Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
2000-08-09
2003-05-27
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C427S305000, C205S126000, C205S187000
Reexamination Certificate
active
06569491
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to dielectric coatings for circuit boards which provide adequate adhesion of copper plate. More particularly, the present invention relates to dielectric materials blended with polybutadiene or polyisoprene which are useful in microvia high density printed circuit boards.
Over the years with changes in sophistication of electrical equipment, manufacturers of printed circuit boards have increasingly been forced into producing high density circuits into smaller packages. In some of the most recent advances, high interconnect density on printed circuit boards is provided by utilizing multi-layer circuits which are separated by a dielectric material. These circuits are interconnected by a high density of holes or vias in the dielectric forming pads, sometimes as much as 100-200 pads per square inch. The formation of such pads may be accomplished by coating an existing circuit board surface having copper leads with a thin layer of dielectric material (i.e., 1-3 thousandths of an inch thick). Thereafter, holes (vias) are drilled with a laser or the like through the dielectric to the copper leads. In order to form a second circuit layer and/or via pads, an electroless copper deposition process is used to coat the dielectric material. This also enters the vias and attaches to the underlying copper leads. Thereafter, an electroplated copper is deposited onto the electroless copper and the printed circuit board can be imaged and etched to provide a new circuit board layer.
These vias may also be produced using photo imaging dielectrics. In this process, rather than drilling holes for via pads with a laser, the holes are imaged in the photo imageable dielectric and developed leaving the vias for forming via pads. Again, electroless copper is plated, followed by electroplated copper, which may be etched to form a circuit if desired. Photoimageable microvia technology is discussed in the article entitled “Photo Defined Vias Enable High Density Design”, McDermott, Electronic Packaging and Production, May 1996, p. 45.
While such processes are known, the present dielectric resins used have been found less than adequate for providing adhesion between the electroless copper plates and the dielectric material which is necessary in these microvia applications. Typical known dielectrics are thermoset epoxies when laser drilling of vias is used. Photo imageable dielectrics may be a combination of thermosettable epoxy and photo setting epoxy acrylates.
Electroless plating onto plastic materials has been known in the art for many years. A typical process includes oxidizing of acrylonitrile butadiene styrene (ABS) plastic moldings. This is typically accomplished in a moderately hot chrome sulfuric acid solution. Typically, the polymers used in the ABS blend are acrylonitrile-butadiene-styrene, styrene-butadiene-rubber blended with acrylonitrile-styrene plastic. It is believed that the oxidant attacks the carbon-carbon double bond of the styrene butadiene based rubber to roughen the surface, which allows the electroless copper to adhere to the surface. The acrylonitrile copolymers are very polar. As stated above, circuit board dielectrics are generally either epoxy compositions or epoxy acrylate based compositions which are of relatively low polarity. In attempts to make these epoxy dielectric resins platable with electroless copper, it has been taught that nitrile rubbers were used in order to render these dielectric materials platable.
An example of the use of the nitrile rubber and its incompatibility with the dielectrics used in the printed circuits applications is U.S. Pat. No. 4,628,022, wherein dicarboxy terminated acrylonitrile-butadiene copolymers (nitrile rubbers) are reacted with epoxy and acrylic acid in order to make this polar material compatible with the low polarity epoxy and acrylic resins used as dielectric materials for printed circuit boards. Such reaction products mixed in the formula make the blend platable after oxidative treatment in a conventional 3-step permanganate process.
U.S. Pat. No. 5,545,510, issued Aug. 13, 1996 to Kukanskis (Mac Dermid, Inc.), discloses compositions containing at least one elastomer selected from the group consisting of copolymers of butadiene with acrylonitrile, carboxy terminated butadiene acrylonitrile and epoxy terminated butadiene acrylonitrile copolymers. The adhesion values reported are 0.5, 3.0 and 4.8 lbs/inch.
Nitrile rubbers separate in phases and were found to be incompatible. Therefore, it has been desirable in the art to provide an improved dielectric material which has superior adhesion properties but which is usable within regularly known epoxy dielectric compositions. These dielectric compositions preferably may be cured to a hard consistency and have a high glass transition temperature, low dielectric constant and low dissipation factor. Additionally, it is preferable to provide a dielectric material which is highly adherent to electroless copper in conventional oxidative processes which provides good adhesion of the plated metal.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a dielectric coating for a circuit board for providing good adherent characteristics for electroless copper. The dielectric coating comprises an aqueous developing epoxy composition, including an amount of butadiene or isoprene modifying agent effective for promoting adhesion of an electroless copper coating. The additives of the present invention must be soluble in the particular epoxy dielectric material utilized. Thus, because of the negative effect of high polarity compositions such as nitrites, this composition is a nitrile-free butadiene or isoprene-type composition.
Further understanding of the present invention will be had by reference to the detailed description of the preferred embodiments set forth below when taken in conjunction with the examples and claims appended hereto.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, there is provided a dielectric coating for a circuit board for providing good adherent characteristics for electroless copper. In its broad aspects, the present invention comprises an epoxy dielectric composition including an amount of a nitrile-free butadiene or isoprene agent. This agent is effective for promoting adhesion of an electroless copper coating. The butadiene or isoprene agent is solubilized in the epoxy dielectric. In accordance with the method aspects of the present invention, a method of producing a multi-layered printed circuit board is provided, which comprises the steps of first providing a circuit board substrate which includes a surface which has circuitry printed thereon. Next, the surface of the board is coated with an effective amount of a solution of a dielectric material comprising an epoxy resin, and an effective amount of a butadiene or isoprene polymer adhesion promoter. Thereafter, the coating is freed from solvent by heating and baking, and textured by permanganate oxidation, and an electroless copper layer is plated onto the coating, followed by electroplated copper.
The board is then imaged by dry film imaging, primary image and unwanted copper is removed to leave vias and circuits. The dielectric material typically includes epoxy resin-type dielectrics, which are known in the art, with a mixture of from about 2% to about 15%, and preferably 4% to 8% of an adhesion promoter. Typically, such epoxy dielectrics include epoxy resins, epoxy curing agents, rheological additives and mineral fillers. These dielectrics are typically applied in 60-85% by weight solutions in glycol ethers or similar solvents. The nitrile-free butadiene or isoprene polymer adhesion promoters are critical to the present invention. These adhesion promoters, without nitrile groups in the molecule, are polarity compatible with the relatively nonpolar epoxy compositions. Thus, they are soluble in the same types of systems as the epoxies. Thus, the absence of nitrile groups is essential to the pr
Enthone Inc.
Senniger Powers Leavitt & Roedel
Talbot Brian K.
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