Abstract:
A substrate for use in a PCB or PWB board having a coreless buildup layer and at least one metal and at least one dielectric layer. The coreless buildup dielectric layers can consist of at least partially cured thermoset resin and thermoplastic resin. The substrate may also contain land grid array (LGA) packaging.

Description:
RELATED APPLICATIONS 
     The present application is related to co-pending U.S. patent application Ser. No. 12/764,993 for CORELESS LAYER BUILDUP STRUCTURE and U.S. patent application Ser. No. 12/764,994 for CORELESS LAYER BUILDUP STRUCTURE WITH LGA, both incorporated by reference herein in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to circuit board manufacturing and preparation and, more specifically, to a structure wherein a core is built up utilizing at least partially to completely advanced thermoset resin or thermoplastics. 
     BACKGROUND OF THE INVENTION 
     A common procedure in circuit board processing involves laminating multiple cores together. However, generally, the cores are not electrically connected via to via during lamination. For example, one method involves first electrically connecting the cores using conductive pads. After lamination, a hole is drilled through the conductive pads and electroplated with copper to form the via. 
     An alternative solution uses conductive adhesive to electrically attach vias during lamination. The conductive adhesive is placed onto a via and electrically connects the vias when the cores are laminated together. However, conductive adhesives contain plate-like structures greater than 0.5 mils in size. These plates tend to clog at the top of the holes. Therefore, the adhesives cannot be used effectively with thicker cores and smaller vias. Additionally, conductive adhesives require precious metal for good connections, making the products more expensive. Finally, a substantial number of manufacturing sites are not equipped to handle conductive adhesives. Consequently, significant costs may be required to modify current manufacturing sites to use conductive adhesives. 
     As a result, there exists a need for a structure and method of attaching cores having vias with conductive surfaces without using a conductive material for the joining process such as that that is currently used. 
     DISCUSSION OF RELATED ART 
     U.S. Pat. No. 6,465,084, by Curcio, et al., granted Oct. 15, 2002, and U.S. Pat. No. 6,638,607, by Curcio, et al., granted Oct. 28, 2003 for METHOD AND STRUCTURE FOR PRODUCING Z-AXIS INTERCONNECTION ASSEMBLY OF PRINTED WIRING BOARD ELEMENTS disclose a method of forming a core for 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. 
     U.S. Pat. No. 6,969,436 by Curcio, et al., granted Nov. 29, 2005 for METHOD AND STRUCTURE FOR PRODUCING Z-AXIS INTERCONNECTION ASSEMBLY OF PRINTED WIRING BOARD ELEMENTS and U.S. Pat. No. 7,303,639, by Curcio, et al., granted Dec. 4, 2007 for METHOD FOR PRODUCING Z-AXIS INTERCONNECTION ASSEMBLY OF PRINTED WIRING BOARD ELEMENTS disclose a method of forming a member to form a composite wiring board. The member includes a dielectric substrate. Adhesive tape is applied to at least one face of said substrate. At least one opening is formed through the substrate extending from one face to the other and through each adhesive tape. An electrically conductive material is dispensed in each of the openings and partially cured. The adhesive tape is removed to allow a nub of the conductive material to extend above the substrate face to form a wiring structure with other elements. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a method and structure of attaching a plurality of cores. A substrate for use in a PC board has a coreless buildup layer and a metal layer and LGA disposed thereon. Optionally, a second metal layer can be provided with a dielectric layer between the two metal layers. 
     A first aspect of the invention is directed to having coreless buildup layers consisting of thermoset resin. Each or alternate buildup layers are partially advanced to process circuitization and subsequently fully cured during final lamination process. Example of buildup layers: resin coated Cu based on filled epoxy or filled PPE, etc. 
     A second aspect of the invention is directed to a method having coreless buildup layers consisting of thermoplastics. Each buildup layer is circuitized and subsequently laminated to get final structure. Example of buildup layers: Polyimide, LCP (liquid crystal polymer) or Teflon® based materials. Buildup layers can also be a mixture of thermoplastics such as LCP and polyimide. Here, LCP will melt and form bonding among the buildups. For LCP and Teflon mixtures, LCP will likewise melt and form bonding among the buildups. 
     A third aspect of the invention is directed to a structure having coreless buildup layers consisting of thermoset and/or thermoplastic resin. Here the thermoset buildup layers are partially advanced to process circuitization and subsequently fully cured during final lamination process. 
     A fourth aspect of the invention is directed to a method having coreless buildup layers consist of thermoset and/or thermoplastic resin. Here thermoset buildup layers are fully cured and circuitized. Thermoplastic will melt and form bonding among the buildups. 
     A fifth aspect of the invention is directed to a method having a metal surface: It can be metal or alloy or their mixture that will diffuse with each other during final bonding. All surfaces, some surfaces, or alternate surface can have low melting point metal or alloy surface finish where low melting melts during or after lamination and form metal-metal bonding. 
     Another aspect of the invention is directed to a structure that consists of at least one joining layer, wherein joining layers will connect multiple signal layers. Joining multiple signal layers and LGA can be a single step process, or a multi step process. 
     Still another aspect of the invention is directed to a structure that consists of at least two different joining layers. The coreless buildup joining layer will be to connect multiple signal layers and coreless buildup layers will connect signal layers to LGA 
     Another aspect of the invention is directed to a structure wherein at least two conducting adhesive filled joining layers are used. One joining layer will connect multiple signal layers and second conducting adhesive filled joining layer will connect signal and coreless buildup layers. Coreless buildup layers will connect the LGA to known in the art Z-interconnection prepared cores using conductive paste or adhesive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings in which: 
         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; 
         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; 
         FIGS. 9-13  show a longitudinal, sectional view, somewhat diagrammatic, of the steps to form a joining member according to another embodiment of the present invention; 
         FIGS. 14 and 15  show the steps of laminating two core members together using a joining member formed according to this invention; 
         FIG. 16  shows a section view of coreless buildup layer stack up; and 
         FIG. 17  shows a plurality of cores attached according to one embodiment of the current invention wherein a metallurgical paste makes an electrical connection between two Z-interconnect vias with conductive surfaces and coreless buildup layers attached thereto. 
     
    
    
     It is noted that the drawings of the invention are not to scale. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. 
     For the sake of clarity and brevity, like elements and components of each embodiment will bear the same designations throughout the description. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the invention, a method and structure are provided for electrically joining a plurality of cores using thermoset resin and/or thermoplastic. 
     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 layers of 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, polytetrafluoroethylene or other suitable well known dielectric. In the embodiment shown in  FIGS. 1-6 , the metal coatings  14  and  16  preferably are copper and, typically, the layer is either one-half ounce copper (17.5 μm), one ounce copper (35 μm thick) or two ounce copper (70 μm thick). However, other thicknesses of copper coatings can be used. 
     As shown in  FIG. 2 , preferably the copper layer  14  is patterned to form circuit traces  18  and the copper layer  16  is patterned to form circuit traces  20 . Any conventional patterning process, such as by using a photoresist, exposing, developing and etching the exposed areas and then stripping the photoresist can be used. 
     As shown in  FIG. 3 , a film in the form of adhesive tape  22  is applied over the circuit traces  18  and the same type of film is applied over the circuit traces  20 . A particularly useful adhesive tape is a polyimide having a silicone adhesive. This is available from Dielectric Polymers, Inc. of Holyoke, Mass. This tape must be compatible with the conductive material and processes associated with the formatting of the core, which will be described presently. Other types of film material may be used, such as plating tapes NT-580, 582, 583, 590 and 590-2 manufactured by Dielectric Polymers, Inc. The tape  22  and  24  should be of a thickness equal to the height that it is desired to have the conductive material extend above the circuit traces  18  and  20 . If a single layer of tape is not sufficient, multiple layers may be used. 
     Referring now to  FIG. 4 , a plurality of holes or openings, two of which are shown at  26 , are drilled through the entire composite, including the adhesive tape  22  and  24 , circuit traces  18  and  20  and the substrate  12 . These holes or openings  26  define the location of the conductive interconnect vias that will be formed. 
     Into the openings  26  is deposited an electrically conductive paste material  28 , as shown in  FIG. 5 . The filling of these openings  26  can be performed by screening, stenciling, flood coating, doctor blading, immersing or injecting. Various types of conductive material may be used. By the term “conductive paste” as used herein is meant an electrically conductive paste composition adapted for use in holes or openings of substrates as well as between conductors which form parts of conductive planes of such a substrate. Such a paste includes at least one organic binder component and, in one embodiment, at least one metallic component including a plurality of “microparticles.” In another embodiment, the paste includes such an organic binder in addition to the aforementioned microparticles. A preferred conductive polymer material is a conductive epoxy sold by National Starch and Chemical Company under the trademark “Ablebond 8175” (This was formerly sold by Ablestik Corporation). “Ablebond 8175” is a silver filled thermosetting epoxy. Following the filling of the holes  26 , as shown in  FIG. 5 , the epoxy is B-staged which entails heating the material to a temperature of about 130° C. until the degree of cure is advanced from about 20% to about 80% complete cure. As will become apparent later, the film material should not be fully cured at this stage since it will be used to adhere to another conductive epoxy in another core element. Alternatively, a solder paste of tin lead, tin lead silver, tin silver copper, tin silver copper antimony or tin bismuth, which are commercially available, can be used and heated to reflow. 
     After the conductive material  28  is partially cured, the adhesive tape  22  and  24  is removed to provide the structure shown in  FIG. 6 . As can be seen in  FIG. 6 , the partially cured conductive material  28  extends above the circuit traces  18  and  20  a distance equal to the thickness of the adhesive tape  22  and  24 . 
     If the copper layers  14  and  16  have not been previously patterned, that can be done at this point. However, in general, it is preferred that the patterning to form the circuit traces  18  and  20  be done, as shown in  FIG. 2 , at that stage in the process so that the conductive material  28  is not subjected to the harsh chemical processes normally encountered in patterning material. 
     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 circuit traces on each of them is slightly different. (In describing the embodiments of  FIGS. 7 and 8 , the letter suffixes a and b are used to denote similar structures in each core element.) As seen in  FIG. 7 , a pre-drilled adhesive 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  28   a ,  28   b  in the two core elements  10   a  and  10   b.    
     Heat and pressure are applied to cause the two core members to bond together, with the Pyralux LF film acting as an adhesive bond material. Also, the fill material  28   a  and  28   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 circuit traces  18   a ,  18   b ,  20   a  and  20   b  on the core element  10   a  and  10   b . Also, the material of the substrate  30  will fill around the circuit traces  18   b  and  20   a . The lamination process also advances the cure of the conductive fill material  28   a  and  28   b  past 80% to the fully cured stage. A specially formulated dicing tape can be used as adhesive tape  22 . An example of suitable dicing tape is Adwill D-series tape provided by Lintec Corporation. These tapes are comprised of a base material, such as PVC (poly vinyl chloride), or PET (polyethylene terephthalate), or PO (polyolefin) with an adhesive film that provides strong temporary adhesion. Alternatively, the adhesive could be provided on other base material, such as polyimide. 
     The adhesive layer provided on the base layer is formulated so that it provides strong initial adhesion but, upon exposure to UV (ultraviolet) radiation, its adhesion is diminished and it can be peeled and released without causing damage or leaving residue on the copper traces  18  or the dielectric layer  12 . In such case, the backing must be transparent to UV radiation. Also, it is to be understood that the tape  22 ,  24  does not need to be a dielectric. For example, a metal foil with an adhesive on one side could be used. This also constitutes a “tape”. (Alternatively, the film material  30  could be a dry film epoxy adhesive which is B-staged, or thermoplastic LCP film or organic pre-preg typically comprising a layer of glass (typically fiberglass) cloth impregnated with a partially cured material, typically a B-staged epoxy resin or other film type adhesive dielectric layers and used to laminate the core elements  10   a  and  10   b  together.) 
     Referring now to  FIGS. 9-13 , another embodiment of the present invention is shown which is useful in forming a joining member. A substrate  10  is provided which is preferably an adhesive dielectric material. For example, this could be an adhesive coated film (such as duPont Pyralux LF, which is a modified acrylic adhesive on a polyimide film) or a B-staged thermoset adhesive (such as IBM Dri-clad glass reinforced high glass transition dielectric material), or other film type adhesive dielectric layers, including materials such as Rogers 2800 Silica filled polytetrafluoroethylene. Thermoset resin coated Silica filled polytetrafluoroethylene or thermoset resin coated liquid crystal polymer (LCP) or LCP-Silica filled polytetrafluoroethylene-LCP or LCP-Polyimide-LCP type materials can also be used. In general, LCP bondply (available from Rogers) can be laminated with polyimide or Silica filled polytetrafluoroethylene to make LCP-Polyimide-LCP or LCP-Silica filled polytetrafluoroethylene-LCP mixed dielectric. 
     A plurality of holes, one of which is shown at  26 , is either mechanically or laser drilled through the substrate  12  and through both of the tapes  22  and  24 , as shown in  FIG. 11 . A conductive material  28  of the same type as described with respect to  FIGS. 1-6  is deposited in the hole  26  by the same techniques as previously described with respect to  FIGS. 1-6 . After the conductive material  28  is remelted or cured, as previously described, the adhesive tapes  22  and  24  are removed to provide a joining member, as shown in  FIG. 13 . Alternatively, tapes  22  and  24  can be CU layer. Cu can be removed by an etching process. Proper conducting paste such as silver-epoxy based paste is etch resistant and generates paste nubs ( FIG. 13 ). 
     In  FIG. 11 a   , the opening  26  is plated, preferably with copper, to form inner conductive layers. A preferred method of accomplishing this is to use a “flash” plating of electroless copper. It is to be understood that forming plated layers  35  is an optional step in forming a substrate as defined herein, but is preferred to further assure sound conductive paths in these portions of the structure. The next step, as also shown in  FIG. 12 , involves the deposition of conductive paste  28  within each of the plated openings  26 . Such deposition may be accomplished using conventional paste printing processes or dispensing through conventional needles. Significantly, the conductive paste as used in this embodiment includes a binder (preferably an organic binder) component and at least one metallic component. As defined herein, this metallic component is in the form of microparticles or nanoparticles or their mixtures, either as flakes or semi-colloidal powders. Metals may include copper, silver, gold, zinc, cadmium, palladium, iridium, ruthenium, osmium, rhodium, platinum, iron, cobalt, nickel, indium, tin, antimony, lead, bismuth and alloys thereof for the microparticles. 
     In one embodiment of the invention, a conductive paste having silver microparticles may be used, the paste including an anhydride epoxide as the organic binder. This paste preferably includes about 88% by weight of the silver microparticles and about 12% by weight of the anhydride epoxide. With the solder added to the microparticles as described below, the resulting paste has a decomposition temperature of approximately 340 degrees C., which, when considering the above high temperature dielectric material, is about ten degrees C. less than the 350 degrees C. temperature the dielectric can withstand during lamination. The average silver particles are each from about 0.01 microns to about 10 microns in diameter. In the case of both flat particles (flakes) and rod-like particles, thicknesses are each from about 0.01 micron to 10 micron. 
     Although only one opening  26  is depicted in  FIG. 11 , this is meant to be representative only. In one example, a total of 2500 openings may be provided within a rectangular layer having dimensions of about 52.5 millimeters (mm) wide by about 52.5 mm long, and a thickness of about 0.175 mm. These 2500 paste filled nubs in  FIG. 13  generate 3-D micro arrays. These kinds of conductive adhesive based 3-D micro array Z-interconnects are used to connect multiple electronic layers. 
       FIGS. 6 and 13  show 3-D micro arrays for connecting several electronic layers starting from chip to board. Adhesives formulated using controlled-sized particles, ranging from nanometer scale to micrometer scale, were used to form micro arrays of contact pads having diameters ranging from 5 μm to 250 μm for internal and external interconnect applications. For example, micro arrays (not shown) with pads having 5-15 micron diameters are suitable for device level interconnects (chip to chip interconnects), whereas 50-75 μm and 250 μm diameters of the pads are suitable for chip carrier and board level interconnects, respectively. 
     As shown in  FIGS. 14 and 15 , a joining member formed according to  FIGS. 9-13  is used to join two printed wiring boards  34 . The dielectric substrate  10  is adhesive for B-staged thermoset resin or thermoplastic polymer acting as a bonding member. Typically, the printed wiring boards will have a dielectric substrate  36  with a plurality of internal conductive planes, one of which is shown at  38 , and plated through holes  40 . However, this is just illustrative as the joining member can be used to join many different types of printed wiring boards, the boards shown in  FIGS. 14 and 15  being merely illustrative. 
     Alternatively, in  FIG. 14 , two printed wiring boards  34  can be flexible substrates and extended beyond the joining layer  10 . In that case, area  34  bonded with dielectric  10  is rigid and the rest of the area is free standing and flexible. In general, II-VI metal layers substrate made with flexible materials produces flexible substrate  34 . One example of such material is sold under the product name “RO2800” dielectric material provided by Rogers Corporation, Rogers, Conn. 
     Again, area  34  can be a substrate having embedded capacitors and resistors. Embedded capacitors can be a high dielectric constant ceramic filled dielectric (e.g., barium titanate filled epoxy) layer. One example of such material is resin coated capacitive materials used as a buildup layer. The resistor can be a multilayer resistor foil laminated with the capacitor dielectric. For example, core can use 25 ohm per square material and 250 ohm per square inch material. This combination enables resistor ranges from 15 ohms through 30,000 ohms with efficient sizes for the embedded resistors. Here, two printed wiring boards  34  having embedded capacitors and resistors are bonded with the dielectric substrate  10 . Adhesive or B-staged thermoset resin or thermoplastic polymer acts as a bonding member. 
     Referring now to  FIG. 16 , a core  100  is shown having a plurality of vias  130 . Core  100  may comprise an epoxy core or any similar structure as commonly known in the art. Core  100  may include one or more planes  120 - 122 , which may include, for example, a power plane, signal plane, or a ground plane. Using via  130  as an example, each via has a conductive surface  135  formed on a surface of core  100 . Conductive surface  135  can comprise a thin layer of any solderable conductive material including, for example, a precious metal or copper. The joining concept for core  100  is to use compression and heating to melt or diffuse the solder to create the laminate, thereby having metallic contacts between planes  120 - 122  while not using a conductive paste. 
     Alternatively, core  100  may include multiple planes and multiple dielectric layers. At least one or multiple dielectrics can be made with thermoplastic polymers. At least one of the thermoplastic layers may be larger than the joining layer and remain as an extended flexible layer. Flexible layer can be a capacitance layer or resistor foil laminated capacitance layer. One example of such flexible capacitance material is sold under the product name “RO2800” dielectric material by Rogers Corporation, Rogers, Conn. 
       FIG. 17  shows a plurality of cores  200  attached according to one embodiment of the current invention wherein a metallurgical paste  210  creates an electrical connection between two vias with conductive surfaces  215  to create Z-axis interconnects  205  as known in the art and also the novel coreless buildup layer  100  and coreless buildup layer with LGA  105  containing individual LGA pads  220  are diffusion bonded to the upper surface  225  and lower surface  230 , respectively. Few or all of the metallurgical paste  210  can be replaced by conductive adhesive to create an electrical connection between two vias with conductive surfaces  215  to create Z-axis interconnects. 
     Since other modifications and changes to the coreless layer buildup will be apparent to those skilled in the art, the invention is not considered limited to the description above for purposes of disclosure, and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
     Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.