Compositions: coating or plastic – Coating or plastic compositions – Metal-depositing composition or substrate-sensitizing...
Reexamination Certificate
2000-01-19
2003-03-04
Klemanski, Helene (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Metal-depositing composition or substrate-sensitizing...
C427S436000
Reexamination Certificate
active
06527840
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a silver alloy plating bath and a method of forming a silver alloy film by means of the silver alloy plating bath, and, more particularly, to a silver alloy plating bath and a method of forming a silver alloy film by means of the silver alloy plating bath, which plating bath does not contain lead, so that it removes a fear of environmental pollution and is also excellent in working health, in which method printed wiring board are not immersed in a molten solder vessel and so will not be injured due to heat, which silver alloy film is hard to oxidize and is excellent in solderability to thereby eliminate defective contact between printed wiring board and parts being mounted thereon, and which silver alloy film obtained is uniform and free of dispersion to be suitable for manufacturing printed wiring board suited to high density mounting.
Generally, various surface treatment methods are used to provide a protective metallic film on conductor surfaces of printed wiring board in order to prevent oxidation of the conductor surfaces of printed wiring board and degradation of the solderability of the conductor surfaces due to corrosion while electronic parts are packaged on the printed wiring board. These surface treatment methods include a solder leveler treatment. The solder leveler treatment is the most common surface treatment method for printed wiring board, and comprises coating a solder resist on portions of a printed wiring board except soldered lands thereof, and then immersing the printed circuit in a solder vessel, which receives therein a molten tin/lead alloy, to adhere a solder to the land portions not covered by the solder resist. Because an amount of solder adhered is too much as it is, the solder is made in the shape of icicle, and forms a. bridge, hot blast is used to blow off a surplus amount of solder upon withdrawal of the printed wiring board so as to leave a suitable amount of solder on-the printed wiring board.
However, because hot blast is used to blow off a surplus amount of solder, pressure of the hot blast causes the solder to be dispersed in thickness. So, problems have been caused such that the resulting printed wiring board having been subjected to surface treatment is not suited to high density mounting, heat will injure the printed wiring board when the printed wiring board is immersed in a molten solder vessel, and lead contained in solder involves harmfulness which will possibly cause environmental pollution and is problematic in working health to be difficult in future use.
Therefore, modification to other treatment methods than the solder leveler treatment has been demanded, and such other treatment methods include, for example, a gold plating treatment. Although such gold plating treatment is advantageous for high density mounting of printed wiring board, the manufacturing cost is problematically high. Accordingly, a silver plating treatment is sometimes performed which is higher in electric conductivity and more advantageous in cost than gold. However, the silver film obtained is unstable and liable to oxidize, and the oxidized silver film is inferior in solderability, so that there is caused a problem of possible defective contacts between printed wiring board and parts being mounted thereon.
The inventors of this application has performed earnest investigation so as to solve the above problems, found that the above problems are solved by depositing on a copper surface silver together with a more noble metal, for example, at least one metal selected from a group consisting of palladium, platinum, gold and rhodium, to form a silver alloy film, and devised the invention.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a silver alloy plating bath and a method of forming a silver alloy film by means of the silver alloy plating bath, which plating bath does not contain lead, so that it removes a fear of environmental pollution and is also excellent in working health, in which method printed wiring board are not immersed in a molten solder vessel and so will not be injured due to heat, which silver alloy film is hard to oxidize and is excellent in solderability to thereby eliminate defective contacts between printed wiring board and parts being mounted thereon, and which silver alloy film obtained is free of dispersion to be suited for manufacturing printed wiring board.
The silver alloy plating bath according to the invention includes at least silver ions and at least one metal ion selected from a group consisting of palladium ions, platinum ions, gold ions and rhodium ions, the silver ions and the at least one metal ion having a concentration of 0.001 to 0.01 mol/l, and a ratio (mol ratio) of the at least one metal to the silver ion being 1:0.01 to 0.1. The method of forming a silver alloy film, according to the invention, has a feature in that the silver alloy plating bath of the invention is used to form a silver alloy film on surfaces of a more base metal than silver by means of displacement plating.
A silver ion source for the silver ions used in the silver alloy plating bath according to the invention suffices to be a water soluble silver salt, examples of which can be silver nitrate, silver acetate, silver sulfate, silver lactate, silver benzoate, silver nitrite, silver carbonate, silver thiocyanate, silver phosphate, silver citrate and the like. Preferably, the silver ions source includes silver nitrate and silver sulfate.
An ion source for palladium ions, platinum ions, gold ions or rhodium ions, used in the silver alloy plating bath according to the invention, suffices to be a water soluble salt, examples of which can be a water soluble palladium salt such as palladium(II) nitrate, palladium(II) sulfate, palladium(II) acetate, tetraamminepalladium(II) nitrate and the like, a water soluble platinum salt such as tetraammine platinum(II) nitrate, diaminedinitroplatinum(II), bisethanol ammoniumplatinum(II) hydroxide and the like, a water soluble gold salt such as potassium dicyanoaurate(I), potassium tetracyanoaurate(III), gold(I)cyanide and the like, and a water soluble rhodium salt such as rhodium(III) nitrate, rhodium(III) acetate and the like. Preferably, the ion source includes palladium(II) nitrate, rhodium(III) nitrate, tetraammineplatinum(II) nitrate and potassium tetracyanoaurate(III).
The silver ion and the at least one metal ion selected from a group consisting of palladium ions, platinum ions, gold ions and rhodium ions, in the silver alloy plating bath according to the invention, have a concentration of 0.001 to 0.01 mol/l. The reason for this is that when the concentration exceeds 0.01 mol/l, the plating bath becomes unstable, and when the concentration is less than 0.001 mol/l, silver and palladium, platinum, gold or rhodium become hard to deposit and the silver alloy film becomes hard to form. The concentration is preferably 0.003 to 0.009 mol/l and is most preferably 0.005 to 0.007 mol/l.
Preferred silver plating baths comprise at least silver ions and at least one metal ion selected from a group consisting of palladium ions, platinum ions, gold ions and rhodium ions, wherein the silver ions and the at least one metal ion have a concentration of 0.003 to 0.009 mol/l, and a mole ratio of the at least one metal ion to the silver ions is 1:0.3 to 0.09. Additional preferred silver plating baths comprise at least silver ions and at least one metal ion selected from a group consisting of palladium ions, platinum ions, gold ions and rhodium ions, wherein the silver ions and the at least one metal ion have a concentration of 0.005 to 0.007 mol/l, and a mole ratio of the at least one metal ion to the silver ions is 1:0.05 to 0.07.
A ratio (mol ratio) of at least one metal ion selected from a group consisting of palladium ions, platinum ions, gold ions and rhodium ions to silver ions in the silver alloy plating bath according to the invention is 1:0.01 to 0.1. The reason for this is that, when the ratio (mol ratio) of the at least one metal ion to th
Hinomiya Koji
Igarashi Kazuo
Kanda Takashi
Corless Peter F.
Edwards & Angell LLP
Frickey Darryl P.
Klemanski Helene
Shipley Company L.L.C.
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