Patent GB-1030273-A (GB)
Title: Improvements in or relating to surface film type electrical elements
IPC: H01C17/04
Filed: 1964-03-04
Abstract: 1,030,273. Machining by electron beams. TELEGRAPH CONDENSER CO. Ltd. March 4, 1964 [March 4, 1963], No. 9056/64. Heading B3V. [Also in Division H1] An electrical element, e.g. a resistor or an inductor, comprises a substrate of electrically insulating material, a layer of electrically conductive material coated on the substrate, and a non-conducting path formed in the layer by volatilization of the electrically conductive material and defining an electrically conductive track extending in the layer between terminals in electrical contact therewith. An apparatus for producing such an element is shown in Fig. 3, the volatilization of the electrically conductive material being effected by means of a electron beam generated by a triode gun 20 and focused so as to converge on an element 10. The leads 18 of the element 10 are held in chocks 21 which are rotated by suitable driving means (not shown), the electron beam being made to scan the length of the element 10 as it is rotated so as to form a spiral non-conducting path. The element 10 is rotated within an air-tight housing 22 in which a vacuum is maintained by a vacuum pump 24 connected to the housing 22 through a conduit 25. A mechanism for advancing and positioning successive elements 10 comprises a chain belt conveyer 26 moving a plurality of paired yokes 27, each of which engages one of the leads 18 of an element 10. Forks 28 are provided to lift the element leads 18 and elevate the elements 10 from the yokes 27 to the chucks 21 for the electron beam treatment and subsequently to lever the elements 10 back to the yokes 27. The width of the electron beam is readily adjustable by the focusing means and accordingly the width of the non- conducting path is adjustable, the pitch of the path being adjustable by relative adjustment of the speed of the scan and the rate of rotation. During the electron beam treatment, the element 10 may be connected by the chucks 21 to a resistance measuring circuit. The start and finish of the non-conducting path are accurately controllable by either deflecting the electron beam on to or away from the element 10 or by focusing or defocusing it at the precisely appropriate instant in time. The electron beam, after vaporizing the conductive coating, strikes the exposed substrate and heats it momentarily to the fusing point so that a skin of glazed material is formed. The surface of the non-conducting path, therefore, is free from irregularities and imperfections and microscopically smooth. The substrate material may be a ceramic such as, for example, beryllia. Beryllia is not suitable for use when conventional methods are used to form the non-conducting path because of the toxicity of the by-products from the worked beryllia. However, beryllia is highly desirable as a substrate material since, because of its high thermal conductivity, it assists in providing a high wattage rating for a given hot spot temperature in a resistor. Cermet films provide desirable surface films for this type of resistor, such films being tenaciously adhesive to the substrate, and a mixture of 65% by weight chromium and 35% by weight silicon monoxide is an example of a suitable cermet material which provides a surface film which can readily be adjusted to a desired final resistance by the above-described method and apparatus. Another good resistance material made useful as an adjustable surface film by this invention is molybdenum disilicide. By way of example, the manufacture of a resistor having a steatite core coated with a nickel alloy is described. After the non-conducting path has been formed, a protective coating may be applied to the resistor by conventional means, for example, a vitreous coating may be applied by dipping or spraying. In a modification of this invention, a ceramic substrate carrying a film of resistive material is overlain with a layer of ceramic to form a covering and the non-conducting path provided as before, the electron beam vaporizing and removing the overlying layer of ceramic at the non-conducting path. In a further modification, a resistor has a protective covering applied by polymerizing a gas in the housing by the action of the electron beam, a cloud of the polymerized product being formed which settles on the resistor. If removed by the cutting action, the layer re-settles after the cutting of the non-conducting path is complete. A surface film type inductor according to this invention is made from a conductive film such as either silver or copper deposited on an elongated non-conducting substrate, such as a round core or a bar-like member, and formed into a number of closely spaced turns by the impingement of an electron beam. Suitable materials are a silver coat on a magnesium oxide core, their coefficients of expansion being similar and the electron beam being particularly effective in removing the silver.