JP7808533B2 - Circuit substrate and method of manufacturing the same - Google Patents

Description

The present invention relates to circuit substrates used in automotive electronic devices, industrial electronic devices, information terminal devices, home appliances, etc., and to methods for manufacturing such substrates.

A conventional circuit substrate, not shown, is formed into a self-capacitance type capacitance sensor that includes, for example, a bendable substrate, a conductive pattern on the surface of the substrate with multiple electrodes and the like arranged thereon, and a protective sheet that is laminated on the surface of the substrate and covers the conductive pattern. After being placed opposite the back side of an operation panel, when a conductive finger selectively approaches multiple electrodes on the conductive pattern via the operation panel, the capacitance between the electrode and the finger changes, detecting touch coordinates (see Patent Documents 1 and 2).

The substrate is formed, for example, from a thick resin film made from polyethylene terephthalate (PET) resin, which not only has high chemical and physical functionality but also stable and uniform quality. The conductive pattern is formed by printing a pattern of a specific conductive material, such as silver ink or carbon nanotubes, on the surface of the substrate, followed by heat treatment to dry and harden it. The protective sheet is not particularly limited, but may be formed, for example, from a transparent sheet made from silicone with weak adhesiveness, which adheres to the surface of the substrate and effectively prevents external dust and other particles from adhering to the conductive pattern.

JP 2017-201272 A JP 2010-244776 A

Conventional capacitance sensors are constructed as described above, and when the substrate is a thick resin film, it becomes difficult to achieve the thinness that has been strongly desired in recent years. Furthermore, when the substrate is a resin film made of polyethylene terephthalate resin, it is not easy to not only thin the sensor, but also to improve its chemical resistance. Furthermore, the resin film becomes brittle, so reinforcement may be required.

Furthermore, when the substrate is a polyethylene terephthalate resin film, when a predetermined conductive material pattern is printed on the surface of the substrate and then heat-treated, the heat treatment is performed at a temperature of 150°C or less, taking into consideration the heat resistance of the polyethylene terephthalate resin. However, if the conductive material is heat-treated at a temperature of 150°C or less, the conductive material may not be able to fully function. For example, if the conductive material is a conductive ink that requires high-temperature heating of 160°C or more to metallize, heat treatment at a temperature of 150°C or less will not prevent the high resistance associated with residual organic matter, making it extremely difficult to achieve a volume resistivity of 20 μΩ-cm or less for the conductive pattern.

Furthermore, if the protective sheet for the capacitance sensor is simply a transparent silicone sheet, when the operation panel is curved like a bow with a high curvature, the protective sheet may not properly conform to the curved surface of the operation panel and fit tightly, or it may become misaligned, causing problems with the attachment of the capacitance sensor.

The present invention has been developed in light of the above, and aims to provide a circuit substrate and a manufacturing method thereof that can be made thinner, have improved chemical resistance and conformability, can improve the functionality of the conductive pattern layer by heat-treating the conductive material at high temperatures, and can eliminate the risk of misalignment causing problems with installation.

In order to solve the above problems, the present invention provides a flexible adhesive sheet including a flexible release layer, an insulating layer releasably formed on the release layer, a conductive pattern layer formed on the insulating layer using at least a silver-containing paste, an adhesive layer adhered to a portion of the conductive pattern layer, and a deformation suppressing layer formed on the remaining portion of the conductive pattern layer,
the release layer is made of a silicone elastomer having a thickness of 10 μm or more and 250 μm or less, the silicone elastomer having a Shore A hardness of 10 or more and 60 or less when measured in accordance with JIS K 6253, a breaking elongation of 100% or more and 1000% or less when measured in accordance with JIS K 6251, and a tensile strength of 4 MPa or more and 12 MPa or less when measured in accordance with JIS K 6251;
The insulating layer is formed by laminating resin to a thickness of 10 μm or more,
The silver-containing paste is characterized by being either a silver complex paste ink or a silver nanopaste ink, the volume resistivity of which decreases when heat-treated at 160°C or higher.

The insulating layer is formed by laminating a thermosetting polyol-based resin, and the Shore D hardness of this resin, measured in accordance with JIS K 6253, can be set to 60 or more and 75 or less, and the flexural modulus, measured in accordance with JIS K 7171, can be set to 500 MPa or more and 2500 MPa or less.

In addition, the silver complex paste ink of the silver-containing paste can be a paste ink having a thixotropy index of 1.7 or more and 1.8 or less, a solid content of 30% wt or more and 33% wt or less, and a volume resistivity of the conductive pattern layer of 10 μΩ cm or less when heat-treated at 160°C or more.
In addition, the silver nanopaste ink of the silver-containing paste can also be a paste ink having a silver concentration of 50 wt % or more as measured by thermogravimetric differential thermal analysis, and a solvent composition of a non-water-soluble solvent or an alcohol/glycol mixed solvent.

The adhesive layer is made of a hot melt adhesive, a thermosetting adhesive, a light curing adhesive, an ultraviolet curing adhesive, or a room temperature curing adhesive, and preferably has a thickness of 12 μm or more.
The deformation suppression layer is preferably made of a resin film having a thickness of 12 μm or more, a Shore A hardness of 10 or more and 60 or less when measured in accordance with JIS K 6253, a breaking elongation of 100% or more and 1000% or less when measured in accordance with JIS K 6251, and a tensile strength of 4 MPa or more and 12 MPa or less when measured in accordance with JIS K 6251.

In order to solve the above-mentioned problems, the present invention provides a method for manufacturing a circuit substrate as described in claim 1 or 2, which comprises: forming a resin insulating layer having a thickness of 10 μm or more on a flexible release layer in a releasable manner; printing at least a silver-containing paste on the insulating layer and heat-treating the printed insulating layer to form a conductive pattern layer; applying an adhesive to a portion of the conductive pattern layer and curing the adhesive to form an adhesive layer; and forming a deformation suppressing layer on the remaining portion of the conductive pattern layer,
the release layer is made of a silicone elastomer having a thickness of 10 μm or more and 250 μm or less, the silicone elastomer having a Shore A hardness of 10 or more and 60 or less when measured in accordance with JIS K 6253, a breaking elongation of 100% or more and 1000% or less when measured in accordance with JIS K 6251, and a tensile strength of 4 MPa or more and 12 MPa or less when measured in accordance with JIS K 6251;
The silver-containing paste is characterized by being either a silver complex paste ink or a silver nanopaste ink, the volume resistivity of which decreases when heat-treated at 160°C or higher.

Here, the silver-containing paste in the claims may be a silver complex paste ink or a silver nanopaste ink, as long as the volume resistivity measured by a four-terminal four-probe method is ^10-5 Ω or less. Furthermore, the multiple electrodes of the conductive pattern layer are appropriately arranged in the XY, X, and Y directions of the insulating layer. When the conductive pattern layer is formed by heat treatment, it is preferable to heat the silver-containing paste to at least 160°C or higher. Furthermore, the circuit substrate may be, for example, a capacitance sensor, a printed wiring board, a flexible substrate, a high-frequency substrate, an antenna, an RFID, or the like, and may be built into the housing of various electrical and electronic devices or attached to the back side of an operation panel, for example.

According to the present invention, no thick substrate is used for the insulating layer, which allows for thinner and lighter circuit boards and greater design freedom for capacitance sensors. Furthermore, because the release layer is made of a silicone elastomer with excellent heat resistance of 200°C or higher, high-temperature drying processes are possible. Furthermore, because this silicone elastomer has excellent flexibility, the circuit board can be appropriately bent and brought into close contact with three-dimensional surfaces, etc.

This invention has the advantage of enabling the circuit substrate to be made thinner and improving its chemical resistance and conformability. It also has the advantage of improving the functionality of the conductive pattern layer by heat-treating the conductive material at high temperatures exceeding 150°C, and eliminating the risk of misalignment causing problems when attaching the circuit substrate.

According to the invention described in claim 2, the insulating layer is formed by laminating a thermosetting polyol-based resin, which is expected to provide excellent heat resistance, chemical resistance, weather resistance, and other properties. Furthermore, the Shore D hardness of the insulating layer, measured in accordance with JIS K 6253, is 60 or more and 75 or less, ensuring the flexibility and pliability of the insulating layer when used on complex or curved surfaces.

According to the invention described in claim 3, the silver complex paste ink of the silver-containing paste has a thixotropy index of 1.7 to 1.8, a solids content of 30% to 33% by weight, and a volume resistivity of the conductive pattern layer of 10 μΩ-cm or less when heat-treated at 160°C or higher, thereby enabling the volume resistivity of the conductive pattern layer to be lower than conventional methods.

According to the invention described in claim 4, the silver nanopaste ink, a silver-containing paste, has a silver concentration of 50 wt% or more when measured by thermogravimetric differential thermal analysis, and is a paste ink whose solvent composition is a non-water-soluble solvent or an alcohol-glycol mixed solvent. This makes it possible to lower the volume resistivity of the conductive pattern layer compared to conventional methods, and also makes it possible to smooth the surface of the conductive pattern layer and stabilize its quality.

1 is an overall perspective view schematically illustrating a capacitance sensor in an embodiment of a circuit board according to the present invention. 1 is a cross-sectional view illustrating a capacitance sensor according to an embodiment of a circuit board according to the present invention; 1 is an exploded perspective view schematically illustrating a capacitance sensor in an embodiment of a circuit board according to the present invention.

A preferred embodiment of the present invention will now be described with reference to the drawings. As shown in FIGS. 1 to 3, the circuit substrate in this embodiment is a self-capacitance type capacitive sensor comprising a flexible release layer 1, a flexible insulating layer 3 removably formed on this release layer 1, a light-transmitting conductive pattern layer 4 formed on this insulating layer 3 using a silver-containing paste, a light-transmitting adhesive layer 8 covering a portion of this conductive pattern layer 4, and a plurality of deformation suppression layers 9 covering the remainder of the conductive pattern layer 4. This contributes to the achievement of Goal 9 of the SDGs (the United Nations' international goals for sustainable development, consisting of 17 global goals and 169 targets (achievement criteria)) adopted at the United Nations Summit.

The release layer 1 is formed into a flat rectangular shape or the like from a silicone elastomer 2 that is flexible and has excellent heat resistance at 200°C to 250°C, weather resistance, flame retardancy, dielectric properties, and electrical insulation, and the surface facing the insulating layer 3 is subjected to a mirror finish or a release treatment as necessary, and is peeled off from the insulating layer 3 when the capacitance sensor is in use. From the viewpoint of contributing to handleability and workability, the silicone elastomer 2 has a thickness of 10 μm to 250 μm, preferably 50 μm to 250 μm, a density measured at ²³ °C in accordance with JIS K 6268 of 1.21 g/ ^cm3 to 1.25 g/ ^cm3 , preferably around 1.24 g/cm3, and a Shore A hardness measured in accordance with JIS K 6253 of 10 to 60, preferably 30 to 60, thereby ensuring flexibility relative to curved surfaces.

The elongation at break of silicone elastomer 2, measured in accordance with JIS K 6251, is preferably 100% to 1000%, and more preferably 500% to 800%, in order to improve mechanical properties. Furthermore, the tensile strength, measured in accordance with JIS K 6251, is preferably 4 MPa to 12 MPa, and more preferably 6 MPa to 12 MPa, in order to achieve both flexibility and strength. Furthermore, to prevent warping and deformation, the tear strength (crescent type), measured in accordance with JIS K 6252, is preferably 8 kN/m to 28 kN/m, and more preferably 13 kN/m to 18 kN/m. These measurements can be performed using measuring instruments and testing machines from M&K Corporation, ITS Japan Co., Ltd., etc.

A specific example of such a silicone elastomer 2 is KE-153U (product name, manufactured by Shin-Etsu Chemical Co., Ltd.).

The insulating layer 3 is made of a specific resin, such as a thermosetting polyol-based resin made from UV-cured acrylic resin, and is laminated onto the surface of the release layer 1 by printing or other methods to form a flat rectangular thin film, providing flexibility for complex uneven and curved surfaces. The reason for using a thermosetting polyol-based resin for the insulating layer 3 is that it is expected to provide excellent heat resistance, chemical resistance, weather resistance, and thermal insulation. To achieve a thinner capacitance sensor, the thickness of the insulating layer 3 should be 10 μm or more, preferably 10 μm to 20 μm, and more preferably 15 μm to 20 μm.

The Shore D hardness of the insulating layer 3, measured in accordance with JIS K 6253, should be 60 to 75, preferably 65 to 75, in order to ensure flexibility on curved surfaces. Furthermore, the flexural modulus, measured in accordance with JIS K 7171, should be 500 MPa to 2500 MPa, preferably 1500 MPa to 2500 MPa.

The conductive pattern layer 4 is formed by directly screen-printing a silver-containing paste onto the surface of the insulating layer 3, followed by heat treatment at a high temperature exceeding 150°C, ie, 160°C or higher, to dry and harden it. Instead of silver ink, a silver complex paste ink or silver nanopaste ink is used as the silver-containing paste, which has a volume resistivity of 20 μΩ cm or less after high-temperature treatment and a volume resistivity of 10 ⁻⁵ Ω or less when measured by the four-terminal four-probe method. This is because, when such a paste ink is heat-treated, the final volume resistivity is about 1/8 or less of that of silver ink.

The silver complex paste ink is preferably an inexpensive paste ink that has a viscosity characteristic (@2s ⁻¹ ) of about 32,000 cPs to about 33,000 cPs when measured with a viscometer (product name: CPA-52Z), a thixotropy index of about 1.7 to about 1.8, and a solids content of about 30% wt to about 33% wt, and that produces a volume resistivity of 10 μΩ·cm or less, preferably 7.5 μΩ·cm or less, of the conductive pattern layer 4 when heat-treated at a high temperature of 160° C. or more. Specific examples of such silver complex paste inks include El-904 (product name, manufactured by Electronix, Inc., USA) and El-909 (product name, manufactured by Electronix, Inc., USA).

In contrast, the silver nanopaste ink preferably has a silver concentration of 50 wt% or more when measured by thermogravimetric differential thermal analysis (TG-DTA) and a solvent composition of a non-water-soluble solvent or an alcohol-glycol mixed solvent. From the perspective of suppressing volume resistivity, the silver concentration of the silver nanopaste ink measured by thermogravimetric differential thermal analysis should be 50 wt% or more and 70 wt% or less, preferably 60 wt% or more and 70 wt% or less. Furthermore, the volume resistivity of the silver nanopaste ink measured by the four-point four-probe method under conditions of baking at 120°C for 30 minutes should be 6.0 μΩ·cm or more and 6.3 μΩ·cm or less, preferably 6.1 μΩ·cm or more and 6.25 μΩ·cm or less.

Specific examples of such silver nanopaste inks include DNS409 (DNS409S) (manufactured by Daicel Corporation: product name Picosil (registered trademark) series), which produces a conductive pattern layer 4 similar to that of metal foil.

The conductive pattern layer 4 includes, for example, a plurality of electrodes 5 laminated in the center of the insulating layer 3 and covered with an adhesive layer 8, a GND electrode laminated on the insulating layer 3, a plurality of conductive lines 6 connected to the plurality of electrodes 5 and covered with a plurality of deformation suppression layers 9, and a plurality of connection terminals 7 formed at the ends of at least some of the plurality of conductive lines 6 and connected to the rigid substrate, and the thickness of these is set in the range of 1 μm to 20 μm.

The multiple electrodes 5 are arranged at a predetermined interval in the longitudinal direction of the insulating layer 3, for example. Each electrode 5 is formed, for example, in a transparent, planar rectangular, diamond, or circular shape, and can be placed in close proximity to a conductor such as an operator's finger via the adhesive layer 8. They function to form capacitance between the electrode 5 and the conductor and detect changes in that capacitance. Each conductive line 6 extends in at least one longitudinal direction of the insulating layer 3, and has a line width of 30 μm to 1000 μm, preferably 30 μm to 500 μm.

The volume resistivity of such a conductive pattern layer 4, when measured in accordance with JIS H 0505 on a pattern 100 μm wide, 10 cm long, and 5 μm thick, is preferably 19 Ω or less, preferably 17 Ω or less, and more preferably 15 Ω or less. Furthermore, the surface roughness Ra of the conductive pattern layer 4, when measured in accordance with JIS B 0651, is preferably 0.1 μm or more and 0.9 μm or less, and preferably approximately 0.1 μm.

The adhesive layer 8 is made of a specific adhesive, such as a hot-melt adhesive, a thermosetting adhesive, a light-curing adhesive, an ultraviolet-curing adhesive, or a room-temperature-curing adhesive. The specific adhesive is screen-printed onto at least a plurality of electrodes 5 of the conductive pattern layer 4, resulting in a flat rectangular laminated bond, which is then attached to the back side of the operation panel, etc. There are no particular limitations on the thickness of this adhesive layer 8, but from the perspective of making the capacitance sensor thinner, a thickness in the range of 12 μm to 100 μm is preferable.

The multiple deformation suppression layers 9 are formed into a planar rectangle using an unstretched resin film such as polyethylene terephthalate resin or polycarbonate resin. They are laminated and bonded to at least the multiple conductive lines 6 of the conductive pattern layer 4 using an acrylic adhesive or the like, and are arranged to sandwich the adhesive layer 8, preventing problems during attachment of the capacitance sensor. Each deformation suppression layer 9 has a thickness of 12 μm or more, preferably 12 μm to 250 μm, a Shore A hardness of 10 to 60 when measured in accordance with JIS K 6253, an elongation at break of 100% to 1000%, preferably 500% to 800%, when measured in accordance with JIS K 6251, and a tensile strength of 4 MPa to 12 MPa when measured in accordance with JIS K 6251.

When manufacturing a capacitance sensor with the above configuration, first prepare a silicone elastomer 2 of a specified size as the release layer 1. Then, a thermosetting polyol-based resin is printed on most of the surface of this silicone elastomer 2, excluding the peripheral edges, and then laminated to form the insulating layer 3 by irradiating it with ultraviolet light or drying and curing it. The thermosetting polyol-based resin is automatically screen-printed using a screen printing device. This is because screen printing is ideal for achieving high quality for the insulating layer 3 and speeding up the process. Because the resin is screen-printed rather than laminating a commercially available thermosetting polyol-based resin film on the surface of the silicone elastomer 2, it is expected that the insulating layer 3 will be significantly thinner.

Once the insulating layer 3 is formed, a silver-containing paste is printed directly onto the surface of this insulating layer 3, and then heat-treated at a high temperature of 160°C or higher to dry and harden, thereby metallizing and forming an array of multiple conductive pattern layers 4. The silver-containing paste is automatically screen-printed using a screen printing device. This is because screen printing is ideal for achieving high-quality conductive pattern layers 4 and speeding up the process. The screen-printed silver-containing paste is then heat-treated, and the heat treatment temperature is adjusted to a high temperature of 160°C or higher, preferably 180°C or higher, more preferably 190°C or higher, and even more preferably around 200°C, in order to reduce the resistance of the conductive pattern layers 4.

Next, a specified adhesive is screen-printed onto the multiple electrodes 5 of the multiple conductive pattern layers 4 in the same manner as above to laminate and bond the optically transparent adhesive layer 8, and a resin film of the deformation suppression layer 9 is laminated and bonded onto the multiple conductive lines 6 of each conductive pattern layer 4 via the adhesive. After that, the release layer 1, insulating layer 3, multiple conductive pattern layers 4, adhesive layer 8, and pair of deformation suppression layers 9 are cut to adjust the appearance, thereby manufacturing the capacitance sensor.

Next, when using a capacitance sensor attached to a curved operation panel, the capacitance sensor is bent onto the back surface of the operation panel, and its adhesive layer 8 and the pair of deformation suppression layers 9 are positioned and brought into contact, and the multiple connection terminals 7 of the conductive pattern layer 4 are connected to the rigid substrate. After that, the release layer 1 is peeled off and removed from the insulating layer 3 of the capacitance sensor, and the capacitance sensor is ready for use. At this point, the insulating layer 3 alone is thin, brittle, and prone to tearing, but because the insulating layer 3 and the pair of deformation suppression layers 9 are integrated, handling is improved and damage to the insulating layer 3 is extremely effectively prevented.

With the above configuration, the insulating layer 3 is formed as a thin film by printing the resin. This eliminates the need for a thick substrate for the insulating layer 3, allowing for thinner and lighter capacitance sensors and improving the design flexibility of the capacitance sensor. Furthermore, if the insulating layer 3 is made of a thermosetting polyol-based resin rather than a polyethylene terephthalate resin film, excellent chemical resistance, weather resistance, and heat insulation can be expected. Furthermore, because the release layer 1 is made of a silicone elastomer 2 with excellent heat resistance of 200°C or higher, high-temperature drying processing is possible, and the conductive pattern layer 4 can be formed by heat-treating the silver-containing paste at a high temperature of 160°C or higher, for example, around 200°C.

This prevents high resistance due to residual organic matter, and allows the volume resistivity of the conductive pattern layer 4 to be 20 μΩ·cm or less, preferably 19 μΩ·cm or less, more preferably 18 μΩ·cm or less, and even more preferably 16 μΩ·cm or less.

Furthermore, because the release layer 1 is made of a silicone elastomer 2 with excellent flexibility and a Shore A hardness of 10 or more and 60 or less, the capacitance sensor can be properly adapted to fit the curved surface of the operation panel, even when the operation panel has a complex three-dimensional curve with a high curvature (e.g., a hollow hemisphere or hemi-ellipse). This effectively eliminates the risk of misalignment interfering with the attachment of the capacitance sensor, and also allows for greater design freedom. Furthermore, because the conductive pattern layer 4 is covered and protected by the adhesive layer 8 and a pair of deformation-suppressing layers 9, it is possible to effectively prevent dust from adhering to the conductive pattern layer 4, thereby preventing performance degradation.

In the above embodiment, the insulating layer 3 is formed by laminating a thin film of a thermosetting polyol-based resin, but this is not limited to this. As long as substantially the same functionality is obtained, a thin film of a resin other than a thermosetting polyol-based resin may also be laminated.

The circuit substrate and manufacturing method thereof according to the present invention are used in the manufacturing fields of automotive electronic devices, industrial electronic devices, information terminal devices, home appliances, etc.

REFERENCE SIGNS LIST 1 Release layer 2 Silicone elastomer 3 Insulating layer 4 Conductive pattern layer 5 Electrode 6 Conductive line 7 Connection terminal 8 Adhesive layer 9 Deformation suppressing layer

Claims (5)

The adhesive tape includes a flexible release layer, an insulating layer releasably formed on the release layer, a conductive pattern layer formed on the insulating layer by laminating at least a silver-containing paste, an adhesive layer adhered to a portion of the conductive pattern layer, and a deformation suppressing layer formed on the remaining portion of the conductive pattern layer,
the release layer is made of a silicone elastomer having a thickness of 10 μm or more and 250 μm or less, the silicone elastomer having a Shore A hardness of 10 or more and 60 or less when measured in accordance with JIS K 6253, a breaking elongation of 100% or more and 1000% or less when measured in accordance with JIS K 6251, and a tensile strength of 4 MPa or more and 12 MPa or less when measured in accordance with JIS K 6251;
The insulating layer is formed by laminating resin to a thickness of 10 μm or more,
A circuit substrate characterized in that the silver-containing paste is either a silver complex paste ink or a silver nanopaste ink whose volume resistivity decreases when heat-treated at 160°C or higher. The circuit substrate of claim 1, wherein the insulating layer is formed by laminating a thermosetting polyol-based resin, and the resin has a Shore D hardness of 60 to 75 when measured in accordance with JIS K 6253, and a flexural modulus of 500 MPa to 2500 MPa when measured in accordance with JIS K 7171. The circuit substrate according to claim 1 or 2, wherein the silver complex paste ink of the silver-containing paste is a paste ink having a thixotropy index of 1.7 to 1.8, a solids content of 30% to 33% by weight, and a volume resistivity of the conductive pattern layer of 10 μΩ-cm or less when heat-treated at 160°C or higher. The circuit substrate according to claim 1 or 2, wherein the silver nanopaste ink of the silver-containing paste has a silver concentration of 50 wt% or more as measured by thermogravimetric differential thermal analysis, and the solvent composition is a water-insoluble solvent or an alcohol/glycol mixed solvent. 3. A method for manufacturing a circuit substrate according to claim 1 or 2, comprising the steps of: forming a resin insulating layer having a thickness of 10 μm or more on a flexible release layer in a releasable manner; printing at least a silver-containing paste on the insulating layer and heat-treating the insulating layer to form a conductive pattern layer; applying an adhesive to a portion of the conductive pattern layer and curing the adhesive to form an adhesive layer; and forming a deformation suppressing layer on the remaining portion of the conductive pattern layer,
the release layer is made of a silicone elastomer having a thickness of 10 μm or more and 250 μm or less, the silicone elastomer having a Shore A hardness of 10 or more and 60 or less when measured in accordance with JIS K 6253, a breaking elongation of 100% or more and 1000% or less when measured in accordance with JIS K 6251, and a tensile strength of 4 MPa or more and 12 MPa or less when measured in accordance with JIS K 6251;
A method for manufacturing a circuit substrate, characterized in that the silver-containing paste is either a silver complex paste ink or a silver nanopaste ink whose volume resistivity decreases when heat-treated at 160°C or higher.