Abstract:
A printed circuit board (B) with a mechanical attachment of connector pins ( 42 a and  42 b) to opposite sides of the same plated through location ( 34 ) maintains electrical isolation between the board sides ( 14  and  16 ). A sequence of controlled depth drilling steps creates a hole ( 34 ) through the printed circuit board (B) with multiple diameters ( 20, 24, 30  and  40 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 60/319,764, filed Dec. 10, 2002, entitled CROSS CONNECT VIA FOR MULTILAYER PRINTED CIRCUIT BOARDS. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    1. Technical Field.  
           [0003]    The invention relates to the field of printed circuit boards and more particularly to a processing method for achieving a mechanical attachment to both sides of a printed circuit board a single plated through hole without electrical conductivity.  
           [0004]    2. Background Art.  
           [0005]    The present invention is a printed circuit board processing technique that allows mechanical attachment of press fit pins to opposite sides of the same plated through location while maintaining electrical isolation between them. This invention is realized through a sequence of controlled depth drilling steps that creates a hole through the printed circuit board with multiple diameters.  
           [0006]    A problem arises when networking topologies, such as cross point switches, are implemented in mid-plane applications. Mid-plane configurations are connectorized from both sides of the printed circuit board. Normally, connectors must be offset from side to side to eliminate mechanical interference so that the pins may be electrically isolated. New high density connector designs are not suitable in these applications because the pin density is too high to allow for a mechanical offset. The present invention eliminates the need to mechanically offset the connectors thus allowing the use of high density connector designs in these applications.  
           [0007]    Mid-plane back-panels are a special back-panel design where there are daughter cards attached to both sides of the back-panel. Mid-planes are typical in large cross point switch designs found in telecommunications and data storage industries.  
           [0008]    A prior method, which could be used to solve this problem, is known in the industry as sequential lamination. Sequential lamination is a process where a partial group of layers is processed through the normal multi-layer lamination process, drilled and plated and then combined with other parts in a subsequent lamination operation to yield the completed assembly.  
           [0009]    However, sequential lamination adds significant cost to the end product through multiple repetitions of drilling, plating, imaging, and lamination processes. These repetitive processes add significant cost to the manufacturing process.  
           [0010]    While the above cited references introduce and disclose a number of noteworthy advances and technological improvements within the art, none completely fulfills the specific objectives achieved by this invention.  
         SUMMARY OF INVENTION  
         [0011]    In accordance with the present invention, a printed circuit board adapted to mount electrical circuitry has at least one base layer and first and second opposing surfaces. A first through passageway having a first passageway diameter extends through the base layer from the first surface to the second surface. A first bore hole having a first bore diameter and concentric with the first through passageway is formed between the first surface and a desired first depth in the base layer between the first and second surfaces. The first bore diameter is preferably greater than the first passageway diameter.  
           [0012]    A second bore hole having a second bore diameter and concentric with the first through passageway is formed or drilled between the second surface and a desired second depth in the base layer between the first and second surfaces. The second bore diameter is greater than the first passageway diameter.  
           [0013]    The first through passageway, first bore hole, and second bore hole form a desired open structure. The open structure is then plated with a desired conductive material, such as copper, silver, or gold.  
           [0014]    A second through passageway concentric with the first through passageway extending through the open structure is then formed in the base layer, and opens the passageway from the first surface to the second surface. The second through passageway having a second passageway diameter at least as large as the first passageway diameter to electrically isolate the first surface from the second surface.  
           [0015]    A known first connector pin compatible with at least a first portion of the open structure is then inserted into the open structure from the first surface. Similarly, a second connector pin compatible with at least a second portion of the open structure is then inserted into the open structure from the second surface.  
           [0016]    These and other objects, advantages and features of this invention will be apparent from the following description taken with reference to the accompanying drawings, wherein is shown the preferred embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0017]    A more particular description of the invention briefly summarized above is available from the exemplary embodiments illustrated in the drawings and discussed in further detail below. Through this reference, it can be seen how the above cited features, as well as others that will become apparent, are obtained and can be understood in detail. The drawings nevertheless illustrate only typical, preferred embodiments of the invention and are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.  
         [0018]    [0018]FIGS. 1A through 1F depict a series of cross sections showing boring and plating operations through a known printed circuit board.  
         [0019]    [0019]FIG. 2 depicts a cross section of a known connector pin being inserted into the circuit board through a prepared via in accordance with the present invention.  
         [0020]    [0020]FIG. 3 is a series of vias prepared in accordance with the present invention in a variety of printed circuit cross-sections. 
     
    
     DETAILED DESCRIPTION  
       [0021]    So that the manner in which the above recited features, advantages, and objects of the present invention are attained can be understood in detail, more particular description of the invention, briefly summarized above, may be had by reference to the embodiment thereof that is illustrated in the appended drawings. In all the drawings, identical numbers represent the same elements.  
         [0022]    A printed circuit board B adapted to mount electrical circuitry  10  has at least one base layer  12  and a first and second opposing surfaces  14  and  16  respectively. A first bore hole  22  having a first bore diameter  24  extends into the base layer  12  to a predetermined first depth  26  from the first surface  14  and between the first surface  14  and the second surface  16 . A first through bore hole or passageway  18  having a first passageway diameter  20  and concentric with the first bore hole  22  is formed extending from the first surface to a predetermined depth  50  from the first surface  14  less than the total thickness  52  of the circuit board B; or, alternatively, the first through passageway  18  may be formed generally extending through the base layer  12  from the first surface  14  to the second surface  16 .  
         [0023]    The first bore diameter  24  is preferably greater than the first through passageway diameter  20 , or in other words, the first through passageway diameter  20  is smaller that the first bore diameter  24 .  
         [0024]    A second bore hole  28  having a second bore diameter  30  and concentric with the first through passageway  18  is formed or drilled between the second surface  16  and a desired second depth  32  in the base layer  12  between the first and second surfaces  14  and  16 . The second bore diameter  30  is greater than the first passageway diameter  20 .  
         [0025]    The depth  50  of the first through hole  18  plus the depth  32  of the second bore hole  28  must equal or exceed the total thickness  52  of the printed circuit board B.  
         [0026]    The first through passageway  18 , first bore hole  22 , and second bore hole  28  comprising a desired open structure  34 . The open structure  34  is then plated with a desired conductive material  36 , such as copper, silver, or gold.  
         [0027]    A second through passageway  38  concentric with the first through passageway  18  extending through the open structure  34  is then formed in the base layer  12  and inures the opening of a passageway from the first surface to the second surface  14  and  16  respectively. The second through passageway  38  having a second passageway diameter  40  at least as large as the first passageway diameter  24  to electrically isolate the first surface  14  from the second surface  16 . The through hole structure  54  thus formed may be electroplated to a specified finished hole diameter to receive the specified compliant connector pin  42 .  
         [0028]    A known first connector pin  42   a  compatible with at least a first portion  44  of the open structure  34  is then inserted into the open structure  34  from the direction of the first surface  14 . Similarly, a second connector pin  42   b  compatible with at least a second portion  46  of the open structure  34  is then optionally inserted into the open structure  34  from the direction of the second surface  16 . The size and depth penetration of the connector pins  42   a  and  42   b  are chosen as desired.  
         [0029]    The connector pins  42   a  and  42   b  generally are known press fit type pins to allow simple mechanical attachment to the printed circuit board B.  
         [0030]    Since the holes formed in the printed circuit board B are normally done with a drill or a laser beam, generally the holes are cylindrical voids  32  formed in the base layer  12  of the circuit board B.  
         [0031]    Referring particularly to FIG. 3, the benefit of the present invention allowing the same x-y location to be electrically isolated from top to bottom of the printed circuit board B. FIG. 3 also illustrates the flexibility of the present invention through varying depths of drill operations allowing hole at one x-y location to be connected to another x-y location on the opposite side of the printed circuit board B and also with circuit boards having multiple layers  12   a ,  12   b , and  12   c  for example and different interior characteristics.  
         [0032]    In the most basic embodiment of the present invention, a hole is partially drilled to a predetermined depth at the required diameter from both sides at the same x-y location. A smaller diameter hole is drilled through the remaining material to allow subsequent chemistry to flow completely through for plating purposes. The small center hole is drilled out in a final step to remove the electrical connection between the two sides.  
         [0033]    Specifically referring to FIGS. 1A through 1F and FIG. 2, the following steps may be followed:  
         [0034]    1. In a first drill operation, the first bore hole  22  is formed to a desired depth  26  in the printed circuit board B.  
         [0035]    2. In a second drill operation the first through passageway  18  is drilled to a predetermined depth  50  from the first surface  14  at the same x-y location as the first drill operation. Diameter  24  is greater than diameter  20 .  
         [0036]    3. In a third drill operation a second bore hole  28  is created to a predetermined depth  32  from the second surface  16  to create a complete opening  34  through the printed circuit board B.  
         [0037]    4. A thin layer or application of an electroless metal, such as copper, may be deposited onto the walls of the drilled or open structure  34  with a known electroless process. Then a thing electroplated copper layer may be plated to provide mechanical integrity to the structure during subsequent processing. Alternatively, another chosen conductor such as copper, silver, gold or the like, may be deposited.  
         [0038]    5. A fourth drill operation drills a second through passageway  38  through the depth of the first operation to desirably remove the copper or other metallic plating. Diameter  40  is preferably slightly larger than diameter  20  to electrically isolate the first surface  14  from the second surface  16 .  
         [0039]    6. The final structure  54  may be electroplated to a specified finished hole diameter to receive the specified compliant connector pin  42 .  
         [0040]    7. Compliant press-fit connector pins  42   a  and  42   b  are inserted from both sides of the printed circuit board B at the same x-y location.  
         [0041]    While the term copper may have been used to describe the present invention, any suitable conductive method or compound may be used as chosen for the specific application.  
         [0042]    Alternatively, a first through passageway  18  having a first passageway diameter  20  is formed first and extends through the base layer  12  from the first surface  14  to the second surface  16 . A first bore hole  22  having a first bore diameter  24  and concentric with the first through passageway  18  is then formed between the first surface  14  and a desired first depth  26  in the base layer  12  between the first and second surfaces  14  and  16 . The first bore diameter  24  is preferably greater than the first passageway diameter  20 .  
         [0043]    A second bore hole  28  having a second bore diameter  30  and concentric with the first through passageway  18  is then formed or drilled between the second surface  16  and a desired second depth  32  in the base layer  12  between the first and second surfaces  14  and  16 . The second bore diameter  30  is greater than the first passageway diameter  20 .  
         [0044]    The remaining steps in this alternative embodiment remains the same as above disclosed.  
         [0045]    The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as in the details of the illustrated construction may be made without departing from the spirit of the invention.