WO2012052832A2

WO2012052832A2 - Electroless nickel plating bath and electroless nickel plating method using same

Info

Publication number: WO2012052832A2
Application number: PCT/IB2011/002514
Authority: WO
Inventors: Takeshi Bessho; Hideo Honma; Mitsuhiro Watanabe; Katsuhiko Tashiro
Original assignee: Toyota Jidosha Kabushiki Kaisha; Kanto Gakuin University
Priority date: 2010-10-21
Filing date: 2011-10-21
Publication date: 2012-04-26
Also published as: WO2012052832A3; JP2012087386A; WO2012052832A8

Abstract

An electroless nickel plating bath that contains a nickel salt, a reducing agent, a stabilizer that contains metal antimony or an antimony compound, and a complexing agent that contains succinic acid or one of malic acid and lactic acid at a chelate molar ratio of 25:75 to 75:25.

Description

ELECTROLESS NICKEL PLATING BATH AND ELECTROLESS NICKEL PLATING

METHOD USING SAME

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to an electroless nickel plating technique, and, more particularly, to an electroless nickel plating bath which enables the formation of a nickel plating film with low stress and an electroless nickel plating method that uses the electroless nickel plating bath.

2. Description of Related Art

[0002] For the purpose of enhancing the stability of the plating solution or improving the plating appearance or the throwing power, organic and/or inorganic additives are often added to the plating solution in an electroless nickel plating bath. In general, addition of an organic additive at an appropriate concentration improves the solution stability, precipitation rate, plating appearance and so on but increases the stress in the resulting plating film. Even if the stress in the plating film is low immediately after the plating, the stress may increase as days go by. An increase in stress in a plating film may cause the plated product to warp or crack. Thus, the stress in a plating film is preferably reduced as much as possible.

[0003] Japanese Patent Application Publication No. 2005-200707 (JP-A- 2005-200707) discloses the fact that addition of a proper amount of molybdenum to an electroless nickel plating bath decreases the stress in the resulting nickel film. Japanese Patent Application Publication No. 2005-264309 (JP-A-2005 -264309) discloses the fact that an electroless nickel plating bath that contains a water-soluble nickel salt, a reducing agent, molybdenum and antimony has high solution stability and can provide a high precipitation rate and good plating appearance without containing a harmful substance such as lead. Japanese Patent Application Publication No. 5-230664 (JP-A-5-230664) discloses the fact that the use of an electroless nickel plating bath that contains a water-soluble antimony or bismuth compound in an amount of 0.01 to 100 ppm in terms of the metal as a plating bath in forming an electroless nickel plating film on aluminum or an aluminum alloy is effective in reducing fine nodules. However, JP-A-2005-264309 and JP-A-5-230664 mention nothing about the stress in the resulting plating film.

SUMMARY OF THE INVENTION

[0004] The present invention provides an electroless nickel plating bath that enables the formation of a nickel plating film with low film stress and an electroless nickel plating method that uses the plating bath.

[0005] The present inventors found that the use of an electroless nickel plating bath that contains metal antimony or an antimony compound as a stabilizer and contains succinic acid of one of malic acid and lactic acid at a predetermined ratio as complexing agents enables the formation of a plating film with low stress.

[0006] A first aspect of the present invention relates to an electroless nickel plating bath. The electroless nickel plating bath contains a nickel salt, a reducing agent, a stabilizer that contains metal antimony or an antimony compound, and a complexing agent that contains succinic acid or one of malic acid and lactic acid at a chelate molar ratio of 25:75 to 75:25.

[0007] In the electroless nickel plating bath, the complexing agent may be present in an amount of 0.3 to 1.3 chelate mol per liter of the electroless nickel plating bath.

[0008] In the electroless nickel plating bath, the complexing agent may be present in an amount of 0.5 to 0.9 chelate mol per liter of the electroless nickel plating bath.

[0009] In the electroless nickel plating bath, the stabilizer may be metal antimony, antimony chloride, antimony acetate, antimony oxide, antimonyl potassium tartrate, or a mixture thereof.

[0010] In the electroless nickel plating bath, the reducing agent may be hypophosphorous acid or sodium hypophosphite. [0011] In the electroless nickel plating bath, the reducing agent may be present in an amount of 0.01 to 0.5 mol per liter of the electroless nickel plating bath.

[0012] In the electroless nickel plating bath, the stabilizer may contain Sb and an amount of Sb is 0.001 to 0.1 mmol per liter of the electroless nickel plating bath.

[0013] A second aspect of the present invention relates to an electroless nickel plating method. The electroless nickel plating method uses the electroless nickel plating bath according to the first aspect of the present invention.

[0014] According to the electroless nickel plating bath and electroless nickel plating method according to each aspect of the present invention, a nickel plating film with low film stress, good appearance and high throwing power can be formed.

DETAILED DESCRIPTION OF EMBODIMENTS

[0015] Description is hereinafter made of an embodiment of the present invention. An electroless nickel plating bath of this embodiment contains a nickel salt, a reducing agent, a stabilizer that contains metal antimony or an antimony compound, and a complexing agent that contains succinic acid or one of malic acid and lactic acid at a chelate molar ratio of 25:75 to 75:25.

[0016] As the nickel salt, a nickel salt that is usually used in an electroless nickel plating bath can be used. Specific examples of the nickel salt include nickel sulfate, nickel chloride, nickel carbonate, nickel acetate, nickel hypophosphite, nickel sulfamate, and nickel citrate. These nickel salts may be used singly or in combination of two or more. The nickel salt is used in an amount in the range of 0.01 to 0.50 mol/L, preferably in the range of 0.01 to 0.30 mol/L, more preferably in the range of 0.05 to 0.20 mol/L.

[0017] Examples of the reducing agent include hypophosphorous acid, sodium hypophosphite, DMAB (dimethyl amino borane), and hydrazine compounds such as hydrazine hydrochloride and hydrazine sulfate. In this embodiment, the use of hypophosphorous acid or sodium hypophosphite as the reducing agent is the most desirable. When hypophosphorous acid or sodium hypophosphite is used as the reducing agent, a Ni-P plating film is obtained. The reducing agent is used in an amount in the range of 0.01 to 0.50 mol/L, preferably in the range of 0.05 to 0.30 mol/L, more preferably in the range of 0.10 to 0.25 mol/L.

[0018] The stabilizer for use in the electroless nickel plating bath of this embodiment contains antimony in the form of metal antimony or an antimony compound. Examples of the antimony compound include antimony chloride, antimony acetate, antimony oxide, antimonyl potassium tartrate, antimonyl-L-tartaric acid, antimonic acid, sodium antimonate, and potassium antimonate. The metal antimonie or antimony compounds may be used as a mixture of two or more kinds thereof. Preferably, the stabilizer is metal antimony, antimony chloride, antimony acetate, antimony oxide, antimonyl potassium tartrate, or a mixture thereof. The stabilizer is used in an amount in the range of 0.001 to 0.10 mmol/L, preferably in the range of 0.005 to 0.075 mmol/L, more preferably in the range of 0.01 to 0.05 mmol/L in terms of Sb (antimony). In this embodiment, antimony chloride (as antimony chloride (III)) or antimonyl potassium tartrate (as antimonyl (III) potassium tartrate-trihydrate) is used as the stabilizer in an amount in the range of 0.1 to 50 mg/L, preferably in the range of 0.1 to 25 mg/L, most preferably in the range of 0.2 to 20 mg/L, in terms of Sb.

[0019] The electroless nickel plating bath of this embodiment contains a complexing agent that contains succinic acid or one of malic acid and lactic acid at a chelate molar ratio of 25:75 to 75:25. The ratio of malic acid or lactic acid to succinic acid is in the range of 30:70 to 70:30, preferably in the range of 35:65 to 65:35, more preferably in the range of 40:60 to 60:40, especially preferably in the range of 45:55 to 55:45 as expressed in chelate molar ratio. As the complexing agent, the combination of malic acid and succinic acid is the most desirable.

[0020] The term "chelate mol (cM)" is known to those skilled in the art as a unit of concentration that is defined in light of the denticity of each complexing agent. The concentration of a complexing agent with a denticity of 1 is equal to the general molar concentration. When the denticity is equal to or greater than 2, the chelate mol is equal to a value obtained by multiplying the molar concentration by the denticity. The use of this unit enables comparison of different complexing agents under generally the same concentration conditions. The denticities of typical complexing agents are as follows. Denticity 1: acetic acid, propionic acid, etc. Denticity 2: lactic acid, glycolic acid, succinic acid, glycine, etc. Denticity 3: malic acid and aspartic acid. DENTICITY 4: citric acid.

[0021] The complexing agent is present in the electroless nickel plating bath in an amount of 0.3 to 1.3 chelate mol per liter of the electroless nickel plating bath, preferably 0.4 to 1.1 chelate mol per liter of the electroless nickel plating bath, more preferably 0.5 to 0.9 chelate mol per liter of the electroless nickel plating bath.

[0022] The complexing agent may contain a compound other than malic acid or lactic acid and succinic acid. Examples of compound that can be used as the additional complexing agent include carboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid and oxalic acid, hydroxy carboxylic acids such as lactic acid, glycolic acid, malic acid and citric acid, and amino acids such as alanine, valine, tyrosine, glycine, aspartic acid and hystidine. In this embodiment, the complexing agent is preferably composed only of malic acid or lactic acid and succinic acid, most preferably of malic acid and succinic acid.

[0023] The electroless nickel plating bath of this embodiment may contain, in addition to the above components, the following components: pH adjuster (potassium hydroxide, aqueous ammonia solution, sodium hydroxide, etc.), brightener (lead, copper, sodium thiosulfate, etc.), surfactant (sodium dodecyl sulfate, PEG IK, etc.), pH buffer (ammonium chloride, ammonium sulfate, boric acid, etc.), and so on.

[0024] This embodiment also relates to an electroless nickel plating method that uses the electroless nickel plating bath that is described above. The electroless nickel plating method of this embodiment method is implemented by impregnating a substrate in the electroless nickel plating bath of this embodiment and optionally heating the electroless nickel plating bath. The substrate is not specifically limited and may be made of either a conductive or nonconductive material. Specific examples of the substrate include those that are made of copper, aluminum, iron, or an alloy thereof, those that are made of a resin, and a silicon wafer on which aluminum has been deposited by sputtering. The substrate is subjected to pretreatments that are known to those skilled in the art, such as degreasing and activation with an acid or alkaline, prior to plating.

[0025] According to the electroless nickel plating bath of this embodiment and the electroless nickel plating method that uses the plating bath, a high-quality nickel plating film which has low film stress and does not develop a warp or cracks can be formed. This embodiment is useful in carrying out nickel plating on silicon wafers, automotive parts, decorative parts, electronic parts, industrial jigs, molds, rolls and so on.

[0026] While the following example describes the present invention in more detail, the present invention is not limited to the example.

[0027] 1. Experimental procedure

A copper plate (50 x 25 mm²) and a commercially available BeCu test strip (effective plating area: 76 x 10 mm²) were prepared as substrates. The copper plate was pre-treated according to the following procedure: 1) alkaline degreasing (NaOH 8g/L, sodium citrate 10 g/L, NAROACTY N-120 (manufactured by Sanyo Chemical Industries, Ltd) 2g/L) at 60°C for 5 minutes, 2) acid activation (10% sulfuric acid) at room temperature for 1 minute, and 3) treatment with a 0.1 g/L PdCl₂ solution at room temperature for 1 minute. The test strip was pre-treated according to the following procedure: 1) acid degreasing (PB-242D (manufactured by EBARA-UDYLITE CO. LTD.)) at room temperature for 5 minutes, 2) acid activation (10% sulfuric acid) at room temperature for 1 minute, and 3) treatment with a 0.1 g/L PdCl₂ solution at room temperature for 5 minutes. After the pretreatment, each of the substrates was subjected to plating without replenishment for 30 minutes, and the precipitation rate and the stress in the plating film were measured.

[0028] The precipitation rate was obtained from the plating time and plating film thickness (the film density was defined to be 7.85 g/cm ) that was calculated based on the difference between the weights of the copper plate before and after the plating that were measured with a precision balance. The stress S (MPa) in the plating film was obtained by setting the test strip after the plating on a strip-type electrodeposit stress tester (683EC analyzer, manufactured by Electrochemical Co., Ltd.) with the ends of its legs spaced apart from the scale, reading the sum of the values between the legs (spread between legs) on the special scale, and substituting the film thickness T (μπι), the spread between legs U, and the test strip factor K in the equation S = 58.2 x UK/T.

[0029] 2. Examination of complexing agent concentration and combination of complexing agents

Plating was carried out according to the conditions that are shown in Table 1 to examine the optimum conditions of the combination of complexing agents and complexing agent concentration.

[0030] [Table 1]

Basic bath composition

Nickel sulfate-hexahydrate 0.10 mol/L

Reducing agent (sodium hypophosphite -monohydrate) 0.175 mol/L

Complexing agent 0.8 to 1.1 cM/L pH adjustor (ammonia water) Proper amount

Stabilizer Not used

Plating conditions

Bath pH 4.60

Bath temperature 80°C

Bath load 0.5 dm²/L

Plating time 30 minutes

[0031] Table 2 shows the results of plating in plating baths that used malic acid and succinic acid, lactic acid, acetic acid or glycine as complexing agents at a chelate molar ratio of 50:50 (in the table, "as plate" means immediately after the plating, and the ratio is chelate molar ratio).

[0032] In the cases of the plating baths that used a combination of malic acid and succinic acid or malic acid and acetic acid as complexing agents, the precipitation rate was relatively high and the stress in the plating film was relatively low ((-) and (+) in the columns of plating film stress represent compressive stress and tensile stress, respectively). However, in the cases of the plating baths that used a combination of malic acid and acetic acid, the stress tended to shift to the tensile side as the total complexing agent concentration increased. On the other hand, when the combination of malic acid and succinic acid was used, such a tendency was not observed. The precipitation rate was the highest when the total complexing agent concentration was 0.8 cM/L. From a comprehensive standpoint, the use of malic acid and succinic acid as complexing agents at a chelate molar ratio of 50:50 with a total complexing agent concentration of 0.8 cM/Lwas determined to be optimum.

[0033] [Table 2]

[0034] Table 3 shows the results of investigation of the precipitation rate, film stress immediately after the plating, and film stress after a heat treatment which was conducted on combinations of various complexing agents at a ratio of 50:50 with the total complexing agent concentration fixed to 0.8 cM/L. The heat treatment was for an accelerated deterioration test to simulate the states of the plating films a few to dozen days later. In the cases of the combinations that included malic acid and the combination of succinic acid and lactic acid, the film stress after the heat treatment was low. However, in the cases of the combination of malic acid and lactic acid and the combination of malic acid and glycine, the precipitation rate was low. From the standpoint of precipitation rate and plating film stress, the combinations of malic acid and succinic acid, malic acid and acetic acid, and succinic acid and lactic acid were determined to be excellent. However, acetic acid has a low chelating ability and acts as a plating rate accelerator, so the use of acetic acid increases the precipitation rate but reduces the stability of the plating bath. The bath stability is more likely to be reduced when the plating bath has been aged because of an increase in by-products and accumulations compared to immediately after the initial make-up thereof. In addition, acetic acid has a problem of odor. Thus, the use of the combination of malic acid and succinic acid or of succinic acid and lactic acid as the complexing agent was determined to be desirable.

[0035] [Table 3]

[0036] 3. Examination of mix ratio of complexing agents

Plating was carried out according to the conditions in Table 1 to examine the optimum conditions of the mix ratio of complexing agents. Table 4 shows the results of investigation of the precipitation rate, film stress immediately after the plating, and film stress after a heat treatment (in the Table, the ratio is chelate molar ratio) which was conducted with various mix ratios between malic acid and succinic acid, which were determined to be a desirable combination of complexing agents.

[0037] As the percentage of succinic acid increased, the precipitation rate increased and the stress tended to shift to the tensile side. The use of malic acid and succinic acid at a chelate molar ratio of 50:50 was determined to be most desirable. A total complexing agent concentration of 0.8 cM/Lwas optimum again.

[0038] [Table 4]

[0039] Examination of stabilizer

Plating was carried out according to the conditions in Table 5 to examine the optimum type and amount of stabilizer.

[0040] [Table 5]

Basic bath composition Nickel sulfate-hexahydrate 0.10 mol/L

Reducing agent (sodium hypophosphite -monohydrate) 0.175 mol/L Complexing agent (malic acid:succinic acid = 50:50) 0.8 cM/L pH adjuster (ammonia water) Proper amount Stabilizer 0 to 20.0 mg/L Plating conditions

Bath pH 4.60

Bath temperature 80°C

Bath load 0.5 dm²/L

Plating time 30 minutes

[0041] Table 6 shows the results of investigation of the precipitation rate, film stress immediately after the plating, and film stress after a heat treatment that was conducted using a plating bath that used bismuth (bismuth nitrate), antimony (antimony chloride (HI)) or thiourea as the stabilizer (the concentrations of bismuth and antimony were in terms of Bi and Sb, respectively), and using a commercially available plating bath (EPITHAS NPR-18, manufactured by C. Uyemura & Co. Ltd.) was used. The use of bismuth or thiourea as a stabilizer was effective in improving the bath stability, plating film appearance and precipitation rate as compared to the case without a stabilizer but resulting in an increase in the film stress after a heat treatment. On the other hand, when antimony was used as a stabilizer, the film stress after a heat treatment was significantly lower as compared to the cases where the plating baths that used other stabilizers and the commercially available plating bath were used.

[0042] [Table 6]

*Heat treatment: at 120°C for one hour [0043] Table 7 shows the results of investigation which was conducted using plating baths that used molybdenum (sodium molybdate dihydrate, Na₂Mo0₄-2H₂0) as a stabilizer. When molybdenum was used, no decrease in film stress after a heat treatment was observed. When the amount of molybdenum was increased, nickel did not precipitate and therefore the plating bath did not fulfill its function.

[0044] [Table 7]

*Heat treatment: at 150°C for five minutes

[0045] Table 8 shows the results of investigation which was conducted using plating baths that used a mixture of antimony (antimonyl III potassium tartrate-trihydrate, CsH4K₂0₁₂Sb₂-3H₂0) and molybdenum (sodium molybdate-dihydrate, Na₂Mo0₄-2H₂0) as a stabilizer. When antimony and molybdenum were mixed, no desirable result was achieved.

[0046] [Table 8]

[0047] 4. Examination of mix ratio of complexing agents used in combination with stabilizer

Plating was carried out according to the conditions in Table 9 to examine the optimum type and amount of complexing agent in the case where antimony (antimonyl III potassium tartrate-trihydrate) is used as a stabilizer.

[0048] [Table 9] Basic bath composition

Nickel sulfate-hexahydrate 0.10 mol/L

Reducing agent (sodium hypophosphite -monohydrate) 0.175 mol/L or

0.208 mol/L

Complexing agent 0.8 cM/L

pH adjuster (ammonia water) Proper amount Stabilizer (antimony*) 0 to 20.0 mg/L

Plating conditions

Bath pH 4.60

Bath temperature 85°C

Bath load 0.5 dm L

Plating time 30 minutes

*C₈H₄K₂0₁₂Sb₂-3H₂0: antimonyl III potassium tartrate-trihydrate

[0049] Table 10 shows the results of investigation of the precipitation rate, film stress immediately after the plating, and film stress after a heat treatment (in the Table, the ratio is chelate molar ratio) which was conducted with various mix ratios between malic acid and succinic acid, which were determined to be a desirable combination of complexing agents.

[0050] Even when a stabilizer (antimony) was used, an increase in the precipitation rate and tendency to shift to the tensile stress were observed as the mix ratio of succinic acid to malic acid increased. In addition, the precipitation rate increased as the concentration of the reducing agent was higher, and the stability of the plating bath decreased despite the fact that the effect on the stress was not very significant. A mix ratio of malic acid:succinic acid = 49:51 with a reducing agent concentration of 0.175 mol/L was determined to be the most desirable.

[0051] [Table 10]

Malic acid : succinic acid = 55:45 9.40 (-) 26.5 (+) 5.9

Heat treatment: at 120°C for one hour

Claims

CLAIMS:

1. An electroless nickel plating bath, comprising:

a nickel salt,

a reducing agent,

a stabilizer that contains metal antimony or an antimony compound, and

a complexing agent that contains succinic acid or one of malic acid and lactic acid at a chelate molar ratio of 25:75 to 75:25.

2. The electroless nickel plating bath according to claim 1,

wherein the complexing agent is present in an amount of 0.3 to 1.3 chelate mol per liter of the electroless nickel plating bath.

3. The electroless nickel plating bath according to claim 2,

wherein the complexing agent is present in an amount of 0.5 to 0.9 chelate mol per liter of the electroless nickel plating bath.

4. The electroless nickel plating bath according to any one of claims 1 to 3, wherein the stabilizer is metal antimony, antimony chloride, antimony acetate, antimony oxide, antimonyl potassium tartrate, or a mixture thereof.

5. The electroless nickel plating bath according to any one of claims 1 to 4, wherein the reducing agent is hypophosphorous acid or sodium hypophosphite.

6. The electroless nickel plating bath according to any one of claims 1 to 5, wherein the reducing agent is present in an amount of 0.01 to 0.5 mol per liter of the electroless nickel plating bath.

7. The electroless nickel plating bath according to any one of claims 1 to 6, wherein the stabilizer contains Sb and an amount of Sb is 0.001 to 0.1 mmol per liter of the electroless nickel plating bath.

8. An electroless nickel plating method that uses an electroless nickel plating bath according to any one of claims 1 to 7.