Printed wiring board structure with integral metal matrix composite core

Abstract: A printed wiring board structure (30) having at least one chip-carrying layer (16) adjacent a core (32) fabricated of a metal matrix (34) having disposed therein continuous pitch based graphite fibers (36). The chip carrying layers (16) and the core (32) have an interface (52) therebetween and are integrally connected to each other through vias (48) plated with an electrically and thermally conductive material (50) to thereby provide a plurality of connection sites along this interface (52). The matrix (34) is preferably fabricated of aluminum. Preferred fibers (36) are fabricated of pitch based graphite. A typically preferred present printed wiring board structure (30) has several circuit layers and two chip-carrying layers (16) each on opposite sides of the core (32), with each of the layers (16) and the core (32) having respective interfaces (52) therebetween wherein each layer is integrally connected to the core (32) at a plurality of connection sites, as required by circuit design, along the respective interfaces. Because of the plurality of connection sites along respective interfaces (52), superior thermal conductivity occurs from the layers (16) to the core (36) since heat travels through these connection sites to effectuate extremely efficient heat transfer and ultimate heat dissipation from the entire printed wiring board structure (30).

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, 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 metallized (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 favourably 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 though 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 follows below. Background art is provided in US-4,609,586, which discloses printed wiring board laminate structures. One embodiment comprises a support fabricated from graphite fibre reinforced metal adhesively secured to a printed wiring board. In a second embodiment, the support comprises a core of graphite fibre reinforced metal or resin sandwiched between two metal sheets. Summary of the Invention The present invention is a printed wiring board structure as defined in Claim 1, comprising at least one chip-carrying layer adjacent a core fabricated of metal matrix having disposed therein pitch based 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. The metal matrix is preferably fabricated of aluminum. 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 layer, 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: Figure 1 is a schematic illustration of a side elevation view of a prior art printed wiring board structure with a core fabricated of copper and molybdenum; Figure 2 is a schematic illustration of a side elevation 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 Figure 3 is a table showing core material construction components and characteristics. Detailed Description of the Preferred Embodiment Referring first to Figure 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. Figure 2 illustrates the preferred embodiment of the present invention. In particular, and similar to the illustration of Figure 1, Figure 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 Figure 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. 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 Figure 3 non-limitedly exemplifies construction variability and selectability, with the fiber grade designations identifying fiber products from Amoco Corporation, Chicago, Illinois. 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 within the scope of the invention as defined in the appended claims.

Claims: A printed wiring board structure (30) comprising at least one chip-carrying layer (16) for carrying at least one chip thereon; characterised in that the structure further comprises: a core (32) adjacent to the chip-carrying layer (16), the core (32) including a metal matrix (34) and a plurality of continuous pitch-based graphite fibres (36) disposed in the metal matrix; and at least one via (48) plated with a thermally and electrically conductive material (50) and extending through the chip-carrying layer (16) and the core (32) to provide a thermally conductive connection between the chip (18) and the core (32). A printed wiring board structure as claimed in Claim 1, wherein the metal matrix of the core (32) is an aluminum material. A printed wiring board structure as claimed in Claim 1, wherein the metal matrix of the core (32) is a copper material. A printed wiring board structure as claimed in any preceding claim, in which there are two said chip-carrying layers (16) on opposite sides of the core (32), and an interface (52) between either side of the core (32) and each of the said chip-carrying layers (16). A printed wiring board structure as claimed in any preceding claim, wherein at least one additional chip-carrying layer (16) is layerly affixed to at least one chip-carrying layer (16) connected to the core (32). A printed wiring board structure as claimed in any preceding claim, in which the metal matrix (34) comprises continuous, substantially unidirecdonally orientated, and uniformly disposed, pitch based graphite fibers. A printed wiring board structure as claimed in Claim 4, wherein there are two said chip-carrying layers (16) at two opposing sides of the core (32) and at least one via (48) extending through the chip-carrying layers (16) and the core (32) to provide electrical connection between the chip-carrying layers (16).

Inventors: KRAMER JESSE J, MADURA JEFFREY R, SMITH CARL R, KRAMER, JESSE J., MADURA, JEFFREY R., SMITH, CARL R.
Applicants: NORTHROP GRUMMAN CORP, NORTHROP GRUMMAN CORPORATION
Application year: 2000
Main classification: H05K1
IPC codes: H05K1