JP2005126773A - Method of preparing plating bath, plating bath, plating method, and method of producing electronic component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a plating bath capable of obtaining a plating film having almost the same film thickness even when continuously used. <P>SOLUTION: A tin salt and a complexing agent are mixed to prepare a mixed solution. Thereafter, the hydrogen ion exponent pH of the mixed solution is adjusted so as to be the first value higher than the acid dissociation exponent PK<SB>a</SB>of the complexing agent. Subsequently, it is allowed to stand for a prescribed time, and thereafter, the hydrogen ion exponent pH of the mixed solution is reduced so as to be the second value lower than the first value. At the time when, in the complexing agent, two or more stages of dissociation reactions occur in the tin salt aqueous solution, the mixed solution is prepared so that the above first value is made higher than the acid dissociation exponent pK<SB>an</SB>in the final stage. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a plating bath, a plating bath, a plating method, and a method for producing an electronic component. More specifically, the present invention relates to a method for producing a plating bath such as a tin plating bath used in an electrolytic plating process for an electronic component. The present invention relates to an obtained plating bath, a plating method using the plating bath, and a method for manufacturing an electronic component such as a multilayer ceramic capacitor obtained using the plating method.

  In electronic parts such as multilayer ceramic capacitors, Ni film is usually formed on the surface of external electrodes mainly composed of Ag, Cu, etc. to prevent solder erosion, and solder wettability is improved and Ni film is oxidized. In order to prevent this, a Sn film or a Sn-Pb film is formed on the surface of the Ni film.

  As this type of plating bath, Sn and Sn—Pb alloy plating solutions containing a stannous salt, a complexing agent such as gluconic acid, and a surfactant have been known (Patent Document 1). ).

In Patent Document 1, by adding a complexing agent to the Sn or Sn-Pb alloy plating solution, Sn ²⁺ forms a stable soluble complex with the complexing agent to improve the stability of the plating bath. .

  Further, when producing a ceramic electronic component such as a multilayer ceramic capacitor using the plating bath, the hydrogen ion exponent pH (hereinafter simply referred to as “pH”) is controlled to 2 to 10, preferably 3 to 7. Accordingly, it is possible to reduce the melting of the ceramic material and the glass component contained in the ceramic body, and it is possible to suppress the quality deterioration of the ceramic electronic component.

JP 57-63689 A

  However, in the above-mentioned Patent Document 1, a pH adjusting agent is added to the plating bath to directly adjust the pH of the plating bath to a desired value suitable for the plating treatment. , “Initial bath”) and plating bath after continuous use (hereinafter referred to as “continuous use bath”) have a drawback that the properties of the plating bath change. For this reason, when electrolytic barrel plating was performed on the object to be plated, the film thickness of the plating film varied between the initial bath and the continuous use bath.

  That is, in electrolytic barrel plating that is frequently used for plating of ceramic electronic parts, usually, an object to be plated and a conductive medium are immersed in a barrel container and energized between an anode and a cathode, thereby performing an electrolytic plating process. Since the conductive medium is generally made of a metal member such as a steel ball, the current density is high, and the current density is low around the object to be plated mainly composed of a ceramic material.

On the other hand, in the case of a plating bath containing a metal salt and a complexing agent, in the initial bath, an anion in the metal salt (for example, SO ₄ ^{2− in} the case of tin sulfate (SnSO ₄ ), tin chloride (SnCl ₂ )). Cl ⁻ ) has a large influence, and these anions exist around Sn ²⁺ , so that Sn ²⁺ cannot form a stable complex with the complexing agent.

Thus, in the initial bath, Sn ²⁺ in the metal salt cannot form a stable complex with the complexing agent, so that Sn is likely to precipitate in a conductive medium having a high current density, and the current density is low. Sn becomes difficult to deposit on the plated product.

  That is, the conventional Sn plating bath as described in Patent Document 1 cannot form a stable complex in the initial bath, so that only a thin Sn film can be formed. In order to obtain an Sn film having the same film thickness as that described above, it is necessary to set the current density high or to increase the plating processing time.

On the other hand, in the continuous use bath, Sn ²⁺ eluted from the anode by electrolytic treatment is coordinated with the complexing agent to form a stable complex. As the complex stabilizes in this way, the plated metal easily deposits not only on the conductive medium having a high current density but also on the object to be plated having a low current density, thereby obtaining a thick Sn film. it can.

  However, when the current density is increased or the plating process time is increased in the continuous use bath, a thick Sn film exceeding the desired film thickness is formed.

  And when the Sn film having a thickness exceeding the desired film thickness is formed in this way, since the hardness of Sn is low, the objects to be plated adhere to each other through the Sn film having such a low hardness. Yield decreases.

  As described above, in the conventional Sn plating bath, the stability of the complex is different between the initial bath and the continuous use bath. Therefore, it is not possible to form a plating film having the same film thickness under the same plating conditions. In order to obtain a plating film, it is necessary to increase the current density or lengthen the plating time in the initial bath, while in the continuous use bath, the current density can be increased or the plating can be performed from the viewpoint of preventing adhesion between electronic components. It is necessary to avoid an increase in processing time.

  That is, in the conventional Sn plating bath, in order to obtain the same film thickness in the initial bath and the continuous use bath, the plating conditions have to be changed. Therefore, the productivity is poor and the high quality with high reliability. There was a problem that it was difficult to obtain electronic parts with high efficiency.

  The present invention has been made in view of such problems, and a plating bath preparation method, a plating bath, a plating method, and a plating bath capable of easily obtaining a plating film having substantially the same film thickness even when continuously used. And it aims at providing the manufacturing method of the electronic component which uses this plating method.

The present inventors have stability of the complex is conducted intensive studies by focusing on that depends on the pH of the plating bath, the pH of the plating bath is higher than the acid dissociation exponent pK _a complexing agent given It was found that the complex can be stabilized even in the initial bath by allowing it to stand for a period of time and then lowering the pH of the plating bath to a predetermined value.

The present invention has been made based on such knowledge, and the method for producing a plating bath according to the present invention is a complex capable of forming a complex between a metal salt aqueous solution containing metal ions and the metal ions. A method for preparing a plating bath containing an agent, wherein a mixed solution is prepared by mixing the aqueous metal salt solution and the complexing agent, and then the hydrogen ion exponent pH of the mixed solution is that of the complexing agent. after adjusted to a first value greater than the acid dissociation index PK _a, then left for a predetermined time, thereafter lower the hydrogen ion exponent pH of the mixed solution to a small second value than the first value It is characterized by that.

In addition, a complexing agent such as citric acid in which a dissociation reaction occurs in multiple stages is often used, but when the complexing agent causes a multi-stage dissociation reaction in a mixed solution in this way, The first value needs to be larger than the acid dissociation index pK _{an at the} final stage.

That is, in the method for producing a plating bath of the present invention, when the complexing agent causes a multi-stage dissociation reaction in the mixed solution, the first value is higher than the acid dissociation index pK _an of the final stage. It is characterized by being large.

  The second value is preferably 3-7.

  As the complexing agent, at least one selected from a polycarboxylic acid such as citric acid, a polyoxymonocarboxylic acid, an aminocarboxylic acid, a lactone compound, and a salt thereof can be used.

  The plating bath according to the present invention is characterized by being produced by the above production method.

  Furthermore, the plating method according to the present invention includes immersing an object to be plated, an anode for supplying metal ions of a plating metal, and a cathode in contact with the object to be plated in the plating bath, and An electric current treatment is performed in between, and a metal film is formed on the surface of the object to be plated.

  In addition, when performing electrolytic barrel plating, it is preferable to mix a conductive medium with the object to be plated in order to ensure conduction between the cathode and the object to be plated.

  That is, the plating method of the present invention is characterized in that a conductive medium formed of a material different from the object to be plated is immersed in the plating bath.

  Furthermore, when performing the said electrolytic barrel plating, it is preferable to mix and carry out a nonelectroconductive medium in a to-be-plated object from a viewpoint of avoiding the overlap of to-be-plated objects.

  That is, the plating method of the present invention is characterized in that a non-conductive medium formed of a material different from the object to be plated is immersed in the plating bath.

  Further, the metal film is characterized by containing tin as a main component.

  In the present invention, the “substance to be plated” means an article to be plated, such as an electronic component, and the plating film is inevitably deposited because the plating treatment is mixed into the plating bath for convenience. Articles to be processed, for example, conductive media such as steel balls, are not included.

  An electronic component manufacturing method according to the present invention is an electronic component manufacturing method for manufacturing an electronic component by performing a plating process on an object to be plated in which a conductive portion is formed on a surface of a component element body. A method is used to form a metal film on the surface of the conductive portion.

According to the method for producing the plating bath, the pH of the mixed solution is adjusted to be a first value larger than the acid dissociation index PK _a of the complexing agent, and then left for a predetermined time, and then the first Since the pH of the mixed solution is lowered so as to be a second value smaller than the value, the complex becomes stable at the initial bath stage. Therefore, the plating bath is not only used in the continuous bath but also in the initial bath. Can also ensure the stability of the complex.

When the complexing agent causes a multi-stage dissociation reaction in the mixed solution as in the case of using citric acid as the complexing agent, the first value is the acid dissociation index pK of the final stage. By adjusting the mixed solution to be larger than _an , a stable complex can be formed at the initial bath stage.

  Further, by setting the second value to pH 3 to 7, the plating bath can maintain weak acidity to neutrality, and even if the object to be plated is mainly composed of a ceramic material or a glass material. It is possible to effectively avoid dissolution of the component to be plated.

  Then, by using the above plating bath and applying an electrolysis treatment between the cathode and the anode and subjecting the object to be plated to electrolytic plating, the desired film can be obtained not only in the continuous use bath but also in the initial bath under the same plating conditions. A thick plating film can be easily formed, whereby a plating film having substantially the same film thickness can be easily obtained in both the initial bath and the continuous use bath.

  Further, by conducting electroplating by immersing a conductive medium and a non-conductive medium in a plating bath, it is possible to ensure conduction between the cathode and the object to be plated while preventing the objects to be plated from overlapping each other. This makes it possible to easily form a desired plating film on the surface of the object to be plated.

  Furthermore, according to the manufacturing method of the electronic component, since the metal film is formed on the surface of the conductive part using the plating method, the plating conditions are substantially the same without changing the plating conditions in the initial bath and the continuous use bath. A metal film having a film thickness can be formed on the conductive portion, and a high-quality electronic component having excellent reliability can be manufactured with high efficiency.

  Next, a method for producing a tin plating bath as the best mode for carrying out the present invention will be described in detail.

  This tin plating bath contains at least a stannous salt as a metal salt and a complexing agent, and is further pH-adjusted by adding a pH adjusting agent, whereby electrolytic plating is performed under the same plating conditions. Even in the initial bath and the continuous use bath, a plating film having substantially the same film thickness can be obtained.

  Here, as the stannous salt, stannous sulfate, stannous chloride, stannous sulfamate, or the like can be used.

  As the complexing agent, an organic acid that forms a stable soluble complex in the Sn plating bath can be used. Specifically, polycarboxylic acid, polyoxymonocarboxylic acid, aminocarboxylic acid, lactone compound, and At least one selected from these salts can be used.

  Here, examples of the polycarboxylic acid include citric acid, malonic acid, maleic acid, succinic acid, tricarballylic acid, lactic acid, malic acid, tartaric acid, phthalic acid, isophthalic acid, 2-sulfoethylamino-N, N-di-acid. Acetic acid, iminodiacetic acid, nitrilotriacetic acid, ethylenediaminetetraacetic acid, triethylenediaminetetraacetic acid can be used, and as polyoxymonocarboxylic acid, for example, gluconic acid, glucoheptonic acid, and glyceric acid can be used.

  Examples of aminocarboxylic acids that can be used include glycine, glutamic acid, aspartic acid, and β-alanine-N, N-diacetic acid. Examples of lactone compounds include glucono-δ-lactone and glucoheptonolactone. Can be used.

  Furthermore, it is also preferable to add a brightener, an antioxidant, or a conductive agent to the plating bath as necessary.

  Here, as the brightener, an anionic, cationic, nonionic or amphoteric surfactant can be used as appropriate.

In addition, an appropriate amount of an antioxidant is appropriately added for the purpose of preventing Sn ²⁺ from being oxidized to Sn ⁴⁺ , and hydroquinone, pyrocatechol, resorcin, ascorbic acid, hydrazine or the like may be used as an antioxidant. it can.

  As the conductive agent, ammonium sulfate or the like can be used.

Next, the method for producing the Sn plating bath will be described in detail in the case of using citric acid (C ₃ H ₄ OH (COOH) ₃ ) as a complexing agent.

  First, a predetermined amount of stannous salt, citric acid, and if necessary, a brightener, an antioxidant, and a conductive agent are weighed and mixed to prepare a mixed solution.

Next, _a pH adjuster is added so that the pH of the mixed solution becomes a value (first value) larger than the acid dissociation index pKa of the final stage in the citric acid dissociation reaction, and the mixture is allowed to stand for a predetermined time.

ここで、第１〜第３段階の酸解離指数ｐＫ_ａｎは、それぞれｐＫ_ａ１が２．８７、ｐＫ_ａ２が４．３５、ｐＫ_ａ３が５．６９であることが知られている。 That is, citric acid exhibits a three-stage dissociation reaction in the Sn plating bath as shown in chemical reaction formulas (A) to (C).

Here, it is known that the acid dissociation indices pK _an of the first to third stages have pK _a1 of 2.87, pK _a2 of 4.35, and pK _a3 of 5.69, respectively.

In the present embodiment, an alkaline reagent such as aqueous ammonia or sodium hydroxide is used as a pH adjuster so that the pH of the mixed solution is larger than the third stage pK _a3 (= 5.69) as the final stage. And the pH (first value) of the mixed solution is set to 7 to 10, for example.

Next, the mixed solution is allowed to stand for a predetermined time, and Sn ²⁺ is coordinated with citric acid to form a stable Sn complex. That is, the mixed solution is allowed to stand for a predetermined time so that the Sn complex becomes stable.

  Thereafter, an acidic pH adjuster such as sulfuric acid or sulfamic acid is added to the mixed solution left for a predetermined time, and the pH of the mixed solution is lowered to the second value to prepare an Sn plating bath.

  Here, the second value varies depending on the type of the complexing agent and the stannous salt, but when the object to be plated contains a ceramic material or a glass material, it is weakly acidic to neutral, specifically 3 to 3. It is preferable to adjust to 7. This is because when the object to be plated contains a ceramic material or a glass material, if the second value is less than 3 or exceeds 7, the component of the object to be plated may be dissolved in the Sn plating bath and cause quality deterioration. .

As described above, in the present embodiment, after adjusting the pH of the mixed solution to be the first value larger than the acid dissociation index PK _a3 of the final stage of citric acid, the mixed solution is left for a predetermined time, and then the mixed solution Since the hydrogen ion exponent pH of the first bath is lowered to the second value smaller than the first value, the Sn complex is stabilized in the initial bath as in the case of the continuous use bath, so that the plating conditions are not changed. In both cases, it is possible to easily form a plating film having substantially the same film thickness in both the initial bath and the continuous use bath.

  Next, a method for manufacturing a multilayer ceramic capacitor as an electronic component using the Sn plating bath will be described in detail.

  FIG. 1 is a cross-sectional view schematically showing an embodiment of a multilayer ceramic capacitor.

In the multilayer ceramic capacitor, internal electrodes 2 (2a to 2f) are embedded in a ceramic body (component body) 1 made of a dielectric ceramic material such as BaTiO ₃ and both ends of the ceramic body 1 are embedded. External electrodes (conductive portions) 3a and 3b are formed, and Ni films 4a and 4b and Sn films 5a and 5b are formed on the surfaces of the external electrodes 3a and 3b.

  Specifically, the internal electrodes 2a to 2f are juxtaposed in the stacking direction, the internal electrodes 2a, 2c, and 2e are electrically connected to the external electrode 3a, and the internal electrodes 2b, 2d, and 2f are external electrodes 3b. And are electrically connected. A capacitance is formed between the opposing surfaces of the internal electrodes 2a, 2c, and 2e and the internal electrodes 2b, 2d, and 2f.

  Hereinafter, a method for producing the multilayer ceramic capacitor will be described in detail.

First, a predetermined dielectric ceramic material such as BaCO ₃ , TiO ₂ , ZrO _2, etc. is mixed and subjected to processes such as pulverization, drying, and calcination, and then a ceramic green sheet (hereinafter referred to as “ceramic sheet”) by a doctor blade method or the like. ).

  Next, a conductive paste for internal electrodes in which an organic component such as glass particles or varnish is contained in a conductive material such as Ag or Cu is prepared. Then, screen printing is performed on the surface of the ceramic sheet using the conductive paste to form a conductive pattern, and then the ceramic sheet on which the conductive pattern is formed is laminated, and the ceramic on which the conductive pattern is not formed A laminated body is formed by sandwiching and pressing with a sheet. Thereafter, the laminate is fired at a predetermined temperature (for example, 900 to 1300 ° C.), barrel-polished, and deburred, whereby the ceramic body 1 is manufactured.

  Next, a conductive paste for an external electrode containing a conductive material such as Ag or Cu is prepared, and the conductive paste is applied to both ends of the ceramic body 1 by a dip method or the like, and then a temperature of 600 to 800 ° C. The baking process is performed with. Thereby, organic components, such as a varnish, burn and burn, and external electrodes 3a and 3b are formed at the both ends. Thereafter, the ceramic body 1 on which the external electrodes 3a and 3b are formed is used as an object to be plated, and Ni plating and Sn plating are performed by electrolytic barrel plating to form Ni coatings 4a and 4b and Sn coatings 5a and 5b. .

  That is, when performing Ni plating, first, for example, the plating tank is filled with a Ni plating bath containing nickel sulfate, nickel chloride and boric acid.

  Next, a non-conductive conductor formed of an object to be plated, a steel ball conductive medium and a resin is accommodated in a barrel container, and then an Ni anode, a cathode, and the barrel container are immersed in a plating tank. The barrel container is rotated, swinged, vibrated, etc., and a current is passed between the anode and the cathode at a predetermined current density for a predetermined time while the object to be plated is brought into contact with the cathode. Ni coatings 4a and 4b are formed on the surfaces of the electrodes 3a and 3b.

  Moreover, when performing Sn plating, a plating tank is first filled with the said Sn plating bath.

  Next, the object to be plated on which the Ni films 4a and 4b are formed, the conductive medium and the non-conductive medium are accommodated in a barrel container, and then the Sn anode, the cathode, and the barrel container are placed in a plating tank. The surface of the Ni coatings 4a and 4b is dipped, and a current is passed between the anode and the cathode for a predetermined period of time while the object to be plated is brought into contact with the cathode by rotating, swinging, vibrating, etc. Then, Sn films 5a and 5b are formed.

  As described above, in the present multilayer ceramic capacitor, the above-described Sn plating bath is used for the electroplating treatment. Therefore, even if the current density is not increased or the plating treatment time is not increased, the continuous use bath can be used from the initial bath time. Sn plating films 5a and 5b having substantially the same film thickness can be formed. In addition, excessively thick Sn coatings 5a and 5b are not formed in the continuous use bath, so that the products can be prevented from sticking to each other in the Sn plating bath, and high quality with excellent reliability. The multilayer ceramic capacitor can be manufactured with high efficiency.

  The present invention is not limited to the above embodiment.

  In the above embodiment, the Sn plating bath has been described. However, the Sn—Pb plating bath, the Sn—Ag plating bath, the Sn—Zn plating bath, the Sn—Cu plating bath, the Sn—Bi plating bath, the Sn—In plating bath, etc. The same can be applied to the Sn alloy plating bath.

  Also applicable to plating baths other than Sn plating bath and Sn alloy bath, such as Au plating bath, Ag plating bath, Pd plating bath, Pt plating bath, Cu plating bath, Ni plating bath, and these metal species. The present invention can be similarly applied to an alloy plating bath having a main component.

  Moreover, although rotating barrel plating was demonstrated to the example as a plating method, the effect similar to this invention can be show | played also about other plating, such as vibration barrel plating and rocking barrel plating.

  Also, the electronic component is not limited to the multilayer ceramic capacitor, and can be applied to an electronic component such as a thermistor or a resistor.

  Next, examples of the present invention will be specifically described.

〔実施例１〕
金属塩として２１０mol／ｍ^３の硫酸第１スズ、錯化剤として４２０mol／ｍ^３のクエン酸水素二ナトリウム、光沢剤として０．１ｋｇ／ｍ^３のアルキルアミン型界面活性剤、導電剤として１００mol／ｍ^３の硫酸アンモニウムを混合させて混合溶液を作製し、ｐＨ調整剤としてアンモニア水を添加し、Ｓｎめっき浴のｐＨを７．０に調整した。 [Preparation of Sn plating bath (including Sn alloy plating bath)]
First, the plating baths of Examples 1 to 4 and Comparative Examples 1 to 4 having the plating compositions shown in Table 1 were prepared.

[Example 1]
Stannous sulfate 210mol / ^{m 3} as a metal salt, citric acid disodium hydrogen of 420mol / ^{m 3} as a complexing agent, alkylamine surfactant of 0.1 kg / ^{m 3} as brighteners, as a conductive agent 100 mol / A mixed solution was prepared by mixing ammonium sulfate of m ³ , and aqueous ammonia was added as a pH adjuster to adjust the pH of the Sn plating bath to 7.0.

That is, diammonium hydrogen citrate, which is a carboxylic acid, causes a dissociation reaction in which proton H ⁺ (H ₃ O ⁺ ) of a carboxyl group (—COOH) is dissociated in a mixed solution, but diammonium hydrogen citrate has a chemical formula ( NH ₄₎ indicated by ₂ HC ₆ H ₅ O _7, because it has three carboxyl groups, dissociates divided into three stages. The respective acid dissociation exponent pK _a of the first stage to third stage _{pK a1} is 2.87, _{pK a2} is 4.35, _{pK a3} is 5.69.

Therefore, in Example 1, aqueous ammonia was added to the mixed solution, and the pH of the mixed solution was adjusted to 7.0 so that the pH was higher than pK _a3 (= 5.69).

  Next, the solution was allowed to stand at a bath temperature of 25 ° C. for 24 hours so that the Sn complex was stabilized, and then sulfuric acid was added to lower the pH to 4.5, whereby the plating bath of Example 1 was prepared.

[Example 2]
Stannous sulfamate of 420mol / ^{m 3} as a metal salt, making malic acid 3360mol / ^{m 3,} a mixed solution by mixing an alkyl amine type surfactant 0.1 kg / ^{m 3} as brighteners as complexing agent Then, sodium hydroxide was added as a pH adjuster to adjust the pH of the Sn plating bath to 6.0.

Namely, malic acid is a carboxylic acid, like hydrogen diammonium citrate, although protons ^{H +} of the carboxyl groups in the mixed solution is caused dissociation reaction of dissociating, malic acid represented by the chemical formula HOOCCH _(OH) CH 2 COOH As shown, it has two carboxyl groups and dissociates in two stages. The acid dissociation index pK _a in the first stage and the second stage has a pK _a1 of 3.24 and a pK _a2 of 4.71.

Therefore, in Example 2, sodium hydroxide was added to the mixed solution, and the pH of the mixed solution was adjusted to 6.0 so that the pH value was higher than the second-stage pK _a2 (= 4.71).

  Next, the solution was left at a bath temperature of 30 ° C. for 5 hours so that the Sn complex was stabilized, and then sulfamic acid was added to lower the pH to 4.5 to prepare the plating bath of Example 2.

Example 3
Sulfamic lead of 200 mol / ^{m 3} of stannous sulfamate, and 1 mol / ^{m 3} as a metal salt, of 600mol / ^{m 3} as a complexing agent for tin glucono -δ- lactone, 6 mol / m as a complexing agent for lead ³ glycine and 0.5 kg / m ³ aminobetaine surfactant as a brightener were mixed to prepare a mixed solution, sodium hydroxide was added as a pH adjuster, and the pH of the Sn alloy plating bath was 10. Adjusted to zero.

  Here, the reason for adjusting the pH of the Sn alloy plating bath to 10.0 is as follows.

Glucono-δ-lactone, which is a complexing agent of tin (the main component metal of the plating bath), is represented by the chemical formula C ₆ H ₁₀ O ₆ and does not have a carboxyl group. However, glucono-δ-lactone is hydrolyzed in the mixed solution and mostly converted to gluconic acid.

On the other hand, gluconic acid is represented by the formula _{CH 2 (OH) (CHOH)} 4 COOH, has one carboxyl group, acid dissociation exponent pK _a is 3.60. In addition, gluconic acid is considered to contribute to the stability of the Sn complex by elimination of hydroxyl group (—OH) protons in a strongly alkaline region of pH 11 or higher.

In a third embodiment, by adding sodium hydroxide to the mixed solution, the pH of the mixed solution to a pH greater value sufficiently than the acid dissociation exponent _pK a of gluconic acid (= 3.60) 10.0 Adjusted.

  Subsequently, it was left to stand at a bath temperature of 25 ° C. for 12 hours so that the Sn complex was stabilized, and then sulfamic acid was added to lower the pH to 4.5, whereby the plating bath of Example 3 was produced.

Example 4
Silver iodide 200 mol / ^{m 3} of stannous sulfate, and 5 mol / ^{m 3} as a metal salt, 1500mol / ^{m 3} as a complexing agent for tin citrate disodium hydrogen 800mol / ^{m 3,} as silver complexing agent Of potassium iodide and 0.5 kg / m ³ of alkyl imidazolium type surfactant as a brightener were mixed to prepare a mixed solution. Ammonia water was added as a pH adjuster to adjust the pH of the Sn alloy plating bath to 6 Adjusted to .5.

As described above, diammonium hydrogen citrate, which is a complexing agent of tin (the main component metal of the plating bath), dissociates in three stages and has a pK _a3 of 5.69. Water was added, and the pH of the mixed solution was adjusted to 6.5 so that the pH value was larger than pK _a3 (= 5.69).

  Next, the plate was allowed to stand at a bath temperature of 40 ° C. for 10 minutes so that the Sn complex was stabilized, and then sulfuric acid was added to lower the pH to 4.5, whereby the plating bath of Example 4 was produced.

[Comparative Example 1]
The same and the same amount of metal salt, complexing agent, brightening agent and conductive agent as in Example 1 are mixed, sulfuric acid or aqueous ammonia is appropriately used as a pH adjuster, and the pH is directly set to 5. without increasing the pH. A zero plating bath was prepared.

[Comparative Example 2]
The same and the same amount of metal salt, complexing agent, and brightening agent as in Example 2 were mixed, and sulfamic acid or sodium hydroxide was appropriately used as a pH adjuster. The pH was directly adjusted to 3.5 without increasing the pH. A plating bath was prepared.

[Comparative Example 3]
The same and the same amount of metal salt, complexing agent and brightening agent as in Example 3 were mixed, sulfamic acid or sodium hydroxide was appropriately used as a pH adjuster, and the pH was directly adjusted to 4.5 without increasing the pH. A plating bath was prepared.

[Comparative Example 4]
The same and the same amount of metal salt, complexing agent, and brightening agent as in Example 4 are mixed, and sulfuric acid or aqueous ammonia is appropriately used as a pH adjusting agent, and the plating is directly at pH 4.5 without increasing the pH. A bath was made.

  Next, an object to be plated having a length of 1.0 mm, a width of 0.5 mm, and a thickness of 0.5 mm was prepared. In addition, this to-be-plated thing forms the thick film electrode (external electrode) which consists of Cu containing a glass component in the both ends of the ceramic element | base body with which the internal electrode was embed | buried.

  Next, this object to be plated was subjected to electrolytic barrel plating by using an Ni plating bath having the following plating composition and applying a current of 8 A for 2 hours to form a Ni film having a thickness of about 3 μm.

[Composition of Ni plating bath]
Nickel sulfate 1000 mol / m ³
Nickel chloride 200mol / m ³
Boric acid (pH buffer) 70 mol / m ³
pH: 4.5
Bath temperature: 60 ° C
Next, the ceramic body on which the Ni film was formed was used as an object to be plated, and electrolytic barrel plating was performed using the plating baths of Examples 1 to 4 and Comparative Examples 1 to 4, that is, a predetermined number of objects to be plated. The product was made into one batch, and an electric current of 8 A was applied for 2 hours for each batch to perform electrolytic barrel plating, and an Sn film was formed on the surface of the object to be plated.

Subsequently, 20 samples were sampled from the samples in the initial bath and the continuous use bath, and the film thickness of the Sn film was measured with a fluorescent X-ray film thickness meter. Here, the initial bath is the Sn plating bath subjected to the first batch treatment, and the continuous use bath is after all Sn ²⁺ in the metal salt contained in the plating bath is replaced with Sn ²⁺ from the anode. It was judged whether it was a bath for continuous use with an electric quantity Q per 1 m ³ .

  That is, the deposition amount w (g) of Sn on the cathode is expressed by Formula (1) according to Faraday's law of electrolysis.

w = (I × t × M) / (Z × F) (1)
Here, I is current (A), t is energization time (sec), M is the atomic weight of Sn (= 18.7), Z is the valence of Sn (= 2), and F is the Faraday constant (= 96500 coulomb). ), And (I × t) represents the quantity of electricity Q.

Therefore, the amount of deposition w on the cathode can be calculated by substituting the amount of electricity Q (current I, energization time t) into Equation (1). And it can be considered that when this precipitation amount w completely replaces the Sn ²⁺ concentration (g / L) in the metal salt, the bath was continuously used. Therefore, the Sn ²⁺ concentration is calculated from the metal salt content of the Sn plating bath, the amount of electricity Q per 1 m ³ corresponding to this Sn ²⁺ concentration is obtained, and the amount of electricity Q is loaded on the Sn plating bath (electrolyzer). When it becomes a continuous use bath. In this example and the comparative example, the continuous use bath was set after the time when the amount of electricity reached 300 [A · min / m ³ ].

この表２から明らかなように比較例１〜４は、めっき浴を直接所定のｐＨに設定しているため、連続使用浴では４．０〜４．１μｍの膜厚を有するＳｎ皮膜が得られたのに対し、初期浴では２．４〜２．７μｍと薄い膜厚のＳｎ皮膜しか得られなかった。これは初期浴ではＳｎ錯体は安定化していないため、連続使用浴のような厚膜のＳｎ皮膜を形成することができないためと思われる。 Table 2 shows the film thickness measurement results in each example and comparative example.

As is apparent from Table 2, in Comparative Examples 1 to 4, the plating bath is set directly at a predetermined pH, so that a Sn coating having a film thickness of 4.0 to 4.1 μm is obtained in the continuous use bath. On the other hand, in the initial bath, only a thin Sn film of 2.4 to 2.7 μm was obtained. This is presumably because the Sn complex is not stabilized in the initial bath, so that a thick Sn film as in the continuous use bath cannot be formed.

Examples 1-4 In contrast, after the pH of the plating bath once raised above acid dissociation exponent of the last stage pK _an, complexing agents and left for a predetermined time, since reduced to a predetermined value, Sn The complex was stable from the state of the initial bath, and as a result, an Sn film having the same film thickness (4.0 to 4.1 μm) as that of the continuous use bath could be obtained even in the initial bath.

It is sectional drawing which showed typically the multilayer ceramic capacitor as a ceramic electronic component manufactured using the plating bath of this invention.

Explanation of symbols

3a, 3b External electrode (conductive part)
5a, 5b Sn coating (metal coating)

Claims (10)

A method for producing a plating bath containing a metal salt aqueous solution containing metal ions and a complexing agent capable of forming a complex between the metal ions,
Wherein after the metallic salt aqueous solution and the complexing agent by mixing to prepare a mixed solution, so that the hydrogen ion exponent pH of the mixed solution becomes a first value larger than the acid dissociation exponent pK _a of the complexing agent And then leaving it for a predetermined time, and then reducing the hydrogen ion exponent pH of the mixed solution to a second value smaller than the first value. 2. The plating according to claim 1, wherein when the complexing agent causes a multi-stage dissociation reaction in the mixed solution, the first value is larger than the acid dissociation index pK _an of the final stage. How to make a bath.   The method for producing a plating bath according to claim 2, wherein the second value is 3 to 7.   The complexing agent includes at least one selected from polycarboxylic acids, polyoxymonocarboxylic acids, aminocarboxylic acids, lactone compounds, and salts thereof. 4. A method for producing a plating bath according to any one of 3 above.   A plating bath produced by the production method according to claim 1.   An object to be plated, an anode for supplying metal ions of a plating metal, and a cathode in contact with the object to be plated are immersed in the plating bath according to claim 5, and an energization treatment is performed between the anode and the cathode. A plating method comprising forming a metal film on the surface of the object to be plated.   The plating method according to claim 6, wherein a conductive medium formed of a material different from the object to be plated is immersed in the plating bath.   The plating method according to claim 6 or 7, wherein a non-conductive medium formed of a material different from the object to be plated is immersed in the plating bath.   The plating method according to claim 6, wherein the metal film contains tin as a main component. An electronic component manufacturing method for manufacturing an electronic component by performing a plating process on an object to be plated formed on the surface of a component element body,
10. A method of manufacturing an electronic component, wherein a metal film is formed on a surface of the conductive portion using the plating method according to claim 6.

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