Chemical palladium plating bath

Abstract

The present invention discloses an electroless palladium plating bath comprising at least a palladium compound, a reducing agent, a complex, and a stabilizer. The stabilizer is an organic compound formed by bonding a divalent sulfur compound to a compound having a heterocyclic structure, and the organic compound lacks thiol groups and disulfide bonds. Therefore, the stabilizer can suppress the reduction in palladium precipitation on nickel-plated films while improving the stability of the plating bath.

Description

Technical Field

[0001] The invention relates to a chemical palladium plating bath. Background Art

[0002] In the electronics industry, electroless nickel (Ni) / electroless palladium (Pd) / displacement gold (Au) plating (ENEPIG) is used as a surface treatment method for printed circuit boards (PCBs), IC package mounting areas, and terminals. Using the ENEPIG process, a coating is obtained by sequentially stacking an electroless nickel film, an electroless palladium film, and a displacement gold film.

[0003] The palladium film exhibits good electrical conductivity and excellent corrosion resistance. It also has the function of preventing the base nickel from diffusing to the gold surface due to thermal history. Therefore, the palladium film plays an important role in the above-mentioned ENEPIG process.

[0004] Here, the plating bath is generally required to have excellent stability. Conventional electroless palladium plating baths use ethylenediaminetetraacetic acid or its salts as a stabilizer, but the plating bath is prone to spontaneous decomposition, resulting in insufficient stability.

[0005] Therefore, a chemical palladium plating bath to which an organic compound containing divalent sulfur is added has been proposed, and it is described that the stability of the plating bath is improved by using the organic compound containing divalent sulfur (for example, see Patent Document 1).

[0006] Patent Document 1: Japanese Patent No. 3972158 Summary of the Invention

[0007] -Technical problem to be solved by the invention-

[0008] Here, the above-mentioned conventional plating bath has a problem in that the stability of the plating bath is improved by adding an organic compound containing divalent sulfur, but the precipitation of palladium on the nickel plating film is reduced.

[0009] In recent years, the formation of nickel-plated films containing phosphorus (P) (nickel-plated films with a phosphorus concentration of 4-8%) has increased the guaranteed operating temperature of the equipment being plated. As the guaranteed operating temperature of such equipment increases, there is a growing demand for nickel-plated films with a low phosphorus content (nickel-plated films with a phosphorus concentration of less than 4%) that can be formed in equipment operating at high guaranteed operating temperatures. However, the aforementioned existing plating baths suffer from a particularly serious problem: palladium deposition is significantly reduced on these low-phosphorus nickel-plated films. Therefore, there is a pressing need for the development of electroless palladium plating baths capable of forming low-phosphorus nickel-plated films.

[0010] The present invention has been made to solve the above-mentioned problems, and its object is to provide an electroless palladium plating bath that can suppress the reduction in the precipitation of palladium on the nickel plating film and improve the stability of the plating bath.

[0011] -Technical solutions for solving technical problems-

[0012] To achieve the above-mentioned object, the electroless palladium plating bath of the present invention contains at least a palladium compound, a reducing agent, a complex and a stabilizer. The stabilizer is an organic compound formed by bonding a divalent sulfur compound and a compound having a heterocyclic structure, and the organic compound does not have a thiol group and a disulfide bond.

[0013] -Effects of the Invention-

[0014] According to the present invention, it is possible to suppress a decrease in the precipitation of palladium on the nickel plating film and to improve the stability of the plating bath. DETAILED DESCRIPTION

[0015] Next, the electroless palladium plating bath of the present invention will be described.

[0016] <Electroless Palladium Plating Bath>

[0017] The chemical palladium plating bath of the present invention contains a palladium compound, a reducing agent, a complex and a stabilizer.

[0018] (Palladium compounds)

[0019] A palladium compound is a source of palladium ions used to produce a palladium plating bath. Any palladium compound is acceptable as long as it is water-soluble. Examples of such palladium compounds include inorganic water-soluble palladium salts such as palladium chloride, palladium sulfate, and palladium acetate; and organic water-soluble palladium salts such as tetraamminepalladium hydrochloride, tetraamminepalladium sulfate, tetraamminepalladium acetate, tetraamminepalladium nitrate, and ethylenediaminepalladium chloride. These palladium compounds can be used alone or as a mixture of two or more.

[0020] The concentration of palladium ions in the electroless palladium plating bath is not particularly limited, but if the palladium ion concentration is too low, there is a situation where the precipitation rate of the plated film is significantly reduced. Therefore, the palladium ion concentration is preferably more than 0.1g / L, more preferably more than 0.3g / L, and more preferably more than 0.5g / L. If the palladium ion concentration is too high, there is a situation where the physical properties of the epithelium are reduced due to abnormal precipitation, etc. Therefore, the palladium ion concentration is preferably less than 10g / L, more preferably less than 5g / L, and more preferably less than 3g / L.

[0021] It should be noted that the palladium ion concentration can be measured by atomic absorption spectrometry (AAS) using an atomic absorption spectrophotometer.

[0022] (reducing agent)

[0023] The reducing agent has the function of precipitating palladium in the electroless palladium plating bath. Various known reducing agents can be used as the reducing agent, and examples thereof include formic acid or its salts, hydrazines, hypophosphorous acid or its salts, phosphorous acid or its salts, amine borane compounds, boron hydride compounds, formalin, ascorbic acid or its salts, and the like.

[0024] Examples of the salt include alkali metal salts such as potassium and sodium; alkaline earth metal salts such as magnesium and calcium; ammonium salts; quaternary ammonium salts; and amine salts containing primary to tertiary amines.

[0025] Examples of the amine borane compound include dimethylamine borane (DMAB) and trimethylamine borane (TMAB); examples of the borohydride compound include alkali metal borohydride salts such as sodium borohydride (SBH) and potassium borohydride (KBH).

[0026] It should be noted that, from the perspective of achieving both plating bath stability and plating film deposition, formic acid or its salts (eg, sodium formate) are preferably used among the above reducing agents.

[0027] The content of the reducing agent in the electroless palladium plating bath can be appropriately adjusted (the amount when used alone, the total amount when two or more are mixed) taking into account the precipitation rate during the plating process and the stability of the plating bath. The lower limit of the reducing agent content is preferably 1 g / L or more, more preferably 3 g / L or more, even more preferably 5 g / L or more, and particularly preferably 10 g / L or more. The upper limit of the reducing agent content is preferably 100 g / L or less, more preferably 80 g / L or less, and even more preferably 50 g / L or less.

[0028] (Complex)

[0029] The complex mainly has the function of stabilizing the solubility of palladium in the electroless palladium plating bath. As the complex, various well-known complexes can be used, for example, at least one selected from ammonia and amine compounds, and more preferably an amine compound. Examples of amine compounds include methylamine, dimethylamine, trimethylamine, benzylamine, methylenediamine, ethylenediamine, ethylenediamine derivatives, tetramethylenediamine, diethylenetriamine, ethylenediaminetetraacetic acid (EDTA) or alkali metal salts thereof, EDTA derivatives, glycine, and the like. It should be noted that the above-mentioned complexes can be used alone or in combination of two or more.

[0030] Taking into account the stabilization of the above-mentioned palladium solubility, the content of the complex in the electroless palladium plating bath can be appropriately adjusted (the amount when used alone is the individual amount, and the total amount when two or more are mixed). The lower limit of the content of the complex is preferably 0.1 g / L or more, more preferably 1 g / L or more, and even more preferably 3 g / L or more. The upper limit of the content of the complex is preferably 15 g / L or less, and more preferably 10 g / L or less.

[0031] (Stabilizer)

[0032] The stabilizer is added for the purpose of improving the stability of the plating bath, improving the appearance after plating, and adjusting the formation rate of the plated film. In the chemical palladium plating bath of the present invention, an organic compound represented by the following formula (1) in which a divalent sulfur compound (a compound containing divalent sulfur) and a compound having a heterocyclic structure are bonded can be used.

[0033] [Chemical Formula 1]

[0034] R1-R2(1)

[0035] (In the formula, R1 is a compound having a heterocyclic structure, R2 is a divalent sulfur compound, and R1-R2 represents an organic compound without a thiol group and a disulfide bond.)

[0036] Examples of the compound R1 having a heterocyclic structure include imidazole, tetrahydroimidazole, imidazoline, oxadiazole, oxazine, thiadiazole, thiazole, tetrahydrothiazole, tetrazole, triazine, triazole, piperazine, piperidine, pyrazine, pyrazole, pyrazolidine, pyridine, pyridazine, pyrimidine, pyrrole, pyrrolidine, benzothiazole, benzimidazole, isoquinoline, thiophene, tetrahydrothiophene, pentamethylene sulfide, and the like, and their derivatives.

[0037] Examples of the divalent sulfur compound R2 include thiadiazole, thiazole, tetrahydrothiazolidine, benzothiazole, thiophene, tetrahydrothiophene, methyl mercaptan, benzenethiol, pentamethylene sulfide, dimethyl sulfide, methylmercaptan, ethyl mercaptan, allyl mercaptan, thiopropionic acid, thioacetic acid, methyl ethyl sulfide, 1-propanethiol, 2-propanethiol, 2-aminoethanethiol, 2-mercaptoethanol, 4-mercaptopyridine, dimethyl sulfoxide, tetrahydrothiazolidine, methyl mercaptan acetate, ethyl sulfide, methylpropyl sulfide, 1-butanethiol, thioglycolic acid, 2-(methylthio)ethanol, 3-mercapto-1-propanol, 2-methylthiazoline, cyclopentanethiol, 2-methyltetrahydrothiophene, pentamethylene sulfide, thiomorpholine, S-methylthiopropionic acid, 3-mercaptopropionic acid, and derivatives thereof.

[0038] Examples of the stabilizer represented by formula (1) include 2-(4-thiazolyl)benzimidazole, 2-(methylthio)benzimidazole, 2-(methylthio)benzothiazole, (2-benzothiazolylthio)acetic acid, 3-(2-benzothiazolylthio)propionic acid, 2-(methylthio)pyridine, (4-pyridylthio)acetic acid, 4,4'-dipyridyl sulfide, 2-methylthio-4-hydroxypyrimidine, S-methylthiobarbituric acid, 4-amino-6-chloro-2-(methylthio)pyrimidine, 5-(methylthio)-1H-tetrazole, 5-(ethylthio)-1H-tetrazole, N-(phenylthio)phthalimide, and 5-(methylthio)thiophene-2-carboxaldehyde. The stabilizers may be used alone or in combination of two or more. The chemical formulas of the stabilizers are shown below.

[0039] [Chemical Formula 2]

[0040]

[0041] [Chemical Formula 3]

[0042]

[0043] In the electroless palladium plating bath of the present invention, the organic compound (R1-R2) used as a stabilizer includes an organic compound in which the divalent sulfur compound R2 bonded to the heterocyclic compound R1 is derived from a thiol group (—SH)-containing compound.

[0044] More specifically, for example, the aforementioned 2-(methylthio)benzimidazole is an organic compound (R1-R2) formed by bonding benzimidazole (R1) to methylthiol (R2). As shown in the chemical formula above, the R1-R2 structure does not contain a thiol group (-SH), whereas R2 (methylthiol) before bonding to R1 does contain a thiol group (-SH). Therefore, R2 bonded to R1 is derived from a compound (methylthiol) containing a thiol group (-SH). The same is true for 2-(methylthio)benzothiazole (R1: benzothiazole, R2: methylthiol) and 2-(methylthio)pyridine (R1: pyridine, R2: methylthiol).

[0045] For example, (2-benzothiazolylthio)acetic acid, in its R1-R2 state (R1: benzothiazole, R2: thioacetic acid), as shown in the chemical formula above, does not have a thiol group (-SH). However, R2 (thioacetic acid) before bonding to R1 does have a thiol group (-SH). Therefore, R2 bonded to R1 is derived from a compound (thioacetic acid) containing a thiol group (-SH). The same is true for (4-pyridylthio)acetic acid (R1: pyridine, R2: thioacetic acid).

[0046] For example, 3-(2-benzothiazolylthio)propionic acid, in the R1-R2 state (R1: benzothiazole, R2: thiopropionic acid), does not have a thiol group (-SH) as shown in the above chemical formula. However, R2 (thiopropionic acid) before bonding to R1 does have a thiol group (-SH). Therefore, R2 bonded to R1 is derived from a compound (thiopropionic acid) containing a thiol group (-SH).

[0047] For example, 4,4'-dipyridyl sulfide does not have a thiol group (-SH) in the R1-R2 state (R1: pyridine, R2: 4-mercaptopyridine), as shown in the above chemical formula. However, R2 (4-mercaptopyridine) before bonding to R1 does have a thiol group (-SH). Therefore, R2 before bonding to R1 is derived from a compound (4-mercaptopyridine) containing a thiol group (-SH).

[0048] As mentioned above, the addition of a divalent sulfur-containing organic compound improves the stability of the plating bath, but there is a problem of reduced palladium deposition on nickel-plated films. In particular, palladium deposition on nickel-plated films with low phosphorus content is significantly reduced.

[0049] Therefore, the inventors of the present application have studied the above-mentioned problem and found that by using a stabilizer formed by an organic compound, the reduction in the precipitation of palladium on the nickel-plated film can be suppressed and the stability of the plating bath can be improved; the organic compound is an organic compound formed by bonding a divalent sulfur compound and a compound having a heterocyclic structure (i.e., the above-mentioned R1-R2).

[0050] The inventors of the present application have also discovered that when a compound containing a thiol group or a disulfide bond is used, which is an organic compound formed by bonding a divalent sulfur compound to a compound having a heterocyclic structure, the thiol group or disulfide bond deteriorates due to redox reactions in the plating bath (i.e., reactions in which the disulfide bond is generated by oxidation of the thiol group and the thiol group is generated by reduction of the disulfide bond). Consequently, not only does the palladium precipitation property change, but the stability of the plating bath is also reduced.

[0051] Specifically, an organic compound formed by bonding a divalent sulfur compound to a compound having a heterocyclic structure is used as a stabilizer. This organic compound does not have a thiol group or a disulfide bond, thereby achieving both palladium precipitation and bath stability.

[0052] It is also possible to deposit palladium on a minute portion of a nickel-plated film having a low phosphorus content.

[0053] The content of the stabilizer in the electroless palladium plating bath can be appropriately adjusted (the amount when used alone is the amount alone, and the total amount when two or more are mixed) taking into account the precipitation of palladium during the plating process and the stability of the plating bath. The lower limit of the content of the stabilizer is preferably 0.01 mg / L or more, more preferably 0.03 mg / L or more, and even more preferably 0.05 mg / L or more. The upper limit of the content of the stabilizer is preferably 10 mg / L or less, more preferably 5 mg / L or less, and even more preferably 1 mg / L or less.

[0054] (Other ingredients)

[0055] In addition to the above-mentioned components, various additives commonly used in the field of plating baths may be added to the electroless palladium plating bath of the present invention. Examples of such additives include pH adjusters, buffers, and surfactants.

[0056] A pH adjuster is an additive that adjusts the pH of the plating bath. Examples of such agents include acids such as hydrochloric acid, sulfuric acid, nitric acid, citric acid, malonic acid, malic acid, tartaric acid, and phosphoric acid, and bases such as sodium hydroxide, potassium hydroxide, and aqueous ammonia. These pH adjusters may be used alone or in combination of two or more.

[0057] If the pH value is too low, the palladium deposition rate tends to decrease; if the pH value is too high, the stability of the electroless palladium plating bath decreases. Therefore, the pH value of the electroless palladium plating bath of the present invention is preferably 4 to 10, more preferably 5 to 8.

[0058] A buffering agent having a buffering effect may also be added. Examples of such buffering agents include citric acid such as trisodium citrate dihydrate, carboxylic acids such as tartaric acid, malic acid, and phthalic acid, phosphoric acid such as orthophosphoric acid, phosphorous acid, hypophosphorous acid, and pyrophosphoric acid, or their potassium salts, sodium salts (such as trisodium phosphate dodecahydrate), and ammonium salts, boric acid, and tetraboric acid. It should be noted that the above buffering agents may be used alone or in combination of two or more.

[0059] A surfactant may be added as needed for the purpose of improving stability, preventing pitting, improving the appearance of the plating, etc. The surfactant is not particularly limited, and nonionic, cationic, anionic, and amphoteric surfactants can be used.

[0060] (use)

[0061] The electroless palladium plating bath of the present invention can be used, for example, to form a laminated film comprising a palladium-plated film and a gold-plated film. The substrate for forming the palladium-plated film is not particularly limited; examples include various well-known substrates such as aluminum (Al) or aluminum alloys, copper (Cu) or copper alloys, and a coating formed by coating the substrate with a metal that catalyzes the reduction and precipitation of the palladium-plated film, such as iron (Fe), cobalt (Co), nickel (Ni), copper, zinc (Zn), silver (Ag), gold, platinum (Pt), or alloys thereof. Even non-catalytic metals can be used as the substrate to be plated using various methods.

[0062] Furthermore, the electroless palladium plating bath of the present invention can be applied to the ENEPIG process. In the ENEPIG process, for example, a stacked coating (electroless nickel / palladium / gold coating) comprising a nickel coating, the palladium coating, and a gold coating thereon can be formed on aluminum or an aluminum alloy, or copper or a copper alloy forming an electrode. It should be noted that each coating can be formed using conventional methods.

[0063] Next, a method for producing a laminated plated film having a palladium plated film formed using the electroless palladium plating bath of the present invention under the above-mentioned ENEPIG process will be described. It should be noted that the conditions for forming the palladium plated film are not limited thereto and can be appropriately modified according to known techniques.

[0064] The plating conditions and plating apparatus when using an electroless nickel plating bath for chemical nickel plating are not particularly limited, and various known methods can be appropriately selected. For example, the object to be plated can be brought into contact with an electroless nickel plating bath at a temperature of 50 to 95°C for about 15 to 60 minutes. The thickness of the nickel plating film can be appropriately set according to the required properties and is generally about 3 to 7 μm. In addition, various known compositions such as nickel-phosphorus alloys and nickel-boron (B) alloys can be used for the electroless nickel plating bath.

[0065] The plating conditions and apparatus for electroless palladium plating using the electroless palladium plating bath of the present invention are not particularly limited, and various known methods can be appropriately selected. For example, an article to be plated, having a nickel plating film formed thereon, can be brought into contact with an electroless palladium plating bath at a temperature of 50 to 95°C for approximately 15 to 60 minutes. The thickness of the palladium plating film can be appropriately set depending on the desired properties and is generally approximately 0.001 to 1.0 μm.

[0066] The plating conditions and apparatus for electroless gold plating using an electroless gold plating bath are not particularly limited, and various known methods can be appropriately selected. For example, an object having a palladium-plated film formed thereon can be brought into contact with an electroless gold plating bath at a temperature of 40 to 90°C for approximately 3 to 20 minutes. The thickness of the gold-plated film can be appropriately set depending on the desired properties and is generally approximately 0.001 to 2 μm.

[0067] The electroless palladium plating bath of the present invention is also useful for electronic device components having a plated film. Examples of such electronic device components include chip components, quartz oscillators, bump electrodes, connectors, lead frames, ring materials, semiconductor packages, printed circuit boards, and other electronic device components.

[0068] <Palladium coating>

[0069] The palladium-plated film of the present invention can be obtained using the chemical palladium plating bath of the present invention. The palladium-plated film includes both pure palladium films and palladium alloy films containing alloy components. This is because the palladium-plated film may contain elements other than palladium depending on the type of reducing agent used. It should be noted that the film may also contain components derived from the various additives mentioned above. The residues in the palladium-plated film consist of palladium and unavoidable impurities.

[0070] For example, when formic acid or its salts, hydrazine or its salts are used as reducing agents, a pure palladium film can be obtained. In contrast, when a phosphoric acid compound such as hypophosphite or phosphite is used as a reducing agent other than the formic acid or its salts, a palladium-plated film containing phosphorus can be obtained. When a boron compound such as an amine borane compound or a hydrogen boron compound is used, a palladium-plated film containing boron can be obtained. When both the phosphoric acid compound and the boron compound are used, a palladium-plated film containing both phosphorus and boron can be obtained.

[0071] (Example)

[0072] Hereinafter, the present invention will be described in more detail based on Examples and Comparative Examples, but the present invention is not limited to the following Examples.

[0073] (Examples 1 to 18, Comparative Examples 1 to 8, Reference Example 1)

[0074] (Preparation of Plating Bath)

[0075] A palladium compound (palladium salt), ethylenediamine as a complex, trisodium citrate dihydrate as a buffer, sodium formate as a reducing agent, and a stabilizer were mixed and stirred to the concentrations shown in Tables 2 to 4 to prepare plating baths for Examples 1 to 18, Comparative Examples 1 to 8, and Reference Example 1 (an example without a stabilizer). The plating bath temperature (plating treatment temperature) was set to 60°C, and the pH was set to 6.0.

[0076] The chemical formula of each stabilizer used in Comparative Examples 1 to 8 is shown below.

[0077] [Chemical Formula 4]

[0078]

[0079] (Pre-processing)

[0080] Before forming the chemical plating film, the substrate was subjected to the pretreatment steps 1 to 5 shown in Table 1 in sequence.

[0081] Step 1: The substrate (Si, TEG wafer) was degreased and cleaned using MCL-16 (manufactured by Uemura Industry Co., Ltd., trade name: EPITHAS (registered trademark) MCL-16).

[0082] Step 2: Next, an acid pickling treatment was performed using a 30% by mass nitric acid solution, thereby forming an oxide film on the surface of the substrate.

[0083] Step 3: Next, the substrate was subjected to a primary zinc immersion treatment (zincification) using MCT-51 (manufactured by Uemura Industry Co., Ltd., trade name: EPITHAS (registered trademark) MCT-51).

[0084] Step 4: Next, the Zn-substituted film was peeled off by pickling using a 30% by mass nitric acid solution, and an oxide film was formed on the surface of the substrate.

[0085] Step 5: Next, the substrate was subjected to a secondary zinc immersion treatment using MCT-51 (manufactured by Uemura Industry Co., Ltd., trade name: EPITHAS (registered trademark) MCT-51).

[0086] (Plating treatment)

[0087] Next, the substrate that had undergone the above-described pretreatment was subjected to the plating treatment step 6 shown in Table 1, thereby forming an electroless nickel plating film on the substrate. More specifically, an electroless nickel plating treatment was performed using a nickel plating bath (manufactured by Uemura Industry Co., Ltd., trade name: NIMUDEN (registered trademark) NPR-18) to form an electroless nickel plating film containing phosphorus (a nickel plating film having a phosphorus concentration of 4 to 8%) on the substrate. Furthermore, an electroless nickel plating treatment was similarly performed using a nickel plating bath (manufactured by Uemura Industry Co., Ltd., trade name: NIMUDEN (registered trademark) NLL-1) to form a nickel plating film with a low phosphorus content (a nickel plating film having a phosphorus concentration of less than 4%) on the substrate.

[0088] Next, the substrate on which the above-mentioned nickel-plated film had been formed was subjected to the plating treatment step 7 shown in Table 1 (chemical plating treatment using a palladium plating bath in Examples 1 to 18, Comparative Examples 1 to 8, and Reference Example 1), and a palladium-plated film was formed on the surface of the nickel-plated film (100μm×100μm solder pad and 2mm×3mm solder pad) on the substrate.

[0089] [Table 1]

[0090]

[0091] (Measurement of Palladium Plating Film Thickness)

[0092] Next, the film thickness of the palladium plating film formed on each pad was measured using a fluorescent X-ray measuring instrument (manufactured by Fischer Instrumentation, trade name: XDV-μ). The above results are shown in Tables 2 to 4.

[0093] (Evaluation of bath stability)

[0094] The presence or absence of palladium particles in the palladium plating bath after the electroless palladium plating treatment was visually observed and evaluated based on the following criteria. The above results are shown in Tables 2 to 4.

[0095] ○: Even after one week has passed since the plating treatment, no precipitation of palladium particles was observed.

[0096] ×: Within one week after the plating treatment, precipitation of palladium particles was confirmed.

[0097] [Table 2]

[0098]

[0099] [Table 3]

[0100]

[0101] [Table 4]

[0102]

[0103] Tables 2-3 show that in Examples 1-18, an organic compound formed by bonding a divalent sulfur compound to a compound having a heterocyclic structure, and lacking a thiol group or disulfide bond, was used as a stabilizer. The palladium film thickness on the nickel-plated films (100 μm × 100 μm pads and 2 mm × 3 mm pads) in Examples 1-18 was maintained at the same level as the palladium-plated film thickness in Reference Example 1, which did not contain a stabilizer. Even with the use of a stabilizer, a decrease in palladium precipitation was suppressed. In particular, palladium precipitation was found to be as sufficient on low-phosphorus nickel-plated films (those with a phosphorus concentration of less than 4%) as on phosphorus-containing electroless nickel-plated films (those with a phosphorus concentration of 4 to 8%), with the two depositing to the same degree.

[0104] Even after one week had passed since the plating treatment, no precipitation of palladium particles was observed in the plating bath, indicating that the stability of the plating bath was excellent.

[0105] On the other hand, Table 4 shows that in Comparative Examples 1-3, a divalent sulfur compound not bonded to a compound having a heterocyclic structure was used as a stabilizer; in Comparative Examples 4, 6, and 8, an organic compound comprising a divalent sulfur compound bonded to a compound having a heterocyclic structure, and having a thiol group, was used as a stabilizer; and in Comparative Examples 5 and 7, an organic compound comprising a divalent sulfur compound bonded to a compound having a heterocyclic structure, and having a disulfide bond, was used as a stabilizer. In Comparative Examples 1-3, Comparative Examples 4, 6, 8, and Comparative Examples 5 and 7, no palladium was deposited on the low-phosphorus nickel plating film (100 μm × 100 μm pad). Within one week after the electroplating process, palladium particles were observed deposited in the plating bath, indicating a lack of bath stability.

[0106] Industrial Applicability

[0107] The present invention is particularly suitable for use in a chemical palladium plating bath used in a stacked film having a palladium plating film and a gold plating film, an ENEPIG process, and the like.

Claims

1. A chemical palladium plating bath comprising at least a palladium compound, a reducing agent, a complex and a stabilizer, characterized in that: The stabilizer is an organic compound formed by bonding a divalent sulfur compound with a compound having a heterocyclic structure, and the organic compound does not have a thiol group and a disulfide bond. The stabilizer is 2-(4-thiazolyl)benzimidazole, 2-(methylthio)benzimidazole, 2-(methylthio)benzothiazole, (2-benzothiazolylthio)acetic acid, 3-(2-benzothiazolylthio)propionic acid, 2-(methylthio)pyridine, (4-pyridylthio)acetic acid, 4,4'-dipyridyl sulfide, 2-methylthio-4-hydroxypyrimidine, S-methylthiobarbituric acid, 4-amino-6-chloro-2-(methylthio)pyrimidine, 5-(methylthio)-1H-tetrazole, 5-(ethylthio)-1H-tetrazole, 5-(benzylthio)-1H-tetrazole, N-(phenylthio)phthalimide or 5-(methylthio)thiophene-2-carboxaldehyde.

2. The chemical palladium plating bath according to claim 1, wherein: The concentration of the stabilizer is 0.01-10 mg / L.

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