Stannous solutions containing hydroxy carboxylic acid ions their preparation and their use in plating tin on conductive surfaces particularly on aluminum

Claim 2

An aqueous solution according to claim 2 wherein said inorganic acid anions are chloride ions and wherein said solution further comprises from about 0.5 to 10 grams per liter of a compound selected from the group consisting of hydroquinone, o-cresotinic acid, beta-naphthol, ethylene gylcol, propylene glycol, water-soluble polyalkylene glycols and formamides.

Claim 3

An aqueous solution according to claim 2 which further comprises from 2 to 10 grams per liter of dimethyl formamide.

Claim 4

An aqueous solution according to claim 2 which further comprises from about 0.5 to 10 grams per liter of boric acid.

Claim 5

An aqueous solution according to claim 1 wherein said sugar acid is a monocarboxylic polyhydroxy aliphatic acid of 5 to 7 carbon atoms and 4 to 6 hydroxyl groups, and is present in an amount between about 2.2 and about 3 moles per mole of tin in the solution; wherein said base is selected from the group consisting of ammonia and alkylolamines of 2 to 8 carbon atoms and is present in an amount sufficient to give a pH between about 6.5 and 7.5.

Claim 6

A process for preparing a tin electrolyte solution which comprises dissolving in water (a) about 50 to 100 grams per liter of a tin salt selected from the group consisting of stannous chloride, stannous sulfate, stannous fluoborate and stannous fluoride, (b) 1.8 to 3 mole equivalents of a chelating acid selected from the group consisting of glyceric acid and C5 to C7 hydroxy carboxylic aliphatic acids containing 4 to 6 hydroxyl groups and 1 to 2 carboxyl groups per molecule per mole of stannous salt, and (c) sufficient base selected from the group consisting of ammonia and alkylolamines of 2 to 8 carbon atoms to adjust the pH of the solution to between about 6 and 8.

Claim 7

A process according to claim 7 wherein said tin salt is stannous chloride, said sugar acid is gluconic acid and said base is ammonia.

Claim 8

A process for preparing a tin electrolyte solution which comprises dissolving stannous chloride or sulfate in water, mixing the resulting stannous salt solution with ammonium hydroxide to precipitate stannous hydroxide from solution, mechanically separating the precipitated stannous hydroxide from the solution, redissolving the separated stannous hydroxide in an aqueous solution containing about 2.0 to 2.6 moles gluconic acid per mole of stannous hydroxide, and adding ammonium hydroxide to the resulting stannous gluconate solution in an amount sufficient to produce a substantially neutral solution characterized by a pH between about 6 and 8.

Claim 9

A process according to claim 9 wherein the substantially neutral stannous gluconate solution is concentrated by evaporation.

Claim 10

A process according to claim 9 wherein light colored stannous hydroxide is formed by adding the stannous salt solution to an agitated aqueous ammonia solution which contains sufficient ammonium hydroxide to convert the stannous salt to a precipitate of stannous hydroxide and give a pH between about 8.5 and 10 in the supernatant liquid.

Claim 11

A process according to claim 9 wherein about 2 to 5 moles ethylene glycol is added to the stannous salt solution prior to mixing it with ammonia.

Claim 12

A process for plating tin on an aluminum or aluminum alloy substrate which comprises immersing a tin anode and said substrate as a cathode in a plating solution which is essentially free from alkali metals and comprises stannous ions in a concentration of from about 10 to 70 grams tin per liter and, as an essentially sole chelating agent, a chelating sugar acid in a concentration equivalent of at least 1.8 mole equivalents of acid per mole of tin and a sufficient amount of ammonium hydroxide to give a pH between about 6 and 8.

Claim 13

A process according to claim 13 wherein said sugar acid is a C5 to C7 monocarboxylic polyhydroxy aliphatic acid and electric current is set up between it and said anode to give a cathode current density of between about 10 and 100 amperes per square foot.

Claim 14

A process according to claim 14 wherein said plating solution comprises stannous ions in a concentration of from about 30 to 60 grams tin per liter and ions of gluconic acid in a concentration equivalent to between about 2.2 and 4 mole equivalents of acid per mole of tin.

Claim 15

A process according to claim 15 wherein an initial thin tin layer is deposited on said aluminum substrate from said plating solution by immersion and galvanic current action and further tin is deposited on said initial layer electrolytically in a system wherein said immersed aluminum substrate is connected as an anode and tin metal is connected as a cathode and current is passed through the system at a cathode current density of between about 10 and 30 amperes per square foot.

Claim 16

A process according to claim 15 wherein the tin anode and the aluminum cathode have a ratio of surface areas of between about 2:1 and 3:1.

Claim 17

A process according to claim 15 wherein the plating solution further comprises small effective amounts of ammonium chloride as a conductivity salt and a formamide as a brightener.

Claim 18

A process according to claim 15 wherein the substrate is an aluminum wire and the current density is between about 40 and 100 amperes per square foot.