Resistor substrate production method and variable resistor

Abstract

A resistor substrate production method according to the present invention comprises: a coating step for coating an ink that contains a solvent, a thermosetting resin, and conductive particles onto a substrate and forming a conductive coating film; a drying step for drying a workpiece, which is obtained by providing the conductive coating film on the substrate, at a second temperature that is lower than a first temperature at which a curing reaction of the thermosetting resin progresses, and removing the solvent; a hot pressing step for sandwiching the workpiece after the drying step between a first press platen and a second press platen, which are kept at the first temperature, pressing the workpiece, and partially curing the thermosetting resin; and a firing step for placing the workpiece after the hot pressing step into a firing furnace, firing the workpiece at the first temperature, and completing the curing of the thermosetting resin. In the hot pressing step, pressing is performed after a cushioning sheet that is softer than the pressing surface of the first press platen has been placed between the pressing surface of the first press platen and the workpiece. Therefore, the produced resistor substrate has excellent wear resistance with respect to sliding.

Description

Method for manufacturing a resistor substrate and variable resistor

[0001] The present invention relates to a method for manufacturing a resistor substrate and a variable resistor used, for example, in a variable resistor used in a position sensor.

[0002] In a rotary variable resistor in which the substrate side rotates, an inner electrode portion and an outer electrode portion made of a conductive pattern are formed in a ring shape or an arc shape on the inner peripheral side and the outer peripheral side of the substrate, and a resistance pattern is formed in an arc shape between the inner electrode portion and the outer electrode portion. One end of the resistance pattern is connected to the inner electrode portion, and the outer electrode portion is connected to the other end. Although the principle is the same, there is also a type in which the substrate side is fixed and the slider is slid.

[0003] On the other hand, three sliders are provided on the fixed part side, and each slider can slide on the surfaces of the inner electrode portion, the resistance pattern, and the outer electrode portion, respectively. When the substrate is moved (rotated) with a predetermined voltage applied between the inner electrode portion and the outer electrode portion through the slider, a resistance value that changes in proportion to the moving distance (rotation angle) can be taken out as a voltage from the remaining slider.

[0004] Patent Document 1 discloses a variable resistor provided with a substrate having a resistance pattern and an electrode pattern, and a fixed part having sliders that slide on the resistance pattern and the electrode pattern, respectively. The substrate is formed of a thermoplastic substrate, the electrode pattern is made of a conductive coating film containing silver powder printed on the substrate, and the conductive coating film is subjected to hot pressing. Patent Document 2 discloses an operating device using a linear reciprocating type (slide type) variable resistor as part of a sensor for measuring the tilting operation (tilting direction, tilting angle) of a lever.

[0005] Japanese Unexamined Patent Application Publication No. 2004-288924 International Publication No. 2021 / 246003

[0006] In the above-described variable resistor, the resistance pattern on the substrate (resistor substrate), like the electrode pattern, is manufactured by applying a heat press to a conductive coating containing conductive particles, which is then heated by pressing a surface plate against it. When the resistance pattern wears down due to contact between the resistance pattern and the slider, the thickness of the resistance pattern decreases locally, changing the resistance value per unit length. In the case of a rotary type, this changes the relationship between the rotation angle of the resistor substrate relative to the fixed part and the output voltage, affecting the basic function of the variable resistance value (controlling the output by changing the rotation angle). In the case of a sliding type (linear reciprocating type), this changes the relationship between the position of the slider relative to the fixed part of the resistor substrate and the output voltage, affecting the basic function of the variable resistance value (controlling the output by changing the relative position of the slider). Therefore, in order to improve the wear resistance of the resistor substrate with the resistance pattern against sliding, hard graphite may be used as the conductive particles contained in the conductive coating for forming the resistance pattern, or wear-resistant reinforcing materials may be additionally contained in the conductive coating.

[0007] In such cases, the variations and undulations in the surface roughness of the conductive coating used to form the resistance pattern become particularly pronounced. Therefore, when a surface plate is pressed against the conductive coating for hot pressing, variations occur in the pressure the conductive coating receives from the surface plate, resulting in variations in the mechanical properties of the resistance pattern obtained by hot pressing. This variation in the mechanical properties of the resistance pattern was a factor that destabilized the wear resistance of the resistive substrate of the variable resistor due to sliding.

[0008] In view of the current situation, the present invention aims to provide a method for manufacturing a resistor substrate with excellent wear resistance due to sliding, and a variable resistor equipped with a resistor substrate with excellent wear resistance to sliding.

[0009] In one embodiment, the present invention provides a method for manufacturing a resistor substrate, comprising: a coating step of applying an ink containing a solvent, a thermosetting resin, and conductive particles onto a substrate to form a conductive coating film; a drying step of drying a workpiece having the conductive coating film on the substrate at a second temperature lower than a first temperature at which the curing reaction of the thermosetting resin progresses, in order to remove the solvent; a hot pressing step of pressing the workpiece after the drying step between a first press platen and a second press platen held at a first temperature to partially cure the thermosetting resin; and a firing step of placing the workpiece after the hot pressing step in a firing furnace and firing it at a first temperature to complete the curing of the thermosetting resin, wherein in the hot pressing step, a buffer sheet softer than the press surface of the first press platen is interposed between the press surface of the first press platen and the workpiece when pressing.

[0010] The cushioning sheet interposed between the pressing surface of the first press platen and the workpiece deforms to conform to the undulations of the conductive coating on the workpiece, thus reducing variations in the pressure applied to the conductive coating on the workpiece during the hot pressing process. As a result, even when the conductive particles contained in the ink include hard materials such as graphite, the surface roughness of the resistance pattern formed by the curing of the thermosetting resin of the conductive coating is small. Therefore, when the resistance pattern slides against the metal contact member which acts as the slider, wear of the resistance pattern is suppressed, resulting in a resistor substrate with excellent wear resistance.

[0011] In the above manufacturing method, the cushioning sheet may be attached to the pressing surface of the first press platen. In the hot pressing process, when the cushioning sheet comes into contact with the workpiece, the temperature of the cushioning sheet is the same as that of the first press platen, so the processing time of the hot pressing process can be shortened.

[0012] In the above manufacturing method, it is preferable that the cushioning sheet be made of silicone rubber.

[0013] In the above manufacturing method, it is preferable that the cushioning sheet has a thickness of 0.3 to 3.0 mm and a durometer (Type A) rubber hardness measured in accordance with JIS K6253:2012 of 50° to 80°.

[0014] In the above manufacturing method, a release film may be interposed between the cushioning sheet and the workpiece. This makes it easier to peel the workpiece from the cushioning sheet when the pressure is released during the hot pressing process. This release film may be made of polyphenylene sulfide.

[0015] In the above manufacturing method, the ink may contain graphite in which the cumulative 50% particle size D50 in the cumulative particle size distribution from the small particle size side is 1 μm or more and 40 μm or less, in a volume-based particle size distribution obtained by wet measurement with water as the dispersion medium using laser diffraction / scattering method.

[0016] In another aspect, the present invention provides a variable resistor comprising a substrate, a resistor formed on the substrate comprising a thermosetting resin and conductive particles dispersed in the thermosetting resin, and a metal contact member sliding on the resistor, wherein the arithmetic mean roughness Ra of the resistor, as defined in JIS B0601:1994, is 0.13 μm or less.

[0017] In the above-described variable resistor, the maximum height Ry of the resistor as defined in JIS B0601:1994 may be 1.2 μm or less.

[0018] In the above-described variable resistor, the resistor may contain graphite in which the cumulative 50% particle size D50 in the cumulative particle size distribution from the small particle size side, obtained by wet measurement using laser diffraction and scattering with water as the dispersion medium, is 1 μm or more and 40 μm or less.

[0019] In another aspect, the present invention provides a variable resistor comprising a substrate, a resistor formed on the substrate comprising a thermosetting resin and conductive particles dispersed in the thermosetting resin, and a metal contact member sliding on the resistor, wherein a first roughness, which is the arithmetic mean roughness Ra as defined in JIS B0601:1994 and obtained by measuring the peaks of the undulations on the surface of the resistor, and a second roughness, which is the arithmetic mean roughness Ra obtained by measuring the peaks of the undulations on the surface of the resistor, satisfy the following formula (1): 0.6 ≤ (first roughness / second roughness) ≤ 1 (1)

[0020] In the above-described variable resistor, the conductive particles may include graphite in which the cumulative 50% particle diameter D50 in the cumulative particle size distribution from the small particle size side, obtained by wet measurement using laser diffraction / scattering with water as the dispersion medium, is 1 μm or more and 40 μm or less.

[0021] According to the present invention, even if the conductive coating on the surface of a workpiece subjected to a hot pressing process has undulations, all parts can be pressed. As a result, the surface roughness of all parts of the conductive coating is improved by hot pressing, and the wear resistance of the resistance pattern obtained from the conductive coating against sliding can be further enhanced.

[0022] This is a perspective view of a variable resistor according to one embodiment of the present invention. This is an explanatory diagram of the sliding structure of a variable resistor according to one embodiment of the present invention. This is a perspective view of the resistive substrate of a variable resistor according to one embodiment of the present invention. This is a diagram showing the output characteristics of the variable resistor in Figure 1. This is a process diagram for explaining the manufacturing method of a resistive substrate according to one embodiment of the present invention. This is a volume-based particle size distribution obtained by wet measurement using laser diffraction / scattering with water as the dispersion medium for an example of graphite that can be used in the manufacturing method of a resistive substrate according to one embodiment of the present invention. This is an explanatory diagram of the configuration of equipment used in the hot pressing process included in the manufacturing method of a resistive substrate according to one embodiment of the present invention. This is an explanatory diagram of the hot pressing process included in the manufacturing method of a resistive substrate according to one embodiment of the present invention. This is an observation image of the resistor (Example) of a resistive substrate manufactured by the manufacturing method of a resistive substrate according to one embodiment of the present invention. This is an observation image of the resistor (Comparative Example 1) of a resistive substrate manufactured by the conventional manufacturing method of a resistive substrate (with a hot pressing process but without a buffer sheet). This is an observation image of the resistor (Comparative Example 2) of a resistive substrate manufactured by the conventional manufacturing method of a resistive substrate (without a hot pressing process). This is a table showing the measurement results of the surface roughness of the resistors according to the Example and Comparative Example 1 and Comparative Example 2. This is a table summarizing the results shown in Figure 12A. This is a table showing the filtering wave undulation measurement results of resistors for the example and Comparative Example 1 and Comparative Example 2. This is a table summarizing the results shown in Figure 13A. This is a table showing the surface roughness measurement results of the peaks and valleys of the resistor for Comparative Example 1. This figure shows the measurement results of the surface shape after the sliding test of the hot-pressed resistor with cushioning material for the example. This figure shows the measurement results of the surface shape after the sliding test of the hot-pressed resistor without cushioning material for Comparative Example 1. This figure shows the measurement results of the surface shape after the sliding test of the resistor without hot pressing for Comparative Example 2. This is a table showing the relationship between the thickness of the cushioning material (cushioning sheet) and the surface roughness of the manufactured resistor in the method for manufacturing a resistor substrate according to one embodiment of the present invention. This is a table showing the relationship between the rubber hardness of the cushioning material (cushioning sheet) and the surface roughness of the manufactured resistor in the method for manufacturing a resistor substrate according to one embodiment of the present invention. This is a table showing the relationship between the arrangement of the cushioning material (cushioning sheet) and the surface roughness of the manufactured resistor in the method for manufacturing a resistor substrate according to one embodiment of the present invention.

[0023] Embodiments of the present invention will be described below with reference to the accompanying drawings. In each drawing, the same component is given the same number, and its description is omitted. Reference coordinates are shown in each drawing as appropriate to indicate the positional relationship of each component.

[0024] Figure 1 is a perspective view of a variable resistor according to one embodiment of the present invention. Figure 2 is an explanatory diagram of the sliding structure of the variable resistor according to one embodiment of the present invention. Figure 3 is a perspective view of the resistive substrate of the variable resistor according to one embodiment of the present invention. Figure 4 is a diagram showing the output characteristics of the variable resistor of Figure 1.

[0025] A variable resistor 1 according to one embodiment of the present invention shown in Figure 1 is used, for example, as part of a sensor that measures the tilting operation (tilting direction, tilting angle) of a lever of an operating device disclosed in Patent Document 2. The variable resistor 1 comprises a resistor substrate 3 on which resistance patterns 3b1, 3b2, and 3b3 (which may be collectively referred to as "resistance pattern 3b" in the following description), a first holder 2A on which a first metal contact member 6A having a contact portion 6c at each end is provided, and a second holder 2B on which a second metal contact member 6B having a contact portion 6c at each end is provided. In the following description, the first metal contact member 6A and the second metal contact member 6B may be collectively referred to as "metal contact member 6".

[0026] In the variable resistor 1, a sliding structure is formed in which the contact portion 6c and the resistance pattern 3b come into contact and slide against each other. Specifically, the sliding structure associated with the first metal contact member 6A moves back and forth (slides) in the Y1-Y2 direction, and the sliding structure associated with the second metal contact member 6B moves back and forth (slides) in the X1-X2 direction.

[0027] The resistor substrate 3 has an insulating substrate 3s made of a polyester resin such as polyethylene terephthalate (PET), a polyamide resin, a polyimide resin, and a resistor pattern 3b. In this embodiment, the substrate 3s is a flexible film, and as a specific example, it is made of a PET film with a thickness of 100 μm or less. The resistor pattern 3b is a conductive coating film containing a thermosetting resin and conductive particles that has been heat-pressed, and its manufacturing method will be described later.

[0028] The metal contact member 6 is made of phosphor bronze with nickel plating, then gold plating on its surface, and finally the tip is bent into a roughly circular shape. The metal contact member 6 has elasticity in the Z1-Z2 direction shown in the figure, and is capable of compressing the surface of the resistance pattern 3b.

[0029] It is possible to obtain a voltage corresponding to the position of the metal contact member 6 sliding on the resistance pattern 3b, that is, a voltage corresponding to the relative position of the metal contact member 6 with respect to the resistor substrate 3. The solid line in Figure 4 shows the relationship between the output voltage V0 and the relative position of the metal contact member 6 (both ends are normalized to 0 and 100). The two are basically directly proportional, and in Figure 4, as a specific example, the output voltage V0 is adjusted to be 0V when the relative position is 0, and to be 5.0V when the relative position is 100.

[0030] Figure 5 is a process diagram illustrating a method for manufacturing a resistor substrate according to one embodiment of the present invention. The manufacturing method according to this embodiment includes: a coating step of applying an ink containing a solvent, a thermosetting resin, and conductive particles onto a substrate to form a conductive coating film; a drying step of drying the workpiece 3W, which has the conductive coating film provided on the substrate, at a second temperature lower than the first temperature at which the curing reaction of the thermosetting resin progresses, in order to remove the solvent; a hot pressing step of pressing the workpiece 3W after the drying step between a first press platen and a second press platen held at a first temperature to partially cure the thermosetting resin; and a firing step of placing the workpiece 3W after the hot pressing step into a firing furnace and firing it at a first temperature to complete the curing of the thermosetting resin. Each step will be described in detail below.

[0031] The steps shown in Figure 5, from step S101 to step S104, are general processes, such as those described in Patent Document 1. An ink containing conductive particles such as silver particles for forming wiring (silver ink for wiring) is applied to the substrate 3s by screen printing to form a conductive coating for wiring (step S101), and this conductive coating for wiring is dried to evaporate the solvent contained in it (step S102).

[0032] Next, an ink for sliding purposes (sliding silver ink) is applied by screen printing onto a conductive coating for wiring that contains silver particles and is made of a thermosetting resin material (phenolic resin with relatively high hardness) to form a sliding conductive coating (step S103), and this sliding conductive coating is dried to evaporate the solvent contained in it (step S104).

[0033] If the resistor substrate 3 has a carbon electrode configuration, carbon ink for forming the carbon electrode is further applied to the conductive coating film for wiring by screen printing to form the carbon electrode (step S105), and the carbon electrode coating film is dried to volatilize the solvent contained in it (step S106).

[0034] Next, in the coating process, an ink for forming the carbon resistors that constitute the resistance pattern 3b (resistive ink) is applied by screen printing to the substrate on which the conductive coating for wiring, the conductive coating for sliding, and the conductive coating for carbon electrodes are provided, thereby forming a conductive coating (step S107). The coating method is not limited to screen printing, and other coating methods such as inkjet printing may also be used.

[0035] Resistive ink is an ink containing a solvent, a thermosetting resin, and conductive particles (carbon material). The type of solvent is not limited as long as it can dissolve the thermosetting resin before curing, and examples include organic solvents such as carbitol. Examples of conductive particles include graphite (graphite particles) which also function as a reinforcing material, and carbon black such as acetylene black, Ketjen black, and furnace black, which are finer and softer than graphite but have excellent conductivity. Figure 6 shows the particle size distribution of an example of graphite suitable as conductive particles for resistive ink. The conductive particles may be composed of one type of material or multiple types of materials.

[0036] Figure 6 shows the volume-based particle size distribution obtained by wet measurement using laser diffraction and scattering with water as the dispersion medium for an example of graphite that can be used in a method for manufacturing a resistor substrate according to one embodiment of the present invention. In the particle size distribution shown in Figure 6, the cumulative 10% particle size D10 in the cumulative particle size distribution from the small particle size side is 2.48 μm, the cumulative 50% particle size D50 in the cumulative particle size distribution from the small particle size side is 11.69 μm, and the cumulative 90% particle size D90 in the cumulative particle size distribution from the small particle size side is 43.46 μm.

[0037] Thus, in the manufacturing method according to this embodiment, when the resistive ink contains graphite, it is preferable that the cumulative 50% particle size D50 in the cumulative particle size distribution from the small particle size side, obtained by wet measurement using laser diffraction / scattering with water as the dispersion medium, be 1 μm or more and 40 μm or less. Note that when acetylene black and carbon black are measured in the same manner, the cumulative 90% particle size D90 is about 500 nm, which is much smaller than that of graphite.

[0038] The thermosetting resin contained in the resistance ink is not particularly limited. Specific examples of thermosetting resins include resol-type phenolic resins, mixtures of resol-type phenolic resin and phenoxy resin, and novolac-type phenolic resins, but other thermosetting resins such as epoxy resins and polyester resins (including curing agents) may also be used. The thermosetting resin may consist of one type of resin or a mixture of multiple types. The resistance ink may contain a curing agent for curing the thermosetting resin, may contain reinforcing materials consisting of conductive particles and insulating particles, and may contain other components such as colorants and viscosity modifiers.

[0039] The proportions of thermosetting resin, conductive particles, solvent, and other components as optional in the resistive ink are not limited. Examples, though not limited, include 5-10 wt% conductive particles (carbon black), 10-30 wt% reinforcing particles (graphite), 30-50 wt% thermosetting resin, and 30-40 wt% solvent.

[0040] Next, as a drying step, the substrate 3s, i.e., the workpiece 3W, on which the conductive coating film is applied, is dried at a second temperature lower than the first temperature at which the curing reaction of the thermosetting resin contained in the conductive coating film proceeds, thereby removing the solvent (step S108). The first and second temperatures should be appropriately set depending on the type and composition of the thermosetting resin. When the thermosetting resin is a phenolic resin, the first temperature is about 170°C, and the second temperature should be a temperature sufficiently lower than 170°C, for example, 120°C to 150°C.

[0041] In the process diagram shown in Figure 5, the coating step (step S109) in which the resistive ink is further applied to the workpiece 3W, and the drying step (step S110) in which the newly formed conductive coating film on the workpiece 3W is dried are performed again. Although these steps are not essential, by performing the coating and drying steps multiple times in this manner, the thickness of the conductive coating film on the substrate can be increased, and as a result, the resistance value of the resistive pattern 3b obtained from the conductive coating film can be increased, or the wear resistance of the resistive substrate 3 can be improved.

[0042] Once a conductive coating pattern of the desired thickness is formed on the substrate, a hot pressing process is performed (step S111). Figure 7 is an explanatory diagram of the configuration of the equipment used in the hot pressing process of the method for manufacturing a resistor substrate according to one embodiment of the present invention.

[0043] In Figure 7, the conductive coating 3b0 is provided on the D1 side of the resistor substrate 3, which is one side in the pressurizing direction (D1-D2 direction). The first press platen 11 is located on the D1 side of the resistor substrate 3, and the second press platen 12 is located on the D2 side of the substrate. Both press platens are made of steel and have a heating mechanism inside, maintaining a first temperature. By moving these press platens close together and pressing the workpiece 3W, which includes the resistor substrate 3 and the conductive coating 3b0, between the first press platen 11 and the second press platen 12, the shape of the conductive coating 3b0 on the workpiece 3W is adjusted (formed), and the thermosetting resin contained in the conductive coating 3b0 is partially cured. In the configuration shown in Figure 7, the D2 side is facing downward in the vertical direction.

[0044] In the manufacturing method according to this embodiment, a buffer sheet 21 that is softer than the pressing surface (the surface on the D1 side) of the first press platen 11 is interposed between the pressing surface (the surface on the D1 side) of the first press platen 11 and the workpiece 3W, and pressure is applied. As a result, even when the conductive particles contained in the resistive ink include a hard material such as graphite, the resistive pattern 3b formed by curing the conductive coating film 3b0 has a small surface roughness, that is, a smooth surface. Therefore, when the resistive pattern 3b and the metal contact member 6 slide, wear of the resistive pattern 3b is suppressed, and a resistor substrate 3 with excellent wear resistance can be obtained.

[0045] As shown in FIG. 4, the variable resistor 1 of the linear reciprocating type (slide type) including the resistor substrate 3 is adjusted such that the output voltage V0 is 5.0 V when the relative position is 100. However, when the resistive pattern 3b wears due to sliding with the metal contact member 6, a large amount of noise is generated when the metal contact member 6 is slid due to the influence of unevenness and wear powder generated on the surface of the resistive pattern 3b, and the output becomes unstable. Therefore, it is preferable that the wear of the resistive pattern 3b of the resistor substrate 3 is small.

[0046] In the manufacturing method according to this embodiment, the buffer sheet 21 is attached to the pressing surface (the surface on the D1 side) of the first press platen 11. In this case, since the buffer sheet 21 and the first press platen 11 are integrally heated, when the buffer sheet 21 contacts the workpiece 3W (in a preferred example, a peeling film 40 is located on the buffer sheet 21 as described later), the temperature of the buffer sheet 21 (the peeling film 40 when the peeling film 40 is provided) is substantially the same as the temperature of the first press platen 11. Therefore, the processing time of the hot press process can be shortened.

[0047] The buffer sheet 21 is not limited in material as long as it has appropriate heat resistance that can withstand the heating and pressing in the hot press process, is harder than the conductive coating film 3b0, and is softer than the pressing surface (the surface on the D1 side) of the first press platen 11. As an example where the material of the buffer sheet 21 is not limited, silicone rubber can be mentioned.

[0048] The thickness of the buffer sheet 21 should be appropriately set according to the curing characteristics of the conductive coating film 3b0 containing a thermosetting resin, the heating and pressing conditions in the hot press process, the material of the buffer sheet 21, and the like. If the thickness of the buffer sheet 21 is excessively thin, it may be difficult to obtain the effect of providing the buffer sheet 21. On the other hand, if the thickness of the buffer sheet 21 is excessively thick, the degree to which the pressing force of the first press platen 11 is relaxed by the buffer sheet 21 becomes large, so it becomes necessary to take measures such as particularly increasing the pressing force of the first press platen 11, and it may be difficult to increase the production efficiency. As an example where the thickness of the buffer sheet 21 is not limited, it can be set to 0.3 to 3 mm.

[0049] The hardness of the buffer sheet 21 should also be appropriately set in relation to other conditions, similar to the thickness. If the buffer sheet 21 is excessively soft or excessively hard, it may be difficult to obtain the effect of providing the buffer sheet 21. As an example where the hardness of the buffer sheet 21 is not limited, it can be set to 50° to 80° as the durometer (type A) rubber hardness measured in accordance with JIS K6253:2012.

[0050] In the manufacturing method according to this embodiment, as a preferred example, a peeling film 40 is interposed between the buffer sheet 21 and the workpiece 3W. By providing the peeling film 40, when the pressure is released in the hot press process, it becomes easy to peel the workpiece 3W from the buffer sheet 21. The material of the peeling film 40 is not limited as long as it has appropriate heat resistance and peelability. As an example where the material of the peeling film 40 is not limited, polyphenylene sulfide can be mentioned.

[0051] In the manufacturing method according to this embodiment, a buffer sheet 22 is also provided between the surface of the substrate 3s on the side where the conductive coating film 3b0 is not provided (the surface on the D2 side) and the second press platen 12. By providing the buffer sheet 22 in addition to the buffer sheet 21, the smoothness of the obtained resistance pattern 3b may be improved, and a resistor substrate 3 with more excellent wear resistance may be obtained. The description of the material, thickness, and hardness of the buffer sheet 22 is omitted because they are the same as those of the buffer sheet 21.

[0052] Figure 8 is an explanatory diagram of the hot pressing process included in a method for manufacturing a resistor substrate according to one embodiment of the present invention. Figure 8 shows the effect of the buffer sheet 21 on the conductive coating 3b0. The upper section shows the case when the buffer sheet 21 is not provided, and the lower section shows the case when the buffer sheet 21 is provided. The following states are shown from right to left: (State 1) The state in which the heating and pressing member comes into contact with the conductive coating 3b0 and the hot pressing process begins. (State 2) The stage during the hot pressing process in which the conductive coating 3b0 is deforming and hardening is progressing due to the heating and pressing member. (State 3) The stage in which the hot pressing process is completed just before the depressurization of the press platen is started for the conductive coating 3b0 after heating and pressing under predetermined conditions is completed. (State 4) The state in which the depressurization of the press platen is completed and the heating and pressing member is separated from the partially hardened conductive coating 3b0.

[0053] When the buffer sheet 21 is not provided (upper section), the member that is heated and pressurized in state 1 (hereinafter also referred to as the "contact member") is the release film 40 in contact with the first press platen 11. When the buffer sheet 21 is provided (lower section), the contact member in state 1 is the release film 40 in contact with the buffer sheet 21. The buffer sheet 21 is attached to the first press platen 11 in advance, and the thickness of the release film 40 that contacts it is also thin at 100 μm, so the temperature of the buffer sheet 21 and the release film 40 is substantially the same as that of the first press platen 11. Therefore, in state 1, there is no particular difference in the conductive coating 3b0 between the upper and lower sections. That is, the convex portions 31 of the relatively protruding regions (peak portions 3bh) on the surface of the conductive coating 3b0 are in contact with the contact member, and the convex portions 32 of the relatively recessed regions (valley portions 3bv) on the surface of the conductive coating 3b0 are not in contact with the contact member.

[0054] In state 2, as the heat pressing process progresses, the contact member becomes closer to the conductive coating 3b0 than in state 1. As a result, the protrusions 31 of the peak portion 3bh are pressed in, the height of the protrusions 31 decreases, and the surface roughness of the peak portion 3bh improves. This trend is basically the same in both cases where the cushioning sheet 21 is not provided (upper part of Figure 8) and when the cushioning sheet 21 is provided (lower part of Figure 8).

[0055] From this state 2 onward, there is a difference in the degree of pressure between when the cushioning sheet 21 is not provided (upper part of Figure 8) and when the cushioning sheet 21 is provided (lower part of Figure 8), and this tendency is particularly pronounced in the valley portion 3bv. When the cushioning sheet 21 is not provided (upper part of Figure 8), even as the heat pressing process progresses, it is difficult for the valley portion 3bv to come into contact with the contact member (the release film 40 in contact with the first press platen 11), and the curing reaction of the thermosetting resin progresses due to the heat conducted from the peak portion 3bh that is in contact with the contact member (the release film 40 in contact with the first press platen 11) and the heat conducted from the second press platen 12. For this reason, in the valley portion 3bv, the degree of protrusion of the convex portion 32 does not change significantly, and the curing reaction of the thermosetting resin proceeds.

[0056] In contrast, when a buffer sheet 21 is provided (lower part of Figure 8), the release film 40 in contact with the buffer sheet 21, which is located between the first press platen 11 and the conductive coating 3b0, is compressed, particularly in the region in contact with the peak portion 3bh. As a result, the buffer sheet 21, heated to a predetermined temperature, also comes into contact with the valley portion 3bv of the conductive coating 3b0. Heat and pressure from the first press platen 11 are transmitted from the buffer sheet 21 in contact with the valley portion 3bv, directly promoting the curing reaction of the thermosetting resin in the valley portion 3bv, and pushing the convex portion 32 of the valley portion 3bv of the conductive coating 3b0 towards D2. Consequently, when a buffer sheet 21 is provided (lower part of Figure 8), the surface roughness of the valley portion 3bv of the conductive coating 3b0 remains high compared to when a buffer sheet 21 is not provided (upper part of Figure 8). (The surface roughness before hot pressing is reflected.)

[0057] As this process progresses and the hot pressing treatment is completed, in state 3, when the cushioning sheet 21 is provided (lower part of Figure 8), the degree of protrusion of the convex portions 31 and 32 on the entire surface of the conductive coating 3b0 is reduced. In contrast, when the cushioning sheet 21 is not provided (upper part of Figure 8), even after the hot pressing treatment is completed, the convex portions 32 of the valley portions 3bv of the conductive coating 3b0 do not deform properly, and the degree of protrusion remains high.

[0058] Therefore, in state 4, when the hot pressing process is completed and the contact member is separated from the conductive coating 3b0, if the buffer sheet 21 is provided (lower part of Figure 8), the surface roughness of the entire surface of the conductive coating 3b0 is reduced and smoothed, whereas if the buffer sheet 21 is not provided (upper part of Figure 8), there are parts (valleys 3bv) where the surface roughness is not reduced.

[0059] Once the above hot pressing process is complete, the workpiece 3W, on which the partially cured conductive coating 3b0 of the thermosetting resin is applied to the substrate 3s, is placed in a firing furnace and fired at a first temperature at which the curing reaction of the thermosetting resin proceeds, thereby completing the curing of the thermosetting resin (step S112). The first temperature does not need to be exactly the same as the first temperature of the press platen in the hot pressing process; it is sufficient if it is a temperature at which the curing reaction of the thermosetting resin proceeds. The firing time in the firing process is set appropriately so that the curing of the thermosetting resin is completed. For example, if the time for the hot pressing process is sufficiently long, the firing time can be short, and furthermore, if the entire thermosetting resin is cured in the hot pressing process, the firing process is unnecessary. If the number of workpieces 3W that can be processed simultaneously in the firing process is greater than the number of workpieces 3W that can be processed simultaneously in the hot pressing process, it is preferable, as it is more productive, to partially cure the thermosetting resin in the hot pressing process without completing the curing reaction in the hot pressing process, and to complete the curing reaction in the firing process.

[0060] In this way, a resistor substrate 3 having a resistor pattern 3b made of resistors is manufactured.

[0061] The following describes the results of observation and evaluation of the resistors of the resistor substrate 3 manufactured by the manufacturing method according to this embodiment (Example), the resistors of the resistor substrate manufactured by a process that performs a hot pressing process without using a buffer sheet 21 (Comparative Example 1), and the resistors of the resistor substrate manufactured by completing the curing reaction of the thermosetting resin by a firing process alone without a hot pressing process (Comparative Example 2).

[0062] In each example, as shown in Figure 5, the resistive ink was applied and dried twice. In both applications, the resistive ink contained a novolac-type phenolic resin as a thermosetting resin and graphite and acetylene black as conductive particles having the particle size distribution shown in Figure 6. The drying conditions were 120°C (second temperature) for 10 minutes.

[0063] In the hot pressing process of Example 1, cushioning sheets 21 and 22 were used. The material of the cushioning sheets 21 and 22 used was silicone rubber, with a durometer (Type A) rubber hardness of 60° measured in accordance with JIS K6253:2012, and a thickness of 5 mm. The conditions for the hot pressing process were a load of approximately 4 kN, and pressurization and heating were performed at 170°C (first temperature) for 1 minute.

[0064] The firing conditions were 170°C (first temperature) for 20 minutes. In Comparative Example 1, the buffer sheet 21 was not used during the hot pressing process. In Comparative Example 2, the firing process was performed immediately after the drying process without performing the hot pressing process.

[0065] Figure 9 is an observation image of a resistor (example) on a resistor substrate manufactured by the resistor substrate manufacturing method according to one embodiment of the present invention. Figure 10 is an observation image of a resistor (comparative example 1) on a resistor substrate manufactured by the conventional resistor substrate manufacturing method (with a hot pressing process but without a buffer sheet). Figure 11 is an observation image of a resistor (comparative example 2) on a resistor substrate manufactured by the conventional resistor substrate manufacturing method (without a hot pressing process).

[0066] The observation image of the resistor according to the embodiment shown in Figure 9 shows a low degree of contrast, confirming that the resistor of Example 1 has good overall surface roughness (smoothness). The observation image of the resistor according to Comparative Example 1 shown in Figure 10 shows a higher degree of contrast compared to the observation image in Figure 9, and areas with particularly high contrast were observed, as shown by the dashed circles in Figure 10. This area is the valley portion 3bv, indicating large irregularities. Such areas can generate locally large frictional resistance when in contact with the metal contact member 6, and can become the starting point for wear of the resistor. Such areas were not observed in the resistor according to the embodiment, suggesting that the resistor according to the embodiment is less prone to wear than the resistor according to Comparative Example 1.

[0067] The observation image of the resistor according to Comparative Example 2 shown in Figure 11 showed that the entire surface had irregularities equivalent to or greater than those of the valley portion 3bv in Figure 10. Furthermore, as shown in Figure 11, a gap G was observed between the hardened resin and the graphite in the resistor according to Comparative Example 2. Because the graphite adjacent to this gap G is relatively weakly held by the resin, this graphite may detach from the resistance pattern 3b shortly after the start of sliding when the resistance pattern 3b made of the resistor slides against the metal contact member 6. A recess is created in the part of the resistance pattern 3b where the graphite has detached, and this recess can become the starting point for wear.

[0068] Figure 12A is a table showing the measurement results of the surface roughness of the resistors for the Examples and Comparative Examples 1 and 2. Figure 12B is a table summarizing the results shown in Figure 12A. Figure 13A is a table showing the measurement results of the filtered wave undulation of the resistor patterns for the Examples and Comparative Examples 1 and 2. Figure 13B is a table summarizing the results shown in Figure 13A. The surface shape of each resistor was measured using a surface roughness measuring instrument "Surfcom 480A" manufactured by Tokyo Seimitsu Co., Ltd., and roughness parameters specified in JIS B0601:1994 were obtained. The measurement length was 3 mm, and the filter pass wavelength used for filtering wave undulation measurement was 2.5 μm or greater.

[0069] For each example, five measurement points were arbitrarily selected, and Figure 12A shows the measurement results for arithmetic surface roughness Ra and maximum height Ry at each measurement point, as well as the average, maximum, and minimum values. In Figure 12A, "with hot pressing" refers to Example Example, "without hot pressing" refers to Comparative Example 2, and "hot pressing without cushioning material" refers to Comparative Example 1. The same applies to the tables in the following figures.

[0070] As shown in Figures 12A and 12B, it was confirmed that the resistor according to the example has a smoother surface shape and smaller values ​​for both arithmetic surface roughness Ra and maximum height Ry compared to the resistor according to the comparative example. This is thought to be because, in the manufacturing method of the example, the cushioning sheet 21 appropriately deforms the convex portion 32 of the valley portion 3bv during the hot pressing process. The arithmetic mean roughness Ra of the resistor, as defined in JIS B0601:1994, is preferably 0.13 μm or less, and the maximum height Ry, as defined in JIS B0601:1994, is preferably 1.2 μm or less.

[0071] In the measurement of filtered wave swell, five measurement points were arbitrarily selected for each example, and Figure 13A shows the measurement results of arithmetic surface roughness Wa and maximum height WCM at each measurement point, as well as the average, maximum, and minimum values.

[0072] As shown in Figures 13A and 13B, the resistor according to the example had smaller values ​​for both arithmetic surface roughness Wa and maximum height WCM than the resistor according to the comparative example, confirming that it had a better surface shape. In the manufacturing method according to the example, the cushioning sheet 21 not only appropriately deforms the convex portion 32 of the valley portion 3bv during the hot pressing process, but the deformation of the cushioning sheet 21 during pressurization also smooths the shape of the entire peak portion 3bh and the entire valley portion 3bv, which may result in a smaller filtering wave undulation of the resistor according to the example.

[0073] Figure 14 is a table showing the surface roughness measurement results for the peaks and valleys of the resistor according to Comparative Example 1. Regarding the shape of the peak portion 3bh and the valley portion 3bv, the roughness parameters of the peak portion (peak portion 3bh) and valley portion (valley portion 3bv) of the resistor were measured for Comparative Example 1. As shown in Figure 14, for the resistor according to Comparative Example 1, after hot pressing, the degree of improvement in the surface roughness of the valley portion 3bv was lower than the degree of improvement in the surface roughness of the peak portion 3bh, and as a result, the value of the surface roughness parameter of the valley portion 3bv became relatively larger. In the case of Example 1, after hot pressing, it was difficult to visually distinguish between the peak portion 3bh and the valley portion 3bv because it was sufficiently flattened. In the resistor of Example 1, the surface roughness of the valley portion 3bv was also appropriately improved in the hot pressing process, so it is thought that the identifiability between the peak portion 3bh and the valley portion 3bv decreased.

[0074] The results shown in Figure 14 indicate that it is preferable for the first roughness, which is the arithmetic mean roughness Ra obtained by measuring the peaks of the undulations on the surface of the heat-pressed resistors constituting the resistance pattern 3b, and the second roughness, which is the arithmetic mean roughness Ra obtained by measuring the peaks of the undulations on the surface of the resistors constituting the resistance pattern 3b, to satisfy the following equation (1): 0.6 ≤ (first roughness / second roughness) ≤ 1 (1)

[0075] In the resistor according to Comparative Example 1, the ratio of the first roughness to the second roughness was 0.55, calculated from the results shown in Figure 14. In contrast, in the resistor according to the Example, the peak portion 3bh and the valley portion 3bv could not be distinguished, so it can be said that the ratio of the first roughness to the second roughness was substantially 1.

[0076] Figure 15 shows the measurement results of the wear depth measured from the surface shape after the sliding test of the resistor according to the embodiment. Figure 16 shows the measurement results of the surface shape after the sliding test of the resistor according to Comparative Example 1. Figure 17 shows the measurement results of the surface shape after the sliding test of the resistor according to Comparative Example 2.

[0077] For each resistor in each example, a sliding test was conducted using a simple sliding test machine, with two lines of sliding tests each having a reciprocating sliding distance of 2.46 mm, under a load of 7 gf, and with approximately 5 million sliding cycles.

[0078] As shown in Figure 15, the wear depth of the resistor according to the example was about 1 μm, while in Comparative Example 1, as shown in Figure 16, the wear depth was about 3 to 4 μm. The surface shape of the resistor in Example 1 was better than that of the resistor in Comparative Example 1 in terms of both surface roughness and filtering undulation, which is thought to have made it less prone to wear. As shown in Figure 17, the wear depth of Comparative Example 2 was about 6 μm. From the above wear tests, it was confirmed that a certain improvement in wear resistance can be obtained even with a resistor substrate manufactured through a hot pressing process without using the buffer sheet 21, but more importantly, it was confirmed that the wear resistance of the resistor substrate 3 manufactured through a hot pressing process using the buffer sheet 21 was significantly improved.

[0079] Figure 18 is a table showing the relationship between the thickness of the buffer material (buffer sheet) and the surface roughness of the manufactured resistor in a method for manufacturing a resistor substrate according to one embodiment of the present invention. Figure 19 is a table showing the relationship between the hardness of the buffer material (buffer sheet) and the surface roughness of the manufactured resistor in a method for manufacturing a resistor substrate according to one embodiment of the present invention. Figure 20 is a table showing the relationship between the arrangement of the buffer material and the surface roughness of the manufactured resistor in a method for manufacturing a resistor substrate according to one embodiment of the present invention. The influence of the buffer sheet (buffer material) on the surface shape of the manufactured resistor will be explained below using Figures 18 to 20.

[0080] First, under the manufacturing conditions of Example 1 (using cushioning sheet 21 and not using cushioning sheet 22), the thickness of cushioning sheet 21 was set to 1 mm, 2 mm, 3 mm (same conditions as Example 1), 4 mm, and 5 mm, while other manufacturing conditions were the same as in Example 1, to manufacture a resistor substrate 3 (Example 2).

[0081] As a result, as shown in Figure 18, both the arithmetic surface roughness Ra and the maximum height Ry increased as the thickness of the cushioning sheet 21 increased. Since the cushioning sheet 21 deforms preferentially to other components in a pressurized and heated environment, the pressurizing force from the first press platen 11 is partially mitigated by the cushioning sheet 21. Therefore, it was suggested that as the thickness of the cushioning sheet 21 increases, the pressure on the conductive coating film weakens, and the degree of improvement in the surface smoothness of the resistor after the heat press treatment decreases.

[0082] Furthermore, as the thickness of the cushioning sheet 21 increases, the degree of heat transfer from the first press platen 11 to the cushioning sheet 21 weakens, and as a result, the temperature of the surface of the cushioning sheet 21 that is in contact with the conductive coating may be lower than the temperature of the first press platen 11. Therefore, when the thickness of the cushioning sheet 21 is large, it may be preferable to take measures such as increasing the pressure applied to the first press platen 11 or increasing the heating temperature compared to when the cushioning sheet 21 is relatively thin.

[0083] Next, under the manufacturing conditions of Example 1 (using cushioning sheet 21 and not using cushioning sheet 22), the rubber hardness of cushioning sheet 21 was set to 50°, 60° (same conditions as Example 1), and 80°, while other manufacturing conditions were the same as in Example 1, to manufacture a resistor substrate 3 (Example 3).

[0084] As a result, as shown in Figure 19, the values ​​of the surface roughness parameters (arithmetic surface roughness Ra and maximum height Ry) were smaller when the rubber hardness was 60° than when the rubber hardness was 50° or 80°. This result indicates that there is a particularly good range for the rubber hardness of the cushioning sheet 21. If the rubber hardness of the cushioning sheet 21 is too low, the cushioning sheet 21 may deform excessively, the pressing force may decrease relatively, and the degree of adjustment of the surface shape of the conductive coating may decrease. If the rubber hardness of the cushioning sheet 21 is too high, the cushioning sheet 21 may not deform properly, the pressing force on the valley portion 3bv may decrease relatively, and the degree of adjustment of the surface shape of the conductive coating in the hot pressing process may decrease.

[0085] Next, we compared the case where only the cushioning sheet 21 is used (same manufacturing conditions as in Example 1) with the case where the cushioning sheet 22 is also used (Example 3). In Example 3, the resistor substrate 3 was manufactured under the same manufacturing conditions as in Example 1, except that the cushioning sheet 22 was used.

[0086] As a result, as shown in Figure 20, the surface roughness parameters (arithmetic surface roughness Ra and maximum height Ry) were slightly smaller when the cushioning sheet 22 was also used compared to when only the cushioning sheet 21 was used. This result indicates that using the cushioning sheet 22 is preferable.

[0087] The embodiments described above are provided to facilitate understanding of the present invention and are not intended to limit it. Accordingly, each element disclosed in the above embodiments is intended to include all design modifications and equivalents that fall within the technical scope of the present invention.

[0088] 1: Variable resistor 2A: First holder 2B: Second holder 3: Resistor substrate 3W: Workpiece 3b: Resistor pattern 3b0: Conductive coating 3b1, 3b2, 3b3: Resistor pattern 3bh: Peak portion 3bv: Valley portion 3s: Substrate 6: Metal contact member 6A: First metal contact member 6B: Second metal contact member 6c: Contact portion 11: First press platen 12: Second press platen 21, 22: Cushioning sheet 31, 32: Protrusion 40: Release film G: Gap

Claims

1. A method for manufacturing a resistor substrate, comprising: a coating step of applying an ink containing a solvent, a thermosetting resin, and conductive particles onto a substrate to form a conductive coating film; a drying step of drying a workpiece having the conductive coating film on the substrate at a second temperature lower than a first temperature at which the curing reaction of the thermosetting resin progresses to remove the solvent; a hot pressing step of pressing the workpiece after the drying step between a first press platen and a second press platen held at a first temperature to partially cure the thermosetting resin; and a firing step of placing the workpiece after the hot pressing step in a firing furnace and firing it at a first temperature to complete the curing of the thermosetting resin, wherein in the hot pressing step, a buffer sheet softer than the press surface of the first press platen is interposed between the press surface of the first press platen and the workpiece to apply pressure.

2. The method for manufacturing a resistor substrate according to claim 1, wherein the cushioning sheet is attached to the pressing surface of the first press platen.

3. The method for manufacturing a resistor substrate according to claim 1, wherein the cushioning sheet is made of silicone rubber.

4. The method for manufacturing a resistor substrate according to claim 1, wherein the cushioning sheet has a thickness of 0.3 to 3.0 mm and a durometer (Type A) rubber hardness measured in accordance with JIS K6253:2012 is 50° to 80°.

5. The method for manufacturing a resistor substrate according to claim 1, wherein a release film is interposed between the cushioning sheet and the workpiece.

6. The method for manufacturing a resistor substrate according to claim 5, wherein the release film is polyphenylene sulfide.

7. The method for manufacturing a resistor substrate according to claim 1, wherein the ink contains graphite in which the cumulative 50% particle size D50 in the cumulative particle size distribution from the small particle size side is 1 μm or more and 40 μm or less in a volume-based particle size distribution obtained by wet measurement with water as the dispersion medium using laser diffraction and scattering method.

8. A variable resistor comprising: a substrate; a resistor formed on the substrate, comprising a thermosetting resin and conductive particles dispersed in the thermosetting resin; and a metal contact member that slides on the resistor, wherein the arithmetic mean roughness Ra of the resistor, as defined in JIS B0601:1994, is 0.13 μm or less.

9. The variable resistor according to claim 8, wherein the maximum height Ry of the resistor as defined in JIS B0601:1994 is 1.2 μm or less.

10. The variable resistor according to claim 8, wherein the resistor contains graphite in which the cumulative 50% particle size D50 in the cumulative particle size distribution from the small particle size side is 1 μm or more and 40 μm or less in a volume-based particle size distribution obtained by wet measurement with water as the dispersion medium using laser diffraction and scattering method.

11. A variable resistor comprising: a substrate; a resistor formed on the substrate comprising a thermosetting resin and conductive particles dispersed in the thermosetting resin; and a metal contact member sliding on the resistor, wherein the first roughness, which is the arithmetic mean roughness Ra as defined in JIS B0601:1994 and obtained by measuring the peaks of the undulations on the surface of the resistor, and the second roughness, which is the arithmetic mean roughness Ra obtained by measuring the peaks of the undulations on the surface of the resistor, satisfy the following formula (1): 0.6 ≤ (first roughness / second roughness) ≤ 1 (1) 12. The variable resistor according to claim 11, wherein the conductive particles include graphite, the cumulative 50% particle diameter D50 in the cumulative particle size distribution from the small particle size side in the volume-based particle size distribution obtained by wet measurement using laser diffraction and scattering with water as the dispersion medium is 1 μm or more and 40 μm or less.

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