EP0025220A1

EP0025220A1 - Additive-free hard gold electroplating and resulting product

Info

Publication number: EP0025220A1
Application number: EP80105298A
Authority: EP
Inventors: Daniel Robert Blessington; Reginald Russ Buckley; Frederick Bayard Koch; Yutaka Okinaka; Richard Sard
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1979-09-06
Filing date: 1980-09-05
Publication date: 1981-03-18
Also published as: JPS5644791A; KR830003601A

Abstract

This invention relates to articles at least a portion of a surface of which includes an electroplated gold coating having a Knoop hardness number of at least 100 and a process of producing the same. The coating, which may be called "Additive-Free Hard Gold", consists of gold with additional metal ingredients in the deposit, especially such hardening nonnoble metals as Co, Ni, Ca and As, being not greater than 0.1 weight percent and the smoothness of the coating over at least 95 percent of a major surface does not deviate from flat by more than 0.5µm within a sampling area of 5µm square. The gold deposit is produced by electroplating with a fluid which contains gold and cyanide; the electroplating conditions are selected such thatthe pH of the fluid is 7.5 or less, the content of hardening nonnoble metals, if present, in the fluid is in an amount of no more than 0.5 percent of the gold content. the temperature of the fluid in the vicinity of the surface is no greater than 50°C, the flow rate of the fluid in the vicinity of the surface is at least 50 cm/sec and the current flow resulting from maintaining the surface cathodic relative to an anode is at a maximum value not greater than 0.9i

where is defined as the mass transport limited current which is the current beyond which further increase results in no further increase in gold plating rate. The resulting "Additive-Free Hard Gold" is suitable for use in electrical switch contacts and other devices, as well as in ornamentation, requiring hardness and wear characteristics generally associated with so-called "hard gold".

Description

Background of the Invention

A. Technical Field

The invention is concerned with gold electroplating procedure and resulting product. More specifically, the invention is concerned with gold electroplate of hardness and wear characteristics generally associated with so-called "hard gold". Such gold is of significance in electrical switch contacts, and in a variety of other devices, as well as in ornamentation.

B. History

The noble metal elements, particularly gold, have found extensive use because of their resistance to corrosion. Gold, generally as electroplated, is of extreme device interest as contact surface in electrical switches of a variety of designs. Aside from silicon and its related compounds, gold is the one element common to large-scale integrated circuits. Due, again, to corrosion resistance and, consequently, retention of initial appearance, gold has been used for ornamentation since ancient times. This field, to a large extent, too, is preempted by electroplating.
Elemental gold is a physically soft material with generally poor wear qualities. Twenty-four karat (pure) gold is impractical for most uses. A variety of alloying elements improve these qualities without significantly affecting corrosion resistance, and resultant alloys are in extensive use.
For electrical purposes, "hard" gold generally takes the form of gold of reasonable purity containing tenths of a percent of an alloying ingredient. Alloying ingredients in prevalent use, particularly in electroplated hard gold, are cobalt and nickel. Decades of use have yielded a sophisticated technology with baths of well- controlled composition and long shelf life, and with processes characterized by reliability and high throughput.
Bath compositions generally contain gold and other metallic ingredients in complexed or ionic form with the gold introduced often as a potassium or sodium cyanide salt. For the most part, hard gold bath compositions are buffered to acid or near neutral pH values, for example, in citrate or phosphate systems. Initially introduced to avoid substrate attack, for example, on phenol formaldehyde impregnated circuit boards, such buffered baths have other advantages and are in general use.
In so active a field, it is to be expected that research and development continue apace. Problems, once considered tolerable, are of increasing concern with the trend toward miniaturization. For electrical contact purposes, for example, contact resistance is a function of and is strongly dependent upon closely controlled alloy metal concentration. Further, it has been observed that contact resistance, at an acceptable level on a freshly plated structure, deteriorates with age. Other studies are directed at economic implications in processing, so that there is a constant emphasis on improving throughput and, generally, on reducing cost.
Work by H. Reinheimer, reported in 1974 (121, Journal of the Electrochemical Society, 490, 1974), while generally directed to studies relating to "soft" or pure gold electrodeposit, is relevant. Studies revealed that deposits from pure gold bath were of increasing hardness as the temperature of the bath was reduced from usual plating temperature of 60-90 degrees C. The significance of this finding was likely not overlooked by those concerned with hard gold. Process simplification offered by elimination of alloying ingredients in the production of hard gold electroplate would significantly advance the field. H. Reinheimer's work did not, however, result in acceptance of nonalloyed hard gold. For example, the nonalloyed hard gold is unacceptable for demanding electrical use due to surface roughness.

Summary of the Invention

"Additive-Free Hard Gold" hereinafter referred to as (AFHG) electroplate produced in accordance with this invention from buffered cyanide bath under specified plating conditions satisfies demanding requirements, for example, for electrical switch use. AFHG electroplate of the invention is characterized by a high degree of surface smoothness as well as values both of physical hardness and initial contact resistance comparable to cobalt hardened gold electroplate. Aging, either shelf or in use, results in little change in contact resistance relative to that experienced with cobalt hardened gold--an advantage likely attributable to the aging mechanism for cobalt hardened gold electroplate (which invokes the surface oxidation of initially present or migrating cobalt to form the resistive compound CoO). As expected, some cost reduction results from simplification of the bath chemistry.
Like usual soft gold electroplating, plating efficiency is high. Typical values are at least twice that of plating from usual additive hardened gold bath solutions. Under many circumstances, increased limiting current conditions are permitted so that actual plating rate and, therefore, throughput may be increased.
That condition, required for all purposes in accordance with the invention and representing the major departure from the above-mentioned Reinheimer work, is a high degree of agitation. Preferred processing conditions result in turbulence.
It will be seen from the Detailed Description that surface smoothness which typifies the invention may result under a variety of processing conditions. From the economic standpoint, significance is attached to those conditions which permit high throughput. Preferred embodiments are selected accordingly and, generally, contemplate a degree of agitation necessary to produce turbulence. Alternative embodiments rely on macroscopic disturbance which, like conventional turbulence, results in permissible increase in current density. Such alternatives include actual brushing of the surface being plated, ultrasonic agitation of the workpiece, moving fins, and designs in which the workpiece is not immersed but is wetted by bath fluid propelled by nozzles or other means.
For most immersion "apparatus", i.e., apparatus in which the workpiece is at all times immersed in the bath, contemplated flow conditions of the bath fluid in the vicinity of the workpiece is at about 50 cm/sec or preferably 100 cm/sec or higher. Nonimmersion apparatus may permit equivalent agitation at lower bulk liquid flow. Where nonconventional means of agitation are provided, i.e., nonimmersion apparatus, fins, or other provision for "spoiling" streamline flow, bulk flow may be lower. Other preferred processing parameters are set to satisfy high throughput. These include gold concentration of at least 5 g/1 expressed as elemental gold, plating current density greater than 5 mA/cm² and a temperature range of from about 0-50 degrees C..

Brief Description of the Drawing

FIG. 1 is a cross-sectional view of one type of plating apparatus suitable for practice of the invention;
FIG. 2 is a schematic view of a "strip" plater line suitable for use in accordance with the invention, particularly where very high throughput is desired;
FIG. 3 is a cross-sectional view of the high speed plating cell included in the line of FIG. 2; and
FIG. 4, on coordinates of Knoop hardness on the ordinate and temperature on the abscissa, is a plot relating those two characteristics for plating produced in accordance with the invention under a set of conditions in which other relevant parameters were maintained constant. Detailed Description

I. THE DRAWING

FIG. 1 depicts a "rug" electroplating apparatus 1 consisting of recepticle 2 containing electroplating bath 3. Workpiece to be plated is depicted as 4 which may be a front elevational view of the first of a series of separable connectors to be plated on both of faces 5 and 6. Bath is partly carried and partly projected at surfaces 5 and 6 by means of rotating electrodes 7 and 8, each provided with looped pile fabric surfaces 9 and 10, respectively. Stop 11 serves as a support for workpiece 4 and may also assure an appropriate path for bath carried by electrodes 7 and 8. Jaws 12 and 13 adjustable as schematically depicted by the double-headed arrows are tipped by titanium members 14 and 15. Jaws 12 and 13 are biased cathodic relative to rotating electrodes 7 and 8 by means not shown and thereby serve both as guides and electrical contacts for workpiece 4.
FIG. 2 is a schematic representation of a typical strip plating line 20. As depicted, pieceparts on a continuous strip 21 are fed through the line by capstans 22 and 23. Processing positions 24-28 are defined by partitions within common member 29. Each reservoir is provided with means for circulating fluid contents--such means including schematically depicted outlet path 30 and inlet path 31 with such paths connecting processing positions 24-28 with reservoirs 32-36. In a typical strip plating line 20, processing positions may include electrocleaning at 24 (workpieces reverse biased with the processing fluid typically consisting of a strong alkali solution), electropolishing at 25, (again, with the workpiece anodic, and with the fluid typically consisting of a highly viscous acid (such as phosphoric acid), nickel plating at 26, gold strike plating at 27, possibly by AFHG plating, but here using a highly dilute solution and high current density to cause high hydrogen evolution and to "scrub" the surface to be plated and, finally, the inventive AFHG plating at 28.
FIG. 3 depicts an in-line plating cell 40 suitable for incorporation at 28 in FIG. 2. As shown, the cell consists of recepticle 41, containing a plenum 42, which carries a moving stream of plating solution 43. Fluid 43 is maintained under pressure to result in jet 44 emanating from slit 45. Control means for positioning of jet 44 include movable plate 46 provided with vertical adjusting means 47, as well as plenum 48 which carries pressurized gas 49 to result in stream 50, which emanates through slit 51. The workpiece 52 depicted may, again, be regarded as a separable contact which is maintained in position, while moving by walls 53 and 54, as well as roller support 55. As shown, workpiece 52 is to be plated only on lower portion of surface 56. Overplating on surface 57 is prevented by means of gas stream 50. In operation, workpiece 52 is biased cathodically relative to plenum 42 which is biased anodically both by means not shown. Recirculating means also not shown includes reservoir 58.
FIG. 4, on coordinates of Knoop hardness under 25 grams load on the ordinate, and plating temperature in degrees Celsius on the abscissa, is a typical curve form showing the relationship between those two parameters. The high temperature limit of 70 degrees C shown is outside the inventive scope and is barely sufficient to result in a hardness of value of 90. The low temperature limit of zero degrees C is not limiting in these terms.

II. PROCESS PARAMETERS

A. General

All processing in accordance with the invention presupposes conditions yielding a hard plate--i.e., a plate of at least 90 and preferably at least 100 on the Knoop hardness scale. This desideratum requires only operation at or below 50 degrees C or preferably at or below 45 degrees C. Other variables are considered in terms of surface smoothness--i.e., a surface topology in which elevation variations are no greater than about 0.5 pm total peak-to-valley as measured over a sample area 5 pm square. In these terms, such processing variables and convenient units are:

Plating current density, i (mA/cm²)

Mass transport limiting current density (the total current beyond which gold deposition rate does not increase, i₁ (mA/cm²)

Concentration of gold, C (g/1)
Linear flow speed, u (cm/sec)
Temperature, T (degrees C).

i_l= k_Cu^x(_T + a)Y

Mass transport limiting current density i₁ may be determined from the relationship above. Inventive conditions depend upon a plating current density i which is significantly lower than i_l. The numerical value of the fraction i/i₁ must be maintained below about 0.9 to avoid rough and/or discolored plating. Description is in terms of maximum permitted current on the basis of this fractional value with this maximum permissible current being defined as i_max, i.e., i_max= 0.9 i₁. Under any set of parameters i may be equal to or less than i_max, i.e., i = i₁ and, in fact, lesser currents may result in further improvement in smoothness.
Certain practical limits may be set on operating parameters. These are based on a plating rate of at least 0.01 pm/sec and include:

iplating current density, = 5 mA/cm².
For most practical purposes, lower currents result in uneconomically low plating rates. Further, such lower currents may favor preferential deposition and result in surface features which interfere with desired smoothness.
u, agitation expressed as linear flow speed,
= 25 cm/sec. Such flow rates are attainable in advanced production equipment. Under usual conditions, lower agitation rates do not result in required surface smoothness for plating current density values, i, at 5 mA/cm2_.
C = 5 g/1 (expressed as grams of elemental gold per liter of bath solution). Lower gold concentration results in a plating current density value, i_max, of less than about 5 mA/cm² for high throughput under most conditions.
T = ₀degrees C. Substantially lower temperatures result in lowered diffusion coefficients in the delta layer and in lowered kinematic viscosities to preclude high throughput under most conditions.

B. Preferred Process Parameters

This section is largely in terms of a permissible throughput corresponding with a plating rate of at least 0.01 pm/sec.
1 Agitation: For most commercial processing, at least on a regular basis, it is expected that processing will take the form of throughput values which approach permissible limits for particular equipment. Equipment limitations are most significant in terms of agitation. Preferred embodiments are described in terms of agitation resulting in flow rates of at least 25 cm/sec or at least about 50 or 100 cm/sec in order of increasing preference.
This most significant parameter is, to some extent, dependent upon apparatus design. The general purpose of agitation is to increase the transport limited current so as in turn to decrease the fraction represented by the actual current density divided by the transport limited current density. As discussed, requisite surface smoothness results from a lessening value of this fraction. Where no unusual provision is made for disturbing streamline flow, desired smoothness is realized only with for flow rates of at least 50 cm/sec or preferably at least 100 cm/sec. Such rates are used in apparatus such as that depicted in FIG. 3. Provision of baffles, fins, ultrasonic vibration, etc. permits attainment of desired surface smoothness for somewhat lower bulk flow rates and for this purpose a minimum of about 25 cm/sec is generally prescribed. Use of nonimmersion equipment, such as that depicted in FI_G. 1, may permit such low bulk flow rate or even lower values. Here the general mechanism by which bath fluid is transported to the workpiece gives rise to the requisite degree of agitation.
In common with prior art usage, agitation defines that condition which obtains at some distance from the surface being plated. This distance corresponds with the thickness of the "hydrodynamic boundary layer." Use of "flow rate" or "bulk flow rate" refers to flow rates as measured outside of this boundary layer. Measurement at a distance of a millimeter is appropriate.
2 Temperature: As noted, the maximum permissible temperature remains at about 50 degrees C. Higher temperatures result in coatings of lowered hardness values. A low temperature limit of about 0 degrees C is considered a practical limit due to lowered efficiency, as well as reduced throughput. The maximum temperature is somewhat arbitrary, since further decrease results in increased hardness. A preferred maximum temperature is 45 degrees C.
3 Plating Rate: This paramater is not truly an independent variable but results directly from the current density and plating efficiency. Preferred embodiments are considered to correspond with a plating rate of at least about 0.03 pm/sec. Experimentally, this plating rate was found to correspond with a current density of about 50 mA/cm² _.
4 Current Efficiency: Typically, values are on a level of at least 85 percent for a fresh bath. Actual measured values have been as high as 95 percent and higher. Indicated current efficiencies represent a significant advance over usual additive hardened gold plating which are operated at typical values of 50 percent and below.
Throughput: In actual experience, permitted maximum throughput, relative to that for additive hardened gold, has been realized. For given apparatus, maximum throughput for equivalent coating quality has been as much as twice or three times that for cobalt hardened gold. Preferred processes in accordance with the invention take advantage of this throughput advantage.

III. BATH COMPOSITION

A Gold: Gold, always measured in terms of equivalent elemental content, is desirable at a level of 25 grams/liter or higher. In common with other plating processes, gold content may become limiting for high throughput; and, accordingly, the solubility limit may be approached. All results obtained have been from cyanide solution and the invention is discussed in terms of alkali metal gold cyanide regardless of the form in which gold is initially introduced. The alkali metal is usually potassium so that gold content is considered to be in the nominal form KAu(CN)₂. Lithium salts are an order of magnitude more soluble than potassium salts and may be particularly useful for very high throughput processes.
As discussed in Section VI, cyanide is considered essential for AF_HG plating and at least some amount of cyanide must be present in the bath to realize required hardness. For these purposes, it is considered that the amount of cyanide must be such as to satisfy the nominal formula KAu(CN)₂ for at least 50 percent of the gold in the bath.
B pH Modifier: Useful results in accordance with the inventive teaching have been realized only at bath pH below about 9.0 with a preference for a pH maximum of about 7.5. As discussed in Section IV, pH is a relevant quantity for ideal plating morphology. While smooth coatings may result at somewhat higher pH values, practical buffer systems are largely within a range defined by that maximum and by a minimum of about 3.0. A system considered particularly desirable depends upon presence of phosphate ion. Accepted buffer systems are known. K^H ₂ ^P0 ₄ has a natural buffer pH range over 4.3 - 4.5 (for 100 g/1 equivalent KH₂P0₄). With added KOH, the range may be shifted in the alkaline direction so that--e.g., addition of 28 g/1 KOH results in a pH range of 6.8 to 7.2. The term "buffer" is not intended to serve as a rigorous, mechanistic description. Ranges set forth whether "buffered" or not are conveniently maintained in bath compositions indicated.
Phosphate ion has the desired effect of cleansing the bath of nonnoble metal ingredients. Such contaminants are precipitated out in the form of phosphates. Other acidifying ingredients may otherwise be substituted in whole or in part. Such subsitutions include citrate ion, possibly considered present as potassium citrate. Use of citrate ion, again, as optionally supplemented by a soluble base, such as KOH, results in an easily maintained pH range of from 3.0 to 6.0.
C Reducing Agent: as in other gold plating, formation of Au(III) is a contributor to diminishing plating efficiency. In the inventive processes, permitted throughput may also suffer. Addition of reducing agent may lessen this source of inefficiency. Reducing agents should be chosen so as to avoid contamination to levels above those indicated in section D below. Hydrazine hydrate has been slowly added as Au(III) forms to maintain initial plating efficiency. Use of excess hydrazine, however, results in reduction of Au(I) and, consequently, in decomposition of the bath. In one series of experiments, rejuvenation, after four turnovers, was effected by addition of the analytically determined amount of 85 percent aqueous hydrazine hydrate required to reduce Au(CN)₄ to Au(CN)2 Four hour treatment at 75 degrees C was sufficient to lessen the 30 percent accumulation of Au(III) to 0.4 percent. Subsequent plating was at the 94 percent plating efficiency realized in the fresh bath. Addition of this or other reducing agent may be made a part of the regular replenishment procedure.
D Other Ingredients: Generally, other ingredients are not desirable for the generic process. Plating efficiency and throughput are a direct consequence of minimal action contamination so that inclusions of this nature, at least in significant amount, are undesired from an economic standpoint. Ions other than gold fall into three categories of diminishing consequence and, therefore, increasing tolerable concentration. The categories are:

1 Hardening ingredients of the corroding type-- e.g., cobalt, nickel, (which result in contact resistance increase upon aging due to oxidation and therefore of particular consequence in electrical apparatus which is not hermetically sealed). Permitted content is desirably < 0.01 g/1 for an assumed gold content of 20 g/1 generally < 0.05 weight percent in the deposited film.
2 Hardening noncorroding ingredients--e.g., silver--may impair plating efficiency but are not harmful in the plating in small quantities. Bath compositions of up to about 0.1 g/1 for gold content of 20 g/1 to yield platings up to 0.5 weight percent impurity which do not differ in any important characteristic from those produced from baths in which such ingredients are at an undetectable level.
3 Nonhardening ingredients may generally be present in the bath to any amount based on specific intended application of the resultant coating. Accordingly, the potassium generally included in the soluble gold species, as well as the pH modifier, is of no consequence in the coating. Calcium is of consequence only in very large amount and may be included up to 1.0 g/1 or ⁵ weight percent based on gold. Other ingredients, intentional or unintentional, may generally be included as desired.

IV APPARATUS: Suitable apparatus is exemplified by that depicted in FIGS. 1, 2, and 3. Other apparatus permitting agitation and plating rates set forth may be used. Common to other gold plating processes, gold may be introduced as a soluble bath constitutent or as an anode constituent. It has been noted that loss in efficiency results from oxidation of Au(I) -----> Au(III). Au(III) accumulation is significantly lessened by use of gold anode, with the probable mechanism based on anode gold dissolution preferential to oxidation. Anode dissolution was found to be impractically fast with loss of the anode after 100 hours of continuous plating for apparatus such as that depicted in FIG. 1 under one set of conditions. An alternative designed to lessen Au(III) accumulation depends upon use of a ruthenium oxide/titanium oxide catalyzed anode. The electrode structure is primarily titanium, which is coated with a mixture of the two oxides and is known as "dimensionally stable anode". (See U. S. Patent No. 4,067,783 issued January 10, 1978 (Okinaka et al Case 10-3). In one set of experiments using a dimensionally stable anode, Au(III) never exceeded 5 percent of total gold and current efficiency remained above 87 percent over four bath turnovers. An alternative structure is coated with iridium oxide and tantalum oxide.

V EXPERIMENTAL PROCEDURE AND RESULTS

Introduction

Information presented in this section is distilled from extensive experimentation. It includes plating under conditions outside the scope of the invention as defined by the claims. This information is significant in understanding the mechanism and boundary conditions which apply.
Reported data was derived from samples plated under two sets of conditions:

(i) low speed plating, and
(ii) high speed plating.

²

A Hardness

(1) Low Speed Plating: Hardness measurements were taken using a hardness tester with a Knoop indenter at 25 gram load on gold plated copper foils. Plating current was at 10 mA/cm². Data is presented in tabular form:
It is seen from TABLE I that hardness is relatively independent of plating temperature in the range of 10-45 degrees C. An increase in plating temperature to 70 degrees C results in a,significant fall off in hardness to a value of about 100.
Hardness stability was measured by one hour anneals at temperatures of 250, 350, and 450 degrees C. There was no measurable softening at 250 degrees C and only at 450 degrees C was softening significant.
(2) High Speed Plating: Hardness measurements were made on 25 µm thickness sections. As for low speed plating, hardness is relatively independent of temperature below about 45 degrees C. Data is in tabular form:
From TABLE II, it is seen that hardness is generally within the device significant range 140-200 KHN. From the semibright appearance reported, it appears that a current density of 153 mA/cm² exceeds the critical value of approximately 0.9 i₁ at 30^oC.

B Wear

(1) Low Speed Plating: Wear response was measured with crossed rods as follows:
Two 2 millimeter (0.08") diameter rods of hard copper plated with 2.5 pm gold were mounted at right angles. The lower rod was cycled under a 200 gram load to produce a wear track of about 4 millimeter (5/32"). After 500 cycles samples were removed and the width of the wear tracks was measured using an optical microscope. Type of wear was characterized qualitatively in terms of:

(1) galling
(2) brittle fracture
(3) excellent resistance to wear.

Data presented is illustrative of extensive experimentation. Failure was evident only at 70°C.
(2) High Speed Plating: Plated test connectors were assembled in a comb. Plating thickness was at a nominal minimum of 2.5 pm. The comb was mated two hundred times with a female. The comb was then cleaned in boiling trichloroethane and was tested for wear induced porosity. Thickness was measured at three locations by beta backscatter. The mean thickness of all measurements was 3 ± 0.25 pm. Of twenty-five specimens, all but five were free of pores. Three had one pore each, and two had two poreseach. Reported results are at least as good as those measured on a control sample identical to that described but plated with a popular alloy hardened gold.

C. Contact Resistance

(I) Low Speed Plating: Measurements were conducted on freshly prepared samples using a four wire probe system. Samples were copper coupons, as described above. Thermal aging studies were conducted at 150 degrees C following which resistance measurements were, again, made with the same apparatus. Initial values were at 0.9-l.5µm ; after 168 hours were at 1.1-1.7mΩ; and after 504 hours were at 1.1-3.OmΩ.
Values both as initially conducted and after aging were significantly better than those measured on alloy hardened gold as plated under optimum conditions from a commercial bath composition. The aging temperature of 150 degrees C is regarded as a severe test used only for development purposes.
(2) High Speed Plating: Three connectors were plated at 45 degrees C, 170 mA/cm² to a thickness of 2.5 µm. Measurements were as described under (I) above. Following initial measurement ranges, specimens were aged at 150 degrees C for 168 hours. Resistance measurements in milliohms as initially deposited and after aging were 1.4-2.6 and 1.2-4.0.
Data developed was better than that for similarly prepared specimens plated from commercial alloy hardened gold. As expected, improvement after thermal aging was pronounced, reflecting absence of hardening agent which as oxidized is considered the most significant contributor to increased contact resistance.

VI. MECHANISTIC CONSIDERATIONS

While the invention is described in terms of readily measured parameters, some further understanding may result from consideration of postulated mechanism.

A. Hardness

Prior workers have recognized formation of AuCN during gold electroplating: Electrochimica Acta, 173, 18, pp. 829-834 J. Electrochemical Society, 119, pp. 672-677, _J. Electroanalytical Chem., 40 (1972) pp. 113-120, J. Electroanalytical Chem., 99, (1979) p. 341, 346).
Study of specimens prepared by AFHG have revealed presence of AuCN in amount which increases with hardness and at a level such as to indicate more than one formula unit per grain. It may be postulated that this compound is responsible for stabilization of small grains which are, in turn, responsible for hardness. Stabilization likely results from initiation of grain boundaries with each AuCN center due to lattice mismatch. It is unnecessary to relate AuCN count to nucleation of grains but a thermodynamically consistent mechanism may be proposed. Relatively high free energy at each AuCN site may give rise to nucleation at such site so that the compound serves not only to stabilize grains but is responsible for initial grain growth. Softening of coatings at increasing temperature is, in any event, quite consistent with the expected temperature dependence of adsorption. Softening at high pH values is consistent with known solubility of AuCN so that such grain nucleator/stabilizer is not retained in the coating.

B. Smoothness

Coating growth at currents closely approaching i₁, the mass transport limiting current, may result in i_l controlled deposition. Current reduction to levels of 0.9 i_l and lower, permit equilibration at the interface so that coating is not i_i limited but is controlled by random processes which tend both to prevent nodules and/or minimize nodule contribution to surface topology. Under these circumstances, growth is both lateral and in the direction of the concentration gradient within the delta layer with any nodules thickening at rates presumed at least equal to that resulting in vertical growth.

VII. THE ELECTROPLATE

Electroplate of the invention is characterized by three parameters:

(a) Hardness: Generally measured in terms of Knoop hardness (KHN). A KHN of 100 is sufficient to differentiate from usual soft gold which, as generally conceived, is no harder than 80. A preferable lower limit is 120. For many switch device purposes, where long life is significant, hardness may be specified in terms of some higher minimum value. Various device designs require KHN values of 130 and 140. Such hardness numbers are readily available in accordance with the invention.
(b) Composition: Inventive electroplate in accordance with the invention is characterized by freedom from metallic hardening ingredients other than gold to a total level of about 0.1 weight percent based on the total weight of the electroplate. Metals in common use as hardening ingredients are cobalt, nickel, cadmium, and arsenic. It is expected that commercial utilization will take the form of electroplate which is free of all metals other than gold to the same total maximum content of 0.1 weight percent on the same basis and this represents a preferred embodiment. Coatings produced experimentally have evidenced a total freedom of all metal ingredients including noble metals, other than gold, to the same maximum level of 0.1 weight percent on the same basis.
(c) Smoothness: The inventive advance is meaningfully described in terms of this parameter. The invention makes possible AFHG electroplate which is free of topological deviations as measured in a direction normal to the electroplate surface to a maximum of 0.5 ym from nominal within 5 pm square sample areas. To allow for possible edge deviations, total sample area is specified as totaling at least 95 percent of an entire face surface plated. In plating, surfaces which are essentially continuous, either flat or curved, the smoothness standard described may be applicable to the entirety of the region.

Claims

1. An article at least a portion of which includes an electroplated gold coating having a Knoop hardness number of at least 100,
CHARACTERIZED IN THAT
the said coating consists essentially of gold with additional metal ingredients being not greater than 0.1 weight percent, and the smoothness of this coating over at least 95 percent of a major face thereof does not deviate from flat by more than 0.5pm within a sampling area of 5pm square.

2. An article according to claim 1,
CHARACTERIZED IN THAT
the total content of at least one of cobalt, nickel, calcium and arsenic in the coating is restricted to a maximum of 0.1 weight percent.

3. Process for electroplating a surface of at least a portion of an article with a gold-containing deposit having a Knoop hardness number of at least 100, in accordance with which the said surface is made cathodic relative to an anode, and both the said surface and the said anode are wetted by a gold-containing aqueous ionic fluid containing cyanide in an amount such that total number of cyanide units, with each cyanide unit containing a single carbon atom and a single nitrogen atom, however charged, at least equals the total number of gold atoms in the fluid, however charged, in solution, said fluid defining a path for ionic transport between the said surface and the said anode,
CHARACTERIZED IN THAT
to produce on the said surface a gold deposit which has said hardness but contain no more than 0.1 weight percent of additional metal ingredients, especially such hardening ingredients as cobalt, nickel, calcium and arsenic, and which are smooth over at least 95 percent of a major face thereof, as defined by a maximum tolerable deviation, from flat, of 0.5pm within a sampling area of 5µm square, the plating conditions are adjusted so that

a) said fluid has a pH of 7.5 or less and contains a total amount of hardening nonnoble metal, however charged, which does not exceed 0.5 percent of the said gold content of the said fluid,

b) the temperature of the said fluid in the vicinity of the said surface is maintained at a temperature which is not greater than 50 degrees C,

c) the flow rate of the said fluid in the vicinity of the said surface is at least 50 cm/sec, and

d) a current flow resulting from maintaining the said surface cathodic relative to the said anode is at a maximum value no greater than 0.9 i₁ where i_I is defined as the mass transport limited current which is the current beyond which further increase results in no further increase in gold plating rate.

4. Process according to claim 3,
CHARACTERIZED BY
selecting the said temperature to be no greater than 45 degrees C.

5. Process according to claim 3,
CHARACTERIZED BY
selecting the said flow rate to be at a value sufficient to result in turbulence or-macroscopic disturbance.

6. Process according to claim 3,
CHARACTERIZED BY
selecting the said flow rate to be at least 100 cm/sec.

7. Process according to claim 3,
CHARACTERIZED BY
selecting the gold content of the said fluid expressed as grams of elemental gold per liter of fluid to be at least 5 g/l, preferably at least 20 g/l.

8. Process according to claim 3,
CHARACTERIZED BY
providing cyanide in an amount which is essentially sufficient to at least satisfy the nominal composition Au(CN)^- ₂.

9. Process according to claim 3,
CHARACTERIZED BY
conducting said plating with a plating current density of at least 5 mA/cm² and with a gold concentration of at least 5 g/1.

10. Process according to claim 3,
CHARACTERIZED BY
conducting said plating with a plating rate of at least 0.01 µm/sec.