Patent Publication Number: US-2012031764-A1

Title: Microcrystalline-to-amorphous gold alloy and plated film, and plating solution for those, and plated film formation method

Description:
TECHNICAL FIELD 
     The present invention relates to a mixed microcrystalline-amorphous gold alloy plated film that is useful as a plated film for a terminal of an electronic equipment component and has excellent electrical properties and mechanical properties, an electroplating solution that can form this mixed microcrystalline-amorphous gold alloy plated film, and an electroplating method employing this electroplating solution. 
     BACKGROUND ART 
     As an electrical contact material for, in particular, parts where high reliability is required in an electrical/electronic component connector, a miniature electromechanical relay, a printed wiring board, etc., a gold plated film called a hard gold plated film is currently widely used. The hard gold plated film has cobalt, nickel, etc. added to gold, and its film hardness is improved without reducing the intrinsically good electrical conductivity and chemical stability of gold. This hard gold plated film has a fine structure in which gold microcrystals (20 to 30 nm) are aggregated, and it is surmised that, in accordance with this fine structure, a hardness (on the order of Hk 170 as a Knoop hardness) that is the minimum requirement for obtaining the abrasion resistance required for the contact material is obtained. 
     On the other hand, accompanying the recent reduction in size of electronic components, the size of an electrical contact has been miniaturized, a plated film formed on such a microcontact is also made thin and small in size, and there is a demand for further improvement in hardness in order to obtain high abrasiveness. 
     Furthermore, it is thought that, in the near future, the size of a contact will become close to the size of the above-mentioned microcrystals of the hard gold plated film; when a hard gold plated film like the one mentioned above is formed on such a fine contact, since the absolute number of microcrystals forming a film becomes small, it is predicted that it will not be possible to obtain the same degree of durability as is the case when a hard gold plated film is formed on the order of size of contact that is currently used. The present inventors have invented an amorphous gold alloy plated film that is formed so as to have a homogeneous amorphous phase without having microcrystals (e.g. Patent Documents 6 to 8). However, for the purpose of obtaining one that has improved hardness while the intrinsically good specific resistance and chemical stability of gold are maintained to such a degree that there are no problems in practice, there is still room for improvement. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Prior art document information related to the present invention is as follows.
     Patent Document 1: JP, A, 60-33382   Patent Document 2: JP, A, 62-290893   Patent Document 3: Japanese Patent No. 3452724   Patent Document 4: Japanese Patent No. 3983207   Patent Document 5: JP, A, 2004-300483   Patent Document 6: JP, A, 2006-241594   Patent Document 7: JP, A, 2007-92157   Patent Document 8: JP, A, 2007-169706   

     Non-Patent Documents 
     
         
         Non-Patent Document 1: S. Kawai, ‘Study of Deposition Structure of Gold-nickel Alloy Plating’, Kinzoku Hyomen Gijutsu, 1968, Vol. 19, No. 12, p. 487-491 
         Non-Patent Document 2: Y. Shimizu and one other, ‘Electromicroscopic Study of Electrodeposited Au—Ni alloy Fine Structure and Phase’, Kinzoku Hyomen Gijutsu, 1976, Vol. 27, No. 1, p. 20-24 
         Non-Patent Document 3: T. Watanabe, ‘Fine Plating—Techniques of Structural Control of Plated Film and Analytical Methods Therefor’, Technical Information Institute, 2002, February, p 256-262 
         Non-Patent Document 4: T. Omi and two others, ‘Increase in W content of Ni-—W Alloy Plated Film and Film Properties’, Kinzoku Hyomen Gijutsu, 1988, Vol. 39, No. 12, p. 809-812 
         Non-Patent Document 5: T. Watanabe, ‘Mechanism of Formation of Amorphous Alloy by Plating Method’, Hyomen Gijutsu, 1989, Vol. 40, No. 3, p. 21-26 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The present invention has been accomplished in light of the above-mentioned circumstances, and it is an object thereof to provide a mixed microcrystalline-amorphous gold alloy plated film having improved hardness and excellent abrasion resistance while having good electrical conductivity and chemical stability, an electroplating solution that can form this mixed microcrystalline-amorphous gold alloy plated film, and an electroplating method employing this electroplating solution. 
     Means for Solving the Problems 
     While carrying out an intensive investigation in order to accomplish the above-mentioned object, the present inventors have carried out a study with the expectation that, with regard to the fine structure of a plated film that does not lower the hardness even for a microcontact, compared with a crystalline structure, an amorphous phase structure can improve the hardness and abrasion resistance while the intrinsically good specific resistance and chemical stability of gold are maintained to such a degree that there are no problems in practice, but since the electron mean free path is shorter than in a crystalline film, electroconductivity is low, and cracks are easily generated in a plated film by internal stress; it has been found that, in accordance with electroplating using an electroplating solution having good liquid stability, containing at a predetermined concentration a gold cyanide salt, and a nickel salt and/or a cobalt salt, and preferably further containing a complexing agent such as an organic acid, an inorganic acid, or a salt thereof and ammonia or ammonium ion, a mixed microcrystalline-amorphous gold alloy plated film that is formed so that a microcrystalline phase and an amorphous phase are mixed is surprisingly obtained, and this film has improved hardness while the intrinsically good specific resistance value and chemical stability of gold are maintained to such a degree that is useful in practice, and as a result of further investigation the present invention has been accomplished. 
     That is, the present invention provides (1) a mixed microcrystalline-amorphous gold alloy plated film that is formed so that a microcrystalline phase and an amorphous phase are mixed, (2) an electroplating solution, having good liquid stability, containing a gold cyanide salt at a concentration of 0.0001 to 0.4 mol/dm 3  on a gold basis, and a nickel salt at a concentration of 0.001 to 0.5 mol/dm 3  on a nickel basis and/or a cobalt salt at a concentration of 0.001 to 0.5 mol/dm 3  on a cobalt basis, and preferably further containing a complexing agent such as an organic acid, an inorganic acid, or a salt thereof at a concentration of 0.001 to 2.0 mol/dm 3  and ammonia or ammonium ion at a concentration of 0.001 to 5.0 mol/dm 3 , and (3) an electroplating method in which a mixed microcrystalline-amorphous gold alloy plated film is formed on an article to be plated using this electroplating solution. 
     Effects of the Invention 
     The mixed microcrystalline-amorphous gold alloy plated film of the present invention is formed so that a microcrystalline phase and an amorphous phase are mixed; as a result the hardness is improved while the intrinsically good specific resistance value and chemical stability of gold are maintained to a degree that is useful in practice, and it is useful as a contact material for an electrical/electronic component such as a relay. It is known that, in general, in the constituent case of a crystalline film formed from microcrystals, when the crystalline particles reduce in size, the hardness increases up to a certain limit (e.g. about 4 nm in the case of nickel), but when the crystalline particles further reduce in size the hardness is degraded. Although there are no actual measurements if the generalization is applied to gold, it has been confirmed for the first time that, in accordance with the present invention, which has achieved for the first time a mixed microcrystalline-amorphous crystalline film for gold, the mixed microcrystalline-amorphous gold alloy plated film solves all such problems, has high electrical conductivity, and is resistant to cracking, thus making it fully applicable as a microcontact material for an electrical/electronic component such as a connector or a relay. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  A diagram showing XRD patterns of mixed microcrystalline-amorphous gold alloy plated films obtained in Examples 1, 2, 3, 4, and 5 and gold alloy plated films obtained in Comparative Examples 1 and 2. 
         FIG. 2  A diagram showing a TEM image (100,000×) of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 1. 
         FIG. 3  A diagram showing a TEM image (1,000,000×) of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 1. 
         FIG. 4  A diagram showing a TREED pattern of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 1. 
         FIG. 5  A diagram showing a TEM image (500,000×) of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 2. 
         FIG. 6  A diagram showing a TEM image (1,000,000×) of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 2. 
         FIG. 7  A diagram showing a TREED pattern of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 2. 
         FIG. 8  A diagram showing a TEM image (300,000×) of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 3. 
         FIG. 9  A diagram showing a TEM image (1,000,000×) of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 3. 
         FIG. 10  A diagram showing a THEED pattern of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 3. 
         FIG. 11  A diagram showing a TEM image (200,000×) of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 4. 
         FIG. 12  A diagram showing a TEM image (700,000×) of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 4. 
         FIG. 13  A diagram showing a TREED pattern of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 4. 
         FIG. 14  A diagram showing a TEM image (400,000×) of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 5. 
         FIG. 15  A diagram showing a TEM image (1,000,000×) of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 5. 
         FIG. 16  A diagram showing of a TREED pattern of the mixed microcrystalline-amorphous gold alloy plated film obtained in Example 5. 
         FIG. 17  A diagram showing a TEM image (1,000,000×) of the amorphous gold alloy plated film obtained in Comparative Example 1. 
         FIG. 18  A diagram showing a THEED pattern of the amorphous gold alloy plated film obtained in Comparative Example 1. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     The present invention is explained below in further detail. 
     The mixed microcrystalline-amorphous gold alloy plated film of the present invention is formed so that a microcrystalline phase and an amorphous phase are mixed. 
     The mixed microcrystalline-amorphous gold alloy plated film of the present invention contains nickel and/or cobalt in gold, its fine structure is a structure in which a microcrystalline phase and an amorphous phase are mixed, and in accordance with these characteristics, a good specific resistance value and chemical stability as well as high hardness can be achieved compared with an amorphous gold alloy plated film having a pure amorphous structure. Such a structure in which a microcrystalline phase and an amorphous phase are mixed can be confirmed by an X-ray diffraction (XRD) pattern, a transmission electron microscope (TEM) image, and a transmission high energy electron diffraction (THEED) image. 
     From the viewpoint of maintaining high hardness, it is preferable that the mixed microcrystalline-amorphous gold alloy plated film of the present invention has an average microcrystal particle size of no greater than 30 nm, particularly no greater than 20 nm, and more particularly no greater than 15 nm. 
     Furthermore, from the viewpoint of maintaining properties (good specific resistance value and chemical stability) that are intrinsic to gold or high hardness, which is not possessed by a conventional gold or gold alloy plated film, it is preferable that the mixed microcrystalline-amorphous gold alloy plated film of the present invention has a microcrystal volume fraction of 10% to 90%, and particularly 15% to 60%. 
     In accordance with the present invention, a mixed microcrystalline-amorphous gold alloy plated film having excellent hardness and specific resistance, such that the Knoop hardness is at least Hk 180, particularly at least Hk 220, further at least Hk 300, and yet further at least Hk 350, and the specific resistance is no greater than 200 μΩ·cm, particularly no greater than 150 μΩ·cm, and yet further no greater than 100 μΩ·cm can be obtained. Furthermore, with regard to the mixed microcrystalline-amorphous gold alloy plated film of the present invention, the structure in which a microcrystalline phase and an amorphous phase are mixed will not be changed (that is, crystallization progressing and the microcrystal average particle size or volume fraction increasing) by annealing at no greater than 300° C. (kept for 1 hour). 
     Since the mixed microcrystalline-amorphous gold alloy plated film of the present invention has characteristics of high hardness, which is not possessed by a conventional gold or gold alloy plated film, together with its excellent specific resistance value and chemical stability, it is effective as a conductive contact such as a terminal of an electrical/electronic component such as an electromagnetic switch, a breaker, a thermostat, a relay, a timer, various types of switches, or a printed wiring board. 
     The mixed microcrystalline-amorphous gold alloy plated film of the present invention may be represented by the compositional formula Au 100-x-y M x C y , wherein Au or M is a main component, inevitable impurities may be contained, M is Ni and/or Co, C is carbon, 1 atom %≦x≦80 atom %, and 1 atom %≦y≦30 atom %. 
     The mixed microcrystalline-amorphous gold alloy plated film of the present invention may be formed by electroplating using an electroplating solution containing a gold cyanide salt, and a nickel salt and/or a cobalt salt. 
     This electroplating solution contains a gold cyanide salt, and a nickel salt and/or a cobalt salt; specific examples of the gold cyanide salt include gold potassium cyanide, gold sodium cyanide, and gold lithium cyanide, specific examples of the nickel salt include nickel sulfate and nickel nitrate, and specific examples of the cobalt salt include cobalt sulfate and cobalt nitrate. The gold cyanide salt concentration of the plating solution is 0.0001 to 0.4 mol/dm 3  on a gold basis, preferably 0.001 to 0.2 mol/dm 3 , and more preferably 0.01 to 0.1 mol/dm 3 , the nickel salt concentration is 0.001 to 0.5 mol/dm 3  on a nickel basis, and preferably 0.01 to 0.2 mol/dm 3 , and the cobalt salt concentration is 0.001 to 0.5 mol/dm 3  on a cobalt basis, and preferably 0.01 to 0.2 mol/dm 3 . The ratio [(Ni+Co)/Au] of nickel and/or cobalt to gold in the plating solution is preferably in the range of 0.01 to 300 as a molar ratio, and more preferably 1 to 30. 
     Furthermore, this electroplating solution preferably further contains a complexing agent. Examples of this complexing agent include an organic acid, inorganic acid, or salt thereof that has a complexing action and a pH buffering action, and examples of the organic acid, inorganic acid, and salt thereof include citric acid, tartaric acid, malic acid, pyrophosphoric acid, phosphoric acid, sulfamic acid, and sodium, potassium, and ammonium salts thereof. It is preferable that the concentration of the complexing agent in the plating solution is 0.001 to 2.0 mol/dm 3 , particularly 0.01 to 1.0 mol/dm 3 , and more particularly 0.1 to 0.3 mol/dm 3 . The ratio [complexing agent/(Ni+Co)] of the complexing agent to nickel and/or cobalt in the plating solution is preferably in the range of 0.01 to 100 as a molar ratio, and more preferably 1 to 4. 
     Furthermore, this electroplating solution preferably further contains ammonia or ammonium ion. Specific examples of the ammonia or ammonium ion include aqueous ammonia, ammonium sulfate, and an ammonium salt of the complexing agent. It is preferable that the concentration of ammonia or ammonium ion in the plating solution is 0.001 to 5.0 mol/dm 3 , and particularly 0.01 to 2.0 mol/dm 3 . This ammonia is heavily involved in the crystallization state of a plated film, such as the average particle size of the crystalline phase or the microcrystalline (or amorphous) volume fraction, and the stability of a plating bath. 
     It is preferable that this electroplating solution preferably has a pH of 3 to 11, particularly a pH of 5 to 9, and more particularly a pH of on the order of 6. For adjustment of the pH, a conventionally known pH adjusting agent such as aqueous ammonia or potassium hydroxide may be used. 
     Furthermore, this electroplating solution may contain as necessary various types of additives such as a surfactant and a solvent for the purpose of improving the gloss, preventing pits, imparting electrical conductivity, imparting buffering properties, increasing the range of current density that can be used, promoting the deposition rate, improving heat resistance, improving wettability, etc. as long as the film physical properties of a plated film (microcrystalline volume fraction and average particle size, XRD pattern peak half-width, Knoop hardness, specific resistance) and film composition are not greatly affected (ref. e.g. JP, A, 7-11476, JP, A, 2004-76026, JP, A, 2006-37164). 
     The electroplating conditions are not particularly limited, but it is desirable that the plating temperature is 20° C. to 95° C., and particularly 50° C. to 90° C. The cathode current density depends on the composition of the plating solution and is not particularly limited, and a mixed microcrystalline-amorphous gold alloy plated film may be obtained in both a low current density region (e.g. at least 1 mA/cm 2  but less than 10 mA/cm 2 ) and a high current density region (e.g. greater than 10 mA/cm 2  but no greater than 200 mA/cm 2 ). Furthermore, as an anode, an insoluble anode such as platinum may be used. Moreover, nickel and/or cobalt may be used as an anode. On the other hand, as an article to be plated, a metal material such as copper or nickel used for electrical wiring can be cited. This metal material may be one formed as a base layer on a metal substrate or a non-metal substrate. Although stirring may or may not be present, plating is preferably carried out under stirring, and electric current may be applied by pulse current. 
     The present invention is specifically explained below by reference to Examples and Comparative Examples, but the present invention is not limited to the Examples below. In the Examples, the method and conditions for each of the analyses and measurements were as follows. 
     Crystallinity and Crystal Particle Size 
     XRD method CuKα (40 kV/40 mA) using a RINT 2100-Ultima+manufactured by Rigaku Corporation, or 
     TEM and THEED methods, acceleration voltage 200 V, bright-field image using an HF-2200 manufactured by Hitachi High-Technologies Corporation 
     Volume Fraction 
     TEM method and THEED method, acceleration voltage 200 V, bright-field image using an HF-2200 manufactured by Hitachi High-Technologies Corporation 
     Metal Composition 
     EDXRF method using an SEA 5100 manufactured by SII Nanotechnology Inc. 
     Non-Metal Element Measurement 
     EMIA-920V manufactured by Horiba, Ltd., TC-436 manufactured by LECO USA 
     Knoop Hardness 
     Measured in accordance with JIS Z2251: load 5 gf, load duration 30 sec., 30 μm thick plated film formed on copper plate 
     Specific Resistance 
     Measured in accordance with JIS K7194 (four-point probe method) using a K-705RS manufactured by Kyowariken Co., Ltd. 
     Example 1 
     A mixed microcrystalline-amorphous gold alloy plated film (film thickness 1 μm) was formed on a copper plate having a purity of 99.96% at a temperature of 70° C. and a current density of 10 mA/cm 2  using an electroplating solution containing 0.035 mol/dm 3  of KAu(CN) 2 , 0.076 mol/dm 3  of NiSO 4 .6H 2 O, and 0.21 mol/dm 3  of triammonium citrate and having a pH adjusted to 6 with KOH and sulfuric acid. As the anode, a platinum-coated titanium electrode (mesh form) was used, and the plating bath was vigorously stirred during plating. 
     The mixed microcrystalline-amorphous gold alloy plated film thus obtained was analyzed by XRD, TEM, and THEED. An XRD pattern is shown in  FIG. 1 , and TEM images and a THEED pattern are shown in  FIGS. 2 to 4 . A broad peak having a peak half-width of 1 degree or greater, which is characteristic of being microcrystalline or amorphous, was observed in the XRD pattern at around 2θ=40 degrees. Furthermore, in the TEM image a state in which crystal fringes characteristic of being crystalline and an irregular structure characteristic of being amorphous are mixed could be observed. In the THEED pattern, a state in which a diffraction spot characteristic of being crystalline and a halo ring characteristic of being amorphous are mixed could be observed. From these results, it can be seen that the plated film obtained had a mixed microcrystalline-amorphous structure. Furthermore, as a result of examining the TEM image, the average particle size of the microcrystals was found to be 10 nm, and the volume fraction of the microcrystalline phase was 50%. Separately, the compositional analysis, Knoop hardness, and specific resistance of the mixed microcrystalline-amorphous gold alloy plated film obtained were measured. As metal elements gold was detected at a content of 41.2 atom % and nickel was at 46.0 atom %, and as a non-metal element carbon was detected at a content of 12.8 atom %. The Knoop hardness was Hk 347, and the specific resistance was 89 μΩ·cm. 
     Example 2 
     Plating was carried out in the same way as for Example 1 except that n-propanol was added at 20 vol %, and the plated film thus obtained was subjected to XRD, TEM, and THEED analyses. An XRD pattern is shown in  FIG. 1 , and TEM images and a THEED pattern are shown in  FIGS. 5 to 7 . A broad peak having a peak half-width of 1 degree or greater, which is characteristic of being microcrystalline or amorphous, was observed in the XRD pattern at around 2θ=40 degrees. Furthermore, in the TEM image a state in which crystal fringes characteristic of being crystalline and an irregular structure characteristic of being amorphous are mixed could be observed. In the THEED pattern, a state in which a diffraction spot characteristic of being crystalline and a halo ring characteristic of being amorphous are mixed could be observed. From these results, it can be seen that the plated film obtained had a mixed microcrystalline-amorphous structure. Furthermore, as a result of examining the TEM image, the average particle size of the microcrystals was found to be 10 nm, and the volume fraction of the microcrystalline phase was 50%. Separately, the compositional analysis, Knoop hardness, and specific resistance of the mixed microcrystalline-amorphous gold alloy plated film obtained were measured. As metal elements gold was detected at a content of 48.1 atom % and nickel was at 38.1 atom %, and as a non-metal element carbon was detected at a content of 13.8 atom %. The Knoop hardness was Hk 348, and the specific resistance was 89 μΩ·cm. 
     Example 3 
     Plating was carried out in the same way as for Example 1 except for a concentration of citric acid of 0.143 mol/dm 3 , a concentration of ammonia of 1.2 mol/dm 3 , and electroplating being carried out alternatingly at current densities of 1 mA/cm 2  (application time 50 sec) and 10 mA/cm 2 (application time 5 sec) without a gap, and the plated film thus obtained was subjected to XRD, TEM, and THEED analyses. An XRD pattern is shown in  FIG. 1 , and TEM images and a THEED pattern are shown in  FIGS. 8 to 10 . A broad peak having a peak half-width of 1 degree or greater, which is characteristic of being microcrystalline or amorphous, was observed in the XRD pattern at around 2θ=40 degrees. Furthermore, in the TEM image a state in which crystal fringes characteristic of being crystalline and an irregular structure characteristic of being amorphous are mixed could be observed. In the TREED pattern, a state in which a diffraction spot characteristic of being crystalline and a halo ring characteristic of being amorphous are mixed could be observed. In the case of constant-current plating, only a crystalline phase was obtained at a current density of 1 mA/cm 2 , and only an amorphous phase was obtained at 10 mA/cm 2 . From these results, it can be seen that the plated film obtained by pulse plating had a mixed microcrystalline-amorphous structure. Furthermore, as a result of examining the TEM image, the average particle size of the microcrystals was found to be 10 nm, and the volume fraction of the microcrystalline phase was 60%. Separately, the compositional analysis, Knoop hardness, and specific resistance of the plated film obtained were measured. As metal elements gold was detected at a content of 47.4 atom % and nickel was at 47.0 atom %, and as a non-metal element carbon was detected at a content of 5.6 atom %. The Knoop hardness was Hk 222, and the specific resistance was 57 μΩ·cm. 
     Example 4 
     Plating was carried out in the same way as for Example 1 except for a concentration of citric acid of 0.143 mol/dm 3 , a concentration of ammonia of 1.2 mol/dm 3 , and a current density of 50 mA/cm 2 , and a plated film obtained by subjecting the amorphous gold alloy plated film thus obtained to an annealing treatment at an annealing temperature (holding temperature) of 400° C., a rate of temperature increase of 10° C./min, temperature held for 1 hour, under an air atmosphere was subjected to XRD, TEM, and THEED analyses. An XRD pattern is shown in  FIG. 1 , and TEM images and a THEED pattern are shown in  FIGS. 11 to 13 . A broad peak having a peak half-width of 1 degree or greater, which is characteristic of being microcrystalline or amorphous, was observed in the XRD pattern at around 2θ=40 degrees. Furthermore, in the TEM image a state in which crystal fringes characteristic of being crystalline and an irregular structure characteristic of being amorphous are mixed could be observed. In the THEED pattern, a state in which a diffraction spot characteristic of being crystalline and a halo ring characteristic of being amorphous are mixed could be observed. From these results, it can be seen that the plated film obtained had a mixed microcrystalline-amorphous structure. Furthermore, as a result of examining the TEM image, the average particle size of the microcrystals was found to be 15 nm, and the volume fraction of the microcrystalline phase was 60%. 
     Example 5 
     A mixed microcrystalline-amorphous gold alloy plated film (film thickness 1 μm) was formed on a copper plate having a purity of 99.96% at a temperature of 70° C. and a current density of 10 mA/cm 2  using an electroplating solution containing 0.035 mol/dm 3  of KAu(CN) 2 , 0.076 mol/dm 3  of CoSO 4 .7H 2 O, and 0.1 mol/dm 3  of citric acid.H 2 O, having an ammonia concentration of 0.44 mol/dm 3 , and having a pH adjusted to 6 with KOH and sulfuric acid. As the anode, a platinum-coated titanium electrode (mesh form) was used, and the plating bath was vigorously stirred during plating. 
     The mixed microcrystalline-amorphous gold alloy plated film thus obtained was analyzed by XRD, TEM, and TREED. An XRD pattern is shown in  FIG. 1 , and TEM images and a TREED pattern are shown in  FIGS. 14 to 16 . A broad peak having a peak half-width of 1 degree or greater, which is characteristic of being microcrystalline or amorphous, was observed in the XRD pattern at around 2θ=40 degrees. Furthermore, in the TEM image a state in which crystal fringes characteristic of being crystalline and an irregular structure characteristic of being amorphous are mixed could be observed. In the TREED pattern, a state in which a diffraction spot characteristic of being crystalline and a halo ring characteristic of being amorphous are mixed could be observed. From these results, it can be seen that the plated film obtained had a mixed microcrystalline-amorphous structure. Furthermore, as a result of examining the TEM image, the average particle size of the microcrystals was found to be 5 nm, and the volume fraction of the microcrystalline phase was 15%. Separately, the compositional analysis and Knoop hardness of the mixed microcrystalline-amorphous gold alloy plated film obtained were measured. As metal elements gold was detected at a content of 36.4 atom % and cobalt was at 40.6 atom %, and as a non-metal element carbon was detected at a content of 23.0 atom %. The Knoop hardness was Hk 180. 
     Comparative Example 11 
     Plating was carried out in the same way as for Example 1 except for a concentration of citric acid of 0.143 mol/dm 3  and a concentration of ammonia of 0.46 mol/dm 3 , and the plated film thus obtained was subjected to XRD, TEM, and TREED analyses. An XRD pattern is shown in  FIG. 1 , and a TEM image and a THEED pattern are shown in  FIGS. 17 and 18 . A broad peak having a peak half-width of 1 degree or greater, which is characteristic of being amorphous, was observed in the XRD pattern at around 2θ=40 degrees. Furthermore, in the TEM image an irregular structure characteristic of being amorphous could be observed, but a regular structure such as a grain boundary or crystal fringes could not be observed. In the TREED pattern, a halo ring characteristic of being amorphous could be observed. From these results, it can be seen that the plated film obtained had a homogeneous amorphous structure without having microcrystals. Furthermore, the compositional analysis, Knoop hardness, and specific resistance of the plated film obtained were measured. As metal elements gold was detected at a content of 15.2 atom % and nickel was at 67.5 atom %, and as a non-metal element carbon was detected at a content of 17.3 atom %. The Knoop hardness was Hk 435, and the specific resistance was 251 μΩ·cm. 
     Comparative Example 2 
     A hard gold plated film (film thickness 1 μm) was formed on a copper plate having a purity of 99.96% at a temperature of 30° C. and a current density of 10 mA/cm 2  using an electroplating solution containing 0.04 mol/dm 3  of KAu(CN) 2 , 0.0085 mol/dm 3  of NiSO 4. 6H 2 O, 0.5 mol/dm 3  of citric acid.H 2 O, and 0.7 mol/dm 3  of KOH and having a pH adjusted to 3.5 with sulfuric acid. As the anode, a platinum-coated titanium electrode (mesh form) was used, and the plating bath was gently stirred during plating. 
     The hard gold plated film thus obtained was analyzed by XRD, TEM, and TREED. An XRD pattern is shown in  FIG. 1 . A sharp peak due to Au(111) was observed in the XRD pattern at around 2θ=38 degrees. Furthermore, it was confirmed from a TEM image and a THEED pattern that it was crystalline. From these results, it can be seen that the plated film obtained had a polycrystalline structure without having an amorphous phase. Furthermore, as a result of calculating from the XRD pattern, it was found that the average particle size of the crystals was 13 nm. Separately, the compositional analysis, Knoop hardness, and specific resistance of the plated film obtained were measured. As metal elements gold was detected at a content of 96.5 atom % and nickel was at 0.77 atom %, and as a non-metal element carbon was detected at a content of 2.7 atom %. The Knoop hardness was Hk 160, and the specific resistance was 17 μΩ·cm. 
     In the XRD patterns shown in  FIG. 1 , a sharp peak appearing at around 2θ=50° is due to copper of the substrate. 
     Furthermore, it can be seen that whereas the Knoop hardness of additive-free hard gold (AFHG), nickel hard gold (NiHG), and CoHG, which are considered to have high hardness among gold plated films, is on the order of less than Hk 200, the Knoop hardness of the mixed microcrystalline-amorphous gold alloy plated film of Example 1 has a high hardness corresponding to 2 to 3 times the above.