Patent Publication Number: US-6340796-B1

Title: Printed wiring board structure with integral metal matrix composite core

Description:
FIELD OF THE INVENTION 
     This invention relates in general to surface mount and plated through hole technology printed wiring boards, and in particular to a printed wiring board structure interfaced with an integral core fabricated of a metal matrix with pitch based graphite fibers therein disposed, with the board structure and core integrally connected to each other at a plurality of connection sites along the interface thereof. 
     BACKGROUND OF THE INVENTION 
     Printed wiring board structures generally are constructed of a core upon which one or more layers carrying ceramic chips or leaded components are laminated. These circuit and chip-carrying layers usually are of metalized (copper clad) polymeric construction such as a polyamide, and can be stacked above and below the core material. In order to provide efficient operation within the wiring board structure, the core should perform favorably with respect to tensile modulus, thermal conductivity, and thermal expansion. 
     A common core construction now employed is a metal core fabricated as a layer of molybdenum having on each surface thereof a respective layer of copper. While this copper-molybdenum-copper core is satisfactory with respect to tensile modulus, thermal conductivity, and thermal expansion considerations, the weight of this prior-art core can be a significant disadvantage in weight-sensitive applications. Conversely, of course, any replacement core material whose attributes include light weight must still provide satisfactory strength, low coefficient of thermal expansion and heat-response characteristics in order to qualify for wiring board construction. 
     In view of the above described requirements, it is apparent that a need is present for a printed wiring board structure having a core fabricated to meet weight and expansion restraints while providing efficient wiring board performance. Accordingly, a primary object of the present invention is to provide an integrated printed wiring board structure with a core exhibiting high tensile modulus, high thermal conductivity, low coefficient of thermal expansion, and light weight while being compatible with both surface mount and plated through hole technology components and corresponding circuitry. 
     Another object of the present invention is to provide such a printed wiring board structure whose core construction includes a metal matrix having disposed therein pitch based graphite fibers. 
     These and other objects of the present invention will become apparent throughout the description thereof which now follows. 
     SUMMARY OF THE INVENTION 
     The present invention is a printed wiring board structure comprising at least one chip-carrying layer adjacent a core fabricated of a metal matrix having disposed therein graphite fibers. The chip-carrying layer and the core have an interface therebetween and are integrally connected to each other through vias plated with a thermally and electrically conductive material to thereby provide a plurality of connection sites along this interface. A metal matrix is preferably fabricated of aluminum. Preferred fibers are fabricated of pitch based graphite. A typically preferred present printed wiring board structure has two chip-carrying layers each on opposite sides of the core, with each of the layers and the core having respective interfaces therebetween wherein each layer is integrally connected to the core at a plurality of connection sites along the respective interfaces. Additional circuit layers can be affixed to those layers, allowing layer interconnection, and/or interface with the core, thereby increasing circuit density of a wiring board structure. The present printed wiring board structure possesses high tensile modulus, low coefficients of thermal expansion, and light weight to thereby provide versatility in printed wiring board structural utility and placement. Because of the plurality of connection sites along respective interfaces of the core and adjacent circuit and chip-carrying layers, superior thermal conductivity occurs from the layers to the core since heat travels through these connection sites to effectuate extremely efficient heat transfer and ultimate heat dissipation from the entire printed wiring board structure. In this manner reduced operating temperatures are accomplished to thereby yield higher reliability, improved dynamic load, improved thermal fatigue life of solder joints, and accommodation of more circuitry per unit area. In addition, weight savings of 30% to 40% can be realized over current printed wiring board structures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An illustrative and presently preferred embodiment of the invention is shown in the accompanying drawings in which: 
     FIG. 1 is a schematic illustration of a cut-away sectional view of a prior art printed wiring board structure with a core fabricated of copper and molybdenum; 
     FIG. 2 is a schematic illustration of a cut-away sectional view of a printed wiring board structure with a core fabricated of an aluminum metal matrix having therein disposed pitch based graphite fibers according to the present invention; and 
     FIG. 3 is a table showing core material construction components and characteristics. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, a typical prior-art printed wiring board structure  10  is shown. This printed wiring board structure  10  is configured with a conventional card rail  12 , and has positioned on either side of a core  14  a plurality of polyamide circuit layers  16  including chip components  18  as known in the art. This prior art core  14  has a molybdenum center layer  20  with respective copper layers  22 ,  24  disposed above and below the molybdenum center layer  20 . As earlier noted, the copper-molybdenum-copper core  14  is generally satisfactory with respect to tensile modulus, thermal conductivity, and thermal expansion considerations, but is of a weight that can be highly disadvantageous depending upon particular applications. Plated through holes  15  are shown with hole fill  17  disposed thereabout through the core  14 . Conventional blind vias  19  are shown. 
     FIG. 2 illustrates the preferred embodiment of the present invention. In particular, and similar to the illustration of FIG. 1, FIG. 2 shows a printed wiring board structure  30  configured with a conventional card rail  12 , a novel core  32 , and polyamide circuit layers  16  including chip components  18 . The core  32  is fabricated of a metal matrix  34  having disposed therein continuous pitch-based graphite fibers  36  substantially unidirectionally oriented and generally uniformly disposed throughout the matrix  34 . A core  32  of this construction has a coefficient of thermal expansion (CTE) between about 3 and 6 ppm/° C. and supports the low CTE required for surface mount technology solder joint reliability. The respective chip-carrying layers  16  and core  32  of the embodiments of FIG. 2 have interfaces  52  therebetween and are electrically connected to each other (as required) through vias  48  extending between the layers  16  and core  32  which are plated with thermally conductive material  50  to thereby provide a plurality of connection sites along this interface to accommodate heat transfer to the core  32  for dissipation therefrom as described above and in addition to thermal vias  50 . 
     Operationally, the preferred embodiment  30  has favorable characteristics with respect to high tensile modulus, high thermal conductivities, low coefficients of thermal expansion, and light weights. The pitch-based graphite fiber segments  36  of the core  32  especially provide extremely high tensile modulus, high thermal conductivity, low density, and selectable coefficient of thermal expansion of the core  32  obtainable by varying the fiber volume, fiber grade and fiber orientation during core manufacture. The table shown in FIG. 3 non-limitedly exemplifies construction variability and selectability, with the fiber grade designations identifying fiber products from Amoco Corporation, Chicago, Ill. Because of the plurality and conductivity of connections along the respective interfaces  52 , the present invention provides novel temperature control and circuit density since every connection results in heat transfer to the core  32 . 
     While an illustrative and presently preferred embodiment of the invention has been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.