Compositions: coating or plastic – Coating or plastic compositions – Metal-depositing composition or substrate-sensitizing...
Reexamination Certificate
2000-01-18
2001-12-18
Klemanski, Helene (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Metal-depositing composition or substrate-sensitizing...
C106S001250
Reexamination Certificate
active
06331201
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to bismuth coating protection for copper-containing surfaces and is more particularly concerned with improved compositions and methods for providing protective bismuth metal platings on such surfaces.
The present invention constitutes a further development from the invention described in commonly assigned International Patent Application No. PCT/US95/03574, published under publication No. WO 95/25008 and incorporated herein by reference.
Briefly stated, the prior application describes compositions, and methods of use, for immersion plating bismuth metal onto copper-containing surfaces, such as copper surfaces of printed circuit boards (copper pads for receiving surface-mount electronic components, copper-plated through holes for receiving the leads of pin-in-hole type electronic components, etc.). Immersion plating is a well known process in which a dissolved metal is displaced out of solution by a more active (less noble) base metal from a workpiece surface that is contacted with a plating medium containing the dissolved metal. Displacement occurs due to a difference between the electropotentials of the plating metal and the base metal, with the displaced metal depositing on and coating the workpiece surface. The workpiece may be contacted with the plating medium by any suitable contacting method, such as dipping, spraying, or flood coating.
Immersion-plated bismuth metal coatings provide excellent protection of copper-containing surfaces against tarnishing, corrosion, and other agents that can impair surface quality. As a protectant for copper surfaces of printed circuit boards, for example, immersion-plated bismuth metal coatings have been found to constitute an effective means of preserving solderability prior to and during processing of the boards for circuit assembly. They also exhibit a high surface energy which promotes wetting during soldering, and a high degree of flatness which is desirable for mounting surface-mount electronic components. As contrasted with lead-tin protective coatings commonly used in the electronics industry, protective bismuth metal coatings are not subject to degradation by the formation of intermetallics with copper. Nor do they exhibit the problems of oxidation, volitization on heating, and poor solderability (especially poor hole-fill for wave soldering) that typify less widely used organic protective coatings.
The coating compositions of the prior application are acidic aqueous systems containing dissolved bismuth and halide, such as chloride or bromide. The bismuth is preferably provided as a constituent of a bismuth salt, and the halide as a constituent of the acid (i.e., a halogen acid). The acid solubilizes the bismuth salt, and the halide shifts the relative electropotentials of the bismuth and copper to enable the plating reaction to proceed.
In a preferred mode, the invention of the prior application also utilizes an iodide, such as an iodide salt or an organic iodide, as a complexing agent to promote adhesion of the bismuth metal coating to the surface being plated. The iodide may be provided as an ingredient of the plating bath, or as part of a pre-plating bath with which the surface is treated prior to immersion plating.
SUMMARY OF THE INVENTION
In accordance with the present invention, it has been discovered that effective adhesion of immersion-plated bismuth metal to copper-containing surfaces (including copper surfaces) can be achieved through the use of a sulfur-containing ligand—for example, a thiourea derivative or a thione compound—as a completing agent to promote bismuth adhesion. It has also been discovered that, relative to iodide as an adhesion-promoting agent, significant advantages may be obtained with a sulfur-containing ligand. These advantages include one or more of improved tarnish resistance of the bismuth metal plating (both during and after rinsing and drying), greater coating thickness of the plating, and lower operating temperature of the plating bath.
Thus, in one of its principal aspects, the present invention provides a coating composition that is effective for immersion plating a coating of bismuth metal onto a copper-containing surface, and that comprises a moderately to strongly acidic mixture containing dissolved bismuth, halide, and a sulfur-containing ligand as a complexing agent.
In another of its principal aspects, the present invention provides a moderately to strongly acidic coating composition that is effective for immersion plating a coating of bismuth metal onto a copper-containing surface, and that comprises a salt of bismuth solubilized in a halogen acid, and a sulfur-containing ligand as a complexing agent.
According to still further aspects of the invention, methods of protecting a copper-containing surface are provided, in which the surface is contacted with a composition according to either of the aspects described above, to form a bismuth metal coating on the surface.
Yet another aspect of the invention relates to a method that is broadly applicable to immersion bismuth plating processes to enhance the thickness of the bismuth coating. As will be explained hereinafter, it has been discovered that by pre-cleaning of the surface to be plated with a nitric acid based pre-cleaner, it is possible to significantly increase the immersion bismuth coating thickness relative to processes that do not involve such a pre-cleaning agent.
Still another aspect of the present invention relates to a generally applicable method that improves the tarnish resistance of a bismuth metal coating deposited from an acidic plating bath. As will be explained hereinafter, it has been found that by contacting the plated bismuth coating with an alkaline composition that neutralizes acid residues on the coating, the susceptibility of the coating to tarnishing is greatly reduced.
The aforementioned and other aspects of the present invention will be more fully appreciated from the following detailed description.
DETAILED DESCRIPTION OF THE INVENTION
Immersion bismuth plating compositions in accordance with the present invention are moderately to strongly acidic systems containing, as principal ingredients, dissolved bismuth, halide, sulfur-containing ligand, and water (preferably deionized). The bismuth is conveniently provided in the form of a bismuth salt, such as bismuth trioxide, bismuth chloride, etc. The bismuth salt is dissolved by the acid to form bismuth ions that can be deposited on a copper-containing surface. The preferred concentration of bismuth salt is from about 0.5 to about 20 grams per liter of the plating composition. The halide may be provided as a constituent of the acid, hydrochloric acid being preferred. Another suitable halogen acid is hydrobromic acid. The halide is present in an amount effective to cause the bismuth to plate substantially uniformly over the entire surface to be coated, and is preferably present in an amount substantially in excess of the stoichiometric amount of bismuth. The use of a non-halogen acid, such as sulfuric acid, would require a suitable supplemental source of halide, as will be appreciated by those skilled in the art.
As previously stated, the plating compositions of the invention are moderately to strongly acidic. The pH is most preferably less than 1, but the invention may be practiced with compositions of higher pH, although the plating rate will probably be reduced. The pH should ordinarily be less than about 3. A pH exceeding 4 is not preferred because, at such levels, satisfactory bismuth plating rates and coating thicknesses may not be achieved. The preferred low pH formulations in accordance with the invention are characteristically stable and therefore do not require the addition of stabilizers, such as ammonium ion and the like.
The sulfur-containing ligand is preferably constituted by a thiourea derivative, a thione compound, or a combination thereof. Thiourea per se is not preferred due to toxicity concerns. The sulfur-containing ligand is present in the plating composition in an amount effective to pro
McGrath Peter T.
Owei Abayomi I.
Fry's Metals, Inc.
Klemanski Helene
Miles & Stockbridge P.C.
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