Abstract:
A fixture attached to the rear of the base plate of a semi automatic screen printer includes an element holder having a vertical recess or pocket for receiving an assembly, such as a substrate, of LTCC laminate material lengthwise therein so as to expose only one side surface to the printing head which would otherwise be prevented because of the height restrictions imposed by the printer head.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application is related to U.S. Ser. No. 10/718,805 (Northrop Grumman Case No. 000408-078) entitled “Fabrication of LTCC T/R Modules With Multiple Cavities And An Integrated Ceramic Ring Frame” filed on Nov. 24, 2003 in the name of Rena Y. Jacobson et al., and which is assigned to the assignee of the present invention and which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates generally to side printing on a laminated assembly of low temperature co-fired ceramic (LTCC) material, and more particularly to a method and apparatus for side printing on an LTCC substrate for microwave transmit/receive (T/R) modules which are used in connection with an active aperture of a phased array radar system.  
         [0004]     2. Description of Related Art  
         [0005]     The conventionally used screen printers cannot accommodate the newly designed LTCC substrates because of height restrictions associated with the screen printers and because of the currently used hold down mechanisms for the substrates.  
         [0006]     Conventional T/R module packages include a structure utilizing a multi-cavity and a multi-layer substrate as shown, for example, in U.S. Pat. No. 6,114,986, comprised of high temperature co-fired ceramic (HTCC) layers including, for example, black alumina ceramics and tungsten. The ceramic layers have outer surfaces including metallization patterns of ground planes and conductors as well as feedthroughs or vertical vias formed therein for providing three dimensional routing of both RF and DC signals. Active circuit components comprised of integrated circuit chips are located in the various cavities of the substrate. The active circuit components are used for implementing suitable control and generating functions of RF signals to and from respective antenna elements. In addition, such HTCC designs utilize a nickel/gold plating approach.  
         [0007]     However, a newly designed LTCC substrate such as shown and described in related application Ser. No. 10/718,805, does not accept the nickel/gold plating approach because such LTCC design, including T/R modules incorporating the same, include highly complex packages of up to 20 or more layers of tape and have multiple cavities with different floor thicknesses, through-holes, and an integrated ring frame in the package as required, for example, by an LTCC T/R module utilized in a radar system.  
         [0008]     The newly designed LTCC T/R module referred to above utilizing such a complex package design requires that RF lines extend to the outer surface (periphery) of the LTCC substrate. The RF lines are then connected to connector pins through a brazing operation. In order for a print to be added to the RF lines, metallization must be deposited and fired on the outer side wall of the LTCC substrate. Side printing requires deposition of brazeable metallization; however, LTCC material does not accept the standard HTCC approach of using nickel/gold plating. Furthermore, attempts have been made to use standard screen printers to do side printing, however, the standard screen printers cannot accommodate a part standing on its end or a part having a thickness greater than 1″. As such, in using standard screen printers, there is no room for the part to stand nor is there any currently available method of hold down for parts thicker than 1″. Therefore, in order for the side of the LTCC substrate to be printed on, a new and improved print fixture is needed and comprises the subject matter of this invention.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, it is an object of the present invention to provide an apparatus for performing the difficult task of side printing a LTCC substrate for a T/R module design using a standard MPM screen printer.  
         [0010]     It is another object of the present invention to provide an improved print fixture, e.g., a workholder, for accommodating an LTCC substrate, wherein the improved workholder is capable of being used within standard screen printers.  
         [0011]     These and other objects are achieved in the present invention by an improved apparatus and method for side printing the newly designed LTCC substrates, for a T/R module, by the addition of a modified workholder to, for example, an MPM SP-1500 Semi-Automatic Screen Printer.  
         [0012]     Such a modified workholder takes advantage of the free space at the back of the standard workholder base in an MPM SP-1500 Semi-Automatic Screen Printer. As a result, the 2″ height of the newly designed LTCC T/R module can be placed in the depth of this free space, allowing the top of the substrate to sit flush with a printing surface of the printer. This arrangement allows easy clearance of the height restrictions imposed by the printer head. In other words, the present invention allows an add-on fixture/workholder, having a recess or a pocket for the LTCC substrate to sit in, to be added to the base plate of an existing screen printer.  
         [0013]     As a result, side printing capabilities, relating to brazeable metallization, are achieved by the present invention wherein connector pins are attached thereto. In general, brazing metallization is also used to attach heat sinks and covers or lids. A result of the present invention, an LTCC substrate for a T/R module is provided which is relatively simple, elegant and light in weight.  
         [0014]     In forming a newly designed LTCC substrate, such as shown in related application Ser. No. 10/718,805, typically a number of LTCC tape layers are initially divided into multiple stacks of tape layers, e.g., four stacks or sections, based on a final cavity routing configuration of each stack. One of the stacks preferably comprises a set of layers for the formation of a ceramic ring frame. Each stack is then separately tack laminated, followed by cavity patterns being machined therein using a router device whereby a large number of layers are routed in a single pass, forming cavities having side walls which are relatively smooth and straight as opposed to the known prior art method of cutting individual layers with a laser, for example, which results in the formation of jagged side walls. Following cavity routing, all of the stacks are laid up on a base plate including a set of tooling pins for providing alignment of the stacks. The lay up is completed with a placement of a top mylar template followed by a copper template. The assembly is next covered with an expandable latex sheet that has been lightly coated with a graphite aerosol for aiding the removal of the latex sheet without metal lift-off after lamination. The assembly is then placed in a lamination fixture and isostatically laminated for a specific period of time at a predetermined pressure and temperature, typically 4000-5000 psi at 72° C. for 15 minutes, resulting in a structure in the form of laminated panel. The panel is then allowed to cool, removed from the fixture where it is green cut to remove tooling holes and then fired with a designated firing profile. A plurality of mutually identical newly designed LTCC substrates for a T/R module are then diced, i.e. cut, from the panel and post fire printed as required.  
         [0015]     Due to the design of conventional screen printers, e.g., the MPM SP-1500 printer, for example, the printing of LTCC substrates incorporating the above noted design was limited to a certain range of the overall thickness of the substrate or board. For instance, the allowable thickness of the substrates were limited to less than 1″ maximum. Substrates with a thickness greater than 1″ were not able to access the print position inside of the standard screen printing machine due to the fact that the printer head was in the way. However, the newly designed LTCC substrate for the T/R module is approximately 2″ in height. As a result, the present invention is directed to a fixture or workholder that can accommodate the newly designed LTCC 2″ substrate for side printing.  
         [0016]     Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be noted, however, that the detailed description and specific examples, while indicating the preferred method of the invention, are provided by way of illustrations only, since various changes, alterations and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the following detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The present invention will become more fully understood when considered in conjunction with the accompanying drawings, which are provided by way of illustration and thus are not meant to be considered in a limiting sense, and wherein:  
         [0018]      FIG. 1  is a front elevational view of a standard MPM SP-1500 printer;  
         [0019]      FIG. 2  illustrates how a top edge of a 2″ substrate for a T/R module when stood on its end for side printing does not clear the printer head of the MPM SP-1500 printer shown in  FIG. 1 ;  
         [0020]      FIG. 3  is illustrative of a space behind the base plate of the printer shown in  FIG. 1  for accommodating a fixture for holding the substrate shown in  FIG. 2  in accordance with the present invention;  
         [0021]      FIG. 4  is a perspective view of the preferred embodiment of the fixture for holding the substrate in accordance with the subject invention;  
         [0022]      FIG. 5  illustrate the substrate positioned in the fixture shown in  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     The subject invention is directed to side printing on LTCC substrates of transmit/receive (T/R) modules utilized in connection with the phased array radar system.  
         [0024]     Referring now to the drawings, and more particularly to  FIG. 1 , shown thereat is a conventional MPM SP-1500 printer semi-automatic screen printer  10  which includes, among other things, a generally rectangular bed  12 , a workholder base plate  14  which is mounted on rails  16  and  18  and is moved forwardly and rearwardly by a motor  20 . The base plate  14  shown in  FIG. 1  is fitted with a porous vacuum plate  22  which is moved under a print head assembly  24  when lowered. Reference numeral  26  denotes a control console for the printer.  
         [0025]     In a desire to use the printer  10  for printing on one side edge located at one end of a substrate  30  such as shown, for example, in related application Ser. No. 10/718,805 referenced above,  FIG. 2  discloses that the height of the substrate  30  when stood on its end, does not clear the bottom surface of the printer head  32 .  
         [0026]     In an effort to overcome the height problem of the substrate  30  in relation to the printer head  32 , a region  34  behind the base plate  14 , shown in  FIG. 3 , was noted to provide a free space for accommodating a fixture  34 , such as shown in  FIG. 4 , for holding the substrate  30  in a general vertical position while exposing the surface  36  of the side edge  28  because due to the design of the SP-1500 printer, the printing of substrates is limited to a certain range of overall thicknesses and/or length of the substrate or board. Allowable thicknesses of substrates is limited to less than 1″ maximum. Substrates with thicknesses greater than 1″ were not able to exit the print position inside the machine for printing due to the print head being in the way. Thus a substrate for a T/R module with its desired side for gold printing in the proper printing position is approximately 2″ inches in height will not clear the print entrance. As a result, the workholder base plate had to be modified or a new workholder made. This resulted in the development of the fixture  34  for holding the substrate  30  in a vertical position as shown in  FIG. 4 .  
         [0027]     The fixture  34  is comprises a structure having three main parts: a mounting plate  38 , a substrate holder  40  and a switch extension piece  42 . The mounting plate  38  is dimensioned so as to substantially match a top surface of the base plate  14  and attachable thereto so that the entire assembly can be moved as a unit in and out from under print head  24  for a printing operation on the surface  36  of the substrate  30 .  
         [0028]     Attached to the rear end of the mounting plate  34  is the substrate holder  40  which includes an elongated transverse L-shaped bar type element  44 , as shown in  FIG. 4 , having a rectangular recess or pocket  46  for vertically receiving a substrate  30  therein so that only the surface  36  is flush with the surface of the mounting plate  38 . The substrate  30 , when inserted into the pocket  46 , is held into place by a spring loaded block  48  and a rear plate member  50 . This arrangement allows the substrate  30  to be held firmly in place for accurate printing. Both the mounting plate  38 , the element  44 , and the rear plate member  50  include generally rectangular cut-out regions  52  and  54  for enabling manual insertion and removal of the substrate  30  from the pocket  46 .  
         [0029]     The third part of the fixture  34  shown in  FIG. 4  is the switch extension piece  42  which is shown attached to an outer end portion of an L-shaped bar element  44  of the substrate holder  40 . This part is crucial for contacting a pressure switch (not shown) at the back of the printer. The pressure switch, when depressed, stops the motor  20  and thus the rearward movement of the base plate  14  and the attached fixture  34 . The length of the extension determines the general position where the surface  36  of the substrate  30  will stop with respect to the printer head  24 . Without the extension  42 , the pressure switch would be depressed too late, and the substrate would be pushed all the way to the back of the printing machine, where printing would be impossible.  
         [0030]     It should be noted, however, that in an alternate modification, not shown, the switch extension piece  42  can be attached to the base plate  14  shown in  FIG. 3 . Such an arrangement offers the advantage of holding the base plate  14  in place while allowing the substrate  30  to be maneuvered into a proper print position.  
         [0031]     Thus what has been shown and described is a side print fixture which provides a relatively inexpensive method of side printing on LTCC substrates with a conventional MPM screen printer including substrates for T/R modules which include pin and shroud connections on the front side edge.  
         [0032]     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