Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
2001-07-26
2003-09-23
Lovering, Richard D. (Department: 1712)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C106S031920, C106S472000, C106S478000, C422S041000, C427S430100, C516S038000
Reexamination Certificate
active
06623787
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
The invention described herein relates to aqueous carbon based dispersions. In particular, it relates to improving the stability of an alkaline carbon based aqueous dispersion while it is exposed to air during use.
Aqueous carbon dispersions, based on carbon black, graphite, or both types of particles, are used in a variety of commercial applications. The use of aqueous carbon black dispersions as inks for printing and black toner for electrostatic imaging is described in U.S. Pat. Nos. 6,171,382 and 5,759,728, respectively. The use of aqueous graphite-based dispersions as lubricants is described in U.S. Pat. No. 5,030,367. In addition, both carbon black and graphite based aqueous dispersions are used in coating printed circuit boards for electroplating as described in U.S. Pat. Nos. 5,389,270; 5,476,580; 5,632,927; 5,690,805; 5,725,807; 5,800,739; 6,037,020; and 6,171,468. Each identified patent is incorporated herein by reference for the disclosure of uses of carbon-based aqueous dispersions. Other graphite and carbon dispersions are described in U.S. Pat. Nos. 4,622,107; 4,622,108; 4,631,117; 4,718,993; 4,874,477; 4,897,164; 4,964,959; 5,139,642; 5,536,386; 5,632,927; and 5,759,378.
In coating printed circuit boards, the carbon particles are used to produce an electrically conductive coating that acts as a seed layer upon which metals can be electroplated. The electroplating process thus selectively forms a metal interconnect between two separate circuits. A variety of aqueous dispersions of carbon particles, including carbon black and graphite, are commercially available for this use. One commercially available aqueous dispersion of carbon for this use is sold under the trademark SHADOW® by Electrochemicals Inc., Maple Plain, Minn.
Aqueous carbon dispersions are unstable during use. The viscosity of the dispersion increases with time. The carbon particles eventually flocculate and settle, thus rendering the dispersion ineffective for coating a substrate. The dispersion must then be replaced before additional substrates can be coated. The limited lifetime of carbon based aqueous dispersions slows production and increases the cost of producing printed circuit boards.
While the invention is not limited by the accuracy of this theory, the inventors believe that at least one cause of this instability problem is interaction between an alkaline ingredient in the carbon dispersion and the atmosphere. For example, carbon dioxide in ambient air can react with ammonia or a hydroxide ion in the dispersion to produce carbonates—ionic salts which change the pH of the carbon dispersion and increase the ionic strength of the solution, making it less stable. Other examples of reactive gases are given in the detailed description of this specification.
One approach to this instability problem has been adding a stabilizer that binds the carbon particles to prevent the dispersion breakdown. However, U.S. Pat. No. 5,800,739 in column 3, lines 11-20, teaches, “It has been found that when a stabilizer is added to the dispersion to enhance stability, the stabilizer envelopes [sic] the particles within the dispersion and forms an electrically insulative coating over the particles. Therefore when the particles are coated onto a substrate, the particles which are initially poorly conductive become less conductive because of the insulating layer of the stabilizer thereby further limiting the commercial use of the same.” Also in U.S. Pat. No. 5,800,739, the inventors disclose a stabilizing agent that is said to improve the stability of the dispersion while maintaining the conductivity of the resulting coatings.
BRIEF SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to improve the stability of carbon-based aqueous dispersions during use.
The preceding object, or one or more other objects which will become plain upon consideration of the present specification, are satisfied at least in part by the invention described herein.
One aspect of the invention is a method to stabilize an aqueous dispersion of carbon that is susceptible to a viscosity increase during use. The dispersion of carbon initially has a pH of at least 7.5. The alkalinity of the dispersion is reduced, and in turn, the susceptibility of the dispersion to a viscosity increase is reduced. The method is applicable to dispersions containing a variety of carbon types including carbon black or graphite particles.
In another aspect of the invention the ammonia content in the aqueous dispersion of carbon is reduced without substantially raising the ionic strength. The concentration of ammonia is reduced by adding an agent to reduce the pH or by forcing the ammonia out of solution. An example of forcing the ammonia out of solution is purging the solution with an inert fluid such as nitrogen or air depleted of carbon dioxide.
In another aspect of the invention, the aqueous dispersion containing carbon is isolated from reactive atmospheric gas during use. The aqueous dispersion reacts with atmospheric gas. There are many ways the dispersion can be isolated from the reactive gases. One example is to reduce the headspace above the bath, during use. In another example, the headspace is filled with an inert fluid such as nitrogen, argon, or air depleted of carbon dioxide.
Still another aspect of the invention is a substantially stabilized aqueous dispersion of carbon having a viscosity of less than about 20 cps and a conductivity of less than about 3 mS.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
Not applicable.
DETAILED DESCRIPTION OF THE INVENTION
While the invention will be described in connection with one or more 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 stabilizing an aqueous dispersion containing carbon particles. A detailed description of the ingredients in a dispersion of carbon is given. Also, methods of stabilizing the dispersion of carbon are given.
Definitions
The phrase ‘susceptible to viscosity increase’ is used throughout this patent. It should be understood to mean the viscosity of a dispersion of carbon would increase upon exposure to reactive atmospheric gas. The phrase ‘reactive atmospheric gas’ is used here to mean any gas from the immediate surroundings of the dispersion of carbon that may cause an increase in the ionic strength or viscosity of the dispersion. When an aqueous dispersion of carbon with pH at least around 8 is exposed to an atmosphere containing a reactive gas it will absorb the reactive gas causing an increase in viscosity or ionic strength. After some time enough reactive gas will be absorbed to destabilize the dispersion, causing the particles of carbon to flocculate and settle, thus rendering the dispersion less useful for coating substrates. The atmosphere contains a variety of reactive gases that may destabilize the dispersion including, but not limited to, carbon dioxide, oxygen, carbon monoxide, ozone, nitrogen dioxide, nitrous oxide, and sulfur dioxide. A viscosity-unstable dispersion therefore is meant as any dispersion of carbon that is susceptible to a viscosity increase upon exposure to a reactive atmospheric gas.
Viscosity is used throughout this patent to mean the resistance a dispersion presents to flow when a given shear force is applied. It is typically measured in cPs or centipoises and can be defined as grams per centimeter second ((g/cm·sec) and is measured using a variety of techniques including, but not limited to commercial viscometers. Examples of commercial viscometers include capillary flow viscometers and rotational viscometers. A typical aqueous dispersion of carbon, having pH of at least around 7.5 has a viscosity no greater th
Bernards Roger F.
Bowers, Jr. Joseph S.
Electrochemicals Inc.
Lovering Richard D.
McAndrews Held & Malloy Ltd.
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