Abstract:
Terminal pins comprising a core of a first electrically conductive material selectively coated with a layer of a second electrically conductive material for incorporated into feedthrough filter capacitor assemblies are described. The feedthrough filter capacitor assemblies are particularly useful for incorporation into implantable medical devices such as cardiac pacemakers, cardioverter defibrillators, and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from U.S. Provisional Patent Application Ser. No. 61/354,747 filed Jun. 15, 2010. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to a hermetic feedthrough terminal pin, assembly, preferably of the type incorporating a filter capacitor. More specifically, this invention relates to terminal pins comprising a refractive metal core in which an electrically conductive second. metal is selectively coated to provide a cost effective terminal pin of increased solderability for incorporation into a feedthrough filter capacitor assembly, particularly of the type used in implantable medical devices such as cardiac pacemakers, cardioverter defibrillators, and the like, to decouple and shield internal electronic components of the medical device from undesirable electromagnetic interference (EMI) signals. The terminal pin feedthrough assembly provides a hermetic seal that prevents passage or leakage of fluids into the medical device. 
         [0004]    2. Prior Art 
         [0005]    Feedthrough assemblies are generally well known in the art for use in connecting electrical signals through the housing or case of an electronic instrument. For example, in an implantable medical device, such as a cardiac pacemaker, defibrillator, or neurostimulator, the feedthrough assembly comprises one or more conductive terminal pins supported by an insulator structure for passage of electrical signals from the exterior to the interior of the medical device. The conductive terminals are fixed into place using a gold brazing process, which provides a hermetic seal between the pin and insulative material. Conventionally, the terminal pins have been composed of platinum or combination of platinum and iridium. Platinum and platinum-iridium alloys are biocompatible, have good mechanical strength, which adds to the durability of the feedthrough. However, platinum is a precious metal that creates a manufacturing cost barrier. 
         [0006]    The replacement of platinum and platinum alloys by refractive metals such as niobium, molybdenum and tungsten offers several advantages. First, these refractive metals have a significant cost advantage over platinum. Secondly, these refractive metals are generally known to be biocompatible. Finally, previous research has shown that after high temperature brazing, there is no significant degradation in the mechanical properties of these refractive metals, in comparison to platinum. 
         [0007]    However, these refractive metals are susceptible to surface oxidation. Surface oxidation generally inhibits the ability of these metals to be joined to other materials, particularly other electrically conductive metals. What is needed, therefore, is a biocompatible, mechanically robust, cost effective terminal pin that can be readily joined to other metals. The present invention provides embodiments by which a terminal pin of a cost effective core metal is selectively coated with a metal that can be more readily joined to other electrically conductive metals. 
       SUMMARY OF THE INVENTION 
       [0008]    In a preferred form, a feedthrough filter capacitor assembly according to the present invention comprises an outer ferrule hermetically sealed to either an alumina insulator or fused glass dielectric material seated within the ferrule. The insulative material is also hermetically sealed to at least one terminal pin. That way, the feedthrough assembly prevents leakage of fluid, such as body fluid in a human implant application, past the hermetic seal at the insulator/ferrule and insulator/terminal pin interfaces. 
         [0009]    According to the invention, the terminal pin of a feedthrough assembly, and preferably of a feedthrough filter capacitor assembly, is composed of a refractive metal core in which a layer of a non-refractive electrically conductive second metal is selectively contacted to the surface of the pin core. In a preferred embodiment, the terminal pin comprises a core of tantalum, niobium, molybdenum or alloy thereof. A layer of a second non-refractive electrically conductive metal, such as palladium, platinum, gold or silver, is selectively applied to a portion or portions of the surface of the core metal. In that respect, the application of the second electrically conductive metal is an alternative solderable, oxidation resistant material that provides a considerably less expensive terminal pin than conventional platinum or platinum-iridium terminal pins while still achieving the same benefits of biocompatibility, good mechanical strength, solderability and a reliable hermetic feedthrough seal. 
         [0010]    These and other objects and advantages of the present invention will become increasingly more apparent by a reading of the following description in conjunction with the appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a perspective view of a feedthrough assembly embodying the novel features of the present invention. 
           [0012]      FIG. 2  is cross-sectional view of the feedthrough assembly of the present invention taken along line  2 - 2  of  FIG. 1 . 
           [0013]      FIG. 3  is a side view of a preferred embodiment of a selectively coated terminal pin. 
           [0014]      FIG. 4  is a cross-sectional view of a coated portion of the terminal pin taken along line  4 - 4  of  FIG. 3 . 
           [0015]      FIG. 5  is a cross-sectional view of the feedthrough assembly of the present invention taken along line  5 - 5  of  FIG. 2 . 
           [0016]      FIG. 6  is a cross-sectional view of the feedthrough assembly taken along line  6 - 6  of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0017]    Referring now to the drawings,  FIGS. 1 and 2  show an internally grounded feedthrough capacitor assembly  10  comprising a feedthrough  12  supporting a filter discoidal capacitor  14 . The feedthrough filter assembly  10  is useful with medical devices, preferably implantable devices such as pacemakers, cardiac defibrillators, cardioverter defibrillators, cochlear implants, neurostimulators, internal drug pumps, deep brain stimulators, hearing assist devices, incontinence devices, obesity treatment devices, Parkinson&#39;s disease therapy devices, bone growth stimulators, and the like. The feedthrough  12  portion of the assembly  10  includes terminal pins  16  that provide for coupling, transmitting and receiving electrical signals to and from a patient&#39;s heart, while hermetically sealing the interior of the medical instrument against ingress of patient body fluids that could otherwise disrupt instrument operation or cause instrument malfunction. While not necessary for accomplishing these functions, it is desirable to attach the filter capacitor  14  to the feedthrough  12  for suppressing or decoupling undesirable EMI signals and noise transmission into the interior of the medical device. 
         [0018]    More particularly, the feedthrough  12  of the feedthrough filter capacitor assembly  10  comprises a ferrule  18  defining an insulator-receiving bore surrounding an insulator  20 . Suitable electrically conductive materials for the ferrule  18  include titanium, tantalum, niobium, stainless steel or combinations of alloys thereof, the former being preferred. The ferrule  18  may be of any geometry, non-limiting examples being round, rectangle, and oblong. A surrounding flange  22  extends from the ferrule  18  to facilitate attachment of the feedthrough  12  to the casing (not shown) of, for example, one of the previously described implantable medical devices. The method of attachment may be by laser welding or other suitable methods. 
         [0019]    The insulator  20  is of a ceramic material such as of alumina, zirconia, zirconia toughened alumina, aluminum nitride, boron nitride, silicon carbide, glass or combinations thereof. Preferably, the insulating material is alumina, which is highly purified aluminum oxide, and comprises a sidewall  24  extending to a first upper side  26  and a second lower side  28 . The insulator  20  is also provided with bores  30  that receive the terminal pins  16  passing there through. A layer of metal  32 , referred to as metallization, is applied to the insulator sidewall  24  and the sidewall of the terminal pin bores  30  to aid a braze material  34  in hermetically sealing between the ferrule  18  and the insulator  20  and between the terminal pins  16  and the insulator  20 , respectively. 
         [0020]    Suitable metallization materials  32  include titanium, titanium nitride, titanium carbide, iridium, iridium oxide, niobium, tantalum, tantalum oxide, ruthenium, ruthenium oxide, zirconium, gold, palladium, molybdenum, silver, platinum, copper, carbon, carbon nitride, and combinations thereof. The metallization layer may be applied by various means including, but not limited to, sputtering, electron-beam deposition, pulsed laser deposition, plating, electroless plating, chemical vapor deposition, vacuum evaporation, thick film application methods, and aerosol spray deposition, and thin cladding. 
         [0021]    Non-limiting examples of braze materials include gold, gold alloys, and silver. Then, if the feedthrough  12  is used where it will contact bodily fluids, the resulting brazes do not need to be covered with a biocompatible coating material. In other embodiments, if the brazes are riot biocompatible, for example, if they contain copper, they are coated with a layer/coating of biocompatible/biostable material. Broadly, the biocompatibility requirement is met if contact of the braze/coating with body tissue and blood results in little or no immune response from the body, especially thrombogenicity (clotting) and encapsulation of the electrode with fibrotic tissue. The biostability requirement means that the braze/coating remains physically, electrically, and chemically constant and unchanged over the life of the patient. 
         [0022]    In an embodiment of the present invention, the terminal pins  16  ( FIGS. 3 ,  4 ,  5  and  6 ) comprise a terminal pin core  16 B of a first electrically conductive material and an exterior outer coating  16 A of a second electrically conductive material. In a more preferred embodiment of the invention, the terminal pins  16  comprise a core  168  of a refractive metal and an exterior outer coating  16 A comprising palladium and its alloys. Non-limiting examples include pure palladium and alloys comprising from about 50% to about 99% palladium along with other elements including those from the platinum group such as ruthenium, rhenium, and iridium, or refractory metals such as molybdenum, and boron, and combinations thereof. 
         [0023]    Mechanical properties of the terminal pin  16  can be tailored to a desired mechanical performance by adjusting the amounts of the elemental additions in the palladium alloy. For example, age hardening can be improved by increasing the amount of ruthenium. Other additions to the palladium alloy such as platinum, gold, copper, and zinc, for example, increase the alloy&#39;s ability to achieve a higher tensile strength. 
         [0024]    As previously mentioned, the terminal pin core  16 B is comprised of a refractive metal. A refractory metal is herein defined as a metal that is resistant to heating and has a melting temperature greater than about 1,800° C. Non-limiting examples of refractory metals include niobium, molybdenum, tantalum, tungsten, rhenium, titanium, vanadium, zirconium, hafnium, osmium, iridium, and alloys thereof. In a more preferred embodiment, the terminal pin core  16 B comprises niobium and niobium alloys. However, an alternative embodiment, the terminal pin core  168  may comprise nickel-titanium (NITINOL®, titanium, particularly beta titanium, titanium alloys, stainless steel, palladium and palladium alloys, and combinations thereof. 
         [0025]    In a preferred embodiment, the external outer coating  16 A comprises an alternative electrically conductive metal. Non-limiting examples of this alternative second conductive metal comprise platinum, gold, silver, nickel and combinations thereof. 
         [0026]    In a preferred embodiment, this second electrically conductive metal may have a surface  25  that is readily joinable to other materials, particularly electrically conductive metals. These material-joining processes may include soldering, welding and/or brazing. Preferably, the surface  25  of the second metal is “wettable” to tin based solders, such as Sn63/Pb37 and the like. A “wettable” surface is herein defined as the ability of a material to adhere to the surface. 
         [0027]    In a preferred embodiment, as shown in  FIGS. 1 ,  3  and  4 , the external outer coating  16 A of the second electrically conductive metal is selectively applied at discrete locations to a surface  23  of the terminal pin core  16 B. Preferably the external outer coating  16 A of the second electrically conductive metal is applied to a discrete portion or portions of the surface  23  of the terminal pin core  16 B. These portions may include but are not limited to a distal end portion  21 , a central portion  19  and/or a proximal end portion  17  of the terminal pin core  16 B. 
         [0028]    The means of coating may include sputtering, cladding, and or plating. The coating may be applied through a process of sputtering, electron-beam deposition, pulsed laser deposition, plating, electroless plating, chemical vapor deposition, vacuum evaporation, thick film application methods, aerosol spray deposition, and thin cladding. 
         [0029]    Such a preferred embodiment of selectively applying the exterior outer coating  16 A enhances electrical conduction and retards oxidation of the surface  23  of the terminal, pin core  16 B within these regions  17 ,  19 ,  21 . Selectively applying the external outer coating  16 A to the core  16 B allows for improved design and manufacturing flexibility. For example, the coating  16 A may be precisely applied to the surface  23  of the core  16 B after high temperature processing. This feature is beneficial the external outer coating  16 A can be tailored to meet the dimensions of the joining metal. Furthermore, the application of the external outer coating  16 A to a discrete portion of the core  16 B further reduces cost of manufacture. 
         [0030]    In a preferred embodiment, as illustrated in  FIGS. 1 ,  3 , the portions  17 , 19 , 21  of the external outer coating  16 A may not be limited to a single external outer coating  16 A composition. For example, the proximal end portion  17  of the terminal pin core  16 B may be coated with a metal that is of a composition that is different then the coating  16 A comprising the distal end portion  21  and/or the central. portion  19 . This feature of designing an external outer coating  16 A of multiple compositions allows for custom tailoring of electrical or joining properties. The proximal and distal end portions  17 ,  21 , each have a length of about 5 percent to about 15 percent of the total length of the terminal pin  16  may be coated with an outer external coating  16 A of composition “A” which is preferable for soldering. The central portion  19  of the terminal pin core  16 B, located within the capacitor  14 , and having a length of from about 10 percent to about 40 percent of the total length of the terminal pin  16 , may be coated with composition “B” which is readily joined to the metallization material by soldering, and the like, to provide improved electrical conduction or EMI filtration performance. 
         [0031]    For example, it is known that refractive metals such as niobium, tungsten and molybdenum readily oxidize. This means that when it is used as a terminal pin material, secondary operations are necessary in order to effect a hermetic braze with low equivalent series resistance (ESR) characteristics. Providing a palladium outer coating  16 A over a niobium core  16 B in an evacuated atmosphere prior to formation of niobium oxide ensures that the thusly constructed terminal pin can be directly brazed into the insulator  20 . 
         [0032]    Although the terminal pin  16  is shown having a circular cross-section, that is not necessary. The terminal pin.  16  can have other cross-sectional shapes including square, triangular, rectangular, and hexagonal, among others. Nonetheless, the core  16 B has a diameter of from about 0.002 inches to about 0.020 inches and the outer coating  16 A has a thickness of from about 0.5μ inches to about 0.002 inches. 
         [0033]    Up to now, terminal pins for feedthrough assemblies used in implantable medical devices, and the like, have generally consisted of platinum. However, replacement of platinum and platinum alloys by such alternative metals as palladium and its alloys offers several advantages. For one, the density of platinum is 21.45 g/cc in comparison to palladium at 12.02 g/cc. Both of these materials are priced by weight, but used by volume. Therefore, palladium has significant cost advantage over platinum. Secondly, palladium has comparable electrical conductivity to platinum (platinum=94.34 l/mohm-cm, palladium=94.8 l/mohm-cm and gold=446.4 l/mohm-cm). Thirdly, palladium and platinum have significantly equivalent mechanical properties. After high temperature brazing, there is no significant degradation of mechanical properties such as strength and elongation. Fourthly, palladium is both solderable and weldable. Fifthly, palladium has good radiopacity characteristics. This is an important consideration for viewing the terminal pin during diagnostic scans such as fluoroscopy. Lastly, but every bit as important, palladium is biocompatible. Previous research indicates a variety of positive biocompatibility studies (both soft tissue and bone) for all elements used. Palladium and its alloy additives are regarded as chemically inactive. 
         [0034]    As further shown in  FIGS. 1 and 3 , the feedthrough filter capacitor  10  includes the filter capacitor  14  that provides for filtering undesirable EMI signals before they can enter the device housing via the terminal pins  16 . The filter capacitor  14  comprises a ceramic or ceramic-based dielectric monolith  36  having multiple capacitor-forming conductive electrode plates formed therein. The capacitor dielectric  36  preferably has a circular cross-section matching the cross-section of the ferrule  18  and supports a plurality of spaced-apart layers of first or “active” electrode plates  38  in spaced relationship with a plurality of spaced apart layers of second or “ground” electrode plates  40 . The filter capacitor  14  is preferably joined to the feedthrough  12  adjacent to the insulator side  26  by an annular bead  42  of conductive material, such as a solder or braze ring, or a thermal-setting conductive adhesive, and the like. The dielectric  36  includes lead bores  44  provided with an inner surface metallization layer. The terminal pins  16  pass there through and are conductively coupled to the active plates  38  by a conductive braze material  46  contacting between the terminal pins  16  and the bore metallization. In a similar manner, the ground plates  40  are electrically connected through an outer surface metallization  48  and the conductive material  42  to the ferrule  18 . 
         [0035]    It is appreciated that various modifications to the invention concepts described herein may be apparent to those of ordinary skill in the art without departing from the scope of the present invention as defined by the appended claims.