Compositions – Etching or brightening compositions – Inorganic acid containing
Reexamination Certificate
1997-10-01
2001-07-10
Talbot, Brian K. (Department: 1762)
Compositions
Etching or brightening compositions
Inorganic acid containing
Reexamination Certificate
active
06258294
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the removal of solder and tin films from printed circuit boards. More particularly, the present invention relates to a new and improved method and composition for stripping solder and tin films and the underlying tin-copper alloy from the copper substrate of a printed circuit board while providing the surrounding laminate with enhanced insulation resistance.
BACKGROUND OF THE INVENTION
A typical printed circuit board has a copper conductor pattern on an insulating support. Tin of solder is applied onto the copper substrate, typically by electroplating. A standard solder film is nominally 0.0003 inches thick and a standard tin film has approximately the same or slightly less thickness. After the tin/solder film is applied to the board, a thin film of copper-tin alloy forms between the copper and the film. This thin film of copper-tin alloy is typically 0.000002 to 0.000004 inches thick. The copper-tin alloy thickness increases in thickness over time.
As used in the specification and claims, the word solder includes the various low melting point alloys and elements used for electrical soldered connections and for copper etching masks or resists. The majority of such coatings comprise various tin-lead alloys, but can also include alloys containing silver, bismuth, cadmium, indium, and other metals. Such films are produced using various methods, including chemical plating, chemical deposition, chemical displacement and immersion in a melt.
In the manufacturing process, the tin or solder film is stripped from the copper substrate. Generally, two types of compositions have been generally used in the past for tin and solder stripping. The most widely used prior compositions were based on an acid solution containing hydrogen peroxide and fluoride. In recent years, formulations based on nitric acid solutions containing ferric ions have become widely used commercially.
The prior art peroxide-fluoride solutions are undesirable because there is an exothermic reaction during stripping which heats the solution to a temperature which decomposes the unstable peroxide and makes the solution unusable. Hence, the solution requires cooling during use. Also, the peroxide-fluoride solutions are slower in operation than the nitric acid solutions.
Nitric acid solutions eliminate the problems associated with the peroxide-fluoride solutions. Early nitric acid based solutions comprised a two-solution system. The first acidic solution contained nitric acid to strip the tin or solder. The second acidic solution contained ferric chloride, ammonium persulfate, a mixture of sulfuric acid and hydrogen peroxide, or an acidic peroxide-fluoride mixture to dissolve the tin-copper alloy.
Further, the basic compositions and methods for single bath and spray nitric acid/ferric stripping are now well described in prior art patents. For example, the composition of U.S. Pat. No. 4,713,144 utilizes a combination of nitric, ferric, and sulfamic acid which quickly strips both the tin/solder film and the tin-copper alloy while leaving the copper surface bright, shiny and uniform.
Printed circuits in the telecommunications industry must have extremely high resistance insulating materials between the printed circuits to prevent the electrical signals from crossing over to adjoining circuits. The cross-over of electrical signals to adjoining circuits which can result from a low insulation resistance can cause, among other things, the ability to hear another person's phone conversation when you are using a phone.
Incomplete copper stripping or etching as well as the presence of a wide variety of metal species or other contaminants left on the printed circuit board after tin or solder stripping can result in a relatively low insulation resistance. Current nitric acid/ferric solutions are not formulated to maximize the insulation resistance between printed circuit traces.
Accordingly, it is an object of the present invention to provide a new and improved stripping composition and method of stripping tin or solder which provides the printed circuit board with enhanced insulation resistance.
This and other objects, advantages, features and results of the present invention will more fully appear in the course of the following description.
SUMMARY OF THE INVENTION
It is a principal object of the present invention to provide a metal-dissolving liquid for stripping tin or solder and the underlying tin-copper alloy from the copper substrate of a printed circuit board.
It is another object of the present invention to provide a stripping composition and method of stripping tin or solder which provides the printed circuit board with enhanced insulation resistance.
It is still another object of the present invention to provide an aqueous solution that can be directly sprayed onto a circuit board to strip tin or solder and the underlying tin-copper alloy from the copper substrate of a printed circuit board.
In brief, the composition of the present invention for removing tin and solder and the underlying tin-copper alloy from the copper substrate of a printed circuit board includes an aqueous solution of nitric acid in an amount sufficient to dissolve solder, ferric ion in an amount sufficient to dissolve tin-copper alloy, and halide ions in an amount to significantly improve the insulation resistance of the printed circuit board. More specifically, the liquid composition of the present invention consists essentially of an aqueous solution of 5 to 50 percent by volume of 69% by weight nitric acid aqueous solution, 1 to 50 percent by volume of 45% by weight ferric nitrate, 0.1% by weight to saturation of halide ions, and the balance water.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The composition of the present invention is an aqueous solution with nitric acid, ferric nitrate, and a source of halide ions as essential ingredients.
It is known that nitric acid will dissolve tin or solder. It is also known that ferric nitrate will dissolve copper and tin-copper alloys. Further, it is known that acidic peroxide-fluoride tin and/or solder strippers provide improved insulation resistance over nitric acid based tin and/or solder strippers. It is not known that the act of combining nitric acid, ferric nitrate, and halide ions in a single formulation will result in the creation of a nitric acid based stripping solution capable of providing enhanced insulation resistance to printed circuits. The composition of the invention in the ranges set forth in the following paragraphs produces the desired reactions in the desired sequence to achieve the desired stripping result.
The metal-dissolving liquid consists essentially of an aqueous solution of nitric acid in an amount sufficient to dissolve tin and/or solder, ferric nitrate in an amount sufficient to dissolve tin-copper alloy, and a source of halide ions in an amount sufficient to remove metal and other contaminants from the circuit board thereby substantially improving the printed circuits insulation resistance.
For practical purposes, the concentration of nitric acid must be limited on the high end to avoid possible damage to printed circuit board substrates and to the equipment in which the tin or solder stripping solutions are used. It has been found that there is no practical need to exceed concentrations of nitric acid higher than about 50 percent by volume of 69% by weight nitric acid. Concentrations considerably less than 40 percent by volume yield useful tin/solder stripping solutions.
The concentration of nitric acid is limited on the low end by the time that may be allotted for the solder stripping operation, and by the total amount of solder a given volume of solder stripping solution may be expected to strip before it becomes spent. If neither of these considerations is of importance, the nitric acid content can be as low as 5 percent by volume of 69% by weight nitric acid or less and still effectively strip tin and/or solder from copper.
Nevertheless, the concentration of nitric acid is an important factor in determinin
Fakler John T.
Johnson, II Todd
Morton International Inc.
Talbot Brian K.
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