US3405381A - Thin film resistor - Google Patents

Description

Oct. 8, 1968 F, z-ADMAN ET AL.

THIN FILM RESISTOR Filed May 4, 1965 2 Sheets-Sheet 1 rmvh,

ATTORNEY f Oct. 8, 1968 F. zANDMAN ET AL 3,405,381

THIN FILM RESISTOR Filed May 4, 1965 2 Sheets-Sheet 2 gran ahi. 'ay

afnam Arronne United States Patent O M 3,405,381 THIN FILM RESISTOR Felix Zandman, Villanova, and Branin A. Boyd, Malvern, Pa., assignors, by mesne assignments, to Vishay Interteclinology, Inc., Malvern, Pa., a corporation f Pennsylvania Filed May 4, 1965, Ser. No. 453,098 6 Claims. (Cl. 338-254) ABSTRACT 0F THE DISCLOSURE A precision resistor having a predetermine-d pattern of thin metallic film supported on a substrate and oppositely disposed coating positioned on each side of the substrate to eliminate the effect of dimensional changes.

Background ofthe invention The present invention relates to electrical components and especially resistors. It is particularly concerned with high-precision components, such as resistors capable of very high stability and a low, closely controlled temperature coeflicient of resistance.

In the past, attempts have been made to construct precision resistors by winding wire of a specially selected alloy upon a rigid for-m. The alloy selected was a material having a low temperature coeiiicient of resistivity. It was found that variations of temperature caused some dimensional variation in the form, and accordingly caused changes in the dimensions of the resistive wire element. These resulted in undesirable changes of resistance. Such units not only have disadvantages of sensitivity to ambient temperature but also yare difficult and expensive to construct and to adjust to predetermined precise resistance values. Furthermore, it is very difficult to construct them in such a way as to avoid appreciable inductance, Moreover, they lack compactness. Any attempt at miniaturization greatly increases the difficulty of manufacture and hence the cost of such resistors.

One approach toward eliminating the effect of dimensional changes in the support for a precision alloy wire resistor has been the elimination of rigid support for the wire. The resistive wire was suspended loosely in a mass within lan oil-filled chamber, in order that it would be free from any stresses. Such an arrangement, however, is difficult to make and presents problems with respect to sealing. Also, it involves special problems of insulating the portions of the resistive wire from each other.

It is also known to produce -a resistor by photo-etching of a metal lilm on a substrate. However, such a resistor, especially in small sizes, introduces problems of temperature compensation and stability.

Summary of the invention An object of the present invention is to provide a precision electrical component such as a resistor overcoming the foregoing disadvantages.

It is an object of this invention to provide a precision resistor which is reliable, economic-al to construct, capable of being made in extremely small sizes, substantially free of inductance, .possessed of great short-term and long-term electrical and mechanical stability, and having a predetermined tightly controlled temperature coeliicient of resistance.

It is a further object to provide a resistor capable of yielding extremely high stability with respect to variations of ambient temperature and moisture.

In accordance with the present invention, a high-precision resistor is constructed by supporting a thin film of a selected metal alloy upon a substrate having known physical properties, the substrate being many times thicker 3,405,381 Patented Oct. 8, 1968 ICC than the metallic film (preferably, of the order of to 1,000 times thicker). The metallic film is caused to have a predetermined pattern, such that electric current flows along a conducting path of very great length and extremely small width, this pattern preferably including a great number of parallel narrow linear path portions in a planar array. The side of the substrate having thereon the resistor film of predetermined pattern is coated with an epoxy resin, and the opposite si-de of the substrate is similarly coated to equal thickness with epoxy resin of the same kind. These oppositely `disposed coatings of epoxy resin provide symmetry with respect to their mechanical effects upon the substrate, so as to avoid any tendency to cause bending thereof. Similar balance can be obtained with unequal coatings of different characteristics. Moreover, the epoxy resin on the conductive layer reinforces the resistive path and protects it from mechanical damage, and also protects it from any skin acids which might otherwise reach it in the course of being handled.

Brief description of the drawings An embodiment of a resistor in accordance with the present invention is illustrated in the accompanying drawings, wherein:

FIG. l is a plan view of the resistor;

FIG. 2 is a cross-sectional view of said resistor taken on the line 2-2;

FIG. 3 is a magnified cross-sectional view showing in greater detail the encircled portion of FIG. 2;

FIG. 4 is a magnied fragmentary view diagrammatically illustrating one way in which lthe path resistance rnay be adjusted; and

FIGS. 5 and 6- are a perspective view with part broken away and a cross-sectional view, respectively, of an encapsulated resistor embodiment of the present invention.

Description of the preferred embodiments In a preferred embodiment, `a resistor o'f the present invention, as illustrated in FIGS. l, 2 and 3, comprises a substrate 11 having an etched-pattern resistor layer of bulk metal iilrn 12 fixed to one surface thereof, Coatings 13 and 14 of a hard epoxy resin are `applied to the resistor surface and the opposite surface of the substrate. The two opposite epoxy coatings are so related to each other `as to result in a sandwich which does not bend or warp as a consequence of changes of temperature or moisture absorption by the coatings.

The substrate 11 may be made of glass having a ternperature coeiiicient of expansion of the order of 3 parts per million per degree F. In an illustrative resistor unit, substrate 11 may be of the order of 1A inch by 1A inch with a thickness of 0.04 inch.

The bulk metal film 12 may be made from a resistive alloy such as one of the Nichrome alloys, wherein nickel and chromium are the principal metals. This film may 1be of the order of 0.0001 inch thick.'

The metallic lilm 12 is photo-etched to a pattern which establishes a narrow conductive path of much greater total length than the dimensions of the face of substrate 11. This step may be carried out after the film has been bonded to the substrate as by the laver 13 of plastic thereunder (FIG. 3), o-r it may be carried out when the metallic film is on a thinsupport suc-h as plastic layer 13 but before being bonded to substrate 11. To perform the photo-etching, the `face of the thin alloy lm opposite the layer 13 is covered with a photosensitive masking medium such as Kodak Photo-Resist (KPR). By means of a photographic exposure and development', the KPR is retained in contact with the surface of the iilm only in the desired resistor pattern and is removed from those portions where the alloy lrn is to be etched away. An

etching process is then used to remove the exposed portions of the thin alloy film.

-One example of a suitable pattern for the film after etching is shown in FIG. 1. The junctions o'f the fiexible leads and 20 are shown `at 16 and 17. These may be welded junctions, with reinforcing epoxy mounds. As the pattern appears in FIG. 1, a very low resistance path exists between these two junctions, extending -along the left side, across the top, and down the right side of film 12 as shown in said illustration. By extending the long, narrow gap 18 along the dotted line 18 to the upper edge of the film, as by cutting with a scribing tool or with a' fine abrasive cutter, the current is required to flow through narrow linear path portions 19 and 21. Upon extending the next long, narrow -gap 22 to the upper edge of the pattern as by scribing along dotted line 22', the current path is further lengthened to pass through narrow linear portions 23 an-d 24. With this process, as many of the long, narrow portions -as desired may be included in the current path between terminal areas 16 and 17. As will be apparent, in order to include in the current path a great number of the long, narrow portions similar to portions 19, 21, 23 and 24, a transverse cut may be extended `across several of the gaps such as gaps 18 and 22 near theirends.

As illustrated in the right-hand side of FIG. 1, the pattern in which the film is exposed and etched may include several wider portions and portions of shorter lengths, so that an operator 4is enabled to complete the steps of bringing about the desired resistance value with the inclusion of one or more of the lower-resistance increments as may be needed. Examples of such wider and shorter portions in the film pattern for various amounts of incremental resistance are indicated at 27, 28, 29, 31, 32

and 33.

With the alloy film etched in the desired pattern and bonded to one surface of the substrate 11 as by a suitable epoxy bonding cement 13', further epoxy material 13 is added, covering the surface of the metal film. This precedes the adjusting of the resistance by cutting, referred to above. FIG. 4 illustrates the protective reinforcement provided by the epoxy plastic material 13, 13 which embeds the metallic film 12. As the cut is being made through the outermost plastic 13 and at least part way through the alloy film 12, either with a stylus or with an abrasive cutter or other tool, the plastic holds the film 12 in position along the boundaries of the cut and resists any tendency for the film to be detached from the substrate.

The upper protective epoxy coating comprising layers 13 and 13 may have a total thickness of 0.001 inch. There is also applied to the opposite face of the substrate 11 a further epoxy coating 14. Epoxy coating 13, 13 and the opposite epoxy coating 14 may be of equal thickness and identical characteristics in order that the stress contributions which they make to the flat surfaces of the glass substrate shall be balanced, and shall not tend to cause bending or warping of the glass substrate. Along with this elimination of bending, any tendency toward long-term dimensional instability due to stress relaxation is substantially overcome. Alternatively, the same resultl can be obtained by coatings of different material characteristics provided that their thicknesses are properly related.

The glass substrate has a temperature coefficient of expansion of the order of 3 parts per million per degree F. The epoxy or other plastic coatings 13, 13', and 14 on top and bottom of the glass substrate lia-ve a much higher temperature coefficient of expansion, of the order of 40 parts per million per degree F. Furthermore, said epoxy coatings tend to expand or contract as their moisture content varies. Hence, the balanced application of the epoxy or other plastic to both sides prevents it from causing bending 0f the device.

The'modulus of elasticity of the glass substrate is many times higher than that of the epoxy material. Hence, the expansion and contraction of the unit in length and width are determined mainly by the temperature coefficient of expansion of the glass. Inasmuch as the total thickness of the epoxy layers in the described embodiment is of the order of one-twentieth the thickness of the glass substrate, and the modulus of elasticity of the epoxy is of the order of one-thirtieth of the modulus of elasticity of the glass substrate, the tendency of the epoxy to expand with vtemperature by a factor of ten ti-mes greater than the expansion of the glass is made comparatively small by the relative thinness of the epoxy material and its far lower modulus of elasticity.

The resistive alloy film, etched in its predetermined pattern and bonded to the glass substrate, being of the order of one hundredth to one thousandth the thickness of the glass, exerts minimal influence upon the dimensional responsiveness of the unit to the changes of temperature and moisture. The changes of resistance in the path ultimately determined in the patterned film between the junctions 16 and 17 is infiuenced by the `following factors:

(a) The temperature coefficient of resistivity of the alloy of which the patterned metal film is comprised;

(b) The elongation and narrowing and consequent increase of resistance of the alloy film caused by the expansion of the symmetrically coated substrate with increase of temperature (and conversely, the compression and broadening of the alloy film when the symmetrically coated substrate co-ntracts with decreasing temperature);

(b") The variation of resistance as a function of the stress produced in the alloy film when the symmetrically coated substrate expands or contracts with changes of temperature.

As will be readily apparent, the factors b' and b above represent the resultant effect of the forces produced in the substrate and the forces produced in the coatings thereon.

By the selection of a nickel chromium alloy with such minor alloy components as to provide a desired curve of resistivity versus temperature and a desired temperature coefficient of expansion, the resistor may be made to have a reliable temperature coefiicient of resistivity as low as l part per million per degree C. in the vicinity of a desired design temperature such as 25 C., and to have an extremely low overall temperature coefiicient of resistivity throughout a range from C. to +175 C. In general, the alloy consisting primarily of nickel and chromium will have a greater temperature coefiicient of expansionA than the glass substrate. Hence, with increasing temperature, as the glass substrate elongates and carries with it the alloy film layer, the alloy film is subjected to compressive stress. Conversely, as the glass substrate contracts with decreasing temperature and the alloy layer tends to undergo greater contraction, the resistive metallic film which is bonded to the glass and constrained to duplicate the contraction of the glass is subjected to tensile stress.

Provided that the net sum effect of the resistance change component due to changing stress in the alloy film and the resistance change component due to expansion or contraction of the film is substantially equal to the temperature c0- efiicient of resistivity of the alloy under stress-free conditions, and of opposite sign, the overall temperature coefficient of resistivity of the device is substantially zero. Since the last-named factor is not linear, the device will have a predictable variation of its temperature coefficient of resistivity throughout the design temperature range.

For most applications, it is desirable to have the precision resistor encapsulated. As shown in FIGS. 5 and 6, the resistor made in accordance with the present invention may be encapsulated in a plastic or metal housing 36 wherein suitable potting material or materials are included to embed one or more resistor units such as that described above. In order to protect the resistor unit against yany substantial mechanical forces exerted by or through the potting material, the resistor unit is provided with a sheath of soft rubber, polyurethane foam, or other very soft material. Such soft material 37 may be used alone, filling the space inside case 36 surrounding the coa-d substrate, if desired; alternatively, the soft material 37 may in turn be surrounded by a hard filler 38 such as an epoxy.

The soft material 37, with a thickness preferably many times greater than the thickness of the epoxy layers on the substrate, serves as a protective cushion by virtue of its very low modulus of elasticity.

As shown in FIGS. 5 and 6, the flexible ribbon conductors and 20 from the resistor element extend to terminals 39 and 41. By virtue of the soft cushion 37 and the fiexible conductors 15 and 20, the dimensional changes in the hardened potting material, which may be of the order of five to ten times greater than the dimensional changes which the resistor unit itself tends to undergo, are isolated and prevented from forcing the resistor unit to depart from its design characteristics.

The illustrated application of the principles of the invention to the making of a precision resistor demonstrates many important advantages. With the extremely compact construction, minimum values of inductance and distributed capacitance are achieved. A very wfde range of resistance values is readily provided. A given patternetched metal film is conveniently adjusted precise'y to any desired resistance value within a very wide range.

Substantial heating of the resistor by electric power can be tolerated without damage. Since the resistor is miniature, it is suitable for use in applications where space is at a premium. It may be included as a part of a miniaturized circuit. Other portions of the circuit may be built directly upon the substrate of the resistor. Two or more resistors may be made on a common substrate. Where desired, a second resistor or resistance path may be disposed on the face of Ia substrate opposite the face on which the first resistance path is established.

Where space permits and greater power dissipation capability may be desired, the elements of the resistor may be made larger. The principles of the present invention may be used in a device having a thickness of an .appreciable fraction of an inch, with a substrate tace area which may be as great as a square inch or greater. As will also be apparent, the substrate may be a metal body, the resistive metallic film being insulated therefrom by plastic layer 13 in contact with the substrate.

As many changes may be made and many widely different embodiments may be constructed without departing from the spirit of the present invention, the accompanying drawings and description are intended not by way of limitation but rather illustration of one form which may be taken by the invention, the scope of the invention being indicated by the claims.

What is claimed is:

1. A stable electrical component comprising a rigid substrate having two opposed flat surfaces, a metallic layer bonded to one of said fiat surfaces and having a tortuous path configuration for vconduction of electric current through a total path distance many times longer than the dimensions of the at surface to which it is bonded, said metallic layer being an alloy film of the order of one-hundredth to one-thousandth the thickness of said substrate and yhaving appreciable resistivity, and coatings of epoxy resin disposed on the two opposed surfaces of said substrate, one of said epoxy coatings abutting and embedding said alloy lm and the other epoxy coating abutting the opposite flat surface of said substrate, said epoxy coatings being thicker than said metallic layer, their thickness being of a lesser order of magnitude than the thickness of said substrate, the temperature coefcient of resistivity of said alloy in the temperature range from -55 to +175 C. being approximately equal to and tending to compensate for the incremental change of resistance thereof induced by the changing contraction and expansion and consequent changes of stress which the film udergoes as the substrate with its coatings eX- pands and contracts with changes of temperature.

2. A stable electrical component as defined in claim 1, wherein said substrate comprises a body of glass whose thickness is between 0.005 inch and 0.5 inch.

3. A stable electrical component as defined in claim 1, wherein said film comprises. an alloy comprising nickel and chromium as the principal constituents.

4. A stable electrical component las defined in claim 1, wherein said substrate comprises a body of glass whose thickness is between 0.02 inch and 0.1 inch, said metallic layer comprising an alloy film of the order of magnitude of 100 millionths inch thickness wherein nickel and chromium are the predominant constituents totalling appreciably more than of the metallic content, said epoxy coatings being of the order of ten times the thickness of the metallic layer.

5. A stable electrical component as defined in claim 1, wherein said substrate comprises a metal body, and said metallic layer is bonded thereto by a bonding material which insulates it from said substrate.

6. A stable, high-precision electrical component cornprising a substrate, a metallic resistive path bonded to one face of said substrate for conduction of electric current, said metallic resistive Ipath being of a much thinner order of magnitude than said substrate, anda pair of hard coatings symmetrically disposed about said substrate, said coatings being bonded to opposite faces of said substrate whereby one of said coatings embeds said metallic resistive path and said pair of coatings maintain balance with respect to their response to changes of temperature and moisture, further including a housing surrounding said substrate with its metallic film and coatings, said housing being spaced from said substrate, means filling the space between the housing and the coated substrate, said means including a pliable cushion of material having a very low modulus of elasticity, said cushion of material substantially enclosing said coated substrate .and being in contact therewith, whereby said coated substrate is enabled to expand or contract independently of said housing, an electrical conductor means extending from predetermined junction regions on said metallic film to the exterior of said housing, said conductor means including fiexible conductor portions within said housing.

References Cited UNITED STATES PATENTS 2,751,477 6/ 1956 Fitzgerald 338--275 X 2,822,524 2/1958 Williston 174-52 X 2,939,807 6/1960 Needham 338-258 X LEWIS H. MYERS, Primary Examiner.

E. GOLDBERG, Assistant Examiner.

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