US 3722085 A
A film-type power resistor consists of a ceramic wafer or washer having a central opening, one side of such washer being printed at its outer and inner edges with generally annular traces of termination metal. An electrically resistive film is printed on such one side over all portions of, and between, the termination metal, excepting for predetermined adjacent end portions which are connected to radially outwardly extending terminal lugs or leads. In one embodiment, the metal traces are continuous, whereas in another embodiment they are interrupted in order to create series-related resistor portions. The washer (and its associated films) are mounted over a metal base and are embedded in a thermosetting synthetic resin, the base having an upwardly extending central post which passes through the opening in the washer to the upper surface of the resin.
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Description (OCR text may contain errors)
United States Patent [191 Caddock METHOD OF MAKING FILM-TYPE POWER RESISTORS  Inventor: RichardCaddock, 640 Sandalwood Court, Riverside, Calif. 92507  Filed: Sept. 23, 1971  Appl. No.: 183,113
Related US. Application Data  Division of Ser. No. 40,308, May 25, 1970, Pat. No. 3,649,944, which is a continuation-in-part of Ser. No. 847,783, July 18, 1969, abandoned, which is a continuation-in-part of Ser. No. 820,538, April 30, 1969,
 US. Cl. ..29/620, 29/613, 29/621  Int. Cl ..H0lc 7/00, HOlc 17/00  Field of Search ..29/620, 621, 613; 338/308, 338/309, 295, 260, 275, 320, 51, 226, 328,
Primary Examiner--Charles W. Lanham Assistant Examiner-V. A. Dipalma Attorney Richard L. Gausewitz [451 Mar. 27, 1973 ABSTRACT A film-type power resistor consists of a ceramic wafer or washer having a central opening, one side of such washer being printed at its outer and inner edges with generally annular traces of termination metal. An electrically resistive film is printed on such one side over all portions of, and between, the termination metal, excepting for predetermined adjacent end portions which are connected to radially outwardly extending terminal lugs or leads. In one embodiment, the metal traces are continuous, whereas in another embodiment they are interrupted in order to create series-related resistor portions. The washer (and its associated films) are mounted over a metal base and are embedded in a thermosetting synthetic resin, the base having an upwardly extending central post which passes through the opening in the washer to the upper surface of the resin.
are masked. Thereafter, terminal lugs or leads are connected to such terminal regions.
19 Claims, 10 Drawing Figures PATENTEUmzmrs 3,7 2,0 5
SHEET 10F 2 I NVEN TOR F/CA/AED E CQDDOCK A r Tom/5m.
METHOD OF MAKING FILM-TYPE POWER RESISTORS CROSS REFERENCES TO RELATED APPLICATIONS This application is a division of patent application Ser. No. 40,308, now U.S. Pat. No. 3,649,944 filed May 25, 1970, by the present applicant, for Film-Type Power Resistor, and Method of Making the Same. Said application Ser. No. 40,308 is, in turn, a continuationin-part of application Ser. No. 847,783, filed July 18, 1969, for Power Resistors, now abandoned. Application Ser. No. 847,783, is a continuation-in-part of application Ser. No. 820,538, filed Apr. 30, 1969, for Power Resistors, also now abandoned.
BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the field of film-type power resistors, of the type described in copending patent application Ser. No. 40,281, filed May 25, 1970, and of methods of making them.
2. Description of Prior Art It has previously been known to provide resistors of the film type and incorporating divergent radial flow of electric current, and it has also been known to effect overprinting of terminal film traces with film-type resistor material. However, the prior art has not achieved planar power resistors characterized by divergent radial flow and having adjacent outwardly extending terminal elements, central openings for stack or other mounting to a chassis, a wide range of predetermined resistance values ranging from (for example) one-tenth square to one square, and other advantages. In addition, the prior art has not suggested combining suchpower resistor elements with metal bases, particularly in combination with upwardly extending central posts, and molded and embedded in masses of thermosetting synthetic resin.
Relative to the method, the prior art has employed numerous ways to vary the value of a filmtype resistor. These have included changing the quantity of conductive material in the film in proportion to the quantity of nonconductive material therein, applying films of different thicknesses to the substrate, employing abrasiveblast or other means to notch completely through portions of the film to thereby adjust the resistance of the film, and changing the materials employed. It has also been known to apply a resistive film to a cylindrical substrate and then uniformly abrade the substrate by means of an abrasive jet in order to regulate the resistance of the film, but such abrading of the cylindrical substrate was not commercially practical or successful.
The prior art has not provided, insofar as applicant is informed, a'method of manufacture whereby a film of resistive material is overprinted on termination metal on a planar substrate of insulating material, following which the film is uniformly abraded in order to achieve a desired close tolerance relative to resistance value. In addition, the prior art has not provided such a method in combination with a masking method for end terminations, and wherein the mask serves additionally to protect leads to an ohmmeter which determines when the desired resistance has been achieved. Furthermore, the art has not provided such a method in combination with the attaching of radially outwardly extending terminal lugs or leads.
SUMMARY OF THE INVENTION The power resistor of the invention includes a wafer or washer (substrate) formed of insulating material and having a central hole therethrough, and also having annular traces of termination metal film on one surface thereof and adjacent, respectively, the inner and outer marginal edges of the wafer. In accordance with one embodiment, the annular termination traces are interrupted only at the terminal ends thereof, and to which radially outwardly extending terminal lugs or leads are attached. In another embodiment, the annular termination traces are interrupted at predetermined points creating series-related resistors. On such one surface of the washer is provided, over and between the annular termination traces except at the terminal ends thereof, a film of electrically resistive materiaL ln the second of the above-mentioned embodiments, the film is provided with radial gaps to achieve the indicated series relationship. The resistor element is mounted on a metal base having an upwardly extending post which passes through the hole in the wafer or washer, and the wafer, terminal lugs or leads, etc., are molded in a thermosetting'synthetic resin.
In accordance with the method, the planar surface of an insulating substrate is provided with termination traces of metal or other highly conductive material, following which a film of resistive material is overprinted on such traces and on other portions of the substrate.
' The substrate and associated films are then fired to cure the resistive material, following which a jet of abrasive material is employed to effect uniform abrasion of all portions of the resistive film. The abrading step is continued until the desired resistance is achieved, such resistance being determined by connecting an ohmmeter or the like to the terminal ends-of the termination traces at a region which is masked to shield the same from the abrasive blast. Terminal lugs or leads are then connected to the terminal ends of the traces, and extend radially outwardly.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view, with portions broken away, showing a first embodiment of a power resistor manufactured in accordance with the present invention;
FIG. 2 is a top plan view of the resistive element incorporated in the embodiment of FIG. 1;
FIG. 2a is a view corresponding to FIG. 2 but omitting a showing of the resistive film, so that only the substrate and the termination traces are shown;
' FIG. '3 is a view corresponding to FIG. 2 but showing a second embodiment of the invention, wherein a plu- 1 FIG. 7 schematically represents the masking of the terminal ends of the termination traces, the connection of the ohmmeter or the like to such terminal ends at a region below the mask, and the uniform abrading of the resistive film by means of ajet of abrasive material; and
FIG. 8 represents the element manufactured in accordance with FIGS. -7, but after completion of a subsequent step whereby terminal lugs are riveted or otherwise secured to such element.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In copending patent application Ser. No. 40,281, filed May 25, 1970, for Resistor with Heat Dissipating Means, there is described one form of a film-type resistor which generates a higher temperature at the center of the device than at the periphery thereof, with consequent favorable temperature gradient and high rate of dissipation of the generated heat. The present application describes (FIGS. 14, inclusive) several other types of film resistors which generate favorable temperature gradients, and which are employed for different ranges of resistance values. In addition, the present application describes (FIGS. 5-8) a method of manufacturing the specified film-type resistors and also other forms of film-type resistors having planar surfaces.
Referring first to FIG. 1, the illustrated power resistor is identical to one described in detail in said copending patent application filed on May 25, 1970, particularly relative to FIGS. 6, 7, 9 and thereof, except for the configurations of the resistive film and of the associated termination traces (of film). Thus, the resistor comprises a disc-shaped metal base 1, preferably formed of anodized aluminum, the upper edge of which is radially inwardly indented to form a smaller-diameter cylindrical wall 2 having a plurality of recesses 3 spaced circumferentially therearound. A wafer or washer 4 of ceramic, such as aluminum oxide or beryllium oxide, has planar upper and lower surfaces, the latter being disposed in flatwise heat-transfer relationship to the planar upper surface of base 1.
The resistive film provided on the upper surface of wafer 4, and which is described below relative to FIG. 2, is associated with film-type termination traces or strips which connect, respectively, with radially-outwardly extending terminal lugs 5 and 6. These are, respectively, riveted to wafer 4 by means of rivets 7 and 8 as described in detail relative to FIG. 10 of the cited copending application of May 25, l970. In place of the terminal lugs Sand 6, the terminal conductors may comprise flexible leads enclosed in insulating material, as described relative to FIGS. 17 and 18 of the cited copending application. The lugs orleads may also be directly brazed to the terminal ends of the termination traces. 1
The metal base 1 is integral with an upwardly extending tubular post 9 having a central opening 10 therethrough, the latter communicating with a corresponding opening in the center of the base 1. Thus, a mounting bolt may be inserted clear through the base 1 and post 9 for mounting of the resistor to an underlying metal chassis plate, and also for mounting of a plurality of resistors in stacked relationship and in alternation with heat-transfer fins.
A mass 11 of thermosetting synthetic resin, preferably a silicone thermosetting resin, is molded around post 9 and over washer 4 and its associated termination and resistive films, and also over the inner ends of the terminal lugs 5 and 6. The resin mass 11 extends downwardly around the smaller-diameter cylindrical wall 2 and into the recesses 3 to effect locking of the resin relative to the base, against both rotational and axial movements. The outer wall of the resin mass 11 is preferably cylindrical, and the upper surface of the resin is perpendicular to the axis of post 9 and flush with the upper end of the post. Such upper surface of the resin mass is parallel to the planar lower surface of base 1, to thereby facilitate stacking of the assemblies as indicated above.
The wafer or washer 4 has a central opening 12 therein to receive the post 9. Thus, in addition to its parallel and planar upper and lower surfaces, each wafer or washer 4 has concentric cylindrical outer and inner edge surfaces 13 and 14, respectively. The outer cylindrical surface, No. 13, is the peripheral surface or edge and is concentric with the inner edge surface 14 which marks the inner boundary or margin of the wafer.
A major aspect of the present invention relates to film and termination trace configurations whereby a very large proportion of the upper surface of wafer 4 may be covered with resistive film, while still creating a wide range of resistance values. In addition, and very importantly, the configurations are such that the terminal lugs 5 and 6 (or corresponding leads) may be located adjacent each other on one side of the resistor (and on one side of post 9) and may both extend radially outwardly, there being no need for central connections.
The films described herein create radial conduction paths for the electrical current, and (additionally) result in divergent flow or fanning-out of the radially flowing current. The result is a markedly higher current density at the portions of the film closer to central opening 12. Therefore, and since the power varies with the square of the current, but only linearly with the resistance, it follows that more power is present adjacent the opening 12 than adjacent the peripheral edge 13. The temperature generated at the central region is thus higher than at the periphery, with consequent favorable temperature gradient as stated in the copending patent application filed on May 25, 1970.
FIGS. 2 and 2a relate to a first configuration of the divergent radial-flow film resistor. Such FIGS. 2 and 2a illustrate the element and associated termination and resistive films incorporated in the completed resistive device of FIG. 1.
The resistive film 15 of the embodiment of FIGS. 1, 2 and 2a is generally C-shaped, there being a gap 16 in the otherwise annular configuration. The inner edge of the film 15 is shown at 17 and is spaced outwardly a slight distance from the inner boundary 14 of wafer 4. correspondingly, the outer edge 18 of film 15 is spaced somewhat inwardly from the peripheral edge 13 of the wafer.
Film 15 is adherently provided on the upper surface of wafer 4 by any one of various techniques, including painting, spraying, etc. Preferably, film 15 is appliedby silk screening and as indicated hereinafter relative to FIG. 5. The resistive film, which is preferably a complex metal oxide in a glass matrix, is fired and otherwise treated (after the screen-printing operation) as stated in the method portions of this specification.
As best shown in FIG. 2a, outer and inner concentric annular termination traces or strips 19 and 20, respectively, are provided on the upper surface of wafer 4 beneath film (such film being screened over the traces 19 and 20). Each of such traces l9 and 20 is continuous except in the vicinity of gap 16 in the film. At such gap, one end of the inner trace 20 bends outwardly at 21 and terminates at a hole 22 through the wafer 4. correspondingly, the other end of the outer trace l9 bends inwardly at 23 and terminates at a hole 24 in wafer 4. Such holes are adapted to receive the above-indicated rivets 7 and 8, respectively. Each trace end portion 21 and 23 ends in a circular trace region (which forms the terminal end of the termination trace) around the associated hole 22 or 24, to thereby increase the degree of electrical contact between terminal lugs 5 and 6 (or the flexible leads) and the traces.
The traces or strips 19 and 20, etc., are films of metal or other highly conductive material, as indicated in the method portions of this specification.
With the described configurations of film l5 and of traces l9 and 20, the length of the current path between the terminals is equal to the radial distance between inner trace 20 and outer trace 19. Because of the generally annular configuration of the resistor, the current diverges as it flows outwardly (or converges as it flows inwardly). The distances shown in FIG. 2 are approximately such that the' indicated radial path is only about one-tenth of the average circumference of the annular film (excluding the gap 16). Accordingly,
the described configuration produces about one-tenth square of resistive film. Since the pattern produces about one-tenth square, and assuming that the thickness and other characteristics of the film are such that the film has a resistance value of 100 ohms per square, then the configuration of FIGS. 2 and 2a produces a resistance value of approximately 10 ohms.
EMBODIMENTS OF FIGS. 3, 3a AND 4 In the embodiment of FIGS. 3, 3a and 4, all components correspond to the ones described in the embodiment of FIGS. 1, 2 and 2a except as specifically stated. Thus, there are outer and inner concentric traces overprinted by a resistive film, but such traces and film are interrupted in a predetermined manner as stated below.
As best illustrated in FIG. 3a, the outer trace or strip has a first section, numbered 26, which extends for more than one-half a circle, namely about 200. A second section 27 of the outer trace is separated from the first section by a gap 28 and extends for a much smaller distance, for example through an angle of about 100. The end of second section 27, remote from gap 28, turns inwardly at 29 and extends to a hole 30 in wafer 4, namely the hole for rivet 7 or for a connector to a flexible lead.
Referring next to the inner trace, this comprises a portion 31 which extends from a hole 32 in wafer 4 and then connects to a relatively short trace portion 33 (extending, for example, through about 90). There is then a gap 34, located radially inwardly from an intermediate region of trace section 26. A second inner trace portion 35 extends, for example, through about 200. An intermediate region of such second portion 35 of the inner trace is disposed radially inwardly from gap 28 in the outer trace.
A resistive film 36 is printed over the specified traces, in interrupted C-shaped configuration (the gap in the C being at the terminal region and indicated at 37), as shown in FIG. 3.
Film 36 is also interrupted (formed with a radial gap) at the region of gap 28 and radially inwardly thereof, but the film is not interrupted at any region above the second trace portion 35. Stated otherwise, a film region 38 is provided (FIG. 3) above trace portion 35 and radially inwardly from gap 28 in the outer trace. Thus, in the region 38 the resistive film is overprinted on the inner trace 35, and the parallel combination of such overprint 38 and the inner trace 35 form the sole electrical connection between two adjacent segments 36a and 36b of film 36.
Another radial gap, numbered 39, is provided in film 36 between portions 36b and 360 at the region of, and radially outwardly from, gap 34 in the inner trace. However, the resistive material is overprinted on allof the outer trace portion 26, including the part indicated at 40 in FIG. 3. Thus, the parallel combination of the resistive overprint 40 and the underlying region of outer trace 26 form the sole connection between the two adjacent segments 36b and 36c of resistive film 36.
It will thus be seen that the pattern of FIG. 3 results in three segments of resistive film, each being such that there is a divergent radially outward or radially inward flow of the electrical current. Assuming, for example, that the terminal connected to the wafer 4 at hole 30 is the positive terminal, current flows through trace portion 29 to trace section 27 and thus to the outer edge of film section 360, then flows radially inwardly in convergent manner to the left-most region (FIG. 3) of second trace portion 35, then flows through the parallel-related overprint 38 and trace portion 35 to the rightmost region of trace portion 35, then flows radially outwardly in divergent manner through film section 36b to the left-most region of trace 26, then flows through the overprint 40 and adjacent region of outer trace portion 26 to the right-most region of the latter trace portion, then flows radially inwardly in convergent manner through resistive film section 360 to trace portion 33, and then flows through trace section 31 to the terminal connected to the wafer at hole 32. I
The three segments of radial conduction 36a, 36b and 36c are thus in series-circuit relationship relative to each other. Each segment 36a, 36b and 36c has a length" that is approximately one-third of its width, so that each segment provides approximately one-third square. The three segments being connected in series, it follows that the total is approximately one square, which is equal to about ohms if I00 ohms per square resistive film material is employed.
The radial-flow embodiments described above are characterized by very high rates of heat dissipation, by an efficient use of the upper surface of the wafer 4, and by terminations which permit radially outwardly extending conductors (such as terminal lugs 5 and 6) to be utilized.
DESCRIPTION OF THE METHOD The method of the invention will be described relative to the manufacture of the element shown in FIG. 3 (the embodiment of FIGS. 3, 3a and 4) but it is to be understood that such method may also be performed relative to the embodiment of FIGS. 1, 2 and 2a, relative to the embodiment shown in the cited copending application filed on May 25, 1970, and relative to various other film-type resistor configurations wherein the film is provided on a planar surface.
As the first step in the method, the ceramic wafer 4 is formed in a press and in the illustrated annular shape, having the central opening 12. Such press preferably incorporates means to form the holes 30 and 32 and also the counterbores described relative to FIGS. 7 and 10 of the cited copending application filed May 25, 1970. As above indicated, the ceramic may be, for example, aluminum oxide or beryllium oxide. After pressing, the ceramic wafer is fired.
The next step in the method is to provide the termination traces, such as 26, 27, 33 and 35, on the upper planar surface of wafer 4 and as best illustrated in FIG. 3a. This may be done by silk-screen printing techniques in the general manner shown schematically in FIG. except using a screen which corresponds to the indicated termination traces instead of to the openings for the resistive film. Alternatively, the traces may be applied by vacuum deposition, or by using a ceramic matrix print having the metal incorporated therein. After the trace metal is applied, by any of the indicated (or other) methods, the wafer or base is fired to further bond the metal thereto. The firing may be effected, for example, in the oven shown schematically in FIG. 6.
The metal of the termination traces or strips should have good electrical conductor characteristics and may be, for example, either gold or a gold-platinum alloy.
As the next step in the method, and as indicated schematically in FIG. 5, the resistive film, such as 36, is provided on the base or wafer 4 and in overprinted relationship to all portions of the terminal traces except at the gap 37 in the resistive film. The resistive film may be applied by silk screening, painting, spraying, etc.
FIG. 5 shows a mask 41 having screen openings 42a, 42b and 42c therein and corresponding, respectively, to the film segments 36a, 36b and 36c (and interconnecting portions 38 and 40) shown in FIG. 3. The screen in openings 42a, 42b and 42c may be, for example, 250 mesh (U.S. Standard Sieve Series). Alternatively, and when a thicker deposit of resistive film is required, as relative to the low-resistance embodiment of FIGS. 1, 2 and 2a, the size of the screen may be 100 mesh.
After application 'of the resistive film as indicated relative to FIG. 5, the wafer 4 with films thereon is again fired in an oven, for example the oven 43 schematically represented in FIG. 6 and which contains a heat source schematically indicated at 44.
Referring next to FIG. 7, a segment-shaped mask 45 is mounted over the gap 37 in the resistive film, that is to say over the exposed terminal ends 29 and 31 (FIG. 3a) of the traces, in order to prevent the abrasive blast from abrading the trace metal at such termination ends. The present method further employs the use of probes (electrically conductive) which are mounted on the underside of mask 45 (in mutually insulated relationship relative to each other). The probes connect, respectively, to trace portions 29 and 31 (and associated circular trace ends, adjacent holes 30 and 32), when the mask 45 is in position.
Probes 46 and 47 connect to the terminals of an ohmmeter represented schematically at 48 and which is indicated as having a readout needle 49. The ohmmeter 48 may be a Wheatstone bridge (or other) arrangement electrically associated with the abrasive-blast means, described in the following paragraph, to automatically terminate the abrasive blast when the resistive value of the film 36 on wafer 4 reaches the desired tolerance range.
The abrasive blast is delivered from a nozzle represented schematically at 50 (and which connects to a suitable source of abrasive), emanating therefrom in the form of ajet 51 the diameter of which, at the substrate or wafer 4, is approximately equal to the distance between the inner diameter of wafer 4 and the outer diameter thereof. Means, not shown, are provided to scan or move the nozzle relative to the wafer in such manner that all portions of the resistive film on wafer 4 are uniformly abraded. The scanning means may move the nozzle relative to the wafer, or may move the wafer and associated mask 45 while leaving the nozzle 50 stationary.
The abrasive blast or jet 51 from nozzle 50 is directed against the planar upper surface of resistive film 36 (and the planar upper surface of wafer 4) in a direction perpendicular to such upper surface. There fore, the abrasive particles bounce backwardly and may be readily retrieved by vacuum or other means.
After the desired resistive value has been achieved,
as indicated by ohmmeter 48 or an equivalent circuit, it I is merely necessary to connect to wafer 4 the terminal lugs 5 and 6 (or corresponding flexible leads) by means of the rivets 7 and 8. Such terminal lugs may be, for example, gold-plated brass or Monel. There is shown in FIG. 8 the completed resistive element and associated terminals. Such completed resistive element is then, in accordance with the next step, mounted on base 1 (FIG. 4). The thermosetting synthetic resin 11 is then molded over the resistive element, as described in detail in the copending patent application filed on May 25, 1970, in order to complete the resistor.
The resistive film configuration shown in the embodiment of FIGS. 3, 3a and 4 may produce a resistance range, for example, of from ohms to 1,600 ohms (or a much different range depending upon the type of resistive material, etc.). The resistive film shown relative to the embodiment of FIGS. 1, 2 and 2a may produce a resistance value in the range of about 10 ohms to about ohms (or other range where different materials, etc., are employed).
The use of the term wafer, as employed in the present specification and/or claims, is not intended to denote or imply that the insulating substrate for the resistive and metal films is circular or round.
1. A method of manufacturing a resistor, which comprises:
providing an electrically insulating substrate having a substantially planar surface,
providing on said surface, in adherent relationship thereon, termination traces of highly conductive material,
providing a resistive film on said surface in adherent relationship thereon and in electrical contact with said traces,
abrading said film to reduce the thickness thereof but without abrading completely through any substantial portion thereof, and
continuing said abrading step until the electrical resistance of said film is within a predetermined range of values.
2. The invention as claimed in claim 1, in which said method further comprises performing said abrading step by directing against said resistive film, and in substantially perpendicular relationship relative thereto, a jet of abrasive material.
3. The invention as claimed in claim 1, in which said method further comprises performing said abrading step by directing a jet of abrasive material against said film, and scanning said jet and film relative to each other in a manner effecting uniform abrasion of substantially said entire film.
4. The invention as claimed in claim 3, in which said method further comprises directing said jet perpendicularly to said film.
5. The invention as claimed in claim 1, in which said method further comprises employing a ceramic as said substrate, employing metal films as said termination traces, employing a film of complex metal oxides in a glass matrix as said resistive film, and firing said resistive film after application thereof to said surface.
6. The invention as claimed in claim 1, in which said method further comprises the step of electrically connecting terminal conductors to said termination traces, and causing said terminal conductors to extend outwardly from said surface in a direction generally parallel thereto.
7. The invention as claimed in claim 1, in which said method further comprises overprinting said resistive film on at least major portions of said termination traces, whereby to protect said major portions of said traces from abrasion as the result of said abrading step.
8. A method of manufacturing a power resistor, which comprises:
providing a ceramic washer having a central opening and having planar upper and lower surfaces which are parallel to each other,
adherently applying termination traces to said upper surface in the form of metal film regions located at at least a predetermined portion of the circumference of said washer,
adherently applying a resistive film to said upper surface and in electrical contact with said traces, firing said washer and the films adhered thereto, directing a jet of abrasive material against a portion of said resistive film and uniformly scanning said jet and film relative to each other to effect uniform reduction in the thickness of said film throughout substantially all portions thereof, and
continuing said abrading step until the electrical resistance of said film is within a predetermined range of values.
9. The invention as claimed in claim 8, in which said method further comprises directing said jet against said film in a direction perpendicular to said film.
10. The invention as claimed in claim 8, in which said method further comprises employing a ceramic washer having a circular peripheral edge which is concentric with a circular inner edge, the latter edge defining said central opening, employing as said jet a jet having a diameter, at said surface, substantially equal to the radial distance between said inner edge and said peripheral edge, and scanning said jet relative to said resistive film in uniform manner to effect uniform reduction in the thickness of said entire resistive film.
11. The invention as claimed in claim 10, in which said method further comprises directing said jet against said film in a direction perpendicular to said film.
12. The invention as claimed in claim 8, in which said method further comprises providing a mask over said predetermined portion of the circumference of said washer, and performing said abrading step by directing a jet of abrasive material against the upper surface of said washer at all portions thereof except at said mask.
13. The invention as claimed in claim 12, in which said method further comprises making electrical connections to said termination traces at the region beneath said mask, and effecting connection of said electrical connections to a resistance-measuring means whereby to determine when said abrading step has effected a sufficient reduction in the thickness of said resistive filmto cause the resistance thereof to be within said predetermined range of values.
14. The invention as claimed in claim 8, in which said method further comprises connecting radially outwardly extending electrical conductors to said washer at said predetermined portion of the circumference thereof and in electrical contact with said termination traces at said predetermined portion.
15. The invention as claimed in claim 14, in which said connections are effected by means of rivets, and in which said conductors are caused to lie generally in the plane of said resistive film.
16. The invention as claimed in claim 8, in which said method further comprises employing as said washer a ceramic washer, and employing as said resistive film a complex metal oxide in a glass matrix.
17. The invention as claimed in claim 8, in which said method further comprises overprinting said resistive film on at least substantial portions of said termination traces, whereby to protect said termination traces against said jet of abrasive material and thus prevent abrading of said traces.
18. The invention as claimed in claim 8, in which said method further comprises causing said termination traces to include concentric inner and outer trace portions which extend, respectively, around the inner edge of said washer in encompassing relationship relative to said opening therein and around the outer edge of said washer, causing said trace portions to each have at least one end extending to said predetermined portion of the circumference of said washer, and providing said resistive film on said surface in electrical contact with said trace portions extending about said opening whereby to permit radial flow of current between said inner and outer trace portions and through said resistive film.
19. The invention as claimed in claim 18, in which said method further comprises effecting interruption of said resistive film and at least one of said trace portions at a region remote from said predetermined portion of