Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
1998-08-12
2001-10-02
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C427S098300, C427S304000, C427S305000, C427S307000, C427S336000, C427S437000, C427S443100
Reexamination Certificate
active
06296897
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for fabricating printed circuit boards, cards and chip carriers and is especially directed to a method for plating a metal layer on the circuitized portions of an already circuitized substrate. More particularly, the method of the present invention reduces bridging that can occur during the plating.
BACKGROUND OF THE INVENTION
In the manufacture of printed circuit cards and boards, a dielectric sheet material is employed as the substrate. A conductive circuit pattern is provided on one or both of the major surfaces of the substrate. The conductive pattern can be formed on the surface of the substrate using a variety of known techniques. These known techniques include the subtractive technique, where a layer of for example copper is etched to form the desired circuit pattern, the EDB (electroless direct bond) technique, where copper is electrolessly plated directly on the surface of the substrate in the desired pattern, and the peel-apart technique, where the desired circuit pattern is plated up from a thin layer of peeled-apart copper. Since the substrate employed is a dielectric, when it is desired to plate directly on the surface of the substrate, various techniques for seeding or catalyzing the substrate are used.
Furthermore, in various situations, it is desirable to selectively plate on the metallic surfaces (usually copper) as opposed to the dielectric surfaces of the substrate. This is especially true for plating copper areas that are to be used for electrical connection. For example, it is common practice to overplate copper lines with a barrier layer, typically a metal such as nickel followed by a second overplating with a precious metal such as gold, palladium or rhodium. Examples of such processes are disclosed in U.S. Pat. Nos. 4,940,181 and 5,235,139, disclosures of which are incorporated herein by reference.
However, there is a tendency for the nickel and/or precious metal to plate not only on the already present circuit lines, but also to deposit on a portion of the dielectric substrate or insulator located between lines. This problem is especially pronounced when dealing with very fine lines that are only separated by very small intervals. For example, circuit boards having surface conductive paths whose spacing is 50 microns or below have a tendency to suffer from bridging or short circuiting due to the subsequent plating of the nickel and/or precious metal. It would therefore be desirable to provide a process for plating only already circuitized lines whereby the problem of bridging is significantly reduced if not entirely eliminated.
SUMMARY OF THE INVENTION
The present invention is concerned with a method that significantly reduces the problem of extraneous plating in areas between circuitry on a circuitized substrate. More particularly, it has been found according to the present invention that treating an already circuitized substrate with a swelling agent and with a composition of an alkaline permanganate, a chromate or chlorite significantly reduces if not entirely eliminates the bridging problem. Accordingly, the method of the present invention relates to fabricating a printed circuit board which comprises providing a circuitized substrate and treating the circuitized substrate with a swelling agent. The substrate is then treated with a composition of an alkaline permanganate, a chromate or chlorite followed by applying a metal layer on the treated circuitized substrate to coat the circuitized portion of the substrate.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from the following detailed description, wherein it is shown and described only the preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the description is to be regarded as illustrative in nature and not as restrictive.
BEST AND VARIOUS MODES FOR CARRYING OUT INVENTION
The process of the present invention is applicable to treating a variety of circuitized dielectric (non-conductor) substrates. Dielectric substrates described in the prior art, including thermoplastic and thermosetting resins, may be plated in accordance with the present invention.
Typical thermosetting polymeric materials include epoxy, phenolic base materials, and polyamides. Combinations of epoxies with cyanates, BT (bismalemide) resins or polyimides are also quite suitable. The dielectric materials may be molded articles of the polymers containing fillers and/or reinforcing agents, such as glass-filled epoxy or phenolic base materials. Examples of some phenolic type materials include copolymers of phenol, resorcinol and cresol. Examples of some suitable thermoplastic polymeric materials include polyolefins such a polypropylene, polysulfones, polycarbonates, nitrile rubbers, ABS polymers and fluorinated polymeric materials such as polytetrafluoroethylene.
More typically, the dielectric substrates employed are FR-4 epoxy compositions. A typical FR-4 epoxy composition contains 70-90 parts of brominated polyglycidyl ether of bisphenol-A and 10-30 parts of tetrakis(hydroxypenyl)ethane tetraglycidyl ether cured with 3 to 4 parts of dicyandiamide and 0.2 to 0.4 parts of a tertiary amine, all parts being parts by weight per 100 parts of resin solids.
Another typical FR-4 epoxy composition contains:
A) about 25 to about 30 parts by weight of a tetrabrominated diglycidyl ether of bisphenol-A, having an epoxy equivalent weight of about 350 to about 450;
B) about 10 to about 15 parts by weight of a tetrabrominated diglycidyl ether of bisphenol-A having an epoxy equivalent weight of about 600 to about 750; and
C) about 55 to about 65 parts by weight of at least epoxidized non-linear novolak, having at least terminal epoxy groups, along with suitable curing and/or hardening agents.
Another typical FR-4 epoxy composition contains about 70 to about 90 parts of brominated polyglycidyl ether of bisphenol-A, and 10 to 30 parts of tetrakis(hydroxyphenyl)ethane tetraglycidyl ether cured with 0.8 to 1 part of 2-methyl imidazole.
Still another FR-4 epoxy composition employs tetrabromo bisphenol-A as curing agent, along with 2-methyl imidazole as the catalyst.
The desired pattern of circuit line is provided on the substrate employing any of the well known copper plating techniques. A typical example is a subtractive copper plating technique such as that disclosed in Principles of Electronic Packaging by Seraphim, Laski & Li, McGraw-Hill, 1989, disclosure of which is incorporated herein by reference. Other metals include nickel and cobalt.
Since the present invention is especially concerned with plated features that are relatively close to each other, the spacing between adjacent lines is typically about 75 microns or less, a particular example being about 28 microns.
According to the present invention, the now circuitized substrate is treated with a swelling agent. The swelling agent is typically an organic swelling agent and is usually present as a composition in order along with an alkaline reacting substance. Examples of suitable organic swelling material include monohydric alcohols, di- and polyhydric alcohols, ethers, cyclic ethers, lactones, and pyrrolidones. Suitable alcohols include methanol, ethanol, isopropanol, alkanol amines, 1,2,3-propane triol, 1-hexanol, ethylene glycol, ethanol amine, 1,6-hexane diol, triethylene glycol, 2,2-dimethyl-1,3-propane diol, 1,2-propane diol, 1,5-pentane diol, 1,4-butane diol, 1,3-butane diol and 1,2-butane diol.
Suitable ethers include propylene glycol monomethyl ether, propylene glycol monopropyl ether and diethylene glycol monobutyl ether (butyl “carbitol”). A suitable pyrrolidone includes N-methyl pyrrolidone and a suitable lactone includes gamma-butyrolactone.
Preferred swelling agents
Konrad John Joseph
Papathomas Konstantinos I.
Wells Timothy Leroy
Wilson James Warren
Connolly Bove Lodge & Hutz
Fraley Lawrence R.
International Business Machines - Corporation
Talbot Brian K.
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