US3749650A - Method of electrodepositing gold alloys - Google Patents

Abstract

GOLD ALLOYS OF IMPROVED MECHANICAL AND ELECTRICAL PROPERTIES ARE ELECTRODEPOSITED CATHODICALLY FROM AQUEOUS ELECTROLYTES ON CONDUCTIVE ARTICLES BY MEANS OF CYCLICALLY VARYING POTENTIALS, THE POTENTIAL IN A FIRST PERIOD OF EACH CYCLE HAVING A DURATION OF AT LEAST 0.1 SECOND BEING EQUAL TO THE DEPOSITION POTENTIAL OF THE ALLOY COMPONENTS, THE POTENTIAL IN A SECOND PERIOD OF 10**-3 TO 10**-4 SECOND BEING MUCH HIGHER THAN IN THE FIRST PERIOD, AND THE SECOND PERIOD BEING FOLLOWED BY A THIRD PERIOD OF APPROXIMATELY 10**-1 TO 10**-3 SECOND IN WHICH THERE IS NO SIGNIFICANT POTENTIAL BETWEEN THE CATHODE AND THE ELECTROLYTE.

Description

July 31, 1973 M. DETTKE ETAL 3,749,650

Mmiion OF ELECI'IRODEIOSITING 001,1) MJLOYS Fil'ed A '1 14, 1972 pm g5 as ./L

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United States Patent Office 3,749,650 METHOD OF ELECTRODEPOSITING GOLD ALLOYS Manfred Dettke, Trautenstrasse 24, 1 Berlin 31, Germany; Rolf Ludwig, Bulowstrasse 19-22, 1 Berlin 30, Germany; and Wolfgang Riedel, Ludwigsfelder Strasse 7, 1 Berlin 37, Germany Filed Apr. 14, 1972, Ser. No. 244,209 Claims priority, application Germany, Apr. 24, 1971, P 21 21 150.7 Int. Cl. C23b /42 US. Cl. 204-44 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the electrodeposition of gold alloys from aqueous electrolytes, and particularly to a method of improving the properties of electrodeposited gold alloys by modifying the potential applied to the electrodes during deposition.

it is known to produce gold-copper alloy deposits of improved brightness by periodically reversing the potential applied to the electrodes. It is a serious shortcoming of this known method that the amount of metal deposited per unit of current passing through the bath is relatively small since some of the metal deposited cathodically is again dissolved when the object to be plated becomes the anode. It has also been proposed to employ pulsating direct current in which pulses of a strength sufficient to deposit all constituents of the alloy alternate with pulses of at least twice the current strength. The last-mentioned method does not permit the deposition of alloys having uniform composition. The alloys produced do not show significantly improved hardness, wear resistance, or elongation as compared to conventionally produced alloys.

It is a primary object of this invention to provide a method of electrodepositing gold alloys of improved crystal structure resulting in better hardness, wear resistance, and elongation and of uniform composition without loss in the deposition efiiciency of the applied current.

The improved binary, ternary, and quaternary alloys of the invention are deposited on immersed conductive objects from aqueous baths by closely controlled cycles of potential pulses, each cycle consisting of a pulse of high voltage and a duration of to 10- second, which is preceded by a period of deposition potential lasting 0.1 second or longer, and is followed by a period of about 10- to 10" second during which the potential is much lower than the deposition potential and practically zero, the several potentials being applied to the immersed object as the cathode and to the electrolyte.

Best results have been achieved when the time integral of the high voltage pulse in each group is smaller than 5X10 voltseconds, the magnitude of the pulse is greater by about 1 to 5 volts than the deposition voltage, and is preferably between 2 and 8 times that voltage. The highvoltage pulses of the several groups may be uniform or 3,749,650 Patented July 31, 1973 further modulated. The potential changes generally described above have been found to polarize the free and complex metal ions in the electrolyte in such a manner as to produce the results indicated above.

The desired sequence of applied potentials is achieved by means of a step generator capable of producing stepped voltages of predetermined duration in a cycle including, for example, ten stages differing from each other in duration and amplitude. In the attached drawing:

FIG. 1 is a block diagram of a suitable generator; and

FIG. 2 diagrammatically illustrates the changes in the output potential of the generator as a function of time.

Referring initially to FIG. 1, there is seen a generator whose principal element is a decade counter 1 equipped with a 1-of-10 decoder. The decoder outputs control semi-conductor switches (field effect transistors) T to T through an adapter circuit 2, the switches being arranged at the signal inputs of an integrator 3- and of an output signal amplifier 4. The switches T to T select the duration of intervals t to r which are each infinitely variable by means of potentiometers P to P The simultaneously selected respective switches T to T select the amplitudes of the output signals during the corresponding interval, the amplitudes being continuously adjustable by means of potentiometers P to P An integrator input current I defined by the position of the associated potentiometer P is associated with each semi-conductor switch T The integrator output voltage U s satisfy the equation that is, I determines the slope of U s.

When each of the switches T to T is closed, the switch T controlled by the adapter, is to be closed for a period which is short as compared to the time of increase of U,,s in order to discharge a capacitor C. From the moment at which a switch T is closed, there elapses a time t defined by the associated potentiometer P until U s becomes equal to the threshold potential of a comparator 5. At this moment, the comparator furnishes a pulse to the decade counter 6 and advances the counter by one unit, that is, the next output of the decoder is activated. The semi-conductor switches are operated by the adapter circuit so that they set the duration and the output signal amplitude of the subsequent interval. The integrator, comparator, counter, and decoder operate in a closed loop so that a new cycle of stepped potentials starts after each group of ten pulses at the counter input. The counter is readily started in position 8 of a singlepole, double-throw switch by means of a gate 7 in the input circuit of the counter and stopped in position 9.

The potential pattern so produced is seen in FIG. 2.

It is essential for the change in polarisation that the high-potential pulses A A A act on the electrolyte for a duration of 10- to 10* second and that the periods of a potential not significantly dilferent from zero potential t t t extend over l0 to 10- second.

The high-voltage pulses employed according to the invention do not generate current variations according to Ohms law but merely cause so-called non-Faraday currents which transfer or polarize the ions and complexes present in the cathode film, but do not discharge ions in the electrolyte. These unsteady potential changes are produced in the manner indicated, the potential of the Faraday or deposition current being followed briefly by a potential peak which generates a non-Faraday current.

In the example of a pattern of potential changes illustrated in FIG. 2, A A and A are the deposition potentials for the desired alloy composition, their magnitudes being merely presented by way of example. The effective deposition periods are indicated as t t and t A A and A are the potential peaks of the non-Faraday currents which must satisfy the relationship:

The periods of the potential peaks A A A are approximately to 10* second and are indicated at t t r The periods of zero or practically zero potential t t t having a duration of about 10 to 10- second follow the peak potentials.

While A A A need not be zero, they must be much smaller than A A A The periods of deposition potential t t t must be longer than the periods of practically zero potential t t t and the latter must be much longer than the periods of the potential peaks t t t The effective deposition periods 2 t are to be selected so that the thickness of the electrodeposit during each individual period should not exceed 300 angstrom units. The necessary deposition periods are of the order of 0.1 to about 100 seconds and thus much longer than the periods of peak potential. Good results are generally obtained when the growth of the electrodeposit in each deposition period is of the order of 50 angstrom units.

Each cycle of three groups, as illustrated in FIG. 2, is separated from the next cycle by a period of approximately 10* second in which the potential amplitude A is of the same order as A A A and approximately Zero. The period is provided merely to fit the pattern to the available apparatus which includes a decade counter.

While a cycle of three groups of potential steps has been illustrated in FIG. 2, the number of groups in the cycle is not critical, and good results can be achieved with repeating cycles having two or four groups, each group consisting of a period of deposition voltage, a short potential peak, and a period of zero or practically zero voltage. The deposition voltages A A A, need not be constant in the manner illustrated, but may additionally be modulated by the use of an alternating current generator which superimposes a sine, triangle, or square wave pattern on the basic potential.

There is no current reversal during any cycle so that deposited alloy is not again dissolved.

Gold alloy electrodeposits formed with the pattern of potentials exemplified in FIG. 2 have been found to have improved chemical, physical, and mechanical properties due to the structure of the metal. The binary goldcopper alloys of the invention are distinguished by superior conductivity and low surface contact resistance. The ternary gold-copper-cadmium alloy deposits are extremely ductile even in heavy coatings and are bright, and the quaternary gold-silver-nickel-palladium alloys have surprising wear resistance.

The electrolytes employed contain alkali metal dicyanoaurates and one or more complex bound elements of Groups IVa, Va, Ib, III], or VIII of the Periodic Table, or mixtures thereof.

Sources of metals to be deposited and their suitable concentrations in the electrolytes are listed below, the concentrations being expressed in milligram atom per liter:

The electrolytes additionally may contain the usual conductive salts and butters such as the following whose names are followed by preferred concentrations in mole/ liter:

potassium cyanide 0.05-1.0; potassium dihydrogen phosphate 0.1-2.0; dipotassium hydrogen phosphate 0.05- 2.0; dipotassium dihydrogen diphosphate 0.05-2.0; tetrapotassium diphosphate ODS-2.0; potassium carbonate 0.05-1.0.

The gold alloy deposits prepared according to the method of the invention have been used successfully in the electronics industry for contacts and for printed circuits, and in the jewelry trade.

The following examples further illustrate the invention:

EXAMPLE 1 An aqueous electrolyte was prepared to contain:

gold as KAu(CN) :6.0-8.0 mg. atom/liter copper as -K Cu(CN) 150-240 mg. atom/liter potassium cyanide: 60-90 millimole per liter For each specific run, the composition was held practically constant and the temperature was held at :1 C. at a chosen value between 60 and C. The generator was set for delivering a pattern corresponding to that of FIG. 2 with the following values:

tinguished by unusually good electrical conductivity and low contact resistance. They were eminently suitable for guilding electronic components and for printed circuits. They contained intermetallic compounds not known heretofore such as Cu Au.

Typical electrodeposited copper-gold alloys of the invention differ from conventionally deposited alloys of the same composition as follows:

Alloy Conven- 0f the tlonal invention Hardness, Viekers 16 260-340 350-450 Elongation, percent 0. 5-1. 6 2. 0-3. 5 Abrasion loss (4,500 double strokes), 111g- 75 Porosity, m 2. b 1. 5

The effects of process variables on the color of goldcopper alloys are well known in the art, and the commonly accepted rules of electrolyte composition for producing a more red or a more yellow shade are substantially applicable to the method of this invention. A rich gold color is obtained at the mid-points of the composition ranges indicated above, that is, at 7 mg. atom/liter gold, 195 mg. atom/liter copper, 75 millimole/liter potassium cyanide and a temperature of 65:1 C.

EXAMPLE 2 gold as KAu(CN) 65-100 mg. atom/liter copper as K Cu(CN) 150-300 mg. atom/liter cadmium as K Cd(CN) 0.35-0.90 mg. atom/liter potassium cyanide: 60-85 millimole/liter The generator was set for the following values:

The temperature of the electrolyte was held at i-l C. between 50 and 75 C. for controlling the color of the deposit which could also be varied by increasing the values of A A A up to 2.0, 1.7, and 2.5 v. respectively. Depending on the specific conditions chosen, the alloys contained 70%-85% gold, 12%-22% copper, and 3%- 8% cadmium. They were extremely ductile even when deposited heavily, were bright, and had an elongation of up to 15%. The Vickers hardness of some of the deposits was as high as 400 -kp./mm. The electrodeposited alloy Au75-Cu22-Cd3 had an electrical conductivity of 6.7 10* ohmr cm.- as compared to a value of 5.5 10- ohmcm.- for a thermally produced alloy of the same composition.

EXAMPLE 3 A quaternary gold silver nickel-palladium electrodeposit was obtained from an aqueous electrolyte containing:

gold as KAu(C-N) 10-50 mg. atom per liter palladium as K Pd(ON) 5-25 mg. atom/liter silver as KAg(CN) 4.5-15.0 mg. atom/liter nickel as K Ni(CN) 30-150 mg. atom/liter potassium cyanide: 50-100 millimole/ liter The generator was set for the following program:

Secs.

t 1O- t 0.25 t3 10 t9 10"" The electrolyte was used within the temperature range of '60-75 C. The deposits formed were bright to have a reflectance of as compared to mirror bright silver. Their color was closely similar to that of silver. They had compositions within limits of 83 %-90% gold, 7%-11% silver, 0.5%-1.0% palladium, and 2.5 %-5.0% nickel. Their wear resistance exceeded that of pure gold deposits by a factor of about 50. They showed little microstress in layers up to 8 m. and were ductile so as to make them eminently suitable for plating jewelry.

In an analogous manner, the following gold alloys were prepared:

Au-Pd-Cd-Zn Au-Cu-Pb Au-Ag-Co Au-Cd-Sb Au-Cd-As Au-Cu-Sn Au-Ag-Ni The components of the electrolytes employed and their concentrations have been indicated above. It will be noted that complex potassium salts were listed throughout, but it will be understood that sodium salts may be employed as well.

Alloys containing signi-ficant amounts of more than three minor constituents in combination with gold are not commercially useful at this time, and no attempt has been made to produce them according to this invention. However, there is no reason why such multi-component alloys should not be electrodeposited from suitable electrolytes by the pulsed current of the instant invention.

It should be understood, therefore, that the foregoing disclosure relates only to preferred embodiments of the invention, and that it is intended to cover all changes and modifications of the examples of the invention herein chosen for the purpose of the disclosure which do not constitute departures from the spirit and scope of the invention set forth in the appended claims.

What is claimed is:

1. A method of electrodepositing a gold alloy on an electrically conductive object which comprises:

(a) immersing said object in an aqueous electroplating electrolyte containing dissolved sources of the components of said alloy; and

(b) establishing a cyclically varying potential between said object as the cathode and said electrolyte, each cycle of varying potential including:

(1) a first period of at least 0.1 second in which the deposition potential of said sources is maintained,

(2) a second period of approximately 10 to 10 second in which a potential much higher than during said first period is maintained, and

(3) a third period of approximately 10* to 10' second in which the potential between said object and said electrolyte is not significantly different from zero, said first, second and third periods immediately succeeding each other.

2. A method as set forth in claim 1, wherein said components are members of the group consisting of gold, silver, copper, zinc, cadmium, arsenic, antimony, tin, lead, cobalt, nickel, and palladium.

3. A method as set forth in claim 2, wherein said components are present in said electrolyte as cyanide complexes when they are members of the group consisting of gold, silver, copper, zinc, cadmium, cobalt, nickel, and palladium, and are present at hexahydroxide complexes when members of the group consisting of arsenic, antimony, tin, and lead.

4. A method as set forth in claim 2, wherein said alloy essentially consists of a major amount of gold and of a minor amount of one to three other members of said group.

5. A method as set forth in claim 1, wherein the integral of said much higher potential with respect to time is smaller than 5 10- volt-second.

6. A method as set forth in claim 1, wherein said much higher potential is about 1 to 5 volts higher than said deposition potential.

7. A method as set forth in claim 1, wherein said components are elements of Groups IVa, Va, Ib, IIb, and VIII of the Periodic Table of Elements.

References Cited UNITED STATES PATENTS Wohlwill 204DIG. Huggins 204DIG. Chester 20443 Rockafellow 204DIG Rockafellow 204DIG.

10 GERALD L. KAPLAN, Primary Examiner US. Cl. X.R.

20443 G, 228, DIG. 9

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