Title:
CONTINUOUS PLATING METHOD
United States Patent 3622470
Abstract:
A method of continuously plating aluminum wire or strip stock. The stock passes through an alkaline etch bath, an acid oxidizing bath, a conditioner bath containing tin, and a bronze-plating bath in which bronze is plated on the aluminum. The conditions of concentration, temperature, and solution movement are all controlled so as to achieve continuous plating of bronze while feeding the stock at a relatively high speed. Following the bronze plating, further plating and/or drawing steps may be carried out.
US Patent References:
Process and apparatus for electroplating
Andersen - September 1934 - 1974441
Electrodeposition of nickel
Struyk et al. - September 1950 - 2523161
Bearing
Schultz - February 1952 - 2586099
Method of etching aluminum and aluminum base alloys
Dvorkovitz - September 1953 - 2650875
Electroplating on aluminum wire
Fenster - June 1959 - 2891309
Process and apparatus for electroplating
Andersen - September 1934 - 1974441
Electrodeposition of nickel
Struyk et al. - September 1950 - 2523161
Bearing
Schultz - February 1952 - 2586099
Method of etching aluminum and aluminum base alloys
Dvorkovitz - September 1953 - 2650875
Electroplating on aluminum wire
Fenster - June 1959 - 2891309
Application Number:
04/826381
Publication Date:
11/23/1971
Filing Date:
05/21/1969
View Patent Images:
Export Citation:
Assignee:
Wire & Strip Platers, Inc. (Marlette, MI)
Primary Class:
Other Classes:
205/241, 205/185, 205/176, 205/213, 204/207
International Classes:
C25D5/44; C25D7/06; C25D5/34; C23B5/50; C23B5/62
Field of Search:
204/27,33,44,49,40
US Patent References:
| 3108006 | Plating on aluminum | October 1963 | Kenedi et al. |
Primary Examiner:
Mack, John H.
Assistant Examiner:
Solomon I, W.
Claims:
Having thus described my invention, I claim
1. A method of continuously plating the surfaces of aluminum wire or strip stock, comprising the steps of:
2. feeding the aluminum wire or strip stock to a series of baths,
3. passing said stock through an aqueous alkaline etch bath containing alkali hydroxide in sufficient concentration and temperature to strip substantially all of the oxide from the aluminum surfaces of said stock,
4. passing said stock through an aqueous rinse bath to remove the alkali hydroxide from the stock,
5. passing said stock through an aqueous acid bath containing nitric or hydrochloric acid and a fluoride compound in sufficient concentrations to provide an oxide layer on the aluminum surfaces of said stock and remove foreign matter,
6. passing said stock through an aqueous rinse bath to remove the acid from the stock,
7. passing said stock through an aqueous conditioning bath containing sodium or potassium stannate in sufficient concentration to condition the aluminum surfaces of the stock, which is substantially about 5 ounces of tin per gallon of solution contained in a stannate compound, and sufficient concentration of alkali hydroxide to remove the oxide from the aluminum surfaces of the stock sufficiently to permit subsequent bronze plating and substantially prevent immersion plating of the tin from the stannate on the aluminum surfaces, and,
8. passing said stock as a cathode directly into an aqueous bronze-plating bath, without rinsing, containing an alkali hydroxide in substantially the same concentrations as in said conditioning bath, and sufficient concentrations of tin in a stannate compound and copper in a cyanide compound to plate bronze on the conditioned aluminum surface of said stock while maintaining a cathode current density of less than 500 amperes per square foot.
9. The plating method as claimed in claim 1 including the further step of passing said stock as cathode through a nickel-plating bath comprised of an aqueous solution of nickel fluoborate while maintaining in said nickel-plating bath a cathode current density sufficient to plate nickel and less than 500 amperes per square foot.
10. The plating method as claimed in claim 2 including the further step of passing said stock as cathode through a further plating bath comprised of a fluoboric solution of a material selected from the group consisting of tin, copper, lead-tin, lead, and nickel to plate the selected material on said stock.
1. A method of continuously plating the surfaces of aluminum wire or strip stock, comprising the steps of:
2. feeding the aluminum wire or strip stock to a series of baths,
3. passing said stock through an aqueous alkaline etch bath containing alkali hydroxide in sufficient concentration and temperature to strip substantially all of the oxide from the aluminum surfaces of said stock,
4. passing said stock through an aqueous rinse bath to remove the alkali hydroxide from the stock,
5. passing said stock through an aqueous acid bath containing nitric or hydrochloric acid and a fluoride compound in sufficient concentrations to provide an oxide layer on the aluminum surfaces of said stock and remove foreign matter,
6. passing said stock through an aqueous rinse bath to remove the acid from the stock,
7. passing said stock through an aqueous conditioning bath containing sodium or potassium stannate in sufficient concentration to condition the aluminum surfaces of the stock, which is substantially about 5 ounces of tin per gallon of solution contained in a stannate compound, and sufficient concentration of alkali hydroxide to remove the oxide from the aluminum surfaces of the stock sufficiently to permit subsequent bronze plating and substantially prevent immersion plating of the tin from the stannate on the aluminum surfaces, and,
8. passing said stock as a cathode directly into an aqueous bronze-plating bath, without rinsing, containing an alkali hydroxide in substantially the same concentrations as in said conditioning bath, and sufficient concentrations of tin in a stannate compound and copper in a cyanide compound to plate bronze on the conditioned aluminum surface of said stock while maintaining a cathode current density of less than 500 amperes per square foot.
9. The plating method as claimed in claim 1 including the further step of passing said stock as cathode through a nickel-plating bath comprised of an aqueous solution of nickel fluoborate while maintaining in said nickel-plating bath a cathode current density sufficient to plate nickel and less than 500 amperes per square foot.
10. The plating method as claimed in claim 2 including the further step of passing said stock as cathode through a further plating bath comprised of a fluoboric solution of a material selected from the group consisting of tin, copper, lead-tin, lead, and nickel to plate the selected material on said stock.
Description:
BACKGROUND OF THE INVENTION
Although processes or methods for plating metal on aluminum have been proposed previously, to the knowledge of applicant, there has been no process available for continuous plating of aluminum wire or strip with bronze at a relatively high speed; say as high as 50 feet of stock per minute or even greater. In order to provide such a continuous plating method, the conditions should be based on a long range forecast of consumption of stock by the user. A speed of wire or strip feed may then be selected so as to meet the needed production within a minimum space. For example, plating only one strand of eight gauge aluminum wire at 100 feet per minute produces 60 pounds of wire per hour; plating one strand of 14 gauge aluminum wire at 100 feet per minute produces 22.5 pounds per hour; and plating one strand of 20 gauge aluminum wire at 100 feet per minute produces 5.6 pounds per hour. Having fixed the production rate, the length and width of the plating line may be fixed, the width being selected so as to accommodate the number of wires to be plated. The number of baths to be used is selected, and the length of each bath is selected to allow the wire or strip to remain in each bath long enough to accomplish the required chemical action at the feeding speeds to be used. Since the time for which the stock remains in any one bath is limited, concentration, temperature, and solution movement in the bath are the factors or parameters which can be controlled to control reaction kinetics. Current density may also be controlled where relevant. The parameters of batch aluminum plating processes would not apply to a high-speed process such as is needed for economical continuous plating of aluminum wire and strip.
Accordingly, it is an object of the present invention to provide a method or process for continuous plating of aluminum wire and strip on an economical basis.
Another object of the invention is to provide an aluminum-plating process in which parameters such as temperature, concentrations, and solution movement are selected and controlled to permit plating of aluminum wire and strip moving at a relatively high speed.
A further object of the invention is to provide a method of continuously plating aluminum wire and strip stock in which etching, oxidizing, conditioning, and bronze-plating steps are carried out by passing the stock through successive baths, with any given portion of the stock remaining in each bath only a short time.
Another object of the invention is to condition the aluminum wire or strip, plate bronze thereon to provide a metallurgical bond between the bronze and the aluminum, and then plate a selected metal on the bronze to provide a desired coating, suitable metals being tin, copper, lead, lead-tin alloy, or nickel.
Still another object of the invention is to plate metals on aluminum wire as just described in the last preceding paragraph, and then draw the wire to reduce its size.
A further object is to continuously plate layers of bronze, then nickel, and then tin, lead or lead-tin alloy on aluminum wire or strip to provide plated material which is solderable and will withstand prescribed solderability tests.
Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.
ON THE DRAWINGS
The two figures of the drawings show a plating line for continuous plating of aluminum wire or strip in accordance with the invention.
GENERAL DESCRIPTION
FIGS. 1 and 2 illustrate a processing line for continuously plating wire or strip stock in accordance with one embodiment of the invention. These figures are schematic and are not intended to show the details of processing equipment since such equipment does not form a part of the present invention.
The stock to be plated is represented in FIGS. 1 and 2 by the line 10 which may be wire or strip stock. The stock 10 passes through a series of tanks designated 12, 14, 16, 18, 20, 22, and 24. Contact is made to the stock as by means of rollers or other suitable contact means where the stock is to be electroplated. At least one tray for holding a bath is provided in each of these tanks, there being two trays 26 and 28 in tank 12, two trays 30 and 32 in tank 14, two trays 34 and 36 in tank 16, one tray 38 in tank 18, one tray 40 in tank 20, one tray 42 in tank 22, and three trays 44, 46, and 48 in tank 24. The stock also passes through rinse tanks 50, 52, 54, and 56 as shown, and it may be noted that there is no rinse tank between tanks 16 and 18 nor between tanks 18 and 20. Anodes are used in the electroplating trays, the anode being of the metal being plated. The stock is the cathode in these trays.
The first tank 12 contains an alkaline etching liquid. Some of this liquid is continuously moved through the trays 26 and 28 by means of a pump P, and the liquid overflows from the trays through suitable outlets back to the tank 12. Thus, each of the trays 26 and 28 comprises an alkaline etching bath. A suitable composition for the etching liquid is:
Sodium or potassium 3-8 ounces per gallon of hydroxide solution Sodium or potassium 1-4 ounces per gallon of gluconate solution
Any suitable wetting agent may also be added to the etching bath, if desired. The balance of the bath is water. The temperature of the etching baths 26 and 28 is preferably in the range from 150° to 170° F. For high speed plating, the stock remains in each of the baths 26 and 28 a time no greater than 10 seconds for each bath, a suitable range being 3 to 10 seconds. The etching baths 26 and 28 serve to remove oxide from the aluminum stock to expose the metal.
The term aluminum as used herein is intended to include pure aluminum, commercial grades of aluminum containing normal amounts of impurities, and alloys of aluminum in which aluminum is a major ingredient.
The aluminum stock 10 passes through rinsing bath 50 to remove excess etching liquid and passes from there into baths 30 and 32 in tank 14.
Tank 14 contains an oxidizing liquid which is preferably aqueous hydrochloric or nitric acid containing a fluoride compound. Some of the oxidizing liquid is continuously moved through trays 30 and 32 by means of a pump P and liquid overflows from trays 30 and 32 through suitable outlets back to the tank 14. Thus, the trays 30 and 32 contain the oxidizing baths through which the stock 10 passes. The composition of the bath may be 50 percent nitric acid or 50 percent hydrochloric acid plus a fluoride salt such as ammonium bifluoride. Hydrofluoric acid can be used in place of the fluoride salt. Where ammonium bifluoride is used, the concentration is preferably in the range from 4-8 ounces per gallon of solution. The temperature of each of the baths 30 and 32 is maintained in the range from 70°-90° F. The stock 10 remains in each of the baths 30 and 32 for a time no greater than 10 seconds where high production rates are required. A suitable range for this immersion time is 3 to 10 seconds.
The baths 30 and 32 serve to reoxidize the aluminum somewhat for the subsequent treating steps. The stock then passes through the rinsing bath 52 which removes excess oxidizing liquid, and from there passes to trays or baths 34 and 36 in tank 16.
Tank 16 contains a conditioning liquid which conditions the stock for a bronze plating step which is to take place in tank 18. Some of this conditioning liquid is continuously moved through trays 34 and 36 by means of a pump P, and the liquid overflows from trays 34 and 36 through suitable outlets back to the tank 16. The conditioning ingredient of the baths 34 and 36 is a stannate compound which is preferably sodium stannate or potassium stannate. The baths 34 and 36 also preferably contain sodium or potassium gluconate and sodium or potassium hydroxide. A suitable composition for the baths 34 and 36 is:
Sodium or potassium 12 to 16 ounces per gallon stannate of solution Sodium gluconate 1 to 3 ounces per gallon of solution Sodium or potassium 1 to 3 ounces per gallon hydroxide of solution Balance water
The baths 34 and 36 should contain at least 5 ounces of tin in the form of a stannate compound per gallon. The tin does not plate out to any substantial extent on the aluminum surface, but merely conditions that surface to make it receptive to the bronze plating which is carried out in tanks 18 and 20. The sodium or potassium gluconate ingredient of the baths 34 and 36 is not believed to be essential, but is helpful to keep the aluminum surface clean by chelating alloying constituents. The temperature of the baths 34 and 36 is preferably maintained in the range from 80° to 100° F. The stock 10 remains in the baths 34 and 36 a time no greater than 10 seconds for high production rates, and a suitable range for this immersion time is 1 to 10 seconds.
The stock 10 passes from tank 16 without any intervening rinsing bath to the baths 38 and 40 of tanks 18 and 20 respectively. Tanks 18 and 20 each contain a bronze-plating solution. This is an aqueous solution comprising in ounces per gallon of solution (a) at least 5 ounces of tin contained in a stannate compound, (b) at least 2.5 ounces of copper contained in a cyanide compound, and (c) at least 1 ounce of alkali hydroxide. The solution is moved through trays 38 and 40 by means of pumps P and the solution is allowed to overflow from the trays 38 and 40 through suitable outlets back to the tanks 18 and 20.
A suitable composition for the bronze plating solution is:
Sodium or potassium 12-16 ounces per gallon stannate Sodium or potassium 3-41/2 ounces per gallon cyanide 2KCN . CuCN 12-15 ounces per gallon
The balance of the solution is water.
The solution is preferably maintained in a temperature range from 90° to 120° F. A cathode current density of 100-600 amperes per square foot is maintained. The stock remains in each of the baths 38 and 40 for a time no greater than 10 seconds for high production rates, and a suitable range for this immersion time is 3 to 10 seconds.
These high current densities are required to plate a suitable thickness of bronze on the aluminum stock. A suitable thickness is 0.000030 inch.
The bronze plate should have good adherence to the aluminum, and good adherent platings of bronze have been achieved with the processing conditions outlined above. The bronze plate provides a metallurgical bond to the aluminum, and makes it possible to plate other metals on the stock in subsequent processing steps. An example of such processing steps is illustrated by the tank 24 in the drawing. One of the most desirable materials to plate on the wire in the tank 24 would be a solderable metal or alloy such as tin or lead-tin alloy. However, it has been found that in order for the tin or lead-tin material to be bonded in a manner such that the resulting product will meet the solderability test known as a solder pot test, it has been found to be desirable to plate a barrier layer of nickel on the bronze plated stock before the tin or lead-tin material is plated in tank 24. Therefore, the processing line illustrated in the drawing also includes a nickel plating tank 22 having a tray 42 therein. It is to be understood that for some applications it would not be essential to use the nickel plating tank 22. Thus, it is possible to plate a metal such as tin, copper, lead-tin, lead or nickel on the bronze-plated wire without any intervening barrier if there is no need to pass a solderability test such as the solder pot test. Where a nickel barrier layer is used, it serves to prevent the bronze from dissolving in the metal or alloy plated in tank 24 and thus impairing the metallurgical bond.
Where a nickel-plating bath is used, the solution is preferably an aqueous solution of nickel fluoborate. A suitable composition for this bath is two parts of nickel fluoborate to one part of water. The pH of the solution is preferably maintained at about 3 to 41/2. The stock is the cathode, and a cathode current density of 100 to 500 amperes per square foot is maintained. The temperature of the nickel-plating bath is preferably maintained in the range from about 100° to 120° F. The stock passes through the nickel-plating bath in a time no greater than 10 seconds for high volume production.
From the nickel plating bath 42, the stock passes through a rinsing bath 56 to the baths 44, 46, and 48 provided in the plating tank 24. As previously mentioned, any of several metals or alloys may be plated on the stock in the tank 24, and no attempt will be made to illustrate all of the plating solutions which are possible. Preferably, the selected metal should be plated from a fluoboric solution of the metal.
It is to be noted that in the foregoing description, the stock passes through each of the baths in a time of 10 seconds or less. For some applications, it may be possible to increase this time, but in general the parameters of the process are selected such that successful plating cannot be achieved at extremely slow speeds of the stock.
EXAMPLE 1
The stock in this example was 0.040 inch thick by 2 inches wide aluminum strip of the type known as alloy No. 1100-0. The speed of the stock was 50 ft./min. The stock passed through trays 26 and 28 containing the etching liquid in a time of 10 seconds, and the temperature of these baths was maintained at 155° F. The composition of these baths was in accordance with the previous description. The stock passed through each of the oxidizing baths 30 and 32 in a time of 10 seconds for each bath, and the temperature of these baths was maintained at about 75° F. The composition of the oxidizing bath 30 and 32 and other processing conditions were in accordance with the previous description.
The stock passed through baths 34 and 36 in a time of 10 seconds each, these baths being maintained at a temperature of about 95° F. The baths 34 and 36 contained about 5.5 ounces of tin metal in the form of potassium stannate per gallon of solution.
The stock passed through each of the bronze-plating baths 38 and 40 in a time of 10 seconds, and a current density of 270 a./square foot was used as the cathode current density. The temperature of the baths 38 and 40 was maintained at about 100° F. The baths 38 and 40 contained about 3 oz./gal. of copper in the form of copper cyanide, about 4 oz./gal. of free cyanide, about 5.5 oz./gal. of tin in the form of potassium stannate, and about 1.5 oz./gal. of free caustic in the form of potassium hydroxide. A nickel plating bath 22 was not used in this example, but rather the material was passed directly into tank 24 where tin was plated on the stock. A tin metal concentration of about 11 oz./gal. was used at a temperature of 90° F. and a cathode current density of 180 a./square foot.
With these processing conditions, the resulting product was adherent, and could be soldered satisfactorily by normal soft soldering techniques.
EXAMPLE 2
In this example, eight gauge aluminum wire was passed through the processing line at a speed of 100 ft./min. The wire passed through the etching baths 26 and 28 in a time of 10 seconds for each bath. The temperature of the etching baths was maintained at about 155° F. The composition of the etching baths was the same as in example 1.
The wire passed through the oxidizing baths 30 and 32 in about 10 seconds for each bath, and the temperature of these baths was maintained at about 75° F. The oxidizing baths contained 50 percent by volume nitric acid and 4 oz./gal. of ammonium bifluoride. The balance of the solution was water.
The stock passed through the conditioner baths 34 and 36 in a time of 5 seconds for each bath, and the temperature of these baths was maintained at 90° F. These baths contained about 6.5 oz./gal. of solution of tin metal in the form of potassium stannate.
The wire passed through the bronze plating baths 38 and 40 in a time of 10 seconds for each bath, and the baths were maintained at a temperature of 95° F. A cathode current density of 540 a./square foot was used. The composition of the baths was the same as in example 1. The nickel-plating bath 42 was not used.
The wire passed through baths 44, 46, and 48 in a time of 5 seconds each (total immersion time 15 seconds), and the temperature of the plating solution was 85° F. A cathode current density of 200 a./square foot was used. The material plated on the wire was tin-lead alloy, and this alloy was plated from a fluoboric solution of standard composition.
The resulting product had an adherent plating, and good solderable coatings were obtained.
EXAMPLE 3
In this example, 20 gauge aluminum wire alloy identified as alloy No. 1245 was passed through the processing line at a speed of 200 ft./min. The stock passed through the etching baths 26 and 28 in a time of 5 seconds for each bath. The temperature of the baths was maintained at about 170° F. The compositions of the baths were in accordance with the previous description.
The wire passed through the oxidizing baths 30 and 32 in a time of 5 seconds for each bath, and the other processing conditions were the same as for example 1.
The wire passed through the conditioning baths 34 and 36 in a time of 5 seconds for each bath, the temperature of these baths was maintained at 100° F. The other processing conditions were the same as for example 1.
The stock passed through the bronze plating baths 38 and 40 in a time of 5 seconds for each bath, and the temperature of these baths was maintained at about 110° F. A cathode current density of 1,080 a./square foot was used. The composition of the baths was about 3.5 oz./gal. of copper metal, about 6.5 oz./gal. of tin metal, about 4.5 oz./gal. of free cyanide in the form of potassium cyanide and about 1.7 oz./gal. of free caustic in the form of potassium hydroxide. The balance of the solution was water.
The stock passed through the nickel plating bath 42 in a time of 2.5 seconds, and the bath was maintained at 115° F. A cathode current density of 300 a./square foot was used. The composition of the nickel-plating bath was in accordance with the previous description.
The wire passed through the tin-plating baths 44, 46, and 48 in a time of 7.5 seconds, and the temperature of these baths was about 100° F. The cathode current density was 200 a./sq. foot. The metal plated on the wire in the baths 44, 46, and 48 was tin, and a standard tin fluoborate plating solution was used.
The resulting product had highly adherent coatings, and coatings were obtained which were easily solderable by normal soft soldering techniques. These coatings also withstood a solder pot test which is more rigid than normal soft soldering techniques.
After plating of the metal on the stock, it is possible to draw the stock to reduce its size. In experimental runs, a reduction of four gauge sizes after the plating process has been carried out has been successfully achieved.
To determine whether good solderable coatings have been obtained, certain tests can be carried out. When a nickel barrier layer is not used, a coating of tin or lead-tin alloy can be provided on bronze-plated aluminum stock which is solderable by normal soft soldering techniques. The solderability can be checked by soldering a joint between two pieces of stock using a solder such as 60-40 lead-tin alloy at a temperature of 240°-350° F. The joint is then stressed to the breaking point, and if a break occurs in the solder rather than between solder and aluminum, the solder joint was sound and the coatings are satisfactory.
Where soldering is to be carried out at high temperature or with prolonged exposure to heat, a test known as the solder pot test is used. This test is described in U.S. military specification MIL-STD 202 C dated Sept. 12, 1963. In general, soldering is carried out at a temperature in the range from 350° to 500° F. (say 450° F.) for 10 seconds, and the results are satisfactory if good wetting of the solder on the stock is achieved.
Although processes or methods for plating metal on aluminum have been proposed previously, to the knowledge of applicant, there has been no process available for continuous plating of aluminum wire or strip with bronze at a relatively high speed; say as high as 50 feet of stock per minute or even greater. In order to provide such a continuous plating method, the conditions should be based on a long range forecast of consumption of stock by the user. A speed of wire or strip feed may then be selected so as to meet the needed production within a minimum space. For example, plating only one strand of eight gauge aluminum wire at 100 feet per minute produces 60 pounds of wire per hour; plating one strand of 14 gauge aluminum wire at 100 feet per minute produces 22.5 pounds per hour; and plating one strand of 20 gauge aluminum wire at 100 feet per minute produces 5.6 pounds per hour. Having fixed the production rate, the length and width of the plating line may be fixed, the width being selected so as to accommodate the number of wires to be plated. The number of baths to be used is selected, and the length of each bath is selected to allow the wire or strip to remain in each bath long enough to accomplish the required chemical action at the feeding speeds to be used. Since the time for which the stock remains in any one bath is limited, concentration, temperature, and solution movement in the bath are the factors or parameters which can be controlled to control reaction kinetics. Current density may also be controlled where relevant. The parameters of batch aluminum plating processes would not apply to a high-speed process such as is needed for economical continuous plating of aluminum wire and strip.
Accordingly, it is an object of the present invention to provide a method or process for continuous plating of aluminum wire and strip on an economical basis.
Another object of the invention is to provide an aluminum-plating process in which parameters such as temperature, concentrations, and solution movement are selected and controlled to permit plating of aluminum wire and strip moving at a relatively high speed.
A further object of the invention is to provide a method of continuously plating aluminum wire and strip stock in which etching, oxidizing, conditioning, and bronze-plating steps are carried out by passing the stock through successive baths, with any given portion of the stock remaining in each bath only a short time.
Another object of the invention is to condition the aluminum wire or strip, plate bronze thereon to provide a metallurgical bond between the bronze and the aluminum, and then plate a selected metal on the bronze to provide a desired coating, suitable metals being tin, copper, lead, lead-tin alloy, or nickel.
Still another object of the invention is to plate metals on aluminum wire as just described in the last preceding paragraph, and then draw the wire to reduce its size.
A further object is to continuously plate layers of bronze, then nickel, and then tin, lead or lead-tin alloy on aluminum wire or strip to provide plated material which is solderable and will withstand prescribed solderability tests.
Other objects of this invention will appear in the following description and appended claims, reference being had to the accompanying drawings forming a part of this specification wherein like reference characters designate corresponding parts in the several views.
ON THE DRAWINGS
The two figures of the drawings show a plating line for continuous plating of aluminum wire or strip in accordance with the invention.
GENERAL DESCRIPTION
FIGS. 1 and 2 illustrate a processing line for continuously plating wire or strip stock in accordance with one embodiment of the invention. These figures are schematic and are not intended to show the details of processing equipment since such equipment does not form a part of the present invention.
The stock to be plated is represented in FIGS. 1 and 2 by the line 10 which may be wire or strip stock. The stock 10 passes through a series of tanks designated 12, 14, 16, 18, 20, 22, and 24. Contact is made to the stock as by means of rollers or other suitable contact means where the stock is to be electroplated. At least one tray for holding a bath is provided in each of these tanks, there being two trays 26 and 28 in tank 12, two trays 30 and 32 in tank 14, two trays 34 and 36 in tank 16, one tray 38 in tank 18, one tray 40 in tank 20, one tray 42 in tank 22, and three trays 44, 46, and 48 in tank 24. The stock also passes through rinse tanks 50, 52, 54, and 56 as shown, and it may be noted that there is no rinse tank between tanks 16 and 18 nor between tanks 18 and 20. Anodes are used in the electroplating trays, the anode being of the metal being plated. The stock is the cathode in these trays.
The first tank 12 contains an alkaline etching liquid. Some of this liquid is continuously moved through the trays 26 and 28 by means of a pump P, and the liquid overflows from the trays through suitable outlets back to the tank 12. Thus, each of the trays 26 and 28 comprises an alkaline etching bath. A suitable composition for the etching liquid is:
Sodium or potassium 3-8 ounces per gallon of hydroxide solution Sodium or potassium 1-4 ounces per gallon of gluconate solution
Any suitable wetting agent may also be added to the etching bath, if desired. The balance of the bath is water. The temperature of the etching baths 26 and 28 is preferably in the range from 150° to 170° F. For high speed plating, the stock remains in each of the baths 26 and 28 a time no greater than 10 seconds for each bath, a suitable range being 3 to 10 seconds. The etching baths 26 and 28 serve to remove oxide from the aluminum stock to expose the metal.
The term aluminum as used herein is intended to include pure aluminum, commercial grades of aluminum containing normal amounts of impurities, and alloys of aluminum in which aluminum is a major ingredient.
The aluminum stock 10 passes through rinsing bath 50 to remove excess etching liquid and passes from there into baths 30 and 32 in tank 14.
Tank 14 contains an oxidizing liquid which is preferably aqueous hydrochloric or nitric acid containing a fluoride compound. Some of the oxidizing liquid is continuously moved through trays 30 and 32 by means of a pump P and liquid overflows from trays 30 and 32 through suitable outlets back to the tank 14. Thus, the trays 30 and 32 contain the oxidizing baths through which the stock 10 passes. The composition of the bath may be 50 percent nitric acid or 50 percent hydrochloric acid plus a fluoride salt such as ammonium bifluoride. Hydrofluoric acid can be used in place of the fluoride salt. Where ammonium bifluoride is used, the concentration is preferably in the range from 4-8 ounces per gallon of solution. The temperature of each of the baths 30 and 32 is maintained in the range from 70°-90° F. The stock 10 remains in each of the baths 30 and 32 for a time no greater than 10 seconds where high production rates are required. A suitable range for this immersion time is 3 to 10 seconds.
The baths 30 and 32 serve to reoxidize the aluminum somewhat for the subsequent treating steps. The stock then passes through the rinsing bath 52 which removes excess oxidizing liquid, and from there passes to trays or baths 34 and 36 in tank 16.
Tank 16 contains a conditioning liquid which conditions the stock for a bronze plating step which is to take place in tank 18. Some of this conditioning liquid is continuously moved through trays 34 and 36 by means of a pump P, and the liquid overflows from trays 34 and 36 through suitable outlets back to the tank 16. The conditioning ingredient of the baths 34 and 36 is a stannate compound which is preferably sodium stannate or potassium stannate. The baths 34 and 36 also preferably contain sodium or potassium gluconate and sodium or potassium hydroxide. A suitable composition for the baths 34 and 36 is:
Sodium or potassium 12 to 16 ounces per gallon stannate of solution Sodium gluconate 1 to 3 ounces per gallon of solution Sodium or potassium 1 to 3 ounces per gallon hydroxide of solution Balance water
The baths 34 and 36 should contain at least 5 ounces of tin in the form of a stannate compound per gallon. The tin does not plate out to any substantial extent on the aluminum surface, but merely conditions that surface to make it receptive to the bronze plating which is carried out in tanks 18 and 20. The sodium or potassium gluconate ingredient of the baths 34 and 36 is not believed to be essential, but is helpful to keep the aluminum surface clean by chelating alloying constituents. The temperature of the baths 34 and 36 is preferably maintained in the range from 80° to 100° F. The stock 10 remains in the baths 34 and 36 a time no greater than 10 seconds for high production rates, and a suitable range for this immersion time is 1 to 10 seconds.
The stock 10 passes from tank 16 without any intervening rinsing bath to the baths 38 and 40 of tanks 18 and 20 respectively. Tanks 18 and 20 each contain a bronze-plating solution. This is an aqueous solution comprising in ounces per gallon of solution (a) at least 5 ounces of tin contained in a stannate compound, (b) at least 2.5 ounces of copper contained in a cyanide compound, and (c) at least 1 ounce of alkali hydroxide. The solution is moved through trays 38 and 40 by means of pumps P and the solution is allowed to overflow from the trays 38 and 40 through suitable outlets back to the tanks 18 and 20.
A suitable composition for the bronze plating solution is:
Sodium or potassium 12-16 ounces per gallon stannate Sodium or potassium 3-41/2 ounces per gallon cyanide 2KCN . CuCN 12-15 ounces per gallon
The balance of the solution is water.
The solution is preferably maintained in a temperature range from 90° to 120° F. A cathode current density of 100-600 amperes per square foot is maintained. The stock remains in each of the baths 38 and 40 for a time no greater than 10 seconds for high production rates, and a suitable range for this immersion time is 3 to 10 seconds.
These high current densities are required to plate a suitable thickness of bronze on the aluminum stock. A suitable thickness is 0.000030 inch.
The bronze plate should have good adherence to the aluminum, and good adherent platings of bronze have been achieved with the processing conditions outlined above. The bronze plate provides a metallurgical bond to the aluminum, and makes it possible to plate other metals on the stock in subsequent processing steps. An example of such processing steps is illustrated by the tank 24 in the drawing. One of the most desirable materials to plate on the wire in the tank 24 would be a solderable metal or alloy such as tin or lead-tin alloy. However, it has been found that in order for the tin or lead-tin material to be bonded in a manner such that the resulting product will meet the solderability test known as a solder pot test, it has been found to be desirable to plate a barrier layer of nickel on the bronze plated stock before the tin or lead-tin material is plated in tank 24. Therefore, the processing line illustrated in the drawing also includes a nickel plating tank 22 having a tray 42 therein. It is to be understood that for some applications it would not be essential to use the nickel plating tank 22. Thus, it is possible to plate a metal such as tin, copper, lead-tin, lead or nickel on the bronze-plated wire without any intervening barrier if there is no need to pass a solderability test such as the solder pot test. Where a nickel barrier layer is used, it serves to prevent the bronze from dissolving in the metal or alloy plated in tank 24 and thus impairing the metallurgical bond.
Where a nickel-plating bath is used, the solution is preferably an aqueous solution of nickel fluoborate. A suitable composition for this bath is two parts of nickel fluoborate to one part of water. The pH of the solution is preferably maintained at about 3 to 41/2. The stock is the cathode, and a cathode current density of 100 to 500 amperes per square foot is maintained. The temperature of the nickel-plating bath is preferably maintained in the range from about 100° to 120° F. The stock passes through the nickel-plating bath in a time no greater than 10 seconds for high volume production.
From the nickel plating bath 42, the stock passes through a rinsing bath 56 to the baths 44, 46, and 48 provided in the plating tank 24. As previously mentioned, any of several metals or alloys may be plated on the stock in the tank 24, and no attempt will be made to illustrate all of the plating solutions which are possible. Preferably, the selected metal should be plated from a fluoboric solution of the metal.
It is to be noted that in the foregoing description, the stock passes through each of the baths in a time of 10 seconds or less. For some applications, it may be possible to increase this time, but in general the parameters of the process are selected such that successful plating cannot be achieved at extremely slow speeds of the stock.
EXAMPLE 1
The stock in this example was 0.040 inch thick by 2 inches wide aluminum strip of the type known as alloy No. 1100-0. The speed of the stock was 50 ft./min. The stock passed through trays 26 and 28 containing the etching liquid in a time of 10 seconds, and the temperature of these baths was maintained at 155° F. The composition of these baths was in accordance with the previous description. The stock passed through each of the oxidizing baths 30 and 32 in a time of 10 seconds for each bath, and the temperature of these baths was maintained at about 75° F. The composition of the oxidizing bath 30 and 32 and other processing conditions were in accordance with the previous description.
The stock passed through baths 34 and 36 in a time of 10 seconds each, these baths being maintained at a temperature of about 95° F. The baths 34 and 36 contained about 5.5 ounces of tin metal in the form of potassium stannate per gallon of solution.
The stock passed through each of the bronze-plating baths 38 and 40 in a time of 10 seconds, and a current density of 270 a./square foot was used as the cathode current density. The temperature of the baths 38 and 40 was maintained at about 100° F. The baths 38 and 40 contained about 3 oz./gal. of copper in the form of copper cyanide, about 4 oz./gal. of free cyanide, about 5.5 oz./gal. of tin in the form of potassium stannate, and about 1.5 oz./gal. of free caustic in the form of potassium hydroxide. A nickel plating bath 22 was not used in this example, but rather the material was passed directly into tank 24 where tin was plated on the stock. A tin metal concentration of about 11 oz./gal. was used at a temperature of 90° F. and a cathode current density of 180 a./square foot.
With these processing conditions, the resulting product was adherent, and could be soldered satisfactorily by normal soft soldering techniques.
EXAMPLE 2
In this example, eight gauge aluminum wire was passed through the processing line at a speed of 100 ft./min. The wire passed through the etching baths 26 and 28 in a time of 10 seconds for each bath. The temperature of the etching baths was maintained at about 155° F. The composition of the etching baths was the same as in example 1.
The wire passed through the oxidizing baths 30 and 32 in about 10 seconds for each bath, and the temperature of these baths was maintained at about 75° F. The oxidizing baths contained 50 percent by volume nitric acid and 4 oz./gal. of ammonium bifluoride. The balance of the solution was water.
The stock passed through the conditioner baths 34 and 36 in a time of 5 seconds for each bath, and the temperature of these baths was maintained at 90° F. These baths contained about 6.5 oz./gal. of solution of tin metal in the form of potassium stannate.
The wire passed through the bronze plating baths 38 and 40 in a time of 10 seconds for each bath, and the baths were maintained at a temperature of 95° F. A cathode current density of 540 a./square foot was used. The composition of the baths was the same as in example 1. The nickel-plating bath 42 was not used.
The wire passed through baths 44, 46, and 48 in a time of 5 seconds each (total immersion time 15 seconds), and the temperature of the plating solution was 85° F. A cathode current density of 200 a./square foot was used. The material plated on the wire was tin-lead alloy, and this alloy was plated from a fluoboric solution of standard composition.
The resulting product had an adherent plating, and good solderable coatings were obtained.
EXAMPLE 3
In this example, 20 gauge aluminum wire alloy identified as alloy No. 1245 was passed through the processing line at a speed of 200 ft./min. The stock passed through the etching baths 26 and 28 in a time of 5 seconds for each bath. The temperature of the baths was maintained at about 170° F. The compositions of the baths were in accordance with the previous description.
The wire passed through the oxidizing baths 30 and 32 in a time of 5 seconds for each bath, and the other processing conditions were the same as for example 1.
The wire passed through the conditioning baths 34 and 36 in a time of 5 seconds for each bath, the temperature of these baths was maintained at 100° F. The other processing conditions were the same as for example 1.
The stock passed through the bronze plating baths 38 and 40 in a time of 5 seconds for each bath, and the temperature of these baths was maintained at about 110° F. A cathode current density of 1,080 a./square foot was used. The composition of the baths was about 3.5 oz./gal. of copper metal, about 6.5 oz./gal. of tin metal, about 4.5 oz./gal. of free cyanide in the form of potassium cyanide and about 1.7 oz./gal. of free caustic in the form of potassium hydroxide. The balance of the solution was water.
The stock passed through the nickel plating bath 42 in a time of 2.5 seconds, and the bath was maintained at 115° F. A cathode current density of 300 a./square foot was used. The composition of the nickel-plating bath was in accordance with the previous description.
The wire passed through the tin-plating baths 44, 46, and 48 in a time of 7.5 seconds, and the temperature of these baths was about 100° F. The cathode current density was 200 a./sq. foot. The metal plated on the wire in the baths 44, 46, and 48 was tin, and a standard tin fluoborate plating solution was used.
The resulting product had highly adherent coatings, and coatings were obtained which were easily solderable by normal soft soldering techniques. These coatings also withstood a solder pot test which is more rigid than normal soft soldering techniques.
After plating of the metal on the stock, it is possible to draw the stock to reduce its size. In experimental runs, a reduction of four gauge sizes after the plating process has been carried out has been successfully achieved.
To determine whether good solderable coatings have been obtained, certain tests can be carried out. When a nickel barrier layer is not used, a coating of tin or lead-tin alloy can be provided on bronze-plated aluminum stock which is solderable by normal soft soldering techniques. The solderability can be checked by soldering a joint between two pieces of stock using a solder such as 60-40 lead-tin alloy at a temperature of 240°-350° F. The joint is then stressed to the breaking point, and if a break occurs in the solder rather than between solder and aluminum, the solder joint was sound and the coatings are satisfactory.
Where soldering is to be carried out at high temperature or with prolonged exposure to heat, a test known as the solder pot test is used. This test is described in U.S. military specification MIL-STD 202 C dated Sept. 12, 1963. In general, soldering is carried out at a temperature in the range from 350° to 500° F. (say 450° F.) for 10 seconds, and the results are satisfactory if good wetting of the solder on the stock is achieved.