Base material, method for producing same, and wiring board

Abstract

Provided is a base material with excellent bonding reliability. A base material 100 according to the present invention has a first main surface 101 on which a conductor layer is formed and a second main surface 102 that is in a front-back relationship with the first main surface 101. The base material 100 includes an internal region 103 containing a fluororesin as a main component, and a surface region 104 formed on the internal region 103, the surface region 104 including the first main surface 101 and containing a fluororesin as a main component. When the material strength of the base material 100 in the direction perpendicular to the first main surface 101 is defined as the base material peeling strength, the base material peeling strength of the surface region 104 is greater than the base material peeling strength of the internal region 103.

Description

Substrate, manufacturing method thereof, and wiring board

[0001] The present invention relates to a substrate and a wiring board having excellent bonding reliability, and to a method for manufacturing the same.

[0002] A material that forms the base on which other layers are formed is called a substrate, and such substrates are widely used in various industries. The mechanical, physical, and chemical properties of a substrate have a significant impact on the performance and reliability of a device that incorporates the substrate, so the substrate structure is determined so as to simultaneously satisfy multiple independent characteristics required of the device.

[0003] For example, in the case of a substrate that constitutes the dielectric layer of a wiring board, a conductor layer is formed on the surface to form the wiring board. This substrate is required to simultaneously satisfy multiple independent required characteristics of the wiring board, such as heat resistance, insulation, chemical stability, dimensional stability, ease of forming the conductor layer, and bonding reliability. Substrates that satisfy these various required characteristics have been used, such as those in which various fillers are dispersed in polymers made of epoxy resin, polyimide, liquid crystal polymer, etc.

[0004] Recently, the increase in transmission speeds has required electronic devices and the wiring boards included therein to be able to handle even higher frequencies, such as frequencies exceeding 30 GHz or 80 GHz. In addition to the various properties described above, the substrates used in such wiring boards are also strongly required to have electrical properties important for high-frequency transmission, such as a low dielectric constant and transmission loss. One solution to this requirement is a fluororesin such as polytetrafluoroethylene (hereinafter referred to as PTFE). Fluororesin materials not only have excellent heat resistance, electrical insulation, and chemical stability, but also inherently possess many advantageous properties for dielectric layers in high-speed transmission, such as a low dielectric constant and a low dielectric loss tangent.

[0005] However, even with fluororesin materials, simply replacing existing resins makes it difficult to simultaneously satisfy multiple required characteristics for wiring boards. Patent Document 1 discloses a fluororesin printed circuit board (PCB) having at least one layer of nonwoven fabric formed by fusion-bonding fluororesin short fibers randomly oriented in both the longitudinal and transverse directions, and an insulating layer substantially composed of fluororesin alone. Because the fluororesin short fibers are randomly oriented in both the longitudinal and transverse directions, the thermal expansion coefficient is reduced due to the anisotropy of the thermal expansion coefficient of the short fibers. Since the nonwoven fabric is essentially composed of fluororesin alone, the fluororesin printed circuit board's dielectric constant and dielectric loss tangent are low values ​​inherent to fluororesin. However, since heat treatment at a temperature above the melting point of PTFE is required to obtain a nonwoven fabric composed solely of PTFE, controlling the thermal expansion coefficient is more difficult than without such heat treatment. In other words, the printed circuit board disclosed in Patent Document 1 leaves room for improvement in achieving both electrical properties and thermal expansion coefficient control.

[0006] In addition, the adhesiveness between the substrate and other layers formed on the surface can also be an important requirement. For example, attempts have been made to improve the adhesiveness between a substrate mainly composed of fluororesin and a conductor pattern formed on the surface.

[0007] Patent Document 2 states that dielectric substrates made of fluororesin have low surface energy due to their high chemical stability, making adhesion to other materials extremely difficult, and discloses that the surface of a dielectric substrate containing fluororesin is subjected to atmospheric pressure plasma treatment to introduce peroxide radicals, followed by a copper-containing film formation process in which a copper-containing composition is applied and heated to modify the surface. This causes peroxide radicals to form on the surface of the dielectric substrate containing fluororesin, and these peroxide radicals bond with metal ions of various metal-containing compositions, thereby increasing the adhesive strength of the metal film to the substrate surface.

[0008] Patent Document 3 discloses a technique for forming a base metal layer having a predetermined structure by dry plating such as sputtering, and then forming a copper conductor layer of a desired thickness on the base metal layer, in order to address the problem that the anchor effect used in bonding between the metal and the insulating film cannot be expected because the metal-insulating film interface of a copper clad laminate (CCL) obtained by a metallizing method is smooth, and therefore the adhesive strength of the interface is not sufficiently exhibited.

[0009] All of these merely aim to improve the adhesive strength at the surface of the substrate or insulating film, i.e., at the interface between the substrate or insulating film and the metal film, and do not disclose any configuration for suppressing material failure of the substrate, which may become apparent after the interfacial bonding strength has been ensured.

[0010] JP 2002-76544 A JP 2017-43829 A JP 2010-253693 A

[0011] In view of the above circumstances, the invention described in the claims of this application aims to provide a substrate that has high bonding reliability between the substrate and another layer formed on the surface of the substrate and simultaneously satisfies other properties required of the substrate, a device using the substrate, and methods for manufacturing the same.

[0012] A substrate according to one aspect of the present invention is, for example, a substrate having a first main surface on which a conductor layer is formed and a second main surface that is opposite to the first main surface, the substrate including an internal region whose main component is a fluororesin and a surface region formed on the internal region, which includes the first main surface and is also made of a fluororesin, and when the material strength in a direction perpendicular to the first main surface of the substrate is defined as the substrate peel strength, the substrate peel strength of the surface region can be greater than the substrate peel strength of the internal region.

[0013] Furthermore, a substrate according to one aspect of the present invention can be, for example, a substrate having a first main surface on which a conductor layer is formed and a second main surface that is opposite to the first main surface, the substrate including an internal region whose main component is a fluororesin and a surface region formed on the internal region, which includes the first main surface and is also made of a fluororesin as a main component, and the crystallinity of the surface region, expressed as the ratio of the peak area of ​​the crystalline component to the total peak area by X-ray diffraction, can be a substrate in which the crystallinity of the surface region is smaller than that of the internal region.

[0014] Furthermore, a substrate according to one aspect of the present invention can be, for example, a substrate having a first main surface on which a conductor layer is formed and a second main surface that is opposite to the first main surface, the substrate including an internal region whose main component is a fluororesin and a surface region formed on the internal region, including the first main surface, and whose main component is a fluororesin, the fluororesin being polytetrafluoroethylene, and when an endothermic reaction observed near 345°C in a DSC curve obtained by differential scanning calorimetry of the internal region and the surface region of the substrate is defined as a first endothermic reaction, the heat of fusion of the first endothermic reaction of the surface region can be a substrate in which the heat of fusion of the first endothermic reaction of the surface region is smaller than the heat of fusion of the first endothermic reaction of the internal region.

[0015] Furthermore, a substrate according to one aspect of the present invention can be, for example, a substrate having a first main surface and a second main surface that is opposite to the first main surface, the substrate including an internal region and a surface region that is formed on the internal region and includes the first main surface, the substrate being a laminate of multiple thin films made of a fluororesin, the multiple thin films including a first thin film located in the internal region and a second thin film located in the surface region, the multiple thin films made of a fluororesin being polytetrafluoroethylene, the first thin film being made of unsintered expanded polytetrafluoroethylene, and the second thin film being made of sintered polytetrafluoroethylene.

[0016] Furthermore, a wiring board according to an aspect of the present invention can be, for example, a wiring board including the above-described base material and a conductor layer formed on the first main surface of the base material.

[0017] A method for manufacturing a substrate according to one aspect of the present invention can be, for example, a method for manufacturing a substrate that includes preparing a plurality of polytetrafluoroethylene thin films, stacking the plurality of polytetrafluoroethylene thin films, and applying pressure in the thickness direction, wherein the plurality of polytetrafluoroethylene thin films include a first polytetrafluoroethylene thin film and a second polytetrafluoroethylene thin film, and when the material strength in the thickness direction of the thin films is defined as the thin film peel strength, the second polytetrafluoroethylene thin film has a higher thin film peel strength than the first polytetrafluoroethylene thin film.

[0018] Furthermore, a method for manufacturing a substrate according to one aspect of the present invention can be a method for manufacturing a substrate that includes, for example, extruding a polytetrafluoroethylene resin to prepare a polytetrafluoroethylene sheet, stretching the polytetrafluoroethylene sheet to prepare a plurality of stretched polytetrafluoroethylene thin films, preparing a first stretched polytetrafluoroethylene thin film without firing a portion of the plurality of stretched polytetrafluoroethylene thin films, firing another portion of the plurality of stretched polytetrafluoroethylene thin films to prepare a second stretched polytetrafluoroethylene thin film, stacking the first stretched polytetrafluoroethylene thin film and the second stretched polytetrafluoroethylene thin film, and pressing them in the thickness direction.

[0019] These substrates, wiring boards, and methods for manufacturing substrates provide a substrate, wiring board, and method for manufacturing substrates that simultaneously satisfy multiple independent required properties, including bonding reliability, while maintaining the inherent excellent properties of the polymers that make up the substrates by controlling the state of the polymers that make up the substrates.

[0020] 1(a) is a schematic diagram showing a substrate according to a first embodiment of the present invention; FIG. 1(a) is a plan view of the substrate as seen from above, and FIG. 1(b) is a cross-sectional view as seen from the side; FIG. 1(b) is a diagram showing DSC curves of PTFE thin films having different crystal structures; FIG. 1(c) is a schematic diagram showing a substrate according to a fourth embodiment of the present invention; FIG. 1(d) is a schematic diagram showing one aspect of the wiring board of the present invention; FIG. 1(e) is a diagram for explaining an example of a manufacturing flow of the substrate of the present invention;

[0021] The following describes embodiments of the substrate and its manufacturing method according to the invention as claimed in the present application, as well as a wiring board having the substrate as a dielectric layer. The embodiments described below do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the solution of the invention. Furthermore, each embodiment and each embodiment in the examples can be freely combined within the scope that does not lose the technical significance of the present invention.

[0022] First Embodiment FIG. 1 is a schematic diagram of a substrate 100 according to a first embodiment of the present invention. FIG. 1( a) is a schematic diagram of the substrate surface of the substrate 100 viewed vertically from above, and FIG. 1( b) is a schematic diagram showing a cross section of the substrate 100 viewed from the side of the substrate. In this specification, the direction perpendicular to the substrate surface is referred to as the thickness direction of the substrate. Similarly, for a wiring board, the direction perpendicular to the substrate surface in the wiring board is referred to as the thickness direction of the wiring board. The thickness directions of the substrate and the wiring board are represented as the Z-axis direction in the Cartesian coordinate system shown in the figures of this specification. Similarly, in this specification, the direction parallel to the substrate surface is referred to as the surface direction of the substrate, and similarly, for a wiring board, the direction parallel to the substrate surface in the wiring board is referred to as the surface direction of the wiring board. The surface directions of the substrate and the wiring board are represented as arbitrary directions on a plane including the X-axis and the Y-axis in the Cartesian coordinate system shown in the figures of this specification. Furthermore, in this specification, terms such as parallel and perpendicular do not require a geometrical level. Unless otherwise specified and unless there is any technical contradiction, for example, parallel may include a case where the angle between two lines (or planes) is within the range of 0 degrees ± 3 degrees. Similarly, perpendicular may include a case where the angle between two lines (or planes) is within the range of 90 degrees ± 3 degrees.

[0023] The substrate 100 has a first main surface 101 and a second main surface 102 that is opposite to the first main surface 101. FIG. 1( a) is a plan view of the first main surface 101 of the substrate 100. Note that the ordinal numbers, such as "first" for the first main surface 101 and "second" for the second main surface 102, do not indicate an order but are simply used to distinguish between the two main surfaces. The ordinal numbers used hereinafter in this specification are also based on this concept.

[0024] As shown in FIG. 1B , the substrate 100 of this embodiment has an internal region 103 and a surface region 104 formed on the internal region 103 and including a first main surface 101. The internal region 103 and the surface region 104 of the substrate 100 can be made primarily of the same organic polymer. The substrate 100 configured in this manner can suppress deterioration of characteristics due to the organic polymer constituting the surface region 104, even when the surface region 104 is formed on the internal region 103. For example, in this embodiment, both the internal region 103 and the surface region 104 can be made primarily of a fluororesin. In the substrate 100 having this configuration, the internal region 103 and the surface region 104 are made primarily of a material other than a fluororesin, thereby providing the excellent electrical characteristics inherent to fluororesins.

[0025] When the material strength in a direction perpendicular to the first main surface 101 is defined as the substrate peel strength, the substrate peel strength of the surface region 104 of the substrate 100 is greater than the substrate peel strength of the internal region 103. That is, the substrate 100 comprises the internal region 103 and the surface region 104, which is mainly composed of the same organic polymer as the internal region 103 and has a substrate peel strength greater than that of the internal region 103. The substrate 100 having such a configuration can simultaneously satisfy multiple required properties, including bonding reliability at the bonding interface, while maintaining the inherent properties of the polymer that constitutes the substrate.

[0026] The present inventors have discovered that by controlling the molecular state of the organic polymer that constitutes the substrate, it is possible to independently control one property from another, even while maintaining the same main component. For example, in a substrate primarily composed of a fluororesin, differences in the orientation of the fluororesin do not significantly affect the electrical properties of the substrate, but do significantly affect some of the substrate's physical properties. By utilizing this, it is possible to impart desired physical properties to the substrate while maintaining the inherent electrical properties of the fluororesin. Furthermore, it has been discovered that by controlling the molecular state according to the position within the substrate, it is possible to provide a substrate that simultaneously satisfies even more independent required properties. For example, a structure in which either the internal region or the surface region of the substrate each plays a role in contributing more to property improvement than the other can result in greater overall property improvement than a uniform structure. For example, in a wiring board using a fluororesin base material, by providing a surface region with a property in which the material strength in the thickness direction is relatively large and providing an internal region with a property in which the material strength in the thickness direction is relatively small but which contributes to other properties, it is possible to simultaneously satisfy many more required properties than a fluororesin substrate that does not have such control, while maintaining the electrical properties that the fluororesin inherently possesses.

[0027] An example of a property to which the internal region contributes relatively greatly is the dimensional stability of the substrate and wiring board. Here, dimensional stability includes small dimensional changes in the substrate due to temperature changes or external forces. As an indicator of dimensional changes in the substrate or board due to temperature changes, the rate of change in length in response to an increase in temperature, i.e., the coefficient of linear expansion, is generally used. Hereinafter, in this specification, the coefficient of linear expansion may also be simply referred to as CTE. Other examples of properties that are preferably preferentially carried out by the internal region include control of the elastic modulus of the wiring board to which the substrate is applied, control of thickness, and ensuring insulation reliability between conductor layers.

[0028] For example, a substrate 100 is provided that has a first main surface 101 on which a conductor layer is formed and a second main surface 102 that is opposite to the first main surface 101, wherein the substrate 100 includes an internal region 103 whose main component is a fluororesin, and a surface region 104 that is formed on the internal region 103, includes the first main surface 101, and is also made of a fluororesin as a main component, and wherein when the material strength in a direction perpendicular to the first main surface 101 of the substrate 100 is defined as the substrate peel strength, the substrate peel strength of the surface region 104 is greater than the substrate peel strength of the internal region 103.The substrate 100 thus provided simultaneously has excellent electrical properties brought about by the low dielectric constant, low dielectric tangent, and other properties of the fluororesin, bonding reliability with the conductor layer brought about by the surface region with increased material peel strength, and other properties such as a low CTE brought about by the internal region.

[0029] The substrate 100 can be a sheet-like insulating material extending in the surface direction of the substrate. Other layers can be formed on the surface of the substrate, i.e., the first and / or second main surfaces. Examples of such layers include conductor layers containing metal or other materials. For example, in a wiring board, when the conductor layer includes multiple conductor patterns for achieving desired electrical connections, the substrate 100 can support multiple electrically independent conductor patterns. These conductor patterns can be formed directly on the first and / or second main surfaces 101 and 102. In other words, in a wiring board, the first and / or second main surfaces 101 and 102 of the substrate 100 can serve as a bonding interface with the conductor patterns. Furthermore, in a wiring board having multiple conductor layers in the thickness direction, the substrate 100 serves as the smallest dielectric layer that physically and electrically separates one conductor layer from another conductor layer adjacent to the one conductor layer in the thickness direction.

[0030] Although the internal region 103 and the surface region 104 are shown as rectangular regions indicated by dashed lines in FIG. 1( b), their respective formation ranges are not limited to these rectangular regions. The internal region 103 and the surface region 104 are preferably formed so as to extend continuously in a direction parallel to the first main surface 101, i.e., in the plane direction of the substrate 100. The internal region 103 and / or the surface region 104 are preferably formed over the entire area in the plane direction of the substrate 100, but may be formed with a portion missing. Even if a portion is missing, it is preferable that the internal region 103 and the surface region 104 include an overlapping region when viewed in the thickness direction of the substrate, as shown by the two rectangular regions indicated by dashed lines in FIG. 1( b). The internal region 103 and the surface region 104 may be spaced apart from each other, as shown in FIG. 1( b), or may be formed so as to contact each other. The substrate may further have a surface region 105 including the second main surface 102. In this case, the internal region 103 can be formed between the surface region 104 and the surface region 105.

[0031] In this embodiment, the inner region 103 and the surface region 104 of the substrate 100 may both be primarily composed of fluororesin. In this specification, the term "primary component" refers to the component that occupies the largest amount in the composition, and this also applies elsewhere unless otherwise specified. If the fluororesin ratio is defined as the weight ratio of fluororesin to the weight of all constituent materials constituting a certain region, then in this embodiment, the fluororesin ratios of the inner region 103 and the surface region 104 may each be 90 wt % or more. In this case, it is preferable that the fluororesin ratio of the entire substrate 100 is also 90 wt % or more. From the viewpoint of the transmission characteristics of a wiring board including the substrate 100, the fluororesin ratio of the inner region 103 is preferably 95 wt % or more, and more preferably 97 wt % or more. Similarly, the fluororesin ratio of the surface region 104 is preferably 95 wt % or more, and more preferably 97 wt % or more. The fluororesin ratio of the entire substrate 100 is preferably 95 wt % or more, and more preferably 97 wt % or more. Furthermore, in this embodiment, the internal region 103 and the surface region 104 of the substrate 100 can both be made primarily of perfluoroalkoxyalkane (hereinafter referred to as PFA), a fluororesin. Based on the same concept as above, if the weight ratio of PFA to the weight of all constituent materials constituting a certain region is defined as the PFA ratio, in this embodiment, the PFA ratios of the internal region 103 and the surface region 104 can each be 90% by weight or more. In this case, it is preferable that the PFA ratio in the entire substrate 100 is also 90% by weight or more. From the viewpoint of the transmission characteristics of a wiring board including the substrate 100, the PFA ratio in the internal region 103 is preferably 95% by weight or more, and more preferably 97% by weight or more. Similarly, the PFA ratio in the surface region 104 is preferably 95% by weight or more, and more preferably 97% by weight or more. The PFA ratio in the entire substrate 100 is preferably 95% by weight or more, and more preferably 97% by weight or more. In this embodiment, the inner region 103 and the surface region 104 of the substrate 100 may both contain PTFE, which is a fluororesin, as a main component.If the PTFE ratio is defined as the weight ratio of PTFE to the weight of all constituent materials constituting a certain region, in this embodiment, the PTFE ratios of the inner region 103 and the surface region 104 can each be 90 wt% or more. In this case, the PTFE ratio in the entire substrate 100 is preferably 90 wt% or more. From the viewpoint of the transmission characteristics of a wiring board including the substrate 100, the PTFE ratio in the inner region 103 is preferably 95 wt% or more, and more preferably 97 wt% or more. Similarly, the PTFE ratio in the surface region 104 is preferably 95 wt% or more, and more preferably 97 wt% or more. The PTFE ratio in the entire substrate 100 is preferably 95 wt% or more, and more preferably 97 wt% or more.

[0032] The fluororesin ratio of the inner region 103 and the fluororesin ratio of the surface region 104 may be different, but preferably the fluororesin ratio of the surface region is 0.95 to 1.05 times, more preferably 0.97 to 1.03 times, of the fluororesin ratio of the inner region. It is particularly preferable that the main component of the inner region and the main component of the surface region have the same chemical composition. In this specification, "having the same chemical composition" means that the respective ratios of chemical components, such as elements and compounds, constituting each region are equal. Note that "equal ratios of chemical components" does not necessarily mean a perfect mathematical match, and includes cases where the difference between the composition ratios of the chemical components constituting the inner region and the composition ratios of the chemical components constituting the surface region is 2% by weight or less. In addition, unavoidable differences in terminal functional groups and impurities are acceptable.

[0033] In particular, for substrates applied to wiring boards including high-frequency transmission paths, it is particularly preferable that the internal region 103 and the surface region 104 are each composed substantially of fluororesin. It is even more preferable that the entire substrate 100 is composed substantially of fluororesin. Such a configuration makes it possible to provide a wiring board with excellent high-frequency transmission characteristics that are extremely difficult to achieve with substrates primarily composed of other resins. In this specification, "composed substantially of a specific resin" does not exclude cases in which impurities are contained, and means, for example, that the specific resin accounts for 99% by weight or more. In other words, it is preferable that the substrate 100 is composed to contain as few components other than fluororesin as possible, including CTE control materials made of materials such as glass and ceramics whose CTE is smaller than that of fluororesin.

[0034] The other layer formed on the surface of the substrate can be, for example, a conductor layer made of metal in the case of a wiring substrate. Examples of metals include single metals such as copper, silver, gold, tin, nickel, and aluminum, or alloys made of multiple single metals. As will be described in detail later, a copper film formed by rolling or plating is particularly suitable. The other layer formed on the surface of the substrate is not limited to the above, and can also be, for example, a different resin layer, a layer made of a sintered body of conductive or non-conductive fine particles, or a resin layer with such fine particles dispersed therein. In plan view, the other layer may be formed over the entire surface of the substrate, or may be formed only partially, and may have a predetermined shape. Examples of predetermined shapes include wiring patterns that realize arbitrary electrical connection or insulation, patterns shaped like symbols for recording / displaying information, and patterns that control physical properties such as the flexural modulus of the substrate. Forming these other layers directly on the surface of the substrate can suppress deterioration of properties other than bonding reliability, for example, due to the presence of an adhesive layer.

[0035] The present inventors conducted the peel strength test described below using samples in which a metal layer was formed on a fluororesin substrate with different formation conditions. The results confirmed that in this bonding system, the failure morphology included a mixture of interfacial delamination between the metal layer and the substrate and material failure of the substrate. Furthermore, it was confirmed that a level where interfacial delamination was suppressed and material failure was prominent indicated a relatively high peel strength. Therefore, even for substrates made of materials that are considered relatively difficult to adhere to, such as fluororesin, it is possible to ensure interfacial adhesive strength equal to or greater than the material strength through appropriate processing and configuration. Furthermore, it was discovered that, in this state, improving the material strength of the substrate can further improve bonding reliability. At least for substrates primarily composed of fluororesin, when the substrate has interfacial adhesive strength sufficient to include material failure on the fracture surface formed in the peel strength test, increasing the material strength of the substrate in the thickness direction near the substrate surface is effective in improving bonding reliability.

[0036] The fracture morphology can be confirmed by optical microscopic observation of the bonded surface of the metal layer with the substrate after peeling. When the metal layer is completely exposed at the bonded surface of the metal layer with the substrate, i.e., the bonded interface with the substrate before peeling, it can be determined that interfacial peeling has occurred. When a portion of the substrate constituent material adheres to the bonded surface of the metal layer with the substrate, it can be determined that material fracture has occurred. The substrate constituent material that adheres during material fracture is often a very thin resin film attached to the metal layer. Therefore, increasing the material strength of the substrate against peel stress, i.e., external stress acting to peel off a portion of the substrate in the thickness direction of the substrate, at least in the shallow region of the substrate surface, is effective in improving the bonding reliability of the substrate. Note that no material fracture of the metal layer was observed, so in this specification, material fracture refers to material fracture of the substrate unless otherwise specified.

[0037] In this specification, the material strength in the thickness direction of the substrate is referred to as substrate peel strength. Substrate peel strength can be easily evaluated by measuring the amount of substrate carried away by another layer formed on the substrate surface to be evaluated when the other layer is peeled away by external stress. The application and peel conditions are not necessarily limited to specific ranges, but care should be taken to minimize the difference in conditions between comparison targets. For evaluation, the other layer can be a cellophane tape with an adhesive layer formed on one side. For this application, it is recommended to use a jig or other device to control the pressure, pressure application time, temperature, etc. Adhesion conditions can be adjusted depending on the tape type and substrate surface, but they must be such that material failure occurs in some or all evaluation levels, with a portion of the substrate being carried away by the tape. Regarding peel strength, it is possible to set conditions suitable for inter-level comparison using a jig or other device, but existing equipment used for peel strength testing, described below, can also be used. The amount of substrate carried away by the adhesive layer can be determined by the change in tape weight, or visual judgment using a microscope is also acceptable. The evaluation surface can be prepared using a microtome or by flat-polishing a resin-encapsulated substrate sample, but if a semi-finished product from the substrate manufacturing process, such as a thin film (described below), is available, comparative evaluation using this will enable simpler and more stable measurements.

[0038] Even when the same polymer material is used as the main component, different substrate peel strengths can be achieved by controlling the substrate formation conditions. When the substrate peel strength due to the formation conditions is unknown, the above-described evaluation can be performed on a plurality of substrate candidates formed under different formation conditions, and the substrate 100 can be constructed such that the substrate formed under the formation conditions that showed a relatively low substrate peel strength is used for the inner region 103, and the substrate formed under the formation conditions that showed a relatively high substrate peel strength is used for the surface region 104. The substrate formed under the formation conditions that show a low substrate peel strength and that can be used for the inner region 103 can be selected from those that have more advantageous properties other than substrate peel strength than the substrate formed under the formation conditions that show a high substrate peel strength and that can be used for the surface region 104. In particular, a substrate with a low level of substrate peel strength is preferably one that can compensate for properties that would be disadvantageous if the substrate peel strength were high, i.e., properties that are in a trade-off relationship with the substrate peel strength. Examples of materials with a low level of substrate peel strength include materials with a relatively low CTE in the surface direction of the substrate, or materials with a relatively high Young's modulus, dielectric strength, and / or material strength in the surface direction of the substrate, compared to materials with a high level of substrate peel strength.

[0039] When the main component of the substrate is a fluororesin, the fluororesin may contain PTFE. Alternatively, the fluororesin may contain PFA. PTFE may be a homopolymer of tetrafluoroethylene (hereinafter referred to as TFE), or it may be a modified PTFE containing a small amount of other monomers. Examples of small amounts of monomers other than TFE contained in modified PTFE include chlorotrifluoroethylene (CTFE), hexafluoropropylene (HFP), and perfluoroalkyl vinyl ether (PPVE). One or more of these monomers may be contained in combination. These fluororesins are among the materials with the smallest dielectric tangents among various materials, and by using them appropriately as the dielectric layer of a wiring board, it is possible to achieve excellent transmission characteristics that are difficult to achieve with other materials. In particular, PTFE can obtain a unique microporous structure consisting of nodes and fibrils by stretching, which can be utilized to control the specific orientation state. In combination with such control, a substrate with useful properties can be constructed. PTFE that has been stretched is called expanded PTFE, expanded PTFE, or EXPANDED PTFE, and is widely used in the clothing industry. The average molecular weight of fluororesin is 1 x 10 ６ More than 1×10 is preferable. ７ Fluororesins having such average molecular weights have the advantage that their properties as substrates or wiring boards tend to be more stable than low-molecular-weight fluororesins, even at high temperatures, particularly at or above the crystalline melting temperature.

[0040] The relative permittivity (εr) of the substrate 100 at 30 GHz and 22°C can be 2.3 or less, preferably 2.1 or less. The dielectric loss tangent (tanδ) under the same conditions can be 0.001 or less, preferably 0.0003 or less, and particularly preferably 0.0002 or less. Furthermore, even at 50 GHz and 110 GHz, the substrate 100 has a relative permittivity (εr) of 2.1 or less and a dielectric loss tangent (tanδ) of 0.0002 or less at 22°C, enabling low-loss signal transmission in high frequency bands that are very difficult to achieve with other materials. It is preferable that a wiring board using the substrate 100 as a dielectric layer also has the same characteristics as above. Such a wiring board can be obtained by including or minimizing constituent materials other than the substrate 100 and conductors.

[0041] The thickness of the substrate 100, i.e., the size in the Z direction in FIG. 1(b), is preferably 0.01 mm or more, more preferably 0.05 mm or more, from the viewpoints of ease of handling and insulation reliability when formed into a wiring substrate. An excessively thick substrate may result in a loss of flexibility and may impose design constraints, so a thickness of 2 mm or less is preferable, more preferably 0.2 mm or less. The substrate 100 is preferably configured to have a uniform thickness within its surface. Here, a uniform thickness includes a state in which, when thickness measurements are performed at 12 or more randomly placed points on the substrate surface, each measured value is distributed within a range of 0.95 to 1.05 times the average measured value. Regardless of thickness, the substrate 100 is preferably also uniform within its surface in terms of resin composition, hardness, bending strength, hue, saturation, brightness, and the like.

[0042] The substrate 100 can be rectangular in plan view. The length of the short side can be determined appropriately based on the manufacturing equipment for the substrate and / or wiring board and the efficiency of obtaining the final product, but may be, for example, 2 mm or more, preferably 50 mm or more, and more preferably 300 mm or more. While increasing the width often improves productivity, there are cases where the requirements for manufacturing equipment become more sophisticated, so the width of the short side can be, for example, 1500 mm or less, or even 700 mm or less. The length of the long side of the substrate 100 can be 1 time or more of the short side, or even 1.2 times or more of the short side. From the viewpoint of productivity, the length of the long side is preferably 20 times or more of the short side, and more preferably 200 times or more. A substrate with such a large aspect ratio can be handled as a roll wound around a reel or bobbin for supply and recovery before and after the desired processing steps, making it suitable for expanded-scale production.

[0043] The thickness of the surface region 104 of the substrate 100 is preferably 20 μm (micrometers) or less, and more preferably 6 μm or less. This is because material failure in peel strength tests often occurs at a very shallow position, making the thickness unnecessary. For example, if the surface region 104 is a region in which the substrate peel strength is increased by controlling the molecular crystal structure, other properties such as CTE are often relatively inferior compared to the internal region 103. In this case, thinning the surface region 104 is advantageous for maintaining the excellent properties of the substrate 100 as a whole. The thickness of the surface region 104 can be 20% or less of the thickness of the substrate 100, preferably 10% or less, and particularly preferably 5% or less. If the substrate 100 includes a surface region 105, the surface region 105 can have a similar configuration to the surface region 104. Furthermore, the thickness of the surface region 104 is preferably 95% to 105% of the thickness of the surface region 105, and more preferably 97% to 103%.

[0044] The base material 100 may have a controlled coefficient of linear expansion so as to be suitable for a wiring substrate. The coefficient of linear expansion of the base material 100 in any direction on the base material surface of the base material 100 is referred to as CTE. ＸThe coefficient of linear expansion of the substrate 100 in the direction perpendicular to the substrate surface and the arbitrary direction is defined as CTE Ｙ Then, CTE Ｘ and CTE Ｙ In both cases, the coefficient of linear expansion in the thickness direction of the substrate 100 in the same temperature range is preferably 80 ppm / °C or less, more preferably 50 ppm / °C or less, and particularly preferably 30 ppm / °C or less. Ｚ Then, CTE Ｚ is CTE Ｘ and CTE Ｙ The coefficient of linear expansion may be larger than the average value of the above. The coefficient of linear expansion is in the temperature range of 30°C to 250°C. Unless otherwise specified, the same applies elsewhere in this specification. When the substrate 100 has the above CTE, it is preferable that the internal region 103 and the surface region 104 are each composed substantially of fluororesin. By adopting such a configuration, a substrate can be provided that combines particularly excellent electrical properties and a low CTE at a high level.

[0045] When the linear expansion coefficients of the inner region and the surface region are defined in the same manner as the linear expansion coefficient of the substrate, the CTE of at least the inner region 103 is Ｘ and CTE Ｙ The average value of Ｘ and CTE Ｙ It is preferable that the CTE is smaller than the average value of the CTE in the XY direction of the entire substrate 100. This allows the inner region to mainly control the CTE in the XY direction of the entire substrate 100, and the surface region to mainly control the bonding reliability with other layers on the substrate surface, so that the state of each molecule can be made to take a form appropriate for its role. Ｚ is the CTE in the inner region Ｘ and CTE Ｙ The CTE in the surface region may be greater than the average value of Ｚ is the CTE in the surface region Ｘ and CTE Ｙ The CTE in the surface region may be larger or smaller than the average value of Ｚ is the CTE of the inner region Ｚ It may be smaller.

[0046] Second Embodiment In this embodiment, a substrate 100 is provided that has a first main surface 101 and a second main surface 102 that is opposite to the first main surface 101. The substrate 100 includes an internal region 103 primarily composed of an organic polymer, and a surface region 104 formed on the internal region 103, including the first main surface 101, and primarily composed of the same organic polymer as the organic polymer, wherein the crystallinity of the organic polymer constituting the surface region 104 is lower than the crystallinity of the organic polymer constituting the internal region 103. Other configurations of the second embodiment can be selected and combined as appropriate from one or more of the various configurations of the first embodiment. A substrate of this embodiment having such a configuration can simultaneously satisfy both bonding reliability and other required characteristics.

[0047] Here, crystallinity refers to the proportion (%) of crystalline part in the whole including crystalline part and amorphous part.By controlling extrusion, rolling and / or stretching, the molecular axis of the organic polymer that constitutes the substrate can be preferentially oriented in the surface direction of the substrate, and can obtain a sheet-like substrate with relatively high crystallinity.This substrate, especially the substrate with a very high proportion of molecules whose molecular axis is parallel to the surface direction of the substrate, such as stretched PTFE, can have a higher crystallinity than unstretched PTFE, and can be made into a substrate with excellent dimensional stability, which can be suppressed from deformation caused by the stress from inside / outside the substrate in the direction parallel to the stretching direction.

[0048] However, the inventors of the present application have found that substrates with such high crystallinity may have reduced substrate peel strength. One possible mechanism, but not limited to this, is that the van der Waals forces required to separate one molecule from another parallel molecule are easier to break than the covalent bonds required to cut the molecule in the molecular axis direction. In contrast, in this embodiment, the surface region 104 of the substrate 100 has a lower crystallinity of the organic polymer constituting it than the internal region 103. That is, compared to the internal region 103, the surface region 104 is thought to have more entanglement of molecules with molecular axes parallel to the substrate surface, and a smaller proportion of molecules. By achieving this state, the material strength in the surface region 104 in the direction perpendicular to the substrate surface can be increased.

[0049] For example, when the main component of both the internal and surface regions of the substrate 100 is PTFE, each of these two regions may have a crystalline portion and an amorphous portion. Furthermore, it is preferable that at least the crystalline portion of the internal region contains more PTFE crystals with molecular axes parallel to the surface of the substrate than the surface region. In this configuration, by making the crystallinity of the surface region smaller than that of the internal region, the internal region with a relatively high crystallinity improves the dimensional stability of the substrate, while the surface region with a relatively low crystallinity improves the bonding reliability with other layers formed on the surface of the substrate. The crystalline portion of the surface region may also contain PTFE crystals with molecular axes parallel to the surface of the substrate. Even in this case, by making the crystallinity of the surface region smaller than that of the internal region, a substrate can be provided that simultaneously satisfies both bonding reliability and other required properties.

[0050] The crystallinity of polymeric materials can be measured using XRD (X-ray diffraction) and DSC (differential scanning calorimetry). XRD, for example, uses a wide-angle X-ray diffractometer to irradiate X-rays at a predetermined angle relative to the analysis surface of a film sample, and the reflected energy is measured to obtain an X-ray diffraction pattern. According to Bragg's diffraction theory, diffraction occurs at angles corresponding to the distance between regularly spaced crystals, resulting in strong reflected energy. The resulting diffraction pattern can provide various information about the crystalline state of the sample. In the X-ray diffraction pattern of PTFE, the peak near 2θ = 16° is a halo pattern representing the amorphous region, and the peak near 2θ = 18° is a diffraction peak for the crystalline region. The crystallinity can be calculated from the area ratio of these peaks.

[0051] The degree of crystallinity can be determined by DSC, for example, by calculating the heat of fusion of the object to be measured from the area of ​​the endothermic peak of crystalline melting that appears on the DSC curve, and then deriving the ratio of this heat of fusion to the heat of fusion of the completely crystalline material, i.e., the heat of fusion at 100% crystallinity. For example, the heat of fusion of the completely crystalline material of PTFE can be calculated as 92.9 J / g.

[0052] Stretching PTFE with a given average molecular weight increases the crystallinity, while a temperature history including a temperature exceeding the crystalline melting temperature decreases the crystallinity. Therefore, the crystallinity can be controlled by adjusting the average molecular weight of the PTFE material, the stretching conditions, and the heat treatment conditions. For example, when the substrate is sufficiently thick, a semi-finished substrate with a uniform crystallinity distribution in the thickness direction is prepared, and then the surface region of the substrate is heated to a temperature exceeding the crystalline melting temperature of PTFE, while the inner region is heat-treated so as not to exceed the crystalline melting temperature of PTFE, thereby obtaining a substrate with a desired crystallinity distribution. Alternatively, a substrate with a desired crystallinity distribution can be obtained by preparing thin films with different crystallinities in advance, for example, by the above-mentioned means, and then laminating them to form a substrate.

[0053] (Third Embodiment) In this embodiment, a substrate 100 is provided that has a first main surface 101 and a second main surface 102 that is opposite to the first main surface 101, the substrate 100 including an inner region 103 primarily composed of PTFE and a surface region 104 formed on the inner region 103, including the first main surface 101, and primarily composed of PTFE, and the first heat of fusion calculated from the area of ​​an endothermic peak in a temperature range of 339°C to 355°C in DSC is defined as the first heat of fusion, and the first heat of fusion of the surface region 104 is smaller than the first heat of fusion of the inner region 103. In this specification, "near 345°C" means a temperature range of 339°C to 355°C. Alternatively, in a modified example of the third embodiment, there is provided a substrate 100 having a first main surface 101 and a second main surface 102 opposite to the first main surface 101, the substrate 100 including an inner region 103 mainly composed of PTFE and a surface region 104 formed on the inner region 103, including the first main surface 101 and mainly composed of PTFE, wherein the second heat of fusion is defined as the heat of fusion calculated from the area of ​​an endothermic peak in a temperature range of 315°C to 340°C in DSC, and the second heat of fusion of the surface region 104 is greater than the second heat of fusion of the inner region 103. In this specification, "near 327°C" refers to a temperature range of 315°C to 340°C. Alternatively, in another modification of the third embodiment, there is provided a substrate 100 having a first main surface 101 and a second main surface 102 that is opposite to the first main surface 101, the substrate 100 including an inner region 103 primarily composed of PTFE and a surface region 104 formed on the inner region 103, including the first main surface 101, and primarily composed of PTFE, wherein, when the third heat of fusion calculated from the area of ​​an endothermic peak in a temperature range of 370°C or higher and 390°C or lower in DSC is defined as the third heat of fusion, the third heat of fusion of the surface region 104 is smaller than the third heat of fusion of the inner region 103. In this specification, "near 380°C" means a temperature range of 370°C or higher and 390°C or lower.

[0054] In the third embodiment and its modified example, other configurations can be selected and combined as appropriate from one or more of the various configurations of the first and second embodiments.

[0055] The crystalline portion of the PTFE substrate is believed to contain one or more of the following three crystalline structures. The first is a crystalline structure with a crystalline melting temperature around 345°C, and this crystal is referred to as ECC in this specification. ECC is believed to be, for example, an extended chain crystal or a similar structure. A substrate containing a large amount of ECC can be obtained, for example, by using a PTFE fine powder containing a large amount of ECC and forming it within a temperature range not exceeding the crystalline melting temperature of ECC. In a DSC curve, ECC is observed as an endothermic peak with a peak position within a temperature range of 339°C to 355°C, and its heat of fusion (hereinafter referred to as the first heat of fusion) can be calculated from the area of ​​the endothermic peak.

[0056] The second is a crystalline structure with a crystalline melting temperature around 327°C, and this crystal is referred to as FCC in this specification. FCC is considered to be a folded crystal or a structure similar thereto. A substrate containing a large amount of FCC can be obtained, for example, by including a thermal history during or after the formation of the substrate containing a large amount of ECC, such that the substrate temperature exceeds 345°C, which is the crystalline melting temperature of ECC. FCC is observed as an endothermic peak in a temperature range of 315°C to 340°C in a DSC curve, and its heat of fusion (hereinafter referred to as the second heat of fusion) can be calculated from the area of ​​the endothermic peak.

[0057] The third is a crystalline structure with a crystalline melting temperature around 380°C, and this crystal is referred to as HECC in this specification. Like ECC, HECC has an extended chain crystal structure or a similar structure, but is considered to be more strongly oriented than ECC. A substrate containing a large amount of HECC can be obtained, for example, by stretching the substrate in the plane direction at a high stretch ratio during or after the formation of the above-mentioned ECC-rich substrate. Because HECC disappears due to thermal history exceeding the crystalline melting temperature of HECC, when obtaining a substrate containing a large amount of HECC, it is recommended to increase the stretch ratio and not exceed 380°C. HECC is observed as an endothermic peak in a DSC curve within the temperature range of 370°C to 390°C, and its heat of fusion (third heat of fusion) can be calculated from the area of ​​the endothermic peak.

[0058] As described above, the dimensional stability and bonding reliability of the substrate can be controlled by the orientation direction of the molecular axes of the resin constituting the substrate. Based on this, these two required substrate properties can be controlled by the ratio of the proportions of each crystal in each region of the substrate. More specifically, by configuring the internal region to contain a large amount of ECC and / or HECC oriented in the substrate surface direction and the surface region to contain less of these than the internal region, a substrate can be provided that simultaneously satisfies bonding reliability and other required properties. Alternatively, a similar substrate can be provided by configuring the internal region to contain fewer folded crystals and the surface region to contain more folded crystals than the internal region.

[0059] The substrate of this embodiment has a configuration in which the first heat of fusion calculated from the area of ​​an endothermic peak in the temperature range of 339°C or higher and 355°C or lower is smaller in the surface region 104 than in the internal region 103. A modified version of the substrate of this embodiment has a configuration in which the second heat of fusion calculated from the area of ​​an endothermic peak in the temperature range of 315°C or higher and 340°C or lower is larger in the surface region 104 than in the internal region 103. A modified version of the substrate of this embodiment has a configuration in which the third heat of fusion calculated from the area of ​​an endothermic peak in the temperature range of 370°C or higher and 390°C or lower is smaller in the surface region 104 than in the internal region 103. The substrate of this embodiment, which includes one or more of these configurations, can have a structure in which a crystalline structure that relatively contributes to dimensional stability of the substrate is arranged in the internal region, and a crystalline structure that relatively contributes to improving material strength in the thickness direction is arranged in the surface region.

[0060] Figure 2 shows the DSC curves for films A2, B2, and C2 made from three different types of PTFE. Measurements were performed using a NETZSCH JAPAN DSC3200 in accordance with Japanese Industrial Standard JIS K 7122. Film A2 is a strongly oriented PTFE film formed at a temperature not exceeding 345°C and stretched at an area ratio of 30 or more. Films B2 and C2 were obtained by heating film A2 at 360°C, and film C2 was maintained at 360°C for a longer period of time than film B2. Curves (A2), (B2), and (C2) in Figure 2 are the DSC curves for films A2, B2, and C2, respectively.

[0061] Curve (A2) has a large endothermic peak near 345°C, indicating crystalline melting of ECC. Curve (C2) has a large endothermic peak near 326°C, indicating crystalline melting of FCC. It is believed that film A2 was heated to 360°C for a certain period of time, causing most of the ECC to melt and then recrystallize as FCC. Curve (B2) has a broad peak near 332°C. In film B2, only a portion of the ECC has melted and recrystallized, and it is believed to be in a mixed state of ECC and FCC. The heats of fusion of the ECC, FCC, and HECC in each film can be determined from the respective peak areas. Note that curve (B2) is a curve that combines the ECC peak and the FCC endothermic peak, but the respective heats of fusion may be calculated by arithmetically separating the peaks.

[0062] For example, focusing on ECC, the amount of ECC in a film can be determined by the first heat of fusion, which is calculated from the area of ​​the endothermic peak within a temperature range of 339°C to 355°C. In each film in Figure 2, film A2 has a large endothermic peak within this temperature range. In contrast, film B2 has a relatively small peak within this temperature range, and film C2 has no peak within this temperature range, or a peak so small that it is difficult to distinguish. In such a case, a layer of PTFE corresponding to film A2 is preferably disposed within the substrate so as to form the interior region of the substrate, and a layer of PTFE corresponding to film B2, preferably film C2, is disposed so as to form the surface region of the substrate, thereby providing a substrate that simultaneously satisfies both bonding reliability and other required properties.

[0063] In this specification, the heat of fusion calculated from the area of ​​an endothermic peak within a specific temperature range refers to the heat of fusion calculated from the area of ​​the entire endothermic peak having a peak position within that specific temperature range. That is, even if the base of the target peak extends outside the specific temperature range, the base is included in the calculation. However, if the peak is extremely broad or is considered to overlap with other peaks, the relative magnitude of the area within that specific temperature range may be considered to be the relative magnitude of the heat of fusion.

[0064] Fourth Embodiment In this embodiment, the internal region of the substrate is a laminate of a plurality of thin films, and other configurations can be the same as those of any of the first, second, and third embodiments.

[0065] FIG. 3 is a schematic diagram showing a cross section of a substrate 100 according to a fourth embodiment of the present invention, viewed from the lateral direction of the substrate. As shown in the figure, the substrate 100 has a first main surface 101 and a second main surface 102 that is opposite to the first main surface 101. The substrate 100 also has an internal region 103 and a surface region 104 formed on the internal region 103 and including the first main surface 101. As shown in the figure, the substrate 100 includes multiple thin films, which are configured as a laminate of these multiple thin films. The internal region 103 of the substrate includes multiple thin films 110 (first thin films). Preferably, each of the multiple thin films 110 that make up the internal region 103 is bonded in contact with each other without an adhesive layer. In this case, the bonding surface is an interface shared by the surfaces of these two thin films 110.

[0066] The surface region 104 including the first main surface 101 of the substrate includes a thin film 150 (second thin film). The substrate may further include a surface region 104 including a second main surface 102 and another thin film 150 constituting the surface region 104. From the viewpoint of suppressing warping of the substrate, the thin film 150 on the first main surface side and the other thin film 150 on the second main surface side preferably have similar mechanical properties and are of the same thickness. The thin film 110 and thin film 150 constituting the internal region 103 are preferably bonded in contact with each other without an adhesive layer therebetween.

[0067] In this embodiment, the relative relationship between the inner region 103 and the surface region 104 or 105 as a laminate of multiple thin films 110 can be the same as any one or more of the configurations in the above-described embodiments. For example, as in the first embodiment, the substrate peel strength of the surface region can be greater than the substrate peel strength of the inner region. The same applies to the second and subsequent embodiments, that is, the crystallinity and heat of fusion can be configured to have a predetermined relationship.

[0068] In this embodiment, the internal region 103 of the substrate 100 is configured as a laminate of multiple thin films 110. In this configuration, the required properties of the substrate 100 can be controlled by the total thickness according to the number of thin films 110, making it possible to more directly and easily control properties with high stability and reproducibility. Furthermore, a surface region 104 made of a thin film is formed on the internal region 103. In this configuration, the molecular orientation and crystalline state of the polymer resins that make up the internal region and the surface region can be controlled independently, making it possible to more directly and easily control properties with high stability and reproducibility, including the thickness of each region.

[0069] The multiple thin films 110 that make up the internal region 103 may have anisotropy in CTE in the substrate surface direction (in the XY direction in the figure). In this case, by stacking one thin film 110 with another thin film 110 in a manner that crosses the anisotropy, the CTE of the substrate surface direction of the substrate formed as a stack of these thin films may be provided with the desired anisotropy.

[0070] In the stacked state, the thickness of each thin film 110 can be defined as the distance between the bonding interfaces. The multiple thin films 110 constituting the internal region may be composed of multiple thin films 110 having the same thickness, or may be composed of a combination of thin films 110 having different thicknesses. In either case, it is preferable that the average thickness of the multiple thin films 110 constituting the internal region is greater than that of the thin films 150 constituting the surface region.

[0071] The thin films 110 and 150 may differ only in the orientation and crystalline state due to the temperature conditions during formation, and in the resulting differences in various properties. In this case, it is preferable that the thin films 110 and 150 have similar configurations for some or all of the characteristics and properties that do not fall under the above-mentioned differences. For example, a certain thin film can be designated as the thin film 110, and then heat-treated at a temperature above the crystalline melting temperature to obtain the thin film 150. In this case, as described above, differences in the orientation and crystalline state occur between the thin films 110 and 150, resulting in differences in properties such as dimensional stability and material strength through the thickness. However, the composition, density, or electrical properties of the thin films 110 and 150 can be similar. For example, when the thin film is primarily composed of a fluororesin, the fluororesin ratio of the thin film 150 can be 0.95 to 1.05 times, preferably 0.97 to 1.03 times, of the fluororesin ratio of the thin film 110. It is particularly preferable that the primary component of the inner region and the primary component of the surface region have the same chemical composition.

[0072] In such a substrate 100, the thin film 110 can be unsintered expanded PTFE. The thin film 150 can be formed by sintering the unsintered expanded PTFE, or by coating, drying, and sintering a PTFE dispersion on a support film such as polyimide to form a thin film. When the thin film 150 is formed on a support film, the thin film 110 can be laminated thereon, and then the support film can be removed to form the substrate 100. Here, "unsintered" and "sintered" are determined based on the presence or absence of a thermal history exceeding the crystalline melting temperature of the crystal with the greatest heat of fusion among the crystals of the polymer resin that constitutes the main component of the thin film. When the thin film 110 is expanded PTFE, heat treatment at a temperature exceeding the crystalline melting temperature of extended-chain ECC (ECC), 345°C, can be considered sintering. In a typical DSC curve, unsintered expanded PTFE exhibits an endothermic peak with a maximum heat of fusion near 345° C., while sintered expanded PTFE exhibits an endothermic peak with a maximum heat of fusion near 327° C. Furthermore, sintered expanded PTFE does not exhibit an endothermic peak with a maximum heat of fusion near at least 345° C., but depending on the degree of sintering, it may exhibit a broad endothermic peak at a temperature greater than 327° C. and less than 345° C., for example, near 332° C.

[0073] 4 is a schematic diagram of a wiring board 200 having a substrate 100 according to an embodiment of the present invention as a dielectric layer. Any of the substrates 100 described so far can be used for the wiring board 200. FIG. 4(a) is a schematic diagram of the wiring board 200 viewed from a direction perpendicular to the substrate surface, and FIG. 4(b) is a schematic diagram of the wiring board 200 viewed from the cross-sectional direction of the wiring board at position B-B' in FIG. 4(a). The wiring board 200 can be a flexible substrate that can be bent freely.

[0074] The wiring substrate 200 has a plurality of conductor patterns. The plurality of conductor patterns include one or more of pads 210, wiring 220, vias 230, antenna pads 240, and a ground plane 250. Other electronic components are electrically connected to the pads 210. The vias 230 form electrical connections including in the thickness direction of the wiring substrate (the Z direction in the figure). In particular, they form electrical connections between conductor patterns formed on the upper surface of a dielectric layer and conductor patterns formed on the lower surface of the dielectric layer.

[0075] In this embodiment, the wiring substrate 200 includes wiring 220, and an electrical signal having a frequency of, for example, 50 GHz or higher is transmitted through the wiring 220. The width of the wiring may be 1 μm to 400 μm, or 10 μm to 30 μm. The thickness of the wiring may be 1 μm to 40 μm, or 7 μm to 25 μm.

[0076] The wiring board 200 includes a base material 100, and the above-described multiple conductor patterns are formed on a first main surface and / or a second main surface of the base material 100. In this embodiment, the wiring board 200 including the base material 100 shown in FIG. 3 as a dielectric layer is exemplified, but the base material 100 according to other embodiments may also be used as a dielectric layer. The wiring board 200 may further include a protective layer (not shown) made of an insulating material formed on the above-described multiple conductor patterns. The base material 100 of the invention described in the present application can also improve bonding reliability to the protective layer.

[0077] The wiring board 200 includes a dielectric layer made of the base material 100. The dielectric layer functions as a base for forming conductor layers on its upper and / or lower surfaces, and also functions as a support material that supports the conductor layers and the multiple conductor patterns in each conductor layer so that they are electrically independent from each other.

[0078] In this embodiment, the dielectric layer significantly reduces the deterioration of electrical properties due to dielectric materials other than the composite fluororesin, allowing the wiring board 200 to fully utilize the excellent electrical properties of polymer resin materials such as fluororesin. In particular, even signals with extremely high frequencies of 50 GHz or higher can be transmitted with extremely low loss. Furthermore, the thermal expansion coefficient is kept low by the orientation and crystalline state of the resin that constitutes the substrate 100, providing the wiring board 200 with excellent physical properties, including significantly reduced deformation and warping due to temperature changes.

[0079] It is preferable that the wiring substrate 200 does not contain a CTE control material or contains only a small amount of it. In particular, when a wiring transmitting a high-frequency signal and a ground plane with a fixed potential formed below the wiring form a microstrip line (not shown), it is preferable that the dielectric layer between the wiring and the ground plane does not contain a CTE control material. The dielectric layer between the wiring and the ground plane is preferably composed solely of a thin film primarily composed of a fluororesin, and more preferably substantially solely of a fluororesin. Alternatively, in a cross-sectional view perpendicular to the substrate surface of the wiring substrate 200, the area ratio of the CTE control material in the dielectric layer between the high-frequency signal transmission wiring and the ground plane may be 10% or less, or even 1% or less. Furthermore, the wiring substrate 200 may not include a reinforcing layer that enhances the dimensional stability of the wiring substrate and contains 20% or more by weight of a CTE control material. Examples of CTE control materials include compounds containing oxygen and / or nitrogen. Examples of CTE control materials include amorphous silica, zirconium tungstate phosphate, crystalline silica, glass, zeolite, ceramic, titanium oxide, boron nitride, and the like, and mixtures thereof.

[0080] There are a D1 direction, which is an arbitrary direction, and a D2 direction (neither of which is shown) perpendicular to the D1 direction on the substrate surface of the wiring substrate 200, and the linear expansion coefficient of the base material 100 of the wiring substrate 200 in the D1 direction is defined as CTE. Ｄ１ , the linear expansion coefficient of the base material 100 of the wiring substrate 200 in the D2 direction is the CTE of the wiring substrate Ｄ２ When this is done, CTE Ｄ１ CTE Ｄ２The value divided by (hereinafter referred to as specific CTE Ｄ１ / CTE Ｄ２ The specific CTE is 0.5 or more and 2 or less. Ｄ１ / CTE Ｄ２ is preferably 0.7 or more and 1.3 or less, and particularly preferably 0.8 or more and 1.2 or less. When measuring the linear expansion coefficient of the base material 100 of the wiring board 200, the measurement can be carried out after removing the conductor layer by etching or the like. Ｄ１ and CTE Ｄ２ and σ are preferably 80 ppm / °C or less, more preferably 60 ppm / °C or less, even more preferably 50 ppm / °C or less, even more preferably 40 ppm / °C or less, and particularly preferably 30 ppm / °C or less. In this case, in a cross-sectional view perpendicular to the substrate surface of the wiring board 200, the area ratio of the CTE control material in the dielectric layer excluding the conductor is preferably 10% or less, and particularly preferably 1% or less. By adopting such a configuration, a wiring board can be provided that achieves both particularly excellent electrical properties and a low CTE at a high level.

[0081] The wiring substrate 200 may further include a second dielectric layer (not shown) and a third conductor layer (not shown) formed on the second dielectric layer. In this case, it is preferable that the second dielectric layer also uses the base material 100 according to any one of the embodiments of the present invention.

[0082] (Method of Manufacturing Substrate) An example of a method of manufacturing a substrate according to an embodiment of the present invention will now be described. FIG. 5 shows an outline of the manufacturing flow of the substrate. The manufacturing flow of the substrate can be broadly divided into a group of steps for preparing a sheet (S1 to S5 in the figure), steps for preparing a plurality of thin films with different properties (S6 and S7 in the figure), and a group of steps for laminating these thin films to form the substrate (S7 and S8 in the figure). The number of steps and embodiments of each step are not limited to those described below, and each step can be performed in multiple steps or combined with other steps before or after it, as long as the technical significance of each step is not lost.

[0083] First, a sheet containing fluororesin as a main component is formed by the following steps: The fluororesin sheet can be formed by a known method.

[0084] (Material Preparation) Prepare the resin particles and auxiliary agents that will form the main components of the substrate. For example, particles primarily composed of fluororesin can be used as the resin particles. Fine powder made of unsintered PTFE is suitable. While commercially available PTFE fine powders can be used, it is preferable to select one with a small average particle size in order to control the thin film to have the desired thermal expansion characteristics. A preferred average particle size is 250 μm or more and 900 μm or less. The apparent density of the PTFE fine powder is, for example, 0.3 g / ml or more and 0.8 g / ml or less. Commercially available auxiliary agents can also be used. For example, petroleum-based solvents with an initial boiling point (IBP) of 150°C or more and 250°C or less can be used.

[0085] (Blending) Resin particles and auxiliary agents are mixed at a predetermined weight ratio to prepare a paste of resin particles. For example, a V blender or a rocking mixer can be used for mixing.

[0086] (Preforming) The prepared paste is compression-molded to form a preform. Including the subsequent extrusion step, the treatment is carried out at a temperature lower than the initial boiling point of the auxiliary agent, for example, at room temperature, to suppress volatilization of the auxiliary agent.

[0087] (Extrusion) The formed preform is passed through an extrusion die while applying pressure to form a film-like sheet with a longitudinal direction. The width and thickness of the sheet are controlled by the extrusion conditions, particularly the shape of the die. In the process after extrusion, continuous processing may be applied while feeding the sheet in the longitudinal direction. Hereinafter, the feeding direction when continuously processing a sheet or thin film is referred to as MD (Machine Direction), and the width direction of the sheet perpendicular to the MD is referred to as TD (Transverse Direction).

[0088] (Rolling) If necessary, the outer shape of the formed sheet can be adjusted to a desired thickness by applying pressure by sandwiching the formed sheet from above and below with metal rolls, etc. The dimensional change of the sheet that occurs here is not limited to the thickness, and may also involve changes in the overall length of the sheet (i.e., the size of the substrate in the MD) and width (i.e., the size of the substrate in the TD).

[0089] (Drying) The sheet is heated to volatilize and remove the auxiliary agent remaining in the sheet. The efficiency of removing the auxiliary agent can be increased by increasing the drying temperature, but it is preferable to control the temperature so that it does not exceed the crystalline melting temperature of the resin material that constitutes the main component of the sheet. In this way, it is possible to prepare a highly pure, long sheet, for example, a long sheet mainly composed of fluororesin or a long sheet essentially consisting of fluororesin.

[0090] (Stretching) Next, the long sheet is stretched. A tensile stress is applied to the dried sheet at a temperature lower than the crystalline melting temperature of the fluororesin constituting the sheet, thereby stretching the sheet in its plane. The direction of stress application, i.e., the stretching direction, may be MD, TD, or another direction, and may be a single direction or multiple directions. When stretching in multiple directions, stretching in a single direction may be performed individually and combined, or stretching may be performed simultaneously in different directions. The stretching ratio in a certain direction may be, for example, 1 to 50 times, or may be higher. Here, the stretching ratio is the value obtained by dividing the length after stretching by the length before stretching. When the product of the stretching ratio in MD and the stretching ratio in TD is defined as the areal stretching ratio, the areal stretching ratio is preferably 10 times or more, and more preferably 100 times or more.

[0091] In particular, PTFE can be stretched to form a thin film with a porous structure consisting of nodes and fibrils. The thickness of the thin film at this stage may be 0.005 mm or more and 3 mm or less, and preferably 0.01 mm or more and 1 mm or less. The density of the thin film at this stage is, for example, 0.1 g / cm ３ 0.7g / cm or more ３ or less, 0.2 g / cm ３ 0.5g / cm or more ３ It is preferable that:

[0092] Even before stretching, the fluororesin molecules that make up the sheet are thought to have a configuration in which they are preferentially oriented in the plane direction of the sheet as a result of shape changes caused by, for example, extrusion or rolling, but at this stage they have not yet achieved a sufficiently suppressed low CTE. By further stretching this sheet as described above, the orientation becomes even more pronounced, and the CTE of the thin film in the stretching direction can be reduced.

[0093] In particular, fibril formation contributes to the orientation of expanded PTFE. It is believed that fibrils are composed of linear molecular chains drawn from nodes. Such linearly extending chains are less likely to stretch in the direction of extension than folded chains. As a result, a thin film can be obtained with significantly increased mechanical strength in the stretching direction and a sufficiently controlled CTE. In expanded PTFE thin films with such extremely high orientation, the material strength in the orientation direction, i.e., in the plane direction of the thin film, is enhanced, while the material strength in the direction perpendicular to the plane of the thin film is thought to be relatively weak.

[0094] (Preparation of Multiple Thin Films) Next, multiple thin films are prepared. The multiple thin films include a first thin film applied to the internal region of the substrate and a second thin film applied to the surface region of the substrate. The first thin film and the second thin film are composed of the same main component but have at least different mechanical properties. For example, the second thin film may have a higher material strength in the thickness direction than the first thin film. When the material strength in the thickness direction of the thin film is defined as the thin film peel strength, the thin film peel strength can be easily evaluated using a method similar to the substrate peel strength described above. The first thin film and the second thin film are not limited thereto, and each thin film can be selected so that the substrate formed therefrom has the configuration of each of the above-described embodiments of the substrate. In other words, the relationship between the crystallinity and / or heat of fusion, etc. between the first thin film and the second thin film may be the same as the relationship between the crystallinity and / or heat of fusion, etc. between the internal region and the surface region of the substrate.

[0095] The preparation of two or more thin films with different properties can be carried out by applying different conditions to one or more of the steps from material preparation to stretching, or by applying different heat treatment conditions to the formed thin films, or by combining these. For example, different grades of PTFE fine powder can be used for the first thin film and the second thin film, or the shape change rate can be changed in one or more of the steps of extrusion, rolling, and stretching, or the processing temperature or speed can be changed. The first thin film can also be baked to form the second thin film, which enables efficient and flexible use of materials for forming the substrate.

[0096] The heat treatment temperature for obtaining the second thin film by baking is preferably a temperature at which at least a portion of the crystals contained in the polymer resin that is the main component of the thin film melts. When the main component of the thin film is PTFE, the heat treatment temperature can be 345°C or higher, preferably 355°C or higher. If the temperature is too high, there is a risk of melting the strongly oriented crystalline HECC, so the heat treatment temperature is preferably less than 390°C, more preferably less than 370°C.

[0097] A vacuum press can be used to heat-treat the second thin film. In this method, pressure and heat can be applied simultaneously by sandwiching the thin film between two press surfaces controlled at a predetermined temperature. This method allows for direct contact between the heated press surfaces and the substrate, which allows for excellent temperature control and high productivity in generating crystalline melting.

[0098] Alternatively, the second thin film may be heat-treated using a thermal fluid or radiation. For example, the second thin film can be heat-treated in a non-contact manner and with minimal external physical stress by being placed in a heated gas and / or passing through a heating furnace equipped with infrared heating means. Such heating methods have the advantage of being easily combined with other processes, such as a stretching process. Furthermore, while press heating can potentially cause the formation of an embrittlement layer or a damaged layer on the surface of the thin film due to the pressure of the press, heat treatment methods using a thermal fluid or radiation have the advantage of being able to perform heat treatment without such risks.

[0099] (Thin Film Lamination) Next, the first thin film and the second thin film are laminated to obtain a substrate with the desired properties. While the number of laminated thin films in each region of the substrate is not limited, a preferred configuration is one in which an internal region is formed from multiple first thin films, and one second thin film is formed on each surface of one or both sides of the internal region. The total thickness of the first thin films used for lamination is preferably greater than the total thickness of the second thin films. The total thickness may be adjusted by adjusting the number or thickness of each thin film. It is preferable that the second thin film is not included in the internal region. However, when obtaining a thick substrate, for example, two substrates each having an internal region and surface regions formed on both sides of the internal region may be bonded together to form a thick substrate having a thin film identical to the surface region inside.

[0100] The first and second thin films to be prepared may or may not have anisotropy in the in-plane CTE. When applying a thin film with anisotropy in the in-plane CTE, it is possible to suppress the anisotropy of the in-plane CTE of the final substrate by crossing the anisotropy directions of the thin films constituting the substrate, while placing the second thin film in the desired layer, thereby further achieving high bonding reliability. A thin film with anisotropy in the in-plane CTE may be prepared using, for example, the method described in Japanese Patent Application No. 2022-211414, filed in Japan on December 28, 2022. The contents of Japanese Patent Application No. 2022-211414 are incorporated herein by reference and may be incorporated herein by reference.

[0101] After forming the laminate, it is also possible to modify only the shallow surface region using known surface heating means or the like to obtain a substrate consisting of an internal region and a surface region, each with the desired properties. However, by preparing two thin films with different properties in advance as described above and laminating them to form a substrate, it is possible to accurately and stably control the properties of each region. In particular, when the thickness of the substrate is thin, the internal region and the surface region can be given significant differences in properties through independent processes, and the advantage of accurately controlling their thickness is extremely significant.

[0102] (Compression) The laminate including the first thin film and the second thin film obtained by the above-mentioned superposition may be further compressed. A thin film laminate in which multiple thin films are simply stacked may have insufficient adhesive strength between the thin films as is. In the compression process, the laminate is compressed in the thickness direction to strengthen the bonds between the thin films. Furthermore, when the thin film is made of expanded PTFE, air spaces may be present between the nodes and fibrils. Densifying the thin film by compression can improve the dimensional stability of the substrate.

[0103] Compression may be performed, for example, by sandwiching the thin film laminate in a press and applying pressure, or by passing the thin film laminate between one or more pairs of rolls and applying pressure continuously. The temperature during compression can be, for example, 40°C or higher and 340°C or lower, and is preferably a temperature lower than the crystalline melting temperature of ECC in the inner region. The pressure during compression is appropriately adjusted based on the thickness and density of the substrate to be obtained, and examples include a range of 0.5 MPa or higher and 400 MPa or lower. Compression changes the pore size and distribution within the porous PTFE thin film, resulting in increased density. In the case of a PTFE substrate, its density is 1.9 g / cm. ３ 2.4g / cm or more ３ In this manner, a substrate having an exposed surface of the second thin film as a bonding surface with another layer can be obtained.

[0104] (Conductor Layer Formation) If necessary, a conductor layer made of, for example, a metal-containing film may be formed on one or both surfaces of the obtained substrate to form a conductor layer-formed substrate. A conductor layer-formed substrate in which a copper film with excellent electrical properties is applied as the conductor layer is suitably used as a member for manufacturing a wiring board.

[0105] The metal film may be formed by forming a separately formed thin metal film on the substrate by thermocompression bonding or the like, or by growing the metal film on the substrate. The former is exemplified by a method in which copper foil is superimposed on the substrate and pressed or pressed between rolls. The latter is exemplified by a method in which a seed layer is formed on the substrate by dry or wet bonding, and a copper film is grown on the seed layer by electrolytic copper plating using the seed layer as a power supply layer. The substrate 100 of the present invention can be obtained with improved bonding reliability, particularly by the latter method.

[0106] As a pretreatment for forming such a conductor layer, the surface of the substrate may be subjected to plasma treatment. Plasma treatment improves the adhesion between the substrate 100 and the conductor pattern constituent material, particularly suppressing interfacial peeling. However, at the same time, it may sever some of the linear or side chains of the fluororesin, forming an embrittlement layer on the surface of the substrate 100. Even if an embrittlement layer is formed in the substrate 100 by plasma treatment, the strengthened surface region of the substrate 100 can suppress the effect of the embrittlement layer on the substrate strength. Other pretreatment methods that can be used include wet etching and irradiation of electromagnetic waves such as monochromatic light.

[0107] In this way, a conductor layer-forming substrate can be obtained. In all steps from blending to conductor layer formation, it is preferable to control that at least the inner region and the first thin film that will become the inner region do not exceed the crystalline melting temperature of the applied resin material. For example, in the case of PTFE, it is preferable to control that it does not exceed 345 ° C throughout these steps.

[0108] (Method for manufacturing wiring board) Next, an example of a method for manufacturing a wiring board using the above-mentioned substrate will be described. A substrate is prepared, and a conductor pattern is formed on the surface of the substrate by a known method. The conductor pattern is preferably formed by a subtractive method in which a copper film previously formed on the surface of the substrate is etched away except for areas to be left as the conductor pattern. An additive method may also be used in which a conductor layer is formed by building up by plating or the like only in the areas to be the conductor pattern. Photolithography techniques using a resist containing a photosensitive resin can be applied to define the etching and plating areas.

[0109] In the case of a double-sided substrate, after forming a via hole penetrating the substrate, a via conductor can be formed in the via hole to form an electrical connection between the conductor pattern on the upper side of the substrate and the conductor pattern on the lower side of the substrate. The via conductor can be formed by plating or filling with a conductive paste, for example. If necessary, a protective insulating layer can be formed on the conductor pattern to protect the conductor pattern.

[0110] Example 1: Polyflon F106 (product name) manufactured by Daikin Industries, Ltd. was used as the PTFE fine powder, and a sheet made of PTFE was prepared by known means. This PTFE was stretched to prepare an unsintered expanded PTFE thin film, in which the molecular axes of the molecules were controlled to be parallel to the substrate surface in large numbers. Next, a portion of the prepared unsintered expanded PTFE thin film was sintered. A vacuum press was used for the sintering to prepare a sintered PTFE thin film with a thermal history of approximately 360°C. A predetermined number of unsintered thin films were then stacked on top of each other, and a sintered thin film was then placed on top of the stack and compressed to prepare a substrate with an unsintered thin film formed in the inner region and a sintered thin film formed in the surface region. A conductor layer made of copper was formed on the surface of this substrate by plating to form a conductor layer-formed substrate, and patterning was performed by etching away a portion of the formed copper foil to prepare a substrate with a predetermined pattern formed on the surface. (Example 2) A substrate having a predetermined pattern formed on its surface was prepared under the same conditions as in Example 1, except that the expanded PTFE thin film was sintered by passing it through heated gas heated to 360° C. (Comparative Example) A substrate having a predetermined pattern formed on its surface was prepared under the same conditions as in Example 1, except that all the thin films to be laminated were composed of unsintered expanded PTFE thin films.

[0111] (CTE Measurement) CTE measurements were performed on samples cut out from these substrates. The CTE of each substrate was 60 ppm / °C or less in both MD and TD, confirming that the substrates were fluororesin substrates with suppressed thermal expansion, even though they did not contain a CTE control material such as glass cloth. CTE measurements were performed by mechanical thermal analysis (TMA) in accordance with ISO 11359-2. In the TMA method, the measurement sample is set on a tensile probe, and the sample is held under a constant load while undergoing two cycles of temperature increase and decrease. The CTE is determined by measuring the change in length due to expansion of the measurement sample during the second temperature increase. CTE is calculated using the following formula (1): CTE = (ΔL / L0) × (1 / ΔT) × 10 ６ Equation (1) Here, ΔL is the amount of change in length of the measurement sample when the temperature changes from T1 to T2, L0 is the length of the measurement sample before measurement, and ΔT is the amount of change in temperature (T2 - T1) °C. The measurement device used was a TMA4000SE manufactured by NETZSCH JAPAN, and measurements were taken under the following conditions: Measurement temperature range: room temperature to 250 °C Heating rate: 20 °C / min Tensile load: 0.5 gf Measurement sample shape: rectangular, 15 mm long x 5 mm wide (sample dimensions including chuck: 20 mm long x 5 mm wide) The CTE was calculated in the temperature range from 30 °C to 250 °C in the long side direction of the measurement sample.

[0112] Next, the bond reliability between the substrate and the conductor pattern formed on the substrate surface was evaluated by a peel strength test. The test conditions were in accordance with Japanese Industrial Standard JIS C6481. The peel strength of the comparative example was approximately 1.5 N / cm, while the peel strength of Example 1 was approximately 4-5 N / cm, and furthermore, the peel strength of Example 2 was approximately 9-10 N / cm. It was confirmed that the level of having a sintered PTFE thin film on the surface region significantly improved the bond reliability.

[0113] As described above, according to the claimed invention, by controlling the state of the polymer constituting the substrate depending on the position within the substrate, it is possible to provide a substrate that simultaneously satisfies multiple independent required characteristics while maintaining the inherent properties of the polymer constituting the substrate. For example, it is possible to improve the bondability with a conductor pattern formed on the surface while maintaining the excellent electrical properties of an organic polymer resin such as a fluororesin. Although the present invention has been described above based on the embodiments, the present invention is not limited to the above embodiments. Various modifications can be made to the above embodiments within the scope of the same or equivalent to the present invention.

[0114] The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit and scope of the present invention, all of which are included in the technical concept of the present invention.

[0115] DESCRIPTION OF SYMBOLS 100: Substrate 101: First main surface 102: Second main surface 103: Internal region 104: Surface region 105: Surface region 110, 150: Thin film 200: Wiring substrate 210: Pad 220: Wiring 230: Via 240: Antenna pad 250: Ground plane

Claims

1. A substrate having a first main surface on which a conductive layer is formed and a second main surface that is in a front-back relationship with the first main surface, The substrate comprises an internal region mainly composed of fluororesin and a surface region formed on the internal region, including the first main surface, and mainly composed of fluororesin. When the material strength in the direction perpendicular to the first main surface of the substrate is defined as the substrate peel strength, the substrate peel strength in the surface region is greater than the substrate peel strength in the internal region.

2. A substrate having a first main surface on which a conductive layer is formed and a second main surface that is in a front-back relationship with the first main surface, The substrate comprises an internal region mainly composed of fluororesin and a surface region formed on the internal region, including the first main surface, and mainly composed of fluororesin. Crystallinity, expressed as the ratio of the peak area of ​​the crystalline component to the total peak area obtained by X-ray diffraction, A substrate in which the degree of crystallinity of the surface region is smaller than the degree of crystallinity of the internal region.

3. A substrate having a first main surface on which a conductive layer is formed and a second main surface that is in a front-back relationship with the first main surface, The substrate comprises an internal region mainly composed of fluororesin and a surface region formed on the internal region, including the first main surface, and mainly composed of fluororesin. The aforementioned fluororesin is polytetrafluoroethylene, When the endothermic reaction observed around 345°C in the DSC curve obtained by differential scanning calorimetry in the internal and surface regions of the substrate is defined as the first endothermic reaction, A substrate in which the heat of fusion of the first endothermic reaction in the surface region is smaller than the heat of fusion of the first endothermic reaction in the internal region.

4. A substrate having a first main surface and a second main surface that is in a front-and-back relationship with the first main surface, The substrate includes an internal region and a surface region formed on the internal region and including the first main surface. The substrate is a laminate of multiple thin films made of fluororesin. The plurality of thin films include a first thin film located in the internal region and a second thin film located in the surface region. The plurality of thin films made of the fluororesin are polytetrafluoroethylene, The first thin film is made of uncalcined stretched polytetrafluoroethylene, and the second thin film is a substrate made of calcined polytetrafluoroethylene.

5. The substrate according to any one of claims 1 to 4, wherein the proportion of fluororesin in the total constituent materials constituting the internal region and the surface region is 90% by weight or more, respectively.

6. The substrate according to any one of claims 1 to 4, wherein the proportion of polytetrafluoroethylene in the total constituent materials constituting the internal region and the surface region is 90% by weight or more, respectively.

7. The substrate according to any one of claims 1 to 4, wherein the main component of the internal region and the main component of the surface region have the same chemical composition.

8. The substrate according to any one of claims 1 to 4, wherein the internal region and the surface region are each substantially composed of fluororesin only.

9. The substrate according to any one of claims 1 to 4, wherein the internal region and the surface region are each substantially composed of only polytetrafluoroethylene.

10. The substrate according to any one of claims 1 to 4, wherein, at 30 GHz and 22°C, the relative permittivity of the substrate is 2.3 or less, and the dielectric loss tangent is 0.001 or less.

11. The substrate according to any one of claims 1 to 4, wherein, at 30 GHz and 22°C, the relative permittivity of the substrate is 2.1 or less, and the dielectric loss tangent is 0.0003 or less.

12. In a temperature range of 30°C to 250°C, the coefficient of linear expansion of the substrate in any direction on the substrate surface is defined as CTE. Ｘ The coefficient of linear expansion of the substrate in the direction perpendicular to the arbitrary direction on the substrate surface is defined as CTE. Ｙ In that case, the CTE Ｘ and CTE Ｙ The substrate according to any one of claims 1 to 4, wherein each of the particles is 80 ppm / °C or less.

13. In a temperature range of 30°C to 250°C, the coefficient of linear expansion of the substrate in any direction on the substrate surface is defined as CTE. Ｘ The coefficient of linear expansion of the substrate in the direction perpendicular to the arbitrary direction on the substrate surface is defined as CTE. Ｙ In that case, the CTE Ｘ and CTE Ｙ The substrate according to any one of claims 1 to 4, wherein each of the particles is 50 ppm / °C or less.

14. A wiring board comprising a substrate according to any one of claims 1 to 4 above, and a conductive layer formed on the first main surface of the substrate.

15. Prepare multiple polytetrafluoroethylene thin films, A method for manufacturing a substrate, comprising stacking a plurality of polytetrafluoroethylene thin films and applying pressure in the thickness direction, The plurality of polytetrafluoroethylene thin films include a first polytetrafluoroethylene thin film and a second polytetrafluoroethylene thin film. When the material strength in the thickness direction of the thin film is defined as the thin film peel strength, The method for producing a substrate in which the second polytetrafluoroethylene thin film has a thinner film peel strength higher than the first polytetrafluoroethylene thin film.

16. A polytetrafluoroethylene sheet is prepared by extruding polytetrafluoroethylene resin. Multiple stretched polytetrafluoroethylene thin films are prepared by stretching the aforementioned polytetrafluoroethylene sheet. A first stretched polytetrafluoroethylene thin film is prepared without firing a portion of the plurality of stretched polytetrafluoroethylene thin films. A second stretched polytetrafluoroethylene thin film is prepared by calcining another portion of the aforementioned plurality of stretched polytetrafluoroethylene thin films. A method for manufacturing a substrate, comprising laminating the first stretched polytetrafluoroethylene thin film and the second stretched polytetrafluoroethylene thin film and applying pressure in the thickness direction.

17. The method for manufacturing a substrate according to claim 15, wherein the lamination of the plurality of polytetrafluoroethylene thin films includes arranging the first polytetrafluoroethylene thin film inside the substrate and arranging the second polytetrafluoroethylene thin film on the surface of the substrate.

18. The method for manufacturing a substrate according to claim 16, wherein the lamination of the plurality of stretched polytetrafluoroethylene thin films includes arranging the first stretched polytetrafluoroethylene thin film inside the substrate and arranging the second stretched polytetrafluoroethylene thin film on the surface of the substrate.

19. The degree of crystallinity, expressed as the ratio of the peak area of ​​the crystalline component to the total peak area obtained by X-ray diffraction, The substrate according to claim 1, wherein the degree of crystallinity of the surface region is smaller than the degree of crystallinity of the internal region.

20. The fluororesin is polytetrafluoroethylene, When the endothermic reaction observed around 345°C in the DSC curve obtained by differential scanning calorimetry in the internal and surface regions of the substrate is defined as the first endothermic reaction, The substrate according to claim 1, wherein the heat of fusion of the first endothermic reaction in the surface region is smaller than the heat of fusion of the first endothermic reaction in the internal region.

21. The substrate is a laminate of multiple thin films made of fluororesin, The plurality of thin films include a first thin film located in the internal region and a second thin film located in the surface region. The plurality of thin films made of the fluororesin are polytetrafluoroethylene, The substrate according to claim 1, wherein the first thin film is made of uncalcined stretched polytetrafluoroethylene, and the second thin film is made of calcined polytetrafluoroethylene.