US3755089A

US3755089A - Method of gold plating

Info

Publication number: US3755089A
Application number: US00200045A
Authority: US
Inventors: F Rapids
Original assignee: RAPID ELECTROPLATING PROCESS I
Current assignee: RAPID ELECTROPLATING PROCESS I; RAPID ELECTROPLATING PROCESS INC US
Priority date: 1971-11-18
Filing date: 1971-11-18
Publication date: 1973-08-28
Anticipated expiration: 1990-08-28
Also published as: CA1001580A

Abstract

Method of gold plating particularly suited for the gold plating of localized conductive areas such as contacts on printed circuit boards, wherein the surface to be plated is first cleaned and activated without any current being applied by rubbing onto said surface a non-displacement type of gold electrolyte solution carried by absorbent material forming part of a gold applicator assembly that includes a gold anode. A plating current is then passed through said electrolyte solution during continued rubbing to effect the plating of gold on said surface as the cathode. The preferred electrolyte is a water solution of an alkali metal gold cyanide containing substantially no free cyanide or alkali metal cyanide in excess of the cyanide bound to the gold present in said water solution. The method is self-indicating in that so long as the plating maintains a superficial dark or brownish coloration throughout the plating cycle, the gold electrodeposit is of satisfactory quality and is firmly adherent, whereas if the gold electrodeposit exhibits a clear gold color, or if no plating takes place, that indicates a poor or broken plating connection in the plating circuit. Rubbing of the electrodeposit with the applicator during the plating operation, if there are poor or broken plating connections, quickly removes the superficial dark coloration of the electrodeposit and imparts thereto a clear gold color of substantially the same clarity of gold color as that of the gold applicator used. If this occurs, it is only necessary to correct the bad plating connection and continue plating.

Description

United States Patent 11 1 Rapids METHOD OF GOLD PLATING [75] Inventor: Felix R. Rapids, Chicago, 111.

Assignee: Rapid Electroplating Process, Inc.,

Chicago, Ill.

22 Filed: Nov. 18, 1971 [21] Appl. No.: 200,045

Related US. Application Data [63] Continuation-impart of Ser. No. 2,982, Jan. 15, 1970,

abandoned. I

52] us. c1 ..204/15, 2 04/46, 204/224 [51] Int. Cl. C23b 5/48, C23b 5/24, 1323p H02 [58] Field of Search 2'04/15, 224 R, 43, 204/44, 46

[56] References Cited I UNITED STATES PATENTS 1,552,591 9/1925 Batenburg 20'4 224 R 1,787,431 1/1931 Batenburg... 204/224 R 2,061,591 11/1936 Rapids-....- 204/224 R 2,958,928 ll/l960 Bain, Jr. et al. 204/15 2,812,299 ll/l957 Volk ..-'204/44 OTHER PUBLICATIONS Modern Electroplating, (1963) pp. 207-223 Primary ExaminerThomas Tufariello J. Arthur Gross et a].

2nd Edition, Lowenheim Aug. 28, 1973 [57] ABSTRACT Method of gold plating particularly suited for the gold plating of localized conductive areas such'as contacts on printed circuit boards, wherein the surface to be plated is first cleaned and activated without any current being applied by rubbing onto said surface a nondisplacement type of gold electrolyte solution carried the gold electrodeposit is of satisfactory quality and is firmly adherent, whereas if the gold electrodeposit exhibits a clear gold color, or if no plating takes place,

that indicates a poor or broken plating connection in the plating circuit. Rubbing of the electrodeposit with the applicator during the plating operation, if there are poor or broken plating connections, quickly removes the superficial dark coloration. of the electrodeposit and imparts thereto a clear gold color of substantially the same clarity of gold color as that of the gold applicator used. If this occurs, it is only necessary to correct the bad plating connection and continue plating.

6 Claims, 3 Drawing Figures o c Q 0 l3 I l /20 Patented Aug. 28, 1973 3,755,089

@ oogo D INVENTOR. F54 /x K. FA '5 M firrnm ,wh-rw l w a y ATTORNEYS METHOD OF GOLD PLATING RELATED INVENTIONS This is a continuation-in-part of my pending applicationSer. No. 2,982, filed Jan. 15, 1970 and now abandoned.

DESCRIPTION or THE PRIOR ART ried out without the use 'of plating baths, wherein a nondisplacement type alkali metal gold cyanide electrolyte is used that contains no free acid and no free alkali metal cyanide, and in accordance with which gold electrodeposits are obtained which are self-indicating as to their satisfactory or unsatisfactory character fo use as electrical connections or contacts.

SUMMARY OF THE INVENTION The present invention provides an improved, rapid and, more convenient method for electrodepositing gold on surface areas of electrically conductive members to provide new contacts or to repair old contacts for use in an electrical circuit, such as a printed circuit board having copper or other electrically'conductive members. i t

The process requires no stationary plating'tank or. other stationary equipment but involves manually rubbing the surface to beplated, both before and during I plating, with an electrolyte applicator that suitably comprises an absorbent, inert material for carrying the gold electrolyte disposed on or about a gold-surfaced anode of suitable form and design for manual handling. The absorbent material, which maybe a fabric of natural fibers, such as cotton, or of inert artificial-fibers, such as polyacrylonitrile, is wetted with an electrolyte of my invention particularly adapted for gold plating. A non-corrosive electrolyte, incapable of displacement plating, is used, such as an aqueous solution consisting essentially of water and an alkali metal gold cyanide in which there is no substantial excess of any alkali metal cyanide or freecyanide, over and beyond that bound to or held by the gold as a part of the gold cyanide moiety.

One of the imprtant uses of this plating method is the repair and maintenance of gold platings on.electronic and other contact surfaces. This includes printed cir- .cuits with as many as two dozen or more individual The situation is further complicated by the fact that this type of plating or replating must usuallybe done by operators with little or no knowledge of electroplating. No equipment or method other than through actual use has previously been available that would indicate the quality of the gold plating on each contact.

My present invention provides a gold plating electrolyte and method that:

' only one single, relatively harmless, non-corrosive solution that can be used practically anywhere without danger to operator or to critical equipment.

type that can be rubbed on areas to be plated, with a manual gold applicator, without current, as the final cleaning and activating operation before plating to ensurefirm adhesion.

3. Is self-indicating in that the maintencance of a su perficial dark or brownish coloration throughout the plating cycle establishes that satisfactory gold plating is being carried out, whereas no plating, or a clear gold color of plating on any contact or contacts, immediately indicates a bad or broken plating connection which must be corrected to ensure good plating. The rubbing of the applicator over a gold plated surface during the plating operation will remove any superficial dark coloration and expose the underlying clear gold color of contacts or areas having bad'or broken platingconnections, but will not remove such-superficial dark coloration-during plating if the plating circuit is in good operating condition.

4.. Ensures purity of the gold plating and permits the use of more rigid metals, such asnickel, for the backing or reinforcing of pure gold-surfaced anodes, because such more rigid metals will not codeposit with the gold from my solution operated under the conditions of my method.

5. My gold electrolyte'is a non-displacement type, consisting of sodium or potassium gold cyanide or 'a combination of both sodium and potassium gold cyanides in water with possibly a small amount of wetting agent. Commercial gold cyanide and acid goldplating solutions cannot be used for carrying out my method. This holds true for the gold plating electrolyte of aforementioned Underwood patent.

6. My electrolyte solution when used with the gold applicator utilizes current densities of from 400 to 700 amperes or more/sq. ft. and relatively high voltages of about 8 to 16 volts (direct current).

It is therefore an'object of this invention to provide an improved method of gold plating to form gold contacts of high electrical-conductivity and of optimum thickness, flexibility and adherence, wherein plating'is carried out to a point that is self-indicated by the appearance of the electrodeposited gold as having a superficial dark or brownish coloration in striking contrast to the clear gold color exhibited by the goldsurfaced anode of the gold applicator assembly.

It is a further important object of this invention to provide a reliable method of forming gold plating that provides good electrical connections and that can be distinguished from insufficiently plated contacts or areas by their dark appearance as contrasted with the clear gold eolor of the plating that indicates poor and- /or inadequate connections in the plating, circuit.

Other and further important objects of the invention will become apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawing, although variations Requires in order to obtain a good gold plating,

2. Employs a gold solution of a non-displacement and modifications may be effected without departing from the spirit and scope of the novel concepts of the disclosure, and in which: 7

FIG. 1 of the drawing illustrates somewhat schematically equipment suitable for carrying out my gold plating method as applied to the gold plating of contacts on a printed circuit board;

FIG. 2 is an enlarged fragmentary plan view of the conductive areas that have been gold plated, showing the difference in appearance between contacts properly plated in accordance with my invention and others not properly plated; and

FIG. 3 is a back view of a preferred shorting strip for use with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In order to prepare the surface for carrying out the method of my invention in the gold plating of an electrical contact or other conductive surface, it is essential that the surface be properly cleaned and otherwise prepared for receiving the electrodeposition of gold over the localized areas that are to fonn electrical connections or contacts in an electrical circuit. Preparation of the surface is accomplished prior to the application thereto of the gold electrolyte by scouring the surface with steel wool, such as 3/0 or 4/0 steel wool, to thoroughly polish the same. Such steel wool is commercially available. After polishing, the surface is wiped clean with a lint-free cloth. The surfaces to be plated, which may be copper, brass, or nickel, or'even ferrous metal surfaces, should be thoroughly polished in this manner over the area to be plated prior to the application of any electrical plating current.

With the current still off, a film of my gold plating solution, or electrolyte, is rubbed over the polished surface area by the use of the gold electrolyte applicator later to be described, indicated by the reference numeral 11. Such rubbing action activates'the surface for subsequent electrodeposition of gold, without, however, any objectionable displacement plating taking place prior to energizing the plating circuit.

As shown on the drawings, the reference numeral indicates a tray, basin or other portable receptacle over which or in connection with which my plating method can suitably be carried out. The use of a plating tank, as such, is obviated entirely. The receptacle 10 serves as a portable reservoir for an electrolyte solution capable of depositing gold, and/or for collecting any excess electrolyte during the application thereof to electrically conductive material, but the applicator ll wetted with the electrolyte from that in the receptacle 10, serves to apply the electrolyte to the conductive surfaces that are to be plated.

For the purpose of this invention, I prefer to use a gold plating solution which is slightly on the alkaline side, that is non-corrosive, and that consists of a solution of sodium gold cyanide, or of potassium gold cyanide, or of a mixture of sodium gold cyanide and of po tassium gold cyanide, equivalent to ten grams of sodium gold cyanide (Au Na (CN),) in 16 fluid ounces of water, but as little as 2.5 grams of sodium gold cyanide per 16 fluid ounces of water can be used satisfactorily. Also, there may be an excess of the selected alkali metal gold cyanide in the gold plating bath over that which can be completely dissolved therein, since solution will then take place as gold is plated out in using the bath.

Instead of sodium gold cyanide, a molecularly equivalent weight of potassium or other alkali metal gold cyanide can be used but the resulting electrolyte should contain substantially no free cyanide and no excess of alkali metal cyanide over that bound to or held by the gold cyanide in combination therewith, which combination of gold and alkali metal cyanide is referred to herein as an alkali metal gold cyanide moiety. A solution of 10 grams per 16 fluid ounces of water of sodium gold cyanide and of 3 grams of potassium gold cyanide is most preferred.

The following are analyses of satisfactory sodium and potassium gold cyanide moieties, respectively, expressed in percentages by weight:

Percent by weight 1 Potassium cyanide (K CN).

Electrolytic gold cyanide solutions made with either the sodium or potassium or other alkali metal cyanide or mixtures of sodium and potassium gold cyanides should not contain any free or uncombined cyanides, or any excess of an alkali metal cyanide, since free alkali metal cyanides cause displacement plating, which is poorly adherent to the underlying stratum of conductive metal and which is likely to result in the giving off of fumes that are corrosive and noxious. Wetting agents are not usually needed, but anionic wetting agents can be included in the alkali metal gold cyanide solutions if desired.

The avoidance of these undesirable results and the portability of the equipment make my method and electrolyte most suitable for gold plating contacts or other electrical connections in confined spaces, such as those in astronautical vehicles, space ship capsules and the like.

The electroplating system of the present invention includes a goldsurfaced electrode'lla in the applicator assembly 11. Said electrode 11a can suitably be formed of a conductive, relatively rigid strip of metal, such as nickel to give the desired rigidity, with a strip of relatively pure gold welded to the working surface of the electrode. A handle 12 is secured to the electrode 11a to permit the operator to manipulate the electrode along any preselected area of the surface to be coated. The electrode 11a is encased in an absorbent sleeve 13 of an non-conductive woven or non-woven fabric .such as cotton, polyacrylonitrile, glass fibers, or similar in ert, porous, fluid-absorbent and retaining material. It is thisabsorbent but inert and non-conductive fabric 13 that prevents direct contact between the anode 1 lo and the surface area 16 to be plated and thus prevents a short in the plating circuit. The fabric 13, during the plating operation, is manually held against and rubbed over the surface area 16, with the result that the plating circuit between the gold surfaced electrode 1 1a and the surface area 16 to be plated is very short, indeed.

' direct current.

The invention as shown in the drawings contemplates providing a gold plating on a laminated printed circuit board 15 having conductive areas 16 composed of copper, nickel or the like disposed in a prearranged pattern on the surface. Contacts 16a are disposed at one edge of the board for connecting the board electrically with other circuits. In order to insure electrical continuity during plating to all of the contacts 16a on the board, a shorting strip 17 (FIG. 3) composed of an electrically conductive metal is laid across the surface of the board so that there is an electrically continuous path through all of the circuit elements 16a at the time electrodeposition takes place. Such strip, as shown in FIG. 3, conveniently consists of a strip of aluminum foil 17a secured to a strip of pressure-sensitive tape 17b with one edge of the foil flush with an edge of the tape. Alternatively, contact of small isolated areas on the printed circuit board can be made by connecting the electrical lead from the power source to a pointed piece of electricallyconductive metal thatcan be held r against the edge of the contact area by hand.

Prior to starting my electroplating process, the area to be plated should preferably be polished with steel wool to remove any inert matter or burrs or other foreign material. After wiping the surface clean, a clip 18a can be fastened to the shorting strip 17 for providing electrical current to all the contacts 16a simultaneously.

An electrical conductor 19 connected by a' clip 18 to the electrode 11 and an electrical conductor 20 connected to the clip 18a are secured to the proper terminals of a source of unidirectional current, such as a DC generator 22, batteries, a rectifier, or other source of In normal operation, the voltage from the generator to other source 22 will be adjusted to about 8 to 16 volts, or broadly in the range from 3 to 16 volts. With the electrode Hz: the anode and the contacts 16a to be plated the cathode, and with the sleeve 13 thoroughly wetted with the electrolyte solution, the sleeve is rubbed against the areas to be plated while passing between the anode and cathode a unidirectional current having a current density equivalent to about 100 to 750 amps/sq. ft. or higher, but usually averaging from 400 to 700 amps/sq. ft. to effect the plating out of gold over the contact areas 16a. v

Instead of rubbing the gold electrolyte solution onto the surface areas to be plated by means of the gold applicator, but without current, prior to'actual plating, the final cleaning and activating-operation can be .effected'to ensure satisfactory adhesion of the plating by temporarily reversing the current and applying 1 to volts of reverse current per square inch for 2 or 3 seconds, and then turning the switch to plating position, at a voltage of 8 to 12 volts and proceeding with the plating in the manner already described. Plating is continued if it has a dark or brownish superficial appearance, until the desired thickness of plating has been obtained. This self-indicating appearance assures that a properly conductive, firmly adherent gold plated contact has been attained. Normally, reaching such an end-point takes place immediately upon energizing the plating circuit. Desired thickness of gold plating can be accomplished by the continued passage of a plating current and continued application or a reapplication of the electrolyte by rubbing the electrolyte-saturated sleeve of the gold applicator assembly over the already plated surface. After the plating has been completed the work is rinsed with plain warm or cold, and wiped.

The improved method of my invention is applicable to plating gold or copper, brass and/or nickel surfaces provided the surfaces have been thoroughly cleaned and polished in the manner earilier described herein.

water,

' After being gold plated, the work should be rinsed with plain water, either hot or cold, and wiped dry. If a' bright finish is desired, it is merely necessary to polish the dark-appearing gold plating lightly with a 3/0 or 4/0 steel wool and wipe the surface clean with a clean, lintfree cloth. There is, however, no advantage except in appearance in this polishing step, since the darkappearing gold electrodeposits formed by properly carrying out the gold plating are just as suitable and just as efficient as those that have been polished.

On the other'hand, if during the electroplating step, the gold electrodeposit remains or becomes clear gold color, like the gold surface on the anode of the applicator assembly, or changes from a superficial dark appearance to a clear gold color, that means that the platgold plating to the work even when carried out by'inexperienced operators. Theprocess of my invention makes possible the safe repair and maintenance of printed circuits and other electronic contacts in confined spaces of missiles, aerospace equipment, wave guides and other areaswhich are difficultly accessible. Neither corrosive fumes nor corrosive liquids are generated or caused to bepresent during the operation of the herein-described method The process of the invention can also be used in repair and maintenance work to fill in areas where the previously applied coatings have become depleted.

In FIG. 2, certain of the contacts 16a, indicated by the letter B, are shown as having bright gold-colored surfaces, while others of said contacts, indicated by the letter D have a superficial brownish-colored appearance. The B-designatedcontacts, exhibiting a cleargold color, are not of satisfactory quality or thickness if such clear gold color develops and persists asthe direct and inherent result of the plating operations without any polishing of the surfaces of the'gold plate-having taken place.

On the other hand, if the surfaces of the contacts 16a become and stay brownish-coloreddu'ring plating, like those designated by the letter D, the built-up gold contacts are then of such high quality as to meet military specifications, such as MIL-045204, and stay firmly adherent to the under structure even when flexed. It of the desired thickness, say, at least 0.00005 inches, the contacts are satisfactory in the as-plated state for use without further processing, but for appearances sake the surfaces can be lightly polished to present a clear gold surface. The description of the D contacts as dark", or brownish is in contrast to the clear yellow color. characteristic of gold, and is very marked and easily observable, as is alsothe contrast between the superficial brownish coloration of the'D contacts and the characteristically gold appearance of the gold on the surface 11a of the applicator electrode 1 l. The use of my non-displacement type of gold electrolye combination with the relatively close proximity, usually about 1/16 inch, of the surface of the gold anode to the surface area 16a to be plated, insures a satisfactory gold plating of said surface area with a superficial brownishcolored gold layer, provided the plating circuit is properly completed to include all of the areas to be plated. Such completion of the electrical circuit through the assembled printed circuits and component assemblies, when including many discrete contact areas, can sometimes be best accomplished preparatory to, or during actual plating, by pressing a flexible mass of 1/10 or 2/0 steel wool against all of the contact areas 16a simultaneously by means of an electrically conductive strip or bar of copper or brass. Such a strip or bar is used in lieu of the shorting strip 17 of FIG. 3 to short the contact areas 16a.

In the practice of the method of my invention, so long as the gold plating is of a brownish color, the plating is satisfactory and can be continued until the desired thickness of the plating has been reached. Since under these conditions the rate of deposition remains uniform, a given thickness of electrodeposit will be obtained in a certain length of plating time. Thickness of the plating can thus be calculated directly from the length of time of plating and the known rate of electrod'eposition of a unit thickness of gold.

I claim as my invention:

l. The method of electrodepositing gold on an area of an electrically conductive metal surface to provide a firmly adherent high quality gold plate over said area,

which comprises establishing a plating current between said surface and a manually guided pure gold anode through a gold electrolyte solution carried by a nonconductive, absorbent fabric between said anode and said surface, effecting wetting and rubbing contact between said absorbent material and said surface as the cathode to effect gold plating of said area,

said anode having a backing of a more rigid metal than gold incapable of codepositing with the gold from said gold electrolyte solution,

said electrolyte solution consisting essentially of an aqueous solution of an alkali metal gold cyanide moiety containing substantially no free cyanide and substantially no free alkali metal cyanide not bound to the gold present in said gold cyanide moiety, and

continuing said gold plating to produce a gold plate having a superficial brownish coloration in contrast to the clear gold coloration characteristic of gold,

. said brownish color persisting during such continued plating, and

stopping said plating when the desired thickness of said gold plate has been reached.

2. The method as defined by claim 1, wherein said plating circuit carries a current density of at least about amps/sq. ft. and is at a voltage of from about 8 to 16 volts.

3. The method as defined by claim 2, wherein, the current density is from 400 to 700 amps/sq. ft. and higher.

4. The method as defined by claim 1, wherein said surface area to be gold plated includes many discrete area portions, and all of said area portions are simultaneously electrically connected during plating by a flexible mass of steel wool held thereagainst and contacting said areas.

5. A non-displacement type of gold plating electrolyte solution consisting essentially of an aqueous solution of an alkali metal gold cyanide moiety containing substantially no free cyanide and substantially no free alkali metal cyanide not bound to the gold present in a concentration molecularly equivalent to from about 2.5 grams to about 10 grams of sodium gold cyanide, (Au Na (CN),), per 16 fluid ounces, said aqueous solution containing substantially no free cyanide and substantially no free alkali metal cyanide, and substantially all of the cyanide present including the alkali metal cyanide being bound to said gold.

6. The method of electrodepositing gold on an area of an electrically conductive metal surface to provide a firmly adherent high quality gold plate over said area, which comprises establishing a plating current of at least 400 amps/sq.

ft. density between said surface and a manually guided, effectively pure gold anode through a gold electrolyte solution carried by a nonconductive, absorbent fabric enveloping said anode and maintained in pressure contact with both said anode and said surface, effecting wetting and rubbing contact between said absorbent fabric and said surface with said surface constituting the cathode to effect gold plating of said area,

said anode having a backing of a more rigid metal than gold incapable of codepositing with the gold from said gold electrolyte solution; said electrolyte solution consisting essentially of an aqueous solution of an alkali metal gold cyanide moiety containing substantially no free cyanide and substantially no free alkali metal cyanide not bound to the gold present in said gold cyanide moiety, and containing no addition agent of any kind, and continuing said gold plating to produce a gold plate having a superficial brownish coloration in contrast to the clear gold coloration characteristic of gold,

said brownish coloration persisting during such continued plating, and

stopping said plating whenthe desired thickness of said gold plate has been reached.

Claims

2. The method as defined by claim 1, wherein said plating circuit carries a current density of at least about 100 amps./sq. ft. and is at a voltage of from about 8 to 16 volts.
3. The method as defined by claim 2, wherein, the current density is from 400 to 700 amps./sq. ft. and higher.
4. The method as defined by claim 1, wherein said surface area to be gold plated includes many discrete area portions, and all of said area portions are simultaneously electrically connected during plating by a flexible mass of steel wool held thereagainst and contacting said areas.
5. A non-displacement type of gold plating electrolyte solution consisting essentially of an aqueous solution of an alkali metal gold cyanide moiety containing substantially no free cyanide and substantially no free alkali metal cyanide not bound to the gold present in a concentration molecularly equivalent to from about 2.5 grams to about 10 grams of sodium gold cyanide, (Au Na (CN)2), per 16 fluid ounces, said aqueous solution containing substantially no free cyanide and substantially no free alkali metal cyanide, and substantially all of the cyanide present including the alkali metal cyanide being bound to said gold.
6. The method of electrodepositing gold on an area of an electrically conductive metal surface to provide a firmly adherent high quality gold plate over said area, which comprises establishing a plating current of at least 400 amps./sq. ft. density between said surface and a manually guided, effectively pure gold anode through a gold electrolyte solution carried by a nonconductive, absorbent fabric enveloping said anode and maintained in pressure contact with both said anode and said surface, effecting wetting and rubbing contact between said absorbent fabric and said surface with said surface constituting the cathode to effect gold plating of said area, said anode having a backing of a more rigid metal than gold incapable of codepositing with the gold from said gold electrolyte solution; said electrolyte solution consisting essentially of an aqueous solution of an alkali metal gold cyanide moiety containing substantially no free cyanide and substantially no free alkali metal cyanide not bound to the gold present in said gold cyanide moiety, and containing no addition agent of any kind, and continuing said gold plating to produce a gold plate having a superficial brownish coloration in contrast to the clear gold coloration characteristic of gold, said brownish coloration persisting during such continued plating, and stopping said plating when the desired thickness of said gold plate has been reached.