Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
1999-08-20
2002-04-30
Parker, Fred J. (Department: 1762)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C427S229000, C427S261000, C427S287000, C106S001180, C106S001210, C430S119880, C430S126200
Reexamination Certificate
active
06379745
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to compositions which can be used to apply conductors to electronic components such as printed circuit boards and semiconductors, particularly, to compositions which can be applied and converted to solid conductors at temperatures below 450° C.
2. Related Art
A common method for printed circuit fabrication process is subtractive or semi-additive processes in which conductors are formed by etching away unwanted copper. A fully additive process would have many advantages over the subtractive or semi-additive methods. The primary problem in providing a wholly additive process for producing printed circuitry is the requirement for high electrical conductivity with low enough curing temperature to be compatible with the polymer-based circuit boards. Another major problem is making connections to the additive traces, preferably by conventional soldering. Present technology includes low cure temperature conductive epoxies and transient liquid phase materials which produce traces with poor electrical conductivity and poor solderability or high temperature thick film inks which produce traces with good electrical conductivity and good solderability but which are limited to ceramic substrates. These small, expensive and specialized substrates are required to withstand the thick film ink firing temperatures of more than 650° C. and usually above 850° C. A method which could duplicate the performance of thick film inks but on polymer-based substrates at 250 to 350° C. would permit application of this technology broadly in the $27 billion rigid circuit board industry and the $2.5 billion flexible circuit industry, worldwide.
“Thick film” technology is routinely practiced to produce hybrid circuits on ceramic substrates. R. W. Vest, “Electronic Ceramics”, R. Breckenridge, ed., 1991. The conductor patterns are created by silk screening or stencil printing thick film pastes or inks onto ceramic substrates and firing them at temperatures of 850 to 1100° C. to reduce the metal-containing inks to metal. An example of such inks are silver-palladium compositions which have recently been reviewed by Wang, Dougherty, Huebner and Pepin, J. Am. Ceram. Soc. 77(12), 3051-72 (1994). Typically thick film inks contain metal powders, an inorganic glass binder and a vehicle consisting of. a polymer binder and a solvent. The vehicle provides the correct consistency for screen printing and consists typically of a polymer such as ethyl cellulose, hydrogenated rosin or polyacrylics dissolved in a low volatility solvent. Common solvents are terpineol, dibutyl carbitol and various glycol ethers and esters. The inks are applied to ceramic substrates by screen printing, dried to drive off the solvent and heat treated, usually in a belt furnace, to decompose the polymer binder and fuse the metal and the inorganic glass binder. The glass phase provides the bond to the substrate which is usually alumina, and the metal provides the electrical conductivity. Typically the conductors have a striated cross section with layers of glass alternating with layers of metal. The glass tends to concentrate at the ceramic interface and the metal at the air interface. The conductivity is typically one half to one quarter that of the bulk metal.
A number of thick film compositions contain surfactants to improve screenability and stability of the metal powder dispersions. Often these surfactants are metallo-organic compounds such as soaps of carboxylic acids. These are convenient in that they will decompose at relatively low temperature to deposit the metal or its oxide which can perform a useful function in the fired conductor.
U.S. Pat. No. 5,071,826 issued on Dec. 10, 1991 and U.S. Pat. No. 5,338,507 issued on Aug. 16, 1994 to J. T. Anderson, V. K. Nagesdh and R. C. Ruby, disclose the addition of silver neodecanoate to superconducting oxide mixtures in which the neodecanoate is decomposed to the metal at 300° C. to coat the superconducting grains with silver. The coated grains are then sintered and oxidized at 600-800° C. to produce an oxide superconductor of enhanced strength and critical current.
The addition of titanate to thick film conductors by decomposition of an organo-metallic titanate is described by K. M. Nair in U.S. Pat. No. 4,381,945 issued on May 3, 1983.
U.S. Pat. No. 4,599,277 issued on Jul. 8, 1986 to J. M. Brownlow which discloses adding organo-metallic compounds to thick film inks to increase the densification temperature of the metal to match that of the ceramic substrate at 850-950° C., the inverse of the process required to apply conductors to polymer circuits at low temperatures.
Conventional thick film paste compositions containing silver flake, glass flit and silver resinates, which are carboxylic acid soaps, as well as surfactants such as Triton X 100, were described in U.S. Pat. No. 5,075,262, issued on Dec. 24, 1991 and U.S. Pat. No. 5, 183,784, issued on Feb. 2, 1993 to M. N. Nguyen and coworkers. The objective was to eliminate the preliminary drying step after printing, and the resinate was said to promote adhesion and minimize cracks and voids in bonding semiconductor dies to a ceramic substrate at 350-450° C.
V. K. Nagesh and R. M. Fulrath were issued U.S. Pat. No. 4,130,671 on Dec. 19, 1978 which was assigned to U.S. Department of Energy. It discloses a similar composition of glass frit and silver resinate which was decomposed at low temperature to provide silver-coated glass particles similar to the superconductor of Anderson above. The particles were applied to a substrate either before or after decomposition of the resinate and fired in an oxidizing atmosphere at 500 to 700° C. to provide a conductor of metal-coated glass particles.
Still other conventional thick film compositions of glass and metal powders in an organic vehicle but without the resinate are described in U.S. Pat. Nos. 5, 250,229 and 5,378,408.
To create a low temperature analog of the thick film process, it will be necessary to find a new mechanism to obtain adhesion and cohesion of the deposited metal which can operate at temperatures below 450° C., which is the extreme upper temperature limit that most polymers can tolerate. The use of inorganic glass powder binders which are universally used in conventional thick film inks is not possible in this application because none of them melt a low enough temperature, and the glass will not bond to the metal or to the polymer substrates.
Other approaches to this objective have been described. The most common one is the creation of electrically conductive inks or pastes by incorporating metal powder, usually silver powder, in an organic matrix, the so-called “Polymer Thick Film ” materials. This is a major industry with products available from Ablestik, AIT, Hokurika, M-Tech, Thermoset, Epoxy Technology and Ferro, among others. These materials can be printed on circuit boards, and they have good adhesion. An example of the application of this technology was described in an article by K. Dreyfack in Electronics 52(17), 2E-4E, 1979, on Societie des Produits Industrielles ITT's silk screening silver and graphite-based conductors of this type onto rigid and flexible circuits. One problem with this approach is that the inks conduct by random contacts between powder grains in the organic matrix, and the conductivity is poor. Typical values of the resistivity, which is the reciprocal of conductivity, are 40 to 60 microohm cm, compared to bulk silver at 1.59 microohm cm and high temperature thick film conductors at 3-6 microohm cm. Still more disturbing is the fact that the electrical conductivity is not constant with time. The conductivity depends on adventitious contacts between individual metal grains which are prone to be made and broken randomly as the trace is heated and cooled and particularly as it is exposed to moisture and other environmental influences. Another major problem with polymer thick film materials is that because of their organic content, they are not solderable.
A typical resin-bonde
Jablonski Gregory A.
Kydd Paul H.
Richard David L.
Nissim Stuart H.
Parelec, Inc.
Parker Fred J.
Woodbridge Richard C.
Woodbridge & Associates PC
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