Brandt, Ivan L. (Fox Point, WI)
Von Alten, Theodor (Grafton, WI)

05/347883

04/30/1974

04/04/1973

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Allen-Bradley Company (Milwaukee, WI)

338/262

338/329, 338/274, 338/327, 338/276, 338/273

H01C1/144; H01C1/14; H01C1/14

338/262,273,274,276,322,327,329 174/74R 317/260 29/621

Goldberg E. A.

1. In a fixed resistor having a tubular core, a resistive substance on the core, and lead wires inserted in openings of and projecting from the ends of the core, the combination comprising:

2. A resistor as in claim 1, with said low resistance terminating layer

3. A resistor as in claim 2, wherein said silver in glass mix enters the opening at each end of said cylindrical core, each lead wire head fits snugly within the silver in glass mix, and the second solder of said solder bond is dispersed between the lead wire heads and silver in glass mix within the core openings and also between the lead wire collars and

4. A resistor as in claim 3 wherein the lead wire heads are knurled and in

5. A resistor as in claim 1, wherein the low melting solder is substantially 60 percent lead and 40 percent tin, and the high melting

6. A resistor as in claim 2 in which the silver in glass mix contains palladium of 20 percent by weight or less of the silver, glass and

7. A resistor as in claim 1 having a synthetic coating of copolymerized epoxy resin and phenolic resin surrounding the layer of cermet resistance

8. A resistor as in claim 7 having a thin glaze of glass interposed between the cermet resistance material and the copolymerized synthetic coating.

9. A resistor as in claim 2 having a nickel film over the silver in glass mix and a silver film over the nickel before bonding of the higher melting solder, and a bond combining the solder, nickel and silver of the silver

10. A resistor as in claim 7 having an outer jacket of epoxy resin surrounding the synthetic coating of copolymerized epoxy and phenolic resins.

BACKGROUND OF THE INVENTION

The present invention relates to the field of small, fixed resistors such as used in electronic and control circuits.

For many years fixed resistors used in radio, communications and other electronic circuits fell into two principal categories. There were wire wound resistors comprised of many turns of fine wire to develop the necessary resistance. Such resistors could be made with quite precise values, small change in resistance value with temperature variation, and substantial power ratings. They had the disadvantages of short life, inductive characteristics, and limitation in the range of resistance value. The second category was carbon resistors, wherein the resistance was developed either by dispersing carbon particles in a phenolic resin, known as the carbon composition resistor, or by pyrolyzing a thin film of carbon material on an insulating substrate such as glass. Carbon resistors have been low cost, they can be readily produced in both low and high resistance ranges, and the carbon composition resistor in particular has not been susceptible to open circuits.

Metal film resistors were also introduced, and they came into increased usage in and after the late 1940's. They complemented rather than supplanted the inexpensive, mass produced carbon types. Chromium, chromium-nickel, and chromium-cobalt films have been typical, and also a variety of other metals and their oxides have been used as resistance materials. Generally, metal film constructions, while more expensive, maintain their resistance values within narrower tolerances over a period of time, exhibit low temperature coefficients, have low noise levels, and better withstand adverse ambient conditions. They have found application in circuits requiring greater precision, or in special circumstances requiring the enhanced stability which they can provide, and metal film resistors fit into semi-precision and precision classifications. Tin oxide film resistors are made at low cost to be competitive with the carbon types of resistors, and have found a market in applications where precision is not as important as in many scientific and military uses.

In about the mid-1960's the cermet type resistor was introduced. The resistance material in these resistors constitutes a metal system dispersed in glass. There is variation in the compositions used to prepare a cermet, but they are in general formed from a combination of an oxide of a transition metal, such as palladium, and a conductive metal, such as silver. They are not limited to this particular combination and in preparation of a cement the metals are powdered and mixed with finely ground glass. This mixture is in turn combined with a liquid carrier to develop a thixotropic paste. This paste is applied to an insulating substrate, such as a ceramic, and the carrier is driven off by heating. Next, a firing is undertaken in which the metal and glass constituents are combined into a complex material of metal alloy and metal oxide particles dispersed within the glass. The particles are in electrical contact with one another to form a resistance path through the glass matrix, and the particle density and their interface determine the resistance value. Such cermet films are physically rugged and can be produced over a wide range of resistance values. Cermet chemistry has advanced, and the characteristics of precise value, small temperature coefficient of resistance, satisfactory life at higher temperatures, low noise, and stability under changing voltage and adverse ambient conditions are now attainable.

Cermets have been used largely for potentiometers, and thus cermet inks have been applied primarily to flat surfaced substrates over which potentiometer brushes ride. The smooth, hard glassy surface of a cermet may be ideal for making contact with a sliding potentiometer brush, but cermets also exhibit characteristics that make them desirable for small, fixed resistors. Such a resistor is characterized by a small body supporting the resistance material to which a pair of lead wires are permanently affixed. The lead wires must have a very firm and strong union with the resistor body, and it is desirable that the entire construction should be producible at low cost. The present invention provides novel connections between the lead wires and the resistor body for facilitating the use of cermets in fixed resistors, and has for a principal objective the incorporation of the advantages of cermet materials into low cost, fixed resistors.

SUMMARY OF THE INVENTION

The present invention relates to a fixed, cermet resistor, and a method for making such resistor, and the invention more specifically resides in a cylindrical core with openings at opposite ends, a cermet deposited along the outer core surface to form a resistor body with the core, a conductive terminating layer containing a metal-glass mix deposited on the core ends that diffuses with the cermet to function as a continuum thereof, and solder coated lead wires inserted into the substrate openings which have collar portions abutting the core ends, and which have a second, high melting temperature solder forming a union between the lead wires and the conductive terminating layer.

Fixed resistor construction requires a permanent, mechanically strong attachment for the lead wires that does not generate noise, or other electrical disturbance. The junction between the lead wires and the resistive element should also be of a low resistance that does not create hot spots, or interfere with the calibration of the resistor. For cermet potentiometers, the use of a silver-glass mix, with some minor amount of palladium, as a low resistance continuation from the cermet to the point where attachment is made with terminals is of proven utility. For fixed resistors, however, in which extended lead wires are subjected to much more severe bending, twisting, and other abusive forces, attachment of lead wires to a low resistance end termination on the resistor body poses a particular problem. The area for attachment is small, and the region at the ends of the resistor body where attachment is made is unprotected. To obtain a strong and reliable connection between a resistor body and lead wires, the present invention preferably bonds the lead wires to a low resistance terminating material on the resistor core by using a unique solder bond, and in supplementation thereto relying upon an interfitting mechanical union between each lead wire and the resistor core upon which the cermet is deposited.

The insertion of the head end of a lead wire into an end opening of a ceramic core with a close fit, but not too tight to caUse fracture of the core, to establish a mechanical connection between the parts is well known. U.S. Pat. Nos. 2,597,338; 2,977,561 and 3,012,214 disclose this construction, and No. 2,597,338 also shows a radial collar at the head end which abuts the end of the ceramic core. Inserting head ends of lead wires by an axial movement into the ends of a resistor body is also a common practice in the manufacture of carbon composition resistors, as shown in U.S. Pat. Nos. 2,261,916; 2,271,774 and 3,238,490. It is one of the objectives of the present invention to utilize such technique in the manufacture of cermet resistors. The attainment of an adequate mechanical and electrical bond between the lead wires and the resistive cermet layer on the resistor core, cannot, however, rest in a mere tight fit of the parts. The present invention uses a unique solder type bond together with the mechanical insertion of a lead wire head into a resistor core to obtain a union between the lead wire and both the resistance material and the core supporting the resistance material.

In some resistor constructions lead wires have typically been made of drawn copper to which is applied a solder coating of 60 percent lead and 40 percent tin. This solder melts at about 183°C. and plays the role of presenting a solderable surface for making an electrical and physical connection into a circuit. The solder coating is applied prior to attachment of the lead wires to the resistor body, and manufacturing processes must not injure or destroy this solder coating during assembly with the resistor core. In the present invention, heat is required to bond the lead wires in place with the core, and to make an electrical connection with the cermet resistance material. The bond is made with a solder having a higher melting temperature than the 60-40 solder, so that it will not remelt when the resistor is subsequently connected into an electrical circuit. Care is exercised in applying heat locally to the joint between the lead wire and resistor body, so as not to cause the 60-40 solder covering the external lead wire to melt or run. Such care to melt only a higher melting temperature solder has not been exercised in prior solder connections for lead wires. For example, in U.S. Pat. No. 2,977,561 a hard solder connection for a lead wire with a head inserted into a ceramic core is carried out by induction heating, but there is no mention of localizing the heat to avoid melting a solder coat on the lead wires, nor is any such coat mentioned. In U.S. Pat. No. 2,987,813 a resistance heater is discussed which generates heat locally at the end of a resistor body to solder an end cap in place, but no mention is made of a solder coat on the lead wires. Then too, when lead wires are attached by compression welding, as in U.S. Pat. No. 3,329,922, the heat is very local and of short generation. In some disclosures, as in U.S. Pat. No. 2,997,979 no mention is made how heat is applied to a solder that connects a lead wire in place, and in others, such as U.S. Pat. No. 2,213,067 the heat is of a low order insufficient to melt a lead wafer in the vicinity of heat application.

The low resistance termination system at the end of the ceramic core can take alternate forms. In one, the electrical termination at the ends of the body comprises multiple layers of deposited metals with which the high melting temperature solder on the lead wires alloys. After depositing and firing the cermet on the sides of the resistor core, a silver-glass mix with some palladium is applied to the core ends to have a low resistance continuation of the cermet. This low resistance material is similar to that heretofore used in the manufacture of cermet potentiometers. Then, a nickel coating is applied to the silver-glass mix, and finally a very thin silver film is plated over the nickel to inhibit oxide formation which would impair soldering to the nickel. To this multiple layer surface lead wires are bonded with the high temperature melting solder, and the silver film, nickel and solder alloy with one another to form a secure mechanical bond with good electrical qualities. This bond is supplemented by a mechanical fit of the lead wire ends in openings of the resistor core.

In an alternate form, the low resistance end termination takes the form of a silver-glass-palladium mix, coated upon the core ends. The palladium content is uniquely high in amount to inhibit the silver from coagulating, or balling-up, in subsequent heating steps that occur in applying the cermet material. This silver-glass-palladium mix is heated to secure the materials to the core, and to form a uniform layer. Then the cermet is applied to the core sides, and firing of the cermet at elevated temperatures develops a resistive film having a secure ohmic bond to the low resistance end termination. The lead wires are subsequently attached by endwise insertion of the heads into openings in the core, and a solder bond is obtained by localized heating of a high temperature solder, similarly as in the embodiment of the preceding paragraph.

The invention also provides a protective conformal coating of a unique synthetic material applied over the cermet. In one embodiment this synthetic material functions as the final protective coating for the resistor body. In a second embodiment, it is applied as an inner layer in an early stage of manufacture that in turn is subsequently covered by an outer coating of a second synthetic, such as an epoxy resin. In either version it serves as a protection for the cermet material and it is formulated to have a relatively light degree of adhesion with the cermet surface, so that upon polymerization of its constituents forces are not developed which might tear the cermet from the body core, or cause a rupture in the cermet layer. To achieve this goal the coating is formulated of an epoxy and a phenol which exhibits a light, but tenacious adhesion with the resistive cermet film that is less than usual coating compounds such as the epoxy materials.

It is an object of this invention to provide a fixed resistor incorporating a cermet resistance material, and in this connection to have a union between the lead wires and the resistor body that is physically strong to withstand bending, twisting and pull forces to which the lead wires may be subjected.

It is a further object to provide a union between the lead wires and resistor body that does not create electrical noise or hot spots due to any excessive localized resistance.

It is another object to provide a solder connection between a resistor body and associated lead wires, and a method for making the connection, that does not disrupt a solder coating along the lengths of the lead wires.

It is another object to utilize an endwise insertion of lead wire heads into a resistor body for a resistor of cermet material in which the physical union is combined with a strong solder bond.

It is still another object to provide an inexpensive fixed resistor comprised largely of ceramic and metallic materials that are not combustible. In this vein, it is a purpuse to reduce the susceptibility to fire that is more pronounced in other types of resistor constructions.

It is another object to provide an inexpensive, fixed resistor that is precise in its resistance value, that is stable over a long life, and that varies minimally with changes in temperature and ambient conditions.

The foregoing and other objects and advantages of the invention will appear from the following description. In the description reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration and not of limitation two preferred embodiments of the invention. Such embodiments do not represent the full scope of the invention, but rather the invention may be employed in a variety of forms, and reference is made to the claims herein for interpreting the breadth of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view partially in cross section of a fixed resistor embodying the present invention,

FIG. 2 is a schematic view of a step in the process of the manufacture of the resistor of FIG. 1 in which lead wires are about to be inserted into the resistor body,

FIG. 3 is an enlarged view in perspective of the head end of a lead wire as used in the practice of the invention,

FIG. 4 is a schematic view of a soldering jig used in the process of the manufacture of a resistor of the invention,

FIG. 5 is an end view partially in section of the jig of FIG. 4,

FIG. 6 is a view in partial cross section of a resistor comprising a second embodiment of the present invention, and

FIG. 7 is a view in cross section of the resistor of FIG. 5 in a partially completed state of manufacutre.

DESCRIPTION OF FIRST PREFERRED EMBODIMENT

Referring to FIG. 1 of the drawings, there is shown a fixed resistor 1 with a cermet layer 2 forming the resistive element of the resistor. This resistor 1 has an insulating substrate for supporting the cermet layer 2 that is in the form of a circular, cylindrical core 3. This substrate, or core 3 is preferably composed of a ceramic material such as alumina comprising approximately 96 percent aluminum oxide, or it may be composed of some other material used in resistor manufacture that exhibits desired insulating characteristics. The core 3 is characterized by a central opening 4 that extends through its entire length, and it can be formed into its tube-like configuration by either extrusion or pressing. If the material is an alumina it is sintered after forming to form a hard base, similarly as prior resistor cores as exemplified in U.S. Pat. No. 3,329,922.

In the manufacture of the resistor 1 of FIG. 1 an initial step is to terminate the core ends with a thin coating 5 of low resistance material to which lead wires are to be subsequently attached. To develop the low resistance end coating 5 a mixture is prepared of finely ground glass frit, silver flakes, palladium and an organic carrier. Such mixture exhibits an ink-like consistency, and is applied by contacting the core ends against a turning wheel dipping into the ink. First, one core end will be coated in this fashion and by proper control of the viscosity of the ink it will travel a short distance into the central opening 4 of the core 3 and also around the outer circular, cylindrical surface of the core 3 for a short distance from the core end, all as illustrated in FIG. 1. Air is then blown through the core opening 4 from the other end to blow out excessive ink from the opening 4, so as to leave a residual coating 5 inside the opening 4 which will not obstruct entry of lead wires in subsequent steps of manufacture. The applied ink is then dried to drive off the organic carrier, and after drying a like coating is applied to and dried on the opposite end of the ceramic core 3.

After the low resistance end coating 5 is dried on each end of the core 3 it is heated to about 650°C. to melt the glass and obtain a glass-silver mix in which the silver particles are uniformly dispersed and in contact with one another to provide an end termination for the resistor body that is of low, or negligible, resistance. Also, at this temperature the low resistance glass-silver mix will fuse into a firm bond with the surface of the alumina core 3.

The low resistance end coating 5 must be able to withstand subsequent firing temperatures which will occur after the deposition of the cermet paste, from which the cermet layer 2 is formed. It is important that the silver in the coating 5 will not melt during such firing of the cermet, for then it might coagulate, or ball up, so as to lose the necessary continuum of electrical conduction through the coating 5. To obtain a satisfactorily high melting point for the silver an amount of palladium is introduced into the end coating 5, and the amount of palladium is balanced against the selection of a cermet material that can be fired at a relatively low temperature. In this manner the amount of palladium introduced into the coating 5 is minimized.

The glass selected for the coating 5 may be a lead-boro-silicate glass that starts to flow at about 550°C. and which will not decompose until about 1,000°C. It is prepared as a very fine frit that will pass a 360 mesh screen and a typical glass composition may be about 48 percent lead oxide, 28 percent silicon oxide, 10 percent barium oxide, 4 percent titanium oxide, 4 percent cadmium oxide, 4 percent aluminum oxide, 2 percent copper oxide and traces of bismuth oxide, calcium oxide and magnesium oxide.

The silver flakes and the palladium are also finely ground to pass a 360 mesh screen, and the metals and glass are mixed with an organic carrier and binder. The glass, silver and organic materials may be obtained as DuPont Silver Paste 8706 in which the silver comprises about 66 to 69 percent of the mixture, the glass about 3.7 to 5.9 percent, and the remainder are organic constituents. To this silver paste palladium is added in the amount of up to 20 percent by weight of the silver, glass and palladium content, depending upon the degree of control desired over inhibiting the silver from migrating in subsequent steps of manufacture.

The material for the thin cermet layer 2 is first prepared as a paste, as hereinbefore discussed, and mixes within the skill of the art may be utilized. This cermet paste may be applied to the outer surface of the core 3 by rolling the core across an applicator, and it is then heated to drive away the organic carrier forming a part of the paste. Next, the cermet coated core 3 is placed in a furnace and fired at about 900°C. to melt the glass constituent, disperse the metal materials throughout the glass, and to develop a metal and metal oxide system within the glass that exhibits the characteristic cermet resistive qualities. A particular objective is to formulate the cermet 2 so that it may be fired at a temperature that will not melt the silver-glass-palladium end coating 5, but will diffuse into the end coating 5 to obtain a good electrical union between the coating 5 and the cermet layer 2 at the ends of the core 3 where the cermet 2 overlaps the coating 5.

In order to obtain a broad range of resistance values for resistors of the invention it is desirable to formulate a number of cermet pastes exhibiting different ohms per square. For example, in order to offer a range of resistance values up to 22 megohms in resistors having bodies only 0.270 inch long and 0.093 inch in diameter, a range of 5 cermet pastes having 3 ohms per square, 100 ohms per square, 1,000 ohms per square, 10,000 ohms per square, and 100,000 ohms per square have been selected. To obtain a final calibration of the resistance value of a resistor a helical groove is cut in the cermet surface, similarly as in the manufacture of other cylindrical type resistors having carbon and metal films. The cermets having the lower ohms per square have a higher percentage of metal content in the glass, and for these cermet formulations the surface of the cermet layer 2 may have a slight roughness. To alleviate this roughness a thin glass glaze 6 may be applied over the cermet layer 2. Such a glaze 6 may be a finely ground lead-boro-silicate glass mixed in an organic carrier that is applied to the resistor body similarly as the cermet paste was applied. This glass is then dried to eliminate part of the organic carrier and is fired at about 600°C. to obtain a hard, glazed, smooth outer surface. At such a temperature, the temperature coefficient of resistance and other characteristics of the cermet are not disturbed. It has been discovered that such a glass glaze materially enhances the life of a resistor and stability with changes in ambient conditions. After the glaze is applied, then the step of resistance calibration may be undertaken by cutting a helix in the cermet layer 2, and as stated above usual apparatus known in the art may be employed for this purpose.

The next procedure is to attach a pair of lead wires 7 to the resistor body. As shown in FIG. 3, the inner, or head, end 8 of a lead wire 7 is knurled to develop a diamond embossed surface. A radial extending, circumferential collar 9 is formed in each lead wire 7 adjacent the inner end of the knurling which separates the head end 8 from the long, thin shank portion 10 of the lead wire 7. The lead wires 7 are also coated with a low melting solder comprised of 60 percent lead and 40 percent tin, or similar compositions, such a coating being common in the art and functioning to present an adherable surface for solder when attaching the lead wires into an electrical circuit. Such a 60-40 solder customarily melts at about 183°C., and it is a particular purpose of the invention to provide a method of attaching lead wires to their resistor body by a soldering process in which this 60-40 solder is not disturbed. Also, it is a purpose to accomplish this objective with a fairly heavy coat of 60-40 solder of an order of 0.0015 inch thick.

A thin coat of a second solder that melts at a higher temperature is deposited on the knurled lead wire heads 8. Such a layer of solder paste is indicated in FIGS. 1-3 by stippling and the designating reference numeral 11. This high melting temperature solder may be a 90-10 solder comprised of 90 percent lead and 10 percent tin with which is mixed a "white" inert rosin to function as a flux. Such a solder customarily melts at about 307°C.

The next step is to insert the heads 8 of the lead wires 7 into the central opening 4 of the core 3, and this step is illustrated in FIG. 2. The resistor body comprising the core 3 with its cermet layer 2, with or without the glaze 6, and the low resistance end coating 5 is placed in a cylindrical opening 12 of a die block 13. The diameter of the opening 12 conforms closely to that of the outer surface of the resistor body, so that it is accurately positioned. At each end of the opening 12 there is inserted a centrally bored, circular, cylindrical driving pin 14, and in the central bore of each pin 14 there is loaded one of the lead wires 7. The head end 8 of each lead wire 7 faces the central opening 4 of the core 3 and is accurately aligned therewith, so that it may be translated into the opening 4. The die block 13 is heated to raise the temperature to about 180°C., and the lead wires will be at a lower temperature that is below the melting point of the 60-40 solder that coats their full length. This heat is sufficient to soften the "white" rosin flux of the 90-10 solder. The driving pins 14 are now brought toward the resistor body, and the knurled, solder coated head ends 8 are driven into the central opening 4 of the core 3 so that the collars 9 abut against the end coatings 5. The diameter of the knurled, head ends 8 with the solder coat 11 is carefully dimensioned with respect to the core opening 4 to have a friction or interference fit in which the knurling is slightly upset to develop a firm mechanical adherence. In addition, the rosin of the 90-10 solder functions as a temporary adhering agent to attach the lead wires 7 to the resistor body with sufficient strength for subsequent steps of manufacture. The interference fits will also translate some of the 90-10 solder onto the faces of the lead wire collars 9. Upon fastening the lead wires 7 in the resistor core 3 the driving pins 14 are withdrawn, and then the resistor unit is removed from the die block 13. This procedure of joining lead wires 7 with the resistor body is similar to steps described in U.S. Pat. Nos. 2,271,774 and 2,302,564, in which headed lead wires are driven into phenol formaldehyde composition type resistor bodies employing dispersed carbon particles as a resistance material.

The next step is to heat the regions occupied by the 90-10 solder to melt this solder for obtaining a strong bond between the lead wires 7 and the resistor body. Such bond will also exhibit good electrical conduction between the lead wires 7 and the low resistance end coatings 5. A preferred procedure that may be employed for this ensuing step is illustrated in FIGS. 4 and 5. A holding jig 15 is provided which has a base block 16 upon which a pair of soldering iron arms 17 are mounted. The arms 17 are formed of flat, heat conductive stock, and the upper ends are notched with V-shaped slots 18 to receive resistor lead wires 7 for a group of three resistors 1. One of the arms 17 is fixed in position, and the other arm 17 is pivotally mounted and is biased by a spring 19 to have its upper end pivot toward the fixed arm. In this fashion, the arms 17 will bear firmly against the lead wire collars 9 of resistors inserted in the jig 15, and heat applied to the arms 17 will be transmitted from the arms 17 directly to the collars 9 and the 90-10 solder.

To supply heat for melting the 90-10 solder a set of three focused heat lamps 20 are disposed about the holding jig 15 to apply heat to the sides of each of the two soldering iron arms 17 and to the resistor bodies located between the arms 17. In this manner, heat is conducted to the regions of the 90-10 solder, while at the same time the shanks of the lead wires 7 are kept relatively cool, so that the 60-40 solder will not melt. The 90-10 solder has its temperature raised to the melting point, so that solder bonds are formed with the low resistance end coatings 5. By a capillary action the 90-10 solder will run along the entire interface of the collars 9 and the coatings 5, and also between the knurled head ends 8 and the portions of the coatings 5 that extend into the core opening 4. The solder will not migrate down the ceramic surface of the opening 4, because of the non-wetting characteristics of the alumina comprising the core 3. Thus, shorting between the lead wires 7 is avoided.

The jig 15 is attached to a link of a conveyor chain 21 by means of a machine screw 22. By connecting a number of jigs 15 to an endless chain a conveyor can be provided for conveniently transporting resistors 1 into the region of the heat lamps 20, and the lamps 20 can be arranged for more effective heating. For example, the resistors 1 may first be brought beneath a first upper heat lamp 20 that is focused for preheating the resistor bodies, and then the resistors 1 may be transported into a position between a set of three lamps 20, as particularly shown in FIG. 5. In this position the side lamps 20 are focused to heat a horizontal line along the soldering iron arms 17. The heat will migrate upwardly through the arms 17 and into the lead wire collars 9 at the vicinity of the 90-10 solder. Heat will also migrate downward into the block 16 which functions as a heat sink. By proper positioning of the parts of the structure heat can be pumped into the soldering iron arms 17 with a high temperature at the line of focus, in the order of 800°-1,300°F., to establish a steep heat gradient across the arms 17 that causes a rapid transfer of heat into the locality of the 90-10 solder. At the same time the 60-40 solder on the long shanks of the lead wires 7 is maintained beneath 183°C. and the coating of this 60-40 solder is left undisturbed. This result is achievable with a relatively thick 60-40 coat, as well as with a very thin coat. For purposes of illustration, only one jig 15 is shown in FIG. 4, but in actual practice they would be mounted immediately adjacent one another to have a conveyor without interruption.

The remaining step is to apply a protective, conformal coating 23 to the resistor, such as is illustrated in FIG. 1. Such coating must adhere firmly, but should not have such strong adhesion with the cermet layer 2 that it may tear or rupture the cermet, particularly during the stage of polymerization of the coating material. To achieve this goal it has been discovered that an epoxy resin polymerized with a phenolic resin will exhibit desirable characteristics. To these resins there is added a filler and solvents to develop a consistency suitable for application and a coloring pigment may also be added. The mixture is applied to coat the surface areas as indicated in FIG. 1, and then surface heat is applied to initiate polymerization and to cure the resin. Several layers may be applied in this fashion to develop a final coating 21 of desired thickness. An example of a formulation for the coating 23 is as follows:

Portion (Parts by weight) Epi Rez - 520C (epoxy resin) 450 Eppon 1007 (epoxy resin) 450 Bakelite 3485 (phenolic resin) 300 Silica "O" (filler) 875 Cellosolve Acetate (ethylene glycol monoethyl ether ethyl acetate) 950 Methyl Ethyl Ketone 216 Alpha Terpenol 20 Pigment Dragenfeld 10363 60 10390 60

In the foregoing example, the phenolic resin is 25 percent by weight of the total resin content. This amount can vary from about 15 percent to 50 percent. Another characteristic of the above formulation is a heavy loading of a filler in the form of a silica to give body and a low coefficient of expansion. A 30 percent to 65 percent by weight of the filler and resins is a desirable range for the amount of the filler.

DESCRIPTION OF SECOND PREFERRED EMBODIMENT

In the embodiment of FIGS. 1-3 the cermet layer 2 was applied to the core 3 after the low resistance end terminating coating 5 was applied. The coating 5 had a substantial palladium content to allow the silver and glass of the coating 5 to withstand the firing temperature of the cermet 2 without coagulation of the silver. An alternative arrangement is illustrated in FIGS. 6 and 7, in which a cermet layer 24 is first applied to a substrate, or core, 25, and thereafter the low resistance end termination is applied.

FIG. 7 shows the second embodiment in partially completed form. Along the entire length of the outer, cylindrical surface of the tubular core 25 a cermet paste is applied and fired at temperatures that can be higher than for the first embodiment, because of the absence of any end termination, the firing melting the glass particles, dispersing the metal materials throughout the glass and developing a resistive characteristic, similarly as in the first embodiment. By firing at the proper temperature the temperature coefficient of resistance for the completed cermet may be held to a low value, such as within 50 ppm per degree centigrade over a substantial temperature range. Such manufacture of the cermet is within the skill of that art, and in of itself is not a part of the present invention. The desired temperature coefficient of resistance is also achieved in the first embodiment of the invention.

Next, a silver-glass low resistance terminating layer 26 is applied to each end of the core 25. The silver-glass material may be the same as in the end coating 5 of the first embodiment, except for the palladium content. The need of palladium for inhibiting silver agglomeration during firing a cermet is eliminated, but a low content of palladium, such as five percent by weight of the total mix excluding the carrier and solvents, is still beneficial to inhibit silver migration during usage of the completed resistor. The silver-glass mix is heated at about 650°-750°C. to melt the glass and disperse the silver therein, and also to soften the glass within the cermet layer 24 to mix at the interface with the low resistance layer 26 to obtain good electrical conduction between the layers. This temperature of heating is maintained at a low enough value not to upset the chemistry of the cermet layer 24.

In the resistor body of FIG. 7, the next step in the manufacture was cutting a helix to adjust the resistance to desired value. If desired, this step can be deferred until later in the process of making the resistor. It is, however, one of the more costly steps, and from a cost standpoint is preferably carried out late in the manufacturing process. As mentioned above, cutting a helix is a common practice, and is exemplified in U.S. Pat. No. 3,329,922 for the manufacture of a metal film resistor and in U.S. Pat. No. 2,597,338 for a carbon film resistor.

At this stage in the manufacture of the resistor of FIGS. 6 and 7 a resinous protective undercoating 27 is applied over the cermet 24, which leaves the end terminating layer 26 exposed. This undercoating 27 is formulated of an epoxy-phenolic mix like that of the conformal coating 23 in the first embodiment. Hence, the undercoating 27 has a relatively low adhesion with the cermet 24, so that it will not rupture or pull the cermet 24 from the ceramic core 25.

After the application of the protective undercoating 27 a very thin film of nickel 28 is applied over the low resistance terminating layer 26 by electroplating. The "barrel" coating method of plating is preferable for this step, and the purpose of the nickel 28 is to provide a better surface for making a solder connection with the lead wires that will be subsequently attached, than the surface presented by the silver-glass of the layer 26. The silver-glass of this embodiment, having been fused at a lower temperature than in the first embodiment, does not have the same satisfactory solderable characteristic, and to improve the solderability the nickel film 28 is laid down. To protect the nickel 28 from oxidation, which would detract from its solderable character, a flash coating of silver 29 is applied over the nickel 28, and this may also be deposited in place by the "barrel" electroplating technique.

The next step is to attach a pair of lead wires 30 to the ends of the resistor body. This is done in the same manner as for the first embodiment, and the wires 30 are shown in place in FIG. 6. After attachment of the wires 30 a final insulating resin 31 is applied over the undercoating 27 and around the resistor body ends, as is also illustrated in FIG. 6. This resin 31 is preferably an epoxy formulated similarly as other epoxy coatings for electrical components, and it is heated to polymerize the material to obtain a hard, thin jacket enveloping the body proper of the resistor.

For both embodiments disclosed herein, a fixed cermet resistor is provided which has a low resistance end termination characterized by a silver in glass coating which extends between the resistive cermet and the attached lead wires. A mechanical and electrical bond for the lead wires is comprised of a solder connection utilizing a high temperature melting solder separate and distinct from a low melting temperature solder coating the entire lead wire length. The dual solders perform their functions of setting up physical attachments at different temperatures. A soldering step is provided for the higher melting solder in which heat is localized, so as not to disturb the lower melting solder.

A collar is provided as part of the lead wire heads to present a solderable surface, and heat may be conducted directly to this collar by conduction through soldering iron members which are in direct contact with the collar. The soldering iron members also function to isolate the lead wire shanks from heat applied to the resistor bodies during the soldering step. Thus, the invention employs a dual solder system for effecting a termination for the lead wires of a fixed, cermet resistor.