Method For Making A Resistor

Abstract

This disclosure provides a method for making a resistor comprising at least one resistor film and electrode films adhered to the inside surface of a hollow substrate such as one having a cylindrical form. The method includes attaching a flexible transfer film having resistor and electrode pastes thereon to the inside surface of the substrate and burning out the transfer film so as to fix the pastes onto the inside surface as films. The method, therefore, is very easy to carry out and makes it possible to obtain a desired pattern of resistor film or films easily.

Description

This invention relates to a method for making a resistor comprising at least one resistor film and electrode films adhered to the inside surface of a hollow substrate.

A few types of resistors of this kind have already been developed. This type of resister will hereinafter be called the "inside type." It is known that resistors of the inside type are superior to a resistor comprising a resistor film and electrode films adhered to the outside surface of an insulating substrate such as one having a cylindrical form, which type is called the "outside type", in that the hollow substrate acts as a film supporter and as a protector for protecting the resistor films from environmental conditions.

One inside type resistor is made as follows. A resistor film is deposited on the entire inside surface of an insulating hollow substrate such one having a cylindrical form by spraying raw resistor liquid, in the case of a metal oxide, or vacuum-evaporating metal or alloy in the case of a metal or alloy resistor film. Then, the resistor film on portions of the hollow substrate are removed by cutting, e.g., spirally, or in a similar manner.

In this method, however, there are disadvantages that it is very difficult to remove portions of the resistor film to leave a complicated pattern of the resistor film and a complicated cutting method is required for making a desired pattern and trimming the resistance value. These cutting steps are very difficult because they must be carried out on the inside surface of the hollow substrate. Moreover, waste of resistor material caused by the cutting step cannot be overlooked, and the production yield thereof is very low.

However, a complicated pattern of the resistor film is very much desired, e.g., obtaining a desired resistance value, providing a resistor film which does not suffer a surface corona discharge, obtaining plural resistor elements, controlling the temperature distribution of the resistor, and so forth. These are even more desirable features where the resistor is used under high power consumption conditions.

Therefore, a principal object of this invention is to provide a method for making a resistor comprising at least one resistor film and electrode films adhered to the inside surface of a hollow substrate, in which it is very easy to carry out the method, it is very easy to make a desired pattern of the resistor film, and the method requires a small amount of resistor material and makes passible inexpensive production and high production yield.

Another object of this invention is to provide a method for making such a resistor which is stable with respect to environmental conditions such as humidity.

These and other objects and the features of this invention will become apparent upon consideration of the following detailed description taken together with the accompanying drawings, in which:

FIG. 1 is a perspective view of a transfer film on a carrier sheet and having one pattern of resistor paste and electrode paste applied thereto;

FIG. 2 is a perspective view, partially cut away, of a heat-resistant hollow substrate having a cylindrical form having a transfer film attached thereto;

FIG. 3 is a cross-sectional view of a resistor which is made by the method of this invention and which has further been air-tightly sealed with a further coating on the inside surface;

FIG. 4 is a cross-sectional view of a resistor made by the method of this invention which is air-tightly sealed and is provided with a further coating on the inside surface of the end caps;

FIG. 5 is a perspective view of a transfer film having another pattern of resistor paste thereon;

FIG. 6 is a graph showing the surface temperature distribution in the direction of the length of a resistor made by the method of this invention using a pattern of resistor film as shown in FIG. 5; and

FIG. 7 is a perspective view of a transfer film having still another pattern of resistor paste and electrode paste thereon.

Referring to FIGS. 1 and 2, the method of this invention comprises: applying a resistor paste 4 and an electrode paste 3 in a given pattern on one surface of a flexible transfer film 2; attaching the flexible transfer film 2 to an inside surface 6 of a hollow heat-resistant substrate 7; heating the heat-resistant substrate 7 with the transfer film 2 attached thereto so as to burn out the transfer film 2 and to transfer the resistor paste 4 and electrode paste 3 to the inside surface 6 of the heat-resistant substrate 7 as at least one resistor film and electrode films, respectively; and connecting electrical conductors 10 to the electrodes.

FIG. 1 shows one example of a substantially flat transfer film 2 having a given pattern of resistor paste 4 and electrode paste 3 applied thereto. These pastes can be applied by using any techniques. For example, well known methods such as a screen printing method, a spraying method, a brushing method, a gravure printing method and a relief printing method can be used. It is clear that it is very easy to apply the resistor and electrode pastes even in a complicated pattern on a substantially flat surface of the transfer film, compared with the direct application of the pastes to the inside surface of a hollow substrate. Particularly, the method of this invention exhibits its advantage when the substrate is a hollow substrate, the inside surface of which is curved, such as a substrate having a cylindrical form or an elliptical form. When the inside surface of the hollow substrate is irregularly curved, it is especially difficult to cut the resistor film coated on the entire inside surface by using a conventional method as mentioned beforehand. However, the method of the present invention can be easily performed even in the case of such an irregularly curved inside surface. The material which can be used for the transfer film is a film that can carry the resistor paste without damaging the pattern of the resistor paste even when it is rolled or bent so that it can be attached to the inside surface of the hollow substrate and that can be burned out by a heating step carried out at a high temperature. It is further preferred that the transfer film becomes sticky when subjected to the heating step until the transfer film burns out (e.g., during heating at a temperature of 70.degree. to 200.degree.C) in order to adhere the transfer film to the inside surface of the hollow substrate in a stable manner and in order that the resultant resistor and electrode films be very strongly fixed to the inside surface. Preferred materials usable for the transfer film include e.g. a sythetic resin such as vinyl, polystyrole, polyethylene, butyrates, cellulose, derivatives of cellulose and mixtures of these materials. The range of the thickness of the transfer film depends on the material used for the transfer film. However, it should be noted that a transfer film having a thickness which is too small is not suitable because of its mechanical weakness and a transfer film having a thickness which is too great is also not suitable because it is very difficult to attach such a thick transfer film to the inside surface of the hollow substrate along the curve of the inside surface and also because such a thick transfer film does not burn out easily during the heating step. Usually, the preferred range of the thickness of the transfer film is between 1 micron and 100 microns. For example, a polyvinyl butyrate resin film having a thickness of 5 to 50 microns is very suitable for the transfer film.

If necessary, the resistor paste and the electrode paste in the given pattern on the transfer film can be checked for pin holes and cracks. This is a step which is almost impossible to carry out in the conventional method as described above, because the desired pattern of a resistor film is made after a complete resistor film is provided; on the inside surface of a hollow substrate according to the conventional method. In order to make possible such a check during the carrying out of the method according to this invention, it is preferred that the transfer film be light transparent, although such is not an essential condition. If such a check is carried out, the production yield which is attributable to this invention and which is much higher than that of the conventional method can be still further increased.

In order to make it easy to hold the transfer film temporarily before it is attached to the inside surface of a hollow substrate, a carrier sheet 1 as shown in FIG. 1 can be used, although it is not necessary to use such a carrier sheet. The assembly of a carrier sheet 1 and a transfer film 2 on said carrier constitutes a transfer sheet 5. All that is required with the transfer sheet is that it be easy to strip the transfer film from the carrier sheet manually or mechanically. If a check of the resistor paste and electrode paste in the given pattern is required while they are still on the transfer sheet, it is preferred that the transfer film and the carrier sheet both be light transparent, although such is not required.

Any resistor pastes which can be coated on a transfer film in a given pattern and can be fixed to the substrate by the heating step can be used for the resistor pastes in the method of this invention. For example there can be used a resistor paste mainly comprising conductive particles and glass frit dispersed in a liquid vehicle and in which the conductive particles are preferably of metal or metal oxide powder, or a mixture or alloy of these powders such as silver, palladium, gold, palladium oxide, ruthenium oxide, indium oxide or cadmium oxide. One typical resistor paste preferred in the method of this invention comprises, as solid ingredients, 55 to 90 percent by weight of a mixture consisting essentially of 20 to 92 percent by weight of finely divided CdO and 8 to 80 percent by weight of finely divided glass frit, and 10 to 45 percent by weight of liquid vehicle, which resistor paste is very stable during the heating step.

The thickness of a resistor paste applied to the flexible transfer film according to this invention is preferably 1 to 100 microns in order to obtain good electric stability and good results in the successive steps. For obtaining such a thickness, a very good method for applying the resistor paste to the transfer film is a screen printing method.

As for the electrode paste, any known electrode pastes can be used. In forming the pattern of pastes, it is necessary that a portion of the electrode paste be in contact with the resistor paste.

FIG. 2 shows is a perspective view, partially broken away, of a hollow substrate 7 having a cylindrical form having a transfer film already attached thereto.

With respect to the hollow substrate 7, not only such the cylindrical substrate shown but any hollow substrate having an irregularly curved inside surface can be used. Any heat resistant material which can withstand the heat during the heating step and having a electrically insulating characteristic can be used for the hollow substrate. For example, ceramics and heat-resistant glasses such as alumina, forsterite, mullite, zirconia, beryllia steatite, Pylex (trade name: Corning Glass Works) and dehydro ceramic glass can be used as a material for the substrate.

In attaching the transfer film to the inside surface 6, it is necessary that the side of the transfer film which is opposite to the side thereof having the resistor paste and electrode paste thereon be against the inside surface 6. If the transfer film is attached to the inside surface 6 with the side of the transfer film having the pastes thereon against the inside surface 6, then good adhesion of the pastes to the inside surface can not be obtained.

Any suitable technique can be used for attaching the transfer film to the inside surface 6 of the hollow substrate along the curve of the inside surface. One method is to use an electrostatic attractive force, in which an electrostatic charge of one polarity is applied to the inside surface 6 and an electrostatic charge of the opposite polarity is applied to the transfer film. Thus, the inside surface and the transfer film attract each other. Another method is to attach the transfer film mechanically and forcibly to the inside surface 6. Still another method which is very preferable is to use the surface tension of a thin liquid layer interposed between the inside surface 6 and the transfer film. For inserting the transfer film into the hollow of the substrate, a rod or something like that which is smaller in cross-section than the hollow and can be inserted in the hollow may be necessary.

In carrying out the heating step, the heating temperature and heating time depend on the kinds of materials used for the transfer film, substrate and pastes and also on the desired resistance values. What is required is that the heating step burns out the transfer film and changes the resistor paste and the electrode paste to at least one rigid resistor film having a desired resistance value and electrode films, respectively. As for the heating temperature, therefore, the maximum heating temperature is usually higher than 500.degree.C and lower than 900.degree.C. As for the heating time, it is difficult and is almost useless to express it in figures, because it depends very much on the factors as set forth above. Usually, a heating time of at least a few minutes is required for heating at a maximum temperature.

Electrical conductors 10 can then be connected to the electrode films.

The thus formed resistor has a resistance value deviating very little from the desired value. The deviation is much less than in the case of the conventional method.

FIG. 3 is a cross-sectional view of a resistor which is thus formed, and in which the inside surface of the heat-resistant substrate is further coated with a thin layer of organic resin 8 so as to cover the resistor film and the electrode films for protecting the resistor film from corona discharge more completely than when the effect of corona discharge is reduced by designing a suitable pattern of resistor film and so forth for that purpose. The material of the thin layer of organic resin can be e.g., epoxy, phenol, meramin and silicon resin. Furthermore, in FIG. 3, the hollow substrate is air-tightly sealed at both end openings by end caps 9. The end caps can be of metal and are connected to electrical conductors (lead wires) 10. This air-tight sealing is to improve the electrical characteristics with respect to such conditions as humidity and load life of the resultant resistor. However, the provision of these organic resin layer 8 and the end caps 9 is not required for the resistor to fall within the inventive scope of this invention. Moreover, in order to further improve the air-tight sealing, conductive organic paints or solder can be used to fill the gaps between each end cap and the inside surface of the substrate.

Further, when protection of the resistor from corona discharge produced at, e.g., the high voltage side and at the electrical conductors 10 is required, rubber seal caps 11 can be provided as shown in FIG. 4.

By designing the pattern of the resistor paste on the transfer film in a manner as shown in FIG. 5, the voltage load life characteristics of the resistor can be further improved when it is applied to e.g. a hollow substrate having a cylindrical form. The pattern of FIG. 5 is designed in a manner so that the density of the path of the resistor paste is higher towards the ends than towards the center. The curve B of FIG. 6 represents the temperature distribution of a resistor in a cylindrical substrate and having a pattern as shown in FIG. 5, while the curve A of FIG. 6 represents the temperature distribution of a resistor in a cylindrical substrate and having a uniform pattern as shown in FIG. 1. The curve B is flat, while the curve A is not. Therefore, the resistor having the characteristic represented by the curve B has a longer voltage load life in this respect. The pattern of FIG. 5 can be formed just as easily as the pattern of FIG. 1. However, the pattern as of FIG. 5 is difficult to form by the conventional method as described above.

Moreover, referring to FIG. 7, a plurality of resistor film segments 4 and electrode film segments 3 can be made as easily as forming the pattern of FIG. 1. However, a pattern such as that of FIG. 7 is very difficult or almost impossible to form by the conventional method as described above.

The following Examples are given to illustrate the invention. In the Examples, all the possible combinations of the materials usable for the method of this invention are not illustrated, because it would require an unduly large number of pages to illustrate all the possible combinations. However, since the point of this invention is in the method, this invention should not be limited to the materials used in the Examples. Similarly, the conception of the method of this invention should not be limited to the specific details of the method described in the Examples.

EXAMPLE 1

A flexible transfer film was placed on a carrier paper having a thickness of about 150 microns. A conventionally prepared glass frit having a composition of 70 percent by weight of PbO, 8 percent by weight of ZnO, 12 percent by weight of PbF.sub.2 and 10 percent by weight of B.sub.2 O.sub.3 was pulverized. The pulverized glass frit was mixed with CdO powder in a ratio of 40 percent by wieght of glass frit and 60 percent by weight of CdO. This mixture was mixed with a liquid vehicle consisting of 20 percent by weight of cellulose acetate butyrate and 80 percent by weight of carbitol acetate to form a resistor paste having a composition consisting of 74 percent by weight of the mixture of the glass frit and CdO and 26 percent by weight of the liquid vehicle.

The resistor paste was applied by using a screen printing method to one surface of the flexible transfer film in a pattern in which the parts of the pattern were at a uniform pitch as shown in FIG. 1 and having a total path 225mm long and 1.5mm wide. The transfer film was polyvinyl butylate resin having a thickness of 15 microns and was light transparent. The resistor paste had a viscosity of 1200 poises and the surface resistivity of the resistor paste was adjusted to 10 kilo ohms per square by heating. The printed paste was then dried in an oven at 60.degree.C for 30 minutes for evaporating off the liquid vehicle. Thereafter, an electrode paste mainly comprising Ag-Pd (No. 8151 DuPont, U.S.A.) was applied to the transfer film having the printed resistor paste thereon in a form as shown in FIG. 1 and then was dried in a manner the same as mentioned above in connection with the resistor paste. Then, the transfer film was stripped from the carrier sheet manually. The resistor paste on the transfer film was checked for pin holes, scratches and uniformity of thickness mainly by eye with the aid of a light beam.

A hollow substrate having a cylindrical form (inside diameter: 12mm; outside diameter: 18mm; length: 100mm) of forsterite ceramic was prepared. The transfer film was then wound on the surface of an assist rod having a diameter of 10mm in a manner such that the surface of the transfer film having the resistor paste thereon was toward the assist rod. The inside surface of the hollow substrate was wetted with a thin water layer. The assist rod was inserted into the hollow of the substrate and was touched to the inside surface of the hollow substrate and then was rotated slowly with the assist rod being in contact with the inside surface so as to attach the transfer film to the curved inside surface with the aid of the thin water layer in a manner such that the surface of the transfer film having the resistor paste thereon did not touch the inside surface. Then, the substrate was heated in an oven at a temperature of 90.degree.C for 30 minutes so as to soften the transfer film and increase the adherence of the transfer film to the inside surface of the hollow substrate. Thereafter, the substrate with the transfer film was heated in a tunnel furnace at a maximum temperature of 760.degree.C for 10 minutes. Thus, a resistor film and electrode films fixed in a desired pattern to the inside surface of the hollow substrate were obtained.

Then, the inside surface of the hollow substrate except for a portion of the electrode films was coated with a silicon resin film by a dipping method and cured at a temperature of 180.degree.C for 2 hours for the purpose of resin curing. Then, metal end caps each having a lead wire therein were fitted tightly on both ends of the substrate with the lead wires connected to the electrode films on the inside of the substrate.

The resistance of this resistor was 1550 kilo ohms.

The voltage load life characteristic of this resistor was measured and compared with that of a conventional outside type resistor which had the same resistance (1550 kilo ohms) and had the same size as the resistor made by the method of this invention, with the results shown in Table 1.

TABLE 1. Load life test

(for 1000 hours at 70.degree.C under 6 watts)

Initial Resistance resistance change (kilo ohms) (%) This invention 1550 -0.5 Outside type 1550 -4.8

EXAMPLE 2

A resistor was prepared in a similar manner to that in Example 1. The difference between Example 2 and Example 1 was in the structure of the air-tight sealing and electrical insulation at both ends of the hollow substrate as shown in FIG. 4. Both end portions of the resistor were sealed air-tightly by coating conductive silver paste thereon. Thereafter, insulation caps of silicon rubber were fitted on the both ends. A lead wire with an insulation coating extended outwardly through the center portion of each cap.

A humidity test of this resistor was carried out. The result was compared with that of a resistor which was not air-tightly sealed, with the results shown in Table 2. A voltage load life test of the resultant resistor was carried out. The result was compared with that of a conventional outside type resistor, with the results shown in Table 3.

TABLE 2. Humidity test

(90% R.H. at 60.degree.C for 1000 hours)

Initial Resistance Sealing resistance change (kilo ohms) (%) air-tight 1550 0.3 not air-tight 1550 2.3

TABLE 3. Load life test

(under 90% R.H. and 6 watts at 60.degree.C for 1000 hours)

Initial Resistance resistance change (kilo ohms) (%) This invention 1550 -0.2 Outside type 1550 -3.5

EXAMPLE 3

A resistor was prepared in a similar manner to Example 1. The difference between Example 3 and Example 1 was in the pattern of the resistor paste. In this Example 3, a pattern as shown in FIG. 5 was employed. The pitch of the parts of the path of the pattern was three times higher at an end part than at the center portion as shown in FIG. 5. By changing not only the pitch but also the width of the pitch, the length of the path at the center portion was about three-fifths of the total length of the path. The temperature distribution of this resistor was measured under an applied power of 6 watts. The maximum temperature measured was 90.degree.C and was located at the center part of the resistor. A resistor having the same size but having a uniform pattern as shown in FIG. 1 had a maximum temperature of 108.degree.C under the same conditions.

EXAMPLE 4

A resistor was prepared in a similar manner to that in Example 1. The difference between Example 4 and Example 1 was in the pattern of the resistor paste and the electrode paste. The pattern was that of FIG. 7. Thus, a plurality of resistor film segments and electrode film segments were formed. Four insulated lead wires were soldered to the electrode film segments.

Claims

What we claim is:

1. A method for making a resistor comprising: applying a resistor paste and an electrode paste in a given pattern of at least one resistor film with two electrode films in contact therewith on one surface of a flexible transfer film which will burn out when heated; attaching said flexible transfer film to an inside surface of a hollow heat-resistant substrate; heating said heat-resistant substrate with said transfer film therein so as to burn out said transfer film and to transfer said resistor paste and said electrode paste to said inside surface of said heat-resistant substrate in the form of said at least one resistor film and said electrode films, respectively; and connecting electrical conductors to said electrode films.

2. A method as claimed in claim 1, wherein said transfer film is placed on a carrier sheet prior to having the pastes applied thereto to form a transfer sheet and is separated from said carrier sheet after the pastes have been applied thereto and prior to being attached to said inside surface of said hollow heat resistant substrate.

3. A method as claimed in claim 1, wherein a flexible synthetic resin film is used for said transfer film.

4. A method as claimed in claim 3, wherein a synthetic resin film having a thickness of 1 to 100 microns is used for said flexible synthetic resin film.

5. A method as claimed in claim 3, wherein a film consisting essentially of polyvinyl butylate resin having a thickness of 5 to 50 microns is used for said flexible synthetic resin film.

6. A method as claimed in claim 1, wherein said resistor paste and said electrode paste are applied to said flexible transfer film by a screen printing method.

7. A method as claimed in claim 1, wherein said transfer film is attached to said inside surface of said hollow heat-resistant substrate with the side of said transfer film which is opposite to the side thereof having said resistor paste and said electrode paste thereon against the inside surface of said heat-resistant substrate.

8. A method as claimed in claim 1, wherein said transfer film is attached to said inside surface of said hollow heat-resistant substrate by placing a thin liquid layer on the inside surface of said hollow heat-resistant substrate and adhering said transfer film to the inside surface by the surface tension of the thin liquid layer.

9. A method as claimed in claim 1, wherein said heat-resistant substrate with said transfer film thereon is heated at a temperature higher than 500.degree.C.

10. A method as claimed in claim 1, wherein the inside surface of said heat-resistant substrate is further coated with a thin layer of organic resin after the heating step so as to cover said resistor film and said electrode films therewith.

11. A method as claimed in claim 1, wherein said heat-resistant substrate has an inside surface which is curved.

12. A method as claimed in claim 1, wherein said heat-resistant substrate has a cylindrical form.

13. A method as claimed in claim 1, further comprising air tightly sealing both end openings of the heat-resistant substrate after the heating step.

14. Method as claimed in claim 1, wherein a paste comprising cadmium oxide powder, a glass frit and a liquid vehicle is used as a material for said resistor paste.

15. Method as claimed in claim 1, wherein said electrode paste is applied to said transfer film in a pattern for forming plural resistor film segments.