Low dielectric substrate material

Abstract

A low dielectric substrate material (1) includes: a metal layer (2), and a porous resin layer (3) disposed on one face of the metal layer (2) in the thickness direction. The porous resin layer (3) includes: a first region (31), a second region (32), a third region (33), and a fourth region (34) arranged in order in a direction away from the metal layer (2) when the porous resin layer (3) is equally divided into four in the thickness direction. The first region (31) has a plurality of closed cell structures (30) independent of each other in a resin matrix (35), and the average of the aspect ratio AR of the plurality of closed cell structures (30) in the first region (31) is 0.80 or more and 1.20 or less. The average of the aspect ratio AR is the ratio (L1 / L2) of the length L1 of the closed cell structure (30) in a direction orthogonal to the thickness direction to the length L2 of the closed cell structure (30) in the thickness direction in a cross-sectional view.

Description

Technical Field

[0001] This invention relates to low-dielectric substrate materials. Background Technology

[0002] Low-dielectric substrate materials having a metal layer and a porous resin layer disposed on one side of the metal layer in the thickness direction are known (for example, see Patent Document 1 below).

[0003] Low-dielectric substrate materials are processed into flexible wiring boards, for example.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2019-123851 Summary of the Invention

[0007] The problem the invention aims to solve

[0008] Excellent processability is required for low-dielectric substrate materials. However, there are limitations to meeting the above-mentioned physical properties requirements for the low-dielectric substrate material of Patent Document 1.

[0009] This invention provides a low-dielectric substrate material with excellent processability.

[0010] Solution for solving the problem

[0011] The present invention (1) includes a low dielectric substrate material comprising: a metal layer and a porous resin layer disposed on one side of the aforementioned metal layer in the thickness direction. The aforementioned porous resin layer includes: a first region, a second region, a third region, and a fourth region arranged sequentially in a direction away from the aforementioned metal layer when the aforementioned porous resin layer is divided into four equal parts in the thickness direction. At least the aforementioned first region has a plurality of mutually independent closed-cell structures in the resin matrix. The average of the length-to-diameter ratio of the plurality of aforementioned closed-cell structures in the aforementioned first region, that is, the average length-to-diameter ratio (L1 / L2) of the ratio of the length L1 of the aforementioned closed-cell structure in the direction orthogonal to the thickness direction in the cross-sectional view to the length L2 of the aforementioned closed-cell structure in the thickness direction, is 0.80 or more and 1.20 or less.

[0012] The effects of the invention

[0013] The low-dielectric substrate material of the present invention has excellent processability. Attached Figure Description

[0014] Figure 1 This is a cross-sectional view of one embodiment of the low-dielectric substrate material of the present invention.

[0015] Figure 2 This is a cross-sectional view of a deformed example of a low-dielectric substrate material.

[0016] Figure 3 This is a processed image of a SEM photograph of the low-dielectric substrate material of Example 1.

[0017] Figure 4 This is a processed image of a SEM photograph of the low-dielectric substrate material in Example 2.

[0018] Figure 5 Image processing diagram of SEM image of the low dielectric substrate material of Comparative Example 1.

[0019] Figure 6 Image processing diagram of SEM image of the low dielectric substrate material of Comparative Example 2. Detailed Implementation

[0020] 1. Low-dielectric substrate material 1

[0021] Reference Figure 1 An embodiment of the low-dielectric substrate material of the present invention will be described.

[0022] The low-dielectric substrate material 1 has a thickness. The low-dielectric substrate material 1 extends along a surface direction. The surface direction is orthogonal to the thickness direction. The low-dielectric substrate material 1 has a plate shape. The low-dielectric substrate material 1 is flexible. The thickness of the low-dielectric substrate material 1 is not particularly limited. The thickness of the low-dielectric substrate material 1 is, for example, 5 μm or more, and for example, 2,000 μm or less.

[0023] 1.1 Layer structure of low-dielectric substrate material 1

[0024] The low-dielectric substrate material 1 has a metal layer 2, a porous resin layer 3, and an outer layer 4 sequentially disposed on one side facing the thickness direction. That is, the low-dielectric substrate material 1 includes: a metal layer 2, a porous resin layer 3 disposed on one side of the metal layer 2 in the thickness direction, and an outer layer 4 disposed on one side of the porous resin layer 3 in the thickness direction. In this embodiment, it is preferable that the low-dielectric substrate material 1 only includes a metal layer 2, a porous resin layer 3, and an outer layer 4.

[0025] 1.2 Metal Layer 2

[0026] Metal layer 2 is disposed at the other end of the low-dielectric substrate material 1 in the thickness direction. Metal layer 2 forms the other side of the low-dielectric substrate material 1 in the thickness direction. Metal layer 2 extends in the surface direction. Specifically, metal layer 2 is a metal thin film. Examples of metals include copper, iron, silver, gold, aluminum, nickel, and their alloys (stainless steel, bronze). Copper is a preferred metal. The thickness of metal layer 2 is, for example, 0.1 μm or more, preferably 1 μm or more, and also, for example, 100 μm or less, preferably 50 μm or less.

[0027] 1.3 Porous resin layer 3

[0028] The porous resin layer 3 is in contact with one surface of the metal layer 2 in the thickness direction. The porous resin layer 3 has a thickness. The porous resin layer 3 extends in the surface direction. The porous resin layer 3 has a plurality of independent closed-cell structures 30 in the resin matrix throughout the thickness direction. The closed-cell structures 30 are completely covered by the resin matrix and do not communicate with adjacent pore structures. In addition, the interior of the closed-cell structure 30 is not an open pore structure facing the metal layer 2 or the outer layer 4.

[0029] When the porous resin layer 3 is divided into four equal parts along its thickness direction, the parts facing away from the metal layer 2 sequentially include: a first region 31, a second region 32, a third region 33, and a fourth region 34. In the porous resin layer 3, the first region 31, the second region 32, the third region 33, and the fourth region 34 are arranged sequentially facing one side of the thickness direction.

[0030] 1.3.1 Region 1, 31

[0031] The first region 31 is located on the far side of the porous resin layer 3 in the thickness direction. The first region 31 is in contact with one surface of the metal layer 2 in the thickness direction. The first region 31 has the aforementioned closed-cell structure 30.

[0032] The average aspect ratio of the multiple closed-cell structures 30 in region 1 31 is greater than 0.80 and less than 1.20.

[0033] When the average aspect ratio AR of the multiple closed-cell structures 30 in region 1 is less than 0.80 or greater than 1.20, the processability of the low-dielectric substrate material 1 decreases. Processability includes the property that the precursor film is cured by heating before complete curing (described later), and that the properties of the low-dielectric substrate material 1 are not easily changed even when it is pressed. The properties include the thickness of the porous resin layer 3, the dielectric constant, and / or the dielectric loss tangent.

[0034] The aforementioned aspect ratio AR is the ratio of the length L1 of the closed-cell structure in the orthogonal direction to the length L2 of the closed-cell structure 30 in the thickness direction when the porous resin layer 3 is observed in cross-section. The orthogonal direction is perpendicular to the thickness direction, equivalent to... Figure 1 The left and right directions.

[0035] The average aspect ratio AR of the plurality of closed-cell structures 30 in the first region is preferably 0.85 or more, more preferably 0.90 or more, and even more preferably 0.95 or more. It is also preferably 1.15 or less, more preferably 1.10 or less, and even more preferably 1.05 or less. When the average aspect ratio AR of the plurality of closed-cell structures 30 in the first region is below the aforementioned upper limit or above the aforementioned lower limit, the processability of the low-dielectric substrate material 1 can be further improved.

[0036] The average length of the long side of the closed-cell structure 30 is, for example, more than 1 μm and less than 100 μm. It should be noted that the average length of the long side of the closed-cell structure 30 is equivalent to the pore diameter when the average aspect ratio AR is 1.

[0037] It should be noted that, in addition to the closed-hole structure 30, the first region 31 may also include the aforementioned open-hole structure.

[0038] 1.3.2 Region 2, 32 to Region 4, 34

[0039] Region 2, Region 33, and Region 4 have the same configuration as Region 1 31 described above. In each of Regions 2, Region 32, Region 33, and Region 4 34, the average aspect ratio AR of the plurality of closed-cell structures 30 is, for example, 0.80 or more and 1.20 or less.

[0040] From the viewpoint of the manufacturing method described later, among the first region 31 to the fourth region 34, the average aspect ratio AR of the first region 31 tends to deviate from 1.0. However, as in this invention, for the first region 31, by making the average aspect ratio AR of the first region 31 above and below 0.80 and below 1.20, the average aspect ratio AR of the second region 32 to the fourth region 34 will naturally also be above 0.80 and below 1.20.

[0041] Furthermore, the average length of the long side of the closed-hole structure 30 in regions 2 to 4 is, for example, the same as or greater than that in region 1.

[0042] 1.3.3 Material of Porous Resin Layer 3

[0043] The porous resin layer 3 is made of resin. The resin is not limited. Specifically, examples of resins include polycarbonate resin, polyimide resin, fluorinated polyimide resin, epoxy resin, phenolic resin, urea resin, melamine resin, diallyl phthalate resin, silicone resin, thermosetting urethane resin, fluoropolymer (including polytetrafluoroethylene (PTFE)), and liquid crystal polymer (LCP). These can be used alone or in combination of two or more. Among the above-mentioned resins, polyimide resin is preferred. Details of the physical properties and manufacturing methods of polyimide resins are described, for example, in WO2018 / 186486 and Japanese Patent Application Publication No. 2020-172667. It should be noted that polyimide resin is sometimes simply referred to as polyimide.

[0044] 1.3.4 Other physical properties of porous resin layer 3

[0045] The mass reduction rate of the porous resin layer 3 after heating at 450°C for 1 hour is, for example, 3.0% by mass or less, preferably 2.0% by mass or less, more preferably 1.8% by mass or less, and also, for example, 0.1% by mass or more, preferably 1.5% by mass or more.

[0046] When the mass reduction rate of the porous resin layer 3 is below the aforementioned upper limit, variations in the dielectric constant and / or dielectric loss tangent of the porous resin layer 3 during heating and / or pressing can be suppressed. The method for measuring the mass reduction rate of the porous resin layer 3 is described in the examples described later.

[0047] When the porous resin layer 3 is formed of polyimide, the imidization rate of the porous resin layer 3 is, for example, 0.920 or more, preferably 0.950 or more.

[0048] When the imidization rate of the porous resin layer 3 is above the aforementioned lower limit, it is possible to suppress the variation of the dielectric constant and / or dielectric loss tangent of the porous resin layer 3 during hot working. The method for determining the imidization rate of the porous resin layer 3 is described in the following examples.

[0049] The porosity of the porous resin layer 3 is, for example, 50% or more, preferably 60% or more, and more preferably 70% or more. It should be noted that the porosity of the porous resin layer 3 is, for example, less than 100%, and further less than 99%. When the porous resin layer 3 is formed of polyimide, the porosity is calculated according to the following formula.

[0050] Porosity (%) = (1 - specific gravity of porous resin layer 3 / specific gravity of polyimide) × 100

[0051] The dielectric constant of the porous resin layer 3 at a frequency of 10 GHz is, for example, 1.63 or less, and also, for example, 1.55 or more. The dielectric constant of the porous resin layer 3 is determined by the resonator method.

[0052] The dielectric loss tangent of the porous resin layer 3 at a frequency of 10 GHz is, for example, 0.006 or less, preferably 0.005 or less, more preferably 0.004 or less, even more preferably 0.003 or less, and also, for example, 0.002 or more. The dielectric loss tangent of the porous resin layer 3 is measured by the resonator method.

[0053] The thickness of the porous resin layer 3 is, for example, 2 μm or more, preferably 5 μm or more, and also, for example, 1,000 μm or less, preferably 500 μm or less.

[0054] 1.4 Outer layer 4

[0055] An outer layer 4 is disposed at one end of the low-dielectric substrate material 1 in the thickness direction. The outer layer 4 forms one surface of the low-dielectric substrate material 1 in the thickness direction. The outer layer 4 extends along the surface direction. The outer layer 4 is in contact with one surface of the porous resin layer 3 in the thickness direction. For example, the outer layer 4 is a dense film formed of the same resin as the porous resin layer 3. The thickness of the outer layer 4 is, for example, 1 μm or more, and also, for example, 50 μm or less. The porosity of the outer layer 4 is 0.1% or less, and further, 0%.

[0056] 1.5 Method for manufacturing low-dielectric substrate material 1

[0057] The manufacturing method of low dielectric substrate material 1 is described.

[0058] First, prepare metal layer 2.

[0059] Next, a varnish comprising the above-mentioned resin precursor, a pore-forming agent, a nucleating agent, and a solvent is prepared. Then, the varnish is applied to one side of the metal layer 2 in the thickness direction to form a coating film. The types and mixing ratios of the pore-forming agent, nucleating agent, and solvent in the varnish are described, for example, in WO2018 / 186486.

[0060] The case where the resin is polyimide will be explained. The precursor of polyimide is, for example, the reaction product of a diamine component and an acid dianhydride component. Examples of diamine components include aromatic diamines and aliphatic diamines. From the viewpoint of obtaining a tensile modulus below the aforementioned upper limit, aromatic diamines are preferred as diamine components.

[0061] Furthermore, the diamine component and the acid dianhydride component can be used alone or in combination. Specifically, as the diamine component, the use of aromatic diamines alone is preferred.

[0062] Examples of aromatic diamines include the first diamine, the second diamine, and the third diamine.

[0063] The first diamine contains an aromatic ring. Examples of first diamines include phenylenediamine, dimethylphenylenediamine, and ethylmethylphenylenediamine. Phenylenediamine is preferred. Examples of phenylenediamines include o-phenylenediamine, m-phenylenediamine, and p-phenylenediamine. p-phenylenediamine is preferred. p-phenylenediamine is sometimes simply referred to as PDA.

[0064] The second diamine contains multiple aromatic rings and ether bonds disposed between them. Examples of second diamines include oxodiphenylamine. Examples of oxodiphenylamines include 3,4'-oxodiphenylamine and 4,4'-oxodiphenylamine. 4,4'-oxodiphenylamine (also known as 4,4-diaminodiphenyl ether) is preferred. 4,4'-oxodiphenylamine is sometimes simply referred to as ODA.

[0065] The third diamine contains multiple aromatic rings and ester bonds disposed between them. Examples of third diamines include aminophenylaminobenzoate, and more preferably 4-aminophenyl-4-aminobenzoate. 4-aminophenyl-4-aminobenzoate is sometimes simply referred to as APAB.

[0066] It should be noted that, in addition to the first to third diamines, other examples of aromatic diamines include 4,4'-methylenediphenylamine, 4,4'-dimethylenediphenylamine, 4,4'-trimethylenediphenylamine, and bis(4-aminophenyl) sulfone.

[0067] The aforementioned diamine components can be used alone or in combination. Preferably, combinations of a first diamine, a second diamine, and a third diamine are preferred diamine components. More preferably, combinations of p-phenylenediamine (PDA), 4,4'-oxodiphenylamine (ODA), and 4-aminophenyl-4-aminobenzoic acid ester (APAB) are preferred.

[0068] The molar fraction of the first diamine in the diamine component is, for example, 10 mol% or more, preferably 20 mol% or more, and also, for example, 70 mol% or less, preferably 65 mol% or less. The molar fraction of the second diamine in the diamine component is, for example, 5 mol% or more, preferably 10 mol% or more, and also, for example, 40 mol% or less, preferably 30 mol% or less. The molar fraction of the third diamine in the diamine component is, for example, 5 mol% or more, for example, 10 mol% or more, and also, for example, 40 mol% or less, preferably 30 mol% or less.

[0069] In addition, relative to the total of 100 moles of the first and second diamines, the moles of the third diamine are, for example, 5 moles or more, preferably 10 moles or more, more preferably 20 moles or more, and, for example, 100 moles or less, preferably 50 moles or less, more preferably 30 moles or less.

[0070] The dianhydride component is not limited. For example, the dianhydride component may contain an aromatic ring. Examples of aromatic ring-containing dianhydrides include, for example, aromatic tetracarboxylic acid dianhydrides. Examples of aromatic tetracarboxylic acid dianhydrides include, for example, benzophenone tetracarboxylic acid dianhydrides, biphenyl tetracarboxylic acid dianhydrides, biphenyl sulfone tetracarboxylic acid dianhydrides, and naphthalene tetracarboxylic acid dianhydrides. These can be used alone or in combination. As an aromatic ring-containing dianhydride, biphenyl tetracarboxylic acid dianhydrides are preferably included. Examples of biphenyl tetracarboxylic acid dianhydrides include, for example, 3,3'-4,4'-biphenyl tetracarboxylic acid dianhydrides, 2,2'-3,3'-biphenyl tetracarboxylic acid dianhydrides, 2,3,3',4'-biphenyl tetracarboxylic acid dianhydrides, and 3,3',4,4'-diphenyl ether tetracarboxylic acid dianhydrides. As a biphenyl tetracarboxylic acid dianhydride, 3,3'-4,4'-biphenyl tetracarboxylic acid dianhydrides are preferably included. It should be noted that 3,3'-4,4'-biphenyltetracarboxylic acid dianhydride is sometimes simply referred to as BPDA.

[0071] The ratio of diamine to dianhydride is adjusted, for example, by making the molar amounts of the amino group (-NH2) of the diamine and the molar amounts of the anhydride group (-CO-O-CO-) of the dianhydride equal.

[0072] For the preparation of the polyimide precursor, the above-mentioned diamine component, the above-mentioned acid dianhydride component, and solvent are mixed to prepare a varnish. The varnish is then heated to prepare a precursor solution. Next, a nucleating agent and a pore-forming agent are mixed into the precursor solution to prepare a porous precursor solution.

[0073] Subsequently, the porous precursor solution is coated onto one side of the metal layer 2 in the thickness direction to form a coating film.

[0074] Subsequently, the coating is dried by heating, thereby forming a precursor film. Through the aforementioned heating, the solvent is removed, and a precursor film with a phase-separated structure of a polyimide precursor and a porousing agent, using a nucleating agent as the core, is prepared. Furthermore, by heating the coating, an outer layer 4 is formed on one side of the precursor film in the thickness direction. The heating temperature is, for example, 160°C or less. The heating time is, for example, 1000 seconds or less.

[0075] Subsequently, for example, a pore-forming agent is extracted (extracted or removed) from the precursor film using supercritical extraction with supercritical carbon dioxide as a solvent. This forms a porous resin layer 3 before curing.

[0076] Subsequently, the precursor film is cured by heating. That is, the imidization of the precursor film is carried out. The temperature is, for example, 340°C or higher, preferably 350°C or higher, more preferably 360°C or higher, and also, for example, 410°C or lower, preferably 390°C or lower, more preferably 380°C or lower, and even more preferably 370°C or lower. When the temperature is above the lower limit, the mass reduction rate of the porous resin layer 3 can be reduced, thereby appropriately increasing the imidization rate, and a porous resin layer 3 having the above-mentioned aspect ratio AR is formed. When the temperature is below the upper limit, a porous resin layer 3 having the above-mentioned aspect ratio AR is formed.

[0077] Thus, a porous resin layer 3 made of polyimide is formed after curing.

[0078] Through the above, low-dielectric substrate material 1 is manufactured.

[0079] 1.6 Applications of Low-Dielectric Substrate Material 1

[0080] The application of the low-dielectric substrate material 1 is not limited. The low-dielectric substrate material 1 is, for example, processed into a flexible wiring board. In this case, the low-dielectric substrate material 1 is, for example, pressed along the thickness direction.

[0081] Additionally, before or after pressing, the metal layer 2 is etched to form a pattern.

[0082] 2. Effects of the first implementation method

[0083] In this low-dielectric substrate material 1, the average aspect ratio of the plurality of closed-hole structures 30 in the first region 31 is 0.80 or more and 1.20 or less, thus exhibiting excellent processability.

[0084] Specifically, it can suppress the thickness variation of the porous resin layer 3 before and after pressing during the processing of the low dielectric substrate material 1, thereby suppressing the variation of the dielectric constant and / or dielectric loss tangent of the porous resin layer 3.

[0085] 3. Variations

[0086] In the following variations, the same reference numerals are used to mark the same components and processes as in the first embodiment, and detailed descriptions are omitted. Furthermore, unless otherwise specified, the variations can achieve the same effects as the first embodiment. Moreover, the first embodiment and its variations can be appropriately combined.

[0087] 3.1 First Variation

[0088] Alternatively, the precursor film before heating (curing) can be treated as a porous resin layer 3.

[0089] The low-dielectric substrate material 1, which has the aforementioned precursor film as a porous resin layer 3, can suppress the thickness variation caused by subsequent heating (curing).

[0090] 3.2 Second Variation

[0091] In the first variation, such as Figure 2 As shown, the low-dielectric substrate material 1 may not have an outer layer 4, but may further have other metal layers 5. The other metal layers 5 are disposed at the other end of the low-dielectric substrate material 1 in the thickness direction.

[0092] In the porous resin layer 3, another metal layer 5 is disposed on one surface of the porous resin layer 3. Specifically, the other metal layer 5 is in contact with one surface of the porous resin layer 3. The other metal layer 5 extends along the surface direction. The other metal layer 5 has the same configuration as the metal layer 2.

[0093] The average aspect ratio AR of each of the two regions (the region adjacent to metal layer 2 and the region adjacent to other metal layers 5) can be calculated as follows: the average of the lower of these two regions is defined as the average aspect ratio AR of the first region 31. Figure 2 In the text, the case where the region adjacent to metal layer 2 is region 31 is recorded outside the parentheses. Figure 2 In the text, the case where the region adjacent to other metal layers 5 is designated as region 31 is described in parentheses.

[0094] 3.3 Other variations

[0095] Although not shown, the low-dielectric substrate material 1 has a metal layer 2 and a porous resin layer 3, but does not have an outer layer 4 or other metal layers 5.

[0096] Although not shown, the low-dielectric substrate material 1 includes a metal layer 2, a porous resin layer 3, an outer layer 4, and other metal layers 5.

[0097] Although not shown in the figure, the outer layer 4 can be disposed on one side and the other side of the porous resin layer 3 in the thickness direction.

[0098] Although not shown in the figure, the adhesive layer can be disposed on one side and the other side of the porous resin layer 3 in the thickness direction.

[0099] Although not shown in the figure, the outer layer 4 can be disposed on one side or the other side of the porous resin layer 3 in the thickness direction.

[0100] Example

[0101] The following examples and comparative examples illustrate the present invention in more detail. It should be noted that the present invention is not limited by the examples and comparative examples. Furthermore, the specific numerical values ​​of mixing ratios (including proportions), physical property values, parameters, etc., used in the following description can replace the upper limit (defined as "below" or "less than") or lower limit (defined as "above" or "more than") of the corresponding mixing ratios (including proportions), physical property values, parameters, etc., described in the above-described "Specific Embodiments".

[0102] 1. Manufacturing of low-dielectric substrate material 1

[0103] Example 1

[0104] A diamine component solution was prepared by dissolving 0.66 mol of p-phenylenediamine (PDA) (the first diamine), 0.22 mol of 4,4'-oxodiphenylamine (ODA) (the second diamine), and 0.22 mol of 4-aminophenyl-4-aminobenzoate (APAB) (the third diamine) in N-methyl-2-pyrrolidone (NMP). Next, 1.00 mol of 3,3'-4,4'-biphenyltetracarboxylic acid dianhydride (BPDA) was added to the diamine component solution, and the mixture was stirred at 80°C. After stirring was stopped, the mixture was allowed to cool naturally to prepare a polyimide precursor solution.

[0105] The solid component concentration of the polyimide precursor solution is 13% by mass.

[0106] To a polyimide precursor solution of 100 parts by mass, 150 parts by mass of polyoxyethylene dimethyl ether (Nippon Oil-made grade: MM400) with a weight average molecular weight of 400 as a pore-forming agent and 3 parts by mass of PTFE powder with a particle size of less than 1 μm as a nucleating agent were added, and the mixture was stirred to obtain a transparent, homogeneous solution. To the obtained solution, 4 parts by mass of 2-methylimidazolium as an imidization catalyst were added to prepare a varnish. The varnish was coated onto a metal layer 2 formed from a copper thin film to form a coating film, and dried by heating at 120–160°C for 540 seconds, thereby removing NMP. Thus, a polyimide precursor film with a thickness of approximately 50 μm was formed on one side of the metal layer 2 in the thickness direction.

[0107] Subsequently, a supercritical extraction method using supercritical carbon dioxide as a solvent was employed to promote the extraction and removal of the pore-forming agent, phase separation of residual NMP, and the formation of pores (closed-pore structures). This yielded a porous precursor film. Next, the precursor film was heated at 360°C to undergo imidization. This resulted in a low-dielectric substrate material 1.

[0108] Example 2, Comparative Example 1, and Comparative Example 2

[0109] The same procedure as in Example 1 was followed to obtain low-dielectric substrate material 1. The heating temperature during imidization was changed as shown in Table 1.

[0110] 2. Evaluation

[0111] Regarding the low-dielectric substrate material 1 of each embodiment and comparative example, the following matters were evaluated. The results are recorded in Table 1.

[0112] 2.1 Mass reduction rate of porous resin layer 3

[0113] 2 mg of porous resin layer 3 was collected to obtain the sample. The sample was heated at 450 °C for 1 hour. The mass reduction rate of the sample after heating was calculated based on the following formula.

[0114] [Mass of sample before heating - Mass of sample after heating] / (Mass of sample after heating) × 100

[0115] 2.2 Imidification rate of porous resin layer 3

[0116] Using the apparatus and method described below, the imidization rate of the porous resin layer 3 is calculated in the form of the intensity ratio of the peaks obtained by infrared absorption spectroscopy (IR).

[0117] Device: Nicolet IR-200

[0118] Measurement conditions: ATR, 128 cumulative measurements, 2 cm resolution -1

[0119] Imidination strength ratio: based on 1773cm -1 and 3064cm -1 The strength ratio was measured. Next, the porous resin layer 3 (heated at 420°C) of Comparative Example 2 was used as a fully cured product, and the imidization rate of Example 1, Example 2 and Comparative Example 1 was calculated based on the imidization strength ratio of Comparative Example 2 (i.e., 1).

[0120] 2.3 Average aspect ratio AR of closed-cell structure 30 in region 1 31

[0121] Using the following apparatus and conditions, a cross-sectional SEM observation of the low-dielectric substrate material 1 was performed to obtain an SEM image. The aspect ratio AR of all closed-hole structures 30 observed in region 31 of the SEM image was determined, and their average was calculated.

[0122] SEM device: SU8020, manufactured by Hitachi Corporation

[0123] Measurement conditions: accelerating voltage 2.0kV

[0124] Observation magnification: ×750

[0125] 2.4 Thickness of porous resin layer 3

[0126] The thickness of the porous resin layer 3 was measured using a thickness gauge (HKT-1200, manufactured by Fujiwork Co., Ltd.).

[0127] 2.5 Variation in the thickness of porous resin layer 3

[0128] The low-dielectric substrate material 1 was heated at 450°C for 1 hour. The thickness of the low-dielectric substrate material 1 after heating was determined. Then, the change in the thickness of the low-dielectric substrate material 1 after heating was determined.

[0129] Next, the heated low-dielectric substrate material 1 is pressed at 5 MPa for 5 minutes. The thickness of the pressed low-dielectric substrate material 1 is then determined. Finally, the variation in the thickness of the pressed low-dielectric substrate material 1 is calculated.

[0130] 2.6 Dielectric constant and dielectric loss tangent of porous resin layer 3

[0131] The dielectric constant and dielectric loss tangent of the porous resin layer 3 at 10 GHz were determined using the following apparatus and method.

[0132] Device: PNA network analyzer (manufactured by Agilent Technologies)

[0133] The result is obtained using the Split Dielectric Resonator (SPDR) method.

[0134] [Table 1]

[0135] Table 1

[0136]

[0137] It should be noted that the above-described invention is provided as an illustrative embodiment of the present invention, but it is merely an example and not intended to be limiting. Modifications of the invention that will be obvious to those skilled in the art are included in the foregoing claims.

[0138] Industrial availability

[0139] The low-dielectric substrate material is processed into a flexible wiring board.

[0140] Explanation of reference numerals in the attached figures

[0141] 1. Low-dielectric substrate materials

[0142] 2 Metal layer

[0143] 3 Porous resin layer

[0144] 5 Metal Layers

Claims

1. A low-dielectric substrate material comprising: a metal layer and a porous resin layer disposed on one side of the metal layer in the thickness direction. The porous resin layer is formed of polyimide. The imidization rate of the porous resin layer is 0.920 or higher. The porous resin layer comprises: When the porous resin layer is divided into four equal parts along its thickness direction, the first, second, third, and fourth regions are arranged sequentially in the direction away from the metal layer. At least the first region has multiple independent closed-cell structures in the resin matrix. The average length-to-diameter ratio (L1 / L2) of the plurality of closed-cell structures in the first region, which is the ratio of the length L1 of the closed-cell structure in the direction orthogonal to the thickness direction in the cross-sectional view to the length L2 of the closed-cell structure in the thickness direction, is 0.80 or more and 1.20 or less.

Images