Nonwoven fabric for circuit boards, prepreg for circuit boards using same, and circuit board using same

Abstract

To provide a nonwoven fabric for circuit boards with improved smoothness, suppressed high prices, and thinner thickness.SOLUTION: A nonwoven fabric for a circuit board is composed of a paper-making structure containing multiple thick fibers in which the thickest part of the fiber diameter is 5 μm or more and multiple fine fibers in which the thickest part of the fiber diameter is less than 5 μm. The average fiber length of the thick fibers is longer than that of the fine fibers and the number of the fine fibers is greater than the number of the thick fibers. The thick fibers have a flat shape having a long axis and a short axis, and the short axis is oriented in the thickness direction of the paper-making structure.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a nonwoven fabric for circuit boards, a prepreg for circuit boards using the same, and a circuit board using the same. [Background technology]

[0002] As circuit boards become more densely mounted, the prepregs for circuit boards used in these circuit boards are required to have a smoother surface. That is, a circuit pattern such as a copper foil is arranged on the surface of the prepreg for circuit boards, and if the surface smoothness of the prepreg for circuit boards is low, the circuit pattern may break or the through-holes may vary greatly. Therefore, in the past, the smoothness of the nonwoven fabric for circuit boards itself used to make the prepreg for circuit boards has been increased. Specifically, the smoothness of nonwoven fabric for circuit boards is improved by producing the fabric from fine denier aramid fibers having a fiber diameter of 10 μm or less, preferably 8 μm or less (this is referred to as the first conventional example, and a similar structure is found, for example, in Patent Document 1 below). It is also known that attempts are made to make nonwoven fabrics for circuit boards thinner by using only flattened, short aramid fibers to produce the nonwoven fabrics for circuit boards (this is referred to as the second conventional example, and a similar configuration is found, for example, in Patent Document 2 below). It is also known that by using flattened short fibers, a nonwoven fabric for circuit boards having a high bulk density can be obtained even with a low basis weight (this is referred to as the third conventional example, and a similar structure is found, for example, in Patent Document 3 below). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2004-51951 A [Patent Document 2] JP 2003-49388 A [Patent Document 3] JP 2006-200066 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the first conventional example, the nonwoven fabric for circuit boards is made of fine denier aramid fibers having a fiber diameter of 10 μm or less, preferably 8 μm or less, without using thick fibers, and the smoothness of the nonwoven fabric for circuit boards can be improved. However, if the nonwoven fabric for circuit boards is made only of thin fibers having a fiber diameter of 10 μm or less, preferably 8 μm or less, the productivity of the thin fibers is low and the fabric is expensive, so the nonwoven fabric for circuit boards is expensive. Naturally, the prepreg for circuit boards using the nonwoven fabric for circuit boards and the circuit boards using the prepreg for circuit boards are expensive. In addition, since the strength of the nonwoven fabric made only of thin fibers is reduced, it is necessary to increase the amount of resin contained in the nonwoven fabric for circuit boards and to cover the front and back surfaces in order to increase the strength of the prepreg for circuit boards, which results in thickening of the prepreg for circuit boards and the circuit boards. In the second conventional example, since only the flattened short aramid fibers are used, it is possible to achieve a thin nonwoven fabric for circuit boards. However, if wet papermaking is performed using only the short flattened aramid fibers, the amount of the short flattened aramid fibers that falls off the wire during the wet papermaking increases, resulting in a decrease in yield and an increase in costs. Furthermore, in the third conventional example, by using flattened short fibers, it may be possible to obtain a nonwoven fabric for circuit boards having a high bulk density even at a low basis weight. However, as in the second conventional example, this also increases the amount of short, flattened aramid fibers that fall off the wire during wet papermaking, resulting in a decrease in yield and an increase in costs. Therefore, an object of the present invention is to prevent the prices of a nonwoven fabric for circuit boards, a prepreg for circuit boards using the same, and a circuit board using the same from increasing, and to achieve a thinner nonwoven fabric for circuit boards, a prepreg for circuit boards using the same, and a circuit board using the same. [Means for solving the problem]

[0005] In order to achieve the above-mentioned object, the nonwoven fabric for circuit boards according to the first aspect of the present invention is a nonwoven fabric for circuit boards made of a paper structure including a plurality of thick fibers each having a maximum fiber diameter of 5 μm or more and a plurality of thin fibers each having a maximum fiber diameter of less than 5 μm, wherein the average fiber length of the thick fibers is longer than the average fiber length of the thin fibers, the number of the thin fibers is greater than the number of the thick fibers, and the thick fibers have a flat shape having a major axis and a minor axis, and the minor axis is oriented in the thickness direction of the paper structure. In the nonwoven fabric for circuit boards according to the second aspect of the present invention, the minor axis diameter of the thick fibers is 10 μm or less. In the nonwoven fabric for circuit boards according to a third aspect of the present invention, the fiber diameter of the major axis of the thick fibers is 5 μm or more and 20 μm or less. In the nonwoven fabric for circuit boards according to a fourth aspect of the present invention, the thick fibers and thin fibers are aramid fibers. In the nonwoven fabric for circuit boards according to the fifth aspect of the present invention, the ratio of the total area occupied by thick fibers in the CD cross section of the nonwoven fabric is 10% or less. A prepreg for circuit boards according to a sixth aspect of the present invention is obtained by impregnating the above nonwoven fabric for circuit boards with a resin. A prepreg for circuit boards according to a seventh aspect of the present invention has a prepreg thickness that is 120% or more and 300% or less of the thickness of the nonwoven fabric for circuit boards. The circuit board according to an eighth aspect of the present invention comprises the above-mentioned prepreg for circuit board, on the surface of which a circuit pattern is provided. Effect of the Invention

[0006] By providing a nonwoven fabric for circuit boards having the specified characteristics of the present invention, it is possible to improve smoothness, prevent a rise in cost, and achieve a thinner product. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 shows a cross section of a circuit board according to one embodiment of the present invention. [Diagram 2] FIG. 2 shows a prepreg obtained by impregnating the nonwoven fabric of the present invention with resin and forming it into a flat plate. [Diagram 3] FIG. 3 shows a scanning electron microscope photograph of a nonwoven fabric of the present invention. [Figure 4] FIG. 4 shows a flat thick fiber (length A in the planar direction, length B in the thickness direction) having a major axis and a minor axis in the nonwoven fabric of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] An embodiment of the present invention will be described in detail below. Fig. 1 shows a cross section of a circuit board 10 according to an embodiment of the present invention, in which a plurality of layers of prepregs 20 are stacked in the thickness direction on a core material 50 and compressed together to form a plate-like structure. As shown in Fig. 1, a circuit pattern 30 is provided on the front and back surfaces of each prepreg 20, and the circuit patterns 30 on the front and back surfaces of each prepreg 20 are appropriately electrically connected via through holes 40 provided from the front surface to the back surface. In addition, the circuit patterns 30 provided on the upper and lower prepregs 20 are appropriately electrically connected via the through holes 40.

[0009] As shown in Fig. 2, each prepreg 20 is a flat plate-shaped nonwoven fabric containing thick fibers 21 and thin fibers 23 impregnated with resin 25, and the circuit patterns are formed on the front and back sides of the nonwoven fabric, and through holes are formed on the front and back sides of the land portions provided at appropriate positions of the circuit patterns. As shown in the scanning electron microscope photograph of Fig. 3, the nonwoven fabric of the present invention is made by intertwining thick fibers and thin fibers, and is characterized in that the thick fibers are longer than the thin fibers. As shown in Fig. 4, the thick fibers 21 have a flat shape having a long axis and a short axis, and the short axis is oriented in the thickness direction of the paper structure. In contrast, the number of thin fibers is far greater than that of the thick fibers, and the thick fibers and thin fibers are intertwined to form the nonwoven fabric 27.

[0010] The nonwoven fabric for circuit boards of the present invention comprises a paper structure including a plurality of thick fibers each having a maximum fiber diameter of 5 μm or more and a plurality of thin fibers each having a maximum fiber diameter of less than 5 μm, the average fiber length of the thick fibers being longer than the average fiber length of the thin fibers, the number of the thin fibers being greater than the number of the thick fibers, and the thick fibers each having a flat shape with a major axis and a minor axis, with the minor axis oriented in the thickness direction of the paper structure.

[0011] As described above, the present invention uses not only a plurality of fine fibers whose thickest fiber diameter is less than 5 μm, but also a plurality of thick fibers whose thickest fiber diameter is 5 μm or more. This makes it possible to prevent the cost of the nonwoven fabric for circuit boards from increasing, and also to increase the strength of the nonwoven fabric for circuit boards, compared to the case where only expensive thin fibers are used. In addition, by using fine fibers and thick fibers longer than the fine fibers in wet papermaking, it is possible to avoid an increase in costs. In other words, if wet papermaking is performed using only short fine fibers, the amount of fine fibers that fall off the wire during the wet papermaking process increases, and the yield rate decreases, but by using fine fibers and thick fibers longer than the fine fibers, the thin fibers become entangled with the thick fibers and the thick fibers remain on the wire, reducing the amount of fibers that fall off the wire and improving the yield rate.

[0012] Furthermore, in the present invention, the number of thick fibers is less than that of thin fibers, and the thick fibers are flat with long and short axes, with the short axis oriented in the thickness direction of the paper structure, so that the thickness of the nonwoven fabric for circuit boards can be reduced and the smoothness of the nonwoven fabric for circuit boards can be improved, which in turn improves the smoothness of the nonwoven fabric for circuit boards, the prepreg for circuit boards using the nonwoven fabric, and the circuit boards using the nonwoven fabric. In addition, in the present invention, a paper structure is made using not only a plurality of fine fibers whose thickest fiber diameter is less than 5 μm, but also a plurality of thick fibers whose thickest fiber diameter is 5 μm or more, so that the tensile strength as a nonwoven fabric for circuit boards is increased. As a result, the amount of resin required when making a prepreg for circuit boards can be reduced, making it possible to achieve a thinner structure.

[0013] In the present invention, the thickest part (long axis) of the fiber diameter of the thick fiber is 5 μm or more, preferably 7 μm or more, more preferably 9 μm or more, and most preferably 11 μm or more. In addition, the upper limit of the fiber diameter of the long axis of the thick fiber can be appropriately set by a person skilled in the art, but is preferably 25 μm or less, more preferably 20 μm or less, and most preferably 15 μm or less. The fiber diameter of the long axis of the thick fiber is preferably 5 μm or more and 20 μm or less, more preferably 9 μm or more and 15 μm or less. In particular, by setting the fiber diameter of the long axis of the thick fiber to 5 μm or more and 20 μm or less, the smoothness of the circuit board can be improved. If the thickest part of the fiber diameter does not contain thick fibers of 5 μm or more, the tensile strength of the nonwoven fabric is insufficient, paper breaks occur during papermaking or prepreg production, and stable production cannot be achieved. In addition, if the fiber diameter is large, the surface of the substrate is blocked by fibers, making it difficult for the resin to impregnate the inside of the substrate, which may cause voids or reduce productivity. Furthermore, the thickness dimension of the nonwoven fabric becomes relatively large, so that the nonwoven fabric for circuit boards becomes thick.

[0014] In addition, the fiber diameter of the thinnest part (minor axis) of the thick fiber can be appropriately set by a person skilled in the art within a range less than the fiber diameter of the major axis described above, but it is preferably 12 μm or less, more preferably 10 μm or less, and most preferably 8 μm or less, provided that it is less than the fiber diameter of the major axis. By setting the fiber diameter of the minor axis of the thick fiber to the above value, it is possible to make the circuit board thinner. If the fiber diameter of the minor axis is thick, the nonwoven fabric for circuit boards containing it will become thicker and will not be able to meet the recent demand for thinner fibers.

[0015] In addition, the thickest fiber diameter of the fine fibers of the present invention is less than 5 μm, more preferably less than 3 μm, and most preferably less than 1 μm. If the thin fibers having the thickest fiber diameter of less than 5 μm are not included, the number of fibers per unit weight will be reduced, resulting in fewer intertwining points between fibers, resulting in insufficient sheet strength, paper breakage during papermaking or prepreg production, and reduced smoothness of the nonwoven fabric for substrates.

[0016] The average fiber length of the thick fibers and thin fibers of the present invention can be determined by a person skilled in the art based on technical common sense by actually measuring the fiber lengths of a specific number of fibers, and for example, the fiber lengths of any 50 fibers contained in an image obtained by disintegrating a nonwoven fabric in water and drying the fibers on a slide are measured and counted to obtain an average value. If the average fiber length of the thick fibers is shorter than the average fiber length of the thin fibers, the amount of fibers that fall off the wire during papermaking increases, resulting in a decrease in yield and an increase in costs.

[0017] The number of thick fibers and thin fibers can also be compared by, for example, actually counting the number of fibers identified in a certain range of an actual photographed image. The number of thin fibers is preferably at least twice the number of thick fibers, more preferably at least 2.5 times, and most preferably at least 3 times. If the number of thin fibers is less than the number of thick fibers, the number of fibers per unit weight is reduced, resulting in fewer intertwining points between fibers, which in turn leads to insufficient sheet strength and paper breaks during papermaking or prepreg production.

[0018] In the nonwoven fabric paper structure of the present invention, the thick fibers having a flat shape (having a long axis and a short axis) are characterized in that the short axis is oriented in the thickness direction of the paper structure (nonwoven fabric). As long as the short axis can be oriented in the thickness direction of the paper structure (nonwoven fabric) in the actually produced nonwoven fabric, the method of controlling the orientation of the fibers is not particularly limited, and a person skilled in the art can adopt an appropriate method based on technical common sense in response to various nonwoven fabric manufacturing methods. For example, in the papermaking method, fibers are dispersed in water and dehydrated from below through a net, and the fibers are deposited on the net. At that time, the fiber cross section basically faces the direction with the larger filtration resistance, so if flat fibers are used as the thick fibers, the long side of the fiber cross section naturally faces horizontally, and a nonwoven fabric in which the short axis is oriented in the thickness direction of the paper structure can be obtained.

[0019] The orientation of the minor axis in the thickness direction can be confirmed, for example, by measuring the length A in the planar direction of the nonwoven fabric and the length B in the thickness direction of the nonwoven fabric of at least 10 thick fibers in a cross section cut in the transverse direction (CD direction) of the nonwoven fabric, and finding the average value of A>B. The number of thick fibers measured to find the average values ​​of A and B may be even greater; for example, the A and B dimensions of all thick fibers present in one visual field of a scanning microscope photograph of the cross section of the nonwoven fabric may be measured, or the same operation may be performed in multiple visual fields, and the average values ​​of the A and B dimensions of all measured thick fibers may be calculated.

[0020] In addition, the ratio of the total area occupied by thick fibers in the CD cross section of the nonwoven fabric (hereinafter also referred to as the thick fiber ratio) can be, for example, 15% or less, but is more preferably 10% or less. Here, the CD cross section of the nonwoven fabric refers to a cross section in the manufacturing direction of the nonwoven fabric. In addition, the cross-sectional area of ​​the CD cross section includes not only the cross-sectional areas of the thick fibers and the thin fibers but also the gaps between the fibers, that is, it represents "the cross-sectional area of ​​all fibers including the thick fibers and the thin fibers" + "the area of ​​the gaps (parts in the cross section where no fibers exist)". If the ratio of the total area occupied by thick fibers in the CD cross section of the nonwoven fabric is 10% or less, the smoothness of the circuit board can be improved. If the ratio of thick fibers is greater than 10%, the nonwoven fabric for circuit boards containing it becomes thick, and it cannot meet the thinning demanded in recent years. In addition, a relative decrease in the ratio of thin fibers deteriorates the surface roughness, and the number of fibers per unit weight decreases, reducing the number of intertwining points between fibers, resulting in insufficient strength, which causes paper breakage during papermaking or prepreg production. The above ratio can be determined, for example, by measuring the total number of cross-sectional areas of thick fibers in a cross section of the nonwoven fabric cut in the transverse direction (CD direction) and dividing the total number by the cross-sectional area of ​​the nonwoven fabric for circuit boards.

[0021] The thick and thin fibers of the present invention are preferably aramid fibers. If the thick and thin fibers are aramid fibers, thin and thick fibers can be easily produced. The aramid fibers suitable for the nonwoven fabric for circuit boards of the present invention can be para-aramids such as polyparaphenylene terephthalamide, poly(paraphenylene-4,4'-biphenylene-dicarboxylic acid amide), poly(paraphenylene-2,6-naphthalene dicarboxylic acid amide) or those copolymerized with a certain amount (for example, less than 10% by weight) of diamine or diacid chloride, but for example, para-aramid fibers mainly composed of polyparaphenylene terephthalamide and spun with polyvinylpyrrolidone are preferred. By adding polyvinylpyrrolidone, the plasticity of the fibers is increased, and the fibers are easily deformed by pressure, etc., making it easier to obtain flat fibers. By beating the para-aramid fibers, a nonwoven fabric for circuit boards suitable for the present invention can be obtained. The process of turning fibers into pulp is called refining, and the machine that performs the beating is called a refiner. Generally, the purpose of beating is to swell the fibers, cut the fibers, and fibrillate the fibers. Beating that mainly cuts the fibers is called free beating, and beating that mainly fibrillates the fibers is called viscous beating. The type of refiner, blade shape, operating conditions, etc. that are selected vary depending on which beating method is adopted. For example, conventional equipment such as single disc refiners, double disc refiners, and PFI mills can be used for beating without any particular restrictions. For example, when a PFI mill is used, the para-aramid fiber is mixed with pure water to adjust the sample concentration to 5%, and beaten in a standard PFI mill (manufactured by Kumagai Riki Kogyo Co., Ltd.), which is a beater conforming to JIS P8221-2 "Pulp-beating method-Part 2: PFI mill method" (ISO5264-2), with a gap between the roll and the housing of 0.1 mm, to perform wet papermaking, thereby obtaining a nonwoven fabric for circuit boards suitable for the present invention. In the present invention, the beating equipment and beating conditions can be appropriately selected by a person skilled in the art and are not particularly limited.

[0022] The nonwoven fabric for circuit boards of the present invention can be made into a wet nonwoven fabric by any method such as spunlace manufacturing method, air-through manufacturing method, papermaking method, etc., but for example, it is preferable to make a wet nonwoven fabric formed into a sheet by a papermaking method. In the papermaking method, fibers are dispersed in water, dehydrated from below through a net, and the fibers are deposited on the net. In this case, since the fiber cross section basically faces the side with the larger filtration resistance, if flat fibers are used as thick fibers, the fiber cross section will naturally have the long side oriented horizontally, and a nonwoven fabric with the short axis oriented in the thickness direction of the paper structure can be obtained. When forming a sheet by the papermaking method, Fourdrinier papermaking, short net papermaking, cylinder net papermaking, and combinations thereof can be used without any particular limitation. Fourdrinier papermaking or short net papermaking layers are preferred because they are easy to capture very thin and short fibers. In addition, after forming the sheet, the thickness and porosity may be adjusted by calendaring.

[0023] In addition, a prepreg for a circuit board is produced by impregnating the nonwoven fabric for a circuit board of the present invention with a resin. By impregnating the nonwoven fabric for a circuit board with a resin, a prepreg for a circuit board can be easily produced, and the strength of the board can be increased. The prepreg for a circuit board obtained by impregnating the nonwoven fabric for a circuit board of the present embodiment with a resin can generally be produced by impregnating the nonwoven fabric for a circuit board with an epoxy resin or a phenol resin and curing the nonwoven fabric by applying heat. For example, by adding a thermosetting resin such as an epoxy resin, a phenol resin, a polyurethane resin, or a melamine resin, the adhesiveness with the copper foil is improved, so that a water-dispersed binder made of these thermosetting resins may be added. However, the resin to be impregnated is not limited to the above examples, and any resin that is usually used may be used.

[0024] The prepreg for circuit boards can have a prepreg thickness of 120% or more and 300% or less of the thickness of the nonwoven fabric for circuit boards, more preferably 130% or more and 200% or less, and most preferably 140% or more and 180% or less. By setting the prepreg thickness as described above, it is possible to make the circuit board thinner. If the prepreg thickness is thinner than 120%, the fibers on the surface of the nonwoven fabric for circuit boards are exposed on the prepreg surface, and the smoothness is deteriorated, which may cause breaks in the circuit pattern or large variations in the through holes. If the prepreg thickness is thicker than 300%, it cannot meet the thinning demanded in recent years.

[0025] The above prepreg can be suitably used as a substrate for a circuit board and a laminate. Such a substrate for a circuit board and a laminate can be produced by a commonly used method. That is, the prepreg of the present invention is laminated as an insulating layer on a core material, and a conductor layer made of a metal foil is laminated as a circuit pattern to produce a laminate for a circuit. As the core material, a resin plate with metal foil attached to both sides, a metal plate, etc. can be used. As the metal foil, metal plate, and metal foil of the conductor layer to be attached to the resin plate, for example, gold, silver, copper, nickel, aluminum, etc. can be used, but they are not limited to the above examples, and any one that is usually used may be used. By providing a circuit pattern on the surface of the prepreg for a circuit board of the present invention by a conventional method, a thin, smooth, and strong circuit board can be produced.

[0026] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these embodiments. EXAMPLES

[0027] Specific measurements of the properties of the nonwoven fabric for circuit boards of this embodiment (hereinafter also simply referred to as "nonwoven fabric") were carried out under the following conditions and by the following method. [Cross-sectional structure] The nonwoven fabric was cut in the cross direction (CD direction) using a sharp blade, and the cross section was photographed using a scanning electron microscope (model number JCM-IT200, manufactured by JEOL Ltd.). The length of the fiber cross section in the nonwoven fabric in the plane direction of the nonwoven fabric was designated A, and the length in the thickness direction of the nonwoven fabric was designated B. In addition, fibers in the nonwoven fabric in the photograph with the thickest fiber diameter of the cross section of the fiber included in the nonwoven fabric were designated as thick fibers, and fibers with the thickest fiber diameter of the cross section of the fiber included in the nonwoven fabric were designated as thin fibers, and the number of each was counted. The total number of cross-sectional areas of the thick fibers was then measured and divided by the cross-sectional area of ​​the nonwoven fabric for circuit boards to determine the ratio of the total area occupied by the thick fibers (hereinafter also referred to as the thick fiber ratio). The photographs and counts were taken at 10 random locations for each example, and the average values ​​were used to calculate the individual values. The above dimensions A and B were measured for all thick fibers present in the field of view (magnification 2000x (photographed area 64μm×48μm)) at each location, and the average values ​​of the A and B dimensions of all the measured thick fibers were calculated. [Average fiber length] The nonwoven fabric was disintegrated in water, the fibers were dried on a slide, and the fibers were photographed using a microscope (model number VHX-6000 (manufactured by Keyence Corporation)). The fiber lengths of 50 randomly selected thick and thin fibers in each example were measured for the fibers contained in the photographed images, and the average fiber length was calculated using the average count. [Surface roughness] The nonwoven fabric and prepreg surfaces were measured for the average deviation of surface roughness using a surface tester (model number KES-FB4 (manufactured by Kato Tech Co., Ltd.)) and used as an index of smoothness. The measurement was performed three times for each example, and the average value of the counts was used as the surface roughness. [Basic weight] The basis weight of the nonwoven fabric for circuit boards in an absolute dry state was measured according to the method specified in JIS C 2300-2 "Cellulose paper for electrical use - Part 2: Test method" 6 Basis weight. Thickness Using a micrometer with a measuring surface diameter of 6.35 mm and a pressure of 155 kPa ± 15 kPa, the thickness of one piece of nonwoven fabric for circuit boards and one piece of prepreg were measured at any five points, and the thickness was calculated using the average value of the counts.

[0028] [Example 1] Para-aramid fibers (polyparaphenylene terephthalamide; 1.7 dtex) were beaten and then Fourdrinier-printed to produce a nonwoven fabric. Microscopic observation of the cross section of the nonwoven fabric revealed that the major axis of the flat shape of all thick fibers was oriented in the plane direction of the nonwoven fabric. A was 12.1 μm, B was 6.2 μm, the average fiber length of the thick fibers was 0.9 mm, the average fiber length of the thin fibers was 0.4 mm, the number of thick fibers per field of view was 3.0, and the number of thin fibers (fiber diameters of all fibers clearly less than 5 μm in microscopic observation, the same applies to the following examples) was 10 or more, resulting in a nonwoven fabric for circuit boards. This nonwoven fabric for circuit boards had a thickness of 25 μm and a basis weight of 10.5 g / m 2 The thick fiber ratio was 4.2%, and the surface roughness was 0.71 μm.

[0029] [Example 2] The beating and pressing conditions for the para-aramid fiber were changed from those in Example 1, and the paper was made into a fourdrinier paper, followed by calendaring, to obtain a nonwoven fabric for circuit boards in which the major axes of the flattened thick fibers were all oriented in the plane direction of the nonwoven fabric, A being 13.2 μm, B being 5.7 μm, the average fiber length of the thick fibers being 0.9 mm, the average fiber length of the thin fibers being 0.4 mm, the number of thick fibers per photographed field being 2.6, and the number of thin fibers being 10 or more. This nonwoven fabric for circuit boards has a thickness of 10 μm and a basis weight of 6.8 g / m 2 The thick fiber ratio was 9.7%, and the surface roughness was 0.63 μm.

[0030] [Example 3] The beating conditions and pressing conditions of the para-aramid fibers were changed from those of Example 1, and a fourdrinier paper was made to obtain a nonwoven fabric for circuit boards in which the major axes of the flattened thick fibers were all oriented in the plane direction of the nonwoven fabric, A was 10.9 μm, B was 9.3 μm, the average fiber length of the thick fibers was 1.0 mm, the average fiber length of the thin fibers was 0.4 mm, the number of thick fibers per photographed field was 3.4, and the number of thin fibers was 10 or more. This nonwoven fabric for circuit boards had a thickness of 28 μm and a basis weight of 11.1 g / m. 2 The thick fiber ratio was 9.1%, and the surface roughness was 0.77 μm.

[0031] [Example 4] The beating and pressing conditions of the para-aramid fiber were changed from those of Example 1, and the paper was made into a fourdrinier paper, followed by calendaring, to obtain a nonwoven fabric for circuit boards in which the major axes of the flattened thick fibers were all oriented in the plane direction of the nonwoven fabric, A being 20.9 μm, B being 7.1 μm, the average fiber length of the thick fibers being 1.1 mm, the average fiber length of the thin fibers being 0.5 mm, the number of thick fibers per photographed field being 3.1, and the number of thin fibers being 10 or more. This nonwoven fabric for circuit boards had a thickness of 22 μm and a basis weight of 16.0 g / m 2 The thick fiber ratio was 14.1%, and the surface roughness was 0.69 μm.

[0032] [Comparative Example 1] The beating conditions and pressing conditions of the para-aramid fibers were changed from those of Example 1, and a fourdrinier paper was made to obtain a nonwoven fabric for circuit boards in which the major axes of the flattened thick fibers were all oriented in the plane direction of the nonwoven fabric, A was 8.9 μm, B was 9.6 μm, the average fiber length of the thick fibers was 1.0 mm, the average fiber length of the thin fibers was 0.4 mm, the number of thick fibers per photographed field was 2.4, and the number of thin fibers was 10 or more. This nonwoven fabric for circuit boards had a thickness of 30 μm and a basis weight of 10.5 g / m. 2 The thick fiber ratio was 3.2%, and the surface roughness was 0.95 μm.

[0033] [Comparative Example 2] A raw material consisting of a mixture of 70% by mass of para-aramid fiber and 30% by mass of aramid fibrids was Fourdrinier-made paper and then calendered to obtain a nonwoven fabric for circuit boards in which the major axes of the flattened thick fibers were oriented in the plane direction of the nonwoven fabric, A being 12.5 μm, B being 12.0 μm, the average fiber length of the thick fibers being 3.0 mm, the number of thick fibers per field of view being 8.7, and the number of thin fibers being 0. This nonwoven fabric for circuit boards had a thickness of 52 μm and a basis weight of 34.2 g / m. 2 The thick fiber ratio was 72%, and the surface roughness was 1.89 μm.

[0034] [Comparative Example 3] An attempt was made to make Fourdrinier paper using a raw material that did not contain thick fibers and that had been modified from the beating conditions of the para-aramid fibers in Example 1, but paper breaks occurred frequently and stable product production was not possible.

[0035] Next, the nonwoven fabric for circuit boards produced in the above Examples and Comparative Examples was impregnated with epoxy resin and then dried to obtain a prepreg. The thickness of the prepreg was adjusted by changing the pressing pressure during the resin impregnation. [Example 5] A prepreg was obtained by the above method using the nonwoven fabric for circuit boards produced in Example 1. The produced prepreg had a thickness of 40 μm (160% of the substrate thickness) and a surface roughness of 0.45 μm.

[0036] [Example 6] A prepreg was obtained by the same method as in Example 5 except for the pressing pressure, using the nonwoven fabric for circuit boards produced in Example 1. The produced prepreg had a thickness of 27 μm (108% of the substrate thickness) and a surface roughness of 0.65 μm.

[0037] [Example 7] A prepreg was obtained by the same method as in Example 5 except for the pressing pressure, using the nonwoven fabric for circuit boards produced in Example 1. The produced prepreg had a thickness of 83 μm (332% of the substrate thickness) and a surface roughness of 0.49 μm.

[0038] [Comparative Example 4] A prepreg was obtained by the same method as in Example 5 using the nonwoven fabric for circuit boards produced in Comparative Example 1. The produced prepreg had a thickness of 45 μm (150% of the base material thickness) and a surface roughness of 0.71 μm.

[0039] The configurations and evaluation results of various physical properties of the nonwoven fabrics for circuit boards of each of the above-described Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 1, and the configurations and evaluation results of various physical properties of the prepregs of each of the above-described Examples 5 to 7 and Comparative Example 4 are shown in Table 2.

[0040] [Table 1]

[0041] [Table 2]

[0042] The thick fiber of Comparative Example 1 has a length B in the thickness direction of the nonwoven fabric that is longer than the length A in the planar direction of the nonwoven fabric, and the short axis of the fiber cross-sectional length is not oriented in the thickness direction of the nonwoven fabric. Therefore, compared with each example, the surface roughness is worse. Also, when comparing Example 5 with Comparative Example 4, the prepreg of Comparative Example 4 using the nonwoven fabric of Comparative Example 1 has a worse surface roughness. From this, it can be seen that it is preferable that the thick fiber has a flat shape having a long axis and a short axis, and the short axis is oriented in the thickness direction of the paper structure. In Comparative Example 2, a raw material containing no fine fibers and mixed with aramid fibrids as a binder was used. Compared to Example 1, the thickness was increased and the surface roughness was worsened because no fine fibers were included. In addition, we attempted to manufacture a 50 μm nonwoven fabric for circuit boards using only unbeaten para-aramid fibers that did not contain fine fibers, but the wet paper strength was insufficient and stable manufacture was not possible. This shows that nonwoven fabrics for circuit boards preferably contain fine fibers with a maximum fiber diameter of 5 μm or less.

[0043] In Comparative Example 3, an attempt was made to produce a nonwoven fabric for circuit boards using a beaten raw material that did not contain thick fibers, but since the raw material did not contain thick fibers, the wet paper strength was weak and it was not possible to stably produce a nonwoven fabric for circuit boards. A comparison of Comparative Example 3 with each Example shows that it is preferable to contain thick fibers with a maximum fiber diameter of 5 μm or more. In the nonwoven fabric for circuit boards in Example 4, the fiber diameter of the major axis of the flat thick fibers was large at 20.9 μm, and therefore the resin impregnation during prepreg formation was poorer than in Example 2. In addition, the thick fiber ratio was large at 14.1%, and therefore the surface roughness was poorer than in Example 2. From this, it is clear that it is preferable that the fiber diameter of the major axis of the thick fibers is smaller than 20 μm and that the thick fiber ratio is 10% or less.

[0044] Comparing Example 5 and Example 6, it can be seen that Example 5 has a smaller surface roughness. This is thought to be because in Example 6, the thickness of the prepreg was too thin compared to the thickness of the substrate, so that the fibers of the substrate surface were exposed on the prepreg surface. In addition, the surface roughness of Example 7 is about the same as that of Example 5, but the thickness is increased to 83 μm, which causes problems such as increased costs due to the increased amount of resin and difficulty in responding to thinner designs. From the above comparison with Examples 5, 6, and 7, it is clear that the prepreg thickness is preferably 120% or more and 300% or less of the thickness of the nonwoven fabric for circuit boards.

[0045] As described above, according to the present embodiment, it is possible to provide a nonwoven fabric for circuit boards that is capable of improving smoothness, suppressing an increase in cost, and achieving a reduction in thickness. Furthermore, by using this nonwoven fabric for circuit boards, it is possible to improve the smoothness of the prepreg for circuit boards and the circuit board using the same, suppress an increase in cost, and achieve a reduction in thickness. [Explanation of symbols]

[0046] 10 Circuit Board 20 Prepreg 21 Thick Fiber 23 Fine Fiber 25 Resin 27 Nonwoven fabrics 30 Circuit Pattern 40 through hole 50 Core material

Claims

1. A nonwoven fabric for circuit boards comprising a paper structure including a plurality of thick fibers having a maximum fiber diameter of 5 μm or more and a plurality of thin fibers having a maximum fiber diameter of less than 5 μm, The average fiber length of the thick fibers is longer than the average fiber length of the thin fibers, The number of the thin fibers is greater than the number of the thick fibers, The thick fibers have a flat shape having a long axis and a short axis, the fiber diameter of the long axis is 5 μm or more and 20 μm or less, and the short axis is oriented in the thickness direction of the paper structure.

2. 2. The nonwoven fabric for circuit boards according to claim 1, wherein the minor axis diameter of the thick fibers is 10 μm or less.

3. 3. The nonwoven fabric for circuit boards according to claim 1, wherein the thick fibers and thin fibers are aramid fibers.

4. 4. The nonwoven fabric for circuit boards according to claim 1, wherein the ratio of the total area occupied by the thick fibers in the CD cross section of the nonwoven fabric is 10% or less.

5. A prepreg for circuit boards, comprising the nonwoven fabric for circuit boards according to any one of claims 1 to 4 impregnated with a resin.

6. 6. The prepreg for circuit boards according to claim 5, wherein the prepreg has a thickness of 120% or more and 300% or less of the thickness of the nonwoven fabric for circuit boards.

7. A circuit board comprising the prepreg for circuit boards according to claim 5 or 6, on the surface of which a circuit pattern is provided.

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