Abstract:
A method of forming a laminated composite printed wiring structure of a plurality of at least three superimposed subcomposites having organic substrates is provided. Via openings in the subcomposite structures having conductive paste therein are positioned to align with openings in at least one adjacent subcomposite structure also filled with conductive paste that is to be joined thereto. Printed wiring is provided on at least one face of one subcomposite structure. A fixture with pins which extends through index openings in the composites are provided to mount masks for screening paste and stacking of the composites is provided. After screening of paste, and partially curing of the paste, in each composite, a group of composites is placed on the fixture and the pastes are fully cured to form a unitary structure.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to forming multi-level, organic printed wiring boards and, more particularly, to a method of joining subcomposite structures utilizing electrically conductive paste for z-axis interconnects to form printed wiring boards, such as chip carriers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Organic based printed wiring boards are conventionally made up of a plurality of individual elements or subcomposite structures joined together to provide various levels of wiring and power planes on the surfaces of the elements or subcomposites and interconnections between the various wiring levels, such interconnections between the various wiring and power plane 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 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. There have been various prior art proposals for forming the z-axis interconnect to eliminate the drilling of the final structure. However, in some instances, it is difficult to precisely align the z-axis interconnects of the subcomposite structures in certain types of very dense circuitry type chip carriers. Thus, it is desirable to provide a technique for forming z-axis interconnects of subcomposites forming a printed circuit board or printed wiring board which provides accurate alignment of the z-axis interconnects as well as reliable connections of the z-axis interconnects at the various interfaces which allow processing of parts according to similar processing techniques in other technologies  
         SUMMARY OF THE INVENTION  
         [0003]    According to the present invention, a method of forming a laminated composite printed wiring structure of a plurality of at least three superimposed subcomposites is provided. The method includes providing a plurality of organic, dielectric subcomposite structures, each having opposed faces and a plurality of through via openings therein extending between the faces. The via openings in the subcomposite structures are positioned to align with openings in at least one adjacent subcomposite structure that is to be joined thereto. Printed wiring is provided on at least one face of one subcomposite structure and at least one power plane is also provided in at least one subcomposite structure. Each via opening is filled with a conductive paste material that can be subsequently hardened and cured with the conductive paste material extending beyond at least one face of one subcomposite structure. A plurality of aligned index openings are provided in each subcomposite structure which will cooperate with fixture pins to align the filled via holes of the various subcomposite substructures where required when in superimposed relationship. The index openings also cooperate with pins to align the subcomposites for filling through screens, stencils, or masks. The via holes in the subcomposites are filled with the subcomposites, each being held in a fixture with a mask also being held in the fixture. An adhesive is provided between adjacent subcomposite structures, the adhesive having openings for the conductive paste. Conductive paste is less than the fully cured so that the paste in adjacent layers can bond together when fully cured. The subcomposite structures are laid up with the adhesive material disposed there between in superposed relationship on a fixture including elements extending through said index openings to align the subcomposite structure with the conductive paste and adjacent openings in said subcomposite structures in contact with each other. The conductive paste is then cured fully and the laminate wiring structure is formed from the superimposed subcomposite structures. In one embodiment, the subcomposite structures are fully circuitized before being laid up and formed into a composite laminate, using conventional circuitizing techniques and, in another embodiment, conductive paste is utilized on at least some faces of the subcomposite structures to provide the circuitization prior to being laid up in a laminate structure. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is a side elevational view, partly in section and somewhat schematic, showing one subcomposite element for use in forming a printed wiring structure according to one embodiment of the present invention;  
         [0005]    [0005]FIG. 2 is a view similar to FIG. 1 of another subcomposite element for use in forming the printed wiling structure according to said one embodiment of the present invention;  
         [0006]    [0006]FIG. 3 is a plan view of a card showing several subcomposite elements being formed on a single card and which elements can be cut from the cards to form the individual elements;  
         [0007]    [0007]FIG. 4 is a perspective view, somewhat diagrammatic, showing a fixture and mask for filling a subcomposite element with conductive paste;  
         [0008]    [0008]FIG. 5 is a view similar to FIG. 1 showing the vias filled with a conductive paste;  
         [0009]    [0009]FIG. 6 is a view similar to FIG. 2 showing the vias filled with a conductive paste;  
         [0010]    [0010]FIG. 7 is a somewhat schematic view of a series of elements comprised of subcomposite elements shown in FIGS. 5 and 6 being laid up in a fixture to form a composite laminate printed wiring board;  
         [0011]    [0011]FIG. 8 is a view similar to FIG. 1 of one element of a subcomposite structure for use in forming a laminated wiring board according to another embodiment of the present invention;  
         [0012]    [0012]FIG. 9 is a view similar to FIG. 2 of subcomposite structure for use in forming a laminated printed wiring board according to the other embodiment of this invention;  
         [0013]    [0013]FIG. 10 is a view of the subcomposite structure of FIG. 8 having the vias filled with conductive paste and circuit lines screened thereon;  
         [0014]    [0014]FIG. 11 is the subcomposite structure of FIG. 9 having the vias filled with conductive paste and the outer layers of copper foil etched therefrom;  
         [0015]    [0015]FIG. 12 shows subcomposite structures of FIGS. 10 and 11 laid up for lamination to form a laminated printed wiring structure; and  
         [0016]    [0016]FIG. 13 is a view similar to FIG. 12 but utilizing the subcomposite structures of FIGS. 10 and 11 laid up for lamination to a composite wiring structure utilizing separate adhesive materials between said subcomposite structures. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    The present invention provides two separate embodiments of a technique for forming a composite wiring structure utilizing conductive paste to form z-axis interconnections of the various subcomposite structures forming the laminated printed wiring structure. In one embodiment, which is described herein as the first embodiment, relatively conventional techniques are utilized to circuitize the various organic subcomposite structures which are utilized to form the laminated printed wiring structure; and then the printed wiring structure is formed by laminating these subcomposites together utilizing conductive paste in the plated through holes of each of the subcomposite structures to provide the z-axis via interconnects between adjacent subcomposite structures.  
         [0018]    In another embodiment, hereinafter referred to as the second embodiment, organic subcomposites are provided which have circuitry formed thereon by conductive paste techniques, as well as utilizing conductive paste to form Z-axis interconnect vias through openings in adjacent subcomposite structures.  
         [0019]    First Embodiment  
         [0020]    For the first embodiment of the present invention, two different subcomposite elements are shown in FIGS. 1 and 2. In FIG. 1, a  2 S 1 P subcomposite element is shown, and in FIG. 2, a  0 S 1 P subcomposite element is shown (according to convention,  2 S 1 P refers to two signal planes and one power plane, and the  0 S 1 P refers to no signal plane and one power plane).  
         [0021]    As seen in FIG. 1, the  2 S 1 P subcomposite designated by the reference character  10  is provided with a power plane  12  surrounded by an organic dielectric material  14 . The dielectric material can be any conventional organic material that is cured for making printed wiring structures. Particularly useful are silica filled polytetrafluoroethylene (PTFE); fiberglass or aromatic polyamid (or aramid) fiber or expanded PTFE reinforced epoxy, cyanate ester, BT (bismaleimide triazine) prepreg or laminate, example FR-4; or combination of hydrocarbon based thermosets and thermoplastics such as Rogers RO4350 by the Rogers Corporation; or suitable organic based, inorganic or inorganic filled or reinforced composite laminate material. The subcomposite  10  has opposite sides  16  and  18 , with signal circuitry  20  and  21  formed on sides  16  and  18 , respectively, of the subcomposite structure  10 . The subcomposite structure  10  also includes a plurality of plated through holes  22 . Subcomposite  10  is formed with a plurality of registration openings  24 , the location and purpose of which will be described presently.  
         [0022]    The  0 S 1 P subcomposite designated by the reference character  26  includes a power plane  28  surrounded by dielectric material  30 , such as a filled PTFE or FR-4, or polyamids or Rogers R04350 by Rogers Corp., a hydrocarbon having thermoset or thermoplastic, or other suitable organic material, having opposite sides  32  and  34 . A plurality of through holes  35  are formed therethrough. The subcomposite  26  has a plurality of registration openings  36 , again the purpose and location of which will be described presently. The subcomposite structures  10  and  26  are preferably produced using traditional printed wiring board technology which does not need to be described in detail. Generally, this technology involves the steps of laminating, drilling, hole cleaning, plating and circuitization. Also, in the case of each subcomposite  10 ,  26 , this is preferably done in a large card  37 , as shown in FIG. 3, from which the subcomposites can be cut. The cards  37  to form subcomposites illustrated in FIG. 3 are for subcomposite structure  10 . However, the subcomposites  26  can be formed in the same way. As can be seen, the registration openings  24  are located at the four comers of each of the subcomposite structures  10 . The same is true for the registration openings  36  in subcomposite  26 .  
         [0023]    Once the circuitized subcomposite structures  10  and  26  have been fully formed and cut from the card  37 , the plated through openings  22  are filled with an electrically conductive paste, which is done in the following manner. A paste filling fixture  38  is provided which has a plurality of upwardly extending rods or other similar locating devices  39  located to engage the registration openings, either  24  or  36 , in the subcomposite structures  10  and  26 . The subcomposite structures  10  and  26  are individually placed on the fixtures  38  with the rods  39  extending through either the openings  24  or  36  depending upon which subcomposite structure is to be filled. A mask  40 , having registration holes (unnumbered) and openings  41  therein, is provided with the mask placed on the rods  39  over a subcomposite  10  or  24  so that the openings  41  therein align with each of the openings  24  or  35 , whichever subcomposite is being filled. Conductive paste  42  (FIGS. 5 and 6) is then applied over the mask  41  by squeegees, or injection techniques or other suitable means so that it will go through the openings  41  and into the openings  22  or  35  in either the subcomposite structure  10  or  26 . The conductive paste is applied in a flowable form so that it can flow through the openings  41  and into the openings  22  or  35  and thereafter be either dried or cured (hereinafter collectively referred to as cured) in steps that will be described presently. Particularly useful pastes which can be used are Ablestik 8175 manufactured by the Ablestik Corporation or CB100 manufactured by E. I. du Pont de Nemours and Company. These are conductive epoxies which can be cured to various states of curing and are applied in the uncured state. It is to be understood, however, that other types of conductive paste can be used and these are merely illustrative.  
         [0024]    [0024]FIGS. 5 and 6 show the subcomposite structures  10  and  26 , respectively, with openings  22  and  35  filled with conductive adhesive  42  such as Ag, Sn/Pb or Sn/Bi coated or mixed with Cu particles in an organic thermosetting resin such as an epoxy. At this stage, the conductive adhesive  42  is either left uncured or only partially cured (e.g. B-stage cured in the case of epoxies) so that when the subcomposite structures are superposed and joined (as will be described presently), the paste  42  in the openings in adjacent subcomposites  10 ,  24  will join together to form a coherent. cohesive and electrically conductive bond or union.  
         [0025]    Referring now to FIG. 7, a somewhat diagrammatic showing of the lay up of alternating  2 S 1 P and  0 S 1 P subcomposites is shown to form a composite wiring board. (It should be noted that one of the subcomposites is designated as  10 ′. This is a slightly modified form of subcomposite  10 , formed according to the teachings of patent application Ser. No. 09/871,555, filed May 31, 2001, entitled “METHOD FOR FILING HIGH ASPECT RATIO VIA HOLES IN ELECTRONIC SUBSTRATES AND THE RESULTING HOLES”, Attorney Docket No. END920000119US1 (IEN-10-5560). The various subcomposite structures  10  (or  10 ′) and  26  are placed alternately over alignment pins  52 , with dielectric adhesive sheets  44  interposed between each pair of adjacent subcomposite structures  10  (or  10 ′) and  26 , with the dielectric adhesive sheets  44  having openings  46  therein, each aligned with adjacent conductive paste fillers  42  in the subcomposite structures  10  (or 10 ′) and  26 , as shown in FIG. 7. Also, optionally, copper coated dielectric materials  48  can be placed on opposite sides of the entire composite structure. These copper coated dielectric sheets  48  eventually can be patterned and formed to electrical circuitry in a conventional manner. Particularly useful are silica filled polytetrafluoroethylene (PTFE); fiberglass or aromatic polyamid (or aramid) fiber or expanded PTFE reinforced epoxy, cyanate ester, BT (bismaleimide triazine) prepreg or laminate, example FR-4; or combination of hydrocarbon based thermosets and thermoplastics such as Rogers RO4350 by the Rogers Corporation; or suitable organic based, inorganic or inorganic filled or reinforced composite laminate material. Heat and pressure are then applied to the stacks of subcomposite structures  10  (or  10 ′) and  26 , preferably the heat being at about 185° C. for about 90 minutes (for epoxies), which will cause the adjacent subcomposite structures  10  (or  10 ′) and  26  to bond together due to the action of the adhesive sheets  44 , and the interface of the adjacent, electrically conductive paste  42  will bond together and the entire electric column of the joined conductive paste  42  in all of the subcomposites will be fully or C-cured (in the case of epoxies) forming a fully conductive z-axis connection between adjacent subcomposite structures.  
         [0026]    It is to be understood that the illustration of the two particular subcomposite structures  10  and  26  is merely illustrative and various different subcomposite structures can be utilized to form different types of composite printed wiring structures.  
         [0027]    Referring now to FIGS.  8 - 13 , another embodiment of the present invention is shown; this is referred to as the second embodiment.  
         [0028]    Second Embodiment  
         [0029]    Referring now to FIGS. 8 and 9, again subcomposite structures, which will eventually be formed into a composite wiring structure, are shown. The subcomposite of FIG. 8, will eventually be formed into a  1 S 1 P subcomposite and of FIG. 9 will be formed into a  0 S 1 P subcomposite. Referring now to FIG. 8, a subcomposite structure  60  is shown having a copper core  62  and an organic dielectric material  64  surrounding the core  62 . Again, the dielectric materials that may be particularly useful are silica filled polytetrafluoroethylene (PTFE); fiberglass or aromatic polyamid (or aramid) fiber or expanded PTFE reinforced epoxy, cyanate ester, BT (bismaleimide triazine) prepreg or laminate, example FR-4; or combination of hydrocarbon based thermosets and the rmoplastics such as Rogers RO4350 by the Rogers Corporation; or suitable organic based, inorganic or inorganic filled or reinforced composite laminate material. The subcomposite has opposite faces  66  and  68 , with openings  70  extending therethrough. This subcomposite will eventually be circuitized, as will be described presently, to form a  1 S 1 P subcomposite, although it is to be understood that a  2 S 1 P plane could also be formed.  
         [0030]    In FIG. 9, a subcomposite structure  74  is shown, which has a copper core  76 , organic dielectric material  78 , such as silica filled polytetrafluoroethylene (PTFE); fiberglass or aromatic polyamid (or aramid) fiber or expanded PTFE reinforced epoxy, cyanate ester, BT (bismaleimide triazine) prepreg or laminate, example FR-4; or combination of hydrocarbon based thermosets and thermoplastics such as Rogers RO4350 by the Rogers Corporation; or suitable organic based, inorganic or inorganic filled or reinforced composite laminate material surrounding the copper core  76 , and copper foil  80  and  82  on opposite sides of the dielectric material. Openings  84  extend through the entire subcomposite structure  74 .  
         [0031]    As in the previous embodiment, the subcomposite  60  has registration opening  87  therein and the subcomposite  74  has registration openings  89  therein. The subcomposites are preferably made in multiples on a card as in the previous embodiment, and are cut from the card. In this embodiment, as in the previous embodiment, each of the subcomposite structures  60  and  74  is placed on the fixture  38  with the pins  39  extending through the registration openings  87  or  89  and the mask  40  put in place. However, in this case, the mask  40  has openings, not only for providing conductive paste  42  in the openings  70  and  84 , but also additional openings to allow the conductive paste  42  to be applied to the face  66  of the subcomposite  60  to form the signal traces  90  as shown in FIG. 10. Thus, in this case, the circuitization of the subcomposite  60  takes place by the use of conductive paste rather than by conventional and traditional circuitization techniques. Also entire power and ground planes can be applied by printing through masks or the like.  
         [0032]    In this embodiment, copper foil  80  and  82  on the opposite sides of the subcomposite  74  are etched away to leave the conductive paste  42  having exposed protrusions  92  on both sides thereof. The pastes are then B-cured (if epoxies).  
         [0033]    At this point, the subcomposites  60  and  74  are stacked alternately on the pins  52  of the fixture in contact with each other as shown in FIG. 12, and the entire stack structure put under pressure and heated to about 185° C. for about 90 minutes to C cure the adhesive, and laminate the stack and to C-cure the conductive paste forming circuitry  90  to form continues electrically conducting paths.  
         [0034]    [0034]FIG. 13 shows a variation of the second embodiment wherein separate dielectric sheets  46  are used between adjacent subcomposites  60  and  74 , just as in the previous embodiment.  
         [0035]    Accordingly, the preferred embodiments have been described. With the foregoing description in mind, however, it is understood that this description is made only by way of example, that the invention is not limited to the particular embodiments described herein, and that various rearrangements, modifications, and substitutions may be implemented without departing from the true spirit of the invention as hereinafter claimed.