JP7530759B2 - Electroless palladium plating bath - Google Patents

Description

The present invention relates to an electroless palladium plating bath.

In the electronics industry, for example, the electroless nickel (Ni) / electroless palladium (Pd) / immersion gold (Au) method (Electroless Nickel Electroless Palladium Immersion Gold: ENEPIG) is used as a surface treatment method for circuits on printed circuit boards, mounting parts and terminal parts of IC packages, etc. By using this ENEPIG process, a plating film is obtained in which an electroless nickel plating film, an electroless palladium plating film, and an immersion gold plating film are applied in sequence.

In addition, the palladium coating has good electrical conductivity, excellent corrosion resistance, and the ability to prevent the nickel base from diffusing onto the gold surface due to thermal history, so it plays an important role in the ENEPIG process described above.

Generally, plating baths are required to have excellent stability. In conventional electroless palladium plating baths, ethylenediaminetetraacetic acid or its salts have been used as stabilizers, but the plating baths are prone to spontaneous decomposition, which creates the problem of insufficient stability.

Therefore, an electroless palladium plating bath containing an organic compound containing divalent sulfur has been proposed. It is described that the use of this organic compound containing divalent sulfur improves the stability of the plating bath (see, for example, Patent Document 1).

Patent No. 3972158

In the conventional plating baths described above, the stability of the plating bath is improved by adding an organic compound containing divalent sulfur, but this can cause a problem in that the deposition of palladium on the nickel plating film is reduced.

In addition, in recent years, as the guaranteed operating temperatures of devices that could previously be handled by nickel plating films containing phosphorus (P) (nickel plating films with a phosphorus concentration of 4 to 8%) have risen, there has been an increasing demand for nickel plating films with a low phosphorus content (nickel plating films with a phosphorus concentration of less than 4%) that can be used with devices with high guaranteed operating temperatures. However, the above-mentioned conventional plating baths have a problem in that the deposition of palladium on nickel plating films with a low phosphorus content is significantly reduced, and there is a strong demand for the development of an electroless palladium plating bath that can be used with nickel plating films with a low phosphorus content.

In view of the above problems, the present invention aims to provide an electroless palladium plating bath that can suppress the decrease in the deposition properties of palladium on nickel plating films and improve the stability of the plating bath.

In order to achieve the above object, the electroless palladium plating bath of the present invention is a plating bath containing at least a palladium compound, a reducing agent, a complexing agent, and a stabilizer, and is characterized in that the stabilizer is an organic compound in which a divalent sulfur compound is bonded to a compound having a heterocyclic structure, and the organic compound does not have a thiol group or a disulfide bond.

According to the present invention, it is possible to suppress the decrease in the precipitation property of palladium on the nickel plating film and to improve the stability of the plating bath.

The electroless palladium plating bath of the present invention is described below.

<Electroless palladium plating bath>
The electroless palladium plating bath of the present invention is a plating bath containing a palladium compound, a reducing agent, a complexing agent, and a stabilizer.

(Palladium compounds)
The palladium compound is a palladium ion supply source for obtaining palladium plating. The palladium compound may be water-soluble, and examples thereof include inorganic water-soluble palladium salts such as palladium chloride, palladium sulfate, and palladium acetate, and organic water-soluble palladium salts such as tetraamine palladium hydrochloride, tetraamine palladium sulfate, tetraamine palladium acetate, tetraamine palladium nitrate, and dichlorodiethylenediamine palladium. These palladium compounds may be used alone or in combination of two or more.

The palladium ion concentration in the electroless palladium plating bath is not particularly limited, but if the palladium ion concentration is too low, the deposition rate of the plating film may decrease significantly, so 0.1 g/L or more is preferable, 0.3 g/L or more is more preferable, and 0.5 g/L or more is even more preferable. Also, if the palladium ion concentration is too high, abnormal deposition may cause the film properties to decrease, so 10 g/L or less is preferable, 5 g/L or less is more preferable, and 3 g/L or less is even more preferable.

The palladium ion concentration can be measured by atomic absorption spectrometry (AAS) using an atomic absorption spectrophotometer.

(Reducing Agent)
The reducing agent has the effect of precipitating palladium in the electroless palladium plating bath. As the reducing agent, various known reducing agents can be used, for example, formic acid or its salt, hydrazines, hypophosphorous acid or its salt, phosphorous acid or its salt, amine borane compounds, hydroboron compounds, formalin, ascorbic acid or its salt, etc.

Examples of the above-mentioned salts 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 including primary to tertiary amines.

Examples of amine borane compounds include dimethylamine borane (DMAB) and trimethylamine borane (TMAB), and examples of borohydride compounds include alkali metal borohydride salts such as sodium borohydride (SBH) and potassium borohydride (KBH).

Of these reducing agents, it is preferable to use formic acid or a salt thereof (e.g., sodium formate) from the viewpoint of achieving both stability of the plating bath and deposition properties of the plating film. These reducing agents may be used alone or in combination of two or more kinds.

The content of the reducing agent in the electroless palladium plating bath (the amount of the reducing agent when used alone, and the total amount when used in combination of two or more types) may be adjusted appropriately taking into consideration the deposition rate during plating and the stability of the plating bath, with the lower limit being 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.

(Complexing Agent)
The complexing agent mainly has the effect of stabilizing the solubility of palladium in the electroless palladium plating bath. As the complexing agent, various known complexing agents can be used, for example, at least one selected from the group consisting of ammonia and amine compounds, more preferably an amine compound. Examples of the amine compound include methylamine, dimethylamine, trimethylamine, benzylamine, methylenediamine, ethylenediamine, ethylenediamine derivatives, tetramethylenediamine, diethylenetriamine, ethylenediaminetetraacetic acid (EDTA) or its alkali metal salts, EDTA derivatives, glycine, etc. In addition, these complexing agents may be used alone or in combination of two or more.

The content of the complexing agent in the electroless palladium plating bath (the amount of the complexing agent when used alone, and the total amount when used in combination of two or more) may be adjusted appropriately taking into consideration the stabilization of the solubility of palladium described above, and the lower limit 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 complexing agent is preferably 15 g/L or less, and more preferably 10 g/L or less.

(Stabilizer)
The stabilizer is added for the purposes of improving the stability of the plating bath, improving the appearance after plating, adjusting the speed of plating film formation, and the like. In the electroless palladium plating bath of the present invention, an organic compound in which a divalent sulfur compound (a compound containing divalent sulfur) is bonded to a compound having a heterocyclic structure, as shown in the following formula (1), can be used.

[Chemical formula 1]
R ₁ -R ₂ (1)
(In the formula, R ₁ is a compound having a heterocyclic structure, R ₂ is a divalent sulfur compound, and R ₁ -R ₂ represent an organic compound having no thiol group or disulfide bond.)

Examples of the compound _R1 having a heterocyclic structure include compounds having a nitrogen-containing or sulfur-containing heterocyclic structure, such as imidazole, imidazolidine, imidazoline, oxadiazole, oxazine, thiadiazole, thiazole, thiazolidine, tetrazole, triazine, triazole, piperazine, piperidine, pyrazine, pyrazole, pyrazolidine, pyridine, pyridazine, pyrimidine, pyrrole, pyrrolidine, benzothiazole, benzimidazole, isoquinoline, thiophene, tetrahydrothiophene, and pentamethylene sulfide, and derivatives thereof.

Examples of the divalent sulfur compound R ₂ include thiadiazole, thiazole, thiazolidine, benzothiazole, thiophene, tetrahydrothiophene, methanethiol, benzenethiol, pentamethylene sulfide, dimethyl sulfide, methyl mercaptan, ethyl mercaptan, allyl mercaptan, thiopropionic acid, thioacetic acid, ethyl methyl sulfide, 1-propanethiol, 2-propanethiol, 2-aminoethanethiol, 2-mercaptoethanol, 4-mercaptopyridine, dimethyl sulfoxide, thiazolidine, thioacetate S-methyl, ethyl sulfide, methyl propyl sulfide, 1-butanethiol, thioglycolic acid, 2-(methylthio)ethanol, 3-mercapto-1-propanol, 2-methylthiazoline, cyclopentanethiol, 2-methyltetrahydrothiophene, pentamethylene sulfide, thiomorpholine, thiopropionic acid S-methyl, 3-mercaptopropionic acid, and derivatives thereof.

Examples of stabilizers represented by the above 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-pyrimidinol, 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. These stabilizers may be used alone or in combination of two or more. The chemical formulas of these stabilizers are shown below.

In the electroless palladium plating bath of the present invention, the organic compound (R ₁ -R ₂ ) used as a stabilizer includes an organic compound in which a divalent sulfur compound R ₂ bonded to a compound R ₁ having a heterocyclic structure is derived from a compound containing a thiol group (—SH).

More specifically, for example, the above-mentioned 2-(methylthio)benzimidazole is an organic compound (R ₁ -R ₂ ) in which benzimidazole, R ₁ , and methanethiol, R ₂ , are bonded together, and in the state of R ₁ -R ₂ , as shown in the above chemical formula, it does not have a thiol group (-SH), but R ₂ (methanethiol) before bonding with R ₁ has a thiol group (-SH), so R ₂ bonded with R ₁ is derived from a thiol group (-SH)-containing compound (methanethiol). The same is true for 2-(methylthio)benzothiazole (R ₁ : benzothiazole, R ₂ : methanethiol) and 2-(methylthio)pyridine (R ₁ : pyridine, R ₂ : methanethiol).

For example, (2-benzothiazolylthio)acetic acid does not have a thiol group (-SH) as shown in the above chemical formula in the state of R ₁ -R ₂ (R ₁ : benzothiazole, R ₂ : thioacetic acid), but R ₂ (thioacetic acid) before bonding with R ₁ has a thiol group (-SH), so R ₂ bonded to R ₁ is derived from a compound (thioacetic acid) containing a thiol group (-SH). The same is true for (4-pyridylthio)acetic acid (R ₁ : pyridine, R ₂ : thioacetic acid).

Furthermore, for example, 3-(2-benzothiazolylthio)propionic acid does not have a thiol group (-SH) in the state of R ₁ -R ₂ (R ₁ : benzothiazole, R ₂ : thiopropionic acid) as shown in the above chemical formula, but R ₂ (thiopropionic acid) before bonding with R ₁ has a thiol group (-SH), so R ₂ bonded to R ₁ is derived from a thiol group (-SH)-containing compound (thiopropion).

Furthermore, for example, 4,4'-dipyridyl sulfide does not have a thiol group (-SH) in the state of R ₁ -R ₂ (R ₁ : pyridine, R ₂ : 4-mercaptopyridine) as shown in the above chemical formula, but R ₂ (4-mercaptopyridine) before bonding with R ₁ has a thiol group (-SH), so R ₂ bonded to R ₁ is derived from a thiol group (-SH)-containing compound (4-mercaptopyridine).

As mentioned above, the incorporation of an organic compound containing divalent sulfur improves the stability of the plating bath, but there is a problem in that the deposition of palladium on the nickel plating film is reduced. In particular, there is a problem in that the deposition of palladium on nickel plating films with a low phosphorus content is significantly reduced.

Therefore, the present inventors have investigated the above problems and found that by using a stabilizer consisting of an organic compound in which a divalent sulfur compound is bonded to a compound having a heterocyclic structure (i.e., the above-mentioned R ₁ -R ₂ ), it is possible to suppress the deterioration of the deposition ability of palladium on the nickel plating film and to improve the stability of the plating bath.

The inventors also discovered that when organic compounds having a thiol group or disulfide bond, in which a divalent sulfur compound is bonded to a compound having a heterocyclic structure, are used, the thiol group or disulfide bond is altered by an oxidation-reduction reaction in the plating bath (i.e., a reaction in which a disulfide bond is generated by the oxidation reaction of a thiol group, and a thiol group is generated by the reduction reaction of the disulfide bond), causing a change in the deposition properties of palladium and a decrease in the stability of the plating bath.

In other words, by using an organic compound in which a divalent sulfur compound is bonded to a compound having a heterocyclic structure as a stabilizer, and which does not have a thiol group or a disulfide bond, it is possible to achieve both the deposition properties of palladium and the stability of the plating bath.

It also becomes possible to deposit palladium even in very small areas on nickel plating films that have a low phosphorus content.

The content of the stabilizer in the electroless palladium plating bath (the amount of the stabilizer alone when used alone, and the total amount when used in combination of two or more) may be adjusted appropriately taking into consideration the precipitation properties of palladium during plating and the stability of the plating bath, and the lower limit 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 stabilizer content is preferably 10 mg/L or less, more preferably 5 mg/L or less, and even more preferably 1 mg/L or less.

(Other ingredients)
In addition to the above-mentioned components, the electroless palladium plating bath of the present invention may contain various additives that are commonly used in the field of plating baths, such as pH adjusters, buffers, and surfactants.

The pH adjuster is an additive that has the effect of adjusting the pH of the plating bath, and examples thereof include acids such as hydrochloric acid, sulfuric acid, nitric acid, citric acid, malonic acid, malic acid, tartaric acid, and phosphoric acid, and alkalis such as sodium hydroxide, potassium hydroxide, and aqueous ammonia. These pH adjusters may be used alone or in combination of two or more types.

In addition, if the pH is too low, the palladium deposition rate is likely to decrease, and if the pH is too high, the stability of the electroless palladium plating bath may decrease, so the pH of the electroless palladium plating bath of the present invention is preferably 4 to 10, and more preferably 5 to 8.

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 acids such as orthophosphoric acid, phosphorous acid, hypophosphorous acid, and pyrophosphoric acid, and potassium salts, sodium salts (e.g., trisodium phosphate dodecahydrate), phosphates such as ammonium salts, boric acid, and tetraboric acid. These buffering agents may be used alone or in combination of two or more.

Surfactants are added as necessary to improve stability, prevent pitting, improve plating appearance, etc. There are no particular limitations on the surfactant, and various types of nonionic, cationic, anionic, and amphoteric surfactants can be used.

(Application)
In addition, the electroless palladium plating bath of the present invention can be used, for example, for laminated plating films having a palladium plating film and a gold plating film. The base on which the palladium plating film is formed is not particularly limited, and examples thereof include various known substrates such as aluminum (Al) and aluminum-based alloys, copper (Cu) and copper-based alloys, and plating films coated with metals that are catalytic for the reduction precipitation of the palladium plating film, such as iron (Fe), cobalt (Co), nickel (Ni), copper, zinc (Zn), silver (Ag), gold, platinum (Pt), and alloys thereof. In addition, even metals that are not catalytic can be used as the plated object by various methods.

The electroless palladium plating bath of the present invention can also be applied to the ENEPIG process. In the ENEPIG process, for example, a laminated plating film (electroless nickel/palladium/gold plating film) having a nickel plating film, the above palladium plating film, and then a gold plating film on top of that is obtained on the aluminum or aluminum-based alloy, or copper or copper-based alloy that constitutes the electrode. Each plating film can be formed by a method that is normally used.

Next, a method for producing a laminated plating film having a palladium plating film formed using the electroless palladium plating bath of the present invention based on the above-mentioned ENEPIG process will be described. Note that the conditions for forming the palladium plating film are not limited to these and can be changed as appropriate based on known techniques.

When electroless nickel plating is performed using an electroless nickel plating bath, the plating conditions and plating equipment are not particularly limited, and various known methods can be selected as appropriate. For example, the object to be plated may 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 may be set appropriately depending on the required characteristics, and is usually about 3 to 7 μm. Various known compositions such as nickel-phosphorus alloy and nickel-boron (B) alloy can be used for the electroless nickel plating bath.

The plating conditions and plating equipment used 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, the workpiece on which the nickel plating film is formed can be contacted with the electroless palladium plating bath at a temperature of 50 to 95°C for about 15 to 60 minutes. The thickness of the palladium plating film can be appropriately set depending on the required characteristics, and is usually about 0.001 to 1.0 μm.

When electroless gold plating is performed using an electroless gold plating bath, the plating conditions and plating equipment are not particularly limited, and various known methods can be selected as appropriate. For example, the object to be plated, on which a palladium plating film has been formed, can be brought into contact with an electroless gold plating bath at a temperature of 40 to 90°C for about 3 to 20 minutes. The thickness of the gold plating film can be set appropriately depending on the required characteristics, and is usually about 0.001 to 2 μm.

The electroless palladium plating bath of the present invention is also useful for applications involving electronic device components having a plating film. Examples of such electronic device components include chip parts, quartz crystal oscillators, bumps, connectors, lead frames, hoop materials, semiconductor packages, printed circuit boards, and other parts that make up electronic devices.

<Palladium plating film>
The palladium plating film of the present invention is obtained by using the electroless palladium plating bath of the present invention described above, and includes both a pure palladium film and a palladium alloy plating film containing alloy components. This is because the palladium plating film may contain elements other than palladium depending on the type of reducing agent used. In addition, components derived from the various additives described above may also be included. The balance of the palladium plating film is palladium and unavoidable impurities.

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

The invention of this application will be explained in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

(Examples 1 to 18, Comparative Examples 1 to 8, Reference Example 1)
(Preparation of plating bath)
Plating baths for Examples 1 to 18, Comparative Examples 1 to 8, and Reference Example 1 (an example not containing a stabilizer) were prepared by mixing and stirring a palladium compound (palladium salt), a complexing agent (ethylenediamine), a buffer agent (trisodium citrate dihydrate), a reducing agent (sodium formate), and a stabilizer to the concentrations shown in Tables 2 to 4. The plating bath temperature (plating temperature) and pH were set to 60° C. and 6.0, respectively.

The chemical formulas of the stabilizers used in Comparative Examples 1 to 8 are shown below.

(Pretreatment)
Before forming the electroless plating film, the substrate was subjected to the pretreatment steps 1 to 5 shown in Table 1 in order.

Step 1: The substrate (Si, TEG wafer) was degreased and cleaned using MCL-16 (manufactured by Uemura Kogyo Co., Ltd., product name: Epitas (registered trademark) MCL-16).

Step 2: Next, an acid pickling process was performed using a 30% by weight nitric acid solution to form an oxide film on the substrate surface.

Step 3: Next, the substrate was subjected to a primary zincate treatment using MCT-51 (manufactured by Uemura Kogyo Co., Ltd., product name: Epitas (registered trademark) MCT-51).

Step 4: Next, the substrate was pickled using a 30% by weight nitric acid solution to remove the Zn-substituted film and form an oxide film on the substrate surface.

Step 5: Next, a secondary zincate treatment was performed on the substrate using MCT-51 (manufactured by Uemura Kogyo Co., Ltd., product name: Epitas (registered trademark) MCT-51).

(Plating process)
Next, the substrate that had been subjected to the above-mentioned pretreatment was subjected to plating process step 6 shown in Table 1 to form an electroless nickel plating film on the substrate. More specifically, electroless plating was performed using a nickel plating bath (manufactured by Uemura Kogyo Co., Ltd., product name: Nimden (registered trademark) NPR-18) to form an electroless nickel plating film containing phosphorus (nickel plating film having a phosphorus concentration of 4 to 8%) on the substrate. Similarly, electroless plating was performed using a nickel plating bath (manufactured by Uemura Kogyo Co., Ltd., product name: Nimden (registered trademark) NLL-1) to form a nickel plating film with a low phosphorus content (nickel plating film having a phosphorus concentration of less than 4%) on the substrate.

Next, the substrate on which the nickel plating film was formed was subjected to plating process step 7 shown in Table 1 (electroless plating using the palladium plating bath of Examples 1 to 18, Comparative Examples 1 to 8, and Reference Example 1), forming a palladium plating film on the surface of the nickel plating film on the substrate (100 μm x 100 μm pad and 2 mm x 3 mm pad).

(Measurement of thickness of palladium plating film)
Next, the thickness of the palladium plating film formed on each of the pads was measured using an X-ray fluorescence measuring device (manufactured by Fisher Instruments, product name: XDV-μ). The results are shown in Tables 2 to 4.

(Evaluation of Bath Stability)
After the electroless palladium plating process, the palladium plating bath was visually observed for the presence or absence of precipitation of palladium particles, and was evaluated according to the following criteria. The results are shown in Tables 2 to 4.
◯: No deposition of palladium particles was observed even after one week had passed since the plating treatment.
×: Precipitation of palladium particles was confirmed within one week after plating.

As shown in Tables 2 and 3, in Examples 1 to 18 in which an organic compound in which a divalent sulfur compound is bonded to a compound having a heterocyclic structure and which does not have a thiol group or a disulfide bond was used as a stabilizer, the thickness of the palladium plating film on the nickel plating film (100 μm x 100 μm pad and 2 mm x 3 mm pad) was maintained similar to that of the palladium plating film in Reference Example 1, which did not contain a stabilizer, and it can be seen that even when a stabilizer was used, the decrease in the deposition property of palladium was suppressed. In particular, it can be seen that palladium was sufficiently deposited even on nickel plating films with a low phosphorus content (nickel plating films with a phosphorus concentration of less than 4%) as well as on electroless nickel plating films containing phosphorus (nickel plating films with a phosphorus concentration of 4 to 8%).

In addition, even after one week had passed since the plating process, no precipitation of palladium particles was observed in the plating bath, demonstrating the excellent stability of the plating bath.

On the other hand, as shown in Table 4, in Comparative Examples 1 to 3 in which a compound having a heterocyclic structure and no divalent sulfur compound was used as a stabilizer, Comparative Examples 4, 6, and 8 in which an organic compound having a heterocyclic structure and a divalent sulfur compound bonded thereto, which has a thiol group, was used as a stabilizer, and Comparative Examples 5 and 7 in which an organic compound having a heterocyclic structure and a divalent sulfur compound bonded thereto, which has a disulfide bond, was used as a stabilizer, no palladium was precipitated at all on the nickel plating film (100 μm x 100 μm pad) with a low phosphorus content. In addition, precipitation of palladium particles was confirmed in the plating bath within one week after plating, indicating that the plating bath was poorly stable.

The present invention is particularly suitable for use in laminated plating films having palladium plating films and gold plating films, and in electroless palladium plating baths used in the ENEPIG process, etc.

Claims (2)

A plating bath containing at least a palladium compound, a reducing agent, a complexing agent, and a stabilizer,
the stabilizer is an organic compound in which a divalent sulfur compound is bonded to a compound having a heterocyclic structure, the organic compound having no thiol group or disulfide bond,
the compound having a heterocyclic structure is at least one selected from the group consisting of imidazole, imidazolidine, imidazoline, oxadiazole, oxazine, thiadiazole, thiazole, thiazolidine, tetrazole, triazine, triazole, piperazine, piperidine, pyrazine, pyrazole, pyrazolidine, pyridine, pyridazine, pyrimidine, pyrrole, pyrrolidine, benzothiazole, benzimidazole, isoquinoline, thiophene, tetrahydrothiophene, and pentamethylene sulfide ;
The electroless palladium plating bath is characterized in that the divalent sulfur compound is at least one selected from the group consisting of thiadiazole, thiazole, thiazolidine, benzothiazole, thiophene, tetrahydrothiophene, methanethiol, benzenethiol, pentamethylene sulfide, dimethyl sulfide, methyl mercaptan, ethyl mercaptan, allyl mercaptan, thiopropionic acid, thioacetic acid, ethyl methyl sulfide, 1-propanethiol, 2-propanethiol, 2-aminoethanethiol, 2-mercaptoethanol, dimethyl sulfoxide, thiazolidine, S-methyl thioacetate, ethyl sulfide, methyl propyl sulfide, 1-butanethiol, 2-(methylthio)ethanol, 3-mercapto-1-propanol, 2-methylthiazoline, cyclopentanethiol, 2-methyltetrahydrothiophene, pentamethylene sulfide, thiomorpholine, and S -methyl thiopropionate. The electroless palladium plating bath according to claim 1, characterized in that the concentration of the stabilizer is 0.01 to 10 mg/L.