Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
1999-06-03
2002-08-27
Wong, Edna (Department: 1741)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S503000
Reexamination Certificate
active
06440331
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
The present invention generally relates to a composition and method for making an initially electrically nonconductive surface electrically conductive by applying an electrically conductive coating from an aqueous carbon dispersion. One aspect of the present invention relates more particularly to such a method for making the initially nonconductive through hole and via walls of printed wiring boards electrically conductive, so they can be electroplated. (“Via” as used herein refers either to a through hole or to an open or blind via, however formed. Vias and through holes can be formed by drilling, by laser or plasma ablation, additively (as from a photoresist), or in any other way presently known or discovered in the future.)
Conductive graphite and carbon black aqueous dispersions are used to provide a conductive coating on via walls and other nonconductive surfaces. Such dispersions, methods for using such dispersions to coat vias, and improved printed wiring boards fabricated by using such dispersions are defined in U.S. Pat. Nos. 5,476,580, 5,389,270, and 5,725,807 respectively issued to Thorn et al. on Feb. 14, 1995, Dec. 19, 1995, and Mar. 10, 1998. These patents referred to in the preceding sentence are incorporated herein by reference in their entireties. A graphite composition, cleaners, conditioners, and other materials and directions needed to practice these patents are available under the trademark SHADOW(& from Electrochemicals Inc., Maple Plain, Minn. Other carbon dispersions containing carbon black or graphite are described, for example, in U.S. Pat. No. 5,139,642.
A problem with the current electroless plating processes is that “blowholes” occasionally develop after the via wall has received a conductive coating, has been electroplated with copper, and is then suddenly heated, as by contacting it with molten solder. (Soldering is carried out by coating the via walls and other conductive surfaces of a printed wiring board with hot, molten metal to make electrical connections by wetting and filling the spaces between the conductive via surfaces and the leads of electrical components which have been inserted through the vias. A properly soldered via is filled with solder.) If there are any gaps or voids in the plated copper in the via, moisture can enter the substrate through the gaps. Soldering heats the plated via walls very quickly. The hot solder rapidly vaporizes any moisture in the substrate, which can blow some or all of the solder out of the hole and breach the copper layer. The result is a blowhole or a partially filled or empty hole, any of which is counted as a soldering defect.
The problem of blowholes in vias made electrically conductive by electroless plating, and the solution to blowholes when that technology is used, are described in a series of articles published in CIRCUIT WORLD, Vol. 12 No. 4 (1986), Vol. 13 No. 1 (1986), and Vol. 13 Nos. 2-3 (1987), under the common title,
Blowholing in PTH Solder Fillets.
A related article is C. Lea,
The Harmfulness of Blowholes in PTH Soldered Assemblies,
CIRCUIT WORLD, Vol.16, No.4, (1990). All the articles in this paragraph are incorporated herein by reference in their entirety for their discussion of blowholes in electroless copper technology.
Recently, the present inventors have discovered that blowholes can occasionally be a problem for vias that have been made conductive, to facilitate electroplating, by applying certain aqueous carbon-based conductive compositions. Thus, a need has arisen to solve the problem of blowholes when a carbon-based conductive coating is used to make via walls electrically conductive to facilitate electroplating.
One alternative method for reducing the formation of blowholes is with ultrasonic treatment of the carbon dispersion, the conditioner bath, or other treatment baths used to prepare for and deposit the conductive carbon coating. This method has solved the blowhole problem for many applications.
Another continuing challenge is how to increase the conductivity of the carbon coating that is deposited on the nonconductive surface. The graphite compositions disclosed in the Thorn et al. patents cited above provide coatings with markedly improved conductivity, which results in faster electroplating and provides other benefits. For some applications, however, further improvements in conductivity would be useful.
BRIEF SUMMARY OF THE INVENTION
One object of the present invention is a conductive coating that can be deposited on a non-conductive surface to make it electrically conductive.
Another object of the present invention is to provide a conductive carbon coating with improved conductivity over coatings of palladium, electroless carbon, carbon black, or graphite provided by prior through hole coating compositions and processes.
An additional object of the present invention is a process for applying a uniform coating of carbon particles and a second conductive material on a non-conductive surface. As used herein, a “uniform” coating is one essentially free of excess conductive coating buildup so that the coating has a substantially consistent thickness after plating.
Still another object of the present invention is a conductive composition that can be electroplated to a previously non-conductive surface, such as a printed wiring board, and adhere to a through hole wall better than coatings of palladium, electroless carbon, carbon black, or graphite provided by prior through hole coating process and compositions.
A still further object of the invention is to provide a conductive through hole coating which results in the formation of essentially no blowholes when soldered.
One aspect of the invention which satisfies one or more of the foregoing objects is a composition including electrically conductive carbon particles, a second conductive material, a water dispersible binding agent, and an aqueous dispersing medium. Enough of the carbon particles are present to provide an electrically conductive coating when the composition is applied to a substrate. The electrically conductive carbon particles can be, for example, graphite, carbon black, or combinations of the two. Enough of the second conductive material is present to provide an improved electrically conductive coating when the composition is applied to a substrate. The second conductive material can be a substrate coated with a metal, a metal, a metal oxide, or a powder containing tin oxide and antimony. Each metal recited in the preceding sentence can optionally be aluminum or nickel. Enough of the binding agent is present to bind the carbon particles and the second conductive material to the substrate.
Another aspect of the invention is a composition including electrically conductive carbon particles as previously defined, a second conductive material as previously defined, a water dispersible binding agent as previously defined, and an aqueous dispersing medium as previously defined. Enough of the carbon particles and second conductive material are present to provide an electrically conductive coating which can be electroplated to an originally nonconductive printed wiring board surface, resulting in soldering of said surface substantially without blowholes.
Still another aspect of the present invention is a conductive composition stated in terms of the solids found in a coating applied from the compositions described above. The coating includes from about 2.0 to 99.6% by weight of electrically conductive carbon particles and from about 0.4 to 98% of a second conductive material. The second conductive material can be a substrate coated with a metal, a metal powder, or a powder including tin oxide and antimony.
The compositions above can be used to coat a via surface or other nonconductive substrate with an electrically conductive composition to facilitate electroplating. These compositions can be used to coat an electronic equipment housing to dissipat
Carano Michael V.
Polakovic Frank
Electrochemicals Inc.
McAndrews Held & Malloy Ltd.
Wong Edna
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