CYANIDE-BASED SILVER ALLOY ELECTROPLATING SOLUTION

Abstract

The present invention provides a cyanide-based silver alloy electroplating solution characterized by containing 10 to 100 g/L of silver cyanide complex in terms of silver, 5 to 300 g/L of electroconductive salt, 0.1 to 10 g/L of germanium compound in terms of germanium, and 1 to 100 g/L of a coordinating polymer additive.

Claims

1. A cyanide-based silver alloy electroplating solution containing 10 to 100 g/L of a silver cyanide complex in terms of silver, 5 to 300 g/L of an electroconductive salt, 0.1 to 10 g/L of a germanium compound in terms of germanium, and 1 to 100 g/L of a coordinating polymer additive.

2. The cyanide-based silver alloy electroplating solution according to claim 1, wherein the electroconductive salt contains at least one selected from among a cyanogen salt, a phosphate, a pyrophosphate, a nitrate, a citrate, a tartrate, a sulfate, and boric acid and a salt thereof.

3. The cyanide-based silver alloy electroplating solution according to claim 1, wherein the germanium compound contains at least one selected from among germanium dioxide, germanium halide, tetraalkoxy germanium, germanium sulfide, and germanic acid and a salt thereof.

4. The cyanide-based silver alloy electroplating solution according to claim 1, wherein the coordinating polymer additive is at least one selected from among polyacrylic acid, polyethyleneimine, and a copolymer containing them in a structure thereof.

Description

DESCRIPTION OF EMBODIMENTS

[0022] A silver electroplating solution according to the present invention contains a silver cyanide complex as a silver salt, an electroconductive salt, a germanium compound, and a coordinating polymer additive. Hereinbelow, each of the components of the silver electroplating solution according to the present invention will be described.

Silver Cyanide Complex

[0023] The cyanide-based silver alloy electroplating solution according to the present invention can use a known silver cyanide complex as a silver source without limitation. Examples of the silver cyanide complex include silver cyanide, silver potassium cyanide, and silver sodium cyanide.

[0024] The concentration of the silver cyanide complex is 10 to 100 g/L, preferably 20 to 70 g/L as a silver ion concentration. If the silver ion concentration is less than 10 g/L, there is a case where deposition efficiency reduces and a resulting silver coating cannot have a desired thickness. On the other hand, if the silver ion concentration exceeds 100 g/L, the amount of the silver salt to be lost by taking out of the plating solution by an object to be plated is large, which is economically disadvantageous.

Electroconductive Salt

[0025] The kind of the electroconductive salt contained in the cyanide-based silver alloy electroplating solution according to the present invention is not particularly limited as long as it has electric conductivity in an aqueous solution. The electroconductive salt preferably contains at least one selected from among a cyanogen salt, a phosphate, a nitrate, a citrate, a tartrate, a sulfate, and boric acid and a salt thereof to industrially stably use the electroconductive salt and economically produce the plating solution. In addition, a soluble organic acid salt is also preferred. These may be used singly or in combination of two or more of them. Examples of the cyanogen salt include potassium cyanide and sodium cyanide. Examples of the phosphate include potassium phosphate, sodium phosphate, and ammonium phosphate. Examples of the pyrophosphate include potassium pyrophosphate, sodium pyrophosphate, and ammonium pyrophosphate. Examples of the nitrate include potassium nitrate, sodium nitrate, and ammonium nitrate. Examples of the citrate include potassium citrate, sodium citrate, and ammonium citrate. Examples of the tartaric acid include potassium tartrate, sodium tartrate, and sodium potassium tartrate. Examples of the sulfate include potassium sulfate, sodium sulfate, and ammonium sulfate. Examples of the boric acid and the salt thereof include boric acid, sodium borate, and potassium borate.

[0026] The concentration of the electroconductive salt in the cyanide-based silver alloy electroplating solution according to the present invention is 5 to 300 g/L, preferably 50 to 250 g/L, more preferably 100 to 240 g/L. If the concentration of the electroconductive salt is less than 5 g/L, electric resistance of the plating solution is excessively high, and therefore plating cannot be performed at an appropriate cathode current density.

Germanium Compound

[0027] The germanium compound contained in the cyanide-based silver alloy electroplating solution according to the present invention is a compound containing germanium. Particularly, germanium dioxide, germanium halide, tetraalkoxy germanium, germanium sulfide, and germanic acid and a salt thereof are preferred. Examples of the germanate include sodium germanate and potassium germanate.

[0028] The concentration of the germanium compound in the cyanide-based silver alloy electroplating solution according to the present invention is 0.1 to 10 g/L, preferably 1 to 6 g/L as a germanium concentration. If the content of the germanium compound is out of the above concentration range, there is a case where a lustrous silver coating cannot be obtained or plating cannot be performed at an appropriate cathode current density.

Coordinating Polymer Additive

[0029] The coordinating polymer additive in the cyanide-based silver alloy electroplating solution according to the present invention is at least one selected from among polyacrylic acid, polyethyleneimine, and a copolymer containing them in a structure thereof, and is preferably polyacrylic acid or polyethyleneimine. The molecular weight of the coordinating polymer additive is not particularly limited, but the coordinating polymer additive generally has a number-average molecular weight of about 300 to 5000000.

[0030] The concentration of the coordinating polymer additive in the cyanide-based silver alloy electroplating solution according to the present invention is 1 to 100 g/L, preferably 2 to 84 g/L. If the concentration of the coordinating polymer additive is less than 1 g/L, there is a case where germanium cannot sufficiently be co-deposited. If the concentration of the coordinating polymer additive exceeds 100 g/L, there is a case where the viscosity of the plating solution excessively increases so that plating cannot be performed at an appropriate cathode current density or the amount of the plating solution to be taken out increases.

Other Components

[0031] In order to reduce viscosity to prevent unevenness of a silver coating, the cyanide-based silver alloy electroplating solution according to the present invention may contain, in addition to the above-described components, a component such as a surfactant without impairing the object of the present invention. Examples of the surfactant include an anionic surfactant such as sodium polyoxyethylene alkyl ether sulfate and a nonionic surfactant such as a polyoxyethylene alkyl ether condensate.

[0032] The cyanide-based silver alloy electroplating solution according to the present invention may contain neither a selenium compound nor a sulfur compound (except for germanium sulfide and the surfactant described above). Specifically, the cyanide-based silver alloy electroplating solution according to the present invention may contain neither a selenium compound such as potassium selenium cyanide, selenium cyanide, selenious acid, selenic acid oxide, or selenium oxide nor a sulfur compound such as carbon disulfide, thiourea, thiolactic acid, thiouracil, thiobarbituric acid, cysteine, cystine, thioacetic acid, or mercaptobenzothiazole.

[0033] The concentration of the selenium compound and the sulfur compound in the cyanide-based silver alloy electroplating solution according to the present invention is preferably less than 1 g/L, more preferably less than 0.1 g/L as a selenium concentration and a sulfur concentration. Even more preferably, the cyanide-based silver alloy electroplating solution according to the present invention contains substantially no selenium compound and sulfur compound (less than 0.01 g/L).

[0034] A solvent used for the cyanide-based silver alloy electroplating solution according to the present invention is water and may contain a water-based solvent (solvent dissolved in water at an added concentration).

[0035] The cyanide-based silver alloy electroplating solution according to the present invention can be produced by dissolving the above-described components in the solvent. The order of dissolving is not particularly limited. The cyanide-based silver alloy electroplating solution according to the present invention may be in a concentrated state (including a solvent-free state) during distribution or storage. Alternatively, the cyanide-based silver alloy electroplating solution according to the present invention may be distributed or stored without dissolving some of the components therein and used just after dissolving these components therein.

EXAMPLES

[0036] Hereinbelow, the present invention will specifically be described with reference to Examples. The present invention is not limited to these Examples.

[0037] As an object to be plated, a copper plate of 0.1 dm.sup.2 was used. First, the copper plate was subjected to degreasing treatment using an alkaline degreasing solution and then neutralized with dilute sulfuric acid. Then, a matte copper coating of about 1.7 .Math.m was formed in a cyanide bath. Then, a silver coating of about 0.1 .Math.m was formed in a cyanide-based strike bath.

[0038] Plating solutions of Examples 1 to 12 and Comparative Examples 1 to 5 were prepared to have compositions shown in Tables 1 and 2 (in all of the plating solutions, the balance was water). The object to be plated was immersed in one liter of each of the prepared plating solutions, subjected to silver electroplating under conditions shown in Tables 1 and 2 until the thickness of a silver coating became 20 .Math.m, washed with clean pure water, and then dried.

[0039] The thus obtained silver coatings of Examples 1 to 12 and Comparative Examples 1 to 5 were subjected to evaluation of appearance and measurement of hardness. The appearance herein is an appearance visually observed. The appearance was evaluated according to the following criteria: o The silver coating had a lustrous appearance without plating unevenness; and x The silver coating had an appearance other than the appearance evaluated as o. The hardness herein is micro-Vickers hardness obtained by keeping a test force of 10 g for 10 seconds using an ultra-micro hardness tester MVK-H300 manufactured by Mitutoyo Corporation. The hardness was determined by performing measurement five times and averaging three measurement results other than the minimum value and the maximum value.

TABLE-US-00001 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 Example 10 Example 11 Example 12 Plating solution composition (g/L) Silver cyanide complex Silver potassium cyanide (in terms of Ag) 20 20 30 40 50 60 60 40 40 40 40 40 Electroconductive salt Potassium cyanide 120 120 150 150 200 220 220 180 180 180 180 Potassium phosphate 30 100 Potassium nitrate 20 10 Potassium citrate 20 10 Potassium tartrate 40 10 Potassium sulfate 40 10 Boric acid 10 10 Germanium compound Germanium dioxide (in terms of Ge) 1.0 1.0 2.0 4.0 4.0 6.0 4.0 4.0 Potassium germanate (in terms of Ge) 6.0 4.0 4.0 4.0 Coordinating polymer additive Polyacrylic acid 2.0 2.0 4.0 6.0 4.0 4.0 1.0 2.0 4.0 Polyethyleneimine 80 80 60 60 60 60 60 Plating conditions Liquid temperature (°C) 25 25 25 25 25 25 25 25 25 30 30 30 Current density (ASD) 2 2 3 3 4 5 5 4 4 4 4 2 Plating time (min) 30 30 20 20 17 14 14 17 17 17 17 30 Evaluations Appearance O O O O O O O O O O O O Hardness (HV) 180.40 188.20 187.30 180.50 182.50 184.60 188.00 184.70 182.70 185.30 183.70 184.20

TABLE-US-00002 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Comparative Example 5 Comparative Example 6 Comparative Example 7 Plating solution composition (g/L) Silver cyanide complex Silver potassium cyanide (in terms of Ag) 60 30 60 60 60 60 60 Electroconductive salt Potassium cyanide 30 Potassium phosphate 30 Potassium nitrate 30 Potassium citrate 30 Potassium tartrate 30 Potassium thiocyanate 10 50 Potassium sulfate 10 Boric acid 10 10 10 Germanium compound Germanium dioxide (in terms of Ge) 2.0 4.0 2.0 4.0 Potassium germanate (in terms of Ge) 2.0 4.0 4.0 Additive Propionic acid 4.0 4.0 Ethylenediamine 60 60 Plating conditions Liquid temperature (°C) 60 60 60 60 60 60 60 Current density (ASD) 50 70 70 70 50 50 70 Plating time (min) 8.0 5.7 5.7 5.7 8.0 8.0 57 Evaluations Appearance × × × × × × × Hardness (HV) 108.5 111.8 105.6 117.5 110.4 123.5 109.8

[0040] All of the silver coatings obtained in Examples 1 to 12 had a hardness of 180.0 or more. These silver coatings had a silver white color and an excellent appearance without unevenness. Bath stability was also excellent.

[0041] All of the silver coatings obtained in Comparative Examples 1 to 7 had a hardness of 130.0 or less. These silver coatings basically had a brown matte color and partially had a semilustrous appearance, and therefore the appearance thereof was poor due to unevenness. The hardness was measured in a semilustrous portion for convenience of measurement. Bath stability was excellent.