Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly alloy coating
Reexamination Certificate
2000-10-19
2002-08-20
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly alloy coating
C205S244000, C205S254000
Reexamination Certificate
active
06436269
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the electrodeposition of tin-zinc alloys. The invention also relates to a plating bath for depositing tin-zinc alloy coatings on various substrates.
BACKGROUND OF THE INVENTION
The electrodeposition of tin and various tin alloys has been widely employed to protect steel and similar metal materials from corrosion, or to improve the solderability of the metals.
Tin-zinc alloy plating baths have been described in the prior art. The use of citric acid or salts of citric acid, and ammonium salts in the electroplating baths is known. In U.S. Pat. No. 4,163,700, it has been suggested even when tin-zinc electroplating baths containing citric acid are used, there is still a disadvantage in that when a metallic ion concentration in the bath gradually increases as the charged current is increased, an insoluble substance (considered to be a stannate or other metallic salt) is formed on the anode of tin or tin alloy and then released from the cathode to be plated which gives an undesirable effect on the plated surface. Accordingly, in the '700 patent, the patentee suggests the tin or tin alloy electroplating baths which contain citric acid or its salt and an ammonium salt can be improved by adding to the bath, at least one saturated hydroxy carboxylic acid or its salt, other than citric acid, and/or at least one saturated dibasic carboxylic acid or its salt.
U.S. Pat. No. 4,168,223 also describes an electroplating bath for depositing tin or a tin alloy such as a tin-zinc alloy with satisfactory brightness. The electroplating bath comprises a bath having a pH value ranging from 4 to 8 and containing citric acid or its salts, an ammonium salt, and a water-soluble polymer as a brightener. The bath may further comprise an aldehyde compound as a co-brightener. The water-soluble polymers useful as brighteners in these plating baths include polyoxyethylenes, derivatives thereof, or the reaction products of an epoxy compound with ethylene glycol, propylene glycol or glycerine.
U.S. Pat. No. 5,618,402 describes a tin-zinc alloy electroplating bath which comprises a water-soluble stannous salt, a water-soluble zinc salt, and amphoteric surfactant, and water. The amphoteric surfactant useable in the plating baths include those of imidazoline, betaine, alanine, glycine and amide types. The baths also may contain hydroxy carboxylic acids such as citric acid or gluconic acid.
Zinc alloy plating baths containing a quaternary ammonium polymer are described in U.S. Pat. No. 5,405,523. The electroplating baths described in the '523 patent comprise zinc ions, alloy metal ions of a metal of the first transition series of the Periodic Table and a quaternary ammonium polymer as a brightener. The electroplating baths can be either alkaline baths having a pH in the range of from about 9 to 13 or acid baths having a pH in the range of from 3 to 7. The quaternary ammonium polymers useful in the plating baths include a ureylene quaternary ammonium polymer, an iminoureylene quaternary ammonium polymer or a thioureylene quaternary ammonium polymer.
SUMMARY OF THE INVENTION
The present invention relates to an aqueous plating bath for electrodeposition of tin-zinc alloys comprising at least one bath-soluble stannous salt, at least one bath soluble zinc salt, and a quaternary ammonium polymer selected from a ureylene quaternary ammonium polymer, an iminoureylene quaternary ammonium polymer or a thioureylene quaternary ammonium polymer. The plating baths also may contain one or more of the following additives: hydroxy polycarboxylic acids or salts thereof such as citric acid; ammonium salts; conducting salts; aromatic carbonyl-containing compounds; polymers of aliphatic amines such as a poly(alkyleneimine); and hydroxyalkyl substituted diamines as metal complexing agents. The plating baths of this invention deposit a bright and level deposit, and they can be adapted to provide plated alloys having high tin concentration over a wide current density range.
DETAILED DESCRIPTION OF THE INVENTION
The aqueous plating baths of the present invention comprise an aqueous composition comprising stannous ions in the form of at least one bath-soluble stannous salt, zinc ions in the form of at least one bath soluble zinc salt, and a quaternary ammonium polymer selected from a ureylene quaternary ammonium polymer, an iminoureylene quaternary ammonium polymer or a thioureylene quaternary ammonium polymer. In one embodiment, the baths also contain at least one hydroxy polycarboxylic acid such as citric acid. If the acidity of the bath falls below the desired operating range of from about 4 to about 8, or from 5 to about 7, the pH can be increased by the addition of ammonium hydroxide or an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide.
The plating bath of the present invention generally will contain stannous ion at concentrations of from about 1 g/l to about 100 g/l, and zinc ions at a concentration of from about 0.2 to about 80 g/l. In another embodiment, the plating baths will contain from about 5 g/l to about 40 g/l of stannous ions and from about 5 to about 50 g/l of zinc ions. In yet another embodiment, the plating bath may contain from about 10 to about 20 g/l of stannous ion and from about 10 to about 40 g/l of zinc ions. Throughout this written description of the invention, the range and ratio limits may be combined.
The stannous ion may be in the form of a soluble salt such as stannous sulfate, stannous chloride, stannous fluoride, stannous sulfamate, stannous acetate, stannous oxide, stannous methane sulfonate etc. The zinc ion may be present in the bath in the form of a soluble salt such as zinc sulfate, zinc chloride, stannous fluoride, zinc fluoroborate, zinc sulfamate, zinc acetate, etc. Mixtures of the stannous salts and/or zinc salts may be utilized to provide the desired concentration of tin and zinc. In one embodiment, the stannous ions and the zinc ions are both present as the chloride salts.
The relative and total amounts of stannous ions and zinc ions in the electroplating baths of the invention will influence the tin-zinc alloy distribution, and the appearance of the deposit. When a plating bath is too low in total metal (e.g., less than 5 g/l) gas streaking in the high current density range (greater than 40 ASF to 20 ASF) and the mid current density range (20 ASF to 8 ASF) will result. The ratio of metals also effects the alloy content. For example, other things being equal, a bath containing a greater amount of tin will deposit an alloy containing higher tin than a plating bath containing a lower concentration of tin.
The plating baths of the present invention can be utilized to deposit tin-zinc alloys comprise about 50%w up to about 95%w or more of tin and about 5%w up to about 50%w of zinc. Alloys containing about 70-80% of tin and 20 to 30% of zinc are useful in corrosion prevention (for example, of fasteners) and alloys containing about 90% tin and about 10% zinc are useful in electronic applications where soldering is required.
The electroplating baths of the present invention also contain at least one quaternary ammonium polymer which may be a ureylene quaternary ammonium polymer, an iminoureylene quaternary ammonium polymer or a thioureylene quaternary ammonium polymer. The amount of the quaternary ammonium polymer included in the tin-zinc alloy plating baths is an amount sufficient to provide desired improvements in the deposited tin-zinc alloy such as reduced burning of the high current density deposits, and improved grain refinement. When used with brightener compositions such as aromatic aldehydes and ketones (described more fully below), improved brightness is obtained. Generally, the tin-zinc alloy plating baths will contain from about 0.2 to about 2.5 g/l of the quaternary ammonium polymer. In another embodiment the bath will contain from about 0.5 to about 2.0 g/l of the quaternary ammonium polymer.
Quaternary ammonium polymers which are useful in the present invention may be prepared by condensation copolymerization
Capper Lee Desmond
Opaskar Vincent C.
Atotech Deutschland GmbH
Leader William T.
Valentine Donald R.
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