Abstract:
Proposed are an iridium plating solution capable of easily forming an iridium plating coat in which the occurrence of cracks is suppressed as much as possible and an iridium plating method. In the present invention, the iridium plating solution uses an iridium compound obtained by adding, to an iridium (III) complex salt containing a halogen as an anionic component, one or more compounds selected from the following group and by stirring the resulting mixture, the group consisting of a saturated monocarboxylic acid, a saturated monocarboxylic acid salt, a saturated dicarboxylic acid, a saturated dicarboxylic acid salt, a saturated hydroxycarboxylic acid, a saturated hydroxycarboxylic acid salt, an amide and urea, wherein the iridium plating solution includes at least one or more of Fe, Co, Ni and Cu.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an iridium plating solution and a method of plating using the same. 
         [0003]    2. Description of the Related Art 
         [0004]    Iridium is a metal having high hardness and additionally exhibiting excellent corrosion resistance to high-concentration acids, aqua regia and halogens. Accordingly, the application fields of iridium include hardening agents for specific metals and catalysts as well as ornaments, and further iridium is industrially widely used as anticorrosives and materials such as electric contact materials. 
         [0005]    As an iridium plating solution in such applications of iridium, known is an iridium plating solution using an iridium compound obtained by adding, to an iridium (III) complex salt containing a halogen as an anionic component, one or more compounds selected from the following group and by stirring the resulting mixture, the group consisting of a saturated monocarboxylic acid, a saturated monocarboxylic acid salt, a saturated dicarboxylic acid, a saturated dicarboxylic acid salt, a saturated hydroxycarboxylic acid, a saturated hydroxycarboxylic acid salt, an amide and urea (see Japanese Patent Application Laid-Open No. 6-316786). 
         [0006]    The above-described iridium plating solution is effectively used as a practical iridium plating solution being stable and hardly decomposable and having a high current efficiency and a high plating rate. 
         [0007]    Even such an excellent iridium plating solution, however, is pointed out as follows with respect to the plating properties and conditions thereof. For example, when the iridium plating solution is used for electronic parts for use in electric connection such as lead pins (see Japanese Patent Application Laid-Open No. Hei 7-21867), cracks occur in the iridium plating coat to cause a phenomenon in which electric properties are not sufficiently satisfied as the case may be. In the electronic parts such as the lead pins, usually iridium plating coats are applied to rhodium substrates; however, as the price of rare metals such as rhodium is escalated, a countermeasure is investigated in which the amount of rhodium used in the substrate is reduced and plating processing is performed so as for the iridium plating coat to be thick. When such thick iridium plating coats are formed, conventional iridium plating solutions lead to particularly remarkable occurrence of cracks and electric properties are not sufficiently satisfied as the case may be. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention has been achieved under the above-described circumstances, and an object of the present invention is to propose an iridium plating solution capable of easily forming an iridium plating coat in which the occurrence of cracks is suppressed as much as possible and an iridium plating method. 
         [0009]    In the present invention, the iridium plating solution uses an iridium compound obtained by adding, to an iridium (III) complex salt containing a halogen as an anionic component, one or more compounds selected from the following group and by stirring the resulting mixture, the group consisting of a saturated monocarboxylic acid, a saturated monocarboxylic acid salt, a saturated dicarboxylic acid, a saturated dicarboxylic acid salt, a saturated hydroxycarboxylic acid, a saturated hydroxycarboxylic acid salt, an amide and urea, wherein the iridium plating solution includes at least one or more of Fe, Co, Ni and Cu. The presence of at least any metal of Fe, Co, Ni and Cu in the plating solution effectively suppresses the occurrence of cracks in the iridium plating coat. 
         [0010]    In the iridium plating solution of the present invention, the content of at least one or more of Fe, Co, Ni and Cu is preferably 0.01 g/L to 10 g/L. When the concerned content is less than 0.01 g/L, cracks tend to occur, and when the concerned content exceeds 10 g/L, crystal growth is unstable. 
         [0011]    Any metal of Fe, Co, Ni and Cu is preferably contained as a soluble metal salt in the plating solution. 
         [0012]    In the iridium plating solution in the present invention, iridium is preferably contained in a content of 1 to 200 g/L, and more preferably in a content of 10 to 20 g/L in terms of the metal iridium concentration. When the iridium concentration is less than 1 g/L, the upper limit of the current density is small to make the iridium plating solution hardly practically usable, and when the iridium concentration is larger than 200 g/L, the iridium plating solution is saturated to make iridium insoluble and at the same time, the cost is expensive to be practically inappropriate. Examples of the adoptable iridium (III) complex salt include hexachloroiridic (III) acid salt, a hexabromoiridic (III) acid salt and a hexafluoroiridic (III) acid salt, and preferably sodium hexabromoiridate (III) and sodium hexachloroiridate (III). 
         [0013]    Further, one or more compounds selected from the following group are added preferably in a content of 0.001 to 1.0 mol/L and more preferably in a content of 0.01 to 0.2 mol/L, the group consisting of a saturated monocarboxylic acid, a saturated monocarboxylic acid salt, a saturated dicarboxylic acid, a saturated dicarboxylic acid salt, a saturated hydroxycarboxylic acid, a saturated hydroxycarboxylic acid salt, an amide and urea. Examples of the compounds adoptable as such compounds include acetic acid, disodium malonate and oxalic acid, and preferably disodium malonate. The reasons for setting the addition amount of such a compound or such compounds at 0.001 to 1.0 mol/L are such that when the addition amount is less than 0.001 mol/L, the effect due to the addition is hardly exhibited, and when the addition amount exceeds 1.0 mol/L, the deposition is disturbed. 
         [0014]    The iridium plating solution according to the present invention may contain, where necessary, a buffering agent for regulating the pH, such as boric acid and sulfamic acid. 
         [0015]    The iridium plating method according to the present invention is applied under the operation conditions that the pH is set at 1 to 8, the temperature is set at 50 to 98° C. and the current density is set at 0.01 to 3.0 A/dm 2 , and preferably under the conditions that the pH is set at 4 to 6, the temperature is set at 80 to 90° C. and the current density is set at 0.1 to 0.8 A/dm 2 . The pH is set at 1 to 8 because when the pH is lower than 1, the upper limit of the current density is small to make the plating method impractical, and when the pH is higher than 8, a hydroxide is produced to cause precipitation. When the temperature is lower than 50° C., deposition is made to extremely hardly occur, and when the temperature is higher than 98° C., practically unpreferably evaporation of water is vigorous. When the current density is lower than 0.01 A/dm 2 , the deposition rate is extremely small, and when the current density is higher than 3.0 A/dm 2 , the generation of hydrogen occurs to prevent the deposit from being deposited. 
         [0016]    According to the present invention, it is possible to form an iridium plating coat in which the occurrence of cracks is suppressed as much as possible. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is an observation photograph of a plated surface in Example 1; 
           [0018]      FIG. 2  is an observation photograph of a plated surface involving no metal addition; 
           [0019]      FIG. 3  is an observation photograph of a plated surface in Example 2; 
           [0020]      FIG. 4  is an observation photograph of a plated surface for a Co content of 20.0 g/L in Example 2; 
           [0021]      FIG. 5  is an observation photograph of a plated surface in Example 3; 
           [0022]      FIG. 6  is an observation photograph of a plated surface for a Ni content of 15.0 g/L in Example 3; 
           [0023]      FIG. 7  is an observation photograph of a plated surface in Example 4; and 
           [0024]      FIG. 8  is an observation photograph of a plated surface in Example 5. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Hereinafter, the embodiments of the present invention are described in detail with reference to Examples. 
       Example 1 
       [0026]    In Example 1, a case where Fe was added to the iridium plating solution is described. The solution composition of Example 1 was as follows. 
         [0027]    Sodium hexabromoiridate (III): 15 g/L (in terms of iridium metal) 
         [0028]    Boric acid: 40 g/L 
         [0029]    Disodium malonate: 0.02 mol/L 
         [0030]    Iron sulfate heptahydrate: 0.01 g/L (in terms of iron metal) 
         [0031]    In Example 1, the iridium plating solution used an iridium compound obtained by adding, to the above-described sodium hexabromoiridate (III), disodium malonate as a dicarboxylic acid salt and by stirring the resulting mixture with a magnetic stirrer for 1 hour while the temperature of the mixture was being maintained at 85° C. by using a laboratory water bath. Iron sulfate heptahydrate was added to the resulting iridium plating solution to allow the plating solution to contain Fe in a content of 0.01 g/L. 
         [0032]    Then, a 2 cm×2 cm brass test piece was subjected to a gold strike plating treatment to form a 1.0-μm thick gold plating coat, and then subjected to a formation of a 3.0-μm thick iridium plating coat. The plating conditions were such that the pH was set at 3.5 to 4.0, the solution temperature was set at 80 to 85° C. and the current density was set at 0.5 A/dm 2 . 
         [0033]    The plating properties and conditions of the coated iridium plating coat were observed by using a metallograph (magnification: 400×). The results thus obtained are shown in  FIG. 1 . 
         [0034]    Additionally, for comparison, a sample was prepared in which an iridium plating coat was formed by using a iridium plating solution prepared without adding any one of Fe, Co, Ni and Cu. The plating conditions were set as the same as the plating conditions in the case where Fe was contained. The results thus obtained are shown in  FIG. 2 . 
         [0035]    As shown in  FIG. 2 , it was verified that the sample coated with the iridium plating solution not containing Fe underwent the occurrence of a large number of cracks on the surface thereof. On the other hand, as shown in  FIG. 1 , in the case of the iridium plating solution which was allowed to contain Fe, almost no cracks were identified. 
         [0036]    Additionally, the Fe content was varied to be 0.005 g/L, 0.01 g/L, 0.5 g/L, 5.0 g/L and 10 g/L, and thus the crack occurrence conditions were examined; consequently, the occurrence of cracks was observed for 0.005 g/L, but no occurrence of cracks was observed for the Fe contents of 0.01 g/L or more. 
       Example 2 
       [0037]    In Example 2, a case where Co was added to the iridium plating solution is described. The solution composition of Example 2 was as follows. 
         [0038]    Sodium hexabromoiridate (III): 15 g/L (in terms of iridium metal) 
         [0039]    Boric acid: 40 g/L 
         [0040]    Disodium citrate: 0.05 mol/L 
         [0041]    Cobalt sulfate heptahydrate: 0.5 g/L (in terms of cobalt metal) 
         [0042]    In Example 2, the iridium plating solution used an iridium compound obtained by adding, to the above-described sodium hexabromoiridate (III), disodium citrate as a hydroxycarboxylic acid salt and by stirring the resulting mixture with a magnetic stirrer for 1 hour while the temperature of the mixture was being maintained at 85° C. by using a laboratory water bath. Cobalt sulfate heptahydrate was added to the resulting iridium plating solution to allow the plating solution to contain Co in a content of 0.5 g/L. 
         [0043]    Then, a 2 cm×2 cm brass test piece was subjected to a gold strike plating treatment to form a 1.0-μm thick gold plating coat, and then subjected to a formation of a 3.0-μm thick iridium plating coat. The plating conditions were such that the pH was set at 3.5 to 4.0, the solution temperature was set at 80 to 85° C. and the current density was set at 0.5 A/dm 2 . 
         [0044]    The plating properties and conditions of the coated iridium plating coat were observed by using a metallograph (magnification: 400×). The results thus obtained are shown in  FIG. 3 . 
         [0045]    As shown in  FIG. 3 , in the case of the iridium plating solution which was made to contain Co, almost no cracks were identified. 
         [0046]    Additionally, the Co content was varied to be 0.005 g/L, 0.01 g/L, 0.5 g/L, 5.0 g/L and 10 g/L, and thus the crack occurrence conditions were examined; consequently, the occurrence of cracks was observed for 0.005 g/L, but no occurrence of cracks was observed for the Co contents of 0.01 g/L or more. 
         [0047]    Further, the Co content was set at 20.0 g/L, and the plating properties and conditions were observed by using a metallograph (magnification: 400×). 
         [0048]    The results thus obtained are shown in  FIG. 4 . With the Co content of 20.0 g/L, no normal deposition was attained. 
       Example 3 
       [0049]    In Example 3, a case where Ni was added to the iridium plating solution is described. The solution composition of Example 3 was as follows. 
         [0050]    Sodium hexabromoiridate (III): 15 g/L (in terms of iridium metal) 
         [0051]    Boric acid: 40 g/L 
         [0052]    Oxalic acid: 0.05 mol/L 
         [0053]    Nickel sulfate hexahydrate: 0.5 g/L (in terms of nickel metal) 
         [0054]    In Example 3, the iridium plating solution used an iridium compound obtained by adding, to the above-described sodium hexabromoiridate (III), oxalic acid as a dicarboxylic acid and by stirring the resulting mixture with a magnetic stirrer for 1 hour while the temperature of the mixture was being maintained at 85° C. by using a laboratory water bath. Nickel sulfate hexahydrate was added to the resulting iridium plating solution to allow the plating solution to contain Ni in a content of 0.5 g/L. 
         [0055]    Then, a 2 cm×2 cm brass test piece was subjected to a gold strike plating treatment to form a 1.0-μm thick gold plating coat, and then subjected to a formation of a 3.0-μm thick iridium plating coat. The plating conditions were such that the pH was set at 3.5 to 4.0, the solution temperature was set at 80 to 85° C. and the current density was set at 0.5 A/dm 2 . 
         [0056]    The plating properties and conditions of the coated iridium plating coat were observed by using a metallograph (magnification:  400 ×). The results thus obtained are shown in  FIG. 5 . 
         [0057]    As shown in  FIG. 5 , in the case of the iridium plating solution which was made to contain Ni, almost no cracks were identified. 
         [0058]    Additionally, the Ni content was varied to be 0.005 g/L, 0.01 g/L, 0.5 g/L, 5.0 g/L and 10 g/L, and thus the crack occurrence conditions were examined; consequently, the occurrence of cracks was observed for 0.005 g/L, but no occurrence of cracks was observed for the Ni contents of 0.01 g/L or more. 
         [0059]    Further, the Ni content was set at 15.0 g/L, and the plating properties and conditions were observed by using a metallograph (magnification: 400×). The results thus obtained are shown in  FIG. 6 . With the Ni content of 15.0 g/L, no normal deposition was attained. 
       Example 4 
       [0060]    In Example 4, a case where Cu was added to the iridium plating solution is described. The solution composition of Example 4 was as follows. 
         [0061]    Sodium hexabromoiridate (III): 15 g/L (in terms of iridium metal) 
         [0062]    Boric acid: 40 g/L 
         [0063]    Acetic acid: 0.02 mol/L 
         [0064]    Copper sulfate pentahydrate: 0.01 g/L (in terms of copper metal) 
         [0065]    In Example 4, the iridium plating solution used an iridium compound obtained by adding, to the above-described sodium hexabromoiridate (III), acetic acid as a monocarboxylic acid and by stirring the resulting mixture with a magnetic stirrer for 1 hour while the temperature of the mixture was being maintained at 85° C. by using a laboratory water bath. Copper sulfate pentahydrate was added to the resulting iridium plating solution to allow the plating solution to contain Cu in a content of 0.01 g/L. 
         [0066]    Then, a 2 cm×2 cm brass test piece was subjected to a gold strike plating treatment to form a 1.0-μm thick gold plating coat, and then subjected to a formation of a 3.0-μm thick iridium plating coat. The plating conditions were such that the pH was set at 3.5 to 4.0, the solution temperature was set at 80 to 85° C. and the current density was set at 0.5 A/dm 2 . 
         [0067]    The plating properties and conditions of the coated iridium plating coat were observed by using a metallograph (magnification: 400×). The results thus obtained are shown in  FIG. 7 . 
         [0068]    As shown in  FIG. 7 , in the case of the iridium plating solution which was made to contain Cu, almost no cracks were identified. 
         [0069]    Additionally, the Cu content was varied to be 0.005 g/L, 0.01 g/L, 0.5 g/L and 1.0 g/L, and thus the crack occurrence conditions were examined; 
         [0070]    consequently, the occurrence of cracks was observed for 0.005 g/L, but no occurrence of cracks was observed for the Cu contents of 0.01 g/L or more. 
       Example 5 
       [0071]    In Example 5, a case where Co was added to the iridium plating solution is described. The solution composition of Example 5 was as follows. 
         [0072]    Sodium hexachloroiridate (III): 5 g/L (in terms of iridium metal) 
         [0073]    Boric acid: 20 g/L 
         [0074]    Disodium malonate: 0.10 mol/L 
         [0075]    Cobalt sulfate heptahydrate: 0.5 g/L (in terms of cobalt metal) 
         [0076]    In Example 5, the iridium plating solution used an iridium compound obtained by adding, to the above-described sodium hexachloroiridate (III), disodium malonate as a dicarboxylic acid salt and by stirring the resulting mixture with a magnetic stirrer for 1 hour while the temperature of the mixture was being maintained at 85° C. by using a laboratory water bath. Cobalt sulfate heptahydrate was added to the resulting iridium plating solution to allow the plating solution to contain Co in a content of 0.5 g/L. 
         [0077]    Then, a 2 cm×2 cm brass test piece was subjected to a gold strike plating treatment to form a 1.0-μm thick gold plating coat, and then subjected to a formation of a 3.0-μm thick iridium plating coat. The plating conditions were such that the pH was set at 3.5 to 4.0, the solution temperature was set at 80 to 85° C. and the current density was set at 0.2 A/dm 2 . 
         [0078]    The plating properties and conditions of the coated iridium plating coat were observed by using a metallograph (magnification: 400×). The results thus obtained are shown in  FIG. 8 . 
         [0079]    As shown in  FIG. 8 , in the case of the iridium plating solution which was made to contain Co, almost no cracks were identified. 
       Example 6 
       [0080]    In Example 6, a case where Ni was added to the iridium plating solution and the plating conditions were varied is described. The solution composition of Example 6 was as follows. 
         [0081]    Sodium hexabromoiridate (III): 10 g/L (in terms of iridium metal) 
         [0082]    Boric acid: 30 g/L 
         [0083]    Oxalic acid: 0.05 mol/L 
         [0084]    Nickel sulfate hexahydrate: 0.5 g/L (in terms of nickel metal) 
         [0085]    In Example 6, the iridium plating solution used an iridium compound obtained by adding, to the above-described sodium hexabromoiridate (III), oxalic acid as a dicarboxylic acid and by stirring the resulting mixture with a magnetic stirrer for 1 hour while the temperature of the mixture was being maintained at 85° C. by using a laboratory water bath. Nickel sulfate hexahydrate was added to the resulting iridium plating solution to allow the plating solution to contain Ni in a content of 0.5 g/L. 
         [0086]    Then, a 2 cm×2 cm brass test piece was subjected to a gold strike plating treatment to form a 1.0-μm thick gold plating coat and then subjected to a formation of a 3.0-μm thick iridium plating coat, and thus the deposition efficiency was measured. The plating conditions were such that the pH was set at 2.0 to 8.5, the solution temperature was set at 40 to 95° C. and the current density was set at 0.01 to 2.0 A/dm 2 . 
         [0087]    The deposition efficiency was measured when the solution temperature was set at 85° C., the current density was set at 0.5 A/dm 2  and the pH was varied. 
         [0088]    When the pH was 0.5, the deposition efficiency was found to be 0% and no deposition occurred. When the pH was 3.0, the deposition efficiency was found to be 85% and no cracks were identified. When the pH was 4.0 to 7.0, the deposition efficiency was 95% to 100% and no cracks were identified. Further, when the pH was 8.5, the hydroxide precipitate occurred. 
         [0089]    Next, the deposition efficiency was measured when the current density was set at 0.5 A/dm 2 , the pH was set at 3.5 and the bath temperature was varied. 
         [0090]    When the bath temperature was 40° C., the deposition efficiency was found to be 0% and no deposition occurred. When the bath temperature was 50° C., the deposition efficiency was found to be 35% and cracks were identified. When the bath temperature was 60° C. to 70° C., the deposition efficiency was found to be 40% to 60% and no cracks were identified. When the bath temperature was 80° C. to 95° C., the deposition efficiency was found to be 90% to 100% and no cracks were identified. When the bath temperature was increased to 99° C., the evaporation of water from the plating bath was vigorous and it was difficult to perform stable plating treatment. 
         [0091]    Next, the deposition efficiency was measured when the bath temperature was set at 85° C., the pH was set at 3.5 and the current density was varied. 
         [0092]    When the current density was 0.01 A/dm 2 , the deposition efficiency was found to be 50% and no cracks were identified. When the current density was 0.02 A/dm 2  to 1.0 A/dm 2 , the deposition efficiency was found to be 90% to 100% and no cracks were identified. When the current density was 1.5 A/dm 2 , the deposition efficiency was found to be 60% and no cracks were identified. When the current density was 3.0 A/dm 2 , the deposition efficiency was found to be 20% and cracks were identified. When the current density was increased to 3.5 A/dm 2 , hydrogen was evolved and no normal deposition was attained. 
         [0093]    It is possible to easily form an iridium plating coat in which the occurrence of cracks is suppressed as much as possible.