Patent Publication Number: US-6992544-B2

Title: Shielded surface mount coaxial connector

Description:
TECHNICAL FIELD 
   The invention relates to radio frequency (RF) and microwave circuits and systems. In particular, the invention relates to coaxial connectors used with planar circuits operating at RF and microwave frequencies. 
   BACKGROUND ART 
   High frequency devices, circuits and subsystems, such as those operating at radio frequency (RF) and microwave frequency ranges, are often manufactured as or using a planar circuit. The planar circuits, typically referred to as ‘printed circuit boards’ (PCBs), frequently are interconnected with one another using coaxial cables. Coaxial connectors at an interface between a PCB and the coaxial cable enable the individual PCB to be connected and disconnected during assembly and/or test, as well as for maintenance and replacement purposes once the PCB has been deployed. A variety of classes or series of standard and semi-custom coaxial connectors are readily available and in widespread use including, but not limited to, SMA, SMB, SMC, SSMA, 3.5-mm, and 2.4-mm, 1.85-mm connectors. In general, each of the various coaxial connector series is available in a variety of styles, each style being adapted to a particular application and/or circuit-mounting configuration. 
   Among the coaxial connector styles used in conjunction with high frequency PCBs are surface-mountable styles often referred to as ‘surface mount’ (SMT) connectors.  FIG. 1A  illustrates a perspective view of a typical, conventional SMT coaxial connector  10  that emphasizes an end (hereinafter the end view is referred to as being ‘top-oriented’).  FIG. 1B  illustrates a perspective view of the SMT connector  10  of  FIG. 1A  that emphasizes an opposite end (hereinafter the opposite end view is referred to as being ‘bottom-oriented’).  FIG. 2  illustrates a cross sectional view of the conventional SMT connector  10  of  FIG. 1A  attached to a PCB  11 , the connector  10  being interfaced with a microstrip transmission line  24  on the PCB  11 . 
   The conventional SMT connector  10  illustrated in  FIGS. 1A ,  1 B and  2  comprises a connector shell or barrel  12 , a connector base  14 , a center pin  16 , and a dielectric pin support  18 . The base  14 , connected to a first end  12   a  of the shell  12 , comprises a flange  20  and a plurality of spacer legs or stand-offs  22 . The center pin  16  is mounted in and extends a length of a through hole of the shell  12 . The shell through hole runs axially through the shell  12  and through the flange  20  from a second or connector end  12   b  of the shell  12  to an outer surface  19  of the flange  20 . The center pin  16  is supported in the through hole by the dielectric pin support  18 . Together, the through hole through the shell  12  and the flange  20  along with the center pin  16  therethrough form a coaxial transmission line. The center pin  16  extends axially beyond the outer surface  19  of the flange  20  a distance equivalent to a length of the spacer legs  22 . Typically, the connector  10  is interfaced to the PCB  11  by soldering a connection end  16   a  of the center pin  16  to a transmission line  24  of the PCB  11  and soldering or otherwise electrically connecting the spacer legs  22  to a ground plane  28  of the PCB  11 . 
   The presence of the spacer legs  22  creates a gap  30  between the outer surface  19  of the flange  20  and a top surface of the PCB  1 . The gap  30  enables a solder joint  26  at the connection end  16   a  of the center pin  16  to be cleaned and inspected during manufacturing. In addition, the gap  30  insures that expansion of the dielectric pin support  18  during solder reflow will not interfere with proper solder attachment of the center pin  16 . In particular, the gap  30  accommodates any expansion of the dielectric pin support  18  such that the connector  10  does not lift off of the PCB  11  surface during soldering. 
   Unfortunately, the presence of the gap  30  results in a signal path discontinuity experienced by a signal traveling between the connector  10  and the transmission line  24  of the PCB  11 . In particular, the signal path discontinuity exists in the SMT connector  10  transmission line between the outer surface  19  of the flange  20  and the PCB  11  surface where the center pin  16  is attached to the transmission line  24  of the PCB  11 . In addition, a solder joint or fillet  26  formed when the center pin  16  is soldered to the transmission line  24  tends to exacerbate the discontinuity associated with the gap  30 . 
   Ultimately, the discontinuity associated with the gap  30  and solder fillet  26  leads to unwanted or spurious electromagnetic radiation (EM) from the interface between the connector  10  and the PCB  11 . In addition, the discontinuity associated with the gap  30  and solder fillet  26  manifests itself as an impedance mismatch, thereby introducing unwanted signal reflections in the signal path passing through the connector  10  and to the PCB  11 . The signal reflections can and often do interfere with a performance of a device or system that employs conventional SMT connectors. 
   Accordingly, it would be advantageous to have an SMT connector that minimized spurious EM radiation and minimized a signal path discontinuity and associated impedance mismatch associated with interfacing the SMT connector to a PCB. Such a coaxial connector would address a longstanding need in the area of surface-mountable connectors for RF and microwave applications. 
   SUMMARY OF THE INVENTION 
   The present invention provides a shielded, coaxial connector interface for planar circuits operating in the radio frequency (RF) and microwave frequency ranges. In particular, a shielded, surface-mountable (SMT), coaxial connector, a system for removably connecting and a method of interfacing for RF and microwave circuit and device applications are provided. The shielded SMT coaxial connector connection electromagnetically shields an interface between the connector and a planar circuit, such as a printed circuit board (PCB), to which the connector is attached. In addition to providing a shielded interface, the present invention also reduces an impedance mismatch associated with attaching the connector to the PCB relative to an impedance mismatch associated with an attachment without the present invention. The present invention is applicable to a wide variety of standard and semi-custom connector classes including, but not limited to SMA, SMB, SMC, 3.5-mm, 2.4-mm, 1.85-mm, and 1.0-mm series connectors. 
   In an aspect of the present invention, a surface-mountable (SMT) coaxial connector is provided. The SMT coaxial connector comprises an electromagnetic shield that shields an interface created between the coaxial connector and a planar circuit when the connector is attached to the planar circuit. The shield comprises a mounting end of the connector that is annular in shape and coplanar with a connection end of a coaxial transmission line of the connector. The coplanar mounting end and the connection end of the transmission line are adjacent to the interface. Depending on the embodiment, the coaxial connector of the present invention either alternatively comprises or additionally comprises an impedance mismatch reducer that reduces an impedance mismatch between the coaxial transmission line and a transmission line of the planar circuit at the interface. The coaxial transmission line is an air dielectric transmission line or air-line at and adjacent to the interface. The impedance mismatch reducer comprises an accommodation for a fillet of conductive attachment material used to attach the air-line to the planar circuit, such that an overall diameter of the airline remains constant. 
   In other aspects of the present invention, a system for removably connecting to an RF or microwave device is provided. The system comprises the surface mountable coaxial connector of the present invention, and further comprises a multilayer planar circuit and a mounting footprint on an exposed surface of the multilayer planar circuit that is adapted to accept the coaxial connector. Moreover, a method of interfacing a coaxial connector to a printed circuit board is provided. The method comprises electromagnetically shielding a coaxial transmission line at an interface created between a coaxial connector and a printed circuit board when the connector is attached to the printed circuit board. The method further comprises accommodating a fillet of conductive attachment material within a mean diameter of the coaxial transmission line. Advantageously, the shielding provided by the SMT connector according to the present invention reduces spurious electromagnetic radiation from the interface between the connector and PCB. Additionally, the present invention reduces an impedance discontinuity at the interface, the discontinuity being association with connector attachment. Certain embodiments of the present invention have other advantages in addition to and in lieu of the advantages described hereinabove. These and other features and advantages of the invention are detailed below with reference to the following drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, where like reference numerals designate like structural elements in the different drawing figures, and in which: 
       FIG. 1A  illustrates a perspective end view of a typical, conventional surface-mountable (SMT), coaxial connector. 
       FIG. 1B  illustrates a perspective end view of the SMT connector illustrated in  FIG. 1A  from an opposite end. 
       FIG. 2  illustrates a cross-sectional view of the conventional SMT connector illustrated in  FIG. 1A  attached to a PCB. 
       FIG. 3A  illustrates a perspective end view of a shielded, surface-mountable (SMT) coaxial connector according to an embodiment of the present invention. 
       FIG. 3B  illustrates a perspective end view of the shielded, surface-mountable (SMT) coaxial connector embodiment illustrated in  FIG. 3A  from an opposite end. 
       FIG. 3C  illustrates a magnified view of a portion of the end view illustrated in  FIG. 3B  that is within a dashed circle labeled  3 C. 
       FIG. 4  illustrates a cross-sectional view of an embodiment of a shielded SMT coaxial connector according to the present invention. 
       FIG. 5  illustrates a magnified cross-sectional view of a portion of the shielded SMT coaxial connector illustrated in  FIG. 4  enclosed within a dashed circle labeled  5 . 
       FIG. 6  illustrates a perspective view of an embodiment of a connector pin of the shielded SMT coaxial connector according to the present invention. 
       FIG. 7  illustrates a cross-sectional view of the shielded SMT coaxial connector illustrated in  FIG. 4  attached to an exemplary printed circuit board (PCB). 
       FIG. 8  illustrates a magnified cross-sectional view of a portion of the attached shielded SMT coaxial connector illustrated in  FIG. 7  enclosed within a dashed circle labeled  8 . 
       FIG. 9  illustrates a cross-sectional view of an embodiment of a two-piece shielded SMT coaxial connector according to the present invention. 
       FIG. 10  illustrates a cross-sectional view of another embodiment of the two-piece shielded SMT coaxial connector according to the present invention. 
       FIG. 11  illustrates a perspective view of an embodiment of a system for removably connecting to an RF or microwave device using a shielded SMT coaxial connector according to the present invention. 
       FIG. 12  illustrates a surface view of an embodiment of a PCB mounting footprint adapted to a shielded, SMT coaxial connector according to the present invention. 
       FIG. 13  illustrates a cut-away, perspective view of the PCB mounting footprint illustrated in FIG.  12 . 
       FIG. 14  illustrates a flow chart of a method of interfacing according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 3A  illustrates a perspective end view of a shielded, surface-mountable (SMT) coaxial connector  100  according to an embodiment of the present invention. The end view illustrated in  FIG. 3A  is referred to as ‘top-oriented’ herein also.  FIG. 3B  illustrates a perspective end view of the shielded (SMT) coaxial connector  100  embodiment illustrated in  FIG. 3A  from an opposite end of the connector  100 . The opposite end view illustrated in  FIG. 3B  is referred to as ‘bottom-oriented’ herein also.  FIG. 3C  illustrates a magnified view of a portion of the end view illustrated in  FIG. 3B  that is within a dashed circle labeled  3 C. The view illustrated in  FIG. 3C  is of a surface  111  of the opposite end of the shielded SMT connector  100 . The portion of the surface  111  illustrated in  FIG. 3C  includes an exit end of a coaxial transmission line of the connector  100 .  FIG. 4  illustrates a cross-sectional view of an embodiment of the shielded SMT coaxial connector  100  according to the present invention. 
   The present invention provides a connector interface to a printed circuit board (PCB) or equivalent device realized as a planar circuit for communicating high frequency signals to and from the PCB. In particular, the shielded SMT coaxial connector  100  facilitates removably connecting a coaxial cable or another appropriately ‘connectorized’ device to the coaxial connector  100 . By high frequency, it is meant that the shielded SMT coaxial connector  100  accommodates electromagnetic (EM) signals having frequencies in the radio frequency (RF) and microwave frequency ranges. Moreover according to the present invention, the interface provided is electromagnetically shielded and exhibits an impedance discontinuity associated with the interface that is reduced, and preferably minimized, relative to an interface therebetween without using the present invention. 
   The shielded SMT coaxial connector  100  comprises an electrically conductive shell  110  having a connector portion  112 , and a shield or base portion  114 . The connector portion  112  is located adjacent to a mating end  113  of the shell  110 . The connector portion  112  is adapted for being removably connected to a mating connector (not illustrated). The mating connector may be on an end of a coaxial cable, such as a semi-rigid coaxial cable, for example. The connector portion  112  is configurable as either a ‘female’ connector or a ‘male’ connector according to the present invention. The connector portion  112  is illustrated and represented hereinbelow as a female connector, for discussion purposes only. In particular, the representation of the connector portion  112  according to the present invention as a female connector is not intended to limit the scope of the present invention. 
   In addition, the connector portion  112  may conform to or be adapted to mate with any standard or non-standard RF or microwave coaxial connector configuration known in the art. For example, the connector portion  112  may conform to any one of the standard microwave coaxial connector configurations or classes including, but not limited to, an SMA connector, a 3.5-mm connector, a 2.4-mm connector, a 1.85-mm connector, a 1.0-mm connector and a 0.6-mm connector. One skilled in the art is familiar with a wide variety of such connector classes in addition to those listed above, all of which are within the scope of the present invention. The skilled artisan may readily realize the connector portion  112  according to the present invention in any of the connector classes known in the art without undue experimentation. 
   The base portion  114  is located adjacent to a mounting end  115  of the shell  110 , the mounting end  115  being distal (i.e., opposite) to the mating end  113 . The mounting end  115  provides means for mounting or attaching the coaxial connector  100  to a PCB. In some embodiments, the means for mounting comprises an annular-shaped flange  119 , as illustrated in  FIGS. 3A and 3B . The annular flange  119  may be soldered, attached with conductive epoxy, or otherwise affixed to a mounting surface or a mounting footprint of the PCB to attach the coaxial connector  100  thereto. Moreover, unlike conventional SMT connectors, the annular flange  119  may be affixed to the PCB all the way around a circumference of the annular flange  119 . The entire circumferential attachment of the annular flange  119  contrasts with conventional SMT connectors that employ standoffs or legs as discrete points distributed around a base of the connector for mounting the connector to a PCB. 
   The shell  110  is tubular having an approximately central through hole  118  that extends through the shell  110  along a longitudinal axis of the shell  110 . In particular, the hole  118  extends from the mating end  113  through a length of the connector portion  112  and the base portion  114  to the mounting end  115  of the shell  110 . The hole  118  preferably is located at or near a central longitudinal axis of the shell  110  and preferably has a substantially cylindrical shape. Thus, the shell  110  is a hollow tube having an inner surface that is cylindrical and has an inner diameter. The inner diameter of the inner surface of the shell  110  may either vary or be constant along a length of the shell  110 . 
   The shielded SMT coaxial connector  100  further comprises a connector pin  120  and a pin support  130 . The connector pin  120  is electrically conductive, and is located in and is coaxial with the hole  118 . Preferably, the connector pin  120  is approximately centrally located in the through hole  118 , and therefore, the connector pin  120  may be referred to herein as ‘enter pin  120 ’ without limiting the scope of the invention to only a centrally located connector pin. Acting together, the shell  110  and the center pin  120  function as a high frequency, coaxial waveguide or transmission line that supports electromagnetic signal propagation through the connector  100  in the form of electromagnetic waves. As a coaxial transmission line, the connector  100  supports signal propagation as a transverse electromagnetic (TEM) wave. 
     FIG. 6  illustrates a perspective view of an embodiment of the center pin  120  of the shielded SMT coaxial connector  100  according to the present invention. As illustrated in  FIG. 6 , the center pin  120  has a generally extended cylindrical shape a diameter of which is determined by a desired impedance of the coaxial transmission line according to well-known design equations for such transmission lines. As such, the diameter of the center pin  120  may vary along a length of the center pin  120  depending on a diameter of the inner surface of the shell  110  and a dielectric constant of a material present between the center pin  120  and the inner surface of the shell  110 . The center pin  120  has a connection end  122  and a mating end  123 , wherein the mating end  123  is distal or opposite to the connection end  122 . 
   In some embodiments according to the present invention, the center pin  120  comprises a stop  124  and a stepped end portion  126 . The stop  124  is a portion of the center pin  120  that has a larger diameter than a remainder of the center pin  120 . A portion of the center pin  120  between the stop  124  and the mating end  123  is adapted to receive the pin support  130 . In some embodiments, the stop  124  comprises a flange or shelf formed as a part of the center pin  120 . 
   Advantageously, the stop  124  helps to prevent the center pin  120  from being pulled away from a surface of a PCB during mounting of the connector  100 . For example, heating of the dielectric pin support  130  during soldering may cause the pin support  130  to expand. The stop  124  keeps the pin  120  from being pulled up and into the pin support  130  as a result of the heat related expansion, for example. 
   The stepped end portion  126  is a portion of the center pin  120  adjacent to the connection end  122  of the center pin  120 . The stepped end portion  126  has a reduced width or diameter compared to a width or diameter of a pin portion  125  of the center pin  120  immediately adjacent to the stepped end  126 , which is between the stepped end  126  and the stop  124 . In some embodiments, the width or diameter of the stepped end portion  126  is reduced compared to a remainder of the connector pin  120 . A ratio of the width or diameter of the stepped end  126  to the diameter of the pin  120  in the adjacent pin portion  125  beyond the stepped end  126  may be determined by an estimate of a thickness of a conductive attachment material such as, but not limited to, a solder, that is likely to accumulate at the stepped end portion  126  during connector  100  attachment. In other words, the determined ratio may accommodate the attachment material such that a resulting combined diameter of the attachment material and stepped end  126  is substantially similar to the diameter of the adjacent pin portion  125 . 
   Advantageously, the stepped end portion  126  helps to control the overall diameter of a combination of the conductive attachment material and the center pin  120  in a vicinity of an attachment between the connector  100  and a PCB. In particular, the stepped end portion  126  advantageously enables the application of a sufficient amount of conductive attachment material to the center pin  120  to insure a secure and robust attachment of the center pin  120  to the PCB. Moreover, due to the stepped end portion  126 , the overall diameter of the combination of attachment material and center pin  120  may be made to approximate the diameter of the center pin  120  at the adjacent portion  125 . In essence, the stepped end portion  126  enables the diameter of the center pin  120  at the adjacent portion  125  to be carried or continued all the way to the connection end  122  without sacrificing a robustness of the conductive connection of the center pin  120  to the PCB. 
   For example, consider an application that employs a solder to attach the center pin  120  to a PCB and assume that a solder fillet having approximately 0.3-mm to 0. 0.4-mm thickness is desired and expected. Moreover, assume that the center pin  120  in the pin portion  125  between the stepped end portion  126  and the stop  124  has a diameter of 1.02-mm. In this example, the exemplary stepped end portion  126  may have a diameter of approximately 0.35-mm or about one third the diameter of the pin portion  125 . The expected solder fillet thickness will result in an overall thickness of the solder and center pin  120  at the stepped end portion  126  that is approximately equal to the diameter of the adjacent pin portion  125 . Thus, when the center pin  120  is soldered to the PCB, the combination of the solder fillet and the stepped end portion  126  will present a relatively small impedance discontinuity or mismatch while still insuring that the center pin  120  is adequately secured to the PCB. 
   The center pin  120  may further comprise a knurled, fluted or splined portion  128 . The splined portion  128  is preferably located in a portion of the center pin  120  corresponding to a location of the pin support  130 . The splined portion  128  assists in retaining or securing the center pin  120  within the pin support  130 . In particular, the splined portion  128  helps to prevent the center pin  120  from rotating during repeated mating and unmating of the connector  100  with a complimentary connector at the mating end  123 . 
   Preferably in addition to preventing rotation, the splined portion of  128  also allows material of the pin support  130  to expand along the center pin  120  in a direction that is essentially away from the connection end  122  of the center pin  120 . Expansion in a direction away from the connection end  122  is hereinafter referred to as ‘upward expansion’ without limitation to the scope of the present invention. Expansion of the pin support  130  material may occur during heating cycles associated with attachment of the connector  100 , for example. Such upward expansion of the pin support  130  material facilitated by the splined portion  128  reduces a chance that the expansion of the material will result in the connection end  122  being pulled away from the PCB during connector attachment. 
   In addition to employing the splined portion  128  to retain the center pin  120  within the pin support, any of various captivation means known in the art may be employed to retain the pin support  130  within the shell  110  of the connector  100 . In particular, use of such captivation means may further reduce an incidence of center pin  120  rotation during connector  100  mating and unmating. Specifically, use of the captivation means may prevent the pin support  130  from rotating thereby preventing the secured center pin  120  from rotating. All such means of pin support  130  captivation are within the scope of the present invention. 
   For example, a pair of ‘dimple-like’ side crimps  117  may be used to secure the pin support  130  within the shell  110 . Other captivation means including, but not limited to, epoxy captivation and the use of formed barbs on the inner surface of the shell may be used instead of or in addition to the exemplary side crimps  117 . Moreover, if the side crimps  117  or other captivation means are located at or near a base end of the pin support  130  adjacent to the connection end  122  of the center pin  120 , advantageous essentially upward expansion of the pin support  130  material may be further facilitated. Thus, a combined use of the splined portion  128  to secure the center pin  120  within the pin support  130  and the use of side crimps  117  or other captivation means at the base end of the pin support  130  to secure the pin support in the shell  110  advantageously further reduces the chance of the center pin  120  being pulled away from the PCB due to pin support  130  material expansion. 
   The center pin  120  may further comprise a mating portion  129  adjacent to the mating end  123  of the center pin  120 . The mating portion  129  may have any one of a variety of mating configurations. The connector portion  112  of the shell  110  may be any one of a variety of connector classes. The connector class of the connector portion  112  dictates a specific configuration of the mating portion  129  of the center pin  120 . For example, the mating portion  129  of a female, SMA connector class of the connector portion  112  may comprise a socket with four to six circumferentially array ‘fingers’. The socket and fingers of the example are adapted to receive a mating pin of a male, SMA mating connector (not illustrated). 
   Referring again to  FIG. 4 , the pin support  130  comprises a rigid or semi-rigid insulating or dielectric material that extends from the center pin  120  to an inner surface of the shell  110  within a space created by the hole  118 . The pin support  130  supports the center pin  120  at or near a center of the hole  118 . In some embodiments, the pin support  130  may extend along a substantial portion of the length of the hole  118  within the connector portion  112  of the shell  110  of the coaxial connector  100  as illustrated in FIG.  4 . In particular, the space within a substantial portion of the length of the hole  118  within the connector portion  112  may be essentially filled with a low-loss dielectric material such as, but not limited to, Teflon®. Teflon® is a trade name for polytetrafluoroethylene (PTFE), registered to E. I. Du Pont De Nemours and Company Corporation, 101 West 10th St., Wilmington, Del., 19898. The presence of the dielectric material in the space between the center pin  120  and the inner surface of the shell  110  serves to support the center pin  120  and thus acts or functions as the pin support  130 . 
   For example, an embodiment of the coaxial connector  100  consistent with the aforementioned SMA connector class may have such an extended pin support  130  made of Teflon®. The pin support  130  for such an embodiment may be formed into a cylindrical ‘bead’ having an approximately central hole therethrough. Ideally, the central hole in the Teflon® bead is slightly smaller than a diameter of the center pin  120 . To assemble the exemplary coaxial connector  100 , the center pin  120  is inserted into the hole of the Teflon® bead. The assembly comprising the center pin  120  and the Teflon® bead pin support  130  thus crested is inserted into and secured within the hole  118  in the connector portion  112  of the shell  110 . The pin support  130  is secured in the shell  110  using the exemplary side crimps  117  as illustrated in FIG.  4 . One skilled in the art is familiar with Teflon® beads used as pin supports for SMA connectors and can readily apply such familiarity to the manufacture of the shielded SMT coaxial connector  100  according to the present invention. 
   In other embodiments (not illustrated), the pin support  130  may be confined to a small portion of the length of the hole  118  within the connector portion  112 . Moreover, there may be more than one pin support  130 . In particular, in such embodiments, a total length of the pin support(s)  130  along the center pin  120  may be minimized to a total length capable of adequately supporting the center pin  120  given a particular implementation of the pin support  130 . Minimizing the length of the pin support  130  tends to reduce an effect that the support  130  has on a propagating electromagnetic wave passing through the connector  100 . For example, an embodiment of the shielded SMT coaxial connector  100  of the present invention consistent with a 3.5-mm or 2.4-mm class of connectors may employ a pin support  130  having a minimized length to facilitate operation at frequencies up to and beyond 40 GHz. 
   As used herein, a coaxial transmission line in which the space between an inner and outer conductor (e.g., the space within the hole  118  surrounding the center pin  120 ) is substantially filled with a dielectric material, such as Teflon®, is referred to as a ‘dielectric-filled’ coaxial transmission line. Similarly, a coaxial transmission line in which the space between the inner and outer conductor is filled by a gas, for example air, is called an ‘air dielectric’ coaxial transmission line or more simply an ‘air-line’. Thus in some embodiments, the coaxial transmission line within the connector portion  112  may be one or both of an air-line and a dielectric-filled coaxial transmission line. For example, the coaxial transmission line of the embodiment illustrated in  FIG. 4  is a dielectric-filled coaxial transmission line throughout the connector portion  112 . On the other hand, a portion of the coaxial transmission line within the base portion  114  and adjacent to the mounting end  115  of the coaxial connector  100  is an air-line. The air-line of the base portion  114  extends to and exits at the mounting end  115 . Thus as illustrated in FIG.  3 C and  FIG. 4 , the center pin  120  is surrounded by a gas and not surrounded by a solid dielectric material at or in a vicinity of the mounting end  115  of the coaxial connector  100  according to some embodiments. 
     FIG. 5  illustrates a magnified cross-sectional view of a portion of the shielded SMT coaxial connector  100  illustrated in  FIG. 4  that is enclosed within a dashed circle labeled  5  in FIG.  4 . The magnified view of the portion illustrates the connection end  122  of the center pin  120  and the air-line coaxial transmission line within the base portion  114  of the coaxial connector  100 . In particular,  FIG. 5  illustrates the air-line of the base portion  114  extending from an end of the pin support  130  within the connector portion  112  to the mounting end  115 . The air-line comprises the stepped end portion  126  and the immediately adjacent pin portion  125  of the center pin  120 . Preferably, the annular flange  119  completely surrounds and shields the center pin  120  within the base portion  114 . More preferably, the connection end  122  of the center pin  120  is essentially coplanar with a bottom surface  111  of the annular flange  119 . 
   Advantageously, the use of an air-line within the base portion  114  minimizes a deleterious mechanical effect that an expansion of the dielectric of the pin support  130  might have on a conductive connection between the PCB and the center pin  120 . Furthermore, a continuation of the coaxial transmission line as an air-line through the base portion  114  and to the mounting end  115  of the connector  100  provides shielding of the interface between the PCB and the connector  100 . In particular, the presence of the coplanar annular flange  119  shields the center pin  120 . The shielding provided by the present invention significantly reduces spurious EM radiation from and associated with the interface between the connector and the PCB compared to conventional SMT connectors known in the art. In addition, a relatively large and essentially continuous attachment surface afforded by the annular flange  119  of the coaxial connector  100  provides a highly secure and rugged means of attaching the coaxial connector  100  to the PCB. 
     FIG. 7  illustrates a cross-sectional view of an embodiment of the shielded SMT coaxial connector  100  illustrated in  FIG. 4  attached to an exemplary PCB  150 . The attachment is made using a solder or similar eutectic bonding material. As illustrated in  FIG. 7 , the exemplary PCB  150  is a multilayer PCB having a first surface, a second surface that is opposite to the first surface, and a buried stripline transmission line  156 . The first surface has a first or ‘top’ ground plane  152 , the second surface has a second or ‘bottom’ ground plane  154  and the buried stripline transmission line  156  is located between the ground planes  152 ,  154 . The PCB  150  further has a blind via  158  that extends from the buried stripline to the first surface. The blind via  158  connects a solder pad  160  on the first surface to the stripline  156 . The blind via  158  and the solder pad  160  are electrically isolated from the ground planes  152 ,  154 . The coaxial connector  100  is attached to the top ground plane  152  by soldering, or otherwise electrically and mechanically affixing the annular flange  119  of the connector  100  to the top ground plane  152 , for example. The center pin  120  may be soldered or otherwise electrically and mechanically affixed to the solder pad  160  to complete the attachment of the shielded SMT coaxial connector  100 . 
     FIG. 8  illustrates a magnified cross-sectional view of a portion of the attached shielded SMT coaxial connector  100  illustrated in  FIG. 7  enclosed within a dashed circle labeled  8 . The portion illustrated in  FIG. 8  depicts a solder connection between the connection end  122  of the center pin  120  and the solder pad  160  on the first surface of the PCB  150 . In particular, a solder fillet  162  is illustrated bridging between the center pin  120  and the solder pad  160  at the stepped end portion  126  of the pin  120 . Note that a diameter of the combination of the center pin  120  in the stepped end portion  126  and the solder fillet  162  is approximately equal to the diameter of the center pin  120  in the adjacent pin portion  125 . In particular, when solder flows into the stepped end portion  126  during soldering, the resulting combined diameter of the solder fillet  162  and center pin  120  at the stepped end portion  126  advantageously closely approximates the diameter of the center pin  120  in the immediately adjacent pin portion  125 . Such a novel accommodation of the solder fillet  162  within the diameter of the adjacent pin portion  125  achieves a substantially constant air-line diameter and greatly reduces a potential for introducing an impedance mismatch discontinuity associated with solder attachment of the center pin  120  of the shielded SMT coaxial connector  100 . 
     FIG. 9  illustrates a cross-sectional view of an embodiment of a two-piece shielded SMT coaxial connector  100 ′ according to the present invention. The two-piece connector  100 ′ comprises a shell  110 ′, the shell  110 ′ comprising a connector assembly  112 ′ and a base assembly  114 ′, wherein the connector assembly  112 ′ and base assembly  114 ′ are separable from one another. The constituent elements of the connector assembly  112 ′ and base assembly  114 ′ are essentially those described for the connector portion  112  and base portion  114 , respectively, of the shielded SMT coaxial connector  100  hereinabove. While having essentially the same constituent elements, there are several notable exceptions necessitated by the ‘separable nature’ of the two-piece connector embodiment  100 ′. 
   A primary exception is that the center pin  120 ′ of the two-piece connector  100 ′ is a two-piece center pin  120 ′ comprising a connector assembly pin  120   a ′ and a base assembly pin  120   b ′. The connector assembly pin  120   a ′ and base assembly pin  120   b ′ provide means for cooperatively engaging one another. For example, the connector assembly pin  120   a ′ may comprise a socket  182  while the base assembly pin  120   b ′ may comprise a plug  184 , the socket  182  and plug  184  being adapted to cooperatively engage. Those skilled in the art are familiar with other means for cooperatively engaging pins together, all of which are also within the scope of the present invention. 
   In addition, the connector assembly  112 ′ and base assembly  114 ′ of the two-piece connector  100 ′ provide means for cooperatively engaging or connecting to one another. For example,  FIG. 9  illustrates an embodiment of the connector assembly  112 ′ having a set of screw threads  172  on an outer surface of the connector assembly  112 ′. Similarly, the base portion  114 ′ of the embodiment illustrated in  FIG. 9  has a set of screw threads  174  on an inner surface of a cavity  176  in the base assembly  114 ′. The screw threads  172  are complementary to the screw threads  174 . Thus, when the connector assembly  112 ′ is received by the cavity  176  of the base assembly  114 ′, the two sets of screw threads  172 ,  174  cooperatively engage, thereby providing a mechanical and electrical connection between the assemblies  112 ′,  114 ′ of the two-piece shell  110 ′. Those skilled in the art are familiar with other means for cooperatively engaging the shell assemblies together, all of which are also within the scope of the present invention. Likewise, the connector assembly pin  120   a ′ and a base assembly pin  120   b ′ are cooperatively engaged when the assemblies  112 ′,  114 ′ are connected together. 
     FIG. 10  illustrates a cross-sectional view of another embodiment of the two-piece shielded SMT coaxial connector  100 ″ according to the present invention. The embodiment illustrated in  FIG. 10  differs from the shielded SMT connector  100 ′ illustrated in  FIG. 9  in a few aspects. The shielded SMT connector  100 ″ comprises a shell  110 ″ having a base assembly  114 ″ and a connector assembly  112 ″. The connector assembly  112 ″ is similar to the connector assembly  112 ′ described above except that the connector assembly  112 ″ further comprises a shield portion  113 ″. The shield portion  113 ″ fits around the base assembly  114 ″ of the shell  110 ″ when the connector assembly  112 ″ and base assembly  114 ″ are cooperatively engaged together. In the two-piece shielded SMT coaxial connector  100 ′ described above, the base assembly  114 ′ comprises a circumferential flange, similar to the flange  119  described above for the shielded SMT coaxial connector  100 . However in the two-piece embodiment  100 ″ illustrated in  FIG. 10 , the shield portion  113 ″ comprises the circumferential flange and the base assembly  114 ″ does not and need not include such a circumferential flange. The shield portion  113 ″ provides EM shielding for the shielded SMT coaxial connector  100 ″ in the base portion. Unlike other embodiments disclosed hereinabove, the base assembly  114 ″ advantageously need not completely surround a base assembly pin  120   b ″ since shielding is provided by the shield portion  113 ″ of the connector assembly  112 ″. 
   Also illustrated in  FIG. 10  is that the means for cooperatively engaging the connector pin portions  120   a ″ and  120   b ″ of the connector pin  120 ″ employs a complementary socket and plug embodiment, similar to the socket  182  and plug  184  of the two-piece connector embodiment  100 ′ illustrated in FIG.  9 . However,  FIG. 10  illustrates the socket and plug embodiment in reversed pin portions compared to that illustrated in FIG.  9 . This illustration is for exemplarily purposes only and not by way of limitation. 
   Both of the two-piece embodiments of the shielded SMA coaxial connector  100 ′,  100 ″ comprise the connector pin  120 ′,  120 ″ with a stepped end portion, an immediately adjacent pin portion and a stop that are similar or equivalent to the stepped end portion  126 , the immediately adjacent pin portion  125  and the stop  124  of the connector pin  120  for the one-piece shielded SMA coaxial connector  100 , as described above. Therefore, the two-piece connector embodiments  100 ′,  100 ″ have all of the features and advantages of achieving an essentially constant air-line diameter in the base assembly  114 ′,  114 ″ that are described above for the stepped end portion  126  and the solder fillet  162  when the respective connector  100 ′,  100 ″ is attached to a PCB. 
   The shell  110 ,  110 ′,  110 ″ is preferably fabricated from an electrically conductive material. More preferably, the conductive material, such as a metal that is readily machined, is employed to facilitate fabrication of the various portions of the shell  110 ,  110 ′,  110 ″. For example, a metal such as, but not limited to, Stainless Steel, Iron-Nickel, Copper, Tungsten or Brass, or any other metal conventionally used in fabricating high frequency coaxial connectors may be used. Alternatively, the shell  110 ,  110 ′,  110 ″ may be fabricated from an electrically non-conductive material. When a non-conductive material is employed, an electrically conductive coating is deposited on a surface of the shell  110 ,  110 ′,  110 ″ during fabrication to render the shell  110 ,  110 ′,  110 ″ electrically conductive. 
   For high frequency applications of the one-piece connector  100  and/or the two-piece connectors  100 ′,  100 ″, especially above about 1 GHz, an outer surface of the shell  110 ,  110 ′,  110 ″, as well as an inner surface of the shell  110 ,  110 ′,  110 ″ created by the hole  118 , are preferably plated with a material, such as gold (Au), to improve conductivity and control or minimize corrosion. In some embodiments, additional plating layers are applied before the gold (Au) layer is applied to facilitate adhesion or improve plating reliability. For example, the shell  110 ,  110 ′,  110 ″ may be plated with an undercoat of nickel (Ni) prior to being plated with gold (Au). 
   The use of plating for improving conductivity (i.e., decreasing ohmic loss) and/or for controlling corrosion in high frequency coaxial connectors is well known to one skilled in the art. A choice of the conductive material for the shell  110 ,  110 ′,  110 ″ and/or the use of a particular type of plating are not intended to limit the scope of the present invention. One skilled in the art is familiar with a wide range of materials used for fabricating and/or plating high frequency connectors that are suitable for use in fabricating the shell  110 ,  110 ′,  110 ″ of the present connectors  100 ,  100 ′,  100 ″. All such materials and platings are within the scope of the present invention. 
   The center pin  120 ,  120 ′,  120 ″ is an electrical conductor, preferably a metal. The center pin  120 ,  120 ′ may be fabricated from an electrically conductive material or a non-conductive material by machining, stamping or forming. The non-conductive material is further plated with an electrically conductive plating. For example, the center pin  120 ,  120 ′,  120 ″ may be fabricated by machining a metal such as, but not limited to, beryllium-copper, brass, KOVAR™, tungsten or molybdenum preferably plated with gold (Au). KOVAR™, a registered trademark for a nickel-cobalt-iron alloy, is registered to Westinghouse Electric &amp; Manufacturing Company, Pittsburgh, Pa. In particular, Tungsten and Molybdenum generally possess a high strength enabling them to survive fabrication and repeated mating and un-mating during operational use of the connector  100 ,  100 ′,  100 ″. Preferably, the center pin  120 ,  120 ′,  120 ″ is gold (Au) plated along the entire length of the pin  120 ,  120 ′,  120 ″. While several suitable metal materials are listed for the connector pin  120 ,  120 ′,  120 ″ hereinabove by way of example, the listed exemplary materials are not intended to limit the scope of the present invention in any way. Those skilled in the art are aware of other materials that are useful for the connector pin  120 ,  120 ′,  120 ″, all of such other materials are also within the scope of the present invention. 
   As mentioned hereinabove, a main criterion for choosing the dielectric material for the pin support  130  of the shielded SMT coaxial connector  100 ,  100 ′,  100 ″ is whether or not the material can adequately support the center pin  120 ,  120 ′,  120 ″ while simultaneously producing a minimal loss in, or disruption of, the TEM wave propagating through the connector  100 ,  100 ′,  100 ″. Dielectric materials including, but not limited to, borosilicate glass, alumina ceramic and various glass-ceramic materials, such as Macor™, may be used for the pin support  130  as an alternative to a dielectric material such a Teflon® mentioned previously herein. Macor™ is a trademark for unworked or semi-worked glass-ceramic materials, registered to Corning Glass Works, Houghton Park, N.Y., 14830. 
   In another aspect of the invention, a system  200  for removably connecting to an RF or microwave device fabricated in or on a multilayer printed circuit board using a shielded SMT coaxial connector is provided.  FIG. 11  illustrates a perspective view of an embodiment of the system  200  for removably connecting to an RF or microwave device according to the present invention.  FIG. 12  illustrates a surface view of an embodiment of a PCB mounting footprint  230  adapted to the shielded, SMT coaxial connector according to the present invention.  FIG. 13  illustrates a cutaway, perspective view of the PCB mounting footprint  230  illustrated in FIG.  12 . 
   The system  200  comprises a shield SMT coaxial connector  210 , a multilayer printed circuit board (PCB)  220  and a mounting footprint  230  on a first or ‘top’ surface or layer  222  of the PCB  220 . The shield SMT connector  210  may be any of the shielded SMT coaxial connector  100 ,  100 ′,  100 ″ embodiments described hereinabove. The multilayer PCB  220  comprises a planar transmission line  224  connected to the mounting footprint  230 . The planar transmission line  224  is located on a layer  226  below the top layer  222 . The mounting footprint  230  is adapted to accept the shielded SMT coaxial connector  210  and provides means for mounting the shield SMT coaxial connector  210  to the PCB  220  and means for electrically interfacing the connector  210  to the transmission line  224  of the PCB  220 . 
   The mounting footprint  230  of the system  200  comprises an annular ring-shaped pad  234 . The annular pad has a plurality of vias  236  arranged through the annular pad  234  and an approximately centrally located void that electrically isolates the annular pad  234 . The mounting footprint  230  further comprises a center pad  232  that is located in the central void. The center pad  232  and the annular pad  234  are provided as an electrically conductive material on the top surface  222  of the PCB  220 . For example, the center pad  232  and the annular pad  234  may be etched copper foil bonded to the top surface  222 , wherein the etching is used to define a shape of the pads  232 ,  234 . A blind via  238  or another equivalent means for electrical connection connects the center pad  232  to the transmission line  224 . The center pad  232  is electrically isolated from the annular pad  234 . The annular pad  234  is preferably electrically connected to and more preferably, continuous with a first ground plane  228  on the top surface  222  of the PCB  220 . 
   Each of the vias of the plurality  236  is a hole passing from the top surface  222  to a second or ‘bottom’ surface  223  of the PCB  220 . As the name might imply, the bottom surface  223  is opposite to the top surface  222 . Preferably, the vias  236  are plated with a conductive material on an inside surface and otherwise provide an opening between the top surface  222  and the bottom surface  223 . The plurality of vias  236  is arranged in an annular pattern within the annular pad  234 . 
   In some embodiments, a second ground plane  229  is located on the bottom surface  223 . In such embodiments, the vias  236  may be electrically connected to the second ground plane  229 . In other embodiments, one or more additional ground planes (not illustrated) are provided between the first ground plane  228  and the second ground plane  229 . In these embodiments, the vias  236  may be electrically connected to one or more of the additional ground planes in addition to or instead of being connected to the second ground plane  229 . 
   Moreover, an additional ring of vias (not illustrated) may be employed concentrically between the vias  236  and a boundary of the central void. The additional ring of vias may be used to help compensate or ‘match’ an impedance of the blind via  238  to an impedance of one or both of the transmission line  224  and the shield SMT coaxial connector  210 . When the additional ring of vias are used, the vias  236  essentially serve to provide shielding for the system  200  while the additional ring of vias provides impedance matching. Furthermore, other matching structures (not illustrated) such as holes in the first ground plane  228 , holes in the second ground plane  229 , holes in the additional ground planes, and various stubs and coupled sections on the transmission line  224  may be employed to help with impedance matching. One of skilled in the art is familiar with a wide variety of impedance matching techniques that may be used all of which are within the scope of the present invention. 
   The system  200  is assembled by applying a conductive attachment material such as, but not limited to, a solder material to the center pad  232  and to the annular pad  234 . Alternatively or in addition, the conductive attachment material may be applied to a connector pin and an annular flange-mounting surface of the coaxial connector  210 . The coaxial connector  210  is then placed in contact with the PCB  220  and aligned with the footprint  230 . The aligned connector  210  has the flange-mounting surface aligned with the annular pad  234  and the connector pin aligned with the center pad  232 . In the case of solder, the solder may be reflowed to attach the connector  210  to the PCB  220 . Advantageously, the plurality of vias  236  allow excess attachment material to move out from between the flange and the annular pad  234  facilitating attachment while reducing a possibility of a short circuit being created between the center pad  232  and the annular pad  234 . For example, when solder is used as the conductive attachment material, excess solder tends to flow into the open vias  236  during solder reflow. 
   The presence of the plurality of vias  236  through their collective action with respect to excess solder also advantageously and unexpectedly assists in aligning the coaxial connector  210  during reflow and in adding mechanical strength to a bond between the connector  210  and the PCB  220 . In particular, surface tension of a solder fillet formed along the boundary of the annular pad  234  at the central void preferentially aligns the connector  210  to the annular pad  234  during solder reflow. Moreover, removal of excess solder from between the coaxial connector  210  and the annular pad  234  by the plurality of vias  236  tends to leave a relatively thin solder bondline. Thin solder bondlines are known to be generally stronger than thick bondlines or layers of solder. Furthermore, presence of solder within the vias  236  increases the strength of the bond between the annular pad  234  and the coaxial connector  210  attached thereto. Essentially, the solder within the vias  236  enables the vias  236  to act as rivets through the PCB  220 . The annular pad  234  and coaxial connector  210  are effectively ‘riveted’ to the PCB  220  thereby increasing the overall strength of the system  200 . 
   Accordingly, the system  200  advantageously achieves a substantially constant air-line diameter in the connector  210  at or adjacent to the connector/PCB interface using the conductive attachment material and all of the advantages described above for such a constant air-line diameter. In addition, the plurality of vias  236  provides a coaxial ground structure within the PCB  220  and that helps to shield the system  200  and minimize a transitional impedance discontinuity between the coaxial connector  210  and the transmission line  224 . 
   In another aspect of the invention, a method  300  of interfacing to a printed circuit board (PCB) is provided.  FIG. 14  illustrates a flow chart of the method  300  of interfacing according to an embodiment of the present invention. The method  300  of interfacing comprises shielding  310  a portion of a coaxial transmission line in a surface-mountable coaxial connector. The portion of the coaxial transmission line that is shielded  310  is a connector portion adjacent to a mounting surface of the PCB, when the connector is attached to the PCB. In particular, shielding is essentially continuous from the connector portion of the connector to a mounting surface of the connector, such that no gaps are present between the connector mounting surface and the mounting surface of the PCB once the connector is attached to the PCB. The connector is attached to the PCB with a conductive attachment material. The method  300  further comprises accommodating a fillet of the conductive attachment material. The center pin of the connector includes a pin end portion with a reduced diameter at a pin end thereof, and an immediately adjacent pin portion, both within the shielded connector portion of the connector. The pin end of the pin end portion is adjacent to the mounting end of the connector. The diameter of the pin end portion is reduced relative to the immediately adjacent pin portion. The adjacent pin portion is opposite to the pin end that is adjacent to mounting end of the connector. In particular, when the attachment material is applied to the connector pin of the connector during connector attachment, the attachment material is accommodated such that a mean diameter of the fillet plus the center pin along a length of the fillet is approximately equal to the diameter of the adjacent pin portion of the connector pin. The method optionally further comprises enabling excess attachment material to move out of a space between an attachment flange of the connector and an attachment footprint on the PCB during connector attachment. 
   Thus, there has been described a shielded SMT coaxial connector, a system using a shielded SMT coaxial connector, and a method of interfacing a shielded surface-mountable coaxial connector to a printed circuit board. It should be understood that the above-described embodiments are merely illustrative of some of the many specific embodiments that represent the principles of the present invention. Those skilled in the art can readily devise numerous other arrangements without departing from the scope of the present invention.