Abstract:
An apparatus and process for adding electrolytically dissolved tin to the electrolyte solution of a tin plating cell is described. The tin plating process cell has an insoluble anode. In conventional plating processes, this requires the addition of tin salts to the process cell electrolyte. The tin salts represent a substantial cost, both in term of materials and waste removal. The present plating apparatus includes a secondary cell, separate from the main process plating cell, which has a dedicated rectifier, and in which a soluble tin anode and a cathode are separated by a perm-selective ion exchange membrane. The anode compartment of the secondary cell is hydraulically connected to the process cell and serves to continuously add tin to the plating process, as needed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority on provisional application Ser. No. 60/113,322, filed Dec. 22, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a metal plating process and, in particular, to a novel process and apparatus for plating a workpiece with tin. 
     2. Prior Art 
     When insoluble anodes are used in a methane sulfonic acid (MSA) based tin plating process, or other tin electroplating process such as those provided with a tin sulfate or a tin fluoroborate compound, all of the plated tin is derived from the dissolved tin salt. In other words, the tin MSA, tin fluoroborate or tin sulfate is the sole source for the plated tin. The tin compound represents a significant cost to the plating process. Additionally, the acidity of the plating bath builds up over time, necessitating periodic bailouts. After the bailout, tin MSA or the other tin bearing salt and organic additives must be added back into the plating bath. Also, the bailed out solution is a waste product which must be treated. These are all steps in a conventional tin plating procedure which add cost to the final product. 
     The present invention, as an improvement on the prior art plating process, eliminates, or greatly reduces the need to periodically add a tin salt to the plating bath, and for removing and treating the acid built up in the plating bath. 
     SUMMARY OF THE INVENTION 
     According to the present invention, tin in the process cell, which is used up or plated out during the plating operation, is replenished with tin metal from a secondary cell. The secondary cell is hydraulically connected to the process cell. Tin metal costs significantly less, i.e., about 85% less, than tin MSA. Acid bailout and the costs associated with organic additives are eliminated or reduced and, consequently, waste treatment costs are significantly reduced. A further advantage of the present invention is that the plating process operates at a relatively constant concentration of tin and acid. 
    
    
     These and other objects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and the appended drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic representation of the plating process of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawing, FIG. 1 illustrates the process of the present invention including a primary or process, electroplating cell  10  and a secondary cell  12 . The process cell comprises an insoluble anode  14  and a cathode workpiece  16  electrically connected to each other by a circuit including a main or process rectifier  18  and an electrolyte bath  20 . A preferred material for the insoluble anode is titanium coated with iridium oxide and/or ruthenium oxide, commercially available from Eltech Inc. under the designation DSA. An alternate material for the insoluble anode is platinum plated titanium. 
     The secondary cell  12  comprises a soluble anode  22  and a tin sheet cathode  24  electrically connected to each other by a circuit including a secondary rectifier  26  and activating electrolyte. Titanium, tin, stainless steel, copper, copper alloys, steel, ferrous alloys and nickel alloys are materials which are also useful for the secondary cathode. The soluble anode  22  and the stainless steel cathode  24  are separated from each other by a permeability-selective (perm-selective) ion exchange membrane  28 , which essentially segregates the secondary cell into two compartments, one being the anolyte compartment  30  and the other the catholyte compartment  32 . The perm-selective membrane  28  may be of the cationic or anionic type. 
     The process cell  10  is in fluid flow communication with the secondary cell  12  by a conduit  34  and circulation pump  36  which circulates the electrolyte in the anolyte compartment  30  of the secondary cell  12  to the process cell  10 . A second, overflow conduit  38  drains from the process cell  10  to the anolyte compartment  30  of the secondary cell  12 . 
     During the electroplating process of the present invention, oxygen is produced and released at the insoluble anode  14  of the process cell  10 , as shown by equation I, and tin, provided by the tin salt in the electrolyte  20 , is reduced to tin metal at the cathode workpiece  16 , as shown by equation II. 
     
       
         H 2 O→½O 2 +2H + +2e  I 
       
     
     
       
         (CH 3 SO 2 O) 2 Sn+2H + 2e→Sn 0 +2CH 3 SO 2 OH  II 
       
     
     In the hydraulically connected anolyte compartment  30  of the secondary cell  12 , tin is dissolved from the soluble anode  22 , thereby replenishing the tin concentration in the anolyte compartment. The anolyte compartment  30  of the secondary cell  12  is in fluid flow communication with the process cell  10 . According to the present invention, the electrolyte having the replenished tin concentration from the anolyte compartment is moved to the process cell  10  through conduit  34  by pump  36 . The anolyte electrolyte consumes the acid produced at the insoluble anode  14  in the process cell, as shown by equation III. The tin methane sulfonic acid formed in the anolyte compartment  30  of the secondary cell  12  is substantially prevented from flowing into the catholyte compartment  32  by the perm-selective ion exchange membrane  28 . In the catholyte compartment  32 , water is dissociated producing hydrogen gas and hydroxyl ions, as shown by equation IV. 
     A perm-selective ion exchange membrane useful with the present invention is a cationic ion exchange membrane which allows only about 10% of the tin dissolved from the anode  22  into the anolyte  30  to migrate to the catholyte compartment  32  to be deposited on the cathode  24 , as shown by equation V. A preferred cation exchange membrane is of a perfluorinated ion exchange polymer reinforced with a support cloth of polytetrafluoroethylene. Such membranes are commercially available from DuPont under the NAFION designation. A perfluorinated ion exchange polymer membrane is permeable to cations and polar compounds, but almost completely rejects anions and nonpolar species. Anionic membranes are also useful with the present invention. Therefore, it is contemplated by the scope of the present invention that any cationic or anionic type membrane capable of preventing at least about 90% of the tin produced by the soluble anode from migrating to the secondary cathode is suitable. 
     
       
         Sn 0 +2CH 3 SO 2 OH→(CH 3 SO 2 O) 2 Sn + +2H + +2e  III 
       
     
     
       
         2H 2 O+2e→H 2 +2OH  IV 
       
     
     
       
         (CH 3 SO 2 O) 2 Sn+2H + +2e→Sn 0 +2CH 3 SO 2 OH  V 
       
     
     In a conventional tin plating process, using insoluble anodes, the tin concentration in the electrolyte  20  gradually declines as the acid concentration increases, as shown by equation II. The diminished tin must be replaced by adding tin methane sulfonic acid or some other salt such as tin sulfate or tin fluoroborate. Tin methane sulfate is about seven to eight times as expensive as tin metal, based on the tin value. Furthermore, the increasing acid concentration of the electrolyte  20  necessitates a periodic partial bailout of the process cell  10  to reduce acid concentration to a desired level. In addition to there being a cost associated with waste treatment of the removed acid, the bailouts also remove a portion of the organic additives used for grain refinement and brightening of the deposited tin layer. 
     The plating process of the present invention overcomes the drawbacks inherent in conventional plating processes by replacing at least 90% of the tin needed to maintain the electrolyte  20  of the process cell  10  at an operable tin concentration with tin from the anolyte compartment  30  of the secondary cell  12 . The costs incurred in operating the secondary cell  12  are predominantly those associated with the rectifier connected to the electrodes  22 ,  24  of the secondary cell  12  and the energy cost of the pump  36  circulating the electrolyte between the process cell  10  and the hydraulically connected secondary cell  12 . There is also a relatively small cost for recycling the tin deposited on the cathode  24  of the secondary cell  12 . 
     A further advantage of the present electroplating process is that the concentrations of tin and acid in the anolyte compartment  30  and the catholyte compartment  32  are controllable by adjusting the current output of the rectifier  26  connected to the electrodes  22 ,  24  of the secondary cell  12 . To maintain the tin and acid concentrations at a desired level, rectifier  26  is adjusted to provide a current output equal to the sum of the current output of the process rectifier  18  and the amperes required to deposit the small amount of tin plated on the cathode  24  of the secondary cell  12 . To increase the tin concentration and reduce the acid concentration, this current is raised, and to decrease the tin concentration and increase the acid concentration, this current is lowered. 
     Accordingly, it is an important aspect of the present invention that the anolyte compartment of the secondary cell  12  is hydraulically connected to the process cell  10  in order to bring the tin enriched solution from the anolyte compartment  30  to the process cell  10  and return the tin depleted solution back to the anolyte compartment  30 . The catholyte compartment  32  of the secondary cell  12  is isolated by the perm-selective ion exchange membrane  28  and contains water and methane sulfonic acid. Water is periodically added to make up for the water dissociated at the cathode  24 . 
     A further embodiment of the present invention includes a controller  40  which senses the output current of the process cell rectifier  18 , and then delivers to the secondary rectifier  26  an amount of current expressed as a percentage of the process cell rectified current. This percentage is selected so that the tin and acid concentrations in the electrolyte of the process cell  10  are maintained at a desired concentration, regardless of variations in current output from the process rectifier  18 . The controller  40  regulates the process rectifier  18  output based on the area of the cathode workpiece  16  to be plated and the desired current density. 
     According to another embodiment of the present invention, the plating process is useful with soluble tin anode systems when the anode current efficiency in the process cell is less than the cathode current efficiency. This would have an economic advantage in terms of making up for depleted tin in the process cell. In this case, the secondary rectifier amperage is set to a value chosen to compensate for the difference between the anode current efficiency and the cathode current efficiency in the process cell, and the amount of current requirement to plate any tin on the secondary cell, i.e., about 10% due to the permeability of the ion exchange membrane  28 . It should be pointed out that the anode current efficiency in the process cell with the insoluble anode is zero with respect to tin. 
     It is appreciated that various modifications to the present inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the herein appended claims.