US3474375A

US3474375A - Precision resistor

Info

Publication number: US3474375A
Application number: US741409A
Authority: US
Inventors: John P Smith Jr
Original assignee: Vishay Intertechnology Inc
Current assignee: Vishay Intertechnology Inc
Priority date: 1968-07-01
Filing date: 1968-07-01
Publication date: 1969-10-21
Anticipated expiration: 1986-10-21
Also published as: FR1594338A; DE1900425C3; DE1900425B2; DE1900425A1; GB1219515A

Description

PRECISION RESISTOR Filed July n.,

ATTORNEYS United States Patent O U.S. Cl. 338-231 4 Claims ABSTRACT F THE DISCLOSURE A stable precision resistor which includes a resistor element comprising a resistive metal lm, having elastic properties which obey Hooks law in tension and compression, on a rigid dielectric substrate which is mounted inside a hermetically sealed oil containing housing.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is related to the commonly assigned applications of Felix Zandman and Branin A. Boyd, Ser. No. 453,098, led May 4, 1965 and of John P. Smith IR., tiled concurrently herewith.

BACKGROUND OF THE INVENTION The present invention relates to electrical resistors, and especially to resistors of extremely high precision and stability.

An object of the invention is to provide resistors of such extraordinary stability with respect to a wide range of ambient conditions and such permanence of characteristics as to be suited for use as resistance standards and to serve other exacting needs.

Various attempts have been made in the past to construct high precision resistors of specially prepared wire. Winding the wire on spools introduced problems of shortterm and long-term stability of the spools themselves, as to their mechanical and electrical properties. In attempts to obviate spool difficulties, the wire has in some cases been loosely rbunched in a container, rather than being wound on a spool. Special alloys have been used, and attempts have been made to minimize the strains in the wire. Problems of strength and permanence of structure arise with such resistors. Furthermore, reactance problems are encountered at high frequencies, because of inductance of the wire, varying with the positioning thereof, and capacitance between the portions of the wire.

In the copending allowed application of Felix Zandman and Branin A. Boyd, cited above, a resistor element is described which overcomes many of the disadvantages of the best resistors formed of wire. A dielectric substrate, such as a thin glass wafer which may be of the order of onefourth inch square, is provided with a resistive metallic lm in which a relatively long resistive path is established. As there described, epoxy coatings may be applied to such a substrate, and may be symmetrically disposed to avoid warping tendencies. By taking into account the `ditferent temperature coetlicients of expansion of the epoxy coatings and the substrate, and appropriately relating thereto the temperature coefficient of resistivity of the resistive metallic lm, the resulting uncased resistor element may be made to have an overall temperature coeicient of Aresistivity of the order of three parts per million per degree centigrade over a wide temperature range such as the range from 25 C. to 125 C. Also described in said application of Zandman and Boyd is the encapsulation of such a resistor element in a plastic, ceramic, or metal housing, which is impermeable to vapor transmission, wherein the resistor unit itself is surrounded by a sheath of rubber, polyurethane foam or other soft material, t-he 3,474,375 Patented Oct. 21, 1969 ICC remainder of the space within the housing being filled with a hard ller such as an epoxy.

The resultant encased resistor is characterized by shortterm and long-term stability characteristics greatly superior to those of other types of resistors. Along with its extremely low temperature coefficient of resistivity, it possesses stability with respect to the effects of moisture and pressure on the housing which is high enough for many exacting requirements. In accordance with an important feature of the present invention, a resistor element consisting of a lm on a substrate, which may be constructed as described in said prior application Ser. No. 453,098 is so housed as to isolate it completely from the effects of moisture and variations of pressure on the case, In order to achieve this goal, the resistor element consisting of the substrate and the resistive metallic lm thereon, which has elastic properties obeying Hooks law in tension and compression, is immersed within a selected oil and housed in a hermetically sealed casing, the arrangement of the resistor element being such as to minimize the transmission of any mechanical forces to the element from the housing.

DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS The invention will now be described in reference to the appended drawings, wherein FIG. l is an exploded view of one embodiment of the present invention and FIG. 2 is a detailed view showing arrangement of an enclosure element applicable to the structure of FIG. l; FIG. 3 is a view of the completely 'assembled resistor unit lcorresponding to FIGS. 1 and 2; FIG. 4 is a cross-sectional View of an alternative embodiment of the resistor unit arranged to include a plurality of resistor wafer elements; FIGS. 5 and 6 are an isometric view and a bottom view, respectively, of a multi-cavity dielectric body included in the apparatus of FIG. 4; FIG. 7 is an isometric view of a third embodiment of the invention, a portion of the case being broken away to expose the interior to view; and FIG. 8 is an exploded view of the apparatus shown in FIG. 7.

Referring now to FIG. l, a metallic cylinder 11 is arranged to receive two end pieces, 13 and 14, each of which consists of a glass disk having a surrounding metal ring fixed to its periphery by a glass-to-metal seal. Each disk also includes a metal eyelet centrally located therein, and similarly bonded to the glass by a glass-to-metal seal. The surrounding metal ring of each end of the disk is arranged to be bonded by solder at its periphery to an end of the metal cylinder 11. Preferably, a minute shoulder (not shown) is formed in each end of the metal cylinder 11 to facilitate the accurate positioning of the end disk with its surrounding metal ring. The central eyelet of each end disk is also arranged to project a short distance from the Surface of the glass and is prepared to be bonded by solder to a terminal wire lead of the unit.

As one example of mutually compatible glass and metal materials, borosilicate glass can be used with kovar metal.

The assembly to be housed within the cylinder 11 between the end disks 13 and 14 consists of a very small printed circuit board 21 upon which is supported, by its

flexible leads

15 and 20, a resistor element 22 comprising ya rigid dielectric substrate having a resistive metallic lm, having elastic properties which obey Hooks law in tension and compression, atlixed thereon. The arrangement and construction of the resistive element 22, arranged for a relatively long conductive path between the junctions of the

flexible leads

15 and 20* is described in the copending Zandman and Boyd application, Ser. No. 453,098.

Also fixed to the printed circuit board 21 are

wires

26 and 27 which are arranged to serve the dual purposes of supporting the printed circuit board 21, and constituting the ultimate terminal Wires or lead wires of the completed resistor.

The enclosure, consisting of the metal cylinder 11 and the end disks 13 and 14 peripherally bonded to the cylinder and centrally bonded to the

lead wires

26 and 27, respectively, is almost entirely filled with a suitable oil, a suitable material for this purpose having been found to be Dow Corning #200 Silicone Oil. A very small pocket of gas, such as dry air, is provided in order to accommodate differential expansions or contractions of the housing and the oil contained therein.

The steps involved in assembling the resistor of FIGS. 1, 2, and 3 will now be described. Initially, one of the end disks 13 is inserted in cylinder 11 and its metal ring is soldered in place, making a seal around the periphery. The resistor unit 22 is connected to the printed circuit board 21, the end of flexible lead 1S being soldered to lthe printed circuit conductor which extends to lead 26, and the end of flexible lead being soldered to the printed circuit conductor leading to the junction of lead 27. The leads 1S and 20 are bent after being bonded to the respective conductor portions of the printed circuit board 21. Preferably, before inserting the assembly including resistor 22, printed circuit board 21, and the ultimate resistor terminal leads 26 and 27 into the cylinder 11, this cylinder is lined with a thin layer of Teflon insulating material to insure against any accidental electrical contact between either of the units 21 or 22 and the inner cylindrical wall of the housing.

The lead wire 26 is inserted through the eyelet of disk 13 as the sub-assembly, including the printed circuit board 21 and the resistor 22, is moved to the left and into the interior of the cylinder 11. When the sub-assembly, including parts 21 and 22, is approximately centrally located within the cylinder 11, a soldered junction is formed between the central eyelet of disk 13 and the lead 26, completing hermetic sealing of the left end of the resistor unit.

Next, the opposite end disk 14 is slipped over the terminal lead wire 27 and moved into position in the end of cylinder 11 opposite the first end disk 13. The peripheral metal ring of end disk 14 is soldered to the cylinder 11 around its entire periphery by dipping this end of the cylinder (along with the projecting terminal lead 27) into a hot solder bath. During this operation, the solder is prevented from bonding the wire 27 to the eyelet of the right hand disk 14 by the heated air escaping through said eyelet from the interior of the cylinder 11.

The unit as thus far assembled has the same appearance essentially as shown in FIG. 3, the only things lacking at this stage being the almost complete filling of oil in the solder junction between terminal lead line 27 and the eyelet of end disk 14. The structure is then placed within an evacuation chamber in which is a reservoir of the silicone oil. In that chamber, the air contained within the structure is substantially completely exhausted, and by virtue of immersion of the unit in the silicone, restoration of atmospheric pressure causes the silicone oil to be drawn into the housing substantially filling it. Before making the final solder junction between the terminal lead wire 27 and the righthand end disk 14, the temperature of the unit is elevated to approximately 125 C., at which temperature the silicone is expanded to a greater than normal volume. Thereafter, cooling of the unit to room temperature results in the ingress of a very small amount of air, to provide the desired pocket for expansion and contraction. The product is completed by forming the solder junction between the terminal lead wire 27 and the eyelet of end disk 14.

For some ranges of resistance, and for increased heat dissipation capacity, it is desirable in some instances to use several resistor elements within the housing defined by cylinder 11 and end disks 13 and 14. In FIGS. 4, 5, and 6, a modification is illustrated wherein there are provisions for connecting three separate resistor units, each like the resistor element 22 in FIG. l, in a series and housing them within the oil bath in the enclosure defined by cylinder 11 and end disks 13 and 14. For this purpose, a plastic block 41 having a plurality of cavities 42, 43, and 44, may be provided and so arranged as to be inserted within the metal cylinder 11. The plastic block 41, shown in cross-section in FIG. 4 and its oblique and bottom views in FIGS. 5 and 6-, respectively, provides for the positioning of a plurality of the resistor elements in respective ones of its cavities, and for the connection of their respective flexible leads through holes in the bottom of the block 41 to a pattern of printed circuit conductors on the bottom of said block (FIG. 6). As shown in FIG. 6, the pattern of conductors on the bottom of the block 41 may be so arranged as to cause the plurality of resistor elements to be connected in series with each other between the left hand terminal lead 26 and the right hand terminal lead 27.

The internal dimensions of the cavities 42, 43 and 44 are substantially greater than the respective dimensions of the resistor element, such as the resistor element 22 of FIG. 4. Accordingly, when the resistor element is properly positioned and installed within its cavity of the block 41, it is surrounded entirely by oil, and is free from any mechanical forces exerted by the block 41 or any portion of the enclosures cylindrical or end parts.

In FIGS. 7 and 8 is shown an alternative construction of a resistor in accordance with this invention. Referring now to FIGS. 7 and 8, a single-cavity of a molded plastic container 51 is arranged to accommodate a single film-onsubstrate wafer resistor 22, the cavity again being substantially larger in all its dimensions than the dimensions of the resistance wafer element. The housing for the resistor of FIGS. 7 and 8 comprises a relatively narrow metal cup 52 open only at its lower end, and an end closure unit 53 comprising a glass body 54 having two

eyelets

56 and 57 bonded thereto by glass-to-metal seals, along with a peripheral metal bond 58 also joined to the glass in a glass-to-metal seal.

In this form of a resistor, the terminal lead wires 26' and 27 are both extended downward from the bottom of the resistor unit parallel to each other.

The single-cavity body 51, unlike the molded plastic unit 41 on the multi-cavity resistor embodiment, may omit any printed wiring pattern and rely instead upon mere passages or bottom holes through which the resistor terminal leads are to extend.

The assembly of FIGS. 7 and 8 is made up by rst bonding the ends of the relatively short

flexible leads

15 and 20 from the iilm-on-substrate resistor element 22 to the upper ends of the terminal lead wires 26' and 27. These terminal lead wires are -then passed downward through the cavity of the molded body 51, and extended through the holes in the bottom of said body. The terminal lead wires 26 :and 27' are then passed through the

metal eyelets

56 and 57 which are sealed to the glass 54 of the end closure unit 53.

A very small layer of Teflon, or other suitable insulating material, (not shown) is inserted within the metal can 52 and made to lie against the closed end thereof. The molded body 51 having the resist-o-r wafer element 22 enclosed therein is -moved up into the interior of the can 52, and the end closure unit 53 is next brought into a position on the lower end of the can 52, where it is ready to be soldered in place.

The unit is Ithen soldered by dip soldering, to form a continuous and complete bond 4between the lower end of the can 52 and the metal ring 58 bonded to the periphery of the lower end closure unit 53 and forming a part thereof'. In this dip-soldering process, one of the eyelets in lthe lower end of the closure unit will be solder-bonded to the terminal lead wire passing therethrough, but the other will be kept open by the emerging air due to the rising temperature within the can 52. Again, the process of substantial evacuation is followed by filling with the silicone filler at the elevated temperature, after which the silicone contracts leaving a small void sufficient to allow for the differential expansion and contraction of the case and the silicone filling. The final step, in this embodiment, as in the previous embodiment, is the soldering of the open eyelet to the terminal wire lead passing therethrough.

The resultant resistor in any of the embodiments illustrated in the drawings is better capable of standing shock and vibration than any of the loosely wound wire precision resistors, and is at least as good as the spoolwound precision resistors. In contrast to both such types of wire resistors, it has a minimum of reactive effect, its inductance being typically as low as or lower than 1/10 microhenry, and its distributed capacitance being typically as low as or lower than 1/2 micromicrofarad. By virtue of such extremely low reactance factors, the resistor in accordance with the present invention remains at a substantially unity power factor at frequencies far greater than at the frequencies up to which the resistors constructed of wire may be used, with -or without attempts at inductance cancellation arrangements.

Whereas, the prior form of wafer resistor, embedded in a soft cushion which in turn is within an epoxy filling in a case having epoxy end seal or seals, has some susceptibility to moisture effect and to variations of pressure ent invention is rendered independent of surrounding have as much as 50 p.p.m. change of resistance due to moisture even steam under pressure) and independent of unit is used, the resistor unit in accordance with the presof the medium (frequently the atmosphere) in which the encased wafer resistor, with its resistive film path, could pressure of 'the surrounding medium. The former type of change of pressure, and as much as 400 p.p.m. change of resistance with moisture under extremely adverse condi- `ti-ons. In contrast, the resistor in accordance with the present invention demonstrates insufficient change of resistance in response `to either of these effect-s to be measurable with extremely sensitive measuring equipment.

Also, while the change of resistance value with aging (for example, aging on the shelf) is less for the previously described epoxy-sealed resistor from that which applies to an unencased wafer resistor unit, the resistor constructed in accordance with the present invention demonstrates such extraordinary freedom from a drift with aging as to show no measurable a-mount on equipment arranged for checking to the resolution of one p.p.m. In this respect, also, the precision resistor in accordance with the present invention not only is at least an order of magnitude'better than the encased wafer resistor heretofore developed, but also is an order of magnitude better than the precision resistors which have been made using specially prepared wire, either loosely inserted or spooled.

The resistor, in accordance with the present invention, has the properties of a resistor standard. Inasmuch as 'the resistor is extremely compact, it is possible to prepare -a versatile, multivalue resistor standard by the use of switching means, as set forth in the above-identified ap plication of John P. Smith, Jr.

It will be understood that various modifications can be made without departing from the invention.

What is claimed is:

1. An ultra stable precision resistor comprising:

a housing impermeable to vapor transmission;

at least two resistor terminal wires extending outward from said housing;

at least one of said wires being insulated from said housing;

a resistor element within said housing including a rigid dielectric substrate having la resistive metal film thereon and a pair of flexible leads connected to said resistive film and extending from said substrate;

said resistive film having elastic properties which obey Hooks law in tension and compression;

means including said flexible leads for establishing a resistive path between said two resistor terminal wires through said resistive film;

means for hermetically sealing said housing at at lea-St one end thereof and affording insulated support for at least one of said resistor terminal wires;

said means including a glass portion sealed by glass-tometal seals at its periphery and at the point of support of the resistor terminal wire;

and an oil medium sealed within said housing surrounding said resistor element and filling the majority of the enclosed volume.

2. An ultra stable precision resistor as defined in claim 1, wherein said housing is cylindrical and said resistor terminal wires extend axially in opposite directions from said housing, said means for hermetically sealing said housing including a pair of symmetrically arranged end closure units each comprising a glass disk with peripheral and central metal-to-glass seals.

3. An ultra stable resistor as defined in claim 1, wherein said housing is a cup and said -means for hermetically sealing said housing comprises a glass body through which both said resistor terminal wires extend.

4. A precision resistor comprising:

-a metal housing;

two resistor terminals including at least one resistor terminal conductor extending outward from said housing and insulated therefrom;

a resistor element within said housing including a rigid dielectric substrate having a resistive metal film thereon and a pair of connecting leads extending from said substrate for establishing a circuit path through said film;

means providing conductive paths between said connecting leads and Said two resistor terminals;

means for hermetically sealing an end of said housing and affording an insulated support for said resistor terminal conductor, said last named means including a glass portion sealed by metal-to-glass seals both at its periphery and at the point of support of the resistor terminal conductor;

and an oil medium sealed within said housing surrounding said resistor element and incompletely filling the enclosed volume.

References Cited FOREIGN PATENTS 1,111,248 4/ 1968 Great Britain.

ELLIOT A. GOLDBERG, Primary Examiner U.S. Cl. X.R.