Patent Publication Number: US-2006003642-A1

Title: Plating of brazed rf connectors for t/r modules

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates generally to the fabrication of transmit/receive (T/R) modules used in connection with the aperture of an active electronically scanned array, and more particularly to the plating of RF connectors brazed onto the front side of high temperature co-fired ceramic (HTCC) substrate of a T/R module package.  
      2. Description of the Prior Art  
      The process of brazing RF connectors onto the side of an HTCC package involves re-flowing the high temperature alloy of copper and silver (Cusil) to provide a robust mechanical attachment of the center pin and shroud of the RF connector to a substrate member of a T/R module shown, for example, in U.S. Pat. No. 6,114,986, entitled “Dual channel Transmit/Receive Module For An Active Aperture Of A Radar System”. The materials involved in this operation, Kovar for the center pin and shroud and Cusil brazing material, are prone to corrosion and therefore require gold plating to prevent corrosion in any non-hermetic environment. Plating of this part involves first nickel plating and then gold plating. Any non-plated area on the center pin area can result in corrosion of the center pin and loss of the RF signal, resulting in a catastrophic failure of the entire T/R module package.  
      The present state of the art for fabrication/assembly of a connector onto a T/R module package typically involves brazing the center pin and shroud of the connector as stated above. The shroud is currently an elongated solid structure including a plurality of generally cylindrical apertures through which the center pins project and which is required to connect the ground signal of the mating structure, i.e., the connector. During the plating process, the module package is placed in plating baths of nickel followed by gold. For proper plating, i.e., complete plating to occur, the solution must flow throughout the interior of the connector shroud. However, this does not often occur due to the inherent physical features of the inner walls of the connector shroud. It has been found that due to the lack of good plating solution flow throughout the interior of the RF connector, a relatively large number of packages have been rejected because of unplated areas around the connector center pins. The pins do not have proper plating, for example, where the pin is brazed to the ceramic package. This plating deficiency occurs as a result of a lack of flow of the plating solution within the connector.  
      Since plating is performed at the end of the package fabrication process, a rejection of the package results in a loss at the most expensive point in the process of a finished product. Thus an unreasonable number of failures in a production system using this type of RF interconnect can result in a relatively large increase in the overall cost of acceptable or “good” T/R module packages.  
     SUMMARY  
      Accordingly, it is the primary object of the present invention to provide an improvement in the fabrication of a T/R module.  
      It is another object of the present invention to improve the plating flow of plating material throughout the interior of an RF connector for a T/R module package during fabrication of a T/R module.  
      It is a further object of the present invention to provide a plating process wherein the plating fluid is made to more easily contact and flow around all of the interior metallized surfaces of an RF connector shroud.  
      These and other objects are achieved by initially providing openings such as slots or holes in the shroud of an RF connector of a T/R module package in the region of the connector pins prior to being brazed to a ceramic substrate so as to subsequently allow plating solution to flow freely throughout the interior of the RF connector and particularly around the connector pins. The slots or holes are large enough to allow proper plating of the interior of the connector but small enough to prevent any radiation through the connector to the exterior of the T/R module package.  
      Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific example, while illustrating the preferred embodiment of the invention, it is given by way of illustration only, since various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     DESCRIPTION OF THE DRAWINGS  
      The present invention will become more fully understood from the detailed description provided hereinafter in the accompanying drawings which are given by way of illustration only, and thus are not meant to be limitative of the present invention, and wherein:  
       FIG. 1  is a perspective view of a transmit/receive (T/R) module including a connector shroud fabricated in accordance with the subject invention;  
       FIG. 2  is a top plan view of the T/R module shown in  FIG. 1 ;  
       FIG. 3  is a side plan view of the T/R module shown in  FIG. 1 ;  
       FIG. 4  is a bottom plan view of the T/R module shown in  FIG. 1 ;  
       FIG. 5  is a front planar view of the RF connector shown in  FIG. 1 ; and  
       FIG. 6  is a partial longitudinal cross-sectional view of the connector shown in  FIG. 5  taken along the lines  6 - 6  thereof. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring now collectively to the drawings and more particularly to  FIGS. 1-4 , shown is a microwave transmit/receive (T/R) module  10  utilized in connection with an active electronically scanned array. The details of such a T/R module is shown and described in the above-referenced U.S. Pat. No. 6,114,986. What is common between the T/R module  10  of the subject invention and that shown and described in U.S. Pat. No. 6,114,986, is the use of a dielectric substrate  12  onto which an RF connector assembly  14  is brazed. The substrate  12  constitutes a multi-level configuration of high temperature co-fired ceramic (HTCC) in which active and passive circuit elements, not shown, are located to provide routing of RF signals between respective antenna elements of the array and the circuit elements as well applying DC power supply voltages. To protect the interior of the substrate  12 , a cover plate  16  is secured to the top of the substrate. A heat sink plate  18  is also provided on the bottom of the substrate  12  as shown in  FIG. 4  for transferring heat generated by the active components, located interiorally of the substrate into an external heat sink, also not shown. Reference numerals  20  and  22  shown in  FIG. 4  at the rear end of the module  10  denote metal tabs which are used for coupling DC signals to the various active components within the substrate  12 .  
      The present invention is directed to the connector assembly  14  which is shown in  FIG. 5  comprising a blind mate press-on RF connector assembly including six identical RF coaxial connector elements  24   1  . . .  24   6  arranged linearly within and across a metal shroud  28 .  
      As noted above and referring now to  FIGS. 5 and 6 , fabrication of the T/R module  10  involves brazing center pins  26   1  . . .  26   6  and a prefabricated shroud  28  to the front side  30  of a ceramic HTCC substrate  12  as shown, for example, in  FIG. 1 , and involves the application of a copper/ silver alloy to provide a mechanical attachment. As noted, Kovar is used for attaching the center pins  26   1  . . .  26   6  while Cusil is used for brazing the shroud to the substrate surface  30 .  
      The assembled elements are next placed in electrolytic plating baths, first of nickel and then of gold. It can be seen by reference to  FIG. 6  that the structural features of the interior of the six connectors  24   1  . . .  24   6  in the shroud  28  each include, for example as shown with respect to connector  24   6 , a flared front opening  32   6 , a rear section  34   6  separated by an opening  36   6  through which the center pin  26   6  passes. It can be readily seen that proper plating flow throughout the interior of the RF shroud  28  and the connector pins  26  can be less than desired, particularly as it pertains to the flow of plating material to the surfaces within the rear section  34   6  around the center pin  26 .  
      Accordingly, this invention is directed to the solution of the plating problem noted above by fabricating generally rectangular slots or holes in the shroud  28  above and below the location of each RF connector  24   1  . . .  24   6 . This is shown in  FIGS. 2-5  by upper and lower slots  38   1  . . .  38   6  and  40   1  . . .  40   6 . The pairs of upper and lower slots  38   1 ,  40   1  . . .  38   6 ,  40   6  allow for plating solution to flow more freely throughout the interior of the RF connector shroud  28  and particularly into the interior spaces  34   1  . . .  34   6 . The holes or slots  38   1  . . .  38   6  and  40   1  . . .  40   6 , moreover, are sized to prevent RF leakage, but large enough so as not to trap air bubbles inside the shroud  28 .  
      Since the functions of an RF connector in a T/R module are to provide a low loss connection between two different structures and to provide RF isolation, that is to prevent the RF signal from leaking out, the first of these functions is not affected by the slots as the center conductor is not changed and where the slots are made to be below waveguide cut-off. The second function of the connector is potentially affected due to the holes placed in the shroud but since they are below waveguide cut-off there is relatively no evenescient mode radiation.  
      The slots or holes  38   1  . . .  38   6  and  40   1  . . .  40   6  are fabricated by machining operation which is performed prior to brazing the connector shroud  28  onto the ceramic substrate  12 . It should be noted that the added cost of the additional machining operation is practically negligible when compared to the loss of the entire package, particularly where T/R modules in acceptable condition numbering in the thousands must be produced and delivered.  
      Thus what has been shown and described is a connector shroud brazed on the forward surface of a dielectric substrate which permits plating fluid in an electrolytic plating process to move readily to contact and flow by all the interior metallized surfaces including the connector pins, as opposed to prior art connector shrouds which tend to inhibit fluid from reaching all the surfaces due to the closed nature of the shroud itself.  
      The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.