Patent Publication Number: US-2016230287-A1

Title: Reductive electroless gold plating solution, and electroless gold plating method using the plating solution

Description:
TECHNICAL FIELD 
     The invention disclosed in the present filing relates to an electroless gold plating solution, an electroless gold plating method using the electroless gold plating solution, and a plated product obtained by plating by the electroless gold plating method. More specifically, the invention relates to reductive electroless gold plating technology capable of plating directly a surface of a plating target. 
     BACKGROUND ART 
     In recent years, while requirements for higher performance and higher multi-function of electronic devices have been raised, further downsizing has been demanded on printed wiring boards used in these electronic devices. In order to cope with the downsizing, micronization of circuit patterns is progressing and along with the micronization of circuit patterns, advanced mounting technologies have been demanded. Generally in the field of printed wiring boards, as technologies of joining mounting components and terminal components, technologies using soldering and wire bonding have been established. 
     For the purpose of securing the connection reliability of these junctions using soldering and wire bonding, plating is undergone as the surface treatment for wiring pads that are mounting portions and terminal portions of circuits on printed wiring boards. The plating includes a technology of carrying out nickel plating, palladium plating and gold plating in order on a circuit pattern formed of a metal having a low electric resistance such as copper. A plated nickel film is to prevent the erosion of a copper circuit by soldering; and a plated palladium film is to prevent the diffusion of nickel constituting the plated nickel film to a plated gold film. Then, the plated gold film is formed in order to provide excellent wetting performance of a solder, realizing a low electric resistance. 
     As conventional technologies of the above-mentioned plating technology, there are, for example, Patent Literature 1 to Patent Literature 3 described below. An electroless gold plating method described in Patent Literature 1 is a method of forming a plated gold film on nickel by using an electroless gold plating solution containing a reducing agent, and involves formation of an immersion plated gold film as a catalyst for electroless gold plating on nickel. 
     Further an electroless gold plating method described in Patent Literature 2 is a method of forming an electroless plated gold film of a plated film laminate in which an electroless plated nickel film is formed on a surface to be plated of an electronic component through a catalyst; an electroless plated palladium film is formed on the electroless plated nickel film; and the electroless plated gold film is further formed on the electroless plated palladium film, and involves the formation of the electroless plated gold film by first electroless gold plating using an electroless gold plating bath containing a water-soluble gold compound, a complexing agent, formaldehyde and/or a formaldehyde bisulfite salt adduct, and a specific amine compound. 
     Further a reductive deposition-type electroless gold plating solution for a palladium film described in Patent Literature 3 is an electroless gold plating solution enabling the direct formation of a plated gold film on the palladium film, and is composed of an aqueous solution containing a water-soluble gold compound, a reducing agent and a complexing agent, wherein at least one compound selected from the group consisting of formaldehyde bisulfites, Rongalite and hydrazines is contained as the reducing agent. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] 
     Japanese Patent Laid-Open No. 05-222541 
     [Patent Literature 2] 
     Japanese Patent Laid-Open No. 2008-266668 
     [Patent Literature 3] 
     Japanese Patent Laid-Open No. 2008-174774 
     SUMMARY OF INVENTION 
     Technical Problem 
     The electroless gold plating method of the Patent Literature 1 has such a problem gold dissolves and corrodes substrate nickel, and nickel thereby diffuses in a plated gold film, however, since the immersion plated gold film is formed by depositing gold by utilizing the difference in redox potential between nickel that is a substrate and gold ions in the plating bath. When nickel diffuses in the plated gold film, there arises a problem that the gold-gold junction strength in wire bonding decreases. In order to prevent such disadvantages, Patent Literature 1 forms an electroless plated gold film on an immersion plated gold film to make the gold film thickness large to thereby suppress a decrease in wire bondability. The technology poses a problem of causing a rise in cost and poor productivity, however, since formation of the immersion plated gold film is essentially needed. 
     Further in the cases of the electroless gold plating method described in Patent Literature 2 described above and of using a reductive deposition-type electroless gold plating solution for a palladium film described in Patent Literature 3 described above, although the corrosion of nickel that is a substrate metal is enabled to be suppressed, since the electroless gold plating bath contains formaldehyde or a formaldehyde bisulfite salt adduct, which is strongly toxic, it becomes difficult for the safety in plating work to be secured. 
     Hence, in markets, there has been raised the requirement for an electroless gold plating solution capable of suppressing the corrosion of a substrate metal and realizing excellent wire bondability and containing no hazardous substances. 
     Solution to Problem 
     As a result of diligent studies in order to solve the above-mentioned problem, the present inventors have arrived at providing an electroless gold plating solution, an electroless gold plating method and a plated product, which are shown below. 
     The reductive electroless gold plating solution according to the present invention is used for the formation of an electroless plated gold film on a surface of a plating target, and contains a water-soluble gold compound, citric acid or a citrate salt, ethylenediaminetetraacetic acid or an ethylenediaminetetraacetate salt, hexamethylenetetramine, and a chain polyamine having an alkyl group having 3 or more carbon atoms and 3 or more amino groups. 
     The reductive electroless gold plating solution according to the present invention has preferably a pH of 7.0 to a pH of 9.0. 
     In the reductive electroless gold plating solution according to the present invention, the chain polyamine is preferably 3,3′-diamino-N-methyldipropylamine or N,N′-bis(3-aminopropyl)ethylenediamine. 
     The reductive electroless gold plating solution according to the present invention preferably further contains a thallium compound as a deposition accelerator. 
     The method of electroless gold plating according to the present invention includes forming an electroless plated gold film on a surface of a plating target using the above-mentioned reductive electroless gold plating solution. 
     In the electroless gold plating method according to the present invention, one of copper, palladium, gold and nickel is preferably present on the surface of the plating target. 
     Further in the electroless gold plating method according to the present invention, the surface of the plating target preferably includes an electroless plated palladium film formed on a surface of an electroless plated nickel film. 
     The plated product according to the present invention is obtained by electroless gold plating by the above-mentioned electroless gold plating method. 
     Advantageous Effects of Invention 
     When the reductive electroless gold plating solution of the present invention contains a water-soluble gold compound, citric acid or a citrate salt, ethylenediaminetetraacetic acid or an ethylenediaminetetraacetate salt, hexamethylenetetramine, and a chain polyamine having an alkyl group having 3 or more carbon atoms and 3 or more amino groups, it becomes easy for a plated gold film to be thickly deposited on a surface of a plating target. 
     Further even in the case where a plated nickel film/plated palladium film/plated gold film installed on an electric connection site is formed by using the reductive electroless gold plating solution of the present invention, the plated gold film can be formed quickly on the surface of the plated palladium film without being influenced by the film thickness of the plated palladium film. Further when the reductive electroless gold plating solution of the present invention is used, even in the case where an electroless plated gold film is formed on a surface of an electroless plated palladium film formed on a surface of an electroless plated nickel film, the dissolution of nickel can greatly be suppressed as compared with the case where an immersion plated gold film is formed, and the diffusion of nickel in the plated gold film is enabled to be prevented. Hence, when the reductive electroless gold plating solution of the present invention is used, a plated gold film capable of realizing high junction reliability of wire bonding can be obtained. 
     Further since the reductive electroless gold plating solution of the present invention has higher stability as a solution as compared with conventional electroless gold plating solutions and contains neither formaldehyde nor formaldehyde bisulfite salt adduct, which is strongly toxic, it becomes easy for the safety in plating work to be secured. 
     Additionally, in the reductive electroless gold plating solution of the present invention, since the deposition reaction of gold occurs only on the surface of gold, palladium, nickel, copper or the like, which can become a catalytic nucleus, and does not occur on portions having no catalytic nucleus, the selective deposition property is excellent. Therefore, the plating solution can avoid the formation of a plated gold film on portions having no need of the deposition of gold, and is beneficial in that the raw material can be saved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a graph showing a relation between the plating time and the plated film thickness of a reductive electroless plated gold film of an Example sample group 1A. 
         FIG. 2  is a graph showing a relation between the plating time and the plated film thickness of a reductive electroless plated gold film of Example 2. 
         FIG. 3  is diagrams showing relations between the film thickness of a substrate plated palladium film and the deposition rate of a plated gold film in the case of using electroless gold plating solutions of Example 1 and Comparative Example 1. 
         FIG. 4  shows electron microscope photographs (×10,000 and ×30,000) of a reductive electroless plated gold film of an Example sample 1A-2. 
         FIG. 5  shows electron microscope photographs (×30,000) of reductive electroless plated gold films of an Example sample 2-2 and Comparative Example 2. 
         FIG. 6  shows an electron microscope photograph (×5,000) of the surface of a plated nickel film after peeling-off of a reductive electroless plated gold film and an electroless plated palladium film from a plated film of an Example sample 1A-2. 
         FIG. 7  shows electron microscope photographs (×3,000) of surfaces of plated nickel films after peeling-off of reductive electroless plated gold films from plated films of an Example sample 2-2 and Comparative Example 2. 
         FIG. 8  is a cross-sectional observation photograph (×30,000) of a reductive electroless plated gold film of an Example sample 1A-6. 
         FIG. 9  shows electron microscope photographs (×500) of an end portion and a central portion of a plated product in which a plated film was formed under the same condition as in an Example sample 1A-6. 
         FIG. 10  is a diagram showing relations of nickel dissolution amounts in gold plating solutions in the case of using electroless gold plating solutions of Example 1 and Comparative Example 1. 
         FIG. 11  is a diagram showing deviations of the film thicknesses of electroless plated gold films of Example 2 and Comparative Example 2. 
         FIG. 12  is a diagram showing the wire bonding performance of electroless plated gold films of Example 2 and Comparative Example 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the reductive electroless gold plating solution, the electroless gold plating method using the plating solution, and the plated product plated by the method, respectively, according to the present invention will be described. 
     1. The Reductive Electroless Gold Plating Solution According to the Present Invention 
     The reductive electroless gold plating solution according to the present invention is used for the formation of an electroless plated gold film on a surface of a plating target, and contains “a water-soluble gold compound”, “citric acid or a citrate salt”, “ethylenediaminetetraacetic acid or an ethylenediaminetetraacetate salt”, “hexamethylenetetramine”, and “a chain polyamine having an alkyl group having 3 or more carbon atoms and 3 or more amino groups”. Hereinafter, the respective components will be described. 
     (1) A Water-Soluble Gold Compound 
     As a water-soluble gold compound used for the reductive electroless gold plating solution according to the present invention, any water-soluble gold compound of cyanide-based gold salts and non-cyanide-based gold salts as long as the compound is soluble in the plating solution and can provide a specific concentration can be used. As specific water-soluble gold compounds of cyanide-based gold salts, potassium gold cyanide, sodium gold cyanide and ammonium gold cyanide can be exemplified. Further as specific water-soluble gold compounds of non-cyanide-based gold salts, chloroaurate salts, gold sulfite salts and gold thiosulfate salts can be exemplified. Among these, potassium gold cyanide is especially preferable. Further the water-soluble gold compounds may be used singly or in a combination of two or more. Here, the water-soluble gold compounds are not limited to the gold compounds exemplified here. 
     The concentration of a water-soluble gold compound in the reductive electroless gold plating solution according to the present invention is preferably 0.0025 mol/L to 0.0075 mol/L. This is because when the concentration of a water-soluble gold compound is lower than 0.0025 mol/L, the deposition rate of a plated gold film is slow and a desired-thickness plated gold film is hardly obtained. This is because when the concentration of a water-soluble gold compound is higher than 0.0075 mol/L, there arises a risk that the stability of the plating solution decreases, and the high concentration is an economical drawback. 
     (2) Citric Acid or a Citrate Salt 
     The reductive electroless gold plating solution according to the present invention contains citric acid or a citrate salt. These citric acid and citrate salt are used as a complexing agent capable of forming a complex with gold ions. The concentration of citric acid or a citrate salt in the reductive electroless gold plating solution according to the present invention is preferably 0.05 mol/L to 0.15 mol/L. This is because when the concentration of citric acid or a citrate salt used as a complexing agent is lower than 0.05 mol/L, gold deposits in the plating solution and the solution stability is inferior; and this is because when being more than 0.15 mol/L, the complex formation excessively progresses and the deposition rate of gold decreases, and a desired-thickness plated gold film is hardly obtained. 
     (3) Ethylenediaminetetraacetic Acid (EDTA) or an Ethylenediaminetetraacetate Salt 
     The reductive electroless gold plating solution according to the present invention contains ethylenediaminetetraacetic acid (EDTA) or an ethylenediaminetetraacetate salt. These ethylenediaminetetraacetic acid and ethylenediaminetetraacetate salt are complexing agents used by in combination with the above-mentioned citric acid or citrate salt. The concentration of ethylenediaminetetraacetic acid or an ethylenediaminetetraacetate salt in the reductive electroless gold plating solution according to the present invention is preferably 0.03 mol/L to 0.1 mol/L. This is because when the concentration of ethylenediaminetetraacetic acid or an ethylenediaminetetraacetate salt used as a complexing agent is lower than 0.03 mol/L, gold deposits in the plating solution and the solution stability is inferior; and this is because when the concentration is more than 0.1 mol/L, the complex formation excessively progresses and the deposition rate of gold decreases, and a desired-thickness plated gold film is hardly obtained. 
     (4) Hexamethylenetetramine 
     The reductive electroless gold plating solution according to the present invention contains hexamethylenetetramine. The hexamethylenetetramine is used as a reducing agent which reduces gold ions in the plating solution and causes gold to deposit on a surface of a plating target. 
     The concentration of hexamethylenetetramine in the reductive electroless gold plating solution according to the present invention is preferably 0.003 mol/L to 0.009 mol/L. This is because when the concentration of hexamethylenetetramine is lower than 0.003 mol/L, the deposition rate of a plated gold film is slow and a desired-thickness plated gold film is hardly obtained; and this is because when the concentration of hexamethylenetetramine is higher than 0.009 mol/L, the reduction reaction rapidly progresses and the gold salt in the plating solution may abnormally deposit, and the solution stability is inferior and the high concentration is an economical drawback. 
     (5) A Chain Polyamine 
     Further the reductive electroless gold plating solution according to the present invention contains a chain polyamine having an alkyl group having 3 or more carbon atoms and 3 or more amino groups. The chain polyamine is an amine compound which acts as a reduction auxiliary agent assisting the reduction of gold ions in the plating solution. As the chain polyamine, specifically, there can be used 3,3′-diamino-N-methyldipropylamine, N,N′-bis(3-aminopropyl)ethylenediamine and the like. This is because these are especially preferable from the viewpoint of the performance of an obtained plated film, and the economic efficiency. 
     The concentration of the chain polyamine in the reductive electroless gold plating solution according to the present invention is preferably 0.02 mol/L to 0.06 mol/L. By making the concentration of a chain polyamine in the range of 0.02 mol/L to 0.06 mol/L, a high deposition rate can be maintained without affecting the substrate metal film thickness. Further the throwing power of a plated gold film can be improved and the plated gold film can have a large thickness of 0.2 lam or larger. Further the solution stability can be greatly enhanced. 
     (6) Other Components 
     The reductive electroless gold plating solution according to the present invention, in addition to a water-soluble gold compound, citric acid or a citrate salt, ethylenediaminetetraacetic acid or an ethylenediaminetetraacetate salt, hexamethylenetetramine, and a chain polyamine having an alkyl group having 3 or more carbon atoms and 3 or more amino groups, as described above, may contain a deposition accelerator. The deposition accelerator used here includes thallium compounds and lead compounds. It is preferable to use a thallium compound from the viewpoint of making a thick plated gold film. 
     The concentration of a thallium compound as the deposition accelerator in the reductive electroless gold plating solution according to the present invention is preferably 1 mg/L to 10 mg/L. When the concentration of a thallium compound as the deposition accelerator is lower than 1 mg/L, it becomes difficult to make a plated gold film thick. Further when the concentration of a thallium compound as the deposition accelerator is higher than 10 mg/L, making the thickness larger than that cannot be accomplished and the high concentration would be an economical drawback. 
     The reductive electroless gold plating solution according to the present invention, in addition to the above-mentioned essential components, may contain additives such as a pH regulator, an antioxidant, a surfactant and a brightening agent. 
     The pH regulator is not especially limited, but includes potassium hydroxide, sodium hydroxide, an ammonia water solution, sulfuric acid and phosphoric acid. In the reductive electroless gold plating solution according to the present invention, by using a pH regulator, the pH is preferably maintained at 7.0 to 9.0. This is because when the pH of the reductive electroless gold plating solution is lower than 7.0, it becomes easy for the plating solution to be decomposed; and when the pH is higher than 9.0, the plating solution becomes too stable and the plating deposition rate becomes slow, and it needs to take much time for a plated gold film to be made thick. Further by regulating the pH condition at 7.0 to 9.0, even a plating target constituted from a material weak to alkali can be plated. Further as the additives such as an antioxidant, a surfactant and a brightening agent, known ones can be used. 
     (7) The Plating Condition 
     The gold plating condition using the reductive electroless gold plating solution according to the present invention is not especially limited, but the solution temperature is preferably 40° C. to 90° C., and especially preferably 75° C. to 85° C. The plating time also is neither especially limited, but 1 min to 2 hours is preferable, and 2 min to 1 hour is especially preferable. 
     Since the reductive electroless gold plating solution according to the present invention comprises, as essential components, a water-soluble gold compound, citric acid or a citrate salt, ethylenediaminetetraacetic acid or an ethylenediaminetetraacetate salt, hexamethylenetetramine, and a chain polyamine having an alkyl group having 3 or more carbon atoms and 3 or more amino groups as described above, it becomes easy for a plated gold film to be thickly deposited on a surface of a plating target by the electroless plating method. 
     Further even in the case where a plated nickel film/plated palladium film/plated gold film installed on an electric connection site is formed, the plated gold film can be formed quickly on the surface of the plated palladium film without being influenced by the film thickness of the plated palladium film by using the reductive electroless gold plating solution according to the present invention. Further even in the case where an electroless plated gold film is formed on a surface of an electroless plated palladium film formed on a surface of an electroless plated nickel film, by using the reductive electroless gold plating solution of the present invention, the dissolution of nickel can greatly be suppressed as compared with the case where an immersion plated gold film is formed, and the diffusion of nickel in the plated gold film can be prevented. Therefore, when the reductive electroless gold plating solution of the present invention is used, a plated gold film capable of realizing high junction reliability of wire bonding can be provided. 
     Further the reductive electroless gold plating solution of the present invention is high in the solution stability as compared with conventional electroless gold plating solutions. For example, in the case where continuous plating is carried out while a plating solution is being replenished, the metal turnover (MTO, in which the case where gold in a plating solution in making-up of an electrolytic bath is all deposited is taken as 1 turn) is used as an index of outwearing of the plating solution. Whereas MTO is 2.0 to 3.0 turns in the case of conventional reductive electroless gold plating solutions, an MTO of 5.0 turns or more is enabled to be realized in the reductive electroless gold plating solution of the present invention. 
     Further since the reductive electroless gold plating solution of the present invention contains neither formaldehyde nor a formaldehyde bisulfite salt adduct, which is strongly toxic and which is contained in conventional reductive electroless gold plating solutions, it becomes easy for the safety in plating work to be secured. 
     Additionally, with the reductive electroless gold plating solution of the present invention, since the deposition reaction of gold occurs only on the surface of gold, palladium, nickel, copper or the like, which can become a catalytic nucleus, and does not occur on portions having no catalytic nucleus, the selective deposition property is excellent. Therefore, the plating solution can avoid the formation of a plated gold film on portions having no need of the deposition of gold, and is beneficial in that the raw material can be saved. 
     2. The Electroless Gold Plating Method 
     Then, the electroless gold plating method according to the present invention will be described. The electroless gold plating method according to the present invention uses one of the reductive electroless gold plating solutions described above and carries out electroless gold plating on a surface of a plating target to thereby form a plated gold film. The electroless gold plating method carries out plating by a method of immersing a plating target in an electroless gold plating solution as in usual reductive electroless plating methods. 
     In the electroless gold plating method according to the present invention, it is preferable that one of copper, palladium, gold and nickel is present on a surface of a plating target, which is an object of the treatment. The presence form thereof may be any one as long as one of copper, palladium, gold and nickel is present on a surface of a plating target. It is more preferable to use particularly a plating target itself constituted from copper or a plating target having any film composed of copper, palladium, gold, nickel or an alloy containing these metals on the surface of the plating target. The alloy containing these metals may include, for example, gold cobalt. Gold, palladium, nickel, copper or an alloy containing these metals becomes a substrate metal for electroless gold plating in the present invention, and exhibits a catalytically active effect to hexamethylenetetramine as a reducing agent contained in the above-mentioned reductive electroless gold plating solution. It is preferable that as the film to be formed on a surface of a plating target, particularly an electroless plated palladium film, an immersion plated gold film or a plated copper film is used. For example, in the case where mounting portions and terminal portions of circuits of printed wiring boards have undergone electroless nickel plating on their surfaces, it is preferable that an electroless plated palladium film is formed on the surface of the electroless plated nickel film. This is because when the plated nickel film has a plated palladium film formed on its surface, it is especially effective in that the plated nickel film is prevented from being diffused in the plated gold film. 
     3. The Plated Product 
     Then, the plated product according to the present invention will be described. The plated product according to the present invention is characterized in that a surface of a plating target undergoes electroless gold plating by using one of the above-mentioned electroless gold plating solutions and by the above-mentioned electroless gold plating method. It is preferable to make a surface of a plating target undergo electroless gold plating particularly by using the reductive electroless gold plating solution having a pH of 7.0 to 9.0. Further, the presence form thereof may be any one as long as one of copper, palladium, gold and nickel is present on a surface of a plating target. It is more preferable to use particularly a plating target itself constituted from copper or a plating target having any film composed of copper, palladium, gold, nickel or an alloy containing these metals on the surface of the plating target. It is preferable that as the film to be formed on a surface of a plating target, particularly an electroless plated palladium film, an immersion plated gold film or a plated copper film is used. It is preferable that a plating target including an electroless plated palladium film on its surface is particularly one having an electroless plated nickel film as an underlayer of the electroless plated palladium film formed on its surface. This is because the plating using the above-mentioned reductive electroless gold plating solution can especially suitably be used for the formation of plated films of electric connection sites. 
     The embodiments according to the present invention described hitherto are one aspect of the present invention, and it is natural that changes and modifications may suitably be made without departing from the gist of the present invention. 
     The present invention will be described more specifically hereinafter by referring to Example 1 and Example 2 of plated gold films fabricated by using the reductive electroless gold plating solution of the present invention, Comparative Example 1 of a plated gold film fabricated by using an immersion electroless gold plating solution, and Comparative Example 2 of a plated gold film fabricated by using a conventional reductive electroless plating solution. Note that, it should be mentioned by way of caution that the present invention is not limited to Examples described in the below. 
     Example 1 
     In Example 1, by using a reductive electroless gold plating solution to which the present invention was applied and using a copper plate as a substrate, plated films composed of an electroless plated nickel film/electroless plated palladium film/electroless plated gold film were formed on the substrate. 
     Preparation of the reductive electroless gold plating solution: The composition of the reductive electroless gold plating solution used in the present Example is shown in the below. The plating condition (pH, solution temperature) is shown together with the composition. 
     Potassium gold cyanide: 5 mmol/L 
     Dipotassium ethylenediaminetetraacetate: 0.03 mol/L 
     Citric acid: 0.15 mol/L 
     Hexamethylenetetramine: 3 mmol/L 
     3,3′-diamino-N-methyldipropylamine: 0.02 mol/L 
     Thallium acetate: 5 mg/L 
     pH: 8.5 
     Solution temperature: 80° C. 
     Fabrication of plated films: Samples with a plated film as Example 1 were composed of an Example sample group 1A to an Example sample group 1D. These Example sample group 1A to Example sample group 1D were divided according to differences in electroless plated palladium film thickness. 
     The Example sample group 1A was composed of an Example sample 1A-1 to an Example sample 1A-6; and the each Example sample was made by forming an electroless plated nickel film of 5 μm in film thickness on the surface of the copper plate, and thereafter forming an electroless plated palladium film of 0.1 μm in film thickness on the surface of the electroless plated nickel film. Thereafter, a reductive electroless plated gold film was formed on the surface of the electroless plated palladium film by using the above-mentioned reductive electroless gold plating solution according to the condition of a corresponding plating time. Specifically, in the Example sample 1A-1 to the Example sample 1A-6, the plating time conditions in the reductive electroless plated gold film formation were made to be 10 min, 20 min, 30 min, 40 min, 50 min and 60 min, respectively, to thereby obtain samples with a plated gold film. 
     The Example sample group 1B was composed of an Example sample 1B-1 to an Example sample 1B-6, and fabricated as in the Example sample group 1A, except that the film thickness of the electroless plated palladium film was 0.2 μm. Here, in each of the Example sample 1B-1 to the Example sample 1B-6, the plating time conditions in the reductive electroless plated gold film formation were made different, as in the Example sample 1A-1 to the Example sample 1A-6. 
     The Example sample group 1C was composed of an Example sample 1C-1 to an Example sample 1C-6, and fabricated as in the Example sample group 1A, except that the film thickness of the electroless plated palladium film was 0.4 μm. Here, in each of the Example sample 1C-1 to the Example sample 1C-6, the plating time conditions in the reductive electroless plated gold film formation were made different, as in the Example sample 1A-1 to the Example sample 1A-6. 
     The Example sample group 1D was composed of an Example sample 1D-1 to an Example sample 1D-6, and fabricated as in the Example sample group 1A, except that the film thickness of the electroless plated palladium film was 0.6 μm. Here, in each of the Example sample 1D-1 to the Example sample 1D-6, the plating time conditions in the reductive electroless plated gold film formation were made different, as in the Example sample 1A-1 to the Example sample 1A-6. 
     Example 2 
     In Example 2, by using the reductive electroless gold plating solution as in Example 1 and using a copper plate as a substrate, plated films composed of an electroless plated nickel film/immersion electroless plated gold film/reductive electroless plated gold film were formed on the substrate. Samples with a plated film as Example 2 were composed of an Example sample 2-1 to an Example sample 2-6. The Example sample 2-1 to the Example sample 2-6 were each made by forming an electroless plated nickel film of 5 μm in film thickness on the surface of the copper plate, and thereafter forming an immersion electroless plated gold film of 0.07 μm in film thickness on the surface of the electroless plated nickel film. Thereafter, a reductive electroless plated gold film was formed on the surface of the immersion electroless plated gold film by using the above-mentioned reductive electroless gold plating solution according to the condition of a corresponding plating time. Here, in each of the Example sample 2-1 to the Example sample 2-6, the plating time conditions in the reductive electroless plated gold film formation were made different, as in the Example sample 1A-1 to the Example sample 1A-6. 
     COMPARATIVE EXAMPLES 
     Comparative Example 1 
     In Comparative Example 1, by using an immersion electroless gold plating solution and using a copper plate as a substrate as in Example 1, plated films composed of an electroless plated nickel film/electroless plated palladium film/electroless plated gold film were fabricated on the substrate. 
     Preparation of the immersion electroless gold plating solution: The composition of the immersion electroless gold plating solution used in Comparative Example 1 is shown in the below. The plating condition (pH, solution temperature) is shown together with the composition. 
     Potassium gold cyanide: 10 mmol/L 
     Ethylenediaminetetraacetic acid: 0.03 mol/L 
     Citric acid: 0.15 mol/L 
     Thallium acetate: 50 mg/L 
     pH: 4.5 
     Solution temperature: 80° C. 
     Fabrication of plated films: Samples with a plated film as Comparative Example 1 are composed of a comparative sample group 1A to a comparative sample group 1D. These comparative sample group 1A to comparative sample group 1D are divided according to differences in electroless plated palladium film thickness. 
     The comparative sample group 1A was composed of a comparative sample 1A-1 to a comparative sample 1A-6; and the each comparative sample was made by forming an electroless plated nickel film of 5 μm in film thickness on the surface of the copper plate, and thereafter forming an electroless plated palladium film of 0.1 μm in film thickness on the surface of the electroless plated nickel film. Thereafter, an immersion electroless plated gold film was formed on the surface of the electroless plated palladium film by using the above-mentioned immersion electroless gold plating solution according to the condition of a corresponding plating time. Specifically, in the comparative sample 1A-1 to the comparative sample 1A-6, the plating time conditions in the immersion electroless plated gold film formation were made to be 10 min, 20 min, 30 min, 40 min, 50 min and 60 min, respectively, to thereby obtain samples with a plated gold film. 
     The comparative sample group 1B was composed of a comparative sample 1B-1 to a comparative sample 1B-6, and fabricated as in the comparative sample group 1A, except that the film thickness of the electroless plated palladium film was 0.2 μm. Here, in each of the comparative sample 1B-1 to the comparative sample 1B-6, the plating time conditions in the immersion electroless plated gold film formation were made different, as in the comparative sample 1A-1 to the comparative sample 1A-6. 
     The comparative sample group 1C was composed of a comparative sample 1C-1 to a comparative sample 1C-6, and fabricated as in the comparative sample group 1A, except that the film thickness of the electroless plated palladium film was 0.4 μm. Here, in each of the comparative sample 1C-1 to the comparative sample 1C-6, the plating time conditions in the immersion electroless plated gold film formation were made different, as in the comparative sample 1A-1 to the comparative sample 1A-6. 
     The comparative sample group 1D was composed of a comparative sample 1D-1 to a comparative sample 1D-6, and fabricated as in the comparative sample group 1A, except that the film thickness of the electroless plated palladium film was 0.6 μm. Here, in each of the comparative sample 1D-1 to the comparative sample 1D-6, the plating time conditions in the immersion electroless plated gold film formation were made different, as in the comparative sample 1A-1 to the comparative sample 1A-6. 
     Comparative Example 2 
     In Comparative Example 2, by using a conventional reductive electroless gold plating solution and using a copper plate as a substrate as in Example 2, plated films composed of an electroless plated nickel film/immersion electroless plated gold film/conventional reductive electroless plated gold film were formed on the substrate. 
     Preparation of the conventional reductive electroless gold plating solution: The composition of the reductive electroless gold plating solution used in Comparative Example 2 is shown in the below. The plating condition (pH, solution temperature) is shown together with the composition. 
     Potassium gold cyanide: 0.015 mol/L 
     Potassium cyanide: 0.03 mol/L 
     Sodium hydroxide: 0.8 mol/L 
     Dimethylamine borane: 0.2 mol/L 
     Lead compound: 5 mg/L (in terms of lead) 
     pH: 13 
     Solution temperature: 70° C. 
     Fabrication of a plated film: In Comparative Example 2, an electroless plated nickel film of 5 μm in film thickness was formed on the surface of a copper plate, and thereafter, an immersion electroless plated gold film of 0.05 μm in film thickness was formed on the surface of the electroless plated nickel film. Thereafter, by using the above-mentioned reductive electroless gold plating solution, a reductive electroless plated gold film of 0.20 μm in film thickness was formed on the surface of the immersion electroless plated gold film. 
     [Evaluations] 
     Then, the plated gold films in Example 1 and Example 2 fabricated by using the reductive electroless gold plating solution of the present invention were evaluated for the deposition rate, the surface form and the like. Hereinafter, these evaluations will be described specifically, if required, by comparing Example 1 and Example 2 with Comparative Example 1 of the plated gold films fabricated by using the immersion electroless gold plating solution and Comparative Example 2 of the plated gold film fabricated by using the conventional reductive electroless plating solution. 
     Deposition rate: There is shown in  FIG. 1  a relation between the plating time and the plated film thickness of the plated gold films of the Example sample group 1A (the Example sample 1A-1 to the Example sample 1A-6) in Example 1 using the reductive electroless gold plating solution according to the present invention. There is similarly shown in  FIG. 2  a relation between the plating time and the plated film thickness of the plated gold films of Example 2 (the Example sample 2-1 to the Example sample 2-6) using the reductive electroless gold plating solution according to the present invention. Here, in  FIG. 2 , there is shown an electron microscope photograph (×10,000) of the plated gold film of the Example sample 2-2 obtained by making the plating time to be 20 min. 
     It is confirmed from  FIG. 1  that the plated gold film formed on the surface of the electroless plated palladium film by using the above-mentioned reductive electroless gold plating solution was formed stably at a rate of 0.15 μm/30 min without being influenced by the thickness of the plated gold film formed. 
     It is confirmed from  FIG. 2  that the reductive plated gold film formed on the surface of the immersion electroless plated gold film by using the above-mentioned reductive electroless gold plating solution was formed stably at a rate of 0.17 μm/30 min without being influenced by the thickness of the plated gold film formed. 
     Influence of the thickness of the electroless plated palladium film on the deposition rate of the plated gold film: Then, comparing Example 1 with Comparative Example 1, there will be described the influence of the thickness of the electroless plated palladium film on the deposition rate of the plated gold film.  FIG. 3  shows a relation between the film thickness of the electroless plated palladium film and the deposition rate of the plated gold film in the Example sample group 1A (the Example sample 1A-1 to the Example sample 1A-6) to the Example sample group 1D (the Example sample 1D-1 to the Example sample 1D-6) each in which the plated gold film was formed on the surface of the electroless plated palladium film by using the reductive electroless gold plating solution.  FIG. 3  also shows, together with the relation, a relation between the film thickness of the electroless plated palladium film and the deposition rate of the plated gold film in the comparative sample group 1A (the comparative sample 1A-1 to the comparative sample 1A-6) to the comparative sample group 1D (the comparative sample 1D-1 to the comparative sample 1D-6) each in which the plated gold film was formed on the surface of the electroless plated palladium film by using the immersion electroless gold plating solution. 
     It is found from  FIG. 3  that in the plated gold films formed by using the immersion electroless gold plating solution of the comparative sample group 1A to the comparative sample group 1D, the thicker the plated palladium film being the substrate metal, the more the deposition rate of the plated gold film decreased. By contrast, it can be confirmed that the plated gold films formed by using the reductive electroless gold plating solution of the Example sample group 1A to the Example sample group 1D were formed at a stable rate irrespective of the thickness of the plated palladium film being the substrate metal. 
     Surface form of the plated gold film: Then, the surface form of the plated gold film formed on the surface of the electroless plated palladium film by using the reductive electroless gold plating solution of the present invention was observed.  FIG. 4  shows electron microscope photographs (×10,000 and ×30,000) of the plated gold film surface of the Example sample 1A-2 in which the reductive electroless plated gold film was formed in a film thickness of 0.1 μm out of Example 1. The surface form of the reductive electroless plated gold film formed on the surface of the immersion electroless plated gold film by using the reductive electroless gold plating solution of the present invention was further observed.  FIG. 5  shows an electron microscope photograph (×30,000) of the plated gold film surface of the Example sample 2-2 in which the reductive electroless plated gold film was formed in a film thickness of 0.13 μm out of Example 2. As a comparison, there was observed the surface form of the reductive electroless plated gold film formed on the surface of the immersion electroless plated gold film by using the conventional reductive electroless gold plating solution.  FIG. 5  shows an electron microscope photograph (×30,000) of the plated gold film surface of Comparative Example 2 in which the reductive electroless plated gold film was formed in a film thickness of 0.13 μm. 
     It can be confirmed from  FIG. 4  and  FIG. 5  that the electroless plated gold film was densely formed not only by using the reductive electroless gold plating solution of the present invention, but by using the conventional reductive electroless gold plating solution. 
     Surface form after peeling-off of the electroless plated gold film: Further there were observed the surface forms of the plated nickel films after the electroless plated gold film or the electroless plated gold film and the electroless plated palladium film were peeled off from the each plated film shown in  FIG. 4  and  FIG. 5 .  FIG. 6  shows an electron microscope photograph (×5,000) of the plated nickel film surface after the electroless plated gold film and the electroless plated palladium film were peeled off from the state of  FIG. 4 .  FIG. 7  shows electron microscope photographs (×3,000) of the plated nickel film surfaces after the electroless plated gold film was peeled off from the state of  FIG. 5 . 
     As apparent from  FIG. 6  and  FIG. 7 , in any of Examples and Comparative Examples formed by using the reductive electroless gold plating solutions, no local corrosion of the plated nickel film was observed. 
     Cross-sectional form of the plated film: Then, the cross-section of the plated film having the layer structure of the electroless plated nickel film/electroless plated palladium film/electroless plated gold film of Example 1, in which the plated gold film was formed on the surface of the electroless plated palladium film by using the reductive electroless gold plating solution of the present invention was observed.  FIG. 8  shows a cross-sectional photograph (×30,000) of the plated film of the Example sample 1A-6, in which the reductive electroless plated gold film was formed in a film thickness of 0.3 μm. It can be confirmed from  FIG. 8  that the electroless plated gold film formed by using the above-mentioned reductive electroless gold plating solution was formed uniformly on the surface of the plated palladium film.
 
Selective deposition property of the plated gold film: Then, there are shown in  FIG. 9  electron microscope photographs (×500) of an end portion and a central portion of a plated product whose plated film was formed under the same condition as in the Example sample 1A-6 out of Example 1, in which the plated gold film was formed on the surface of the electroless plated palladium film by using the reductive electroless gold plating solution of the present invention. It can be confirmed from  FIG. 9  that the electroless plated gold films were formed uniformly similarly in the end portion and the central portion of the plated product. Hence, it can be said also from the photograph of  FIG. 9  that the reductive electroless gold plating solution of the present invention was excellent in the selective deposition property of the electroless plated gold film.
 
Influence of the nickel dissolution in the gold plating solution: Then, with respect to Example 1, in which the plated gold film was formed on the surface of the electroless plated palladium film by using the reductive electroless gold plating solution of the present invention, there was investigated the influence of the dissolution of the electroless nickel into the reductive electroless gold plating solution. Specifically, there was examined the dissolution amount of the substrate nickel into the electroless gold plating solution in the case where 1 g of gold was deposited on the surface of the electroless plated palladium film, by using ICP. As a comparison, also Comparative Example 1 using the immersion electroless gold plating solution was examined as in Example 1.  FIG. 10  shows a dissolution amount of the electroless nickel of Example 1 using the reductive electroless gold plating solution and a dissolution amount of the substrate nickel of Comparative Example 1 using the immersion electroless gold plating solution.  FIG. 10  indicates, for the either case, a value when the dissolution amount of Ni into the gold plating solution in the case where 1 g of gold was deposited was examined by using ICP.
 
     From  FIG. 10 , in Comparative Example 1 in which 1 g of the plated gold film was deposited by using the immersion electroless gold plating solution, Ni used as the substrate metal dissolved out in 162 ppm into the immersion electroless gold plating solution. By contrast, in Example 1 in which 1 g of the plated gold film was deposited by using the reductive electroless gold plating solution of present application, Ni used as the substrate metal dissolved out in 0.2 ppm only into the reductive electroless gold plating solution. 
     From the results of the evaluation tests, it can be said that the reductive electroless gold plating solution according to the present application could greatly suppress the dissolution of the substrate nickel through the plated palladium film as compared with the case of forming the immersion plated gold film, and nickel was enabled to be prevented from diffusing into the plated gold film. 
     Deviation in film thickness of the plated gold film: Then, there was investigated the deviation in film thickness of the plated gold film formed by using the reductive electroless gold plating solution on the surface of the immersion electroless plated gold film. Here, there was examined the film thickness of the reductive electroless plated gold film of the Example sample 2-2 of Example 2 as an example using the reductive electroless gold plating solution according to the present invention. As a comparison, there was examined the film thickness of the reductive electroless plated gold film of Comparative Example 2 using the conventional reductive electroless gold plating solution. For the each case, the results by the examination of the film thickness of 20 points are collectively shown in Table 1. Further  FIG. 11  shows the deviation states. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Example 2 
                   
               
               
                   
                 (Example 
                 Comparative 
               
               
                   
                 sample 2-2) 
                 Example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Plating Time 
                 20 min 
                 1.5 min 
               
            
           
           
               
               
               
               
            
               
                 Film 
                 Average Value (μm) 
                 0.199 
                 0.206 
               
               
                 Thickness 
                 Maximum Value (μm) 
                 0.204 
                 0.218 
               
               
                   
                 Minimum Value (μm) 
                 0.194 
                 0.182 
               
               
                   
                 Maximum − Minimum 
                 0.01  
                 0.036 
               
               
                   
                 (μm) 
               
               
                   
                 Standard Deviation 
                 0.004 
                 0.013 
               
               
                   
               
            
           
         
       
     
     The average value of the film thickness of the electroless plated gold film of the Example sample 2-2 using the reductive electroless gold plating solution according to the present invention was 0.199 μm; the difference between the maximum value and the minimum value was 0.01 μm; and the standard deviation was pretty much as low as 0.004. By contrast, the average value of the film thickness of the electroless plated gold film of Comparative Example 2 using the conventional reductive electroless gold plating solution was 0.206 μm; the difference between the maximum value and the minimum value was 0.036 μm; and the standard deviation was 0.013. Hence, it is found that by using the reductive electroless gold plating solution according to the present invention, as compared with the case where the conventional reductive electroless gold plating solution was used, there was provided the electroless plated gold film having a low deviation in a considerably high level, that is, being uniform, across the entire region. From the results, by using the reductive electroless gold plating solution according to the present invention, the entire of the surface of the plating target was enabled to undergo plating more uniformly and the quality could be improved. Further since the electroless plated gold film could be formed in a required thickness, the formation of the electroless plated gold film exceeding the required thickness was suppressed and an excess burden of gold was enabled to be greatly reduced. 
     Wire bonding performance of the plated gold film: Then, there was investigated the wire bonding performance of the plated gold film formed by using the reductive electroless gold plating solution according to the present invention. There was examined the strength of wire bonding of the reductive electroless plated gold film of the Example sample 2-2 of Example 2 as an example using the reductive electroless gold plating solution according to the present invention. As a comparison, there was examined the strength of wire bonding of the reductive electroless plated gold film of Comparative Example 2 using the conventional reductive electroless gold plating solution. Specifically, a gold wire of 25 μm in wire diameter was joined to the reductive electroless plated film of the Example sample 2-2 and Comparative Example 2 each by using a wire bonding apparatus; the wire was pulled by a pull tester and the strength of the wire bonding was examined. For the each case, 20 points were examined and the maximum value, the minimum value and the average value of the wire bonding strength were determined. The examination results are shown in  FIG. 12 . 
     The maximum value of the wire bonding strength of the electroless plated gold film of Example 2 (the Example sample 2-2) using the reductive electroless plating solution according to the present invention was 6.0 gf; the minimum value thereof was 4.8 gf; and the average value thereof was 5.3 gf. Then, the maximum value of the wire bonding strength of the electroless plated gold film of Comparative Example 2 using the conventional reductive electroless plating solution was 6.0 gf; the minimum value thereof was 4.8 gf; and the average value thereof was 5.3 gf. From these results, it is found that the electroless plated gold film obtained by using the reductive electroless plating solution according to the present invention provided an excellent wire bonding strength, almost the same as the case using the conventional reductive electroless plating solution. Hence, it can be said that the reductive electroless gold plating solution of the present invention enables to provide a plated gold film capable of realizing the high junction reliability of wire bonding. 
     INDUSTRIAL APPLICABILITY 
     The reductive electroless gold plating solution of the present invention greatly suppresses the dissolution of the substrate metal such as nickel and palladium, and enables the plated gold film to be deposited at a high deposition rate in a thick deposition on the surface of the substrate metal. Hence, the present invention enables the plated gold film high in the wire bonding junction reliability to be provided.