JP4465068B2 - Method for forming silver-tin alloy plating layer - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming a silver-tin alloy plating layer. More specifically, the present invention relates to a method for forming a silver-tin alloy plating layer which does not contain lead and is easy to manage a bath, which is used for joining printed wiring boards and electronic components.
[0002]
[Prior art]
For bonding to printed wiring boards, electronic parts, etc., low-cost solders excellent in electrical characteristics and solderability, that is, tin-lead alloys are widely used.
However, solder contains a large amount of lead. Lead is regarded as a problem from the viewpoints of work environment conservation and the natural environment, and it is coated with a low melting point alloy film that is based on tin and does not contain lead. A simple method of forming on a processed material is desired.
In recent years, as a method for forming such a low-melting point alloy film containing no lead, as a tin alloy plating solution containing no lead, a tin-silver alloy plating solution, a tin-bismuth alloy plating solution, a tin-copper alloy plating solution, etc. The method of using is known. In the tin-silver alloy plating solution, since silver is a noble metal with respect to tin, silver ions are reduced by oxidation of tin ions to form metalized silver precipitates. There are problems such as difficulty in use. In addition, in order to precipitate tin and silver having significantly different precipitation potentials as an alloy, a complexing agent such as potassium iodide or gluconic acid is added, and the silver precipitation potential is shifted in a base direction to co-relate with tin. There is a method that enables analysis, but in the low current density portion, the silver content tends to increase. According to the equilibrium diagram of tin-silver, when the silver content is 3.5% by weight or more, the higher the silver content, the higher the melting point of the film, and the lower the current density in the through-hole portion of the printed wiring board. Therefore, there is a problem that the silver content is increased and the solder wettability is lowered. In addition, if the deposition potential shifts to the base due to the addition of the complexing agent, the current efficiency may be reduced, and the complexing agent also imposes a heavy burden on the wastewater treatment.
[0003]
On the other hand, bismuth in the tin-bismuth alloy plating solution and copper in the tin-copper alloy plating solution have better bath stability than the tin-silver alloy plating solution because the deposition potential for tin is closer to that of silver. The bismuth or copper content at each current density is also less variable. Further, since the addition of a complexing agent is unnecessary or the amount added can be suppressed, the decrease in precipitation efficiency is small, and the burden on waste liquid treatment can be reduced. Furthermore, since the tin-silver alloy film has excellent characteristics such as film bonding strength, it is most promising as a bonding material for electronic components and printed wiring boards. On the other hand, the characteristics of the tin-bismuth alloy film have the disadvantage that it is very brittle, and the tin-copper alloy film has a high melting point. Therefore, when mounting an electronic component plated with this film on a printed wiring board, the heating temperature If the height is high, there is a risk of causing thermal damage to the electronic component and the printed wiring board.
By the way, the above-mentioned problems of these tin-bismuth alloy films and tin-copper alloy films are known to exhibit performance superior to that of tin-silver alloy plating films by precipitating silver in the films. However, an alloy plating bath in which tin and silver are present in the same plating solution is unstable, and a ternary alloy plating bath in which tin, silver and bismuth or copper are added in the same plating solution is also unstable. At the same time, management is very cumbersome and impractical.
[0004]
Tin-silver alloy plating film such as tin-silver alloy, tin-bismuth-silver alloy, tin-copper-silver alloy plating film shows excellent performance such as melting point and bonding strength, and does not contain lead. Although it is promising as an alternative to the tin-lead alloy plating film, the plating solution is unstable and difficult to use for a long time because the deposition potential of tin and silver is very different. However, there are many problems such as requiring very complicated bath management.
[0005]
[Problems to be solved by the invention]
Accordingly, an object of the present invention is to provide a method for forming a silver-tin alloy plating layer that can be easily managed without using lead.
[0006]
[Means for Solving the Problems]
As a result of intensive studies to achieve the above-mentioned problems, the inventor, on a metal substrate, a silver plating film, a tin plating film, a tin-bismuth alloy plating film, or a tin-copper alloy plating film in this order, Alternatively, in the reverse order, the formed two-layer plating film is found to be able to form a tin-silver alloy plating film, preferably by heating, and the present invention has been achieved.
That is, the present invention deposits (1) a silver plating film and (2) a tin, tin-bismuth, or tin-copper alloy plating film in this order or vice versa on a metal substrate. The present invention relates to a method for forming a silver-tin alloy layer.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
The silver-tin alloy plating layer of the present invention is an alternative to solder in the field where solder is used. Therefore, as long as it is in such a field, it can be applied to various metal substrates. Specifically, examples of the metal substrate include a printed wiring board, a steel plate, a copper alloy plate, etching or press-molded body thereof, and the like.
For a composite substrate of an organic base material such as a printed wiring board and a metal substrate, it is preferable to remove the residual oil or resist using an aqueous cleaning agent such as an acidic cleaning agent in advance. As this acidic detergent, an aqueous solution obtained by adding a small amount of a surfactant to sulfuric acid, phosphoric acid, formic acid or sulfamic acid alone or in a mixture can be used.
For other metal substrates, it is preferable to remove oils and fats in advance using an aqueous cleaner such as an organic solvent or an alkali cleaner.
As the organic solvent, various solvents can be used as long as they are used in such pretreatment. Preferable solvents include, for example, chlorofluorocarbon-free solvents such as HFC, alcohol solvents such as ethanol and isopropanol, and the like.
As the alkaline cleaner, for example, caustic soda, sodium carbonate, silicate soda, phosphate soda, etc., alone or as a mixture, an aqueous solution to which a small amount of surfactant is added can be used.
[0008]
Further, the surface of the metal substrate may be cleaned by a physical method such as brushing or honing.
It is preferable that after the metal substrate is further cleaned, the oxide film is removed with dilute sulfuric acid or dilute hydrochloric acid.
Thus, for example, a silver plating process is first performed on the metal base material that has been subjected to a cleaning process or the like. The silver plating process may be an electroplating process or an electroless plating process. As these electric or electroless plating treatment methods, known treatment methods can be employed.
Next, tin, tin-bismuth alloy or tin-copper alloy plating is performed on the formed silver plating film by electroplating.
These tin, tin-bismuth alloy or tin-copper alloy plating treatments are known, and various conditions, methods, and the like can be employed.
[0009]
In the present invention, the silver plating film and the tin, tin-bismuth alloy or tin-copper alloy plating film formed in this way are allowed to stand at room temperature or the like, so that the metals constituting both films diffuse mutually. By silver alloying, a silver-tin alloy plating layer is formed.
Preferably, it is appropriate to promote alloying by heating both coatings. The heating temperature is, for example, 80 to 190 ° C., preferably 100 to 170 ° C., and the treatment time is usually 10 minutes to 10 hours, preferably 30 minutes to 8 hours. In addition, even if it is a case where both membrane | film | coats fuse | melt, if it is a short time, it will not be a problem in particular. For example, the heating temperature is 200 to 300 ° C., preferably 230 to 270 ° C., and the treatment time is, for example, 1 to 60 seconds, preferably 10 to 40 seconds.
[0010]
In the present invention, since both plating films are alloyed by heat treatment, both plating films may be formed at an arbitrary film thickness ratio depending on the desired silver content.
In particular, in the film after alloying, considering solder joint strength, solder wettability, etc., silver is suitably 2 to 10% by mass, preferably 3 to 5% by mass, for example. In addition, the film thickness ratio is suitably silver plating film: tin, tin-bismuth or tin-copper alloy plating film = 1: 99 to 8:92, preferably 2:98 to 4:96.
The film thickness of the plating film is not particularly limited, and may be in a range that can function as a solderable film. Usually, the silver plating film and tin, tin-bismuth, or tin-copper alloy are used. The total film thickness of both layers with the plating film is, for example, 1 to 20 μm, preferably 2 to 10 μm.
[0011]
It is an object of the present invention to form and alloy a two-layer plating film consisting of a silver plating film and tin, a tin-bismuth alloy or a tin-copper alloy plating film as a lead-free solder joint material. However, the present invention is not limited to the two-layer plating film, and includes a case where a silver plating film and a tin, tin-bismuth or tin-copper alloy plating film are alternately formed to form a multilayer plating film. That is easy to understand. In addition, as the order of forming the plating layer, the lowermost layer does not need to be a silver plating film, but silver is sulfided or oxidized by applying tin, tin-bismuth alloy or tin-copper alloy plating on the outermost surface. Therefore, solder wettability equivalent to or better than that of a tin plating film can be ensured.
[0012]
In the present invention, the plating solution used for forming the silver plating film, the tin plating film, the tin-bismuth alloy plating film and the tin-copper alloy plating film is not particularly limited, and a conventionally known one can be used.
Specifically, the silver plating solution has a silver ion concentration of 1 to 60 g / L, preferably 2 to 40 g / L, and as a cyan bath or a solution not containing cyan, a thiocyan bath, an iodide bath, There is a thiosulfuric acid bath. In the tin plating solution, the tin ion concentration is, for example, 10 to 100 g / L, preferably 20 to 70 g / L, and there are a methanesulfonic acid bath, a sulfuric acid bath, and a chloride bath. In the tin-bismuth alloy plating solution and the tin-copper alloy plating solution, the bismuth ion concentration is 0.5 to 40 g / L, preferably 1 to 30 g / L, and the copper ion concentration is 0.3 to 20 g / L. L, preferably 0.5 to 10 g / L, and there is a methanesulfonic acid bath and a sulfuric acid bath.
[0013]
【The invention's effect】
Since the method for forming a silver-tin alloy plating layer of the present invention does not use lead, which is a harmful component, the safety is high. The formed silver plating film and the tin, tin-bismuth or tin-copper alloy plating film are preferably alloyed easily by heat treatment, and the formed alloy film is excellent in solder wettability. It is highly useful as a low-temperature bonding material.
In addition, since the tin-silver alloy plating film of the present invention is produced by two layers of a silver plating film and a tin or tin alloy plating film, silver and tin, silver and tin and bismuth, or silver and tin and copper are the same. It is possible to obtain a plating film which is superior in the stability of the bath than the alloy plating solution contained in the plating solution and has a stable metal content.
[0014]
【Example】
Reference example 1
A silver plating solution and a tin plating solution having the following composition were prepared and plated on an iron-nickel alloy lead frame.
Silver plating solution Silver cyanide (as metallic silver) 40g / L
Potassium cyanide (as free potassium cyanide) 78g / L
Potassium carbonate 10g / L
Distilled water remaining
Tin plating solution Methanesulfonic acid 150g / L
Tin methanesulfonate (as divalent tin) 20g / L
Polyoxyethylene polyoxypropylene glycol 2g / L
(Ethylene oxide: 10 mol, propylene oxide: 4 mol)
Distilled water remaining [0015]
Plating process Alkaline degreasing (5 A / dm ² , 30 seconds, 60 ° C.)
Sodium hydroxide 40 g / L,
Sodium carbonate 25 g / L,
Sodium metasilicate 10 g / L,
Dioxyethylene benzyl alcohol ether 1g / L
Water-washing 20% hydrochloric acid-immersed water-washing silver plating (1 A / dm ² , 30 seconds, 25 ° C.)
Washed tin plating (10 A / dm ² , 120 seconds, 45 ° C.)
Washing and drying heat treatment (170 ° C, 1 hour)
[0016]
Example 2
A silver plating solution and a tin plating solution having the following composition were prepared and plated on a phosphor bronze connector.
Silver plating solution Silver cyanide (as metallic silver) 40g / L
Potassium cyanide (as free potassium cyanide) 78g / L
Potassium carbonate 10g / L
Distilled water remaining
Tin plating solution Methanesulfonic acid 150g / L
Tin methanesulfonate (as divalent tin) 60g / L
Acetaldehyde 0.2g / L
1-naphthylaldehyde 0.1 g / L
Polyoxyethylene nonylphenyl ether 2g / L
(Ethylene oxide: 12 mol)
Distilled water remaining [0017]
Plating process Alkali degreasing water washing 10% sulfuric acid immersion water washing nickel plating (2A / dm ² , 5 minutes, 55 ° C)
Silver plating (10A / dm ² , 1 second, 25 ° C)
Washed tin plating (10 A / dm ² , 40 seconds, 20 ° C.)
Washing and drying heat treatment (120 ° C, 1 hour)
[0018]
Example 3
A silver plating solution and a tin plating solution having the following composition were prepared and plated on an iron-nickel alloy lead frame.
Silver plating solution Silver cyanide (as metallic silver) 4g / L
Potassium cyanide (as free potassium cyanide) 10g / L
Potassium carbonate 10g / L
Distilled water remaining
Tin plating solution Sulfuric acid 150g / L
Stannous sulfate (as divalent tin) 20g / L
Polyoxyethylene nonylphenyl ether 2g / L
(Ethylene oxide: 9 mol)
Distilled water remaining [0019]
Plating process Alkali degreasing water washing 20% hydrochloric acid water washing tin plating (1 A / dm ² , 600 seconds, 25 ° C.)
Washed silver plating (0.5 A / dm ² , 60 seconds, 25 ° C.)
Washed tin plating (1 / dm ² , 600 seconds, 25 ° C)
Washing and drying heat treatment (120 ° C, 1 hour)
[0020]
Example 4
A silver plating solution and a tin-bismuth alloy plating solution having the following composition were prepared and plated on an iron-nickel lead frame.
Silver plating solution Silver cyanide (as metallic silver) 4g / L
Potassium cyanide (as free potassium cyanide) 10g / L
Potassium carbonate 10g / L
Distilled water remaining
Tin-bismuth alloy plating solution Methanesulfonic acid 150 g / L
Methanesulfonate tin (as divalent tin) 25g / L
Bismuth methanesulfonate (as metal bismuth) 1.5g / L
Polyoxyethylene nonylphenyl ether 2g / L
(Ethylene oxide: 10 mol)
Catechol 0.1g / L
[0021]
Plating process Alkaline degreasing water washing 20% hydrochloric acid water washing silver plating (0.5 A / dm ² , 60 seconds, 25 ° C.)
Washed tin-bismuth alloy plating (2 A / dm ² , 10 minutes, 35 ° C.)
Washing and drying heat treatment (120 ° C, 2 hours)
[0022]
Reference Example 5
A silver plating solution and a tin-copper alloy plating solution having the following composition were prepared and plated on an iron-nickel alloy lead frame.
Silver plating solution Silver cyanide (as metallic silver) 4g / L
Potassium cyanide (as free potassium cyanide) 10g / L
Potassium carbonate 10g / L
Distilled water remaining
Tin-copper alloy plating solution Methanesulfonic acid 130g / L
Tin methanesulfonate (as divalent tin) 60g / L
Methanesulfonic acid copper (as metallic copper) 1g / L
Lauryldimethylaminoacetic acid betaine 2g / L
Polyoxyethylene nonylphenyl ether 3g / L
(Ethylene oxide: 12 mol)
Hydroquinone 0.1g / L
[0023]
Plating conditions Alkali degreasing water washing 20% hydrochloric acid water washing silver plating (0.5 A / dm ² , 60 seconds, 25 ° C.)
Washed tin-copper alloy plating (15 A / dm ² , 80 seconds, 35 ° C.)
Washing and drying heat treatment (170 ° C, 1 hour)
[0024]
Reference Example 6
An iron-nickel alloy lead frame was plated using a silver plating solution and a tin-copper alloy plating solution having the composition of Reference Example 5.
Plating conditions Alkaline degreasing water washing 20% hydrochloric acid water washing silver plating (0.5 A / dm ² , 12 seconds, 25 ° C.)
Washed tin-copper alloy plating (15 A / dm ² , 15 seconds, 35 ° C.)
Washing and drying heat treatment (250 ° C, 20 seconds)
[0025]
Example 7
A silver plating solution and a tin-copper alloy plating solution having the following composition were prepared and plated on an iron-nickel alloy lead foam.
Silver plating solution Silver nitrate (as metallic silver) 8g / L
Potassium thiocyanate 110g / L
pH 8.0 (adjusted with potassium hydroxide)
Tin-copper alloy plating solution Methanesulfonic acid 130g / L
Tin methanesulfonate (as divalent tin) 60g / L
Methanesulfonic acid copper (as metallic copper) 1g / L
Lauryldimethylaminoacetic acid betaine 2g / L
Polyoxyethylene nonylphenyl ether 3g / L
(Ethylene oxide: 12 mol)
Hydroquinone 0.1g / L
[0026]
Plating conditions Alkaline degreasing water washing 20% hydrochloric acid water washing silver plating (0.5 A / dm ² , 12 seconds, 25 ° C.)
Washed tin-copper alloy plating solution (15 A / dm ² , 15 seconds, 35 ° C.)
Washing and drying heat treatment (120 ° C, 1 hour)
[0027]
All of the obtained plating films had a good matte or glossy appearance. The solder wettability of these plating films was evaluated by a meniscograph method using a solder checker. The measurement conditions of the meniscograph are as follows.
Meniscograph measurement condition 1
Solder bath Sn: Pb = 60: 40 eutectic solder bath temperature 230 ° C.
Immersion depth 2mm
Immersion speed 10mm / sec Immersion time 5sec Flux Rosin-based inactive type [0028]
Meniscograph measurement condition 2
Solder bath Sn: Ag: Cu = 95.8: 3.5: 0.7
Bath temperature 245 ° C
Immersion depth 2mm
Immersion speed 10mm / sec Immersion time 5sec Flux Rosin-based inactive type [0029]
Table 1 shows the results obtained by the above test. From these results, the zero cross times were all less than 1 second, and sufficiently satisfied the solder wettability required for electronic components and the like.
[0030]
[Table 1]

Claims (2)

(1) A silver plating film and (2) a tin, tin-bismuth, or tin-copper alloy plating film are deposited in this order or vice versa on a metal substrate, and the silver plating is further performed. A silver-tin alloy plating layer comprising a step of heating both the coating (1) and the tin, tin-bismuth, or tin-copper alloy plating coating (2) to 80 to 120 ° C. Method to form. The method according to claim 1, wherein the step of heating both coatings to 80 to 120 ° C. is performed for 10 minutes to 10 hours.