Direct metallization process

Details Direct metallization process Direct metallization process

1997-11-21

2001-01-09

Wong, Edna (Department: 1741)

Electrolysis: processes, compositions used therein, and methods

Electrolytic coating

Coating selected area

C205S159000, C205S125000, C205S162000, C205S164000, C205S166000, C427S122000, C427S443200, C427S430100, C427S421100, C427S427000

Reexamination Certificate

active

06171468

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention is directed to printed wiring boards having recesses, or other non-conductive substrates, made electrically conductive by applying a coating containing carbon, preferably graphite, to an initially nonconductive through hole or other substrate.
Additional background information may be found from page 2, line 1, to page 10, line 21, of U.S. Ser. No. 08/471,871, filed Jun. 7, 1995, now U.S. Pat. No. 5,690,805. That text is hereby incorporated by reference in the present disclosure.
SUMMARY OF THE INVENTION
An object of the present invention is to improve the deposition of a thin and uniform coating of graphite or carbon black (which are referred to in this specification either together or separately as “carbon”) particles on a non-conductive surface, such as the wall of a through hole, “via” (which is a blind hole), another type of recess, or some other surface.
As used herein, a “uniform” coating is one essentially free of excess conductive coating composition build up, particularly at the opening(s) of a recess, so the coating has a substantially uniform thickness at the mouth and in the interior of the recess, as viewed under a 50× magnification of a cross-section of a recess after plating.
Still another object of the present invention is to provide an electroplated conductive through hole coating which is capable of withstanding the solder shock test.
A still further object of the invention is to provide a conductive carbon coating with a low resistivity.
Other objects of the invention will become apparent to one skilled in the art who has the benefit of this specification and the prior art.
One aspect of the present invention, which meets one or more of the above objects, is a method of applying an electrically conductive carbon coating to a non-conductive surface. A substrate is provided having at least one non-conductive surface. An example of such a substrate is the wall of a through hole or via drilled or otherwise formed in the non-conductive substrate for a printed wiring board. A liquid dispersion of electrically conductive carbon is also provided. The carbon dispersion has a mean particle size no greater than about 50 microns.
The carbon dispersion is applied to the non-conductive surface to form a substantially continuous, electrically conductive carbon coating. The carbon in the dispersion is present in an amount effective to provide an electrically conductive coating.
Next, the conductive carbon coating is fixed on the (formerly) nonconductive surface. (“Fixing” is defined below in the detailed description.) Typically, fixing is carried out after the carbon dispersion is applied, without drying the carbon coating first.
Fixing may be accomplished in a variety of different ways. For example, the fixing step can be carried out by applying a fixing liquid to the carbon-coated surface. One example of a suitable fixing liquid is a dilute aqueous acid maintained at a pH between about 0.01 and about 6. Other suitable fixing liquids are described later in this specification. For another example, fixing may be carried out by removing the excess carbon dispersion with an air knife or other source of compressed air.
The fixing process removes excessive carbon composition deposits, and thus smooths the carbon coating on the recess surfaces by eliminating lumps and by making the coating more uniform. Certain liquid fixing agents also crosslink the first monolayer of carbon which is directly attached to the substrate or an aqueous organic binding agent associated with the coating. Crosslinking these coatings improves adhesion without interfering with microetching. Microetching selectively removes the carbon coating from metallic surfaces, without removing it from nonmetallic surfaces such as the substrate of a printed wiring board.
An alternative aspect of the invention is a method of applying a conductive graphite coating to a non-conductive surface. A substrate having at least one recess having a non-conductive surface is coated with a synthetic graphite dispersion to form a substantially continuous, electrically conductive graphite coating having an electrical resistivity of less than about 1000 ohms.
In still another alternative aspect of the invention the graphite dispersion contains from about 1% to about 7% by weight of electrically conductive graphite (whether natural or synthetic).
In yet another alternative aspect of the invention the graphite dispersion contains from about 0.01% to about 10% by weight of a water dispersible organic binding agent.
DETAILED DESCRIPTION OF THE INVENTION
While the invention will be described in connection with one or more preferred embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention includes all alternatives, modifications, and equivalents as may be included within the spirit and scope of the appended claims.
The present invention is carried out by providing and using a conductive carbon dispersion. A detailed description of the ingredients of the dispersion and how it is made and used follows. Additional disclosure is provided from page 14, line 9, to page 58, line 9, of U.S. Ser. No. 08/471,871, filed June 7, 1995, now U.S. Pat. No. 5,690,805. That text is hereby incorporated by reference in the present disclosure.
Carbon
One component of the present conductive compositions is electrically conductive carbon, for example, carbon black, graphite, or combinations of the two.
The electrically conductive carbon particles should be present in an amount effective to provide an electrically conductive coating when the composition is applied to a substrate. The carbon may be present as from about 0.1 to about 20% by weight, alternatively from about 0.5 to about 10% by weight, alternatively from about 1% to about 7% by weight, alternatively from greater than about 4% to about 6.5% by weight of the composition.
The carbon may have a mean particle size within the range from about 0.05 to about 50 microns, alternatively from about 0.3 to 1.0 microns, alternatively from about 0.7 to about 1.0 microns. From the perspective of performance and ease of dispersion, particles from the smaller end of the size range are preferred. However, the smaller particles, particularly graphite particles, are more costly. The inventors have found it unnecessary to obtain graphite having mean particle sizes substantially less than one micron, contrary to the conventional belief that extremely fine graphite is necessary.
The carbon black useful herein can be substantially as described in U.S. Pat. No. 5,139,642. The carbon black description of that patent is hereby incorporated herein by reference in its entirety. Several commercial carbon blacks contemplated to be useful herein include CABOT MONARCH 1300, sold by Cabot Corporation, Boston, Mass.; CABOT XC-72R Conductive, from the same manufacturer; ACHESON ELECTRODAG 230, sold by Acheson Colloids Co., Port Huron, Mich.; COLUMBIAN RAVEN 3500, made by Columbian Carbon Co., New York City, N.Y.; and other conductive carbon blacks having similar particle sizes and dispersion characteristics.
The graphite useful herein can be substantially as, described in U.S. Pat. No. 5,139,642. The graphite description of that patent is hereby incorporated herein by reference in its entirety. In the present compositions, the graphite may be either synthetic or naturally occurring. Accordingly, suitable commercial graphite and graphite dispersions contemplated to be useful herein include: ULTRAFINE GRAPHITE, sold by Showa Denko K. K., Tokyo, Japan; AQUADAGE E; MICRO 440, sold by Asbury Graphite Mills Inc., Asbury, N.J.; GRAPHITE 850, also sold by Asbury; GRAFO 1204B, sold by Metal Lubricants Company, Harvey, Ill.; GRAPHOKOTE 90, sold by Dixon Products, Lakehurst, N.J.; NIPPON AUP (0.7 micron), sold by Nippon Graphite Industries, Ltd., Ishiyama, Japan; and others having similar electrical and dispersion characteristics.
Aqueous dispersions of carbon black or graphite are well known in the art and in related arts, su

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