JP2008536011A - Electrolyte and method for tin-bismuth alloy layer deposition - Google Patents

Abstract

Electrolyte and method for deposition (precipitation) of tin-bismuth alloy layer United States Patent Application 20070227735 Kind Code: A1 The present invention relates to one or more alkyl sulfonic acids and / or alkanol sulfonic acids, one or more soluble tin (II) salts, one type. An acidic electrolyte for the deposition of tin-bisas alloys is described, comprising the above soluble bismuth (III) salt, one or more nonionic surfactants and one or more thiazole and / or thiadiazole compounds. In addition, the method using the electrolyte, the coating obtained using the method, as well as the use of the electrolyte for electronic component coating are also made possible.
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Description

  The present invention relates to an acidic electrolyte for tin-bismuth alloy deposition, a method using the electrolyte, a coating obtained using the method, and the use of the electrolyte for coating electronic components.

  The manufacture of electrical and electronic assemblies often requires the use of components having a coating with an electrolytically deposited tin-lead layer for functional reasons. Even after very long storage periods, these coatings ensure that the parts can be easily soldered.

  The use of lead in the manufacture of electrical and electronic equipment nevertheless poses a risk of environmental contamination with lead whenever such equipment is stored in a garbage dump. The corrosion process causes the lead from the solder joint to be converted to a water-soluble form, which in the long term results in groundwater contamination. From 1 July 2006, the European Union Directive no longer allows the use of lead-containing solder and electrodeposited tin-lead layers with few exceptions (Electrical and electronic equipment by the European Parliament and the European Council). January 27, 2003 Directive 2002/95 / EC on restrictions on the use of certain hazardous substances in Japan.

  An electrodeposited pure tin layer or a tin alloy layer such as tin copper (copper: 1-2% by mass), tin silver (silver: 2-5% by mass, tin bismuth (bismuth: 2-4% by mass), In the process, pure tin coating is a simple technology and at first glance it is also an economical alternative to the tin-lead layer, the risk of whisker formation cannot be completely eliminated, Whisker is defined as a single crystal of acicular tin, which usually has a diameter of a few micrometers, but can reach a length of a few millimeters. As a result of the increasing pace of miniaturization, whisker formation can cause shorting of nearby conductive components, which can cause malfunctions in electrical and electronic equipment, with considerable economic losses. .

  Whisker formation in the tin layer can be prevented by co-deposition of trace amounts of further metal. Co-deposition of trace amounts of lead (at least 3% by weight) has proven to be an effective process here. As explained above, however, this co-deposition is only allowed for a few applications.

  The deposition of a tin-bismuth alloy containing about 2-4% by weight bismuth has also been found to be a reliable measure to prevent whiskers. Larger amounts of bismuth are inconvenient because the resulting toughness of the deposited layer is reduced, so there is a risk that cracks will appear in the coating when the pins of an electronic component are mechanically processed. . As a result, characteristics such as solder ease and corrosion resistance of the layer deteriorate. When less than 2% by weight of bismuth is co-deposited, whisker formation is not completely excluded.

  Appropriate industrially feasible methods of tin bismuth deposition have already been used in the manufacture of electronic products, for example EP 0255558, EP 0715003, JP 2983548 and JP It is described in the specification of Japanese Patent No. 2132894.

Due to the significant difference in the electrochemical potential of tin (−0.12 volts) and bismuth (+0.35 volts), bismuth present in dissolved form as Bi ³⁺ in the electrolyte is more electronegative during charge exchange. Deposit on tin anode.

This reaction occurs according to the following formula:
3Sn ⁰ + 2Bi ³⁺ → 3Sn ²⁺ + 2Bi ⁰ ↓
The bismuth deposited during this charge exchange reaction is deposited in the form of a porous sponge-like coating that spontaneously peels off from the anode, so that the electrolyte penetrates and hence the part is also coated. In this case, these bismuth particles are also incorporated into the electrodeposition layer, thus giving rise to a highly dendritic layer which is technically useless. This can result in production discontinuation as well.

  In addition, the charge exchange reaction continually deprives the electrolyte of dissolved bismuth, so bismuth must be fully replenished, thus increasing process costs. In order to ensure the required range of bismuth co-deposition of 2-4% by weight, it is necessary to subject the bismuth content in the electrolyte to constant analysis and to compensate for the amount of bismuth accordingly. This requires a significant amount of time and effort.

  If loss of bismuth due to the charge exchange reaction can be prevented, a combination of an automatic metering pump and an ampere clock may be able to replenish the required amount of bismuth compound according to the amount of current flowing. The method is simple to implement and consequently becomes more economical. In this case, more accurate bismuth analysis in the electrolyte is simply required at less frequent intervals.

  A further drawback associated with the above charge exchange reaction is that this reaction can be performed whenever the coating operation is interrupted, for example as a result of a system malfunction, and whenever the part is immersed in the electrolyte for a short time without a current load. As well as on parts already coated. This can cause black stains on the surface. If electroplating is subsequently continued, a so-called sandwich coating occurs. The tin bismuth layer produced by the charge exchange reaction is formed between two electrolytically deposited tin-bismuth layers. When the pins are mechanically processed, this layer arrangement can lead to crack formation, can cause the layers to flake off and can be poor solder wettability.

In order to prevent the deposition of bismuth as a result of the charge exchange reaction, it has been proposed to use an electrolyte consisting of methanesulfonic acid and tin and bismuth salts of the acid, the electrolyte being an unsaturated carboxylic acid as an organic additive. And nonionic surfactants from the group of polyethylene oxide / polypropyloxide copolymers (US 2003/0132122). In the case of this electrolyte, bismuth deposition certainly decreases considerably during charge exchange, but it is impossible to achieve the required 2% Bi co-deposition in the coating. Even if as much as 10% by weight Bi is present in the electrolyte relative to the total metal content, the bismuth content in the deposited layer is only about 1.5-2% by weight.
European Patent No. 0255558 European Patent No. 0715003 Japanese Patent No. 2983548 Japanese Patent No. 2132894 US Patent Application Publication No. 2003/0132122

The object on which the present invention is based is therefore to produce an available electrolyte for depositing a tin-bismuth alloy having an alloy content of at least 2-4% by weight, in which the current is interrupted. If done, bismuth is not deposited on the already coated surface during charge exchange.

  The purpose is to provide one or more alkyl sulfonic acids and / or alkanol sulfonic acids, one or more soluble tin (II) salts, one or more soluble bismuth (III) salts, one or more nonionic surfactants, and one This is achieved by an aqueous acidic electrolyte for depositing a tin-bismuth alloy comprising the above thiazole compound and / or thiadiazole compound.

  Any of the compounds contained in the electrolyte will cause a shift of the bismuth electrode potential in the negative direction, as would be expected by one skilled in the art as a precondition to prevent the deposition of noble metals on more electronegative metals. Therefore, the solution for the above purpose using the electrolytes of the present invention is therefore surprising as far as the person skilled in the art is concerned. Even when combined, the components contained in the electrolyte do not shift the equilibrium potential to the cathode. Avoiding the deposition of bismuth on the newly deposited tin-bismuth surface is therefore not accountable for electrochemical conditions. The combination of materials at the time of use may interfere with the electroless permeation process, but there may be an inhibitory effect such as not interfering with bismuth electrolyte deposition.

No particular restrictions are placed on the thiazole or thiadiazole compound. The term “thiazole compound” is intended to define any substituted or unsubstituted, saturated or unsaturated thiazole compound. Possible examples of thiazole compounds are: benzothiazole, 2-aminobenzothiazole, 6-aminobenzothiazole, 2-hydroxybenzothiazole, 6-hydroxybenzothiazole, 2-methylbenzothiazole, 2-mercaptobenzothiazole, 6-methoxy -2-aminobenzothiazole and 2-thiazoline thiol-2. Of these, 2-mercaptobenzothiazole is particularly preferred. The term “thiadiazole compound” is intended to define any substituted or unsubstituted, saturated or unsaturated thiadiazole compound. Possible examples of thiadiazole compounds are: 2-amino-5-methyl-1,3,4-thiadiazole, 2-amino-5-mercapto-1,3,4-thiadiazole, 2-mercapto-5-methyl-1, Examples include 3,4-thiadiazole, 2-amino-1,3,4-thiadiazole and 2,5-dimercapto-1,3,4-thiadiazole. The thiazole and thiadiazole compounds can be used by themselves or in combination with each other.

  The concentration of the thiazole and / thiadiazole compound in the electrolyte is preferably 5 to 1,000 mg / l electrolyte, particularly preferably 50 to 500 mg / l electrolyte.

Any known nonionic surfactant can be used as the nonionic surfactant. Of these, specific references are block copolymers and copolymers of ethylene and propylene oxide. Particular preference is given to the general formula H— (OCH ₂ —CH ₂ ) m — (OCH (CH ₃ ) —CH ₂ ) n —OH having the weight average molecular weight of 2,000 to 10,000, n represents an integer of 5 to 60), and is a polyethylene oxide / polypropylene oxide copolymer. In an even more preferred embodiment, the nonionic surfactant has a cloud point of> 30 ° C.

  The concentration of the nonionic surfactant in the electrolyte according to the invention is preferably in the range 1-50 g / l, more preferably 2-20 g / l, particularly preferably 5-10 g / l. .

Tin (II) may be present in the electrolyte as a salt of mineral acid, alkyl sulfonic acid or alkanol sulfonic acid. Examples of mineral acid salts include sulfate and tetrafluoroborate. Preferred alkyl sulfonates include, for example, methane sulfonate, ethane sulfonate, n- and iso-propane sulfonate, methane disulfonate, ethane disulfonate, 2,3-propane disulfonate, and 1,3-propane disulfonate. Suitable alkanol sulfonates include 2-hydroxyethane sulfonate, 2-hydroxypropane sulfonate, 3-hydroxypropane sulfonate, and the like. Particularly preferred is tin (II) methanesulfonate.

  Tin (II) salts, calculated as tin (II), are present in the electrolyte in an amount of preferably 5 to 200 g / l electrolyte, particularly preferably 10 to 100 g / l electrolyte.

  The bismuth (III) salt can be added to the electrolyte in any form that allows for sufficient solubility in Bi (III) ions within the electrolyte. The addition can be carried out in the form of a dissolved Bi (III) salt or in the form of a solid compound that is converted into a soluble Bi (III) compound in the electrolyte in the presence of the free acid. Examples of suitable Bi (III) salts are salts of mineral acids such as chloride, tetrafluoroborate, nitrate, methanesulfonate, ethanesulfonate, methanedisulfonate, ethanedisulfonate, 2,3-propanedisulfonate and Alkyl sulfonic acid salts such as 1,3-propanedisulfonate, and alkanol sulfonic acid salts such as 2-hydroxyethane sulfonate and 3-hydroxypropane sulfonate. Particularly preferred is bismuth (III) methanesulfonate. When Bi (III) is added in solid form, bismuth (III) carbonate or bismuth (III) oxide is preferably used. These compounds dissolve in the electrolyte in the presence of alkali and / or alkanol sulfonic acids to form soluble Bi (III) compounds.

  The concentration of bismuth depends on the amount of Bi required in the resulting tin-bismuth alloy and the selected tin (II) salt concentration. The electrolyte preferably comprises bismuth (III) salt at a concentration that allows deposition of a tin-bismuth alloy having an alloy composition of 2-4% Bi. In this case, the concentration of Bi ions is preferably 3-6%, particularly preferably 5%, with respect to the selected tin (II) concentration.

  Alkyl sulfonic acids and alkanol sulfonic acids preferably have 1 to 10 carbon atoms, particularly preferably 1 to 5 carbon atoms. As an example, methanesulfonic acid, ethanesulfonic acid, n-propanesulfonic acid, iso-propanesulfonic acid, methanedisulfonic acid, ethanedisulfonic acid, 2,3-propanedisulfonic acid and 1,3-propanedisulfonic acid are alkylsulfones. Present as an acid. Examples of suitable alkanol sulfonic acids include 2-hydroxyethane sulfonic acid, 2-hydroxypropane sulfonic acid and 3-hydroxypropane sulfonic acid.

  The alkyl and / or alkanol sulfonic acid is preferably present at a concentration in the range of 50 to 300 g / l electrolyte, preferably 100 to 200 g / l electrolyte.

  In addition, to avoid tin oxidation, the electrolyte may contain conventional antioxidants such as mono- or polyphenolic compounds such as catechol, hydroquinone and phenolsulfonic acid. The use of catechol is preferred. The concentration of these antioxidants can be 50 to 2,000 mg / l electrolyte, preferably 500 to 1,000 mg / l.

  The electrolyte may also include various additives commonly used in acidic electrolytes for the deposition of tin alloys, such as grain refiners, wetting agents, brighteners and the like.

The amount of grain refiner is preferably present in an amount of 0.1 to 50 g / l electrolyte, preferably 1 to 10 g / l electrolyte.

  The wetting agent may be present in an amount of 0.1 to 50 g / l electrolyte, preferably 0.5 to 10 g / l electrolyte.

  The pH of the acidic electrolyte is preferably 0 to <1.

  In addition, the present invention is the basis for providing a method for electrolytic coating of a substrate with a tin-bismuth alloy, the method coating a novel electrolyte, a metal anode and a cathode made of a substrate to be coated. The present invention is also the basis for providing a coating obtained by use of this method.

The current density can be 0.1 A / dm ² (barrel or rack plating) up to 100 A / dm ² (high speed system).

  The temperature of the electrolyte is preferably in the range of 0 to 70 ° C., particularly preferably in the range of 20 to 50 ° C.

  Any material commonly used in the manufacture of electronic components such as copper, copper-containing alloys, nickel-iron alloys (e.g., alloy 42) or nickel plating materials may be present as the substrate to be coated.

  The electrolyte of the present invention can be used for coating electronic parts.

  The invention will now be described on the basis of the following examples and comparative examples.

Examples An electrolyte was prepared containing:
150 g / l methanesulfonic acid, 70% by weight
80 g / l tin [as Sn (CH ₃ SO ₃ ) ₂ ]
3.5 g / l bismuth [as Bi (CH ₃ SO ₃ ) ₃ ]
1 g / l catechol 4 g / l EO / PO block copolymer (BASF Pluronic PE6400, molecular weight about 2,900)
1 g / l methacrylic acid 200 mg / l 2-mercaptobenzothiazole

In this electrolyte, the copper sheet was coated under the following conditions.
Temperature: 40 ° C
Current density: 10 A / dm ²
Movement: Magnetic stirring, 700rpm
Duration: 2 minutes

  The tin-bismuth coating obtained in this way had a layer thickness of 10 μm with a Bi (remaining Sn) alloy content of 2.8%.

In order to test the deposition of bismuth during charge exchange, further test sheets were coated under the same conditions. After 2 minutes deposition, the current was turned off and the sheet was held in the electrolyte for 30, 60, 120 and 180 seconds, respectively. The alloy composition was measured by X-ray fluorescence analysis.

  All surfaces had a uniform semi-glossy metallic appearance.

Comparative Example Deposition from an electrolyte having the following composition was carried out under the same conditions as in Example 1.
150 g / l methanesulfonic acid, 70% by weight
80 g / l tin [as Sn (CH ₃ SO ₃ ) ₂ ]
3.5 g / l bismuth [as Bi (CH ₃ SO ₃ ) ₃ ]
2-naphthol ethoxylate having 1 g / l catechol 5 g / l 12EO group (Lugaluban BNO-12 from BASF)
0.5 g / l naphthalene sulfonic acid formaldehyde condensate (BASF's Tamol NN4501)

These sample surfaces deposited by this method showed a significant dark discoloration depending on the duration after dead exposure. After 180 seconds of continuous exposure, the surface was deep black.

Claims (12)

One or more alkyl sulfonic acids and / or alkanol sulfonic acids,
One or more soluble tin (II) salts;
One or more soluble bismuth (III) salts,
An aqueous acidic electrolyte for the deposition of tin-bismuth alloys comprising one or more nonionic surfactants and one or more thiazole compounds and / or thiadiazole compounds. The thiazole compound is benzothiazole, 2-aminobenzothiazole, 6-aminobenzothiazole, 2-hydroxybenzothiazole, 6-hydroxybenzothiazole, 2-methylbenzothiazole, 2-mercaptobenzothiazole, 6-methoxy-2- The electrolyte of claim 1 selected from aminobenzothiazole and 2-thiazolinethiol-2. The thiadiazole compound is 2-amino-5-methyl-1,3,4-thiadiazole, 2-amino-5-mercapto-1,3,4-thiadiazole, 2-mercapto-5-methyl-1,3,4. The electrolyte according to claim 1, wherein the electrolyte is selected from thiadiazole, 2-amino-1,3,4-thiadiazole and 2,5-dimercapto-1,3,4-thiadiazole. The electrolyte according to any one of claims 1 to 3, wherein the thiazole compound and / or thiadiazole compound is present in the electrolyte at a concentration of 5 to 1,000 mg / l. The electrolyte according to any one of claims 1 to 4, wherein the nonionic surfactant is a polyethylene oxide / polypropylene oxide copolymer. The polyethylene oxide / polypropylene oxide copolymer has the general formula H— (OCH ₂ —CH ₂ ) m— (OCH (CH ₃ ) —CH ₂ ) n—OH, having a molecular weight of 2,000 to 10,000. 6. The electrolyte according to claim 5, wherein m and n are polyethylene oxide / polypropylene oxide copolymers represented by the formula:   The electrolyte of claim 6, wherein the polyethylene oxide / polypropylene oxide copolymer has a cloud point of> 30 ° C. The electrolyte according to any one of claims 1 to 7, wherein one or more polyphenol compounds are further present. The electrolyte according to claim 8, wherein the polyhydric phenol compound is catechol. 10. A method of electrolytically coating a substrate with a tin-bismuth alloy, wherein the coating further uses a metallic tin anode by passing direct current through the electrolyte according to any one of claims 1-9. And by using a cathode made of a substrate to be coated. A coating obtainable by the method of claim 10. Use of the electrolyte according to any one of claims 1 to 9 for coating electronic components.