Patent Publication Number: US-2021172084-A1

Title: Method for reproducing plating solution

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Japanese Patent Application No. 2019-220573, filed on Dec. 5, 2019, the entire disclosure of which as is incorporated by reference herein in its entirety. 
     BACKGROUND 
     The present invention relates to a method for reproducing a plating solution. 
     In integrated circuit (IC) packages, for example, solder paste and solder balls have widely been used for connection with ICs. In recent years, electrode areas have been microminiaturized, and thus it has been difficult for the conventional solder paste and the like to cope with the situation. 
     Given these circumstances, as a measure therefor, a method is proposed which uses an electrolytic tin plating solution or an electrolytic tin alloy plating solution for formation of bumps of semiconductor chips. More specifically, a plating solution is proposed which contains a soluble salt containing a stannous salt, and an acid selected from an organic acid and an inorganic acid or a salt thereof, two kinds of surfactants, namely, an amine-based surfactant and a nonionic surfactant, and an additive such as indole, for example. It is shown that use of such a plating solution achieves height uniformity of solder bumps over a wide current density range, and prevents or reduces occurrence of voids during formation of the bumps (see, e.g., Japanese Unexamined Patent Publication No. 2018-162512). 
     BRIEF SUMMARY 
     In the plating solution for bump formation, an additive such as indole is used as in Japanese Unexamined Patent Publication No. 2018-162521 because a plated coating is required to be uniform and to have a flat surface. Unfortunately, when electrolytic treatment is performed on the plating solution, this additive changes to form impurities (a water-insoluble substance as a byproduct accompanying decomposition of the additive), and consequently, it is disadvantageously impossible to prevent or reduce occurrence of voids within the plated coating after reflow. 
     In view of the foregoing problems, it is therefore an object of the present invention to provide a method for reproducing a plating solution, the method including removing impurities from a plating solution to prevent or reduce occurrence of voids within a plated coating after reflow. 
     To achieve the above object, a method for reproducing a plating solution according to the present invention includes bringing a plating solution containing a leveler into contact with silica particles with an average particle diameter of 500 μm or less to remove impurities from the plating solution. 
     The present invention can remove impurities from a plating solution and can thus prevent or reduce occurrence of voids within a plated coating after reflow. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic diagram for illustrating a method for reproducing a plating solution of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS 
     The following describes a method for reproducing a plating solution of the present invention. 
     Plating Solution to be Treated 
     A plating solution to which the method of reproduction of the present invention is applied is not limited to a particular plating liquid. Examples thereof include an electrolytic tin plating solution and an electrolytic tin alloy plating solution for use in formation of connection bumps. 
     The electrolytic tin plating solution or the electrolytic tin alloy plating solution can contain a tin salt as a compound as a tin ion supply source. In one preferred embodiment, a stannous salt (tin salt(II)) and a stannic salt (tin salt(IV)) are used. 
     The stannous salt (tin salt(II)) is not limited to a particular stannous salt. Examples thereof include tin(II) alkane sulfonates such as tin(II) methane sulfonate, tin(II) organic sulfonates such as tin(II) alkanol sulfonates such as tin(II) isethionate, tin(II) sulfate, tin(II) borofluoride, tin(II) chloride, tin(II) bromide, tin(II) iodide, tin(II) oxide, tin(II) phosphate, tin(II) pyrophosphate, tin(II) acetate, tin(II) citrate, tin (II) gluconate, tin(II) tartrate, tin(II) lactate, tin(II) succinate, tin(II) sulfamate, tin(II) formate, and tin(II) silicofluoride. 
     The stannic salt (tin salt(IV)) is not limited to a particular stannic salt. Examples thereof include sodium stannate and potassium stannate. In one preferred embodiment, tin(II) alkane sulfonates such as tin(II) methane sulfonate and tin(II) organic sulfonates such as tin(II) alkanol sulfonates such as tin(II) isethionate are used. 
     The concentration of the tin salt (the concentration as Sn 2+ ) is preferably 5 g/L or more, more preferably 10 g/L or more in order to reduce the occurrence of burning and scorching in the film. The concentration of the tin salt is preferably 120 g/L or less, more preferably 90 g/L or less in order to improve the stability of a plating bath and reduce the occurrence of precipitates. Being such a concentration is advantageous also in view of costs. 
     As the tin salt, a low-concentration lead tin salt with a lead (Pb) concentration of 1.0 ppm or less can also be used. Use of the low-concentration lead tin salt can achieve low-concentration lead. 
     The electrolytic tin alloy plating solution can contain a silver salt as a compound as a silver ion supply source. This silver salt is not limited to a particular silver salt. Examples thereof include silver organic sulfonates, silver sulfate, silver borofluoride, silver chloride, silver bromide, silver iodide, silver oxide, silver phosphate, silver pyrophosphate, silver acetate, silver citrate, silver gluconate, silver tartrate, silver lactate, silver succinate, silver sulfamate, silver formate, and silver silicofluoride. Among these, in one particularly preferred embodiment, silver organic sulfonates are used. 
     The concentration of the compound as the silver ion supply source (the concentration as Ag + ) is preferably 10 mg/L or more, more preferably 20 mg/L or more in order to easily control the plating solution. The concentration of the compound as the silver ion supply source is 1,000 mg/L or less, more preferably 500 mg/L or less in view of costs. 
     The electrolytic tin alloy plating solution may further contain a compound as a copper (Cu) ion supply source. Addition of the compound as the copper ion supply source can form a film of a Sn—Ag—Cu ternary alloy. 
     As the copper ion supply source, a copper salt can be used and this copper salt is not limited to a particular copper salt. Examples thereof include copper organic sulfonates, copper sulfate, copper borofluoride, copper chloride, copper bromide, copper iodide, copper oxide, copper phosphate, copper pyrophosphate, copper acetate, copper citrate, copper gluconate, copper tartrate, copper lactate, copper succinate, copper sulfamate, copper formate, and copper silicofluoride. Among these, in one particularly preferred embodiment, copper organic sulfonates are used. 
     The concentration of the compound as the copper ion supply source (the concentration as Cu + ) is preferably 10 mg/L or more, more preferably 50 mg/L or more in order to easily control the plating solution. The concentration of the compound as the copper ion supply source is preferably 5,000 mg/L or less, more preferably 2,000 mg/L or less in view of bath stability. 
     The electrolytic tin plating solution or the electrolytic tin alloy plating solution contains a leveler in order to improve the uniformity of a plated coating and the flatness of the surface shape thereof. As this leveler, a nitrogen-containing aromatic compound is used, for example. Examples of this nitrogen-containing aromatic compound include indole, pyridine, pyrazine, pyrimidine, pyridazine, triazine, tetrazine, acridine, quinoline, isoquinoline, quinoxaline, quinazoline, cinnoline, indazole, imidazole, benzimidazole, isoindole, benzothiazole, phenazine, iminostilbene, quinaldine, purine, 1,10-phenanthroline, carbazole, acrinol, benzotriazole, benzoxazole, and derivatives thereof. 
     The concentration of the nitrogen-containing aromatic compound is preferably 0.001 g/L or more, more preferably 0.01 g/L or more in order to improve the flatness of the surface shape of the film. The concentration of the nitrogen-containing aromatic compound is preferably 20 g/L or less, more preferably 10 g/L or less in view of costs. 
     The electrolytic tin plating solution or the electrolytic tin alloy plating solution may contain any of an inorganic acid, an organic acid, and a water-soluble salt thereof. Addition of the acid or water-soluble salt thereof allows the pH of the surface of an object to be plated and a tin surface or Sn—Ag alloy surface serving as the plated coating to be made uniform, thus achieving uniform surface electric potential. This can reduce co-deposition of lead. 
     The acid or water-soluble salt thereof is not limited to particular one. Examples thereof include sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, sulfamic acid, organic sulfonic acids (alkane sulfonic acids such as methane sulfonic acid and alkanol sulfonic acids such as isethionic acid), and carboxylic acids (aromatic carboxylic acids, aliphatic saturated carboxylic acids, and amino carboxylic acids). Neutralization salts of these water-soluble salts can also be used as needed. Among these, in one preferred embodiment, methane sulfonic acid, which is easy to handle, is used. 
     The concentration of the acid or water-soluble salt thereof is preferably 35 g/L or more, more preferably 50 g/L or more in order to improve the stability of the plating solution and reduce occurrence of precipitates. Such a concentration is advantageous also in view of lead precipitation potential. The concentration of the acid or water-soluble salt thereof is preferably 500 g/L or less, more preferably 300 g/L or less, even more preferably 200 g/L or less in view of costs. 
     The electrolytic tin plating solution or the electrolytic tin alloy plating solution may contain a surfactant. As the surfactant, one or more selected from an anionic surfactant, a cationic surfactant, and a nonionic surfactant can be used. Among these, in one preferred embodiment, the nonionic surfactant is used, and in one more preferred embodiment, an alkylene oxide-based one is used. Addition of the surfactant allows the current density of the object to be plated and a tin crystal surface serving as the plated coating to be made uniform, thus maintaining uniform precipitation potential on the surface. This can reduce co-deposition of lead. 
     The alkylene oxide-based surfactant is not limited to a particular surfactant. Examples thereof include polyoxyethylene alkyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene alkyl amines, polyoxyethylene alkyl amides, polyoxyethylene aliphatic esters, polyoxyethylene polyhydric alcohol ethers, ethylene oxide-propylene oxide block copolymerized compounds, ethylene oxide-propylene oxide random copolymerized compounds, and propylene oxide polymerized compounds. Among these, in one preferred embodiment, polyoxyethylene alkylphenyl ethers are used. 
     The concentration of the surfactant is preferably 0.05 g/L or more, more preferably 0.5 g/L or more. Being such a concentration, even when plating is performed with a high current density in order to reduce a plating time, can reduce occurrence of burning and scorching in parts with a high current density. The concentration of the surfactant is preferably 100 g/L or less in order to reduce the occurrence of color unevenness caused by blackening of the plated coating. 
     The electrolytic tin plating solution or the electrolytic tin alloy plating solution contains an acid or water-soluble salt thereof. The acid or water-soluble salt thereof is one or more acids or water-soluble salts thereof selected from sulfuric acid, hydrochloric acid, nitric acid, phosphoric acid, sulfamic acid, organic sulfonic acids, and carboxylic acids or salts thereof. 
     The electrolytic tin plating solution or the electrolytic tin alloy plating solution can contain an organic solvent, an antioxidant, and a chelating agent. The organic solvent is not limited to a particular organic solvent. Examples thereof include monohydric alcohols such as methanol and 2-propanol and dihydric alcohol such as ethylene glycol, diethylene glycol, and triethylene glycol. The antioxidant is not limited to a particular antioxidant. Examples thereof include catechol, hydroquinone, 4-methoxyphenol, and ascorbic acid. The chelating agent is not limited to a particular chelating agent. Examples thereof include oxalic acid, succinic acid, malonic acid, glycolic acid, gluconic acid, gluconolactone, glycine, ethylenediamine acetic acid, pyrophosphoric acid, and tripolyphosphoric acid. 
     When the plated coating is formed using the electrolytic tin plating solution or the electrolytic tin alloy plating solution, in one preferred embodiment, the pH of the plating solution is strongly acidic. Temperature when the plated coating is formed is not limited to a particular temperature. In one preferred embodiment, the temperature is 20° C. or more and 40° C. or less. Current density when the plated coating is formed is preferably 0.1 A/dm 2  or more, more preferably 0.5 A/dm 2  or more and is preferably 20 A/dm 2  or less, more preferably 10 A/dm 2  or less. 
     The electrolytic tin plating solution or the electrolytic tin alloy plating solution can be used for formation of plated bumps of semiconductor chips and package substrates, for example. In forming the plated bumps, reflow treatment may be performed after a plated coating with a certain size is formed at a certain position. This reflow treatment is not limited to particular reflow treatment and can be performed using a normal reflow apparatus. 
     Method for Reproducing Plating Solution 
     As described above, when electrolytic treatment is performed using the electrolytic tin plating solution or the electrolytic tin alloy plating solution containing the leveler such as indole, there is a problem in that caused by decomposition of the leveler, the impurities (the water-insoluble substance as a byproduct accompanying decomposition of the leveler, for example) are generated in the plating solution, and consequently, voids occur within the plated coating after reflow. 
     Given these circumstances, the inventors of the present invention have studied the above problem to find out that the plating solution containing the impurities is circulated to be brought into contact with silica particles (SiO 2 ), whereby the impurities can be removed from the plating solution. 
     The following specifically describes the method for reproducing a plating solution of the present invention with reference to the accompanying drawing.  FIG. 1  is a schematic diagram for illustrating the method for reproducing a plating solution of the present invention. 
     As illustrated in  FIG. 1 , the method for reproducing a plating solution of the present invention circulates a plating solution  2  (that is, the electrolytic tin plating solution or the electrolytic tin alloy plating solution containing the leveler such as indole described above) contained in a plating tank  1  by a pump  3  and brings the plating solution  2  into contact with silica particles  5  contained in an impurity removing device  4  disposed in a circulating path of the plating solution  2  to remove impurities  6  from the plating solution  2 . 
     More specifically, first, the plating solution  2  is circulated to be brought into contact with the silica particles  5  to cause the leveler within the plating solution  2  to adsorb onto the silica particles  5 . Next, electrolytic treatment is performed with the plating solution  2  in the plating tank  1  to form the plated bumps described above, for example. In this process, as described above, the impurities  6  are generated in the plating solution  2  caused by decomposition of the leveler; the plating solution  2  is constantly circulated, whereby as illustrated in  FIG. 1 , the impurities  6  are brought into contact with the silica particles  5  and are adsorbed onto the silica particles  5  instead of the leveler. 
     Consequently, the silica particles  5  can remove the impurities  6  from the plating solution  2 , and consequently, this can prevent or reduce occurrence of voids within the plated coating after reflow. 
     As illustrated in  FIG. 1 , the plating solution  2  from which the impurities  6  are removed is circulated by the pump  3  to be returned again to the plating tank  1 . 
     The impurity removing device  4  is not limited to a particular device. Examples thereof include a bag filter. An opening of this bag filter is not limited to a particular value. In one preferred embodiment, the opening is 4 μm or less in order to surely prevent an outflow of the silica particles contained in the filter (that is, filter passing) caused by circulation of the plating solution. 
     In the present invention, an average particle diameter of the silica particles is 500 μm or less in order to surely perform adsorption of the impurities. When the average particle diameter is larger than 500 μm, the pore size of the silica particles is large, and selective adsorptivity for the impurities is lost. That is to say, this is because when the size of the silica particles increases, matter other than the impurities is also adsorbed and removed, which may make removal of the impurities difficult (that is, reduce impurity removal performance). 
     In order to surely prevent the filter passing described above, in one preferred embodiment, the lower limit value of the average particle diameter of the silica particles is larger than the opening of the bag filter described above; more specifically, in one preferred embodiment, the lower limit value is 20 μm or more. 
     The “average particle diameter” referred to in this specification indicates a 50% particle diameter (D50) and can be measured with, e.g., a particle size distribution measurement apparatus using the laser Doppler method (manufactured by Nikkiso Co., Ltd., Nanotrac (registered trademark) particle size distribution measurement apparatus UPA-EX150). 
     EXAMPLES 
     The following describes the invention according to the present application more specifically based on examples and comparative examples. The present invention is not limited to the following examples at all. 
     Example 1 
     Formation of Plated Coating First, a base material was subjected to electrolytic nickel plating (electrolytic nickel plating solution: manufactured by C. Uyemura &amp; Co., Ltd., product name: Thrunic AMT, liquid temperature: 50° C., current density: 1 A/dm 2 , plating time: 10 minutes). 
     Next, mixing was performed so as to contain tin(II) alkane sulfonate as a tin salt of 70 g/L as tin (Sn 2+ ), methane sulfonic acid as an organic acid of 100 g/L, polyoxyethylene bisphenol A ether as a surfactant of 50 g/L, and indole of 5 g/L, and the mixture was stirred to prepare an electrolytic tin plating solution of the present example. 
     Next, this electrolytic tin plating solution was contained in a plating tank. While the electrolytic tin plating solution was circulated using a pump, using the electrolytic tin plating solution, a tin plated coating was formed on the surface of nickel as the base material with a liquid temperature of 30° C. and a current density of 4 A/dm 2 . 
     A bag filter as an impurity removing device (manufactured by Eaton, product name: LCR-113-TO1E-401, opening: 4 μm) was disposed in a circulating path of the plating solution. Silica particles with an average particle diameter of 50 μm (manufactured by Evonik Industries, product name: Sipernat 50) in an amount of 1 kg was contained in the bag filter. A tin plated coating was formed while the electrolytic tin plating solution was circulated using a pump to bring the electrolytic tin plating solution into contact with the silica particles. 
     Void Evaluation 
     After being reflowed at 260° C., the obtained tin plated coating was evaluated for the presence or absence of voids with an X-ray nondestructive inspection apparatus (manufactured by Nordson Dage, product name: XD7600NT Diamond FP). The X-ray nondestructive inspection apparatus was set to give a tube voltage of 60 kV and an output of 1.5 W. Table 1 lists a result of the foregoing. 
     Example 2 
     A tin plated coating was formed, and void evaluation was performed in a manner similar to Example 1 except that the average particle diameter of the silica particles was changed to 120 μm (product name: CARPLEV XR manufactured by Evonik Industries was used). Table 1 lists a result of the foregoing. 
     Example 3 
     A tin plated coating was formed, and void evaluation was performed in a manner similar to Example 1 except that the average particle diameter of the silica particles was changed to 300 μm (product name: Nipsil AQ manufactured by Tosoh Silica Corporation was used). Table 1 lists a result of the foregoing. 
     Comparative Example 1 
     A tin plated coating was formed, and void evaluation was performed in a manner similar to Example 1 except that the bag filter containing the silica particles was not disposed in the circulating path of the plating solution, and the plating solution was not brought into contact with the silica particles. Table 1 lists a result of the foregoing. 
     Comparative Example 2 
     A tin plated coating was formed, and void evaluation was performed in a manner similar to Example 1 except that silica particles with an average particle diameter of larger than 500 μm (manufactured by Fuji Silysia Chemical Ltd., product name: Fuji silica gel ID40) was used in place of the silica particles with an average particle diameter of 50 μm. Table 1 lists a result of the foregoing. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                   
                 Compar- 
                 Compar- 
               
               
                   
                   
                   
                   
                 ative 
                 ative 
               
               
                   
                 Example 1 
                 Example 2 
                 Example 3 
                 Example 1 
                 Example 2 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Presence or 
                 Present 
                 Present 
                 Present 
                 Absent 
                 Present 
               
               
                 Absence of 
               
               
                 Silica 
               
               
                 Treatment 
               
               
                 Average 
                 50 
                 120 
                 300 
                 None 
                 &gt;500 
               
               
                 Particle 
               
               
                 Diameter 
               
               
                 of Silica 
               
               
                 Particles 
               
               
                 (μm) 
               
               
                 Occurrence 
                 Not 
                 Not 
                 Not 
                 Occurred 
                 Occurred 
               
               
                 of Voids 
                 Occurred 
                 Occurred 
                 Occurred 
               
               
                   
               
            
           
         
       
     
     As listed in Table 1, in Examples 1 to 3, the tin plated coating was formed while the plating solution containing the leveler was brought into contact with the silica particles with an average particle diameter of 500 μm or less, and it is thus shown that the impurities in the plating solution were removed by the silica particles, and consequently, no voids occurred within the plated coating after reflow. 
     On the other hand, in Comparative Example 1, the plating solution was not brought into contact with the silica particles, and it is thus shown that the impurities were not removed from the plating solution, and voids occurred within the plated coating after reflow. 
     In Comparative Example 2, the average particle diameter of the silica particles was larger than 500 μm, and it is thus shown that the impurities were not removed from the plating solution, and voids occurred within the plated coating after reflow. 
     The method for reproducing a plating solution of the present invention is suitably used in plating solutions for use in formation of plated bumps of semiconductor chips and package substrates in particular.