Abstract:
Terminal pins that include a refractory metal forming a full perimeter weld connected to a terminal block including 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. 61/352,612 filed Jun. 8, 2010. 
    
    
     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. 
     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 diffult 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 interference (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. 
     Traditional methods of joining dissimiliar metals such as a refractive metal to a non-refractive metal, typically result in cracking of the joint. This is particularly the case when niobium and nickel are joined together. Such joint cracks tend to create pathways for the introduction of undesirable debris and contamination. Debris and contamination could enter the assembly and potentially affect the electrical performance of the feedthrough assembly and/or connected device. What is desired is a feedthrough assembly and method of assembly thereof that produces a crack free joining of dissimiliar metals, for example of a refractive metal and a non-refractive metal, particularly the metals niobium and nickel. 
     In conjunction with the difficulties in joining dissimilar metals, other constraints from adjacent materials of the feedthrough assembly present additional difficulties that 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, provide 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, preferably 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 connection of the terminal pin to the terminal block. As such, the present invention relates to a feedthrough assembly and manufacturing process thereof that provides a robust crack free full perimeter joint about the terminal pin to effectively join the dissimilar metals of the terminal pin and terminal block. In addition, joining the terminal pin to the terminal block, 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 view of the filter capacitor assembly shown in  FIG. 1 . 
         FIG. 5  is a magnified top view showing an embodiment of one of the welds of the present invention. 
         FIG. 6  is an illustration of a nickel-niobium binary phase diagram. 
         FIG. 7  shows a cross-sectional illustration of a prior art weld. 
         FIG. 7A  shows a cross-sectional micrograph image of a prior art weld. 
         FIG. 8  illustrates a cross-section of a preferred weld embodiment of the present invention. 
         FIG. 8A  shows a cross-sectional micrograph image of a preferred weld embodiment of the present invention. 
         FIG. 9  illustrates a preferred embodiment of an assembly process of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings,  FIGS. 1 ,  3 , and  9  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. 
     Non-limiting examples of braze materials  38  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 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 chemical 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 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 pins  20  comprise niobium and niobium alloys. 
     As shown in  FIGS. 1-5 ,  8 ,  8 A, and  9 , 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 height  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, 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 an end portion  54  of the terminal pin  20  resides above the topside surface  52  of the terminal block  18  ( FIGS. 3 and 9 ). In a preferred embodiment, the terminal pin  20  resides from about 0.02 mm to about 0.2 mm above the top surface  52  of the terminal block  18 . Although it is preferred that the end portion  54  of the terminal pin  20  is positioned above the topside surface  52  of the terminal block  18 , it is contemplated that the end portion  54  of the terminal pin  20  may be positioned below the top surface  52  of the terminal block  18 . 
     Furthermore, each terminal block  18  is preferably positioned on the topside  56  of a 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), polyether ether ketone (PEEK), low and high density polyethylene, polyethylene chloride, polypropylene, acetal, acetylcellulose, acrylic resin, and polytetrafluoroethylene. In an alternate preferred embodiment, the protective cap  16  may also be composed of a ceramic insulator material. 
     In a preferred embodiment, as shown in  FIGS. 1 ,  3 ,  4 , and  9  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 specific embodiment of joining niobium to that of nickel, it is preferred that a weld  80  of increased niobium content is formed. Such a weld  80  of increased niobium content is preferred because it reduces mechanical stresses within the niobium-nickel weld  80 , thereby increasing the robustness and minimizing weld cracking. 
     It is believed that the increased robustness of the weld  80  of the present invention is attributed to the increased niobium content. As can be seen in the nickel-niobium binary phase diagram, illustrated in  FIG. 6 , an increased niobium content with respect to nickel, reduces the occurrence of inter-metallic phases. As illustrated in the diagram of  FIG. 6 , there are fewer inter-metallic phases, such as Ni 3 Nb 7  and Ni 3 Nb, above about 65 weight percent niobium. 
     In a preferred embodiment, a full perimeter weld  80  is formed between the first metal of the terminal pin  20  and the second metal of the terminal block  18 . More specifically, the weld  80  is formed between the first metal of the terminal pin  20  and terminal block  18 , such that weld encompasses the full perimeter  84  of the terminal pin  20 . This is shown in  FIGS. 1 ,  2 ,  4  and  5 . It is preferred that the weld  80  is formed about the proximal end region  54  of the terminal pin  20 . It is also preferred that the weld  80 , as shown in  FIGS. 1-5 ,  8 ,  8 A, and  9 , is formed of a shape similar to that of a “button”. As illustrated in the cross-sectional view of  FIG. 3 , this “button” shaped weld  80  is formed above the top surface  52  of the terminal block  18 . A “button weld” is herein defined as a weld having the general shape and appearance of that of a button as illustrated in  FIGS. 8 and 8A . 
     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 . In a preferred embodiment, an alloy comprising about 65 weight percent to about 95 weight percent of the first metal is combined with about 35 weight percent to about 5 weight percent of the second metal. In a more preferred embodiment, a weld  80  comprising from about 65 to about 95 weight percent niobium as the first metal is combined with about 35 to about 5 weight percent nickel as the second metal of the terminal block. 
     As previously mentioned, a niobium content of greater than about 65 weight percent provides for a niobium-nickel alloy with few inter-metallic phases. A weld  100  ( FIGS. 7 ,  7 A) comprising inter-metallic phases typically results in an undesirably brittle weld that is prone to cracking.  FIGS. 7 and 7A  illustrate a cross-sectional view of a prior art weld  100  having cracks  102  therewithin. Weld cracks  102 , such as those illustrated and shown in  FIGS. 7 and 7A , are typically formed during prior art joining processes. It is possible that a crack  102  or multiple cracks  102  could propagate through the weld  100 , creating a pathway for the entry of undesirable debris that could disrupt the performance of the feedthrough assembly  10  and/or medical device. In contrast, the weld $0 of the present invention lacks these cracks  102  of the prior art weld  100 , as shown in the cross-sectional views of  FIGS. 8 and 8A . 
     As shown in the illustration and micrograph of  FIGS. 7 and 7A , respectively, prior art weld  100  has an appearance of a flat “nail head” which is unlike the preferred “button” shape of the present invention weld  80 . It is believed that the curved shape of the preferred “button” weld  80  acts as a stress reducer that contributes to the increased robustness of the present weld  80 . 
     Furthermore, as shown in the cross-sectional micrograph image of the present weld  80  of  FIG. 8A , and the illustration of  FIG. 8 , there is a distinct boundary layer  106  positioned on either side of the weld  80 . This boundary layer  106  delineates the first metal of the terminal pin  20  from the second metal of the terminal block  18 . As shown in the illustration and micrograph of  FIGS. 8 and 8A , the “button weld”  80  is distinguished from the prior art weld  100  shown in  FIGS. 7 and 7A  by the presence of the boundary layer  106 , a well defined distinct region comprising a mixture of the first metal of the terminal pin  20  and the second metal of the terminal block  18 . As shown, the boundary layer  106  has a well defined width  82  extending from the top surface  52  of the terminal block  18  to a position distally from the top surface  52 . 
     Unlike the weld  80  of the present invention, the prior art weld  100  as shown in the micrograph of  FIG. 7A , does not have a distinct boundary layer  106 . The prior art weld  100  is characterized by a weld gradient region  104  in which the first metal of the terminal pin  20  appears to gradually diffuse or transition into the second metal of the terminal block  18 . This weld gradient region  104  appears of a distinct shade of grey, contrasting between the darker and lighter shades of grey of the terminal pin  20  and terminal block  18 , respectively. 
     It is believed that the combination of the curved “button” like weld shape and the distinct boundary layers  106  between the first and second metals contributes to the reduced mechanical stress, therefore enabling a crack free weld. In addition, it is believed that the weld gradient region  104  of the prior art weld  100 , comprises undesirable inter-metallic phases that contribute to its brittleness. 
     The present button weld  80  is manufactured during a welding process by a beam  110  of laser energy focused at a center region  114  of the end  54  of the terminal pin  20 , as illustrated in  FIG. 9 . Focusing the laser energy at substantially the center  114  of the terminal pin  20  provides a concentration of heat there that melts and deforms the first metal of the terminal pin  20 . By focusing the heat energy at the center region  114  of the end  54  of the terminal pin  20 , the first metal content of the weld  80  is increased. A sufficient amount of heat is generated to effectively form the alloy joining the two dissimilar first and second metals  20 ,  18  without generating too much heat such that the protective cap  16  and other adjacent materials of the feedthrough assembly  10  are damaged. In addition, focusing the beam of laser energy  110  about the center region  114  of the end  54  of the terminal pin  20 , dissipates the energy away from the protective cap  16  thereby minimizing degradation of the adjacent cap  16 . 
     In a preferred embodiment, a laser welding instrument  108  ( FIG. 9 ) such as a Lasag® model SLS200 is used to join the terminal pin  20  to the terminal block  18 . In a preferred embodiment, a laser pulse frequency of between about 10 Hz to about 30 Hz is used with a pulse width of between about 1.0 ms to about 5.0 ms to thereby generate a welding energy of from about 1.0 J to about 5.0 J to weld the dissimilar metals together. These preferred laser welding parameters provide a full perimeter weld  80  that sufficiently joins the two dissimilar metals of the terminal pin  20  and terminal block  18 . 
     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.