JP7213842B2 - Cyanide electrolytic roughening silver plating solution - Google Patents

Description

The present invention relates to an electrolytic silver plating solution. More specifically, the present invention relates to an electrolytic silver plating solution that uses silver cyanide as a silver source and that provides a highly roughened plating film.

Silver has been widely used in jewelry since ancient times because of its white luster. Among precious metals, silver is produced in relatively large quantities and is inexpensive, so silver plating is still used today for decorative purposes such as silver accessories and tableware. In addition, since silver has the highest electrical conductivity among all metals at room temperature, silver plating is often used for lead frames and substrates for electronic devices such as ICs and transistors. Furthermore, since silver has the highest reflectance of visible light among all metals, silver plating is often used on lead frames and various substrates for light-emitting devices represented by LEDs. In addition, silver plating is also used for bearing parts and applications that utilize the antibacterial properties of silver.

Various efforts have been made in the semiconductor industry to improve the reliability of IC packages. In particular, in order to prevent destruction of IC packages called the popcorn phenomenon, the development of roughening plating has been promoted. By roughening the plating surface of copper or nickel, which is the base plating, we aim to improve the adhesion between the plating and the resin by the anchor effect. For example, Patent Literature 1 describes a technique for improving adhesion by making full use of two layers of roughening plating. However, with these conventional techniques alone, there is the problem that the noble metal plating applied to the outermost surface prevents the anchor effect from being fully exhibited. Therefore, there is a demand for a method of roughening the silver plating itself, which has excellent electrical properties, and applying it to the outermost surface. On the other hand, silver plating has the property of being easily recrystallized by heat. Therefore, in the case of silver plating, there is a possibility that the needle-like plating seen in typical roughened copper plating and roughened nickel plating may lose its anchoring effect due to heat treatment.

A silver plating solution using a sulfur-based compound is conventionally known. Patent Document 2 discloses a silver plating solution containing a thiol or a disulfide compound as a silver stabilizer instead of cyanide. Further, Patent Document 3 discloses a method of stabilizing a plating solution with thiosulfuric acid. Thus, sulfur compounds, particularly thiosulfuric acid, have been used as stabilizers and reducing agents, particularly in cyanide-free silver plating solutions and electroless silver plating solutions. However, since thiosulfuric acid is well-known as an antidote for cyanide compounds, there is no example of using thiosulfuric acid or a salt thereof in a cyanide electrolytic silver plating solution for the purpose of roughening the plating.

International publication number WO2017/077903 Patent No. 6608597 Patent No. 3300519

Due to the circumstances described above, it is not easy to improve the adhesion between noble metals, especially silver, which has excellent electrical properties and solder wettability, and resins with conventional techniques. In addition, needle-like silver has concerns about deterioration of performance due to recrystallization and whiskers. Therefore, it is desired to develop a silver plating solution capable of obtaining a silver film roughened in a non-acicular shape.

SUMMARY OF THE INVENTION An object of the present invention is to provide an electrolytic silver plating solution capable of obtaining a highly roughened silver film.

As a result of intensive research, the present inventor found that a non-acicular, highly roughened silver film can be obtained by adding thiosulfuric acid and/or its salt to an electrolytic silver plating solution, and completed the present invention. came to. The present invention for solving the above problems is described below.

[1] 10 to 100 g/L of silver cyanide or silver cyanide salt in terms of silver;
1 to 200 g/L of an electrically conductive salt;
3 to 500 g/L of thiosulfuric acid and/or a salt thereof;
An electrolytic silver plating solution characterized by containing

[2] The electrically conductive salt is at least one selected from the group consisting of cyanates, phosphates, pyrophosphates, nitrates, citrates, tartrates, thiocyanates, sulfates, and boric acid or salts thereof. The electrolytic silver plating solution according to [1].

[3] The electrolytic silver plating solution according to [1], which has a pH (25°C) of 7.0 to 13.0.

The electro-roughening silver plating solution of the present invention stably provides a highly roughened silver film that is not acicular. Specifically, as shown in FIGS. 1 and 2, a film having a shape in which massive crystals are elongated is obtained. As a result, there is little possibility that the roughened shape will deteriorate due to recrystallization. In addition, large unevenness is generated in the direction in which the plating grows, and a high anchor effect can be obtained when it is adhered to a resin or the like. Furthermore, since stable performance is obtained, manufacturing yield is improved.

3 is a micrograph (370×) of the silver coating obtained in Example 2. FIG. 4 is a micrograph (5000×) of the silver coating obtained in Example 2. FIG. 3 is a micrograph (370 times) of the silver coating obtained in Comparative Example 6. FIG. 4 is a micrograph (5000×) of a silver coating film obtained in Comparative Example 6. FIG. 3 is a micrograph (370 times) of the silver coating obtained in Comparative Example 7. FIG. 4 is a micrograph (5000×) of a silver coating film obtained in Comparative Example 7. FIG.

The electrolytic silver plating solution of the present invention contains 10 to 100 g/L of a silver cyanide complex, 1 to 200 g/L of an electrically conductive salt, and 3 to 500 g/L of thiosulfuric acid or a salt thereof in terms of silver as a silver salt. contains. Each component constituting the electrolytic silver plating solution of the present invention will be described below.

[Silver cyanide complex]
In the electrolytic silver plating solution of the present invention, any known silver cyanide complex can be used without limitation as a silver source. Examples of silver cyanide complexes include silver cyanide, potassium silver cyanide, and sodium silver cyanide.

The concentration of the silver cyanide complex in the electrolytic silver plating solution of the present invention is preferably 10 to 100 g/L, more preferably 20 to 70 g/L, in terms of silver ion concentration. If the silver ion concentration is less than 10 g/L, the deposition efficiency may decrease and a desired silver film thickness may not be obtained. On the other hand, if the silver ion concentration exceeds 100 g/L, the loss of silver salt increases due to the removal of the plating solution by the object to be plated, which is not economical.

[Electrically conductive salt]
The electrically conductive salt to be incorporated in the electrolytic silver plating solution of the present invention is not particularly limited as long as the aqueous solution has electrical conductivity. In order to produce the liquid, it is preferable to contain at least one selected from cyanates, phosphates, nitrates, citrates, tartrates, thiocyanates, sulfates, and boric acid or salts thereof. In addition, soluble organic acid salts are also preferred. These may be used alone or in combination of two or more. Examples of cyanide salts include potassium cyanide and sodium cyanide. Examples of phosphates include potassium phosphate, sodium phosphate and ammonium phosphate. Examples of pyrophosphate include potassium pyrophosphate, sodium pyrophosphate and ammonium pyrophosphate. Examples of nitrates include potassium nitrate, sodium nitrate, and ammonium nitrate. Examples of citrates include potassium citrate, sodium citrate, and ammonium citrate. Examples of tartaric acid include potassium tartrate, sodium tartrate, sodium potassium tartrate and the like. Examples of thiocyanates include potassium thiocyanide and sodium thiocyanide. Examples of sulfates include potassium sulfate, sodium sulfate, and ammonium sulfate. Examples of boric acid or salts thereof include boric acid, sodium borate, potassium borate and the like.

The electroconductive salt concentration in the electrolytic silver plating solution of the present invention is preferably 1 to 200 g/L. If the concentration of the electrically conductive salt is less than 1 g/L, the electrical resistance of the plating solution becomes too high, making it impossible to produce plating with a suitable cathode current density.

[Thiosulfate (salt)]
Thiosulfuric acid and/or a salt thereof (hereinafter abbreviated as "thiosulfuric acid (salt)") to be incorporated in the electrolytic silver plating solution of the present invention specifically includes thiosulfuric acid, potassium thiosulfate, sodium thiosulfate, and the like. exemplified. These may be used alone or in combination of two or more.

The concentration of thiosulfuric acid (salt) in the electrolytic silver plating solution of the present invention is preferably 3 to 500 g/L, more preferably 3 to 300 g/L. When the concentration of thiosulfuric acid (salt) is less than 3 g/L, the shape and size of precipitated silver crystals are not uniform. If the concentration of thiosulfate (salt) exceeds 500 g/L, precipitation may occur.

[Other ingredients]
In the electrolytic silver plating solution of the present invention, in addition to the above components, a component such as a surfactant is added to the extent that the object of the present invention is not impaired in order to reduce the viscosity and suppress the occurrence of unevenness in the silver film. be able to. Examples of surfactants include anionic surfactants such as sodium polyoxyethylene alkyl ether sulfate and nonionic surfactants such as polyoxyethylene alkyl ether condensates.

EXAMPLES The present invention will be specifically described below with reference to examples. The invention is not limited to these examples.

(Examples 1 to 12, Comparative Examples 1 to 7)
A copper plate of 0.1 dm ² was used as the object to be plated. First, a copper plate was degreased with an alkaline degreasing solution, neutralized with dilute sulfuric acid, and then matte copper plated with a thickness of about 1.7 μm using a cyanide bath. After that, about 0.1 μm silver plating was applied using a cyan-based strike bath.

Plating solutions of Examples 1 to 12 and Comparative Examples 1 to 7 were prepared with the compositions shown in Tables 1 and 2. 0.5 L of the prepared plating solution is blown up with a pump, and the plating solution is applied to the masked object to be plated leaving a square of 1 cm square, and the silver film thickness is 4 μm under the conditions described in Tables 1 and 2. Electrolytic silver plating was carried out until the surface became clear, washed with clean pure water, and then dried.

The glossiness and arithmetic mean roughness of the silver films of Examples 1 to 12 and Comparative Examples 1 to 7 obtained as described above were measured. The term "glossiness" as used herein is a numerical value measured with a densitometer ND-11 manufactured by Nippon Denshoku Industries Co., Ltd. Further, the arithmetic mean roughness here is a value of arithmetic mean roughness obtained by analyzing with a shape measuring laser microscope VK-9700 manufactured by KEYENCE CORPORATION at a magnification of 150 times. The measurement results are shown in Tables 1 and 2.

Further, the results of observing the silver films of Example 2 and Comparative Examples 6 and 7 with a scanning electron microscope JSM-IT300HR manufactured by JEOL Ltd. are shown in FIGS. 1 to 6, respectively.

The silver films obtained in Examples 1 to 12 all had a glossiness of 0.0 and an arithmetic mean roughness of 0.30 μm or more. The color tone was white, and the appearance was good without unevenness. Also, as exemplified in FIGS. 1 and 2, lumpy crystals were uniformly precipitated. Bath stability was also good.

The silver films obtained in Comparative Examples 1 to 4 all had a glossiness of 0.1 or more and an arithmetic mean roughness of less than 0.30 μm. The color tone was white, and the appearance was good without unevenness. Bath stability was also good.

The silver films obtained in Comparative Examples 5 and 6 both had a glossiness of 0.0 and an arithmetic mean roughness of less than 0.30 μm. The color tone was white, and the appearance was good without unevenness. Bath stability was also good.
The present inventor found that silver plating was roughened even when iodide salt was used instead of thiosulfate (salt). was precipitated, and the appearance was hardly uniform as far as it was confirmed with an electron microscope.

The silver film obtained in Comparative Example 7 had a glossiness of 0.0 and an arithmetic mean roughness of less than 0.30 μm. The color tone was white, and the appearance was good without unevenness. Also, the crystal sizes were sparse as illustrated in FIGS. Bath stability was good.

Claims (3)

10 to 100 g/L of silver cyanide or silver cyanide salt in terms of silver;
1 to 200 g/L of an electrically conductive salt;
3 to 500 g/L of thiosulfuric acid or a salt thereof;
An electrolytic roughening silver plating solution characterized by containing The electrically conductive salt is at least one selected from the group consisting of cyanates, phosphates, pyrophosphates, nitrates, citrates, tartrates, thiocyanates, sulfates, and boric acid or salts thereof. Item 1. The electrolytic- roughening silver plating solution according to item 1. 2. The electro- roughening silver plating solution according to claim 1, which has a pH (25° C.) of 7.0 to 13.0.

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