Abstract:
A method for making a feedthrough assembly for an implantable electronic medical device comprises providing a metallic ferrule having an outer surface and an aperture defined by an inner lumen surface; providing an insulator, the insulator having a first surface and a second surface. At least one of the first surface and the second surface of the insulator includes a brazing region disposed thereon. The braze material is applied to the brazing region and the insulator is positioned within or around the metallic ferrule such that the positioned insulator brazing region and the metallic ferrule outer surface or inner lumen surface defines a braze gap. The braze gap has a width ranging between 10 μm to 50 μm. The feedthrough assembly is then heated at a temperature conducive to melt the braze material in the braze gap thereby forming a hermetic seal between the ferrule and said insulator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/256,668, filed on Oct. 30, 2009. The entire disclosure of the above application is incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present teachings relate to electrical feedthrough assemblies of hermetically sealed implantable electronic devices. 
       SUMMARY 
       [0003]    The present teachings provide a method for making a feedthrough assembly. The method may include providing a metallic ferrule having an outer surface and a first aperture defined by an inner surface. An insulator may be provided in the first aperture, where the insulator has a first surface separated from the inner surface by a first braze gap, and a second surface defining a second aperture. A conductive element may be provided in the second aperture, where the conductive element is spaced from the insulator by a second braze gap. A braze material may then be applied in the first and second braze gaps and the assembly subsequently heated to braze the ferrule to the insulator, and to braze the conductive element to the insulator. The first and second braze gaps have a width that ranges between 10 and 50 μm, inclusive. 
         [0004]    The present teachings also provide for a medical device. The medical device may include a housing and a connector module for connecting leads to electrical components internal to the housing. A feedthrough assembly located in the connector module connects the leads to the electrical components. The feedthrough may include a metallic ferrule, a conductive member, and an insulator disposed between the metallic ferrule and the conductive member. The insulator may be separated from the metallic ferrule by a first braze gap and may be separated from the conductive member by a second braze gap that are each filled with a braze material that heremetically seals the feedthrough assembly, wherein the first and second braze gaps have a width that ranges between 10 and 50 μm, inclusive. 
         [0005]    Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present teachings. 
     
    
     
       DRAWINGS 
         [0006]    The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present teachings. 
           [0007]      FIG. 1  is a schematic diagram showing in cross-section view a feedthrough assembly according to various embodiments of the present teachings; 
           [0008]      FIG. 2  is a schematic representation of a back-scattered electron image of the braze gap depicting the various phases and compositional arrangement of a titanium ferrule brazed with gold during brazing according to various embodiments of the present teachings; 
           [0009]      FIG. 3  is an electron-probe microanalyzer graph depicting the relative amounts of the various intermetallic phases between a titanium ferrule and a gold braze during brazing between 700 and 1300° C. according to various embodiments of the present teachings. 
       
    
    
       [0010]    Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION 
       [0011]    Exemplary embodiments are provided so that this disclosure will be thorough, and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth, such as examples of specific components, devices and methods, to provide a thorough understanding of embodiments of the present teachings. It will be apparent to those skilled in the art that specific details need not be employed, that exemplary embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the present teachings. In some exemplary embodiments, well-known processes, well-known device structures and well-known technologies are not described in detail. 
       Feedthrough Assemblies for Implantable Medical Devices 
       [0012]      FIG. 1  illustrates an exemplary electronic implantable medical device  100  incorporating a feedthrough assembly  10  according to the present teachings. Medical device  100  may be any type of implantable device and, particularly, may be an implantable pulse generator for a cardiac pacemaker that provides electrical stimulation to an arrhythmic heart or neural tissue, an implantable defibrillator, an implantable cardioverter, an implantable cardiac pacemaker-cardioverter-defibrillator (PCD), an implantable chemical/biochemical sensor (e.g., a glucose sensor), an implantable drug, medicament or metabolite delivery device (e.g., an insulin pump), or an implantable medical device that performs in vivo diagnostic monitoring and telemetry. Regardless, medical device  100  generally includes a medical device housing  102  having a connector module  104  coupled thereto. Connector module  104  electrically couples various internal electrical components (not shown) located within medical device housing  102  to external operational and/or diagnostic systems (not shown) located distal to device  100  through use of leads  106 . Electrical connection of leads  106  to the internal electrical components is accomplished through use of feedthrough assembly  10 . 
         [0013]    An exemplary feedthrough assembly  10  according to the present teachings may include a cylindrical ferrule  11 , a conductive element  50  (e.g. a pin), and a cylindrical insulator  20 . Ferrule  11  includes a ferrule outer surface  12  and a ferrule lumen surface  14  that defines an aperture  13 . Ferrule  11  may be brazed to insulator  20  and, therefore, is separated from insulator  20  by a ferrule-insulator braze gap  16 . Insulator  20  includes an insulator outer surface  18  and an insulator lumen surface  22 . Insulator  20  may be brazed to conductive element  50  and, therefore, is separated from conductive element  50  by an insulator-conductive element braze gap  24 . Braze gaps  16  and  24  are filled with braze material  30 . While the exemplary embodiment in  FIG. 1  shows a cross-section of a cylindrical insulator  20 , a cylindrical ferrule  11 , and a cylindrical conductive element  50 , other shapes can be envisioned and the present teachings should not be limited thereto. Further, although only a single conductive element  50  is illustrated, it should be understood that feedthrough assembly  10  may include a ferrule  11  disposed about a plurality of conductive elements  50 . 
         [0014]    Moreover, other exemplary embodiments of feedthroughs are described in U.S. Pat. No. 4,678,868 issued to Kraska, et al. and entitled “Hermetic electrical feedthrough assembly,” in which an alumina insulator provides hermetic sealing and electrical isolation of a niobium electrical contact from a metal case. Further, for example, a filtered feedthrough assembly for implantable medical devices is also shown in U.S. Pat. No. 5,735,884 issued to Thompson, et al. and entitled “Filtered Feedthrough Assembly For Implantable Medical Device,” in which protection from electrical interference is provided using capacitors and Zener diodes incorporated into a feedthrough assembly. Other implantable feedthrough assemblies useful in the present teachings include those described in U.S. Pat. Nos. 7,164,572, 7,064,270, 6,855,456, 6,414,835 and 5,175,067 and U.S. Patent Application Publication No. 2006/0247714, all commonly assigned and all incorporated herein in their entireties. 
         [0015]    Ferrule  11  may be formed of a conductive material. In some embodiments, the conductive material may be a metallic material including titanium, niobium, platinum, molybdenum, tantalum, zirconium, vanadium, tungsten, iridium, palladium, and any combination thereof. Ferrule  11  may have any number of geometries and cross-sections so long as ferrule  11  is an annular structure such as a ring with a lumen therein to hermetically seal insulator  20 . In some embodiments, ferrule  11  may surround insulator  20  and provide ferrule lumen surface  14  to contact braze material  30  disposed in the ferrule-insulator braze gap  16  to form a hermetic seal. 
         [0016]    Insulator  20  may be formed from a material including an inorganic ceramic material (e.g., sapphire), a glass and/or a ceramic-containing material (e.g., diamond, ruby, crystalline aluminum oxide, and zinc oxide), and an electrically insulative material. Insulator  20  may also be formed of liquid-phase sintered ceramics, co-fired ceramics, a high-temperature glass, or combinations thereof. Insulator  20  may also include a sputtered thin niobium or titanium-niobium coating at least at surfaces  18  and  22 . Because the sputtered niobium coating is thin, the coating is not shown for illustration purposes. Insulator  20  is not limited to any particular configuration for use in feedthrough  10 , so long as insulator  20  accommodates one or more electrically conductive elements  50 . 
         [0017]    Braze material  30  may be formed of a material such as gold. Other materials sufficient to braze ferrule  11  to insulator  20 , and sufficient to braze insulator  20  to conductive element  50 , however, are contemplated. For example, braze material  30  may include materials such as high purity gold, and gold alloys containing silver, copper, tin, and/or zinc without departing from the spirit and scope of the present teachings. The braze material can be reinforced with oxide, carbide, and nitride particles of refractory metals such as molybdenum, tungsten, hafnium, niobium, zirconium 
         [0018]    Conductive element  50  may be formed of materials such as niobium, titanium, niobium-titanium alloy, titanium-6Al-4V alloy, titanium-vanadium alloy, platinum, iridium, molybdenum, zirconium, tantalum, vanadium, tungsten, palladium, nickel super alloy, nickel-chromium-cobalt-molybdenum alloy, and alloys, mixtures, and combinations thereof. 
         [0019]    Feedthrough assembly  10  provides an electrical circuit pathway extending from the interior of hermetically-sealed device housing  102  to an external point outside housing  102  while maintaining the hermetic seal of the housing  102 . The fluid tight hermetic seal is formed by metal braze  30  disposed in ferrule-insulator braze gap  16  and insulator-conductive element braze gap  24  formed between the insulator  20  and the ferrule  11  and between the insulator  20  and conductive element  50 , respectively. A conductive path is provided through feedthrough  10  by conductive element  50 , which is electrically insulated from housing  102 . 
         [0020]    According to the present teachings, there is a narrow requirement for widths of the braze gaps  16  and  24 . Widths of ferrule-insulator braze gap  16  and insulator-conductive element braze gap  24  are controlled to tighter tolerances because if the dimensions are not closely controlled, the volume of the braze gap changes and only small variations of volume can be accommodated by brazing material  30 , such as gold. If the gap volume is too small, oversized braze fillets may be formed and the gold braze can spill. Moreover, a convex shaped braze fillet may exert a strong tensile loading and promote delamination of braze  30 . Further, if the gap volume is too big, the gaps  16  and  24  cannot be filled completely, and the feedthrough  10  will not pass performance requirements. Feedthrough assemblies  10  of the present teachings, therefore, have specified dimensional tolerances for braze gaps  16  and  24  such that ferrule-insulator braze gap  16  and insulator-conductive element braze gap  24  have a width ranging between about 10 μm to about 50 μm, inclusive. In some embodiments, widths of ferrule-insulator braze gap  16  and insulator-conductive element braze gap  24  may be 10 μm, or 20 μm, or 30 μm, or 40 μm, or 50 μm, 
         [0021]    Feedthroughs  10  of the present teachings comprise a ferrule-insulator braze gap  16  and an insulator-conductive element braze gap  24  having widths ranging between about 10 μm to about 50 μm, inclusive, ensure that ferrule  11  and insulator  20  are hermetically adhered by brazing material  30 . In this regard, during the brazing process, instantaneous alloying takes place between the niobium sputter coating disposed on insulator lumen surface  22  of insulator  20  and gold of braze material  30  adjacent pin  20 , and the gold of braze material  30  and titanium of ferrule lumen surface  14 . The concentration of niobium and titanium in the instantaneous alloying depends on the temperature schedule during brazing and on the widths of gaps  16  and  24  between insulator  20  and pin  50  and insulator  20  and ferrule  11 . For example, during brazing, titanium and gold form a series of intermetallic compounds, and a gold mixed crystal phase field showing solid state (ss) solubility of roughly up to 6 mass % titanium as shown in  FIG. 2  and  FIG. 3 . 
         [0022]    When using a metallic ferrule  11  comprising titanium and a braze material  30  of gold, a gold-titanium solid state solution hardened mixture, and some of the titanium-gold intermetallic compounds, which can include TiAu 4 , TiAu 2 , TiAu and Ti 3 Au. These instantaneously formed gold-titanium alloys are considerably stronger than pure gold, and contribute significantly to the mechanical performance of the brazed joint. This increase of strength is reached only if gap  16  is no larger than 50 μm to enable titanium from ferrule  11  to completely diffuse through braze gap  16 , such that the entire braze gap  16  is occupied by the gold-titanium alloy. The local chemical composition of the braze  30  and the mechanical properties can be analyzed by microprobe and nano-indentation, respectively. 
       Methods for Making a Feedthrough Assembly 
       [0023]    With reference again to  FIG. 1 , feedthrough assembly  10  may be manufactured in the following exemplary manner. Insulator  20  can be inserted through ferrule  11  and then conductive element  50  can be inserted through insulator  20  such that gaps  16  and  24  between ferrule  11  and insulator  20 , and between insulator  20  and conductive element  50 , respectively, range between 10 μm to 50 μm, inclusive. Insulator  20  is hermetically bonded to ferrule  11  by placing braze material  30 , for example, gold, in ferrule-insulator braze gap  16 . Conductive element  50  is hermetically bonded to insulator  20  by placing braze material  30  in insulator-conductive braze gap  24 . Feedthrough assembly  10  may then be heated at a temperature (e.g., 700° C.-1300° C.) able to melt braze material  30  in ferrule-insulator braze gap  16  and insulator-conductive element braze gap  24 , thereby forming a hermetic seal between ferrule  11  and insulator  20 , and between insulator  20  and pin  50  having instantaneously formed alloys that are considerably stronger than pure gold. 
         [0024]    Insulators  20  of the present teachings may be made from ceramic materials or biocompatible, high-temperature co-fired alumina. Regardless, insulator  20  should have a very smooth insulator outer surface  18 . Insulators  20  may be manufactured by applying tape casted green sheets of alumina mounted on frames. Through hole vias are punched, the vias can be filled with a platinum metal paste, and surface metallization may be screen printed. Individual sheets can be laminated, and subsequently fired. Next, dicing can be used to separate individual parts. Semi-circular ends can be manufactured by grinding. Infeed ultra-precision grinding can be used to obtain a partially effective insulator surface on a CNC grinder Absolute Grinding Company Inc., Cleveland, Ohio, USA, using a Studer S35 grinder according to manufacturer&#39;s specifications and instructions. Optionally, after surface inspection, cracks in surface  18  greater than 70 μm can be removed by ultrafine polishing. Insulator  20  may also be coated with a metallic film on the insulator outer surface  18  and insulator lumen surface  22  to enable wetting of braze material  30 . 
         [0025]    The insulator outer surface  18  can be polished using any commercial polishing machine, such as a polishing machine commercially available (for example, Struers RotoPol 35, Struers Inc., Cleveland, Ohio, USA) to the desired/specified surface quality (i.e., having no surface cracks at least at insulator outer surface  18  having a crack size greater than the critical flaw size ranging from about 30 μm to about 70 μm). Polishing of insulator  20 , insulator outer surface  18 , or insulator lumen surface  22  can be conducted at a cloth disc rotation of 150 rpm and a sample rotation of 40 rpm, respectively. Diamond suspensions of 3 μm and 0.05 μm (Kemet International Limited, Maidstone, Kent, UK) together with an ethanol-containing high-quality lubricant (DP-Lubrication Blue, Struers Inc., Cleveland, Ohio, USA) can be consecutively applied. The applied contacting force between insulator  20  and the cloth can vary from about 20N to about 80N. The polishing can be conducted for a period of time ranging from about 1 minute to about 60 minutes. 
         [0026]    Outer surface  18  and insulator lumen surface  22  may be polished using a polishing procedure including rough polishing, intermediate polishing, and final polishing using 30 seconds of polishing time at each step. Three different abrasive papers with decreasing abrasiveness can be applied. For example, a 3 μm SiC paper with an air-cushion metal pad may be applied for rough polishing, 0.1 μm diamond paper with a metal-flat pad may be used for intermediate polishing, and 0.05 μm alumina paper with a rubber-flat pad may be used for final polishing. In each step, 91% volume isopropyl alcohol may be applied as a coolant. Sharp edges can further be rounded in a tumbling process. Such a polishing process achieves a surface quality having no cracks larger than the desired critical flaw size of 70 μm or less on the insulator outer surface  18  and insulator lumen surface  22  of the insulator  20  to be brazed. Results of the polishing steps can be verified using any one or more surface analysis tools, including confocal microscopy, electrical capacitance, electron microscopy and interferometer analysis. Cracks smaller than 11 μm may need to be removed if the crack size is greater than the critical flaw size for that particular insulator material when considering the insulator&#39;s acting stress. 
         [0027]    The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention.