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Historical Articles

July, 1954 issue of Plating



Tube Division, Radio Corporation of America, Lancaster, Pa.

This article describes some unusual plating applications used in the production of electron tubes for the television field. The plating applications described are used primarily for functional purposes rather than for the sake of appearance. Special care is taken to exclude contamination from plated coatings used on the interior surfaces of tubes because very small amounts of contaminants may impair the life or performance of the tubes. Care also is taken to insure uniformity of thickness, especially in tubes in which glass-to-metal seals are used. Among the processes described is the plating of fine mesh screens for use in television camera tubes.

Electroplating of tube parts at- the plant of the author’s employer accomplishes such functional objectives as increasing surface conductivity or increasing corrosion resistance or preparing parts for brazing. This paper discusses some unusual applications of plating techniques in the production of tubes for use in television.

Special care must be taken to exclude all impurities from plating baths used for tube parts because the presence of even very small amounts of contaminants the finished assembly may impair tube life or performance. For example, traces of lead in the order of 0.01 per cent in copper plating salts have been known to cause electrical shorts and poisoning of the cathode in power tubes whose parts were plated in a bath made up with those salts. Lead, like zinc and cadmium, is classified as a relatively volatile material. At the low pressures and high temperatures at which most power tubes operate, these metals volatilize and deposit on the cooler nonconducting portions of the tube.

Because it is so important to prevent contamination, the facilities of a complete analytical laboratory are available to the- plating department of the author’s plant. All baths are checked for impurities at least once a month by complete chemical analyses and twice a month by spectrographic analysis. If as little as 0.01 per cent of lead, cadmium, or zinc is found to be present in the plating baths, an attempt is made to remove these impurities by means of low current density electrolysis. In some cases (for example, in a gold plating bath) it has been found more economical to recover the plating metal (gold) and make-up a fresh bath than to try to remove the metallic contamination. Brighteners used in the bright nickel and silver baths also are checked once a month by means of a Hull-cell test. Most baths are filtered continuously, and all are filtered at least once a week.

Several inspectors are employed full time to check quality and uniformity of thickness of the plating. A magnetic type instrument (Magne Gage, manufactured by American Instrument Company, Inc., Silver Spring, Md.) is used to measure thickness when magnetic materials are involved. When this instrument is not adaptable, a cross-sectional method is used.

During the processing of tube parts, the plating must often withstand firing (inert atmosphere heat-treatment) at temperatures as high as 100° C below the melting point of the metal. Because any tendency to peel or blister is readily apparent at these temperatures, variations in the normal cleaning schedules due to substandard’ materials or carelessness will be revealed immediately. These normal cleaning schedules are the same as at any plating establishment and include alkaline electrocleaning, acid pickling, nickel strike for stainless steel, and several others.

Fig 1. Aperture hole having a diameter of 0.031 inch before tumbling, magnification 60X. Fig. 2. Aperture hole have a diameter of 0.031 inch after tumbling, magnification 60X.

The general product line at the plant of the author’s employer can be broken down into three main fields— kinescopes, pickup tubes and photo tubes, and power tubes. The kinescope is the heart of the television set, the tube on which the picture appears. Electroplating on kinescopes represents a large volume of the work performed in the plating department, but requires very little variety.

Metal finishing operations on kinescopes are applied only to the stainless steel parts used in the electron gun and comprise tumbling, bright dipping in acid, electropolishing, and some barrel plating with gold. The parts having apertures through which the electron beam is focused, for example, are tumbled after forming to remove-burrs. Figs. 1 and 2 show the edges of the aperture before and after tumbling. In different kinescopes, these holes vary in diameter from 0.020 inch to 0.400 inch. A very small burr, of the order of 0.005 inch, on the inside of the hole may be enough to interfere with the focusing of the picture.

After the tumbling operation, the gun parts are either bright dipped in acid or fired- in a furnace containing a dry hydrogen atmosphere to remove surface contamination. The dew-point of the hydrogen used is approximately -60° C. Dry hydrogen firing cannot be used on parts which are required to act as supports because the high temperatures (1100° C) tend to recrystallize the metal, and thus anneal or soften it. For some applications, the parts having apertures also are barrel plated with gold to decrease secondary emission. This phenomenon is described later.

In all plating processes on kinescope parts, great care is necessary to exclude impurities from the interior of the tube. The cathode used as an electron emitter in kinescopes is coated with a layer of mixed powdered oxides of barium, strontium, and calcium which is poisoned very easily by slight traces of impurities. As little as 0.5 parts per million of chlorides, for example, may cause slumping emission and poor life in a tube having an oxide coated cathode. When hydrochloric acid pickling or trichlorethylene vapor degreasing is used, therefore, all traces of chlorides must be rinsed from the parts. The rinsing is accom-plished usually by agitation of the parts for periods of 5 to 30 minutes in distilled or deionized water. The time required for rinsing depends upon the complexity of the shape of the part.

Pickup Tubes and Phototubes
”Pickup” tubes are used in television cameras to pick up the images which are seen on the kinescopes. There are several types of pickup tubes, including the image orthicon, which is used for studio and outdoor pickup, the iconoscope, which is used today mainly for movie pickup, and -the vidicon.* The vidicon camera is used in many industries to watch dangerous operations or hard-to-get-at meters, dials, and instruments. Phototubes are included in this general classification because they also contain light-sensitive elements.

The most unique application of plating in television pickup tubes is in the production of fine copper or nickel mesh screens for use in image orthicons and vidicons. These mesh screens are made on a glass master into which two perpendicular sets of grooves have been etched. The number of grooves per inch may be 500 or 1000, depending on the tube type for which the screens are intended. A conductive coating of palladium is sputtered onto the glass master in a vacuum and is then rubbed off the surface of the master to leave a metallic film of palladium in the grooves. The master then is plated in an acid copper plating bath or a bright nickel plating bath, and the plated mesh screen is peeled off and dried. The adherence of the palladium to the glass is adjusted carefully so that the mesh separates easily from the master. The mesh screens are inspected carefully in a dust-free room for holes, tears, and build-up of plating. Because the mesh screen is part of the section of the tube which stores the image until it is picked up by the electron beam, a very- small defect in the plating on the screen or a speck of dust on it is magnified many times in the final picture on the receiver kinescope.. A defective mesh screen would annoy millions of people watching a television program. Figs. 3, 4, and 5 illustrate the relative fineness of these mesh screens.

Fig. 3. 1000-line-per-inch mesh screen,
magnification 50X.
Fig. 4. Nylon stocking, magnification 50X. Fig. 5. Mesh screen ready for mounting in image orthicon

Some types of camera tubes and phototubes contain a photosensitive surface which emits electrons when exposed to light. This surface is produced by the evaporation in a vacuum of a film of silver-bismuth alloy, or its equivalent, onto the base metal or glass. The evaporator consists of a platinum plated molybdenum-tungsten alloy wire. When this wire is plated, an attempt is made to obtain a dark, spongy platinum which can be wet easily by the melted alloy immediately prior to evaporation. A smooth platinum surface causes the molten alloy to ball up and fall off.

In addition to the two plating applications described above, the metal portions of the outer surface of some phototubes are plated with a bright nickel alloy to improve corrosion resistance and appearance.

Power Tubes
The general classification of power tubes is divided into two groups—large power tubes and small power tubes. The large power tubes are used for radio and television transmission or industrial applications, and the small power tubes for such applications as power supplies of radio and television receivers, mobile radio transmitters, and radar equipment. The plating operations for these tubes constitute the major portion of the work done by the plating department.

The grid wires of many power tubes are plated with’ platinum, rhodium, or gold to minimize secondary emission. The grid normally is placed between the cathode and the anode of a tube to control the flow of electrons. When a large number of electrons are flowing, some are bound to strike the closely spaced grid wires and cause emission of secondary electrons The molybdenum wire used for these grids has a high secondary emission characteristic; that is, when bombarded by a primary electron from the cathode, the wire itself can emit many secondary electrons which would interfere with the normal operation of the tube. Plating with platinum, gold, or rhodium changes the secondary emission characteristic of the grid wires and improves the operation of the tubes.

Much plating also is done on large power tubes to increase conductivity both inside and on the surface of the tube. Fig. 6 shows a -transmitting triode which is plated with dull silver on the inside and bright silver on the outside. Because most brighteners for silver plating baths contain some form of sulfide, the type of plating from such baths cannot be used inside these tubes. Recent tests’ with radioactive tracers proved conclusively that a small amount of sulfur as sulfide is coplated with the silver from a bright silver bath. Even this small amount would be enough to impair the operation of the tube.

The plating of external parts of large power tubes such as the triode referred to above presents an interesting problem. After the tube is exhausted (evacuated of air and other gases to a predetermined value) and sealed, it is sent to the plating department for finishing. A typical tube contains glass, which is sensitive to thermal shock, an iron-nickel-cobalt alloy, nickel, cold-rolled steel, brazing alloys, and copper. The cleaning problems prior to plating can be readily appreciated. A previous factory procedure for this type of tube called for a hydrochloric acid pickling process to remove the heat scale produced when the glass was sea-led to the metal at approximately 1000° C. The use of the pickling process gave excellent plating results, but it was discovered that tubes subjected to this treatment became gassy. Laboratory investigation proved that the hydrochloric acid pickling step caused hydrogen penetration and, subsequently, gassy tubes. All large power tubes now are scratch-brushed to remove heat scale prior to plating. Electrocleaning and a nickel strike complete the present cleaning cycle.

Another case of gassy tubes and cracked metal resulted from the silver brazing of parts made from an iron-cobalt-nickel alloy. Laboratory investigation revealed that silver solder penetrated the grains of the iron-cobalt-nickel alloy, causing cracks in the metal and subsequent leaks. It was discovered that a plated layer of nickel of the-order of 0.6 mils (0.0006 inch) would prevent this intergranular penetration during the brazing operation which is performed usually at approximately 800° C. Fig. 7 illustrates the intergranular penetration of the iron-cobalt-nickel alloy by silver solder. Fig. 8 shows the protection afforded by a plated layer of nickel.

Fig. 6. RCA-6161 UHF transmitting tube Fig. 7. Cross section showing penetration of iron-cobalt-nickel alloy by silver solder, magnification 33X. Fig. 8. Cross section showing protection afforded by the use of nickel plating on iron-cobalt-nickel alloy, magnification 33X.

In addition to the processes described above, the plating department performs much electropolishing, electrocleaning of tungsten and molybdenum, and cataphoretic coating of cathodes and filaments.

Electrocleaning is a very efficient process for removing those oxides that are formed when tungsten and molybdenum leads are heat-sealed to glass. Essentially, this process involves the immersion of the parts to be deoxidized in a hot, saturated solution of sodium carbonate and the application of an a’ voltage of approximately 40 volts between the parts and a stainless steel electrode until the parts are completely free of oxides. This cleaning usually takes about one minute depending, of course, upon the depth-of the oxide. The process differs from the normal d-c alkaline electrocleaning process designed to remove dirt, grease, and oils prior to electroplating.

Cataphoretic coating of cathodes and filaments is, to some extent, also an electroplating process. When extremely small particles of aluminum oxide or the carbonates of barium, calcium, and strontium are dispersed in an alcohol solution, the particles assume a positive charge. A d-c voltage is impressed between the cathode or filament and an electrode in the- mixture so that the positively charged particles migrate and coat the negatively charged cathode or filament. A very dense and adherent coating is obtained in this manner.

This article has discussed briefly some of the more unique and unusual plating applications necessary in the production of tubes for television. Many precautions are taken in these applications to prevent contamination from entering the tubes. Most of the plating processes are performed for some necessary reason, and not primarily for appearance. It is hoped that this brief discussion may contribute to a better understanding of some of the plating problems facing the television-tube industry.

The author wishes to express his appreciation to Mr. R. H. Zachariason and Dr. G. S. Briggs of the RCA Tube Department for their many helpful suggestions in the preparation of this manuscript.

1. S. E. Eaton, R. W. Fabian, and E. H. Newton, Metal Finishing 50, 63 64 (December, 1952).

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