US3684666A

US3684666A - Copper electroplating in a citric acid bath

Info

Publication number: US3684666A
Application number: US21216A
Authority: US
Inventors: John K Shaw; John B Romolo; Charles Levy
Original assignee: Pfizer Inc
Current assignee: Pfizer Inc
Priority date: 1970-03-19
Filing date: 1970-03-19
Publication date: 1972-08-15
Anticipated expiration: 1989-08-15
Also published as: DE2104873A1; FR2083400A1; BE762452A; GB1320373A; FR2083400B1

Abstract

A NEW METHOD FOR ELECTROPLATING COPPER ON AN ARTICLE UTILIZING AQUEOUS COPPER SALT-CITRIC ACID SOLUTION IN WHICH CITRIC ACID IS THE PRINCIPAL COMPLEXING AGENT FOR THE COPPER ION IN THE SOLUTION. CUPRIC SALTS ARE PREFERRED IN THIS INVENTION, ESPECIALLY CUCO3-CU(OH)2 AND CUSO4-5H2O, FOR BOTH PLATING SOLUTIONS AND STRIKE SOLUTIONS. FOR PLATING THE PH RANGE IS ABOUT 0.4 TO 8.9, THE CUPRIC ION CONCENTRATION IS ABOUT 16 TO 64 GRAMS PER LITER, AND THE CITRIC ACID CONCENTRATION IS ABOUT 48 TO 192 GRAMS PER LITER. FOR STRIKE SOLUTION THE PH RANGE IS ABOUT 1 TO 12, THE CUPRIC ION CONCENTRATION IS ABOUT 3 TO 16 GRAMS PER LITER, AND THE CITRIC ACID CONCENTRATION IS ABOUT 20 TO 100 GRAMS PER LITER. THE CURRENT DENSITY IS ABOUT 1 TO 40 AMPERES PER SQUARE FOOT FOR BOTH PLATING AND STRIKE SOLUTIONS. THIS NEW METHOD AVOIDS THE HANDLING PROBLEMS OF USING CYANIDE OR MINERAL ACIDS AS THE COPPER COMPLEXING AGENT AND GIVES COMPARABLE PLATING RESULTS.

Description

United States Patent U.S. Cl. 204-40 2 Claims ABSTRACT OF THE DISCLOSURE A new method for electroplating copper on an article utilizing aqueous copper salt-citric acid solutions in which citric acid is the principal complexing agent for the copper ion in the solution. Cupric salts are preferred in this invention, especially CuCO -Cu(OH) and CuSO -5H O, for both plating solutions and strike solutions. For plating the pH range is about 0.4 to 8.9, the cupric ion concentration is about 16 to 64 grams per liter, and the citric acid concentration is about 48 to 192 grams per liter. For strike solutions the pH range is about 1 to 12, the cupric ion concentration is about 3 to 16 grams per liter, and the citric acid concentration is about 20 to 100 grams per liter. The current density is about 1 to 40 amperes per square foot for both plating and strike solutions. This new method avoids the handling problems of using cyanide or mineral acids as the copper complexing agent and gives comparable plating results.

BACKGROUND OF THE INVENTION Metal finishing companies are currently giving serious consideration to treatment of their waste solutions. In particular, most electroplating shops utilize copper cyanide plating baths, for which disposal and break down of the cyanide involves particularly rigorous and expensive treatment methods due to the extremely poisonous nature of cyanide.

Furthermore, when copper is electroplated on a more electronegative metal such as steel or zinc, there is a tendency for the copper to be initially deposited by displacing some of the metal at its surface. This galvanic-type deposit so obtained is nearly always loose and non-adherent, preventing the subsequent electrodeposition of a good copper coating. This necessitates imparting an initial strikecoating of copper from a separate copper bath having a very low copper-ion concentration. The copper electrodeposit is then applied from an ordinary plating bath having much higher copper-ion concentrations.

Although copper baths with cyanide as the major complexing agent for the copper ion provide the best strike and ordinary copper coating, it would be highly desirable to develop the use of equally eilective non-cyanide baths due to the inherent disadvantages of using cyanide-based baths.

Non-cyanide copper plating is not new. Formulations based on copper sulfate and copper fluoborate are used in acid copper plating, and pyrophosphate baths are used commercially for alkaline copper plating. These noncyanide baths, however, do not give equivalent performance to those containing cyanide. Copper sulfate-sulfuric acid solutions cannot be used as strike solutions on steel due to galvanic action which results in a non-adherent, galvanic-type deposit. Furthermore, on zinc and zinc alloys these acid solutions rapidly attack the zinc surface and change the dimensions of the article being plated. Both the fluoborate and pyrophosphate baths require initial cyanide bath copper-strike coatings on steel and zinc diecasts in order to get a consistently good ordinary 3,684,666 Patented Aug. 15, 1972 copper coating from both of these baths. Consequently, although the use of cyanide may be reduced, it has not been eliminated.

Prior art methods of electrodepositing copper on a metallic substrate have utilized citric acid, or a salt thereof, as an additive to copper electroplating solutions to increase the permissible cathode current density range; to increase the conductivity of the bath; to increase the brightness of the cathode deposit; to increase the tolerance of the bath to impurities; and to aid in anode cor IOSlOIl.

Although often heretofore used as an additive, citric acid or its salts have never beiore been used as the grirflcipal complexing agent for copper in electroplating at s.

SUMMARY OF THE INVENTION In contrast to the prior art, it has been surprisingly found that copper can be electrodeposited on an article which is made the cathode by exposing the article to an aqueous cupric salt solution at a pH from about 0.4 to 8.9, the solution containing from about 16 to 64 grams per liter of cupric ion and from about 48 to 192 grams per liter of citric acid, and applying a current density of about 1 to 40 amperes per square foot to obtain the desired coating.

Where the article to be copper-coated consists of a metal that is more electromegative than copper, and which therefore will almost always result in non-adherent ga1vanic-type deposits of copper on the article, an initial strike-coating of copper is imparted on the article prior to electrodepositing the ordinary copper coating on the article. It is again surprisingly found that the copper strike-coating can be imparted on the article by exposing the article to an aqueous cupric salt solution at a pH from about 1 to 12, the solution containing from about 3 to 16 grams per liter of cupric ion and from about 20' to grams per liter of citric acid, and applying a current density from about 1 to 40 amperes per square foot to obtain the desired strike-coating.

Thus, citric acid can be used as the principal complexing agent for copper in strike and plating solutions, giving comparable results to cyanide and acid plating baths but avoiding the disadvantages of their use.

DETAILED DESCRIPTION OF THE INVENTION By using the present invention one major advantage is the elimination of cyanide and pyrophosphate ions in the electroplating bath and in the subsequent rinse water. Thus, waste disposal is made relatively easy in that the citrates are readily biodegradable.

A further advantage is that the copper deposit has inherent brightness as compared to similar deposits obtained from conventional non-proprietary copper electroplating solutions.

Still another advantage of the present invention is the elimination of the need for a cyanide strike bath. Other copper electroplating baths, such as those based on fluoborate or pyrophosphate, require :a cyanide strike before adequate adhesion of the copper plating can be obtained. By this invention it is possible to adjust the quantities of citrate and copper salt to perform both the striking and electroplating.

Furthermore the effect of storing citric acid-based electroplating solutions is insignificant with respect to the copper electrodeposits subsequently obtained using such solutions.

According to the present invention, bright, adherent, and dense coatings of copper can be electrodeposited on an article by exposing the article as the cathode to an aqueous cupric salt solution in which citric acid is the principal complexing agent for the copper in solution. Ordinarily the anode that is contemplated to be used in carrying out the steps of the invention is copper metal.

The article to be electroplated may be composed of any suitable metal such as aluminum, steel, zinc, brass, or other metals and their alloys. Where the article is composed of a metal that is more electronegative than copper, galvanic-type deposits will result on the article surface without the application of any current density. Immersion-type deposits are usually not of good quality and are non-adherent, resulting in a poor quality copper coating when the current density is subsequently applied. This necessitates imparting to the article an initial copper coating, known as a strike-coating, to provide an initial adherent coating and to prevent the formation of an undesirable galvanic-type coating. Thus, it is necessary to impart a strike-coating of copper where the article to be plated is composed of steel or zinc, which are more electronegative than copper; whereas it is not necessary to impart a strike-coating to an article composed of brass. However, the copper galvanic-type coatings encountered where citric acid is the principal complexing agent for the copper in solution, have been found to be surprisingly more adherent than those normally encountered where other complexing agents are used. This copper coating probably occurs due to galvanic displacement by the copper and may even be due to some chemical reduction.

Although any water-soluble cupric or cuprous salt can be used in practicing this invention, it is preferred to use cupric salts and especially preferred to use cupric sulfate (e.g., CuSO and CuSO -H O) or cupric carbonate (e.g., 2CuCO -Cu(OH) and CuCO -Cu(OH) due to their ready availability and price.

The temperature of the plating baths may vary from room temperature to the boiling point of the solution, although for convenience the temperature is kept at room temperature (about 2030 C.).

The optimum current density is dependent on other electrolysis conditions and the composition of the bath. Current densities of about 1 to 40 amperes per square foot have been found satisfactory.

The quantity and quality of the copper coatings attained are strongly dependent upon the relative concentrations of cupric ion and citric acid, as well as the pH of the plating solution. It is preferred to use citric acid in practicing the invention, although the use of citrates is not excluded thereby. The preferred concentration of citric acid in ordinary copper plating baths is from about 48 to 192 grams per liter, and it is most preferred to use 160 to 192 grams per liter. In copper strike-coating baths it is preferred to have a concentration of citric acid from about 20 to 100 grams per liter.

A cupric ion concentration of about 16 to 64 grams per liter is preferred for ordinary plating baths, and a concentration of 50 to 64 grams per liter is most preferred. For strike-coating baths a cupric ion concentration of about 3 to 16 grams per liter is preferred. It is especially preferred to have a cupric ion concentration of about 12 to 16 grams per liter when cupric carbonate is used, and a concentration of about 3 to 5 grams per liter when cupric sulfate is used.

The preferred pH with respect to the quality of the copper deposited will vary somewhat with the concentrations of the bath make-up ingredients. The pH for ordinary copper plating baths can be from about 0.4 to 8.9, with a pH from about 0.4 to 1.2 preferred when cupric sulfate is used and a pH from about 7 to 8 preferred when cupric carbonate is used. For copper strike-coating baths the pH can be from about 1 to 12, and a pH from about 1 to 2 is preferred when cupric sulfate is used and a pH from about 9 to 12 is preferred when cupric carbonate is used.

Although not essential to the use of the invention, sodium sulfate or ammonium citrate can be added to improve the conductivity of the plating bath and boric acid can be added as a buffering agent, as well as a base (e.g., NH OH, NaOH, etc.) for pH control. Of course, this does not exclude the use of other additives to improve the conductivity of the bath or to control the bath pH, which can be determined by someone skilled in the art.

A preferred embodiment of the invention involves using an aqueous copper strike solution containing about 96 grams per liter of citric acid, 20 grams per liter of CuCQ -Cu(OH) adjusting the pH to about 11.6 by addition of NaOH, maintaining it at a current density of 20 to 40 amperes per square foot, and a plating time of 5 minutes.

Another preferred embodiment of the invention involves using an aqueous copper strike solution at a pH of bout 1.6 containing about 24 grams per liter of citric acid, 16 grams per liter of CuSO -5H O, 6 grams per liter of sodium sulfate, 20 grams per liter of boric acid, maintaining the current density at 1-3 amperes per square foot, and a plating time of 5 minutes.

Still another preferred embodiment of the invention involves using an aqueous copper plating solution at a pH of about 0.8 containing about 192 grams per liter of citric acid, 250 grams per liter of CuSO -5H O, maintaining the current density at 20 to 40 amperes per square foot, and a plating time of 60 minutes.

A final preferred embodiment of the invention involves using an aqueous copper plating solution containing about 192 grams per liter of citric acid, grams per liter of CuCO -Cu(OH) adjusting the pH to about 8 by addition of NH OH, maintaining it at a current density of 20 to 40 amperes per square foot, and a plating time of minutes.

The following examples are provided for illustrative purposes and should not be interpreted as limiting the invention, the scope of which is defined by the appended claims.

EXAMPLE I A standard Hull Test Cell is used as the electrolyzing apparatus. The cathode consists of a standard steel panel which is 4" long, 2 /2" wide, and 0.010" thick. A pure copper anode is utilized also.

The cathode is cleaned and prepared according to standard electroplating procedures consisting of degreasing, alkaline cleaning and thorough water rinsing before being electroplated.

The cathode is then exposed to an aqueous copper strike solution at a pH of 1.6 containing 24 grams per liter of citric acid, 16 grams per liter of CuSO -5H O, 6 grams per liter of sodium sulfate, and 20 grams per liter of boric acid. The solution is maintained at room temperature, while the current is I to 3 amperes. After 5 minutes immersion time the cathode is withdrawn from the strike solution and thoroughly rinsed with water, resulting in a bright, thin, adherent copper strike coating.

EXAMPLE II The copper-strike coated steel panel of Example I is then exposed as the cathode to an aqueous copper plating solution containing 95 grams per liter of 192 grams per liter of citric acid, and the pH is adjusted to 8 by addition of NH OH. The solution is maintained at room temperature, while the current density is 20-40 amperes per square foot. After an immersion time of 120 minutes the steel panel is withdrawn from the plating solution resulting in a bright, dense, and adherent copper coating.

EXAMPLE III A standard Hull Test Cell brass panel is cleaned and prepared according to standard electroplating procedures and is exposed as the cathode to an aqueous copper plating solution at a pH of 0.4 containing 250 grams per liter of CuSO -5H O and 192 grams per liter of citric acid, a

temperature of 150 F., and a current of 1 ampere. A good copper plating is attained on the brass panel after an immersion time of 3 minutes, in the Hull Test Cell. Similar results are obtained at a current density of 20-40 amperes per square foot and a pH of 0.8, in larger baths, for longer immersion times. t.

EXAMPLE IV Satisfactory results are obtained following the procedure of Example III except that the plating bath at a pH of 1.2 contains 63 grams per liter of CuSO -5H O and 48 grams per liter of citric acid, and is maintained at a current density of 40 amperes per square foot, and the immersion time is 60 minutes.

' EXAMPLE v Satisfactory results are obtained following the procedure of Example III except that the plating bath at a pH of 8.9 contains 250 grams per liter of CuSO -5H O and 192 grams per liter of citric acid, and is maintained at a current density of 20 amperes per square foot, and the immersion time is 120' minutes.

EXAMPLE VI Similar results are obtained following the procedure of Example I except that the strike solution at a pH of 9.2 contains '29 grams per liter of CuCO -Cu(OH) 86 grams per liter of citric acid, and 61 grams per liter of ammonium citrate, and is maintained at a current density of 40 amperes per square foot, and the immersion time is 60 minutes, ina larger bath.

Thickness tests were determined by means of a Kocour Electronic Thickness Tester Model 955, utilizing the appropriate anodic stripping solution. The tester was first balanced and the sensitivity checked. It was then calibrated against standard panels for copper deposit thickness on steel substrates. Thickness tests were performed on the Hull Cell panels at locations which were indicative of current densities of 4 amps per sq. ft. and amps per sq. ft.

The throwing power measurement is obtained by taking deposit thickness reading at 40 and 4 amps per sq. ft. The relationship normally expected would be a deposit ratio of 10 to 1. The calculation for throwing power in terms of the primary and metal ratios is as follows:

Primary ratiometal ratio Primary ratio The primary ratio is based on the current densities at which the two thicknesses are measured, in our case, 10.

The metal ratio is based on the thickness actually obtained at those current densities. For example, in Table I,

X 100 percent Panel Tl gave a thickness ratio at the two current densi- TABLE I.-THROWING POWER TESTS Plated thickness Solution pH of copper at Percentage im- Solution composi- Plat- 40 a.s.f. 4 a.s.t. prove- Metal Panel No. tion Strike ing (inch) (inch) ment ratio T1 CNST and CNRO- 11. 88 10. 28 0. 00019 0. 00007 73 2. 7 T2. CNST and acid..- 11. 88 0. 48 0. 00021 0. 00005 58 4. 2 T3- V and A 1. 69 0. 75 0. 00022 0. 00005 56 4. 4 T4 H and Q ll. 68 7.73 0.00015 0 00003 50 5. 0

Solution Composition CNST=0.2M copper cyanide, 0.7M sodium cyanide,

0.05 sodium hydroxide, pH 11.9;

CNRO =0.2M copper cyanide, 0.7M sodium cyanide, 0.3M sodium carbonate, 0.16 Rochelle salt, pH 10.3; V

=0.064M copper sulfate, 0125M citric acid, 0.042M sodium sulfate, 0.32M

horie acid, pH 1.7; ACID-=0.8M copper sulfate, 0.5M suliuric acid, pH 0.5; A==1.0M citric acid, 1.0M cop per sulfate, pH 0.8; H=0.5M citric acid, 0.09M copper carbonate, pH 11.6

adjusted by addition of sodium hydroxide; Q=1.0M citric acid, 0.43M copper carbonate, pH 7.8 ad usted by addition of sodium hydroxide.

N orELCopper sulfate means CuSOr-fiHaO; a.s.t. means amperes per square foot.

EXAMPLE v11 Similar results are obtained following the procedure of of Example I except that the strike solution contains 20 grams per liter of CuC0 -Cu(0H) and 96 grams per liter of citric acid, the pH being adjusted to 12 by additionof NaOH, and maintained at a current density of 20-40 amperes per square foot, and the immersion time is 1 minute, in a larger bath.

EXAMPLE VIII Throwing power tests Test panels consisted of Steel Hull Cell panels. According to standard electroplating procedure, four panels were plated, two from conventional baths and two from copper citrate plating solutions. The conventional bath panels were struck from solution composition CNST, and plated from either the copper cyanide-Rochelle salt solution (CNRO) or the acid copper sulfate solution (ACID). Panels T3 and T4 were struck in the (V) and (H) solutions, and plated in the (A) and (Q) solutions, respectively. The details of the solution conditions can be found in Table I. The strike plating was performed in two-liter beakers at three volts for one minute at room temperature. The plating was conducted in Hull Cells at two amperes and room temperature for five minutes.

Copper carbonate means-OuCOr Cu(OH)| EXAMPLE IX Thickness and adhesion tests Additional panels were prepared by electrolyzing conventional and experimental strike: and plating solutions using the techniques described above. These were standard steel Hull Cell panels 0.010 inch thick which were fabricated into bent cathodes to obtain varying current densities along length of immersed cathode. This configuration creates high and low current density areas for evaluation of throwing power, which is the ability of the solution to deposit a relatively uniform thickness over a surface.

The panels rwere cleaned by standard methods and were struck in solutions CNST, (V), and (H), at three to five ivolts at room temperature for one minute. They were then electroplated using solution compositions CNRO', ACID, (A), and (Q) at 20 to 40 amps per sq. ft. at room temperature for thirty minutes. The experimental details are shown in Table H.

The plated panels were then bent at angle approximately one-half inch from the end opposite the bent cathode end, and the bend was observed for lifting of the deposit. The panel was then further bent 9-0 in the same direction, and then bent back to its original flat position.

The bent cathode portion of these same panels were then flattened and thickness tests were performed using the Kocour Tester previously referred to in Example VIII. Five thickness measurement locations were selected for their range of high to low current density areas. These thickness locations are indicated in Table II as No. 1, No. 2, No. 3, No. 4 and No. 5.

8 rent density from about to 40 amperes per square foot until the desired copper strikeecoating has been deposited on said article, and (2) then electrodepositing a second coating of copper on the resulting article which comprises exposing said thin-coated article as the cathode to an aqueous cupric salt solution at a pH from about 0.4 to 8.9, said solution comprising from about 16 to 64 grams TABLE II.THICKNESS AND ADHESION TESTS Average Thickness (inch) Plating Solution rate Panel No. Composition Deposit adhesion No 1 No. 2 No. 3 No. 4 No. 5 (in./hr.2)

B1 CNST and aeid Good 0. 00130 0.00178 0.00058 0.00109 0. 00224 0.00282 CNSI and CNR do 0. 00181 0.00172 0.00106 0.00127 0. 00286 Q, Lifted 3d bend 0 00028 0. 00041 0. 00018 0.00017 0.00039 0.00058 CNSI and CN RO do 0.00111 0.00127 0. 00094 0.00123 0.00222 -A Lifted all bends 0 00055 0. 00008 0.00028 0.00032 0.00082 0. 00106 Good 0. 00041 0. 00024 0.00025 0.00056 0.00071 See Table I for solution composition.

tion composition (Q) was about one-quarter that of conventional plating Ibaths.

Adhesion tests of the deposits plated from copper citrate solutions were not as satisfactory as those obtained from conventional copper plating solutions. Of the ten tests made, only two passed the three bends. These were plated from solution composition (Q). All of the panels plated from solution composition (A) failed at some point. A

contributing factor in these results is that the deposits from solution composition (Q) are thinner than those from solution composition (A).

EXAMPLE X An adherent galvanic copper coating is obtained on a steel substrate utilizing an aqueous solution consisting of 0.064 M copper sulfate, 0.24 M citric acid, 0.042 M sodi urn sulfate, and 0.32 M boric acid. The coating develops at a pH of 1.6 and a temperature of 72 F. in three to fiive seconds. Adhesion of the deposit is determined by rubbing with a finger and with a rubber eraser. Galvanic deposition which is not adherent is observed in similarly constituted solutions containing oxalate or citrate instead of citric acid.

What is claimed is:

1. A method of electroplating copper on an article that exhibits greater electronegativity than copper in an electroplating bath which comprises (1) electrodepositing on said article a thin copper strike-coating which comprises exposing said article as the cathode to an aqueous cupric salt strike solution of CuCO' -Cu(OH) adjusted to a pH per liter of cupric ion and from about 48 to 122 grams per literof citric acid, and applying a current density from about 1 to 40arnperes per square foot until the 'desired copper second coating has been deposited on said article.

2. A method of electroplating copper on an article that exhibits greater electronegativity than copper in an elec troplating bath which comprises (1.) electrodepositingon said article a thin copper strike coating which comprises exposing said article as the cathode to an aqueous cupric saltstri ke solution of CuSO -5H O at a pH of about 1.6 containing about 6 grams per liter of sodium sulfate and 20 grams per liter of boric acid, said strike solution comprising about 4 grams per liter of cupric ion and about 24 gramsper liter of citric acid, and applying a current density from about 1 to 3 amperes per square foot until the desired copper strike-coating has been deposited on said article, and 2) then electrodepositing a second coating of copper on the resulting article which comprises exposing said thin-coated article as the cathode to an aqueous cupric salt solution at a pH from about 0.4 to 89', said solution ion and from about 48 to 192 grams per liter of citric acid,

- and applying a current density from about 1 to 40 amof about 11.6 by the addition of NaOH, said strike solution comprising about 12 grams per liter of cupric ion and about 96 grams per liter of citric acid, and applying a curperes per square foot until the desired copper second coating has been deposited on said article,

References Cited UNITED STATES PATENTS 2,871,172