Abstract:
A method allowing for the inexpensive automated construction of interconnections between circuit boards is provided. According to the present invention, printed circuit pins are inserted in a circuit board from the top (component side). Provided the heads of the pins are thin enough to lie beneath a solder stencil, the pins may be pre-installed on the circuit board and solder applied to the pins at the same time solder is applied to other regions of the board. Thus, known surface mount techniques may be employed to form solder connections between the pins and conductive traces on the circuit board, which facilitates the automation of the previously manual operation of soldering the printed circuit pins separately.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to interconnections for propagating radio frequency (RF) signals between circuit boards. More particularly, the present invention relates to a method for using surface-mount techniques to mount and solder connectors suitable for transmitting RF signals between two circuit boards.  
           [0003]    2. Description of Related Art  
           [0004]    Modern electronic circuits utilize components that operate in the RF domain. It is often necessary to transmit RF signals between two different circuit boards. For example, one may desire to couple the signal from a coplanar waveguide on one circuit board to a coplanar waveguide on another circuit board.  
           [0005]    A coplanar waveguide is a type of high-frequency transmission line formed by placing a central conductor, i.e. a circuit trace, on the surface of one side of a circuit board, between two closely-spaced circuit traces held at ground potential. Additionally, a ground plane is placed on the reverse side of circuit board (opposite the central conductor), in which case the structure then becomes a coplanar waveguide with ground. A coplanar waveguide is a substantially planar analogue of a coaxial cable that has been sliced along its longitudinal axis to reveal a central conductor surrounded by a grounded conductive sheath.  
           [0006]    In order to transmit a signal between coplanar waveguides on two different circuit boards, an interconnection must be provided between the respective central conductors and ground planes on each board. Care must be taken to keep the central conductor path well-shielded to minimize signal attenuation due to radiation of energy into the air. The design of an interconnection must also minimize reflections, another source of signal attenuation. The problems of radiation and reflection are general matters of concern in the design of RF transmission lines, of which coplanar waveguides are just one type.  
           [0007]    Coaxial contacts, such as blind mate connector systems, have been used conventionally to provide an RF path between separate circuit boards or assemblies. However, the blind mate connector systems are bulky and expensive, and thus increase the cost per connection. They also require a manual operation to connect and disconnect them, further increasing the cost of their use.  
           [0008]    Short metal ribbon interconnections have also been used. The metal ribbon interconnections are difficult to handle due to their small size. They also have a further disadvantage of requiring solder applications, often done manually, to complete the RF path. These disadvantages greatly increase the cost of completing each connection. Furthermore, metal ribbon interconnections cannot be easily disassembled. When one desires to disconnect two boards, the metal ribbon interconnections must be unsoldered.  
           [0009]    A printed circuit pin and spring socket system has been used conventionally as a connection system for circuit boards for low-frequency analog, digital and power applications in electronics. A disadvantage of this system is that the pin connector is installed from beneath (i.e., opposite the component side) a circuit board and soldered in place by hand. Automated installation from the side opposite the component side requires complicated and expensive assembly equipment.  
           [0010]    Accordingly, there is a need for an inexpensive method for installing a low-cost RF interconnection for providing a low-radiation, low-reflection RF path between multiple circuit boards or assemblies. There is also a need for an interconnection that may be installed, connected, and disconnected without manual operations.  
         SUMMARY OF THE INVENTION  
         [0011]    In accordance with the teachings of this invention, a method for forming an RF interconnection between circuit boards using pin and socket connectors is provided. As described in more detail below, the present invention provides several distinct advantages over conventional methods of forming RF connections between circuit boards. According to the present invention, printed circuit pins are inserted in a circuit board from the top (component side), which is a step that is easily automated. Another advantage provided by having the pins inserted from the top side of the circuit board is that it is possible to use surface mount techniques to form solder connections between the pins and conductive traces on the circuit board. Provided the heads of the pins are thin enough to lie beneath a solder stencil, the pins may be pre-installed on the circuit board and solder applied to the pins at the same time solder is applied to other regions of the board. This eliminates the conventional manual soldering operations that are usually performed after other components have already been soldered in place. Reducing the conventional two-step, partially manual soldering routine to a single automated step provides a significant advantage of manufacturing efficiency.  
           [0012]    The printed circuit pins used in connection with the present invention include a shaft located at the distal end of the printed circuit pin and a head at the proximal end. The shaft has a cylindrical cross-section and is dimensioned to be inserted through a conductively plated bore through a circuit board and into a socket connector. The head is wider than the plated bore through the circuit board. Thus, the head prevents the printed circuit pin from passing completely though the bore when the pin is inserted. The shaft and head are joined by a friction segment and taper region which form the intermediate portion of the printed circuit pin. The friction segment is dimensioned to fit snugly in the plated bore through the circuit board. The friction segment thus provides an interference fit through friction between the printed circuit pin and the plated bore through the PC board.  
           [0013]    The assembly method includes a step of inserting the printed circuit pin in a bore through a circuit board. The insertion may be done by machine. The interference fit described above keeps the printed circuit pin in position while succeeding steps involving known surface mount techniques are applied to the circuit board. A solder stencil is laid over the circuit board after pins are inserted. Solder paste is applied to the solder stencil which contains several apertures through which solder paste is coated on regions of the circuit board lying beneath the apertures. Specifically, there are some apertures lying over the heads of the printed circuit pins, and the pins are consequently coated with solder paste. The solder paste is then heated and reflowed, and next cooled to complete the electrical connection between the printed circuit pin and a conductive surface surrounding the bore through the circuit board.  
           [0014]    On another circuit board, solder paste is applied to regions surrounding bores through the circuit board. Socket connectors are then inserted through the bores. The solder paste is heated and reflowed, and then cooled to complete the electrical connection between sockets and a conductive surface surrounding the bores through the circuit board.  
           [0015]    Once the pins and sockets are installed on their respective circuit boards, the interconnection is completed by mating the pins with the sockets. The result is a low-radiation, low-reflection RF path between the circuit board wherein the interconnection is integrally a part of the circuit boards.  
           [0016]    A more complete understanding of the invention will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheet of drawings which will first be described briefly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1A depicts a perspective view of two circuit boards having RF conductive traces on the surface thereof and corresponding bores through which printed circuit pins sockets may be inserted for forming an interconnection;  
         [0018]    [0018]FIG. 1B depicts view of conductive regions plated on the top, component-side surface of a circuit board in relation to a conductively plated bore;  
         [0019]    [0019]FIG. 1C depicts a view of the conductive regions plated on the bottom surface of a circuit board in relation to a conductively plated bore;  
         [0020]    [0020]FIG. 2 depicts a side view of a completed interconnection;  
         [0021]    [0021]FIG. 3A depict a perspective view of a solder stencil overlaying a corresponding circuit board;  
         [0022]    [0022]FIG. 3B depicts a side view of a solder stencil overlaying a corresponding circuit board taken along line  1 - 1  of FIG. 3A;  
         [0023]    [0023]FIG. 4 depicts a perspective view of a printed circuit pin which may be used in connection with the present invention;  
         [0024]    [0024]FIG. 5 depicts a view of the printed circuit pin taken along line  2 - 2  of FIG. 4;  
         [0025]    [0025]FIG. 6A depicts a side view of a printed circuit pin partially inserted in a bore through a PC board;  
         [0026]    [0026]FIG. 6B depicts a side view of a printed circuit pin fully inserted in a bore through a PC board with solder paste applied to its head;  
         [0027]    [0027]FIG. 6C depicts a side view of a printed circuit pin after solder paste applied to its head has been heated and reflowed;  
         [0028]    [0028]FIG. 7A depicts a side view of a socket contact partially inserted in a bore through a PC board with solder paste applied to the PC board adjacent to the bore;  
         [0029]    [0029]FIG. 7B depicts a side view of a socket contact fully inserted in a bore through a PC board after solder paste has been heated and reflowed;  
         [0030]    [0030]FIG. 8A depicts an alternative embodiment of a printed circuit pin which may be used in connection with the present invention;  
         [0031]    [0031]FIG. 8B depicts a view of the printed circuit pin taken along line  3 - 3  of FIG. 8A;  
         [0032]    [0032]FIG. 8C depicts an alternative embodiment of a printed circuit pin which may be used in connection with the present invention;  
         [0033]    [0033]FIG. 8D depicts a view of the printed circuit pin taken along line  4 - 4  of FIG. 8C. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]    The present invention satisfies the need for a method for forming an inexpensive interconnection providing an RF path between for multiple circuit boards or assemblies. The present invention also satisfies the need for an interconnection that may be installed, connected, and disconnected without manual operations. In the detailed description that follows, it should be appreciated that like element numerals are used to describe like elements illustrated in one or more of the figures.  
         [0035]    Referring now to FIGS.  1 A-C, a first circuit board  10  is shown. A first RF conductive trace  12  is disposed on a component-side surface  11  of the circuit board  10 . The conductive trace  12  lies between two electrical ground regions  13  formed from conductive traces disposed on a component-side surface  11  of the circuit board  10 . Additionally, an electrical ground plane  17  is disposed on the side of the circuit board  10  opposite the component-side surface  11 . It is well known in the art that an RF signal may be propagated along the RF conductive trace  12 . A second circuit board  20  is also shown with an RF conductive trace  22  and electrical ground regions  23  formed from conductive traces disposed on a component-side surface  21  of the second circuit board  20 .  
         [0036]    The circuit boards  10 ,  20  have conductively plated bores  14 ,  24  through them. The conductive plating  15 ,  25  lining the bores  14 ,  24  is electrically connected to the RF conductive traces  12 ,  22  on the circuit boards  10 ,  20 . As illustrated in FIGS. 1B and 1C, the conductive plating  15  may have an component-side extremity  15   a  that is in electrical contact with the first RF conductive trace  12 . The electrical contact between the conductive plating  15  and the RF conductive trace  12  allows an RF signal to propagate along the RF conductive trace  12  and through the circuit board  10  via the conductive plating  15 . For applications using a coplanar waveguide with ground, the opposite extremity  15   b  of the conductive plating  15  is isolated from the ground plane  17  by a surrounding insulation region  19 . The insulation region  19  prevents the RF signal from being shorted to ground. The second circuit board  20  is similarly constructed. The circuit boards  10 ,  20  also may have additional conductively plated bores  18 ,  28 .  
         [0037]    Referring now to FIG. 2, a completed interconnection is depicted. A printed circuit pin  30  is shown inserted from the component side through circuit board  10  and into socket  50  which has been inserted through circuit board  20 . The pin  30  and socket  50  pass through and are in electrical contact with the plated bores as discussed in connection with FIGS.  1 A-C.  
         [0038]    Referring now to FIGS. 3A and 3B, a solder stencil  40  is shown overlaying a circuit board  10 . The stencil  10  contains apertures  41  and  42  corresponding to regions of the circuit board  10  where solder paste should be deposited. Specifically, the apertures  42  correspond to the regions in which printed circuit pins  30  are located after they are inserted in the circuit board  10 . The apertures  42  allow solder paste to be applied to the heads  35  of the printed circuit pins  30 . There is a thickness  60  of the solder stencil  40 . The heads  35  must be thin enough that they can be accommodated in the thickness  60 . That is, the heads  35  must be able to lie inside the aperture  42  of the solder stencil  40  without interfering with its functioning. For example, the head  35  may be about 0.008 inches thick and the thickness  60  may be about 0.010 inches thick.  
         [0039]    Referring now to FIGS. 4 and 5, an embodiment of the printed circuit pin  30  of the present invention is shown. The printed circuit pin  30  has a proximal end and a distal end joined by an intermediate portion. The printed circuit pin  30  includes a shaft  31  located at the distal end of the printed circuit pin  30  and a head  35  at the proximal end. The shaft  31  and head  35  are joined by a friction segment  33  and taper region  39  which form the intermediate portion of the printed circuit pin  30 . The shaft  31  has a cylindrical cross-section. The diameter and length of the shaft must be such that it may be received in the inner cavity of a socket contact. For example, the diameter may be about 0.025 inches and the length about 0.155 inches. In the embodiment shown in FIGS. 4 and 5, the friction segment  33  has a hexagonal cross-section. Alternative embodiments may use, for example, a regular polygonal cross-section with more or fewer sides. The taper region  39  may angle away from the shaft  31  at about forty-five degrees, although other tapering angles are within the scope of the invention. The portion of the printed circuit pin  30  including the taper region  39  and the friction segment  33  should have a length substantially the same as the thickness of the circuit board  10 , such as about 0.055 inches. The printed circuit pin may be made of a brass alloy covered with a material with high conductivity and resistance to oxidation. For instance, the brass alloy may be covered with a nickel layer and finished with gold.  
         [0040]    The construction of an interconnection will now be described with respect to FIGS.  6 A- 6 C. Referring first to FIG. 6A, the printed circuit pin  30  is shown partially inserted through a bore  14  in the circuit board  10 . The bore  14  is lined with an electrically conductive plating  15 . A conductive trace  12  is in contact with the conductive plating  15 . The friction segment  33  has a diameter slightly larger than the bore  14  so that it fits snugly within the bore  14 , such that edges  37  of the friction segment  33  firmly contact the conductive plating  15  of the bore  14  when the printed circuit pin  30  is inserted. The contact between the edges  37  and the conductive plating  15  creates an interference fit that resists movement of the printed circuit pin  30 . The head  35  is shown to have a diameter slightly larger than that of the bore  14 . This dimension of the head  35  prevents the printed circuit pin  30  from being pushed completely through the bore  14  in the circuit board  10 .  
         [0041]    [0041]FIG. 6B shows a printed circuit pin  30  that has been fully inserted in a circuit board  10 . A solder stencil is positioned over the circuit board  10  so that the aperture  42  is in alignment with the head  35  of the printed circuit pin  30 . Next solder paste  70  is applied to the head  35  and surrounding areas of the printed circuit pin  30  through aperture  42 . The solder paste  70  is then heated causing it to melt and reflow around the head  35 . Referring now to FIG. 6C, the cooled and hardened solder  72  is shown after reflow. The hardened solder  72  provides an electrically conductive path between the conductive trace  12 , the conductive plating  15  and the printed circuit pin  30 .  
         [0042]    Referring now to FIGS. 7A and 7B, solder paste  53  is deposited around a bore  57  through a second circuit board  20 . The bore  57  is lined with an electrically conductive plating  25 . The conductive plating  25  is in electrical contact with a conductive trace  22  on the component-side surface of the circuit board  20 . Next, a socket contact  50  is positioned through the bore  57  and the conductive plating  25  until a shoulder  52  of the socket contact  50  rests substantially flush with the conductive trace  22  and the conductive plating  25 . The solder paste  53  is then heated causing it to melt and reflow around the shoulder  52  of the socket contact  50 . After the solder paste  53  is cooled and hardened, the resulting hardened solder joint  55  provides an electrically conductive path between the conductive trace  22 , the conductive plating  25 , and the socket contact  50 . It will be understood by one skilled in the art that the steps of inserting and soldering the printed circuit pins  30  and the inserting and soldering socket contacts  50  may be performed simultaneously or in succession.  
         [0043]    Referring now again to FIG. 2, a side view of a completed interconnection is shown. The printed circuit pin  30  is shown extending through the first circuit board  10  down into a socket contact  50  which is dimensioned to receive the printed circuit pin  30 . The socket contact  50  is disposed in a bore through the second circuit board  20 .  
         [0044]    Referring now to FIGS.  8 A- 8 D, some alternative embodiments of the printed circuit pin  30  are shown. The essential feature of the printed circuit pin  30  is that it provide an interference fit when positioned within a plated bore  14 ,  24 . The feature may be provided by a wide variety of shapes. For example, FIGS. 8A and 8B depict a printed circuit pin  30  having a friction segment  33  that is square in cross-section. FIGS. 8C and 8D depict a printed circuit pin having a friction segment  33  that has a knurled cross-section.  
         [0045]    A preferred method for constructing an interconnection between microstrip lines on two circuit board having just been described, it should be apparent to those skilled in the art that certain advantages of the system described herein have been achieved. While specific embodiments of the present invention have been described above, it will be apparent that obvious variations and modifications of the present invention will occur to those of ordinary skill in the art from a consideration of the foregoing description.  
         [0046]    For example, an interconnection for microstrip applications has been illustrated, but it should be apparent that the inventive concepts described above would be equally suitable for any application requiring electrical connections between conductive traces on two different circuit boards.  
         [0047]    Moreover, many variations in the shape of the printed circuit pin may be made within the scope of the present invention.  
         [0048]    It is therefore desired that the present invention be limited only by the following claims.