Abstract:
The present invention relates to a method for providing an interconnect between layers of a multilayer circuit board. A first via extending through a total thickness of a first layer is formed. The first via is totally filled with a first solid conductive plug and an end of the first solid conductive plug includes a first contact pad that is in contact with a surface of the first layer. A second via extending through a total thickness of a second layer is formed. The second via totally filling with a second solid conductive plug and an end of the second solid conductive plug includes a second contact pad that is in contact with a surface of the second layer. The second layer is electrically and mechanically coupled to the first layer by an electrically conductive adhesive that is in electrical and mechanical contact with both the end of the first plug and the end of the second plug.

Description:
This application is a divisional of Ser. No. 09/867,312 filed on May 29, 2001 now U.S. Pat. No. 6,504,111. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a method and structure for providing an interconnect between layers of a multilayer circuit board. 
     2. Related Art 
     Typically, high density multilayer circuit boards are constructed of several layers joined by dielectric material to form a stack. Each layer may include an electrically conductive element, such as, a signal plane or a power plane. Layer to layer interconnections may be accomplished using vias, which are typically formed by drilling a hole through layers, followed by plating the wall of the hole with an electrically conductive material. The electrically conductive material along the wall of the via interconnects the conductive elements in the layers. 
     SUMMARY OF THE INVENTION 
     A first general aspect of the present invention provides a structure comprising: 
     a stack comprising an at least one layer; 
     a via opening extending through the at least one layer of the stack; and 
     wherein the via opening is filled with a solid conductive plug. 
     A second general aspect of the present invention provides a method comprising: 
     providing an at least one layer; 
     forming a via opening in the at least one layer; and 
     filling the via opening with a solid conductive plug. 
     A third general aspect of the present invention provides a method comprising: 
     forming a plurality of layers; 
     forming at least one via opening extending through at least one layer; and 
     filling the at least one via opening with a solid conductive plug. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For an understanding of the present invention, reference should be made to the following detailed description taken in connection with the accompanying drawings wherein: 
         FIG. 1  illustrates a cross sectional view of a related art formation of a via opening in a layer; 
         FIG. 2  illustrates the related art layer of  FIG. 1  having a plated coating applied to the wall of the via opening; 
         FIG. 3  illustrates the related art layer of  FIG. 2  having a dielectric material deposited within the via opening; 
         FIG. 4  illustrates a cross sectional view of a via opening in a layer in accordance with the present invention; 
         FIG. 5  illustrates the layer of  FIG. 4  including a solid conductive plug filling the via opening of the present invention; 
         FIG. 6  illustrates a first embodiment of the present invention including an exploded side view of a stack comprising a plurality of layers, a plurality of electrically conductive adhesives, and a dielectric adhesive applied between each layer; and 
         FIG. 7  illustrates a second embodiment of the present invention including the stack of  FIG. 6 , an electronic device connected to a solid conductive plug of a first layer, and an electronic device connected onto an electrically conductive adhesive deposited onto a solid conductive plug of a third layer of the stack. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1–3  illustrate a related art method of forming a via  10  in a layer  12 . The layer  12  may comprise a glass-reinforced epoxy dielectric layer. Typically, a drill, a laser or punch is used to form a via opening  14  in the layer  12 . As illustrated in  FIG. 2 , a conductive plating  16  is deposited on the wall of the via opening  14 . As illustrated in  FIG. 3 , a material  18  may be deposited within the via opening  14  of the via  10 . 
     The present invention eliminates the related art step that requires filling the via opening  14  ( FIG. 3 ) with a material. The via-fill material  18  is typically a polymer that may or may not contain a particulate filler. Depending on the application requirements, the material may be electrically conductive or non-conductive. It serves to reinforce the conductive plating if its mechanical properties are optimized. In addition, it may be overplated with a conductive metal and provide a surface for making connection to a device or to another similar structure in a different layer. 
     Despite these advantages, via-fill material  18  can be difficult to process, especially considering that there may be tons of thousands of vias  10  that require filling a single printed wiring board. 
     The present invention provides a first solid conductive plug  20 A that completely fills a first via opening  14 A in a first via  10 A ( FIG. 5 ). 
       FIG. 4  illustrates a first layer  12 A including the first via opening  14 A. The first via opening  14 A is formed in the first layer  12 A by any suitable means (e.g., drilling, punching, laser, etc.). The first layer  12 A may comprise any suitable material for printed wiring board or chip-carrier dielectric material, such as reinforced or unreinforced materials including epoxy, Bismaleimide-triazine epoxy, cyanate-epoxy blends, flouropolymer dielectrics, etc.) The reinforcing may be fiber, such as glass, or particles, such as silica. Additionally, the first layer  12 A may include polymide films having an adhesive layer on either side such as a polyimide coated with a thermosetting resin, or an aramid impregnated with a thermosetting resin. The first layer  12 A may also have an external metallization layer  15  applied to the layer  12 A ( FIG. 4 ). 
       FIG. 5  illustrates the first solid conductive plug  20 A completely filling the via opening  14 A in the first layer  12 A forming a conductive path to any suitable object  17  (e.g., circuit line, electronic device, etc). The first solid conductive plug  20 A may comprise any suitable conductive material (e.g., copper, gold, etc.). The solid conductive plug  20 A may be formed by any suitable means (e.g., plating, sputtering, etc.). The solid conductive plug  20 A may include a first contact pad  22 A and a second contact pad  22 B. The thickness of solid conductive plug  20 A including the first contact pad  22 A is shown as greater than the thickness of the first layer  12 A. The contact pads  22 A and  22 B are each shown as having a diameter that is greater than the diameter of the solid conductive plug  20 A. The contact pads  22 A,  22 B may be formed by any suitable means, (e.g., additive, pattern plating, reverse pulse plating, etc.). The contact pads  22 A,  22 B are shown as being in direct contact with the first layer  12 A. The first solid conductive plug  20 A provides a solid reliable metallic and mechanically strong via  10 A. 
       FIG. 6  illustrates an exploded view of a stack  24  in accordance with a first embodiment of the present invention including a plurality of layers  12 A– 12 C. The stack  24  may be included in a high density circuit board, or other similar device. The stack  24  may be one or more layers. The stack  24  includes the first layer  12 A, a second layer  12 B, a third layer  12 C, having the first, a second, and a third via opening  14 A,  14 B,  14 C, respectively, formed therein as described above. The first solid conductive plug  20 A, a second solid conductive plug  20 B, and a third solid conductive plug  20 C, are then deposited within the openings  14 A,  14 B,  14 C, respectively. A first electrically conductive adhesive  32 A, a second electrically conductive adhesive  32 B, and a third conductive adhesive  32 C are formed by any suitable means (e.g., screen printing, stenciling a conductive adhesive) above the solid conductive plugs  20 A,  20 B, and  20 C, respectively. The conductive adhesive  32 A,  32 B and  32 C provide a plurality of conductive connections for conductively connecting the solid conductive plugs  20 A,  20 B,  20 C to any suitable adjacent device (e.g., plated via, solid via, chip, etc.). 
     The layers  12 A– 12 C may include a plurality of electrically conductive planes  30 A– 30 C, respectively. The electrically conductive planes  30 A– 30 C may carry any suitable electrical current (e.g., signal, power, etc.). The solid conductive plugs  20 A– 20 C may contact selected conductive planes  30 A– 30 C to provide electrical interconnection between selected conductive planes  30 A– 30 C. 
     Optionally, layers  12 A– 12 C may have additional conductive or metallization layers placed thereon (not shown). The electrically conductive adhesive  32 A is deposited between the contact pads  22 A and  22 C. The electrically conductive adhesive  32 B is deposited between the contact pads  22 D and  22 E. The electrically conductive adhesive  32 C is deposited on the contact pad  22 F. The electrically conductive adhesive  32 A– 32 C may be deposited by any suitable means (e.g., screen printing, stenciling, etc.). The electrically conductive adhesives  32 A– 32 C may be any suitable adhesive, such as, a conductive metal filled thermosetting polymer. Examples include a silver filled thermoset, such as Ablestick 8175 (made by Ablestick), CB-100 (made by Dupont), JM-3200 (made by Johnson Mathey), Polymet-100 (made by Multicore), and Sn/Bi Cu particle composition. The electrically conductive adhesive  32  may be heated for a period of time at a temperature until the degree of cure of the electrically conductive adhesive  32  is advanced. For example Ablestick 8175 may be heated at a temperature around 100° C. until the degree of cure is approximately 30%. As an alternative to a thermosetting adhesive, a thermoplastic electrically conductive adhesive filled with metal conductive particles (e.g., silver, gold, palladium, tin, lead, copper etc.) may be used. In the case of a thermoplastic adhesive, heating after dispense may be required to remove any solvents that are in the adhesive. A further alternative is to avoid adhesives or place a low melting point metal that will form a metallurgical joint with the conductive plug  20  (e.g. using a solder joint or layer). For example, a metal alloy such as a Sn/Pb solder may be used by attachment to the plug  20 . 
     A dielectric bonding layer  36  is deposited between the layers  12  of the stack  24  to join the layers  12 . The dielectric bonding layer  36  may be deposited by any suitable means (e.g., spraying, coating, screening, etc.). The dielectric bonding layer  36  may be any suitable adhesive, such as a solvent thinned thermosetting, or thermoplastic, dielectric polymer. After dispense, the solvent is removed by drying. If the bonding layer  36  is blanket coated on the layer  12 , it must be selectively removed from conductive adhesive  32  by laser ablation, by mechanical drilling, or by selectively exposing, developing and stripping in the case of a photo-sensitive dielectric. As an alternative, the bonding layer  36  may be selectively applied with a mask or screen or stencil, in which case selective removal is obviated. 
     In another embodiment, a dielectric bonding layer  36  is formed by in a free standing manner by aligning or positioning the layer  36  on the layers  12  (i.e. without using deposition techniques). Apertures are formed in the bond film  36  using any suitable material removal technique such as drilling, punching, or selective etching. In the case of a free standing film the bonding layers  36  may be a partially cured thermosetting or polymer a thermoplastic film, and may contain reinforcing particles or fibers. Further, it could be an adhesive coated polyimide layer such as a bondfilm. 
     The stack  24  of  FIG. 6  is then laminated in a laminating press to apply heat and pressure so all layers may be brought into contact so that the adhesive or solder  36  may be used to join the stack together. In the example where the bonding layer is a high glass transition glass reinforced multifunctional epoxy such as IBM Dri-clad, and the electrically conductive adhesive in Ablestick 8175, suitable lamination conditions would be 180° C. for 90 minutes at 400 psi. 
       FIG. 7  illustrates a second embodiment of the present invention. The stack  24  further includes an electronic device  38 A and an electronic device  38 B mounted on the stack  24 . The electronic devices  38 A,  38 B may be any suitable device (e.g., chip, chip carrier, ball grid array, etc.). The electronic device  38 A is connected to the contact pad  22 F of the third solid conductive plug  20 C. The electrically conductive adhesive  32 C connects the electronic device  38 A with the contact pad  22 F. In this embodiment, it would be required to apply the conductive adhesive  32 C subsequent to the aforementioned lamination process. A second electronic device  38 B is connected to the second contact pad  22 B of the first solid conductive plug  20 A by any suitable means (e.g., soldering, conductive adhesive, etc.). 
     The solid conductive plugs  20 A– 20 C provide the benefits of a stronger and more reliable connection compared with the related art with plated wall vias. The solid conductive plugs  20 A– 20 C provide improved heat dissipation and are void free. 
     While embodiments of the present invention have been described herein for purposes of illustration, many modifications and changes will become apparent to those skilled in the art. Accordingly, the appended claims are intended to encompass all such modifications and changes as fall within the true spirit and scope of this invention.