JP5278988B2 - Electroless tin plating bath and electroless tin plating method for electronic parts - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve temporal stability of a mildly acidic or neutral electroless tin plating bath. <P>SOLUTION: The electroless tin plating bath has a pH of 2.5-7 and comprises (A) a soluble stannous salt, (B) at least one acid chosen from an inorganic salt and an organic salt, (C) thioureas, (D) a complexing agent comprising oxycarboxylic acid for stabilizing the bath and (E) a metal ion chosen from the group consisting of chromium, manganese, iron, aluminum, vanadium, zirconium and molybdenum, provided that the amount of the metal ion (E) is 0.01-6.0 mol/L. Due to the coexistence of a specific metal ion such as iron, manganese and chromium at a prescribed concentration with oxycarboxylic acid, the electroless tin plating bath becomes mildly acidic or neutral and shows an excellent temporal stability. <P>COPYRIGHT: (C)2009,JPO&amp;INPIT

Description

  The present invention relates to a weakly acidic to neutral electroless tin plating bath and an electroless tin plating method for an electronic component using the plating bath, and has excellent bath aging stability and erosion of a resin protective coating on the electronic component during plating. Provide the one that can prevent the problem well.

Tin plating is widely used in weak electrical industry and electronic industry parts as a film for improving solderability or etching resist, and electroless tin plating is effective from the viewpoint of uniform film thickness.
For example, Patent Documents 1 to 5 disclose acidic electroless tin plating baths.
Patent Document 1 contains a soluble stannous salt, an acid such as organic sulfonic acid and organic carboxylic acid, thiourea, and perchloric acid, and if necessary, a coating of uniform and fine particles. An electroless tin plating bath to which a water-soluble zirconium salt can be added in order to obtain (Claims, upper left column on page 3). As said carboxylic acid, tartaric acid, malic acid, a citric acid, gluconic acid, etc. are described (the 2nd page lower left column).

  The above-mentioned Patent Document 2 is an electroless tin plating bath that contributes to the improvement of the deposition rate and the bonding property of the plating film by adding a metal salt within a range of a predetermined oxidation-reduction potential within a range in which a tin alloy is not formed. Item 1), iron, chromium, zinc, bismuth, indium and the like are exemplified as the predetermined metal (paragraph 13).

  Patent Document 3 describes an electroless tin plating bath containing a soluble salt of a specific metal selected from the group consisting of bismuth, indium, lead, and antimony at a predetermined concentration for the purpose of improving bonding strength and preventing whiskers. (Claim 1, Examples 1 to 5).

  Patent Document 4 describes that an iron compound (for example, potassium ferrocyanide) is added to an electroless plating bath containing a metal compound such as tin and a copper compound (paragraph potassium ferrocyanide) (paragraph). 19).

However, in general, when electroless plating is performed using an electroless tin bath as described in Patent Documents 1 to 4 on a fine pattern such as a printed board, a flexible printed board, or a film carrier, an acidic electroless tin bath is obtained. Therefore, the vicinity of the edge of the solder resist of the electronic component is partly peeled off, and erosion occurs. In particular, the printed circuit board and the flexible printed circuit board are easily eroded, and the copper of the base material may be eluted to reduce the reliability of the product. There is.
Therefore, it is conceivable to avoid the adverse effects due to the erodibility by performing electroless plating on the electronic component using an electroless tin plating bath in a weakly acidic or neutral range.

Patent Documents 5 to 7 disclose weakly acidic or neutral electroless tin plating baths.
That is, the above-mentioned Patent Document 5 uses a polyoxycarboxylic acid such as gluconic acid, tetronic acid, or pentonic acid or a salt thereof instead of organic sulfonic acid as a base acid, to a more preferable pH of about 1.2 to 1.9. It is the adjusted electroless tin plating bath (Claim 1, paragraphs 3-4, 7, 12).

  Patent Document 6 is an electroless tin plating bath having a pH of 6.5 to 11 to which trivalent titanium salt (reducing agent) or citric acid can be added at a predetermined concentration (Claims 4 to 6).

  Patent Document 7 is a displacement plating bath of tin or the like that contains a water-soluble saccharide having a nitrogen atom and preferably has a pH of 6 to 12 (claims 2-3, 8, 9, 17, paragraph 26).

JP-A-2-217478 Japanese Patent Laid-Open No. 10-36973 JP-A-8-296050 JP-A-8-193275 JP-A-4-289178 JP-A-8-60376 JP 2002-180259 A

However, the electroless tin plating bath in the above weakly acidic to neutral range has no harmful effect of eroding the solder resist such as a printed circuit board, but on the other hand, gluconic acid, citric acid, tartaric acid, etc. having a complexing action to divalent tin. Even if the oxycarboxylic acid is contained, the stability over time is low, the precipitation of the tin compound occurs at an early stage, and the plating bath is decomposed. For example, depending on the composition, it may be decomposed within one day. In many cases, it is not practical from the viewpoint of continuous plating.
An object of the present invention is to improve practicability by improving stability over time in an electroless tin plating bath in a weakly acidic to neutral range.

First of all, the present inventors presuppose that this electroless tin plating bath is used on the assumption that a weakly acidic or neutral electroless tin plating bath is used from the viewpoint of reducing the influence on the solder resist when tin plating is performed on a printed circuit board or the like. While adding oxycarboxylic acids such as gluconic acid, citric acid and tartaric acid to the tin bath, we studied the stability of the bath when other components coexist.
As a result, when a specific metal ion such as iron, manganese, chromium, aluminum, zirconium coexists at a predetermined concentration, the stability over time of the plating bath in a weakly acidic or neutral region can be increased, and decomposition can be satisfactorily prevented. Further, the present invention has found that the concentration ratio of the metal ion to the oxycarboxylic acid is limited to a specific range, or that the use of a trivalent or higher oxycarboxylic acid can further contribute to the improvement of the bath stability over time. Was completed.

That is, the present invention 1 includes (A) a soluble stannous salt,
(B) at least one of an inorganic acid and an organic acid;
(C) thioureas,
(D) a bath stabilizing complexing agent comprising oxycarboxylic acid;
(E) containing at least one metal ion selected from the group consisting of chromium, manganese, iron, aluminum, vanadium, zirconium, and molybdenum,
The electroless tin plating bath is characterized in that the content of the metal ion (E) is 0.01 to 6.0 mol / L and has a pH of 2.5 to 7.

The present invention 2 is the electroless tin plating bath according to the present invention 1, wherein the bath stabilizing complexing agent (D) is an oxycarboxylic acid having 3 or more hydroxyl groups .

  Invention 3 is that, in the Invention 1 or 2, the molar ratio of the content of the metal ion (E) and the complexing agent for bath stabilization (D) is E: D = 1: 1-30. This is an electroless tin plating bath.

  Invention 4 is the composition of any one of Inventions 1 to 3, wherein the weight ratio of the component (A), the component (B), and the component (D) is A: B: D = 1: 2-8: It is an electroless tin plating bath characterized by being 2-16.

  Invention 5 is the electroless tin plating bath according to any one of Inventions 1 to 4, wherein the acid (B) is an organic sulfonic acid.

The present invention 6 comprises (A) a soluble stannous salt,
(B) an organic sulfone,
(C) thiourea,
(D) an oxycarboxylic acid having three or more hydroxyl groups ;
(E) contains iron ions,
The electroless tin plating bath is characterized in that the content of iron ions (E) is 0.03 to 3.0 mol / L and the pH is 2.5 to 6.

  Invention 7 is the electroless tin plating bath according to any one of Inventions 1 to 6, further comprising an additive selected from the group consisting of an antioxidant, a surfactant and a pH adjuster. It is.

  Invention 8 is characterized in that a tin film is formed on the circuit of the electronic component in which the circuit is covered with a resin protective film using the electroless tin plating bath according to any one of Inventions 1 to 7 above. This is an electroless tin plating method for electronic parts.

  The present invention 9 is the electroless tin plating method for a printed circuit board according to the present invention 8, wherein the electronic component is a printed circuit board.

(1) Since the oxycarboxylic acid is contained in the electroless tin plating bath in the weakly acidic to neutral range, and specific metal ions such as iron, manganese, chromium, aluminum, and zirconium coexist, the stability of the tin bath over time is improved. In addition, decomposition can be prevented for a long time, and continuous processing of electroless plating can be performed smoothly.
In weakly acidic to neutral tin baths based on soluble stannous salts, acids and thioureas, tin compounds precipitate in a short time even when only oxycarboxylic acid is added. It can be presumed that it was generated by the interaction of tin ion and thiourea, but in the presence of oxycarboxylic acid and a specific metal ion, it seems that the bath is stabilized by preventing such interaction.
Moreover, the said specific metal does not eutect with tin, but can form a tin film smoothly with the electroless plating bath of this invention (refer the below-mentioned test example).

  (2) Since the electroless tin plating bath of the present invention is weakly acidic or neutral having a pH of 2.5 to 7, the tin bath of the present invention is applied to an electronic component (for example, a printed circuit board) in which the circuit is coated with a resin protective film. Even if it is applied, the resin coated on the electronic component (resist of the printed circuit board) is not eroded and the reliability of the electronic component can be kept high.

  In the present invention, first, a weakly acidic to neutral tin bath having a basic composition of a soluble stannous salt (A), an acid (B), and a thiourea (C), an oxycarboxylic acid (for bath stabilization) Complexing agent (D)) and a specific metal ion (E) coexisting with electroless tin plating bath. Second, this weakly acidic or neutral electroless tin bath is made of a protective film made of resin. This is an electroless tin plating method applied to a coated electronic component (such as a printed circuit board).

Any soluble stannous salt (A) may be used as long as it is a compound capable of generating Sn ²⁺ in water, and even a sparingly soluble salt is not excluded. Specifically, stannous borofluoride, stannous sulfate, stannous oxide, stannous chloride, tin pyrophosphate, tin sulfamate, stannate and other inorganic soluble salts, organic sulfonic acid stannous Organic soluble salts such as stannous, stannous sulfosuccinate and stannous aliphatic carboxylic acid.

  Examples of the acid (B) include alkane sulfonic acids, alkanol sulfonic acids, organic sulfonic acids such as aromatic sulfonic acids, organic acids such as aliphatic carboxylic acids, hydrochloric acid, sulfuric acid, borofluoric acid, hydrofluoric acid, peroxy acids, and the like. Examples thereof include inorganic acids such as chloric acid, and as shown in the present invention 5, organic sulfonic acids are preferable.

The alkane sulfonic acid is represented by the chemical formula C _n H _{2n + 1} SO ₃ H (eg, n = 1 to 11), and specifically includes methane sulfonic acid, ethane sulfonic acid, and 1-propane sulfonic acid. Examples thereof include 2-propanesulfonic acid, 1-butanesulfonic acid, 2-butanesulfonic acid, pentanesulfonic acid and the like.

The alkanol sulfonic acid has the formula _{C m H 2m + 1 -CH (} OH) -C p H 2p -SO 3 H ( e.g., m = 0~6, p = 1~5 )
Specifically, 2-hydroxyethane-1-sulfonic acid, 2-hydroxypropane-1-sulfonic acid, 2-hydroxybutane-1-sulfonic acid, 2-hydroxypentane-1-sulfone In addition to acids, 1-hydroxypropane-2-sulfonic acid, 3-hydroxypropane-1-sulfonic acid, 4-hydroxybutane-1-sulfonic acid, 2-hydroxyhexane-1-sulfonic acid and the like can be mentioned.

  Examples of the aromatic sulfonic acid include phenolsulfonic acid, cresolsulfonic acid, naphtholsulfonic acid, sulfosalicylic acid, 1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid, benzenesulfonic acid, toluenesulfonic acid, xylenesulfonic acid, and nitrobenzenesulfonic acid. , Sulfobenzoic acid, diphenylamine-4-sulfonic acid and the like.

  The aliphatic carboxylic acid is preferably a carboxylic acid having 1 to 6 carbon waters, and specific examples include acetic acid, propionic acid, butyric acid, citric acid, tartaric acid, and sulfosuccinic acid.

The thioureas (C) are coordinated with the base metal copper or copper alloy to form complex ions, and the copper electrode potential is shifted in the base direction to promote the chemical substitution reaction with tin. Contained in
These thioureas include thiourea, or 1,3-dimethylthiourea, trimethylthiourea, diethylthiourea (for example, 1,3-diethyl-2-thiourea), N, N′-diisopropylthiourea, Examples include thiourea derivatives such as allylthiourea, acetylthiourea, ethylenethiourea, 1,3-diphenylthiourea, thiourea dioxide, and thiosemicarbazide.
Further, as a compound having a complexing action similar to thioureas, ethylenediaminetetraacetic acid (EDTA), ethylenediaminetetraacetic acid disodium salt (EDTA · 2Na), hydroxyethylethylenediaminetriacetic acid (HEDTA), diethylenetriaminepentaacetic acid (DTPA) ), Triethylenetetramine hexaacetic acid (TTHA), ethylenediaminetetrapropionic acid, ethylenediaminetetramethylenephosphoric acid, diethylenetriaminepentamethylenephosphoric acid, nitrilotriacetic acid (NTA), iminodiacetic acid (IDA), iminodipropionic acid (IDP), amino Trimethylene phosphate, aminotrimethylene phosphate pentasodium salt, benzylamine, 2-naphthylamine, isobutylamine, isoamylamine, methylenediamine, ethylenediamine, tetramethylenediamine, penta Methylenediamine, hexamethylenediamine, diethylenetriamine, tetraethylenepentamine, pentaethylenehexamine, hexaethyleneheptamine, cinnamylamine, p-methoxycinnamylamine and the like are also effective.

Since the present invention is an electroless tin plating bath in a weakly acidic to neutral range, it contains oxycarboxylic acid (D) as a complexing agent for bath stabilization.
Examples of the oxycarboxylic acid (D) include gluconic acid, citric acid, tartaric acid, glucoheptonic acid, malic acid, glycolic acid, lactic acid, trioxybutyric acid, ascorbic acid, tetraoxydecanoic acid, tartronic acid, glyceric acid, citramalic acid, and leucine. Acids, mevalonic acid, pantoic acid, ricinoleic acid, ricinaleic acid, cerebronic acid, quinic acid, and shikimic acid are included, and saccharides such as erythrose are also effective, and these can be used alone or in combination.
As shown in the present invention 2 , in this oxycarboxylic acid, an oxycarboxylic acid having 3 or more hydroxyl groups is preferable . Examples of such oxycarboxylic acid include gluconic acid, trioxybutyric acid , tetraoxydecanoic acid, gluconoheptonic acid, Examples include erythrose.
Gluconolactone and ascorbic acid both have a cyclic ester structure and generate oxycarboxylic acid upon hydrolysis. These gluconolactone and ascorbic acid are also effective as in the oxycarboxylic acid of the present invention.

The present invention is characterized in that the specific metal ion (E) is allowed to coexist in the oxycarboxylic acid (D) at a predetermined concentration to improve the temporal stability of the electroless tin plating bath in a weakly acidic to neutral range.
The metal ion (E) is a metal ion selected from the group consisting of chromium, manganese, iron, aluminum, vanadium, zirconium, and molybdenum.
The metal ion (E) can be used alone or in combination, and its content must be 0.01 to 6.0 mol / L, preferably 0.05 to 1.0 mol / L, more preferably It is about 0.1 to 0.5 mol / L.
If the content of metal ions is less than 0.01 mol / L, the stability with time of the bath is impaired, and if it is more than 6.0 mol / L, the solubility and plating rate of the specific metal salt may be reduced. .
As shown in the present invention 4, the molar ratio of the content of the metal ion (E) and the oxycarboxylic acid (D) is preferably E: D = 1: 1 to 30, more preferably E: D. = 1: 1 to 15. When component (E) is less than the proper amount or component (D) is large, the stability of the bath with time decreases, and when component (E) is more than the proper amount or component (D) is small, component (E ) Metal salt solubility and plating rate may be reduced.

In addition, the electroless tin plating bath of the present invention needs to maintain the pH in a weakly acidic or neutral range from the viewpoint of suppressing the erosion of a solder resist such as a printed circuit board, specifically, a pH of 2.5-7. Electroless tin bath in the region, preferably pH 2.5-6.0.
And, as shown in the present invention 4, from the viewpoint of maintaining good stability over time of the electroless tin plating bath in the weakly acidic to neutral range, the above soluble stannous salt (A), acid (B) and thioureas The weight ratio of the content of (D) is preferably A: B: D = 1: 2 to 8: 2 to 16, more preferably A: B: D = 1: 2 to 8: 2. ~ 8.
Therefore, the present invention 6 shows a typical composition of the electroless tin plating bath. (A) a soluble stannous salt, (B) an organic sulfone, (C) thiourea, and (D) 3 It contains an oxycarboxylic acid having at least one hydroxyl group and (E) iron ion, the content of iron ion (E) is 0.03 to 3.0 mol / L, and pH 2.5 to 6 This is an electroless tin plating bath. Incidentally, gluconic acid is suitable for the oxycarboxylic acid (D) having three or more hydroxyl groups .

In addition to the above components, various additives such as known surfactants, antioxidants, pH adjusters, brighteners and semi-brighteners can be mixed in the electroless tin plating bath of the present invention.
Examples of the surfactant include various nonionic, anionic, amphoteric, and cationic surfactants, which contribute to improving the appearance, denseness, smoothness, and adhesion of the plating film.
Examples of the anionic surfactant include alkyl sulfates, polyoxyethylene alkyl ether sulfates, polyoxyethylene alkyl phenyl ether sulfates, alkyl benzene sulfonates, and alkyl naphthalene sulfonates. Examples of the cationic surfactant include mono-trialkylamine salts, dimethyldialkylammonium salts, and trimethylalkylammonium salts.
Examples of the nonionic surfactant, C ₁ -C ₂₀ alkanols, phenol, naphthol, bisphenol, C ₁ -C ₂₅ alkyl phenols, aryl phenols, C ₁ -C ₂₅ alkyl naphthol, C ₁ -C ₂₅ alkoxyl phosphate ( Salt), sorbitan ester, polyalkylene glycol, C ₁ -C ₂₂ aliphatic amide and the like, and 2-300 mol addition-condensation of ethylene oxide (EO) and / or propylene oxide (PO).
Examples of the amphoteric surfactant include carboxybetaine, imidazoline betaine, sulfobetaine, and aminocarboxylic acid.

Examples of the pH adjuster include various acids such as hydrochloric acid, sulfuric acid, oxalic acid, boric acids, and phosphoric acids, various bases such as potassium hydroxide, sodium hydroxide, ammonia, and amine. The plating bath is well adjusted to a neutral pH range.
The above-mentioned antioxidant is intended to prevent Sn ^{2+ in} the bath, and hypophosphorous acid or a salt thereof, ascorbic acid or a salt thereof, hydroquinone, catechol, resorcin, phloroglucin, cresolsulfonic acid or a salt thereof Phenolsulfonic acid or a salt thereof, catecholsulfonic acid or a salt thereof, hydroquinonesulfonic acid or a salt thereof, hydrazine, and the like.

  Examples of the brightener or semi-brightener include benzaldehyde, o-chlorobenzaldehyde, 2,4,6-trichlorobenzaldehyde, m-chlorobenzaldehyde, p-nitrobenzaldehyde, p-hydroxybenzaldehyde, furfural, 1-naphthaldehyde, 2-naphthaldehyde, 2-hydroxy-1-naphthaldehyde, 3-acenaphthaldehyde, benzylideneacetone, pyridideneacetone, furfuryldenacetone, cinnamaldehyde, anisaldehyde, salicylaldehyde, crotonaldehyde, acrolein, glutaraldehyde, para Various aldehydes such as aldehyde and vanillin, triazine, imidazole, indole, quinoline, 2-vinylpyridine, aniline, phenanthroline, neocuproin, pi Phosphoric acid, thioureas, N- (3-hydroxybutylidene) -p-sulfanilic acid, N-butylidenesulfanilic acid, N-cinnamoylidenesulfanilic acid, 2,4-diamino-6- (2'-methyl) Imidazolyl (1 ')) ethyl-1,3,5-triazine, 2,4-diamino-6- (2'-ethyl-4-methylimidazolyl (1')) ethyl-1,3,5-triazine, 2 , 4-diamino-6- (2′-undecylimidazolyl (1 ′)) ethyl-1,3,5-triazine, phenyl salicylate, or benzothiazole, 2-methylbenzothiazole, 2-aminobenzothiazole, 2-Amino-6-methoxybenzothiazole, 2-methyl-5-chlorobenzothiazole, 2-hydroxybenzothiazole, 2-amino-6-methylbenzothiazole, 2-chlorobenzo Azole, 2,5-dimethyl benzothiazole, benzothiazole such as 5-hydroxy-2-methyl-benzothiazole.

This invention 8 is the electroless tin plating method of the electronic component which forms a tin film on the said circuit of the electronic component of the form which coat | covered the circuit with the resin protective film using the electroless tin plating bath of this invention 1-7. .
Examples of the electronic component in which the above circuit is covered with a resin protective film such as a cover lay, a dry film, or a liquid resist include a printed circuit board and a TAB film carrier. The printed board is a concept including both a rigid printed board and a flexible printed board.
Although the conditions for the electroless tin plating of the present invention are arbitrary, the bath temperature is preferably 45 to 90 ° C, and more preferably 50 to 70 ° C from the viewpoint of increasing the deposition rate.

Examples of the weakly acidic to neutral electroless tin plating bath according to the present invention, evaluation tests of the aging stability of the plating bath, and the corrosion resistance of the printed circuit board will be sequentially described below.
Further, the present invention is not limited to the following examples and test examples, and it is needless to say that arbitrary modifications can be made within the scope of the technical idea of the present invention.

<< Example of electroless tin plating bath >>
Among Examples 1 to 15 below, Examples 7 to 8 and 12 are examples in which specific metal ions are used in combination, and other examples are single use examples. Among these single use examples, Examples 1-3 are examples of trivalent iron ions, Examples 11 are examples of divalent iron ions, Examples 4, 9-10 are examples of manganese ions, Examples 6, 14 -15 are examples of aluminum ions, Example 5 is an example of chromium ions, and Example 13 is an example of zirconium ions.
Moreover, among the following comparative examples 1-5, the comparative example 1 is a blank example which does not contain an oxycarboxylic acid, the comparative example 2 is a blank example which does not contain a specific metal ion, and the comparative example 3 is an appropriate amount of a specific metal ion. An example containing less, Comparative Example 4 is an example containing metal ions (Zn ²⁺ ) other than a specific one, and Comparative Example 5 is a normal acidic electroless plating bath (pH 1 or less) in accordance with Patent Document 2 described above. ).

(1) Example 1
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Gluconic acid: 0.30 mol / L
Fe ₂ (SO ₄ ) ₃ · 12H ₂ O (as Fe ³⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0
In Example 1, the molar ratio of trivalent iron ions (E) to gluconic acid (D) is E: D = 0.22: 0.30 = 1: 1.36. The weight ratio of the soluble stannous salt (A), methanesulfonic acid (B), and gluconic acid (D) is as follows: A: B: D = 0.13: 0.50: 0.30 = 1: 3 .85: 2.36.

(2) Example 2
An electroless tin plating bath was constructed with the following composition.
Stannous ethanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Ethanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Gluconic acid: 0.30 mol / L
Fe ₂ (SO ₄ ) ₃ · 12H ₂ O (as Fe ³⁺ ): 0.01 mol / L
pH (adjusted with NaOH): 4.0

(3) Example 3
An electroless tin plating bath was constructed with the following composition.
p-Phenol sulfonic acid stannous (as Sn ²⁺ ): 0.13 mol / L
p-phenolsulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Gluconic acid: 0.30 mol / L
Fe ₂ (SO ₄ ) ₃ · 12H ₂ O (as Fe ³⁺ ): 1.00 mol / L
pH (adjusted with NaOH): 4.0

(4) Example 4
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Gluconic acid: 0.30 mol / L
MnSO ₄ .H ₂ O (as Mn ²⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0

(5) Example 5
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Citric acid: 0.30 mol / L
CrK (SO ₄ ) ₂ · 12H ₂ O (as Cr ³⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0

(6) Example 6
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Citric acid: 0.30 mol / L
Al ₂ (SO ₄ ) ₃ · 16H ₂ O (as Al ³⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0

(7) Example 7
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Malic acid: 0.30 mol / L
Fe ₂ (SO ₄ ) ₃ · 12H ₂ O (as Fe ³⁺ ): 0.11 mol / L
Al ₂ (SO ₄ ) ₃ · 16H ₂ O (as Al ³⁺ ): 0.11 mol / L
pH (adjusted with NaOH): 4.0

(8) Example 8
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Tetraoxydecanoic acid: 0.30 mol / L
Fe ₂ (SO ₄ ) ₃ · 12H ₂ O (as Fe ³⁺ ): 0.11 mol / L
MnSO ₄ .H ₂ O (as Mn ²⁺ ): 0.11 mol / L
pH (adjusted with NaOH): 4.0

(9) Example 9
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 0.50 mol / L
Gluconic acid: 0.30 mol / L
MnSO ₄ .H ₂ O (as Mn ²⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0

(10) Example 10
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.50 mol / L
Gluconic acid: 0.30 mol / L
MnSO ₄ .H ₂ O (as Mn ²⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0

(11) Example 11
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Citric acid: 0.30 mol / L
FeSO ₄ (as Fe ²⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0

(12) Example 12
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Tartaric acid: 0.30 mol / L
FeSO ₄ (as Fe ²⁺ ): 0.11 mol / L
MnSO ₄ .H ₂ O (as Mn ²⁺ ): 0.11 mol / L
pH (adjusted with NaOH): 4.0

(13) Example 13
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Tartaric acid: 0.30 mol / L
Zirconium sulfate (as Zr ²⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0

(14) Example 14
An electroless tin plating bath was constructed with the following composition.
Stannous borofluoride (as Sn ²⁺ ): 0.13 mol / L
Borohydrofluoric acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Citric acid: 0.30 mol / L
Al ₂ (SO ₄ ) ₃ · 16H ₂ O (as Al ³⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0

(15) Example 15
An electroless tin plating bath was constructed with the following composition.
Stannous sulfate (as Sn ²⁺ ): 0.13 mol / L
Sulfuric acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Citric acid: 0.30 mol / L
Al ₂ (SO ₄ ) ₃ · 16H ₂ O (as Al ³⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0

(16) Comparative Example 1
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
CrK (SO ₄ ) ₂ · 12H ₂ O (as Cr ³⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0

(17) Comparative example 2
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Citric acid: 0.30 mol / L
pH (adjusted with NaOH): 4.0

(18) Comparative Example 3
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Citric acid: 0.30 mol / L
CrK (SO ₄ ) ₂ · 12H ₂ O (as Cr ³⁺ ): 0.005 mol / L
pH (adjusted with NaOH): 4.0

(19) Comparative Example 4
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
Citric acid: 0.30 mol / L
ZnSO ₄ .7H ₂ O (as Zn ²⁺ ): 0.22 mol / L
pH (adjusted with NaOH): 4.0

(20) Comparative Example 5
An electroless tin plating bath was constructed with the following composition.
Stannous methanesulfonate (as Sn ²⁺ ): 0.13 mol / L
Methanesulfonic acid: 0.50 mol / L
Thiourea: 1.00 mol / L
pH: 1.0 or less

Therefore, the stability over time was evaluated for each electroless tin plating bath obtained in Examples 1 to 15 and Comparative Examples 1 to 5.
<< Example of evaluation test of electroless tin plating bath over time >>
After maintaining each electroless plating bath at 60 ° C., the superiority or inferiority of the stability over time of the plating bath was evaluated based on the following criteria.
A: There was no change in the bath even after one month had passed since the bathing.
○: There was no change in the bath when 2 weeks passed after the bathing.
X: The bath decomposed within 1 day after the bathing.

The left column of FIG. 1 shows the test results.
In Comparative Example 1 that does not contain oxycarboxylic acid and Comparative Example 2 that does not contain a specific metal ion, the plating bath decomposed within one day, whereas weak acidity using a combination of oxycarboxylic acid and a specific metal ion. Examples 1-15, which are electroless tin baths, were stable over a period of 2 weeks or over a month.
This confirmed that the coexistence of oxycarboxylic acid and a specific metal ion was necessary to ensure the temporal stability of the weakly acidic electroless tin plating bath.

Moreover, since the evaluation of Comparative Example 3 containing a specific metal ion but having a concentration less than the appropriate amount was x, when comparing Comparative Example 3 with Examples 1 to 15, weakly acidic electroless tin plating In order to improve the aging stability of the bath, the coexistence of the oxycarboxylic acid and a specific metal ion is not sufficient, and the necessity of optimizing the concentration of the metal ion has been confirmed.
Furthermore, since the evaluation of Comparative Example 4 in which a metal ion (Zn ²⁺ ) other than a specific one was added was x, the improvement with time of the weakly acidic electroless tin plating bath was coexistent with oxycarboxylic acid. It was confirmed that it was necessary to specify the type of metal ions to be used.
The acidic electroless tin plating bath (Comparative Example 5) showed good stability over time, but the weakly acidic to neutral electroless tin baths (Examples 1 to 15) of the present invention are also stable. It was found that this conventional acid bath is not inferior.

When the electroless tin baths of Examples 1 to 15 are examined in detail, first, Examples 1 to 3 which contain trivalent ions of Fe ³⁺ , Cr ³⁺ and Al ³⁺ as specific metal ions, or In Examples 5 to 7, it was evaluated as 安定, and it was found that the stability over time was superior to that of Examples containing divalent metal ions.
In Examples 1 to 3, the concentration and type of oxycarboxylic acid (gluconic acid) and the type of metal ion (Fe ³⁺ ) were the same and the concentration of metal ion was changed, but the evaluation of stability over time did not change. It was. The stability over time was not changed in Examples 6 and 14 to 15 containing Al ³⁺ .
Further, in Examples 4 and 9 to 10 which are common in that Mn ²⁺ as a specific metal ion and gluconic acid as an oxycarboxylic acid are contained, only the concentration of thiourea is changed. It didn't change.

Subsequently, the printed circuit board was immersed in each of the electroless tin plating baths of Examples 1 to 15 and Comparative Examples 1 to 5, and the degree of erosion prevention of the solder resist was evaluated.
<< Example of evaluation test for prevention of erosion of printed circuit board >>
That is, after the above electroless tin bath was constructed, FPC (flexible printed circuit board; test board on which a circuit was formed) was immersed for 10 minutes while being held at 60 ° C. to perform electroless plating. And the solder resist (especially edge part) of FPC was visually observed, and the superiority or inferiority of the degree of peeling (erosion prevention) of the solder resist was evaluated based on the following criteria.
○: No change in solder resist.
X: The solder resist was eroded and the end portions of the resist were peeled off.

The right column of FIG. 1 shows the test results.
In Comparative Example 4 which is an acidic electroless tin plating bath, the solder resist was peeled off. However, in Examples 1 to 15 which were weakly acidic (pH 4) electroless tin baths, the solder resist was not changed and peeling was not caused. There wasn't.
As described above, according to the results of the evaluation tests for the stability over time and the erosion prevention, the electroless tin plating bath of the present invention has no erosion to the solder resist and can stabilize the bath for a long time. It is suitable for application of tin plating to electronic parts (printed circuit boards, flexible printed circuit boards, etc.) in a form covered with a protective film.

<< Example of composition analysis test of electroless tin film >>
Next, Example 1 is selected as a representative example of the electroless tin plating bath of the present invention, and a bath temperature of 60 ° C. is formed on a fine pattern of an FPC (circuit-formed test substrate) using the electroless tin bath of Example 1. Then, electroless plating is performed under the condition of plating time of 10 minutes (that is, the same condition as the erosion prevention test), and the obtained tin film is analyzed for film composition using an energy dispersive X-ray analyzer (EDX). Went.
According to the quantitative results, Sn = 100 wt% and Fe = 0 wt%. FIG. 2 is an analysis chart thereof, and a sharp peak of Sn was observed in the film, but no peak of Fe was observed.
Therefore, according to the above quantitative results and chart, the metal does not co-deposit with tin due to ions of a specific metal such as Fe contained in a small amount in the plating bath. It was confirmed that the tin film could be obtained smoothly by the electrolytic plating bath.

It is a graph which shows the evaluation test result of temporal stability about each electroless tin plating bath of Examples 1-15 and Comparative Examples 1-5, and erosion prevention property. 1 is an EDX analysis diagram of a tin film obtained using the electroless tin plating bath of Example 1. FIG.

Claims (9)

(A) a soluble stannous salt;
(B) at least one of an inorganic acid and an organic acid;
(C) thioureas,
(D) a bath stabilizing complexing agent comprising oxycarboxylic acid;
(E) containing at least one metal ion selected from the group consisting of chromium, manganese, iron, aluminum, vanadium, zirconium, and molybdenum,
An electroless tin plating bath, wherein the content of the metal ion (E) is 0.01 to 6.0 mol / L and has a pH of 2.5 to 7. The electroless tin plating bath according to claim 1, wherein the complexing agent (D) for bath stabilization is an oxycarboxylic acid having 3 or more hydroxyl groups .   3. The electroless tin plating according to claim 1, wherein the molar ratio of the content of the metal ion (E) and the complexing agent for bath stabilization (D) is E: D = 1: 1-30. bath.   The weight ratio of the content of component (A), component (B), and component (D) is A: B: D = 1: 2-8: 2-16, The electroless tin plating bath according to any one of the above.   The electroless tin plating bath according to any one of claims 1 to 4, wherein the acid (B) is an organic sulfonic acid. (A) a soluble stannous salt;
(B) an organic sulfone,
(C) thiourea,
(D) an oxycarboxylic acid having three or more hydroxyl groups ;
(E) contains iron ions,
An electroless tin plating bath having a content of iron ions (E) of 0.03 to 3.0 mol / L and a pH of 2.5 to 6.   Furthermore, the additive selected from the group which consists of antioxidant, surfactant, and pH adjuster is contained, The electroless tin plating bath of any one of Claims 1-6 characterized by the above-mentioned.   An electroless tin plating bath according to any one of claims 1 to 7, wherein a tin film is formed on the circuit of an electronic component in which the circuit is covered with a resin protective film. Electroless tin plating method for parts.   9. The electroless tin plating method for a printed circuit board according to claim 8, wherein the electronic component is a printed circuit board.

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