WO1999063553A1

WO1999063553A1 - Thick film resistor compositions for making heat-transfer tapes and use thereof

Info

Publication number: WO1999063553A1
Application number: PCT/US1999/003202
Authority: WO
Inventors: Hisashi Matsuno; Yuichi Tachibana; Keiichiro Hayakawa; Mamoru Murakami; Sadanobu Kawasaki
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1998-05-29
Filing date: 1999-02-16
Publication date: 1999-12-09
Anticipated expiration: 2000-11-29
Also published as: TW444212B

Abstract

To provide a technique capable of obtaining thick-film resistors of any resistance value and shape without a particular need for trimming in order to be able to modify and select the resistance value which is determined by the length, width and thickness of the resistor. A thick-film resistor is produced using a thick-film resistor composition for heat-transfer tape formation, comprising from 54 to 76 % by weight of a mixed inorganic powder containing ruthenium oxide and/or ruthenium pyrochlore oxide powder as the electrically conductive ingredient, and a glass powder composed primarily of PbO, SiO2 and Al2O3, from 14 to 23 % by weight of a resin binder, and an organic vehicle containing from 10 to 23 % by weight of an organic solvent. This composition is coated onto a polyethylene terephthalate base film and dried so as to remove the organic solvent present, in this way obtaining a thick-film resistor-forming heat-transfer tape in which a thick-film resistor layer of a prescribed thickness has been deposited and formed. A base film portion of the resulting heat transfer tape corresponding to a selected thick-film resistance shape is heated, whereupon the above thick-film resistor layer portion melts, and is transferred to and deposited on a dielectric substrate, then subsequently fired, thereby giving the thick-film resistor.

Description

TITLE THICK FILM RESISTOR COMPOSITIONS FOR MAKING HEAT-TRANSFER TAPES AND USE THEREOF FIELD OF THE INVENTION The present invention relates to thick-film resistor compositions wherein a heat-transfer tape composition has been deposited on a base film, and a method for producing thick-film resistors using the heat-transfer tape. This invention relates in particular to the production of thick-film resistors of diverse target resistance values and sizes, and to an effective technique for changing the resistance values of thick-film resistors .

BACKGROUND OF THE INVENTION A resistor film is formed on a dielectric substrate by combining an electrically conductive finely divided powder, in which the conductive ingredients include ruthenium oxide or ruthenium pyrochlore oxide, and glass powder together with an organic vehicle. It is applied by a screen printing process onto a dielectric substrate in a required shape and to a wet thickness of about 30 to 80 μm, then fired at a required temperature. Thick-film resistance electronic components and thick-film hybrid circuits and the like are formed in this way. In prior-art thick-film resistor fabrication methods, coating and firing are carried out in accordance with the resistor paste used so as to give a resistance value lower than the target resistance value, following which the resistor is trimmed using laser light and a kerf is formed in order to increase the resistance value and modify it to the desired value.

In resistors which have been processed and modified in this way, not only are the production costs high, the likelihood of adjustment to the target resistance value in the production process worsens. It is also known that the resistance value yield markedly declines as the resistance value tolerance narrows.

When thick-film resistors are produced in this way by prior-art screen printing methods, possible causes for the increase in the resistance value deviation include defects particular to the printing technology, such as large variations in the resistor thick films owing to the low precision of the printing pattern, and to warpage and waviness of the alumina substrate.

Problems associated with the commonly employed thick-film paste printing methods in which the paste film that will form the resistor is formed on a substrate by screen printing through a mask include the very low resolution of the printed pattern and the fact that, when thick-film resistors of different resistance values or sizes are fabricated, specific masks are required for each such variation. Moreover, there are problems with the quality and reproducibility of the paste films that are formed, and also with the non-uniformity of the film thickness owing to variances in the squeegee pressure and squeegee speed conditions as well as other factors such as the type of paste, the production lot, and the elapsed time during printing.

In order to resolve these problems, it has been found that, when use is made of a known method, such as the formation of a resistor unfired film by slip casting or the like on an organic film followed by punching, to form a resistor by punching a resistor film to the mold dimensions, the resulting resistors have a lower resistance value deviation and Irirnming can be eliminated. This is described in JP-A 57-186301, for example. However, in prior-art methods, it has been impossible to cope with the diversification and increasing fineness in thick-film resistors of differing resistance values and sizes by making use of mechanical punching processes or the like.

Moreover, as described in JP-A 8-222837, for example, there has been proposed a process in which a transfer ink prepared by mixing a resin binder that thermally decomposes when fired at an elevated temperature with an electrically conductive metal powder that forms the wiring pattern on the base material to which transfer is carried out is coated with a coater or the like and dried so as to deposit a heat-transfer ribbon and, using this heat-transfer ribbon and a heat- transfer recording apparatus, a wiring pattern is formed on the base material of the circuit substrate without the need for a screen mask or writing.

However, although this prior-art method discloses that the partial electrical resistance of the wiring pattern can be varied by using heat transfer to form a wiring pattern on the ceramic circuit substrate and by overprinting transfer ink, it does not imply or disclose the formation of thick-film resistors with a high degree of freedom, a high pattern precision, and a uniform-thickness layer.

The present invention was conceived in order to resolve the above- described problems in the prior art. One object of this invention is to provide, in a method for producing thick-film resistor components such as hybrid ICs and chip resistors, a technique which is capable of obtaining thick-film resistors of any resistance value and shape that have a high pattern precision and substantially no variance in the resistor thick film, and which, because it is able to modify and select the resistance value to be set in accordance with the resistor length, width and thickness, makes trimming unnecessary. Another object of the invention is to provide a method for producing thick- film resistors by heat-transferring a thick-film resistor paste onto a dielectric substrate in accordance with a desired resistance value or shape, and with high pattern precision and essentially no film thickness deviation. Yet another object of the invention is to provide heat-transfer sheets which can be obtained by the formation, on a dielectric substrate and with any resistance value and shape, of a resistor paste by heat transfer at a high pattern precision and without film thickness deviation. Still another object of this invention is to provide a thick-film resistor composition which melts in those portions that correspond to heating elements driven for the heat transfer of thick-film resistors having any resistance value and shape, a high pattern precision, essentially no film thickness deviation and requiring no trimming, and which can be transferred and formed on the dielectric substrate to which transfer is carried out.

SUMMARY OF THE INVENTION In order to achieve these objects, the present invention is constituted as follow:

( 1 ) The thick-film resistor composition for heat-transfer tape formation according to the first invention disclosed herein is characterized by comprising from 54 to 76% by weight of a mixed inorganic powder containing ruthenium oxide and/or ruthenium pyrochlore oxide powder as the electrically conductive ingredient and a glass powder composed primarily of PbO, Siθ₂ and AI₂O3, from 14 to 23% by weight of a resin binder, and an organic vehicle containing from 10 to 23% by weight of an organic solvent.

(2) The heat-transfer tape-forming thick-film resistor composition according to the second invention disclosed herein is characterized by, in the heat- transfer tape-forming thick-film resistor composition according to the above- described first invention, there having been added at least one TCR adjustor selected from the group consisting of MnO₂, iθ₂, Nb₂O5, Fe₃O₄, Sb₂θ₃, CuO, Bi₂O₃, PbO, AgO, ZnO, SnO, V₂O₅, A1₂0₃, ZrO₂, SiO₂, Cr₂O₃ and Ta₂O₅.

(3) The thick-film resistor-forming heat-transfer tape according to the third invention disclosed herein is characterized by coating the thick-film resistor composition for heat-transfer tape formation according to the above-described first or second invention onto a polyethylene terephthalate base film having a thickness of 4 to 5 μm, then drying to remove the organic solvent present, such as to deposit and form on the base film a thick-film resistor film having a thickness in a range of about 3 to 12 μm.

(4) The thick-film resistor-forming heat-transfer tape according to the fourth invention disclosed herein is characterized by heating a portion of a second polyethylene terephthalate base film which has been provided in close contact with the thick-film resistor-forming heat-transfer tape according to the above- described third invention on the side where the thick-film resistor layer has been formed, which portion corresponds to a predetermined thick-film resistor shape on the side opposite to the side of the thick-film resistor-forming heat-transfer tape where the thick-film resistor layer has been formed, and by such heating causing the portion of the thick-film resistor layer which has been deposited and formed on this heated base film portion to melt and heat-transfer, such as to form a divided thick-film resistor layer having the specific shape.

(5) The thick-film resistor production method which uses a heat-transfer tape on which has been deposited a thick film resistor-forming composition according to the fifth invention disclosed herein is characterized by comprising the steps of: disposing the thick-film resistor-forming heat-transfer tape according to the above-described third invention on a dielectric substrate such that the thick-film resistor layer of the heat- transfer tape is in contact with the surface of the dielectric substrate; heating a portion of the base film which corresponds to a desired thick-film resistor shape, on the side of the base film in the heat-transfer tape opposite to the side on which the thick-film resistor layer has been formed, so as to melt a portion of the thick-film resistor layer and heat-transfer it onto the dielectric substrate; and firing the portion of the thick-film resistor film that has been separated from the heat-transfer tape and deposited onto the dielectric substrate.

(6) The thick-film resistor production method which uses a heat-transfer tape on which has been deposited a thick-film resistor-forming composition according to the sixth invention disclosed herein is characterized by comprising the steps of: disposing the thick-film resistor-forming heat-transfer tape according to the above-described fourth invention on a dielectric substrate such that the partial thick-film resistor layer which was formed on the base film of the heat-transfer tape and was separated in a specific shape is in contact with the surface of the dielectric substrate heating a portion of the heat-transfer tape base film which corresponds to the thick-film resistance layer portion having a specific shape, on the side of the heat-transfer tape base film opposite to that where the separated partial thick-film resistor layer has been formed so as to melt the separated partial thick- film resistor layer and heat-transfer it onto the dielectric substrate; and firing the partial thick-film resistor layer that has been separated from the heat-transfer tape and deposited onto the dielectric substrate.

(7) The thick-film resistor production method according to the seventh invention disclosed herein comprises using a thick-film resistor-forming heat- transfer tape obtained by coating the thick-film resistor composition for heat- transfer tape formation according to the above-described first or second invention onto a polyethylene terephthalate base film having a thickness of up to 5 μm, then drying to remove the organic solvent present, such as to deposit and form on the base film a thick-film resistor film having a thickness in a range of about 3 to 12 μm; and repeating a first resistor fabrication step in which a thick-film resistor is formed by transferring a selected first shape onto a dielectric substrate then firing, and a second resistor fabrication step in which a thick-film resistor is formed by separately selecting and transferring from the thick- film resistor layer a second shape which differs from the first shape in accordance with the resistance value obtained for the formed resistor, then firing, so as to selectively vary the shape of the thick-film resistor layer transferred from the thick-film resistor layer and thereby fabricate a thick-film resistor having a desired resistance value. DETAILED DESCRIPTION OF THE INVENTION

The main technical concept of this invention is thus to use a heat-transfer technique to bond and form on a dielectric substrate a resistor paste film having a high pattern precision and essentially no resistor film thickness deviation. By selectively controlling and varying as desired the heating region of a heating element which partially heats the heat-transfer tape during heat transfer, any resistor shape can be formed in a very short time and resistors of the desired resistance values can be easily obtained. According to the present invention, thick-film resistors which have been bonded and formed on a dielectric substrate have a very low variance in resistance value and do not require laser trimming modification. Because the resistor thus does not incur damage due to laser trimming, the voltage resistance and noise characteristics are vastly improved.

The resistance value in thick-film resistors generally fluctuates on account of such factors as temperature changes and air flow during firing. Hence, based on the resistance values obtained in resistors fabricated by bonding and forming on a substrate in the above-described manner a thick-film resistor having a prescribed shape then firing under fixed firing conditions, the present invention may also be employed to vary the shape of those thick-film resistors that are heat- transferred and thereby rework resistors in which a target resistance value must be obtained. The resistance value of a resistor may be determined from the following equation.

W x T

In this equation, R is the resistance value (Ω), p is the resistivity of the inorganic solids, which is constant here, L is the length of the resistor (mm), W is the resistor width (mm), and T is the resistor thickness (μm). As is apparent from this, when the length (L) and width (W) of the resistor varies within an allowable range, the thickness (T), length (L) and width (W) of the thick-film resistor obtained by repeatedly heat-transferring, bonding and forming onto the substrate a thick-film resistor layer can be varied at will. Because changes in these dimensions make a relatively large contribution to changes in the resistance values, any resistance value may be obtained for the resistor by appropriately selecting the thickness (T), length (L) and width (W) of the resistor. The invention is described more fully below.

The thick-film resistor compositions used in the present invention, which are heat-transferable and can be applied onto a dielectric substrate, contain inorganic powder serving as the main component thereof electrically conductive ingredients, glass binders and TCR adjustors. When producing the thick-film resistor-forming heat-transfer tape of the invention, the above-mentioned thick- film resistor composition is prepared as an ink, which is then printed and transferred onto a heat-transfer tape base film by a commonly known gravure printing process. The constituent components of this ink are thus the electrically conductive ingredients and glass binders serving as the main ingredients of the above-described thick-film resistor composition. To this are also added TCR adjustors, which are inorganic additives used for reducing the temperature coefficient of resistance (TCR); resin binders which hold the above-mentioned inorganic powder ingredients until the ink film formed after printing and transfer of the ink onto the base film of the heat-transfer tape dries and cures, and which melt upon being heated by a heating element, the heating region of which is controlled and brought into contact with the heat-transfer tape, when the thick- film resistor paste film is heat-transferred onto a dielectric substrate, thereby allowing a film of the resistor paste composed of the above-mentioned inorganic powder to adhere to the dielectric substrate; and an organic solvent for dispersing these ingredients and rendering them into the form of an ink. A. Inorganic Powder

The inorganic powder in the thick-film resistor composition used in this invention is a mixture of electrically conductive ingredients, glass binders and TCR adjustors. Examples include ruthenium oxides and ruthenium pyrochlore oxides.

The preferred ruthenium pyrochlore oxide is lead ruthenate (Pb₂Ru₂O6) because it may easily be obtained in pure form, is not adversely affected by glass binders, has a relatively low TCR, is stable even when heated in air up to about 1,000°C, and is relatively stable even in a reducing atmosphere. Use may also be made of other pyrochlore oxides, such as bismuth ruthenate (Bi₂Ru₂O₇) and bismuth lead ruthenate (Pb[ ₅Bio.₅Ru₂O_{6 5}).

The amount of ruthenium oxides or ruthenium pyrochlore oxides is from 10 to 50% by weight, and preferably from 12 to 40% by weight, based on the combined inorganic powder content. "Combined inorganic powder content", as used herein, refers to the combined amount of the electrically conductive ingredients and glass binder, and, when inorganic additives are also added as TCR adjustors, includes these as well. There is no particular limitation on the specific surface area of the electrically conductive ingredients, although this is preferably from 5 to 25 m²/g for ruthenium oxides, and from 3 to 15 m²/g for ruthenium pyrochlore oxides. In addition, precious metals such as gold, silver, platinum or palladium may be combined with ruthenium oxides or ruthenium pyrochlore oxides as electrically conductive ingredients, and blended into the composition. Various glasses generally used in thick-film resistor compositions may be employed as the glass inorganic binders. For example, glasses may be made of a mixture obtained by mixing a first glass powder containing 30 to 60% by weight of SiO₂, 5 to 30% by weight of CaO, 1 to 40% by weight of B₂O₃, 0 to 50% by weight of PbO, and 0 to 20% by weight of Al₂O₃, in which the combined amount of SiO₂, CaO, B₂O₃, PbO and Al₂O₃ accounts for at least 95% by weight; and a second glass powder containing 50 to 80% by weight of PbO, 10 to 35% by weight of SiO₂, 1 to 10% by weight of Al₂O₃, 1 to 10% by weight of B₂O₃, 1 to 10% by weight of CuO, and 1 to 10% by weight of ZnO, in which the combined amount of PbO, SiO₂, A1₂0₃, B₂O₃, CuO and ZnO accounts for at least 95% by weight.

The glass used as the glass binder in the present invention may be produced by a conventional production method.

The amount of glass binder is 5 to 45% by weight of the first glass and 5 to 45% by weight of the second glass, based on the combined weight of the inorganic powder ingredients. When the content of the first glass powder is greater than the above range, the resistance value becomes high, and when it is below this range, the firing temperature dependence during firing becomes poor. Conversely, when the content of the second glass powder exceeds the above range, the size effect becomes large, and when it falls below this range, the resistance value becomes high.

In addition, the inorganic powders may also include, as the TCR adjustors which can used in this invention, one or more metal oxides selected from among, for example, MnO₂, TiO₂, Nb₂O₅, Fe₃O₄, Sb₂O₃, CuO, Bi₂O₃, PbO, AgO, ZnO, SnO, V₂O₅, Al₂O₃, ZrO₂, SiO₂, Cr₂O₃ and Ta₂O₅. The content of these is from 0 to 5% by weight, based on the combined inorganic powder content. When the amount of TCR adjustors exceeds this upper limit in content, the desired effects of such addition cease and the resistance value increases.

B. Resin Binder The resin binders which may be used in the thick-film resistor compositions of the invention employ a thermoplastic resin required for heat transfer. For reasons having to do with the melting point, viscosity, and the like, these binders are mixtures of synthetic resin and wax. Synthetic resins that may be used include butylated urea, amino-type resins such as melamine resin, vinyl resin systems such as vinyl chloride resins, vinyl acetate resins, vinyl chloride- vinyl acetate copolymers and butyral resins, and cellulose resins such as methyl cellulose, ethyl cellulose, cellulose acetate and nitrocellulose resin. Waxes that may be used include natural waxes and synthetic waxes. Natural waxes that may be used are carnauba wax (melting point, 80-86°C), Japan wax, lanolin, montan wax (melting points, 70-90°C), paraffin waxes (melting points, 45-74°C), and microcrystalline waxes (melting points, 66-93 °C). Of these, carnauba wax is preferable. Examples of synthetic waxes include Fischer-Tropsch wax (melting point, about 100°C), polyethylene wax (high melting point, 100-130°C) and polypropylene waxes (high melting point). The resin binder content is from 14 to 23% by weight, based on the entire thick-film resistor composition, including the inorganic powder and the organic solvent.

C. Organic Solvent

The inorganic powders and resin binder of the thick-film resistor composition used in the present inventions are dispersed in an organic solvent to give an ink-like printable slurry. The content of the organic solvent is 10 to 23% by weight, based on the overall weight of the composition containing the inorganic powders and resin binder. The organic solvent must be capable of suitably wetting the finely divided inorganic powders, and must have a solubility that is capable of fully dissolving the resin binder and a good drying speed at low temperature of 60 to 80°C, for example. Examples of organic solvents having these properties include toluene, ethanol and methyl ethyl ketone.

In the thick-film resistor compositions of the present invention, the content of the above-described mixture of electrically conductive ingredients, glass binders and TCR adjustors is preferably 54 to 76% by weight, based on the total weight of the composition containing the resin binder and the organic solvent. When the content of the mixture of inorganic powders is more than 76% by weight, the resulting slurry viscosity is not suitable for the step in which the thick- film resistor composition is coated onto the base film in the heat-transfer tape. On the other hand, when the content of the inorganic powder mixture is less than 54% by weight, the resistor film thickness after firing becomes too small in the step in which the resistor that has been heat-transferred and formed on a dielectric substrate is fired.

In the thick-film resistor compositions of the invention, the content of the resin binder is preferably from 14 to 23% by weight, based on the total weight of the composition including the inorganic powders and the organic solvent. When the resin binder content in this composition is less than 14% by weight, the amount of wax ingredients present in the resin binder becomes low, resulting in inferior heat-transferability. On the other hand, when the resin binder content in this composition is more than 23 % by weight, the inorganic powder content becomes relatively low, resulting in a small resistor film thickness after firing in the step in which the resistor that has been heat-transferred and formed on the dielectric substrate is fired.

The content of the organic solvent within the inventive thick-film resistor composition preferably falls within a range of 10 to 23% by weight. Outside of this range, a suitable slurry viscosity cannot be obtained in the step in which the resistor composition is coated onto a base film in order to produce the heat- transfer tape. D. Fabrication of Heat-Transfer Tape

The slurry-type thick-film resistor composition ink prepared as described above is printed using a known gravure printing process onto a base film made of a polyethylene terephthalate (PET) resin and having a width of 110 mm and a thickness of 4.5 μm, following which it is dried for about 5 to 10 minutes in a temperature range of 65 to 70°C so as to remove the organic solvent, thereby giving a heat-transfer tape having a thick-film resistor film with a thickness of about 4 μm. The typical range of the thickness of the coated thick film resistor layer is about 3 to 12 μm with the desired resulting thickness of about up to 5 μm.

E. Formation of Thick-Film Resistor by Heat Transfer

Although the elasticity and thickness of the base film in the heat-transfer tape are also factors, the thick-film resistor can generally be transferred directly onto a flat, flexible substrate. The above-described heat-transfer tape and the dielectric substrate to which the resistor paste film is to be bonded are carried synchronously to the print starting position in a known thermal printer head in which heating resistor elements are disposed. Then, with the thermal head pushed against the heat-transfer tape, a predetermined heating resistor element is driven by control signals from an image processing computer connected to the thermal printer and generates heat, by means of which a selected portion of the heat- transfer tape melts and is transferred and attached, in a 0.8-mm square shape, to a dielectric substrate such as an alumina substrate at a facing position. In cases where transfer is carried out onto a substrate which has projecting features or lacks flexibility, indirect transfer must be carried out. A thick-film resistor-forming heat-transfer tape is fabricated in which a selected portion of the above-described resistor film has been transferred, in accordance with a predetermined shape, from the above-described heat-transfer tape on which a resistor film having a thickness of about 4 μm has been formed, and deposited on another polyethylene terephthalate film having a thickness within a range of 25 to 75 μm. Using the resulting heat-transfer tape, the position corresponding to the selected film portion of the above-described thick-film resistor is heated, with a heat roller or the like, on the base film side opposite to the side on which the resistor film has been formed, thereby melting the thick-film resistor and heat-transferring it onto a substrate that has been provided in close contact with the heat-transfer tape.

F. Firing

In order to induce sintering of the electrically conductive ingredients and glass binder in the thick-film resistor composition that has been transferred and attached to the substrate in the above-described manner, firing is carried out with a 30-minute temperature profile that includes 5 minutes at the peak temperature of 850°C, thereby producing the resistor.

G. Formation of Electrodes

Next, a conductor paste obtained by kneading into the form of a paste 65% by weight of silver and 5% by weight of palladium together with a solvent- containing vehicle and an inorganic binder was applied by screen-printing or the like so as to partially overlap the resistor layer that was printed on the substrate, dried (at 150°C for 10 minutes), then heated at 600°C for 30 minutes, thereby printing and forming a top electrode layer (Cl electrode). In this way, thick-film resistor elements can be obtained. When chip resistors are produced, where necessary, a conductive paste using a thermosetting resin or the like may be additionally applied so as to partially overlap the top electrode layer, and cured in a dryer at 150°C and 30 minutes, for example, to form an end electrode (C2 electrode). To protect the circuit, an insulating resin or a low-melting glass powder paste, for example, may be applied or printed, then dried and fired so as to form a cover coat layer.

Examples are given below by way of illustration. Unless noted otherwise, concentrations are expressed in weight percent. WORKING EXAMPLE 1

The mixture indicated below was kneaded for about one hour in a sand mill to prepare an ink-type thick-film resistor composition in which the inorganic powders were uniformly dispersed. Using this ink, a heat-transfer tape was fabricated as described above, and thick-film resistors were formed in a size of 0.8-mm square by heat transfer onto an alumina substrate, then fired in air with a firing profile in which a temperature of 850°C was maintained for 5 minutes, thereby forming a thick-film resistor having a film thickness of 6 μm.

(1) Inorganic Powders 65 wt %

(a) Conductive ingredients 14.4 wt% Ruthenium oxide (specific surface area, 12 m /g) 1.6 wt%

Lead ruthenate (specific surface area, 7 m²/g) 12.8 wt%

(b) Glass Binders 50.0 wt% First glass with average particle size of 1-2 μm 24.4 wt% Second glass with average particle size of 1-2 μm 25.6 wt% (c) TCR Adjustor: 0.6 wt%

Niobium oxide with average particle size of 1-2 μm 0.6 wt%

(2) Resin Binders 18.5 wt%

Carnauba wax 17.6 wt%

Vinyl acetate resin 0.9 wt% (3) Organic Solvent 16.5 wt%

Methyl ethyl ketone 16.5 wt%

The resistance values were measured for 60 sample thick-film resistors having a shape of 0.8 x 0.8 mm formed in the above-described manner, based upon which the resistance value precision (standard deviation/average value x 100) was computed. In addition, the noise for these resistors was measured. Table 1 shows the average value determined based on these measured results.

WORKING EXAMPLE 2 Aside from changing to 12 μm the film thickness of the thick-film resistor formed by repeated transfer, thick-film resistors were produced using a mixture similar to that in Working Example 1 by essentially the same method. The resistance values were measured for 60 sample thick-film resistors thus formed, based upon which the resistance value precision (standard deviation/average value x 100) was computed. In addition, the noise for these resistors was measured. Table 1 shows the average value determined based on these measured results.

COMPARATIVE EXAMPLE A paste was prepared by kneading together, in a three-roll mill, 60% by weight of the same mixed inorganic powder as was used in Working Examples 1 and 2 above and 40% by weight of an organic vehicle composed of terpineol and ethylene cellulose. Using the paste thus obtained, resistors measuring 0.8-mm square were printed and formed on a dielectric substrate by a screen-printing process, then dried at 150°C for 10 minutes, and fired with a 30-minute firing profile at 850°C, thereby forming thick-film resistors. The resistance values for 60 sample resistors obtained in this way were measured in the same way as in the working examples, based upon which the resistance value precision (standard deviation/average value x 100) was computed. In addition, the average values for noise were measured and computed in the same manner as in the working examples. These results are shown in Table 1.

TABLE 1

The results in Table 1 show that the present invention makes it possible to vary the resistance value in accordance with the selected film thickness, without film thickness deviation, over a broad range in film thickness from the film thickness required to avoid substrate-induced effects to the film thickness at which resistance value modification by laser trimming becomes difficult, and to obtain the desired resistance value by such adjustment in the film thickness. The standard deviation/average value for the resistors produced in the working examples was 1.5 to 1.7%, and the 3 x standard deviation/average value was about 4.4 to 5.1%. Hence, a high resistance value precision can be obtained, meaning that the variance in the resistance values for resistors produced according to this invention can be greatly reduced, making it unnecessary to adjust the resistance value by trimming and thus simplifying the production process. Moreover, it is also apparent that good noise characteristics may be obtained.

Because the wax ingredients in the thick-film resistor compositions of the invention have a good ability to wet inorganic powders, they enhance the filling properties of the inorganic powders so that voids and the like are not present in the resistor after firing. This enables lower resistance values to be achieved.

Some of the advantageous effects obtained by the inventions disclosed herein are summarized briefly below.

(1) There is no special need for printing apparatus, trimming, and process management that are specially tailored to the target resistor in order to produce thick-film resistors of any resistance value and shape by the formation of a resistor paste film on a dielectric substrate using heat transfer.

(2) Because thick-film resistors with high-precision patterns and uniform film thickness are produced, no trimming is needed. As a result, damage is not imparted to the resistors, making it possible to obtain, with good reproducibility, thick-film resistors of high reliability and excellent voltage resistance.

(3) The inventive heat transfer-based production method enables the resistor length, width and thickness to be freely modified in a short time. Hence, when if the resistance value changes under the influence of the firing conditions, the target resistance value can be easily obtained.

Claims

1. A thick-film resistor composition for heat-transfer tape formation, comprising, based on total composition, 54 to 76% weight of inorganic materials comprising (1) ruthenium oxide or ruthenium pyrochlore oxide powder or mixtures thereof, (2) glass powder comprising PbO, SiO₂ and Al₂O₃, and (3) at least one inorganic oxide; and 14 to 23% weight resin binder; and 10 to 23% weight organic solvent.

2. The composition of Claim 1 wherein the inorganic oxide is selected from the group consisting of MnO₂, TiO₂, Nb₂O₅, Fe₃O₄, Sb₂O₃, CuO, Bi₂O₃, PbO, AgO, ZnO, SnO, V₂O₅, Al₂O₃, ZrO₂, SiO₂, Cr₂O₃ and Ta₂O₅.

3. A heat-transfer tape wherein the composition of Claim 1 is coated onto a polyethylene terephthalate base film then drying to remove the organic solvent.

4. A heat-transfer tape comprising a second polyethylene terephthalate base film bearing selected portions of the coated thick film resistor layer corresponding to a predetermined thick-film resistor shape by disposing said second polyethylene terephthalate base film on a side of the heat-transfer tape of Claim 3 bearing the coated thick film resistor layer in close contact therewith followed by heating on the side opposite to the side of the heat-transfer tape bearing the coated thick film layer for melting and heat-transferring said predetermined portions of the coated thick film resistor layer thereof.

5. A method for forming a thick film resistor which uses the heat-transfer tape of Claim 3, comprising the steps of: disposing the heat-transfer tape of Claim 3 constituting a base film on a dielectric substrate such that the thick-film resistor layer of the heat-transfer tape is in contact with the surface of the dielectric substrate; heating a portion of the base film which corresponds to a selected thick-film resistor shape, on the side of the base film with the heat-transfer tape opposite to the side on which the thick-film resistor layer has been formed, so as to melt a portion of the thick-film resistor layer and heat-transfer it onto the dielectric substrate; and firing the selected portion of the thick-film resistor film that has been transferred from the heat-transfer tape and deposited onto the dielectric substrate.

6. A method for forming a thick film resistor which uses the heat-transfer tape of Claim 4, comprising the steps of: disposing the heat-transfer tape of Claim 4 on a dielectric substrate such that the partial thick-film resistor layer of the heat-transfer tape that has been separated and formed in a specific shape is in contact with the surface of the dielectric substrate; heating a portion of the heat-transfer tape base film which corresponds to the position where the partial thick-film resistor layer has been formed, on the side of the heat-transfer tape opposite to that where the partial thick-film resistor layer has been formed, so as to melt the separated partial thick-film resistor layer and heat-transfer it onto the dielectric substrate; and firing the partial thick-film resistor layer that has been transferred from the heat-transfer tape and deposited onto the dielectric substrate.

7. A method for forming a thick-film resistor comprising the steps of: preparing a heat-transfer tape bearing a coated thick film resistor layer of the composition of Claim 1 having a thickness in a range of about 3 to 12 μm on a polyethylene terephthalate base film having a thickness of up to 5 μm by drying for removing the solvent after coating; forming a selected first resistor having a first predetermined shape thereof by heating and transferring the first resistor on a dielectric substrate and firing the first resistor deposited on the substrate; forming a selected second resistor having different shape from the first shape by heating and transferring the second resistor on the dielectric substrate, the second shape of resistor being in correlation with resistance of the first resistor by heating and transferring the second resistor on the substrate and firing the second resistor deposited on the; and repeating the first resistor fabrication step and the second resistor fabrication step by selectively varying shapes of the first and second resistor to obtain a thick-film resistor having a desired resistance value.