JP7466348B2 - Anisotropic conductive paste and method for manufacturing electronic substrate - Google Patents

Description

The present invention relates to an anisotropic conductive paste and a method for manufacturing an electronic substrate.

In recent years, connection methods using anisotropic conductive adhesives (anisotropic conductive films, anisotropic conductive pastes) have been used to connect flex boards (flexible wiring boards) and rigid boards (non-flexible wiring boards) and to connect electronic components and wiring boards. For example, when connecting electronic components and wiring boards, anisotropic conductive adhesive is placed between the electronic component on which electrodes are formed and the wiring board on which an electrode pattern is formed, and the electronic component and wiring board are thermocompression-bonded to ensure electrical connection.

As an anisotropic conductive paste, a paste containing solder powder, a thermosetting resin, a carboxylic acid-based curing agent, an imidazole-based curing agent, and an amine-based curing agent has been proposed (for example, Patent Document 1). When an electronic component and a wiring board are thermocompression-bonded, the electrodes of the electronic component and the wiring board to be connected can be solder-joined, and conductivity between these electrodes is ensured. Meanwhile, in the gaps between the electrodes of the electronic component and the gaps between the electrodes of the wiring board, solder powder is embedded in the resin component, ensuring insulation between adjacent electrodes.

JP 2013-045650 A

When using an anisotropic conductive paste as described in Patent Document 1, thermocompression bonding is performed at a high temperature of 180° C. or more to react a thermosetting resin such as an epoxy resin with its curing agent. When performing thermocompression bonding at such a high temperature, gas (mainly low boiling point substances such as water) originating from the wiring board and electronic components is generated in a temperature range lower than the progress of curing of the epoxy resin. Then, this gas is taken into the low viscosity epoxy resin before curing, which causes the problem of voids being generated.
Therefore, in the conventional thermocompression bonding process, it was necessary to volatilize in advance the low boiling point substances that were taken into the wiring board or electronic components as a measure to reduce voids. Specifically, the wiring board or electronic components were pre-baked at a temperature of 150° C. for about 5 to 10 minutes. However, since such a pre-baking process requires a processing time of about 5 to 10 minutes, there is a demand to omit the pre-baking process.

The present invention aims to provide an anisotropic conductive paste and a method for manufacturing an electronic substrate that can further reduce the occurrence of voids when the pre-bake process before the thermocompression bonding process is omitted.

The anisotropic conductive paste according to one embodiment of the present invention contains (A) solder powder, (B) epoxy resin, (C) hardener, (D) activator, (E) pregelling agent, and (F) inorganic filler, and is characterized in that the (F) component contains (F1) halloysite.

In an anisotropic conductive paste according to one embodiment of the present invention, the component (F1) is preferably nanotube-shaped microparticles, the length of the microparticles being 0.1 μm or more and 3 μm or less, and the outer diameter of the microparticles being 20 nm or more and 100 nm or less.
In the anisotropic conductive paste according to one aspect of the present invention, the component (F1) is preferably surface-treated with aminomethylpropanol and ammonium polyacrylate.
In the anisotropic conductive paste according to one aspect of the present invention, the component (E) is preferably polymethacrylate-based organic fine particles.

The method for manufacturing an electronic board according to one embodiment of the present invention is a method for manufacturing an electronic board using the anisotropic conductive paste according to the embodiment of the present invention, and is characterized by comprising a coating step of coating the anisotropic conductive paste onto a wiring board, a preheating step of subjecting the wiring board after the coating step to a heat treatment at a temperature of 90°C or higher and 150°C or lower for 3 seconds or higher and 120 seconds or lower, and a thermocompression step of placing electronic components on the anisotropic conductive paste and thermocompression bonding the electronic components to the wiring board.

The present invention provides an anisotropic conductive paste and a method for manufacturing an electronic substrate that can further reduce the occurrence of voids when the pre-bake process before the thermocompression bonding process is omitted.

First, the anisotropic conductive paste of this embodiment will be described.
The anisotropic conductive paste of the present embodiment contains (A) conductive particles, (B) epoxy resin, (C) curing agent, (D) activator, (E) pregelling agent, and (F) inorganic filler, which are described below. The (F) component contains (F1) halloysite.

[Component (A)]
The solder powder (A) used in this embodiment preferably has a melting point of 240° C. or less, and from the viewpoint of low-temperature processing, more preferably has a melting point of 180° C. or less, and particularly preferably has a melting point of 130° C. or more and 160° C. or less. On the other hand, from the viewpoint of the strength of the solder joint, this solder powder more preferably has a melting point of 180° C. or more.
From the viewpoint of the impact on the environment, the solder powder is preferably lead-free solder powder. Here, lead-free solder powder refers to a powder of a solder metal or alloy to which no lead is added. However, although the presence of lead as an unavoidable impurity in the lead-free solder powder is permitted, in this case, the amount of lead is preferably 100 mass ppm or less.

It is preferable that the (A) component is a metal or alloy consisting of at least one metal selected from the group consisting of tin (Sn), bismuth (Bi), copper (Cu), silver (Ag), antimony (Sb), indium (In), zinc (Zn), and titanium (Ti). For example, tin-based solders include tin-copper-based solders such as Sn-0.7Cu; tin-silver-based solders such as Sn-3.5Ag; tin-silver-copper-based solders such as Sn-3.0Ag-0.5Cu, Sn-3.5Ag-0.7Cu, Sn-1.0Ag-0.7Cu, and Sn-0.3Ag-0.7Cu; tin-silver-bismuth-copper-based solders such as Sn-2.5Ag-1.0Bi-0.5Cu and Sn-1.0Ag-2.0Bi-0.5Cu; tin-silver-bismuth-indium-based solders such as Sn-3.5Ag-0.5Bi-8.0In; and tin-silver-bismuth-indium-based solders such as Sn-1.0Ag-0.7Cu-2.0Bi-0.2In. Examples of the solder include tin-silver-copper-bismuth-indium solders, tin-bismuth solders such as Sn-58Bi, tin-silver-bismuth solders such as Sn-1.0Ag-58Bi, tin-antimony solders such as Sn-5.0Sb, tin-zinc solders such as Sn-9Zn, tin-zinc-bismuth solders such as Sn-8.0Zn-3.0Bi, tin-indium-antimony-zinc solders such as Sn-30In-12Sb-3Zn, tin-bismuth-titanium solders such as Sn-56Bi-4Ti, tin-silver-titanium solders such as Sn-3.5Ag-4Ti, and tin-indium solders such as Sn-52In. Indium-based solders include indium solders of metallic indium, and indium-silver solders such as In-3.0Ag. In addition, the above metals and alloys may further contain, as trace components, iron (Fe), nickel (Ni), cobalt (Co), molybdenum (Mo), phosphorus (P), cerium (Ce), germanium (Ge), silicon (Si), gallium (Ga), aluminum (Al), niobium (Nb), vanadium (V), calcium (Ca), magnesium (Mg), zirconium (Zr), gold (Au), palladium (Pd), platinum (Pt), lead (Pb), and the like. Among these, from the viewpoint of the strength of the solder joint, tin-silver-copper system and tin-silver system are preferable. Furthermore, from the viewpoint of low melting point characteristics, tin-bismuth system, tin-silver-bismuth system, tin-indium system, indium system, indium system, indium-silver system, and the like are more preferable.

The average particle diameter of component (A) is usually 1 μm or more and 40 μm or less, but from the viewpoint of being compatible with electronic boards with narrow pitches of solder pads, it is more preferably 1 μm or more and 20 μm or less, even more preferably 2 μm or more and 15 μm or less, and particularly preferably 3 μm or more and 12 μm or less. The average particle diameter can be measured by a dynamic light scattering type particle diameter measuring device.

The amount of component (A) is preferably 10% by mass or more and 50% by mass or less, more preferably 15% by mass or more and 35% by mass or less, and particularly preferably 18% by mass or more and 25% by mass or less, relative to 100% by mass of the anisotropic conductive paste. If the amount of component (A) is equal to or more than the lower limit, the adhesive strength and conductivity of the resulting anisotropic conductive paste can be ensured. On the other hand, if the amount of component (A) is equal to or less than the upper limit, the insulating properties of the resulting anisotropic conductive paste can be ensured.

[Component (B)]
As the epoxy resin (B) used in this embodiment, a known epoxy resin can be appropriately used. In addition, the component (B) preferably contains (B1) a bisphenol type epoxy resin that is liquid at 25°C and an epoxy resin other than the component (B1) (hereinafter also referred to as component (B2)). In this way, by using the components (B1) and (B2) in combination, it is possible to improve the adhesive strength while maintaining the pregel property during preheating.

Examples of component (B1) include bisphenol A type epoxy resins and bisphenol F type epoxy resins. These may be used alone or in combination of two or more.

Examples of the (B2) component include novolac type epoxy resins (such as biphenyl novolac type epoxy resins, cresol novolac type epoxy resins, and phenol novolac type epoxy resins), crystalline epoxy resins (such as tetrakis(glycidyloxyphenyl)ethane, triglycidyl isocyanurate), biphenyl type epoxy resins, naphthalene type epoxy resins, dicyclopentadiene type epoxy resins, and bisphenol type epoxy resins that are solid at 25°C. Among these, crystalline epoxy resins or naphthalene type epoxy resins are preferred from the viewpoint of heat resistance and the like. These may be used alone or in combination of two or more.

When the (B1) and (B2) components are used in combination, the amount of the (B1) component is preferably 10% by mass or more and 90% by mass or less, and more preferably 30% by mass or more and 70% by mass or less, relative to 100% by mass of the (B) component.

The amount of component (B) is preferably 25% by mass or more and 70% by mass or less, more preferably 30% by mass or more and 60% by mass or less, and particularly preferably 35% by mass or more and 50% by mass or less, relative to 100% by mass of the anisotropic conductive paste. If the amount of component (B) is equal to or less than the lower limit, the adhesive strength of the resulting anisotropic conductive paste can be ensured. On the other hand, if the amount of component (B) is equal to or less than the upper limit, the viscosity of the resulting anisotropic conductive paste can be reduced, ensuring coatability.

[Component (C)]
The curing agent (C) used in this embodiment includes imidazoles, imidazole derivatives, epoxy resin amine adduct curing agents, and latent curing agents. These may be used alone or in combination of two or more. In addition, from the viewpoint of curability and performance of the cured product, it is preferable to use at least one of imidazoles and imidazole derivatives in combination with an epoxy resin amine adduct curing agent. Furthermore, it is particularly preferable to use at least one of imidazoles and imidazole derivatives in combination with an epoxy resin amine adduct curing agent and a latent curing agent.

Examples of imidazoles include 2-phenyl-4-methyl-5-hydroxymethylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole, 2,4-diamino-6-[2'-ethyl-4'-methylimidazolyl-(1')]-ethyl-s-triazine, 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine, 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine isocyanuric acid adduct, 1-cyanoethyl-2-phenylimidazole, 1-cyanoethyl-2-phenylimidazolium trimellitate, 1-cyanoethyl-2-undecylimidazole, and 2,4-diamino-6-[2'-undecylimidazolyl-(1')]-ethyl-s-triazine. Among these, it is preferable to use 2-phenyl-4-methyl-5-hydroxymethylimidazole, 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine isocyanuric acid adduct, 1-cyanoethyl-2-phenylimidazolium trimellitate, etc. These may be used alone or in combination of two or more.
Commercially available imidazoles include 2P4MHZ, 2PHZ-PW, 2E4MZ-A, 2MZ-A, 2MA-OK, 2PZ-CN, 2PZCNS-PW, C11Z-CN, and C11Z-A (product names, manufactured by Shikoku Chemical Industry Co., Ltd., etc.).

The imidazole derivative is obtained by derivatizing imidazoles. Commercially available imidazole derivatives include Sanmide LH-210, Imicure AMI-2, Imicure HAPI, etc. (trade names, manufactured by Air Products Co., Ltd.).
Examples of epoxy resin amine adduct curing agents include Amicure PN-23, PN-F, MY-24, VDH, UDH, PN-31, and PN-40 (trade names, manufactured by Ajinomoto Fine-Techno Co., Ltd.), EH-3615S, EH-3293S, EH-3366S, EH-3842, EH-3670S, EH-3636AS, EH-4346S, and EH-5030S (trade names, manufactured by ADEKA Corporation).
An example of the latent curing agent is Novacure HX-3921HP (trade name, manufactured by Asahi Kasei Corporation).

The amount of component (C) is preferably 5% by mass or more and 35% by mass or less, more preferably 10% by mass or more and 30% by mass or less, and particularly preferably 15% by mass or more and 28% by mass or less, relative to 100% by mass of the anisotropic conductive paste. If the amount of component (C) is equal to or more than the lower limit, the hardening properties of the resulting anisotropic conductive paste can be ensured. On the other hand, if the amount of component (C) is equal to or less than the upper limit, the storage stability of the resulting anisotropic conductive paste can be ensured.

[Component (D)]
The (D) activator used in the present embodiment includes organic acids, organic acid amine salts, non-dissociation type activators, and amine-based activators. These may be used alone or in combination of two or more. Among these, from the viewpoint of environmental protection, it is preferable to use organic acids and organic acid amine salts.

Examples of the organic acid include monocarboxylic acids, dicarboxylic acids, and other organic acids.
Examples of monocarboxylic acids include formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, capric acid, lauric acid, myristic acid, pentadecylic acid, palmitic acid, margaric acid, stearic acid, tuberculostearic acid, arachidic acid, behenic acid, lignoceric acid, and glycolic acid.
Examples of dicarboxylic acids include oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, fumaric acid, maleic acid, tartaric acid, diglycolic acid, etc. Among these, glutaric acid, adipic acid, etc. are preferred.
Other organic acids include dimer acid, levulinic acid, lactic acid, acrylic acid, benzoic acid, salicylic acid, anisic acid, citric acid, and picolinic acid.

The organic acid amine salt is an amine salt of the organic acid. As the amine, any known amine can be used. Such an amine may be an aromatic amine or an aliphatic amine. These may be used alone or in combination of two or more. As such an amine, from the viewpoint of the stability of the organic acid amine salt, it is preferable to use an amine having 3 to 13 carbon atoms, and it is more preferable to use a primary amine having 4 to 7 carbon atoms.
Examples of aromatic amines include benzylamine, aniline, 1,3-diphenylguanidine, etc. Among these, benzylamine is particularly preferred.
Examples of the aliphatic amine include propylamine, butylamine, pentylamine, hexylamine, heptylamine, octylamine, cyclohexylamine, and triethanolamine.

Non-dissociation type activators include non-salt organic compounds in which halogen atoms are bonded by covalent bonds. The halogenated compounds may be compounds formed by covalent bonds of each of the elements chlorine, bromine, and fluorine, such as chlorides, bromides, and fluorides, but may also be compounds having covalent bonds of any two or all of chlorine, bromine, and fluorine. These compounds preferably have polar groups such as hydroxyl groups and carboxyl groups, such as halogenated alcohols and halogenated carboxyl compounds, in order to improve the solubility in aqueous solvents. Examples of halogenated alcohols include brominated alcohols such as 2,3-dibromopropanol, 2,3-dibromobutanediol, trans-2,3-dibromo-2-butene-1,4-diol, 1,4-dibromo-2-butanol, and tribromoneopentyl alcohol, chlorinated alcohols such as 1,3-dichloro-2-propanol and 1,4-dichloro-2-butanol, fluorinated alcohols such as 3-fluorocatechol, and other compounds similar to these. Examples of halogenated carboxyl compounds include iodinated carboxyl compounds such as 2-iodobenzoic acid, 3-iodobenzoic acid, 2-iodopropionic acid, 5-iodosalicylic acid, and 5-iodoanthranilic acid, chlorinated carboxyl compounds such as 2-chlorobenzoic acid and 3-chloropropionic acid, brominated carboxyl compounds such as 2,3-dibromopropionic acid, 2,3-dibromosuccinic acid, and 2-bromobenzoic acid, and other compounds similar to these.

Examples of amine-based activators include amines (polyamines such as ethylenediamine), amine salts (organic acid salts and inorganic acid salts (hydrochloric acid, sulfuric acid, hydrobromic acid, etc.) of amines and amino alcohols such as trimethylolamine, cyclohexylamine, and diethylamine), amino acids (glycine, alanine, aspartic acid, glutamic acid, valine, etc.), and amide compounds. Specific examples include diphenylguanidine hydrobromide, cyclohexylamine hydrobromide, diethylamine salts (hydrochloride, succinate, adipate, sebacate, etc.), triethanolamine, monoethanolamine, and hydrobromides of these amines.

The amount of component (D) is preferably 0.1% by mass or more and 10% by mass or less, more preferably 0.5% by mass or more and 5% by mass or less, and particularly preferably 1% by mass or more and 3% by mass or less, relative to 100% by mass of the anisotropic conductive paste. If the amount of component (D) is equal to or more than the lower limit, the activation effect on the surface of the solder powder can be ensured. Also, if the amount of component (D) is equal to or less than the upper limit, the insulating properties of the resulting anisotropic conductive paste can be ensured.

[Component (E)]
The pre-gelling agent (E) used in this embodiment is organic fine particles that have good compatibility with epoxy resin. Moreover, when this component (E) is heated to a temperature equal to or higher than a predetermined temperature and equal to or lower than the curing temperature of the epoxy resin, the organic fine particles swell, thereby gelling the anisotropic conductive paste. From the viewpoint of suppressing voids, the component (E) is preferably polymethacrylate-based organic fine particles. Moreover, it is preferable that the polymethacrylate-based organic fine particles are hydrophilically treated. Moreover, the component (E) may have a core-shell structure.

The average particle size of component (E) is preferably from 0.1 μm to 5 μm, and more preferably from 0.3 μm to 3 μm. The average particle size can be measured by a dynamic light scattering particle size measuring device.
Commercially available products of component (E) include Zefiac F301, F303, F320, and F351 (product names, manufactured by Aica Kogyo Co., Ltd., etc.).

The amount of component (E) is preferably 2% by mass or more and 25% by mass or less, more preferably 3% by mass or more and 20% by mass or less, and particularly preferably 4% by mass or more and 15% by mass or less, relative to 100% by mass of the anisotropic conductive paste. If the amount of component (E) is equal to or more than the lower limit, the effect of suppressing voids can be further improved. On the other hand, if the amount of component (E) is equal to or less than the upper limit, the adhesive strength and conductivity of the resulting anisotropic conductive paste can be further improved.

[Component (F)]
The inorganic filler (F) used in this embodiment must contain halloysite (F1). The inventors speculate that the reason why the (F1) component can suppress the generation of voids is as follows. That is, it is speculated that the voids are generated due to the moisture that is mainly evaporated from the substrate during thermocompression bonding and is taken into the resin. And, according to this (F1) component, it is possible to suppress the moisture from being taken into the resin during thermocompression bonding, and also to suppress the generation of voids due to the taken-in moisture. In this way, the (F1) component can suppress the generation of voids.

The component (F1) is an alumino-silica clay mineral product (Al ₂ Si ₂ O ₅ (OH) _{4.2H 2} O) and is in the form of nanotube-like fine particles.
The length L of the fine particles is preferably 0.1 μm or more and 3 μm or less, and particularly preferably 0.2 μm or more and 2 μm or less.
The outer diameter D of the fine particles is preferably 20 nm or more and 100 nm or less, and particularly preferably 50 nm or more and 70 nm or less.
The inner diameter of the fine particles is preferably 10 nm or more and 40 nm or less, and particularly preferably 15 nm or more and 30 nm or less.
The aspect ratio (L/D) of the fine particles is preferably 5 or more and 30 or less, and particularly preferably 10 or more and 20 or less.

From the viewpoint of adhesive strength, it is preferable that the (F1) component has been subjected to a surface treatment. Any known surface treatment agent can be used as the surface treatment agent. However, from the viewpoint of compatibility with the (A) and (B) components, it is preferable that the surface treatment agent is a combination of aminomethylpropanol and ammonium polyacrylate.

The amount of the (F1) component is preferably 0.5% by mass or more and 12% by mass or less, more preferably 1% by mass or more and 9% by mass or less, and particularly preferably 2% by mass or more and 7% by mass or less, relative to 100% by mass of the anisotropic conductive paste. If the amount of the (F1) component is equal to or more than the lower limit, the effect of suppressing voids can be further improved. On the other hand, if the amount of the (F1) component is equal to or less than the upper limit, the thixotropy is not too high, resulting in poor coating.

The component (F) may contain an inorganic filler other than the component (F1) (hereinafter also referred to as component (F2)).
Examples of the component (F2) include amorphous silica. Examples of the amorphous silica include Aerosil R974 and Aerosil 200. The component (F2) can prevent the resulting anisotropic conductive paste from bleeding when applied.

The amount of component (F) is preferably 0.5% by mass or more and 12% by mass or less relative to 100% by mass of the anisotropic conductive paste. If the amount of component (F) is equal to or more than the lower limit, sufficient thixotropy is obtained and sagging can be sufficiently suppressed. Also, if the amount of component (F) is equal to or less than the upper limit, the thixotropy is not too high, resulting in poor coating.

The anisotropic conductive paste of this embodiment may contain additives such as a thixotropic agent, a surfactant, an antifoaming agent, a powder surface treatment agent, a reaction inhibitor, and an anti-settling agent, in addition to the components (A) to (F) as necessary. The amount of these additives is preferably 0.01% by mass or more and 10% by mass or less, and more preferably 0.05% by mass or more and 5% by mass or less, relative to 100% by mass of the anisotropic conductive paste.

[Electronic Substrate Manufacturing Method]
Next, a method for manufacturing the electronic substrate according to this embodiment will be described.
The manufacturing method for an electronic board of this embodiment is a manufacturing method for an electronic board using the anisotropic conductive paste of this embodiment described above, and is characterized by comprising: a coating step of coating the anisotropic conductive paste onto a wiring board; a preheating step of subjecting the wiring board after the coating step to a heat treatment at a temperature of 90°C or higher and 150°C or lower for 3 seconds or higher and 120 seconds or lower; and a thermocompression bonding step of placing electronic components on the anisotropic conductive paste and thermocompression bonding the electronic components to the wiring board.
The wiring board may be a known wiring board such as a printed wiring board. The electronic component is not particularly limited as long as it has a plurality of electrodes, and may be, for example, a flexible board. Here, an example will be described in which an anisotropic conductive paste is used to connect the electrodes of a rigid board and a flexible board.

In the coating step, the anisotropic conductive paste is coated on the rigid substrate.
Examples of the coating device used here include a dispenser, a screen printer, a jet dispenser, and a metal mask printer.
The thickness of the coating film is not particularly limited, but is preferably 50 μm to 500 μm, and more preferably 100 μm to 300 μm. If the thickness is less than the lower limit, the adhesive strength when the flexible board is mounted on the electrode of the rigid board tends to decrease, while if the thickness exceeds the upper limit, the paste tends to easily overflow into areas other than the connection parts.

In the preheating step, the wiring board after the coating step is subjected to a heat treatment at a temperature of 90° C. to 150° C. for 3 seconds to 120 seconds.
The heat treatment in the preheating step may be performed within the above range, but may be changed as appropriate from the viewpoint of workability. For example, the heat treatment temperature is preferably 100°C or more and 150°C or less, more preferably 110°C or more and 140°C or less, and particularly preferably 120°C or more and 130°C or less. The heat treatment temperature is preferably lower than the melting point of the solder powder. The heat treatment time is preferably 3 seconds or more and 30 seconds or less, more preferably 5 seconds or more and 20 seconds or less, and particularly preferably 5 seconds or more and 10 seconds or less.
Furthermore, when an anisotropic conductive paste containing solder powder is used, the heat treatment temperature is preferably set to a temperature 1° C. or more lower (more preferably 10° C. or more lower) than the melting point of the solder powder.
In this embodiment, the pre-baking process before the thermocompression bonding process can be omitted by performing the pre-heating process with such a short processing time. That is, in the past, a pre-baking process was required to volatilize low-boiling point substances that are taken into the wiring board or electronic components in advance as a measure to reduce voids. However, when the anisotropic conductive paste of this embodiment is used, the anisotropic conductive paste can be pre-gelled by the pre-heating process. And even if gas (mainly low-boiling point substances such as water) originating from the wiring board or electronic components is generated, it is possible to suppress the gas from being taken into the epoxy resin. Therefore, even if the pre-baking process before the thermocompression bonding process is omitted, the generation of voids can be sufficiently suppressed.

In the thermocompression bonding step, the flexible substrate is placed on the anisotropic conductive paste, and the flexible substrate is thermocompression bonded to the rigid substrate. When an anisotropic conductive paste containing solder powder is used, the temperature during thermocompression bonding is preferably 1° C. or more higher (more preferably 10° C. or more higher) than the melting point of the solder powder.
If the temperature during thermocompression bonding does not satisfy the condition that it is 1° C. or more higher than the melting point of the solder powder, the solder tends not to be melted sufficiently, and therefore a sufficient solder joint cannot be formed between the flexible board and the rigid board, and the conductivity between the flexible board and the rigid board tends to be insufficient.
The temperature during thermocompression bonding is preferably 130°C or higher and 250°C or lower, more preferably 150°C or higher and 240°C or lower, and particularly preferably 180°C or higher and 220°C or lower.
The pressure during thermocompression bonding is not particularly limited, but is preferably 0.05 MPa to 4 MPa, and more preferably 0.1 MPa to 3.5 MPa. If the pressure is equal to or higher than the lower limit, a sufficient solder joint can be formed between the rigid board and the flexible board, and the conductivity between the rigid board and the flexible board can be improved. On the other hand, if the pressure is equal to or lower than the upper limit, excessive stress on the rigid board can be prevented, and dead space can be reduced.
In this embodiment, as described above, the pressure during thermocompression bonding can be set to a lower pressure range than in the case of the method using an anisotropic conductive paste of a conductive filler, and therefore the cost of the device used in the thermocompression bonding process can be reduced.
The time for thermocompression bonding is not particularly limited, but is usually from 1 to 60 seconds, preferably from 2 to 20 seconds, and more preferably from 3 to 15 seconds.

In addition, the connection method using the anisotropic conductive paste of the present invention is not limited to the above connection method, and any modifications or improvements that can achieve the object of the present invention are included in the present invention.

The present invention will now be described in more detail with reference to examples and comparative examples, but the present invention is not limited to these examples. The materials used in the examples and comparative examples are shown below.
(Component (A))
Solder powder: average particle size 12 μm, solder melting point 139° C., solder composition 42Sn/58Bi
(Component (B1))
Epoxy resin A: a mixed liquid epoxy resin of bisphenol F type and bisphenol A type, product name "EPICRON EXA-830LVP", manufactured by DIC Corporation (component (B2))
Epoxy resin B: naphthalene type epoxy resin, product name "EPICRON HP-4032D", manufactured by DIC Corporation (component (C))
Curing agent A: 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine, trade name "Curezol 2P4MHZ", manufactured by Shikoku Chemical Industry Co., Ltd. Curing agent B: epoxy resin amine adduct, "EH-5030S", manufactured by ADEKA Corporation Curing agent C: latent curing agent, trade name "Novacure HX-3921HP", manufactured by Asahi Kasei Corporation (component (D))
Activator: Adipic acid (component (E))
Pregelling agent: Modified polymethacrylic acid ester fine particles (hydrophilic treatment), product name "Zefiac F303", manufactured by Aica Kogyo Co., Ltd. (component (F1))
Inorganic filler A: halloysite (without surface treatment), trade name "DRAGONITE HP:M", manufactured by Fimatec Co., Ltd. Inorganic filler B: halloysite (with surface treatment (aminomethylpropanol and ammonium polyacrylate)), trade name "DRAGONITE APA:M", manufactured by Fimatec Co., Ltd. (component (F2))
Inorganic filler C: amorphous silica, product name "AEROSIL R974", manufactured by Nippon Aerosil Co., Ltd.

[Example 1]
18 parts by mass of epoxy resin A, 18 parts by mass of epoxy resin B, 5 parts by mass of curing agent A, 5 parts by mass of curing agent B, 16 parts by mass of curing agent C, 1 part by mass of activator, 5 parts by mass of inorganic filler A, 2 parts by mass of inorganic filler B, and 10 parts by mass of pre-gelling agent A were charged into a container and pre-mixed with a stirrer, and then mixed and dispersed at room temperature using a three-roll mill to obtain a resin composition.
Thereafter, 64 parts by mass of the obtained resin composition, 16 parts by mass of curing agent C, and 20 parts by mass of solder powder were added to a container, and mixed for 2 hours in a kneader to prepare an anisotropic conductive paste.
Next, a rigid substrate (base material: FR-4, line width: 100 μm, pitch: 200 μm, copper thickness: 18 μm) was prepared, and the obtained anisotropic conductive paste was applied to the comb-shaped electrodes of the rigid substrate using a dispenser (thickness: 0.2 mm). The rigid substrate after application was preheated at a temperature of 130° C. for 10 seconds, and then a flexible substrate (line width: 100 μm, pitch: 200 μm, copper thickness: 12 μm) was placed on the anisotropic conductive paste, and the flexible substrate was thermocompression-bonded to the rigid substrate using a thermocompression bonding device (manufactured by Ohashi Manufacturing Co., Ltd.) under conditions of a temperature of 180° C., a pressure of 2.0 MPa, and a compression time of 15 seconds, to produce a rigid substrate (evaluation substrate) with a flexible substrate.

[Example 2 and Comparative Example 1]
An anisotropic conductive paste was obtained in the same manner as in Example 1, except that the materials were mixed according to the composition shown in Table 1.
Moreover, a rigid board (evaluation board) with a flexible board was produced in the same manner as in Example 1, except that the obtained anisotropic conductive paste was used.

<Evaluation of anisotropic conductive paste>
The anisotropic conductive paste was evaluated (voids, adhesive strength, conductivity, and insulation) by the following methods. The results are shown in Table 1.
(1) Voids The evaluation board was observed with a measuring microscope (Olympus "STM6"), and the area of the overlapping part of the rigid board and the flexible board in the evaluation board and the area of the part where bubbles existed were measured, and the ratio of the bubbles [(area of the part where bubbles exist) / (area of the overlapping part) x 100%] was measured. The evaluation of voids was made according to the following criteria.
A: The ratio of bubbles is 10% or less.
Δ: The ratio of bubbles is more than 10% and 20% or less.
×: The ratio of bubbles is more than 20%.
(2) Adhesive strength (peel strength)
Using a tester (Dage 4000 manufactured by Dage), the flexible board was pulled at a test speed of 50 mm/min so that the angle of the flexible board to the rigid board in the evaluation board was 90 degrees, and the peel strength (unit: N/mm) at that time was measured. The adhesive strength was evaluated according to the following criteria.
⊚: Peel strength is 1.0 N/mm or more.
◯: The peel strength is 0.8 N/mm or more and less than 1.0 N/mm.
Δ: The peel strength is 0.6 N/mm or more and less than 0.8 N/mm.
×: The peel strength is less than 0.6 N/mm.
(3) Continuity The resistance of the evaluation board was measured using a digital multimeter (Agilent Technologies "34410A"). The resistance was measured by connecting terminals to the electrodes of the rigid board and the flexible board. If a resistance value is measured as the measurement standard, it is understood that there is continuity, and it can be determined that a circuit can be formed. The conductivity was evaluated according to the following criteria.
◯: The resistance value is 100 mΩ or less.
Δ: The resistance value is more than 100 mΩ.
×: The resistance value is too high to measure.
(4) Insulation The evaluation board was placed in a thermo-hygrostat chamber at a temperature of 85° C. and a humidity of 85% RH, and a voltage of 15 V was applied to the comb-shaped electrodes of the evaluation board. After 500 hours in the thermo-hygrostat chamber, the partial insulation resistance value (unit: Ω) of the comb-shaped electrodes was measured. The insulation was evaluated according to the following criteria.
◯: The insulation resistance value is 1.0×10 ⁸ Ω or more.
Δ: The insulation resistance value is 1.0×10 ⁶ Ω or more and less than 1.0×10 ⁸ Ω.
×: The insulation resistance value is less than 1.0×10 ⁶ Ω.

As is clear from the results shown in Table 1, when the anisotropic conductive paste of the present invention was used (Examples 1 and 2), the results for voids, adhesive strength, conductivity, and insulation were all good. Therefore, it was confirmed that the anisotropic conductive paste of the present invention can further reduce the occurrence of voids when the pre-bake treatment before the thermocompression bonding step is omitted.
In contrast, when an anisotropic conductive paste not containing halloysite was used (Comparative Example 1), it was found to be inferior in terms of voids, adhesive strength, and insulating properties. The reason for this is presumably that when halloysite is not contained, voids cannot be sufficiently suppressed, and the adhesive strength and insulating properties are reduced due to the voids.

The anisotropic conductive paste of the present invention can be suitably used as a technique for connecting wiring boards together (e.g., a flexible board and a rigid board) or as a technique for connecting electronic components to a wiring board.

Claims (5)

(A) a solder powder, (B) an epoxy resin, (C) a curing agent, (D) an activator, (E) a pregelling agent, and (F) an inorganic filler;
An anisotropic conductive paste, characterized in that the component (F) contains (F1) halloysite (excluding hydrated halloysite) . (A) a solder powder, (B) an epoxy resin, (C) a curing agent, (D) an activator, (E) a pregelling agent, and (F) an inorganic filler;
The (F) component contains (F1) halloysite ,
The component (F1) is surface-treated with aminomethylpropanol and ammonium polyacrylate.
1. An anisotropic conductive paste comprising: The anisotropic conductive paste according to claim 1 or 2 ,
The component (F1) is a nanotube-shaped fine particle,
The anisotropic conductive paste, characterized in that the fine particles have a length of 0.1 μm or more and 3 μm or less, and an outer diameter of the fine particles is 20 nm or more and 100 nm or less. The anisotropic conductive paste according to any one of claims 1 to 3,
The anisotropic conductive paste, wherein the component (E) is polymethacrylate-based organic fine particles. A method for manufacturing an electronic substrate using the anisotropic conductive paste according to any one of claims 1 to 4, comprising the steps of:
a coating step of coating the anisotropic conductive paste on a wiring board;
a preheating step of subjecting the wiring substrate after the coating step to a heat treatment at a temperature of 90° C. or more and 150° C. or less for 3 seconds or more and 120 seconds or less;
a thermocompression bonding step of placing an electronic component on the anisotropic conductive paste and thermocompression bonding the electronic component to the wiring board;
A method for manufacturing an electronic substrate, comprising: