US2585752A - Production of discontinuous, conducting coatings upon insulating surfaces - Google Patents

Description

Feb. 12, 1952 s. o. DoRsT 2,585,752 l PRODUCTION OF DISCONTINUOUS, CONDUCTING COATINGS j UPON INSULATING SURFACES :f Filed May 2e, 1948 JTA/VLE 7 0 DoRT JNVENTOR.

Patented Feb. 12, 1952 PRODUCTION OF DISCONTINUOUS, CON- DUCTING COATINGS UPON INSULATING SUR-FACES Stanley O. Dorst, North Adams, Mass., assignor to Sprague Electric Company, North Adams, Mass., a corporation of Massachusetts Application May 26, 1948, Serial N o. 29,317

6 Claims. (Cl. 2925.42)

My invention relates to a process for producing discontinuous, conducting coatings upon surfaces composed of heat resistant, insulating materials. More particularly, it concerns a process whereby separate, coated metal electrodes are formed upon the surface of a heat resistant, insulating body such as is used, for example, in the production of ceramic condensers and printed circuits.

Ceramic condensers, especially those of the tubular type, are of growing importance in the electrical field, due to the development of high dielectric constant ceramic materials which can be processed without loss of their desirable electrical characteristics. One typical condenser is made by extruding a cylindrical tube of a semi-pasty ceramic mix, drying and firing 'the extruded tube, before or after cutting it into suitable lengths. Each length of tubing is thereafter provided with an outer and an inner electrode, which are highly conducting and integral with the surface of the ceramic. One of the main problems encountered in providing such electrodes arises from the fact that the two electrodes applied to the ceramic surface must be insulated from one another, Le., the conducting coating upon the ceramic tube must be divided.

According to one method developed for applying separate, coated electrodes, a silver lacquer is sprayed or painted only on those portions of the ceramic tube upon which the electrodes are desired by a careful masking operation. Thereafter, the silver layer is fired at elevated temperatures, to remove the binder and give an adherent silver coating on the ceramic. The electrodes thus produced are satisfactory for use without further treatment. However, the silver layer so formed is relatively ,thick and correspondingly expensive.

According to another method, the ceramic body is coated with an insulating lacquer on those areas which are not`to be provided with a metal coating. Thereafter, the uncoated surface is provided with a layer of chemically reduced silver, which layer is in turn electroplated with copper or some other readily solderable material. The insulating lacquer is then removed with a suitable solvent, and terminal wires are soldered to the electroplated layer.

The last mentioned process requires less silver than the one first described, but at the same time is not satisfactory, because of the difficulties encountered in plating the initial silver layer. The inexpensive barrel plating process is unsuitable for this purpose, since it is difficult to 2 make electrical contact to both of the silvered areas of each ceramic body. As a result, many ceramic condensers produced by this process come out with only one electroplated electrode. Another difliculty is that chemically reduced silver is deposited upon the insulated lacquer from which it may be transferred to the underlying ceramic surface, when that lacquer is removed with solvent. This residual silver may cause a low re- -sistance path between the two electrodes.

It is an object of the present invention to overcome the foregoing and related disadvantages of the prior art procedures. A further object is to produce discontinuous, solderable metal coatings on heat resistant, insulating materials. A still further object is to produce separate, coated metal electrodes on ceramic bodies by a simple, inexpensive and novel process. Additional objects will become apparent from the following description and claims.

These objects are accomplished in accordance with my invention by first providing the entire surface of the insulating body with a thin layer of a highly conducting metal, such as silver, platinum and palladium. Selected portions of this metal layer are then masked with an impervious material. Thereafter, the exposed portions of the metal layer are electroplated with a conducting metal, preferably a solderable one. Then the masking material is removed, and the entire body is treated with a selective solvent, which serves to remove those portions of the original, highly conducting metal layer, that were .previously masked by the impervious material.

Thus, a discontinuous, conducting coating of an electroplated metal is produced upon the surface of the insulating body.

In a more restricted sense my invention in concerned with a process for producing separated electrodes on ceramic bodies, which comprises the successive steps of providing the entire ceramic surface with a thin layer of chemically reduced silver, coating those portions of the body surface that are to separate the electrodes with an insulating lacquer, electroplating the exposed, reduced silver layer with a solderable metal, removing the insulating lacquer with a solvent, and then dipping the entire bodies in a molten solder bath. In one of its preferred embodiments, this invention is concerned with a process for producing separated electrodes on a ceramic body, which comprises providing the entire surface of said body with a layer of chemically reduced silver not greater than about 0.0001" in thickness, coating those portions of the body surface which are to separate'the electrodes with an insulating lacquer, electroplating the exposed silver layer with copper to a thickness between about 0.0005" and 0.002", removing the insulating lacquer with a solvent, applying terminals to the electrodes, and dipping the ceramic body in a bath of molten solder selected from the class of those containing lead, tin, bismuth, zinc, copper, cadmium and their alloys.

The initial thin metallic layer is deposited over substantially the entire surface of the insulating base by well known means, such as spraying. sublimation, sputtering, photochemical reduction, etc. A particularly satisfactory method is the so-called chemical reduction method, wherein a silver salt is reduced to produce the silver particles on the surface. By this process, the insides of tubes, etc. may be provided with a conducting coating. Provision of a conducting coating over substantially all of the surface is desirable, as hereinafter explained.

Metals other than silver may be deposited For example, platinum and palladium are suitable.

Thereafter, those portions of the surface which are not to serve as electrodes are masked with a wax, resin, Scotch tape or other impervious material. Resin lacquers are often used for this purpose.

Since the initial layer is very thin and relatively fragile, it is customary to electroplate the electrodes with copper, zinc or some other readily solderable material. The coated base may be subjected to a barrel plating process, wherein a. large number are plated in a single operation.

y'Electrical contact between adjacent bases need be made at but a single point, since the initial conducting layer covers the entire surface. Electroplating will occur only on the exposed metal layer sections, thus producing two or more electrodes as desired.

'I'he masking materials separating the electroplated portions may then be removed, leaving thereunder a conducting metal layer which effectively short circuits the two or more electroplated electrodes. These areas are removed in accordance with my invention by treatment with a selective solvent. By this term, I mean a material which will remove the initial conducting layer preferentially or at a rate such that the latter will be entirely removed without appreciably affecting the electroplated layers. A number of selective solvents which may be employed in accordance with my invention are described in the following paragraphs.

Certain metallic solvents are suitable. Various solders, such as alloys of lead, tin, bismuth, zinc. copper, silver and cadmium, are useful, although the melting point thereof may be appreclably less than the melting point of the conducting layer metal. For example, a chemically reduced layer of silver will dissolve in a solder of 50% lead-50% tin content held at 260 C. The solders will not wet the ceramic surfaces after dissolving the thin metal layer. Other metals and alloys may be used, mercury being representative. Solders are particularly desirable since the removal of the inter-electrode areas may be combined with the soldering of terminal elements to the various electrodes.

The selective solvent may be based on an electrolytic treatment, whereby the metal passes into solution in the ionic state. It is possible to dissolve the initial layer preferentially or at a higher rate than the electroplated layer by suit- The following description will be particularly` concerned with this selective solvent and process, but it is to be understood that the other selective solvents referred to above may be employed on.

a chemically reduced or otherwise produced initial layer.

Theventire surface of the insulating body is provided with a chemically reduced silver layer generally of a thickness less than about 0.0002". Those portions of the body surface that are to separate the conducting coating, are then masked with a layer of material, generally insulating and non-pervious, such as a nitro-cellulose lacquer. Thereafter, the exposed and uninsulated silver surfaces are electroplated with a metal or alloy, copper being representative of the electroplated metal. This plating step can be conducted in a barrel or other massive arrangement, since the insulating bodies may be dumped freely into the electroplating bath, and need contact each other only at a single point where the silver layer is exposed. One such body must, of course, make contact with the cathode terminal of the plating assembly.

The plated body is then treated with a solvent and/or with heat to remove the masking coating. leaving the unplated silver layer exposed. At this point, it is possible mechanically to attach terminal elements, such as tinned copper wires. to the electroplated electrodes. The whole body is then dipped in a molten solder bath. During this step, the electroplated surface and terminal contact become coated with a solder layer, thus permanently connecting any such terminal elements, while the exposed silver layer is dissolved in the molten solder bath, leaving the underlying surface of the insulating body completely uncoated and, therefore, non-conducting. In this manner, the diiliculties attendant with prior processes employing the so-called chemical silver process, are completely overcome.

My invention will be further discussed with reference to the appended drawings in which Figures 1 through 5 represent a ceramic condenser body at various points during the processing sequence of the invention.

More specifically, Figure 1 shows a tubular ceramic condenser body I0, the entire surface of which has been provided with a chemically reduced silver layer Il. The thickness of this layer and of the others is shown greatly exaggerated for purposes of clarity. In Figure 2 a coating of masking lacquer l2 has been applied to those portions which are to separate the Y condenser electrodes. The lacquer or other masking material may be applied with a brush, spray device, marking wheel or by other known methods.

Figure 3 shows the metal electrodes I3 and Il which are electroplated on the exposed surfaces of .the silver layer Il. This plating may be accomplished by the barrel process, in which the bodies to be plated are dumped en masse into a plating barrel containing the appropriate plating solution.

Figure 4 shows the body. after the lacquer coating I2 has been removed, leaving portions of the silver layer Il exposed, and short-circuiting the condenser electrodes I3 and I4. Terminal wires I5 and I8 are shown wrapped around the outer surface of electrodes I3 and I4, respectively. These terminal elements are generally of tinned copper.

Figure 5 shows the condenser after it has been dipped in a molten solder bath. A solder layer I8 covers electrode I3 and bonds terminal I5 thereto, while a solder layer I1 coats electrode I 4 and bonds terminal I6 thereto. The molten solder bath has dissolved and completely removed the exposed silver surface, as indicated at I9 and 20, leaving the ceramic body surface uncoated.

The finished condenser shown in Figure 5 may be dipped in wax, coated with an insulating lacquer or protected by a molded plastic casing, in the usual manner.

While the example illustrated and specifically described above is a tubular ceramic condenser, the invention may also be applied to many other insulating bodies in many different shapes and configurations. Ceramic materials which may be provided with electrodes in accordance with the invention include steatite, titanium dioxide, alkali metal titanates, various glasses, such as the borosilicate glasses, quartz, etc. Other insulating materials include the heat resistant organic or semi-organic resins, such as the polysiloxanes. polyhaloethylenes, etc. The invention is particularly applicable to polytetrahaloethylenes, such as polytetrafluoroethylene, polytetrachloroethyly ene and polychlorotriuoroethylene. These resins do not soften at temperatures below the melting points of many of the coating soldersreferred to herein, and are extremely resistant to the usual solvents. The insulating bodies may be fiat, tubular, rod-shaped and in sheet form, as well as in special congurations for special purposes.

The chemically reduced silver layer may be applied by any suitable method. One highly satisfactory procedure is to dip the insulating body in a mixture of 275 parts of a silvering solution containing 20 grams/ml. of AgNOa, 19 grams/ l. of KOH and 21/ml./ 1. of NHiOH conc., and of 34 parts of a reducing solution containing 100 grams/ 1. of dextrose and 158 ml. of CHaOH. The insulating body is allowed to remain in the mixed solutions for about 5 minutes. After rinsing and drying, the body will have a silver layer about 0.0001" thick thereon.

The electroplating may be done with any suitable conducting metal. Copper is preferred, since it is an excellent electrical conductor and. when clean, readily solderable. Other suitable metals include nickel, cadmium, tin, lead, etc. One satisfactory copper electroplating process employs a bath containing 120 grams of copper cyanide, 135 grams of sodium cyanide and .30 grams of NaOH per liter of water. The bath is held at lI-80 C., and the plating is carried out at a current of 4100 amperes per square foot to be plated, until the desired thickness is built up. A copper thickness of 0.001" is satisfactory for the present purposes, although thicknesses as low as about 0.0002" may be used.

The solder bath is held above the melting point of the particular solder to be used, generally between about and 100 C. above its melting point. The solder should dissolve the silver, and for this purpose, copper, lead. tin, zinc, bismuth, cadmium and silver solders may be used. A suit avoiding a separate step.

It is apparent that the invention is applicable to the preparation of numerous electrical and mechanical devices which require a discontinuous, conducting coating on an insulator. The process of the invention leads to durable, adherent, metal coatings, separated by wholly uncoated portions of the insulating surface.

As many different embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments hereof, except as defined in the appended claims.

I claim:

1. A process for producing discontinuous, conducting coatings upon the surfaces of insulating bodies, which comprises providing the entire surface of the insulating body with a thin layer of a metal of the class consisting of silver, platinum and palladium masking portions of said layer with an insulating coating, electroplating the exposed portions of said layer with a conducting metal of the class consisting of copper, nickel, cadmium, lead and tin, removing the insulating coating, and dipping the entire surface in a bath of molten solder composed of an alloy of at least two metals selected from the group consisting of lead, tin, bismuth, zinc, copper, silver and cadmium, until the newly exposed portions of the thin layer have been dissolved by the solder.

2. A process for producing separated electrodes upon the surfaces of ceramic bodies, which comprises providing the entire ceramic surface with a thin layer of silver, coating those portions of the body surface which are to separate the electrodes with an insulating lacquer, electroplating the exposed portions of the silver layer with a conducting metal of the class consisting of copper, nickel, cadmium, lead and tin, removing the insulating lacquer with a solvent, and then dipping the entire surface in a bath of molten solder composed of an alloy of at least two metals selected from the group consisting of lead, tin. bismuth, zinc, copper, silver and cadmium, until the newly exposed portions of the thin silver layer have been dissolved by the solder.

3. A process for producing separated electrodes on a ceramic body, which comprises providing the entire surface of said body with a layer of chemically reduced silver, not greater than about 0.0001 in thickness, coating those portions of body surface which are to separate the electrodes with an insulating lacquer, electroplating the exposed silver layer with copper to a thickness between about 0.0005" and 0,002, removing the insulating lacquer with a solvent, and dipping the ceramic body in a bath of molten solder composed of an alloy of at least two metals selected from the group consisting of lead, tin, bismuth, zinc, copper, 'silver and cadmium, until the newly exposed portions of the thin silver layer have been dissolved by the solder.

4. A process for producing ceramic condensers, which comprises the process dened in claim 2 wherein tinned copper terminal wires are wound about the electroplated portions that are to serve assuma n W as electrodes, before the ceras Ilcuody is flipped in the molten solder bath, whereby the electroplated portions and the terminal wires are simultaneously coated with a solder layer and thus permanently connected to each other.

5. A process for producing electrical devices, which comprises the process defined in claim 1, wherein terminal elements are mechanically attached to the electroplated portions, before the insulating body is dipped in the molten solder bath. whereby such terminal elements are simultaneously soldered to the electroplated portions.

6. A process for producing separated electrodes I upon the' surface of a ceramic body, which comprises providing the entire surface of said body with a thin layer of silver, coating those portions of the body surface which are to separate the electrodes with an insulating lacquer, electroplating the exposed portions of the silver layer with copper, removing the insulating lacquer with a solvent, and then dipping the ceramic body in a oath of molten solder composed of 8 equal parts by weight of lead and tin, held at about 260 C., until the newly exposed portions of'the thin silver layer have been dissolved by the solder.

. STANLEY O. DORST.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS Name Date Pickard July 21, 1908 1,160, l0 Foersterling et al. Nov. 16, 1915 1,359,338 Dagory Nov. 16, 1920 1,892,755 Scheppmann Jan. 3, 1933 2,062,116 Betterton et al. Nov. 24, 1936 2,161,888 Rearick June 13, 1939 2,220,961 Kern Nov. 12, 1940 2,253,026 Godsey. Jr Aug. 19, 1941 2,398,176 Deyrup Apr. 9, 1946 2,443,119 Rubin June 8, 1948

Claims (1)

1. A PROCESS FOR PRODUCING DISCONTINUOUS, CONDUCTING COATINGS UPON THE SURFACES OF INSULATING BODIES, WHICH COMPRISES PROVIDING THE ENTIRE SURFACE OF THE INSULATING BODY WITH A THIN LAYER OF A METAL OF THE CLASS CONSISTING OF SILVER, PLATINUM AND PALLADIUM MASKING PORTIONS OF SAID LAYER WITH AN INSULATING COATING, ELECTROPLATING THE EXPOSED PORTIONS OF SAID LAYER WITH A CONDUCTING METAL OF THE CLASS CONSISTING OF COPPER, NICKEL, CADMIUM, LEAD AND TIN, REMOVING THE INSULATING COATING, AND DIPPING THE ENTIRE SURFACE IN A BATH OF MOLTEN SOLDER COMPOSED OF AN ALLOY OF AT LEAST TWO METALS SELECTED FROM THE GROUP CONSISTING OF LEAD, TIN, BISMUTH, ZINC, COPPER, SILVER AND CADMIUM, UNTIL THE NEWLY EXPOSED PORTIONS OF THE THIN LAYER HAVE BEEN DISSOLVED BY THE SOLDER.

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