Method And Composition For Producing Bright Palladium Electrodepositions

Abstract

A composition for producing adherent deposits of palladium from a cyanide-free bath uses small amounts of cobalt or nickel or the combination thereof as the brightener. The bath employs as an electrolyte alkali metal and ammonium sulfates, sulfamates, phosphates, nitrates, nitrites and mixtures thereof. Preferably, the brightener concentration is 0.004-0.02 gram mole per liter nickel or 0.002-0.04 gram mole per liter cobalt, or the combination thereof. Cetyltrimethylammonium bromide is an optional component for the bath which is maintained at a pH of 7.5-11.0 and at a temperature of 26.degree.-60.degree. Centigrade. For barrel plating, the current density is 0.05-0.5 amperes per square decimeter and for rack plating the current density is 0.2-2.0 amperes per square decimeter.

Description

BACKGROUND OF THE INVENTION

Various compositions have been proposed for the deposition of palladium and various electrolytes have been utilized to provide sufficient conductivity in such compositions. As is true with many of the noble metals, cyanide electrolytes have enjoyed considerable commerical application, and it is the desire to control pollution problems which has created an increasing trend toward the substitution of other electrolytes.

As is true with many of the noble metals, small amounts of brighteners are highly desirable to effect optimum specular brightness in the electrodeposit. Although some brighteners will produce specular brightness, there is sometimes a tendency for the brightener itself to reduce the ductility of the deposit or to reduce the adhesion to the substrate.

It is an object of the present invention to provide a novel composition for the electrodeposition of palladium in bright, highly adherent coatings upon various types of conductive substrates.

It is also an object to provide such composition which is operable over a wide range of current density and at ambient to low elevated temperatures.

Another object is to provide a method for the electroplating of palladium which is relatively trouble-free and relatively insensitive to minor variations in operating conditions and which will produce bright, highly adherent electrodeposits of palladium.

SUMMARY OF THE INVENTION

It has now been found that the foregoing and related objects may be obtained in an aqueous bath for the electroplating of palladium which consists essentially of 0.02-0.25 gram mole per liter of palladium ion; 0.9-3.6 gram moles per liter of a soluble electrolyte selected from the group consisting of alkali metal and ammonium sulfates, sulfamates, phosphates, nitrates, nitrites and mixtures thereof; and a soluble metallic brightener selected from the group consisting of 0.002-0.04 gram mole per liter of cobalt ion, 0.004-0.02 gram mole per liter of nickel ion and mixtures thereof providing 0.003-0.068 gram mole per liter of the metal ions combined. The bath may contain up to 6.times.10.sup.-.sup.4 gram mole per liter of cetyltrimethylammonium bromide, and the pH thereof is adjusted to be within the range of 7.5-11.0.

The preferred soluble electrolyte is a sulfate salt and potassium sulfate has been found highly advantageous. The preferred compositions employ cetylitrimethylammonium bromide in an amount of at least 3.times.10.sup.-.sup.5 gram mole per liter. The palladium ion is desirably present in the amount of 0.05-0.1 gram mole per liter; the ammonium hydroxide is preferably present in an amount of 1.4-2.1 gram moles per liter; and the electrolyte is desirably present in an amount of 0.2-0.4 gram mole per liter; this preferred bath composition is desirably utilized at a pH of 8.5-9.5.

In the method of electroplating, a workpiece having a conductive surface is immersed in the aqueous bath described hereinbefore while the temperature is maintained at 26.degree.-60.degree. Centigrade. A potential is applied across the workpiece and an anode inert to the bath so as to provide a current density of 0.05-2.0 amperes per square decimeter, and the anode to cathode surface ratio is maintained at about 1.0-5.0:1.0. After application of the potential for a period of time sufficient to develop the desired thickness of palladium deposit, the workpiece is removed from the bath. In barrel plating, the current density is maintained at about 0.05-0.5 amperes per square decimeter and in rack plating the current density is maintained at about 0.2-2.0 amperes per square decimeter. The preferred baths are operated at a temperature of 46.degree.-52.degree. Centigrade under conditions of vigorous agitation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As has been indicated hereinbefore, the compositions of the present invention essentially comprise an aqueous solution of palladium ion, an electrolyte, a brightener selected from the group consisting of cobalt ion, nickel ion or the combination thereof, and ammonium hydroxide sufficient to provide a pH of about 7.5-11.0. A desirable optional additive is cetyltrimethylammonium bromide.

Turning first in detail to the palladium ion, it may be introduced as any soluble compound having a non-interfering anion. Thus, the palladium may be introduced as palladium sulfate, palladium chloride, palladium nitrate, or as a palladium complex such as diaminepalladium hydroxide, dichlorodiaminepalladium or tetraminepalladium chloride. The amount of palladium in the bath may broadly range from 0.02-0.25 gram mole per liter and is preferably within the range of 0.05-0.10 gram mole per liter.

As indicated hereinbefore, the electrolyte may be provided by any one or mixture of alkali metal and ammonium sulfates, sulfamates, phosphates, nitrates and nitrites. Of the various electrolytes, the alkali metal and ammonium sulfates have proven to be most advantageously employed; potassium sulfate is preferred. The concentration of the electrolyte salt may vary from as little as 0.1 gram mole per liter to as much as 0.7 gram mole per liter. Preferably, the concentration of the electrolyte salt is within the range of 0.2-0.4 gram mole per liter.

Ammonium hydroxide is advantageously used to regulate the pH of the bath and is present in an amount of 0.9-3.6 gram moles per liter calculated as ammonium hydroxide exclusive of water. For the preferred compositions, the ammonium hydroxide is present in an amount of 1.4-2.1 gram mole per liter. Most desirably, the ammonium hydroxide is added as a highly concentrated aqueous solution (29 percent by weight) in order to minimize dilution, although less concentrated compositions may also be employed if the water introduced thereby is entered into the calculations as to concentration of the remaining ions.

In the event that it is desired to adjust the pH downwardly (i.e., to make it more acid), and acid having non-interfering anion should be employed. Most conveniently, this is sulfuric acid, although nitric acid, hydrochloric acid and sulfamic acid may be employed.

As has been indicated heretofore, the brightener is a metal ion selected from the group consisting of cobalt, nickel and the combination thereof. As such, the metal ions may be introduced in the form of any soluble compound which will not introduce an interfering anion. For the preferred sulfate baths, these metal ions are introduced as sulfate salts. However, they may also be introduced as chloride or nitrate salts and as complexes such as diaminecobalt chloride, hexaminecobalt chloride, hexaminecobalt nitrate, aquapentaminecobalt chloride, diaquatetraminenickel nitrate, hexaminenickel nitrate and hexaminenickel chloride.

When cobalt is used alone as the brightener, it is present in an amount of 0.002-0.04 gram mole per liter and preferably 0.003-0.02 gram mole per liter. When nickel is used alone as the brightener, it is present in an amount of 0.004-0.02 gram mole per liter and preferably 0.010-0.015 gram mole per liter. When the two metal ions are used in combination with each other, the total amount thereof should provide about 0.5-4.0 grams per liter of metal ion or about 0.003-0.068 gram mole per liter of ions combined. Preferably, the metal ions combined equal 0.8-2.0 grams per liter or about 0.005-0.034 gram mole per liter.

The ratio of palladium ion to cobalt or nickel ion must be fairly closely controlled in order to obtain stress-free deposits. The weight ratio of palladium to cobalt should adjusted to within the range of 10-50 and preferably 10-33. The palladium to nickel metal weight ratio should be adjusted to within the range 7.5-19.2 and preferably 7.5-12.8.

Cetyltrimethylammonium bromide is an optional but desirable additive to effectively eliminate gas pitting in the palladium deposit. The amount of this additive should be closely controlled since excessive amounts will produce excessive foaming and interfere with the plating operation. As a result, the maximum amount which should be employed is about 6.times.10.sup.-.sup.4 gram mole per liter or 0.2 gram per liter. Preferably, the maximum concentration is less than 0.06 gram per liter. An amount of as little as 2.7.times.10.sup.-.sup.5 gram mole per liter and preferably 8.times.10.sup.-.sup.5 gram mole per liter (0.01 and 0.03 gram per liter respectively) will produce highly advantageous results.

The pH of the bath is maintained within the range of 7.5-11.0 and preferably within the range of 8.5-9.5. As indicated hereinbefore, the pH may be adjusted by the addition of ammonium hydroxide or by use of a suitable acid providing a non-interfering anion such as sulfuric acid.

The temperature of the bath should be within the range of 26.degree.-60.degree. Centigrade, and preferably 46.degree.-52.degree. Centigrade. Although the bath may be utilized without agitation, it is desirable to employ agitation and vigorous agitation has been found extremely beneficial. Filtration is highly desirable if pore-free adherent deposits are to be obtained because the presence of any solid contaminants will have a profound effect upon the quality of the deposit. Standard filter cartridges of polypropylene or other filter media may desirably be employed for continuous filtration of the bath.

The anode to cathode surface area ratio should be within the range of 1.0-5.0:1.0 and preferably 2.0:1.0. For rack plating, the current density should be within the range of 0.2-2.0 amperes per square decimenter and preferably about 0.5-1.0 amperes per square decimeter. For barrel plating, the current density should be within the range of 0.05-0.5 amperes per square decimeter and preferably about 0.1-0.3 amperes per square decimeter.

It has been found that the palladium deposits produced by the present invention are relatively stress-free and can be employed without further treatment for the great bulk of intended applications. In some instances, where the workpiece is to be subjected to severe mechanical deformation or to heavy wearing and abrasion, it is desirable to heat treat the workpieces at a temperature of about 300.degree.-450.degree. Centigrade for 1/2 to 3 hours in order to relieve all residual internal stresses. The heat treatment may be conducted either under vacuum or in an inert gas atmosphere.

Various anodes which are inert to the plating bath may be employed and generally such anodes will have a surface of noble metal, although carbon anodes do have limited utility. The preferred anodes are platinum-clad tantalum although gold-clad tantalum, platinum and palladium electrodes have all been employed effectively.

Exemplary of the present invention are the following specific examples:

EXAMPLE ONE

A bath is prepared by adding to deionized water palladium sulfate sufficient to provide 5.0 grams per liter as palladium metal, 80 c.c. per liter of ammonium hydroxide (29 percent by weight NH.sub.3), 1.05 grams per liter cobalt sulfate, 50 grams per liter potassium sulfate and 0.04 grams per liter cetyltrimethylammonium bromide. The resulting bath has a pH of 8.5 and is introduced into a plating cell where the temperature is maintained at 44.5.degree. Centigrade.

A potential of 1.7 volts is applied across a platinum surfaced anode and a Hull cell panel 3 centimeters by 5 centimeters in dimension, the anode to cathode surface area ratio being 3;1. The current density is determined to be 0.5 amperes per square decimeter and the current is continued for 20 minutes during which there is fast agitation of the plating bath.

The panel is then removed and the palladium deposit thus formed is found to have a thickness of 2.5 microns and to be specular bright. Flexing of the panel repeatedly indicates that the deposit is highly adherent.

EXAMPLE TWO

To deionized water are added the various components in amounts suffcient to provide 9.0 grams per liter of palladium determined as the metal, 100 c.c. per liter of ammonium hydroxide (29 percent by weight NH.sub.3), 2.63 grams per liter nickel sulfate, 100 grams per liter potassium sulfate and 0.03 gram per liter cetyltrimethylammonium bromide. The pH of the formulation is 7.9 and the temperature is maintained at 37.8.degree. Centigrade.

A Hull cell panel is used as the cathode and a platinum surfaced anode is employed. A potential of 2.2 volts is applied thereacross during fast agitation of the solution in the cell. The anode to cathode surface area ratio is 3:1 and the current density is 0.5 amperes per square decimeter.

After plating for 20 minutes, the panel is removed and is found to have a specular bright palladium deposit of 1.9 microns thickness. Flexing the panel indicates the deposit to be highly adherent.

EXAMPLE THREE

To evaluate the effect of other electrolytes, a mixed electrolyte is prepared using 25 grams per liter potassium sulfate, 2 grams per liter ammonium nitrate and 1 gram per liter ammonium chloride. The bath contains palladium sulfate sufficient to provide 5 grams per liter of palladium as metal and 60 c.c. per liter ammonium hydroxide (29 percent by weight NH.sub.3) and 0.06 gram per liter cetyltrimethylammonium bromide. Used as the brightener is 0.46 gram per liter cobalt sulfate. The pH of this formulation is 9.9 and the bath is introduced into a test barrel plating installation wherein it is maintained at 49.degree. Centigrade.

The workpieces to be plated are nickel pins about one centimeter in length and about 0.5 millimeters in diameter. The anode to cathode surface area ratio is calculated at 1:1 and a potential of 5 volts is applied to provide a current density of 0.2 amperes per square decimeter. The plating operation is continued for a period of 90 minutes after which the parts are removed and found to have a specular bright deposit of 2.5 microns thickness. The deposit is highly adherent as evidenced by the lack of exfoliation after bending of the pins through 90.degree..

Thus it can be seen from the foregoing detailed specification and examples that the baths of the present invention provide bright, highly adherent electrodeposits on various types of conductive substrates. They are operable over a wide range of current density and at ambient to low elevated temperatures. The plating operation is relatively trouble-free and relatively insensitive to minor variations in operating conditions and will yield deposits which are substantially free from internal stresses.

Claims

Having thus described the invention, we claim:

1. In an aqueous bath for the electroplating of palladium, the combination consisting essentially of:

A. 0.02-0.25 gram mole per liter of palladium ion;

B. 0.9-3.6 gram moles per liter of ammonium hydroxide;

C. 0.1-0.7 gram mole per liter of a soluble electrolyte selected from the group consisting of alkali metal and ammonium sulfates, sulfamates, phosphates, nitrates, nitrites and mixtures thereof;

D. a soluble metallic brightener selected from the group consisting of 0.002-0.04 gram mole per liter of cobalt ion, 0.004-0.02 gram mole per liter of nickel ion and mixtures thereof providing 0.003-0.068 gram mole per liter of metal ions; and

E. up to 6.times.10.sup.-.sup.4 gram mole per liter of cetyltrimethylammonium bromide;

said bath having a pH of 7.5-11.0.

2. The bath in accordance with claim 1 wherein said soluble electrolyte is a sulfate salt.

3. The bath in accordance with claim 2 wherein said soluble salt is potassium sulfate.

4. The bath in accordance with claim 1 wherein said palladium ion is present in an amount of 0.05-0.1 gram mole per liter, said ammonium hydroxide is present in an amount of 1.4-2.1 gram moles per liter, said electrolyte is present in an amount of 0.2-0.4 gram mole per liter, and wherein said bath has a pH of 8.5-9.5.

5. The bath in accordance with claim 4 wherein said electrolyte is a sulfate salt and wherein said cetyltrimethylammonium bromide is present in an amount of at least 3.times.10.sup.-.sup.5 gram per liter.

6. The bath in accordance with claim 1 wherein said cetyltrimethylammonium bromide is present in an amount of at least 3.times.10.sup.-.sup.5 gram mole per liter.

7. The bath in accordance with claim 1 wherein said soluble metallic brightener is a mixture of nickel and cobalt ions.

8. In the method of electroplating palladium deposits upon a workpiece, the steps comprising:

A. immersing a workpiece having a conductive surface in an aqueous bath consisting essentially of:

1. 0.02-0.25 gram mole per liter of palladium ion;

2. 0.9-3.6 gram moles per liter of ammonium hydroxide;

3. 0.1-0.7 gram mole per liter of a soluble electrolyte selected from the group consisting of alkali metal and ammonium sulfates, sulfamates, phosphates, nitrates, nitrites and mixtures thereof;

4. a soluble metallic brightener selected from the group consisting of 0.002-0.04 gram mole per liter of cobalt ion, 0.004-0.02 gram mole per liter of nickel ion and mixtures thereof providing 0.003-0.068 gram mole per liter of metal ions; and

5. up to 6.times.10.sup.-.sup.4 gram mole per liter of cetyltrimethylammonium bromide;

said bath having a pH of 7.5-11.0;

B. maintaining the temperature of said bath at 26.degree.-60.degree. Centigrade;

C. applying a potential across said workpiece and an anode inert to said bath, said potential providing a current density of 0.05-2.0 amperes per square decimeter and the anode to cathode surface ratio being 1.0-5.0:1.0; and

D. removing said workpiece from said bath upon deposition of the desired thickness of palladium upon the surface thereof.

9. The method in accordance with claim 8 wherein said palladium ion is present in an amount of 0.05-0.1 gram mole per liter, said ammonium hydroxide is present in an amount of 1.4-2.1 gram moles per liter, said electrolyte is present in an amount of 0.2-0.4 gram mole per liter, and wherein said bath has a pH of 8.5-9.5.

10. The method in accordance with claim 9 wherein said electrolyte is a sulfate salt and wherein said cetyltrimethylammonium bromide is present in an amount of at least 3.times.10.sup.-.sup.5 gram mole per liter.

11. The method in accordance with claim 8 wherein said solution, workpiece and anodes are placed in contact within a barrel plating installation and wherein the current density is maintained at about 0.05-0.5 amperes per square decimeter.

12. The method in accordance with claim 8 wherein said solution, a workpiece and anodes are placed in contact within a rack plating installation and wherein the current density is maintained at about 0.2-2.0 amperes per square decimeter.

13. The method in accordance with claim 8 wherein the temperature of the bath is maintained at 46.degree.-52.degree. Centigrade and wherein vigorous agitation is employed.

14. The method in accordance with claim 8 wherein the surface of said anode is a noble metal.

15. The method in accordance with claim 8 wherein said soluble salt is potassium sulfate.