Abstract:
Terminal pins that include a refractory metal partially welded to a terminal block of a dissimilar metal incorporated into feedthrough filter capacitor assemblies are discussed. 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 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 61/346,921, filed May 21, 2010. 
    
    
     1. FIELD OF THE INVENTION 
     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 a method of welding two dissimilar metals into feedthrough filter capacitor assemblies, 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. 
     2. PRIOR ART 
     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. 
     Since feedthrough assemblies such as these are implanted in human bodies, it is generally preferred that the materials used to construct such assemblies are biocompatible. These biocompatible materials, although commonly considered to be immune to the human body, generally have different material properties. These differing material properties such as melting temperature, thermal expansion, thermal conductivity and electrical conductivity make these materials difficult to join and construct into a feedthrough assembly. 
     Feedthrough assemblies generally comprise an insulative body, a supporting ferrule, and a plurality of electrically conductive feedthrough terminal pins that are hermetically sealed in the insulative body. In some cases, a capacitor is also incorporated into the assembly to provide protection from electromagnetic interface (EMI). With respect to the present invention, additional metallic terminal blocks, incorporated with a polymeric body, are integrated in the feedthrough assembly. Nevertheless, the electrically conductive feedthrough terminal pins are preferrably electrically connected to these metallic terminal blocks located adjacent the polymeric body. 
     Terminal pins have been composed of niobium and niobium alloys. Niobium and niobium alloys are biocompatible refractory metals that are cost effective. The niobium material provides good mechanical strength and electrical conduction, which adds to the durability and performance of the feedthrough. However the refractive nature of the niobium metal makes it a difficult material with which to join to other metals, particularly non-refractive metals such as nickel. 
     In addition to the difficulties in joining dissimilar metals, other constraints from adjacent materials of the feedthrough assembly present additional difficulties which need to be overcome in constructing feedthrough assemblies. For example, the generally lower melting temperatures of adjacent polymeric bodies provide additional constraining parameters, particularly when they are located adjacent to where dissimilar metals are being joined together. The present invention addresses these problems as it relates to the construction of feedthrough assemblies. The present invention further provides an optimal construction and joining process thereof by which dissimilar metals are joined in the construction of feedthrough assemblies. 
     SUMMARY OF THE INVENTION 
     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. 
     According to the invention, the terminal pin of a feedthrough assembly, and preferably of the feedthrough filter capacitor assembly, is composed of a biocompatible refractive metal, such as niobium. The terminal pin can be a uniform wire-type structure of niobium or an alloy thereof. In that respect, niobium is a corrosion resistant material that provides a more cost effective terminal pin than other conventional metals, such as platinum or platinum-iridium terminal pins. Furthermore, terminal pins composed of niobium achieve the same benefits of biocompatibility, good mechanical strength, electrical conduction and a reliable hermetic feedthrough seal. 
     A plurality of terminal blocks are each preferably positioned in a slot atop a polymeric protective cap which preferably resides within the proximal region of the feedthrough assembly. The plurality of terminal blocks, preferably composed of an electrically conductive metal such as nickel, provides a preferred means of electrically attaching the feedthrough assembly to a medical device. 
     These terminal blocks provide a larger surface area with which to attach electrical connections between the feedthrough assembly and the medical device. The protective cap, composed of a biocompatible polymeric material, electrically insulates each individual terminal block and protects the feedthrough assembly from possible mechanical damage. 
     The specific design parameters and material properties comprising the feedthrough assembly, of the present invention, present particular constraints regarding the connection of the terminal pin to the terminal block. As such, the present invention provides a feedthrough assembly and manufacturing process thereof that effectively joins these two dissimilar metals of the terminal pin and block. Particularly, the joining of the terminal block to the terminal pin, without causing damage to the adjacent polymeric protective cap is discussed. 
     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 
         FIG. 1  is a perspective view of a feedthrough filter capacitor assembly. 
         FIG. 2  is top view of the feedthrough filter capacitor assembly shown in  FIG. 1 . 
         FIG. 3  is cross sectional view of the filter capacitor assembly shown in  FIG. 1 . 
         FIG. 4  is a magnified perspective of the view of the perspective an alternate perspective view of the cutter housing of the present invention. 
         FIG. 5  is a magnified top view showing an embodiment of one of the welds of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings,  FIGS. 1 and 3  show an internally grounded feedthrough capacitor assembly  10  comprising a feedthrough  12  supporting a discoidal filter capacitor  14 , a protective cap  16 , and a plurality of terminal blocks  18 . 
     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  20  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. 
     More particularly, the feedthrough  12  of the feedthrough filter capacitor assembly  10  comprises a ferrule  22  defining an insulator-receiving bore formed by a ferrule sidewall extending from a first ferrule end  22 A to a second ferrule end  22 B, the ferrule sidewall surrounding an insulator  26 . Suitable electrically conductive materials for the ferrule  22  include titanium, tantalum, niobium, stainless steel or combinations of alloys thereof, the former being preferred. The ferrule  22  may be of any geometry, non-limiting examples being round, rectangle, and oblong. A surrounding flange  24  ( FIG. 3 ) extends from the ferrule  22  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. 
     The insulator  26  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  28  extending to a first upper side or end  30  and a second lower side or end  32 . The insulator  26  is also provided with bores  34  that receive the terminal pins  20  passing therethrough. A layer of metal  36 , referred to as metallization, is applied to the insulator sidewall  28  and to the sidewall of the terminal pin bores  34  to aid a braze material  38  in hermetically sealing between the ferrule  22  and the outer sidewall  28  of the insulator  26  and between the terminal pins  20  and the bores  34  of the insulator  26 , respectively. 
     Suitable metallization materials  36  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. Parylene, alumina, silicone, fluoropolymers, and mixtures thereof are also useful metallization materials. 
     Non-limiting examples of braze materials  38  include gold, gold alloys, and silver. Then, if the feedthrough  14  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 not 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. 
     According to one embodiment of the invention, the terminal pins  20  are preferably composed of a first metal comprising a refractory metal. A refractory metal is herein defined as a metal that is resistant to heating and has a melting temperature above 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 pins  20  comprise niobium and niobium alloys. 
     As shown in  FIGS. 1-5 , each terminal pin  20  is received in a throughbore  40  of the terminal block  18 . In a preferred embodiment, a proximal end portion  42  of the terminal pin  20  is received in the throughbore  40  of the terminal block  18 . Terminal blocks  18  have a terminal block length  44 , a terminal block width  46  and a terminal block height  48  ( FIG. 4 ). In a preferred embodiment, the length  44  of the terminal block  18  ranges from about 1 mm to about 5 mm, the width  46  of the terminal block  18  ranges from about 1 mm to about 5 mm and the thickness  48  of the terminal block  18  ranges from about 0.05 mm to about 5 mm. 
     It is preferred that the terminal block  18  is composed of a second metal comprising an electrically conductive metal. Non-limiting examples of conductor block  18  second metals include nickel, titanium, gold, silver, platinum, palladium, stainless steel, MP35N (ASTM Material Designation: 35Co-35Ni-20Cr-10Mo), and alloys thereof. In a more preferred embodiment, terminal blocks  18  are composed of nickel or a nickel alloy. 
     Each throughbore  40  of the terminal block  18  is preferably constructed with a diameter ranging from about 0.01 mm to about 0.10 mm such that the terminal pin  20  can pass therethrough. It is preferred that the terminal pin  20  is positioned such that the bore wall  50  of the terminal block  18  circumferentially surrounds the diameter of the terminal pin  20 . It is further preferred that the proximal end portion  42  of the terminal pin  20  may reside above or below the topside surface  52  of the terminal block  18 . In a preferred embodiment, the terminal pin  20  may reside from about 0.02 mm to about 0.2 mm above or below the top surface  52  of the terminal block  18 . This preferred alignment of the end  54  of the terminal pin  20  to the terminal block  18  reduces mechanical stresses in the joining of the first metal comprising the pin  20  to the second metal comprising the block  18 , thereby increasing the robustness of the joint. 
     Furthermore, each terminal block  18  is preferably positioned on the topside  56  of the protective cap  16 . In a preferred embodiment, the terminal block  18  resides within a slot  58  formed into the topside surface  56  of the protective cap  16  ( FIGS. 1 ,  4 ). Each slot  58  is dimensioned such that the width  46  and length  44  of the terminal block  18  fit within the slot  58 . 
     In addition, the terminal pins  20  are preferably positioned such that they are received through a throughbore  60  of the protective cap  16 . More specifically, the proximal portion  42  of the terminal pin  20  is received through the respective throughbores  60  and  40  of the protective cap  16  and the terminal block  18 . The protective cap  16  is positioned in a more distal location of the terminal pin  20  than the terminal block  18  ( FIG. 3 ). 
     In a preferred embodiment, the protective cap  16  is composed of a biocompatible polymeric material that can withstand temperatures up to about 300° C. It is preferred that the protective cap  16  is composed of a polyoxymethylene copolymer such as CELCON® M450 or HOSTAFORM® C 52021 manufactured by Ticona of Florence, Ky. Other non-limiting materials comprising the protective cap  16  include silicone rubber, acrylonitrile butadiene styrene (ABS) and polyether ether ketone (PEEK). 
     In a preferred embodiment, as shown in  FIGS. 1 ,  3 , and  4 , the protective cap  16  has a height  62  defined by a protective cap sidewall  64  extending from a first protective cap end  66  to a second protective cap end  68 , wherein the terminal pin  20  extends through a protective cap throughbore  60  extending from the first protective cap end  66  to the second protective cap end  68 . As shown in  FIG. 3 , the terminal block  18  is positioned in a stacked relationship on the topside surface  56  of the protective cap  16 . The respective throughbores  60 ,  40  of the protective cap  16  and terminal block  18  are aligned such that the proximal region  42  of the terminal pin  20  resides therethrough. It should be noted however, that the protective cap  16  may or may not be incorporated with a feedthrough assembly  10  comprising a capacitor  14 . 
     In addition, the protective cap  16  is constructed such that a plurality of walls  70  project from the topside surface  56  of the protective cap  16 . More preferably, these walls  70  interconnect at a central junction  72  ( FIG. 4 ). These walls  70  have a preferred wall thickness  74  of about 0.5 mm to about 5 mm, a preferred wall height  76  of about 1 mm to about 10 mm, and a preferred wall length  78  of about 1 mm to about 10 mm. The walls  70  electrically insulate the terminal blocks  18  from each other. 
     In a preferred embodiment, a partial weld  80  is formed between the first metal of the terminal pin  20  and the second metal of the terminal block  18 . More specifically, a portion of the proximal end region  42  of the terminal pin  20  is joined to a portion of the throughbore  40  diameter of the terminal block  18  surrounding the terminal pin  18 . As shown in  FIGS. 1-5 , this partial weld  80  is formed of a shape similar to that of a partial “button”. As illustrated in  FIG. 3 , this partial “button” shaped weld  80  fills a portion of the gap  82  between the terminal block  18  and terminal pin  20 . Alternatively, this partial “button” shaped weld  80  may be formed above the top surface  52  of the terminal block  18 . In this embodiment, the terminal pins  20  protrude above the top surface  52  of the terminal block  18 . 
     In a preferred embodiment, an alloy is formed comprising a mixture of the first metal of the terminal pin  20  and the second metal of the terminal block  18 . This partial weld  80  enables the joining of these two dissimilar metals, the first metal and second metal, of the terminal pin  20  and terminal block  18 , respectively, such that the adjacent protective cap  16  is not deformed or damaged. 
     In a preferred embodiment, heat generated during the welding process is localized to a portion of the terminal pin  20  and terminal block  18 . A sufficient amount of heat is generated to effectively form the alloy joining the two dissimilar first and second metals without generating too much heat such that the protective cap  16  and other adjacent materials of the feedthrough assembly  10  are damaged. 
     In a preferred embodiment, a laser welding instrument such as a Lasag® model SLS200 is used to partially join the terminal pin  20  to the terminal block  18 . In a preferred embodiment, a laser pulse frequency of between about 2 Hz to about 10 Hz is used with a pulse width of between about 0.5 ms to about 2.0 ms generating a welding energy from about 0.5 J to about 2.0 J is used to weld the dissimilar metals together. These preferred laser welding parameters provide a weld  80  that sufficiently joins the two dissimilar metals of the terminal pin  20  and the terminal block  18  such that the adjacent protective cap  16  is not damaged or deformed. 
     In a preferred embodiment, illustrated in  FIGS. 1 ,  2  and  4 , a partial weld  80  covering about 10 percent to about 80 percent of the perimeter  83  ( FIG. 5 ) of the terminal pin  20  is joined to the terminal block  18 . In a more preferred embodiment, about 20 percent to about 60 percent of the perimeter  83  of the terminal pin  20  is welded to the terminal block  18 .  FIG. 2  further illustrates these featured embodiments of the partial weld  80 . A partial weld of about 80 percent is identified as  80 A, a 60 percent partial weld embodiment is identified as  80 B, a 40 percent partial weld embodiment is identified as  80 C, and a 20 percent partial weld is identified as  80 D. 
     These partial weld parameters provide a weld of sufficient strength and robustness that allows for the joining of the refractory metal, the first metal, of the terminal pin  20  to that of the terminal block  18 . In addition, these metals are joined without generating enough heat to deform the adjacent polymeric protective cap  16 . Furthermore, these preferred partial welding embodiments minimize the amount of heat transferred into the terminal block  18 . Minimizing heat transfer into the terminal block  18  minimizes heat radiated out of the terminal block  18 , to thereby help prevent degradation of the protective cap  16 . Therefore, the preferred partial weld process of the present invention minimizes heat absorption and heat radiation from the terminal block  18 . On the other hand, excessive radiated heat may contribute to thermal degradation of the adjacent materials of the feedthrough assembly  10 . 
     In a preferred embodiment, the unwelded portion of the partial weld  80  faces the central junction as shown in  FIGS. 1 ,  2   4  and  5 . This further helps prevent damage to the protective cap  16  as there is very little, if any, heat radiated into a corner  85  formed where two walls  70  of the cap  16  meet at the junction  72 . 
     As further shown in  FIGS. 2 ,  4  and  5 , the feedthrough assembly  10  includes the filter capacitor  14  that provides for filtering undesirable EMI signals before they can enter the device housing via the terminal pins  20 . The filter capacitor  14  comprises a ceramic or ceramic-based dielectric monolith  86  having multiple capacitor-forming conductive electrode plates formed therein. The capacitor dielectric  86  preferably has a circular cross-section matching the cross-section of the ferrule  22  and supports a plurality of spaced-apart layers of first or “active” electrode plates  88  in spaced relationship with a plurality of spaced apart layers of second or “ground” electrode plates  90 . The filter capacitor  14  is preferably joined to the feedthrough  12  adjacent to the insulator side  30  by an annular bead  92  of conductive material, such as a solder or braze ring, or a thermal-setting conductive adhesive, and the like. The dielectric  86  includes lead bores  94  provided with an inner surface metallization layer. The terminal pins  20  pass there through and are conductively coupled to the active plates  88  by a conductive braze material  96  contacting between the terminal pins  20  and the bore metallization. In a similar manner, the ground plates  90  are electrically connected through an outer surface metallization  98  and the conductive material  92  to the ferrule  22 . 
     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.