Electroplating Method

Abstract

A cycle of electroplating is repeated in an electroplating bath containing at least one metal ion selected from cobalt, nickel and iron, a buffering agent and a conducting agent by use of a soluble anode. The concentration of the conducting agent in an initially prepared electroplating bath is set at 70 to 95% of a saturated concentration. The electroplating is repeated in such a way that a first replenishment solution containing a buffering agent and a conducting agent at concentrations of 0.5 to 1.2 times the concentrations in the initially prepared electroplating bath and free of the metal ion is added so as to replenish the agents that have been reduced in amount during the course of the electroplating and a concentration of the conducting agent in the electroplating bath after the replenishment of the first replenishment solution is adjusted to 70 to 95% of a saturated concentration thereof. According to the invention, the electroplating can be repeated while keeping the electroplating bath over a long time in a good condition sufficient to provide a plated film having high throwing power and a good appearance, so that there can be stably obtained plated films over a long time without frequent regeneration such as by electrolysis for lowering the concentration of a metal ion in the plating bath, thus being good at economy.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This non-provisional application claims priority under 35 U.S.C. .sctn.119(a) on Patent Application No. 2006-085043 filed in Japan on Mar. 27, 2006, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

[0002] This invention relates to an electroplating method wherein in case where a cycle of electroplating a substrate for plating is repeated by use of a soluble anode in an electroplating bath capable of yielding a film of nickel, cobalt, iron or an alloy thereof having high throwing power, plated films having high throwing power and an good outer appearance can be stably formed over a long time.

BACKGROUND ART

[0003] For an electroplating solution capable of forming plated films having high throwing power, there are known, for example, electroplating solutions containing water soluble salts of nickel, cobalt or iron (hereinafter referred generically to merely as a nickel series metal), conducting agents, buffering agents, halide ions, organic brightening agents and the like (see Japanese Patent Laid-open Nos. Sho 62-103387 and Sho 62-109991).

[0004] Where electroplating is carried out by use of such an electroplating solution and a soluble anode, an anode current efficiency becomes close to 100%, whereas a cathode current efficiency is usually at 95%, thereby causing an efficiency difference to develop. If the plating is continued, a nickel series metal (nickel series metal ion) increases in the electroplating solution. It is known that if the concentration of the nickel-series metal increases in excess, throwing power lowers. In order to keep high throwing power, there has been proposed a method wherein a nickel series metal increased to an extent exceeding an allowable range in the electroplating solution is removed (Japanese Patent Laid-open No. Hei 8-53799).

[0005] In the method, an anodic solution and an electroplating solution are separated from each other by means of a cationic exchange resin membrane, under which electrolysis is carried out by immersing an insoluble anode in the anodic solution and a cathode in the electroplating solution, respectively, thereby causing metal ions in the electroplating solution to be removed by deposition on the cathode as a metal. However, this method needs to stop a plating equipment for 1 to 2 days and treat at a current density of 1 A/dm.sup.2, and it is not good at economy to carry out such a treatment frequently. If the electroplating solution is treated according to the above method, repeated use of the electroplating solution over a long time may lead to a lowering of throwing power of the resulting plated films.

[0006] With the electroplating, there is a so-called "drag-out" phenomenon wherein an electroplating solution is entrained outside a tank along with substrates for plating. By this, concentrations of components other than metal ions vary. In order to carry out the plating repeatedly, individual components reduced in amount by the drag-out are replenished, whereupon precipitates or crystals may occur.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

[0007] Under these circumstances in the art, an object of the invention is to provide an electroplating method wherein in case where a cycle of electroplating a substrate for plating is repeated by use of a soluble anode in an electroplating bath capable of yielding a film of nickel, cobalt, iron or an alloy thereof having high throwing power, the throwing power of the plated films is kept high over a long time without causing disfigurement of the plated films.

MEANS FOR SOLVING THE PROBLEMS

[0008] We made intensive studies in order to solve the above problems and, as a result, found that in order to keep high throwing power in the course of repeated use of a plating bath, it is important not only to suppress a nickel series metal ion from increasing in an electroplating bath, but also to permit a concentration of a conducting agent to be continuedly kept at a high level while suppressing a variation thereof. In this connection, however, where the concentration of the conducting agent in the electroplating bath is kept at a high level, it is liable to cause the conducting agent to be precipitated or crystallized in the electroplating bath and, if precipitation or crystallization of the conducting agent takes place, the resulting plated film becomes poor in appearance.

[0009] It has been found that in order to keep good throwing power and appearance of the plated film over a long time, it is necessary (1) to suppress an increase in concentration of a nickel series metal ion in an electroplating bath, (2) not to make the concentration of a conducting agent in a replenishment solution at a level beyond necessity in order that while keeping the concentration of the conducting agent in the electroplating bath at a high level, a saturated concentration is not exceeded, and (3) to set the concentration of the conducting agent to a level lower that a saturated concentration upon initial preparation of an electroplating bath or after replenishment of a replenishment solution so as not to cause the conducting agent to be precipitated or crystallized owing to the evaporation of water in the electroplating bath.

[0010] In this way, it has been found effective that where a cycle of electroplating a substrate for plating is repeated by use of a soluble anode in an electroplating bath containing at least one metal ion selected from those of cobalt, nickel and iron, a buffering agent, and a conducting agent, the concentration of the conducting agent in an initially prepared electroplating bath is set at a level within a range of 70 to 95% of a saturated concentration, the buffering agent and the conducting agent, each reduced in amount during the repetition of the electroplating, are replenished by adding, to the electroplating bath, a first replenishment solution that contains a buffering agent and a conducting agent each at a concentration of 0.5 to 1.2 times the concentration contained in the initially prepared electroplating bath and is free of the at least one metal ion, and the concentration of the conducting agent in the electroplating bath after the replenishment of the first replenishment solution is adjusted to 70 to 95% of a saturated concentration.

[0011] If the electroplating bath further includes a halide ion, the electroplating is carried out in such a way that the halide ion reduced in amount during the course of the repetition of the plating in the electroplating bath is replenished by adding, to the electroplating bath, the first replenishment solution that further includes a halide ion at a concentration of 0.5 to 1.2 times the concentration in the initially prepared electroplating bath.

[0012] Further, if the electroplating bath further includes an organic brightener, the electroplating is carried in such a way that the organic brightener in the electroplating bath reduced in amount during the course of the repetition of the plating is replenished by adding, to the electroplating bath, the first replenishment solution which further includes an organic brightener at a concentration of 0.5 to 1.2 times that in the initially prepared electroplating bath. The invention has been accomplished based on these findings.

[0013] More particularly, the invention provides:

[1] an electroplating method wherein a cycle of electroplating a substrate for plating is repeated by use of a soluble anode in an electroplating bath containing at least one metal ion selected from those of cobalt, nickel and iron, a buffering agent, and a conducting agent, the method including setting a concentration of the conducting agent in an initially prepared electroplating bath at a level within a range of 70 to 95% of a saturated concentration, replenishing the buffering agent and the conducting agent, each reduced in amount during the repetition of the electroplating, by adding, to the electroplating bath, a first replenishment solution that contains a buffering agent and a conducting agent each at a concentration of 0.5 to 1.2 times the concentration contained in the initially prepared electroplating bath and is free of the at least one metal ion, and adjusting the concentration of the conducting agent in the electroplating bath after the replenishment of the first replenishment solution to 70 to 95% of a saturated concentration, under which the electroplating is repeated; [2] the method of [1] above, wherein the electroplating bath further includes a halide ion, and the halide ion in the electroplating bath reduced in amount during the course of the repetition of the plating is replenished by adding, to the electroplating bath, the first replenishment solution that further includes a halide ion at a concentration of 0.5 to 1.2 times the concentration in the initially prepared electroplating bath; and [3] The method of [1] or [2] above, wherein the electroplating bath further includes an organic brightener, and the organic brightener in the electroplating bath reduced in amount during the course of the repetition of the plating is replenished by adding, to the electroplating bath, the first replenishment solution which further includes the organic brightener at a concentration of 0.5 to 1.2 times that in the initially prepared electroplating bath.

BENEFITS OF THE INVENTION

[0014] According to the invention, the electroplating can be repeated while keeping the electroplating bath over a long time in a good condition sufficient to provide a plated film having high throwing power and a good appearance, so that there can be stably obtained plated films over a long time without frequent regeneration such as by electrolysis for lowering the concentration of a metal ion in the plating bath, thus being good at economy.

BEST MODE FOR CARRYING OUT THE INVENTION

[0015] The invention is now described in more detail.

[0016] The invention relates to a method for repeating a cycle of electroplating a substrate for plating by use of a soluble anode in an electroplating bath containing at least one metal ion selected from those of cobalt, nickel and iron, a buffering agent and a conducting agent. In the method, the electroplating is repeatedly carried out by setting a concentration of the conducting agent in an initially prepared electroplating bath at a level of 70 to 95% of a saturated concentration, replenishing the buffering agent and the conducting agent in the electroplating bath reduced during the repetition of the plating by adding, to the electroplating bath, a first replenishment solution, which is free of the metal ion and contains a buffering agent and a conducting agent at concentrations of 0.5 to 1.2 times those in the initially prepared electroplating bath, respectively, and adjusting the concentration of the conducting agent in the electroplating bath after the replenishment of the first replenishment solution to a level of 70 to 95% of a saturated concentration thereof.

[0017] The electroplating bath, to which the invention is directed, should be one which exhibits high throwing power and includes at least one metal (nickel series metal) ion selected from those of nickel, cobalt and iron, a buffering agent and a conducting agent. Preferably, the bath further includes, aside from the components mentioned above, a halide ion and/or an organic brightener.

[0018] The metal (nickel series metal) ion can be contained in the electroplating bath by use of a water-soluble salt of a nickel series metal such as nickel, cobalt or iron. Examples of the water-soluble salts include sulfates, sulfamates, and halides such as chlorides, bromides and the like. Specific examples include sulfates such as nickel sulfate, ferrous sulfate, cobalt sulfate and the like, sulfamates such as nickel sulfamate, ferrous sulfamate, cobalt sulfamate and the like, and halides such as nickel bromide, nickel chloride, ferrous chloride, cobalt chloride and the like. Of these, sulfates such as nickel sulfate, ferrous sulfate, cobalt sulfate and the like and sulfamates such as nickel sulfamate, ferrous sulfamate, cobalt sulfamate and the like are preferred. These water-soluble nickel series metal salts should preferably be used at a concentration of 5 to 400 g/liter, more preferably from 5 to 200 g/liter. It will be noted that when a halide is used as a water-soluble nickel series metal salt, part of all of such a halide ion as described hereinafter may be simultaneously contained in the electroplating solution.

[0019] The electroplating bath containing such a water-soluble nickel series metal salt contains an ion of the nickel series metal. For the nickel series metal ion, a nickel ion, cobalt ion or iron ion may be contained singly or in combination of two or more. It is preferred that the concentration of the nickel series metal ion in the electroplating bath is within a range of 1 to 20 g/liter.

[0020] For a buffering agent, examples include organic acids such as malic acid, succinic acid, acetic acid, tartaric acid, ascorbic acid, citric acid, lactic acid, pyruvic acid, propionic acid, formic acid and the like, salts of these organic acids, amine compounds such as ethylenediamine, triethanolamine, ethanolamine and the like, boric acid, and the like. These may be used singly or in combination. Preferably, boric acid, citric acid or salts thereof are used. The concentration of the buffering agent in the electroplating bath is within a range of from 10 to 100 g/liter, preferably from 20 to 80 g/liter.

[0021] The conducting agent is added separately from the above-mentioned water-soluble metal salt and the buffering agent. For the conducting agent, a water-soluble salt of a metal selected from alkali metals, alkaline earth metals and aluminum is preferred. Examples include halides of alkali metals, alkaline earth metals or aluminum, e.g. chlorides such as lithium chloride, sodium chloride, potassium chloride, magnesium chloride, aluminum chloride and the like, and bromides such as sodium bromide, potassium bromide, magnesium bromide, aluminum bromide and the like. In addition, there are favorably used sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, aluminum sulfate and the like, methanesulfonates such as sodium methanesulfonate, potassium methanesulfonate and the like. These may be used singly or in combination of two or more. Where it is desired to obtain a plated film whose color tone is excellent, it is preferred to use sulfates such as lithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate, aluminum sulfate and the like, sulfamates such as sodium sulfamate, potassium sulfamate and the like, and methanesulfonates such as sodium methanesulfonate, potassium methanesulfonate and the like.

[0022] The concentration of the conducting agent contained in an initially prepared electroplating bath is within a range of from 70 to 95%, preferably from 80 to 90%, of a saturated concentration at a plating temperature. If the concentration is smaller than 70%, high throwing power cannot be kept. Over 95%, when the concentration of the conducting agent in a first replenishment solution described hereinafter is high, a precipitate or crystals are liable to occur in the electroplating bath upon replenishment of the conducting agent. It will be noted that where a halide is used as a conducting agent, part or all of a halide described hereinafter may be simultaneously contained in the electroplating bath.

[0023] With respect to a halide ion, where a halide of a nickel series metal is used as a water-soluble nickel series metal salt or a halide is used as a conducting agent, such a halide yields a halide ion in the electroplating bath, so that the electroplating bath eventually contains the halide ion. On the other hand, where no halide ion is yielded in the electroplating bath from a water-soluble nickel series metal salt or conducting agent, an anode-dissolving agent that is made of a halide salt may be added to so as to contain a halide ion in the bath. It is to be noted that even when a nickel series metal halide is used as a water-soluble nickel series metal salt or a halide is used as a conducting agent, an anode-dissolving agent may be added.

[0024] For the anode-dissolving agent, there can be used halides of alkali metals, alkaline earth metals or aluminum including chlorides such as lithium chloride, sodium chloride, potassium chloride, magnesium chloride, aluminum chloride and the like, and bromides such as sodium bromide, potassium bromide, magnesium bromide, aluminum bromide and the like.

[0025] It should be noted that the concentration of a halide in the electroplating bath is within a range of from 5 to 150 g/liters, preferably from 10 to 100 g/liter, as including those ions derived from a water-soluble nickel series salt or a conducting agent.

[0026] Moreover, if necessary, surface active agents such as an anionic surface active agent may added. Further, organic brighteners such as saccharin, sodium naphthalenedisulfonate, sodium naphthalenesulfonate, sodium allyl sulfonate, butynediol, propargyl alcohol, coumarin, formalin and the like may be added to the electroplating bath. Each may be contained in an amount of 0.01 to 0.5 g/liter.

[0027] In the practice of the invention, the electroplating bath should preferably be acidic in nature, with the pH being within a range of from 2 to 6, preferably from 3 to 5.

[0028] The electroplating bath containing such a nickel series metal, a buffering agent, a conducting agent and a halide ion as set out hereinabove permits high throwing power. For instance, it is preferred that when measurement is carried out using a Haring cell at a distance ratio between two cathode plates and an anode plate, the throwing power (T) represented by the following equation is 35% or over

T(%)=[(P-M)/(P+M-2)].times.100

wherein T is throwing power, P is at 5 (distance ratio between the anode and the cathode) and M is a ratio by weight of a plated film deposited on two cathodes.

[0029] In the present invention, using a soluble anode, e.g. an anode made of nickel, cobalt, iron or an alloy thereof, a cycle of electroplating a substrate for plating is repeated, for example, at a cathode current density of 0.01 to 5 A/dm.sup.2 at a plating temperature of 10 to 70.degree. C., if necessary, while agitating appropriately by a known method. During the electroplating, a buffering agent and a conducting agent reduced in amount by the repetition of the plating in the electroplating bath are, respectively, replenished by adding, to the electroplating bath, a first replenishment solution that contains a buffering agent and a conducting agent at concentrations of 0.5 to 1.2 times those in an initially prepared electroplating solution, respectively, and is free of a nickel series metal ion.

[0030] As mentioned above, the first replenishment solution contains the buffering agent and the conducting agent at 0.5 to 1.2 times, preferably 0.8 to 1.05 times, the concentrations of the agents in the initially prepared electroplating bath and is free of a nickel series metal ion. Since this first replenishment solution is free of a nickel series metal ion, a nickel series metal in the electroplating bath is not increased at all by the supply of the replenishment solution. This makes it possible to suppress a lowering of the throwing power to an extent as small as possible as would otherwise occur by an increase in concentration of the nickel series metal ion in the electroplating solution. If the concentration is smaller than 0.5 times the concentration of the initially prepared electroplating bath, the feed of the replenishment solution increases, resulting in much labor for concentration of water such as by evaporation. In contrast, when the concentration exceeds 1.2 times the concentration of the initially prepared electroplating bath, a precipitate or crystals are liable to occur when a replenishment solution is supplied to the electroplating solution. It will be noted that the times for the concentration of a buffering agent should preferably be the same as the times for the concentration of a conducting agent.

[0031] Specific examples of these buffering and conducting agents are those as exemplified with respect to the initially prepared electroplating bath. Especially, it is preferred to use the same agents as those used in the initially prepared electroplating bath, respectively.

[0032] If a halide ion is contained in an initially prepared electroplating bath, a halide ion is preferably added to the first replenishment solution. In this connection, the concentration of the halide ion is within a range of from 0.5 to 1.2 times, preferably from 0.8 to 1.05 times, the concentration in the initially prepared electroplating bath. Especially, the times for this concentration are preferably the same as the times for the concentration of a conducting agent.

[0033] It will be noted that the halide ion can be added to the first replenishment solution by adding a halide as such a conducting agent as set out above and/or a halide as a conducting agent, and where a water-soluble nickel series metal halide is used as a supply source of a nickel series metal ion at the initial time of preparation of an electroplating bath, a halide serving as a conducting agent and/or a halide serving as an anode dissolving agent is added in a given amount in place of a water-soluble nickel series metal halide, thereby permitting the halide ion to be set at a concentration thereof, to which an amount of the halide ion derived from the water-soluble nickel series metal salt in the initially prepared electroplating bath is added, without addition of a nickel series metal ion. Specific examples of the halide serving as a conducting agent and the halide serving as a conducting agent are those indicated as contained in the initially prepared electroplating bath, and it is preferred to use the same halide as used in the initial electroplating bath.

[0034] Where an organic brightener is contained in the initially prepared electroplating bath, it is preferred to add an organic brightener to the first replenishment solution. In this case, the concentration of the organic brightener can be at a level of from 0.5 to 1.2 times, preferably, from 0.8 to 1.05 times, the concentration in the initial electroplating bath. More preferably, the times for the concentration are the same as those of the conducting agent. For the organic brightener, mention is made of those exemplified as contained in the initial electroplating bath. It is preferred to use the same material as contained in the initial electroplating bath.

[0035] Without replenishing an organic brightener by addition to the first replenishment solution for supply to an electroplating bath, separate replenishment of an organic brightener is possible by use of a second replenishment solution containing an organic brightener alone at a concentration of 20 to 2,000 times, preferably 50 to 1,000 times, the concentration in the initial electroplating bath.

[0036] This is for the reason that although the buffering agent, conducting agent and halide ion are reduced in amount mainly by "drag-out" the organic brightener is reduced not only by "drag-out", but also by entrainment in a plated film and partial removal with a filter, so that there is some case where it is preferred to supply the brightener separately from the buffering agent, conducting agent and halide ion.

[0037] If an amount of a nickel series metal ion reduced by "drag-out" is large with a shortage of the nickel series metal ion, a third replenishment solution containing a nickel series metal ion can be added. For the third replenishment solution, there can be used, for example, a solution that contains such a water-soluble nickel series metal salt as set out hereinabove and, if necessary, an acid or alkali added for adjustment of pH. Examples of the water-soluble nickel series metal salts are those exemplified as contained in an initially prepared electroplating bath. Preferably, those other than halides such as, for example, sulfates, sulfamates and the like are preferred. It will be noted that the concentration of a nickel series metal ion in the third replenishment solution should preferably be within a range of 40 to 100 g/liter.

[0038] The pH values of the first, second and third replenishment solutions are each from 2 to 6, preferably from 3 to 5.

[0039] The replenishment solutions may be replenished to an electroplating bath in a plating tank where reduced such as by "drag-out". The replenishment solutions can be properly selected depending on the following standards. More particularly, when an increase in amount (I.sub.M) at a given unit (time) and a reduction in amount (D.sub.M) of a metal caused such as by "drag-out" at the given unit (time), both of which are caused by a difference between a cathode current efficiency and an anode current efficiency, is such that I.sub.M.ltoreq.D.sub.M, the first replenishment solution alone or the first and second replenishment solutions are used, and if I.sub.M<D.sub.M, the first and third replenishment solutions or the first, second and third solutions are used.

[0040] The amounts of the replenishment solutions may be determined according to any of methods including (1) a method wherein the concentrations of a nickel series metal ion, a buffering agent, a conducting agent, a halide ion, and, if necessary, an organic brightener in the electroplating bath after repetition of the plating are periodically analyzed at every given unit (time or the like) prior to supply, thereby determining the amounts depending on the results of the analysis, and (2) a method wherein an increase or decrease in concentration of individual components in the electroplating bath is measured, for example, according to a line test (real machine test) and variations of the components are used to determine the amounts at every given unit (time or the like). The given unit (time) is preferably within a range of 1 to 200 hours.

[0041] The concentration of a conducting agent in the electroplating bath after replenishment of the replenishment solution or solutions is adjusted to a level of 70 to 95% of a saturated concentration thereof. In case where the concentration of a conducting agent in the electroplating bath after replenishment of the replenishment solution is within such a range as indicated above only by replenishment of the replenishment solution, electroplating can be carried out in a condition of the bath where the replenishment solution is replenished. If, however, such a range as indicated above is not met in a condition of replenishing a replenishment solution, the concentration is adjusted to be within such a range as indicated above by addition of water or by removal of water through evaporation.

EXAMPLES

[0042] Examples and Comparative Examples are shown to particularly illustrate the invention, which should not be construed as limiting to the following examples.

Example 1

[0043] 1000 liters of electroplating bath A indicated in Table 1 was placed in a plating tank, and a substrate for plating having a plating surface of 100 dm.sup.2 was placed in the electroplating bath, under which plating operations were repeated under conditions of 55.degree. C. and 1 A/dm.sup.2 for 20 minutes while sucking air in the vicinity of the bath solution surface.

[0044] In every 12 hours (upon 36 repetition cycles of plating), water was added to the electroplating bath to adjust the volume of the electroplating bath to 1000 liters and concentrations of a nickel ion and sodium sulfate were, respectively, measured according to a hydrometric method. Because the concentration of the nickel ion increased over the case where the bath was initially prepared, replenishment solution A indicated in Table was not used but replenishment solution B was replenished in such a way that a total amount of sodium sulfate was same as at the time of initial preparation of the bath and water was evaporated to return the volume of the electroplating bath to 1000 liters, followed by re-starting the plating. The results of evaluation of throwing power and outer appearance of plated films obtained after repetition cycles of 500, 1,000, 1,500, 2,000, 2,500 and 3,000 are shown in Table 3. The evaluation methods are set out below.

[0045] Throwing Power

[0046] The plating bath was transferred to a Haring cell and measurement was carried out at a distance ratio between two cathode plates and an anode plate, under which the case where throwing power (T) represented by the following equation is not lower than 35% or over is evaluated as "good" and the case where the uniformity is smaller than 35% is evaluated as "bad".

T(%)=[(P-M)/(P+M-2)].times.100

wherein T is throwing power, P is at 5 (distance ratio between the anode and the cathode) and M is a ratio by weight of a plated film deposited on two cathodes.

[0047] Film Appearance

[0048] The resulting plated film was visually observed whereupon the case where a plated appearance in the Hull cell test (capable of observing from a high current density portion to a low current density portion) is uniform without involving a considerable degree of appearance irregularity is evaluated as "good" and the case where the appearance is non-uniform and irregular with respect to the plated appearance is as "bad".

Comparative Example 1

[0049] The electroplating was repeated in the same manner as in Example 1 except that replenishment solution C indicated in Table 2 was used in place of replenishment solution B and the throwing power of the resulting plated films in all the cycles was evaluated along with the plated film appearance. The results are shown in Table 3.

Comparative Example 2

[0050] The electroplating was repeated in the same manner as in Example 1 except that replenishment solution D indicated in Table 2 was used in place of replenishment solution B and the throwing power of the resulting plated films in all the cycles was evaluated along with the plated film appearance. The results are shown in Table 3.

Example 2

[0051] The electroplating was repeated in the same manner as in Example 1 except that replenishment solution E indicated in Table 2 was used in place of replenishment solution B and the throwing power of the resulting plated films in all the cycles was evaluated along with the plated film appearance. The results are shown in Table 3.

Comparative Example 3

[0052] Replenishment solution F indicated in Table 2 was replenished in place of replenishment solution B, but the replenishment solution could not be replenished at one time in such a way that a total amount of sodium sulfate was the same as that at the time of initial preparation of the bath.

Test Example 1

[0053] 1,000 liters of electroplating bath B indicated in Table 1 was placed in a plating tank, into which a substrate for plating having a plating surface of 100 dm.sup.2 was placed and plating operations were repeated under conditions of 55.degree. C. and 1 AS/dm.sup.2 for 20 minutes while sucking air in the vicinity of a plating bath surface.

[0054] Water was added to the electroplating bath, with which plating was repeated 50 cycles, in order to adjust the volume of the electroplating bath to 1,000 liters. Thereafter, the concentration of sodium sulfate was measured according to a hydrometric method. Replenishment solution B was replenished such that a total amount of sodium sulfate was the same as at the time of initial preparation of the bath, and no crystal were found in the electroplating bath.

Test Example 2

[0055] The replenishment solution was replenished in the same manner as in Test Example 1 except that replenishment solution E indicated in Table 2 was used in place of replenishment solution B, revealing that no crystals were found in the electroplating bath.

Test Example 3

[0056] The replenishment solution was replenished in the same manner as in Test Example 1 except that replenishment solution G indicated in Table 2 was used in place of replenishment solution B, revealing that crystals were found in the electroplating bath.

TABLE-US-00001 TABLE 1 Electroplating bath A Electroplating bath B Nickel sulfate hexahydrate 54 g/liter 54 g/liter (as Ni) (12 g/liter) (12 g/liter) Sodium sulfate anhydride 285 g/liter 335 g/liter (ratio to a saturation at 55.degree. C.) (80%) (95%) Boric acid 45 g/liter 45 g/liter Sodium chloride (as Cl ion) (11 g/liter) (11 g/liter) Anionic surface active agent 0.05 g/liter 0.05 g/liter pH 4.2 4.2

TABLE-US-00002 TABLE 2 Replenishment solution A B C D E F G Concentration ratio -- 1 1 1.5 0.7 0.3 -- Nickel sulfate hexahydrate 360 -- 54 -- -- 16 65 (g/liter) Sodium sulfate anhydride -- 285 285 428 200 86 342 (g/liter) Boric acid -- 45 45 67.5 31.5 13.5 54 (g/liter) Sodium chloride (as Cl ion) -- 11 11 16.5 7.7 3.3 13.2 (g/liter) Anionic surface active agent -- 0.05 0.05 0.075 0.035 0.015 0.06 (g/liter) pH 3.8 3.8 3.8 3.8 3.8 3.8 3.8

TABLE-US-00003 TABLE 3 Plating cycles 500 1,000 1,500 2,000 2,500 3,000 Example 1 Throwing good good good good good good power Appearance of good good good good good good plated film Comparative Throwing good bad bad bad bad bad Example 1 power Appearance of good good good good good good plated film Comparative Throwing good good bad bad bad bad Example 2 power Appearance of bad bad bad bad bad bad plated film Example 2 Throwing good good good good good good power Appearance of good good good good good good plated film

[0057] Japanese Patent Application No. 2006-085043 is incorporated herein by reference.

[0058] Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.

Claims

1. An electroplating method wherein a cycle of electroplating a substrate for plating is repeated by use of a soluble anode in an electroplating bath containing at least one metal ion selected from those of cobalt, nickel and iron, a buffering agent, and a conducting agent, the method comprising: setting a concentration of said conducting agent in an initially prepared electroplating bath at a level within a range of 70 to 95% of a saturated concentration; replenishing said buffering agent and said conducting agent, each reduced in amount during the repetition of the electroplating, by adding, to said electroplating bath, a first replenishment solution that contains a buffering agent and a conducting agent each at a concentration of 0.5 to 1.2 times the concentration contained in the initially prepared electroplating bath and is free of said at least one metal ion; and adjusting the concentration of said conducting agent in said electroplating bath after the replenishment of said first replenishment solution to 70 to 95% of a saturated concentration under which the electroplating is repeated.

2. The method according to claim 1, wherein said buffering agent and said conducting agent used in said first replenishment solution are, respectively, the same as those used in said initially prepared electroplating bath.

3. The method according to claim 1, wherein said electroplating bath further comprises a halide ion, and the electroplating is carried out in such a way that a halide ion in the electroplating bath reduced in amount during the course of the repetition of the plating is replenished by adding, to said electroplating bath, said first replenishment solution that further comprises a halide ion at a concentration of 0.5 to 1.2 times the concentration in the initially prepared electroplating bath.

4. The method according to claim 3, wherein said halide ion used in said first replenishment solution is the same as said halide ion used in said initially prepared electroplating bath.

5. The method according to claim 1, wherein said electroplating bath further comprises an organic brightener, and the electroplating is carried in such a way that said organic brightener reduced in amount during the course of the repetition of the electroplating in said electroplating bath is replenished by adding, to said electroplating bath, said first replenishment solution further comprising an organic brightener at a concentration of 0.5 to 1.2 times that in the initially prepared electroplating bath.

6. The method according to claim 5, wherein said organic brightener used in said first replenishment solution is the same as said organic brightener used in said initially prepared electroplating bath.