[1]United States Patent Office 31307,134 3,307,134 ENCAPSULATED IMPEDANCE ELEMENT Edward M. Griest, Painted Post, N.Y., assignor to Corning Glass Works, Corning, N.Y., a corporation of New York 5 Original application Dec. 14, 1959, Ser. No. 859,342. Divided and this application Apr. 2, 1963, Ser. No. 270,507 2 Claims. (C]. 338-237) This invention relates to impedance devices and more 10 partic,ularly to a method of encapsulating or hermetically sealing impedance elements and the resulting strlicture. This application is a division of application Serial No. 859,342, filed December 14, 1959, now abandoned. . 15 Impedance devices, such as resistors, capacitors, or inductors are usually encapsi-ilated to provide the element with a thermal barrier, or to protect the element from attack by excessive moisture, excessive heat or damage by corrosion or to perform the function of electrically in- 20 sulating the element from adjacent elements or, in certain applications, all functions rnay be served. While I shall describe my novel ,process and tha product resulting therefrom in terms of forming an encapsulated resistor, the preferred embod@'@ment, it is to be understood 2 5 that I do not wish to be so limited since the invention is not restricted solely to resistors. The prior art methods of resistor encapsulation, fall into two general categories, the first of -which is a potting method whereby the resistance element is coated with an 30 appreciably thick layer of pottin- material. The pottidg material is usually in a fluid -or semi-fluid state when initially applied to the resistance element, and is subsequently allbwed to harden -a@bout the body of the element to @proiiide the ne@essary protective coating. The 35 other method is one where the resistance element is hermetically sealed in a container that may be either evacuated gr filled with an in6rt atmosphere. In cithef case, all known metbods of sealed resistor fabrication call for the prior formation or manufacture Of 40 the com-plete iesistor and then the subsequent step of either potting or sealing. Both methods have serious drawbacks particiilarly where small, precision, close tolerance, low ohmic, resistance elements are required, in t@at the resistance 6lement is subjected to the relatively 4 5 high temperature of either the potting compound or the encapsulating sealing flame. It is this exposure to relatively high temperature that causes a radical change in resistance. In many instances, this variation may be great enough to cause the resulting resistance to exceed the 50 allowable tolerance and hence be rejected. A high order of rejection brings about @a relatively low selection rate thereby raising the unit cost of the acceptable items. I have found that by combining the resistor fabricatin,a steps with the sealing ste@p, I am able to at)preciably in- 55 crease the selection rate and produce a resistor noted by its high order of accuracy and its relatively low manufacturing cost. It is therefore an important object of my invention to provide a hermetically sealed resistor. 60 Another object of the present invention is to provide a hermetically sealed resistor noted by its high order of accuracy. Still another object of the present invention is to provide a hermetically sealed resistor noted by its reproduci65 bility and relatively high selection rate. A further object of the present invention is to provide a hermetically sealed resistor that is noted by its ease of manufacture. The features of my invention which I believe to be 70 novel are set forth with particularity in the app6nded claims. My invention itself, however, both as to its Patented Feb. 28, 1967 2 organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following descri-ption taken in conjunction with the accompanying drawings in which, FIG. 1 represents an exploded cross-sectional representation of one end of a resistance element indicating the components necessary to form an encapsulated resistor; and FIG. 2 represents a cross sectional representation of one end of my completed resistor in accordance with my invention. Reierring now to FIGS. I and 2 there is shown resistor body 14 having for example, an electroconductive film 16 deposited on the surface thereof. For a clear understanding of film 16, its cbaracteristics and one example of its method of application, reference is made to U.S. Patents Nos. 2,564,706 and 2,564,707 issued in the name of John M. Mochel and assigned to the same assignee as the subject application. Adjacent the end of resistor body 14 is disc 20 to which lead 24 has been welded or affixed in any of many wellknown manners. Affixed to the end of disc 20 facing the adjacent end of resistor body 14 is a ceramic frit 18 having conductive particles of silver, for example, embedded therein. This silver-bearing frit has preferably been affixed to the end of resistor body 14 after film 16 has been placed thereon and it should be noted that a portion of frit 18 covers film 16. In both instances, the silver-bearin.@ frits 18 have been baked on to improve the adhering qualities and in preparation for the subsequent operations. Spaced about lead 24 is a toroidally shaped fusable end portion or b-@ad 26, tof glass or the like material, having a coefficient of thermal expansion compatible with that of lead 24. While bead 26 is herein depicted as toroidally shaped, it will be obvious to those skilled in the art that this bead may be either spherical or shaped like a washer, that is it may be flat. In any event, the outside diameter of bead 26 should be only slightly smaller than the inside diameter of the encapsulating sleeve 12 and should also have a roefficient of thermal expansion compatible with the material used for sleeve 12. Sleeve 12 may be formed of glass or the like material. In accOTdance with the teachings of my invention a suitable resistance element (14, 16) is cut to length to provide the required resistance. If necessary, film 16 may be appropriately spiralled to achieve higher resistances. The ends of the resistance blank are then dipped in a silver-bearing ceramic frit IS which is then baked thereon. Frit 18 is preferably in the form of a slurry and consists of a low melting ceramic binder mixed intimately with silver particles, all of which is in suspension in an organic vehicle such as turpentine. I find that the slurry that has particular utility in this connection, utilizes a ceramic binder having a particle size that will pass through a loo mesh screen yet will be held on a 200 mesh screen, In a preferred fgrm, the total composition of the slurry is about 68 grams of silver, about 10-20 grams of a fritted glass consisting of about 82% PbO, 8% ZnO, 7% B203 and 2% SiO2 with both the frit and shver suspended in about 34 ml. of an organic vehicle such as turpentine. The next step consists of placing the fusable end portion or bead 26 about lead 24 and applying sufficient heat to fuse the bead 26 to lead 24. After applying frit 18 to disc 20 and heating to insure that the frit 18 adheres to disc 20, the lead subassembly is ready for use. The next step consists of placing the surface of frit IS of the lead subassembly in fusable relationship with the surface of frit 18 of the resistance blank and placing sleeve 12 in a spaced relationship, abo@ut the resistance elenient (14, 16) and bead 26. An appropriate sealing flame is then applied to the juncture of sleeve 12 and bead 26 to form the jtinction seal 28 (FIG. 2).
[2]3 At this point it should be noted that during the scaling operation the heat generated by the flame is conducted through sleeve 12, bead 26 and lead 24 to disc 20. Disc 20 is thus raised to a temperature which is s@ufficiently highe.r than the softening point of frit 18 to cause both 5 surfaces of ceramic. frit 18 to become fused. This is a radical departure from what is presently known in the art. The trend in the industry is to conduct heat away from the resistance elements to, avoid injury thereto. Conversely, I choose to utilize the'heat generated by 10 the encapsulating step to perform the funetion of fusing disc 20 to the resistance element (14, 16) by the use of the intervening conductive ceramic. frit 18. By performing the fusing and sealing steps at the same time I am thus able to avoid the need to subject the 15 resistance element to excessive temperatlires during both the resistor fabricating step and again during the encapsulating step. While I have shown and described my invention in terms of forining and sealing only one end of the resistor, 20 it will be obvious to those skil@led in the art that the same steps may be simultaneously -or subsequently applied to the other end to form the completed resistor. Also, while I have desrribed my resistance elements (14, 16) in terms of electroconductively coated resistors, it is obvious that 25 other typesof resistors, such as wire wound resistors, may be here employed equally as well. Other embodiments of my invention that will now bec,ome apparent, reside in the substitution of either a capacitive or an inductive impedance element for the 30 previously dese-ribed resistive impedance element. In the case of the capacitor, the foil or plate ends as well as one end of both terminal leads are coated with the conductive frit and, as in the case of the - resistance blank, the fusion of the foil to the lead takes place during 35 the sealing step. Similarly, to encapsulate an inductive impedance, wire is wound about a form and the wire ends and the form ends are both coated over with the conductive frit. Thereafter, the fusion of the terrninal lead to the inductor is 40 accomplished during the sealing step and the process is identical with the previously disclosed with regard to the resistance element. While I have described what is present@ly considered the preferred embodiments of my invention, it will be @15 obvious to those skilled in the art that various other c.han,,-es and modifications may be made therein without departing from the inventive concept, and it is, th@refore, aimed in the appended iclaims to cover all such changes and modifications as fall within the true spirit and scope 50 of my invention. What is
