The general structures and manufacturing processes for electronic packages are described in, for example, Donald P. Seraphim, Ronald Lasky, and Che-Yo Li, Principles of Electronic Packaging, McGraw-Hill Book Company, New York, N.Y., (1988), and Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), both of which are hereby incorporated herein by reference.
As described by Seraphim et al., and Tummala et al., an electronic circuit contains many individual electronic circuit components, e.g., thousands or even millions of individual resistors, capacitors, inductors, diodes, and transistors. These individual circuit components are interconnected to form the circuits, and the individual circuits are interconnected to form functional units. Power and signal distribution are done through these interconnections. The individual functional units require mechanical support and structural protection. The electrical circuits require electrical energy to function, and the removal of thermal energy to remain functional. Microelectronic packages, such as, chips, modules, circuit cards, and circuit boards, are used to protect, house, cool, and interconnect circuit components and circuits.
Within a single integrated circuit, circuit component to circuit component and circuit to circuit interconnection, heat dissipation, and mechanical protection are provided by an integrated circuit chip. The chip is referred to as the "zero order package." This chip or zero order package enclosed within its module is referred to as the first level of packaging.
There is at least one further level of packaging. The second level of packaging is the circuit card. A circuit card performs at least four functions. First, the circuit card is employed because the total required circuit or bit count to perform a desired function exceeds the bit count of the first level package, i.e., the chip, and multiple chips are required. Second, the circuit card provides for signal interconnection with other circuit elements. Third, the second level package, i.e., the circuit card, provides a site for components that are not readily integrated into the first level package, i.e., the chip or module. These components include, e.g., capacitors, precision resistors, inductors, electromechanical switches, optical couplers, and the like. Fourth, the second level package provides for thermal management, i.e., heat dissipation.
In most applications, and especially personal computers, high performance workstations, mid range computers, and main frame computers, there is a third level of packaging. This is the board level package. The board contains connectors to accept a plurality of cards, circuitization to provide communication between the cards, I/O devices to provide external communication, and, frequently, sophisticated thermal management systems.
The basic process for polymer based composite package fabrication is described by George P. Schmitt, Bernd K. Appelt and Jeffrey T. Gotro, "Polymers and Polymer Based Composites for Electronic Applications" in Seraphim, Lasky, and Li, Principles of Electronic Packaging, pages 334-371, previously incorporated herein by reference, and by Donald P. Seraphim, Donald E. Barr, William T. Chen, George P. Schmitt, and Rao R. Tummala, "Printed Circuit Board Packaging" in Tummla and Rymaszewski, Microelectronics Packaging Handbook, pages 853-922, also previously incorporated herein by reference.
In the normal process for package fabrication a fibrous body, such as a non-woven mat or woven web, is impregnated with a laminating resin. This step includes coating the fibrous body with, for example, an epoxy resin solution, evaporating the solvents associated with the resin, and partially curing the resin. The partially cured resin is called a B-stage resin. The body of fibrous material and B stage resin is called a prepreg. The prepreg, which is easily handled and stable, may be cut into sheets for subsequent processing.
Typical resins used to form the prepreg include epoxy resins, cyanate ester resins, polyimides, hydrocarbon based resins, and fluoropolymers. One prepreg is the FR-4 prepreg. FR-4 is a fire retardant epoxy-glass cloth material, where the epoxy resin is the diglycidyl ether of 2,2' - bis(4-hydroxyphenyl) propane. This epoxy resin is also known as the diglycidyl ether of bisphenol-A, (DGEBA). The fire retardancy of the FR-4 prepreg is obtained by including approximately 15-20 weight percent bromine in the resin. This is done by partially substituting brominated DGEBA for the DGEBA.
Other epoxy resin formulations useful in providing prepregs include high functionality resins, such as epoxidized cresol novolacs, and epoxidized derivatives of triphenyl methane. The multifunctional epoxy resins are characterized by high glass transition temperatures, high thermal stability, and reduced moisture take up.
Still other epoxy resins are phenolic cured epoxies, as Ciba-Giegy RD86-170(.TM.), Ciba-Giegy RD87-211(.TM.), Ciba-Giegy RD87-212(.TM.), Dow Quatrex(.RTM.) 5010(.TM.), Shell Epon(.RTM.) 1151(.TM.), and the like. These epoxies are mixtures of epoxies, with each epoxy having a functionality of at least 2, a phenolic curing agent with a functionality of at least 2, and an imidazole catalyst.
Cyanate ester resins are also used in forming prepregs. One type of cyanate ester resin includes dicyanates mixed with methylene dianiline bis-maleimide. This product may be further blended with compatible epoxides to yield a laminate material. One such laminate material is a 50:45:5 (parts by weight) of epoxy: cyanate: maleimide. Typical of cyanate ester resins useful in forming prepregs is the product of bisphenol-A dicyanate and epoxy, which polymerizes during lamination to form a crosslinked structure.
A still further class of materials useful in forming prepregs for rigid multilayer boards are thermosetting polyimides. While thermosetting polyimides exhibit high water absorption, and high cost, they have good thermal properties and desirable mechanical properties. The preferred polyimides for prepreg use are addition products such as polyimides based on low molecular weight bis-maleimides.
By way of contrast, another type of polyimides, useful in forming stable, flexible films for flexible circuits are higher molecular weight polymers derived from dianhydrides and diamines.
One condensation polyimide based on diphenylene dianhydride is described in U.S. Pat. No. 4,725,484 to Kiyoshi Kumagawa, Kenji Kuniyasu, Toshiyuki Nishino, and Yuji Matsui for DIMENSIONALLY STABLE POLYIMIDE FILM AND PROCESS FOR PREPARATION THEREOF. This patent describes a copolymer of 3,3',4,4'- biphenyltetracarboxylic anhydride and p-phenylene diamine, commercially known as Upilex(.RTM.) S.
As noted above, the thermosetting polyimides may be reinforced with fibers to form rigid circuit boards. For example, thermosetting polyimide reinforced with polytetrafluoroethylene fibers are described in U.S. Pat. No. 4,772,509 to Ichiro Komada and Minoru Hatakeyama for PRINTED CIRCUIT BOARD BASE MATERIAL.
The polyimide may be an intermediate layer of the core, between a substrate and the circuitization, as described, for example in U.S. Pat. No. 4,705,720 of Ernst F. Kundinger, Erich Klimesch, Hans-Georg Zengel, and Jeffrey D. Lasher for FLEXIBLE MULTI-LAYER POLYIMIDE LAMINATES. Kundinger et al. describe a flexible circuit package having a substrate, for example, a metallic substrate, fully encapsulated in a fully reacted, fully aromatic condensation type polyimide, to which the circuitization is joined by an adhesive. Kundinger et al.'s adhesives include polyacrylates, polysulfones, epoxies, fluoropolymers, silicones, and butyl rubbers.
It is to be understood that processing of rigid boards and flexible circuits are very different manufacturing processes. In the case of manufacturing flexible circuits, subsequent processing includes circuitization, that is, the formation of a Cu signal pattern or power pattern on the film. Circuitization may be additive or subtractive. If a direct metal deposition process, such as chromium or copper sputtering is not used an adhesive is generally required between the circuitization and the flexible carrier film, especially in the case of polyimide films. Processing of flexible films is most often handled by roll to roll processes with a film width of ten inches or less.
Various adhesives have been reported for laminating copper circuitization to polyimide flexible circuit films. For example, U.S. Pat. No. 4,634,631 to Samuel Gazit and Cathy Fleischer for FLEXIBLE CIRCUIT LAMINATE AND METHOD OF MAKING THE SAME describes the use of a microglass reinforced fluoropolymer as a dielectric adhesive for flexible polyimide circuits.
Kundinger et al., U.S. Pat. No. 4,725,720, noted above, describe the use of adhesives such as polyacrylates, polysulfones, epoxies, fluoropolymers, silicones, and butyl rubbers to obtain adhesion between a polyimide layer and the circuitization.
U.S. Pat. No. 4,725,504 to Phillip D. Knudsen and Daniel P. Walsh for METAL COATED LAMINATE PRODUCTS MADE FROM TEXTURED POLYIMIDE FILM describes the use of electroless Ni or Co to bond the Cu circuitization to a polyimide layer of a printed circuit board.
U.S. Pat. No. 3,717,543 to James R. Sinclair, and Gordon M. Ellis, Jr. for LAMINATIONS OF POLYIMIDE FILMS TO LIKE METALS AND/OR METAL FOILS describes an adhesive for bonding polyimide to either a metal, as a circuitization layer, or to another polyimide film. The adhesive is an acrylic-epoxy copolymer, as an ammoniated acrylic and a low molecular weight epichlorohydrin bisphenol A copolymer.
U.S. Pat. No. 3,904,813 to Gaylord L. Groff for ADHESIVE FOR METAL CLAD SHEETING describes an adhesive that is the reaction product of a carboxyl terminated polymer (as azelaic acid and neopentyl glycol) and a high molecular weight polymeric reaction product of bisphenol A and epichlorohydrin.
U.S. Pat. No. 4,762,747 to Jong-Min Liu, Fu-Lung Chen, and Yeong-Cheng Chiou for SINGLE COMPONENT AQUEOUS ACRYLIC ADHESIVE COMPOSITIONS FOR FLEXIBLE PRINTED CIRCUITS AND LAMINATES MADE THEREFROM describes a printed circuit board having an adhesive layer on polyimide. The adhesive contains (1) an acrylonitrile, (2) an alkyl acrylate or methacrylate, (3) an oxirane containing polymerizing ethylenic monomer, (4) a hydroxyl or amide containing acrylate or methacrylate, and (5) styrene.
U.S. Pat. No. 4,627,474 to Ki-Soo Kim for POLYIMIDE FILM/METAL FOIL LAMINATION describes the use of an unreinforced epoxy adhesive to laminate metal foils to a polyimide film.
By way of contrast with the above described process for flexible circuits, the rigid multilayer composite printed circuit package is fabricated by interleaving cores (including signal cores, signal/signal cores, power cores, power/power cores, and signal/power cores) with additional sheets of prepreg, and surface circuitization. After lamination, hole drilling, photolithography, and plating processes are repeated, a rigid multilayer composite is obtained. The size of the rigid multilayer composite can reach 670 square inches (24 inches by 28 inches) or more, with thicknesses of 250 mils or more.
The initial fabrication of the cores is carried out with large area laminates, i.e., up to thirty six inches or more, which are subsequently cut into smaller area assemblies. It is particularly important that these laminates neither delaminate nor tear during fabrication. Moreover, the complex sequence of these steps combined with the chemically and physically aggressive nature of many of these steps results in product compromise and loss. Moreover, increasing clock rates require low impedance, low dielectric constant, thin cores. Thus, there is a clear need for a thin, rugged, thermally stable microelectronic package core, with matched coefficients of thermal expansion between the layers, that is fabricated of materials that are adapted for ease of manufacturing, including high peel strength, tear resistance and toughness while allowing the production of large surface area thin cores, i.e., less then 8 mils thick, by the use of conventional rigid core fabrication processes.