Abstract:
An improved feed-through RF connector uses structural materials with coefficients of thermal expansion selected to enhance the reliability of a hermetic seal. The design of the connector and the selection of materials facilitate easy installation and help avoid cyclic fatigue and cracks that could result in a loss of hermetic seal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to provisional patent application 60/763,572 by inventor Edward A. Taylor filed on Jan. 31, 2006, entitled “Hermetically Sealed Coaxial Type Feed-Through RF Connector”, the contents of which are incorporated herein by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention is directed to electrical connectors and, more particularly, to the field of coaxial type feed-through RF connectors that require hermetic sealing. 
   2. Description of the Prior Art 
   The prior art will be discussed in conjunction with  FIGS. 1-3 . 
     FIG. 1  is a diagrammatic cross-sectional illustration of a conventional coaxial feed-through RF connector, having an RF signal ground-providing Kovar shell that projects outwardly beyond the surface of an aluminum housing having a bore in which the RF connector is soldered. For the RF connector shown in  FIG. 1 , as well as the connectors of  FIGS. 2-5 , to be described, it is to be understood that each of the illustrated components thereof is cylindrically symmetrical about that connector&#39;s longitudinal axis. The RF connector of  FIG. 1  includes a longitudinal signal pin  10 , which lies along a longitudinal axis  12  of the connector, and has a first, generally central portion  11  hermetically bonded to a coaxial bore  21  of a generally cylindrical dielectric (typically glass) spacer  20 . 
   The outer cylindrical surface of the glass dielectric spacer  20  is contiguous with and hermetically bonded to a reduced diameter portion  31  of a surrounding conductive (metallic) shell  30 , that serves as the RF signal ground for RF center pin  10 . The RF ground-providing shell  30  is configured and sized to be soldered within a step-shaped connector support bore  40  of the connector&#39;s support housing  50 , and to project outwardly beyond a first surface  51  thereof. A forward or distal end  13  of the signal pin  10  projects into an interior hollow bore  35  of the conductive shell  30  which, like the pin  10 , is preferably made of relatively low CTE conductive ferrous material, such as Kovar which has a coefficient of thermal expansion of substantially 5.2 PPM/° C., so that it and the pin may be readily hermetically bonded to the glass/dielectric spacer  20 , which has a similarly low CTE, that is compatible with that of Kovar. 
   The step-shaped bore  40  of the housing  50  extends from the first surface  51  thereof to a second, opposite surface  52  of the support housing, and includes respectively different diameter bore portions that are successively contiguous with one another and the first and second surfaces of the housing. In order to conform with the stepped configuration of the bore  40 , the reduced diameter portion  31  of the shell  30  is sized to be inserted into and disposed adjacent to the interior surface of a reduced diameter portion  41  of the bore  40 , so that a first, relatively narrow, cylindrical gap  45  is defined between the outer sidewall of the reduced portion  31  of the shell  30  and the interior surface of the reduced diameter portion  41  of the bore  40 . 
   In addition, shell  30  has a relatively wide diameter portion  32 , that adjoins the relatively narrow diameter portion  31  thereof, and forms a second, relatively thin, annular gap  46 , that is contiguous with the first, relatively narrow, cylindrical gap  45 , and is formed between the bottom surface of the relatively wide diameter portion  32  of the shell and the annular surface of a step portion  42  of the bore  40  that connects the reduced diameter portion  41  of the bore to a relatively wide diameter portion  43  thereof. The shell  30  is conductively and fixedly retained within the step-shaped bore  40  by means of solder joint  60 . This solder joint is produced by flowing solder material into the gaps  45  and  46  from a ring or annular-shaped solder preform, that has been inserted into an annular cavity  65  formed between the outer sidewall of the relatively wide diameter portion  32  of the shell  30  and the inner sidewall of the relatively wide diameter portion  43  of the bore  40 . 
   A second portion  14  of the connector&#39;s center pin  10  passes through a relatively narrow diameter portion  44  of the step-shaped bore  40 , which extends between a relatively shallow, circular depression or counterbore  47 , at the bottom of the reduced diameter portion  41  of the bore  40 , and the second surface  52  of the housing  50 , and terminates at an exterior end  15 . Counterbore  57  serves as a break for solder travel, by increasing the solder&#39;s propagation distance, which reduces capillary action, so that the solder will not travel along the surface of the bottom of the reduced diameter portion  41  of the bore  40 , but rather will remain confined within the gaps  46  and  45  forming solder joint  60 . 
     FIG. 2  shows the architecture of a second type of conventional coaxial feed-through RF connector, the components of which are installed at a bottom portion of a threaded connector support bore that extends into the housing from a first surface thereof, so as to allow an associated externally threaded RF connector, such as one that terminates the end of a section of RF cable, to be screwed into the threaded surface of the bore and engage the RF signal center pin installed therein. As such, the conductive material of the housing forms part of the RF signal ground that surrounds the RF signal pin. 
   More particularly, like the first type of prior art coaxial RF connector shown in  FIG. 1 , the coaxial feed-through RF connector of  FIG. 2  includes a longitudinal (Kovar) signal pin  10 , which is colinear with the connector&#39;s longitudinal axis  12 , and is hermetically bonded to a coaxial bore  21  of a generally cylindrical dielectric (glass) spacer  20 . Rather than being hermetically bonded to an RF ground-providing metallic shell that projects outwardly from the support housing, as in the RF connector architecture of  FIG. 1 , the outer cylindrical surface of the glass spacer  20  of the RF connector of  FIG. 2  is hermetically bonded to a surrounding metallic (e.g., Kovar) cylindrical ferrule  70 . Ferrule  70 , which serves as the RF signal ground, is installed within a cylindrical recess  80  beneath the bottom portion  91  of a threaded connector-support bore  90 , that is formed (e.g., machined) into the housing  50  from the first surface  51  thereof. The forward or distal end  13  of the signal pin  10  projects from the glass spacer  20  into the interior hollow portion  95  of the threaded bore  90 . 
   Similar to the relatively narrow diameter portion of the RF signal ground-providing shell of the coaxial feed-through RF connector of  FIG. 1 , the outer diameter of the ferrule  70  is slightly less than the diameter of the cylindrical recess  80 , so that a relatively narrow, cylindrical gap  75  is formed therebetween. The RF signal ground-providing ferrule  70  of the connector of  FIG. 2  is conductively and fixedly retained within recess  80  by means of solder joint  85  formed in the cylindrical gap  75 . Solder joint  85  not only serves to physically affix the RF signal pin support structure within the housing, but provides an ohmic connection between the Kovar ferrule  70  and the aluminum housing  50 , so that the housing provides part of the RF signal ground surrounding the RF signal pin  10 . The solder joint  85  is produced by flowing solder material into the cylindrical gap  75  from a ring or annular-shaped solder preform, that has been inserted into an annular depression  66  that is contiguous with the cavity  80  and the bottom portion  91  of the threaded bore  90 . 
   Also similar to the RF connector of  FIG. 1 , in the RF connector architecture of  FIG. 2 , a second portion  14  of the RF signal pin  10  passes through a relatively narrow diameter bore  100  in the housing  50 , which extends between a relatively shallow, circular counterbore  102  at the bottom of the recess  80  and the second surface  52  of the housing, and terminates at an exterior end  15 . As described previously, such a counterbore effectively prevents solder from traveling along the bottom of the recess  80 , so that the solder remains confined within the relatively narrow, cylindrical gap  75 , forming the intended solder joint  85 . 
   In each of the coaxially configured RF connectors shown in  FIGS. 1 and 2 , the only reliable hermetic seals are those provided by the hermetic bond between the Kovar RF center pin and the glass spacer, and the hermetic bond between the glass spacer and the Kovar material of a surrounding RF signal ground-providing cylinder (shell  30  in  FIG. 1 , and ferrule  70  in  FIG. 2 ). On the other hand, the solder joint that has been formed between the Kovar ferrule and the aluminum housing can be expected to suffer cyclic fatigue, producing cracks that will propagate and cause the solder joint to lose whatever temporary hermeticity it may have possessed when initially formed. This failure of such a solder joint is due to the substantial mismatch between the CTEs of Kovar and aluminum. 
   Still, if the connectors are relatively small sized, and the solder joints between metals having substantially different CTEs are formed in a dependable and repeatable manner, the types of connectors shown in  FIGS. 1 and 2  are sometimes considered to be ‘sufficiently’ hermetically sealed, so as to conform with some industry standards. Namely, in some applications that require a hermetically sealed connector, the connectors of  FIGS. 1 and 2 , which are not reliably hermetically sealed structures, may be employed as an alternative to the preferred device. 
   One prior art approach to resolve the above-described CTE mismatch problem, that leads to solder joint fatigue and loss of any hermeticity that the solder joints of an RF connector may initially provide, involves laser-welding the RF signal ground-providing Kovar ferrule, to which the glass spacer supporting the Kovar RF signal pin is hermetically bonded, to a coaxial sleeve made of a dissimilar metal (e.g., aluminum), that has the same CTE as the (aluminum) support housing. The coaxial sleeve is made of Kovar and aluminum. Kovar ferrule is welded to Kovar portion of coaxial sleeve and the dissimilar metal (aluminum) coaxial sleeve is laser welded to a connector retention bore in the aluminum housing. One portion of the coaxial sleeve has the same CTE as the ferrule and the other portion of the sleeve has the same CTE as the housing. The sleeve is a transition joint for the Kovar feed thru to the aluminum housing. In such an alternative RF connector structure, the laser welds, which form individual hermetic seals, make up for the lack of reliable hermeticity of the solder joints employed in the RF connector architectures of  FIGS. 1 and 2 , so that the resulting RF connector is completely and reliably hermetically sealed to the aluminum support housing. 
   An example of a prior art RF connector architecture employing such laser-welds to hermetically join a dissimilar metal coaxial sleeve to the RF signal ground-providing (Kovar) cylinder surrounding the (Kovar) center pin, and to hermetically join the dissimilar metal coaxial sleeve to a connector retention bore in the (aluminum) housing, is diagrammatically illustrated in  FIG. 3 . As shown therein, like the coaxial feed-through RF connectors of  FIGS. 1 and 2 , the coaxial feed-through RF connector of  FIG. 3  has a longitudinal (Kovar) RF signal pin  10  disposed along the connector&#39;s longitudinal axis  12 , and hermetically bonded to a coaxial bore  21  of a generally cylindrical dielectric (glass) spacer  20 . The glass spacer  20  abuts against the bottom portion  109  of an electrically conductive grounding spring  110 . Grounding spring  110  is installed at the bottom  112  of a cylindrical recess  114  that is contiguous with and extends beneath the bottom portion  116  of a bore  120  formed into the housing  50  from top surface  51 . 
   The forward or distal end  13  of the RF signal pin  10  projects from the glass spacer  20  into a hollow interior portion  122  of a threaded interior surface  124  of a cylindrical sleeve  125 . Cylindrical sleeve,  125  includes a first, metallic sleeve portion  126 , made of a metal (e.g., aluminum) that may be readily metallurgically joined with (e.g., welded) by way of a (laser) weld joint  132  to the metal (e.g., aluminum) of the housing  50 . Sleeve  125  further includes a second, metallic sleeve portion  127 , that adjoins the first sleeve portion  126 , and is made of a metal, such as Kovar, that may be readily (laser) welded at  133  to a metallic (e.g. Kovar) ferrule  128 , which is coaxially adjacent to the second sleeve portion  127 . The first, metallic sleeve portion  126  is metallurgically joined to the second, metallic sleeve portion  127  by way of an explosion weld joint  130  therebetween. Kovar ferrule  128 , has a lower projection portion  129  and is hermetically bonded to the outer surface of the glass spacer  20 . In the connector&#39;s installed position, the lower projection portion  129  of the Kovar ferrule  128  is urged against the bottom portion  109  of the grounding spring  110 , so that the bottom portion  109  of the grounding spring  110  is firmly captured between the Kovar ferrule  128  and the bottom  112  of the bore  120 . In addition, an upper portion  111  of the grounding spring  110  abuts against a bottom surface  131  of the Kovar ferrule  128 . As a consequence, the grounding spring  110  provides a secure RF ohmic signal ground connection between the Kovar ferrule  128  and the conductive material of housing  50 . 
   The outer diameters of the sleeve  126  and the ferrule  128  are slightly less than the diameter of the cylindrical bore  120 , so that, once the Kovar sleeve portion  127  of sleeve  125  and the Kovar ferrule  128  have been welded together at laser weld joint  133 , they may be readily inserted into the cylindrical bore  120 . After being inserted into the bore  120 , the combined (explosion-welded) sleeve and ferrule structure is hermetically sealed with the aluminum of the surrounding housing, by laser-welding the (aluminum) sleeve portion  126  of the sleeve  125  to the adjoining portion of the surface  51  of the (aluminum) housing  50 , so as to produce laser-weld joint  132  therebetween. 
   Now, although explosion- and laser-welds, such as those employed in the coaxial RF connector architecture of  FIG. 3 , may be employed to form hermetic seals between RF connector components having dissimilar CTEs, and thereby remedy problems associated with the use of solder joints, such as the formation and propagation of cracks in the joints as result of cyclic fatigue, the processing techniques necessary to form such welds are relatively complicated, which makes the connectors expensive and often increase the size of the connectors. 
   PROBLEMS OF THE PRIOR ART 
   Connectors of the prior art have difficulty forming a reliable hermetic seal within a bore of an electronics containing support housing made of a relatively high co-efficient of thermal expansion (CTE) material. In addition, the connectors must provide a reliable electrical ground. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates in general to a coaxial feed-through radio frequency (RF) connector, having a configuration and containing structural materials that enable the connector to be reliably hermetically sealed within a bore of an electronics-containing support housing made of a relatively high coefficient of thermal expansion (CTE) material (such as aluminum), by means of a relatively simple solder joint. To this end, the coaxial type feed-through RF connector of the invention employs an RF signal ground-providing shell, formed of the combination of a stainless steel sleeve, that is generally flush with the top of the support housing, and a Kovar ferrule joined with the stainless steel sleeve. 
   Because the CTE (17.5) of the stainless steel sleeve is sufficiently close to the relatively high CTE (22) of aluminum, soldering the stainless steel sleeve to the aluminum housing is sufficient to provide a reliable hermetic seal between the connector and the housing. Moreover, the slightly higher value of the CTE of aluminum relative to the value of the CTE of stainless steel causes the solder joint to retain the stainless steel sleeve under a slight compression, which is desirable for maintaining the reliability of the hermetic seal. The adjoining Kovar ferrule is also connected to (an interior region of) the housing by means of a solder joint; although this solder joint is non-hermetic, it provides a secure ohmic RF signal ground connection between the housing and the RF connector&#39;s conductive shell. A ground spring as shown in  FIG. 6  can also be employed. 
   In a first embodiment, the shell&#39;s stainless steel sleeve is adjacent to the sidewall of a bore formed within, and flush with the outer surface of a raised cylindrical land portion of the aluminum support housing. In a second embodiment, the internal surface of the stainless steel sleeve is threaded and, when inserted into a connector retention bore, is adjacent to the sidewall of the bore and flush with the surface of the housing. Threading the internal surface of the stainless steel sleeve allows an associated externally threaded RF connector, such as one that terminates the end of a section of RF cable, to be screwed into the sleeve and engage an RF signal center pin hermetically bonded to a dielectric spacer, that is also hermetically bonded to the Kovar ferrule at the bottom of the bore. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic cross-sectional illustration of a first type of conventional coaxial feed-through RF connector, having an RF signal ground-providing Kovar shell that projects outwardly beyond the surface of an aluminum housing in a bore of which the RF connector is soldered; 
       FIG. 2  is a diagrammatic cross-sectional illustration of a second type of conventional coaxial feed-through RF connector, which is installed within a partially threaded bore of a support housing, the threaded portion of the bore allowing an externally threaded RF connector to be screwed into the bore and engage the connector&#39;s RF signal center pin; 
       FIG. 3  is a diagrammatic cross-sectional illustration of a third type of conventional coaxial feed-through RF connector, which employs laser-welds to hermetically join a dissimilar metal coaxial sleeve to an RF signal ground-providing cylinder surrounding the center pin, and to hermetically join the dissimilar coaxial sleeve to an RF connector retention bore of an aluminum support housing; 
       FIG. 4  is a diagrammatic cross-section of a first embodiment of a coaxial type feed-through RF connector of the present invention, having an RF signal ground-providing shell containing a stainless steel sleeve and an adjoining Kovar ferrule, and being hermetically sealed by a solder joint to the sidewall of a bore within a raised cylindrical land portion of an aluminum support housing; 
       FIG. 5  is a diagrammatic cross-section of a second embodiment of a coaxial type feed-through RF connector of the present invention having an RF signal ground-providing shell containing an internally threaded stainless steel sleeve and an adjoining Kovar ferrule, and being hermetically sealed by a solder joint to the sidewall of a bore within an aluminum support housing; and 
       FIG. 6  is a diagrammatic cross-section of a modification of the embodiment of the coaxial type feed-through RF connector shown in  FIG. 5 , employing a grounding spring in lieu of a solder joint to provide a secure ohmic RF signal ground connection between the connector shell and its surrounding conductive housing. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In accordance with the present invention, the drawbacks of conventional coaxial feed-through RF connector architectures, including the problems of cyclic fatigue in solder joints used to join metallic components having substantially different CTEs, and processing complexity and relatively high cost associated with using explosion and laser welding techniques to join dissimilar metallic components, described above, are effectively obviated by a new and improved coaxial type feed-through RF connector structure, that employs an RF signal ground-providing shell that contains both a Kovar ferrule (to provide a hermetic seal with a glass spacer, in which a Kovar RF signal pin hermetically retained) and a stainless steel sleeve (that is laser-welded, rather than soldered, to the Kovar ferrule, due to the substantial difference in the CTEs of Kovar and stainless steel). 
   Because the CTE of the stainless steel sleeve is substantially greater than the CTE of Kovar, it is sufficiently close to the relatively high CTE of aluminum, to enable a reliable hermetic seal to be achieved between the connector&#39;s RF signal ground-providing shell and the sidewall of a bore within an aluminum support housing, by means of a relatively simple, and inexpensive solder joint formed within a narrow cylindrical gap between the aluminum of the sidewall of the bore and the stainless steel of the sleeve portion of the RF signal ground-providing shell. 
   A diagrammatic cross-section of a first embodiment of a coaxial type feed-through RF connector of the present invention is shown in  FIG. 4  as comprising a generally cylindrical, electrically conductive (e.g., metallic) shell  200 , having an outer sleeve  202  and an inner ferrule  204  that is laser-welded to the outer sleeve at laser-weld joint  206 . Once the sleeve  202  has been laser-welded to the inner ferrule, the shell  200  is subjected to a precious metal (electro-)plating process (which typically involves plating an initial thin layer of nickel, followed by plating a thin layer of gold on the nickel plate), to make the shell wettable to solder that will be used to join respective spaced apart surface portions of the aluminum sidewall of the bore to each of the outer sleeve  202  and the ferrule  204 . 
   The shell  200  surrounds one or more RF signal pins, such as a RF signal center pin  208 , that is coaxial with the axis  210  of the RF connector, and is hermetically bonded to a bore  211  through a glass spacer  213 . The outer surface of the glass spacer  213  is hermetically bonded to the interior sidewall  215  of ferrule  204 . Each of the ferrule  204  and the RF pin  208  is preferably made of a material such as Kovar, having a CTE proximate to that of glass dielectric material of the spacer  213 , so that the glass spacer  213  may be readily hermetically bonded with the ferrule and the RF pin. The shell&#39;s outer sleeve  202  is sized to fit within a main portion  212  of a bore  214  formed within a raised cylindrical land portion  216  of an aluminum housing  220 , the land portion  216  projecting beyond a first surface  218  of the housing. Specifically, the shell&#39;s outer sleeve  202  has an outer diameter that is only slightly less than the inner diameter of the main portion  212  of the bore  214 , so that a relatively narrow cylindrical gap  222  is formed between the outer surface of the sleeve  202  and the interior sidewall of the main portion  212  of the bore. 
   The outer sleeve is preferably made of a material, such as stainless steel, that has a coefficient of thermal expansion (CTE) proximate or relatively close to that of the housing. These two aspects of the sleeve (its size and material) relative to the metal of the sidewall of the bore allow the sleeve to be reliably hermetically sealed within the bore  214  by means of a relatively simple, upper solder joint  224  that is formed within the narrow cylindrical gap  222  between the sleeve  202  and the sidewall of the bore  214 . As in the connectors of  FIGS. 1 and 2 , upper solder joint  224  may be readily formed by flowing solder into the cylindrical gap  222  from an annular-shaped solder preform. The solder preform is placed into an annular cavity  226 , that is formed in the raised cylindrical land portion  216  of the aluminum housing between the outer sidewall  228  of the stainless steel sleeve  202  and the sidewall  230  of an annular recess  232 , in the top surface  234  of raised cylindrical land portion  216  of the aluminum housing, that is contiguous with the bore  214 . From the preform in the cavity  226 , the solder flows into the gap  222 . To constrain the depth to which solder flows down through the cylindrical gap  222 , an annular recess  236  is formed in the outer sidewall  228  of the sleeve  202 , interrupting further solder travel, so as to maintain a specific volumetric quantity of solder within cylindrical gap  222 . Because the CTE (22) of the aluminum housing  220  is slightly higher than the CTE (17.5) of the stainless steel sleeve  202 , the solder joint  224  retains the stainless steel sleeve  202  within the bore  214  under a slight compression, which is desirable for maintaining the reliability of the hermetic seal. 
   As described above, due to the substantial mismatch between their respective CTEs, the stainless steel sleeve  202  is laser-welded to the inner ferrule  204 , in order to provide a hermetic seal therebetween. For this purpose, a first end of the ferrule  204  adjoining the sleeve  202  includes a ring-shaped flange  238 , which has a diameter proximate that of the outer sleeve  202 . In order to provide a secure RF signal ground connection for the RF connector, a second or lower end  240  of the ferrule  204  is sized to be inserted into and form a relatively narrow cylindrical gap  242  with the interior sidewall of a reduced diameter, bottom portion  244  of the bore  214 . Similar to the cylindrical gap  224  between the outer sleeve  202  and the bore  214 , the relatively narrow cylindrical gap  242  between the second end  240  of the ferrule  204  and the interior sidewall of the reduced diameter, bottom portion  244  of the bore  214  enables the ferrule to be conductively joined to the (aluminum) housing material surrounding the bore, by means of a relatively simple solder joint  246  formed along the narrow cylindrical gap  242  between the ferrule  204  and the sidewall of the bottom portion  244  of the bore. 
   The solder joint  246  may be formed by flowing solder into the cylindrical gap  242  from an annular-shaped solder preform, that has been placed in an annular cavity  248  formed between the sidewall of the bore  214  and the outer sidewall of the ferrule  204 , and contiguous with the gap  242 . From the solder preform that has been placed in the cavity  248 , solder flows down into the gap  224 . A counterbore  250  is formed adjacent to the floor of the bore  214  beneath the glass spacer  213 , to prevent solder that has flowed into the gap  242 , where the solder joint  246  is intended, from traveling along the bottom of the bore  214 . 
   Because the CTE (22) of the aluminum housing  220  is substantially higher than the CTE (5.2) of the Kovar ferrule  204 , the lower solder joint  246  does not, nor is it intended to, form a hermetic seal between the RF connector and the support housing; a reliable hermetic seal therebetween is provided by way of the upper solder joint  224 , as described above. Instead, the purpose of the lower solder joint  246  is to provide a secure ohmic RF signal ground connection between the shell  200  and the surrounding aluminum housing  220 . Rather than form a solder joint, such as that shown at  246  between the ferrule  204  and the bottom portion  244  of the bore, to provide a secure ohmic RF signal ground connection between the shell  200  and the surrounding aluminum housing, a grounding spring, configured and installed in the manner of the connector of  FIG. 3 , described previously, may be employed. 
   A diagrammatic cross-section of a second embodiment of a coaxial type feed-through RF connector of the present invention is shown in  FIG. 5  as comprising a generally cylindrical, electrically conductive (e.g., metallic) shell  300 , having an outer sleeve  302  and an inner ferrule  304  that is laser-welded to the outer sleeve at laser-weld joint  338 . Once the stainless steel sleeve  302  has been laser-welded to the ferrule  304 , the shell is subjected to a precious metal (electro-)plating process as described above for the embodiment of  FIG. 4  so as to make the shell wettable to solder that will be used to join respective spaced apart surface portions of the aluminum sidewall of the bore to each of the outer sleeve  302  and the ferrule  304 . 
   The shell&#39;s outer sleeve  302  has a threaded interior surface  307 , that allows an externally threaded RF connector to be screwed into the shell and engage one or more RF signal pins, such as the single RF signal center pin  308  shown in these figures, that is coaxial with the axis  310  of the RF connector, and is hermetically bonded to a coaxial bore  311  through a glass spacer  313 . The outer surface of the glass spacer  313  is hermetically bonded to the interior sidewall  315  of the ferrule  304 . As in the embodiment of  FIG. 4 , each of the ferrule  304  and the RF pin  308  is preferably made of a material such as Kovar, having a CTE proximate to that of glass dielectric material of the spacer  313 , so that the glass spacer  313  may be readily hermetically bonded with the ferrule and the RF pin. 
   The shell&#39;s internally threaded outer sleeve  302  is sized to fit within a main portion  312  of a bore  314  that extends into an aluminum housing  316  from a first surface  318  thereof. As in the embodiment of  FIG. 4 , the sleeve  302  has an outer diameter that is only slightly less than the inner diameter of the main portion  312  of the bore  314 , so that a relatively narrow cylindrical gap  320  is formed between the outer surface of the sleeve  302  and the interior sidewall of the main portion  312  of the bore  314 . 
   Like the embodiment of  FIG. 4 , the outer sleeve  302  is preferably made of a material, such as stainless steel, that has a coefficient of thermal expansion (CTE) proximate or relatively close to that of the housing, so as to allow the sleeve to be reliably hermetically sealed within the bore  314  by means of a relatively simple, upper solder joint  322  formed along the narrow cylindrical gap  320  between the outer sleeve  302  and the adjacent sidewall of the bore  314 . As in the connector of  FIG. 4 , the upper solder joint  322  may be formed by flowing solder into the cylindrical gap  320  from an annular-shaped solder preform, that has been placed into an annular cavity  324  formed between the outer sidewall  326  of the stainless steel sleeve  302  and the sidewall  328  of an annular recess  330 , in the top surface  318  of the aluminum housing, that is contiguous with the bore  314 . From the preform that has been placed in the annular cavity  324 , solder flows into the gap  320 . To constrain the depth to which solder flows down along the cylindrical gap  320 , an annular recess  332  is formed in the outer sidewall  326  of the sleeve  302 , thereby interrupting further solder travel, and ensuring that a specific volumetric quantity of solder is maintained within the cylindrical gap  320 . Again, because the CTE (22) of aluminum is slightly higher than the CTE (17.5) of stainless steel, the stainless steel sleeve  302  is retained by the solder joint  322  under a slight compression which, as noted above, is desirable for maintaining the reliability of the hermetic seal. 
   As with the case of the connector of  FIG. 4 , due to the substantial mismatch between their respective CTEs, the stainless steel sleeve  302  is laser-welded to the inner ferrule  304 , in order to provide a hermetic seal therebetween. For this purpose, a lower end portion  334  of the sleeve  302  has a recess or depression  336  of a diameter and depth that substantially correspond to the diameter and thickness, respectively of the ferrule  304 , so as to accommodate a relatively ‘snug’ insertion of the ferrule  304  into the recess  336  of the sleeve  302 , ferrule being laser-welded to the sleeve along their adjoining surfaces at the outer edge of the recess, as shown by weld-joint  338 . 
   In order to provide a secure RF signal ground connection for the RF connector, a lower, reduced diameter portion  340  of the ferrule  304  is sized to be inserted into and form a relatively narrow cylindrical gap  342  with the interior sidewall of a reduced diameter, bottom portion  344  of the bore  314 . Similar to the cylindrical gap  320  between the outer sleeve  302  and the bore  314 , the relatively narrow cylindrical gap  342  between the lower, reduced diameter portion  340  of the ferrule  304  and the interior sidewall of the reduced diameter, bottom portion  344  of the bore  314  enables the ferrule to be conductively joined to the (aluminum) housing material surrounding the bore, by means of a relatively simple, lower solder joint  346  formed along the narrow cylindrical gap  342  between the ferrule  304  and the sidewall of the bottom portion  344  of the bore. 
   As in the embodiment of  FIG. 4 , the solder joint  346  may be formed by flowing solder into the cylindrical gap  342  from an annular-shaped solder preform, that has been placed in an annular cavity  348 , that is formed between the sidewall of the bore  314  and the outer sidewall of the ferrule  304 , and is contiguous with the gap  342 . From this preform, solder flows down into the gap  324  to the bottom portion  344  of the bore  314 . A counterbore  350  is formed adjacent to the bottom floor of the bore  314  beneath the glass spacer  313 , to prevent solder that has flowed into the gap  342 , where the solder joint  346  is intended, from traveling along the bottom of the bore  314 . 
   Again, like the embodiment of  FIG. 4 , because the CTE (22) of the aluminum housing  316  is substantially higher than the CTE (5.2) of the Kovar ferrule  304 , the lower solder joint  346  does not, nor is it intended to, form a hermetic seal between the RF connector and the support housing; a reliable hermetic seal therebetween is provided by way of the upper solder joint  322 , as described above. Instead, the purpose of the lower solder joint  346  is to provide a secure ohmic RF signal ground connection between the shell&#39;s Kovar ferrule  304  and surrounding aluminum housing  316 . Also, as in the embodiment of  FIG. 4 , described above, rather than using a solder joint, such as the lower solder joint shown at  346  between the Kovar ferrule  304  and the bottom portion of the bore  314 , to provide a secure ohmic RF signal ground connection between the shell  300  and the surrounding aluminum housing  316 , the connector architecture of  FIG. 5  may be modified to employ a grounding spring of the type shown in  FIG. 3 , described previously. 
   Such a modification is diagrammatically shown in  FIG. 6 , wherein, as in the case of the grounding spring of the connector of  FIG. 3 , glass spacer  313  abuts against a bottom portion  409  of an electrically conductive grounding spring  410 . Grounding spring  410  is installed at the bottom  412  of cylindrical recess  414  that is contiguous with and extends beneath the bottom portion  416  of the bore  314  formed into the housing  316  from its top surface  318 . In the connector&#39;s installed position, the reduced diameter portion  340  of the Kovar ferrule  304  is urged against the bottom portion  409  of the grounding spring  410 , so that the bottom portion  409  of the grounding spring  410  is firmly captured between the Kovar ferrule  304  and the bottom  412  of the recess  414 . In addition, an upper portion  411  of the grounding spring  410  abuts against a bottom surface  431  of the Kovar ferrule  304 . As a result, grounding spring  410  provides a secure RF ohmic signal ground connection between Kovar ferrule  304  and the conductive material of the housing  316 . 
   As will be appreciated from the foregoing description, the lack of reliable hermeticity in solder joints used to join metals with substantially different CTEs, such as those employed in RF connector structures of the types shown in  FIGS. 1 and 2 , and the relatively complicated and costly processing techniques required to produce explosion- and laser-welds employed in RF connector structures of the type shown in  FIG. 3 , are effectively obviated by the coaxial feed-through RF connector of the present invention, which has a configuration and contains structural materials that enable the connector to be reliably hermetically sealed within a bore of an electronics-containing support housing made of a relatively high coefficient of thermal expansion (CTE) material (such as aluminum), by means of a relatively simple solder joint. 
   In particular, the coaxial feed-through RF connector according to the present invention employs an RF signal ground-providing shell, that combines a stainless steel sleeve with an adjoining Kovar ferrule. The stainless steel sleeve provides the shell with a conductive material having a CTE (17.5) that is sufficiently close to the relatively high CTE (22) of aluminum, so as to enable the connector to be reliably hermetically sealed with the housing, by mean of a relatively simple solder joint formed between the stainless steel sleeve portion of the shell and the aluminum housing. Moreover, the slightly higher value of the CTE of the aluminum housing relative the value of the CTE of the stainless steel sleeve causes the solder joint therebetween to retain the stainless steel sleeve under a slight compression, which is desirable for maintaining the reliability of the hermetic seal. The adjoining Kovar ferrule is also ohmically connected to the housing, as by way of a solder joint or grounding spring. Although this ohmic connection is non-hermetic, it provides a secure ohmic RF signal ground connection between the housing and the RF connector&#39;s conductive shell. 
   While I have shown and described several embodiments in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art, and I therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.