or titanium. Finally, an electrolyte  38  is disposed between and in contact with both anode  30  and cathode coating  36  thus providing a current path between anode  30  and coating  36 . As stated previously, while capacitors such as the one shown in FIG. 1 were generally acceptable for defibrillator applications, optimization of the device is limited by the constraints imposed by the cylindrical design. 
     FIGS. 2,  3 , and  4  are front, side, and top cross-sectional views respectively of a flat electrolytic capacitor, also in accordance with the teachings of the prior art, designed to overcome some of the disadvantages associated with the electrolytic capacitor shown in FIG.  1 . The capacitor of FIGS. 2,  3 , and  4  comprises an anode  40  and a cathode  44  housed inside a hermetically sealed casing  46 . The capacitor electrodes are activated and operatively associated with each other by means of an electrolyte contained inside casing  46 . Casing  46  includes a deep drawn can  48  having a generally rectangular shape and comprised of spaced apart side-walls  50  and  52  extending to and meeting with opposed end walls  54  and  56  extending from a bottom wall  58 . A lid  60  is secured to side-walls  50  and  52  and to end walls  54  and  56  by a weld  62  to complete an enclosed casing  46 . Casing  46  is made of a conductive metal and serves as one terminal or contact for making electrical connections between the capacitor and its load. 
     The other electrical terminal or contact is provided by a conductor or lead  64  extending from within the capacitor through casing  46  and, in particular, through lid  60 . Lead  64  is insulated electrically from lid  60  by an insulator and seal structure  66 . An electrolyte fill opening  68  is provided to permit the introduction of an electrolyte into the capacitor, after which opening  68  is closed. Cathode electrode  44  is spaced from the anode electrode  40  and comprises an electrode active material  70  provided on a conductive substrate. Conductive substrate  70  may be selected from the group consisting of tantalum, nickel, molybdenum, niobium, cobalt, stainless steel, tungsten, platinum, palladium, gold, silver, cooper, chromium, vanadium, aluminum, zirconium, hafnium, zinc, iron, and mixtures and alloys thereof. Anode  40  may be selected from the group consisting of tantalum, aluminum, titanium, niobium, zirconium, hafnium, tungsten, molybdenum, vanadium, silicon, germanium, and mixtures thereof. A separator structure includes spaced apart sheets  72  and  74  of insulative material (e.g. a microporous polyolefinic film). Sheets  72  and  74  are connected to a polymeric ring  76  and are disposed intermediate anode  40  and coated side-walls  50  and  52  which serve as a cathode electrode. 
     As already mentioned, the above described capacitors present certain concerns with respect to device size and manufacturing complexity. In contrast, FIGS. 5,  6 , and  7  are front cross-sectional, side cross-sectional, and scaled cross-sectional views of an electrolytic capacitor suitable for use in an implantable medical device in accordance with a first embodiment of the present invention. As can be seen, one or more layers of an insulative polymer separator material  142  (e.g. micro-porous IFRE or polypropylene) are heat sealed around a thin, D-shaped anode  140  (e.g. tantalum) having an anode lead wire  144  (e.g. tantalum) embedded therein. Capacitor grade tantalum powder such as the “NH” family of powders may be employed for this purpose. These tantalum powders have a charge per gram rating of between approximately 17,000 to 23,000 microfarad-volts/gram and have been found to be well suited for implantable cardiac device capacitor applications. Tantalum powders of this type are commercially available from HC Starck, Inc. located in Newton, Mass. 
     Before pressing, the tantalum powder is typically, but not necessarily, mixed with approximately 0 to 5 percent of a binder such as ammonium carbonate. This and other binders are used to facilitate metal particle adhesion and die lubrication during anode pressing. The powder and binder mixture are dispended into a die cavity and are pressed to a density of approximately 4 grams per cubic centimeter to approximately 8 grams per cubic centimeter. After pressing, it is sometimes beneficial to modify anode porosity to improve conductivity within the internal portions of the anode. Porosity modification has been shown to significantly reduce resistance. Macroscopic channels are incorporated into the body of the anodes to accomplish this. Binder is then removed from the anodes either by washing in warm deionized water or by heating at a temperature sufficient to decompose the binder. Complete binder removal is desirable since residuals may result in high leakage current. Washed anodes are then vacuum sintered at between approximately 1,350 degrees centigrade and approximately 1,600 degrees centigrade to permanently bond the metal anode particles. 
     An oxide is formed on the surface of the sintered anode by immersing the anode in an electrolyte and applying a current. The electrolyte includes constituents such as water and phosphoric acid and perhaps other organic solvents. The application of current drives the formation of an oxide film that is proportional in thickness to the targeted forming voltage. A pulsed formation process may be used wherein current is cyclically applied and removed to allow diffusion of heated electrolyte from the internal pores of the anode plugs. (See U.S. patent application Ser. No. 10/261,066 filed Sep. 30, 2002 entitled “Method and Apparatus for Maintaining Energy Storage in an Electrical Storage Device” the teachings of which are hereby incorporated by reference.) Intermediate washing and annealing steps may be performed to facilitate the formation of a stable, defect free, oxide. 
     Layers of cathode material  146  are deposited on the inside walls of a thin, shallow drawn, D-shaped casing  148  (e.g. titanium) having first and second major sides and a peripheral wall, each of which have an interior surface. The capacitive materials may be selected from those described above or selected from the group including graphitic or glassy carbon on titanium carbide, carbon and silver vanadium oxide on titanium carbide, carbon and crystalline manganese dioxide on titanium carbide, platinum on titanium, ruthenium on titanium, barium titanate on titanium, carbon and crystalline ruthenium oxide on titanium carbide, carbon and crystalline iridium oxide on titanium carbide, silver vanadium oxide on titanium, and the like. 
     Anode  140  and cathode material  146  are insulated from each other by means of a micro-porous polymer separator material such as a PTFE separator of the type produced by W. L. Gore, Inc. located in Elkton, Md. or polypropylene of the type produced by Celgard, Inc. located in Charlotte, N.C. Separators  146  prevent physical contact and shorting and also provide for ionionic conduction. The material may be loosely placed between the electrodes or can be sealed around the anode and/or cathode. Common sealing methods include heat sealing, ultra sonic bonding, pressure bonding, etc. 
     The electrodes are housed in a shallow drawn, typically D-shaped case (e.g. titanium) that may have a material thickness approximately 0.005 to 0.016 inches thick. A feed-through  150  is comprised of a ferrule  154  (e.g. titanium), a terminal lead wire  152  (e.g. tantalum), and an insulator  156  (e.g. a polycrystalline ceramic polymer, non-conducting oxides, conventional glass, etc.) is bonded to ferrule  154  and lead wire  152 . Sealed anode  140  is inserted into a cathode coated case and a spacer ring is inserted around the periphery of the anode to secure the position of the anode within the case. A J-shaped feed-though lead wire  152  is electrically coupled to anode lead wire  144  as, for example, by resistance or laser welding. In accordance with an aspect of the present invention, lead wire  152  may be joined to anode lead wire  144  without the necessity for a J-shaped bend as will be fully described hereinbelow and as represented in FIG.  8 . The lid is positioned and secured in the case by welding. 
     After assembly and welding, an electrolyte is introduced into the casing through a fill-port  160 . The electrolyte is a conductive liquid having a high breakdown voltage that is typically comprised of water, organic solvents, and weak acids or of water, organic solvents and sulfuric acid. Filling is accomplished by placing the capacitor in a vacuum chamber such that fill-port  160  extends into a reservoir of electrolyte. When the chamber is evacuated, pressure is reduced inside the capacitor. When the vacuum is released, pressure inside the capacitor re-equilibrates, and electrolyte is drawn through fill-port  160  into the capacitor. 
     Filled capacitors are aged to form an oxide on the anode leads and other areas of the anode. Aging, as with formation, is accomplished by applying a current to the capacitor. This current drives the formation of an oxide film that is proportional in thickness to the targeted aging voltage. Capacitors are typically aged approximately at or above their working voltage, and are held at this voltage until leakage current reaches a stable, low value. Upon completion of aging, capacitors are re-filled to replenish lost electrolyte, and the fill-port  160  is sealed as, for example, by laser welding a closing button or cap over the encasement opening. 
     FIGS. 8 and 9 are a side cross-sectional and scaled cross-sectional views of an electrolytic capacitor suitable for use in an implantable medical device in accordance with a further embodiment of the present invention. In this case, cathode material is deposited on two substrates  146  (e.g. titanium) which are separate from encasement  148 . Before cathode deposition, the substrates may be chemically or mechanically modified to increase surface area. Methods suitable for this purpose include, but are not limited to, etching, abrasion, and medium blasting. 
     As previously described, an insulative separator is heat sealed around a thin, D-shaped, anode (e.g. tantalum) having an imbedded lead wire  144  (e.g. tantalum) imbedded therein as is shown in FIGS. 8,  9 , and  11 . Anode  140  is sandwiched between two cathodes  146 . Additional layers of insulative separator material  162  are utilized between encasement  148  and cathodes  146  so as to prevent unwanted electrical contact between the cathodes and the encasement sidewalls. Alternatively, the cathodes may be sealed in separators thus eliminating the need for a separator on the anode. Of course, encasement  148  may be utilized as a cathode terminal by simply electrically coupling cathodes  146  to the encasement Insulative material  162  may comprise polymer sheets, formed polymer caseliners, polymer coated cases, sputtered insulating oxides, etc. As already described in connection with FIGS. 5,  6 , and  7 , the electrode stack shown in FIG. 9 is inserted into the encasement, and the embedded anode lead wire is resistance or laser welded to a feedthrough lead wire. A cover is assembled over the electrode stack, and the assembly is completed by means of, for example, laser welding. 
     As stated previously, the outer metal encasement structure of a known planar capacitor generally comprises two symmetrical half shells that overlap and are then welded along their perimeter seam to form a hermetic seal. Such a device is shown in FIG.  10 . That is, the encasement comprises a case  164  and an overlapping cover  166 . A separator sealed anode  168  is placed within case  164 , and a polymer spacer ring  170  is positioned around the periphery of anode assembly  168 . Likewise, a metal weld ring  172  is positioned around the periphery of spacer ring  170  proximate the overlapping portion  174  of case  164  and cover  166 . The overlapping portions of case  164  and cover  166  are then welded along the perimeter seam to form a hermetic seal. 
     This technique presents certain concerns relating to both device size and manufacturing complexity. The use of overlapping half-shields results in a doubling of the encasement thickness around the perimeter of the capacitor thus reducing the available interior space for the anode. Thus, for a given size anode, the resulting capacitor is larger. Furthermore, space for anode material is reduced due to the presence of weld ring  172  and insulative polymer spacer ring  170 . This device is more complex to manufacture and therefore more costly. 
     FIG. 11 is a cross-sectional view illustrating one of the novel aspects of the present invention. In this embodiment, the encasement is comprised of a shallow drawn case  176  and a cover or lid  178 . This shallow drawn encasement design uses a top down welding approach. Material thickness is not doubled in the area of the weld seam as was the situation in connection with the device shown in FIG. 10 thus resulting in additional space for anode material. 
     Cover  178  is sized to fit into the open side of shallow drawn metal case  176 . This results in a gap (e.g. from 0 to approximately 0.002 inches) in the encasement between case  176  and cover  178  that could lead to the penetration of the weld laser beam thus potentially damaging the capacitor&#39;s internal components. To prevent this, a metalized polymeric weld ring is placed or positioned around the periphery of anode  168 . Weld ring  180  is somewhat thicker than the case to cover gap  182  to maximize protection. Metalized weld ring  180  may comprise a polymer spacer  186  having a metalized surface  184  as shown. Metalized weld ring  180  provides for both laser beam shielding and anode immobilization. The metalized polymer spacer  180  need only be thick enough to provide a barrier to penetration of the laser beam and is sacrificial in nature. This non-active component substantially reduces damage to the active structures on the capacitor. 
     Metalized polymer spacer  180  is placed around the perimeter of anode  168  during assembly and may be produced my means of injection molding, thermal forming, tube extrusion, die cutting of extruded or cast films, etc. Spacer  180  may be provided through the use of a pre-metalized polymer film. Alternatively, the metal may be deposited during a separate process after insulator production. Suitable metallization materials include aluminum, titanium, etc. and mixtures and alloys. 
     FIG. 12 is a cross-sectional view illustrating an alternative to the embodiment shown in FIG.  11 . Again, the encasement comprises a case  176  and a cover or lid  178  resulting in gap  182 . The anode assembly  168  is positioned within the encasement as was the situation in FIG.  11 . To protect the capacitor&#39;s internal components from damage due to the weld laser beam, a metalized tape  184  is positioned around the perimeter of anode  168 . 
     The embodiments shown in FIGS. 11 and 12 not only have space saving aspects in the encasement design, but the components are simple and inexpensive to produce. The top down assembly facilitates fabrication and welding processes. The thinness of the weld ring/spacer  180  or metalized tape  184  reduces the need for additional space around the perimeter of the capacitor thus improving energy density. The design lends itself to mass production methods and reduces costs, component count, and manufacturing complexity. 
     As stated previously, a major disadvantage of prior art electrolytic capacitors resides in the complexity of producing a proper weld joint between the anode terminal and the feedthrough terminal. That is, the anode lead wire (e.g. tantalum) is imnbedded into the anode and is joined, by means of, for example, laser welding, to the feedthrough lead wire. This is typically accomplished by forming a “J” shape with each of the lead wires, pressing them together, and laser welding the interface. This is either performed prior to welding the feedthrough ferrule into the encasement, or alternatively, sufficient space must be provided to facilitate clamping and welding when the anode assembly is in the encasement. The former approach results in significant manufacturing complexity while the latter negatively impacts the size of the capacitor. 
     FIG. 13 illustrates a first technique for coupling an anode lead wire to a feedthrough lead wire which substantially avoids the above noted problems. Referring to FIG. 13 a weld block (e.g. tantalum)  190  is positioned at the internal end of feedthrough  154  such that feedthrough lead wire  152  is in electrical engagement therewith. In this case, feedthrough  154  is made of an insulative material (e.g. a glass or polymer) in order to electrically isolate anode lead wire  144  from encasement  148 . When anode  140  having anode lead wire  144  imbedded therein is positioned within encasement  148 , anode lead wire  144  rests, in part, on weld block  190 . Electrical coupling between anode lead wire  144  and weld block  190  may be accomplished by techniques such as laser welding, parallel gap welding, etc. The need for bends in the lead wires has been eliminated thus reducing fixturing and manufacturing complexity. In fact, weld block  190  may contain locating or holding features (e.g. grooves) that entirely eliminate the requirement for fixturing. Weld block  190  may be provided with appropriate insulation to prevent shorting to encasement  148 . The size of the capacitor is reduced because it is no longer necessary to provide internal bends and weld joints in the lead wires. The reliability of the resulting structure is improved because the use of a polymeric feedthrough permits the utilization of materials that are more stable, and the elimination of internal weld joints reduces manufacturing costs and complexity. 
     FIG. 14 illustrates another arrangement for electrically coupling the anode to an external lead wire. As was the case previously, a feedthrough ferrule  154  is positioned within encasement  148  and is configured such that feedthrough lead wire  152  is accessible from the exterior of capacitor encasement  148 . A weld block  190  is positioned at the internal end of feedthrough ferrule  154  in electrical engagement with lead wire  152 . In this case however, anode  140  is likewise provided with weld block  192 , and electrical coupling between weld block  192  and weld block  190  is accomplished by means of, for example, a conductive ribbon  194  electrically coupled, as for example by welding, to both anode weld block  192  and feedthrough weld block  190 . 
     FIG. 15 illustrates yet another arrangement for electrically coupling anode lead wire  144  to feedthrough lead wire  152 . An intermediate weld block (e.g. tantalum)  196  is provided within capacitor encasement  148  and is configured such that an internal end of a lead wire  152  and an end of anode lead wire  144  contact weld block  196  when anode  140  is positioned within encasement  148 . Anode lead wire  144  and feedthrough lead wire  152  are then electrically coupled to weld block  196  using any known technique such as laser welding. 
     Yet another arrangement for coupling anode lead wire  144  to feedthrough lead wire  152  utilizes a small sleeve or piece of tubing  198  made of an electrically conductive material (e.g. tantalum). Referring to FIG. 16, anode lead wire  144  and feedthrough lead wire  152  are received within sleeve  198  and may be electrically coupled thereto by means of, for example, welding or crimping. An opening  200  may be provided in sleeve  198  so as to permit additional welding along the length of the leads. Sleeve  198  provides the necessary fixturing for lead wires  144  and  152 , and due to it&#39;s small size, the use of sleeve  198  is conducive to device downsizing. Sleeve  198  may be imbedded in anode  140  eliminating the need for anode lead wire  144  as is shown in FIG.  17 . Alternatively, sleeve  198  may be integrally coupled or formed with feedthrough ferrule  154  as is shown in FIG.  18 . 
     Thus, there has been provided an electrolytic capacitor that is not only easier and less costly to manufacture, but one which may be made smaller for a given capacitance. The inventive capacitor is therefore suitable for use in implantable medical devices such as defibrillators, even as such devices become smaller and smaller.