Abstract:
A printed wiring board (PWB) comprises an inner high density circuit routing component laminated within the wiring board, and plated through holes electrically coupling the inner high density circuit to a conductive pattern located on a surface of the PWB. A preferred method for forming such a PWB comprises: providing a built up high density routing component; providing two wiring boards; laminating the built up routing component between the two wiring boards; and using plated through holes to electrically connect a conductive portion of each of the wiring boards to the built up routing component.

Description:
FIELD OF THE INVENTION 
     The field of the invention is printed wiring board fabrication. 
     BACKGROUND OF THE INVENTION 
     Printed wiring boards are known electronic component structures which contain copper strips or paths arranged in a conductive pattern on a relatively flat electrically insulative structure or base. The copper strips form current-conducting paths, the electric wiring, by means of which other electrical components thereafter mounted on the board may receive or pass electrical current to other electrical components, similarly mounted, or from a power source. These circuit/wiring boards are notoriously old and well-known, having achieved wide application in all kinds of electronic devices, including radios, televisions, and computers, to name a few. 
     One wiring board structure is referred to as a two-sided plated through-hole board. The board contains electrical conductors on its front and back surfaces, and at least some of those conductors on opposite sides are electrically inter-connected by means of the plated “through-hole” connections. The through-hole is a hole formed through the board between intercepting conductors on the opposite board sides and the hole wall is plated with metal electrically connecting the two conductors. A number of these wiring boards may be laminated or bonded together to form what is known as a multi-layer printed wiring board, and in such multi-layer wiring board through-holes drilled through the layers after the layers are aligned may form electrical connections to different conductive layers in the bonded stack. A less complex printed wiring board is a single-sided plated through-hole wiring board in which a through-hole is drilled through the board and plated with metal even though a conductor circuit pattern appears on only one board surface. 
     Unfortunately, increasing the number of layers in a multi-layer board tends to cause difficulties in processing and to reduce yields. In particular, drilling and electro-plating through holes becomes more difficult as layer counts increase. Thus, there is a continuing need for newer assemblies and methods for producing assemblies having lower layer counts. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to methods and devices relating to the fabrication of wiring boards having a built up layer laminated within the board. More specifically, a preferred printed wiring board (PWB) comprises an inner high density circuit routing component laminated within the wiring board, and plated through holes electrically coupling the inner high density circuit to a conductive pattern located on a surfaces of the PWB. A preferred method for forming such a PWB comprises: providing a high density routing component having an exposed conductive pattern on at least one surface; providing wiring board layers; laminating the high density routing component between the two wiring board layers; electrically connecting a conductive portion of each of the wiring board layers to the high density routing component. 
     Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded cutaway view of a wiring board assembly embodying the invention. 
     FIG. 2 is a non-exploded cutaway view of a wiring board assembly embodying the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1 and 2, a printed wiring board assembly comprises a high density circuit routing component  100   a , first standard layer  200 , and a second standard layer  300 . Routing component  100  is laminated between standard layers  200  and  300  within insulating and bonding layers  410  and  420  being used between routing component  100  and standard layers  200  and  300 . Plated through holes  510  and  520  electrically couple routing component  100  to standard layers  200  and  300 . 
     Routing component  100  comprises a core  110 , a first conductive pattern  120 , a second conductive pattern  130 , a first dielectric layer  140 , a second dielectric layer  150 , a third conductive pattern  160 , and a fourth conductive pattern  170 . Core  110  preferably comprises 0.002 to 0.004 core thickness, glass reinforced PWB material. This core layer can also utilize one of many different resin systems and glass styles available on the market today including but not necessarily limited to FR4, BT, cyanate ester, and polyimide. The core  110  is processed using conventional processing methodologies in order to form a two-sided board having conductor layers  120  and  130  on each side of the core. 
     Once the conductor layers are formed on the core, a treatment is applied to the copper conductors of layers  120  and  130  that promotes high adhesive qualities for subsequent dielectric layers  140  and  150  that will be applied to the outer layers of the core. 
     Dielectric layers  140  and  150  are applied to opposite sides of core  110  so as to bond to and/or substantially encapsulate conductive patterns  120  and  130 . These dielectric layers can be one of numerous readily available materials such as resin coated copper (RCC), dry film, or glass reinforced pre-preg with copper foil. Dielectric layers  140  and  150  are preferably dry film dielectric layers laminated to the outside conductive patterns/layers  120  and  130  and surfaces of the core  110 . 
     After dielectric layers  140  and  150  are applied to core  110 , vias  181 ,  182  are mechanically drilled, lased, or otherwise created in the dielectric layers, with such vias  181 ,  182  extending from the outside surface of the dielectric layers to the underlying conductors of conductive patterns  120  and  130  on the core  110  as well as through the entire laminate of core  100 , patterns  120  and  130 , and dielectric layers  140  and  150 . The entire panel of the combined dielectric layers and core is then metalized using conventional processes (electroless copper or direct plate systems) which coats the surface of the outer layers  140  and  150  of dielectric as well as the blind and through hole via holes such as  181  and  182 . 
     Conductive patterns  160  and  170  are then formed on top of the electroless copper deposit using conventional photo lithography methods consisting of application of dry film resist, exposure using a photo tool (artwork), electroplating, chemical stripping of the resist, and etching away of the excess (electroless) copper from the surface of the panel. 
     After formation of patterns  160  and  170 , patterns  160  and  170  as well as the surfaces of dielectric layers  140  and  150  may be treated to promote adhesion with layers laminated to them such as  410  and  420 . In regard to treatment of dielectric layers  140  and  150 , treatment preferably comprises chemically roughening the surfaces. 
     It is contemplated that higher density circuitry can be formed through the previously described process because (a) the copper on the surface is extremely thin allowing the etching process to form extremely fine conductors, and (b) the surface of the dry film dielectric material is extremely flat allowing intimate contact of the photo resist and artwork photo tool to the surface of the panel. 
     Once the aforementioned processes are complete, the structure can be laminated into a conventional lay-up of a PWB. 
     Standard layer  200  comprises a core  210  and at least one conductive pattern  220 . As with core  100 , this core layer may utilize glass reinforced PWB material, or may utilize one of many different resin systems and glass styles available on the market today including but not necessarily limited to FR4, BT, cyanate ester, and polyimide. The core  200  is processed using conventional processing methodologies in order to form a board having at least one conductor layer/conductive pattern  220  on a surface of the core. Standard layer  200  will typically comprise two conductive patters on opposing sides and will typically have a lower density than routing component  100 . 
     Standard layer  300  comprises a core  310  and at least one conductive pattern  320 . As with cores  100  and  200 , this core layer may utilize glass reinforced PWB material, or may utilize one of many different resin systems and glass styles available on the market today including but not necessarily limited to FR4, BT, cyanate ester, and polyimide. The core  300  is processed using conventional processing methodologies in order to form a board having at least one conductor layer/conductive pattern  320  on a surface of the core. 
     It is contemplated that having an ultra high density on routing component  100  allows for the overall reduction in layers in the PWB, and that the overall reduction in layers in the PWB results in a decrease in drilling problems related to thick, high layer count parts, easier electroplating of the through holes  510  and  520  of the final laminate, and increased yields. 
     The density of routing component  100  is ultra high in relation to standard layers  200  and  300 . It is preferred that the density of routing layers  160  and  170  be in the range 10 microns to 40 microns, more preferably in the range 20 microns to 30 microns, and most preferably about 25 microns. It is contemplated that in many embodiments the density of routing component  100  will be at least 4 times greater than that of layers  200  and  300 . Density, as used herein, refers to the width of lines and spaces. 
     It is also contemplated that laminating the high-density layers of routing component  100  within the overall package of the PWB allows for the utilization of the entire panel area to be used for high-density conductor routing rather than only portions of the area as would be the case if discreet “patches” were mounted to the surface of the PWB. 
     Thus, specific embodiments and applications of printed wiring board assemblies and assembly methods have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.