Abstract:
A process for producing a substrate  1  having a base-metal plating layer, which includes an immersion step for immersing the substrate  1  in a plating solution contained in a plating tank  33,  to thereby form a base-metal plating layer; a washing step for removing the substrate  1  from the plating tank  33,  transferring the substrate  1  to a washing tank, and washing the substrate  1;  and a cooling step for applying a cooling liquid to the substrate  1  during at least a portion of the period during which the substrate is transferred to a position where the washing step is carried out after completing the immersion step, to thereby cool the substrate  1.  An apparatus for carrying out the above process is also disclosed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a process for producing a substrate including a plating layer and to a plating apparatus for forming a plating layer of a substrate; and, more particularly, to a process for producing a substrate including a base-metal plating layer and to a plating apparatus for forming a base-metal plating layer.  
           [0003]    2. Description of the Related Art  
           [0004]    Conventionally, a substrate including a layer plated with a base metal such as Ni has been known. For example, in a wiring substrate on which an IC chip is to be provided, usually, an Ni-plating layer (base-metal plating layer) is formed on a connection terminal formed from Cu, and an Au-plating layer is formed on the Ni-plating layer in order to prevent oxidation of the Ni-plating layer.  
           [0005]    Such a plating layer of a wiring substrate is formed as follows. A wiring substrate on which an Ni-plating layer is to be formed is subjected to Pd activation treatment before plating, and then the wiring substrate is washed with water.  
           [0006]    After washing with water, the wiring substrate is immersed for a predetermined period of time in an Ni-plating solution contained in an Ni-plating tank, to thereby form on a connection terminal of the wiring substrate an Ni-plating layer of predetermined thickness.  
           [0007]    Specifically, as shown in FIG. 6, after being washed with water, wiring substrates  101  are transferred horizontally to a position above an Ni-plating tank  111  by means of a transfer mechanism  103 . The transfer mechanism  103  includes a rack  105  for holding a number of the wiring substrates  101 , a vertical transfer mechanism  107  for transferring the rack  105  vertically, and a horizontal transfer mechanism  109  for transferring the rack  105  horizontally.  
           [0008]    Subsequently, from the position above the Ni-plating tank  111 , the rack  105  is moved downward by means of the vertical transfer mechanism  107 , and the rack  105  (along with the wiring substrates  101 ) is immersed for a predetermined period of time in an Ni-plating solution contained in the Ni-plating tank  111 .  
           [0009]    After an Ni-plating layer is formed on each of the wiring substrates  101 , the rack  105  is removed from the Ni-plating tank  111  by means of the vertical transfer mechanism  107 . Subsequently, the rack  105  is transferred, by means of the horizontal transfer mechanism  109 , to a position above a washing tank  113  containing washing water.  
           [0010]    Thereafter, the rack  105  is moved downward by means of the vertical transfer mechanism  107 , and the rack  105  is immersed for a predetermined period of time in the water contained in the washing tank  113 , to thereby wash the wiring substrates  101 .  
           [0011]    After the substrates are washed, the rack  105  is removed from the washing tank, and transferred to a position above an Au-plating tank (not illustrated) containing an Au-plating solution. Subsequently, the rack  105  is immersed for a predetermined period of time in the Au-plating solution contained in the Au-plating tank, to thereby form an Au-plating layer on the Ni-plating layer formed on each of the wiring substrates  101 .  
           [0012]    Thereafter, the rack  105  is removed from the Au-plating tank, transferred to the subsequent washing tank (not illustrated) containing washing water, and immersed in the water contained in the washing tank, to thereby wash the wiring substrates  101 . Subsequently, the wiring substrates  101  are dried. Thus, the Ni-plating layer and Au-plating layer are formed on a connection terminal of each of the wiring substrates  101 .  
           [0013]    3. Problems to be Solved by the Invention  
           [0014]    However, in the aforementioned production process, a certain time elapses from the time the rack  105  (along with the wiring substrates  101 ) is removed from the Ni-plating tank  111  until the rack  105  is immersed in the water contained in the washing tank  113 . In addition, when the Ni-plating layer is formed, the wiring substrates  101  are heated, since the Ni-plating solution contained in the Ni-plating tank  111  is heated at about 70-90° C. Therefore, during transfer of the wiring substrate  101 , the Ni-plating layer formed on the wiring substrate  101  is exposed to air, while the temperature of the layer is maintained at a high level.  
           [0015]    Consequently, during transfer of the wiring substrate  101 , the Ni-plating solution which remains on the substrate may oxidize, leading to problems such as blackening of the surface of the Ni-plating layer and formation of a thin oxidation film. In such cases, when solder bumps are formed on the connection terminal of the wiring substrate  101  on which the Ni-plating layer has been formed, the bonding strength between the Ni-plating layer and the solder may be lowered, thereby lowering the bonding reliability.  
         SUMMARY OF THE INVENTION  
         [0016]    In view of the foregoing, an object of the present invention is to provide a process for producing a substrate which prevents oxidation of residual Ni-plating solution and accompanying problems, including discoloration of a base-metal plating layer and lowering of bonding strength; and a plating apparatus therefor.  
           [0017]    The present invention achieves the above object by providing a process for producing a substrate including a base-metal plating layer, which process comprises an immersion step for immersing a substrate in a plating solution contained in a plating tank, to thereby form a base-metal plating layer on a substrate; a washing step which comprises removing the substrate from the plating tank, transferring the substrate to a washing tank, and washing the substrate; and a cooling step which comprises applying cooling liquid to the substrate during at least a portion of the period during which the substrate is being transferred to the washing tank after completing the immersion step, to thereby cool the substrate.  
           [0018]    According to the present invention, while the substrate which has undergone the immersion step is removed from the plating tank and is being transferred to a position where the washing step is carried out, cooling liquid is applied to the substrate, to thereby cool the substrate. When cooling liquid is applied during the substrate transfer step, the period of time during which the heated substrate (as heated in the plating tank) is exposed to air is shortened.  
           [0019]    Consequently, during transfer of the substrate, plating solution which remains on the substrate is less likely to oxidize, thereby making the substrate less prone to accompanying problems, including blackening of the surface of the plating layer and formation of a thin oxidation film. Therefore, when solder bumps are formed on the plating layer, the bonding strength between the plating layer and the solder is not lowered, and bonding reliability can be enhanced.  
           [0020]    The cooling liquid is not particularly limited, so long as the solution can cool the substrate. For example, the cooling liquid may be water, pure water, a plating solution, or a washing solution. The temperature of the cooling liquid is not particularly limited, so long as the cooling liquid can cool the substrate to a temperature so as to suppress reaction of the plating solution which remains on the substrate. The temperature of the cooling liquid may be ambient temperature. The cooling liquid itself may be cooled. Alternatively, the cooling liquid may be heated to some extent, so long as the temperature of the cooling liquid is lower than that of the plating solution contained in the plating tank.  
           [0021]    The cooling step may be carried out continuously over the entire substrate-transfer period during which the substrate is transferred from the plating tank to the subsequent washing tank, or during a portion of the transfer period.  
           [0022]    In the cooling step of the aforementioned process for producing a substrate, preferably, the cooling liquid is applied to the substrate while the substrate is being removed from the plating tank, to thereby cool the substrate.  
           [0023]    In this case, the substrate is cooled simultaneously with removal from the plating tank, thereby minimizing the time during which the plating layer of the heated substrate is exposed to air at high temperature during transfer of the substrate from the plating tank to the subsequent washing tank.  
           [0024]    Consequently, the occurrence of problems including oxidation of the surface of the plating layer can be more reliably prevented, and thus a highly reliable substrate can be produced.  
           [0025]    In the cooling step of the aforementioned processes for producing a substrate, preferably, the cooling liquid is sprayed obliquely upward, and, when falling as a result of spraying, the cooling liquid is applied to the substrate from above the substrate. When the cooling liquid is sprayed in such a manner, the cooling liquid is easily atomized in air. Therefore, the cooling liquid can be uniformly applied to the substrate, to thereby cool the substrate uniformly.  
           [0026]    Moreover, the temperature of the cooling liquid is preferably ambient temperature. This is because, when at ambient temperature, the cooling liquid can cool the substrate satisfactorily, thereby preventing problems including oxidation of the plating layer. In addition, employing a cooling liquid of ambient temperature obviates a step specifically for cooling or heating the cooling liquid, and thus costs incurred by the cooling step can be reduced.  
           [0027]    In the aforementioned processes for producing a substrate, the cooling liquid is preferably pure water.  
           [0028]    In a preferred embodiment of the present invention, pure water is used as the cooling liquid. Therefore, an apparatus for applying the cooling liquid to the substrate can be simplified, as compared with the case in which a plating solution, a washing solution, or a similar solution is used as the cooling liquid. Moreover, in the case in which pure water is used as the cooling liquid, when the cooling liquid is applied to the substrate to thereby cool the substrate above the plating tank, and the cooling liquid falls from the substrate into the tank and is mixed with the plating solution, the plating solution is not adversely affected. Similarly, use of pure water as the cooling liquid is advantageous, because when the cooling liquid is applied to the substrate to thereby cool the substrate above the washing tank, and the cooling liquid falls from the substrate into the tank, the washing solution in the tank is not adversely affected. In addition, even when the cooling liquid falls from the substrate onto the floor, the cooling liquid is relatively easily disposed of.  
           [0029]    In the aforementioned processes for producing a substrate, the base-metal plating layer is preferably an Ni-plating layer predominantly containing Ni.  
           [0030]    According to the present invention, an Ni-plating layer predominantly containing Ni is formed in the immersion step. When the Ni-plating layer is formed on the substrate, while the substrate is transferred from the plating tank to the washing tank, the surface of the Ni-plating layer is easily oxidized, particularly at high temperature. Therefore, as described above, when the cooling step is carried out during transfer of the substrate, and the substrate is cooled by the cooling liquid, occurrence of problems, including oxidation of the surface of the Ni-plating layer, can be effectively prevented.  
           [0031]    The present invention also provides a plating apparatus for forming a base-metal plating layer on a substrate, which comprises a plating tank for storing a plating solution; a washing tank for storing a washing solution; transfer means for removing from the plating tank a substrate on which a base-metal plating layer has been formed, transferring the substrate to the washing tank, and immersing the substrate into a washing solution contained in the washing tank; and cooling means for applying a cooling liquid to the substrate removed from the plating tank.  
           [0032]    According to the present invention, the plating apparatus includes the plating tank, the washing tank, the transfer means, and the cooling means. When the substrate on which the base-metal plating layer has been formed is removed from the plating tank, the cooling means allows the cooling liquid to be applied to the substrate so that the substrate is cooled.  
           [0033]    Thus, when the substrate is cooled while being removed from the plating tank, during transfer of the substrate from the plating tank to the washing tank, the time during which the heated substrate is exposed to air at high temperature is shortened. Consequently, during transfer of the substrate, residual plating solution is less likely to oxidize, thereby preventing accompanying problems, including blackening of the surface of the plating layer. Therefore, when solder bumps are formed on the plating layer, the bonding strength between the plating layer and the solder is not lowered, and bonding reliability can be enhanced.  
           [0034]    In the cooling means of the aforementioned plating apparatus, preferably, the cooling liquid is sprayed obliquely upward, and, when falling as a result of spraying, the cooling liquid is applied to the substrate from above the substrate, thus effecting cooling of the substrate. Therefore, since the sprayed cooling liquid is easily atomized in air, the cooling liquid can be uniformly applied to the substrate, to thereby cool the substrate uniformly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    [0035]FIG. 1 is an explanatory view showing an acid-degreasing tank through a sixth washing tank of a plating apparatus according to an embodiment of the present invention.  
         [0036]    [0036]FIG. 2 is an explanatory view showing an Ni-plating tank through a tenth washing tank of the plating apparatus according to an embodiment of the present invention.  
         [0037]    [0037]FIG. 3 is an explanatory view showing the vicinity of the Ni-plating tank and the seventh washing tank of the plating apparatus according to an embodiment of the present invention.  
         [0038]    [0038]FIG. 4 is a flowchart showing the step for forming an Ni-plating layer and an Au-plating layer of the process for producing a substrate according to an embodiment of the present invention.  
         [0039]    [0039]FIG. 5 is a flowchart showing the step for forming the Ni-plating layer of the process for producing a substrate according to an embodiment of the present invention.  
         [0040]    [0040]FIG. 6 is an explanatory view showing the vicinity of an Ni-plating tank and a washing tank of a plating apparatus according to the conventional art. 
     
    
     DESCRIPTION OF REFERENCE NUMERALS  
       [0041]    [0041] 1 : Wiring substrate (substrate)  
         [0042]    [0042] 11 : Plating apparatus  
         [0043]    [0043] 13 : Transfer mechanism (transfer means)  
         [0044]    [0044] 15 : Rack  
         [0045]    [0045] 17 : Vertical transfer mechanism  
         [0046]    [0046] 19 : Horizontal transfer mechanism  
         [0047]    [0047] 33 : Ni-plating tank  
         [0048]    [0048] 34 : Seventh washing tank  
         [0049]    [0049] 37 : Au-plating tank  
         [0050]    [0050] 38 : Ninth washing tank  
         [0051]    [0051] 41 : Cooling mechanism (cooling means)  
         [0052]    [0052] 43 : Spray tube  
         [0053]    [0053] 44 : Spray hole  
         [0054]    [0054] 45 : Spray nozzle  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0055]    An embodiment of the present invention will next be described with reference to the drawings. However, the present invention should not be construed as being limited thereto.  
         [0056]    In relation to a plating apparatus  11  of the present embodiment, FIG. 1 is an explanatory view showing an acid-degreasing tank  21  through a sixth washing tank  31 ; FIG. 2 is an explanatory view showing an Ni-plating tank  33  through a tenth washing tank  39 ; and FIG. 3 is an explanatory view showing the vicinity of an Ni-plating tank  33  and a seventh washing tank  34 .  
         [0057]    The plating apparatus  11  includes a transfer mechanism (transfer means)  13  for transferring wiring substrates  1 . The transfer mechanism  13  includes a rack  15  for holding a number of the wiring substrates  1 , a horizontal transfer mechanism  19  for transferring the rack  15  horizontally, and a vertical transfer mechanism  17  for transferring the rack  15  vertically.  
         [0058]    As shown in FIG. 1, the plating apparatus  11  also includes an acid-degreasing tank  21  for acid-degreasing the wiring substrates  1  to thereby remove stains from the substrates  1 , and a first washing tank  22  and a second washing tank  23  for washing residual acid off the wiring substrates  1  which have undergone acid-degreasing. The plating apparatus  11  also includes a sulfuric acid tank  25  for washing the wiring substrates  1  with sulfuric acid, and a third washing tank  26  and a fourth washing tank  27  for washing residual sulfuric acid off the wiring substrates  1  which have undergone washing with sulfuric acid. The plating apparatus  11  also includes a Pd activation treatment tank  29  for subjecting the wiring substrates  1  to Pd activation treatment before plating, and a fifth washing tank  30  and a sixth washing tank  31  for washing the wiring substrates  1  which have undergone Pd activation treatment.  
         [0059]    As shown in FIG. 2, the plating apparatus  11  also includes an Ni-plating tank  33  for forming an Ni-plating layer containing Ni-P on the wiring substrates  1 , and a seventh washing tank  34  and an eighth washing tank  35  for washing residual Ni-plating solution off the wiring substrates  1  which have undergone Ni-plating. The plating apparatus  11  also includes an Au-plating tank  37  for forming an Au-plating layer on the Ni-plating layer of the wiring substrates  1 , and a ninth washing tank  38  and a tenth washing tank  39  for washing residual plating solution off the wiring substrates  1  which have undergone Au-plating.  
         [0060]    Of these tanks, the Ni-plating tank  33  includes a cooling mechanism (cooling means)  41 , as shown in FIGS. 2 and 3. The cooling mechanism  41  includes two spray tubes  43  having a plurality of spray holes  44  for spraying pure water in an oblique upward direction of ambient temperature serving as a cooling liquid (in the present embodiment, the pure water is sprayed at an inclination of about 85° with respect to the horizontal plane) in a fountain-like manner. These spray tubes  43  are provided in the vicinity of the upper periphery of the Ni-plating tank  33  so as to be parallel to the transfer direction of the rack  15 . The cooling mechanism  41  also includes a plurality of spray nozzles  45  for atomizing and jetting the cooling liquid in a substantially horizontal direction. These spray nozzles  45  are provided along specific sides of the upper periphery of the Ni-plating tank  33 , the sides being perpendicular to the transfer direction of the rack  15 .  
         [0061]    According to the present embodiment, water serving as a washing solution is contained in the first washing tank  22 , the third washing tank  26 , the fifth washing tank  30 , the seventh washing tank  34 , and the ninth washing tank  38 , each of the washing tanks being provided immediately after one of the tanks in which the wiring substrates  1  are treated with a chemical solution. Pure water serving as a washing solution is also contained in the second washing tank  23 , the fourth washing tank  27 , the sixth washing tank  31 , the eighth washing tank  35 , and the tenth washing tank  39 , each of the washing tanks being provided immediately after one of the above washing tanks. The temperature of the washing solutions contained in the above washing tanks exclusive of the eighth washing tank  35  and the tenth washing tank  39  is ambient temperature. The washing solutions contained in the eighth washing tank  35  and the tenth washing tank  39  are heated.  
         [0062]    A process for producing a wiring substrate  1  making use of the plating apparatus  11  will next be described with reference to the drawings. However, the present invention should not be construed as being limited thereto.  
         [0063]    Firstly, a wiring substrate  1  on which an Ni-plating layer is to be formed is produced. The wiring substrate  1  used in the present embodiment includes a main surface and a back surface and has a substantially plate-like shape. The main surface includes a Cu connection terminal exposed on the surface and formed at a position to which the connection terminal of an IC chip is to be connected. The back surface includes a Cu connection terminal exposed thereon and formed at a position to which the connection terminal of a motherboard is to be connected. The wiring substrate  1  is produced by laminating resin insulating layers and conductive layers alternately by a known technique.  
         [0064]    Subsequently, a predetermined number of the wiring substrates  1  are placed in the rack  15  of the transfer mechanism  13 . Using the procedure shown in the flowcharts of FIGS. 4 and 5, an Ni-plating layer is formed on each of the connection terminals on the main and back surfaces of the individual wiring substrates  1 , and an Au-plating layer is formed on the Ni-plating layer.  
         [0065]    After a predetermined number of the wiring substrates  1  are placed in the rack  15 , in step S 1 , the wiring substrates  1  are subjected to acid-degreasing, to thereby remove stains such as oil from the substrates  1 .  
         [0066]    Specifically, as shown in FIG. 1, the rack  15  containing the wiring substrates  1  is transferred horizontally to a position above the acid-degreasing tank  21  by means of the horizontal transfer mechanism  19  of the transfer mechanism  13 . Subsequently, the rack  15  is moved downward by means of the vertical transfer mechanism  17  of the transfer mechanism  13 , and the wiring substrates  1  are immersed in an acid-degreasing solution contained in the acid-degreasing tank  21 , to thereby subject the substrates  1  to acid-degreasing. Thereafter, the rack  15  is removed from the acid-degreasing tank  21  by means of the vertical transfer mechanism  17 , and is transferred horizontally to a position above the first washing tank  22  (the subsequent tank) by means of the horizontal transfer mechanism  19 .  
         [0067]    Subsequently, in step S 2 , the acid-degreasing solution, which has been applied onto the wiring substrates  1  in step S 1 , is washed off the substrates  1 . In step S 3 , the wiring substrates  1  are washed further.  
         [0068]    Specifically, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in a washing solution contained in the first washing tank  22 , to thereby wash the substrates  1 . Subsequently, the rack  15  is removed from the first washing tank  22  by means of the vertical transfer mechanism  17 , and is transferred horizontally to a position above the second washing tank  23  by means of the horizontal transfer mechanism  19 . Then, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in a washing solution contained in the second washing tank  23 , to thereby wash the substrates  1 . Thereafter, the rack  15  is removed from the second washing tank  23  by means of the vertical transfer mechanism  17 , and is transferred to a position above the sulfuric acid tank  25  (the subsequent tank) by means of the horizontal transfer mechanism  19 .  
         [0069]    Subsequently, in step S 4 , the wiring substrates  1  are treated with sulfuric acid, to thereby remove oxidation film from the surface of the Cu terminals formed on the main and back surfaces of the individual substrates  1 . Removal of the oxidation film enables uniform application of Pd onto the Cu terminals in the below-described step S 7 .  
         [0070]    Specifically, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in sulfuric acid contained in the sulfuric acid tank  25 . Thereafter, the rack  15  is removed from the sulfuric acid tank  25  by means of the vertical transfer mechanism  17 , and is transferred to a position above the third washing tank  26  by means of the horizontal transfer mechanism  19 .  
         [0071]    Subsequently, in step S 5 , residual sulfuric acid, which has been applied onto the wiring substrates  1  in the step S 4 , is washed off the substrates  1 . In step S 6 , the wiring substrates  1  are washed further.  
         [0072]    Specifically, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in a washing solution contained in the third washing tank  26 , to thereby wash the substrates  1 . Subsequently, the rack  15  is removed from the third washing tank  26  by means of the vertical transfer mechanism  17 , and is transferred horizontally to a position above the fourth washing tank  27  by means of the horizontal transfer mechanism  19 . Then, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in a washing solution contained in the fourth washing tank  27 , to thereby wash the substrates  1 . Thereafter, the rack  15  is removed from the fourth washing tank  27  by means of the vertical transfer mechanism  17 , and is transferred to a position above the Pd activation treatment tank  29  (the subsequent tank) by means of the horizontal transfer mechanism  19 .  
         [0073]    Subsequently, in step S 7 , prior to plating, Pd, serving as nuclei for forming the below-described Ni-plating layer, is deposited onto the surface of the connection terminals formed on the main and back surfaces of the individual wiring substrates  1 .  
         [0074]    Specifically, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in a Pd activation solution, to thereby subject the wiring substrates  1  to Pd activation treatment. Thereafter, the rack  15  is removed from the Pd activation treatment tank  29  by means of the vertical transfer mechanism  17 , and is transferred to a position above the fifth washing tank  30  by means of the horizontal transfer mechanism  19 .  
         [0075]    Subsequently, in step S 8 , residual Pd activation solution, which has been applied onto the wiring substrates  1  in step S 7 , is washed off the substrates  1 . In step S 9 , the wiring substrates  1  are washed further.  
         [0076]    Specifically, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in a washing solution contained in the fifth washing tank  30 , to thereby wash the substrates  1 . Subsequently, the rack  15  is removed from the fifth washing tank  30  by means of the vertical transfer mechanism  17 , and is transferred to a position above the sixth washing tank  31  by means of the horizontal transfer mechanism  19 . Then, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in a washing solution contained in the sixth washing tank  31 , to thereby wash the substrates  1 . Thereafter, the rack  15  is removed from the sixth washing tank  31  by means of the vertical transfer mechanism  17 , and is transferred to a position above the Ni-plating tank  33  (the subsequent tank) by means of the horizontal transfer mechanism  19 .  
         [0077]    Subsequently, in step S 10 , an Ni-plating layer containing Ni-P and having a predetermined thickness is formed on the surface of the connection terminals on the main and back surfaces of the wiring substrates  1 .  
         [0078]    The subroutine of step S 10  is shown in the flowchart of FIG. 5. In step S 101 , the rack  15  is moved downward by means of the vertical transfer mechanism  19 , and the wiring substrates  1  are immersed in an Ni-P plating solution for a predetermined time. After the rack  15  is immersed in the plating solution contained in the Ni-plating tank  33 , in step S 102 , a timer is set for measuring a predetermined plating time. In step S 103 , the wiring substrates  1  are immersed in the plating solution for a predetermined plating time. After the plating time elapses, step S 104  is carried out. In step S 104 , a timer is set for measuring a showering time. Subsequently, in step S 105 , a cooling liquid is sprayed through the spray holes  44  and the spray nozzles  45  of the cooling mechanism  41 .  
         [0079]    Subsequently, in step S 106 , the rack  15  is removed from the Ni-plating tank  33  by means of the vertical transfer mechanism  17 . When the rack  15  is removed from the tank  33 , the cooling liquid is applied to the wiring substrates  1  as they move upward, to thereby cool the substrates  1 . In step S 107 , removal of the rack  15  is completed. In step S 108 , the rack  15  is transferred to the seventh washing tank  34  (the subsequent tank) by means of the horizontal transfer mechanism  19 . In step S 109 , a determination is made as to whether or not a predetermined showering time has elapsed. When the showering time has elapsed, spraying of the cooling liquid from the cooling mechanism  41  is stopped in step S 10 A. Thereafter, leaving the subroutine of the step S 10 , the production of the wiring substrates  1  proceeds according to the main routine.  
         [0080]    Conventionally, wiring substrates  1  having been heated in the Ni-plating tank  33  are transferred, without being cooled as described above, to the seventh washing tank  34 , while the heated substrates  1  are exposed to air. Therefore, during transfer of the substrates  1 , residual plating solution on the substrates  1  may oxidize, resulting in problems such as blackening of the surface of the Ni-plating layer or formation of a thin oxidation film.  
         [0081]    However, according to this embodiment, while the wiring substrates  1  are removed from the Ni-plating tank  33  and transferred to the seventh washing tank  34 , the substrates  1  are cooled by applying the cooling liquid thereto. Therefore, the time during which the substrates  1  heated in the Ni-plating tank  33  are exposed to air at high temperature is shortened, and thus the aforementioned problems can be prevented.  
         [0082]    In addition, the wiring substrates  1  are cooled by applying the cooling liquid simultaneously with removal from the Ni-plating tank  33 . Therefore, the time during which the substrates  1  of high temperature are exposed to air is minimized, and thus the aforementioned problems can be prevented most reliably.  
         [0083]    In the present embodiment, as shown in FIG. 3, the cooling liquid is sprayed obliquely upward from the spray holes  44  of the spray tubes  43 , and the falling cooling liquid is applied to the wiring substrates  1 . Therefore, the cooling liquid is easily atomized, and thus the solution can be uniformly applied to the substrates  1 , to thereby cool the substrates  1  uniformly.  
         [0084]    In addition, since the cooling liquid is used at ambient temperature, a step for cooling or heating the cooling liquid is not required, and thus costs incurred by the cooling step can be reduced.  
         [0085]    In this embodiment, pure water is used as the cooling liquid. Therefore, an apparatus for applying the cooling liquid to the wiring substrates  1  can be simplified, as compared with the case in which an Ni-plating solution or a washing solution is used as the cooling liquid. Although the cooling liquid applied to the wiring substrates  1  falls into the Ni-plating tank  33  and is mixed with the plating solution, the plating solution is not adversely affected, since pure water is used as the cooling liquid.  
         [0086]    In the present embodiment, the plating layer is formed on the wiring substrates  1  from Ni-P predominantly containing Ni. The Ni-plating layer predominantly containing Ni is easily oxidized at high temperature while the substrates  1  are transferred from the Ni-plating tank  33  to the seventh washing tank  34 . Therefore, provision of the cooling step during transfer of the substrates  1  as described above enhances the effect of preventing, among other problems, oxidation of the surface of the Ni-plating layer.  
         [0087]    The sprayed cooling liquid (pure water) falls into the Ni-plating tank  33  and is mixed with the plating solution. Consequently, the concentration of the plating solution is lowered. However, according to the present embodiment, when the concentration of the plating solution drops below a predetermined level, fresh plating solution is added to the diluted plating solution such that the concentration of the plating solution is maintained at a constant level. Meanwhile, when the temperature of the plating solution drops below a predetermined level as a result of mixing of the cooling liquid with the plating solution, the plating solution is heated such that the temperature of the solution is maintained at a constant level.  
         [0088]    When the sprayed cooling liquid falls into the Ni-plating tank  33  and is contained therein, the level of the plating solution increases. However, according to the present embodiment, an outlet (not illustrated) is provided in the Ni-plating tank  33 , and thus when the level of the solution rises above a predetermined level, the excess plating solution is discharged through the outlet. Therefore, the level of the plating solution can be maintained at a constant level.  
         [0089]    Subsequently, as shown in FIG. 4, in step S 11 , residual plating solution which remains on the wiring substrates  1  is washed off. In step S 12 , the wiring substrates  1  are washed further.  
         [0090]    Specifically, the rack  15  having been transferred horizontally to a position above the seventh washing tank  34  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in a washing solution contained in the seventh washing tank  34 . Subsequently, the rack  15  is moved upward by means of the vertical transfer mechanism  17 , and is transferred to a position above the eighth washing tank  35  by means of the horizontal transfer mechanism  19 . Then, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in a washing solution contained in the eighth washing tank  35 , to thereby be washed. Thereafter, the rack  15  is moved upward by means of the vertical transfer mechanism  17 , and is transferred to a position above the Au-plating tank  37  (the subsequent tank) by means of the horizontal transfer mechanism  19 .  
         [0091]    Subsequently, in step S 13 , an Au-plating layer of predetermined thickness is formed on each of the Ni-plating layers formed on the connection terminals on the main and back surfaces of the individual wiring substrates  1 .  
         [0092]    Specifically, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in an Au-plating solution for a predetermined period of time, to thereby form an Au-plating layer on the substrates  1 . Thereafter, the rack  15  is removed from the Au-plating tank  37  by means of the vertical transfer mechanism  17 , and is transferred to a position above the ninth washing tank  38  by means of the horizontal transfer mechanism  19 .  
         [0093]    Subsequently, in step S 14 , residual Au-plating solution which remains on the wiring substrates  1  is washed off. In step S 15 , the wiring substrates  1  are washed further.  
         [0094]    Specifically, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in a washing solution contained in the ninth washing tank  38 . Subsequently, the rack  15  is removed from the ninth washing tank  38  by means of the vertical transfer mechanism  17 , and is transferred to a position above the tenth washing tank  39  by means of the horizontal transfer mechanism  19 . Then, the rack  15  is moved downward by means of the vertical transfer mechanism  17 , and the wiring substrates  1  are immersed in a washing solution contained in the tenth washing tank  39 , to thereby be washed. Thereafter, the rack  15  is removed from the tenth washing tank  39  by means of the vertical transfer mechanism  17 , and is transferred by means of the horizontal transfer mechanism  19 .  
         [0095]    While the wiring substrates  1  are transferred from the Au-plating tank  37  to the ninth washing tank  38 , the above-described cooling step is not carried out. This is because, unlike a base metal, Au, which is a precious metal, is not susceptible to chemical reaction, and thus the surface of the Au-plating layer is not easily oxidized even when the wiring substrates  1  are transferred while being heated. Therefore, provision of the aforementioned cooling step is not necessary.  
         [0096]    Subsequently, in step S 16 , the wiring substrates  1  are dried, and production of the wiring substrates  1  is completed.  
         [0097]    The wiring substrates  1  are not prone to problems, including blackening of the surface of the Ni-plating layer (between the Ni-plating layer and the Au-plating layer) and formation of an oxidation film. Therefore, even when solder bumps are formed on the connection terminals on the main and back surfaces of the individual wiring substrates  1 , reliability of the bonding between the solder and the Ni-plating layer on each of the connection terminals can be enhanced.  
         [0098]    The present invention has been described by way of the above embodiment, which should not be construed as limiting the invention thereto, and it is to be understood that modifications and variations may be made without departing from the spirit and scope of the present invention.  
         [0099]    In the aforementioned embodiment, while the wiring substrates  1  are transferred from the Ni-plating tank  33  to the seventh washing tank  34 , the cooling liquid is applied to the substrates  1  only at a time when the rack  15  is removed from the Ni-plating tank  33 , to thereby cool the substrates  1 .  
         [0100]    However, while the wiring substrates  1  are transferred from the Ni-plating tank  33  to the seventh washing tank  34 , the cooling liquid may be applied to the substrates  1  continuously. However, in this case, some of the cooling liquid falls onto the floor. Therefore, as described in the above embodiment, the cooling liquid is preferably applied to the substrates  1  only at a time when the rack  15  is being removed from the Ni-plating tank  33 . Since satisfactory cooling of the substrates  1  is attained so long as oxidation of the surface of the Ni-plating layer can be prevented, the cooling liquid is preferably applied to the substrates  1  only for a period of time during which the rack  15  is being removed from the Ni-plating tank  33 , from the viewpoint of economy in the amount of the cooling liquid employed.  
         [0101]    While the wiring substrates  1  are transferred from the Ni-plating tank  33  to the seventh washing tank  34 , the cooling liquid may be applied to the substrates  1  during a period of time when the rack  15  transferred horizontally to a position above the seventh washing tank  34  is being moved downward into the tank  34 . In this case, it is preferable to prevent falling of the cooling liquid into the Ni-plating tank  33  and dilution of the plating solution with the cooling liquid. However, in this case, while the heated wiring substrates  1  are transferred from the Ni-plating tank  33  to a position above the seventh washing tank  34 , the substrates  1  are exposed to air, and thus the surface of the Ni-plating layer is easily oxidized. Therefore, as described above in the present embodiment, simultaneous cooling of the wiring substrates  1  and removal of rack  15  from the Ni-plating solution is best from the viewpoint of prevention of oxidation of the Ni-plating layer.  
         [0102]    In connection with the above embodiment, a process for producing a multi-resin-layer-made wiring substrate in which a number of resin insulating layers and conductive layers are laminated has been described. However, the present invention can be applied to any substrate such as a ceramic substrate, so long as an Ni-plating layer is formed on the substrate.  
         [0103]    In the embodiment of the present invention, the Ni-plating layer containing Ni-P is employed as a base-metal plating layer. When the present invention is applied to the case in which a layer plated with a base metal other than Ni-P (e.g., Ni-B) is employed, effects similar to those obtained in the above embodiment can be obtained, since such a plating layer is easily oxidized at high temperature while a substrate including the layer is transferred from a plating tank to the subsequent washing tank.  
         [0104]    This application is based on Japanese Patent Application No. 2000-183126, filed Jun. 19, 2000, the disclosure of which is incorporated herein by reference in its entirety.