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

Abstract

A nonwoven fabric for a circuit substrate, which is composed of a papermaking structure body including a plurality of thick fibers having a thickest part of a fiber diameter of 5 μm or more and a plurality of thin fibers having a thickest part of a fiber diameter of less than 5 μm, characterized in that 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 more than the number of the thick fibers, and 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 papermaking structure body.

Description

Technical Field

[0001] This invention relates to nonwoven fabrics for circuit boards, prepregs for circuit boards using the same, and circuit boards using the same. Background Technology

[0002] With the increasing density of circuit board mounting, there is a need to improve the smoothness of the surface of the circuit board prepreg used in manufacturing these circuit boards. This is because circuit patterns, such as copper foil, are arranged on the surface of the circuit board prepreg, and if the surface smoothness of the prepreg is low, broken lines in the circuit patterns or larger deviations in vias can occur. Therefore, in the past, improving the smoothness of the nonwoven fabric used to manufacture the circuit board prepreg has been a priority.

[0003] Specifically, nonwoven fabrics for circuit boards are preferably made of fine denier aramid fibers with a fiber diameter of 10 μm or less, preferably 8 μm or less, to improve their smoothness (this is expressed as a first prior example, and as a similar structure, there is, for example, the following Patent Document 1).

[0004] In addition, it is known that nonwoven fabrics for circuit boards can be made by using only flattened short aramid fibers, thereby achieving thinness of nonwoven fabrics for circuit boards (this is expressed as a second prior example, and as a similar structure, there is, for example, the following Patent Document 2).

[0005] Furthermore, it is also known that nonwoven fabrics for circuit boards can be obtained with high packing density even with low basis weight by using flattened short fibers (this is expressed as the third prior example, and as a similar structure, there is, for example, the following Patent Document 3).

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2004-51951

[0009] Patent Document 2: Japanese Patent Application Publication No. 2003-49388

[0010] Patent Document 3: Japanese Patent Application Publication No. 2006-200066

[0011] Summary of the invention

[0012] The problem that the invention aims to solve

[0013] In the aforementioned first prior example, instead of using coarse fibers, fine denier aramid fibers with a diameter of 10 μm or less, preferably 8 μm or less, are used to make the nonwoven fabric for the circuit board, thus improving its smoothness. However, when the nonwoven fabric for the circuit board is made solely from fine fibers with a diameter of 10 μm or less, preferably 8 μm or less, the production efficiency of fine fibers is poor and they are expensive, so the conclusion is that the nonwoven fabric for the circuit board is expensive. Of course, both the circuit board prepreg and the circuit board using the circuit board nonwoven fabric are expensive. In addition, the strength of the nonwoven fabric composed only of fine fibers decreases, so in order to improve the strength of the circuit board prepreg, it is necessary to include it in the circuit board nonwoven fabric, and also increase the amount of resin covering the front and back sides, resulting in the circuit board prepreg and the circuit board becoming thicker.

[0014] Furthermore, in the second prior example, only flattened short aramid fibers are used, thus enabling the nonwoven fabric used as a circuit board to be thinner. However, if wet papermaking is performed using only short, flattened aramid fibers, the amount of short, flattened aramid fibers detaching from the wire bundle increases during wet papermaking, leading to a deterioration in yield and increased costs.

[0015] Furthermore, in the third prior example, by using flattened short fibers, a nonwoven fabric for circuit boards with high packing density can be obtained even with low basis weight. However, similar to the second prior example, during wet papermaking, the amount of short, flattened aramid fibers falling off the wire bundle increases, resulting in a deterioration in yield and an increase in cost.

[0016] Therefore, the object of the present invention is to suppress the increase in price of nonwoven fabrics for circuit boards, prepregs for circuit boards using the same, and circuit boards using the same, and to achieve thinning of nonwoven fabrics for circuit boards, prepregs for circuit boards using the same, and circuit boards using the same.

[0017] Technical means for solving problems

[0018] To achieve the above objectives, the first invention provides a nonwoven fabric for a circuit board, comprising a papermaking structure containing a plurality of coarse fibers with a maximum fiber diameter of 5 μm or more and a plurality of fine fibers with a maximum fiber diameter of less than 5 μm, wherein the average fiber length of the coarse fibers is longer than the average fiber length of the fine fibers, the number of fine fibers is greater than the number of coarse fibers, and the coarse fibers are flat in shape having a long axis and a short axis, with the short axis oriented in the thickness direction of the papermaking structure.

[0019] In addition, a second invention of the present invention provides a nonwoven fabric for a circuit board, wherein the fiber diameter of the short axis of the coarse fiber is set to 10 μm or less.

[0020] In addition, a third invention of the present invention provides a nonwoven fabric for a circuit board, wherein the diameter of the long axis of the coarse fiber is set to be 5 μm or more and 20 μm or less.

[0021] In addition, the fourth invention of the present invention provides a nonwoven fabric for a circuit board, wherein the coarse fibers and fine fibers are aramid fibers.

[0022] In addition, the fifth invention of the present invention provides a nonwoven fabric for a circuit board, wherein the proportion of coarse fibers in the CD cross section of the nonwoven fabric to the total area is set to 10% or less.

[0023] In addition, the sixth invention of the present invention provides a prepreg for a circuit board, wherein the nonwoven fabric for the circuit board is impregnated with resin.

[0024] In addition, the seventh invention of the present invention provides a prepreg for a circuit board, wherein the thickness of the prepreg is set to be 120% or more and 300% or less of the thickness of the nonwoven fabric for the circuit board.

[0025] Furthermore, the eighth invention of the present invention provides a circuit board on which a circuit pattern is provided on the surface of the prepreg fabric of the circuit board.

[0026] Invention Effects

[0027] By employing a nonwoven fabric for circuit boards with the predetermined features of the present invention, smoothness can be improved, price can be reduced, and thinning can be achieved.

[0028] Brief description of the attached diagram

[0029] Figure 1 A cross-section of a circuit board according to one embodiment of the present invention is shown.

[0030] Figure 2 The invention illustrates a prepreg formed by impregnating the nonwoven fabric of the present invention with resin to form a flat sheet.

[0031] Figure 3 A scanning electron microscope image of the nonwoven fabric of the present invention is shown.

[0032] Figure 4 The nonwoven fabric of the present invention shows coarse fibers with a flat shape having a long axis and a short axis (length A in the planar direction and length B in the thickness direction).

[0033] Detailed Implementation of the Invention

[0034] The following describes in detail one embodiment of the present invention.

[0035] Figure 1The diagram shows a cross-section of a circuit board 10 according to one embodiment of the present invention. It is a plate-like structure formed by stacking multiple layers of prepreg fabric 20 on a core material 50 in the thickness direction and compressing them into a single unit. For example... Figure 1 As shown, circuit patterns 30 are provided on the front and back sides of each prepreg 20. Each prepreg 20 is electrically connected to the circuit patterns 30 on its front and back sides via through holes 40 provided from the front to the back side. Furthermore, the circuit patterns 30 provided between the upper and lower prepreg 20 are electrically connected via the through holes 40.

[0036] Regarding each of the prepreg fabrics 20, as follows: Figure 2 The nonwoven fabric containing coarse fibers 21 and fine fibers 23 is impregnated with resin 25 and formed into a flat plate. Circuit patterns are formed on both the front and back sides as described above. Through-holes are provided on the front and back sides of pads located at appropriate positions in the circuit patterns. Additionally, as shown... Figure 3 As shown in the scanning electron microscope image, the nonwoven fabric of the present invention is characterized by being made by intertwining coarse and fine fibers, wherein the length of the coarse fibers is longer than the length of the fine fibers. Furthermore, as... Figure 4 As shown, the coarse fiber 21 has a flat shape with a long axis and a short axis, with the short axis oriented in the thickness direction of the papermaking structure. In contrast, the number of fine fibers is much greater than the number of coarse fibers. The coarse and fine fibers are intertwined to form the nonwoven fabric 27.

[0037] The nonwoven fabric for the circuit board of the present invention is composed of a papermaking structure comprising a plurality of coarse fibers with a maximum fiber diameter of 5 μm or more and a plurality of fine fibers with a maximum fiber diameter of less than 5 μm. The average fiber length of the coarse fibers is longer than the average fiber length of the fine fibers, and the number of fine fibers is greater than the number of coarse fibers. Furthermore, the coarse fibers are flat in shape with a long axis and a short axis, and the short axis is oriented in the thickness direction of the papermaking structure.

[0038] As described above, in this invention, not only are multiple fine fibers with the thickest part of the fiber diameter less than 5 μm used, but also multiple coarse fibers with the thickest part of the fiber diameter greater than 5 μm are used. Therefore, compared to the case of using only expensive fine fibers, it is possible to suppress the increase in the price of nonwoven fabric for circuit boards, thereby improving the strength of nonwoven fabric for circuit boards.

[0039] Furthermore, by using both fine fibers and coarse fibers that are longer than the fine fibers in wet papermaking, increased costs can be avoided. In other words, when wet papermaking is performed using only short fine fibers, the amount of fine fibers falling off the bundle increases, leading to a deterioration in yield. However, by using both fine fibers and coarse fibers that are longer than the fine fibers, the fine fibers become entangled with the coarse fibers, while the coarse fibers remain in the bundle, thereby reducing the amount of fibers falling off the bundle and improving the yield.

[0040] Furthermore, in this invention, the number of coarse fibers is less than the number of fine fibers, and the coarse fibers are flat with a long axis and a short axis, with the short axis oriented in the thickness direction of the papermaking structure. Therefore, the thickness of the nonwoven fabric used as a circuit board can be reduced, and the smoothness of the nonwoven fabric used as a circuit board can be improved. In addition, at the same time, the smoothness of the nonwoven fabric used as a circuit board, the prepreg used as a circuit board, and the circuit board used as a circuit board can be improved.

[0041] Furthermore, in this invention, a papermaking structure is used that employs not only multiple fine fibers with the thickest part of the fiber diameter less than 5 μm, but also multiple coarse fibers with the thickest part of the fiber diameter greater than 5 μm. As a result, the tensile strength of the nonwoven fabric used as a circuit board is also increased. Consequently, the amount of resin required for prepreg fabrication of circuit boards can be reduced, and thinner fabrics can be achieved.

[0042] In this invention, the coarse fiber has a maximum fiber diameter (long axis) of 5 μm or more, preferably 7 μm or more, more preferably 9 μm or more, and most preferably 11 μm or more. Furthermore, the upper limit of the fiber diameter of the long axis of the coarse fiber can be appropriately set by those skilled in the art, but 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 coarse 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 coarse fiber to 5 μm or more and 20 μm or less, the smoothness of the circuit board can be improved. Without coarse fibers with a maximum fiber diameter of 5 μm or more, the tensile strength of the nonwoven fabric is insufficient, leading to paper breakage during papermaking and prepreg manufacturing, making stable manufacturing impossible. Furthermore, if the fiber diameter is large, the substrate surface becomes clogged with fibers, making it difficult for resin to impregnate the substrate, sometimes resulting in voids or deteriorating productivity. In addition, the thickness dimension of the nonwoven fabric also increases, so the nonwoven fabric used for circuit boards becomes thicker.

[0043] Furthermore, the fiber diameter of the finest portion (short axis) of the coarse fiber can be appropriately set by those skilled in the art within a range smaller than the fiber diameter of the long axis, but preferably 12 μm or less, more preferably 10 μm or less, and most preferably 8 μm or less, provided it is smaller than the fiber diameter of the long axis. By setting the fiber diameter of the short axis of the coarse fiber to the above values, it is possible to achieve a thinner circuit board. When the fiber diameter of the short axis becomes thicker, the nonwoven fabric containing it in the circuit board becomes thicker, which cannot meet the thinning requirements of recent years.

[0044] Furthermore, the coarsest portion 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. Without fine fibers whose coarsest portion is less than 5 μm, the number of fibers per unit weight decreases, resulting in fewer entanglement points between fibers, insufficient sheet strength, and paper breaks during papermaking and prepreg fabric manufacturing. In addition, the smoothness of the nonwoven fabric used for the substrate decreases.

[0045] The average fiber length of the coarse and fine fibers of this invention can be determined by those skilled in the art based on practical measurement of the fiber length of a specific number of fibers, for example, by dissociating the nonwoven fabric in water and measuring the length of any 50 fibers in an image captured after drying the fibers on a sample, using the average of the counts. When the average fiber length of the coarse fibers is shorter than that of the fine fibers, the amount of fibers detached from the bundle increases during papermaking, leading to a deterioration in yield and increased costs.

[0046] Furthermore, regarding the number of coarse and fine fibers, for example, by actually counting the number of fibers identified within a certain range of the actual photographed image, it is also possible to compare the number of coarse and fine fibers. The number of fine fibers is preferably at least twice the number of coarse fibers, more preferably at least 2.5 times, and most preferably at least three times. If the number of fine fibers is less than the number of coarse fibers, the number of fibers per unit weight decreases, resulting in fewer entanglement points between fibers, insufficient sheet strength, and paper breakage during papermaking and prepreg manufacturing.

[0047] In the papermaking structure of the nonwoven fabric of the present invention, the flat-shaped (having a long axis and a short axis) coarse fibers are characterized in that the short axis is oriented in the thickness direction of the papermaking structure (nonwoven fabric). As long as the short axis can be oriented in the thickness direction of the papermaking structure (nonwoven fabric) in the actually manufactured nonwoven fabric, the method of controlling the fiber orientation is not particularly limited, and appropriate methods can be adopted by those skilled in the art based on common technical knowledge, corresponding to various manufacturing methods of nonwoven fabrics. For example, in the papermaking process, when fibers are dispersed in water and dehydrated from bottom to top through a web, the fibers are deposited on the web. At this time, the fiber cross-section is basically oriented towards the side with greater filtration resistance, so when flat fibers are used as coarse fibers, the long side of the fiber cross-section naturally becomes transverse, and a nonwoven fabric with the short axis oriented in the thickness direction of the papermaking structure can be obtained.

[0048] The minor axis orientation in the thickness direction can be confirmed, for example, by taking a cross-section of the nonwoven fabric cut along the transverse direction (CD direction) and calculating the average value of the nonwoven fabric planar length A and nonwoven fabric thickness direction length B of the cross-section containing at least 10 coarse fibers, where A > B. The number of coarse fibers measured to calculate the average value of A and B can also be greater. For example, the A and B dimensions of all coarse fibers present in each field of view of a scanning microscope image of the nonwoven fabric cross-section can be measured. Furthermore, after performing the same operation in multiple fields of view, the average value of the measured A and B dimensions of all coarse fibers can be calculated.

[0049] Furthermore, the proportion of coarse fibers in the CD section of the nonwoven fabric (hereinafter also referred to as the coarse fiber proportion) can be set to 15% or less, but more preferably 10% or less. Here, the CD 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 section includes not only the cross-sectional area of ​​coarse and fine fibers, but also the voids between fibers, that is, it represents "the area of ​​the cross-section including all fibers including coarse and fine fibers" + "the area of ​​voids (the part of the cross-section where no fibers are present)". As long as the proportion of coarse fibers in the CD section of the nonwoven fabric is set to 10% or less, the smoothness of the circuit board can be improved. When the proportion of coarse fibers is greater than 10%, the nonwoven fabric for the circuit board containing it becomes thicker, so it cannot meet the thinning requirements of recent years. In addition, the proportion of fine fibers decreases, thereby worsening the surface roughness, or reducing the number of fibers per unit weight, reducing the entanglement points between fibers, resulting in insufficient strength, and causing paper breakage during papermaking and prepreg manufacturing. The aforementioned ratio can be obtained, for example, by measuring the total cross-sectional area of ​​the coarse fibers in a cross-section cut along the transverse (CD direction) of the nonwoven fabric and dividing it by the cross-sectional area of ​​the nonwoven fabric used for the circuit board.

[0050] The coarse and fine fibers of the present invention are preferably aromatic polyamide fibers. As long as the coarse and fine fibers are aromatic polyamide fibers, they can be easily produced. Aromatic polyamide fibers suitable for the circuit board nonwoven fabric of the present invention can be made from para-aromatic polyamides such as poly(p-phenylene-4,4'-biphenyl-dicarboxamide) and poly(p-phenylene-2,6-naphthalenedicarboxamide), or fibers copolymerized with a certain amount (e.g., less than 10% by weight) of diamine or diacyl chloride. However, para-aromatic polyamide fibers spun with poly(p-phenylene) as the main component and polyvinylpyrrolidone are preferred. The addition of polyvinylpyrrolidone increases the plasticity of the fiber, making it easier to deform under pressure and obtain flat fibers. By pulping the para-aromatic polyamide fibers, a nonwoven fabric suitable for the circuit board of the present invention can be obtained. The process of turning fibers into pulp is called refining, and the refining machine is called a refiner. Generally speaking, the purpose of refining is threefold: fiber swelling, fiber cutting, and fiber fibrillation. Refining that primarily focuses on fiber cutting is called free refining, while refining that primarily focuses on fiber fibrillation is called viscous refining. Depending on the refining method used, the type of refiner, blade shape, and operating conditions will vary. For example, for refining, traditional equipment such as single-disc refiners, double-disc refiners, and PFI refiners can be used without particular limitation. For example, when using a PFI mill, in a standard PFI mill (manufactured by Kumagai Riki Kogyo Co., Ltd.) that conforms to JISP8221-2 "Pulp - Beating Methods - Part 2: PFI Mill Method" (ISO5264-2), the para-aramid fibers containing deionized water and prepared to a sample concentration of 5% are beaten with a gap of 0.1 mm between the roll and the shell, and wet papermaking is performed, thereby obtaining a nonwoven fabric suitable for circuit boards according to the present invention. In the present invention, the beating equipment and beating conditions can be appropriately selected by those skilled in the art and are not particularly limited.

[0051] The nonwoven fabric for circuit boards of the present invention can be made into a wet nonwoven fabric by any method such as hydroentangling, hot air forming, or papermaking, but it is preferred, for example, to be made into a wet nonwoven fabric formed by papermaking sheets. In the papermaking process, when fibers are dispersed in water and dehydrated from bottom to top through a web, the fibers are deposited on the web. At this time, the fiber cross-section is basically oriented towards the side with high filtration resistance, so when flat fibers are used as coarse fibers, the long side of the fiber cross-section naturally becomes transverse, and a nonwoven fabric with the short axis oriented in the thickness direction of the papermaking structure can be obtained. When forming sheets by papermaking, long-wire papermaking, short-wire papermaking, rotary papermaking, and combinations thereof can be used without particular limitation. Long-wire papermaking or short-wire papermaking layers are preferred because they easily capture very fine and short fibers. In addition, the thickness and porosity can be adjusted by calendering after the sheet is formed.

[0052] Furthermore, a prepreg for a circuit board is produced by impregnating the nonwoven fabric for a circuit board of the present invention with resin. Impregnating the nonwoven fabric for a circuit board with resin makes it easy to produce the prepreg for a circuit board, thereby improving the strength of the board. The prepreg for a circuit board impregnated with resin using the nonwoven fabric for a circuit board of this embodiment can generally be formed by impregnating it with epoxy resin or phenolic resin and then curing it by heating. For example, when thermosetting resins such as epoxy resin, phenolic resin, polyurethane resin, or melamine resin are added, the adhesion to copper foil is improved, so a water-dispersible binder containing these thermosetting resins can also be added. However, the impregnated resin is not limited to the examples above; any commonly used resin can be used.

[0053] Regarding the prepreg for the circuit board, the thickness of the prepreg can be set to 120% or more and 300% or less of the thickness of the nonwoven fabric for the circuit board, 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, the circuit board can be made thinner. When it is thinner than 120%, the fibers on the surface of the nonwoven fabric for the circuit board are exposed on the surface of the prepreg, the smoothness deteriorates, resulting in broken lines in the circuit pattern or larger deviations in the through holes. When it is thicker than 300%, it cannot meet the thinning requirements of recent years.

[0054] The aforementioned prepreg fabric can be suitably used as a substrate for circuit boards and as a laminate. The related circuit board substrate and laminate can be manufactured using conventional methods. That is, for the core material, the prepreg fabric of the present invention can be laminated as an insulating layer, and then, as a circuit pattern, a conductive layer including metal foil can be laminated to manufacture a circuit laminate. As the core material, a core material with metal foil tightly attached to both sides of a resin board, a metal plate, etc., can be used. For example, gold, silver, copper, nickel, aluminum, etc., can be used as the metal foil, metal plate, and conductive layer attached to the resin board, but it is not limited to the above examples; any commonly used material can be used. By setting the circuit pattern on the surface of the circuit board prepreg fabric of the present invention using conventional methods, a thin, smooth, and strong circuit board can be manufactured.

[0055] The present invention will be described in more detail below through embodiments, but the invention is not limited to these solutions. Example

[0056] The specific determination of the properties of the nonwoven fabric (hereinafter also referred to as "nonwoven fabric") for the circuit board in this embodiment was carried out according to the following conditions and methods.

[0057] [Cross-sectional structure]

[0058] Using a sharp blade, the nonwoven fabric was cut transversely (CD direction), and the cross-section was photographed using a scanning electron microscope (model JCM-IT200 (manufactured by Nippon Electronics Corporation)).

[0059] The length of the fiber cross-section in the nonwoven fabric plane direction is designated as A, and the length in the nonwoven fabric thickness direction is designated as B. Furthermore, fibers with a maximum diameter of 5 μm or more in the fiber cross-section of the nonwoven fabric are designated as coarse fibers, and fibers with a maximum diameter of less than 5 μm are designated as fine fibers. The number of each type is counted. Then, the total cross-sectional area of ​​the coarse fibers is measured and divided by the cross-sectional area of ​​the circuit board nonwoven fabric, which is defined as the proportion of coarse fibers to the total area (hereinafter also referred to as the coarse fiber ratio).

[0060] Furthermore, regarding the imaging and counting, for each example, the measurements were taken at any 10 locations, and the average of the counts was used to calculate the values. Regarding the dimensions A and B mentioned above, the dimensions A and B of all coarse fibers present in each field of view (magnification 2000x (image area 64μm × 48μm)) at each location were measured, and the average of the measured dimensions A and B of all coarse fibers was calculated.

[0061] [Average fiber length]

[0062] The nonwoven fabric was dissociated in water, and the fibers were dried on the sample. The fibers were then photographed using a miniature oscilloscope (model VHX-6000, manufactured by Keyence). For each example, the lengths of 50 coarse and 50 fine fibers in the captured images were measured, and the average fiber length was calculated using the average of the counts.

[0063] Surface roughness

[0064] For the surfaces of nonwoven fabrics and prepregs, a surface testing machine (model KES-FB4, manufactured by Katotech) was used to measure the average deviation of surface roughness as an indicator of smoothness. Furthermore, for each example, the measurement was performed three times, and the average of the counts was used as the surface roughness.

[0065] [Basis Weight]

[0066] The basis weight of the nonwoven fabric for circuit boards in a completely dry state was determined according to the method specified in "JIS C 2300-2 Electrical Cellulose Paper - Part 2: Test Methods 6 Basis Weight".

[0067] [thickness]

[0068] Using a micrometer with a measuring surface diameter of 6.35 mm and a pressure of 155 kPa ± 15 kPa, measure the thickness of any 5 points on a circuit board made of nonwoven fabric and prepreg. Calculate the thickness using the average of the counts.

[0069] [Example 1]

[0070] After pulping para-aramid fibers (poly(terephthalamide); 1.7 dtex), a wire paper was formed to produce a nonwoven fabric. Microscopic observation of the cross-section of the nonwoven fabric revealed that the long axes of the coarse fibers were all flat and oriented towards the plane of the nonwoven fabric. A fiber diameter of 12.1 μm and a fiber diameter of 6.2 μm were obtained. The average fiber length of the coarse fibers was 0.9 mm, and the average fiber length of the fine fibers was 0.4 mm. Each field of view contained 3.0 coarse fibers and more than 10 fine fibers (with a diameter significantly less than 5 μm under microscopic observation, also applicable to subsequent examples) for use in circuit boards. The thickness of this circuit board nonwoven fabric was 25 μm, and its basis weight was 10.5 g / m³. 2 The coarse fiber ratio is 4.2%, and the surface roughness is 0.71μm.

[0071] [Example 2]

[0072] After changing the pulping and pressing conditions of the para-aramid fibers from Example 1 to perform wire-wire papermaking, calendering was performed, resulting in the long axis of the coarse fibers, all of which are flat, being oriented towards the nonwoven fabric plane. This resulted in A being 13.2 μm, B being 5.7 μm, an average fiber length of 0.9 mm for the coarse fibers, an average fiber length of 0.4 mm for the fine fibers, and a nonwoven fabric for circuit boards with 2.6 coarse fibers and 10 or more fine fibers per field of view. The thickness of this nonwoven fabric for circuit boards is 10 μm, and its basis weight is 6.8 g / m³. 2 The coarse fiber ratio is 9.7%, and the surface roughness is 0.63μm.

[0073] [Example 3]

[0074] The papermaking process for the para-aramid fibers was modified from Example 1 by changing the beating and pressing conditions. The coarse fibers, all with flat long axes oriented towards the nonwoven fabric plane, resulted in A being 10.9 μm and B being 9.3 μm. The average fiber length of the coarse fibers was 1.0 mm, and the average fiber length of the fine fibers was 0.4 mm. The number of coarse fibers per field of view was 3.4, and the number of fine fibers was 10 or more. This circuit board nonwoven fabric has a thickness of 28 μm and a basis weight of 11.1 g / m³. 2 The coarse fiber ratio is 9.1%, and the surface roughness is 0.77μm.

[0075] [Example 4]

[0076] After changing the pulping and pressing conditions of the para-aramid fibers from Example 1 to perform wire-wire papermaking, calendering was performed, resulting in the long axis of the coarse fibers, all of which are flat, being oriented towards the nonwoven fabric plane. This yielded a fiber length of A = 20.9 μm, B = 7.1 μm, an average fiber length of 1.1 mm for the coarse fibers, an average fiber length of 0.5 mm for the fine fibers, and a nonwoven fabric for circuit boards with 3.1 coarse fibers and 10 or more fine fibers per field of view. The thickness of this nonwoven fabric for circuit boards is 22 μm, and its basis weight is 16.0 g / m³. 2 The coarse fiber ratio is 14.1%, and the surface roughness is 0.69μm.

[0077] [Comparative Example 1]

[0078] The papermaking process for the para-aramid fibers was modified from Example 1 by changing the beating and pressing conditions. The coarse fibers, all with flat long axes oriented towards the nonwoven fabric plane, resulted in A = 8.9 μm and B = 9.6 μm. The average fiber length of the coarse fibers was 1.0 mm, and the average fiber length of the fine fibers was 0.4 mm. The number of coarse fibers per field of view was 2.4, and the number of fine fibers per field of view was 10 or more. This circuit board nonwoven fabric has a thickness of 30 μm and a basis weight of 10.5 g / m³. 2 The coarse fiber ratio is 3.2%, and the surface roughness is 0.95μm.

[0079] [Comparative Example 2]

[0080] After the raw material, consisting of 70% by mass of para-aramid fibers and 30% by mass of aramid slivers, is processed into a wire mesh and then calendered, the long axes of the coarse fibers, which are all flat, are oriented towards the nonwoven fabric plane. This results in a nonwoven fabric for circuit boards with A = 12.5 μm, B = 12.0 μm, an average fiber length of 3.0 mm, 8.7 coarse fibers per field of view, and 0 fine fibers. The thickness of this nonwoven fabric for circuit boards is 52 μm, and its basis weight is 34.2 g / m³. 2 The coarse fiber content is 72%, and the surface roughness is 1.89μm.

[0081] [Comparative Example 3]

[0082] An attempt was made to use coarse fibers that did not contain pulping conditions modified from those of para-aramid fibers in Example 1 for wire papermaking, but paper breakage occurred frequently, and product manufacturing could not be carried out stably.

[0083] Next, after impregnating the nonwoven fabric for the circuit board produced in the above embodiments and comparative examples with epoxy resin, it is dried to obtain a prepreg. Furthermore, the thickness of the prepreg is adjusted by changing the pressing pressure during resin impregnation.

[0084] [Example 5]

[0085] Using the nonwoven fabric for circuit boards produced in Example 1, a prepreg was obtained by the above method. The thickness of the prepreg was 40 μm (substrate thickness ratio 160%), and the surface roughness was 0.45 μm.

[0086] [Example 6]

[0087] Using the nonwoven fabric for circuit boards prepared in Example 1, a prepreg was obtained by the same method as in Example 5, except for the pressing pressure. The thickness of the prepreg was 27 μm (substrate thickness ratio 108%), and the surface roughness was 0.65 μm.

[0088] [Example 7]

[0089] Using the nonwoven fabric for circuit boards prepared in Example 1, a prepreg was obtained by the same method as in Example 5, except for the pressing pressure. The thickness of the prepreg was 83 μm (substrate thickness ratio 332%), and the surface roughness was 0.49 μm.

[0090] [Comparative Example 4]

[0091] Using the nonwoven fabric for the circuit board prepared in Comparative Example 1, a prepreg was obtained by the same method as in Example 5. The thickness of the prepreg was 45 μm (substrate thickness ratio 150%), and the surface roughness was 0.71 μm.

[0092] Table 1 shows the structure and evaluation results of the nonwoven fabric for the circuit board of each of Examples 1 to 4 and Comparative Examples 1 to 3 described above, and Table 2 shows the structure and evaluation results of the prepreg of each of Examples 5 to 7 and Comparative Example 4.

[0093] [Table 1]

[0094]

[0095] [Table 2]

[0096]

[0097] In Comparative Example 1, the thickness direction length B of the coarse fiber nonwoven fabric is longer than the planar direction length A, and the shape is not such that the short axis of the fiber cross-section is oriented in the thickness direction of the nonwoven fabric. Therefore, when compared with the other embodiments, the surface roughness deteriorates. Furthermore, when comparing Comparative Example 5 and Comparative Example 4, the surface roughness of the prepreg in Comparative Example 4, which uses the nonwoven fabric of Comparative Example 1, deteriorates. It can be concluded that the coarse fiber is preferably a flat shape having a long axis and a short axis, with the short axis oriented in the thickness direction of the papermaking structure.

[0098] Comparative Example 2 did not contain fine fibers; instead, a raw material mixed with aramid fibers was used as the binder. When compared to Example 1, it was thicker but did not contain fine fibers, resulting in a deterioration in surface roughness.

[0099] Furthermore, an attempt was made to manufacture a 50μm nonwoven fabric for circuit boards using only unpulped para-aramid fibers that do not contain fine fibers, but the wet paper strength was insufficient, and stable manufacturing was not achieved. Therefore, it is understood that nonwoven fabrics for circuit boards preferably contain fine fibers with a maximum fiber diameter of 5μm or less.

[0100] In Comparative Example 3, an attempt was made to produce nonwoven fabric for circuit boards using pulping materials that did not contain coarse fibers. However, due to the lack of coarse fibers, the wet paper strength was weak, and the nonwoven fabric for circuit boards could not be stably manufactured. A comparison of Comparative Example 3 with the various embodiments shows that it is preferable to use coarse fibers with a maximum fiber diameter of 5 μm or more.

[0101] Regarding the nonwoven fabric for the circuit board in Example 4, the long axis diameter of the flat, coarse fibers is as large as 20.9 μm. Therefore, compared with Example 2, the resin impregnation during prepreg fabrication is worse. Furthermore, the proportion of coarse fibers is as large as 14.1%, so the surface roughness is worse compared with Example 2. Therefore, it can be seen that the long axis diameter of the coarse fibers is smaller than 20 μm, and the proportion of coarse fibers is preferably 10% or less.

[0102] Comparing Example 5 and Example 6, it can be seen that Example 5 has a smaller surface roughness. This is believed to be because in Example 6, the prepreg is too thin relative to the thickness of the substrate, so the fibers on the substrate surface are exposed on the surface of the prepreg. In addition, Example 7 has the same surface roughness as Example 5, but the thickness is increased to 83 μm, so it is difficult to cope with the increased cost and thinner profile caused by the increased resin content.

[0103] As can be seen from the comparison with Examples 5, 6 and 7, the thickness of the prepreg is preferably 120% or more and 300% or less of the thickness of the nonwoven fabric for the circuit board.

[0104] As explained above, according to this embodiment, smoothness can be improved, thereby suppressing price increases, and a nonwoven fabric for circuit boards that achieves thinness can be provided. Furthermore, by using this nonwoven fabric for circuit boards, the smoothness of the prepreg for circuit boards and the circuit boards using it can be improved, price increases can be suppressed, and thinness can be achieved.

[0105] Symbol Explanation

[0106] 10 Circuit board

[0107] 20 Prepreg fabric

[0108] 21. Crude fiber

[0109] 23 fine fibers

[0110] 25 resin

[0111] 27 Nonwoven Fabric

[0112] 30 circuit patterns

[0113] 40 through hole

[0114] 50 core material

Claims

1. A nonwoven fabric for a circuit board, comprising a papermaking structure containing a plurality of coarse fibers with a maximum diameter of 5 μm or more and a plurality of fine fibers with a maximum diameter of less than 5 μm, characterized in that, The average fiber length of the coarse fiber is longer than that of the fine fiber. The number of fine fibers is greater than the number of coarse fibers. The coarse fibers are flat and have a long axis and a short axis, with the short axis oriented in the thickness direction of the papermaking structure. The coarse and fine fibers are aromatic polyamide fibers. The short axis of the coarse fiber has a fiber diameter of less than 10 μm. Wherein, the long axis of the coarse fiber has a fiber diameter of 5 μm or more and 20 μm or less, and Among them, the coarse fibers in the CD section of the nonwoven fabric account for less than 10% of the total area.

2. A prepreg for a circuit board, wherein the nonwoven fabric for a circuit board according to claim 1 is impregnated with resin.

3. The prepreg for circuit boards according to claim 2, wherein, The thickness of the prepreg is more than 120% and less than 300% of the thickness of the nonwoven fabric used for circuit boards.

4. A circuit board, wherein a circuit pattern is provided on the surface of the prepreg fabric of the circuit board according to claim 2 or 3.

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