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 . in relation to a plating apparatus 11 of the present embodiment , fig1 is an explanatory view showing an acid - degreasing tank 21 through a sixth washing tank 31 ; fig2 is an explanatory view showing an ni - plating tank 33 through a tenth washing tank 39 ; and fig3 is an explanatory view showing the vicinity of an ni - plating tank 33 and a seventh washing tank 34 . 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 . as shown in fig1 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 . as shown in fig2 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 . of these tanks , the ni - plating tank 33 includes a cooling mechanism ( cooling means ) 41 , as shown in fig2 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 . 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 . 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 . 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 . 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 fig4 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 . 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 . specifically , as shown in fig1 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . the subroutine of step s 10 is shown in the flowchart of fig5 . 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 . 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 . 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 . 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 . 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 . in the present embodiment , as shown in fig3 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 . 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 . 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 . 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 . 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 . 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 . subsequently , as shown in fig4 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 . 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 . 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 . 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 . 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 . 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 . 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 . subsequently , in step s 16 , the wiring substrates 1 are dried , and production of the wiring substrates 1 is completed . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 .