Abstract:
A feedthrough terminal pin assembly comprises an outer ferrule hermetically sealed through a braze joint to an insulator seated within the ferrule is described. The insulator is also hermetically brazed to at least one terminal pin. The terminal pin is provided with a braze retention structure such as an annular groove that prevents braze material from filleting past the groove. Similarly, either the ferrule or the insulator is provided with a retention structure such as an annular groove that prevents braze material spill out from the insulator/ferrule interface. In that manner, the braze retention structures keep braze material from accumulating in unwanted areas where it could adversely affect hermeticity as well as proper attachment of an EMI filter to the feedthrough assembly.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to a hermetic feedthrough terminal pin assembly, preferably of the type incorporating a capacitor filter. More specifically, this invention relates to feedthrough terminal pin capacitor filter 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. The feedthrough assembly provides a hermetic seal that prevents passage or leakage of fluids into the medical device.  
         [0003]     2. Prior Art  
         [0004]     Feedthrough terminal pin 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, a defibrillator, and the like, the feedthrough assembly comprises one or more conductive terminal pins supported by an insulator structure seated in the ferrule. Suitable materials may for the ferrule include titanium, tantalum, niobium, stainless steel or combinations of alloys thereof. The ferrule may be of any geometry including round, rectangle, and oblong.  
         [0005]     The terminal pins are typically composed of platinum or a combination of platinum and iridium and provide for passage of electrical signals from the exterior to the interior of the medical device. Platinum and platinum-iridium alloys are biocompatible materials that create a hermetic seal through a gold brazing process that seals any gap between the terminal pin and the supporting insulator. A gold brazing process is also used to hermetically seal any gap between the supporting ferrule and the insulator.  
         [0006]     However, too much braze material at the insulator/ferrule interface or at the insulator/terminal pin interface can cause excess tensile stresses leading to cracking of the ceramic insulator with possible subsequent loss of hermeticity. Another problem with hermetic seals at the feedthrough interfaces occurs if the space allotted for the braze material is insufficient to hold or receive its volume. In that case, the braze material can spill out of its interface channel. In addition to causing a weak braze joint and potentially compromising hermeticity, braze spill out presents an esthetic condition that adversely impacts scrap. Also, if an EMI filter is subsequently attached to the feedthrough to attenuate unwanted EMI interference, a flat attachment surface, free of braze spill out, is needed.  
         [0007]     Other problems related to excessive braze material at the feedthrough interfaces include unwanted wetting of critical areas. One place this occurs is at the feedthrough perimeter where the ferrule is laser welded to the device shield. Excessive braze material at this interface can compromise the hermetic seal between the ferrule and the device shield.  
         [0008]     A second area of concern is on the surface of the insulator between the terminal pins. Over wetting in this area can result in excessive braze material between two terminal pins. This can shorten the pin-to-pin distance which, under high voltage conditions, can cause arcing, or in the case of a filtered feedthrough, premature dielectric breakdown. Another problem is that excessive flow of braze material on the insulator surface can sporadically dewet, then solidify leaving behind small braze balls on the insulator. This unwanted material can also cause arcing or dielectric breakdown under high voltage conditions.  
         [0009]     Wetting of the terminal pin below the insulator/terminal pin interface creates another potential problem area. The flex circuit or wire bond substrate leading to the device control circuitry is subsequently welded to the terminal pin here and electrical shorting can occur should the flex circuit come into contact with this excess braze material.  
         [0010]      FIGS. 1 and 2  show a feedthrough  10  compromised by excess braze to further illustrate some of these problems. The feedthrough terminal pin assembly  10  comprises a so-called unipolar configuration having a terminal pin  12  extending through a bore in an electrically insulating or dielectric material such as an alumina or fused glass type or ceramic-based insulator  14 , hereinafter collectively referred to as an insulator, nested within a ferrule  16 . A layer of metal may be applied to the surface of the insulating material, referred to as metallization, to aid in the creation of a brazed hermetic seal. Metallization materials include titanium, niobium, tantalum, gold, molybdenum, silver, platinum, copper, or combinations thereof. The metallization layer may be applied by various processes including sputtering, e-beam deposition, jet vapor deposition, pulsed laser deposition, chemical vapor deposition, plating, electro-less plating or cladding.  
         [0011]     The ferrule  16  comprises a cylindrically-shaped body  18  having an upper annular flange  20  extending outwardly along a plane generally perpendicular to the longitudinal axis of the ferrule body. The ferrule body  18  comprises a cylindrically-shaped outer wall sized to snuggly fit in an opening  22  provided in the device shield  24  with the flange  20  resting on an outer surface thereof.  
         [0012]     The ferrule body  18  also has a cylindrically-shaped inner wall extending to a lower inwardly-extending annular lip  26  that is sized to receive the insulator  14  in a snug-fitting relationship. That way, the lower end surface  28  of the insulator  14  is coplanar with the lower end surface  30  of the ferrule body. With the insulator  14  seated in the ferrule  16  in this manner, an annulus  32  is formed between them extending along the length of the cylindrically-shaped inner wall of the ferrule body  18  from the annular lip  26  to an annular cut-out channel  34  where the flange  20  meets the body.  
         [0013]     A ring-shaped braze pre-form (not shown) is received in the cut-out. Non-liming examples of braze material include gold, particularly gold alloys, and silver. Heating the pre-form causes it to melt and flow into the annulus  32  between the insulator  14  and the ferrule body  18 . Upon cooling, the resulting braze  36  now hermetically seals the insulator  14  to the ferrule  16  along the entire length of the annulus  32  and the cut-out  34 . However, too much braze material was used and some has spilled out of the cut-out  34 , collecting on the upper surface of the ferrule  16 . This is shown as braze spill out  36 A in  FIG. 2 .  
         [0014]     The insulator further comprises a bore  38  that is sized to provide an annulus  40  between it and the terminal pin  12 . A frusto-conically shaped cut-out  42  is provided in the upper surface  44  of the insulator in communication with the annulus  40 . A ring-shaped braze pre-form (not shown) is received in this cut-out  42 . As with the previously described ferrule braze pre-form, heating the terminal pin pre-form causes it to melt and flow into the annulus  40  between the insulator  14  and the terminal pin  1   2 . Upon cooling, the resulting braze  46  hermetically seals the terminal pin  12  to the insulator  14  along the entire length of the annulus  40 . However, too much braze material was used and some has filleted part way up the terminal pin past the upper surface  44  of the insulator  14 . This filleting is caused by the difference in the coefficients of friction of the braze material  46  and the terminal pin  12 . Braze filleting is undesirable for a number of reasons. Foremost is that it can impair proper attachment of an EMI filter or a molded header assembly to the terminal pin  12  at the upper surface of the insulator. Excess braze material on the terminal pin is also aesthetically unacceptable.  
         [0015]     A weld  48  hermetically seals the flange perimeter to the shield  24 .  
         [0016]     Therefore, there is a need for feedthrough structures that prevent filleting and spill out of braze materials into areas where they can compromise hermeticity as well as prevent the proper attachment of EMI filters and header assemblies to the medical device at the feedthrough. The present feedthroughs provide just such structures.  
       SUMMARY OF THE INVENTION  
       [0017]     In a preferred form, the feedthrough terminal pin assembly comprises an outer ferrule hermetically sealed through a braze joint to an insulator seated within the ferrule. The insulator is also hermetically brazed to at least one terminal pin. That way, the feedthrough assembly prevents leakage of fluid, such as patient body fluid in a human implant application, past the hermetic seal at the insulator/ferrule interface and at the insulator/terminal pin interface.  
         [0018]     A filter capacitor having first and second sets of conductive electrode plates embedded within an insulative or dielectric body such as a monolithic ceramic body is mounted to the ferrule. One set of the conduction plates is electrically connected to the terminal pin while the other set is electrically connected to the grounded ferrule. The filter capacitor prevents unwanted EMI signals from transmitting via the terminal pins into the interior of the medical device. In order for the filter capacitor to function, it must be properly seated on the ferrule and possibly the insulator. Braze material filleted onto the terminal pin and braze spill out can hinder EMI filter attachment.  
         [0019]     However, according to the present invention, the terminal pin is provided with a braze retention structure such as an annular groove that prevents braze material from filleting past the groove. Similarly, either the ferrule or the insulator is provided with a retention structure such as an annular groove that prevents braze material spill out from the insulator/ferrule interface. In that manner, the braze retention structures keep braze material from accumulating in unwanted areas where it could adversely affect hermeticity as well as proper attachment of the EMI filter to the feedthrough assembly.  
         [0020]     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  
       [0021]      FIG. 1  is a side elevational view, partly in cross-section, of a prior art unipolar feedthrough assembly.  
         [0022]      FIG. 2  is a plan view of the prior art feedthrough assembly shown in  FIG. 1 .  
         [0023]      FIG. 3  is a side elevation view, partly in cross-section, showing a feedthrough assembly  100  comprising a ferrule  106  with an internal annular channel  116  and a notched terminal pin  102  for receiving excess braze materials according to the present invention.  
         [0024]      FIGS. 4A and 4B  are cross-sectional views of alternate embodiments of ferrules comprising an internal annular channel for receiving excess braze according to the present invention.  
         [0025]      FIG. 5  is a side elevational view of an alternate embodiment of a terminal pin  170  comprising an annular channel  172  for receiving excess braze according to the present invention.  
         [0026]      FIG. 6  is a side elevational view, partly in cross-section, of an alternate embodiment of a feedthrough assembly  200  comprising an insulator  214  with an annular groove  216  and a terminal pin  226  with an annular groove  232 , both for receiving excess braze according to the present invention.  
         [0027]      FIGS. 7A and 7B  are side elevational views of alternate embodiments of insulators having an annular groove for receiving excess braze material according to the present invention.  
         [0028]      FIG. 8  is a side elevational view, partly in cross-section of another embodiment of a feedthrough assembly according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0029]      FIG. 3  illustrates a feedthrough assembly  100  according to the present invention. The feedthrough assembly  100  is used in a cardiac pacemaker or defibrillator, and the like, for hermetically sealing the interior of the medical instrument against ingress of patient body fluids which could otherwise disrupt instrument operation or cause instrument malfunction. The unipolar feedthrough assembly  100  is similar in structure to the previously described prior art feedthrough assembly  10  except for a unique ferrule structure and terminal pin structure.  
         [0030]     The feedthrough  100  comprises a terminal pin  102  extending through a bore in an insulator  104  seated within a ferrule  106 . The ferrule  106  comprises an annular-shaped body  108  having an upper annular flange  110  extending outwardly along a plane generally perpendicular to the longitudinal axis of the ferrule body. The ferrule body  108  comprises a cylindrically-shaped outer sidewall  108 A that fits snuggly in an opening  112  provided in the device shield  114  with the flange  110  resting on an outer surface thereof.  
         [0031]     Unique to the present invention is a braze retention structure in the form of a sideways facing V-shaped annular groove  116  extending from the inner ferrule sidewall  108 B part way through the sidewall thickness. The V-shaped groove  116  is located adjacent to a lower inwardly-extending annular lip  118  that is sized to receive the insulator  104  in a snug-fitting relationship with the lower insulator surface  120  being coplanar with the lower ferrule surface  122 . With the insulator  104  properly seated in the ferrule  106 , an annulus  124  is formed between them extending along the length of the inner wall  108 B of the ferrule body  108  from the annular lip  118  to an annular cut-out channel  126  where the flange  110  meets the body  108 . The annular cut-out  126  comprises a squared-off lower portion transitioning into a chamfered upper portion. Of course, the cut-out  126  could be completely squared-off or completely chamfered.  
         [0032]     During the brazing process, a ring-shaped braze pre-form (not shown) is received in the annular cut-out  126 . When heated this, pre-form melts and flows into the annulus  124  between the insulator  104  and the ferrule body  108 . Upon cooling, the resulting braze  128  hermetically seals the insulator  104  to the ferrule  106  along the entire length of the annulus  124  and the cut-out  126 . However, the V-shaped groove  116  has taken up some of the braze, which is retained therein. Since there was no retaining structure in the prior art feedthrough  10 , the excess braze material had created braze spilled out  36 A on the upper surface  44  of the ferrule flange  20 .  
         [0033]     An important aspect of the present invention is that the annular groove  116  is a braze retention structure while the annular cut-out  126  is not. The distinction is that the groove  116  is an annular recess positioned at a location between the lower ferrule end surface  122  and an upper ferrule end surface  122 A without being in direct fluid flow communication with either end surface  122 ,  122 A. In contrast, the annular cut-out  126  is in direct fluid flow communication with the upper ferrule end surface  122 A.  
         [0034]     The insulator  104  further comprises a bore  130  that provides an annulus  132  between it and the terminal pin  102 . A frusto-conically shaped annular cut-out  134  is provided in the upper surface  136  of the insulator in communication with the annulus  132 . A ring-shaped braze pre-form (not shown) is received in the cut-out  134 . When heated, this pre-form melts and flows into the annulus  132  between the insulator  104  and the terminal pin  102 . Upon cooling, the resulting braze  138  hermetically seals the terminal pin  102  to the insulator  104  along the entire length of the annulus  132 . A weld  140  hermetically seals the flange perimeter to the shield  114 .  
         [0035]     In a similar manner as the V-shaped groove  116  provided in the ferrule sidewall, the terminal pin  102  is provided with an annular groove  142  extending part way into its diameter. The groove  142  is positioned along the length of the terminal pin  102  so that the braze material partially fills into the groove, but is prevented from filleting past it. This is because the difference in the coefficients of the friction of the braze material and the terminal pin material is not great enough to permit braze from moving along and past the overhang portion  142 A of the groove. In that light, the groove  142  is positioned along the length of the terminal pin  102  at a location that is the maximum height to which it is desired to have the braze material contacting the pin.  
         [0036]      FIGS. 4A and 4B  illustrate alternate embodiments of ferrules according to the present invention.  FIG. 4A  shows a ferrule  150  comprising an annular-shaped body  152  having an upper annular flange  154 . The ferrule body  152  comprises a cylindrically-shaped outer sidewall  156  and an inner sidewall  158 . In this embodiment, a braze retention structure in the form of an annular channel  160  having generally squared off sides extends from the inner sidewall  158  part way through the sidewall thickness. In all other respects, this ferrule  150  is similar to the ferrule  106  described in  FIG. 3 .  
         [0037]      FIG. 4B  shows another embodiment of a ferrule  150 A similar to that shown in  FIG. 4A  except its braze retention structure is in the form of a sideways facing U-shaped or radiused-shaped channel  162  extending from the inner sidewall  158  partway through the sidewall thickness. In each embodiment, it can be seen that the channels  160 ,  162  do not communicate with either the upper or lower end surfaces of their respective ferrules  150 ,  150 A.  
         [0038]      FIG. 5  shows an alternate embodiment of a terminal pin  170  that is useful with the feedthrough  100  of  FIG. 3 . Terminal pin  170  is similar to terminal pin  102  except the annular groove  172  serving as the braze retention structure has a sideways facing U-shape or radiused-shape.  
         [0039]      FIG. 6  illustrates another embodiment of a feedthrough  200  according to the present invention. The feedthrough  200  comprises a ferrule  202  having a cylindrically-shaped body  204  meeting an inwardly extending annular flange  206  at its lower end and an outwardly extending annular flange  208  at its upper end. The ferrule body  204  is received in an opening  210  in the device shield  212  in a snug-fitting relationship with the flange  208  resting on the outer surface thereof.  
         [0040]     The insulator  214  is a cylindrically-shaped member that is received in the ferrule  202 , resting on the inner flange  206 . The insulator  214  is provided with a braze retention structure in the form of an annular channel  216  located between the opposed insulator ends without being in direct fluid flow communication with either of them. The annular channel  216  has a generally squared off sides extending part way into the thickness of the insulator.  
         [0041]     An annulus  218  between the ferrule body  204  and the insulator  214  extends to an annular cut-out  220  where the outer flange  208  meets the body  204 . A ring-shaped braze pre-form (not shown) is received in the cut-out. When heated, this pre-form melts and flows into the annulus  218  between the insulator  214  and ferrule body  204 . Upon cooling, braze material  222  hermetically seals between the insulator  214  and the ferrule  202  along the annulus  218  and cut-out  220 . However, the annular channel  216  has taken up some braze material, which is retained therein. Since there was no retaining structure in the prior art feedthrough  10  ( FIGS. 1 and 2 ), the excess braze material had created braze spill out  36 A on the upper surface  44  of the ferrule flange  20 .  
         [0042]     A second braze pre-form (not shown) is positioned at the annulus  224  between the insulator  214  and a terminal pin  226  received in a bore  228  extending along the length of the insulator. Upon heating, braze material flows down the annulus  224  to create a hermetic seal there. However, some braze material  230  creeps up the terminal pin  226  above the upper surface  232  of the insulator. In a similar manner as the groove  216  provided in the insulator  214 , the terminal pin  226  is provided with a braze retention structure in the form of an annular groove  234  extending part way into its diameter. The groove  234  is positioned along the length of the terminal pin  226  so that the braze material partially fills into the groove, but is prevented from filleting past it. In that light, the groove  234  is positioned along the length of the terminal pin  226  at a location that is the maximum height to which it is desired to have the braze material contacting the pin.  
         [0043]      FIGS. 7A and 7B  illustrate alternate embodiments of insulators according to the present invention.  FIG. 7A  shows an insulator  250  comprising a cylindrical sidewall  252  extending between upper and lower end surfaces  254 ,  256 . A braze retention structure in the form of a sideways facing V-shaped groove  258  is provided in the sidewall  252  without being in direct fluid flow communication with either end. It serves the same function as the groove  216  shown with insulator  214  in  FIG. 6 . Similarly,  FIG. 7B  illustrates an insulator  250 A similar to that shown in  FIG. 7A  except the groove  260  has a sideways facing U- or radiused-shape.  
         [0044]      FIG. 8  illustrates another embodiment of a feedthrough  300  according to the present invention. The feedthrough  300  comprises a ferrule  302  having a cylindrically-shaped body  304  meeting an inwardly extending annular flange  306  at its lower end and an outwardly extending annular flange  308  at its upper end. The ferrule body  304  is received in an opening  310  in the device shield  312  with the flange  308  resting on the outer surface thereof.  
         [0045]     The insulator  314  is received in the ferrule  302 , resting on the inner flange  306 . An annulus  316  between the ferrule body  304  and the insulator  314  extends to an annular cut-out  318  where the outer flange  308  meets the body  304 . A ring-shaped braze pre-form (not shown) is received in the cut-out. When heated, this pre-form melts and flows into the annulus  316  between the insulator  314  and ferrule body  304 . Upon cooling, braze material  320  hermetically seals between the insulator  314  and the ferrule  302  along the annulus  316  and cut-out  318 .  
         [0046]     Terminal pins  322  and  324  are received in respective a bores  326  and  328  extending along the length of the insulator. Terminal pin  322  is a cylindrically-shaped member having a head  330  at one end while terminal pin  324  has its head at an intermediate location between its ends. In both pins, the heads  330 ,  332  are positioned spaced above an upper end surface  334  of the insulator  314 . Braze pre-forms (not shown) are positioned at the annulus  336  between the insulator  314  and the terminal pin  322  and at an annulus  338  between the insulator and the terminal pin  324 . Upon heating, braze material flows down the annuluses  336 ,  338  to create hermetic seals there. However, some braze material  340 ,  342  creeps up the terminal pins  322 ,  324  above the upper surface  328  of the insulator. The head  330  on terminal pin  322  acts as a braze retention structure that prevents braze material from filleting past it. Similarly, the head  332  on terminal pin  324  acts as a braze retention structure. This is because the differences in the coefficients of friction between the braze material and that of the terminal pins is not so great as to enable braze material to flow past the respective heads  330 ,  332 . Instead, the braze material is captured under the terminal pin heads. Restricting the location of the braze material to directly under the heads helps reduce stress exerted at the edge of the bores  326 ,  328  at the upper insulator end surface  334  by the braze.  
         [0047]     Thus, various embodiments of braze retention structures have been shown and described. In some of the embodiments, the braze retainer comprises an annular groove-type structure that is positioned between the opposed ends of a terminal pin, ferrule or insulator and in direct braze flow communication with an annulus there between. In other embodiments, the braze retention structure comprises a headed terminal pin that confines the braze to a relatively small gap between it and the insulator end surface. In either case, the retaining structures takes up excess braze material that might flow or wet onto surfaces where it is not needed to affect a hermetic seal. Instead, this unwanted braze material can cause a myriad of problems as discussed in the prior art section. The present braze retention structures prevent such problems from occurring.  
         [0048]     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.