JP7742473B2 - Conductive particles, their manufacturing method and conductive material containing them - Google Patents

Description

The present invention relates to conductive particles and conductive materials containing the same.

Conductive particles used as the conductive material in anisotropic conductive materials such as anisotropic conductive films and anisotropic conductive pastes generally have a conductive layer made of metal formed on the surface of core particles, and this conductive layer provides electrical connection between electrodes and wiring.

When connecting electrodes under pressure using such conductive particles, the oxide film that forms on the electrode surface must be removed to ensure electrical continuity, and the conductive particles must be hard enough to withstand the initial pressure. For example, Patent Document 1 discloses conductive particles that can reduce connection resistance and improve connection reliability by incorporating a cross-linkable monomer containing multiple ethylenically unsaturated groups into the core resin particles, thereby keeping the ratio of the compression hardness at 10% compression to the compression hardness at 50% compression within a specific range. However, in the initial stages of connecting electrodes under pressure, the characteristics of the conductive layer, in addition to the characteristics of the core particles, are likely to have an impact.

From this perspective, Patent Document 2 describes conductive particles having a conductive layer including a crystalline layer with a nickel-phosphorus crystal structure, which exhibit a compression hardness of a specific value or greater when compressed by 5%. Furthermore, Patent Document 3 describes conductive particles whose conductive portion has multiple protrusions on the outer surface, in which the proportion of (111) planes in X-ray diffraction of these protrusions is 50% or greater, and whose base has an average maximum diameter of 1 nm or greater and 500 nm or less.

WO2014/007334 JP 2013-214511 A JP 2016-119304 A

Both Patent Documents 2 and 3 aim to reduce connection resistance and increase connection reliability when electrodes are electrically connected by improving the properties of the conductive layer, thereby eliminating the oxide film that forms on the electrode and conductive layer and obtaining conductive particles that increase the contact area between the electrode and conductive particles after connection. However, there is room for further improvement.

Therefore, the object of the present invention is to provide conductive particles that have lower connection resistance and higher connection reliability than conventional particles.

As a result of extensive research aimed at solving the above-mentioned problems, the inventors discovered that conductive particles in which the ratio of the hardness of a conductive layer that is hard enough to remove the oxide film that forms on the electrode during the initial stage of pressure connection of the electrodes to the hardness of the conductive particles after connection satisfies a specific range can reduce connection resistance and also provide excellent connection reliability, leading to the completion of the present invention.

That is, the present invention provides conductive particles having a conductive layer formed on the surface of a core particle, wherein the maximum value of the compression hardness of the conductive particles is 24,000 N/ ^mm2 or more, the compression hardness reaches its maximum value at a compression ratio of less than 5%, the average value of the compression hardness at a compression ratio of 20% or more and 50% or less is 5,000 to 18,000 N/ ^mm2 , and the ratio of the maximum value of the compression hardness to the average value of the compression hardness at a compression ratio of 20% or more and 50% or less is 1.5 or more and 10 or less.

The present invention also provides a method for producing conductive particles in which a conductive layer is formed on the surface of core particles by electroless plating, the method including the step of adding a sulfur compound to the electroless plating reaction solution during the formation of the conductive layer.

The present invention makes it possible to provide conductive particles with low connection resistance and high connection reliability.

FIG. 1 is an SEM image of the conductive particles obtained in Example 1.

The conductive particles of the present invention have a maximum compression hardness (hereinafter sometimes referred to as "K value") of 24,000 N/mm2 or ^more , preferably 29,000 N/ ^mm2 or more, and exhibit a maximum compression hardness at a compression rate of less than 5%, preferably at a compression rate of 1%, 2%, 3%, or 4%. The maximum compression hardness is preferably 50,000 N/ ^mm2 or less.
The compression hardness in the present invention is a value obtained by measuring the load value F (N) when a load is applied to a conductive particle having a radius R (mm) at a loading rate of 2.23 mN/sec using a microcompression tester (e.g., MCTM-500 manufactured by Shimadzu Corporation) and calculating the load value F (N) using the following formula.

Compression hardness (N/mm ² )=(3/√2)×F×S ^−3/2 ×R ^−1/2

Here, the radius R (mm) of the conductive particle is a value calculated from the average particle diameter described below, and the compression ratio is the rate of change in length in the particle diameter direction, and is the ratio of the compression displacement S (mm) to the average particle diameter (mm).

Furthermore, the conductive particles of the present invention have an average compression hardness of 5,000 to 18,000 N/mm ² , preferably 6,000 to 15,000 N/mm ² at a compression rate of 20% to 50%, and the ratio of the maximum compression hardness to the average compression hardness at a compression rate of 20% to 50% is 1.5 to 10, preferably 2 to 8, particularly preferably 3 to 5.
Here, the average value of the compression hardness at a compression ratio of 20% or more and 50% or less is the average value of the K values at compression ratios of 20%, 30%, 40% and 50%.

In the present invention, it is preferable that the ratio of the K value at a compression rate of 2% to the K value at a compression rate of 30% is 1.5 or more and 10 or less, particularly 3 or more and 5 or less, since this results in conductive particles with low connection resistance and high connection reliability.

As described above, the conductive particles of the present invention have the property of being hard when initially compressed, but becoming flexible when further compressed. This makes it possible to sufficiently remove the oxide film that forms on the electrode in the early stages of connecting electrodes under pressure, thereby reducing connection resistance. Furthermore, because they remain flexible after connection under pressure, the contact area with the electrode can be maintained, resulting in excellent connection reliability.

The conductive particles of the present invention are obtained by forming a conductive layer on the surface of a core particle.
The core particles can be inorganic or organic, as long as they are particulate. Examples of inorganic core particles include metal particles such as gold, silver, copper, nickel, palladium, and solder, alloys, glass, ceramics, silica, metal or non-metal oxides (including hydrates), metal silicates including aluminosilicates, metal carbides, metal nitrides, metal carbonates, metal sulfates, metal phosphates, metal sulfides, metal acid salts, metal halides, and carbon. Examples of organic core particles include thermoplastic resins such as natural fibers, natural resins, polyethylene, polypropylene, polyvinyl chloride, polystyrene, polybutene, polyamide, polyacrylate, polyacrylonitrile, polyacetal, ionomers, and polyester, alkyd resins, phenolic resins, urea resins, benzoguanamine resins, melamine resins, xylene resins, silicone resins, epoxy resins, and diallyl phthalate resins. These may be used alone or in combination of two or more.

Instead of the aforementioned inorganic or organic materials, the core particles may be composed of both inorganic and organic materials. When the core particles are composed of both inorganic and organic materials, the inorganic and organic materials may be present in the core particles in a core-shell configuration, such as an inorganic core and an inorganic shell covering the core's surface, or an organic core and an inorganic shell covering the core's surface. Other examples include blend configurations in which inorganic and organic materials are mixed or randomly fused within a single core particle. Core particles composed of both inorganic and organic materials can use the materials constituting the inorganic or organic core particles described above. These core particles may be composed of either a single inorganic or organic material, or a combination of two or more inorganic and organic materials.

The core particles are preferably composed of a resin, and more preferably a thermoplastic resin. Using a core made of such a material can improve the dispersion stability of the particles and also provide appropriate elasticity to improve conductivity when electrically connecting electronic circuits.

When an organic substance is used as the core particles, it is preferable that the core particles have no glass transition temperature or that the glass transition temperature is greater than 100°C, as this facilitates maintaining the shape of the core particles during the anisotropic conductive connection process and the metal film formation process. Furthermore, when the core particles have a glass transition temperature, it is preferable that the glass transition temperature be 200°C or less, as this facilitates softening of the conductive particles during anisotropic conductive connection, increasing the contact area and facilitating electrical continuity. From this perspective, when the core particles have a glass transition temperature, the glass transition temperature is more preferably greater than 100°C and less than 180°C, and particularly preferably greater than 100°C and less than 160°C. The glass transition temperature can be determined, for example, by differential scanning calorimetry (DSC) as the intersection of the tangent to the original baseline and the inflection point in the baseline shift portion of the DSC curve.

When an organic material is used as the core particle, if the organic material is a highly cross-linked resin, the glass transition temperature is barely detectable even when measurements are attempted up to 200°C using the above method. Such particles are also referred to herein as particles without a glass transition point, and such core particles may be used in the present invention. Specific examples of core particle materials without a glass transition temperature include those obtained by copolymerizing the monomers constituting the organic materials exemplified above with a cross-linkable monomer. Examples of crosslinkable monomers include tetramethylene di(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, ethylene oxide di(meth)acrylate, tetraethylene oxide (meth)acrylate, 1,6-hexane di(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, trimeterolpropane tri(meth)acrylate, tetramethylolmethane di(meth)acrylate, tetramethylolmethane tri(meth)acrylate, Examples include polyfunctional (meth)acrylates such as tetramethylolmethane tetra(meth)acrylate, tetramethylolpropane tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, glycerol di(meth)acrylate, and glycerol tridi(meth)acrylate; polyfunctional vinyl monomers such as divinylbenzene and divinyltoluene; silane-containing monomers such as vinyltrimethoxysilane, trimethoxysilylstyrene, and γ-(meth)acryloxypropyltrimethoxysilane; and monomers such as triallyl isocyanurate, diallyl phthalate, diallyl acrylamide, and diallyl ether. Core particles made of such hard organic materials are commonly used, particularly in the COG (chip on glass) field.

There are no particular restrictions on the shape of the core particles. Generally, core particles are spherical. However, the core particles may be in a shape other than spherical, such as fibrous, hollow, plate-like, or needle-like, and may have numerous protrusions on their surface or may be amorphous. In the present invention, spherical core particles are preferred because they have excellent filling properties and are easy to coat with metal.

The conductive layer formed on the surface of the core particles is made of a conductive metal. Examples of metals that can be used to form the conductive layer include gold, platinum, silver, copper, iron, zinc, nickel, tin, lead, antimony, bismuth, cobalt, indium, titanium, antimony, bismuth, germanium, aluminum, chromium, palladium, tungsten, and molybdenum, as well as alloys thereof, and metal compounds such as ITO and solder. Among these, gold, silver, copper, nickel, palladium, and solder are preferred due to their low electrical resistance, and nickel, gold, nickel alloys, and gold alloys are particularly preferred. One type of metal may be used, or two or more types may be used in combination.

The conductive layer may have a single layer structure or a laminated structure consisting of multiple layers. In the case of a laminated structure consisting of multiple layers, it is preferable that the outermost layer is made of at least one material selected from nickel, gold, silver, copper, palladium, a nickel alloy, a gold alloy, a silver alloy, a copper alloy, and a palladium alloy.
The conductive layer does not have to cover the entire surface of the core particle, but may cover only a portion of it. When only a portion of the surface of the core particle is covered, the covered portion may be continuous, or may be discontinuous, for example, in the form of islands.
The thickness of the conductive layer is preferably 0.1 nm or more and 2000 nm or less, and more preferably 1 nm or more and 1500 nm or less. When the conductive particle has protrusions described later, the height of the protrusions is not included in the thickness of the conductive layer referred to here. In the present invention, the thickness of the conductive layer can be measured by cutting the particle to be measured into two pieces and observing the cross-section of the cut surface using an SEM.

The average particle diameter of the conductive particles is preferably 0.1 μm or more and 50 μm or less, more preferably 1 μm or more and 30 μm or less. Having the average particle diameter of the metal-coated particles within this range makes it easier to ensure electrical continuity between opposing electrodes without causing short circuits in a direction different from that between the opposing electrodes. In the present invention, the average particle diameter of the conductive particles is a value measured using a scanning electron microscope (SEM). Specifically, the average particle diameter of the conductive particles is measured using the method described in the Examples. The particle diameter is the diameter of a circular conductive particle image. If the conductive particles are not spherical, the particle diameter refers to the longest length (maximum length) of the line segments intersecting the conductive particle image.

When conductive particles have protrusions on their surfaces, the height of the protrusions is preferably 20 nm to 1,000 nm, more preferably 50 nm to 800 nm. The number of protrusions varies depending on the particle size of the conductive particles, but a number of protrusions per particle of preferably 1 to 20,000, more preferably 5 to 5,000, is advantageous in terms of further improving the conductivity of the conductive particles. Furthermore, the length of the base of the protrusion is preferably 5 nm to 1,000 nm, more preferably 10 nm to 800 nm. The length of the base of the protrusion refers to the length along the surface of the conductive particle at the location where the protrusion is formed, as measured using an electron microscope image of the cross-section of the particle. The height of the protrusion refers to the shortest distance from the base of the protrusion to the apex of the protrusion. If a single protrusion has multiple apexes, the height of that protrusion is the highest apex. The length of the base of the protrusion and the height of the protrusion are the arithmetic average values measured for 20 different particles observed under an electron microscope.

The shape of the conductive particles is not particularly limited, although it depends on the shape of the core particles. For example, they may be fibrous, hollow, plate-like, or needle-like, and may have numerous protrusions on their surface or may be amorphous. In the present invention, spherical shapes or shapes with numerous protrusions on the outer surface are preferred, as they provide excellent packing and connectivity. In particular, shapes with large protrusions on the outer surface tend to result in conductive particles with high compression hardness at the beginning of compression.

Methods for forming a conductive layer on the surface of core particles include dry methods such as vapor deposition, sputtering, mechanochemical methods, and hybridization, and wet methods such as electrolytic plating and electroless plating. A conductive layer may also be formed on the surface of core particles by combining these methods.

In the present invention, forming a conductive layer on the surface of core particles by electroless plating is preferred, as this makes it easy to obtain conductive particles with the desired compression hardness.

Hereinafter, a case where a nickel-phosphorus plating layer is formed as the conductive layer will be described.
When a conductive layer is formed on the surface of a core particle by electroless plating, the surface of the core particle preferably has the ability to capture precious metal ions or is surface-modified to have the ability to capture precious metal ions. The precious metal ions are preferably palladium or silver ions. "Having the ability to capture precious metal ions" means being able to capture precious metal ions as a chelate or salt. For example, when an amino group, imino group, amido group, imido group, cyano group, hydroxyl group, nitrile group, carboxyl group, or the like is present on the surface of the core particle, the surface of the core particle has the ability to capture precious metal ions. When the surface is modified to have the ability to capture precious metal ions, for example, the method described in JP-A-61-64882 can be used.

Such core particles are used to support a precious metal on their surfaces. Specifically, the core particles are dispersed in a dilute acidic aqueous solution of a precious metal salt, such as palladium chloride or silver nitrate. This allows precious metal ions to be captured on the particle surface. A concentration of the precious metal salt in the range of 1×10 ⁻⁷ to 1×10 ⁻² mol per m ² of particle surface area is sufficient. The core particles with the captured precious metal ions are separated from the system and washed with water. Subsequently, the core particles are suspended in water, and a reducing agent is added to reduce the precious metal ions. This allows the precious metal to be supported on the surface of the core particles. Examples of reducing agents that can be used include sodium hypophosphite, sodium borohydride, potassium borohydride, dimethylamine borane, hydrazine, and formalin. A reducing agent is preferably selected from these based on the constituent materials of the desired conductive layer.

Before capturing precious metal ions on the surface of the core particles, they may be subjected to a sensitization treatment that causes tin ions to be adsorbed onto the particle surface. To adsorb tin ions onto the particle surface, for example, surface-modified core particles may be placed in an aqueous solution of stannous chloride and stirred for a predetermined period of time.

The core particles that have been pretreated in this way are then subjected to a process for forming a conductive layer. There are two types of conductive layer formation processes: a process for forming a conductive layer with protrusions, and a process for forming a conductive layer with a smooth surface. First, we will explain the process for forming a conductive layer with protrusions.

In the process of forming the conductive layer having the protrusions, the following first and second steps are performed.
The first step is an electroless nickel plating step in which an aqueous slurry of core particles is mixed with an electroless nickel plating bath containing a dispersant, nickel salt, a reducing agent, a complexing agent, etc. In this first step, the plating bath self-decomposes simultaneously with the formation of a conductive layer on the core particles. Because this self-decomposition occurs near the core particles, the self-decomposition products are captured on the surface of the core particles during the formation of the conductive layer, generating nuclei of microprojections, and simultaneously forming the conductive layer. The generated nuclei of microprojections serve as base points for the growth of protrusions.

In the first step, the aforementioned core particles are thoroughly dispersed in water, preferably in the range of 0.1 to 500 g/L, and more preferably 1 to 300 g/L, to prepare an aqueous slurry. The dispersion operation can be carried out using normal stirring, high-speed stirring, or a shear dispersion device such as a colloid mill or homogenizer. Ultrasound may also be used in combination with the dispersion operation. If necessary, a dispersant such as a surfactant may be added during the dispersion operation. Next, the aqueous slurry of dispersed core particles is added to an electroless nickel plating bath containing nickel salt, a reducing agent, a complexing agent, and various additives, to carry out the first step of electroless plating.

Examples of the dispersant include nonionic surfactants, zwitterionic surfactants, and/or water-soluble polymers. Nonionic surfactants include polyoxyalkylene ether surfactants such as polyethylene glycol, polyoxyethylene alkyl ether, and polyoxyethylene alkylphenyl ether. Zwitterionic surfactants include betaine surfactants such as alkyldimethyl acetate betaine, alkyldimethylcarboxymethyl acetate betaine, and alkyldimethylamino acetate betaine. Water-soluble polymers include polyvinyl alcohol, polyvinylpyrrolidinone, and hydroxyethyl cellulose. These dispersants can be used alone or in combination. The amount of dispersant used varies depending on the type, but is generally 0.5 to 30 g/L relative to the volume of the liquid (electroless nickel plating bath). In particular, a dispersant amount in the range of 1 to 10 g/L relative to the volume of the liquid (electroless nickel plating bath) is preferred from the perspective of further improving the adhesion of the conductive layer.

Nickel salts such as nickel chloride, nickel sulfate, or nickel acetate are preferably used, with a concentration in the range of 0.1 to 50 g/L. Reducing agents can be similar to those used to reduce precious metal ions, as described above, and are selected based on the constituent materials of the target base coating. When a phosphorus compound, such as sodium hypophosphite, is used as the reducing agent, the concentration is preferably in the range of 0.1 to 50 g/L.

Complexing agents include compounds that have a complexing effect on nickel ions, such as carboxylic acids (salts) such as citric acid, hydroxyacetic acid, tartaric acid, malic acid, lactic acid, gluconic acid, or their alkali metal or ammonium salts; amino acids such as glycine; amine acids such as ethylenediamine and alkylamines; other ammonium salts; EDTA; and pyrophosphate (salts). These can be used alone or in combination. The concentration is preferably 1 to 100 g/L, more preferably 5 to 50 g/L. The preferred pH of the electroless nickel plating bath at this stage is 3 to 14. The electroless nickel plating reaction begins rapidly upon addition of the aqueous slurry of core particles and is accompanied by the generation of hydrogen gas. The first step is considered complete when the generation of hydrogen gas has completely ceased.

In the second step, following the first step, electroless nickel plating is performed by using either (i) a first aqueous solution containing one of a nickel salt, a reducing agent, and an alkali, and a second aqueous solution containing the remaining two, or (ii) a first aqueous solution containing a nickel salt, a second aqueous solution containing a reducing agent, and a third aqueous solution containing an alkali, and adding these aqueous solutions simultaneously and over time to the solution from the first step. Adding these solutions restarts the plating reaction, and by adjusting the amounts added, the conductive layer formed can be controlled to the desired thickness. After the addition of the electroless nickel plating solution is complete, stirring is continued while maintaining the solution temperature for a while after hydrogen gas generation has completely ceased to be observed, to complete the reaction.

In the case of (i) above, it is preferable to use a first aqueous solution containing a nickel salt and a second aqueous solution containing a reducing agent and an alkali, but this combination is not limiting. In this case, the first aqueous solution does not contain a reducing agent or alkali, and the second aqueous solution does not contain a nickel salt. The nickel salt and reducing agent may be those described above. The alkali may be, for example, an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide. The same applies to the case of (ii) above.

In the case of (ii) above, the first to third aqueous solutions each contain a nickel salt, a reducing agent, and an alkali, and each aqueous solution does not contain the other two components.

In either case (i) or (ii), the concentration of the nickel salt in the aqueous solution is preferably 10 to 1000 g/L, particularly 50 to 500 g/L. When a phosphorus compound is used as the reducing agent, the concentration of the reducing agent is preferably 100 to 1000 g/L, particularly 100 to 800 g/L. When a boron compound is used as the reducing agent, the concentration is preferably 5 to 200 g/L, particularly 10 to 100 g/L. When hydrazine or a derivative thereof is used as the reducing agent, the concentration is preferably 5 to 200 g/L, particularly 10 to 100 g/L. The concentration of the alkali is preferably 5 to 500 g/L, particularly 10 to 200 g/L.

The second step is carried out continuously after the first step is completed. Alternatively, the first and second steps may be carried out intermittently. In this case, after the first step is completed, the core particles and plating solution are separated by a method such as filtration, and the core particles are dispersed in water to prepare an aqueous slurry. To this, an aqueous solution containing a complexing agent dissolved at a concentration of preferably 1 to 100 g/L, more preferably 5 to 50 g/L, is added, and a dispersant is dissolved at a concentration of preferably 0.5 to 30 g/L, more preferably 1 to 10 g/L, to prepare an aqueous slurry. The second step then involves adding the aqueous solutions described above to the aqueous slurry. In this manner, a conductive layer having protrusions can be formed.

Next, a process for forming a conductive layer with a smooth surface will be described below.
A conductive layer with a smooth surface can be formed by reducing the concentration of nickel salt in the electroless nickel plating bath in the first step of the treatment for forming the conductive layer having protrusions. That is, the nickel salt may be, for example, nickel chloride, nickel sulfate, or nickel acetate, and its concentration is preferably in the range of 0.01 to 0.5 g/L. A conductive layer with a smooth surface can be formed by performing the first and second steps described above without reducing the concentration of nickel salt in the electroless nickel plating bath.

In the present invention, it is preferable to add a sulfur compound during the formation of the conductive layer, since this tends to result in conductive particles with a large compression hardness in the initial stage of compression.
Examples of sulfur compounds include 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, 2-mercaptobenzimidazole, 2-mercapto-1-methylimidazole, thioglycolic acid, thiodiglycolic acid, cysteine, saccharin, thiamine nitrate, sodium N,N-diethyldithiocarbamate, 1,3-diethyl-2-thiourea, dipyridine, N-thiazole-2-sulfamylamide, 1,2,3-benzotriazole-2-thiazoline-2- Examples of sulfur compounds include thiol, thiazole, thiourea, ethylenethiourea, thiozole, sodium thioindoxylate, o-sulfonamidobenzoic acid, sulfanilic acid, acid orange, methyl orange, naphthionic acid, naphthalene-α-sulfonic acid, 1-naphthol-4-sulfonic acid, Scheffer's acid, sulfadiazine, ammonium thiocyanate, potassium thiocyanate, sodium thiocyanate, rhodanine, ammonium sulfide, sodium sulfide, and ammonium sulfate. These sulfur compounds can be used alone or in combination of two or more. The amount of sulfur compound used is preferably an amount that results in a total sulfur compound concentration in the electroless plating reaction solution of 0.01 ppm by mass or more and 100 ppm by mass or less, and more preferably an amount that results in a total sulfur compound concentration in the electroless plating reaction solution of 0.1 ppm by mass or more and 50 ppm by mass or less. If the amount of sulfur compound used is too small, the effect of increasing the compression hardness at the early stage of compression is difficult to achieve, while if the amount is too large, the compression hardness increases from the middle to late stages of compression, which is undesirable.

The sulfur compound may be added during the formation of the conductive layer, but is preferably added during the second step. Starting the addition 5 to 20 minutes after the start of the second step is particularly preferred, as this facilitates increasing the compression hardness at the initial stage of compression. The sulfur compound may be added all at once, or in multiple portions, or continuously, but adding the entire amount at once is preferred, as this facilitates increasing the compression hardness at the initial stage of compression.
In the case of adding the sulfur compound all at once, the addition time is preferably 30 seconds or less, more preferably 15 seconds or less, depending on the scale of the reaction system, for example, when a 1 L reactor is used. There is no lower limit to the addition time, but it is usually 0.1 seconds or more, or 0.5 seconds or more. Adding the sulfur compound within this range makes it easier to increase the compression hardness at the initial stage of compression.
In the present invention, by adding a sulfur compound in this manner in the process of forming the conductive layer having the above-mentioned protrusions, the protrusions tend to become larger, thereby obtaining the effect of further increasing the compression hardness at the initial stage of compression.
In this way, the conductive particles of the present invention are obtained.

When the conductive particles of the present invention are used as a conductive filler in a conductive adhesive, as described below, their surfaces can be further coated with an insulating resin to prevent short circuits between conductive particles. The insulating resin coating is formed so that the surface of the conductive particles is not exposed as much as possible when no pressure is applied, and so that it is broken down by the heat and pressure applied when bonding two electrodes with the conductive adhesive, exposing at least the protrusions on the surface of the conductive particles. The thickness of the insulating resin can be approximately 0.1 to 0.5 μm. The insulating resin may cover the entire surface of the conductive particles, or only a portion of the surface of the conductive particles.

A wide variety of insulating resins known in the art can be used. Examples include phenolic resin, urea resin, melamine resin, allyl resin, furan resin, polyester resin, epoxy resin, silicone resin, polyamide-imide resin, polyimide resin, polyurethane resin, fluororesin, polyolefin resin (e.g., polyethylene, polypropylene, polybutylene), polyalkyl(meth)acrylate resin, poly(meth)acrylic acid resin, polystyrene resin, acrylonitrile-styrene-butadiene resin, vinyl resin, polyamide resin, polycarbonate resin, polyacetal resin, ionomer resin, polyethersulfone resin, polyphenyloxide resin, polysulfone resin, polyvinylidene fluoride resin, ethyl cellulose, and cellulose acetate.

Methods for forming an insulating coating layer on the surface of conductive particles include chemical methods such as coacervation, interfacial polymerization, in situ polymerization, and liquid curing coating; physico-mechanical methods such as spray drying, air suspension coating, vacuum deposition coating, dry blending, hybridization, electrostatic coalescence, melt-dispersion cooling, and inorganic encapsulation; and physico-chemical methods such as interfacial precipitation.

The conductive particles of the present invention obtained in this manner are suitable for use, for example, in anisotropic conductive films (ACFs), heat seal connectors (HSCs), and conductive materials for connecting electrodes of liquid crystal display panels to the circuit board of a driving LSI chip. In particular, the conductive particles of the present invention are suitable for use as conductive fillers in conductive adhesives.

The conductive adhesive is preferably used as an anisotropic conductive adhesive that is placed between two substrates each having a conductive base material and adheres the conductive base material to establish electrical conductivity by applying heat and pressure. This anisotropic conductive adhesive contains the conductive particles of the present invention and an adhesive resin. Any adhesive resin can be used as long as it is insulating and suitable for use as an adhesive resin. It may be either a thermoplastic or thermosetting resin, and preferably one that exhibits adhesive properties upon heating. Examples of such adhesive resins include thermoplastic, thermosetting, and UV-curable types. Other examples include semi-thermosetting types that exhibit intermediate properties between thermoplastic and thermosetting types, and hybrid types that combine thermosetting and UV-curable types. These adhesive resins can be selected appropriately depending on the surface characteristics of the circuit board or other substrate to be adhered and the intended use. In particular, adhesive resins containing thermosetting resins are preferred due to their superior material strength after adhesion.

Specific examples of adhesive resins include those prepared using one or a combination of two or more resins selected from the group consisting of ethylene-vinyl acetate copolymer, carboxyl-modified ethylene-vinyl acetate copolymer, ethylene-isobutyl acrylate copolymer, polyamide, polyimide, polyester, polyvinyl ether, polyvinyl butyral, polyurethane, SBS block copolymer, carboxyl-modified SBS copolymer, SIS copolymer, SEBS copolymer, maleic acid-modified SEBS copolymer, polybutadiene rubber, chloroprene rubber, carboxyl-modified chloroprene rubber, styrene-butadiene rubber, isobutylene-isoprene copolymer, acrylonitrile-butadiene rubber (hereinafter referred to as NBR), carboxyl-modified NBR, amine-modified NBR, epoxy resin, epoxy ester resin, acrylic resin, phenolic resin, and silicone resin as the base resin. Among these, thermoplastic resins such as styrene-butadiene rubber and SEBS are preferred due to their excellent reworkability. Epoxy resin is preferred as a thermosetting resin. Of these, epoxy resin is the most preferred due to its advantages of high adhesive strength, excellent heat resistance and electrical insulation, as well as low melt viscosity, allowing connections to be made at low pressure.

Any commonly used epoxy resin can be used as the epoxy resin, as long as it is a polyhydric epoxy resin containing two or more epoxy groups per molecule. Specific examples include novolak resins such as phenol novolak and cresol novolak; polyhydric phenols such as bisphenol A, bisphenol F, bisphenol AD, resorcinol, and bishydroxydiphenyl ether; polyhydric alcohols such as ethylene glycol, neopentyl glycol, glycerin, trimethylolpropane, and polypropylene glycol; polyamino compounds such as ethylenediamine, triethylenetetramine, and aniline; and polyvalent carboxy compounds such as adipic acid, phthalic acid, and isophthalic acid, reacted with epichlorohydrin or 2-methylepichlorohydrin. Other examples include aliphatic and alicyclic epoxy resins such as dicyclopentadiene epoxide and butadiene dimer diepoxide. These can be used alone or in combination.

In addition, from the perspective of preventing ion migration, it is preferable to use high-purity adhesive resins with reduced impurity ions (such as Na and Cl) and hydrolyzable chlorine, as the various adhesive resins mentioned above.

The amount of conductive particles of the present invention used in an anisotropic conductive adhesive is typically 0.1 to 30 parts by mass, preferably 0.5 to 25 parts by mass, and more preferably 1 to 20 parts by mass, per 100 parts by mass of the adhesive resin component. Keeping the amount of conductive particles within this range prevents increases in connection resistance and melt viscosity, improves connection reliability, and ensures sufficient connection anisotropy.

In addition to the conductive particles and adhesive resin described above, the anisotropic conductive adhesive may contain additives known in the art. The amounts of these additives may also be within the ranges known in the art. Examples of other additives include tackifiers, reactivity aids, epoxy resin curing agents, metal oxides, photoinitiators, sensitizers, curing agents, vulcanizing agents, anti-degradation agents, heat-resistant additives, thermal conductivity enhancers, softeners, colorants, various coupling agents, and metal deactivators.

Examples of tackifiers include rosin, rosin derivatives, terpene resins, terpene phenol resins, petroleum resins, coumarone-indene resins, styrene-based resins, isoprene-based resins, alkylphenol resins, and xylene resins. Examples of reactive auxiliaries, or crosslinking agents, include polyols, isocyanates, melamine resins, urea resins, utropines, amines, acid anhydrides, and peroxides. Any epoxy resin curing agent with two or more active hydrogen atoms per minute can be used without limitation. Specific examples include polyamino compounds such as diethylenetriamine, triethylenetetramine, metaphenylenediamine, dicyandiamide, and polyamidoamine; organic acid anhydrides such as phthalic anhydride, methylnadic anhydride, hexahydrophthalic anhydride, and pyromellitic anhydride; and novolac resins such as phenol novolac and cresol novolac. These can be used alone or in combination. A latent curing agent may also be used if necessary. Usable latent curing agents include, for example, imidazoles, hydrazides, boron trifluoride-amine complexes, sulfonium salts, amine imides, polyamine salts, dicyandiamide, and modified versions of these. These can be used alone or as a mixture of two or more.

The anisotropic conductive adhesive is manufactured using manufacturing equipment commonly used in the relevant technical field. For example, it is manufactured by blending the conductive particles of the present invention and adhesive resin, as well as a curing agent and various additives as necessary, and mixing them in an organic solvent if the adhesive resin is a thermosetting resin, or by melt-kneading them at a temperature above the softening point of the adhesive resin, preferably about 50 to 130°C, and more preferably about 60 to 110°C, if the adhesive resin is a thermoplastic resin. The anisotropic conductive adhesive obtained in this manner can be applied as a coating or in the form of a film.

The present invention will be further described below with reference to examples, but the scope of the present invention is not limited to these examples. The properties in the examples were measured by the following methods.
(1) Compression hardness (K value) of conductive particles
The K value was determined by the above-mentioned method using a microcompression tester (MCTM-500, manufactured by Shimadzu Corporation).
Furthermore, the K value when the compression rate is X% may be expressed as "X% K value."
(2) Average Particle Diameter 200 particles were randomly extracted from a scanning electron microscope (SEM) photograph of the object to be measured, and the particle diameters were measured at a magnification of 10,000 times. The arithmetic mean value was taken as the average particle diameter.

Example 1
(1) Pretreatment Spherical benzoguanamine-based hard resin particles with an average particle size of 3.0 μm were used as core particles. 9 g of the core particles were added to 200 mL of an aqueous conditioner solution ("Cleaner Conditioner 231" manufactured by Rohm and Haas Electronic Materials) while stirring. The concentration of the aqueous conditioner solution was 40 mL/L. Subsequently, the solution was stirred at a liquid temperature of 60°C for 30 minutes while applying ultrasonic waves to the core particles, thereby surface-modifying and dispersing the core particles. This aqueous solution was filtered, and the core particles were repulped and washed once to form a 200 mL slurry. 0.1 g of stannous chloride was added to this slurry. The mixture was stirred at room temperature for 5 minutes to perform a sensitization treatment in which tin ions were adsorbed onto the surface of the core particles. Subsequently, the aqueous solution was filtered, and the core particles were repulped and washed once to form a 200 mL slurry, which was maintained at 60°C. To this slurry, 1.5 mL of a 0.11 mol/L aqueous palladium chloride solution was added. The mixture was stirred at 60°C for 5 minutes, and an activation treatment was performed to capture palladium ions on the surface of the core particles. Subsequently, the aqueous solution was filtered, and the core particles were repulped and washed with hot water once to form a 100 mL slurry. 10 mL of a 0.5 g/L aqueous dimethylamine borane solution was added, and the mixture was stirred for 2 minutes while applying ultrasound to obtain a slurry of pretreated core particles.

(2) Preparation of Plating Bath Three liters of an electroless nickel-phosphorus plating bath was prepared from an aqueous solution containing 5 g/L of sodium tartrate, 2 g/L of nickel sulfate hexahydrate, 10 g/L of trisodium citrate, 0.1 g/L of sodium hypophosphite, and 2 g/L of polyethylene glycol, and the temperature was raised to 70°C.

(3) Electroless Plating Treatment The slurry of the pretreated core particles was added to this electroless plating bath and stirred for 5 minutes, after which it was confirmed that hydrogen bubbling had ceased.
To this slurry, 420 mL of a 224 g/L nickel sulfate aqueous solution and 420 mL of a mixed aqueous solution containing 210 g/L sodium hypophosphite and 80 g/L sodium hydroxide were continuously added in portions at a rate of 2.5 mL/min using a metering pump, and electroless plating was initiated. Ten minutes after the start of plating, 2-mercaptobenzothiazole was added over 1 second to achieve a final concentration of 7.5 ppm by mass in the resulting solution. Forty minutes after the start of plating, the addition rates of the two aqueous solutions were both increased to 4.7 mL/min. After the entire nickel sulfate aqueous solution and the mixed aqueous solution of sodium hypophosphite and sodium hydroxide were added, stirring was continued for 5 minutes while maintaining the temperature at 70°C. The solution was then filtered, and the filtrate was washed three times and dried in a vacuum dryer at 110°C to obtain conductive particles having a conductive layer made of a nickel-phosphorus alloy. The resulting conductive particles had an average particle size of 3.22 μm, a conductive layer thickness of 110 nm, and large protrusions. The SEM image is shown in Figure 1. The obtained conductive particles showed the highest compression hardness at a compression ratio of 3%. Table 1 shows the compression hardness at each compression ratio.

Example 2
Conductive particles were produced in the same manner as in Example 1, except that 2-mercaptobenzoxazole was added instead of 2-mercaptobenzothiazole in (3) the electroless plating treatment. The resulting conductive particles had an average particle size of 3.22 μm, a conductive layer thickness of 110 nm, and large protrusions. The resulting conductive particles showed the highest compression hardness at a compression ratio of 3%. The compression hardness at each compression ratio is shown in Table 1.

Example 3
Conductive particles were produced in the same manner as in Example 1, except that the concentration of nickel sulfate hexahydrate in (2) Preparation of Plating Bath was changed from 2 g/L to 0.1 g/L. The obtained conductive particles had an average particle size of 3.22 μm, a conductive layer thickness of 110 nm, and a smooth shape without protrusions. The obtained conductive particles showed the highest compression hardness at a compression ratio of 3%. The compression hardness at each compression ratio is shown in Table 1.

Comparative Example 1
(1) Pretreatment Spherical benzoguanamine-based hard resin particles with an average particle size of 3.0 μm were used as core particles. 9 g of the core particles were added to 200 mL of an aqueous conditioner solution ("Cleaner Conditioner 231" manufactured by Rohm and Haas Electronic Materials) while stirring. The concentration of the aqueous conditioner solution was 40 mL/L. Subsequently, the solution was stirred at a liquid temperature of 60°C for 30 minutes while applying ultrasonic waves to the core particles, thereby surface-modifying and dispersing the core particles. This aqueous solution was filtered, and the core particles were repulped and washed once to form a 200 mL slurry. 0.1 g of stannous chloride was added to this slurry. The mixture was stirred at room temperature for 5 minutes to perform a sensitization treatment in which tin ions were adsorbed onto the surface of the core particles. Subsequently, the aqueous solution was filtered, and the core particles were repulped and washed once to form a 200 mL slurry, which was maintained at 60°C. To this slurry, 1.5 mL of a 0.11 mol/L aqueous palladium chloride solution was added. The mixture was stirred at 60°C for 5 minutes, and an activation treatment was performed to capture palladium ions on the surface of the core particles. Subsequently, the aqueous solution was filtered, and the core particles were repulped and washed with hot water once to form a 100 mL slurry. 10 mL of a 0.5 g/L aqueous dimethylamine borane solution was added, and the mixture was stirred for 2 minutes while applying ultrasound to obtain a slurry of pretreated core particles.

(2) Preparation of Plating Bath Three liters of an electroless nickel-phosphorus plating bath was prepared from an aqueous solution containing 5 g/L of sodium tartrate, 2 g/L of nickel sulfate hexahydrate, 10 g/L of trisodium citrate, 0.1 g/L of sodium hypophosphite, and 2 g/L of polyethylene glycol, and the temperature was raised to 70°C.

(3) Electroless Plating Treatment The slurry of the pretreated core particles was added to this electroless plating bath and stirred for 5 minutes, after which it was confirmed that hydrogen bubbling had ceased.
To this slurry, 420 mL of a 224 g/L nickel sulfate aqueous solution and 420 mL of a mixed aqueous solution containing 210 g/L sodium hypophosphite and 80 g/L sodium hydroxide were continuously added in portions using a metering pump at an addition rate of 2.5 mL/min, and electroless plating was initiated. After the entire nickel sulfate aqueous solution and the mixed aqueous solution of sodium hypophosphite and sodium hydroxide were added, stirring was continued for 5 minutes while maintaining a temperature of 70°C. The solution was then filtered, and the filtrate was washed three times and dried in a vacuum dryer at 110°C to obtain conductive particles having a conductive layer made of a nickel-phosphorus alloy. The resulting conductive particles had an average particle size of 3.22 μm, a conductive layer thickness of 110 nm, and protrusions. The resulting conductive particles also exhibited the highest compression hardness at a compression ratio of 4%. Table 1 shows the compression hardness at each compression ratio.

Comparative Example 2
(1) Pretreatment: Spherical benzoguanamine-based hard resin particles with an average particle size of 3.0 μm were used as core particles. 9 g of the core particles were added to 200 mL of an aqueous conditioner solution ("Cleaner Conditioner 231" manufactured by Rohm and Haas Electronic Materials) while stirring. The concentration of the aqueous conditioner solution was 40 mL/L. Subsequently, the solution was stirred at 60°C for 30 minutes while applying ultrasonic waves to the core particles, thereby surface-modifying and dispersing the core particles. This aqueous solution was filtered, and the core particles were repulped and washed once to form a 200 mL slurry. 0.1 g of stannous chloride was added to this slurry. The mixture was stirred at room temperature for 5 minutes, and a sensitization treatment was performed to adsorb tin ions onto the surface of the core particles. Subsequently, the aqueous solution was filtered, and the core particles were repulped and washed once to form a 200 mL slurry, which was maintained at 60°C. 1.5 mL of a 0.11 mol/L aqueous palladium chloride solution was added to the slurry. The mixture was stirred at 60° C. for 5 minutes to activate the core particles, capturing palladium ions on their surfaces. The resulting aqueous solution was then filtered, and the core particles were repulped and washed with hot water once to prepare a 100 mL slurry. 10 mL of a 0.5 g/L dimethylamine borane aqueous solution was added, and the mixture was stirred for 2 minutes while applying ultrasonic waves to obtain a slurry of pretreated core particles.

(2) Preparation of Plating Bath Three liters of an electroless nickel-phosphorus plating bath was prepared from an aqueous solution containing 5 g/L of sodium tartrate, 2 g/L of nickel sulfate hexahydrate, 10 g/L of trisodium citrate, 0.1 g/L of sodium hypophosphite, and 2 g/L of polyethylene glycol. 2-mercaptobenzothiazole was added to the solution so that the final solution had a concentration of 7.5 ppm by mass, and the solution was heated to 70°C.

(3) Electroless Plating Treatment The slurry of the pretreated core particles was added to this electroless plating bath and stirred for 5 minutes, after which it was confirmed that hydrogen bubbling had ceased.
To this slurry, 420 mL of a 224 g/L nickel sulfate aqueous solution and 420 mL of a mixed aqueous solution containing 210 g/L sodium hypophosphite and 80 g/L sodium hydroxide were continuously added in portions using a metering pump at an addition rate of 2.5 mL/min, and electroless plating was initiated. After the entire nickel sulfate aqueous solution and the mixed aqueous solution of sodium hypophosphite and sodium hydroxide were added, stirring was continued for 5 minutes while maintaining a temperature of 70°C. The liquid was then filtered, and the filtrate was washed three times and dried in a vacuum dryer at 110°C to obtain conductive particles having a nickel-phosphorus alloy coating. The resulting conductive particles had an average particle size of 3.05 μm, a conductive layer thickness of 25 nm, and protrusions. The resulting conductive particles also exhibited the highest compression hardness at a compression ratio of 5%. Table 1 shows the compression hardness at each compression ratio.

[Evaluation of connection resistance and connection reliability]
Using the conductive particles of the examples and comparative examples, the connection resistance and connection reliability were evaluated by the following methods.
An insulating paste was prepared by mixing an insulating adhesive containing 100 parts by weight of epoxy resin, 150 parts by weight of curing agent, and 70 parts by weight of toluene with 15 parts by weight of the coated particles obtained in the Examples and Comparative Examples. This paste was applied to a silicone-treated polyester film using a bar coater, and then dried to form a thin film on the film. The resulting thin-film-formed film was placed between a glass substrate entirely coated with aluminum and a polyimide film substrate with a 50 μm-pitch copper pattern to prepare a sample for conducting resistance measurement. Electrical connections were made, and the connection resistance of this sample was measured at room temperature (25°C, 50% RH). A lower connection resistance indicates better connection resistance of the conductive particles. The results are shown in Table 2.
In addition, the samples for measuring the electrical resistance were placed in a sealed container and subjected to a pressure cooker test in which they were treated for 10 hours at a temperature of 121°C, a relative humidity of 100%, and a pressure of 2 atmospheres. After the pressure cooker test, the connection resistance of the samples was measured at room temperature (25°C, 50% RH). The smaller the difference in connection resistance before and after the pressure cooker test, the higher the connection reliability of the conductive particles. The results are shown in Table 2.

These results show that the conductive particles obtained in Examples 1 to 3 have lower resistance values than the conductive particles obtained in Comparative Examples 1 and 2. Furthermore, it can be seen that the conductive particles obtained in Examples 1 to 3 maintain good conductivity without an increase in connection resistance value even after the pressure cooker test.

Claims (4)

A method for producing conductive particles, comprising forming a conductive layer on the surface of a core particle by electroless plating,
The conductive particles have a maximum compressive hardness of 24,000 N/ ^mm2 or more, and exhibit a maximum compressive hardness at a compressibility of less than 5%,
The average value of the compression hardness at a compression rate of 20% or more and 50% or less is 5000 to 18000 N/ ^mm2 ,
The ratio of the maximum value of compression hardness to the average value of compression hardness at a compression rate of 20% or more and 50% or less is 3 or more and 5 or less,
A method for producing conductive particles, comprising the step of adding a sulfur compound to the electroless plating reaction solution during the formation of the conductive layer. The method for producing conductive particles according to claim 1, wherein the sulfur compound is at least one selected from the group consisting of 2-mercaptobenzothiazole, 2-mercaptobenzoxazole, and 2-mercaptobenzimidazole. The method for producing conductive particles according to claim 1 or 2, wherein the sulfur compound is added within 30 seconds. The method for producing conductive particles described in any one of claims 1 to 3, wherein the amount of the sulfur compound added is an amount that results in a total sulfide concentration in the electroless plating reaction solution of 0.01 ppm or more and 500 ppm or less.