High Speed Selective Brush Plating of GoldADVANTAGES OVER CONVENTIONAL PLATINGIN SPECIFIC APPLICATIONS

Marv RubinsteinSelectrons Limited, New York

The major proportion of all gold plating will always be carried out in

a conventional bath, but for a substantial number of industrial applica-

tions selective brush plating offers considerable savings.

Most gold platers are familiar with selective plating,a technique also known as doctoring or as brush,swab, touch-up, stylus and tampon plating. Selectiveplating is a true electrodeposition technique, althoughin appearance more closely resembling arc welding.A power supply provides D.C. current through twoflexible cables, the cathodic one clamped to the workand the anodic cable plugged into a special workingtool called a stylus, brush, or spatula. The most up-to-date tool, the stylus, has an anode wrapped withabsorbent cotton or covered with a Dacron jacket.This is immersed in a special high speed selectiveplating solution, or the electrolyte is flowed orpumped to the swab. The wrapped anode, saturatedwith solution, is moved over the surface to be plated,thus completing the circuit, as shown in Figure 1.Gold or other metals are deposited from the swab.

Advantages and LimitationsIn general, given the choice between conventional

bath plating and selective plating, bath plating shouldalways be used unless some special advantage inquality, time, convenience or cost can be achievedusing selective plating. For large scale bulk pro-duction, conventional electroplating is almost alwayscheaper and more convenient.

STYLUS

CARBON ANODE

COTTON PAD

PLATING DEPOSITPOWERSOLUTION

Fig. 1 Schematic diagram illustrating the principleof selective brush plating

In carrying out a cost analysis, however, the platershould take all production factors into account: a truecomparison can only be made if costs such as dis-assembly, reassembly, masking, jigging, packing,transport and paper work are included. The possi-bilities of mechanising or automating selective platingto reduce labour costs should also be considered.

Summarising, probably 95 to 98 per cent of allplating will continue to be done in a bath. In anindustrial society, the remaining 2 to 5 per cent ofapplications, however, constitute a substantial numberof applications where selective plating can save moneyand (perhaps more importantly) time. The table listsnine areas where economies can be realised throughthe use of high speed selective plating.

Printed CircuitsProbably the most widely used application for

selective gold plating is in the manufacture and repairof printed circuits and related printed wiring. GeneralElectric NASA Division in the U.S.A. claimedsavings upwards of $344,000 by using this techniquefor circuit maintenance. Similar savings are docu-mented by such companies as I.B.M., Litton Indus-tries and Collins Radio. Typical printed circuitapplications are as follows:

1 Repair of gold plating on edge contacts or fingersof assembled circuit boards without disassembly.

2 Economical small scale production plating ofspecialised printed circuits in lots up to 300per day.

3 Selective plating of easily contaminated flexiblecircuitry and tape cable.

4 Prototype manufacture.

Edge Connector Contact Tabs

When the gold on edge contacts of assembledcircuit cards has flaked or peeled, it is too late toreplate these in a bath. Immersion in an electrolytewould contaminate assembled components. By short-ing the edge contacts, they can readily be selectively

plated without endangering the assembly (see Figure2). In a similar case, the original plating may beexcellent, but after a number of boards have beenmanufactured and assembled a change in specifica-tions is introduced. The extra thickness of goldrequired may be selectively deposited.

In this procedure all loose or defective electro-plating must first be removed; the edge contacts mustthen be shorted, the area selectively electrocleanedand plated. To avoid endangering neighbouringcomponents rinsing is sometimes carried out with asmall sponge.

Flexible Circuits

Flexible circuits and tape cable, when bath plated,present special problems. Flexing of the circuitimparts microscopic cracks between the conductivecopper and the underlying plastic. With tape cable,sections are often cut to size and the plastic coverremoved at both ends to expose contact areas. Thisresults in weakening the bond between copper andunderlying plastic. For both flexible circuitry andtape cable, the presence of microscopic gaps createsdifficulties. Solutions are 'sucked' into these open-ings and trapped. No amount of rinsing completelyremoves them, and residual plating solutions latercause 'sweating' , corrosion and changes in con-ductivity.

Selective plating avoids this problem. Withoutimmersion, there is no hydrostatic pressure to `push'the solution into the openings. Time required isappreciably less than that for bath plating, and con-sequently there is less time for solution to be entrapped.Finally, the very low free cyanide content (less than0.5 gil) of selective gold plating solutions createsfewer corrosion problems. A number of flexible

Table

Major Families of Selective Brush Plating

Applications

1 Repair of defective or missing electroplate2 Partnership applications (e.g. bath plus selec-

tive plating)3 Components which become contaminated if

immersed in an electrolyte4 Parts too large for readily available facilities5 Parts requiring costly disassembly, reassembly

and/or handling6 Parts needing considerable masking before

bath plating7 Improving adhesion to aluminium, stainless

steel, carbon and refractory metals8 Plating ultra-high strength steels with minimal

hydrogen embrittlement9 On-site plating in , the field

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Fig. 2 A typical printed circuit card from the tabs or con -tacts of which defective gold has heen removed. Thecontacts have then been shorted using a copper tape,conductive on both sides. The circuit is now ready forselective gold plating

circuit and tape manufacturers, with previous rejectrates up to 30 per cent, have reduced this figure tozero.

Electrical Contacts

Replating gold on worn or damaged electrical con-tacts not easily removable provides another excellentapplication. Attempts to replate them in a bath,without disassembly, results in contamination ofneighbouring components as well as waste of gold onnon-essential areas.

A typical contact repair was carried out in theUnited States by Remington Rand Univac. In someof their earlier computer models they experienceddifficulties with changes in electrical conductivitycaused by corrosion after gold plating had worn offthe pyramid-shaped electrical contacts. By the timethis problem was corrected, connections had alreadybeen assembled into vast batteries of electrical con-tacts on computer units in the field.

These studs were selectively plated using a smallhand-held electrical drill with a shaped carbon anodesubstituted for the bit. A Dacron felt liner covered

the end. The anode tip was dipped into gold platingsolution and contact was established with the malestud, while the drill rotated at a preset speed. Asmall foot pedal controlled electrical timer signalledthe length of time required. A skilled operator coulddo thousands of contacts per day, depositing approxi-mately 0.00005 inch of gold on each stud in sixseconds. When a bell rang, the operator wouldmerely re-dip the tip of his anode into the solution,re-establish contact at a new point and restart thedrill.

Some of the added advantages of gold platingconnector pins are indicated by the history of analtimeter servo unit manufactured by Litton Indus-tries. Since this device was to be used in an earlywarning missile system, reliability was of the utmostimportance. Consequently, the U.S. Governmenthad a 23-hour test specification. During testing,a male connector plug was subjected to numerousinsertions and withdrawals. The gold plating fre-quently failed. Previous repair involved removal ofthe plug and replacement with a new assembly.However, specifications state that any time any singlecomponent is replaced, the 23-hour test cycle mustbe repeated. Now, with selective plating, defectiveplate is removed from the pins using a wire brush, andfresh gold is replated without removing the com-ponent. Strictly speaking, no single component hasbeen replaced in the assembly, and there is no needto repeat the lengthy test cycle—a major logisticbenefit.

Improving SolderabilityCertain metals, such as aluminium and stainless

steel, provide soldering difficulties. Contaminatingacid type fluxes, normally required for soldering, areforbidden in the electronics field. Selective platingavoids this difficulty and is widely used to improvesolderability to aluminium and stainless steel, as wellas on carbon, semiconductors and refractory metals.The particular base metal is cleaned, activated and athin film of nickel or cobalt (from 0.0003 to 0.001inch) plated, followed by a flash deposit of approxi-mately 0.00003 inch of gold. Since adhesion of allthese materials to the base is excellent, a gold surfaceintimately bonded to the underlying metal is obtain-ed. Gold can easily be soldered with a neutral flux.

Plastic MouldingWith the increasing use of polyvinyl chloride

(PVC) and polyvinylidene chloride (PVDC) for con-tainers and bottles, special production problemsoccur. These p^lymers liberate minute quantities ofhydrochloric acid, which tend to etch or pit the diecavities. This causes rejects due to drag marks andalso shortens the life of moulds and dies.

A simple solution is selectively to plate these mouldsurfaces with a thin 0.0001 inch deposit of a hard goldalloy (140 to 160 Knoop) highly resistant to hydro-chloric acid attack, as shown in Figure 3. This pre-vents penetration of the hydrochloric acid releasedby the PVC and considerably increases mould life.Selective repairs to the gold plate itself can readily bemade while the mould is still in the machine.

Selective gold plating of moulds provides an addi-tional advantage in situations where the plasticmaterial tends to stick to the mould itself. Teflonmoulding is typical. A flash deposit of gold providesa non-sticking surface. The problem of sticking existsin plastic extrusion as well, and gold flashing ofextruder nozzles for C.B.S. Manufacturing Companyhas provided excellent results in preventing sticking.

Aircraft and Marine Maintenance

Specific uses are found in the maintenance depotsof almost every commercial airline and Air Forcestation in the U.S.A. and Europe. Numerous marinemaintenance depots, both commercial and Naval, arealso starting to find this technique valuable, particu-larly for saving down-time.

Most aircraft and marine plating applicationsinvolve the use of cadmium, nickel, copper and tin.For certain specialised work, however, selective goldplating has also proved valuable. For example, con-tact areas on the slip ring of the ADF Loop Collectoron Douglas DC-8 and Boeing B-720 aircraft havebeen replated with gold, as have numerous similarcomponents.

Gears, bearings, bushing and cylinder liners ofaircraft engine components are often selectively silverplated to prevent galling and fretting or to provide aninterference fit. Selective gold is always used as aflash pre-plate. On the Douglas DC-8, gold-plus-silver has been used to repair piston head grooves onthe main gear door activator piston rod and theinternal diameters of worn gland piston rod bores onaileron hydraulic power unit glands. On Pratt &Whitney JT3D, JT4 and JT8D aircraft engines, gold-plus-silver has selectively built up internal diameterson worn cylinder bushings, cover bushings, corrodedand undersized main accessory drive gear box linerbores and worn seal lands of bearing nozzle assemblies.Similar deposits on outside diameters of bearing oiltransfer tubes, worn journals where adaptor mateswith gear drive, and on pressure ratio control yokeshafts have brought these parts back to originaldimensions or have enabled interference fits to beobtained.

Other important aircraft work for gold platinginvolves improving wear on copper commutators forthe many high speed servo-motors used throughoutan aircraft. A flash of gold over a highly finished

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Fig. 3 A split die for blowmoulding PVC containers re-ceives a final deposition ofhard gold by means of selec-tive plating. By guardingagainst hydrochloric acidreleased during moulding,mould life is considerably ex-tended, down-time is reduced,and a better product at lowercost is achieved

commutator surface, followed by rhodium and a finalflash of gold, not only lengthens life, but radicallyreduces operating temperature and radio interferencecaused by arcing. A typical example is the com-mutator on the booster pump on Sud AircraftCVL-VI-R, where the life of the part was doubled.

In the marine field, similar components have beenrestored and their performance improved by selectivegold, gold-plus-silver, or rhodium-plus-gold plating.An example of a marine application involves replatingof cylinder liner adaptor faces and seatings on NapierDeltic Engines. In order to combat corrosion of thespot faces on the liner, the area is electroplated withtin during production by conventional means. Afterassembly or during repair, it was found that theaddition of 0.0002 inch of gold plate to the tin layerenhanced corrosion resistance. Similar operationshave been carried out on other parts of cylinder liners,such as in the threads and on the seatings for thenozzle air start and blanking plug adapters. Selectiveplating has been approved by Lloyd's Register ofShipping.

As an Underplate

Selectively plated gold is frequently used as anunderplate for silver, rhodium and other metals. Sincesilver deposits by immersion, a strike is required to

prevent the formation of poorly adherent deposits.Selective silver plating does not require a silverstrike; instead, a thin colour flash of gold will providea surface more noble than silver. Silver can then beapplied with excellent adhesion and without im-mersion deposits. This technique has proved sosuccessful that several U.S. companies now specifya dilute cyanide gold bath as a pre-plate prior toconventional silver plating.

Rhodium solutions are highly acidic and tend toattack underlying metals. Such attack not onlydestroys lustre, but contaminates the rhodium solu-tion, making subsequent deposits dark in colour. Thisproblem can be overcome by a nickel underlayer, butnickel in thin deposits tends to be porous; a thin flashof gold serves as a superior pre-plate. Gold has alsobeen used as a pre-plate under nickel to reduceporosity for engineering applications where porosityis detrimental.

Miscellaneous Applications

There exist many other applications for selectivegold plating. Gold plated bus bar connections are notuncommon for specialised types of work. Normally,the connections are tin or silver plated to providepositive contact. However, in a chlorine plant,chlorine gas tends to attack the silver. A deposit of

0.0001 inch of gold on the copper bus bar provides asuperior contact area.

Gold plating has also been used selectively to fillpinholes and reduce porosity. A particular exampleis in a very sensitive low pressure safety valve used ina satellite system. The entire device, fabricated from0.0004 inch thick aluminium stock, was made non-operative by a pinhole. Localised gold plating suc-cessfully closed the hole so that the part could bereturned to service.

In a sealing operation of a different nature, lowtemperature diffusion seals at gold-indium platedinterfaces (equivalent in bond strength to a gold-copper weld) have been obtained at 370 to 400°Fwith a 30-minute time cycle.

SummaryHigh speed selective gold plating provides an addi-

tional tool for the plating industry. While not an

economical procedure for most high productionapplications, it provides a simple ancillary techniquefor applications which involve special problems.

In evaluating this technique, platers should strivenot to restrict their thinking only to retouching anddoctoring. Nor should they conclude a priori that,since selective plating normally requires manualmanipulation, it does not lend itself to some types ofproduction.

If these two prejudices are overcome, intelligentindustrial platers will find selective plating an excel-lent money and time laving device for a wide varietyof applications in electronics, plastic moulding, air-craft and marine maintenance, and numerous otherindustries. All that is required is an awareness ofpossible applications and a willingness to learn theprocedures for a somewhat different approach toelectroplating. As the process becomes more gener-ally known both plater and user should benefit.