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High conductivity inks with improved adhesionRelated Patent Categories: Compositions, Electrically Conductive Or Emissive Compositions, Free Metal Containing, Noble Metal (gold, Silver, Ruthenium, Rhodium, Palladium, Osmium, Iridium, Platinum)High conductivity inks with improved adhesion description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070170403, High conductivity inks with improved adhesion. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATIONS [0001] This is a divisional application of U.S. patent application Ser. No. 10/353,837 filed Jan. 29, 2003, now U.S. Pat. No. ______, the entire disclosure of which is expressly incorporated herein by reference. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to electrically conductive ink compositions and methods of producing these compositions. The compositions include adhesion promoting additives and can be cured to form highly conductive metal traces which have improved adhesion to substrates. [0004] 2. Related Art [0005] Materials for printing electrical circuits on electrical conductor substrates known as PARMOD.RTM. materials are disclosed in U.S. Pat. Nos. 5,882,722, 6,036,889, 6,143,356 and 6,379,745, the entire disclosures of which are expressly incorporated herein by reference. PARMOD.RTM. materials have been developed for printing conductive circuits on polymer or paper substrates such as those used for printed wiring boards, flexible circuits and RFID antennae. Typically, polymer thick film conducting materials are made of individual particles which may be in adventitious contact with each other. In contrast, using PARMOD.RTM. materials and a simple print-and-heat process for "chemical welding" of pure metals, electrical conductors made of a single-phase continuous well-bonded metal trace are produced. PARMOD.RTM. materials also provide a desirable alternative to the conventional thick film compositions that are cured at high temperatures onto ceramic or glass based substrates. PARMOD.RTM. materials are cured at temperatures which polymer and paper based substrates can withstand, and provide electrical conductivity comparable to that of the pure metal and greater than that of polymer thick films. [0006] A significant problem that arises in manufacturing conductive circuits on polymer or paper substrates is inadequate adhesion of the metal coating on the substrates. Yet another difficulty is achieving adequate adhesion while maintaining the desired resistivity properties in the electronic circuit. In general, a separate adhesive layer applied to the substrate surface has been required for sufficient adhesion of PARMOD.RTM. materials to rigid printed circuits (see, e.g., U.S. Pat. No. 6,379,745). For example, polyimide films are first coated with various adhesive layers before copper and silver PARMOD.RTM. compositions are printed on the surface and thermally cured to create flexible printed circuits. Suitable substrates for this purpose include Kapton.RTM. type FN with a FEP Teflon.RTM. coating; Kapton.RTM. types KJ and LJ with low melting polyimide coatings; and polyimide substrates with polyamic acid coatings. Copper PARMOD.RTM. compositions have been printed on rigid polyimide-glass laminates coated with a chain extending polyimide adhesive and thermally cured to create rigid printed circuits (see U.S. Pat. Nos. 6,143,356 and 6,379,745). However, because the adhesive layer infiltrates into the porous metal trace during curing, the curing conditions are predominately dictated by the properties of the adhesive rather than the PARMOD.RTM. materials which can cure at lower temperatures and in shorter times than the adhesive. Thus, adding the adhesive coating diminishes the advantages provided by the PARMOD.RTM. method and compositions. In the case of circuits with drilled holes for through-hole components and vias for electrical connections between layers, coating the holes with adhesive makes it difficult to obtain good bonding to the metal traces. Even if adhesive coatings are selected, suitable adhesive coatings are not widely available on substrates of commercial interest, such as paper and polymer based substrates. In addition, coated substrates are generally more expensive than uncoated substrates. Therefore, although attempts have been made to improve adhesion of conductive coatings, a suitable solution to this problem has not heretofore been developed. [0007] Thus, there is a need for methods and compositions that provide sufficient adhesion of PARMOD.RTM. compositions to substrates of interest, and which retain the highly conductive properties of the PARMOD.RTM. materials. SUMMARY OF THE INVENTION [0008] The present application provides conductive ink compositions into which adhesion promoting compounds are incorporated to improve adhesion of the ink compositions to various substrates in the manufacture of electrical conductors. Accordingly, the invention provides a conductive ink composition comprising a reactive organic medium, metal powder or flake, and an adhesion promoting additive. The ink composition may also include an organic liquid vehicle to facilitate mixing and application of the mixture onto the substrate. The ink compositions may further include other additives commonly used in conductive ink compositions. [0009] Preferably, the reactive organic medium comprises a metallo-organic decomposition compound, an organic reactive reagent which can react with the metal powder or flake to form a metallo-organic decomposition compound, or a mixture thereof. [0010] The compositions of the invention are advantageously applied to low-temperature substrates such as polymer, paper and polyimide-based substrates using any suitable printing technique to provide improved low-temperature substrates with well-adhered traces of high electrical conductivity. [0011] The adhesion promoting additive is a polymer or a primary diamine. Preferably, the adhesion promoting additive is a polymer selected from the group consisting of low T.sub.g polyimides, silicones, fluorocarbons, fluoropolymers, soluble (chain extending) polyimides, polyimideamides, polyamic acids and combinations thereof. The adhesion promoting additive may also be a primary diamine, such as 4,4-(1,3-phenylenedioxy)dianiline (RODA) or oxydianiline (ODA). In addition, the adhesion promoting additive is a polymer selected from the group consisting of polyvinylidene chloride, polyvinyl chloride, polyethylene vinyl chloride, polyester, polyurethane, polymethyl methacrylate, epoxy, and copolymers and mixtures thereof. DETAILED DESCRIPTION OF THE INVENTION [0012] PARMOD.RTM. mixtures contain a reactive organic medium and metal flakes and/or metal powders. The reactive organic medium comprises either a metallo-organic decomposition compound or an organic reagent which can form such a compound upon heating in the presence of the metal flakes and/or metal powders, or a mixture thereof. The ingredients are blended together with organic vehicles, if necessary, to improve viscosity or dispersibility of the ink composition. These ink compositions can be printed on temperature-sensitive substrates, and cured at temperatures low enough so that the substrate is not damaged to form well-consolidated electrical conductors. The curing process occurs in seconds at temperatures as much as 500.degree. C. below the temperatures used for conventional sintering of thick film inks and pastes. During the curing process, material deposited from decomposition of the metallo-organic decomposition compound "chemically welds" the powder constituents of the PARMOD.RTM. mixture together into a solid. A porous but continuous metal trace is produced on the substrate surface having a density approximately half that of bulk metal and an electrical conductivity per unit mass which may be as high as half that of the bulk metal. [0013] The compositions of the present invention comprising PARMOD.RTM. materials include adhesive promoting agents that improve the application of the PARMOD.RTM. materials to various substrates. The adhesive agent is added directly to the PARMOD.RTM. material, which enhances the adhesion of the PARMOD.RTM. material to the substrate and does not significantly interfere with the physical and chemical properties of the conductive PARMOD.RTM. material, e.g., resistivity and conductivity. [0014] The present invention provides a method for incorporating adhesion promoting additives into ink compositions that improves adhesion of the ink compositions to polymer and paper substrates while maintaining high metal conductivity of the compositions after curing. Improved adhesion of the ink compositions on the substrates is observed on both rigid and flexible substrates, such as FR4 epoxy-glass rigid board, high temperature flexible polyimide substrates such as Kapton.RTM. H, as well as on low temperature substrates such as polyester and paper. [0015] According to the present invention, traces with improved adhesion and low resistivity can be obtained by curing at temperatures of about 150.degree. C. in 10 minutes or less. The concentration of the adhesion enhancing additive is low enough to maintain significantly higher conductivity of the resulting metal circuit traces than that found for polymer thick film inks, which typically have resistivities of about 25-50 microohms-cm. [0016] The metal component is present in the composition in an amount of about 1 to 20 times the amount of the metallo-organic decomposition compound. The metal constituent comprises metal powder, metal flakes or a mixture thereof. Suitable metals include copper, silver, gold, zinc, cadmium, palladium, iridium, ruthenium, osmium, rhodium, platinum, iron, cobalt, nickel, indium, tin, antimony, lead, bismuth and mixtures thereof. The metal powders suitable for use in the invention preferably have an average particle size in the range of from about 0.05 to 15 .mu.m. The metal flakes preferably have a major dimension between 2 to 15 micrometers, preferably about 5 micrometers, and a thickness of less than 1 micrometer. Metal powders are typically produced by chemical precipitation of the metal to obtain the desired particle size and degree of purity. Metal flakes can be produced by techniques well known in the art, for example, by milling the metal powder with a lubricant, such as a fatty acid or fatty acid soap. Commercially available metal powders and metal flakes may also be used, including flakes sold for electronic applications as constituents of thick film inks and silver-loaded conductive epoxies. [0017] The reactive organic medium provides the environment in which the metal powder mixture is bonded together to form a well-consolidated conductor. The reactive organic medium has, or can form, a bond to the metal via a hetero-atom. The hetero-atom can be oxygen, nitrogen, sulfur, phosphorous, arsenic, selenium or other nonmetallic element, and preferably is oxygen, nitrogen or sulfur. The hetero-atom bond is weaker than the bonds holding the organic moiety together, and is thermally broken to deposit the metal. In most cases the reaction is reversible, so that acid or other organic residue can react with the metal to reform the metallo-organic compound. The reactive organic medium compositions can be made by methods well known in the art and are capable of decomposition to the respective metals at relatively low temperatures. Reactive organic medium compounds are generally described in, e.g., U.S. Pat. No. 6,379,745. [0018] Many classes of organic compounds can function as the reactive organic medium. The reactive organic medium preferably comprises any metallo-organic compound which is readily decomposable to the corresponding metal, i.e., a metallo-organic decomposition compound, an organic reagent which can react with the metal to produce such a compound, or mixtures thereof. Examples of suitable reactive organic mediums are metal soaps and the corresponding fatty acids. Other examples are metal amines and metal mercapto compounds and their corresponding amino and sulfide precursors. Specific examples of preferred reactive organic medium constituents are the carboxylic acids and the corresponding metallic soaps of neodecanoic acid and 2-ethyl hexanoic acid with silver and copper, such as silver neodecanoate. [0019] The adhesion promoting agent is added to the metal containing ink compositions of the present invention to bind the metallic particles together and to provide significantly enhanced adhesion of the ink compositions to substrates. The adhesion promoting agent is added in an amount 0.05 to 2.0 times that of the metallo-organic decomposition compound. The added adhesion promoting compound does not adversely affect the PARMOD.RTM. cure chemistry process whereby the metal chemically welds into a continuous metal network. As a result, the conductivity of the PARMOD.RTM. materials remains significantly higher than that of polymer thick film inks. [0020] Suitable adhesion promoting agents include polymers, particularly low T.sub.g polyimides, silicones, fluorocarbons and fluoropolymers, soluble (chain extending) polyimides, polyimideamides, polyamic acids and combinations thereof. Suitable adhesion promoting polymers are also disclosed in U.S. Pat. No. 6,143,356. The adhesion promoting additives also include polymers such as polyvinylidene chloride, polyvinyl chloride, polyethylene vinyl chloride, polyester, polyurethane, polymethyl methacrylate, epoxy, and copolymers and mixtures thereof. Suitable adhesion promoting additives also include primary diamines, such as 4,4-(1,3-phenylenedioxy)dianiline (RODA) and oxydianiline (ODA). Examples of combinations of these additives include DARAN.RTM. in combination with an acrylic polymer, DARAN.RTM. and polystyrene-bautadiene, DARAN.RTM. and butyl acrylate-co-methylmethacrylate-co-methacrylic acid, DARAN.RTM. and vinyl acetate, and DARAN.RTM. and polyurethane-polyester. Continue reading about High conductivity inks with improved adhesion... 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