US20050139488A1 - Electrolytic stripping method - Google Patents

Electrolytic stripping method Download PDF

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US20050139488A1
US20050139488A1 US10/972,138 US97213804A US2005139488A1 US 20050139488 A1 US20050139488 A1 US 20050139488A1 US 97213804 A US97213804 A US 97213804A US 2005139488 A1 US2005139488 A1 US 2005139488A1
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silver
copper
electrolytic stripping
electrolytic
plating film
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US10/972,138
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Yoko Ogihara
Masao Nakazawa
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Shinko Electric Industries Co Ltd
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Shinko Electric Industries Co Ltd
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Assigned to SHINKO ELECTRIC INDUSTRIES, CO., LTD. reassignment SHINKO ELECTRIC INDUSTRIES, CO., LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: NAKAZAWA, MASAO, OGIHARA, YOKO
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25F—PROCESSES FOR THE ELECTROLYTIC REMOVAL OF MATERIALS FROM OBJECTS; APPARATUS THEREFOR
    • C25F5/00—Electrolytic stripping of metallic layers or coatings
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L23/00—Details of semiconductor or other solid state devices
    • H01L23/48—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor
    • H01L23/488—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor consisting of soldered or bonded constructions
    • H01L23/495—Lead-frames or other flat leads
    • H01L23/49579—Lead-frames or other flat leads characterised by the materials of the lead frames or layers thereon
    • H01L23/49582—Metallic layers on lead frames
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
    • H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
    • H01L2224/42—Wire connectors; Manufacturing methods related thereto
    • H01L2224/44—Structure, shape, material or disposition of the wire connectors prior to the connecting process
    • H01L2224/45—Structure, shape, material or disposition of the wire connectors prior to the connecting process of an individual wire connector
    • H01L2224/45001—Core members of the connector
    • H01L2224/45099—Material
    • H01L2224/451—Material with a principal constituent of the material being a metal or a metalloid, e.g. boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po), and alloys thereof
    • H01L2224/45138—Material with a principal constituent of the material being a metal or a metalloid, e.g. boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po), and alloys thereof the principal constituent melting at a temperature of greater than or equal to 950°C and less than 1550°C
    • H01L2224/45144—Gold (Au) as principal constituent
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
    • H01L2224/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
    • H01L2224/42—Wire connectors; Manufacturing methods related thereto
    • H01L2224/47—Structure, shape, material or disposition of the wire connectors after the connecting process
    • H01L2224/48—Structure, shape, material or disposition of the wire connectors after the connecting process of an individual wire connector
    • H01L2224/485—Material
    • H01L2224/48505—Material at the bonding interface
    • H01L2224/48599—Principal constituent of the connecting portion of the wire connector being Gold (Au)
    • H01L2224/486—Principal constituent of the connecting portion of the wire connector being Gold (Au) with a principal constituent of the bonding area being a metal or a metalloid, e.g. boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po), and alloys thereof
    • H01L2224/48638—Principal constituent of the connecting portion of the wire connector being Gold (Au) with a principal constituent of the bonding area being a metal or a metalloid, e.g. boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po), and alloys thereof the principal constituent melting at a temperature of greater than or equal to 950°C and less than 1550°C
    • H01L2224/48639—Silver (Ag) as principal constituent
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2224/00—Indexing scheme for arrangements for connecting or disconnecting semiconductor or solid-state bodies and methods related thereto as covered by H01L24/00
    • H01L2224/80—Methods for connecting semiconductor or other solid state bodies using means for bonding being attached to, or being formed on, the surface to be connected
    • H01L2224/85—Methods for connecting semiconductor or other solid state bodies using means for bonding being attached to, or being formed on, the surface to be connected using a wire connector
    • H01L2224/8538—Bonding interfaces outside the semiconductor or solid-state body
    • H01L2224/85399—Material
    • H01L2224/854—Material with a principal constituent of the material being a metal or a metalloid, e.g. boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po), and alloys thereof
    • H01L2224/85438—Material with a principal constituent of the material being a metal or a metalloid, e.g. boron (B), silicon (Si), germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po), and alloys thereof the principal constituent melting at a temperature of greater than or equal to 950°C and less than 1550°C
    • H01L2224/85439—Silver (Ag) as principal constituent
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L24/00—Arrangements for connecting or disconnecting semiconductor or solid-state bodies; Methods or apparatus related thereto
    • H01L24/01—Means for bonding being attached to, or being formed on, the surface to be connected, e.g. chip-to-package, die-attach, "first-level" interconnects; Manufacturing methods related thereto
    • H01L24/42—Wire connectors; Manufacturing methods related thereto
    • H01L24/44—Structure, shape, material or disposition of the wire connectors prior to the connecting process
    • H01L24/45—Structure, shape, material or disposition of the wire connectors prior to the connecting process of an individual wire connector
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/0001—Technical content checked by a classifier
    • H01L2924/00014—Technical content checked by a classifier the subject-matter covered by the group, the symbol of which is combined with the symbol of this group, being disclosed without further technical details
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/01—Chemical elements
    • H01L2924/01012—Magnesium [Mg]
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/01—Chemical elements
    • H01L2924/01019—Potassium [K]
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/01—Chemical elements
    • H01L2924/01028—Nickel [Ni]

Definitions

  • the present invention relates to an electrolytic stripping method.
  • 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.
  • 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.
  • a silver plating film is formed, and the foregoing one end portion of the gold wire is connected to the bonding portion 12 a.
  • a copper plating film is formed on the entire surrounding of the lead frame 10 by electrolytic copper plating.
  • 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.
  • 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 .
  • 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.
  • 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 .
  • 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 .
  • Patent Document 1 JP 59-031900 A (pages 2 to 3)
  • 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.
  • 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.
  • a cathode made of platinum was used as a counter electrode to the lead frame 10 serving as the anode.
  • the invention relates to an electrolytic stripping method comprising electrolytically stripping:
  • 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.
  • 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.
  • the electrolytic stripping liquid to be used in the invention is an electrolytic copper-stripping liquid for electrically stripping a copper plating film
  • 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.
  • 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.
  • cathode bag 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.
  • a lead frame made of an iron based alloy can be suitably used as the member.
  • 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.
  • the stripping rate of leaking silver and a copper plating film is reduced.
  • 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.
  • 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.
  • 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 electrolytic stripping liquid to be used in the invention is a cyan compound-free electrolytic copper-stripping liquid.
  • an electrolytic stripping liquid containing a copper compound or an aromatic nitro compound as a copper-oxidizing agent can be suitably used.
  • a copper ammonium complex is suitable for the copper compound as the copper-oxidizing agent.
  • 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+ .
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • complex stability constant 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.
  • 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
  • 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.
  • K CN stability constant
  • K NH3 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
  • 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.
  • 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 .
  • a cathode which is a counter electrode to the lead frame 10 serving as an anode
  • 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 .
  • 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 .
  • 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 .
  • 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 .
  • 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 .
  • 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.
  • 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 .
  • 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.
  • 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.
  • 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.
  • 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.
  • 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
  • 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
  • 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.
  • 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
  • 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 .
  • 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.
  • a cyan based electrolytic stripping liquid potassium cyanide: 80 g/L, p-nitrobenzoic acid: 10 g/L, pH: 11

Abstract

The present invention provides 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.

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 2C 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(NH3)2]2+, [Cu(NH3)4]2+, and [Cu(NH3)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−<- >Ag(CN)2 −  Formula 1
    Ag++2NH3<->Ag(NH3)2 +  Formula 2
  • In these equilibrium reactions, the stability constant (KCN) 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 (KCN) is 1×1021.
    KCN=[Ag(CN)2 −]/{[Ag+][CN−]2}  Equation 1
  • On the other hand, the stability constant (KNH3) 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 (KNH3) is 1.5×107.
    KNH3=[Ag(NH3)2 +]/{[Ag+][NH3]2}  Equation 2
  • In this way, since KCN is larger than KNH3, 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/dm2) (° 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.

Claims (8)

1. 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.
2. The electrolytic stripping method according to claim 1, wherein the compound capable of forming, with silver, a complex ion that is more easily dissociated than a complex ion of silver and cyan is a compound capable of forming, with silver, a complex ion having a lower complex stability constant than the complex ion of silver and cyan.
3. The electrolytic stripping method according to claim 1, wherein the compound capable of forming, with silver, a complex ion that is more easily dissociated than the complex ion of silver and cyan comprises 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.
4. The electrolytic stripping method according to claim 1, wherein the electrolytic stripping liquid further comprises a copper compound or an aromatic nitro compound as a copper-oxidizing agent and has a pH of from 9 to 12.
5. The electrolytic stripping method according to claim 4, wherein the copper compound is a copper ammonium complex derived from an ammonium source and a copper source.
6. The electrolytic stripping method according to claim 1, wherein the cathode is surrounded by a cathode bag for collecting silver particles comprising the deposited silver.
7. The electrolytic stripping method according to claim 1, wherein the member is a lead frame made of an iron based alloy.
8. The electrolytic stripping method according to claim 1, further comprising
circulating the electrolytic stripping liquid in the vicinity of the cathode through a circulation line equipped with a filter, thereby collecting silver particles comprising the deposited silver.
US10/972,138 2003-12-26 2004-10-22 Electrolytic stripping method Abandoned US20050139488A1 (en)

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US20080112988A1 (en) * 2006-11-14 2008-05-15 Ancient Stones, Inc. Composition and method for cosmetic vitalization and stabilization
US20160053399A1 (en) * 2014-08-25 2016-02-25 Seiko Epson Corporation Forming method and formed article
CN105543948A (en) * 2015-12-23 2016-05-04 苏州卓融新能源科技有限公司 Copper stripping process for dummy plates/corrugated plates for PCB electroplating
US11136681B2 (en) 2015-06-24 2021-10-05 Greene Lyon Group, Inc. Selective removal of noble metals using acidic fluids, including fluids containing nitrate ions
US11193214B2 (en) 2013-12-20 2021-12-07 Greene Lyon Group, Inc. Method and apparatus for recovery of noble metals, including recovery of noble metals from plated and/or filled scrap

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