Silver paste and flexible circuit boards

Abstract

To provide a silver paste or the like which has good screen printability, has few voids, and exhibits low resistance, etc. [Solution] A silver paste containing 90-99% by weight of a plurality of silver particles, 0.5-5% by weight of a thermosetting resin, and 0.5-5% by weight of a solvent, based on the total weight, and having the following configurations (a)-(c): (a) containing first silver particles having an average particle size of 10-500 nm and second silver particles having an average particle size of 2-30 μm; (b) the weight blending ratio of the plurality of silver particles is set to a value within the range of 25 / 75-75 / 25; (c) the amount of voids generated is set to a value of 40 volume % or less.

Description

Technical Field

[0001] The present invention relates to a silver paste and a flexible circuit board. In particular, a silver paste (sometimes referred to as a conductive paste) having good screen printability and the like and exhibiting low resistivity and the like, and a flexible circuit board (sometimes referred to as a printed circuit board) excellent in solder reflow properties and the like using such a silver paste.

Background Art

[0002] Conventionally, in flexible substrates used for electrical wiring in semiconductor devices, household appliances, OA equipment, lighting equipment, automobiles, etc., a silver paste in which a conductive material (silver powder) for forming electrical wiring is dispersed in a thermosetting resin or a solvent has been widely used. With the miniaturization, weight reduction, and thinning of semiconductor devices and the like, and further in order to cope with the use of high currents of 1 A or more, further reduction in resistance and thinning of fine lines are required in the electrical wiring of flexible substrates. Therefore, in such a silver paste, for example, it has good handleability during screen printing, solder reflow properties, etc., and can form fine lines and thin films, etc. On the other hand, even when a high current is applied, it is required to generate less heat and maintain good adhesion and flexibility.

[0003] Here, a silver paste containing three types of predetermined silver particles and a solvent has been proposed in order to increase the density of the silver sintered body even when sintered at low temperature and low pressure (see Patent Document 1). More specifically, it is a silver paste containing predetermined silver particles and a solvent, and includes (A) spherical silver particles having a particle diameter of 1 to 300 nm, (B) single crystal silver particles having a particle diameter of 10 nm to 7 μm, and (C) non-spherical silver particles having a particle diameter of 500 nm to 20 μm. And the protective agent covering the surface of the (A) spherical silver particles is an amine compound or the like, the protective agent of the (B) silver single crystal silver particles is an alcohol compound or the like, and the protective agent of the (C) non-spherical silver particles is an aliphatic monocarboxylic acid having 2 to 20 carbon atoms.

[0004] Furthermore, there have been proposed a conductive paste that has excellent printing characteristics, good adhesion to a substrate, and can obtain a conductor that can be used at a high current even for a long and narrow wiring, and a printed circuit board using the same (see Patent Document 2). More specifically, it is a conductive paste containing a substrate, metal nanoparticles provided on the surface of the substrate and having an average particle diameter of 30 to 600 nm or less, metal particles having an average particle diameter larger than that of the metal nanoparticles, a thermosetting resin having an oxirane ring in the molecule, a curing agent, and a cellulose resin. And a printed circuit board provided with a wiring formed by thermally curing such a conductive paste on a substrate, and such a wiring has a length of 100 mm to 1600 mm, a width of 0.3 to 3 mm, a thickness of 10 to 40 μm, and a resistance value of 1000 mΩ / m or less.

[0005] On the other hand, from the viewpoints of durability and mechanical properties, an electrical connection block including a bus bar structure formed of a conductive metal plate material (sometimes referred to as a bus bar structure) has been proposed (for example, Patent Document 3). More specifically, as shown in FIG. 8(b), a pair of bus bar main body portions 101 and 102 made of a long plate-shaped conductive metal facing each other in the plate thickness direction, an intermediate bus bar main body portion 103 that connects the ends along the longitudinal direction of the pair of bus bar main body portions to each other, and input side terminal portions 113 and 115 and output side terminal portions 107 provided on the pair of bus bar main body portions, a bus bar structure 110 has been proposed.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, the silver paste disclosed in Patent Document 1 has silver particles of three types (A) to (C) having a predetermined particle diameter and different types of protective agents, and it was necessary to use at least these three types of silver particles. Therefore, not only is it difficult to uniformly disperse the three types of silver particles, but also, basically, only a solvent is used, so there is a problem that the screen printability and flexibility are poor. Therefore, when the silver paste is applied and heat-treated (sintered), there is also a problem that cracks are likely to occur due to thermal stress (see FIG. 8(a)). Therefore, variations in resistance characteristics and the like are likely to increase, and it is practically difficult to use it for electrical wiring in automobiles and the like where the usage conditions are severe and a high current is applied for a long time.

[0008] In addition, the conductive paste disclosed in Patent Document 2 requires blending a predetermined amount of cellulose resin in addition to a plurality of types of silver particles, a thermosetting resin having an oxirane ring in the molecule, and its curing agent in order to improve printability. Therefore, the number of types of blending components increases, making it difficult to uniformly mix them. Moreover, when applied to a substrate and heat-treated, there are problems such as a decrease in solder reflowability and adhesion, and an easy variation in resistance value.

[0009] Furthermore, the bus bar structure formed of a conductive metal plate disclosed in Patent Document 3 has a problem of lacking flexibility and light weight. Moreover, when used as an electrical connection block provided with a bus bar structure for a vehicle, there is a problem that it is not easy to increase the length and reduce the weight.

[0010] Therefore, as a result of intensive research by the inventors of the present invention, without substantially blending a cellulose resin, a silver paste or the like containing a relatively large amount of a plurality of types of silver particles (first silver particles and second silver particles) and a predetermined amount of a thermosetting resin is used. By doing so, not only good screen printability and flexibility are achieved, but also in wiring, the generation of voids is suppressed, and excellent volume resistivity, adhesion, light weight, etc. can be obtained. That is, according to the present invention, an object is to provide a silver paste having good screen printability and flexibility, and a flexible circuit board suitable for vehicle bus bars and the like using such a silver paste, even in a case where a high current is applied for a long time and the temperature rise is small.

Means for Solving the Problems

[0011] According to the present invention, there is provided a silver paste containing, based on the total amount (100% by weight), 90 to 99% by weight of a plurality of silver particles, 0.5 to 5% by weight of a thermosetting resin (which may contain a curing agent), and 0.5 to 5% by weight of a solvent, and having the following configurations (a) to (c), and the above-mentioned problems can be solved. (a) As the plurality of silver particles, it includes first silver particles having an average particle diameter of 10 to 500 nm and second silver particles having an average particle diameter of 2 to 30 μm. (b) The weight mixing ratio of the first silver particles / the second silver particles is set to a value within the range of 25 / 75 to 75 / 25. (c) When the silver paste is thermally cured to form a conductive thermoset, the amount of voids generated is set to a value of 40% by volume or less when the total amount of the conductive thermoset is 100% by volume. That is, by configuring the silver paste in this way, the screen printability, flexibility, etc. are good, and the generation of voids during thermal curing is suppressed. Therefore, even in a case where a high current is applied for a long time, the volume resistivity is maintained low, and the adhesion, durability, etc. can also be improved.

[0012] Moreover, when forming the silver paste of the present invention, it is preferable that the melting point of the first silver particles is 180°C or lower and the melting point of the second silver particles is higher than 180°C. By controlling the melting points of the first silver particles and the second silver particles that make up the silver paste in this way, the mixing and dispersibility of the plurality of silver particles during heat curing is improved, the volume resistivity is maintained low, and the generation amount of voids can be further suppressed.

[0013] Moreover, when forming the silver paste of the present invention, it is preferable that a protective agent having a melting point or softening point of 180°C or lower is coated around the first silver particles. By forming the silver paste in this way, the mixing and dispersibility of the plurality of silver particles during heat curing is improved, the volume resistivity is maintained low, and the generation amount of voids can be further suppressed.

[0014] Moreover, when forming the silver paste of the present invention, it is preferable that the first silver particles are in a polyhedral shape and the second silver particles are in a flake shape. By forming the silver paste in this way, the mixing and dispersibility of the plurality of silver particles during heat curing is improved, the volume resistivity is maintained low, and the generation amount of voids can be further suppressed.

[0015] Moreover, when forming the silver paste of the present invention, it is preferable that the viscosity (measurement temperature: 25°C) is in the range of 3,000 to 50,000 mPa·sec. By forming the silver paste in this way, not only is it easy to handle, but even when a simple coating method such as screen printing is used, good printing accuracy and good flexibility can be exhibited.

[0016] Moreover, when forming the silver paste of the present invention, it is preferable that the main component of the thermosetting resin is an epoxy resin, and the silver paste further contains a curing agent and a curing catalyst for the epoxy resin, or either one of them. By configuring the silver paste in this way, good screen printability, flexibility, etc. can be achieved, and excellent economic efficiency and versatility can be demonstrated.

[0017] In addition, when configuring the silver paste of the present invention, when the silver paste is thermally cured to form a conductive thermoset, it is preferable to satisfy the following characteristics (1) to (4). (1) The volume resistivity of the conductive thermoset is set to a value within the range of 3×10 -6 ~8×10 -6 Ω·cm. (2) The first silver particles are welded to each other around the second silver particles, and the second silver particles exist in a mutually independent state. (3) The peel strength of the conductive thermoset with respect to the polyimide film is set to a value within the range of 5 to 25 N / 25 mm. (4) Under the application conditions of direct current 2 A for 2 weeks (336 hours), the temperature rise of the conductive thermoset is set to a value within 15°C. That is, by configuring the silver paste in this way, good screen printability, flexibility, etc. can be achieved, excellent heat resistance and adhesion can be obtained, and a low volume resistivity can be maintained. Therefore, even when used for a flexible circuit board as an automotive electrical wiring or the like, that is, a flexible circuit board for a bus bar, excellent solder flowability can be demonstrated, and even when a high current is applied for a long time, a low volume resistivity can be maintained over a long period of time.

[0018] Another aspect of the present invention is a flexible circuit board sequentially including a resist member, a conductive thermoset derived from a silver paste containing a plurality of silver particles, and a polyimide film from the surface side, wherein the silver paste has the following configurations (a) to (d). It is a flexible circuit board characterized by having. (a) As the plurality of silver particles, it includes first silver particles having an average particle diameter of 10 to 500 nm and flaky second silver particles having an average particle diameter of 2 to 30 μm. (b) Set the weight ratio of the first silver particles to the second silver particles to a value within the range of 25 / 75 to 75 / 25. (c) When the silver paste is thermally cured to form a conductive thermoset, set the amount of voids generated to a value of 40% by volume or less when the total amount of the conductive thermoset is 100% by volume. (d) Set the thickness of the conductive thermoset to a value within the range of 23 to 100 μm. That is, by configuring the flexible circuit board derived from the silver paste in this way, it has good screen printability, flexibility, etc., is excellent in heat resistance and adhesion, and can maintain a low volume resistivity. Therefore, even when used for a flexible circuit board such as a vehicle bus bar, it can exhibit excellent solder reflow properties, and even when a high current is applied for a long time, it can suppress the temperature rise at the electrical junction for a long time.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiments for Carrying Out the Invention

[0020] [First Embodiment] The first embodiment is a silver paste containing 90 to 99% by weight of a plurality of silver particles, 0.5 to 5% by weight of a thermosetting resin, and 0.5 to 5% by weight of a solvent with respect to the total amount (100% by weight), and is characterized by having the following configurations (a) to (c). (a) As the plurality of silver particles, it includes first silver particles having an average particle diameter of 10 to 500 nm and second silver particles having an average particle diameter of 2 to 30 μm. (b) The weight mixing ratio of the first silver particles / the second silver particles is set to a value within the range of 25 / 75 to 75 / 25. (c) When the silver paste is thermally cured to form a conductive thermoset, the amount of voids generated is set to a value of 40% by volume or less when the total amount of the conductive thermoset is 100% by volume. Hereinafter, the mode of the silver paste of the first embodiment will be specifically described with appropriate reference to the drawings.

[0021] 1. Silver Particles (First Silver Particles) (1) Average Particle Size The first silver particles are metal particles mainly composed of silver, and as a representative value in the particle size distribution chart, its average particle diameter (D50) is characterized by being within the range of 10 to 500 nm. The reason is that for the first silver particles in the nanometer order, in relation to the second silver particles in the micron order, the mixing dispersibility becomes good, the generation amount of voids during thermal curing is suppressed, it has excellent heat resistance, and a low volume resistivity can be maintained over a long period of time.

[0022] More specifically, as shown in FIGS. 1(a) to 1(d), since the first silver particles 32 on the nanometer order are uniformly present around the second silver particles 30 on the micron order, in this state, the thermosetting resin 33 can be heat-cured at a predetermined temperature. At the same time, while scattering a protective agent (not shown) present around the first silver particles 32, etc., by mainly melting the first silver particles 32, it is understood that electrical connection is closely made to the second silver particles 30 that maintain the initial shape. That is, by being uniformly heated, the protective agent present around the first silver particles 32 desorbs and scatters outside the system, and the first silver particles 32 selectively melt around the second silver particles 30. Therefore, although the first silver particles 32 are likely to be deformed by melting, since the second silver particles 30 are likely to maintain the initial shape, it is presumed that generation of voids is suppressed in the entire silver paste. Therefore, it is more preferable that the average particle diameter (D50) of the first silver particles be a value within the range of 50 to 400 nm, and it is even more preferable that it be a value within the range of 100 to 300 nm. Regarding the average particle diameter of the first silver particles, in accordance with JIS Z 8901:2006, as the "arithmetic mean value of the particle diameters", it can be actually measured with a scanning electron microscope or measured and calculated by an image processing apparatus.

[0023] (2) Form 1 Also, the form of the first silver particles can be appropriately changed according to the application, etc. For example, it can be at least one of spherical, elliptical, polyhedral, flake-like (lamellar), radial, string-like, pyramid-like, disc-like, perforated, irregular, etc., but usually, it is more preferably polyhedral. And as the first silver particles in the form of a polyhedron, more specifically, it is more preferably in a form mainly composed of at least one polyhedron such as a tetrahedron, pentahedron, hexahedron, octahedron, decahedron, etc. The reason is that by mainly using polyhedra as the first silver particles, it becomes easier to coat them with a predetermined protective agent, and in relation to the form such as the flake shape of the second silver particles, even better mixing and dispersibility is exhibited. Also, if the form of the first silver particles is a polyhedron, usually, it has a plurality of corner portions (vertices) and linear sides connecting them, and as a whole, it can be said that while being small, it approximates a cubic shape or a granular shape. Therefore, the first silver particles fit into the gaps between the generally flat flake-shaped second silver particles, and even if the first silver particles moderately melt and deform, the initial shape of the second silver particles will be maintained. Thus, it can be said that by using the first silver particles mainly composed of polyhedra or the like as the entire silver paste, the effect of suppressing the generation amount of voids is also easily exhibited. Moreover, the first silver particles in the form of polyhedra can be easily pulverized under predetermined conditions using a known pulverizer such as a ball mill, jaw crusher, grinder, or mortar to form predetermined corner portions, etc. Therefore, there is also an advantage that they can be stably and mass-produced easily and are economically advantageous. However, as the first silver particles, it is only necessary to mainly use polyhedral particles, and it is also preferable to contain at least one kind of silver particles having an average particle diameter equivalent to that of polyhedral particles, such as spherical, elliptical, granular, pyramid-shaped, or irregular-shaped particles, in addition to the polyhedral particles. More specifically, it is sufficient to use 50% by weight or more of polyhedral particles with respect to the total amount of the first silver particles, it is more preferable to use 60 - 95% by weight of polyhedral particles, and it can be said that it is even more preferable to use 70 - 90% by weight of polyhedral particles.

[0024] (3) Form 2 Also, regarding the form of the first silver particles, it is preferable that the tap density (TD) measured in accordance with JIS Z 2512:2012 is within the range of 1 - 4.5 g / cm 3 of values. The reason is that when such a tap density (TD) is 1 g / cm 3When it becomes a value less than that, handling becomes difficult, the uniform dispersibility with the second silver particles decreases, the volume resistivity increases, and furthermore, it may become difficult to control the melting point or the like. On the other hand, when such a tap density (TD) is 4.5 g / cm 3 exceeds, again, the uniform dispersibility with the second silver particles decreases, the volume resistivity increases, and furthermore, it may become difficult to control the melting point or the like. Therefore, regarding the form of the first silver particles, it is more preferable that such a tap density (TD) is within the range of 2 to 4.3 g / cm 3 , and it is even more preferable that it is within the range of 3 to 4 g / cm 3 .

[0025] Furthermore, regarding the form of the first silver particles, it is preferable that the BET specific surface area measured in accordance with JIS Z 8830:2013 is within the range of 1 to 5 m 2 / g. The reason for this is that even when such a BET specific surface area becomes a value less than 1 m 2 / g, handling may become difficult, the uniform dispersibility with the second silver particles may decrease, the volume resistivity may increase, and furthermore, it may become difficult to control the melting point or the like. On the other hand, when such a BET specific surface area exceeds 5 m 2 / g, again, the uniform dispersibility with the second silver particles decreases, the volume resistivity increases, and furthermore, it may become difficult to control the melting point or the like. Therefore, regarding the form of the first silver particles, it is more preferable that such a BET specific surface area is within the range of 1.2 to 4 m 2 / g, and it is even more preferable that it is within the range of 1.5 to 3 m 2 .

[0026] (4) Blending amount Also, regarding the blending amount of the first silver particles, usually, it is preferable that the value is within the range of about 22 to 75% by weight with respect to the total amount (100% by weight) of the silver paste (in terms of solid content). The reason for this is that when the compounding amount of the first silver particles is less than about 22% by weight, due to the relationship with the compounding amount of the second silver particles and the like, it becomes difficult to uniformly control the mixing and dispersibility, resulting in an increase in the amount of voids generated or difficulty in exhibiting low resistivity over a long period of time. On the other hand, when the compounding amount of the first silver particles exceeds 75% by weight, due to the relationship with the compounding amount of the second silver particles and the like, conversely, it becomes difficult to uniformly control the mixing and dispersibility, resulting in an increase in the amount of voids generated or difficulty in exhibiting low resistivity over a long period of time. Therefore, it is more preferable that the compounding amount of the first silver particles is within the range of 30 to 70% by weight, and more preferably within the range of 40 to 60% by weight, based on the total amount of the silver paste (in terms of solid content).

[0027] Also, regarding the compounding amount of the first silver particles, it is preferable to determine it in consideration of the compounding amount of the second silver particles described later. That is, in order to exhibit good resistivity, adhesion, handleability, etc., it is preferable to determine the total amount of the compounding amount of the first silver particles and the compounding amount of the second silver particles to be within the range of 90 to 99% by weight based on the total amount of the silver paste. Therefore, it is more preferable to determine the compounding amount of the first silver particles so that the total amount of the compounding amount of the first silver particles and the compounding amount of the second silver particles is within the range of 91 to 98% by weight, and more preferably within the range of 92 to 98% by weight.

[0028] And regarding the compounding amount of the first silver particles, it is preferable to determine it in consideration of the weight ratio with the compounding amount of the second silver particles. That is, it is characterized in that the weight ratio of the compounding amount of the first silver particles / the compounding amount of the second silver particles is within the range of 25 / 75 to 75 / 25. The reason for this is that if this weight ratio becomes smaller than 25 / 75 or larger than 75 / 25, the compounding effect of the first silver particles and the like becomes smaller, and the value of the volume resistivity when the silver paste is thermally cured may become excessively large. Therefore, it is more preferable that the weight ratio of the blending amount of the first silver particles to the blending amount of the second silver particles is within the range of 30 / 70 to 70 / 30, and even more preferably within the range of 35 / 65 to 65 / 35.

[0029] (5) Mechanism of electrical connection Next, referring to FIGS. 1(a) to (b), the mechanism of electrical connection of the first silver particles and the like in the silver paste formed by thermosetting will be described. That is, FIG. 1(a) is a conceptual diagram of a state in which the silver paste 34 is applied relatively thickly (for example, the thickness represented by t1 is 30 μm or more) to a predetermined surface portion of the base material 11 made of a polyimide resin or the like and thermoset. Further, FIG. 1(b) is a conceptual diagram of a state in which the silver paste 34 is applied relatively thinly (for example, the thickness represented by t2 is 15 μm or less) to a predetermined surface portion of the base material 11 made of a polyimide resin or the like and thermoset.

[0030] As understood from FIG. 1(a), when the silver paste 34 is applied relatively thickly and thermoset, the first silver particles 32 such as polyhedrons are judged to be uniformly present in a fused state in the gaps or around the second silver particles 30. That is, as shown in FIG. 2, the second silver particles such as flakes are in a state of standing vertically or slightly inclined, and substantially maintain the initial state, and the first silver particles are electrically connected in a state of being uniformly fused in the gaps or around the second silver particles. In addition, although the silver paste contains a very small amount of thermosetting resin (particularly, high heat resistance and high viscosity are preferable) and a solvent, they do not inhibit the electrical connection between the first silver particles and the second silver particles. Rather, it is presumed that they contribute to maintaining the initial state of the second silver particles.

[0031] On the one hand, as can be understood from FIG. 1(b), even when the silver paste 34 is applied relatively thinly and heat-cured, the first silver particles 32 such as polyhedrons are uniformly present in a fused state around the second silver particles 30 such as flakes formed by horizontal and layered deposition or between the layers.

[0032] Further, FIG. 1(c) shows an example in which a multilayer structure is used in which a low-Tg polyimide resin layer 11a, which is a thermocompression-bondable resin layer, is formed on the upper surface side of the high-Tg polyimide resin layer 11b as the base material 11. Regarding the mechanism of electrical connection of the first silver particles and the like in the silver paste, it can be said that there is no difference from the embodiment which is the thermoset of the silver paste of FIGS. 1(a) to 1(b).

[0033] Furthermore, FIG. 1(d) shows an example of a flexible circuit board 10' in which a predetermined resist member 38 is provided on the uppermost surface side in the drawing for mechanical protection of the wiring 12 derived from the silver paste. Since the resist member 38 is formed in a later process, it can be said that there is no difference from the embodiments of FIGS. 1(a) to 1(b) regarding the mechanism of electrical connection of the first silver particles and the like in the silver paste.

[0034] Therefore, in the conceptual diagrams shown in FIGS. 1(a) to 1(d), when the silver paste 34 is heat-cured at a predetermined temperature and at least the protective agent existing around the first silver particles 32 is scattered, the first silver particles 32 and the second silver particles 30 are electrically connected. That is, as shown in FIG. 2, it is presumed that mainly the first silver particles 32 are melted and are in close contact with the second silver particles 30 for electrical connection with almost no voids generated. Furthermore, it is considered that the protective agent (not shown) covering the periphery of the first silver particles 32 easily detaches and scatters outside the system along the periphery of the second silver particles 30 maintaining the initial state. Therefore, even if the first silver particles 32 are melted and deformed, since the second silver particles 30 are likely to maintain the initial state, it is presumed that the generation of voids is suppressed as shown in FIG. 2.

[0035] Note that FIGS. 3(a) to 3(b) are cross-sectional photographs of the conductive cured products (wiring) derived from the respective silver pastes corresponding to Comparative Example 3 and Comparative Example 4. These Comparative Example 3 and Comparative Example 4 are different from Example 1 and the like in that the blending amount and form of the first silver particles, and further the blending amount and form of the second silver particles are different. Since not only the first silver particles but also the second silver particles are easily deformed, it is estimated that the amount of voids generated increases.

[0036] (6) Additives such as metals In addition, the first silver particles may basically be mainly composed of silver, but it is preferable to blend at least one additive such as a metal selected from the group consisting of gold, silver, copper, platinum, palladium, rhodium, ruthenium, iridium, osmium, tungsten, nickel, tantalum, bismuth, lead, indium, tin, zinc, titanium, and oxides thereof. That is, by blending these additives such as metals within a predetermined range with respect to the total amount of the first silver particles, the volume resistivity, melting point, crystal structure, etc. of the conductive thermosetting product can be adjusted within a predetermined range, and furthermore, silver migration can be suppressed. Therefore, when blending additives other than silver into the first silver particles, although it depends on the use and purpose, etc., the blending amount of the additives such as metals is preferably a value within the range of 0.0001 to 30% by weight with respect to the total amount of the first silver particles, and more preferably a value within the range of 0.001 to 10% by weight.

[0037] (7) Melting point In addition, the melting point of the first silver particles (which may mean the softening point, activation temperature, etc. The same applies hereinafter) is preferably usually less than 180°C. The reason for this is that by using the second silver particles having such a melting point, in relation to the melting point etc. of the first silver particles, it becomes easy to melt sufficiently and easily after thermosetting under predetermined conditions in the vicinity thereof. Therefore, the generation of voids in the thermoset of the silver paste is reduced, resulting in excellent heat resistance and the ability to exhibit even lower resistivity over a long period of time. Therefore, it is more preferable that the melting point of the first silver particles is within the range of 100 to 170 °C, and it is even more preferable that the melting point is within the range of 130 to 160 °C. Regarding the melting point and the like of the first silver particles, they can usually be measured as one of the heat change points using a differential scanning calorimeter (DSC) or a dynamic thermal analyzer (DTA).

[0038] (8) Protective agent Moreover, it is preferable that the first silver particles are coated with a protective agent having a melting point or softening point of 180 °C or lower around them. The reason for this is that by using the silver paste coated with such a protective agent, the melting property and aggregability of the first silver particles are suppressed, and the mixing and dispersibility with the first silver particles can be easily controlled uniformly. Therefore, it is more preferable that the first silver particles are coated with a protective agent having a melting point or softening point within the range of 100 to 170 °C around them, and it is even more preferable that the protective agent has a melting point or softening point within the range of 130 to 160 °C.

[0039] And as the type of the protective agent, it is preferably at least one selected from the group consisting of an amine compound, a carboxylic acid compound, an amino acid compound, an amino alcohol compound, and an amide compound. The reason for this is that by using the first silver particles coated with a protective agent of such a type, the mixing and dispersibility are further excellent, the screen printing property, flexibility, etc. are further good, and it has excellent heat resistance and can exhibit even lower resistivity over a long period of time.

[0040] 2. Silver particles (second silver particles) (1) Average particle size The second silver particles are metal particles mainly composed of silver, and as a representative value in the particle size distribution chart, the average particle diameter (D50) thereof is within the range of 2 to 30 μm. The reason is that silver particles on the micron order, even under predetermined thermosetting conditions, do not melt excessively and retain their initial shape, and as a result, the generation of voids can be suppressed.

[0041] More specifically, as shown in FIGS. 1(a) to (b), it can be said that the first silver particles 32 on the nanometer order are uniformly present around the second silver particles 30 on the micron order. Therefore, in this state, when heat-cured, the protective agent coated around the first silver particles 32 scatters. Then, since the first silver particles 32 mainly melt and are in close electrical connection with the second silver particles 30 that do not melt excessively and retain their initial shape, it is presumed that the generation of voids is suppressed. Therefore, in relation to the melting of the first silver particles, since the generation of voids is likely to be suppressed, it is more preferable that the average particle diameter of the second silver particles is within the range of 3 to 25 μm, and it is even more preferable that the average particle diameter is within the range of 5 to 20 μm. Regarding the average particle diameter of the second silver particles as well, similar to the average particle diameter of the first silver particles, in accordance with JIS Z 8901:2006, it can be actually measured with a scanning electron microscope or measured by an image processing apparatus as the "arithmetic average value of the particle diameters".

[0042] (2) Form 1 Also, the form of the second silver particles can be appropriately changed, but usually, spherical, elliptical, flake-shaped (lamellar), radial, pyramid-shaped, disc-shaped, irregular shapes, etc. are preferable, and in particular, flake-shaped is preferable. The reason is that by using such flake-shaped second silver particles, the mixing and dispersibility are further excellent in relation to the form of the first silver particles. Therefore, the screen printability, flexibility, etc. in the silver paste become better, it has excellent heat resistance, and can exhibit lower resistance over a long period of time. In addition, when the form of the second silver particles is flake-shaped, it is preferable that the surface area is within the range of 0.05 to 0.8 m 2 / g, and more preferably within the range of 0.1 to 0.8 m2 It is more preferable that the value is within the range of / g.

[0043] (3) Form 2 Also, regarding the form of the second silver particles, the tap density (TD) measured in accordance with JIS Z 2512:2012 is 5 to 8 g / cm 3 It is preferable that the value is within the range. The reason for this is that when such tap density (TD) is less than 5 g / cm 3 the handling becomes difficult, the uniform dispersibility with the first silver particles decreases, the volume resistivity increases, and furthermore, it may be difficult to control the melting point and the like. On the other hand, when such tap density (TD) exceeds 8 g / cm 3 the uniform dispersibility with the second silver particles also decreases, the volume resistivity increases, and furthermore, it may be difficult to control the melting point and the like. Therefore, regarding the form of the second silver particles, it is more preferable that such tap density (TD) is within the range of 5.2 to 7 g / cm 3 and it is even more preferable that the value is within the range of 5.5 to 6 g / cm 3 It is more preferable that the value is within the range.

[0044] Furthermore, regarding the form of the second silver particles, the BET specific surface area measured in accordance with JIS Z 8830:2013 is 0.05 to 0.5 m 2 It is preferable that the value is within the range of / g. The reason for this is that even when such BET specific surface area is less than 0.05 m 2 / g, the handling becomes difficult, the uniform dispersibility with the second silver particles decreases, the volume resistivity increases, and furthermore, it may be difficult to control the melting point and the like. On the other hand, when such BET specific surface area exceeds 0.5 m 2 / g, the uniform dispersibility with the first silver particles also decreases, the volume resistivity increases, and furthermore, it may be difficult to control the melting point and the like. Therefore, regarding the form of the second silver particles, such BET specific surface area is 0.1 to 0.45 m 2It is more preferable that the value is within the range of / g, and 0.2 to 0.4 m 2 It is even more preferable that the value is within the range of / g.

[0045] (4) Blending amount Also, regarding the blending amount of the second silver particles, usually, it is preferable that the value is within the range of about 22 to 75% by weight with respect to the total amount (100% by weight) of the silver paste (in terms of solid content). The reason for this is that when the blending amount of the second silver particles is less than about 22% by weight, due to the relationship with the blending amount of the first silver particles, etc., it becomes difficult to uniformly control the mixing and dispersibility, and the amount of voids generated may increase, or it may be difficult to exhibit low resistivity over a long period of time. On the other hand, when the blending amount of the second silver particles exceeds about 75% by weight, due to the relationship with the blending amount of the second silver particles, etc., conversely, it becomes difficult to uniformly control the mixing and dispersibility, and the amount of voids generated may increase, or it may be difficult to exhibit low resistivity over a long period of time. Therefore, it is more preferable that the blending amount of the second silver particles is within the range of 30 to 70% by weight with respect to the total amount of the silver paste (in terms of solid content), and it is even more preferable that the value is within the range of 40 to 60% by weight.

[0046] Also, regarding the blending amount of the second silver particles, it is preferable to determine it in consideration of the blending amount of the first silver particles described above. That is, it is preferable to determine the blending amount of the second silver particles and the total amount of the blending amount of the first silver particles so that the value is within the range of 90 to 99% by weight with respect to the total amount of the silver paste (in terms of solid content), it is more preferable that the value is within the range of 91 to 98% by weight, and it is even more preferable that the value is within the range of 92 to 98% by weight. And regarding the blending amount of the second silver particles, as described later, it is also preferable to determine it in consideration of the weight ratio with the blending amount of the first silver particles.

[0047] Here, referring to FIG. 4, the relationship between the blending amount of the second silver particles and the volume resistivity of the wiring made of the conductive thermosetting resin will be described. That is, on the premise that the total blending amount of the first silver particles / the second silver particles is 90 to 99% by weight, the blending amount of the second silver particles is taken on the horizontal axis of FIG. 4, and the value of the volume resistivity of the wiring is taken on the left axis. Line A is a characteristic curve showing these relationships. From such a characteristic curve (line A), when the blending amount of the second silver particles is within the range of about 22 to 75% by weight, the value of the volume resistivity of the wiring tends to decrease. Therefore, in order to control the value of the volume resistivity of the wiring to be low, on the premise that the total blending amount of the first silver particles and the second silver particles is 90 to 99% by weight, it can be said that it is more preferable that the blending amount of the second silver particles is within the range of 30 to 70% by weight, and it is even more preferable that the blending amount is within the range of 35 to 65% by weight.

[0048] Also, similarly, referring to FIG. 4, the relationship between the blending amount of the second silver particles and the void amount of the conductive thermosetting resin will be described. That is, on the premise that the total blending amount of the first silver particles / the second silver particles is 90 to 99% by weight, the blending amount of the second silver particles is taken on the horizontal axis of FIG. 4, and the value of the void amount of the conductive thermosetting resin is taken on the right axis. Line B is a characteristic curve showing these relationships. From such a characteristic curve (line B), when the blending amount of the second silver particles is within the range of about 22 to 75% by weight, the void amount of the conductive thermosetting resin tends to be relatively low. Therefore, in order to control the void amount of the conductive thermosetting resin to be low, on the premise that the total blending amount of the first silver particles and the second silver particles is 90 to 99% by weight, it can be said that it is more preferable that the blending amount of the second silver particles is within the range of 30 to 70% by weight, and it is even more preferable that the blending amount is within the range of 35 to 65% by weight.

[0049] (5) Mechanism of electrical connection Next, the mechanism of electrical connection of the second silver particles and the like in the silver paste is due to the cooperation with the first silver particles. As described above, the first silver particles exist in a uniformly fused state in the gaps or around the second silver particles maintained in the initial state, and are understood to be electrically firmly connected. Therefore, repeated explanations here shall be omitted.

[0050] (6) Additives such as metals Also, the second silver particles may basically be mainly composed of silver, but preferably contain at least one additive of metals other than silver selected from the group consisting of gold, silver, copper, platinum, palladium, rhodium, ruthenium, iridium, osmium, tungsten, nickel, tantalum, bismuth, lead, indium, tin, zinc, titanium, and their oxides. That is, by blending these additives of metals within a predetermined range with respect to the total amount of the first silver particles, the volume resistivity, melting point, crystal structure, etc. of the electrically conductive thermosetting material can be adjusted within a predetermined range, and furthermore, silver migration can be suppressed. That is, when blending the additives of the second silver particles, although it depends on the use, purpose, etc., the blending amount of the additives of metals is preferably within the range of 0.0001 to 30% by weight, and more preferably within the range of 0.001 to 10% by weight with respect to the total amount of the second silver particles.

[0051] (7) Melting point Also, the melting point of the second silver particles (similarly, it may mean the softening point, activation temperature, etc. as in the case of the first silver particles. The same applies hereinafter) is preferably usually a value of 180°C or higher. The reason for this is that by using the second silver particles with such a melting point, in relation to the form of the first silver particles, it is easier to maintain the form before thermosetting even after thermosetting under predetermined conditions. Therefore, the screen printability, flexibility, etc. in the silver paste become better, it has excellent heat resistance, and can exhibit lower resistance over a long period of time. Therefore, it is more preferable that the melting point of the second silver particles is within the range of 200 to 700°C, and more preferably within the range of 250 to 500°C. Incidentally, the melting point and the like of the second silver particles can usually be measured as one of the thermal change points using a differential scanning calorimeter (DSC) or a dynamic thermal analyzer (DTA).

[0052] (8) Protective agent Also, when a protective agent is used for the second silver particles, the type thereof is preferably different from the protective agent for the first silver particles and is coated with a protective agent having a melting point or softening point exceeding 180°C. The reason for this is that by using the second silver particles coated with such a protective agent having a relatively high melting point or softening point, the melting property and the aggregating property of the second silver particles are suppressed, and it is easy to maintain a predetermined form. Therefore, it is more preferable that the protective agent having a melting point or softening point in the range of 200 to 300°C is coated around the second silver particles, and it is still more preferable that the protective agent having a melting point or softening point in the range of 230 to 260°C is coated. Incidentally, the type of the protective agent is preferably at least one selected from the group consisting of an amine compound, a carboxylic acid compound, an amino acid compound, an amino alcohol compound, and an amide compound.

[0053] 3. Mixing ratio of the first silver grains and the second silver particles Also, the mixing ratio of the first silver particles / the second silver particles is characterized in that it is a value within the range of 25 / 75 to 75 / 25. This is because when such a mixing ratio is within this range, it is possible to obtain a conductive thermosetting resin having few voids, a low volume resistivity, and improved conductivity. More specifically, when the mixing ratio of the first silver particles / the second silver particles is less than 25 / 75, it may be difficult to satisfy the specific resistance of the obtained conductive thermosetting resin. On the other hand, when the mixing ratio of the first silver particles / the second silver particles is higher than 75 / 25, the viscosity of the silver paste decreases, and it may become difficult to satisfy the workability.

[0054] Here, referring to FIG. 5, the relationship between the mixing ratio of the first silver particles / the second silver particles and the volume resistivity of the wiring made of the conductive thermosetting resin will be described respectively. That is, on the premise that the total blending amount of the first silver particles / the second silver particles is 90 to 99% by weight, the blending ratio of the first silver particles / the second silver particles is taken on the horizontal axis of FIG. 5, and the value of the volume resistivity of the wiring is taken on the left axis. Line A is a characteristic curve showing these relationships. From such a characteristic curve (line A), when the blending ratio of the first silver particles / the second silver particles is within the range of 25 / 75 to 75 / 25, the value of the volume resistivity of the wiring tends to decrease. Therefore, in order to lower the value of the volume resistivity of the wiring, on the premise that the total blending amount of the first silver particles / the second silver particles is 90 to 99% by weight, it is more preferable that the blending ratio of the first silver particles / the second silver particles is within the range of 30 / 70 to 70 / 30, and it can be said that it is even more preferable to be within the range of 35 / 65 to 65 / 35.

[0055] 4. Void amount When the silver paste is thermally cured to form a conductive thermoset (such as wiring), the amount of voids generated is characterized by being 40% by volume or less when the total amount of the conductive thermoset is 100% by volume. The reason for this is that when the amount of such voids exceeds 40% by volume, the resistivity value of the conductive thermoset as wiring or the like rapidly increases, and the intended use may be excessively restricted. However, when the amount of such voids is excessively small, the fusion property of the first silver particles with respect to the second silver particles becomes insufficient, and the durability and mechanical properties of the conductive thermoset as wiring or the like may become excessively low.

[0056] Therefore, it is more preferable that the amount of voids generated is within the range of 0.1 to 30% by volume, and even more preferable to be within the range of 1 to 20% by volume when the total amount of the conductive thermoset is 100% by volume. It is presumed that the main cause of the amount of voids generated is that the first silver particles melt, deform in volume, and become porous. In addition, it is also presumed that the protective agent covering the periphery of the first silver particles or the like scatters or thermally decomposes.

[0057] Here, referring again to FIG. 5, the relationship between the mixing ratio of the first silver particles / the second silver particles and the void amount of the conductive thermosetting resin will be described. That is, on the premise that the total mixing amount of the first silver particles / the second silver particles is 90 to 99% by weight, the mixing ratio of the first silver particles / the second silver particles is taken on the horizontal axis of FIG. 5. Similarly, the value of the void amount of the conductive thermosetting resin is taken on the right axis, and line B is a characteristic curve showing these relationships. From such a characteristic curve, when the mixing ratio of the first silver particles / the second silver particles is within the range of 25 / 75 to 75 / 25, the value of the void amount of the conductive thermosetting resin tends to be low. Therefore, in order to suppress the void amount of the conductive thermosetting resin to a low level, on the premise that the total mixing amount of the first silver particles / the second silver particles is 90 to 99% by weight, it can be said that it is more preferable that the mixing ratio of the first silver particles / the second silver particles is within the range of 30 / 70 to 70 / 30, and it is even more preferable that the value is within the range of 35 / 65 to 65 / 35.

[0058] 5. Thermosetting resin (1) Main agent The type of the thermosetting resin is not particularly limited, but usually, it is preferable that at least one epoxy resin selected from the group consisting of bisphenol A type epoxy resin, bisphenol F type epoxy resin, novolac type epoxy resin, glycidylamine type epoxy resin, and aliphatic type epoxy resin is used as the main agent. However, as the thermosetting resin, it is also preferable to directly use a self-crosslinking type thermosetting resin such as a phenolic resin or a silicone resin other than the epoxy resin.

[0059] (2) Curing agent Also, when an epoxy resin or the like is used as the main agent, it is preferable to blend a predetermined amount of a curing agent (including a curing catalyst). That is, such a curing agent is usually preferably one kind alone or a combination of two or more kinds, such as an imidazole-based curing agent, an amide-based curing agent, a phenolic-based curing agent, an amine-based curing agent, an acid anhydride-based curing agent, and a five-membered heterocyclic aromatic compound containing nitrogen. And for such a curing agent, generally, since the curing start temperature is 100 °C or higher, even if a silver paste is prepared and applied to a substrate or the like, it is difficult for curing to start at room temperature, and there is a potentiality that curing is likely to start after firing. Therefore, when manufacturing a flexible printed circuit board, there is an advantage that the application and handling of the silver paste become easier.

[0060] Also, the mixing ratio of the thermosetting resin and the curing agent is preferably an epoxy equivalent (1:1 in molar ratio), but usually, it is preferably in the range of 1:1 to 4:1 or less by weight ratio. The reason for this is that by setting the mixing ratio of the thermosetting resin and the curing agent within the above range, the reactivity between the thermosetting resin and the curing agent is further improved, so that the curing of the silver paste can be promoted. Therefore, it is more preferable that the mixing ratio (weight ratio) of the thermosetting resin and the curing agent is a value within the range of 1:15 to 3:1, and it is even more preferable that the value is within the range of 1:2 to 2:1.

[0061] 6. Solvent The silver paste preferably contains a predetermined amount of a solvent in order to uniformly disperse metal nanoparticles, metal particles, a thermosetting resin, a curing agent, etc. Such a solvent is not particularly limited as long as it can highly disperse metal nanoparticles and metal particles and dissolve the thermosetting resin and the curing agent. Therefore, as such a solvent, it is preferable to use one having a total carbon number of 8 to 16, having a hydroxyl group, and further having a boiling point of 280 °C or lower. More specifically, as the solvent, at least one selected from the group consisting of diethylene glycol monoethyl ether acetate (boiling point 218 °C), terpineol (boiling point 219 °C), dihydroterpineol (boiling point 220 °C), texanol (boiling point 260 °C), 2,4-dimethyl-1,5-pentanediol (boiling point 150 °C), and butyl carbitol (boiling point 230 °C) can be used.

[0062] In addition, as the solvent, at least one selected from the group consisting of isophorone (boiling point 215 °C), ethylene glycol (boiling point 197 °C), butyl carbitol acetate (boiling point 247 °C), and 2,2,4-trimethyl-1,3-pentanediol diisobutyrate (C16, boiling point 280 °C) can also be used.

[0063] In addition, the addition amount of the solvent in the silver paste is not particularly limited, but it is preferably prepared so that the silver paste has a viscosity that can be applied by a method such as screen printing. Therefore, more specifically, the content of the solvent is preferably a value within the range of 0.1 to 20% by weight, more preferably a value within the range of 1 to 15% by weight, and still more preferably a value within the range of 2 to 10% by weight, based on the total amount of the silver paste.

[0064] 7. Additives In order to improve the printing characteristics, conductor characteristics, and adhesion, etc., within a range that does not adversely affect the dispersion stability of the plurality of particles in the silver paste and the performance of the wiring after thermosetting, it is also preferable to blend at least one additive such as an antifoaming agent, a surfactant, and a rheology modifier. In that case, although it depends on the type of additive, etc., the blending amount of the additive is preferably a value within the range of usually 0.01 to 30% by weight, more preferably a value within the range of 0.1 to 20% by weight, and still more preferably a value within the range of 1 to 10% by weight, based on the total amount of the silver paste.

[0065] 8. Viscosity In addition, the viscosity (measurement temperature: 25 °C) of the silver paste is preferably a value within the range of 3,000 to 50,000 mPa·sec. The reason for this is that with such a viscosity, even when a simple coating method such as silk screen is used, good printing accuracy and rapid printability can be obtained for wirings having a predetermined thickness, width, etc. Moreover, when silk screen or the like is used, the miscibility of the plurality of silver particles becomes suitable, and it is easier to exhibit lower resistance over a long period of time. Therefore, it is more preferable that the viscosity of the silver paste is within the range of 9,000 to 30,000 mPa·sec, and it is even more preferable that the viscosity is within the range of 10,000 to 20,000 mPa·sec.

[0066] [Second Embodiment] The second embodiment is a flexible circuit board derived from the silver paste of the first embodiment. A side view is illustrated in FIG. 6(a). A flexible circuit board 10 sequentially includes a resist member 38, a wiring 12 made of a conductive thermosetting material derived from a predetermined silver paste, and a base material 11 from the surface side above the drawing. Further, as shown in the plan view of FIG. 6(b), exposed portions where no resist member exists are provided as electrode pads 52a, 52b, 54a, and 54b in a part of the wiring 12, and a conductive member 55 that is electrically connected to a predetermined electronic component or another electric circuit (not shown respectively) via an electrical connection member such as solder or conductive ink, and another wiring 12 are preferably in a mode where electrical connection can be made. The flexible circuit board of the second embodiment will be specifically described assuming the use of a vehicle bus bar.

[0067] 1. Resist Member As shown in FIGS. 6(a) and 6(b), the flexible circuit board is characterized by including a resist member 38 for covering the surface of the wiring 12 made of a conductive thermosetting material and the like to provide electrical, mechanical, and chemical protection. And as such a resist member (which may also be referred to as an insulating cover or the like), it is preferable to use an insulating film or a resist material having a predetermined thickness.

[0068] More specifically, as the resist member, it is preferable to use a resin such as polyimide (PI), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polypropylene (PP), polybutylene terephthalate (PBT), or polyurethane (PU). Furthermore, as the resist member, it is preferable to use a thermosetting resist or a UV-curable resist. In particular, since it is excellent in adhesion, durability, etc., it is preferable to preferably use an epoxy-based resist or a urethane-based resist.

[0069] Furthermore, it is preferable that an adhesive layer is provided on one or both sides of the resist member and is laminated precisely and firmly on the surface of wiring or the like made of a conductive thermosetting material. Moreover, in order to improve the durability, visibility, etc. of the resist member, inorganic fillers (including glass particles and other ceramic particles, etc.), inorganic fibers, carbon fibers, organic fillers, colorants, metal scavengers, lubricants, coupling agents, etc. are usually blended in the range of 0.1 to 30% by weight with respect to the total amount of the resist member. Regarding the thickness of the resist member, it can be appropriately changed according to the use of the flexible circuit board, etc., but it is preferably a value within the range of usually 20 to 50 μm, more preferably a value within the range of 25 to 45 μm, and even more preferably a value within the range of 30 to 40 μm so as to maintain a predetermined low resistance and not excessively impair the flexibility.

[0070] 2. Conductive thermosetting material derived from silver paste Since the conductive thermosetting material derived from silver paste is substantially the same as the content described in the first embodiment, the repeated description here is omitted. However, it is preferable to appropriately change the types of blending components and the suitable range of viscosity, etc. within the range that does not adversely affect the dispersion stability of multiple particles and the performance of the wiring after thermosetting.

[0071] 3. Substrate (1) Types, etc. First, the types of the substrate in the flexible circuit board will be described. That is, as shown in FIG. 6, the flexible circuit board 10 preferably includes a substrate 11 made of a predetermined resin and wiring 12 made of a patterned conductive thermosetting material. Then, as shown in FIG. 6, a pad electrode 21 of a conductive thermosetting material is provided on the flexible circuit board 10, and it is preferable that the conductive member 55 can be electrically connected via solder or the like.

[0072] Here, although the type of the base material in the flexible circuit board is not particularly limited, usually, it is preferable to use an electrically insulating film or plate material. The reason for this is that such a base material has flexibility, durability, etc., and can cope with bending use, etc., according to the place of use.

[0073] And the predetermined material constituting the base material is not particularly limited. For example, it is preferable to use at least one selected from the group consisting of polyimide (PI), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC), polypropylene (PP), and polybutylene terephthalate (PBT). These predetermined materials are excellent in the balance among electrical characteristics (including dielectric characteristics), mechanical characteristics, heat resistance, transparency, moisture resistance, adhesion, workability, and further, economy, etc.

[0074] Moreover, in order to cope with recent high density, high frequency, etc., it is more preferable that it is a multilayer substrate composed of a 2 - 10 layer base material. For example, as shown in FIG. 6(c), a solution of a thermocompression - bondable aromatic polyimide or its precursor is co - extruded on one side or both sides of a resin layer derived from an aromatic polyamic acid solution of aromatic polyimide, and this is made into a semi - cured state at a predetermined temperature (100 - 200°C), and further heated to a temperature of 300°C or higher to be cured to form a multilayer structure such as a two - layer structure or a three - layer structure. This is more preferable.

[0075] That is, it is preferable that a thermocompression-bondable resin layer (such as a low-Tg polyimide resin layer) made of the same material having a glass transition temperature of less than 300°C, more preferably in the range of 150 to 270°C, is formed on the surface of a heat-resistant resin layer (such as a high-Tg polyimide resin layer) as an intermediate layer having a glass transition temperature of 300°C or higher, more preferably in the range of 350 to 450°C, to form a multilayer structure. The reason for this is that in the case of a multilayer substrate having such a thermocompression-bondable resin layer, particularly a polyimide resin multilayer substrate, when wiring is formed thereon, not only can the wiring be formed with higher accuracy, but also a flexible circuit board having excellent adhesion and heat resistance, etc. with the wiring can be formed.

[0076] In addition, the adjustment of the glass transition temperatures of the heat-resistant resin layer as the intermediate layer and the thermocompression-bondable resin layer can be easily achieved by changing the types of reaction components (such as carboxylic acid compounds and amine compounds) such as polyimide resins, the blending ratios, or the addition amounts of glass transition temperature depressants (such as triphenyl phosphate) (for example, 0.01 to 10% by weight based on the total amount of the blending components).

[0077] Furthermore, in order to improve the shape, heat resistance, or adhesion, etc. of the wiring made of a conductive thermosetting material formed on the base material, it is preferable to perform an annealing treatment on the base material. Although it is also preferable to vary the conditions of such an annealing treatment according to the constituent materials and forms of the base material used, usually, it is preferable to perform a heat treatment at 250°C to 450°C for 30 to 60 minutes.

[0078] (2) Wiring Also, as shown in FIG. 1(b), the conductive thermosetting material is preferably provided as wiring (conductor) 12 on the surface (one side or both sides) of the base material 11, either entirely or in a patterned manner. That is, such wiring, although depending on the application, etc., when patterned in a line shape, usually preferably has a length within the range of 100 mm to 5,000 mm. The reason for this is that when the length of such wiring is less than 100 mm, the usage is overly restricted, and in particular, it may be difficult to apply to large circuit wiring boards such as those for automotive use. On the other hand, when the length of such wiring is excessively long, when applying it to a predetermined location, there may be a high need to apply a high voltage. Therefore, when it exceeds 5,000 mm, problems are likely to occur in terms of low energy and safety, and it may become difficult to apply to automotive use or the like. Therefore, when the wiring is patterned in a line shape, it is more preferable to set the value within the range of 200 to 1,500 mm, and even more preferably within the range of 400 to 1,000 mm.

[0079] Also, similarly, when it is patterned in a line shape, even in a narrow space, it is easy to wire and can promote space saving. Therefore, usually, it is preferable to set the wiring width to a value within the range of 0.3 to 3 mm. Therefore, it is more preferable to set the wiring width to a value within the range of 0.4 to 2 mm, and even more preferably within the range of 0.5 to 1 mm.

[0080] Regarding the width of such wiring, when assuming a plurality of wirings (for example, a first wiring and a second wiring), it is not necessarily required that the line widths of the first wiring and the second wiring be equal. For example, it is preferable to set the width of the first wiring to a value within the range of 1 to 3 mm, and the width of the second wiring to a value within the range of 0.3 to less than 1 mm. The spacing (space) between adjacent multiple wirings can be appropriately changed in consideration of the use, form, constituent material (specific resistance), wiring width, thickness, etc. of the flexible circuit board. Usually, it is preferably set to a value within the range of 0.1 to 1.5 mm, more preferably within the range of 0.15 to 1 mm, and even more preferably within the range of 0.2 to 0.8 mm.

[0081] Also, similarly, when it is patterned in a line shape or a planar shape, it is preferable to set the thickness of the wiring to a value within the range of 10 to 100 μm. That is, in the conventional silver paste mainly composed of nano-sized metal particles, it is difficult to increase the film thickness of the wiring. However, according to the silver paste of the present invention, it is easy to make the thickness of the electrically conductive thermosetting material 10 μm or more, and it is easy to suppress the resistivity of the wiring to a low level. On the other hand, by making the thickness of the electrically conductive thermosetting material 100 μm or less, there is an advantage that good flexibility can be easily obtained and the insulating cover can be easily coated. Therefore, it is preferable that the thickness of the electrically conductive thermosetting material, that is, the thickness of the wiring, is a value within the range of 25 to 80 μm, and it is more preferable that the thickness of the wiring is a value within the range of 30 to 50 μm.

[0082] Therefore, when the electrically conductive thermosetting material is patterned in a line shape or a planar shape to form wiring, usually, its length, width, thickness, etc. are appropriately adjusted so that the resistance value per unit length is preferably a value of 1000 mOhm / m (= mΩ / m) or less. The reason for this is that if the resistance value per unit length is relatively low, even when a high voltage of about 12 V is applied to such wiring, the temperature rise (heat generation) of the electrically conductive thermosetting material is unlikely to occur. Therefore, the electrical loss is small, and it is easy to apply to a wide range of applications including automotive applications where safety is required.

[0083] Therefore, it is more preferable that the resistance value per unit length of the wiring is a value within the range of 100 to 800 mOhm / m, and it is even more preferable that the resistance value is a value within the range of 200 to 600 mOhm / m. In addition, when there are a plurality of wirings with different lengths, widths, thicknesses, and resistance values of the wiring provided entirely or in a patterned manner, regarding the resistance value per unit length of the wiring, it may be within the same range or may have resistance values in different numerical ranges depending on the formation positions of the wirings.

[0084] 4. Characteristics In the flexible circuit board shown in FIG. 6, it is preferable that the silver paste serving as the raw material of the conductive thermosetting material, the conductive thermosetting material (wiring) derived therefrom, and further the flexible circuit board itself satisfy the predetermined conditions shown in Example 1 or the like with respect to at least one of the following characteristics (a) to (g). (a) Excellent screen printability (for example, it is possible to form a coating film of uniform thickness on the base material with a #250 screen.) (b) Low void content (for example, 40% by volume or less of the total amount.) (c) Low volume resistivity (for example, a value of 8×10 -6 Ω·cm or less.) (d) Excellent flexure test (for example, no problem occurs in a flexure test of 1000 times or more.) (e) Temperature rise due to constant energization (for example, 15°C or less.) (f) Excellent solder reflow resistance (for example, there is no thermal degradation or peeling under reflow conditions of a temperature of 270°C and a time of 30 minutes.) (g) Excellent peel strength (for example, 15 N / 25 mm or more.)

[0085] 5. Production Example Next, with reference to FIGS. 7(a) to (e), a production example of the flexible circuit board 10 will be described. 1) First, as shown in FIG. 7(a), a 50-μm-thick polyimide film is prepared as the base material 11. Such a base material 11 is preferably a two-layer polyimide film having an adhesive polyimide layer (thickness: 15 μm, glass transition point: 250°C) as a low Tg polyimide resin layer 11a and a polyimide film (thickness: 35 μm, glass transition point: 350°C) as a high Tg polyimide resin layer 11b (total thickness: 50 μm). And although it is an optional step, it is preferable to perform annealing treatment on such a two-layer polyimide film under predetermined conditions to enhance dimensional stability and heat resistance.

[0086] 2) Next, as shown in FIG. 7(b), it is preferable to screen-print a predetermined silver paste 34, for example, in a line shape (width: 1 mm, length: 100 mm, thickness: 30 μm) on the surface of the polyimide film as the base material 11.

[0087] 3) Next, as shown in FIG. 7(c), the predetermined silver paste 34 is heat-treated using a heat curing device (such as a gear oven) under conditions of, for example, 265° C. for 60 minutes to form a wiring 12 made of a conductive thermoset.

[0088] 4) Next, as shown in FIG. 7(d), a resist member 38 before heat curing is formed on the entire surface or a partial surface of the base material 11 on which the wiring 12 is formed. 5) Finally, as shown in FIG. 7(e), by heat treatment, a flexible circuit board 10 having a predetermined structure including the resist member 38 can be obtained. It should be noted that for the obtained flexible circuit board 10, it is preferable to separately provide a process for inspecting and confirming the formation accuracy of the wiring 12, the peeling force (adhesive force) of the wiring 12 in the state covered with the resist member 38, and the like.

Example

[0089] Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but it is not particularly limited to the descriptions of these examples and the like without any reason.

[0090] [Example 1] 1. Preparation of Silver Paste As raw materials, the following first silver particles, second silver particles, thermosetting resin, curing agent, and solvent were prepared respectively. Next, the raw materials were contained at a predetermined ratio as shown in Table 1, and mixed and dispersed using a stirrer and a roll mill under general mixing conditions to prepare the silver paste of Example 1.

[0091] (First Silver Particles) As the first silver particles, silver nanoparticles having an average particle diameter (D50) of 150 nm, 220 nm, and 300 nm respectively were prepared. And in Example 1, as the first silver particles, polyhedral silver particles with an average particle diameter (D50) of 220 nm (tap density: 3.5 g / cm 3 , BET specific surface area: 2 m 2 / g) were used.

[0092] (The second silver particles) As the second silver particles, silver particles with average particle diameters (D50) of 1 μm, 3 μm, and 9 μm respectively were prepared. And in Example 1, as the second silver particles, flake-shaped silver particles with an average particle diameter (D50) of 9 μm and a thickness of 3 μm (thickness: tap density: 5.8 g / cm 3 , BET specific surface area: 0.3 m 2 / g) were used.

[0093] (Thermosetting resin) As the thermosetting resin, EPICLON EXA-4850 (registered trademark) 828 (manufactured by DIC Corporation), a bisphenol A type epoxy resin, was used. (Hardener) As the hardener, dicyandiamide (DICY7, manufactured by Mitsubishi Chemical Corporation) was used. (Solvent) As the solvent, dibasic acid ester (DBE, manufactured by Sanshin Chemical Industry Co., Ltd.) was used.

[0094] 2. Evaluation of silver paste (1) Evaluation 1 (Viscosity) For the obtained silver paste, in accordance with JIS Z 8803:2011, using a B-type viscometer, the viscosity (measurement temperature: 25 °C) was measured and evaluated according to the following criteria. ◎: Values in the range of more than 11,000 to 30,000 mPa·sec. 〇: Values of more than 9,000 to 11,000 mPa·sec, or more than 30,000 to 50,000 mPa·sec. △: Values of more than 3,000 to 9,000 mPa·sec. ×: Values of 3,000 mPa·sec or less.

[0095] (2) Evaluation 2 (Screen Printability) The obtained silver paste was screen-printed onto a substrate made of a heat-resistant polyimide film (total thickness: 50 μm, glass transition point: 350°C) having an adhesive polyimide layer (thickness: 15 μm, glass transition point: 250°C) using a SUS screen (printing plate) with a mesh size of #250 so that the width of the wiring was 1 mm, the length was 10 cm, and the thickness was 30 μm. The evaluation was carried out according to the following criteria based on the variation in the width of the wiring (the value of the maximum width - minimum width). ◎: The variation in the width of the wiring is within 1 mm ± 10%. 〇: The variation in the width of the wiring is within 1 mm ± 20%. △: The variation in the width of the wiring is within 1 mm ± 30%. ×: The variation in the width of the wiring exceeds 1 mm ± 30%.

[0096] (3) Evaluation 3 (Void Content) Furthermore, the screen-printed substrate was introduced into a gear oven maintained at 265°C and heat-treated for 60 minutes to form wiring derived from a conductive thermosetting resin, and it was used as a measurement sample for a flexible printed circuit board, and the void content was measured. That is, the void content (%) was measured from the cross-sectional photograph of the wiring in the measurement sample and evaluated according to the following criteria. ◎: The value is 10% or less. ○: The value is 40% or less. △: The value is 60% or less. ×: The value exceeds 60%.

[0097] (4) Evaluation 4 (Volume Resistivity) Also, an evaluation sample similar to that in Evaluation 3 was prepared, and the volume resistivity of the wiring was evaluated. That is, in accordance with JIS C 6481, the volume resistivity of the wiring was measured using a four-probe resistance measuring instrument (resistivity measuring instrument Sigma-5+ manufactured by Nippon Precision Circuits Co., Ltd.) and evaluated according to the following criteria. ◎: The value is 4.0×10 -6 Ω·cm or less. ○: The value is 6.0×10 -6 Ω·cm or less. △: 8.0×10 -6 It is a value of 8.0×10 Ω·cm or less. ×: 8.0×10 -6 It is a value exceeding 8.0×10 Ω·cm.

[0098] (5) Evaluation 5 (Electrical Conductivity Testability) In Evaluation 5, after creating evaluation materials similar to those in Evaluation 3, a resist member made of 35-μm-thick epoxy resin was formed entirely by screen printing to obtain an evaluation sample for the electrical conductivity test. Next, using a DC power supply unit, a DC 2A, 2-week electrical conductivity test was conducted on the wiring derived from the silver paste for the evaluation sample for the electrical conductivity test. That is, the difference in wiring temperature before and after the energization process was measured as the temperature rise value, and then the electrical conductivity testability was evaluated according to the following criteria. ◎: The temperature rise value is 15°C or less. ○: The temperature rise value is 30°C or less. △: The temperature rise value is 45°C or less. ×: The temperature rise value exceeds 45°C.

[0099] (6) Evaluation 6 (Adhesion) In Evaluation 6, evaluation materials similar to those in Evaluation 3 were created, and the adhesion of the wiring derived from the silver paste to the base material was evaluated as the peel strength. That is, in accordance with JIS K 6854-1:1999, using a tensile testing machine, the peel strength was measured at a peel rate of 300 mm / min, and the adhesion of the wiring derived from the silver paste was evaluated according to the following criteria. ◎: The peel strength is 15 N / 25 mm or more. ○: The peel strength is 10 N / 25 mm. △: The peel strength is 5 N / 25 mm or more. ×: The peel strength is less than 5 N / 25 mm.

[0100] (7) Evaluation 7 (Solder Reflow Test) In Evaluation 7, after preparing evaluation materials similar to those in Evaluation 3, a resist member made of an epoxy resin with a thickness of 35 μm was formed entirely by screen printing to serve as an evaluation sample for the solder reflow test. For the evaluation sample for the solder reflow evaluation, evaluation samples similar to those in Evaluation 3 were prepared in Evaluation 7, and a solder reflow property test was conducted. That is, for the wiring derived from the silver paste, after applying solder, it was welded with a solder reflow device, the peel strength was measured, and the solder reflow property was evaluated according to the following criteria. ◎: The peel strength is 15 N / 25 mm or more. ○: The peel strength is 10 N / 25 mm or more. △: The peel strength is 5 N / 25 mm or more. ×: The peel strength is less than 5 N / 25 mm.

[0101] (8) Evaluation 8 (Bendability) In Evaluation 8, evaluation samples similar to those in Evaluation 3 were prepared, and the bendability was evaluated. That is, in accordance with JIS C 5016:1994, using a flexure resistance tester, the bendability was evaluated according to the following criteria. ◎: In a bending test of 1000 times or more, the change in the resistance value is within 1% of the initial value. ○: In a bending test of 1000 times or more, the change in the resistance value is within 5% of the initial value. △: In a bending test of 1000 times or more, the change in the resistance value is within 10% of the initial value. ×: In a bending test of 1000 times or more, the change in the resistance value exceeds 10% of the initial value.

[0102] [Examples 2 - 6] In Examples 2 - 6, the average particle diameters (D50) of the first silver particles and the second silver particles, and as shown in Table 1, the blending amounts of the first silver particles and the second silver particles, and furthermore, the blending amounts of the thermosetting resin, the curing agent, and the solvent, etc. were each changed, and silver paste was prepared in the same manner as in Example 1. That is, in Examples 2 - 4, as the first silver particles, polyhedral particles with an average particle diameter of 150 nm (tap density: 3.0 g / cm3 , BET specific surface area: 1.8 m 2 / g), and as the second silver particles, flake-shaped particles with an average particle diameter of 3 μm and a thickness of 1.5 μm (tap density: 4.0 g / cm 3 , BET specific surface area: 0.5 m 2 / g) were used. As shown in Table 1, the compounding amounts of the first silver particles and the second silver particles were changed, and a silver paste was prepared in the same manner as in Example 1. Also, in Examples 5 to 6, as the first silver particles, polyhedral particles with an average particle diameter of 300 nm (tap density: 3.8 g / cm 3 , BET specific surface area: 2.2 m 2 / g), and as the second silver particles, flake-shaped particles with an average particle diameter of 1 μm and a thickness of 0.5 μm (tap density: 4.5 g / cm 3 , BET specific surface area: 0.8 m 2 / g) were used. As shown in Table 1, the compounding amounts of the first silver particles and the second silver particles were changed, and a silver paste was prepared in the same manner as in Example 1.

[0103] [Comparative Examples 1 to 6] In Comparative Examples 1 to 6, the average particle diameters (D50) of the first silver particles and the second silver particles, and as shown in Table 2, the compounding amounts of the first silver particles and the second silver particles, and further, the compounding amounts of the thermosetting resin, the curing agent, and the solvent were changed respectively, and a silver paste was prepared and evaluated in the same manner as in Example 1. That is, in Comparative Example 1, as the silver particles, only polyhedral particles with an average particle diameter of 220 nm (tap density: 3.8 g / cm 3 , BET specific surface area: 1.8 m 2 / g) were used. As shown in Table 2, the other compounding amounts were changed, and a silver paste was prepared and evaluated in the same manner as in Example 1. Also, in Comparative Example 2, as the second silver particles, flake-shaped (thickness: tap density: 5.8 g / cm with an average particle diameter (D50) of 9 μm and a thickness of 3 μm 3 , BET specific surface area: 0.3 m 2 / g) of silver particles were used. As shown in Table 2, the other compounding amounts were changed, and a silver paste was prepared and evaluated in the same manner as in Example 1. In Comparative Examples 3 to 4, as the first silver particles, polyhedral particles with an average particle diameter of 150 nm (tap density: 3.8 g / cm 3 , BET specific surface area: 1.8 m 2 / g), and as the second silver particles, flake-like particles with an average particle diameter of 3 μm and a thickness of 1.5 μm (tap density: 4.0 g / cm 3 , BET specific surface area: 0.5 m 2 / g) were used. As shown in Table 2, the compounding amounts of the first silver particles and the second silver particles were changed, and a silver paste was prepared and evaluated in the same manner as in Example 1. In Comparative Examples 5 to 6, as the first silver particles, polyhedral particles with an average particle diameter of 300 nm (tap density: 3.8 g / cm 3 , BET specific surface area: 2.2 m 2 / g), and as the second silver particles, flake-like particles with an average particle diameter of 1 μm and a thickness of 0.5 μm (tap density: 4.5 g / cm 3 , BET specific surface area: 0.8 m 2 / g) were used. As shown in Table 2, the compounding amounts of the first silver particles and the second silver particles were changed, and a silver paste was prepared and evaluated in the same manner as in Example 1.

[0104]

Table 1

[0105]

Table 2

[0106]

Table 3

[0107]

Table 4

Explanation of Signs

[0108] 10, 10´: Flexible circuit board 11: Base material 11a: Low Tg polyimide resin layer (adhesive polyimide layer) 11b: High Tg polyimide resin layer 12: Wiring 30: Second silver particles 32: First silver particles 33: Thermosetting resin 34: Silver paste 38: Resist member 52a, 52b, 54a, 54b: Electrode pads 55: Conductive member

Claims

1. A silver paste containing, based on the total amount, 90 to 99% by weight of a plurality of silver particles, 0.5 to 5% by weight of a thermosetting resin, and 0.5 to 5% by weight of a solvent, the silver paste being characterized by having the following configurations (a) to (c). (a) The plurality of silver particles include first silver particles having an average particle diameter of 10 to 500 nm and second silver particles having an average particle diameter of 2 to 30 μm. (b) The weight mixing ratio of the first silver particles / the second silver particles is set to a value within the range of 25 / 75 to 75 / 25. (c) When the silver paste is thermally cured to form a conductive thermoset, the amount of voids generated is set to a value of 40% by volume or less when the total amount of the conductive thermoset is 100% by volume.

2. The silver paste according to claim 1, wherein the melting point of the first silver particles is lower than the melting point of the second silver particles.

3. The silver paste according to claim 1, wherein a protective agent having a boiling point of 180°C or lower is coated around the first silver particles.

4. The silver paste according to claim 1, wherein the first silver particles are in a polyhedral shape and the second silver particles are in a flake shape.

5. The silver paste according to claim 1, wherein the viscosity (measurement temperature: 25°C) is set to a value within the range of 3,000 to 50,000 mPa·sec.

6. The silver paste according to claim 1, wherein the main component of the thermosetting resin is an epoxy resin, and further includes a curing agent and a curing catalyst for the epoxy resin, or either one of them.

7. The silver paste according to claim 1, wherein when the silver paste is thermally cured to form a conductive thermoset on a substrate, the following characteristics (1) to (4) are satisfied. (1) The volume resistivity of the conductive thermosetting material is set to a value within the range of 3×10 -6 to 8×10 -6 Ω·cm. (2) The first silver particles are welded to each other around the second silver particles and exist, and the second silver particles exist in a mutually independent state. (3) The peeling force of the conductive thermoset with respect to the substrate is set to a value within the range of 5 to 25 N / 25 mm. (4) Under the application conditions of direct current 2 A for 2 weeks, the temperature rise of the conductive thermoset is set to a value within 15°C.

8. A flexible circuit board sequentially including, from the surface side, a resist member, a conductive thermoset derived from a silver paste containing a plurality of silver particles, and a polyimide film, wherein the silver paste has the following configurations (a) to (d). (a) The plurality of silver particles include first silver particles having an average particle diameter of 10 to 500 nm and second silver particles having an average particle diameter of 2 to 30 μm. (b) The weight mixing ratio of the first silver particles / the second silver particles is set to a value within the range of 25 / 75 to 75 / 25. (c) When the silver paste is thermoset, the amount of voids generated is set to a value of 40% by volume or less when the total amount of the conductive thermoset is 100% by volume. (d) The thickness of the conductive thermoset is set to a value within the range of 23 to 100 μm.

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