JP4618488B2 - Electroless tin plating method - Google Patents

Description

The present invention provides an electroless tin plating method that can prevent the occurrence of spots and uneven color tone in a tin film obtained by electroless plating and can provide an excellent film appearance.

In electroless tin plating, for the purpose of improving the plating film, pretreatment is often performed in advance before electroless plating of an object to be plated using a tin plating bath.
For example, in Patent Document 1, an acidic pretreatment liquid containing a thiourea and a surfactant is brought into contact with a surface to be plated of a copper base material for the purpose of preventing the uneven plating by refining the precipitated crystal. Then, electroless tin plating or solder plating is disclosed.
It is pointed out that this pretreatment liquid is made acidic with acids such as hydrochloric acid, sulfuric acid, organic sulfonic acid, etc., and if the acid concentration in the liquid is too low, the effect of activating the copper substrate surface will be insufficient. (See paragraph 11). The pretreatment liquid for electroless solder plating in Examples 1 and 2 contains methanesulfonic acid or hydrochloric acid (see paragraphs 16 to 22), and the pretreatment liquid for electroless tin plating in Example 3 includes methane. Sulfonic acid is contained (see paragraphs 26-27).
Japanese Patent No. 3026527

When electroless tin plating is performed, the uniformity of the film is improved to some extent when immersed in the pretreatment solution of Patent Document 1 above, but on the other hand, in reality, the film has spots and uneven color tone. It is not easy to reliably suppress the occurrence of the film, and the adhesion of the film is insufficient.
Further, after this pretreatment, when electroless tin plating is performed, for example, if plating with a thickness of about 1 μm or more is performed at a bath temperature of 70 ° C. and a plating time of 5 minutes, the variation in the thickness of the tin film increases, and electronic components, etc. Then there is a problem with quality.
An object of the present invention is to improve the appearance of a film by effectively preventing the occurrence of spots and color tone irregularities mainly in the film during electroless tin plating.

  In the case where electroless tin plating is performed after the object to be plated is preliminarily immersed in a solution containing a sulfur complexing agent such as thiourea, the components of the pretreatment liquid and the appearance of the resulting tin film are superior or inferior. As a result of earnestly studying the relationship with the degree, the presence of the acid in the pretreatment liquid greatly affects the appearance of the tin film obtained by electroless plating. In addition, the present inventors have found out that spots and color tone irregularities are likely to occur in the film, and that it is extremely important not to contain an acid in the pretreatment liquid in order to suppress these appearance defects. In addition, the present inventors completed the present invention by finding that when the pretreatment liquid containing no acid is used, even if thick plating is performed during electroless tin plating, variation in tin film thickness can be reduced.

That is, the present invention 1 includes thioamides, thiodiglycol, bis (dodecaethylene glycol) thioether, bis (pentadecaethylene glycol) thioether, 2- (methylthio) ethanol, DTOD, DTDD, thiodiglycolic acid, and 1,2. -Natural or weakly basic pretreatment liquid containing at least one sulfur complexing agent selected from sulfide compounds composed of bis (hydroxyethylthio) ethane and containing no acid is made of copper or copper alloy after immersing the object to be plated of
Soluble stannous salt, using an electroless tin plating solution containing an acid and sulfur-based complexing agent is an electroless tin plating method and performing electroless plating on the object to be plated.

The present invention 2 is the electroless tin plating method according to the present invention 1, wherein the content of the sulfur complexing agent in the pretreatment liquid is 10 ⁻⁸ mol / L to 4 mol / L.

The present invention 3 is the electroless tin plating method according to the present invention 1 or 2, wherein the pretreatment liquid further contains a surfactant.

Invention 4 is characterized in that, in any of the Inventions 1 to 3, the liquid temperature is 10 to 90 ° C. and the immersion time is 1 second to 20 minutes as the conditions for immersion in the pretreatment liquid. This is an electroless tin plating method.

Similar to Patent Document 1, when electroless tin plating is performed after the object to be plated is preliminarily treated with a pretreatment liquid containing thioureas and containing an acid, stains and color tone unevenness are observed in the obtained plating film. Although it occurs and its degree is reduced compared to the case without pretreatment, it is clearly insufficient to prevent poor appearance of the film.
On the other hand, in the present invention, the object to be plated made of copper or a copper alloy is preliminarily immersed in a pretreatment liquid containing a specific sulfur-based complexing agent such as thioureas and not containing an acid. The tin film obtained by electroless plating can be effectively prevented from causing spots and uneven color tone, and the film appearance is excellent.
In this pre-treatment, it can be estimated that sulfur complexing agent is adsorbed on the surface of the object to be plated and promotes substitutional deposition of tin by the subsequent electroless plating. When the agent is added, the adsorption and development of the sulfur complexing agent becomes smoother due to the decrease in surface tension, and the appearance of the electroless coating is further improved.
In addition, when the pretreatment solution containing no acid of the present invention is used, the adhesion of the tin film obtained by electroless plating to the surface of the object to be plated can be improved, and thick plating is performed during electroless plating. In addition, variation in tin film thickness can be reduced. For this reason, for example, when the pretreatment of the present invention is performed on various electronic components, even if electroless thick plating is performed later, plating is performed while suppressing variations in tin film thickness and improving productivity. The quality of the electronic parts can be maintained.

The present invention is a method of performing electroless tin plating after immersing an object to be plated made of copper or a copper alloy in a pretreatment liquid containing a specific sulfur complexing agent and not containing an acid .

The pretreatment liquid of the present invention is a neutral or weakly basic liquid containing a specific sulfur complexing agent and not containing an acid. Therefore, acidic pretreatment liquid is eliminated.
The above sulfur complexing agent is a compound containing a sulfur atom having a loan pair in the molecule. Therefore, for example, in organic sulfonic acid, the outer electrons of the sulfur atom have an octet structure. Although excluded from the complexing agents of the present invention. However, among the complexing agents, thiodiglycolic acid shown below is an acid, but as shown in the above-mentioned Invention 2, when a small amount of complexing agent is added at the additive level, from the neutral range Since it does not deviate much, it is not excluded from the complexing agent of the present invention.

Specific examples of the sulfur complexing agent effective in the complexing action are as follows, and the specific sulfur complexing agent of the present invention can be selected from these.
(I) Thioamides Thiourea, 1,3-dimethylthiourea, trimethylthiourea, diethylthiourea (for example, 1,3-diethyl-2-thiourea), N, N′-diisopropylthiourea, allylthiourea, Thiourea derivatives such as acetylthiourea, ethylenethiourea, 1,3-diphenylthiourea, thiourea dioxide, thiosemicarbazide
(B) mercaptans thioglycolic _{(SH-CH 2 CH 2 OH} ), thioglycolic acid (HSCH ₂ COOH), mercaptopropionic acid _{(CH 3 CH (SH) COOH} ), mercaptosuccinic acid (HOOCCH ₂ -CH (SH) COOH)

(C) sulfides
(a) Oxyalkylene type aliphatic sulfides having a single or repeated oxyethylene group, oxypropylene group or oxy (hydropropylene group) adjacent to both sides or one side of a mono- or disulfide bond, Include the following compounds.
(1) Bis (triethylene glycol) thioether represented by H— (OCH ₂ CH ₂ ) ₃ —S— (CH ₂ CH ₂ O) ₃ —H
(2) Bis (hexaethylene glycol) thioether represented by H— (OCH ₂ CH ₂ ) ₆ —S— (CH ₂ CH ₂ O) ₆ —H
(3) Bis (decaethylene glycol) thioether represented by H— (OCH ₂ CH ₂ ) ₁₀ —S— (CH ₂ CH ₂ O) ₁₀ —H
(4) Bis (dodecaethylene glycol) thioether represented by H— (OCH ₂ CH ₂ ) ₁₂ —S— (CH ₂ CH ₂ O) ₁₂ —H
(5) Bis (pentadecaethylene glycol) thioether represented by H— (OCH ₂ CH ₂ ) ₁₅ —S— (CH ₂ CH ₂ O) ₁₅ —H
(6) Bis (icosaethylene glycol) thioether represented by H— (OCH ₂ CH ₂ ) ₂₀ —S— (CH ₂ CH ₂ O) ₂₀ —H
(7) Bis (triacontaethylene glycol) thioether represented by H— (OCH ₂ CH ₂ ) ₃₀ —S— (CH ₂ CH ₂ O) ₃₀ —H
(8) Bis (tetracontaethylene glycol) thioether represented by H— (OCH ₂ CH ₂ ) ₄₀ —S— (CH ₂ CH ₂ O) ₄₀ —H
(9) Bis (pentacontaethylene glycol) thioether represented by H— (OCH ₂ CH ₂ ) ₅₀ —S— (CH ₂ CH ₂ O) ₅₀ —H
(10) 2,2′-thiodiglycol represented by HOCH ₂ CH ₂ —S—CH ₂ CH ₂ OH
(11) 3,3′-thiodipropanol represented by HOCH ₂ CH ₂ CH ₂ —S—CH ₂ CH ₂ CH ₂ OH
(12) Bis (ω-hydroxypentaethoxy) disulfide represented by H— (OCH ₂ CH ₂ ) ₅ —S—S— (CH ₂ CH ₂ O) ₅ —H
(13) Bis (ω-hydroxydodecaethoxy) disulfide represented by H— (OCH ₂ CH ₂ ) ₁₂ —SS— (CH ₂ CH ₂ O) ₁₂ —H
(14) Bis (ω-hydroxyicosaethoxy) disulfide represented by H— (OCH ₂ CH ₂ ) ₂₀ —SS— (CH ₂ CH ₂ O) ₂₀ —H
(15) Bis (ω-hydroxypentacontaethoxy) disulfide represented by H— (OCH ₂ CH ₂ ) ₅₀ —S—S— (CH ₂ CH ₂ O) ₅₀ —H
Represented by _{(OC 2 H 4) 10 -S} -S- (C 2 H 4 O) 10 -CH 2 CH (OH) CH 2 OH - (16) H-OCH 2 CH (OH) CH 2 Bis (ω-hydroxymonoglyceroxydecaethoxy) disulfide
(17) S, S′-bis (pentaethylene glycol) ethylene dithioether represented by H— (OC ₂ H ₄ ) ₅ —S—CH ₂ CH ₂ —S— (C ₂ H ₄ O) ₅ —H
_{(18) H- (OC 2 H} 4) 15 -S-CH 2 CH 2 -S- (C 2 H 4 O) 15 S represented by -H, S'-bis (pentadecalactone glycol) ethylene di Thioether
(19) S, S′-bis (triacontaethylene glycol) represented by H— (OC ₂ H ₄ ) ₃₀ —S—CH ₂ CH ₂ CH ₂ —S— (C ₂ H ₄ O) ₃₀ —H Propylene dithioether
(20) Bis (diethylene glycol) thioether represented by H— (OCH ₂ CH ₂ ) ₂ —S— (CH ₂ CH ₂ O) ₂ —H
(21) Bis (monoglycerol) thioether represented by HOCH ₂ CH (OH) CH ₂ —S—CH ₂ CH (OH) CH ₂ OH
(22) Bis (ω-hydroxyhentetracontaethoxy) disulfide represented by H— (OCH ₂ CH ₂ ) ₄₁ —S—S— (CH ₂ CH ₂ O) ₄₁ —H
(23) 3,6-dithio-1,8-octanediol (DTOD) represented by HOCH ₂ CH ₂ —S—CH ₂ CH ₂ —S—CH ₂ CH ₂ OH
(24) S, S′-bis (decaethylene glycol) propylene dithioether represented by H— (OC ₂ H ₄ ) ₁₀ —S—C ₃ H ₆ —S— (OC ₂ H ₄ ) ₁₀ —H
(25) S, S′-bis (pentaethylene glycol) represented by H— (OCH ₂ CH ₂ ) ₅ —S—CH ₂ CH (OH) CH ₂ —S— (CH ₂ CH ₂ O) ₅ —H ) -2-Hydroxypropylene dithioether
(26) S, S′-bis (icosaethyleneglycol) ethylenediene represented by H— (OCH ₂ CH ₂ ) ₂₀ —S—CH ₂ CH ₂ —S— (CH ₂ CH ₂ O) ₂₀ —H Thioether
(27) 2- (methylthio) ethanol represented by CH ₃ —S—CH ₂ CH ₂ OH
(28) 4,7-dithio-1,10-decanediol (DTDD) represented by HOCH ₂ CH ₂ —S—CH ₂ CH ₂ CH ₂ CH ₂ —S—CH ₂ CH ₂ —OH

(b) chain sulfides having a mono- or disulfide bond in the molecule and having one or more basic nitrogen atoms in at least one of the two wing atom groups of the bond; specifically, The compound of this is mentioned.
(1) 2-ethylthioaniline
(2) 2- (2-Aminoethyldithio) pyridine
(3) 2,2'-dithiadiazolyl disulfide
(4) 5,5'-di (1,2,3-triazolyl) disulfide
(5) 2,2'-dipyrazinyl disulfide
(6) 2,2'-dipyridyl disulfide
(7) 2,2'-dithiodianiline
(8) 4,4'-dipyridyl disulfide
(9) 2,2'-diamino-4,4'-dimethyldiphenyl disulfide
(10) 2,2'-dipyridazinyl disulfide
(11) 5,5'-dipyrimidinyl disulfide
(12) 2,2'-di (5-dimethylaminothiadiazolyl) disulfide
(13) 5,5'-di (1-methyltetrazolyl) disulfide
(14) 2,2'-di (1-methylpyrrolyl) disulfide
(15) 2-pyridyl-2-hydroxyphenyl disulfide
(16) 2,2'-dipiperidyl disulfide
(17) 2,2'-dipyridyl sulfide
(18) 2,6-di (2-pyridyldithio) pyridine
(19) 2,2'-dipiperazinyl disulfide
(20) 2,2'-di (3,5-dihydroxypyrimidinyl) disulfide
(21) 2,2'-Diquinolyl disulfide
(22) 2,2'-di {6- (2-pyridyl)} pyridyl disulfide
(23) 2,2'-α-picolyl disulfide
(24) 2,2'-di (8-hydroxyquinolyl) disulfide
(25) 5,5'-Diimidazolyl disulfide
(26) 2,2'-dithiazolyl disulfide
(27) 2-pyridyl-2-aminophenyl disulfide
(28) 2-pyridyl-2-quinolyl disulfide
(29) 2,2'-dithiazolinyl disulfide
(30) 2,2'-di (4,5-diamino-6-hydroxypyrimidinyl) disulfide
(31) 2,2'-di (6-chloropyridyl) tetrasulfide
(32) 2,2'-dimorpholino disulfide
(33) 2,2'-di (8-methoxyquinolyl) disulfide
(34) 4,4'-di (3-methoxycarbonylpyridyl) disulfide
(35) 2-pyridyl-4-methylthiophenyl disulfide
(36) 2-piperazyl-4-ethoxymethylphenyl disulfide
(37) 2,2'-di {6- (2-pyridyldithio) pyridyl} disulfide
(38) 2,2'-Diquinoxalinyl disulfide
(39) 2,2'-dipteridinyl disulfide
(40) 3,3'-Difurazanyl disulfide
(41) 3,3'-Diphenanthrolinyl disulfide
(42) 8,8'-Diquinolyl disulfide
(43) 1,1'-Diphenazinyl disulfide
(44) 4,4'-di (3-carboxylpyridyl) trisulfide
(45) 2,2'-dithiazolinyl disulfide
(46) 2,2'-dipicolyl disulfide
(47) Dimethylaminodiethyl disulfide
(48) 2,2'-diperhydroindolyl disulfide
(49) 6,6'-diimidazo [2,1-b] thiazolyl disulfide
(50) 2,2'-di (5-nitrobenzimidazolyl) disulfide
(51) 2,4,6-tris (2-pyridyldithio) -1,3,5-triazine
(52) 2-Aminoethyl-2'-hydroxyethyl disulfide
(53) Di (2-pyridylthio) methane

(c) Other
(1) thio diglycolic acid _{(HOOCCH 2 -S-CH 2 COOH} )
(2) thiodiglycol _{(HOCH 2 CH 2 -S-CH} 2 CH 2 OH)
(D) Sulfite

The sulfur complexing agent of the present invention is a specific compound selected from the sulfur compounds listed above, among thioamides (thioureas or thioureas of thiourea derivatives), or sulfides. thiodiglycol of bis (dodecamethylene glycol) thioether, bis (pentadecalactone glycol) thioethers, 2- (methylthio) ethanol, DtoD, DTDD, thio diglycolic acid, 1,2-bis (hydroxyethyl) ethane Yes, thiourea or thiourea derivatives are more preferred.
These sulfur complexing agents can be used alone or in combination, and the content in the pretreatment liquid is not particularly limited, but is preferably 10 ⁻⁸ mol / L to 4 mol / L (see the present invention 2), more preferably. Is 0.1 to 1.0 mol / L.

The pretreatment liquid of the present invention may further contain a surfactant. The addition of the surfactant contributes to the smooth adsorption of the sulfur complexing agent on the surface of the object to be plated in the pretreatment.
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, polyoxyethylene polycyclic phenyl ether sulfates, polyoxyethylene allyl ether sulfate sodium salts Alkylbenzene sulfonate, alkyl naphthalene sulfonate, and the like. 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 phosphoric acid (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 amphoteric surfactants include carboxybetaine, imidazoline betaine, sulfobetaine, and aminocarboxylic acid.
The amount of the surfactant added to the pretreatment liquid is suitably 0.001 to 50 g / L, preferably 0.1 to 20 g / L.

As described above, the pretreatment liquid of the present invention does not contain an acid, and therefore the pretreatment liquid needs to exhibit neutrality or weak basicity.
The acid may be an inorganic acid such as sulfuric acid, hydrochloric acid or borohydrofluoric acid, or an organic acid such as organic sulfonic acid or carboxylic acid. However, as described above, even if the sulfur complexing agent belongs to an acid, it may be added if the trace amount is added and the pretreatment liquid remains in the vicinity of neutrality.

The conditions for immersing the object to be plated in the pretreatment liquid are not particularly limited, but as shown in the present invention 5, the liquid temperature is suitably 10 to 90 ° C., preferably 30 to 50 ° C. Further, the immersion time is suitably about 1 second to 20 minutes, preferably 10 seconds to 5 minutes.
In addition, the contact of the pretreatment liquid to the object to be plated is basically immersed in the pretreatment liquid, but instead of this immersion treatment, the pretreatment liquid is sprayed on the object to be plated or applied with a brush or the like. Is not to be excluded.

In the present invention, an object to be plated is immersed in a pretreatment liquid that does not contain an acid and contains a specific sulfur-based complexing agent such as thioureas, and then electroless tin plating is performed.
In this case, if the electroless tin plating is performed after washing the object to be plated after the pretreatment, the components of the pretreatment liquid are not brought into the electroless plating bath, and the resulting film appearance can be formed better. The composition of the electroless tin plating bath is not particularly limited, and an ordinary plating bath can be used. In general, a soluble stannous salt, an acid, and a sulfur complexing agent are basic components.
Examples of the soluble stannous salt include stannous salts of organic sulfonic acids such as methanesulfonic acid, ethanesulfonic acid, 2-propanolsulfonic acid, p-phenolsulfonic acid, stannous borofluoride, sulfosuccinic acid Examples include stannous, stannous sulfate, stannous oxide, and stannous chloride.
Examples of the acid include organic acids such as organic sulfonic acids and aliphatic carboxylic acids, or inorganic acids such as borohydrofluoric acid, silicohydrofluoric acid, sulfamic acid, hydrochloric acid, sulfuric acid, and perchloric acid. The salt can also be used. Examples of acids include organic sulfones such as methanesulfonic acid, ethanesulfonic acid, 2-hydroxyethanesulfonic acid, 2-hydroxypropanesulfonic acid, and phenolsulfonic acid, from the viewpoint of ease of wastewater treatment and solubility of stannous salts. Acid is preferred.
Further, the sulfur-based complexing agent is coordinated to the copper or copper alloy of the object to be plated is a compound which acts to shift the electrode potential of copper in the direction of the less noble, particular to be added to the pretreatment liquid This is basically the same as the sulfur-based complexing agents and other sulfur-based complexing agents listed as many which exhibit an effective complexing action . As this sulfur complexing agent, thioureas are preferable.
On the other hand, when thick plating is performed, hypophosphorous acid is contained as a base acid, inorganic acid or organic acid other than hypophosphorous acid is not contained, and the content of hypophosphorous acid is 1.3 mol. It is effective to use an electroless tin plating bath that is at least / L.
Furthermore, it contains hypophosphorous acid and auxiliary acid as base acid, and auxiliary acid is inorganic acid such as sulfuric acid, hydrochloric acid, borofluoric acid, sulfamic acid other than hypophosphorous acid, organic sulfonic acid, carboxylic acid, sulfosuccinic acid. A plating bath that is an organic acid such as an acid and has a hypophosphorous acid content of 1.3 mol / L or more and an auxiliary acid content of 0.5 mol / L or less is also suitable for thickening. Effective as an electrolytic tin bath. If the auxiliary acid exceeds 0.5 mol / L, the resulting plating film may be stained and uneven in color tone, which may impair the appearance of the film. However, the pretreatment of the present invention suppresses the adverse effects. (See the test example below).

The electroless tin plating bath may further contain various additives such as a surfactant, an antioxidant, a brightener, a semi-brightener, and a pH adjuster.
The surfactant may be the same as the surfactant that can be contained in the pretreatment liquid.
Antioxidants are intended to prevent the oxidation of Sn ^{2+ in} the bath. Hypophosphorous acid or its salt, ascorbic acid or its salt, hydroquinone, catechol, resorcin, phloroglucin, cresolsulfonic acid or its salt, phenol Examples include sulfonic acid or a salt thereof, catechol sulfonic acid or a salt thereof, hydroquinone sulfonic acid or a salt thereof, and hydrazine.

Various electronic parts are suitable for an object to be plated which is subjected to the electroless tin plating method of the present invention (that is, immersed in a pretreatment liquid). Examples of the electronic component include semiconductor devices, printed boards, flexible printed boards, film carriers, ICs, connectors, switches, resistors, variable resistors, capacitors, filters, inductors, thermistors, crystal resonators, lead wires, and the like.

Hereinafter, examples of the electroless tin plating method of the present invention, and evaluation test examples such as visual appearance, adhesion, and degree of film thickness variation during thick plating will be described in order.
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 method including pretreatment >>
Among Examples 1 to 6 below, Examples 1 to 3 are examples in which the concentration of thiourea in the pretreatment liquid is changed, and Examples 4 to 6 are thioureas and nonionic interfaces in the pretreatment liquid. This is an example in which the concentration of the active agent is changed. Among Examples 4 to 6, Example 4 is an example in which thiourea is simply used as a thiourea, Example 5 is an example in which a thiourea derivative is used alone, and Example 6 is a combination of thiourea and a thiourea derivative. It is an example.
On the other hand, among Comparative Examples 1 to 3, Comparative Example 1 is a blank example in which only the electroless tin plating is performed on the object to be plated without pretreatment, and Comparative Example 2 is based on Patent Document 1 described above. Example of electroless plating after treatment with thiourea, nonionic surfactant and acid-containing (and therefore acidic) pretreatment solution, Comparative Example 3 contains thiourea and acid (and therefore acidic) This is an example in which electroless plating is performed after the treatment with the pretreatment liquid.

(1) Example 1
Using a test printed circuit board (supported by the appendix of JIS C5012) as an object to be plated, a pretreatment liquid is prepared with the composition of (a) below, and the object to be plated is immersed in this pretreatment liquid, thereby pretreatment. Went.
Next, after the pretreated object to be plated was washed with water, electroless thick plating was performed using an electroless tin plating bath constructed with the composition (b) below.
The electroless tin plating bath is a plating bath for thickening using hypophosphorous acid as the main base acid and methanesulfonic acid as the rest of the base acid.

(a) Pretreatment solution Thiourea 0.33 mol / L
[Immersion conditions in pretreatment liquid]
Liquid temperature: 40 ° C
Immersion time: 30 seconds
(b) Electroless tin plating bath Stannous methanesulfonate (as Sn ²⁺ ) 30 g / L
Methanesulfonic acid 55g / L
Hypophosphorous acid 150g / L
Thiourea 150g / L
Nonylphenol polyethoxylate (EO12 mol) 5g / L
[Conditions for electroless plating]
Bath temperature: 70 ° C
Plating time: 5 minutes

(2) Example 2
Based on the above Example 1, the pretreatment and electroless tin plating were performed under the same conditions as in Example 1 except that the composition of the pretreatment liquid was adjusted to the following (a). The electroless plating conditions are the same as in Example 1. The same applies to Examples 3 to 6 below.
(a) Pretreatment liquid Thiourea 0.65 mol / L

(3) Example 3
Based on Example 1, the pretreatment and electroless tin plating were performed under the same conditions as in Example 1 except that the composition of the pretreatment liquid was adjusted to the following (a).
(a) Pretreatment liquid Thiourea 1.30 mol / L

(4) Example 4
Based on Example 1, the pretreatment and electroless tin plating were performed under the same conditions as in Example 1 except that the composition of the pretreatment liquid was adjusted to the following (a).
(a) Pretreatment liquid Thiourea 0.65 mol / L
Nonylphenol polyethoxylate (EO10mol) 3.0g / L

(5) Example 5
Based on Example 1, the pretreatment and electroless tin plating were performed under the same conditions as in Example 1 except that the composition of the pretreatment liquid was adjusted to the following (a).
(a) Pretreatment liquid 1,3-dimethylthiourea 0.57 mol / L
Nonylphenol polyethoxylate (EO10mol) 5.0g / L

(6) Example 6
Based on Example 1, the pretreatment and electroless tin plating were performed under the same conditions as in Example 1 except that the composition of the pretreatment liquid was adjusted to the following (a).
(a) Pretreatment solution Thiourea 0.30 mol / L
1,3-dimethylthiourea 0.20 mol / L
Nonylphenol polyethoxylate (EO10mol) 3.0g / L

(7) Comparative Example 1
Based on Example 1, the pretreatment was not performed, and only electroless tin plating was performed under the same conditions as in Example 1.

(8) Comparative example 2
Based on Example 1, the pretreatment and electroless tin plating were performed under the same conditions as in Example 1 except that the composition of the pretreatment liquid was adjusted to the following (a).
(a) Pretreatment liquid Methanesulfonic acid 0.2 mol / L
Thiourea 0.2 mol / L
Nonylphenol polyethoxylate (EO10mol) 0.2g / L

(9) Comparative Example 3
Based on Example 1, the pretreatment and electroless tin plating were performed under the same conditions as in Example 1 except that the composition of the pretreatment liquid was adjusted to the following (a).
(a) Pretreatment liquid Methanesulfonic acid 0.2 mol / L
Thiourea 0.2 mol / L

<< Various evaluation test examples >>
About the plating film obtained by each of the electroless tin plating methods of Examples 1 to 6 and Comparative Examples 1 to 3, as described below, the visual appearance and the film thickness at the time of thick plating were evaluated, and the cellophane tape was used. The adhesion test was performed.
(1) Visual appearance evaluation Each tin plating film obtained by the electroless plating method was visually observed, and the superiority or inferiority of the film appearance was evaluated according to the following criteria.
○: The film had no stains or uneven color, and had a uniform and beautiful appearance.
X: Spots and uneven color tone were observed on the film, and the appearance was poor.

(2) Adhesion evaluation For each tin plating film obtained by the electroless plating method, the grid pattern in the section “8.5 Adhesion” described in “8. Test method for resistance of coating film” of JIS K 5400 A test was conducted according to the tape method, and the adhesion of the plating film was evaluated according to the following criteria.
A: 8 to 10 points.
X: 6 points or less.

(3) Film thickness film From Example 1-6 and Comparative Examples 1-3, Example 2, Example 4, and Comparative Examples 1-2 are extracted, and each electroless plating method of these 4 examples is separately separated. Repeatedly applied to the object to be plated 5 times, and measured the thickness of each tin-plated film using a fluorescent X-ray film thickness meter (SFT3300S; manufactured by Seiko Instruments Inc.). The center film thickness and the variation value from the center value were calculated, and the degree of variation during thick plating was evaluated.

The table below shows the test results.
Visual appearance Adhesiveness Film thickness (μm)
Example 1 ○ ○ −−
Example 2 ○ ○ 1.7 ± 0.3
Example 3 ○ ○ −−
Example 4 ○ ○ 1.5 ± 0.4
Example 5 ○ ○ −−
Example 6 ○ ○ −−
Comparative Example 1 × × 1.9 ± 0.9
Comparative Example 2 × × 1.3 ± 1.0
Comparative Example 3 × × −−

According to the above table, in Comparative Example 1, which is a blank example without pretreatment, spots and color unevenness were found in the obtained film, and the adhesion was also inferior. Further, when the thick plating was performed, the variation in the film thickness was large. Also in the comparative example 2 based on the said patent document 1, the external appearance defect was seen in the film | membrane, and the dispersion | variation in the film thickness in the case of adhesiveness and thick plating was the same level as the comparative example 1. Further, in Comparative Example 3 using an acidic pretreatment liquid containing thiourea, as in Comparative Example 1, the film appearance and adhesion were inferior.
On the other hand, in Examples 1 to 6 using a pretreatment liquid containing thioureas and no acid, there was no stain or uneven color tone, and a film having a uniform and excellent appearance was obtained. Adhesion was also good.
In addition, when thick plating is performed, the variation in film thickness is small, and for example, it has been confirmed that practical thick plating can be performed on various electronic components.

Therefore, Examples 1 to 6 will be examined in detail. First, in Examples 1 to 3 in which the concentration of thiourea in the pretreatment liquid was changed, both the film appearance and the adhesion were excellent, so that the concentration of thiourea in the pretreatment liquid is not limited to a high concentration. It was found that the appearance and adhesion of a tin film obtained by electroless plating can be effectively improved even when it is thin.
In Example 2 using a pretreatment liquid containing only thiourea, a film having a uniform appearance was obtained by electroless plating. However, a pretreatment liquid containing a nonionic surfactant in addition to thiourea was used. In Example 4, the color uniformity and adhesion of the tin film were slightly improved as compared with Example 2. By the way, the film thickness variation in thick plating did not change much.
Furthermore, also in Example 5 using a pretreatment liquid containing a thiourea derivative, the appearance and adhesion of the tin film obtained by electroless plating are the same as in Examples 1 to 4, so It was confirmed that the contained component was effective regardless of thiourea and thiourea derivatives.

  As described above, when Examples 1 to 6 and Comparative Examples 1 to 3 are compared, in order to improve the appearance and adhesion of the tin film obtained by electroless plating, thiourea is included in advance during electroless plating. It has become clear that it is important to immerse in the pretreatment liquid and not to contain an acid in the pretreatment liquid (that is, to treat with a neutral or weakly basic pretreatment liquid). In addition, when the pretreatment liquid of the present invention is used, a tin film with small variations in film thickness can be formed even when thickened by electroless plating after pretreatment, and efficient electroless plating can be performed. Was confirmed. Therefore, the electroless plating method of the present invention is particularly suitable for various electronic components.

Claims (4)

Thioamides, thiodiglycol, bis (dodecaethylene glycol) thioether, bis (pentadecaethylene glycol) thioether, 2- (methylthio) ethanol, DTOD, DTDD, thiodiglycolic acid, 1,2-bis (hydroxyethylthio) An object to be plated made of copper or copper alloy is immersed in a neutral or weakly basic pretreatment solution containing at least one sulfur complexing agent selected from sulfide compounds composed of ethane and not containing acid. rear,
Electroless tin plating method soluble stannous salt, using an electroless tin plating solution containing an acid and sulfur-based complexing agent and performing electroless plating on the object to be plated. The electroless tin plating method according to claim 1, wherein the content of the sulfur complexing agent in the pretreatment liquid is 10 ⁻⁸ mol / L to 4 mol / L. The electroless tin plating method according to claim 1, wherein the pretreatment liquid further contains a surfactant.   The electroless tin plating according to any one of claims 1 to 3, wherein the immersion temperature in the pretreatment liquid is 10 to 90 ° C and the immersion time is 1 second to 20 minutes. Method.