Release film

Abstract

The present invention provides a release film that, when used in a heated press for bonding a coverlay film to a flexible circuit board in which a circuit is exposed, can efficiently prevent the occurrence of unintended recesses in a manufactured flexible printed circuit board, or provides a release film that has superior releasability. A release film according to one application example of the present invention has a release layer and a cushion layer, wherein the release layer is composed of a material that includes a polybutylene terephthalate-based resin, and within a 20 cm × 20 cm area of the cushion layer, the number of foreign bodies with a maximum length of 30 μm or greater is less than 20.

Description

Release film

[0001] The present invention relates to a release film.

[0002] For example, as shown in Patent Document 1, when forming a flexible printed circuit board by adhering a coverlay film to a flexible circuit board with an exposed circuit through a heat press via an adhesive layer provided on the coverlay film, a release film is generally used.

[0003] After adhering the coverlay film to the flexible circuit board, it is required that the release film be peeled off from the formed flexible printed circuit board with excellent releasability.

[0004] More specifically, when peeling the release film from the flexible printed circuit board, it is required that the release film exhibits excellent releasability with respect to the flexible printed circuit board and suppresses the occurrence of folds and breaks in the flexible printed circuit board.

[0005] However, in a conventional release film, when performing the above heat press, local recesses may occur in the manufactured flexible printed circuit board, causing a problem of significantly damaging the appearance of the flexible printed circuit board (see FIG. 4).

[0006] When such recesses occur, it will cause great doubts in the user regarding the quality of the flexible printed circuit board. In addition, when such recesses occur significantly, there is also a concern that it may actually have an adverse effect on the quality of the flexible printed circuit board.

[0007] Further, in a conventional release film, the releasability is insufficient. For example, when the release film comes into contact with the adhesive layer provided on the coverlay film, they may be partially joined, and a part of the release film may remain on the flexible circuit board (for example, on the coverlay film on the flexible circuit board).

[0008] Japanese Patent No. 6470461

[0009] The object of the present invention is to provide a release film that can effectively prevent unintended recesses from forming in the manufactured flexible printed circuit board when used in a heat press to adhere a coverlay film to a flexible circuit board with exposed circuits, or to provide a release film with excellent release properties. In the present invention, it is sufficient if either of these problems is solved.

[0010] These objectives are achieved by the present invention as described in (1) to (14) below. (1) A release film having a release layer and a cushion layer, wherein the release layer is composed of a material containing a polybutylene terephthalate resin, and the number of foreign objects with a maximum length of 30 μm or more within a 20 cm × 20 cm area of ​​the cushion layer is less than 20.

[0011] (2) The release film according to (1) above, wherein the cushion layer is made of a material comprising a first resin component having a melting point of 60°C or more and 90°C or less, and a second resin component having a melting point that is 10°C or more and 50°C or less higher than the melting point of the first resin component.

[0012] (3) A release film having a release layer and a cushion layer, wherein the cushion layer is made of a material containing a first resin component having a melting point of 60°C or more and 90°C or less and a second resin component having a melting point that is 10°C or more and 50°C or less higher than the melting point of the first resin component, and the interlayer strength between the release layer and the cushion layer is 3.0 N / 25 mm or more.

[0013] (4) The release layer is made of a material containing a polybutylene terephthalate resin, as described in (3) above.

[0014] (5) The release layer is the release film according to (1), (2), or (4) above, wherein the release layer comprises polybutylene terephthalate as the polybutylene terephthalate resin, and further comprises at least one of the following: polybutylene terephthalate / polytetramethylene glycol block copolymer, terephthalic acid, and a polybutylene terephthalate copolymer containing a dicarboxylic acid component other than terephthalic acid as constituent monomers.

[0015] (6) A release film according to any one of (2) to (4) above, where X1 [mass%] is the content of the first resin component in the cushion layer and X2 [mass%] is the content of the second resin component, and the relationship 0.80 ≤ X2 / X1 ≤ 1.45 is satisfied.

[0016] (7) The release film according to (2), (3), (4), or (6) above, wherein the MFR of the first resin component is 2.0 g / 10 min or more and 8.0 g / 10 min or less.

[0017] (8) The release film according to any one of (2), (3), (4), (6), or (7) to (4) above, wherein the cushion layer is further made of a material containing a polybutylene terephthalate resin as a third resin component.

[0018] (9) The release film according to (8) above, where X1 [mass%] is the content of the first resin component in the cushion layer and X3 [mass%] is the content of the third resin component, and the relationship 0.25 ≤ X3 / X1 ≤ 0.90 is satisfied.

[0019] (10) The release film according to (8) or (9) above, where X2 [mass%] is the content of the second resin component in the cushion layer and X3 [mass%] is the content of the third resin component, and the relationship 0.20 ≤ X3 / X2 ≤ 1.10 is satisfied.

[0020] (11) The release film according to any one of (1) to (10) above, wherein the cushion layer is made of a material containing at least one of ethylene-methyl methacrylate copolymer, ethylene-vinyl acetate copolymer, ethylene-methyl acrylate copolymer, and ethylene-αolefin copolymer.

[0021] (12) The release film according to any one of (1) to (11) above, wherein the average thickness of the cushion layer is 10 μm or more and 200 μm or less.

[0022] (13) A release film according to any one of (1) to (12) above, wherein the release layer is provided on both sides of the cushion layer.

[0023] (14) The release film according to any one of (1) to (13) above, wherein the average thickness of the release layer is 5 μm or more and 35 μm or less.

[0024] According to the present invention, it is possible to provide a release film that can effectively prevent unintended recesses from forming in the manufactured flexible printed circuit board when used in a heat press to adhere a coverlay film to a flexible circuit board with exposed circuits, or to provide a release film with excellent release properties.

[0025] Figure 1 is a longitudinal cross-sectional view illustrating a manufacturing method for producing a flexible printed circuit board in a multi-layered state. Figure 2 is a longitudinal cross-sectional view showing each step in the manufacturing method for producing a flexible printed circuit board in a multi-layered state. Figure 3 is a longitudinal cross-sectional view showing an embodiment of the release film of the present invention. Figure 4 is a photograph showing unintended recesses that occurred in a flexible printed circuit board when a conventional release film was used to manufacture the flexible printed circuit board.

[0026] Preferred embodiments of the present invention will be described in detail below.

[0027] In the following, we will describe, as an example, the case in which a flexible printed circuit board is manufactured in multiple layers using the release film of the present invention, that is, the case in which the release film of the present invention is used for circuit formation. Furthermore, before describing the release film of the present invention, we will first describe the manufacturing method for manufacturing a flexible printed circuit board in multiple layers.

[0028] [1] Method for Manufacturing Flexible Printed Circuit Boards Figure 1 is a vertical cross-sectional view illustrating a manufacturing method for manufacturing flexible printed circuit boards in a multi-layered stacked state, and Figure 2 is a vertical cross-sectional view showing each step in the manufacturing method for manufacturing flexible printed circuit boards in a multi-layered stacked state. For the purposes of explanation below, the upper side in Figures 1 and 2 will be referred to as "up" or "upper," the lower side as "down" or "downward," the left side as "left," and the right side as "right."

[0029] In this embodiment, the manufacturing method for producing a multi-layer flexible printed circuit board 200 (hereinafter sometimes referred to as "FPC200") comprises: a first step of stacking multiple layers of laminates in which glass cloth 300A, release film 10A, FPC200, release film 10B, and glass cloth 300B, each in the form of a sheet, are stacked in this order; a second step of bonding a coverlay film 220 (hereinafter sometimes referred to as "CL film 220") to the flexible circuit board 210 in the FPC200 by heating and pressing each of the multi-layered laminates; and a third step of releasing the release films 10 (10A, 10B) from the FPC200 to obtain an FPC200 in which the CL film 220 is bonded to the flexible circuit board 210.

[0030] The following describes each step in order. [1-1] First step First, a laminate is stacked in multiple layers, in which glass cloth 300A, release film 10A, FPC 200, release film 10B, and glass cloth 300B, each in a sheet-like form, are stacked in this order (see Figures 1 and 2(a)). The following describes the case where the laminate is stacked in two layers.

[0031] [1-1-1] Step 1-1 First, three flat heating and pressing plates 521 are prepared and arranged so that two gaps are formed in the thickness direction of these plates.

[0032] [1-1-2] Step 1-2 Next, in the two gaps, a sheet-like (film-like) glass cloth 300A, a release film 10A, an FPC 200, a release film 10B, and a glass cloth 300B are placed in this order from top to bottom, overlapping each other. In this step (step 1-2), although the FPC 200 placed in the gap is in a laminated state by overlapping the flexible circuit board 210 and the CL film 220, the flexible circuit board 210 and the CL film 220 are not joined via the adhesive layer 222 provided by the CL film 220.

[0033] As a result, as shown in Figure 1, a laminate is stacked in two layers between the two heat-pressure plates 521, with the glass cloth 300A, release film 10A, FPC 200, release film 10B, and glass cloth 300B being layered in this order.

[0034] This process (steps 1-2) constitutes the process of placing a release film on the object (FPC 200).

[0035] [1-2] Second step Next, the CL film 220 is bonded to the flexible circuit board 210 in the FPC 200 by heating and pressing each of the multi-layered laminates that have been stacked through the first step (see Figures 1 and 2(b)).

[0036] [1-2-1] Step 2-1 First, the heating and pressing plate 521 is heated while the glass cloth 300 (300A, 300B) is in contact with the heating and pressing plate 521.

[0037] As a result, the laminate, which consists of the glass cloth 300A, release film 10A, FPC 200, release film 10B, and glass cloth 300B stacked together, is heated by the heat transferred from the heat-sealing plate 521.

[0038] In this step (step 2-1), the temperature at which the laminate, i.e., the FPC 200, is heated is not particularly limited, but is preferably 100°C or more and 250°C or less, and more preferably 150°C or more and 200°C or less.

[0039] Furthermore, the heating time for the laminate is not particularly limited, but is preferably 40 seconds to 5000 seconds, and more preferably 200 seconds to 4000 seconds. This allows the glass cloth 300A, release film 10A, FPC 200, release film 10B, and glass cloth 300B in the laminate to be heated almost uniformly.

[0040] [1-2-2] Step 2-2 Also, almost simultaneously with the heating of the heating and pressing plate 521 in the above step (Step 2-1), the heating and pressing plate 521 located on the upper side and the heating and pressing plate 521 located on the lower side are brought closer together along their thickness direction (press molding method).

[0041] As a result, in a laminate formed by stacking glass cloth 300A, release film 10A, FPC 200, release film 10B, and glass cloth 300B, which are placed in two gaps formed between three heat-sealing plates 521, the FPC 200 is pressurized through the glass cloths 300A and 300B and the release films 10A and 10B (see Figures 1 and 2(b)).

[0042] As a result, the FPC 200 is heated and pressurized, and in the FPC 200, the overlapping flexible circuit board 210 and the CL film 220 are joined via the adhesive layer 222 of the CL film 220. In other words, the coverlay 221 and the flexible circuit board 210 are joined via the adhesive layer 222. Furthermore, when the FPC 200 is heated and pressurized, that is, when the coverlay 221 and the flexible circuit board 210 are joined via the adhesive layer 222, the release film 10 is embedded in the recess 223 formed in the coverlay 221. Therefore, it is possible to suppress the seepage of adhesive originating from the adhesive layer 222 within the recess 223 (see Figure 2(b)).

[0043] In this step (the second step of the second step), the pressure for pressing the FPC 200 is not particularly limited, but is preferably 0.1 MPa or more and 20.0 MPa or less, and more preferably set to 0.5 MPa or more and 15.0 MPa or less.

[0044] Also, the time for pressing the FPC 200 is not particularly limited, but is preferably 20 sec or more and 5000 sec or less, and more preferably set to 100 sec or more and 4000 sec or less.

[0045] By setting the pressure and time for pressing the FPC 200 within the above ranges respectively, the coverlay 221 and the flexible circuit board 210 can be surely joined via the adhesive layer 222.

[0046] In addition, this step (the second step of the second step) constitutes a step of performing a hot press on the object (FPC 200) on which the release film 10 is disposed. Further, when the coverlay 221 is made of a material containing a semi-cured thermosetting resin, the surface of the object (FPC 200) on which the release film 10 is disposed is constituted by the coverlay 221. And since the release film 10 is used by being overlapped so that the surface on the side of the first release layer 1 described later contacts the surface of this coverlay 221, the shape of the coverlay 221 in which the recess 223 is formed can be maintained by the release film 10, and the thermosetting resin can be cured. Therefore, the coverlay 221 (molded product) can be molded on the flexible circuit board 210 with excellent accuracy.

[0047] In addition, in the above, as described above, it is preferable that the heating of the FPC 200 in the above step (the first step of the second step) and the pressing of the FPC 200 in this step (the second step of the second step) are carried out almost simultaneously. However, it can also be carried out in the order of the first step of the second step and the second step of the second step. However, by carrying out the first step of the second step and the second step of the second step almost simultaneously, the time required for the second step and thus the manufacturing of the FPC 200 can be shortened.

[0048] [1-3] The Third Step Next, the release film 10 (10A, 10B) is released from the FPC 200 to obtain an FPC 200 to which the CL film 220 is bonded to the flexible circuit board 210 (see Fig. 2(c)).

[0049] The release method for releasing the release film 10 from this FPC 200 is not particularly limited. For example, after gripping one end of the release film by hand, a method of peeling it off in a direction of 90° or more and 180° or less is preferably used.

[0050] When excellent releasability is exhibited between the heating and pressure bonding plate 521 and the release film 10 during the peeling of the release film 10 from the FPC 200, the arrangement of the glass cloth 300 between the heating and pressure bonding plate 521 and the release film 10 may be omitted.

[0051] Through the above steps, a manufacturing method of the flexible printed circuit board 200 using the release film 10 is constituted.

[0052] And as the release film 10 applied to the manufacture of this flexible printed circuit board 200, the release film of the present invention is used. That is, as the release film 10, it has a first release layer 1 made of a first thermoplastic resin composition, a cushion layer 3 made of a third thermoplastic resin composition, and a second release layer 2 made of a second thermoplastic resin composition. The first release layer 1, the cushion layer 3, and the second release layer 2 are laminated in this order, and a release film 10 with a small number of foreign matters is used as the release film 10. Hereinafter, the release film 10 to which the release film of the present invention is applied will be described.

[0053] [2] Release Film Fig. 3 is a longitudinal sectional view showing an embodiment of the release film of the present invention.

[0054] (Release Film According to the First Application Example) The release film 10 according to a preferred application example of the present invention has a release layer (first release layer 1, second release layer 2) and a cushion layer 3. And the number of foreign matters having a maximum length of 30 μm or more within a region of 20 cm × 20 cm of the cushion layer 3 is less than 20. Hereinafter, the release film 10 according to such an application example will be described as the release film according to the first application example.

[0055] With the above configuration, when the release film 10 according to the first application example is used in a heat press to adhere a coverlay film 220 to a flexible circuit board 210 with exposed circuits, it is possible to provide a release film 10 that can effectively prevent the formation of unintended depressions in the manufactured flexible printed circuit board (FPC) 200. More specifically, when the cushion layer constituting the release film contains a large number of relatively large foreign objects (foreign objects with a maximum length of 30 μm or more), the shape of the foreign objects is transferred to the manufactured FPC by heat press, making it easy for unintended depressions to occur. In contrast, according to the present invention, such transfer is effectively prevented, making it difficult for unintended depressions to occur. As a result, deterioration of the appearance and quality of the FPC can be suitably prevented. The number of foreign objects can be determined, for example, by visual inspection and then counting only foreign objects with a long side of 30 μm or more by observing the surface and cross-section with a microscope.

[0056] Furthermore, the number of foreign objects may be measured over multiple randomly selected 20 cm x 20 cm areas.

[0057] Examples of foreign matter include lumps (particles) of resin. Generally, such foreign matter is harder than other parts of the cushion layer 3 (parts other than the foreign matter).

[0058] Within a 20 cm x 20 cm area of ​​the cushion layer 3, the number of foreign objects with a maximum length of 30 μm or more should be less than 20, preferably less than 15, more preferably less than 10, and even more preferably less than 5. This allows the aforementioned effects to be exhibited more significantly.

[0059] (Release film according to the second application example) Another release film 10 according to another application example of the present invention has a release layer (first release layer 1, second release layer 2) and a cushion layer 3. The interlayer strength between the release layer and the cushion layer 3 is 3.0 N / 25 mm or more. Hereinafter, the release film 10 according to such an application example will be described as the release film according to the second application example.

[0060] With the above configuration, a release film 10 with excellent release properties can be provided. More specifically, for example, after the coverlay film is bonded to the flexible circuit board, the release film can be peeled off the formed flexible printed circuit board with excellent release properties. As a result, it is possible to effectively prevent any part of the release film from remaining on the flexible circuit board (for example, on the coverlay film on the flexible circuit board).

[0061] The interlaminar strength between the release layer and the cushion layer 3 can be determined by holding one end of the first release layer 1 and the cushion layer 3, creating a peeling opening between the two, and then using a tensile testing machine (A&D Company, Ltd., "TENSILON RTG-1310") to pull the first release layer 1 at a speed of 200 mm / min so that the peeling angle is 180°, and measuring the peel strength (interlaminar strength, N / mm). In the embodiments described later, the value obtained using this method is shown as the interlaminar strength.

[0062] Furthermore, in the release film relating to the second application example, excellent release properties can be achieved even when the release film and the adhesive layer do not come into contact.

[0063] As shown in Figure 3, release layers are provided on both sides of the cushion layer 3. Specifically, a first release layer 1 is provided on one side of the cushion layer 3, and a second release layer 2 is provided on the other side of the cushion layer 3.

[0064] In the release film 10 according to the second application example, by providing release layers on both sides of the cushion layer 3 in this manner, the above-mentioned effects can be obtained, and it is also possible to more effectively prevent the release film 10 (release films 10A, 10B) from being unintentionally bonded to the glass cloth 300A, 300B, etc., by adhesive that has seeped out from the adhesive layer 222. Furthermore, the above-mentioned effects can also be obtained when, for example, the orientation of the release film 10 is changed during use.

[0065] As described above, in the release film 10 according to the second application example, the interlayer strength between the release layer and the cushion layer 3 is 3.0 N / 25 mm or more, preferably 4.0 N / 25 mm or more, more preferably 5.0 N / 25 mm or more, and even more preferably 6.0 N / 25 mm or more. This allows the aforementioned effects to be exhibited more significantly.

[0066] In the release film 10 according to the second application example, it is sufficient that the above relationship is satisfied between at least one of the two release layers and the cushion layer 3. More specifically, in the method of using the release film 10 as described above, it is sufficient that the above relationship is satisfied between the release layer that contacts the CL film 220 (the first release layer 1 in the configuration shown in Figure 2) and the cushion layer 3.

[0067] The following describes each layer that makes up the release film 10 (the release film 10 according to the first and second application examples).

[0068] [2-1] Cushion Layer First, let's explain the cushion layer 3. This cushion layer 3 is positioned as an intermediate layer between the first release layer 1 and the second release layer 2.

[0069] The cushion layer 3 preferably contains at least one type of thermoplastic resin, and more preferably contains multiple types of thermoplastic resins, for the purpose of providing suitable embedding properties for the recesses 223 in the release film 10.

[0070] In particular, the cushion layer 3 may be made of any material, but it is preferable that it is made of a material containing a first resin component having a melting point of 60°C or more and 90°C or less, and a second resin component having a melting point that is 10°C or more and 50°C or less higher than the melting point of the first resin component.

[0071] This makes it possible to more effectively prevent the generation of foreign matter in the cushion layer 3, especially relatively large foreign matter (for example, foreign matter with a maximum length of 30 μm or more), and the aforementioned effects are more pronounced. In addition, the interlayer strength conditions between the release layer and the cushion layer 3 can be more favorably met. Furthermore, the cushioning properties of the cushion layer 3 can be made better, and its conformability to the recesses 223 can be improved.

[0072] When the content of the first resin component in the cushion layer 3 is X1 [mass%] and the content of the second resin component is X2 [mass%], it is preferable that the relationship 0.80 ≤ X2 / X1 ≤ 1.45 is satisfied, more preferably that 0.82 ≤ X2 / X1 ≤ 1.40 is satisfied, and even more preferably that 0.84 ≤ X2 / X1 ≤ 1.30 is satisfied.

[0073] This makes it possible to more effectively prevent the generation of foreign matter in the cushion layer 3, especially relatively large foreign matter (for example, foreign matter with a maximum length of 30 μm or more), and the aforementioned effects are exhibited even more significantly. In addition, the interlayer strength conditions between the release layer and the cushion layer 3 can be more favorably met. Furthermore, the cushioning properties of the cushion layer 3 can be made even better, and its conformability to the recesses 223 can be further improved.

[0074] The melting point of the first resin component is 60°C to 90°C, preferably 62°C to 85°C, more preferably 64°C to 80°C, and even more preferably 66°C to 75°C. This allows the aforementioned effects to be exhibited more significantly.

[0075] The difference between the melting point of the second resin component and the melting point of the first resin component, that is, Tm2 - Tm1, where the melting point of the first resin component is Tm1 [°C] and the melting point of the second resin component is Tm2 [°C], is between 10°C and 50°C, preferably between 12°C and 45°C, more preferably between 14°C and 40°C, and even more preferably between 16°C and 35°C. This allows the aforementioned effects to be exhibited more significantly.

[0076] For the first and second resin components, for example, polyolefin resins can be used.

[0077] Polyolefin resins are not particularly limited and include, for example, polyethylene such as low-density polyethylene and high-density polyethylene, α-olefin polymers such as polypropylene, ethylene-α-olefin copolymers which are copolymers of ethylene and α-olefins other than ethylene such as propylene, 1-butene, 1-pentene, 1-hexene, and 1-octene, copolymers of ethylene and (meth)acrylic acid (ethylene-(meth)acrylic acid copolymer), copolymers of ethylene and (meth)acrylic acid ester (ethylene-(meth)acrylic acid ester copolymer), copolymers of ethylene and vinyl acetate (ethylene-vinyl acetate copolymer), and one or more of these can be used in combination.

[0078] In particular, it is preferable that it be at least one of the following: ethylene-methyl methacrylate copolymer (a copolymer of ethylene and methyl methacrylate), ethylene-vinyl acetate copolymer, ethylene-methyl acrylate copolymer (a copolymer of ethylene and methyl acrylate), and ethylene-α-olefin copolymer.

[0079] This makes it possible to more effectively prevent the generation of foreign matter in the cushion layer 3, especially relatively large foreign matter (for example, foreign matter with a maximum length of 30 μm or more), and the aforementioned effects are exhibited even more significantly. In addition, the interlayer strength conditions between the release layer and the cushion layer 3 can be more favorably met. Furthermore, the cushioning properties of the cushion layer 3 can be made even better, and its conformability to the recesses 223 can be further improved.

[0080] The MFR (melt flow rate) of the first resin component is preferably 2.0 g / 10 min or more and 8.0 g / 10 min or less, more preferably 2.5 g / 10 min or more and 7.5 g / 10 min or less, and even more preferably 3.0 g / 10 min or more and 7.0 g / 10 min or less.

[0081] This results in more stable film formation, keeps the surface roughness of the multilayer film below a certain value, and suppresses the transfer of surface roughness during pressing.

[0082] In this specification, MFR (Melt Flow Rate) means "Melt Mass Flow Rate," which is the melt flow rate measured in accordance with JIS K7210-1:2014 under conditions of a temperature of 190°C and a load of 2.16 kgf.

[0083] The cushion layer 3 may contain, in addition to the first and second resin components described above, a third resin component other than those mentioned above.

[0084] Examples of the third resin component include polyester resins and polyamide resins, but polyester resins are preferred.

[0085] This makes it possible to more effectively prevent the generation of foreign matter in the cushion layer 3, especially relatively large foreign matter (for example, foreign matter with a maximum length of 30 μm or more), and the aforementioned effects are exhibited even more significantly. In addition, the interlayer strength conditions between the release layer and the cushion layer 3 can be more favorably met. Furthermore, the cushioning properties of the cushion layer 3 can be made even better, and its conformability to the recesses 223 can be further improved.

[0086] Examples of polyester resins include polyethylene terephthalate (PET), polycyclohexane terephthalate (PCT), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polycyclohexane dimethylene terephthalate, polypropylene terephthalate, and copolymers thereof. One or more of these can be used in combination.

[0087] Among these, polybutylene terephthalate-based resins (polybutylene terephthalate or copolymers containing polybutylene terephthalate) are preferred.

[0088] This makes the aforementioned effects even more pronounced. In particular, when the release layer (at least one of the first release layer 1 and the second release layer 2) is made of a material containing a polybutylene terephthalate resin (polybutylene terephthalate or a copolymer containing polybutylene terephthalate), these effects are even more pronounced.

[0089] When the content of the first resin component in the cushion layer 3 is X1 [mass%] and the content of the third resin component is X3 [mass%], it is preferable that the relationship 0.25 ≤ X3 / X1 ≤ 0.90 is satisfied, more preferably that 0.26 ≤ X3 / X1 ≤ 0.80 is satisfied, and even more preferably that 0.27 ≤ X3 / X1 ≤ 0.70 is satisfied.

[0090] This makes it possible to more effectively prevent the generation of foreign matter in the cushion layer 3, especially relatively large foreign matter (for example, foreign matter with a maximum length of 30 μm or more), and the aforementioned effects are exhibited even more significantly. In addition, the interlayer strength conditions between the release layer and the cushion layer 3 can be more favorably met. Furthermore, the cushioning properties of the cushion layer 3 can be made even better, and its conformability to the recesses 223 can be further improved.

[0091] When the content of the second resin component in the cushion layer 3 is X2 [mass%] and the content of the third resin component is X3 [mass%], it is preferable that the relationship 0.20 ≤ X3 / X2 ≤ 1.10 is satisfied, more preferably that the relationship 0.22 ≤ X3 / X2 ≤ 1.00 is satisfied, and even more preferably that the relationship 0.24 ≤ X3 / X2 ≤ 0.80 is satisfied.

[0092] This makes it possible to more effectively prevent the generation of foreign matter in the cushion layer 3, especially relatively large foreign matter (for example, foreign matter with a maximum length of 30 μm or more), and the aforementioned effects are exhibited even more significantly. In addition, the interlayer strength conditions between the release layer and the cushion layer 3 can be more favorably met. Furthermore, the cushioning properties of the cushion layer 3 can be made even better, and its conformability to the recesses 223 can be further improved.

[0093] The cushion layer 3 may contain components other than those listed above. Hereinafter, in this section, such components will also be referred to as "other components." Examples of such components include crystal nucleating agents, antioxidants, slip agents, antiblocking agents, antistatic agents, colorants, stabilizers, and the like.

[0094] However, the content of other components in the cushion layer 3 is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less.

[0095] The storage modulus E' of the cushion layer 3 at 150°C is preferably 0.1 MPa or more, more preferably 0.5 MPa to 150 MPa, and even more preferably 1 MPa to 100 MPa.

[0096] This makes it possible to more effectively suppress or prevent a portion of the cushion layer 3 from more effectively protruding from the edge of the release film 10 and adhering to the FPC 200 when embedding the release film 10 into the recess 223 in the second step. Therefore, contamination of the FPC 200 can be more effectively suppressed or prevented. In addition, it becomes possible to more easily peel off the release film 10 in the third step.

[0097] The storage modulus E' of the cushion layer 3 at 150°C can be determined, for example, by preparing a cushion layer 3 with a width of 4 mm and a length of 20 mm in accordance with JIS K7244-4, and measuring it using a dynamic viscoelasticity measuring device (Hitachi High-Tech Science Corporation, "DMA7100") in tensile mode, with a frequency of 1 Hz and a heating rate of 5°C / min.

[0098] The average thickness Tk of the cushion layer 3 is preferably 10 μm or more and 200 μm or less, more preferably 20 μm or more and 180 μm or less, and even more preferably 30 μm or more and 150 μm or less.

[0099] This makes it possible to suppress the release film 10 from becoming excessively thick, while also making it easier to improve the cushioning properties of the cushion layer 3 and improve its ability to conform to the recesses 223.

[0100] [2-2] First release layer Next, the first release layer 1 will be described. The first release layer 1 is laminated on one side of the cushion layer 3. The first release layer 1 constitutes one surface of the release film 10.

[0101] The first release layer 1 is flexible, and in the manufacturing method of the flexible printed circuit board 200 using the release film 10 described above, the release film 10 is superimposed on the CL film 220 of the FPC 200 so that the first release layer 1 is in contact with it. Then, in the second step, when the superimposed flexible circuit board 210 and CL film 220 are joined via the adhesive layer 222, the first release layer 1 is pressed in to conform to the shape of the recess 223 formed by the flexible circuit board 210 and CL film 220, and functions as a protective (cushioning) material to prevent the release film 10 from tearing. Furthermore, in the third step, the first release layer 1 functions as a contact layer to enable the release film 10 to exhibit excellent release properties from the CL film 220 (FPC 200).

[0102] Therefore, when embedding the release film 10 into the recess 223 formed in the FPC 200 in the second step, it is possible to effectively suppress or prevent the adhesive originating from the adhesive layer 222 from seeping out. Furthermore, after the formation of the FPC 200 in which the flexible circuit board 210 and the CL film 220 are joined via the adhesive layer 222 of the CL film 220 in the second step, when peeling the release film 10 from the FPC 200 in the third step, it is possible to effectively suppress or prevent stretching and breakage of the FPC 200.

[0103] The first release layer 1 may be composed of any material, but it usually contains a thermoplastic resin, and it is particularly preferable that it contains a polyester resin. This allows the aforementioned effects to be exhibited more significantly.

[0104] The polyester resin is not particularly limited, but examples include polyethylene terephthalate (PET), polycyclohexane terephthalate (PCT), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polycyclohexane dimethylene terephthalate, polypropylene terephthalate, and copolymers thereof. One or more of these can be used in combination.

[0105] Among these, polybutylene terephthalate resins (polybutylene terephthalate or copolymers containing polybutylene terephthalate) are preferred.

[0106] This allows the aforementioned effects to be exhibited more significantly. In particular, when the cushion layer 3 is made of a material containing polybutylene terephthalate resin (polybutylene terephthalate or a copolymer containing polybutylene terephthalate), these effects are exhibited more significantly.

[0107] Polybutylene terephthalate resins can be polybutylene terephthalate or various copolymers containing polybutylene terephthalate, but it is preferable that they contain polybutylene terephthalate and further contain at least one of the following: polybutylene terephthalate / polytetramethylene glycol block copolymer, terephthalic acid, and a polybutylene terephthalate copolymer containing dicarboxylic acid components other than terephthalic acid as constituent monomers.

[0108] This further enhances the aforementioned effects. In particular, these effects are even more pronounced when the cushion layer 3 is made of a material containing polybutylene terephthalate resin (polybutylene terephthalate or a copolymer containing polybutylene terephthalate).

[0109] The content of the thermoplastic resin in the first release layer 1 is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more. This allows the aforementioned effects to be exhibited more significantly.

[0110] Furthermore, the first release layer 1 may also contain at least one of inorganic particles and organic particles in addition to the thermoplastic resin described above.

[0111] The inorganic particles are not particularly limited, but examples include aluminum hydroxide, magnesium hydroxide, calcium carbonate, magnesium carbonate, calcium silicate, magnesium silicate, calcium oxide, magnesium oxide, alumina, aluminum nitride, aluminum borate whiskers, boron nitride, crystalline silica, amorphous silica, antimony oxide, E glass, D glass, S glass, etc., and one or more of these can be used in combination.

[0112] Furthermore, while the organic particles are not particularly limited, examples include polystyrene particles, acrylic particles, polyimide particles, polyester particles, silicone particles, polypropylene particles, polyethylene particles, fluororesin particles, and core-shell particles, and one or more of these can be used in combination.

[0113] Furthermore, the inorganic and organic particles preferably have an average particle diameter of 3 μm or more and 20 μm or less, and more preferably 5 μm or more and 20 μm or less.

[0114] This makes it relatively easy to set the surface roughness of the surface of the first release layer 1 opposite to the cushion layer 3 within the range described later.

[0115] Furthermore, as the average particle size of inorganic and organic particles, the median diameter (D50), which is determined by measuring the particle size distribution on a volume basis using a laser diffraction particle size distribution analyzer (for example, LA-500 manufactured by HORIBA Corporation), can be used.

[0116] If the first release layer 1 has an uneven surface, it is preferable that the 10-point average roughness (Rz) on the surface is 0.1 μm or more and 20.0 μm or less, and more preferably 1.0 μm or more and 10.0 μm or less. This allows the release film 10 to be released from the FPC 200 (flexible circuit board 210) with excellent release properties. The 10-point average roughness (Rz) can be measured in accordance with JIS B 0601-1994.

[0117] The first release layer 1 may contain components other than those listed above. Hereinafter, in this section, such components will also be referred to as "other components." Examples of such components include crystal nucleating agents, antioxidants, slip agents, antiblocking agents, antistatic agents, colorants, stabilizers, and the like.

[0118] However, the content of other components in the first release layer 1 is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 3% by mass or less.

[0119] The storage modulus E' of the first release layer 1 at 150°C is preferably 50 MPa or more, more preferably 50 MPa to 1000 MPa, and even more preferably 50 MPa to 300 MPa. This allows the aforementioned effects to be exhibited more significantly.

[0120] The storage modulus E' of the first release layer 1 at 150°C can be determined in accordance with JIS K7244-4 by preparing a first release layer 1 with a width of 4 mm and a length of 20 mm, and measuring it using a dynamic viscoelasticity measuring device (Hitachi High-Tech Science Corporation, "DMA7100") in tensile mode, with a frequency of 1 Hz and a heating rate of 5°C / min.

[0121] The average thickness T1 of the first release layer 1 is preferably 5 μm or more and 35 μm or less, more preferably 7 μm or more and 30 μm or less, and even more preferably 10 μm or more and 25 μm or less. This allows the aforementioned effects to be exhibited more significantly.

[0122] Furthermore, as described above, if the surface of the first release layer 1 opposite to the cushion layer 3 has an uneven shape, the thickness of the first release layer 1 shall be measured at the position including the convex portion and at the position including the concave portion.

[0123] The surface roughness Rz of the first release layer 1 is preferably 0.1 μm or more and 5.0 μm or less, more preferably 0.3 μm or more and 3.0 μm or less, and even more preferably 0.5 μm or more and 1.0 μm or less.

[0124] This allows for more effective suppression of the transfer of surface roughness of the release film to the substrate while ensuring release properties.

[0125] The surface roughness Rz can be determined by the measurement method described in the example below ([5-2]).

[0126] [2-3] Second release layer Next, the second release layer 2 will be described. The second release layer 2 is laminated on the other side of the cushion layer 3, that is, on the side of the cushion layer 3 opposite to the first release layer 1. The second release layer 2 constitutes the other surface of the release film 10 (the surface opposite to the surface formed by the outer surface of the second release layer 2).

[0127] The second release layer 2 is flexible and, in the manufacturing method of the flexible printed circuit board 200 using the release film 10 described above, the release film 10 is superimposed on the CL film 220 of the FPC 200 so that the first release layer 1 is in contact with it, and in the second step of this manufacturing method, when the superimposed flexible circuit board 210 and CL film 220 are joined via the adhesive layer 222, the second release layer 2 functions as a layer that transmits force from the heat-pressure plate 521 to the cushion layer 3. Furthermore, in the third step, the second release layer 2 functions as a contact layer to provide excellent release properties between the glass cloths 300A and 300B and the release film 10.

[0128] Furthermore, in the manufacturing method of the flexible printed circuit board 200, the second release layer 2 is in contact with the heat-pressing plate 521 via the glass cloths 300A and 300B. Therefore, in the second step of this manufacturing method, when the FPC 200 is heated and pressed, the second release layer 2 also has the function of transferring heat from the heat-pressing plate 521 to the cushion layer 3.

[0129] The composition and components of the second release layer 2 are preferably the same as those described above for the composition and components of the first release layer 1 in [2-2]. This will yield the same effects as described above.

[0130] The storage modulus E' of the second release layer 2 at 150°C is preferably 50 MPa or more, more preferably 50 MPa to 1000 MPa, and even more preferably 50 MPa to 300 MPa. This allows the aforementioned effects to be exhibited more significantly.

[0131] The storage modulus E' of the second release layer 2 at 150°C can be determined in accordance with JIS K7244-4 by preparing a second release layer 2 with a width of 4 mm and a length of 20 mm, and measuring it using a dynamic viscoelasticity measuring device (Hitachi High-Tech Science Corporation, "DMA7100") in tensile mode, with a frequency of 1 Hz and a heating rate of 5°C / min.

[0132] The average thickness T2 of the second release layer 2 is preferably 5 μm or more and 35 μm or less, more preferably 7 μm or more and 30 μm or less, and even more preferably 10 μm or more and 25 μm or less. This allows the aforementioned effects to be exhibited more significantly.

[0133] Furthermore, if the surface of the second release layer 2 opposite to the cushion layer 3 has an uneven shape, the thickness of the second release layer 2 shall be measured at the position including the convex portion in the case of a convex portion, and at the position including the concave portion in the case of a concave portion.

[0134] Furthermore, it is preferable that the product of the storage modulus E' at 150°C and the average thickness T2 of the second release layer 2 ((storage modulus E' at 150°C) × average thickness T2) is 1000 N / m or more and 5000 N / m or less, and more preferably 1500 N / m or more and 4000 N / m or less. This further enhances the effects described above.

[0135] [2-4] In a release film 10 having a structure in which the first release layer 1, cushion layer 3, and second release layer 2 are laminated as described above, the average thickness Tt is preferably 50 μm or more and 180 μm or less, and more preferably 80 μm or more and 150 μm or less.

[0136] In this embodiment, the release film 10 is composed of a laminate in which a first release layer 1, a cushion layer 3, and a second release layer 2 are stacked in this order. However, the configuration is not limited to this, and the release film may be composed of a laminate that includes an intermediate layer, such as an adhesive layer, placed between the first release layer 1 and the cushion layer 3, and between the second release layer 2 and the cushion layer 3.

[0137] Furthermore, in the third step, if the release film 10 can maintain excellent release properties between the glass cloths 300A and 300B and the release film 10, the second release layer 2 that contacts the glass cloths 300A and 300B may be omitted.

[0138] Furthermore, as mentioned above, in the release film 10 according to the second application example, the interlayer strength between the release layer and the cushion layer 3 is 3.0 N / 25 mm or more. However, in the release film 10 according to the first application example, it is preferable that the interlayer strength between the release layer and the cushion layer 3 is 3.0 N / 25 mm or more, more preferably 4.0 N / 25 mm or more, and even more preferably 5.0 N / 25 mm or more.

[0139] This makes it possible to improve the release properties of the release film 10 according to the first application example. More specifically, for example, after the coverlay film is bonded to the flexible circuit board, the release film can be peeled off the formed flexible printed circuit board with better release properties. As a result, it is possible to more effectively prevent any part of the release film from remaining on the flexible circuit board (for example, on the coverlay film on the flexible circuit board).

[0140] In the release film 10 according to the first application example, if release layers are provided on both sides of the cushion layer 3 shown in Figure 3 (i.e., a first release layer 1 is provided on one side of the cushion layer 3, and a second release layer 2 is provided on the other side of the cushion layer 3), the above-described effects can be obtained, and it is also possible to more effectively prevent the adhesive that has seeped out from the adhesive layer 222 from unintentionally bonding with the release film 10 (release films 10A, 10B). Furthermore, the above-described effects can also be obtained, for example, when the orientation of the release film 10 is changed during use.

[0141] In the release film 10 according to the first application example, it is preferable that the above-described relationship of interlayer strength is satisfied between at least one of the two release layers and the cushion layer 3. More specifically, in the method of using the release film 10 as described above, it is preferable that the above-described relationship is satisfied between the release layer on the side that contacts the CL film 220 (the first release layer 1 in the configuration shown in Figure 2) and the cushion layer 3.

[0142] The release film of the present invention has been described above, but the present invention is not limited thereto.

[0143] For example, in the above embodiment, the case in which the release film of the present invention is applied to a press molding method in which flexible printed circuit boards are stacked in one layer between heat-pressure plates was described. However, the number of stacked flexible printed circuit boards is not limited to one layer, but may be two or more layers.

[0144] Furthermore, although the release film of the present invention is intended to be applied when press-molding is used to press-form a flexible printed circuit board placed between heat-pressure plates, it is not limited to this, and press-molding of the flexible printed circuit board can also be performed, for example, using a roll-to-roll press, or even using a vacuum pressure forming method.

[0145] The present invention will be described in detail below based on examples, but the present invention is not limited thereto.

[0146] [3] Preparation of raw materials The following were prepared as raw materials for manufacturing the release film.

[0147] - Polybutylene terephthalate (manufactured by Mitsubishi Chemical Corporation, "Novaduran 5020") (hereinafter referred to as "hPBT") - Copolymer of polybutylene terephthalate (manufactured by Mitsubishi Chemical Corporation, "Novaduran 5505S") (hereinafter referred to as "coPBT") - Copolymer of polybutylene terephthalate (manufactured by Bell Polyester Products, "P02220") (hereinafter referred to as "C-coPBT") - Polypropylene (manufactured by Sumitomo Chemical Corporation, "FH1016") (hereinafter referred to as "PP") - Low-density polyethylene (manufactured by Sumitomo Chemical Corporation, "FX352", melting point: 70℃, MFR: 4g / 10min) (hereinafter referred to as "LDPE2") - Low-density polyethylene (manufactured by Mitsubishi Chemical Corporation, "KS240T", melting point: 60°C, MFR: 2.2 g / 10 min) (hereinafter referred to as "LDPE4") - Low-density polyethylene (manufactured by Tosoh Corporation, "360", melting point: 106°C, MFR: 1.6 g / 10 min) (hereinafter referred to as "LDPE6") - Ethylene-vinyl acetate copolymer (manufactured by Mitsui Dow Company, "EV360", melting point: 77°C, MFR: 2 g / 10 min) (hereinafter referred to as "EVA") - Ethylene-methyl methacrylate copolymer (manufactured by Sumitomo Chemical Corporation, "WK307", melting point: 80°C, MFR: 7 g / 10 min) (hereinafter referred to as "EMMA1") - Ethylene-methyl methacrylate copolymer (manufactured by Sumitomo Chemical Co., Ltd., "CM-8014", melting point: 79°C, MFR: 4g / 10min) (hereinafter referred to as "EMMA2") - Ethylene-methyl methacrylate copolymer (manufactured by Sumitomo Chemical Co., Ltd., "WH206-F", melting point: 86°C, MFR: 2g / 10min) (hereinafter referred to as "EMMA3") - Ethylene-methyl methacrylate copolymer (manufactured by Sumitomo Chemical Co., Ltd., "WH102", melting point: 90°C, MFR: 0.25g / 10min) (hereinafter referred to as "EMMA4") - Ethylene-methyl methacrylate copolymer (manufactured by Sumitomo Chemical Co., Ltd., "WD106", melting point: 101°C, MFR: 0.32g / 10min) (hereinafter referred to as "EMMA5")

[0148] [4] Production of release film <Example 1> First, a kneaded and ground product of 70 parts by mass of hPBT and 30 parts by mass of coPBT was prepared as a composition for forming the release layer (first release layer and second release layer), and a kneaded and ground product of 35 parts by mass of LDPE2, 40 parts by mass of EMMA4, 15 parts by mass of PP and 10 parts by mass of hPBT was prepared as a composition for forming the cushion layer.

[0149] Next, a first release layer was obtained by forming a film using a release layer composition and an extrusion T-die method.

[0150] Next, the first release layer was subjected to a sequential extrusion T-die method using a composition for forming a cushion layer and a composition for forming a release layer, and each was made into a film. This formed a laminate in which the cushion layer and the second release layer were laminated in this order on the first release layer, resulting in a release film as shown in Figure 3.

[0151] In the obtained release film, the average thickness T1 of the first release layer was 20 μm, the average thickness Tk of the cushion layer was 80 μm, and the average thickness T2 of the second release layer was 20 μm.

[0152] Furthermore, by visually inspecting the surface and then observing the cross-section under a microscope to count only those foreign objects with a longest side of 30 μm or more, the number of foreign objects with a maximum length of 30 μm or more within a 20 cm × 20 cm area was determined to be 12.

[0153] Furthermore, the interlayer strength between the first release layer and the cushion layer, and the interlayer strength between the second release layer and the cushion layer, were both 3.2 N / 25 mm.

[0154] <Examples 2-9, Comparative Examples 1-6> Release films were manufactured in the same manner as in Example 1, except that the compositions for forming the cushion layer and the compositions for forming the release layer were those specified in Table 1.

[0155] Tables 1 to 3 summarize the conditions for the release films of each of the above examples and comparative examples. In the tables, "Number of foreign objects" refers to the number of foreign objects with a maximum length of 30 μm or more within a 20 cm × 20 cm area, determined by visual inspection followed by microscopic observation of the surface and cross-section to count only those foreign objects with a long side of 30 μm or more. The unit is "objects / 20 cm□". In the tables, the unit for melting point is "℃", and the unit for MFR is "g / 10 min". In the tables, "Interlaminar strength" refers to the interlaminar strength between the first release layer and the cushion layer, and the unit is "N / 25 mm".

[0156]

[0157]

[0158]

[0159] [5] Evaluation The release films of each of the above examples and comparative examples were evaluated as follows.

[0160] [5-1] Appearance of FPC The release films of each of the above embodiments and comparative examples 1 to 4 were each 100 mm x 100 mm in size, and a coverlay film (DuPont, "HXC1220") was attached to the flexible circuit board with the adhesive layer of the coverlay film facing the flexible circuit board to form an FPC (laminated body) with a pitch of 50 μm, a width of 50 μm, and a height of 18 μm. Then, the release film was laminated in two layers as shown in Figure 1. For the FPC200, a molding press (KVHCIII, manufactured by Kitagawa Seiki Co., Ltd.) was used to press the FPC200, raising the temperature from room temperature to 180°C in 20 mins, maintaining the temperature at 180°C for 20 mins, and then lowering the temperature from 180°C to room temperature in 20 mins, with a press pressure of 3 MPa. After that, the release film was peeled off by holding one end, and the presence or absence of transfer of surface roughness from the release film and the presence or absence of transfer of unevenness due to foreign matter were visually observed on the surface of the flexible circuit board exposed in the recesses, and evaluated according to the following criteria.

[0161] [Evaluation Criteria] ○: No problems with appearance. ×: Significant indentations were observed on the surface of the FPC, resulting in a poor appearance.

[0162] [5-2] Surface roughness The first release layer of each of the above examples and comparative examples 1 to 4 was measured using a surface roughness measuring device (Mitutoyo Corporation, "SURFTST SJ-210") on the surface exposed on the side opposite to the cushion layer.

[0163] [5-3] Embedding properties of release film For each of the above examples and comparative examples, the release film was set to 100 mm x 100 mm, and an FPC 200 (laminated) was formed by attaching a 200 μm thick coverlay film (manufactured by Arisawa Seisakusho Co., Ltd., "CMA0525") with a φ4 mm hole to a flexible circuit board with the adhesive layer of the coverlay film facing the flexible circuit board. Then, as shown in Figure 1, the release film was pressed into the two-layered FPC using a molding press (manufactured by Kitagawa Seiki Co., Ltd., "KVHCIII") under the conditions of raising the temperature from room temperature to 100°C in 20 mins, holding at 100°C for 20 mins, and lowering the temperature from 100°C to room temperature in 20 mins, with a press pressure of 3 MPa. After that, the embedding height of the release film corresponding to the 4 mmφ hole was measured when the release film was peeled off by holding one end of the release film. The higher this number, the better the embedding performance.

[0164] [5-4] Release properties of release film The release films for each of the above examples and comparative examples were each 100 mm x 100 mm in size. A coverlay film (DuPont, "HXC1220") was attached to a flexible circuit board with the adhesive layer of the coverlay film facing the flexible circuit board to form an FPC (laminated product) with a pitch of 50 μm, a width of 50 μm, and a height of 18 μm. Then, as shown in Figure 1, the release film was pressed onto the FPC 200, which was laminated in two layers, using a molding press (Kitagawa Seiki Co., Ltd., "KVHCIII"), under the conditions of raising the temperature from room temperature to 180°C in 20 mins, holding at 180°C for 20 mins, and then lowering the temperature from 180°C to room temperature in 20 mins, with a press pressure of 3 MPa. After that, one end of the release film was held and peeled off. The ease of peeling off the release film (release properties) was evaluated according to the following criteria.

[0165] [Evaluation Criteria] ○: Can be peeled off when removing the release film. ×: Difficult to peel off when removing the release film due to fusion of cushion layers or stretching or tearing of the release film.

[0166] [5-5] Protrusion of the cushion layer For each of the above examples and comparative examples, the release film was set to 100 mm x 100 mm, and this release film was pressed using a molding press (Kitagawa Seiki Co., Ltd., "KVHCIII") under the following conditions: heating from room temperature to 180°C in 20 mins, holding at 180°C for 20 mins, cooling from 180°C to room temperature in 20 mins, and pressing pressure of 3 MPa. After that, the maximum distance of the cushion layer that protruded from the release layer of the film was measured with a caliper on each of the four sides, and the average value was calculated.

[0167] The evaluation results described above are summarized in Tables 4 and 5. In the tables, the unit for evaluating surface roughness is "μm", the unit for evaluating embedding ability (maximum seepage) is "mm", and the unit for evaluating cushion layer overhang is "mm".

[0168]

[0169]

[0170] As shown in Tables 4 and 5, excellent results were obtained in each of the above embodiments, whereas satisfactory results were not obtained in any of the above comparative examples.

[0171] According to the present invention, it is possible to provide a release film that can effectively prevent unintended recesses from forming in the manufactured flexible printed circuit board when used in a heat press to adhere a coverlay film to a flexible circuit board with exposed circuits, or to provide a release film with excellent release properties. Therefore, the present invention has industrial applicability.

[0172] 1 First release layer 2 Second release layer 3 Cushion layer 10 Release film 10A Release film 10B Release film 200 Flexible printed circuit board (FPC) 210 Flexible circuit board 220 Coverlay film (CL film) 221 Coverlay 222 Adhesive layer 223 Recess 300 Glass cloth 300A Glass cloth 300B Glass cloth 521 Heat-pressed board T1 Average thickness of the first release layer T2 Average thickness of the second release layer Tk Average thickness of the cushion layer Tt Average thickness of the release film

Claims

1. A release film having a release layer and a cushion layer, wherein the release layer is made of a material containing a polybutylene terephthalate resin, and the number of foreign objects with a maximum length of 30 μm or more within a 20 cm x 20 cm area of ​​the cushion layer is less than 20.

2. The release film according to claim 1, wherein the cushion layer is made of a material comprising a first resin component having a melting point of 60°C or more and 90°C or less, and a second resin component having a melting point that is 10°C or more and 50°C or less higher than the melting point of the first resin component.

3. A release film having a release layer and a cushion layer, wherein the cushion layer is made of a material containing a first resin component having a melting point of 60°C or more and 90°C or less, and a second resin component having a melting point that is 10°C or more and 50°C or less higher than the melting point of the first resin component, and the interlayer strength between the release layer and the cushion layer is 3.0 N / 25 mm or more.

4. The release film according to claim 3, wherein the release layer is made of a material containing a polybutylene terephthalate resin.

5. The release film according to claim 1 or 4, wherein the release layer comprises polybutylene terephthalate as the polybutylene terephthalate resin, and further comprises at least one of the following: polybutylene terephthalate / polytetramethylene glycol block copolymer, terephthalic acid, and a polybutylene terephthalate copolymer containing a dicarboxylic acid component other than terephthalic acid as constituent monomers.

6. The release film according to claim 2 or 3, wherein when the content of the first resin component in the cushion layer is X1 [mass%] and the content of the second resin component is X2 [mass%], the relationship 0.80 ≤ X2 / X1 ≤ 1.45 is satisfied.

7. The release film according to claim 2 or 3, wherein the MFR of the first resin component is 2.0 g / 10 min or more and 8.0 g / 10 min or less.

8. The release film according to claim 2 or 3, wherein the cushion layer is further composed of a material containing a polybutylene terephthalate resin as a third resin component.

9. The release film according to claim 8, wherein when the content of the first resin component in the cushion layer is X1 [mass%] and the content of the third resin component is X3 [mass%], the relationship 0.25 ≤ X3 / X1 ≤ 0.90 is satisfied.

10. The release film according to claim 8, wherein when the content of the second resin component in the cushion layer is X2 [mass%] and the content of the third resin component is X3 [mass%], the relationship 0.20 ≤ X3 / X2 ≤ 1.10 is satisfied.

11. The release film according to claim 1 or 3, wherein the cushion layer is made of a material containing at least one of ethylene-methyl methacrylate copolymer, ethylene-vinyl acetate copolymer, ethylene-methyl acrylate copolymer, and ethylene-α-olefin copolymer.

12. The release film according to claim 1 or 3, wherein the average thickness of the cushion layer is 10 μm or more and 200 μm or less.

13. The release film according to claim 1 or 3, wherein the release layer is provided on both sides of the cushion layer.

14. The release film according to claim 1 or 3, wherein the average thickness of the release layer is 5 μm or more and 35 μm or less.

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