Patent Publication Number: US-2005139488-A1

Title: Electrolytic stripping method

Description:
FIELD OF THE INVENTION  
      The present invention relates to an electrolytic stripping method. In more detail, the invention relates to an electrolytic stripping method of stripping: a copper plating film exposed from a silver plating film that partially covers a copper plating film formed on the entire surface of a member; and leaking silver formed on an exposed portion of the copper plating film and having a smaller thickness than the silver plating film, by electrolytic stripping using the copper plating film and leaking silver as an anode.  
     BACKGROUND OF THE INVENTION  
      In a lead frame to be used in semiconductor devices, as illustrated in  FIG. 1 , at a tip portion of each of inner leads  12  constructing a lead frame  10  made of an iron based alloy material such as an iron-nickel alloy (42 alloy) in the side of a die pad  14  (this tip portion will be hereinafter referred simply as “tip portion”), a bonding portion  12   a  to be connected with one end portion of a gold wire, the other end portion of which is connected to a semiconductor element mounted on the die pad  14 , is formed.  
      On such a bonding portion  12   a , a silver plating film is formed, and the foregoing one end portion of the gold wire is connected to the bonding portion  12   a.    
      As illustrated in  FIG. 1 , in forming the die bonding portion  12   a  having a silver plating film formed thereon, at the tip portion of each of the inner leads  12  of the lead frame  10  made of an iron based alloy, a copper plating film is formed on the entire surrounding of the lead frame  10  by electrolytic copper plating.  
      As illustrated in  FIG. 2A , the lead frame  10  having such a copper plating film formed thereon is put between rubber-made mask plates  18   a  and  18   b  in such a manner that only a copper plating film  16  of a portion for forming the bonding portion  12   a  is exposed at a tip portion  20  of the inner lead  12 , and electrolytic silver plating for forming a silver plating film on the exposed copper plating film  16  is carried out.  
      Next, as illustrated in  FIG. 2B , after completion of the electrolytic silver plating, when the mask plates  18   a  and  18   b  are removed, a silver plating film  22  is formed only at the portion for forming the bonding portion  12   a  of the tip portion  20  of the inner lead  12 .  
      Thereafter, the lead frame  10  is dipped in an electrolytic stripping liquid, and the copper plating film  16  which is exposed without being covered by the silver plating film  22  is removed by electrolytic stripping using the lead frame  10  as an anode. Thus, as illustrated in  FIG. 2C , the bonding portion  12   a  comprising the copper plating film  16  and the silver plating film  22  is formed only at the portion for forming the bonding portion  12   a  of the tip portion  20  of the inner lead  12 .  
      Now, in putting the lead frame  10  between the mask plates  18   a  and  18   b  and forming a silver plating film on the exposed copper plating film  16 , leaking silver  24  having a smaller thickness than the silver plating film  22  is also formed at a portion having covered by the mask plates  18   a  and  18   b  as illustrated in  FIG. 2B , due to leakage of an electrolytic silver plating liquid because of strains of the mask plates  18   a  and  18   b,  etc. This leaking silver  24  is also removed by electrolytic stripping of the copper plating film  16 , and as illustrated in  FIG. 2C , the silver plating film  22  is formed only at the portion for forming the bonding portion  12   a  of the tip portion  20  of the inner lead  12 .  
      For electrically stripping the leaking silver  24  and the copper plating film  16  without substantially stripping the silver plating film  22 , an electrolytic stripping liquid containing a cyan compound has been used as described in the following Patent Document 1.  
      Patent Document 1: JP 59-031900 A (pages 2 to 3)  
      As described in Patent Document 1, by dipping the lead frame  10 , on which the silver plating film  22  is formed only at the tip portion  20  of the inner lead  12 , in an electrolytic stripping liquid containing a cyan compound and carrying out electrolytic stripping of copper using this lead frame  10  as an anode and a stainless steel plate as a cathode, it is possible to electrolytically strip the leaking silver  24  and the copper plating film  16  without substantially stripping the silver plating film  22 .  
      However, if the electrolytic stripping liquid containing a cyan compound is continuously used, in electrolytically stripping the leaking silver  24  and the copper plating film  16 , a stripping rate of the copper plating film  16  and the leaking silver  24  is reduced within a relatively short period of time. For this reason, it is required to frequently exchange such an electrolytic stripping liquid.  
      Further, if it is intended to recover silver as a noble metal from the cyan compound-containing electrolytic stripping liquid to be exchanged and disposed, another special recover step is required.  
     SUMMARY OF THE INVENTION  
      Accordingly, an object of the invention is to provide an electrolytic stripping method using an electrolytic stripping liquid which is a cyan compound-free electrolytic stripping liquid and the life of which can be greatly extended as compared to that of cyan compound-electrolytic stripping liquids.  
      Other objects and effects of the invention will become apparent from the following description.  
      To achieve the foregoing objects, the present inventors made extensive investigations. As a result, electrolytic stripping was carried out using, as an anode, the lead frame  10  in which the silver plating film  22  is formed only at the tip portion  20  of the inner lead  12  as illustrated in  FIG. 2B  while using an electrolytic stripping liquid which is a cyan compound-free electrolytic stripping liquid containing triammonium phosphate capable of forming a complex ion of silver and ammonium that is more easily dissociated than a complex ion of silver and cyan. At this time, a cathode made of platinum was used as a counter electrode to the lead frame  10  serving as the anode. As a result, it has been found that silver and copper are deposited on the cathode while electrolytically stripping the leaking silver  24  and the copper plating film  16  without substantially stripping the silver plating film  22 .  
      Further, it has been found that in this way, by carrying out electrolytic stripping while depositing silver and copper on the cathode, the life of the electrolytic stripping liquid is prolonged as compared to that of cyan compound-containing electrolytic stripping liquids, thus leading to accomplishment of the invention.  
      Specifically, the invention relates to an electrolytic stripping method comprising electrolytically stripping: 
          a copper plating film exposed from a silver plating film that partially covers a copper plating film formed on the entire surface of a member; and     leaking silver formed on an exposed portion of the copper plating film and having a smaller thickness than the silver plating film, using the copper plating film as an anode,     wherein the electrolytic stripping is carried out using a cyan compound-free electrolytic copper-stripping liquid, and the electrolytic stripping liquid comprises a compound capable of forming, with silver, a complex ion that is more easily dissociated than a complex ion of silver and cyan, and     wherein silver and copper are deposited on a cathode which is used as a counter electrode to the anode and which is made of a metal that is chemically stable against the electrolytic stripping liquid.        

      In the invention, the compound capable of forming, with silver, a complex ion that is more easily dissociated than a complex ion of silver and cyan is preferably a compound capable of forming, with silver, a complex ion having a lower complex stability constant than the complex ion of silver and cyan.  
      As this compound, at least one compound selected from the group consisting of ammonia water, an ammonium salt, a tartaric acid salt, phosphoric acid, and a citric acid salt can be used.  
      Also, since the electrolytic stripping liquid to be used in the invention is an electrolytic copper-stripping liquid for electrically stripping a copper plating film, it is preferable that the electrolytic stripping liquid contains a copper compound or an aromatic nitro compound as a copper-oxidizing agent and is adjusted so as to have a pH of from 9 to 12.  
      Here, the copper compound is preferably derived from an ammonium source and a copper source capable of forming a copper ammonium complex, added to the electrolytic stripping liquid.  
      Further, by surrounding the cathode by a cathode bag, it is possible to collect silver and copper particles, comprising silver and copper deposited on the cathode, respectively, by the cathode bag; and by taking out the cathode bag from the electrolytic stripping liquid, it is possible to recover silver and copper comprising silver and copper deposited on the cathode.  
      Incidentally, a lead frame made of an iron based alloy can be suitably used as the member.  
      The reasons why the life of cyan compound-containing electrolytic stripping liquids which have been used so far is short may be considered as follows.  
      Since a cyan ion in an electrolytic stripping liquid forms a stable complex ion with a silver ion in the electrolytic stripping liquid, a silver concentration in the electrolytic stripping liquid rises with an increase of the electrolytic stripping treatment amount of leaking silver. As the silver concentration in the electrolytic stripping liquid rises, the stripping rate of leaking silver and a copper plating film is reduced.  
      On the other hand, in the invention, the electrolytic stripping is carried out using an electrolytic stripping liquid which is a cyan compound-free electrolytic copper-stripping liquid containing a compound capable of forming, with silver, a complex ion that is more easily dissociated than a complex ion of silver and cyan, while depositing silver and copper on a cathode used as a counter electrode to a copper plating film and leaking silver serving as an anode.  
      As a result, in accordance with the electrolytic stripping method according to the invention, it is possible to prevent a reduction in the stripping rate of the copper plating film and leaking silver caused by accumulation of silver in the electrolytic stripping liquid; and it is possible to greatly extend the life of the electrolytic stripping liquid as compared to conventional cyan compound-containing electrolytic stripping liquids. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a partial plan view to explain a lead frame as a member to be subjected to electrolytic stripping.  
       FIGS. 2A  to  2 C are partial cross-sectional views to explain partial silver plating to be formed on a lead frame as shown in  FIG. 1  and leaking silver.  
       FIG. 3  is a schematic view to explain one embodiment of an electrolytic stripping device. 
    
    
      The reference numerals used in the drawings denote the followings, respectively. 
           10 : Lead frame      12 : Inner lead      12   a:  Bonding portion      14 : Die pad      16 : Copper plating film      22 : Silver plating film      24 : Leaking silver      30 : Electrolytic stripping liquid      36 : Direct current source      38 : Cathode      40 : Cathode bag      42 : Circulation pump      44 : Circulation line      46 : Filter        

     DETAILED DESCRIPTION OF THE INVENTION  
      The electrolytic stripping liquid to be used in the invention is a cyan compound-free electrolytic copper-stripping liquid. As the electrolytic stripping solution, an electrolytic stripping liquid containing a copper compound or an aromatic nitro compound as a copper-oxidizing agent can be suitably used.  
      For the copper compound as the copper-oxidizing agent, a copper ammonium complex is suitable. Such a copper ammonium complex can be formed by adding, to the electrolytic stripping liquid, ammonia water or an ammonium salt as an ammonium source and copper sulfate, copper carbonate, copper oxalate or copper hydroxide as a copper source.  
      The copper ammonium complex that is formed in the electrolytic stripping liquid having an ammonium source and a copper source added thereto comprises at least one of [Cu(NH 3 ) 2 ] 2+ , [Cu(NH 3 ) 4 ] 2+ , and [Cu(NH 3 ) 6 ] 2+ .  
      As the aromatic nitro compound, chloronitrobenzoic acid, 2-chloro-4-nitrobenzoic acid, o-nitrobenzoic acid, m-nitrobenzoic acid, p-nitrobenzoic acid, ethyl p-nitro-benzoate, and sodium p-nitrobenzoate can be suitably used.  
      An electrolytic stripping liquid which contains an aromatic nitro compound as a copper-oxidizing agent but to which a cyan compound is added cannot be expected to have a prolonged life.  
      The amount of the copper-oxidizing agent used in the electrolytic stripping liquid of the present invention is generally from 1 to 100 g/liter, preferably from 5 to 40 g/liter.  
      Further, it is preferable that such an electrolytic stripping liquid is adjusted so as to have a pH of from 9 to 12. The pH adjustment may be achieved by using ammonia water as the ammonium source to be added as a compound capable of forming a copper ammonium complex, or using a pH adjustor such as sodium hydroxide.  
      Here, in the case where the pH of the electrolytic stripping liquid is less than 9, or in the case where the pH of the electrolytic stripping liquid exceeds 12, the stripping rate of the copper plating film tends to be reduced.  
      A compound capable of forming, with silver, a complex ion that is more easily dissociated than a complex ion of silver and cyan is added to the electrolytic copper-stripping liquid to be used in the invention. Examples of such a compound include compounds capable of forming, with silver, a complex ion having a lower complex stability constant than the complex ion of silver and cyan. Specifically, one or more compounds selected from the group consisting of ammonia water, an ammonium salt, a tartaric acid salt, phosphoric acid, and a citric acid salt can be used.  
      Incidentally, ammonia water or an ammonium salt which is a compound capable of forming a complex ion with silver may be also used as an ammonium source for forming a copper ammonium complex.  
      The amount of the compound capable of forming, with silver, a complex ion that is more easily dissociated than a complex ion of silver and cyan, in the electrolytic stripping liquid is generally from 0.1 mg to 100 g/liter.  
      The term “complex stability constant” as referred to herein means a constant expressing the degree of dissociation of a complex ion. Lower values thereof mean that the formed complex ion is more instable and more easily dissociated.  
      For example, the complex ion of silver and cyan is based on an equilibrium reaction between a silver ion and a cyan ion as shown in the following formula 1, and the complex ion of silver and ammonium is based on an equilibrium reaction between a silver ion and ammonia as shown in the following formula 2. 
 
Ag + +2CN − &lt;- &gt;Ag(CN) 2   −   Formula 1 
 
Ag + +2NH 3 &lt;-&gt;Ag(NH 3 ) 2   +   Formula 2 
 
      In these equilibrium reactions, the stability constant (K CN ) as an index expressing hardness of dissociation of the complex ion of silver and cyan can be expressed by the following equation 1, and its value (K CN ) is 1×10 21 . 
 
K CN =[Ag(CN) 2   − ]/{[Ag + ][CN − ] 2 }  Equation 1 
 
      On the other hand, the stability constant (K NH3 ) as an index expressing hardness of dissociation of the complex ion of silver and ammonium can be expressed by the following equation 2, and its value (K NH3 ) is 1.5×10 7 . 
 
K NH3 =[Ag(NH 3 ) 2   + ]/{[Ag + ][NH 3 ] 2 }  Equation 2 
 
      In this way, since K CN  is larger than K NH3 , the complex ion of silver and ammonium is easily dissociated, as compared to the complex ion of silver and cyan, into a silver ion and ammonia.  
      By removing the silver ion in the electrolytic stripping liquid, the complex ion of silver and ammonium is more easily dissociated into a silver ion and ammonia.  
      The silver ion in the electrolytic stripping liquid can be easily removed by depositing silver on a cathode, which is a counter electrode to the lead frame  10  serving as an anode, in carrying out electrolytic stripping with respect to the copper plating film  16  and leaking silver  24  as illustrated in  FIG. 2B . It is preferable that an electrode made of a metal that is chemically stable against the electrolytic stripping liquid, such as platinum and stainless steel, is used as the cathode.  
      The electrolytic stripping using such an electrolytic liquid can be carried out using a device illustrated in  FIG. 3 . In the device illustrated in  FIG. 3 , the lead frame  10  made of an iron-nickel alloy (42 alloy) as illustrated in  FIG. 1  is dipped as a member, in an electrolytic stripping liquid  30  stored in an electrolytic stripping vessel  32  provided with a stirrer  34 . On the lead frame  10 , the copper plating film  16  is formed on the entire surface thereof by electrolytic copper plating, and the bonding portion  12   a  for connection is formed at the tip portion of each of the inner leads  12 . On the bonding portion  12   a , the silver plating film  22  is formed, and the leaking silver  24  is also present.  
      This lead frame  10  is connected to an anode of a direct current source  36 , and a cathode  38  made of platinum to be connected to a cathode of the direct current source  36  is dipped as a counter electrode of the lead frame  10  (the anode) in the electrolytic stripping liquid  30 . Such a cathode  38  is surrounded by a cathode bag  40 . The cathode bag  40  collects silver and copper particles comprising silver and copper deposited on the cathode  38 , respectively, and the like, and is made of fibers having durability against the electrolytic stripping liquid  30 , such as fibers made of PTFE (polytetrafluoroethylene) or PP (polypropylene) and formed into a bag shape.  
      The electrolytic stripping liquid  30  stored in the electrolytic stripping vessel  32  is circulated by a circulation pump  42 , and a filter  46  for separating particles floating in the electrolytic stripping liquid  30  and the like is provided in the way of a circulation line  44 .  
      By passing a direct current between the lead frame  10  as the anode and the cathode  38 , each dipped in the electrolytic stripping liquid  30  stored in the electrolytic stripping vessel  32  as illustrated in  FIG. 3 , from the direct current source  36 , the copper plating film  16  exposed on the lead frame  10  and the leaking silver  24  are stripped. The silver ion generated in the electrolytic stripping liquid  30  by stripping or the like of the leaking silver  24  does not form a stable complex ion such as a complex ion of silver and cyan. For this reason, the silver ion and the copper ion in the electrolytic stripping liquid  30  become silver and copper particles comprising silver and copper deposited on the cathode  38 , respectively, and the like and are collected by the cathode bag  40 . Fine silver particles and the like which have passed through the cathode bag  40  are collected by the filter  46  provided in the circulation line  44 .  
      In this way, by separating the silver ion in the electrolytic stripping liquid  30  as silver particles comprising silver or the like, it is possible to reduce the silver concentration in the electrolytic stripping liquid. For this reason, the method of the invention is free from the formation of a stable complex ion of cyan and silver in an electrolytic stripping liquid as seen in the conventionally employed electrolytic stripping using a cyan compound-containing electrolytic stripping liquid. Also, according to the method of the invention, it is possible to prevent a reduction in the stripping rate of the copper plating film and the leaking silver due to accumulation of silver in the electrolytic stripping liquid; and it is possible to greatly extend the life of the electrolytic stripping liquid as compared to that of conventional cyan compound-containing electrolytic stripping liquids.  
      In  FIG. 3 , though the metallic lead frame  10  is used as the anode, in the case where a resin substrate is used, it is possible to carry out electrolytic stripping of a copper plating film or the like by using a copper plating film formed by electroless plating on-the entire surface of the resin substrate.  
     EXAMPLES  
      The present invention will be illustrated in greater detail with reference to the following Examples, but the invention should not be construed as being limited thereto.  
     Example 1  
      After forming a copper plating film by electrolytic copper plating on the entire surface of a lead frame made of an iron-nickel alloy (42 alloy), a silver plating film was formed on the copper plating film by electrolytic silver plating.  
      Next, using the device illustrated in  FIG. 3  and using an electrolytic stripping liquid shown in the following Table 1, the copper plating film and the silver plating film formed on the lead frame  10  as the anode were continuously electrically stripped between the anode and the cathode  38  under conditions shown in the following Table 2. At the time when the treatment amount of silver reached 1,000 ppm, the silver concentration in the electrolytic stripping liquid  30  stored in the electrolytic stripping vessel  32  was measured. The results obtained are also shown in Table 2.  
                       TABLE 1                          Electrolytic stripping   Triammonium phosphate:   100 g/L       liquid 1   Copper(II) hydroxide:    10 g/L           25% ammonia water   (for pH adjustment)           pH:   10.0       Electrolytic stripping   Ammonium tartarate:   200 g/L       liquid 2   p-Nitrobenzoic acid:    10 g/L           pH:    9.5       Electrolytic stripping   Triammonium citrate:   150 g/L       liquid 3   Copper(II) hydroxide:    5 g/L           25% ammonia water   (for pH adjustment)           pH:    9.8                  
 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
               
               
                 Electrolytic stripping condition 
                 Silver concentration 
               
            
           
           
               
               
               
               
            
               
                 Kind of 
                 Current 
                 Bath 
                 in electrolytic 
               
               
                 electrolytic 
                 density 
                 temperature 
                 stripping liquid 
               
               
                 stripping liquid 
                 (A/dm 2 ) 
                 (° C.) 
                 (ppm) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Electrolytic 
                 1 
                 30 
                 8 
               
               
                 stripping liquid 1 
               
               
                 Electrolytic 
                 0.5 
                 50 
                 7 
               
               
                 stripping liquid 2 
               
               
                 Electrolytic 
                 3 
                 30 
                 3 
               
               
                 stripping liquid 3 
               
               
                   
               
            
           
         
       
     
      In the electrolytic stripping using any of the electrolytic stripping liquids shown in Table 2, silver in the form of particles or the like was collected by the cathode  38 , the cathode  40  and the filter  46 , and the silver concentration in the electrolytic stripping liquid  30  was low.  
      When the electrolytic stripping was carried out using the electrolytic stripping liquid 1, the silver concentration in the particles or the like which adhered to the cathode  38 , silver concentration in the particles or the like which were collected by the cathode bag  40 , and silver concentration in the particles or the likw which were collected by the filter  46  were measured. The results obtained are shown in Table 3. This silver concentration was measured by dissolving the particles or the like which adhered to the cathode  38  or which were collected by the cathode bag  40  or the filter  46  in 50% nitric acid.  
                                   TABLE 3                                       Adhered to   Collected by   Collected by               cathode 38   cathode 40   filter 46                          Electrolytic   200 ppm   9,700 ppm   70 ppm           stripping liquid 1                      
 
     Example 2  
      The copper plating film and the silver plating film formed on the lead frame  10  were continuously electrically stripped using the electrolytic stripping liquid 1 under the same current density and bath temperature as those in the case of the electrolytic stripping liquid 1 of Example 1 in the same manner as in Example 1, except that the cathode bag  40  was removed. When the treatment amount of silver reached 1,000 ppm, the silver concentration in the electrolytic stripping liquid  30  stored in the electrolytic stripping vessel  32 , silver concentration in the particles or the like which adhered to the cathode  38 , and silver concentration in the particles or the like which were collected by the filter  46  were measured. The results obtained are shown in Table 4. The measurement of the silver concentration was carried out in the same manner as in Example 1.  
                           TABLE 4                           Silver concentration                   in electrolytic   Adhered to   Collected by           stripping liquid   cathode 38   filter 46                  Electrolytic   13 ppm   210 ppm   9,500 ppm       stripping liquid 1                  
 
      As is clear from Table 4, it is understood that likewise Example 1, the silver concentration in the electrolytic stripping liquid  30  was low, and silver was collected by the cathode  38  and the filter  46 .  
     Comparative Example 1  
      The copper plating film and the silver plating film formed on the lead frame  10  were continuously electrically stripped in the same manner as in Example 2 under the same current density and bath temperature as those in the case of the electrolytic stripping liquid 1 of Example 1, except that a cyan based electrolytic stripping liquid (potassium cyanide: 80 g/L, p-nitrobenzoic acid: 10 g/L, pH: 11) was used as the electrolytic stripping liquid. When the treatment amount of silver reached 1,000 ppm, the silver concentration in the electrolytic stripping liquid  30  stored in the electrolytic stripping vessel  32 , silver concentration in the particles or the like which adhered to the cathode  38 , and silver concentration in the particles or the like which were collected by the filter  46  were measured. The results obtained are shown in Table 5. The measurement of the silver concentration was carried out in the same manner as in Example 1.  
                           TABLE 5                           Silver concentration                   in electrolytic   Adhered to   Collected by           stripping liquid   cathode 38   filter 46                  Cyan based   6,100 ppm   550 ppm   0 ppm       electrolytic       stripping liquid                  
 
      As is clear from Table 5, in the cyan based electrolytic stripping liquid, though silver was somewhat deposited on the cathode  38 , the silver was not deposited to such extent that particles or the like collectable by the filter  46  were formed. For this reason, the silver concentration in the electrolytic stripping liquid  30  is high.  
      While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.  
      This application is based on Japanese patent application No. 2003-433701 filed Dec. 26, 2003, the contents thereof being herein incorporated by reference.