JP7413659B2 - Resin compositions and electronic component structures - Google Patents

Description

The present invention relates to a resin composition and an electronic component structure. More specifically, the present invention relates to a resin composition for producing a heat dissipating member, and an electronic component structure comprising a cured product of the resin composition as a heat dissipating layer.

In recent years, the wiring density and the mounting density of electronic components in multilayer wiring boards and semiconductor packages have increased, and semiconductor elements have become highly integrated, and the amount of heat generated per unit area of such heating elements has increased. Therefore, a technology for dissipating heat from a heating element is desired. A common method of heat dissipation is to sandwich a thermally conductive sheet between a heat generating element such as a semiconductor package and a heat dissipating element made of aluminum or copper, and then place the heat conductive sheet in close contact with the heat generating element to transfer the heat from the heat generating element to the outside. It has been adopted. In such a thermally conductive sheet, thermal conductivity is achieved by blending thermally conductive inorganic particles into a resin matrix material. However, there has been a desire for thermally conductive sheets to have even higher levels of thermal conductivity and improved adhesion to heating elements and heat radiating elements.

In response to such demand, for example, in Patent Document 1, in the process of forming a resin composition containing a resin and thermally conductive inorganic particles into a sheet, the viscosity of the resin composition is adjusted to reduce voids generated inside the sheet. A technique has been proposed for obtaining a sheet with excellent thermal conductivity.

JP2012-212727A

However, even if the manufacturing method described in Patent Document 1 is used for resin compositions containing inorganic particles, minute voids (microcracks) may still occur at the interface between the inorganic particles and the resin. Ta. The occurrence of such microcracks may cause a decrease in thermal conductivity, a decrease in dielectric strength, etc. Furthermore, if it is attempted to completely eliminate microcracks, it will be disadvantageous in terms of productivity, production cost, etc. during the manufacture of a thermally conductive sheet.

The present inventors found that if the number of microcracks with a length of 1 μm or more and a width of 0.1 μm or more and 5 μm or less existing in a cured product of a resin composition is within a predetermined range, thermal conductivity and insulation properties are affected. The present inventors have discovered that this material can be suitably used as a heat dissipating material, and have completed the present invention.

According to the present invention, there is provided a resin composition containing a thermosetting resin and a boron nitride filler, wherein the resin composition is applied to a base material, heated and dried, and then subjected to high-pressure pressing to obtain a film thickness of 100 μm or more and 200 μm or more. When the following test film was prepared and the surface of the test film was precisely polished and the resulting polished surface was observed with a scanning electron microscope, no microcracks with a length of 1 μm or more and a width of 1 μm or more were observed on the polished surface. A resin composition having a number of 500 pieces/mm ² or more and 5000 pieces/mm ² or less is provided.

Further, according to the present invention, there is provided an electronic component structure comprising an electronic component and a heat dissipation layer, wherein the heat dissipation layer is made of a cured product of the resin composition, and the number of heat dissipation layers is 500 pieces/mm 2 or more and 5000 pieces/mm ² or more. Provided is an electronic component structure having microcracks of 1 μm or more in length and 1 μm or more in width, with a number of cracks/mm ^{2 or} less.

According to the present invention, there are provided a resin composition that can be suitably used to produce a heat dissipation material with excellent thermal conductivity, and an electronic component structure that includes a heat dissipation layer made of a cured product of the resin composition.

Embodiments of the present invention will be described below.

In this embodiment, the resin composition includes a thermosetting resin and a boron nitride filler. The resin composition of this embodiment is obtained by coating the base material, heating and drying, and pressing under high pressure to prepare a test film with a thickness of 100 μm or more and 200 μm or less, and precision polishing the surface of the test film. When the polished surface is observed with a scanning electron microscope, the number of microcracks with a length of 1 μm or more and a width of 0.1 μm or more and 5 μm or less observed on the polished surface is 500 pieces/mm ² or more and 5000 pieces/mm ² or less. It is.

In this embodiment, the number of microcracks when observed by the above method is 5000 pieces/mm ² or less, preferably 4500 pieces/mm ² or less, and more preferably 4000 pieces/mm ² or less. . Since the number of microcracks is below the above value, when the cured product of this resin composition is used as a heat dissipation layer of an electronic component structure, it has excellent thermal conductivity, and therefore the obtained electronic component structure has excellent Reliable. Further, the lower limit of the microcracks is 500 pieces/mm ² or more, preferably 700 pieces/mm ² or more, and more preferably 1000 pieces/mm ^{2 or} more. The cured product of the resin composition having the number of microcracks in the above range has excellent heat resistance and does not require any special process or equipment to reduce microcracks, so it can be manufactured without reducing production efficiency. can do.

In this embodiment, the number of microcracks present in the cured product of the resin composition is measured by the following method. First, the resin composition of this embodiment is applied to a base material and dried by heating at, for example, 80°C. Next, the coated layer of the resin composition is high-pressure pressed at a temperature of 130° C. to 230° C. and a pressure of 4 MPa to 12 MPa to produce a test film having a thickness of 100 μm or more and 500 μm or less. This test film was precisely polished along the thickness direction using a cross-section polisher (CP), and the resulting polished surface was observed at 1000x magnification using a scanning electron microscope (SEM) to obtain an image. From the captured image, the gap portion and other portions are binarized. Next, voids having a length of 1 μm or more and a width of 0.1 μm or more and 5 μm or less are counted as microcracks. The number of microcracks counted is calculated as the number per unit area.

The number of microcracks in the cured product of the resin composition should be determined by appropriately selecting the type and amount of thermosetting resin contained in the resin composition, the shape of boron nitride, the type and amount of additive used, etc. It can be adjusted by

Thermosetting resins used in the resin composition of this embodiment include cyanate resins, epoxy resins, phenoxy resins, phenol resins, benzoxazine resins, urea resins, melamine resins, unsaturated polyester resins, silicone resins, and polyurethane resins. Examples include. These resins can be used alone or in combination of two or more.

Among them, it is preferable to use epoxy resin, cyanate resin, and phenoxy resin.

More specifically, as the epoxy resin, for example, bisphenol type epoxy resin (e.g., bisphenol A type epoxy resin, bisphenol F type epoxy resin, bisphenol S type epoxy resin, water-added bisphenol A type epoxy resin, dimer acid modified bisphenol type) epoxy resins, etc.), novolak-type epoxy resins (e.g., phenol novolak-type epoxy resins, cresol novolac-type epoxy resins, biphenyl-type epoxy resins, etc.), naphthalene-type epoxy resins, fluorene-type epoxy resins (e.g., bisarylfluorene-type epoxy resins, etc.) ), aromatic epoxy resins such as triphenylmethane type epoxy resins (e.g., trishydroxyphenylmethane type epoxy resins), nitrogen-containing epoxy resins such as triepoxypropyl isocyanurate (triglycidyl isocyanurate), hydantoin epoxy resins, etc. Examples of epoxy resins include aliphatic epoxy resins, alicyclic epoxy resins (such as dicyclocyclic epoxy resins), glycidyl ether type epoxy resins, and glycidylamine type epoxy resins. By using such an epoxy resin, the dispersibility of the boron nitride filler can be improved.

When using an epoxy resin, the blending amount is preferably in the range of 1% by mass to 30% by mass, more preferably in the range of 5% by mass to 28% by mass, based on the entire solid content of the resin composition.

Examples of the phenoxy resin include phenoxy resins having a bisphenol skeleton, phenoxy resins having a naphthalene skeleton, phenoxy resins having an anthracene skeleton, and phenoxy resins having a biphenyl skeleton. Furthermore, a phenoxy resin having a structure having multiple types of these skeletons can also be used. By using a phenoxy resin, the viscosity of the resin composition increases, thereby reducing fluidity and suppressing the occurrence of microcracks. Moreover, when the resin composition of this embodiment is used as a thermally conductive sheet, the adhesion to the heat sink can be improved.

When using a phenoxy resin, its blending amount is preferably from 1% by mass to 30% by mass, more preferably from 5% by mass to 20% by mass, based on the total solid content of the resin composition.

As the cyanate resin, cyanate ester resin can be used.
Specifically, the cyanate ester resins include bisphenol A dicyanate, polyphenol cyanate (oligo(3-methylene-1,5-phenylene cyanate), 4,4'-methylenebis(2,6-dimethylphenyl cyanate), 4, 4'-ethylidene diphenyl dicyanate, hexafluorobisphenol A dicyanate, 2,2-bis(4-cyanate) phenylpropane, 1,1-bis(4-cyanate phenylmethane), bis(4-cyanate-3,5- Difunctional cyanate resins such as dimethylphenyl)methane, 1,3-bis(4-cyanatophenyl-1-(methylethylidene))benzene, bis(4-cyanatophenyl)thioether, and bis(4-cyanatophenyl)ether; phenol Polyfunctional cyanate resins derived from novolak, cresol novolac, phenolic resins containing dicyclopentadiene structures, etc.; prepolymers in which a portion of the above-exemplified cyanate ester resins are triazinized, and the like.
Here, as commercially available cyanate ester resins, for example, PT30, BA230, DT-4000, DT-7000 manufactured by Lonza Japan, etc. can be used.

When using a cyanate resin, its blending amount is preferably in the range of 1% by mass or more and 30% by mass or less, more preferably in the range of 5% by mass or more and 28% by mass or less, based on the entire solid content of the resin composition.

The boron nitride particles used in the resin composition of this embodiment are preferably aggregated particles in which primary particles are aggregated. The primary particles of boron nitride particles can be plate-shaped, scale-shaped, or spherical, and are preferably scale-shaped. The boron nitride particles used in this embodiment are preferably aggregated particles of scaly boron nitride.

The scale-like (or plate-like) primary particles of boron nitride have an average longitudinal length (maximum length in the direction perpendicular to the thickness direction of the scales) of, for example, 1 to 100 μm, preferably 3 to 90 μm. . Further, the average longitudinal length of the boron nitride particles is 5 μm or more, preferably 10 μm or more, more preferably 20 μm or more, particularly preferably 30 μm or more, most preferably 40 μm or more, and usually, for example, It is 100 μm or less, preferably 90 μm or less.

Further, the average thickness of the primary particles of scale-like boron nitride (the length in the thickness direction of the scales, that is, the length in the transverse direction of the particles) is, for example, 0.01 to 20 μm, preferably 0.1 to 15 μm. be.

Further, the aspect ratio (longitudinal length/thickness) of the primary particles of scaly boron nitride is, for example, 2 to 10,000, preferably 10 to 5,000.

The average primary particle diameter measured by the light scattering method is the volume average particle diameter measured by a dynamic light scattering particle size distribution analyzer. If the average primary particle diameter of boron nitride particles measured by a light scattering method is less than the above range, the resulting cured resin composition may become brittle and have poor handling properties.

Examples of commercially available boron nitride particles include, for example, the "PT" series (for example, "PT-110") manufactured by Momentive Performance Materials Japan, and the "Shobie N" manufactured by Showa Denko. Examples include the "UHP" series (for example, "Showbi-N UHP-1", etc.).

In the resin composition of the present embodiment, the amount of the boron nitride filler is preferably 60% by mass or more and 80% by mass or less, more preferably 65% by mass or more and 75% by mass or less, based on the thermosetting resin. It is quantity. Further, in the resin composition of the present embodiment, the volume-based content of the boron nitride filler is 35% by volume or more, preferably 40% by volume or more, preferably 40% by volume or more, based on the total volume of the total solid content of the resin composition. , 50% by volume or more, more preferably 58% by volume or more, and usually, for example, 95% by volume or less.

If the volume-based content of boron nitride particles is less than the above range, sufficient thermal conductivity may not be obtained. On the other hand, if the volume-based content of the boron nitride particles exceeds the above range, the handleability of the resulting cured resin composition may deteriorate.

The blending ratio of the thermally conductive inorganic particles based on mass relative to the total solid content of the resin composition is, for example, 40 to 95 parts by mass, preferably 65 to 90 parts by mass.

It is preferable that the resin composition of this embodiment further contains a leveling agent, a curing agent, an antioxidant, and the like.

In particular, when using an epoxy resin, it is preferable to use a curing agent. As the curing agent, it is preferable to use a curing catalyst or a phenolic curing agent.

Examples of curing catalysts include organic metal salts such as zinc naphthenate, cobalt naphthenate, tin octylate, cobalt octylate, cobalt bisacetylacetonate (II), cobalt trisacetylacetonate (III); triethylamine, tributylamine, Tertiary amines such as 1,4-diazabicyclo[2.2.2]octane; 2-phenyl-4-methylimidazole, 2-ethyl-4-methylimidazole, 2,4-diethylimidazole, 2-phenyl-4 -Imidazoles such as methyl-5-hydroxyimidazole and 2-phenyl-4,5-dihydroxymethylimidazole; triphenylphosphine, tri-p-tolylphosphine, tetraphenylphosphonium/tetraphenylborate, triphenylphosphine/triphenylborane , organic phosphorus compounds such as 1,2-bis-(diphenylphosphino)ethane; phenolic compounds such as phenol, bisphenol A, nonylphenol; organic acids such as acetic acid, benzoic acid, salicylic acid, p-toluenesulfonic acid; etc., or This mixture is mentioned.

When using a curing catalyst, its content is preferably 0.001 parts by mass or more and 1 part by mass or less based on the entire solid content of the resin composition.

Examples of phenolic curing agents include phenol novolak resins, cresol novolac resins, naphthol novolak resins, aminotriazine novolak resins, novolak resins, novolak-type phenolic resins such as trisphenylmethane-type phenol novolak resins; terpene-modified phenolic resins, Modified phenol resins such as cyclopentadiene-modified phenol resin; phenol aralkyl resins having a phenylene skeleton and/or biphenylene skeleton; aralkyl type resins such as naphthol aralkyl resins having a phenylene skeleton and/or biphenylene skeleton; bisphenols such as bisphenol A and bisphenol F Compounds include resol type phenolic resins, etc., and these may be used alone or in combination of two or more. The content of the phenolic curing agent is preferably 0.1% by mass or more and 30% by mass or less, more preferably 0.3% by mass or more and 15% by mass or less, based on the entire solid content of the resin composition.

Examples of the leveling agent include acrylic copolymers and the like. The leveling agent is preferably used in an amount of 2% by mass or less, and more preferably in an amount of 0.01% by mass or more and 1.0% by mass or less, based on the total solid content of the resin composition.

The resin composition of this embodiment may further contain a coupling agent. As the coupling agent, an epoxy silane coupling agent, a cationic silane coupling agent, an aminosilane coupling agent, a titanate coupling agent, a silicone oil type coupling agent, etc. can be used. The amount of the coupling agent blended is preferably 0.1 parts by mass or more and 10 parts by mass or less, particularly preferably 0.5 parts by mass or more and 7 parts by mass or less, based on 100 parts by mass of the boron nitride filler.

The resin composition of this embodiment can be prepared by blending the above-mentioned thermosetting resin, boron nitride filler, other components, and solvent in the above-mentioned proportions and stirring and mixing by a known method.

Examples of the solvent include organic solvents such as ketones such as acetone and methyl ethyl ketone, esters such as ethyl acetate, and amides such as N,N-dimethylformamide. Further, examples of the solvent include aqueous solvents such as water, alcohols such as methanol, ethanol, propanol, and isopropanol.

When stirring and mixing using a solvent, the solvent is removed after stirring and mixing. To remove the solvent, for example, leave to stand at room temperature for 1 to 48 hours, heat at 40 to 100° C. for 0.5 to 3 hours, or reduce pressure of, for example, 0.001 to 50 kPa. Heat at 20-60° C. for 0.5-3 hours under atmosphere.

The resin composition of this embodiment can be molded into a desired shape and used as a heat dissipation material. For example, a thermally conductive sheet can be produced by molding a resin composition into a sheet shape. In one embodiment, a thermally conductive sheet can be obtained by, for example, applying the resin composition in the form of varnish onto a base material, and then heat-treating and drying the resin composition. Examples of the base material include metal foils constituting heat dissipation members, lead frames, removable carrier materials, and the like. Further, heat treatment for drying the resin composition is performed, for example, at 80 to 150° C. for 5 minutes to 1 hour. The thickness of the thermally conductive sheet can be, for example, 60 μm or more and 500 μm or less.

The thermally conductive sheet obtained in this way has excellent thermal conductivity and various electrical properties such as dielectric strength (breakdown voltage), and is used for heat dissipation purposes such as LED heat dissipation boards and heat dissipation materials for batteries. be able to. In one embodiment, the thermally conductive sheet can be used as a material constituting a heat dissipation layer of an electronic component structure including an electronic component and a heat dissipation layer. The thermally conductive sheet, which is a cured product of the resin composition of this embodiment, has microcracks of 500 to 5000 microcracks/ ^mm2 , so it has excellent thermal conductivity and heat resistance ^. Therefore, an electronic component structure including this has excellent reliability.

The embodiments of the present invention have been described above, but these are merely examples of the present invention, and various configurations other than those described above can be adopted.
Examples of embodiments will be described below.
1. A resin composition comprising a thermosetting resin and a boron nitride filler,
The resin composition is applied to a base material, heated and dried, and pressed under high pressure to prepare a test film with a thickness of 100 μm or more and 200 μm or less, and the surface of the test film is precisely polished and the resulting polished surface is scanned. When observed with an electron microscope, the number of microcracks with a length of 1 μm or more and a width of 0.1 μm or more and 5 μm or less observed on the polished surface is 500 pieces/mm ² or more and 5000 pieces/mm ² or less. thing.
2. 1. The amount of the boron nitride filler is 60% by mass or more and 80% by mass or less based on the thermosetting resin. The resin composition described in .
3. The thermosetting resin is at least one selected from the group consisting of cyanate resin, epoxy resin, phenoxy resin, phenol resin, benzoxazine resin, urea resin, melamine resin, unsaturated polyester resin, silicone resin, and polyurethane resin. Including 1. or 2. The resin composition described in .
4. 1. The resin composition further contains an acrylic copolymer. to 3. The resin composition according to any one of.
5. An electronic component structure comprising an electronic component and a heat dissipation layer,
The heat dissipation layer comprises: 1. to 4. Consisting of a cured product of the resin composition according to any one of
An electronic component structure in which the heat dissipation layer has microcracks of 500 to 5000 pieces/mm ² , a length of 1 μm or more ^, and a width of 0.1 μm to 5 μm.

The present invention will be described below with reference to Examples, but the present invention is not limited thereto.

(Preparation of thermally conductive sheet)
For Examples 1 to 3 and Comparative Examples 1 to 2, thermally conductive sheets were produced as follows.
First, according to the formulation shown in Table 1, a thermosetting resin, a curing agent, and an additive were added to methyl ethyl ketone as a solvent, and the mixture was stirred to obtain a solution of a thermosetting resin composition. Next, boron nitride particles were added to this solution and premixed, and then kneaded using three rolls to obtain a resin composition for a thermally conductive sheet in which boron nitride particles were uniformly dispersed. Next, the obtained resin composition for a thermally conductive sheet was aged at 60° C. for 15 hours. Next, the resin composition for a thermally conductive sheet was applied onto the copper foil using a doctor blade method, and then dried by heat treatment at 100°C for 30 minutes to form a B-stage film with a film thickness of 400 μm. A thermally conductive sheet was produced.
The details of each component in Table 1 are as follows.

(thermosetting resin)
Cyanate resin 1: Phenol novolac type cyanate resin (PT-30, manufactured by Lonza Japan Co., Ltd.)
Epoxy resin 1: Epoxy resin having a dicyclopentadiene skeleton (XD-1000, manufactured by Nippon Kayaku Co., Ltd.)

(hardening agent)
Curing agent 1: 2-phenyl-4,5-dihydroxymethylimidazole (2PHZ-PW, manufactured by Shikoku Kasei Co., Ltd.)
Curing agent 2: Trisphenylmethane type phenol novolac resin (MEH-7500, manufactured by Meiwa Kasei Co., Ltd.)

(Boron nitride particles)
Boron nitride particles 1: Agglomerated boron nitride particles produced by the following method.
Boron carbide is nitrided in a nitrogen atmosphere at 1200 to 2500° C. for 2 to 24 hours. Next, diboron trioxide was added to the obtained boron nitride, and this was fired in a non-oxidizing atmosphere at 1200° C. to 2500° C. for 2 to 24 hours to produce agglomerated boron nitride particles.
Boron nitride particles 2: Boron nitride particles produced by the following method.
Boron carbide powder is fired at 1600°C to 2200°C under a nitrogen atmosphere. Next, the obtained fired product is decarbonized by heat treatment at a temperature of 500° C. to 1500° C. in an oxygen-containing gas atmosphere. Next, the product after the decarburization treatment was nitrided at a temperature of 1500° C. to 2200° C. in a nitrogen gas atmosphere to obtain hexagonal boron nitride powder.

(Additive)
Additive 1: Leveling agent (BYK-361N, manufactured by BYK Chemie Japan Co., Ltd.)

(Measurement of number of microcracks)
The B-stage thermally conductive sheet obtained above was heat-treated at 180° C. and 10 MPa for 40 minutes to obtain a cured thermally conductive sheet. This cured product is precisely polished along the thickness direction using a cross-section polisher (CP), and the resulting polished surface is observed at 1000x magnification using a scanning electron microscope (SEM) to obtain an image. From the image, the void area and other areas were binarized. Next, voids having a length of 1 μm or more and a size of 500 to 5000/mm ² were counted as microcracks ^. The number of counted microcracks was calculated as the number per unit area. The results are shown in Table 1.

(thermal conductivity)
The B-stage thermally conductive sheet obtained above was heat-treated at 180° C. and 10 MPa for 40 minutes to obtain a cured thermally conductive sheet. Next, the thermal conductivity (W/(m·K)) in the thickness direction of the cured thermally conductive sheet was measured using a laser flash method. The evaluation criteria are as follows. The results are shown in Table 1.
<Evaluation criteria>
○: 10W/(m・K) or more △: 5W/(m・K) or more, less than 10W/(m・K) ×: Less than 5W/(m・K)

(soldering heat resistance)
After cutting a metal base circuit board into a size of 50 mm x 50 mm using a grinder saw, a sample was prepared by etching with only 1/2 of the copper foil remaining, and evaluated in accordance with JIS C 6481. For evaluation, the presence or absence of any abnormality in appearance was examined after immersing the product in a solder bath at 260° C. to 300° C. for 30 seconds. The evaluation criteria are as follows. The results are shown in Table 1.
<Evaluation criteria>
○: No abnormality ×: Abnormality (there are bulges throughout)

(Moisture absorption solder heat resistance)
A sample was prepared by cutting a metal base circuit board into a size of 50 mm x 50 mm using a grinder saw, and then performing half-sided etching in accordance with JIS C 6481. After being left undisturbed for two days in an environment with a temperature of 40°C and a humidity of 90%, it was floated with the copper foil side down in a solder bath at 260°C to 300°C, and after 30 seconds, the presence or absence of any abnormality in appearance was examined. The evaluation criteria are as follows. The results are shown in Table 1.
<Evaluation criteria>
○: No abnormality ×: Blisters (there are bulges throughout)

(Dielectric breakdown voltage)
The dielectric breakdown voltage of the cured product of the thermally conductive sheet obtained above was measured as follows according to JIS K6911. First, the obtained cured thermally conductive sheet was cut into 30 mm square pieces to obtain test pieces. Furthermore, the obtained test piece was placed between circular electrodes and placed in insulating oil.
Next, using TOS9201 manufactured by Kikusui Denshi Co., Ltd., an alternating current voltage was applied to both electrodes so that the voltage was increased at a rate of 2.5 kV/sec. The voltage at which the test piece broke down was defined as the dielectric breakdown voltage. The evaluation criteria are as follows.
<Evaluation criteria>
○: 8.0kV or more △: 5.0kV or more and less than 8.0kV ×: less than 5.0kV

The thermally conductive sheets produced from the resin compositions of Examples had high thermal conductivity and were excellent in insulation and heat resistance.

Claims (2)

A resin composition comprising a thermosetting resin , a boron nitride filler , and an acrylic copolymer ,
The thermosetting resin includes a cyanate resin and an epoxy resin,
The content of the acrylic copolymer is 0.01% by mass or more and 0.3% by mass or less based on the entire solid content of the resin composition,
The content of the boron nitride filler is 58% by volume or more and 95% by volume or less based on the volume of the total solid content of the resin composition,
The resin composition is applied to a base material, heated and dried, and pressed under high pressure to prepare a test film with a thickness of 100 μm or more and 200 μm or less, and the surface of the test film is precisely polished and the resulting polished surface is scanned. When observed with an electron microscope, the number of microcracks with a length of 1 μm or more and a width of 0.1 μm or more and 5 μm or less observed on the polished surface is 500 pieces/mm ² or more and 5000 pieces/mm ² or less. thing. An electronic component structure comprising an electronic component and a heat dissipation layer,
The heat dissipation layer is made of a cured product of the resin composition according to claim 1 ,
An electronic component structure in which the heat dissipation layer has microcracks of 500 to 5000 pieces/mm ² , a length of 1 μm or more ^, and a width of 0.1 μm to 5 μm.