KR20070120592A - Electrolyte and method for depositing tin bismuth alloy layers - Google Patents

Abstract

The invention relates to an acidic electrolyte for depositing tin bismuth alloys, which comprises one or several alkylsulfonic acids and/or alkanolsulfonic acids, one or several soluble tin (II) salts, one or several soluble bismuth (III) salts, one or several non-ionogenic surfactants and one or several thiazol and/or thiadiazol compounds. The invention also relates to a method which uses said electrolyte, and the layer which can be obtained according to said method, in addition to the use of said electrolyte for coating electronic components.

Description

Plating method of electrolyte and tin-bismuth alloy layer {ELECTROLYTE AND METHOD FOR DEPOSITING TIN BISMUTH ALLOY LAYERS}

The present invention relates to an acid electrolyte for plating a tin-bismuth alloy, a method of using the electrolyte, a coating obtained by the method, and a use of the electrolyte for coating an electronic component.

Since coatings having a tin-lead layer have various functional properties, there are cases where parts including a coating having an electrodeposited tin-lead layer are used in the manufacture of electrical and electronic assemblies. When these coatings are used, soft-soldering can be easily performed on the parts even if the parts are stored for a long time.

However, when lead is used in the manufacture of electrical and electronic devices, there is a risk of environmental pollution due to lead even if such devices are stored in a waste treatment plant. In addition, the lead in the soldered joints is converted into a water soluble form by the corrosion process, resulting in contamination of the groundwater. In view of these problems, the European Union Directive prohibits the use of lead solder and electrodeposited tin-lead layers from 1 July 2006, with a few exceptions. And Directive 27 January 2003 of the European Parliament and of the Council on the Restriction of the Use of Certain Hazardous Substances in Electronic Devices (DIRECTIVE 2002/95 / EC).

In addition to the coatings described above, a pure electrodeposited tin layer, or an electrodeposited tin alloy layer, such as tin-copper (1 to 2% by weight Cu), tin-silver (Ag 2 to 5% by weight) or tin-bismuth (Bi 2) (4 to 4% by weight), such as research on the tin alloy layer is in progress. In terms of process, pure tin coating is a simple technique, and in view of this, it is regarded as an economical technique that can replace the coating having the tin-lead layer. However, there is a problem that whiskers can be produced when using the pure tin coating. Whiskers are defined as needle single crystals of tin. Whiskers typically have a diameter of several micrometers, but their diameter may reach several millimeters. As electronic components become smaller, short circuits between adjacent electrically conductive components can be caused when whiskers are produced. This can lead to significant economic losses, including malfunctions of electrical and electronic devices.

As a method of suppressing the generation of whiskers in the tin layer, there is a method of co-plating a small amount of the second metal. In the above-mentioned co-plating, it has been found that co-plating a small amount (at least 3 wt%) of lead is effective. However, as mentioned above, such co-plating is only allowed for a few uses.

In addition, as another method, the plating of tin-bismuth alloys containing about 2 to 4% by weight of bismuth has proved to be a more preferred method for inhibiting whisker formation. However, when a large amount of bismuth is used, the ductility of the plating layer is lowered. Therefore, when the pins of the electronic component are mechanically operated, cracks may appear in the coating, which is not preferable. As a result, properties such as easy solderability and corrosion resistance of the plating layer are degraded. On the other hand, there is a concern that whiskers may be generated when co-plating Bi of less than 2% by weight.

Commercially suitable methods for the plating of tin-bismuth alloys have already been used for the manufacture of electronic components, and examples of such existing plating methods include European Patent 0 255 558, European Patent 0 715 003, Japanese Patent 2983548 and The method of Unexamined-Japanese-Patent 2132894 is mentioned.

However, since the difference between tin electrochemical potential (-0.12 V) and bismuth electrochemical potential (+0.35 V) is large, bismuth in the form of Bi ³⁺ ions dissolved in the electrolyte has a large electronegativity during charge exchange. Precipitates on the tin anode.

This reaction proceeds according to the following formula:

.

.

The bismuth precipitated during this charge exchange reaction is released from the anode over time, plated in the form of a porous sponge coating, and then passed through the electrolyte, followed by the components to be coated again. In the case described above, bismuth particles may be blended into the electrodeposited layer, thereby forming a dendritic layer which is technically ineffective. This may interrupt manufacturing.

In addition, as a result, the bismuth dissolved in the electrolyte is continuously removed from the electrolyte by the above-described charge exchange reaction, thereby increasing the manufacturing cost. In order to allow the bismuth to be co-plated in an amount of 2 to 4% by weight, it is necessary to continuously analyze the content of bismuth in the electrolyte, and through this analysis, the amount of bismuth lost can be replenished. However, the above method requires a lot of time and effort.

If it is possible to suppress the loss of bismuth caused by the charge exchange reaction, use an automatic metering pump with an amperage (A) -hour (h) meter to charge the required amount of bismuth compound according to the amount of current carrying You can do it. Thus, the method described above may be an easier and more economical way to make up for lost bismuth. In the replenishment method described above, in order to more accurately analyze the bismuth in the electrolyte, it is necessary to analyze the bismuth in the electrolyte more accurately at long time intervals.

Another problem associated with the aforementioned charge exchange reaction is that even if the coating process is interrupted, for example due to a system malfunction, and the parts are immersed in the electrolyte for a short time in the absence of a load of current, the reaction is not only on the tin anode. This also occurs in components that are already coated. For this reason, the surface which contacted electrolyte is discolored black. Then, if electroplating is continued, a so-called sandwich coating is obtained. Between the two electrodeposited tin-bismuth layers, a tin-bismuth layer produced by charge exchange can be formed. If the pin is mechanically operated, cracks may be generated due to the order of the layers described above, whereby the layers may be peeled off, resulting in poor solderability.

In order to suppress the precipitation of bismuth resulting from 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 comprising: unsaturated carboxylic acid as an organic additive; And nonionic surfactants selected from the group consisting of polyethylene oxide / polypropylene oxide copolymers (US Pat. No. 2003/0132122 A1). In the case of the electrolyte according to the above US patent, the precipitation of bismuth was substantially reduced during the charge exchange, but it was not possible to co-plat Bi in an amount of 2% by weight in the coating. In addition, even in the case where 10% by weight of Bi is present in the electrolyte, the content of bismuth in the plating layer is only about 1.5 to 2% by weight.

An object of the present invention is to provide an electrolyte for plating a tin-bismuth alloy having an alloy content of 2 to 4% by weight or more. In the case of using the electrolyte of the present invention, charge exchange is performed even when no electricity is supplied. In the meantime, bismuth does not precipitate on the already coated surface.

Another object of the present invention is also to provide at least one alkylsulfonic acid and / or alkanolsulfonic acid, at least one soluble tin (II) salt, at least one soluble bismuth (III) salt, at least one nonionic surfactant, And an aqueous acid electrolyte for plating tin-bismuth alloy, comprising at least one thiazole and / or thiadiazole compound.

According to the present invention, the respective compounds contained in the electrolyte of the present invention do not shift the electrode potential of bismuth in the negative direction, and this characteristic prevents the precipitation of precious metals in metals having a large electronegativity. It is a surprising result for a person skilled in the art because it is regarded as a necessary condition. Even when the components included in the electrolyte are used in combination, these components do not move the equilibrium potential in the cathode direction. Therefore, the suppression of the deposition of bismuth on the newly plated tin-bismuth surface cannot be explained from an electrochemical point of view. Although inhibitory effects may be seen, ie, electroless cementation can be inhibited by the combination of components in use, electrodeposition of bismuth is not suppressed.

There is no particular limitation on the thiazole or thiadiazole compound which can be used in the present invention. A "thiazole compound" is defined as a saturated or unsaturated thiazole compound which is substituted or unsubstituted. Examples of thiazole compounds usable in the present invention include benzothiazole, 2-aminobenzothiazole, 6-aminobenzothiazole, 2-hydroxybenzothiazole, 6-hydroxybenzothiazole, 2-methylbenzo Thiazole, 2-mercaptobenzothiazole, 6-methoxy-2-aminobenzothiazole and 2-thiazoline thiol-2. Especially, it is preferable to use 2-mercaptobenzothiazole as said thiazole compound. A "thiadiazole compound" is defined as a saturated or unsaturated thiadiazole compound which is substituted or unsubstituted. Examples of thiadiazole compounds usable in the present invention include 2-amino-5-methyl-1,3,4-thiadiazole, 2-amino-5-mercapto-1,3,4-thiadiazole, 2-mercapto-5-methyl-1,3,4-thiadiazole, 2-amino-1,3,4-thiadiazole and 2,5-dimercapto-1,3,4-thiadiazole Can be mentioned. The thiazole compound and thiadiazole compound may be used alone or in combination with other compounds.

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

As said nonionic surfactant, a well-known nonionic surfactant can be used. In particular, the nonionic surfactant should be a copolymer and a block copolymer of ethylene oxide and propylene oxide. Examples of the nonionic surfactant, and 2,000 to 10,000 weight average molecular weight of the general formula _{H- (OCH 2 -CH 2) m} - (OCH (CH 3) -CH 2) n -OH ( where, m and n are 5 Preference is given to using polyethylene oxide / polypropylene oxide copolymers. According to a more preferred embodiment of the present invention, the nonionic surfactant has a cloud point of> 30 ° C.

It is preferable that the density | concentration of the said nonionic surfactant in the electrolyte of this invention is 1-50 g / L, More preferably, it is 2-20 g / L, Especially preferably, it is 5-10 g / L.

The tin (II) may be present in the form of a salt of mineral acid, a salt of alkylsulfonic acid or a salt of alkanolsulfonic acid in the electrolyte of the present invention. Examples of the salt of the inorganic acid include sulfate and tetrafluoroborate. Preferred examples of the salt of the alkyl sulfonic acid include methanesulfonate, ethanesulfonate, n-propanesulfonate, iso-propanesulfonate, methane disulfonate, ethane disulfonate, and 2,3-propane disulfonate. And 1,3-propane disulfonate. Preferred alkanol sulfonates for the present invention include 2-hydroxyethanesulfonate, 2-hydroxypropanesulfonate and 3-hydroxypropanesulfonate. In particular, tin (II) methanesulfonate is preferable.

When the tin (II) salt is calculated in the amount of tin (II), the tin (II) salt is present in the electrolyte of the present invention in an amount of 5 to 200 g / l, preferably 10 to 100 g / present in an amount of l.

The bismuth (III) salt may be added to the electrolyte in any form capable of being sufficiently dissolved as Bi (III) ions in the electrolyte of the present invention. The bismuth (III) salt is in the form of a solid compound which is converted into a soluble Bi (III) compound in the electrolyte in the presence of free acid, or in the form of a dissolved Bi (III) salt in the electrolyte of the present invention. Can be added. Examples of suitable Bi (III) salts include salts of inorganic acids, such as chlorides, tetrafluoroborates or nitrates; Salts of alkylsulphonic acids such as methanesulfonate, ethanesulfonate, methanedisulfonate, ethanedisulfonate, 2,3-propane disulfonate and 1,3-propanedisulfonate; Salts of alkanesulfonic acids, such as 2-hydroxyethanesulfonate and 3-hydroxypropanesulfonate. Especially, bismuth (III) methanesulfonate is preferable as said Bi (III) salt. When the Bi (III) salt is added in solid form, it is preferable to use bismuth (III) carbonate or bismuth (III) oxide. These compounds are dissolved in the electrolyte of the present invention in the presence of alkylsulfonic acid and / or alkanolsulfonic acid to form soluble Bi (III) compounds.

The concentration of bismuth in the present invention depends on the desired Bi content in the obtained tin-bismuth alloy and the concentration of tin (II) salt. The electrolyte preferably contains the bismuth (III) salt in a concentration such that the tin-bismuth alloy having an alloy composition of 2 to 4% Bi can be plated. In this case, the concentration of Bi ions is preferably 3 to 6%, more preferably 5% to the selected tin (II) concentration.

The alkylsulfonic acid and alkanolsulfonic acid preferably have from 1 to 10 carbon atoms, particularly preferably from 1 to 5 carbon atoms. Examples of the alkyl sulfonic acid that can be used include methanesulfonic acid, ethanesulfonic acid, n-propanesulfonic acid, iso-propanesulfonic acid, methanedisulfonic acid, ethanedisulfonic acid, 2,3-propanedisulfonic acid, and 1,3. -Propane disulfonic acid. Illustrative alkanolsulfonic acids suitable for the present invention include 2-hydroxyethanesulfonic acid, 2-hydroxypropanesulfonic acid and 3-hydroxypropanesulfonic acid.

The alkylsulfonic acid and / or alkanolsulfonic acid is preferably present in the electrolyte of the present invention at a concentration of 50 to 300 g / l, more preferably at a concentration of 100 to 200 g / l.

In addition, the electrolyte of the present invention may further include conventional antioxidants such as monovalent or polyvalent phenyl compounds such as catechol, hydroquinone and phenolsulfonic acid in order to inhibit oxidation of tin. It is preferable to use catechol as said antioxidant. The concentration of the antioxidant included in the electrolyte of the present invention may be 50 to 2,000 mg / l, preferably 500 to 1,000 mg / l.

The electrolyte of the present invention may also contain various additives conventionally used in acid electrolytes for tin alloy plating, such as grain-refining additives, wetting agents, and / or brighteners. It may further include.

The additive for crystal refinement is preferably present in the amount of 0.1 to 50 g / l, more preferably in the amount of 1 to 10 g / l in the electrolyte of the present invention.

The wetting agent is preferably present in the electrolyte of the present invention in an amount of 0.1 to 50 g / l, more preferably in an amount of 0.5 to 10 g / l.

It is preferable that pH of the acid electrolyte of this invention is 0-1.

In addition, the present invention provides a method of electrolytically coating a substrate with a tin-bismuth alloy, and a coating obtained by the coating method, and according to the coating method of the present invention, in order to plate the coating on the substrate, An electrolyte, a metal tin anode, and a cathode made of the substrate to be coated are used and a direct current is passed during the process of the present invention.

In the coating method of the present invention, the current density may be 0.1 A / dm ² (when using a barrel plating method or rack plating method) to 100 A / dm ² (when using a high speed system).

It is preferable that it is 0-70 degreeC, and, as for the temperature of the electrolyte of this invention, it is especially preferable that it is 20-50 degreeC.

As the substrate to be coated, materials commonly used in electronic components, such as copper, copper-containing alloys, nickel-iron alloys (such as Alloy 42), or nickel plated materials may be used.

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

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

Example

An electrolyte comprising the following components was prepared:

150 g / l methanesulfonic acid, 70 wt%

80 g / l tin [used as Sn (CH ₃ SO ₃ ) ₂ form]

3.5 g / l bismuth [used as Bi (CH ₃ SO ₃ ) ₃ form]

1 g / l catechol

4 g / l EO / PO block copolymer (Pluronic PE 6400, manufactured by BASF,

Molecular weight: about 2,900

1 g / l methacrylic acid

200 mg / l 2-mercaptobenzothiazole

The copper sheet was coated in the prepared electrolyte under the following conditions:

Temperature: 40 ℃

Current Density: 10 A / dm ²

Agitation: magnetic stirring, 700 rpm

Time: 2 minutes

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

In order to investigate bismuth precipitation during charge exchange, the test sheet was coated again under the same conditions as above. Two minutes after plating, the sheet was left in the electrolyte for 30 seconds, 60 seconds, 120 seconds, and 180 seconds, respectively, with no current supplied. Then, the composition of the alloy was confirmed by X-ray fluorescence analysis.

Duration of dead exposure (seconds) in electrolyte Alloy Composition [wt% Bi] * 0 2.8 30 2.9 60 2.9 120 3.0 180 3.0

* Residual amount of Sn

In this example, all surfaces uniformly exhibited the appearance of semi-gloss metals.

Comparative example

Plating was carried out under the same conditions as in Example 1 from an electrolyte having the following composition:

150 g / l methanesulfonic acid, 70 wt%

80 g / l tin [used as Sn (CH ₃ SO ₃ ) ₂ form]

3.5 g / l bismuth [used as Bi (CH ₃ SO ₃ ) ₃ form]

1 g / l catechol

2-naphthol ethoxylate with 5 g / l 12 EO groups

(Lugalvan BNO-12, manufactured by BASF)

0.5 g / l naphthalenesulfonic acid formaldehyde condensate

(Tamol NN 4501, manufactured by BASF)

Time exposed to electrolyte (seconds) without electricity Alloy Composition [wt% Bi] * 0 2.08 30 2.21 60 2.73 120 4.00 180 6.15

* Residual amount of Sn

The surface of the electrodeposited sample according to the manner described above in this comparative example was discolored to a significantly darker color after exposure to the electrolyte in a non-conductive state as a function of time exposed to the electrolyte. The surface color of the sample after exposure to electrolyte for 180 seconds was velvet black.

Claims (12)

One or more alkylsulfonic acids and / or alkanolsulfonic acids, One or more soluble tin (II) salts, One or more soluble bismuth (III) salts, One or more nonionic surfactants, and Aqueous acid electrolyte for plating tin-bismuth alloy, comprising at least one thiazole and / or thiadiazole compound. The method of claim 1, The thiazole compound is benzothiazole, 2-aminobenzothiazole, 6-aminobenzothiazole, 2-hydroxybenzothiazole, 6-hydroxybenzothiazole, 2-methylbenzothiazole, 2-mer An electrolyte, characterized in that it is selected from captobenzothiazole, 6-methoxy-2-aminobenzothiazole and 2-thiazoline thiol-2. The method of claim 1, 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-thiadiazole, 2-amino-1,3,4-thiadiazole and 2,5-dimercapto-1,3,4-thiadiazole Electrolyte. The method according to any one of claims 1 to 3, And the thiazole and / or thiadiazole compounds are contained in the electrolyte at a concentration of 5 to 1,000 mg / l. The method according to any one of claims 1 to 4, Wherein said nonionic surfactant is a polyethylene oxide / polypropylene oxide copolymer. The method of claim 5, The polyethylene oxide / polypropylene oxide copolymer has a weight average molecular weight of 2,000 to 10,000, and general formula H- (OCH ₂ -CH ₂ ) _m- (OCH (CH ₃ ) -CH ₂ ) _n -OH (m And n is an integer of 5 to 60). The method of claim 6, The polyethylene oxide / polypropylene oxide copolymer is characterized in that the cloud point (cloud point) is> 30 ℃. The method according to any one of claims 1 to 7, An electrolyte further comprising one or more polyhydric phenol compounds. The method of claim 8, The polyhydric phenol compound is catechol, characterized in that the electrolyte. A method of electrolytically coating a substrate with a tin-bismuth alloy, The electrolytic coating method according to any one of claims 1 to 9, wherein the coating is carried out by using a electrolyte prepared from the electrolyte, the metal tin anode, and the cathode prepared to be coated, and passing through a direct current. A coating obtained by the method of claim 10. Use of the electrolyte of claim 1 for coating an electronic component.