Abstract:
A method of forming a core for and forming a composite wiring board. The core has an electrically conductive coating on at least one face of a dielectric substrate. At least one opening is formed through the substrate extending from one face to the other and through each conductive coating. An electrically conductive material is dispensed in each of the openings extending through the conducting coating. At least a portion of the surface of the conductive coating on one face is removed to allow a nub of the conductive material to extend above the substrate face and any remaining conductive material to thereby form a core that can be electrically joined face-to-face with a second core member or other circuitized structure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a divisional of application Serial No. 10/213,646, filed Aug. 6, 2002, now U.S. Pat. No. ______, which in turn is a divisional of application Ser. No. 09/834,281, filed Apr. 12, 2001, now U.S. Pat. No. 6,465,084 B1, both of which are incorporated herein by reference. 
     
    
     
       BACKGROUND INFORMATION  
         [0002]    1. Field of the Invention  
           [0003]    This invention relates generally to a method and structure for producing a Z-axis interconnection of printed wiring board elements and, more particularly, to a method and structure of providing a printed wiring board formed of a plurality of elements which are laminated together to form a printed wiring board having Z-axis interconnections.  
           [0004]    2. Background of the Invention  
           [0005]    Printed wiring boards are conventionally made up of a plurality of individual elements joined together to provide various levels of wiring on the surfaces of the elements and interconnections between the various wiring levels, such interconnection between the various levels often being referred to as Z-axis interconnections. In some conventional techniques for forming such interconnections in the Z-axis, a drilling operation is required after the various elements have been joined together. This requires precise alignment of all of the elements as well as precise drilling of the final structure which creates the possibility of misalignment at least requiring either rework of the board or at most scrapping of the board after it reaches this late assembly stage. Thus, it is desirable to provide elements for forming a printed wiring board and a technique for forming the elements in the printed wiring board which does not require drilling in the final stage but, rather, allows the individual elements to be formed with the components of the Z-axis connection which, when finally joined together, will provide the necessary connection between various layers of metal wiring.  
         SUMMARY OF THE INVENTION  
         [0006]    According to the present invention, a method of forming a core member for a composite wiring board and a method of forming the composite wiring board, as well as the core member of the composite wiring board and the wiring board, are provided. The core member is formed by providing a dielectric substrate having opposite faces and forming an electrically conductive coating on at least one face thereof, preferably by laminating copper on the at least one face. At least one opening is formed through the substrate extending from one face to the other and through each conductive coating. An electrically conductive material is dispensed in each of the openings extending through the conducting coating. At least a portion of the surface of the conductive coating on at least one face is removed to allow a nub of the conductive material to extend above the substrate face and any remaining conductive material to thereby form a core that can be electrically joined face-to-face with a second core member or other circuitized structure. In one embodiment, a second core is then formed in a similar manner and the two cores joined face to face to provide a printed wiring board with electrical interconnections in the Z-axis, i.e. between the circuit traces on opposite faces of the circuit board so formed. In another embodiment, the core is used to join with at least one other circuitized member. The invention also contemplates a core member formed according to this invention and a printed wiring board formed using at least one core member. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0007]    FIGS.  1 - 6  show a longitudinal, sectional view, somewhat diagrammatic, of the steps to form a core member according to one embodiment of the present invention;  
         [0008]    [0008]FIGS. 7 and 8 show the steps of laminating two core members together to form a printed wiring board according to one embodiment of the invention;  
         [0009]    [0009]FIG. 9 shows another starting material for a core member;  
         [0010]    [0010]FIGS. 10 and 11 show another technique of forming core members for joining together;  
         [0011]    [0011]FIGS. 12 and 13 show two core members formed according to the technique shown in FIGS. 10 and 11 joined together;  
         [0012]    [0012]FIG. 14 shows another embodiment of a bonding film; and  
         [0013]    [0013]FIGS. 15 and 16 show another embodiment of a core joining two circuitized structures to form a printed wiring board. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]    Referring now to the drawings and, for the present, to FIGS.  1 - 6 , the successive steps in forming a core member  10  for use in laminating to another core member to form a printed wiring board according to one embodiment of the invention are shown. As can be seen in FIG. 1, the core member  10  includes a dielectric substrate  12  which has metal coatings  14  and  16  on opposite faces thereof. Dielectric substrate  12  can be any conventional dielectric, such as FR4 (a glass reinforced epoxy), polyimide, polytetrafluroroethylene or other suitable well known dielectrics. In the embodiment shown in FIGS.  1 - 6 , the metal coatings  14  and  16  preferably are copper and, typically, the layer is either one ounce copper (35 μm thick) or two ounce copper (70 μm thick). However, other thicknesses of copper coatings can be used.  
         [0015]    As shown in FIG. 2, a plurality of holes, two of which are shown at  18 , are drilled entirely through the substrate  12  and the metal coatings  14  and  16 . Optionally, the holes  18  may be plated with a metal  20 , as shown in FIG. 3, to provide a conductive path between the coatings  14  and  16 . However, with good and complete filling of conductive material, as will be explained presently, the plating  20  may be partially or fully omitted. Plating  20  can be any type of conventional plating to provide the openings with such metal, such as electroless or electrolytic plating.  
         [0016]    As shown in FIG. 4, the openings  18  are filled with a conductive material  24 . Filling can be done by screening, stenciling, flood coating, doctor blading, immersing or injecting. In some case, it may be desirable to heat the adhesive to enhance its flow characteristics and, in some cases, multiple passes might be required to achieve a complete fill of the holes  18 . Suitable fill materials include conductive polymers and polymers filled with conductive particles, such as solder, copper particles, silver particles or plated filler particles or mixtures thereof. The polymer can be thermoset or thermoplastic and can be thinned with a solvent if required. A preferred adhesive material  24  is a conductive epoxy sold by the Ablestik Corporation under the trademark Ablebond 8175 which is a silver filled thermosetting epoxy. Following the filling of the holes as shown in FIG. 4, the epoxy is B-staged, which entails heating the material to a temperature of about 130° C. until the degree of cure of the adhesive is advanced to from about 20% to about 80% of complete cure. As will become apparent later, the fill material  24  should not be fully cured at this state since it will be used to adhere to another conductive epoxy in another core element to form the printed wiring board, all of which will be described presently. If a complete fill of the conductive material results from the filling process, the plating  18  may be omitted. However, plating generally increases the area of contact, increasing reliability and lowering the electrical contact (resistance) and, therefore, is preferred.  
         [0017]    It should be noted that if some residue of the adhesive material  24  remains on the surface of the conductive layers  14  or  16  after filling the holes and after it is B-staged, it can be easily removed by chemical or mechanical polishing processes as are well known in the art. It should be noted that, at this point in the processing, no masks have been required for filling the holes so there are no registrations required, and any film material that may accidentally remain on the surface of the metal can be easily removed by polishing or other operations.  
         [0018]    As seen in FIG. 5, layers of material  14  and  16  are partially etched away to form thinned or circuitized conductive surfaces  26  and  28 . Such partial etching can be performed by the technique described in U.S. patent application Ser. No. 08/968,988, filed Nov. 12, 1997, for “Printed Circuit Board with Continuous Connective Bumps” (IBM Docket No. EN9-97-032). Thus, this technique of etching around the protrusions  24  by “cold” cupric chloride etching provides the ideal uniform height of the fill material  24  above the etched surfaces  26  and  28  on opposite sides of the dielectric material  12 . It is important to note that there are several advantages to the element in the form that it is in, as shown in FIG. 5. The very uniform height of the resulting protrusions of the fill material  24  is a particular advantage. Moreover, there is no fill adhesive residue on the metal surface, and no screening operation is required to form a protruding bump of adhesive.  
         [0019]    Following the general selective etching of the surfaces  26  and  28 , the core is personalized as shown in FIG. 6. In this portion of the personalization, the material  26  connects between the protrusions  24  on one face thereof, whereas the material  28  has been entirely removed from the other face. This personalization preferably is done by known photolithographic processes using either positive or negative photoresist materials. At this point, the core element as shown in FIG. 6 is ready to be laminated with at least one other core element to form a printed wiring structure. It is to be understood that several core elements typically will be laminated together but the invention is being illustrated using only two core elements, as shown in FIGS. 7 and 8.  
         [0020]    As can be seen in FIG. 7, two core elements  10   a  and  10   b  are provided which are to be laminated together. It will be noted that the two core elements  10   a  and  10   b  are very similar except that the personalization on each of them is slightly different in that on the core element  10   a  the personalization  26   a  extends from one fill material  24   a  to another fill material  24   a,  whereas on element  10   b  the personalization  26   b  extends in opposite directions from each of the fill materials  24   b.  As seen in FIG. 7, a pre-drilled bonding film  30 , such as the film sold under the trademark Pyralux LF by Pyralux Corporation, is interposed between the two cores  10   a  and  10   b.  The film  30  has openings  32  drilled therein which are positioned to align with the conductive fill material  24   a,    24   b  in the two core elements  10   a  and  10   b.  Heat and pressure are applied as represented by the arrows in FIG. 8 to cause the two core members to bond together, with the Pyralux LF film acting as an adhesive bond material. Also, the fill material  24   a  and  24   b  in each of the openings in the two core members  10   a  and  10   b  will bond together, as shown in FIG. 8, to form a continuous Z-axis electrical connection between the personalization  26   a  on the core element  10   a  and the personalization  26   b  on the core element  10   b.  The lamination process also advances the cure of the conductive fill material  24   a  and  24   b  past 80% to the fully cured stage. (Alternatively, the film material  30  could be a dry film adhesive which is B-staged and used to laminate the core elements  10   a  and  10   b  together.)  
         [0021]    [0021]FIG. 9 shows the starting configuration of a slightly different embodiment for a core element  10   c.  In this embodiment, the dielectric member  12  is coated with a first metal, such as copper  40  on one face and the same metal  42  on the other face, and a second metal  44  on top of the metal  40 , and metal  46  on top of the metal  42 , which is different from copper. Then, after drilling and optionally plating the drilled holes and filling the holes with conductive material, the metal  44  and  46  is selectively etched, leaving the metal  40 , and which would provide the same configuration as shown in FIG. 5. The metal  44 ,  46  could be copper, chromium or copper-invar-copper, as well as others, which can be etched by standard etching processes.  
         [0022]    FIGS.  10 - 14  show another embodiment or technique for joining two core elements together to form a printed wiring board. In this technique, two cores  10   a  and  10   b  are provided, but instead of a separate dry film  30  interposed between the two members to form a laminate structure, a dielectric adhesive material  48   a  and  48   b  is applied to the face of at least one, and preferably both, of the core elements  10   a  and  10   b.  The adhesive material, which is shown, preferably is a thermoset epoxy such as Epoxy 240 sold by the Dexter Corporation. After the epoxy film  48   b  is coated onto a face, as shown in FIG. 10, it is removed from the surface of the conductive material  24   b,  as shown in FIG. 11. This is preferably by a planar polishing technique. The same technique is performed on core element  10   a.  The two core elements  10   a  and  10   b,  with the epoxy films  48   a  and  48   b  coated thereon, as shown in FIG. 12, are then laminated together as shown in FIG. 13. The laminate structure is very similar to that shown in FIG. 8 with the two epoxy films  48   a  and  48   b  fusing together as film  48 .  
         [0023]    In still another embodiment, the bonding film  30  takes the form of another multi-layer structure, such as a conductive sheet of material  50  having an adhesive coating, such as an epoxy  52  coated thereon, and B-staged, as shown in FIG. 14. The conductive element  50  preferably has holes  54  drilled therein to correspond in location to the conductive fill materials  24   a  and  24   b  so that it can be laminated. The epoxy coating  52  on the conductive element  50  can be eliminated in one or more openings  54 , thus providing contact with the conductive element  50  by the conductive material  24   a,    24   b  in the cores  10   a  or  10   b  (not shown in this Figure).  
         [0024]    [0024]FIGS. 15 and 16 show yet another technique of forming a core member  10  and using the core member to join two circuitized structures to form a printed wiring board. The core member  10  is very similar in construction and technique for forming to that shown in FIGS.  1 - 6 , except that, in this embodiment, all of the metal coatings  14  and  16  on both faces of the substrate are removed so that there is a substrate  12  having holes  18  therein, which holes are plated with conductive metal  20  just as in the embodiments shown in FIGS.  1 - 6 . The conductive material  24  disposed in the holes  18  extends above both faces of the substrate  12 . This core  10  is then used to join two additional wiring circuitized structures  60  and  62 . These two wiring structures  60  and  62  are formed in a manner very similar to the formation of the core  10  except that, instead of partially removing the metal coatings  14  and  16  to provide thinned coating materials  24  and  26  as with the embodiment shown in FIGS.  1 - 6 , the opposite sides of the core members  60  and  62  are personalized so that the conductive epoxy remains at the same height as the copper coatings  14  and  16 . These coatings  14  and  16  are then personalized in the core  60  to provide a circuit trace  64  on one face of the core  60  and a pair of bonding pads  66  on the opposite face of the core  60 . On the core  62 , the metal coatings  14  and  16  are personalized on opposite faces thereof to form pads  68  on one face of the substrate and pads  70  on the other face.  
         [0025]    As shown in FIG. 16, the core  10  is disposed between the two circuitized members  60  and  62 , then laminated by heat and pressure as in the previous embodiment. This will provide a printed wiring board structure having continuity between points (a) and (b) on one side of the board through the circuitization  64  on the opposite side of the printed wiring board.  
         [0026]    It is to be understood, of course, that the hole in the core  10  and in the circuitized structures  60  and  62  may be formed without plating of conductive material thereon, and that the techniques shown in FIGS.  10 - 13  could be used for joining the core to the circuitized structures  60  and  62 .  
         [0027]    This invention permits more direct wiring and avoids the need to run additional wiring to a local plated through hole to adjacent layers. Avoiding a plated through hole also permits the connections to be distributed over the XY plane, eliminating concentration of stress and cracks between the plated through hole and the inner planes.  
         [0028]    A plated through hole tends to be a rigid structure that connects the top and bottom of the composite to the internal planes. The structure of this invention avoids this and uses materials that are generally more compliant. Data has been recorded that shows enhanced reliability during current induced thermal cycles (CITC) testing. Not only is composite drilling not required as in prior art conventional boards, desmear (to swell and remove drill waste) and plating to form the connections between the layers is eliminated.  
         [0029]    While the invention has been described in conjunction with embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing teachings. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.