JP5401714B2 - Prevention method of tin whisker by electroless plating - Google Patents

Description

  The present invention relates to a method for preventing tin whiskers by electroless plating, and provides a method that can easily prevent tin whiskers without generating pinholes on the tin plating surface.

Tin plating films are widely used in parts such as weak electrical industry and electronic industry for purposes such as improving solderability and etching resists, but it is well known that tin whiskers are likely to occur. Tin whiskers cause a short circuit and reduce the reliability of various electronic components such as printed circuit boards and film carriers. Therefore, in recent circumstances where the density and miniaturization of wiring patterns are progressing, this whisker is strongly demanded. Has been.
Methods for preventing tin whiskers include annealing and reflow treatment after plating, adding a small amount of dissimilar metals such as lead and bismuth to the tin-based layer, or forming a base layer of a specific dissimilar metal There is a method of coating a tin film.
Therefore, conventional techniques related to tin whisker prevention are as follows.

In order to prevent whisker by simple operation, the present applicant previously formed a single metal base thin film selected from silver, bismuth, etc. by immersion treatment in Patent Document 1, It was disclosed that a whisker is prevented by forming a tin or tin alloy plating film (upper layer).
Similarly, in Patent Document 2, a single metal base film selected from silver, bismuth and the like is formed on a fine pattern such as a film carrier by displacement plating, and after heating the base film, a tin film (upper layer) ) Is formed by displacement plating to prevent whiskers (Claim 1).

In Patent Document 3, for the purpose of preventing whisker while ensuring solder wettability (paragraphs 12 and 84), a plating A layer (base) / metal X in which metal X and metal Y are mixed on a base material is disclosed. An electronic component material (metal X is tin and metal Y is silver, bismuth, copper, etc.) having a coating B layer (upper layer) is disclosed (claim 1).
The film thickness of the coating B layer (upper layer) is 0.05 to 1.0 μm (Claim 4, Paragraph 38), and whisker may be generated at a thickness of 1.0 μm or more (Paragraph 38). It is described that the thickness of the A layer (base) can be 1 to 50 μm (paragraph 39). In this case, the relationship between the thickness of the plating A layer (base) and the thickness of the coating B layer (upper layer) describes that the plating A layer (base) is formed to be considerably thicker than the coating B layer (upper layer) ( Paragraph 40).

In Patent Document 4, for the purpose of preventing whiskers without going through complicated steps such as melting treatment (paragraphs 1, 6, and 8), 5 wt% or less of metal such as silver or bismuth and 95 wt% or more of tin A method of forming an upper layer film of tin or a tin alloy on an undercoat film of the tin alloy is disclosed (claim 1).
The film thickness of the base film is 0.1 to 20 μm, and the film thickness of the upper film is 0.025 to 5 μm (Claim 2). In this two-layered tin-based film, the upper layer is very It may be thin (paragraph 12), and the upper layer thickness is preferably 50% or less of the underlayer, more preferably 25% or less (paragraph 20).

In Patent Document 5, for the purpose of improving solderability (paragraphs 1, 7, and 41), silver or silver alloy (intermediate layer) / tin or tin alloy (surface layer) / tin alloy containing bismuth or indium (the most A metal coating material comprising a surface layer portion is disclosed (claim 1).
The silver alloy (intermediate layer) contains a tin-silver alloy (paragraph 14), the intermediate layer has a thickness of 0.05 to 10 μm, and the surface layer has a thickness of 0.05 to 20 μm in relation to the reflow treatment (paragraph). 13, 18).

JP 2003-129278 A JP 2003-332391 A Japanese Patent No. 3350026 Japanese Patent Laid-Open No. 2006-9039 Japanese Patent Laid-Open No. 11-229178

The above Patent Documents 1 and 2 form a single metal base film such as silver or bismuth and form a tin film on the base film. For example, electroless plating is performed on a silver base layer. When a tin film is formed by this, pinholes are generated in the upper tin film, and there are problems such as reduced solder wettability and bonding strength.
In Examples 1 and 2 of Patent Document 4, a very thin tin-cobalt alloy film (upper layer; 1.25 μm) is formed on an underlayer of a pure tin film (3 μm). Since it is not a tin film, it is effective for whisker prevention, but the problem that solder wettability is inferior to that of a pure tin layer remains.
Furthermore, in Examples 1 and 2 of Patent Document 3, a tin coating B layer (upper layer: 0.5 μm) is formed using a tin-copper alloy plating A layer (3.21 μm) as a base (paragraph 58). ) Basically, since the underlayer is made thicker and the tin film (upper layer) is made thinner, it takes cost and time to form the underlayer and the upper layer by electroless plating.

  An object of the present invention is to easily prevent the generation of tin whiskers without generating pinholes in the tin coating in a two-layer plating method in which a tin coating is formed on an underlayer.

The present inventors formed a thin tin-silver alloy film instead of the silver film as in the above prior art as the underlayer, and formed a pure tin film on the upper layer thicker than the underlayer, If the silver content of the film is specified within a large appropriate range, it is possible to easily prevent the occurrence of tin whiskers while ensuring good solder wettability and bonding strength without generating pinholes in the upper tin film. In particular, the inventors have found that whisker can be sufficiently prevented even if the undercoat is formed very thin to 0.5 μm or less, and even if the silver ratio in the underlayer is very small.
And based on this knowledge, when the film thickness of the undercoat and the total film thickness of the undercoat and the upper layer are specified, and the composition of silver in the underlayer tin-silver alloy film is specified, the upper tin layer It can effectively prevent tin whisker and pinholes in the film, and by optimizing the composition of the silver in the entire underlayer and upper layer, or by optimizing the film thickness ratio of the underlayer / upper layer, the whisker prevention effect can be improved. The present invention has been completed by finding that it can be improved.

That is, the present invention 1 is to form an undercoat made of an electroless tin-silver alloy plating film on an object to be plated,
A method for preventing a tin whisker that forms an electroless tin plating film (upper layer film) on the base film,
The film thickness of the base film is 0.025 to 0.5 μm, and the total film thickness of the base film and the upper film is 0.1 to 6 μm,
The composition ratio of silver in the underlying alloy film is 5 to 90% by weight,
A method for preventing tin whiskers by electroless plating, wherein the object to be plated is a copper-clad laminated substrate, a chip-on-film (COF) or a TAB film carrier.

The present invention 2 is the method for preventing tin whiskers by electroless plating according to the present invention 1, wherein the weight ratio of silver in the entire coating of the base and the upper layer is 0.1 to 50% by weight.

The present invention 3, in the present invention 1 or 2, tin - and the thickness A of the base film made of silver alloy, the thickness B of the upper layer coating consisting of tin,
A method for preventing tin whiskers by electroless plating, wherein the film thickness ratio of the base film and the upper film is B (upper layer) / A (base) = 1.2-30.

Invention 4 is the prevention of tin whisker by electroless plating, wherein the temperature of the electroless plating bath when forming the undercoat is 5 to 60 ° C. Is the method.

In the present invention, in order to specify the film thickness of the base film and the total film thickness of the base and the upper layer within an appropriate range, and to specify the composition of silver in the base within the appropriate range, pinholes are formed in the upper tin film. Tin whisker can be effectively prevented by a simple method without using a complicated method such as a conventional annealing method, while ensuring good solder wettability, bonding strength and coating appearance without generating them.
In addition, when the composition of silver in the entire film of the base and the upper layer or the film thickness ratio of the base / upper layer is specified within an appropriate range, the generation of whiskers can be better prevented.

The present invention is a tin whisker prevention method for forming a base film by electroless tin-silver alloy plating on an object to be plated, and forming an upper film by electroless tin plating on the base film, the film thickness of the base film, This is a method for preventing tin whiskers by electroless plating using a predetermined electronic component in which the total film thickness of the base and upper layers and the composition ratio of silver in the base alloy film are specified.

The undercoating of the present invention 1 is a tin-silver alloy plating film formed by electroless plating, and the upper film is a tin film formed by electroless plating.
The first feature of the first aspect of the present invention in which the film of the base and the upper layer is formed by electroless plating is that the film thickness of the base and the upper layer is specified in an appropriate range.
That is, the film thickness of the undercoat is 0.025 to 0.5 μm, preferably 0.03 to 0.4 μm. Whiskering can be sufficiently prevented even if the film thickness of the undercoat is as thin as 0.5 μm or less, but if it is thinner than 0.025 μm, the whisker prevention effect is lowered.
The total film thickness of the underlayer and the upper layer is 0.1 to 6 μm, preferably 0.15 to 4 μm, and more preferably 0.2 to 3 μm. If the thickness is less than 0.1 μm, the whisker prevention effect is reduced, and even if the thickness is greater than 6 μm, the whisker prevention effect and solder wettability are not significantly changed by electroless plating, and thickening is not suitable for electroless plating.
Furthermore, as shown in the present invention 3 , the ratio of the film thickness A of the undercoat to the film thickness B of the upper coat is preferably B (upper layer) / A (undercoat) = 1.2 to 30, preferably 2 to 20 More preferred. That is, in the present invention, the basic principle is that the base film (tin-silver alloy film) is very thin and the upper film (tin film) is thicker than the base. In this respect, the base (plating A layer) The design concept of the film thickness ratio is opposite to the aforementioned Patent Document 3 (B / A = 0.001 to 1) in which the thickness of the film is formed to be considerably thicker than the upper layer (coating layer B).

The second feature of the present invention 1 is that the composition of silver in the undercoat made of a tin-silver alloy is specified in an appropriate range.
The silver ratio of the undercoat is 5 to 90% by weight, preferably 7 to 70% by weight, and more preferably 10 to 60% by weight.
As described above, since the base film is thin, it is necessary to increase the silver ratio to ensure the whisker prevention effect. When the silver ratio is 5% by weight or less, the whisker prevention effect is lowered. Furthermore, when the ratio is 5% by weight or less, the silver concentration of the plating bath is too low during electroless plating, and it is necessary to replenish the plating solution frequently. On the other hand, when the silver ratio exceeds 90% by weight, the risk of pinholes occurring in the upper tin film increases as in the case of the silver film.
Incidentally, in Patent Document 4 mentioned above, the metal composition such as silver and bismuth of the undercoat is set to 5% by weight or less.
On the other hand, as shown in the present invention 2 , the weight ratio of silver in the entire film of the base and the upper layer is preferably 0.1 to 50% by weight, more preferably 0.5 to 30% by weight. If it exceeds 50% by weight, the solder wettability is lowered and there is a risk of poor bonding.

The electroless tin-silver alloy plating bath used for the formation of the undercoat of the present invention 1 has a basic composition of a soluble stannous salt, a soluble silver salt, an acid or a salt thereof, and a complexing agent.
The soluble stannous salt and silver salt do not exclude hardly soluble salts, and any salts can be used.
Examples of the soluble stannous salt include a stannous salt of an organic sulfonic acid described later, stannous borofluoride, stannous sulfosuccinate, stannous chloride, stannous sulfate, stannous oxide, and the like. Stannous methanesulfonate, stannous ethanesulfonate, stannous 2-hydroxyethane-1-sulfonate, stannous 2-hydroxypropane-1-sulfonate, p-phenolsulfonic acid stannous Organic sulfonic acid salts such as tin are preferred.
Examples of the soluble silver salt include silver sulfate, silver sulfite, silver carbonate, silver sulfosuccinate, silver nitrate, silver organic sulfonate, silver borofluoride, silver citrate, silver tartrate, silver gluconate, silver oxalate, and silver oxide. Originally, poorly soluble silver chloride can be used. Preferred silver salts include silver methanesulfonate, silver ethanesulfonate, silver 2-propanolsulfonate, silver phenolsulfonate, silver borofluoride and the like.
The converted addition amount of the soluble stannous salt or soluble silver salt as a metal salt is 0.0001 to 200 g / L, preferably 0.1 to 80 g / L.

Examples of the acid constituting the base of the electroless tin-silver alloy plating bath of the present invention include organic sulfonic acids such as alkane sulfonic acid, alkanol sulfonic acid, and aromatic sulfonic acid that are relatively easy to treat waste water, or aliphatic. Organic acids such as carboxylic acids are preferred, but inorganic acids such as borohydrofluoric acid, silicohydrofluoric acid, sulfamic acid, hydrochloric acid, sulfuric acid, and perchloric acid may be used.
The above acid can be used alone or in combination, and the amount of acid added is 0.1 to 300 g / L, preferably 20 to 120 g / L.

As the alkane sulfonic acid, those represented by the chemical formula C _n H _{2n + 1} SO ₃ H (for example, n = 1 to 5, preferably 1 to 3) can be used. Specifically, methanesulfonic acid, ethane In addition to sulfonic acid, 1-propanesulfonic acid, 2-propanesulfonic acid, 1-butanesulfonic acid, 2-butanesulfonic acid, pentanesulfonic acid, and the like, hexanesulfonic acid, decanesulfonic acid, dodecanesulfonic acid and the like can be mentioned.

Examples of the alkanol sulfonic acid include chemical formula C _m H _{2m + 1} —CH (OH) —C _p H _2p —SO ₃ H (for example, m = 0 to 6, p = 1 to 5).
In particular, 2-hydroxyethane-1-sulfonic acid, 2-hydroxypropane-1-sulfonic acid, 2-hydroxybutane-1-sulfonic acid, 2-hydroxypentane-1- In addition to sulfonic acid, etc. 1-hydroxypropane-2-sulfonic acid, 3-hydroxypropane-1-sulfonic acid, 4-hydroxybutane-1-sulfonic acid, 2-hydroxyhexane-1-sulfonic acid, 2-hydroxydecane -1-sulfonic acid, 2-hydroxydodecane-1-sulfonic acid and the like.

  The aromatic sulfonic acid is basically benzene sulfonic acid, alkyl benzene sulfonic acid, phenol sulfonic acid, naphthalene sulfonic acid, alkyl naphthalene sulfonic acid, etc., specifically 1-naphthalene sulfonic acid, 2-naphthalene. Examples include sulfonic acid, toluenesulfonic acid, xylenesulfonic acid, p-phenolsulfonic acid, cresolsulfonic acid, sulfosalicylic acid, nitrobenzenesulfonic acid, sulfobenzoic acid, diphenylamine-4-sulfonic acid, and the like.

  Generally as said aliphatic carboxylic acid, a C1-C6 carboxylic acid can be used. Specific examples include acetic acid, propionic acid, butyric acid, citric acid, tartaric acid, gluconic acid, sulfosuccinic acid, and trifluoroacetic acid.

The complexing agent contained in the electroless tin-silver alloy bath of the present invention 1 functions to stabilize the plating bath and to co-deposit tin and silver. Thioureas, amines, sulfides, mercapto And the like.
The thioureas are thiourea and thiourea derivatives.
Examples of the thiourea derivative include 1,3-dimethylthiourea, trimethylthiourea, diethylthiourea (for example, 1,3-diethyl-2-thiourea), N, N′-diisopropylthiourea, allylthiourea, acetylthiourea, Examples include ethylenethiourea, 1,3-diphenylthiourea, thiourea dioxide, thiosemicarbazide and the like.
Examples of the amines include aminocarboxylic acid compounds such as aminoacetic acid, aminopropionic acid, aminovaleric acid, and amino acids, polyamine compounds such as ethylenediamine and tetramethylenediamine, and aminoalcohol compounds such as monoethanolamine and diethanolamine. .
Among the amines, aminocarboxylic acid compounds include ethylenediaminetetraacetic acid (EDTA), ethylenediaminetetraacetic acid disodium salt (EDTA · 2Na), hydroxyethylethylenediaminetriacetic acid (HEDTA), diethylenetriaminepentaacetic acid (DTPA), Ethylenetetramine hexaacetic acid (TTHA), ethylenediaminetetrapropionic acid, nitrilotriacetic acid (NTA), iminodiacetic acid (IDA), iminodipropionic acid (IDP), metaphenylenediaminetetraacetic acid, 1,2-diaminocyclohexane-N, N , N ′, N′-tetraacetic acid, diaminopropionic acid, glutamic acid, ornithine, cysteine, N, N-bis (2-hydroxyethyl) glycine, and the like.
Among the amines, polyamine compounds and amino alcohol compounds include ethylenediaminetetramethylene phosphate, diethylenetriaminepentamethylene phosphate, aminotrimethylene phosphate, aminotrimethylene phosphate pentasodium salt, monoethanolamine, Diethanolamine, triethanolamine, monopropanolamine, dipropanolamine, tripropanolamine, methylenediamine, ethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, diethylenetriamine, tetraethylenepentamine, pentaethylenehexamine, hexaethyleneheptamine , Cinnamylamine, p-methoxycinnamylamine and the like.
Examples of the sulfides and mercapts include 2,2′-dithiodianiline, dipyridyl disulfide, thiodiglycolic acid, β-thiodiglycol, bis (dodecaethylene glycol) thioether, and 1,2-bis (2-hydroxyethyl). Thio) ethane, 1,4-bis (2-hydroxyethylthio) butane, thioglycol, thioglycolic acid, mercaptosuccinic acid and the like.

  In addition to the above-described components, the electroless tin-silver alloy plating bath of the present invention 1 includes a surfactant, a reducing agent, a concealing complexing agent, a pH adjusting agent, a buffering agent, a smoothing agent, a stress relaxation agent, Of course, additives usually used in plating baths such as brighteners, semi-brighteners and antioxidants can be mixed.

The reducing agent is added for reducing the metal salt and adjusting the deposition rate and the ratio of the precipitated alloy, and phosphoric acid compounds, amine boranes, borohydride compounds, hydrazine derivatives, etc. can be used alone or in combination. .
Examples of the phosphoric acid compounds include hypophosphorous acid, phosphorous acid, pyrophosphoric acid, polyphosphoric acid, or salts of ammonium, lithium, sodium, potassium, calcium and the like.
Examples of the amine boranes include dimethylamine borane, trimethylamine borane, isopropylamine borane, morpholine borane and the like.
Examples of the borohydride compound include sodium borohydride. Examples of the hydrazine derivative include hydrazine hydrate, methyl hydrazine, and phenyl hydrazine.
The addition amount of the reducing agent is 0.1 to 200 g / L, preferably 10 to 150 g / L.

Examples of the surfactant include nonionic surfactants, amphoteric surfactants, cationic surfactants, and anionic surfactants. These various surfactants can be used alone or in combination.
The addition amount is 0.01 to 100 g / L, preferably 0.1 to 50 g / L.

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 alkoxylated phosphoric acid ( salts), sorbitan esters, styrenated phenols, polyalkylene glycols, C ₁ -C ₂₂ aliphatic amines, C ₁ -C ₂₂ 2 to 300 moles of aliphatic amides such as ethylene oxide (EO) and / or propylene oxide (PO) Addition condensation. Accordingly, it may be an EO-only adduct such as a predetermined alkanol, phenol, or naphthol, an adduct of PO alone, or an adduct in which EO and PO coexist. Specifically, α-naphthol or β- Ethylene oxide adducts of naphthol (ie α-naphthol polyethoxylate etc.) are preferred.

C ₁ -C ₂₀ alkanols for addition condensation of ethylene oxide (EO) and / or propylene oxide (PO) include octanol, decanol, lauryl alcohol, tetradecanol, hexadecanol, stearyl alcohol, eicosanol, cetyl alcohol, oleyl alcohol , Docosanol and the like.
Similarly, bisphenols include bisphenol A, bisphenol B, bisphenol F, bisphenol S, and the like.
C ₁ -C ₂₅ alkylphenols include mono, di, or trialkyl substituted phenols such as p-butylphenol, p-isooctylphenol, p-nonylphenol, p-hexylphenol, 2,4-dibutylphenol, 2,4, Examples include 6-tributylphenol, p-dodecylphenol, p-laurylphenol, and p-stearylphenol.
Examples of the arylalkylphenol include 2-phenylisopropylphenol .

Examples of the alkyl group of C ₁ -C ₂₅ alkyl naphthol include methyl, ethyl, propyl, butylhexyl, octyl, decyl, dodecyl, octadecyl, and the like, which may be at any position of the naphthalene nucleus.
C ₁ -C ₂₅ alkoxylated phosphoric acid (salt) is represented by the following general formula (a).
Ra · Rb · (MO) P = O (a)
(In formula (a), R _a and R _b are the same or different C ₁ -C ₂₅ alkyl, provided that one of them may be H. M represents H or an alkali metal.)

Sorbitan esters include mono-, di- or triesterized 1,4-, 1,5- or 3,6-sorbitan, such as sorbitan monolaurate, sorbitan monopalmitate, sorbitan distearate, sorbitan dioleate, Examples include sorbitan mixed fatty acid esters.
C ₁ -C ₂₂ aliphatic amines include saturated and unsaturated fatty acid amines such as propylamine, butylamine, hexylamine, octylamine, decylamine, laurylamine, stearylamine, oleylamine, ethylenediamine, propylenediamine, and the like.
C ₁ -C ₂₂ aliphatic amides include amides such as propionic acid, butyric acid, caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, and behenic acid.

As the cationic surfactant, a quaternary ammonium salt represented by the following general formula (b)
_{(R 1 · R 2 · R} 3 · R 4 N) + · X - ... (b)
(In the formula (b), X is halogen, hydroxy, C ₁ -C ₅ alkanesulfonic acid or sulfuric acid, R ₁ , R ₂ and R ₃ are the same or different C ₁ -C ₂₀ alkyl, and R ₄ is C ₁ -C _{10 represents} alkyl or benzyl.)
Or the pyridinium salt etc. which are represented with the following general formula (c) are mentioned.
R _6- (C ₅ H ₅ N-R ₅ ) ⁺ · X ⁻ (c)
(In formula (c), C ₅ H ₅ N is a pyridine ring, X is halogen, hydroxy, C ₁ -C ₅ alkanesulfonic acid or sulfuric acid, R ₅ is C ₁ -C ₂₀ alkyl, R ₆ is H or C _1. ~C ₁₀ represents an alkyl.)

  Examples of cationic surfactants in the form of salts include lauryl trimethyl ammonium salt, stearyl trimethyl ammonium salt, lauryl dimethyl ethyl ammonium salt, octadecyl dimethyl ethyl ammonium salt, dimethyl benzyl lauryl ammonium salt, cetyl dimethyl benzyl ammonium salt, octadecyl dimethyl Examples include benzylammonium salt, trimethylbenzylammonium salt, triethylbenzylammonium salt, hexadecylpyridinium salt, laurylpyridinium salt, dodecylpyridinium salt, stearylamine acetate, laurylamine acetate, octadecylamine acetate and the like.

  Examples of the anionic surfactant include alkyl sulfates, polyoxyethylene alkyl ether sulfates, polyoxyethylene alkyl phenyl ether sulfates, alkyl benzene sulfonates, (mono, di, tri) alkyl naphthalene sulfonates, and the like. It is done. Examples of the alkyl sulfate include sodium lauryl sulfate and sodium oleyl sulfate. Examples of the polyoxyethylene alkyl ether sulfate include sodium polyoxyethylene (EO12) nonyl ether sulfate and sodium polyoxyethylene (EO15) dodecyl ether sulfate. Examples of the polyoxyethylene alkylphenyl ether sulfate include polyoxyethylene (EO15) nonylphenyl ether sulfate. Examples of the alkyl benzene sulfonate include sodium dodecylbenzene sulfonate. Examples of the (mono, di, tri) alkyl naphthalene sulfonate include sodium dibutyl naphthalene sulfonate.

  Examples of the amphoteric surfactant include carboxybetaine, imidazoline betaine, sulfobetaine, and aminocarboxylic acid. In addition, sulfation of a condensation product of ethylene oxide and / or propylene oxide and an alkylamine or diamine, or a sulfonated adduct can also be used.

  Typical carboxybetaines or imidazoline betaines include lauryldimethylaminoacetic acid betaine, myristyldimethylaminoacetic acid betaine, stearyldimethylaminoacetic acid betaine, coconut oil fatty acid amidopropyldimethylaminoacetic acid betaine, 2-undecyl-1-carboxymethyl- Examples thereof include 1-hydroxyethyl imidazolinium betaine, 2-octyl-1-carboxymethyl-1-carboxyethyl imidazolinium betaine, and the sulfated and sulfonated adducts include sulfated adducts of ethoxylated alkylamines, sulfones. Examples include sodium lauric acid derivative sodium salt.

Examples of the sulfobetaines include coconut oil fatty acid amidopropyldimethylammonium-2-hydroxypropanesulfonic acid, N-cocoylmethyl taurine sodium, and N-palmitoylmethyl taurine sodium.
Examples of the aminocarboxylic acid include dioctylaminoethylglycine, N-laurylaminopropionic acid, octyldi (aminoethyl) glycine sodium salt, and the like.

  The concealing complexing agent is used for the purpose of improving the stability of the bath, and prevents the adverse effect on the plating bath of impurity metal ions eluted from the base metal of the object to be plated such as copper. EDTA, hydroxyethylethylenediaminetriacetic acid (HEDTA), iminodiacetic acid (IDA), iminodipropionic acid (IDP), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), triethylenetetraminehexaacetic acid (TTHA), ethylenediamine, hexa Examples include methylenediamine, diethylenetriamine, citric acid, tartaric acid, succinic acid, malonic acid, glycolic acid, glucoheptonic acid, gluconic acid, glycine, pyrophosphoric acid, tripolyphosphoric acid, 1-hydroxyethane-1,1-bisphosphonic acid, and the like.

When using the electroless tin-silver alloy plating bath of the present invention 1, the bath temperature is preferably 5 ° C to 60 ° C, which is lower than usual.
As described above, in the present invention, an electroless tin - since it is sufficient to form a primer film of the silver alloy in very thin 0.025~0.5μm thickness, it is not necessary to thickly plated by increasing the bath temperature .
Moreover, as a to-be-plated object to which the electroless plating of the present invention is applied, a copper-clad laminated substrate, a chip-on-film (COF) or a TAB film carrier is suitable as shown in the present invention 4 . The copper-clad laminate is a rigid printed circuit board (PCB), a flexible printed circuit board (FPC), a ball grid array (BGA), a chip size package (CSP), or the like.

Hereinafter, an evaluation test on the prevention of occurrence of pinholes and tin whiskers in an upper tin film by forming the film of the present example by applying a two-layer plating of an underlayer and an upper film on an object to be plated by electroless plating of the present invention Examples will be described sequentially.
“Parts” and “%” in the above examples and test examples are basically based on weight.
Needless to say, the present invention is not limited to the following examples and test examples, and can be arbitrarily modified within the scope of the technical idea of the present invention.

<< Example of forming a two-layer film by electroless plating >>
Examples 1-9 The following undercoat tin - an example of a silver alloy film. Examples 4 and 6 are examples in which the film thickness of the undercoat is close to the lower limit (0.025 μm) of the appropriate range of the present invention. Example 7 is an example in which the film thickness of the entire film of the base layer and the upper layer is close to the lower limit (0.1 μm) of the appropriate range of the present invention. Example 8 is an example in which the silver ratio in the undercoat is close to the lower limit (5%) of the appropriate range of the present invention. Example 9 is an example in which the silver ratio in the entire film of the base layer and the upper layer is larger than the upper limit (50%) of the preferable range shown in the present invention 2, and all other examples are examples within the preferable range.
On the other hand, among the following Comparative Examples 1 to 4, Comparative Example 1 is an example in which the base film is a pure silver film. Comparative Example 2 is an example in which the ratio of silver in the undercoat made of a tin-silver alloy exceeds the upper limit (90%) of the appropriate range of the present invention. Comparative Example 3 is an example in which the ratio of silver in the undercoat is less than the lower limit (5%) of the appropriate range of the present invention. Comparative Example 4 is an example in which the film thickness of the undercoat is thinner than the lower limit (0.025 μm) of the appropriate range of the present invention.

(1) Example 1
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.35 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.005 mol / L
Methanesulfonic acid: 0.30 mol / L
Thiourea: 1.40 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 30 ° C
Plating time: 45 seconds
(c) Underlayer: Tin-silver alloy film thickness: 0.08 μm
Silver ratio of tin-silver alloy film: 46.1%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and the upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.30 mol / L
Methanesulfonic acid: 1.00 mol / L
Phenolsulfonic acid: 0.50 mol / L
Thiourea: 2.00 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene dodecyl phenyl ether (EO8 mol): 10.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 15 minutes
(f) Upper layer Total film thickness of the underlayer and upper layer: 1.15 μm
Silver ratio of the entire underlayer and upper layer: 2.3%

(2) Example 2
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.35 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.007 mol / L
p-phenolsulfonic acid: 0.10 mol / L
Methanesulfonic acid: 0.20 mol / L
Thiourea: 1.40 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 30 ° C
Plating time: 45 seconds
(c) Underlayer: Tin-silver alloy film thickness: 0.11 μm
Silver ratio of tin-silver alloy film: 49.8%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and the upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.35 mol / L
Methanesulfonic acid: 0.50 mol / L
2-hydroxyethanesulfonic acid: 1.00 mol / L
Thiourea: 2.30 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene dodecyl phenyl ether (EO8 mol): 15.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 15 minutes
(f) Upper layer Total film thickness of the underlying layer and the upper layer: 1.30 μm
Silver ratio of the entire underlayer and upper layer: 1.7%

(3) Example 3
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath 2-hydroxyethane sulfonate stannous (as Sn ²⁺ ): 0.25 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.006 mol / L
2-hydroxyethanesulfonic acid: 0.40 mol / L
Thiourea: 1.40 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 30 ° C
Plating time: 45 seconds
(c) Underlayer: Tin-silver alloy film thickness: 0.15 μm
Silver ratio of tin-silver alloy film: 52.6%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and the upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous 2-hydroxyethanesulfonate (as Sn ²⁺ ): 0.40 mol / L
2-hydroxyethanesulfonic acid: 1.00 mol / L
Thiourea: 2.60 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene nonylphenyl ether (EO12 mol): 12.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 15 minutes
(f) Upper layer Total film thickness of the underlying layer and the upper layer: 1.50 μm
Silver ratio of the entire underlayer and upper layer: 2.8%

(4) Example 4
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.35 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.004 mol / L
Methanesulfonic acid: 0.30 mol / L
Thiourea: 1.40 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 30 ° C
Plating time: 45 seconds
(c) Underlayer: Tin-silver alloy film thickness: 0.06 μm
Silver ratio of tin-silver alloy film: 32.5%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and the upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous 2-hydroxyethanesulfonate (as Sn ²⁺ ): 0.35 mol / L
2-hydroxyethanesulfonic acid: 1.30 mol / L
Thiourea: 2.00 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene nonylphenyl ether (EO12 mol): 12.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 15 minutes
(f) Upper layer Total film thickness of the underlying layer and the upper layer: 1.25 μm
Silver ratio of the entire underlayer and upper layer: 2.1%

(5) Example 5
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath Stannous 2-hydroxyethanesulfonate (as Sn ²⁺ ): 0.35 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.006 mol / L
2-hydroxyethanesulfonic acid: 0.30 mol / L
Thiourea: 1.40 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 30 ° C
Plating time: 45 seconds
(c) Underlayer Film thickness of tin-silver alloy film: 0.09 μm
Silver ratio of tin-silver alloy film: 45.8%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and the upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous 2-hydroxyethanesulfonate (as Sn ²⁺ ): 0.30 mol / L
2-hydroxyethanesulfonic acid: 1.30 mol / L
Thiourea: 2.50 mol / L
Hypophosphorous acid: 0.70 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 15 minutes
(f) Upper layer Thickness of the entire underlayer and upper layer: 1.56 μm
Silver ratio of the entire underlayer and upper layer: 2.7%

(6) Example 6
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.35 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.004 mol / L
Methanesulfonic acid: 0.30 mol / L
Thiourea: 1.40 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 30 ° C
Plating time: 20 seconds
(c) Underlayer: Tin-silver alloy film thickness: 0.06 μm
Silver ratio of tin-silver alloy film: 30.7%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous 2-hydroxyethanesulfonate (as Sn ²⁺ ): 0.35 mol / L
2-hydroxyethanesulfonic acid: 1.30 mol / L
Thiourea: 1.50 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene nonylphenyl ether (EO12 mol): 12.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 3 minutes
(f) Upper layer Total film thickness of the underlying layer and the upper layer: 0.35 μm
Silver ratio in the entire coating of the underlayer and upper layer: 4.3%

(7) Example 7
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath Stannous 2-hydroxyethanesulfonate (as Sn ²⁺ ): 0.35 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.005 mol / L
2-hydroxyethanesulfonic acid: 0.30 mol / L
Thiourea: 1.40 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 30 ° C
Plating time: 20 seconds
(c) Underlayer thickness of tin-silver alloy film: 0.07 μm
Silver ratio of tin-silver alloy film: 28.2%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and the upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous 2-hydroxyethanesulfonate (as Sn ²⁺ ): 0.35 mol / L
2-hydroxyethanesulfonic acid: 1.30 mol / L
Thiourea: 1.50 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene nonylphenyl ether (EO12 mol): 12.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 3 minutes
(f) Upper layer Total film thickness of the underlying layer and the upper layer: 0.30 μm
Silver ratio of the entire underlayer and upper layer: 3.7%

(8) Example 8
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.35 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.005 mol / L
Methanesulfonic acid: 0.30 mol / L
Thiourea: 1.40 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 30 ° C
Plating time: 45 seconds
(c) Underlayer: Tin-silver alloy film thickness: 0.08 μm
Silver ratio of tin-silver alloy film: 10.1%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.30 mol / L
Methanesulfonic acid: 1.00 mol / L
Phenolsulfonic acid: 0.50 mol / L
Thiourea: 2.00 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene dodecyl phenyl ether (EO8 mol): 10.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 5 minutes
(f) Upper layer Total film thickness of the underlayer and upper layer: 0.85 μm
Silver ratio of the entire underlayer and upper layer: 1.1%

(9) Example 9
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.35 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.015 mol / L
Methanesulfonic acid: 0.30 mol / L
Thiourea: 1.40 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 50 ° C
Plating time: 120 seconds
(c) Underlayer: Tin-silver alloy film thickness: 0.25 μm
Silver ratio of tin-silver alloy film: 80.5%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and the upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.30 mol / L
Methanesulfonic acid: 1.00 mol / L
Phenolsulfonic acid: 0.50 mol / L
Thiourea: 2.00 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene dodecyl phenyl ether (EO8 mol): 10.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 3 minutes
(f) Upper layer Total film thickness of the underlying layer and the upper layer: 0.61 μm
Silver ratio of the entire underlayer and upper layer: 55.3%

(13) Comparative Example 1
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (a) below, after forming an underlayer of a silver film on the object to be plated by electroless silver plating, the following As shown in (b), a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless silver plating An electroless silver plating bath having the following composition (a) is constructed, and a silver plating film (c) below (c) is formed on the object to be plated under the conditions (b) below. Underlayer) was formed.
(a) Electroless silver plating bath Silver methanesulfonate (as Ag ⁺ ): 0.06 mol / L
Methanesulfonic acid: 1.00 mol / L
Thiourea: 0.30 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 5.0 g / L
(b) Immersion conditions Plating bath temperature: 50 ° C
Plating time: 60 seconds
(c) Underlayer Film thickness of silver film: 0.10 μm
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and the upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.30 mol / L
Methanesulfonic acid: 1.00 mol / L
Phenolsulfonic acid: 0.50 mol / L
Thiourea: 2.00 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene dodecyl phenyl ether (EO8 mol): 10.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 15 minutes
(f) Upper layer Total film thickness of the underlying layer and the upper layer: 1.23 μm
Silver ratio of the entire underlayer and upper layer: 1.3%

(14) Comparative example 2
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.15 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.05 mol / L
Methanesulfonic acid: 1.50 mol / L
Thiourea: 2.00 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 30 ° C
Plating time: 45 seconds
(c) Underlayer: Tin-silver alloy film thickness: 0.06 μm
Silver ratio of tin-silver alloy film: 98.1%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and the upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.30 mol / L
Methanesulfonic acid: 1.00 mol / L
Phenolsulfonic acid: 0.50 mol / L
Thiourea: 2.00 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene dodecyl phenyl ether (EO8 mol): 10.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 15 minutes
(f) Upper layer Total film thickness of the underlying layer and the upper layer: 1.25 μm
Silver ratio of the entire underlayer and upper layer: 1.8%

(15) Comparative Example 3
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.35 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.003 mol / L
Methanesulfonic acid: 1.50 mol / L
Thiourea: 2.00 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 30 ° C
Plating time: 45 seconds
(c) Underlayer: Tin-silver alloy film thickness: 0.11 μm
Silver ratio of tin-silver alloy film: 2.8%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and the upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.30 mol / L
Methanesulfonic acid: 1.00 mol / L
Phenolsulfonic acid: 0.50 mol / L
Thiourea: 2.00 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene dodecyl phenyl ether (EO8 mol): 10.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 15 minutes
(f) Upper layer Total film thickness of the underlying layer and the upper layer: 1.30 μm
Silver ratio of the entire underlayer and upper layer: 0.09%

(16) Comparative Example 4
Using COF (Chip on Film; ESPANEX; manufactured by Nippon Steel Chemical Co., Ltd.) as an object to be plated, as shown in (b) below, an underlayer of a tin-silver alloy film is formed on the object by electroless tin-silver alloy plating. Then, as shown in (b) below, a tin film (upper layer) was formed on the underlayer by electroless tin plating.
(A) Formation of an underlayer by electroless tin-silver alloy plating An electroless tin-silver alloy plating bath having the following composition (a) is constructed, and the following (c) ) Tin-silver alloy plating film (underlayer).
(a) Electroless tin-silver alloy plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.35 mol / L
Silver methanesulfonate (as Ag ⁺ ): 0.005 mol / L
Methanesulfonic acid: 0.30 mol / L
Thiourea: 1.40 mol / L
Hypophosphorous acid: 0.75 mol / L
Polyoxyethylene octyl phenyl ether (EO 10 mol): 15.0 g / L
(b) Immersion conditions Plating bath temperature: 30 ° C
Plating time: 3 seconds
(c) Underlayer: Tin-silver alloy film thickness: 0.02 μm
Silver ratio of tin-silver alloy film: 33.3%
(B) Formation of upper layer by electroless tin plating An electroless tin plating bath having the following composition (d) was erected, and a tin plating film (upper layer) was formed on the underlayer under the following conditions (e). In the following (f), the film thickness of the whole film of the base and the upper layer and the ratio of silver are shown.
(d) Formation of upper layer by electroless tin plating bath Stannous methanesulfonate (as Sn ²⁺ ): 0.30 mol / L
Methanesulfonic acid: 1.00 mol / L
Phenolsulfonic acid: 0.50 mol / L
Thiourea: 2.00 mol / L
Hypophosphorous acid: 0.50 mol / L
Polyoxyethylene dodecyl phenyl ether (EO8 mol): 10.0 g / L
(e) Plating conditions Plating bath temperature: 65 ° C
Plating time: 15 minutes
(f) Upper layer Total film thickness of the underlying layer and the upper layer: 1.38 μm
Silver ratio of the entire underlayer and upper layer: 1.1%

<< Evaluation test example of upper tin film >>
As described above, in Examples 1 to 12 and Comparative Examples 1 to 4 described above, each tin film was formed on the upper layer of the base film by electroless plating. The prevention performance of tin whiskers was evaluated as follows.
(1) Pinhole generation prevention performance Using a scanning electron microscope, the surface of the upper tin film immediately after formation was microscopically observed, and the superiority or inferiority of the pinhole generation prevention performance was evaluated according to the following criteria.
A: No pinhole was generated in the upper tin film.
×: Pinholes occurred in the upper tin film.

(2) Tin whisker generation prevention performance Using a scanning electron microscope, after confirming that there is no whisker on the surface of the upper layer immediately after formation, the sample was left at room temperature for 500 hours, and then the same visible part of the sample The whisker generation prevention performance was evaluated based on the following criteria.
○: The number of whiskers generated at the same visually recognized site was zero.
X: A whisker having a length of 5 μm or more was found in the visually recognizing part.

The table below shows the test results.
Pinhole prevention property Whisker prevention Example 1 ○ ○
Example 2 ○ ○
Example 3 ○ ○
Example 4 ○ ○
Example 5
Example 6 ○ ○
Example 7 ○ ○
Example 8
Example 9
Comparative Example 1 × ○
Comparative Example 2 × ○
Comparative Example 3 ○ ×
Comparative Example 4 ○ ×

From the above table, whilst no whisker was observed in the upper tin film in Comparative Example 1 where the base film was a pure silver film, pinholes were generated, and solder wettability and bonding strength during soldering. It has become clear that there is a risk of damage.
In contrast, in Examples 1 to 9 in which the underlayer was a tin-silver alloy film, whiskers and pinholes were not generated in the upper tin film.
Thereby, in order to prevent a tin whisker satisfactorily without generating a pinhole in the upper tin film, it was confirmed that it was necessary to form a tin-silver alloy film instead of a silver film on the underlayer.

Further, even when the tin-silver alloy film is used as the underlayer, in Comparative Example 2 in which the silver ratio of the undercoat exceeds the upper limit (90%) of the appropriate range of the present invention, as in Comparative Example 1 above, Pinholes occurred in the tin film. Conversely, in Comparative Example 3 in which the silver ratio of the base film was less than the lower limit (5%) of the appropriate range, no pinhole was observed in the upper tin film, but tin whiskers were generated.
Therefore, in order to prevent the occurrence of pinholes and whiskers in the upper tin film, it is not sufficient to make the underlayer a tin-silver alloy film, and the silver ratio of the underlayer is specified within the proper range of the present invention. The necessity of making it possible was confirmed.

On the other hand, even if the silver ratio of the tin-silver alloy film of the underlayer is within the proper range of the present invention, in Comparative Example 4 where the film thickness of the underlayer is thinner than the lower limit (0.025 μm) of the proper range of the present invention, As in the case of Comparative Example 3, whiskers were generated in the upper tin film.
Therefore, in order to prevent the occurrence of pinholes and whiskers in the upper tin film, in addition to specifying the silver ratio of the underlayer (tin-silver alloy film) within the proper range of the present invention, The necessity of controlling the film thickness to a predetermined film thickness or more became clear. In addition, even if the film thickness of a base layer is very thin of 0.5 micrometer or less as seen in Examples 1-9 , prevention of whisker generation can be ensured favorably.

Next, when Examples 1 to 9 are examined in detail, in any of the Examples in which the underlayer is a tin-silver alloy film, it is possible to prevent whisker and pinholes in the upper tin film. In this case, even if the film thickness of the underlayer and the film thickness of the entire film of the underlayer and the upper layer change within the appropriate range of the present invention, or the silver ratio of the underlayer changes within the appropriate range of the present invention. Even so, it can be seen that the whisker prevention performance and the pinhole prevention performance can be favorably secured.
In particular, as seen in Example 8, a tin-silver alloy film having a low silver ratio of 10% was formed as an underlayer, or as seen in Examples 4 and 6 , the underlying tin-silver alloy film was formed. It should be noted that both the whisker of the upper tin film and the prevention of pinholes can be secured even when the film is formed with a very thin film thickness of 0.06 μm.
In addition, although the object of this test example is a whisker and pinhole generation prevention evaluation, in addition to these, when the solder wettability of the upper tin film was examined, the ratio of silver in the entire base film and the upper film was 50. % Of the examples (that is, examples satisfying the requirement of the present invention 2 ) were evaluated with better solder wettability than Example 9 that deviated from this requirement.

Claims (4)

After forming a base film made of an electroless tin-silver alloy plating film on the object to be plated,
A method for preventing a tin whisker that forms an electroless tin plating film (upper layer film) on the base film,
The film thickness of the base film is 0.025 to 0.5 μm, and the total film thickness of the base film and the upper film is 0.1 to 6 μm,
The composition ratio of silver in the underlying alloy film is 5 to 90% by weight,
A method for preventing tin whiskers by electroless plating, wherein the object to be plated is a copper-clad laminate, a chip-on-film (COF), or a TAB film carrier. The method for preventing tin whiskers by electroless plating according to claim 1 , wherein the weight ratio of silver in the entire base and upper layer coating is 0.1 to 50% by weight. Tin - a film thickness A of the base film made of silver alloy, the thickness B of the upper layer coating consisting of tin,
The method for preventing tin whiskers by electroless plating according to claim 1 or 2 , wherein the film thickness ratio of the undercoat and the upper coat is B (upper layer) / A (undercoat) = 1.2 to 30. . The method for preventing tin whiskers by electroless plating according to any one of claims 1 to 3 , wherein the bath temperature of the electroless plating bath when forming the base film is 5 to 60 ° C.