JP7764377B2 - Multi-layer polyimide film - Google Patents

Description

The present invention relates to a multilayer polyimide film.

In recent years, demand for flexible printed circuit boards (hereinafter sometimes referred to as "FPCs") has grown in line with the expansion of demand for electronic products, particularly smartphones, tablet PCs, and laptops. In particular, demand for flexible printed circuit boards made from multilayer polyimide films containing a thermoplastic polyimide layer as an adhesive layer is expected to grow further due to their excellent heat resistance and flexibility. Furthermore, with the trend toward lighter, smaller, and thinner electronic devices in recent years, there remains a strong demand for finer FPC wiring.

When fabricating double-sided and multilayer FPCs, metal-clad laminates, consisting of copper or other metal foils laminated to both sides of a polyimide film, are typically used. The first step in FPC manufacturing is drilling holes (hereafter sometimes referred to as "vias") to allow electrical connection between layers. Plating the inner walls of the vias provides electrical continuity between both sides of the wiring board. Via formation processes include the through-hole method, in which a drill or laser is used to drill through the metal foil and insulating layer (polyimide layer) on both sides, and the blind via method, in which the metal foil and insulating layer on one side are cut away with a laser or other tool, leaving the metal foil on the other side. The blind via method is frequently used, particularly for micro-FPCs, to efficiently utilize the surface area.

Conventionally, in this type of via formation process, wet desmearing is performed by treating the laminate with an alkaline potassium permanganate aqueous solution under heating to clean the interior of the hole and the surface of the metal foil after drilling and to remove any resin residue. Polyimide is already susceptible to hydrolysis under alkaline conditions, and laser processing generates residual stress due to localized heating, making the desmearing process after the via formation process prone to defects such as cracks on the inner walls of the vias. Patent Document 1 describes a method of adding a heat treatment process between laser processing and desmearing to remove the residual stress caused by laser processing and suppress the occurrence of defects. Patent Document 2 discloses polyimide that is resistant to alkaline solutions used in the development, etching, and resist stripping processes.

JP 2012-186377 A JP 2017-179148 A

Cracks that occur on the inner walls of vias due to desmearing after laser processing can deform the plated area and reduce connection reliability in subsequent plating processes, or allow chemicals to penetrate into the cracks, reducing insulation reliability, thereby adversely affecting quality. Cracks are more likely to occur when blind vias are formed than when through-holes are formed. Furthermore, the inventors' studies have found that cracks do not occur when the metal foil is removed by etching or other methods without desmearing after laser processing, or when desmearing is performed after the metal foil has been removed without laser processing. Furthermore, the inventors' studies have found that cracks are more likely to occur when the swelling and roughening times are extended during desmearing after laser processing.

As a method for suppressing cracking, adding a heat treatment process between laser processing and desmearing, as disclosed in Patent Document 1, reduces wiring board productivity due to the additional heat treatment process. Furthermore, the method described in Patent Document 1 leaves room for improvement in suppressing cracking on the via inner walls.

Furthermore, while the method described in Patent Document 2 can prevent film tearing in an alkaline environment, there is still room for improvement in preventing cracks from occurring on the inner walls of vias.

The present invention was made in consideration of these problems, and its purpose is to provide a multilayer polyimide film that can suppress the occurrence of cracks on the inner walls of vias during desmearing after laser processing.

To prevent cracks from forming on the via inner walls during desmearing after laser processing, it is important to relieve the stress generated in the polyimide film during laser processing. While using polyimides with a high degree of flexible backbone facilitates stress relief, such polyimides have a much higher linear expansion coefficient than the metal to which they are attached, leading to warping and wrinkling when attached to metal foil. After extensive research, the inventors discovered that by using a polyimide with a specific structure as the non-thermoplastic polyimide core material for the multilayer polyimide film, it is possible to relieve the stress generated in the polyimide film during laser processing while maintaining a linear expansion coefficient similar to that of the metal.

The multilayer polyimide film according to the present invention comprises a non-thermoplastic polyimide layer and a thermoplastic polyimide layer disposed on at least one side of the non-thermoplastic polyimide layer. The non-thermoplastic polyimide contained in the non-thermoplastic polyimide layer comprises tetracarboxylic dianhydride residues and diamine residues. The diamine residues include diamine residues having a biphenyl skeleton, 4,4'-diaminodiphenyl ether residues, and p-phenylenediamine residues. The content of the diamine residues having a biphenyl skeleton is 20 mol% or more and 35 mol% or less of all diamine residues constituting the non-thermoplastic polyimide.

In one embodiment of the multilayer polyimide film, the diamine residue having a biphenyl skeleton is a 4,4'-diamino-2,2'-dimethylbiphenyl residue.

In one embodiment of the multilayer polyimide film, the content of the 4,4'-diaminodiphenyl ether residue is 40 mol% or more and 70 mol% or less of all diamine residues constituting the non-thermoplastic polyimide.

In one embodiment of the multilayer polyimide film, the content of the p-phenylenediamine residues is 5 mol% or more and 50 mol% or less of all diamine residues constituting the non-thermoplastic polyimide.

In one embodiment of the multilayer polyimide film, the tetracarboxylic dianhydride residues include one or more selected from the group consisting of 3,3',4,4'-biphenyltetracarboxylic dianhydride residues and pyromellitic dianhydride residues.

In one embodiment of the multilayer polyimide film, the tetracarboxylic dianhydride residues further contain 4,4'-oxydiphthalic anhydride residues.

In one embodiment of the multilayer polyimide film, the content of the 4,4'-oxydiphthalic anhydride residues is 5 mol% or more and 15 mol% or less of the total tetracarboxylic dianhydride residues constituting the non-thermoplastic polyimide.

In one embodiment of the multilayer polyimide film, the thermoplastic polyimide contained in the thermoplastic polyimide layer has one or more selected from the group consisting of 3,3',4,4'-biphenyltetracarboxylic dianhydride residues and pyromellitic dianhydride residues, and 2,2-bis[4-(4-aminophenoxy)phenyl]propane residues.

In one embodiment of the multilayer polyimide film, the storage modulus of the non-thermoplastic polyimide layer at a temperature of 380°C is less than 0.350 GPa.

In one embodiment of the multilayer polyimide film, the non-thermoplastic polyimide layer has a coefficient of linear expansion at elevated temperatures of 100°C to 200°C of 5.0 ppm/K or more and 19.0 ppm/K or less.

The multilayer polyimide film of the present invention can suppress the occurrence of cracks on the inner walls of vias during desmearing after laser processing without increasing the number of steps in the wiring board manufacturing process.

1 is a cross-sectional view showing an example of a multilayer polyimide film according to the present invention. FIG. 1 is a cross-sectional view showing a metal-clad laminate obtained using an example of the multilayer polyimide film according to the present invention. 1 is an example of a polarizing microscope image used for determining the hole crack test. 10 is another example of a polarizing microscope image used for determining the hole crack test. 10 is another example of a polarizing microscope image used for determining the hole crack test.

Preferred embodiments of the present invention are described in detail below, but the present invention is not limited to these. Furthermore, all academic and patent literature cited in this specification is incorporated herein by reference.

First, let us explain the terminology used in this specification. "Polyimide" is a polymer whose repeating units are structural units represented by the following general formula (1):

In general formula (1), X represents a tetracarboxylic dianhydride residue (a tetravalent organic group derived from a tetracarboxylic dianhydride), and Y represents a diamine residue (a divalent organic group derived from a diamine).

"Biphenyl skeleton" refers to a skeleton with a two-ring structure in which two benzene rings are linked by a single bond. Therefore, diamine residues with a biphenyl skeleton do not include diamine residues with fused rings, such as 9,9-bis(4-aminophenyl)fluorene residues.

Unless otherwise specified, "linear expansion coefficient" refers to the linear expansion coefficient at elevated temperatures from 100°C to 200°C.

"Non-thermoplastic polyimide" refers to polyimide that, when fixed in film form to a metal frame and heated at 450°C for 2 minutes, does not wrinkle or stretch and retains its film shape (flat membrane shape). "Thermoplastic polyimide" refers to polyimide that, when fixed in film form to a metal frame and heated at 450°C for 2 minutes, does not retain its film shape.

The "main surface" of a layered material (more specifically, a non-thermoplastic polyimide layer, a thermoplastic polyimide layer, etc.) refers to the surface perpendicular to the thickness direction of the layered material.

In the following, the compound name may be followed by "system" to refer to the compound and its derivatives collectively.

<Multi-layer polyimide film>
The multilayer polyimide film according to this embodiment comprises a non-thermoplastic polyimide layer and a thermoplastic polyimide layer disposed on at least one surface (one of the main surfaces) of the non-thermoplastic polyimide layer. The non-thermoplastic polyimide contained in the non-thermoplastic polyimide layer comprises a tetracarboxylic dianhydride residue and a diamine residue. The diamine residue comprises a diamine residue having a biphenyl skeleton (a residue derived from a diamine having a biphenyl skeleton), a 4,4'-diaminodiphenyl ether residue, and a p-phenylenediamine residue. The content of the diamine residue having a biphenyl skeleton is preferably 20 mol% or more and 35 mol% or less of all diamine residues constituting the non-thermoplastic polyimide.

Hereinafter, tetracarboxylic dianhydride may be referred to as "acid dianhydride." Diamines having a biphenyl skeleton may be referred to as "BPDI." 4,4'-diaminodiphenyl ether may be referred to as "ODA." p-phenylenediamine may be referred to as "PDA." Furthermore, non-thermoplastic polyimides contained in non-thermoplastic polyimide layers may be referred to simply as "non-thermoplastic polyimides." Thermoplastic polyimides contained in thermoplastic polyimide layers may be referred to simply as "thermoplastic polyimides."

The inventors conducted extensive research into the molecular design of polyimides that can alleviate stress generated in films during laser processing while maintaining the heat resistance (linear expansion coefficient, etc.) required for use in metal-clad laminates. As a result, the inventors discovered that by optimizing the structure of the non-thermoplastic polyimide contained in the multilayer polyimide film, it is possible to suppress the occurrence of cracks on the inner walls of vias during desmearing after laser processing, without making major changes to the wiring board manufacturing process.

[Configuration of multilayer polyimide film]
The structure of the multilayer polyimide film according to the present embodiment will be described below with reference to the drawings. The drawings are primarily schematic illustrations of the individual components for ease of understanding, and the size, number, shape, etc. of the individual components shown may differ from the actual size, number, shape, etc. of the individual components due to the convenience of drawing. Furthermore, in the present specification, for the convenience of explanation, the same components as those in the previously described drawings may be designated by the same reference numerals and their description may be omitted.

Figure 1 is a cross-sectional view showing an example of a multilayer polyimide film according to this embodiment. As shown in Figure 1, the multilayer polyimide film 10 has a non-thermoplastic polyimide layer 11 and a thermoplastic polyimide layer 12 disposed on at least one side of the non-thermoplastic polyimide layer 11. The non-thermoplastic polyimide contained in the non-thermoplastic polyimide layer 11 has tetracarboxylic dianhydride residues and diamine residues. The diamine residues include BPDI residues, ODA residues, and PDA residues. The content of BPDI residues is preferably 20 mol% or more and 35 mol% or less of all diamine residues constituting the non-thermoplastic polyimide contained in the non-thermoplastic polyimide layer 11.

The multilayer polyimide film 10 can suppress the occurrence of cracks on the inner walls of vias during desmearing after laser processing. The reason for this is presumed to be as follows: In the multilayer polyimide film 10, the non-thermoplastic polyimide contained in the non-thermoplastic polyimide layer 11 has a specific range of BPDI residue content, which contains a backbone with a high degree of molecular chain free rotation. Furthermore, the non-thermoplastic polyimide contained in the non-thermoplastic polyimide layer 11 has ODA residues with a bent structure that contributes to the flexibility of the multilayer polyimide film 10, and PDA residues with a rigid structure that contributes to the heat resistance of the multilayer polyimide film 10. As a result, the multilayer polyimide film 10 can relieve stress generated in the film during laser processing while maintaining the heat resistance (linear expansion coefficient, etc.) required for use in metal-clad laminates. Therefore, the multilayer polyimide film 10 can suppress the occurrence of cracks on the inner walls of vias during desmearing after laser processing. To more effectively suppress cracking in the via inner wall, the benzene ring in the BPDI residue preferably has a substituent, more preferably an alkyl group, and even more preferably a methyl group. If the benzene ring in the BPDI residue has a substituent, the symmetry of the primary structure of the polyimide is reduced, which inhibits the packing of polymer chains and further relieves the stress generated in the film during laser processing.

While the multilayer polyimide film 10 shown in FIG. 1 has a thermoplastic polyimide layer 12 on only one side of the non-thermoplastic polyimide layer 11, a thermoplastic polyimide layer 12 may be provided on both sides (both main surfaces) of the non-thermoplastic polyimide layer 11. When a thermoplastic polyimide layer 12 is provided on both sides of the non-thermoplastic polyimide layer 11, the two thermoplastic polyimide layers 12 may contain the same type of thermoplastic polyimide or different types of thermoplastic polyimide. The thicknesses of the two thermoplastic polyimide layers 12 may be the same or different. In the present invention, two or more layers of both the non-thermoplastic polyimide layer 11 and the thermoplastic polyimide layer 12 may be provided. In the following description, "multilayer polyimide film 10" includes a film having a thermoplastic polyimide layer 12 on only one side of the non-thermoplastic polyimide layer 11, a film having a thermoplastic polyimide layer 12 on both sides of the non-thermoplastic polyimide layer 11, and a film having two or more layers of both the non-thermoplastic polyimide layer 11 and the thermoplastic polyimide layer 12.

The thickness of the multilayer polyimide film 10 (total thickness of each layer) is, for example, 6 μm or more and 60 μm or less. The thinner the multilayer polyimide film 10, the easier it is to reduce the weight of the resulting FPC and improve the foldability of the resulting FPC. To make it easier to reduce the weight of the FPC while maintaining mechanical strength and to further improve the foldability of the FPC, the thickness of the multilayer polyimide film 10 is preferably 7 μm or more and 30 μm or less, and more preferably 10 μm or more and 25 μm or less. The thickness of the multilayer polyimide film 10 can be measured using a laser hologram.

To easily achieve a thinner FPC while ensuring adhesion to the metal foil, the thickness of the thermoplastic polyimide layer 12 (or the thickness of each thermoplastic polyimide layer 12 if two or more thermoplastic polyimide layers 12 are provided) is preferably 1 μm or more and 15 μm or less. Furthermore, to easily adjust the linear expansion coefficient of the multilayer polyimide film 10, the thickness ratio of the non-thermoplastic polyimide layer 11 to the thermoplastic polyimide layer 12 (thickness of non-thermoplastic polyimide layer 11 / thickness of thermoplastic polyimide layer 12) is preferably 55/45 or more and 95/5 or less. When multiple non-thermoplastic polyimide layers 11 and multiple thermoplastic polyimide layers 12 are provided, the above thickness ratio refers to the ratio of their total thicknesses. Even if the number of thermoplastic polyimide layers 12 is increased, it is preferable that the total thickness of the thermoplastic polyimide layers 12 does not exceed the total thickness of the non-thermoplastic polyimide layers 11.

To suppress warping of the multilayer polyimide film 10, it is preferable that a thermoplastic polyimide layer 12 be provided on both sides of the non-thermoplastic polyimide layer 11. It is more preferable that a thermoplastic polyimide layer 12 containing the same type of thermoplastic polyimide be provided on both sides of the non-thermoplastic polyimide layer 11. When a thermoplastic polyimide layer 12 is provided on both sides of the non-thermoplastic polyimide layer 11, it is preferable that the thicknesses of the two thermoplastic polyimide layers 12 be the same in order to suppress warping of the multilayer polyimide film 10. Even if the thicknesses of the two thermoplastic polyimide layers 12 are different from each other, warping of the multilayer polyimide film 10 can be suppressed as long as the thickness of the other thermoplastic polyimide layer 12 is in the range of 40% to less than 100% of the thickness of the thicker thermoplastic polyimide layer 12.

To further suppress the occurrence of cracks on the via inner walls during desmearing after laser processing, the storage modulus of the non-thermoplastic polyimide layer 11 at a temperature of 380°C is preferably less than 0.350 GPa, and more preferably less than 0.200 GPa. Furthermore, from the perspective of improving the mechanical strength of the multilayer polyimide film 10 at high temperatures, the storage modulus is preferably 0.010 GPa or greater, and more preferably 0.050 GPa or greater. The storage modulus can be adjusted, for example, by changing the content of BPDI residues. The storage modulus is measured by the same method as in the examples described below or a method equivalent thereto.

When dynamic viscoelasticity measurements are performed on the non-thermoplastic polyimide layer 11, the temperature at which the inflection point of the storage modulus is measured is preferably in the range of 270°C to 340°C, and more preferably in the range of 280°C to 330°C, from the viewpoints of stress relaxation during laser processing and thermal stress relaxation when laminating metal foil. If the temperature at which the inflection point of the storage modulus is measured is within this range, dimensional change at the temperature (e.g., 250°C) at which dimensional change after heating of the flexible metal-clad laminate is evaluated can be suppressed. On the other hand, if the temperature at which the inflection point of the storage modulus is measured is low, the stress generated within the multilayer polyimide film 10 during cooling after laser processing will be reduced.

The linear expansion coefficient of the non-thermoplastic polyimide layer 11 is preferably 5.0 ppm/K to 19.0 ppm/K, more preferably 8.0 ppm/K to 15.0 ppm/K, and even more preferably 9.0 ppm/K to 12.0 ppm/K. If the linear expansion coefficient of the non-thermoplastic polyimide layer 11 is 5.0 ppm/K to 19.0 ppm/K, the linear expansion coefficient of the multilayer polyimide film 10 can be adjusted to, for example, 14.0 ppm/K to 22.0 ppm/K, which is closer to that of copper foil, and preferably 16.0 ppm/K to 20.0 ppm/K, which is even closer to that of copper foil. This reduces the stress generated within the multilayer polyimide film 10 during cooling after laser processing, thereby further suppressing the occurrence of cracks on the via inner walls during desmearing after laser processing. The linear expansion coefficient can be adjusted, for example, by changing the content of residues derived from monomers having a rigid structure (more specifically, PDA residues, etc.) and the content of residues derived from monomers having a bent structure (more specifically, ODA residues, etc.) The linear expansion coefficient can be measured by the same method as in the Examples described below or a method equivalent thereto.

The non-thermoplastic polyimide layer 11 preferably has a slope of 2.0 or greater in the plastic deformation region of its stress-strain curve. When the non-thermoplastic polyimide layer 11 is resistant to plastic deformation and has high yield strength, it exhibits high resistance to tearing in alkaline environments. The plastic deformation region refers to the strain region after the yield point in the stress-strain curve of a polyimide film tensile test. The "resistance to plastic deformation" characteristic refers to a significant increase in stress in the plastic deformation region, or the large stress required for plastic deformation. The "resistance to plastic deformation" characteristic can be measured, for example, by the slope of the plastic deformation region. The slope of the plastic deformation region is the slope of the s-s curve in the plastic deformation region of a graph of the results of measuring tensile properties according to ASTM D882, with the vertical axis representing "stress (unit: MPa)" and the horizontal axis representing "strain (unit: mm)." The slope of the s-s curve in the plastic deformation region can be calculated using the following formula: In the following formula, stress1 is the stress at 10% strain, stress2 is the breaking stress, strain1 is the 10% strain, and strain2 is the breaking strain.

Slope of the s-s curve in the plastic deformation region = (stress2 - stress1) / (strain2 - strain1)

The slope of the plastic deformation region of the non-thermoplastic polyimide layer 11 is preferably 2.0 or greater, more preferably 2.2 or greater, and even more preferably 2.5 or greater. When the slope of the plastic deformation region is 2.0 or greater, a cohesive structure with a high degree of packing of polymer chains is formed, which can suppress the occurrence of cracks even during the continuous FPC processing process. While a higher slope of the plastic deformation region is preferable, in order to suppress the occurrence of springback, etc., the slope of the plastic deformation region is preferably 4.5 or less, and more preferably 4.0 or less.

When manufacturing a metal-clad laminate using the multilayer polyimide film 10, a metal foil 13 is laminated to at least one surface of the multilayer polyimide film 10 (for example, the surface 12a of the thermoplastic polyimide layer 12 in the case of Figure 1). This results in the metal-clad laminate 20 shown in Figure 2. The method for laminating the metal foil 13 to the surface 12a of the thermoplastic polyimide layer 12 is not particularly limited, and various known methods can be used. For example, a continuous processing method using a hot roll laminating device having one or more pairs of metal rolls or a double belt press (DBP) can be used. The specific configuration of the means for performing hot roll lamination is not particularly limited, but it is preferable to place a protective material between the pressure surface and the metal foil 13 to improve the appearance of the resulting multilayer polyimide film 10.

When a thermoplastic polyimide layer 12 is provided on both sides of a non-thermoplastic polyimide layer 11, a double-sided metal-clad laminate (not shown) can be obtained by laminating metal foil 13 to both sides of the multilayer polyimide film 10.

[Elements of multilayer polyimide film]
Next, the elements (constituent elements) of the multilayer polyimide film according to this embodiment will be described in detail.

(Non-thermoplastic polyimide layer)
The non-thermoplastic polyimide contained in the non-thermoplastic polyimide layer has BPDI residues, ODA residues, and PDA residues as diamine residues. In order to further suppress the occurrence of cracks on the inner wall of the via during desmearing after laser processing, the total content of the BPDI residues, ODA residues, and PDA residues relative to all diamine residues constituting the non-thermoplastic polyimide is preferably 50 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, even more preferably 90 mol% or more, and may even be 100 mol%.

Examples of diamines (monomers) for forming BPDI residues include 4,4'-diamino-2,2'-dimethylbiphenyl (hereinafter sometimes referred to as "m-TB"), 4,4'-diaminobiphenyl, 4,4'-diamino-3,3'-dimethylbiphenyl, 4,4'-diamino-2,2'-dimethoxybiphenyl, 4,4'-diamino-3,3'-dimethoxybiphenyl, 3,3',5,5'-tetramethylbenzidine, and 4,4'-bis(4-aminophenoxy)biphenyl. In this embodiment, one or more diamines can be used as the diamine for forming the BPDI residue. To further suppress the occurrence of cracks on the via inner wall during desmearing after laser processing, m-TB is preferred as the diamine (monomer) for forming the BPDI residue. In other words, m-TB residues are preferred as the BPDI residue.

To further suppress the occurrence of cracks on the inner wall of a via during desmearing after laser processing while maintaining the linear expansion coefficient, the content of ODA residues relative to all diamine residues constituting the non-thermoplastic polyimide is preferably 40 mol% to 70 mol%, more preferably 45 mol% to 65 mol%, and even more preferably 50 mol% to 65 mol%. To further suppress the occurrence of cracks on the inner wall of a via during desmearing after laser processing while maintaining the linear expansion coefficient, the content of PDA residues relative to all diamine residues constituting the non-thermoplastic polyimide is preferably 5 mol% to 50 mol%, more preferably 10 mol% to 40 mol%, and even more preferably 15 mol% to 30 mol%.

The non-thermoplastic polyimide may contain diamine residues (other diamine residues) other than BPDI residues, ODA residues, and PDA residues. The diamine (monomer) used to form the other diamine residues is preferably an aromatic diamine with high heat resistance. Specific examples of diamines for forming other diamine residues include 1,3-bis(4-aminophenoxy)benzene, 1,4-bis(4-aminophenoxy)benzene, 4,4'-diaminodiphenylpropane, 4,4'-diaminodiphenylmethane, 4,4'-diaminodiphenyl sulfide, 3,3'-diaminodiphenyl sulfone, 4,4'-diaminodiphenyl sulfone, 3,3'-diaminodiphenyl ether, 3,4'-diaminodiphenyl ether, 1,5-diaminonaphthalene, 4,4'-diaminodiphenyldiethylsilane, 4,4'-diaminodiphenylsilane, 4,4'-diaminodiphenylethylphosphine oxide, 4,4'-diaminodiphenyl N-methylamine, 4,4'-diaminodiphenyl N-phenylamine, 1,3-diaminobenzene, and 1,2-diaminobenzene.

Non-thermoplastic polyimides contain diamine residues as well as acid dianhydride residues. From the perspective of improving heat resistance, aromatic acid dianhydrides are preferred as the acid dianhydride (monomer) used to form the acid dianhydride residues. Furthermore, to further suppress the occurrence of cracks on the via inner walls during desmearing after laser processing, acid dianhydrides (monomers) used to form the acid dianhydride residues are preferred to be those with a biphenyl skeleton. Specific examples of acid dianhydrides (monomers) for forming the acid dianhydride residue include pyromellitic dianhydride (hereinafter sometimes referred to as "PMDA"), 3,3',4,4'-biphenyltetracarboxylic dianhydride (hereinafter sometimes referred to as "BPDA"), 2,3,6,7-naphthalenetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, 2,2',3,3'-biphenyltetracarboxylic dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride (hereinafter sometimes referred to as "BTDA"), 2,2',3,3'-benzophenonetetracarboxylic dianhydride, 4,4'-oxydiphthalic anhydride (hereinafter sometimes referred to as "ODPA"), 3,4'-oxydiphthalic anhydride, 2,2-bis(3,4-dicarboxyphenyl)propane dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl)propane dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride, bis(2,3-dicarboxyphenyl)methane dianhydride, bis(3,4-dicarboxyphenyl)ethane dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, p-phenylene bis(trimellitic acid monoester acid anhydride), ethylene bis(trimellitic acid monoester acid anhydride), bisphenol A bis(trimellitic acid monoester acid anhydride), and derivatives thereof.

From the viewpoint of maintaining the linear expansion coefficient, the acid dianhydride residue is preferably one or more selected from the group consisting of BPDA residues and PMDA residues. Furthermore, in order to further suppress the occurrence of cracks on the inner wall of a via during desmearing after laser processing, the acid dianhydride residue is preferably a BPDA residue having a biphenyl skeleton. When the non-thermoplastic polyimide contains BPDA residues, in order to further suppress the occurrence of cracks on the inner wall of a via during desmearing after laser processing while maintaining the linear expansion coefficient, the content of BPDA residues relative to all acid dianhydride residues constituting the non-thermoplastic polyimide is preferably 10 mol% to 60 mol%, more preferably 20 mol% to 60 mol%, and even more preferably 30 mol% to 60 mol%. When the non-thermoplastic polyimide contains PMDA residues, from the viewpoint of maintaining the linear expansion coefficient, the content of PMDA residues relative to all acid dianhydride residues constituting the non-thermoplastic polyimide is preferably 40 mol% or more and 80 mol% or less, more preferably 40 mol% or more and 75 mol% or less, and even more preferably 40 mol% or more and 70 mol% or less. When the non-thermoplastic polyimide contains BPDA residues and PMDA residues, in order to further suppress the occurrence of cracks on the inner wall of a via during desmearing after laser processing while maintaining the linear expansion coefficient, the total content of BPDA residues and PMDA residues relative to all acid dianhydride residues constituting the non-thermoplastic polyimide is preferably 60 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, and may be 100 mol%.

In order to further suppress the occurrence of cracks on the inner wall of a via during desmear treatment after laser processing, it is preferable that the non-thermoplastic polyimide contain, as acid dianhydride residues, one or more selected from the group consisting of BPDA residues and PMDA residues, and an ODPA residue. When the non-thermoplastic polyimide contains ODPA residues, in order to further suppress the occurrence of cracks on the inner wall of a via during desmear treatment after laser processing, it is preferable that the content of ODPA residues relative to all acid dianhydride residues constituting the non-thermoplastic polyimide be 5 mol% or more and 15 mol% or less. When the non-thermoplastic polyimide contains one or more residues selected from the group consisting of BPDA residues and PMDA residues, and an ODPA residue, in order to further suppress the occurrence of cracks on the inner wall of a via during desmearing after laser processing while maintaining the linear expansion coefficient, the total content of the BPDA residues, PMDA residues, and ODPA residues is preferably 80 mol % or more, more preferably 90 mol % or more, and may be 100 mol %, based on the total acid dianhydride residues constituting the non-thermoplastic polyimide.

To further suppress the occurrence of cracks on the via inner walls during desmearing after laser processing while maintaining a good surface appearance of the metal-clad laminate, it is preferable that the non-thermoplastic polyimide have a segment whose repeating unit is the structural unit represented by the following chemical formula (2). Note that, in this specification, "segment" refers to a polymer chain formed from the same repeating unit that constitutes a block copolymer. Also, in this specification, "block copolymer" includes all aspects of a pure block copolymer, a random block copolymer, and a copolymer having a tapered block structure.

A segment having the structural unit represented by chemical formula (2) as a repeating unit (hereinafter referred to as a "specific segment") can be formed, for example, by sequence polymerization, as described below.

The non-thermoplastic polyimide layer may contain components (additives) other than the non-thermoplastic polyimide. Examples of additives that can be used include dyes, surfactants, leveling agents, plasticizers, silicones, fillers, and sensitizers. The content of non-thermoplastic polyimide in the non-thermoplastic polyimide layer is, for example, 70% by weight or more, preferably 80% by weight or more, more preferably 90% by weight or more, and may be 100% by weight, based on the total weight of the non-thermoplastic polyimide layer.

(Thermoplastic polyimide layer)
The thermoplastic polyimide contained in the thermoplastic polyimide layer has an acid dianhydride residue and a diamine residue. The acid dianhydride (monomer) for forming the acid dianhydride residue in the thermoplastic polyimide can be the same compound as the acid dianhydride (monomer) for forming the acid dianhydride residue in the non-thermoplastic polyimide. The acid dianhydride residue in the thermoplastic polyimide and the acid dianhydride residue in the non-thermoplastic polyimide can be the same or different.

To ensure thermoplasticity, the diamine residues contained in the thermoplastic polyimide preferably have a bent structure. To more easily ensure thermoplasticity, the content of the bent structure diamine residues is preferably 50 mol% or more, more preferably 70 mol% or more, and even more preferably 80 mol% or more, and may even be 100 mol%, based on the total diamine residues constituting the thermoplastic polyimide. Examples of diamines (monomers) for forming the bent structure diamine residues include 4,4'-bis(4-aminophenoxy)biphenyl, 4,4'-bis(3-aminophenoxy)biphenyl, 1,3-bis(3-aminophenoxy)benzene, 1,3-bis(4-aminophenoxy)benzene, and 2,2-bis[4-(4-aminophenoxy)phenyl]propane (hereinafter sometimes referred to as "BAPP"). To more easily ensure thermoplasticity, the diamine residues contained in the thermoplastic polyimide preferably have a BAPP residue.

In order to obtain a thermoplastic polyimide layer that has excellent adhesion to metal foil, it is preferable that the thermoplastic polyimide contains one or more residues selected from the group consisting of BPDA residues and PMDA residues, and BAPP residues.

The thermoplastic polyimide layer may contain components (additives) other than the thermoplastic polyimide. Examples of additives that can be used include dyes, surfactants, leveling agents, plasticizers, silicones, fillers, and sensitizers. The content of the thermoplastic polyimide in the thermoplastic polyimide layer is, for example, 70% by weight or more, preferably 80% by weight or more, more preferably 90% by weight or more, and may be 100% by weight, based on the total weight of the thermoplastic polyimide layer.

In order to particularly suppress the occurrence of cracks on the inner wall of the via during desmearing after laser processing, the multilayer polyimide film according to this embodiment preferably satisfies the following condition 1, more preferably satisfies the following condition 2, even more preferably satisfies the following condition 3, still more preferably satisfies the following condition 4, and particularly preferably satisfies the following condition 5.
Condition 1: The non-thermoplastic polyimide has m-TB residues, ODA residues, PDA residues, BPDA residues, and PMDA residues.
Condition 2: The above condition 1 is satisfied, and the content of ODA residues relative to all diamine residues constituting the non-thermoplastic polyimide is 40 mol % or more and 70 mol % or less.
Condition 3: The above condition 2 is satisfied, and the content of PDA residues relative to all diamine residues constituting the non-thermoplastic polyimide is 5 mol % or more and 50 mol % or less.
Condition 4: The above condition 3 is satisfied, and the non-thermoplastic polyimide is a block copolymer having a specific segment.
Condition 5: The above condition 4 is satisfied, and the non-thermoplastic polyimide further has an ODPA residue.

<Method of manufacturing a multilayer polyimide film and method of manufacturing a metal-clad laminate>
Next, an example of a method for producing the multilayer polyimide film according to this embodiment and an example of a method for producing a metal-clad laminate using the multilayer polyimide film according to this embodiment will be described.

[Method of manufacturing multilayer polyimide film]
(Method for producing polyamic acid)
Any known method or a combination thereof can be used to produce (synthesize) polyamic acid, a precursor of polyimide. A key feature of the polymerization method for producing polyamic acid is the order of monomer addition; controlling this order of monomer addition allows for control of the properties of the resulting polyimide. When synthesizing polyamic acid using diamines and tetracarboxylic dianhydrides, the desired polyamic acid (a polymer of diamines and tetracarboxylic dianhydrides) can be obtained by adjusting the molar ratio of each diamine and the molar ratio of tetracarboxylic dianhydrides (or, if multiple tetracarboxylic dianhydrides are used, the molar ratio of each tetracarboxylic dianhydride). The molar ratio of each residue in the polyimide formed from polyamic acid corresponds, for example, to the molar ratio of each monomer (diamine and tetracarboxylic dianhydride) used in the synthesis of the polyamic acid. The temperature conditions for the reaction between diamines and tetracarboxylic dianhydrides, i.e., the synthesis reaction of polyamic acid, are not particularly limited, but are, for example, in the range of 20°C to 150°C. The reaction time for the synthesis reaction of polyamic acid is, for example, in the range of 10 minutes to 30 hours. In this embodiment, any method for adding a monomer may be used to produce polyamic acid. Typical methods for producing polyamic acid include the following.

Examples of methods for producing polyamic acid include a method of polymerization through the following steps (Aa) and (Ab) (hereinafter sometimes referred to as "polymerization method A").
(A-a): A process for reacting an aromatic diamine with an aromatic acid dianhydride in an organic solvent with an excess of aromatic diamine to obtain a prepolymer having amino groups at both ends. (A-b): A process for polymerizing by adding an aromatic diamine having a different structure from that used in step (A-a) and further adding an aromatic acid dianhydride having a different structure from that used in step (A-a) so that the aromatic diamine and aromatic acid dianhydride are substantially equimolar in all steps.

Another example of a method for producing polyamic acid is a polymerization method (hereinafter sometimes referred to as "polymerization method B") that involves the following steps (Ba) and (Bb).
(B-a): A process for reacting an aromatic diamine with an aromatic acid dianhydride in an organic solvent with an excess of the aromatic acid dianhydride to obtain a prepolymer having acid anhydride groups at both ends. (B-b): A process for polymerizing by adding an aromatic acid dianhydride having a structure different from that used in the process (B-a) and further adding an aromatic diamine having a structure different from that used in the process (B-a) so that the aromatic diamine and aromatic acid dianhydride are substantially equimolar in all processes.

A synthesis method in which the addition order is set so that a specific diamine or specific acid dianhydride selectively reacts with any or specific diamine or any or specific acid dianhydride (for example, the above-mentioned polymerization method A and polymerization method B) is referred to herein as sequence polymerization. Among polymers obtained by sequence polymerization, polymers with two types of segments are called diblock copolymers, and polymers with three types of segments are called triblock copolymers. In contrast, a polymerization method in which the addition order of diamines and acid dianhydrides is not set (a polymerization method in which monomers react with each other randomly) is referred to herein as random polymerization. A polymer obtained by random polymerization is called a random copolymer.

In this embodiment, sequence polymerization is preferred as a polymerization method for obtaining polyimide that is effective in suppressing film tearing while maintaining the properties of flexible metal-clad laminates.

The weight-average molecular weight of the polyamic acid obtained by the above-described polymerization method is preferably in the range of 10,000 to 1,000,000, more preferably 20,000 to 500,000, and even more preferably 30,000 to 200,000. A weight-average molecular weight of 10,000 or greater facilitates the formation of a coating film from the polyamic acid. On the other hand, a weight-average molecular weight of 1,000,000 or less exhibits sufficient solubility in solvents, allowing the formation of a coating film with a smooth surface and uniform thickness using the polyamic acid solution described below. The weight-average molecular weight used here refers to the polyethylene oxide equivalent value measured using gel permeation chromatography (GPC).

When obtaining polyimide, a method of obtaining polyimide from a polyamic acid solution containing polyamic acid and an organic solvent may be used. Examples of organic solvents that can be used in polyamic acid solutions include urea-based solvents such as tetramethylurea and N,N-dimethylethylurea; sulfoxide-based solvents such as dimethyl sulfoxide; sulfone-based solvents such as diphenyl sulfone and tetramethyl sulfone; amide-based solvents such as N,N-dimethylacetamide, N,N-dimethylformamide (hereinafter sometimes referred to as "DMF"), N,N-diethylacetamide, N-methyl-2-pyrrolidone, and hexamethylphosphoric triamide; ester-based solvents such as γ-butyrolactone; alkyl halide solvents such as chloroform and methylene chloride; aromatic hydrocarbon solvents such as benzene and toluene; phenol-based solvents such as phenol and cresol; ketone-based solvents such as cyclopentanone; and ether-based solvents such as tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, dimethyl ether, diethyl ether, diethylene glycol diethyl ether, diethylene glycol dimethyl ether, and p-cresol methyl ether. These solvents are usually used alone, but two or more of them may be used in combination as needed. When polyamic acid is obtained by the above-mentioned polymerization method, the reaction solution (the solution after the reaction) itself may be used as a polyamic acid solution for obtaining a polyimide. In this case, the organic solvent in the polyamic acid solution is the organic solvent used in the reaction in the above-mentioned polymerization method. Alternatively, the solid polyamic acid obtained by removing the solvent from the reaction solution may be dissolved in an organic solvent to prepare a polyamic acid solution.

The polyamic acid solution may contain additives such as dyes, surfactants, leveling agents, plasticizers, silicones, and sensitizers. Fillers may also be added to the polyamic acid solution to improve film properties such as friction, thermal conductivity, electrical conductivity, corona resistance, and loop stiffness. While any filler may be used, preferred examples include fillers made of silica, titanium oxide, alumina, silicon nitride, boron nitride, calcium hydrogen phosphate, calcium phosphate, and mica.

The concentration of polyamic acid in the polyamic acid solution is not particularly limited and is, for example, 5% by weight to 35% by weight, and preferably 8% by weight to 30% by weight, based on the total amount of the polyamic acid solution. A polyamic acid concentration of 5% by weight to 35% by weight provides an appropriate molecular weight and solution viscosity.

(Method for forming a non-thermoplastic polyimide layer)
The method for forming the non-thermoplastic polyimide layer is not particularly limited, and various known methods can be applied. For example, a method for forming a non-thermoplastic polyimide layer (polyimide film) through the following steps i) to iv) can be mentioned.
Step i): A step of reacting an aromatic diamine with an aromatic tetracarboxylic dianhydride in an organic solvent to obtain a polyamic acid solution containing a precursor of a non-thermoplastic polyimide (hereinafter, this may be referred to as a "non-thermoplastic polyamic acid solution"); Step ii): A step of applying a dope liquid containing the non-thermoplastic polyamic acid solution onto a support to form a coating film; Step iii): A step of heating the coating film on the support to form a self-supporting polyamic acid film (hereinafter, this may be referred to as a "gel film"), and then peeling the gel film from the support; Step iv): A step of heating the gel film to imidize the polyamic acid in the gel film and drying it to obtain a polyimide film containing a non-thermoplastic polyimide (a polyimide film that will become a non-thermoplastic polyimide layer in a multilayer polyimide film).

In step ii), the method for applying the dope solution to the support is not particularly limited, and methods using conventionally known application devices such as a die coater, comma coater (registered trademark), reverse coater, or knife coater can be used.

The processes from step ii) onwards can be broadly divided into thermal imidization and chemical imidization. Thermal imidization is a method in which a polyamic acid solution is applied as a dope to a support without using a dehydrating ring-closing agent or the like, and the imidization is promoted by heating. Chemical imidization, on the other hand, is a method in which a polyamic acid solution to which at least one of a dehydrating ring-closing agent and a catalyst has been added as an imidization accelerator is used as a dope to promote imidization. Either method can be used, but chemical imidization offers superior productivity.

As a dehydration ring-closing agent, an acid anhydride such as acetic anhydride is preferably used. As a catalyst, a tertiary amine such as an aliphatic tertiary amine, an aromatic tertiary amine, or a heterocyclic tertiary amine is preferably used.

In step ii), suitable supports for applying the dope solution include glass plates, aluminum foil, endless stainless steel belts, and stainless steel drums. In step iii), heating conditions are set according to the final film thickness and production speed, and the film is partially imidized or dried, after which it is peeled off from the support to obtain a polyamic acid film (gel film).

Next, in step iv), the gel film is heated while the edges are fixed to prevent shrinkage during curing. This removes water, residual solvent, imidization accelerator, etc. from the gel film, completely imidizing the remaining polyamic acid, yielding a polyimide film containing a non-thermoplastic polyimide. Heating conditions can be set appropriately depending on the final film thickness and production speed.

(Method for forming a thermoplastic polyimide layer)
The thermoplastic polyimide layer can be obtained, for example, by applying a polyamic acid solution containing a polyamic acid, a precursor of thermoplastic polyimide, (hereinafter, sometimes referred to as "thermoplastic polyamic acid solution") to at least one side of a polyimide film (non-thermoplastic polyimide layer) obtained using the non-thermoplastic polyamic acid solution described above, followed by the same procedure as in the method for forming the non-thermoplastic polyimide layer (polyimide film) described above. This method results in a multilayer polyimide film having a non-thermoplastic polyimide layer and a thermoplastic polyimide layer disposed on at least one side of the non-thermoplastic polyimide layer. Alternatively, a solution containing a thermoplastic polyimide (thermoplastic polyimide solution) may be used instead of the thermoplastic polyamic acid solution to form a coating film of the thermoplastic polyimide solution on at least one side of the non-thermoplastic polyimide layer, and the coating film may be dried to form a thermoplastic polyimide layer.

Alternatively, for example, a co-extrusion die may be used to form a laminate comprising a layer containing polyamic acid, a precursor of non-thermoplastic polyimide, and a layer containing polyamic acid, a precursor of thermoplastic polyimide. The resulting laminate may then be heated to simultaneously form a non-thermoplastic polyimide layer and a thermoplastic polyimide layer. In this method, by using metal foil as a support, a metal-clad laminate (a laminate of a multilayer polyimide film and metal foil) is obtained upon completion of imidization. To produce a multilayer polyimide film containing three or more polyimide layers, the above-described coating and heating steps may be repeated multiple times, or multiple coating layers may be formed by co-extrusion or continuous coating (continuous casting) and then heated simultaneously. Various surface treatments, such as corona treatment and plasma treatment, may also be applied to the outermost surface of the multilayer polyimide film.

[Metal-clad laminate manufacturing method]
When a metal-clad laminate is manufactured using the multilayer polyimide film obtained by the above-described method, a metal foil is laminated to at least one side of the multilayer polyimide film, as described above. The metal foil is not particularly limited, and any metal foil can be used. For example, metal foils made of copper, stainless steel, nickel, aluminum, and alloys of these metals are preferably used. In addition, while copper foils such as rolled copper foil and electrolytic copper foil are often used in general metal-clad laminates, copper foil is also preferably used in this embodiment.

The metal foil can be surface-treated to adjust the surface roughness, etc., depending on the purpose. Furthermore, an anti-rust layer, heat-resistant layer, adhesive layer, etc. may be formed on the surface of the metal foil. There are no particular restrictions on the thickness of the metal foil, as long as it is thick enough to perform its function adequately depending on the application.

<Processing of metal-clad laminates>
When forming vias using laser processing on metal-clad laminates, the laser is irradiated on the desired area to cut and drill holes in the metal-clad laminate. Through-holes can be formed by penetrating the metal-clad laminate, or blind vias can be formed by removing part of the metal foil on the top surface and then removing only the exposed polyimide layer. When forming blind vias, the top metal foil is removed with a laser, and then the laser output is reduced to remove the polyimide layer, allowing for stable blind via formation.

Known types of lasers can be used. Short-wavelength lasers such as UV-YAG lasers and excimer lasers are preferred because they exhibit extremely high absorption rates for both resins and metals. A widely used method for forming through-holes is to directly drill through-holes. Known methods can be used for desmearing after laser processing, including a swelling process using an alkaline aqueous solution or a solution containing an organic solvent, a roughening process using an alkaline aqueous solution containing sodium permanganate, potassium permanganate, or the like, and a wet desmearing process that includes a neutralization process.

In the case of double-sided metal-clad laminates, the inner walls of the holes are plated after desmearing to provide electrical continuity between both surfaces of the metal-clad laminate. One example of a plating method is to deposit palladium on the inner walls of the holes, and then use the palladium as a nucleus to form an electroless copper plating layer on the inner wall surface. In this case, a plating layer of the desired thickness can be formed using electroless copper plating alone, or a thin electroless copper plating layer can be applied first, followed by electrolytic copper plating to form a plating layer of the desired thickness.

The present invention will be explained in detail below using examples, but the present invention is not limited to these examples.

<Methods for measuring and evaluating physical properties>
First, the storage modulus and linear expansion coefficient of the polyimide film, as well as the evaluation method (hole crack test) for the examples and comparative examples will be described.

[Storage modulus at a temperature of 380°C]
The dynamic viscoelasticity of the polyimide film was measured in an air atmosphere using a dynamic viscoelasticity measuring device ("DM6100" manufactured by Hitachi High-Tech Science Corporation), and the correlation between the storage modulus and the measurement temperature was plotted to read the storage modulus at a measurement temperature of 380° C. The measurement conditions are shown below.
Width of sample (polyimide film): 9 mm
Sample holder (grab) spacing: 20 mm
Measurement temperature range: 0℃ to 440℃
Temperature rise rate: 3°C/min Strain amplitude: 10 μm
Measurement frequency: 1Hz, 5Hz, 10Hz
Minimum tension/compression force: 100 mN
Tension/Compression Gain: 1.5
Initial force amplitude: 100 mN

[Linear expansion coefficient]
Using a thermal analyzer ("TMA/SS6100" manufactured by Hitachi High-Tech Science Corporation), the polyimide film was heated from -10°C to 400°C in a nitrogen atmosphere, then cooled to -10°C, and heated again to 400°C, and the linear expansion coefficient was determined from the amount of strain during the second heating from 100°C to 200°C. The measurement conditions are shown below.
Sample (polyimide film) size: width 3 mm, length 10 mm
Load: 3g (29.4mN)
Temperature increase rate: 10°C/min

[Hole crack test]
A 12 μm thick electrolytic copper foil ("3EC-M3S-HTE" manufactured by Mitsui Mining & Smelting Co., Ltd.) was placed on both sides of the multilayer polyimide film obtained in the examples and comparative examples described below. A protective film ("Apical (registered trademark) 125NPI" manufactured by Kaneka Corporation, thickness: 125 μm) was placed on the outer surface of each electrolytic copper foil. Lamination was performed under the conditions of a lamination temperature of 360 ° C., a lamination pressure of 265 N / cm (27 kgf / cm), and a lamination speed of 1.0 m / min to obtain a flexible copper-clad laminate. The resulting flexible copper-clad laminate was then cut into a rectangular shape measuring 5.0 cm x 20.0 cm to obtain a processing sample. Next, using a UV-YAG laser, blind vias (10 vertical x 10 horizontal = 100 pieces, spaced 1 mm) with a diameter of 75 μm were formed in the processing sample under the laser processing conditions described in Table 1.

The laser-processed samples were then desmeared under the conditions shown in Table 2, and the copper foil was then removed by etching to obtain evaluation samples. The chemical solutions used in the desmearing process were all manufactured by Rohm and Haas Electronic Materials. Water washing processes were also carried out between the swelling and roughening processes, between the roughening and neutralization processes, and after the neutralization process.

The resulting evaluation samples were then observed under a polarizing microscope at 200x magnification under crossed Nicols to determine whether or not cracks were present. Specifically, light leakage around the hole was considered to be a "cracked" condition, and 100 holes were observed. The percentage of cracked holes (crack occurrence rate) was calculated. Figures 3 to 5 show examples of polarizing microscope images used for actual determination. Figure 3 shows an example of a hole without cracks, as there is no light leakage around the hole. Figures 4 and 5 show examples of a hole with cracks, as there is light leakage around the hole. Note that for holes where the degree of light leakage was too weak to determine whether or not a crack was present, the cross section of the hole was observed under an electron microscope to determine whether or not a crack was present.

<Preparation of Polyamic Acid Solution>
The methods for preparing solutions P1 to P12, which are non-thermoplastic polyamic acid solutions, and solution P13, which is a thermoplastic polyamic acid solution, are described below. Solutions P1 to P13 were all prepared in a nitrogen atmosphere at a temperature of 20°C.

[Preparation of solution P1]
A 2-L glass flask was charged with 328.53 g of DMF and 17.70 g of ODA, and then 18.01 g of BPDA was slowly added to the flask while stirring the contents. After visually confirming that the BPDA had dissolved, 4.00 g of PMDA was slowly added to the flask while stirring the contents. After visually confirming that the PMDA had dissolved, the contents of the flask were stirred for an additional 30 minutes. Next, while stirring the contents of the flask, 5.77 g of m-TB was added to the flask, followed by 2.21 g of PDA, followed by 11.42 g of PMDA, and the contents of the flask were stirred for an additional 30 minutes. Next, while stirring the contents of the flask, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.89 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added slowly to the flask to prevent a sudden increase in the viscosity of the flask content. When the viscosity of the flask content at 23° C. reached 2,500 poise, the addition of the PMDA solution and stirring of the flask content were stopped, yielding Solution P1, a non-thermoplastic polyamic acid solution.

The polyimide obtained from the polyamic acid in the resulting solution P1 was confirmed to be non-thermoplastic using the following method. First, 32.5 g of an imidization accelerator consisting of acetic anhydride/isoquinoline/DMF (weight ratio: 11.48/3.40/18.18) was added to 65 g of solution P1 to prepare a dope solution. The dope solution was then degassed while stirring in an atmosphere below 0°C, and then coated onto aluminum foil using a comma coater to form a coating film. The coating film was then heated at 115°C for 100 seconds to obtain a self-supporting gel film. The resulting gel film was peeled from the aluminum foil, fixed to a metal frame, and heated at 250°C for 15 seconds, followed by 79 seconds at 350°C for drying and imidization, yielding a polyimide film with a thickness of 12.5 μm. The obtained polyimide film was fixed to a metal frame and heated at 450°C for 2 minutes, whereupon the shape of the polyimide film (film shape) was maintained. Therefore, the polyimide obtained from the polyamic acid in Solution P1 was a non-thermoplastic polyimide. Regarding Solutions P2 to P12, whose preparation methods are shown below, polyimide films obtained by the same method as the film formation method using Solution P1 were fixed to a metal frame and heated at 450°C for 2 minutes, whereupon the shape of the polyimide film (film shape) was maintained. Therefore, the polyimides obtained from the polyamic acid in Solutions P2 to P12 were all non-thermoplastic polyimides.

[Preparation of solution P2]
A 2-L glass flask was charged with 328.55 g of DMF and 16.32 g of ODA, and then 17.98 g of BPDA was slowly added to the flask while stirring the contents. After visually confirming that the BPDA had dissolved, 2.67 g of PMDA was slowly added to the flask while stirring the contents. After visually confirming that the PMDA had dissolved, the contents were stirred for an additional 30 minutes. Next, while stirring the contents, 7.21 g of m-TB was added to the flask, followed by 2.20 g of PDA, followed by 12.74 g of PMDA, and the contents were stirred for an additional 30 minutes. Next, while stirring the contents, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.89 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added slowly to the flask to prevent a sudden increase in the viscosity of the flask content. When the viscosity of the flask content at 23° C. reached 2,500 poise, the addition of the PMDA solution and stirring of the flask content were stopped, yielding Solution P2, a non-thermoplastic polyamic acid solution.

[Preparation of solution P3]
A 2-L glass flask was charged with 328.78 g of DMF and 17.31 g of ODA, and then 22.70 g of BPDA was slowly added to the flask while stirring the contents. After visually confirming that the BPDA had dissolved, the contents were stirred for an additional 30 minutes. Next, while stirring the contents, 5.65 g of m-TB was added to the flask, followed by 2.16 g of PDA, followed by 11.31 g of PMDA, and the contents were stirred for an additional 30 minutes. Next, while stirring the contents, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.87 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added slowly to prevent a sudden increase in the viscosity of the flask contents. When the viscosity of the content of the flask reached 2,500 poise at 23° C., the addition of the PMDA solution and stirring of the content of the flask were stopped to obtain a non-thermoplastic polyamic acid solution P3.

[Preparation of solution P4]
A 2-L glass flask was charged with 328.41 g of DMF and 16.51 g of ODA, and then 18.20 g of BPDA was gradually added to the flask while stirring the contents. After visually confirming that the BPDA had dissolved, 2.70 g of PMDA was gradually added to the flask while stirring the contents. After visually confirming that the PMDA had dissolved, the contents of the flask were stirred for an additional 30 minutes. Next, while stirring the contents of the flask, 5.83 g of m-TB was added to the flask, followed by 2.97 g of PDA, followed by 12.89 g of PMDA, and the contents of the flask were stirred for an additional 30 minutes. Next, while stirring the contents of the flask, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.90 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added slowly to the flask to prevent a sudden increase in the viscosity of the flask content. When the viscosity of the flask content at 23° C. reached 2,500 poise, the addition of the PMDA solution and stirring of the flask content were stopped, yielding Solution P4, a non-thermoplastic polyamic acid solution.

[Preparation of Solution P5]
A 2-L glass flask was charged with 328.29 g of DMF, 5.90 g of m-TB, 3.75 g of PDA, and 15.30 g of ODA. Then, 18.39 g of BPDA was gradually added to the flask while stirring the contents. After visually confirming that the BPDA had dissolved, 15.75 g of PMDA was gradually added to the flask while stirring the contents. After visually confirming that the PMDA had dissolved, the contents were stirred for an additional 30 minutes. Next, while stirring the contents, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.91 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added gradually to prevent a sudden increase in the viscosity of the flask contents. When the viscosity of the content of the flask reached 2,500 poise at 23° C., the addition of the PMDA solution and the stirring of the content of the flask were stopped, to obtain a non-thermoplastic polyamic acid solution P5.

[Preparation of Solution P6]
A 2-L glass flask was charged with 328.57 g of DMF and 17.64 g of ODA, and then 13.95 g of BPDA was gradually added to the flask while stirring the contents. After visually confirming that the BPDA had dissolved, 4.20 g of ODPA was gradually added to the flask while stirring the contents. After visually confirming that the ODPA had dissolved, 3.99 g of PMDA was gradually added to the flask while stirring the contents. After visually confirming that the PMDA had dissolved, the contents of the flask were stirred for an additional 30 minutes. Next, 5.75 g of m-TB was added to the flask while stirring the contents, followed by 2.20 g of PDA, followed by 11.38 g of PMDA, and the contents of the flask were stirred for an additional 30 minutes. Next, while stirring the contents of the flask, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.89 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added gradually to prevent a sudden increase in the viscosity of the flask contents. When the viscosity of the flask contents at 23°C reached 2,500 poise, the addition of the PMDA solution and stirring of the flask contents were stopped, yielding Solution P6, a non-thermoplastic polyamic acid solution.

[Preparation of Solution P7]
A 2-L glass flask was charged with 328.81 g of DMF and 15.94 g of ODA, and then 17.57 g of BPDA was gradually added to the flask while stirring the contents. After visually confirming that the BPDA had dissolved, 2.60 g of PMDA was gradually added to the flask while stirring the contents. After visually confirming that the PMDA had dissolved, the contents of the flask were stirred for an additional 30 minutes. Next, while stirring the contents of the flask, 9.86 g of m-TB was added to the flask, followed by 0.72 g of PDA, followed by 12.44 g of PMDA, and the contents of the flask were stirred for an additional 30 minutes. Next, while stirring the contents of the flask, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.87 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added slowly to the flask to prevent a sudden increase in the viscosity of the flask content. When the viscosity of the flask content at 23° C. reached 2,500 poise, the addition of the PMDA solution and stirring of the flask content were stopped, yielding Solution P7, a non-thermoplastic polyamic acid solution.

[Preparation of Solution P8]
A 2-L glass flask was charged with 327.90 g of DMF, 4.57 g of m-TB, 5.43 g of PDA, and 14.36 g of ODA. Then, while stirring the contents of the flask, 16.88 g of BPDA was gradually added. After visually confirming that the BPDA had dissolved, 17.83 g of PMDA was gradually added to the flask while stirring the contents. After visually confirming that the PMDA had dissolved, the contents of the flask were further stirred for 30 minutes. Next, while stirring the contents of the flask, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.94 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added gradually to prevent a sudden increase in the viscosity of the flask contents. When the viscosity of the content of the flask reached 2,500 poise at 23° C., the addition of the PMDA solution and the stirring of the content of the flask were stopped, to obtain a non-thermoplastic polyamic acid solution P8.

[Preparation of Solution P9]
A 2-L glass flask was charged with 328.27 g of DMF and 16.71 g of ODA, and then 18.41 g of BPDA was slowly added to the flask while stirring the contents. After visually confirming that the BPDA had dissolved, 2.73 g of PMDA was slowly added to the flask while stirring the contents. After visually confirming that the PMDA had dissolved, the contents were stirred for an additional 30 minutes. Next, while stirring the contents, 4.43 g of m-TB was added to the flask, followed by 3.76 g of PDA, followed by 13.05 g of PMDA, and the contents were stirred for an additional 30 minutes. Next, while stirring the contents, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.91 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added slowly to the flask to prevent a sudden increase in the viscosity of the flask content. When the viscosity of the flask content at 23° C. reached 2,500 poise, the addition of the PMDA solution and stirring of the flask content were stopped, yielding Solution P9, a non-thermoplastic polyamic acid solution.

[Preparation of Solution P10]
A 2-L glass flask was charged with 328.91 g of DMF, 5.27 g of ODA, and 16.20 g of BAPP. Then, 8.48 g of BTDA was gradually added to the flask while stirring the contents. After visually confirming that the BTDA had dissolved, 7.17 g of PMDA was gradually added to the flask while stirring the contents. After visually confirming that the PMDA had dissolved, the contents were stirred for an additional 30 minutes. Next, while stirring the contents, 7.11 g of PDA was added to the flask, followed by 14.92 g of PMDA, and the contents were stirred for an additional 30 minutes. Next, while stirring the contents, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.86 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added slowly to the flask to prevent a sudden increase in the viscosity of the flask content. When the viscosity of the flask content at 23° C. reached 2,500 poise, the addition of the PMDA solution and stirring of the flask content were stopped, yielding Solution P10, a non-thermoplastic polyamic acid solution.

[Preparation of Solution P11]
A 2-L glass flask was charged with 328.49 g of DMF and 12.29 g of ODA, and then 16.06 g of BPDA was slowly added to the flask while stirring the contents. After visually confirming that the BPDA had dissolved, the contents were stirred for an additional 30 minutes. Next, while stirring the contents, 11.58 g of m-TB was added to the flask, followed by 2.21 g of PDA, followed by 16.96 g of PMDA, and the contents were stirred for an additional 30 minutes. Next, while stirring the contents, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.89 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added slowly to prevent a sudden increase in the viscosity of the flask contents. When the viscosity of the content of the flask reached 2,500 poise at 23° C., the addition of the PMDA solution and the stirring of the content of the flask were stopped, to obtain a non-thermoplastic polyamic acid solution P11.

[Preparation of Solution P12]
A 2-L glass flask was charged with 329.58 g of DMF, 7.86 g of m-TB, 12.36 g of ODA, and 8.61 g of 9,9-bis(4-aminophenyl)fluorene (hereinafter sometimes referred to as "BAFL"), and then 16.35 g of BPDA was gradually added to the flask while stirring the contents of the flask. After visually confirming that the BPDA had dissolved, 14.01 g of PMDA was gradually added to the flask while stirring the contents of the flask. After visually confirming that the PMDA had dissolved, the contents of the flask were further stirred for 30 minutes. Next, while stirring the contents of the flask, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 0.81 g, PMDA concentration: 7.2 wt %) was added to the flask. The PMDA solution was added gradually to prevent a sudden increase in the viscosity of the flask contents. When the viscosity of the content of the flask reached 2,500 poise at 23° C., the addition of the PMDA solution and the stirring of the content of the flask were stopped, to obtain a non-thermoplastic polyamic acid solution, Solution P12.

[Preparation of solution P13]
A 2-L glass flask was charged with 673.24 g of DMF and 71.83 g of BAPP, and then 7.72 g of BPDA was gradually added to the flask while stirring the contents. After visually confirming that the BPDA had dissolved, 31.30 g of PMDA was gradually added to the flask while stirring the contents. After visually confirming that the PMDA had dissolved, the contents were stirred for an additional 30 minutes. Next, while stirring the contents, a previously prepared PMDA solution (solvent: DMF, dissolved amount of PMDA: 1.15 g, PMDA concentration: 7.2 wt%) was added to the flask. The PMDA solution was added gradually to prevent a sudden increase in the viscosity of the flask contents. When the viscosity of the flask contents reached 300 poise at 23°C, the addition of the PMDA solution and stirring of the flask contents were stopped, yielding a thermoplastic polyamic acid solution, Solution P13.

The polyimide obtained from the polyamic acid in Solution P13 was confirmed to be thermoplastic using the following method. First, 30.0 g of an imidization accelerator consisting of acetic anhydride/isoquinoline/DMF (weight ratio: 6.89/2.14/20.97) was added to 60 g of Solution P13 to prepare a dope solution. The dope solution was then degassed while stirring in an atmosphere below 0°C, and then coated onto aluminum foil using a comma coater to form a coating film. The coating was then heated at 120°C for 3 minutes to obtain a self-supporting gel film. The resulting gel film was peeled from the aluminum foil, fixed to a metal frame, and heated at 250°C for 1 minute, followed by 200 seconds at 300°C for drying and imidization, yielding a 20.0 μm-thick polyimide film. The obtained polyimide film was fixed to a metal frame and heated at 450°C for 2 minutes, but the shape of the polyimide film (film shape) was not maintained. Therefore, the polyimide obtained from the polyamic acid in Solution P13 was a thermoplastic polyimide.

<Preparation of multilayer polyimide film>
The methods for producing the multilayer polyimide films of Examples 1 to 7 and Comparative Examples 1 to 5 will be described below.

[Example 1]
A dope solution was prepared by adding 32.5 g of an imidization accelerator consisting of acetic anhydride/isoquinoline/DMF (weight ratio: 11.48/3.40/18.18) to 65 g of solution P1. The dope solution was then degassed while stirring in an atmosphere below 0°C, and then coated onto aluminum foil using a comma coater to form a coating film. The coating film was then heated at 115°C for 100 seconds to obtain a self-supporting gel film. The resulting gel film was peeled from the aluminum foil, fixed to a metal frame, and heated at 250°C for 15 seconds, followed by 79 seconds at 350°C for drying and imidization, yielding a polyimide film with a thickness of 12.5 μm. The physical properties of the resulting polyimide film (non-thermoplastic polyimide layer) are shown in Table 4. The "physical properties of the non-thermoplastic polyimide layer" in Table 4 are physical properties measured using a polyimide film having a thickness of 12.5 μm.

Next, solution P13 was diluted with DMF to a solids concentration of 8% by weight to prepare a dope solution. This was then applied to both sides of the polyimide film (polyimide film obtained using solution P1) to form a coating film. The amount of coating was adjusted so that each thermoplastic polyimide layer (adhesive layer) formed would be 3 μm thick. The coating film was then heated at 120°C for 2 minutes, followed by 15 seconds at 350°C for drying and imidization, resulting in the multilayer polyimide film of Example 1. The results of the hole crack test (crack occurrence rate) of the resulting multilayer polyimide film are shown in Table 4. The copper-clad laminate produced during the hole crack test had a good appearance with no wrinkles on the surface.

[Examples 2 to 7 and Comparative Examples 1 to 5]
Multilayer polyimide films of Examples 2 to 7 and Comparative Examples 1 to 5 were obtained in the same manner as Example 1, except that the non-thermoplastic polyamic acid solution shown in Table 4 was used instead of Solution P1. The amount of non-thermoplastic polyamic acid solution used in each of Examples 2 to 7 and Comparative Examples 1 to 5 was 65 g. The results of the hole crack test (crack incidence rate) of the obtained multilayer polyimide films are shown in Table 4. For Examples 2 to 4, Example 6, Example 7, and Comparative Examples 2 to 4, the copper-clad laminates produced during the hole crack test were free of wrinkles or the like, and copper-clad laminates with good appearance were obtained. On the other hand, for Example 5, Comparative Example 1, and Comparative Example 5, wrinkles were present on part of the surface of the copper-clad laminate produced during the hole crack test.

<Evaluation results>
Table 3 shows the materials used in each of Solutions P1 to P13 and their proportions. The molar ratios of each residue in the polyimides obtained using Solutions P1 to P13 were consistent with the molar ratios of each monomer (diamine and tetracarboxylic dianhydride) used. Table 4 shows the type of non-thermoplastic polyamic acid solution used, the physical properties of the non-thermoplastic polyimide layer, and the results of the hole crack test (crack incidence) for each of Examples 1 to 7 and Comparative Examples 1 to 5. In Table 3, a "-" indicates that the corresponding component was not used. In Table 3, the values in the "Dianhydride" column represent the content (unit: mol%) of each dianhydride relative to the total amount of dianhydrides used. The values in the "Diamine" column represent the content (unit: mol%) of each diamine relative to the total amount of diamines used.

In Examples 1 to 7, the non-thermoplastic polyimides contained m-TB residues, which are a type of BPDI residue, as well as ODA residues and PDA residues. In Examples 1 to 7, the content of m-TB residues was 20 mol% or more and 35 mol% or less of all diamine residues constituting the non-thermoplastic polyimides. In Examples 1 to 7, the crack occurrence rate was 50% or less. Note that the non-thermoplastic polyimides in Examples 1 to 4, 6, and 7 were block copolymers having specific segments, while the non-thermoplastic polyimide in Example 5 was a random copolymer.

In Comparative Examples 1 and 2, the content of BPDI residues (m-TB residues) was less than 20 mol% relative to all diamine residues constituting the non-thermoplastic polyimide. In Comparative Example 3, the non-thermoplastic polyimide had no BPDI residues. In Comparative Example 4, the content of BPDI residues (m-TB residues) was greater than 35 mol% relative to all diamine residues constituting the non-thermoplastic polyimide. In Comparative Example 5, the non-thermoplastic polyimide had no PDA residues. In Comparative Examples 1 to 5, the crack occurrence rate exceeded 50%.

The above results demonstrate that the multilayer polyimide film of the present invention can suppress the occurrence of cracks on the inner walls of vias during desmearing after laser processing.

10: Multilayer polyimide film 11: Non-thermoplastic polyimide layer 12: Thermoplastic polyimide layer

Claims (10)

A multilayer polyimide film having a non-thermoplastic polyimide layer and a thermoplastic polyimide layer disposed on at least one surface of the non-thermoplastic polyimide layer,
the non-thermoplastic polyimide contained in the non-thermoplastic polyimide layer has a tetracarboxylic dianhydride residue and a diamine residue,
the diamine residue includes a diamine residue having a biphenyl skeleton, a 4,4'-diaminodiphenyl ether residue, and a p-phenylenediamine residue;
a content of the diamine residues having a biphenyl skeleton in the non-thermoplastic polyimide film relative to all diamine residues constituting the non-thermoplastic polyimide; The multilayer polyimide film described in claim 1, wherein the diamine residue having a biphenyl skeleton is a 4,4'-diamino-2,2'-dimethylbiphenyl residue. A multilayer polyimide film as described in claim 1 or 2, wherein the content of the 4,4'-diaminodiphenyl ether residue is 40 mol% or more and 70 mol% or less of all diamine residues constituting the non-thermoplastic polyimide. A multilayer polyimide film described in any one of claims 1 to 3, wherein the content of the p-phenylenediamine residue is 5 mol% or more and 50 mol% or less of all diamine residues constituting the non-thermoplastic polyimide. A multilayer polyimide film according to any one of claims 1 to 4, wherein the tetracarboxylic dianhydride residues include one or more selected from the group consisting of 3,3',4,4'-biphenyltetracarboxylic dianhydride residues and pyromellitic dianhydride residues. A multilayer polyimide film as described in claim 5, wherein the tetracarboxylic dianhydride residue further contains a 4,4'-oxydiphthalic anhydride residue. The multilayer polyimide film described in claim 6, wherein the content of the 4,4'-oxydiphthalic anhydride residues is 5 mol% or more and 15 mol% or less of the total tetracarboxylic dianhydride residues constituting the non-thermoplastic polyimide. A multilayer polyimide film according to any one of claims 1 to 7, wherein the thermoplastic polyimide contained in the thermoplastic polyimide layer has one or more selected from the group consisting of 3,3',4,4'-biphenyltetracarboxylic dianhydride residues and pyromellitic dianhydride residues, and 2,2-bis[4-(4-aminophenoxy)phenyl]propane residues. A multilayer polyimide film described in any one of claims 1 to 8, wherein the storage modulus of the non-thermoplastic polyimide layer at a temperature of 380°C is less than 0.350 GPa. The multilayer polyimide film according to any one of claims 1 to 9, wherein the non-thermoplastic polyimide layer has a coefficient of linear expansion during heating at a temperature of 100 ° C. to 200 ° C. of 5.0 ppm / K or more and 19.0 ppm / K or less.