Laminated structure and printed board

Abstract

Provided is a laminated structure suitable for manufacturing a printed board, the laminated structure being capable of suppressing peeling of a wiring pattern at the time of application of bending stress to the printed board. The laminate structure includes a resin base material and a metal layer provided on a first surface of the resin base material. The resin base material has a recessed portion opening in the first surface, has a stress relaxation structure on an inner surface of the recessed portion, and a part of the metal layer extends into the recessed portion.

Description

Laminated structure and printed circuit board

[0001] The present invention relates to a laminated structure and a printed circuit board.

[0002] A known method for producing a copper-clad laminate is to press-bond a copper foil to a resin substrate. First, the copper foil is subjected to a roughening treatment to roughen the surface, and then the resin substrate is pressed onto the roughened surface to improve adhesion between the copper foil and the resin substrate. Patent Documents 1 and 2 disclose copper foils that have been subjected to a roughening treatment (roughened copper foils).

[0003] The roughened copper foil of Patent Document 1 is a roughened copper foil having a roughened surface on at least one side, the roughened surface comprising a plurality of primary roughening particles having constricted portions, the primary roughening particles having a plurality of secondary roughening particles smaller than the primary roughening particles on the surface including the constricted portions, and a secondary roughening particle density, which is the value obtained by dividing the number of the secondary roughening particles in the constricted portions by the surface area of ​​the constricted portions, of 9 to 30 particles / μm ２ and the ten-point average roughness Rz of the roughened surface is 0.7 to 1.7 μm.

[0004] The roughened copper foil of Patent Document 2 is a roughened copper foil having, on at least one side thereof, a roughened surface on which a plurality of roughened particles are formed, and the roughened particles have an uneven surface, and the developed area ratio Sdr of the interface of the roughened surface measured using a three-dimensional white light interference microscope is 1000% or more and 5000% or less, and the arithmetic mean roughness Sa of the roughened surface is 0.04 μm or more and 0.6 μm or less.

[0005] In copper-clad laminates produced using roughened copper foils as described in Patent Documents 1 and 2, the adhesion (bonding strength) between the copper foil and the resin substrate is improved by the anchor effect. Such copper-clad laminates are mainly used in the manufacture of printed circuit boards. In the manufacture of printed circuit boards, the copper foil of the copper-clad laminate is patterned by photolithography to form a wiring pattern. When the copper foil is partially removed, the resin surface is exposed, and on the exposed resin surface, a microstructure can be seen that is the transfer of the shape of the roughening particles from the roughening treatment applied to the copper foil.

[0006] In addition, since the roughening treatments in Patent Documents 1 and 2 are performed by plating, a microstructure is formed on the roughened surface of the roughened copper foil, in which a plurality of relatively small secondary roughening particles are arranged on relatively large primary roughening particles. On the resin surface to which the microstructure of the roughened copper foil is transferred, a microstructure is formed, which consists of recesses corresponding to the primary roughening particles and recesses corresponding to the secondary roughening particles.

[0007] Japanese Patent No. 6430092 Japanese Patent Application Laid-Open No. 2022-085378

[0008] Flexible substrates have become widely used in recent years, but in printed circuit boards using flexible substrates, the wiring pattern can peel off when the substrate is subjected to bending stress. Furthermore, with the recent trend toward higher signal frequencies, there is a trend to suppress the degree of surface roughening of copper foil. As a result, the anchor effect is reduced, and the problem of wiring pattern peeling due to bending stress has become more pronounced.

[0009] In Patent Documents 1 and 2, although the bonding strength between the copper foil and the resin substrate is increased by the anchor effect, peeling of the wiring pattern when bending stress is applied to the printed circuit board is not considered.

[0010] Therefore, a first object of the present invention is to provide a laminated structure suitable for manufacturing a printed circuit board, which can suppress peeling of a wiring pattern when a bending stress is applied to the printed circuit board, and a second object of the present invention is to provide a printed circuit board capable of suppressing peeling of a wiring pattern due to bending stress.

[0011] According to one aspect of the present invention, there is provided a laminated structure comprising a resin substrate and a metal layer provided on a first surface of the resin substrate, wherein the resin substrate has a recess that opens to the first surface and has a stress relaxation structure on the inner surface of the recess, and a portion of the metal layer extends into the recess.

[0012] According to another aspect of the present invention, there is provided a printed circuit board comprising a resin substrate and metal wiring provided on a first surface of the resin substrate, wherein the resin substrate has a recess that opens to the first surface, and in a wiring-covered region of the first surface that is covered with the metal wiring, a portion of the metal wiring extends into the recess, and in a surface-exposed region that is exposed from the metal wiring, a stress relaxation structure is provided on the inner surface of the recess.

[0013] A printed circuit board manufactured from the laminated structure of the present invention can suppress peeling of the wiring pattern due to bending stress.

[0014] FIG. 1 is a schematic perspective view of a laminated structure according to embodiment 1. FIG. 2A is a schematic enlarged cross-sectional view of the laminated structure taken along line X-X in FIG. 1. FIG. 2B is a schematic enlarged cross-sectional view of the laminated structure taken along line X-X in FIG. 1. FIG. 3 is a schematic enlarged cross-sectional view of a resin substrate included in the laminated structure of FIG. 2A. FIG. 4A is a schematic enlarged cross-sectional view of the portion surrounded by the dashed line in FIG. 3, and schematically shows a narrow groove formed on the inner surface of a recess. FIG. 4B is a schematic enlarged cross-sectional view illustrating the change in shape of the narrow groove shown in FIG. 4A when a bending stress F1 is applied to the laminated structure shown in FIG. 3. FIG. 4C is a schematic enlarged cross-sectional view illustrating the change in shape of the narrow groove shown in FIG. 4A when a bending stress F2 is applied to the laminated structure shown in FIG. 3. FIG. 5 is a schematic view of an SEM image of a resin substrate included in the laminated structure according to embodiment 1, taken from the top surface. FIG. 6A is a schematic enlarged cross-sectional view illustrating a method for producing a laminated structure according to embodiment 2-1. FIG. 6B is a schematic enlarged cross-sectional view illustrating a method for manufacturing a laminated structure according to embodiment 2-1. FIG. 6C is a schematic enlarged cross-sectional view illustrating a method for manufacturing a laminated structure according to embodiment 2-1. FIG. 7A is a schematic enlarged cross-sectional view illustrating a method for manufacturing a laminated structure according to embodiment 2-2. FIG. 7B is a schematic enlarged cross-sectional view illustrating a method for manufacturing a laminated structure according to embodiment 2-2. FIG. 8A is a schematic enlarged cross-sectional view of a printed circuit board according to embodiment 3. FIG. 8B is a schematic enlarged cross-sectional view of a printed circuit board according to embodiment 3. FIG. 9 is a schematic enlarged cross-sectional view of a modified example of the laminated structure according to embodiment 1.

[0015] Hereinafter, a laminated structure 10 according to a first embodiment of the present invention, a method for manufacturing the laminated structure 10 according to a second embodiment, and a printed circuit board 100 according to a third embodiment will be described with reference to the drawings.

[0016] (Embodiment 1: Laminated structure 10) Fig. 1 is a schematic perspective view of a laminated structure 10 according to an embodiment, Figs. 2A and 2B are schematic enlarged cross-sectional views of the laminated structure 10 in a cross section (a cross section in the thickness direction of the resin substrate 20) taken along line X-X in Fig. 1, and Fig. 3 is a schematic enlarged cross-sectional view of the resin substrate 20 included in the laminated structure 10 of Fig. 2A. As shown in Figs. 1 to 3, the laminated structure 10 includes the resin substrate 20 and a metal layer 40 provided on a first surface 25 of the resin substrate 20.

[0017] 2A , 2B , and 3 , the resin substrate 20 has a recess 24 that opens on the first surface 25. A portion 41 of the metal layer 40 passes through an opening 26 of the recess 24 that opens on the first surface 25 and extends into the recess 24 of the resin substrate 20. The portion 41 of the metal layer 40 can function as an anchor that fixes the metal layer 40 to the resin substrate 20. In other words, the portion 41 of the metal layer 40 can improve the adhesion between the resin substrate 20 and the metal layer 40.

[0018] 2A, 2B, and 3, the portion 41 of the metal layer 40 is usually filled with the metal material that constitutes the metal layer 40. However, air bubbles (or small cavities) may exist inside the portion 41 of the metal layer 40. If the size of the air bubbles (or small cavities) is sufficiently small compared to the size of the portion 41 of the metal layer 40, the air bubbles (or small cavities) may exist.

[0019] 2B , in a cross-sectional view of the resin substrate 20 in the thickness direction (T direction), the width W1 of the part 41 of the metal layer 40 at the position of the first surface 25 of the resin substrate 20 is preferably smaller than the maximum width W2 of the part 41 of the metal layer 40 inside the resin substrate 20. Here, the "width W1" and the "maximum width W2" each define a dimension measured in a direction (width direction: W direction) perpendicular to the thickness direction (T direction) of the resin substrate 20.

[0020] 2B , the "width W1 of the portion 41 of the metal layer 40 at the position of the first surface 25" is substantially equal to the width 26w of the opening 26 of the cavity 24 in the resin substrate 20. In other words, "the width W1 of the portion 41 of the metal layer 40 at the position of the first surface 25 is smaller than the maximum width W2 of the portion 41 of the metal layer 40 inside the resin substrate 20" has almost the same meaning as the width 26w of the opening 26 of the cavity 24 being smaller than the maximum width W2 of the portion 41 of the metal layer 40.

[0021] As described above, the portion 41 of the metal layer 40 can function as an anchor that fixes the metal layer 40 to the resin substrate 20. If the width 26w of the opening 26 is smaller than the maximum width W2 of the portion 41 of the metal layer 40 inside the resin substrate 20, the portion 41 of the metal layer 40 is unlikely to escape from the opening 26. In particular, if the width 26w is significantly smaller than the maximum width W2, the portion 41 of the metal layer 40 cannot escape from the opening 26. Therefore, the anchor effect of the portion 41 of the metal layer 40 is improved, and the adhesion between the resin substrate 20 and the metal layer 40 can be further improved.

[0022] In the laminated structure 10 according to the first embodiment, the resin substrate 20 has a stress relaxation structure on the inner surface 24a of the recess 24. The stress relaxation structure is, for example, a narrow groove 27 provided on the inner surface 24a of the recess 24 (see FIG. 4A). In this specification, the term "narrow groove 27" refers to a structure having a length 27L of 0.05 μm or more and 20 times or more the width 27w of the narrow groove 27 (see FIG. 4A) when viewed from above on the inner surface 24a of the recess 24 (see FIG. 5). As shown in FIG. 5, the narrow groove 27 may not be linear when viewed from above. In this case, the length 27L of the narrow groove 27 refers to the maximum dimension of the continuous narrow groove 27 when viewed from above.

[0023] For example, in the case of a single narrow groove 27i that meanders between both ends, the linear distance between both ends of the narrow groove 27i is the length 27L of the narrow groove 27i. As another example, when multiple narrow grooves 27ii are connected and difficult or impossible to separate, the multiple narrow grooves 27ii are treated as a single unit. A region 27R in which multiple narrow grooves 27ii exist is identified, and the maximum dimension of the multiple narrow grooves 27ii within that region 27R is defined as the length 27L of the multiple narrow grooves 27ii.

[0024] Referring again to FIG. 4A , FIG. 4A is a schematic enlarged cross-sectional view of the portion of the inner surface 24a of the recess 24 of the resin substrate 20 in FIG. 3 that is enlarged and surrounded by a dashed circle. FIG. 4A also shows a narrow groove 27 formed on the inner surface 24a of the recess. In cross-sectional view, the narrow groove 27 has a widest width 27w at the opening 27a, and the width of the narrow groove 27 from the opening 27a to the deepest part of the narrow groove 27 (referred to as the innermost part 27t of the narrow groove 27) is considered to be narrower than the width 27w. For example, as shown in FIG. 4A , the narrow groove 27 may be a V-shaped groove whose width gradually narrows from the opening 27a to the innermost part 27t in cross-sectional view. The inner surface 27b of the narrow groove 27 ０ The surface may be flat, curved, rough, or the like.

[0025] The narrow groove 27 has an inner surface 27b ０ The branch groove may have a branch groove (not shown) formed on the inner surface 27b. ０ The width of the branch groove from the opening to the innermost part of the branch groove is considered to be narrower than the width of the opening. Note that the branch groove is usually smaller in length and in width of the opening than the narrow groove 27.

[0026] Here, the mechanism by which the stress relaxation structure (narrow grooves 27) relaxes the bending stress applied to the resin base material 20 will be described with reference to FIG. 3 and FIGS. 4A to 4C.

[0027] When a bending stress F1 as shown in FIG. 3 is applied to the resin substrate 20 of the laminated structure 10 shown in FIG. 2A, the laminated structure 10 and the resin substrate 20 bend in a downward convex direction, and a compressive stress is applied to the first surface 25 of the resin substrate 20. FIG. 4B shows the change in shape of the narrow groove 27 when the bending stress F1 is applied to the laminated structure 10 and the resin substrate 20. When the first surface 25 is subjected to a compressive stress, a compressive stress f1 is applied to the narrow groove 27 (FIG. 3) provided on the inner surface 24a of the recess 24 opening to the first surface 25 (FIG. 4B). As a result, the opening 27a of the narrow groove 27 is deformed so that its width becomes narrower. In other words, the inner surface of the narrow groove 27 is different from the inner surface 27b when no stress is applied. ０ From the inside surface 27b in the state where the compressive stress f1 is applied, １ is displaced to

[0028] On the other hand, when a bending stress F2 as shown in FIG. 3 is applied to the resin substrate 20 of the laminated structure 10 shown in FIG. 2A, the laminated structure 10 and the resin substrate 20 are curved in a direction that is convex upward, and a tensile stress is applied to the first surface 25 of the resin substrate 20. FIG. 4C shows the change in shape of the narrow groove 27 when the bending stress F2 is applied to the laminated structure 10 and the resin substrate 20. When the first surface 25 is subjected to a tensile stress, a tensile stress f2 is applied to the narrow groove 27. As a result, the opening 27a of the narrow groove 27 is deformed so that its width increases. In other words, the inner surface of the narrow groove 27 is different from the inner surface 27b when the tensile stress f2 is not applied. ０ From the inner surface 27b in the state where tensile stress is applied ２ is displaced to

[0029] In this way, by providing the stress relaxation structure (narrow groove 27) on the inner surface 24a of the recess 24 formed in the resin substrate 20, it is possible to relax the bending stress caused by curvature of the resin substrate 20. When a printed circuit board 100 ( FIG. 8A ) is manufactured from the laminated structure 10 including the resin substrate 20 having the stress relaxation structure, the stress relaxation structure (narrow groove 27) on the inner surface 24a of the recess 24 is preferentially deformed in the surface exposed region 252 of the resin substrate 20 where the first surface 25 is exposed from the metal wiring 400, thereby relaxing the bending stress applied to the printed circuit board 100.

[0030] 8A , recesses 24 are also present in the wiring-covered region 251 (the region where the first surface 25 is covered with the metal wiring 400) of the resin base material 20, and stress relief structures (narrow grooves 27) are present on the inner surfaces 24a thereof. However, because the inner surfaces 24a of the recesses 24 are in contact with the portions 410 of the metal wiring 400, the stress relief structures (narrow grooves 27) in the wiring-covered region 251 are less likely to deform than the stress relief structures (narrow grooves 27) formed in the surface-exposed region 252. In other words, deformation of the wiring-covered region 251 of the first surface 25 due to bending stress is suppressed, and therefore, peeling of the metal wiring 400 formed on the surface of the wiring-covered region 251 from the resin base material 20 can be effectively suppressed.

[0031] Referring again to Figure 4A, the average value (average width) of the width 27w of the narrow groove 27 on the inner surface 24a of the recess is preferably 0.1 nm or more and 100 nm or less, more preferably 0.1 nm or more and 10 nm or less. Within this range, the strength of the resin substrate 20 can be maintained while effectively exhibiting a stress relaxation effect. The average width of the narrow groove 27 refers to the arithmetic average value of the width 27w of the opening 27a of the narrow groove 27 measured at multiple locations. A detailed measurement method will be described below.

[0032] The metal layer 40 is removed from the laminated structure 10 by etching or the like. Thereafter, in order to obtain an SEM image, the first surface 25 of the resin substrate 20 is pretreated (for example, using a JEOL JEC-3000FC and pretreating with Pt coat 20mA-30s). The inner surface 24a of the recess 24 is observed from the first surface 25 side (top side) of the resin substrate 20 using a scanning electron microscope (for example, an ultra-high resolution field emission scanning electron microscope SU8230 manufactured by Hitachi High-Tech Corporation). The conditions for SEM observation are an acceleration voltage of 3 kV and a magnification of 100,000 times. In order to clearly observe the narrow grooves 27 on the inner surface 24a of the recess 24, if necessary, the acquired SEM image may be processed using image processing software (for example, image software built into the scanning electron microscope, or image processing software such as Microsoft Photo 2019) to adjust brightness, contrast, etc.

[0033] FIG. 5 is a schematic diagram of an SEM image of the inner surface 24a of the recess 24 of the resin substrate 20 included in the laminate structure 10 according to embodiment 1, taken from the first surface 25 side (top surface side). Three to five narrow grooves 27 are selected from the obtained SEM image, and width measurements are performed at three locations for each narrow groove. In other words, 9 to 15 width measurement data are obtained from one SEM image. Under similar conditions, SEM images are obtained at different positions on the first surface 25 of the resin substrate 20, and the widths of the narrow grooves 27 are measured. Three or more SEM images are obtained, the widths of the narrow grooves 27 are measured, and the obtained width measurements are arithmetically averaged. In this way, the average width of the narrow grooves 27 is obtained.

[0034] The depth 27d of the narrow groove 27 (see FIG. 4A ) is considered to be approximately the same as the width 27w of the narrow groove 27 at the opening 27a. As will be explained later in the section on the formation mechanism of the narrow groove 27, the narrow groove 27 is formed by the diffusion (penetration) of metal elements constituting the metal layer 40 (more precisely, the second metal layer 40y in FIG. 6C ) from the first surface 25 of the resin substrate 20. The narrow groove 27 is considered to grow isotropically in the width and depth directions, and it is assumed that the width 27w and the depth 27d of the narrow groove 27 are approximately the same. The depth 27d of the narrow groove 27 refers to the distance from the opening 27a of the narrow groove 27 to the innermost portion 27t ( FIG. 4A ). Note that FIG. 4A is a schematic diagram, and therefore the dimensional relationship between the depth 27d and the width 27w of the narrow groove 27 differs from the actual relationship.

[0035] The average value (average depth) of the depth 27d of the narrow grooves 27 is preferably 0.1 nm or more and 100 nm or less, and more preferably 0.1 nm or more and 10 nm or less, similar to the average width of the narrow grooves 27. When the depth 27d is in this range, it is expected that the strength of the resin substrate 20 can be maintained while the stress relaxation effect can be effectively exhibited.

[0036] (Modification) A laminated structure 10β according to a modification of the first embodiment may include metal layers 40, 40β on both sides of the resin substrate 20 (see FIG. 9). The laminated structure 10β shown in FIG. 9 includes a metal layer 40 on the first surface 25 of the resin substrate 20, similar to FIGS. 1 and 2A, and further includes a second metal layer 40β on the second surface 25β opposite the first surface 25. The resin substrate 20 has a second recess 24β opening on the second surface 25β, and a stress relaxation structure (narrow groove 27) on the inner surface of the second recess 24β. A portion 41β of the second metal layer 40β extends into the second recess 24β. The laminated structure 10β is suitable for manufacturing printed circuit boards having metal wiring on both sides.

[0037] (Embodiment 2: Manufacturing Method of Layered Structure 10) A manufacturing method of the layered structure 10 according to embodiment 1 (see FIGS. 1 and 2A) will be described. The manufacturing method of the layered structure 10 is not limited thereto, but two examples can be given. These manufacturing methods will be described in detail in embodiments 2-1 and 2-2, respectively.

[0038] 6A to 6C are schematic enlarged cross-sectional views illustrating a first method for manufacturing the laminate structure 10 according to embodiment 1. The first method for manufacturing the laminate structure 10 includes: 1) a step of surface-modifying the first surface 25 of the resin substrate 20; 2) a step of forming a metal precursor 62 containing a first metal element and a second metal element on the first surface 25 of the resin substrate 20 after the surface modification, thereby preparing a precursor laminate 60; and 3) a step of heating the precursor laminate 60.

[0039] 2A and 3 are formed in the resin substrate 20 by heating the precursor laminate 60 in step 3). The narrow grooves 27 on the inner surface 24a of the recess 24 are formed by modifying the surface of the first surface 25 of the resin substrate 20 in step 1), and then heating the precursor laminate 60 in step 3). Each step will be described below.

[0040] Step 1) Step of Surface Modification of First Surface 25 of Resin Substrate 20 A resin substrate 20 without recesses 24 is prepared, and the first surface 25 is surface modified. One example of surface modification is ultraviolet irradiation (UV irradiation), in which ultraviolet rays (UV) are irradiated onto the first surface 25 of the resin substrate 20 to modify it. By performing this surface modification step before step 3), it is possible to form narrow grooves 27 on the inner surface 24a of the recesses 24 when step 3 is performed. The reason for this is not clear, but it is presumed to be due to the following mechanism.

[0041] When modifying the first surface 25 by UV irradiation, UV reacts with oxygen in the air to generate ozone, which breaks some of the chemical bonds of the molecules of the resin material constituting the resin substrate 20 (depolymerization). In the areas where the chemical bonds are broken, the metal elements constituting the second metal layer 40y are more likely to thermally diffuse in step 3), which will be described later. As a result, cracks (narrow grooves 27) are formed on the first surface 25 of the resin substrate 20. The cracks (narrow grooves 27) caused by the diffusion of the metal elements extend toward the interior of the resin substrate 20 as the diffusion of the metal elements progresses.

[0042] As will be described later, in step 3), recesses 24 are formed on the first surface 25 of the resin substrate 20, and the recesses 24 further expand into the resin substrate 20. At this time, the speed at which the narrow grooves 27 expand inward is greater than or equal to the speed at which the recesses 24 expand, so the narrow grooves 27 do not disappear and remain on the inner surface 24a of the recesses 24. As a result, the resin substrate 20 is formed with recesses 24 that open to the first surface 25, and the narrow grooves 27 are formed on the inner surface 24a of the recesses 24.

[0043] In addition to UV irradiation, surface modification methods include plasma treatment, corona treatment, electron beam irradiation, and treatment using chemicals such as oxidizers. All of these methods modify the first surface 25 of the resin substrate 20 and can break some of the chemical bonds of the molecules of the resin material that makes up the resin substrate 20.

[0044] The depth of modification (modification depth) is preferably a certain depth. The modification depth corresponds to the thickness of the modified layer obtained after modification. The thickness of the modified layer can be, for example, about 0.001 μm to 1 μm. In the case of UV irradiation, it is expected that the modification can be performed relatively deep from the first surface 25 of the resin substrate 20 (that is, the modified layer can be made relatively thick) by using a low-pressure mercury lamp. As a specific example of modification treatment by UV irradiation, a low-pressure mercury lamp (MHU-110BK manufactured by Multiply Corporation, dominant wavelength: 254 nm) is used, and the illuminance is 9.54 mW / cm. ２ For example, irradiation for 10 minutes is performed.

[0045] 6A, in the first manufacturing method according to embodiment 2-1, the metal precursor 62 has a two-layer structure including a first metal layer (referred to as a "barrier film 40x") containing a first metal element and disposed on the resin substrate 20, and a second metal layer 40y containing a second metal element and disposed on the barrier film 40x. In the laminate structure 10 shown in FIGS. 1 and 2A, the barrier film 40x and the second metal layer 40y are collectively referred to as the "metal layer 40."

[0046] The metal precursor 62 is formed by sequentially laminating a barrier film 40x and a second metal layer 40y on the resin substrate 20. The barrier film 40x is formed by forming a film of a metal material containing a first metal element on the first surface 25 of the resin substrate 20 by, for example, electroless plating or sputtering. The second metal layer 40y is formed by forming a film of a metal material containing a second metal element on the surface 40xb of the barrier film 40x by, for example, electroless plating, electroless plating, or sputtering. When forming the barrier film 40x on the resin substrate 20 by electroless plating, it is preferable to pretreat the first surface 25 of the resin substrate 20 by a known pretreatment method such as adding a catalyst.

[0047] The first metal element contained in the barrier film 40x and the second metal element contained in the second metal layer 40y preferably have different diffusion coefficients in the resin substrate 20, and the diffusion coefficient of the first metal element in the resin substrate 20 is smaller than the diffusion coefficient of the second metal element. Because the second metal element has a relatively large diffusion coefficient in the resin substrate 20, the second metal layer 40y containing the second metal element preferentially diffuses into the resin substrate 20 by the heating process. This allows the extension portion 44 (see FIG. 6C ) to be easily formed from the second metal layer 40y. The extension portion 44 corresponds to the portion 41 (see FIG. 2A ) of the metal layer 40 in the first embodiment.

[0048] On the other hand, since the first metal element has a relatively small diffusion coefficient in the resin substrate 20, the barrier film 40x containing the first metal element is less likely to diffuse into the resin substrate 20 even during the heating step, and the film shape is likely to be maintained. That is, even after step 3), the barrier film 40x is likely to remain between the resin substrate 20 and the second metal layer 40y. In other words, the barrier film 40x allows a portion 41 of the second metal layer 40y to diffuse into the resin substrate 20, but prevents the entire second metal layer 40y from diffusing into the resin substrate 20. In this way, the small diffusion coefficient of the first metal element allows the formation of a barrier film 40x with high barrier function.

[0049] The first metal element is preferably one or more selected from the group consisting of Fe, V, Ni, Ti, Ca, Ag, Zn, Al, Mg, Rh, Pt, Au, and Pd, and the second metal element is preferably one or more selected from the group consisting of Co, Mn, and Cu.

[0050] The material for forming the barrier film 40x may consist solely of the first metal element, or may further contain other elements in addition to the first metal element as long as the barrier function of the barrier film 40x is not impaired. The material for forming the second metal layer 40y may consist solely of the second metal element, or may further contain other elements in addition to the second metal element as long as the formation of the narrow groove 27 and the extension 44 is not impaired.

[0051] The average thickness of the barrier film 40x before step 3), i.e., the average thickness when the barrier film 40x is formed, is not particularly limited, but can be, for example, 10 nm to 100 nm. When the thickness of the barrier film 40x is 10 nm or more, the barrier function of the barrier film 40x can be fully exhibited. When the thickness is 100 nm or less, the through-holes 32 penetrating the barrier film 40x can be easily formed in step 3).

[0052] The average thickness of the barrier film 40x after step 3), i.e., the average thickness of the barrier film 40x in the stacked structure 10, is more preferably 0.005 μm or more and 1 μm or less, and particularly preferably 0.01 μm or more and 0.1 μm or less. In measuring the thickness of the barrier film 40x, first, element mapping of the cross-sectional EDX observation of the stacked structure 10 is confirmed, and the range (shape) of the barrier film 30 is identified from the portion (range) containing the first metal element. Next, the thickness of the barrier film 40x is measured at any five points, and the arithmetic mean of the measured values ​​is defined as the average thickness of the barrier film 40x.

[0053] The resin substrate 20 may be made of, for example, acrylonitrile butadiene styrene (ABS), polycarbonate / acrylonitrile butadiene styrene (PC / ABS), acrylonitrile styrene acrylate (ASA), silicone composite rubber-acrylonitrile-styrene (SAS), Noryl, polypropylene, polycarbonate (PC), polycarbonate alloy, acrylonitrile styrene, polyacetate, polylactic acid, polystyrene, polyamide, aromatic polyamide, polyethylene, polyether ketone, polyethylene terephthalate, polybutylene The polymer may include one or more selected from the group consisting of polyethylene terephthalate, polysulfone, polyether ether sulfone, polyetherimide, modified polyphenylene ether, polyphenylene sulfide, polyphenylene oxide, polyamide, polyimide, modified polyimide, epoxy resin, cycloolefin polymer, polynorbornene, perfluoroalkoxy fluoropolymer, polytetrafluoroethylene, vinylidene fluoride, vinyl resin, phenol resin, polyacetal, nylon, liquid crystal polymer, etc., and copolymers of the above polymers.

[0054] Step 3) Heating the Precursor Laminate 60 When the precursor laminate 60 is heated, as shown in FIG. 6B , a portion of the second metal layer 40y penetrates the barrier film 40x, which was a continuous film, and diffuses further into the resin substrate 20. At this time, the diffusion occurs while forming cracks (narrow grooves 27) in the resin substrate 20. When the heating is continued, a portion of the second metal layer 40y diffuses further into the resin substrate 20, and an extension portion 44 as shown in FIG. 6C can be formed.

[0055] The mechanism by which a portion of the second metal layer 40y penetrates the barrier film 40x and diffuses into the resin base material 20 is not clear, but is presumed to be as follows.

[0056] In step 3), the first metal element contained in the barrier film 40x and the second metal element contained in the second metal layer 40y undergo mutual thermal diffusion, forming through-holes 32 in the barrier film 40x. The formation of the through-holes 32 is presumably influenced by structural defects in the barrier film 40x, variations in the size of the crystal grains forming the barrier film 40x, and co-deposition of contaminants in the barrier film 40x (collectively referred to as "structural defects in the barrier film 40x"). It is expected that the diffusion rate of the metal elements between the first metal element and the second metal element is faster in areas where structural defects exist in the barrier film 40x than in areas where structural defects do not exist. The locally promoted diffusion of the metal elements locally thins the barrier film 40x, ultimately forming through-holes 32. Because the structural defects in the barrier film 40x occur randomly within the barrier film 40x, the through-holes 32 are also randomly formed at positions corresponding to the structural defects.

[0057] When the through-holes 32 are formed in the barrier film 40x, a portion of the second metal layer 40y passes through the through-holes 32 and contacts the first surface 25 of the resin substrate 20. The metal elements constituting the second metal layer 40y begin to diffuse into the first surface 25 of the resin substrate 20 softened by heating ( FIG. 6B ), forming narrow grooves 27 on the first surface 25. As the heating time increases, the diffusion of the metal elements progresses, and a portion of the second metal layer 40y extends into the resin substrate 20. The portion of the second metal layer 40y further expands into the resin substrate 20 while pushing out the resin material constituting the resin substrate 20, forming an extension 44 ( FIG. 6C ). Wedge-shaped protrusions that penetrate into the narrow grooves 27 may be present on the surface of the extension 44. It is presumed that this mechanism results in the stacked structure 10 shown in FIG. 6C .

[0058] The heating conditions for step 3) are, for example, a heating temperature higher than (Tm-30)°C and lower than Tm°C, and a heating time of 1 minute or more and 120 minutes or less. Tm (°C) is the melting point of the resin material used in the resin substrate 20. When the resin substrate 20 is formed from a mixed material of multiple resin materials, the melting point of the mixed material is measured or calculated and used as Tm. When the resin substrate 20 is formed by laminating multiple resin sheets made of different resin materials, the melting point of the resin material of the resin sheet in contact with the metal precursor 62 is used as Tm.

[0059] The heating conditions (heating temperature, heating time) in step 3) can affect the dimensions of the through-holes 32 in the barrier film 40x and the dimensions and shape of the recesses 24 in the resin substrate 20 (i.e., the dimensions and shape of the extensions 44 of the second metal layer 40y). By setting the heating conditions within the above ranges, the extensions 44 can be made to have dimensions and shapes suitable for functioning as anchors that fix the second metal layer 40y to the resin substrate 20.

[0060] However, if the heating temperature is too high or the heating time is too long, the barrier film 40x may disappear completely. Without the barrier film 40x, the entire second metal layer 40y may diffuse into the resin base material 20, preventing the formation of a structure having an anchor function (extension 44 in FIG. 6C ), and possibly preventing the effect of improving the adhesion between the resin base material 20 and the second metal layer 40y.

[0061] In this manner, the laminated structure 10 shown in FIGS. 2A and 6C is obtained by the first manufacturing method.

[0062] (Embodiment 2-2: Second Manufacturing Method of Layered Structure 10) The second manufacturing method of the layered structure 10 differs from the first manufacturing method mainly in the structure and method of forming the metal precursor formed in step 2). The rest is the same as the first manufacturing method. The second manufacturing method described in embodiment 2-2 will be described mainly with reference to the differences from the first manufacturing method, and descriptions of matters similar to those in the first manufacturing method (e.g., step 1) may be omitted.

[0063] 7A and 7B are schematic enlarged cross-sectional views illustrating a second manufacturing method of the laminate structure 10. The second manufacturing method of the laminate structure 10 includes, in step 2), a step of heating a precursor laminate 70 including the resin substrate 20 after surface modification of the first surface 25 and a metal precursor 72 containing a first metal element and a second metal element located on the first surface 25 of the resin substrate 20. As shown in FIG. 7A , in the second manufacturing method according to embodiment 2-2, the metal precursor 72 formed in step 2) is a single plating layer disposed on the resin substrate 20. This plating layer contains the first metal element and the second metal element.

[0064] The ratio of the content of the first metal element to the content of the second metal element contained in the metal precursor 72 can be, for example, the content (mass%) of the first metal element: the content (mass%) of the second metal element = 0.1:99.9 to 40:60. The metal precursor 72 consisting of a single plating layer can be formed by electroless plating. Known plating solutions, plating conditions, etc. used in electroless plating can be applied. Prior to electroless plating, it is preferable to pretreat the first surface 25 of the resin substrate 20 by a known pretreatment method, such as adding a catalyst.

[0065] Step 3) is performed under the same conditions as in the first manufacturing step on the precursor laminate 70 obtained by forming a metal precursor 72 consisting of a single plating layer on the first surface 25 of the resin substrate 20. When the precursor laminate 70 is heated, the metal precursor 72 has a two-layer structure including a barrier film 40x containing a first metal element located on the resin substrate 20 and a second metal layer 40y containing a second metal element located on the barrier film 40x (see FIG. 7B ).

[0066] The two-layer structure of the barrier film 40x and the second metal layer 40y can be achieved by a phenomenon in which the first metal element in the metal precursor 72 segregates on the first surface 25 of the resin substrate 20. Although the mechanism of this segregation phenomenon is not clear, it is presumed that the first metal element in the metal precursor 72 diffuses in all directions due to heating and is fixed on the first surface 25 of the resin substrate 20 when it reaches the first surface 25. As a result, the first metal element segregates on the first surface 25 of the resin substrate 20 to form the barrier film 40x.

[0067] By further heating the metal precursor 72, which has now become a two-layer structure consisting of the barrier film 40x and the second metal layer 40y, a through-hole 32 is formed in the barrier film 40x, which was a continuous film, as in the first manufacturing method of embodiment 2-1, and a portion of the second metal layer 40y extends through the through-hole 32 into the interior of the resin substrate 20, forming an extension portion 44 (Figures 6B and 6C).

[0068] In the second manufacturing method, the heating step in step 3) may be divided into two stages (preheating and main heating). First, a preheating step may be performed in which the metal precursor 72, which is composed of a single plating layer, is heated at 190°C or higher and 210°C or lower to form a two-layer structure of the barrier film 40x and the second metal layer 40y. Next, main heating may be performed under the same heating conditions as in step 3) in the first manufacturing method described in embodiment 2-1. The preheating time may be any time, but may be, for example, 30 minutes or longer and 90 minutes or shorter.

[0069] The preferred heating conditions (heating temperature and heating time) for step 3), the preferred relationship between the diffusion coefficients of the first metal element and the second metal element, the types of metal elements suitable for the first metal element and the second metal element, the types of resin materials suitable for the resin substrate 20, etc. are the same as those in the first manufacturing method described in embodiment 2-1.

[0070] In this manner, the laminated structure 10 shown in FIGS. 2A and 6C is obtained by the second manufacturing method.

[0071] 8A and 8B , a printed circuit board 100 manufactured from the laminated structure 10 of the first embodiment will be described. The printed circuit board 100 includes a resin substrate 20 and metal wiring 400 provided on a first surface 25 of the resin substrate 20. The resin substrate 20 has a recess 24 that opens on the first surface 25. The resin substrate 20 includes a wiring-covered region 251 in which the first surface 25 is covered with metal wiring 400, and a surface-exposed region 252 in which the metal wiring 400 has been removed and the first surface 25 is exposed from the metal wiring 400. In the wiring-covered region 251, a portion 410 of the metal wiring 400 extends into the recess 24 through the opening 26. In the surface-exposed region 252, the inner surface 24a of the recess 24 is exposed, and a stress relaxation structure (narrow groove 27) is formed on the inner surface 24a. In the wiring covered region 251 as well, a stress relaxation structure (narrow groove 27) may be present on the inner surface 24a of the recess 24.

[0072] 4A to 4C in the above-described first embodiment, the stress relaxation structure (narrow groove 27) can relieve bending stress generated in the laminated structure 10. When bending stress is applied to the printed circuit board 100 as shown in FIG. 8A , the stress relaxation structure (narrow groove 27) on the inner surface 24 a of the recess 24 in the surface exposed region 252 is preferentially deformed, thereby relieving the bending stress applied to the printed circuit board 100. Even in the wiring covered region 251, even if the stress relaxation structure (narrow groove 27) is present on the inner surface 24 a of the recess 24, the inner surface 24 a is in contact with the part 410 of the metal wiring 400, and therefore is less likely to deform than the stress relaxation structure (narrow groove 27) of the recess 24 formed in the surface exposed region 252.

[0073] As described above, in the printed circuit board 100 according to the third embodiment, the stress relaxation structure (narrow groove 27) of the recess 24 formed in the surface exposed region 252 is preferentially deformed to relieve stress. Because the metal wiring 400 is not present in the surface exposed region 252, even if the stress relaxation structure (narrow groove 27) is deformed, it is unlikely to cause peeling of the metal wiring 400. On the other hand, the stress relaxation structure (narrow groove 27) of the recess 24 formed in the wiring covered region 251 is unlikely to be subjected to bending stress (i.e., the stress relaxation structure (narrow groove 27) is unlikely to deform), so peeling of the metal wiring 400 can be suppressed.

[0074] The printed circuit board 100 of the third embodiment may have branch grooves on the inner surface of the narrow groove 27, similar to the laminated structure 10 of the first embodiment. The shape, average width, average depth, etc. of the narrow groove 27 and the branch grooves are similar to those of the narrow groove 27 and the branch grooves described in the first embodiment, and therefore details thereof will be omitted.

[0075] In the metal wiring 400 of the printed circuit board 100, as in the laminated structure 10 of the first embodiment, there is a preferred relationship between the width W10 and the maximum width W20 of the portion 410. This preferred relationship will be described with reference to FIG. 8B.

[0076] As shown in Figure 8B, in a cross-sectional view of the resin substrate 20 in the thickness direction (T direction), it is preferable that the width W10 (substantially equal to the width 26w of the opening 26) of the portion 410 of the metal wiring 400 at the position of the first surface 25 of the resin substrate 20 is smaller than the maximum width W20 of the portion 410 of the metal wiring 400 inside the resin substrate 20.

[0077] If the width W10 of the portion 410 of the metal wiring 400 (corresponding to the width 26w of the opening 26) is smaller than the maximum width W20 of the portion 410 of the metal wiring 400 inside the resin base material 20, the portion 410 of the metal wiring 400 is unlikely to slip out of the opening 26. In particular, if the width 26w is significantly smaller than the maximum width W20, the portion 410 of the metal wiring 400 cannot slip out of the opening 26. Therefore, the anchor effect of the portion 410 of the metal wiring 400 is improved, and the adhesion between the resin base material 20 and the metal wiring 400 can be improved.

[0078] When the printed circuit board 100 is formed from the laminated structure 10, the portion 410 of the metal wiring 400 of the printed circuit board 100 corresponds to the portion 41 of the metal layer 40 of the laminated structure 10, and the width W10 and maximum width W20 of the portion 410 of the metal wiring 400 correspond to the width W1 and maximum width W2 of the portion 41 of the metal layer 40, respectively. Furthermore, as with the laminated structure 10 of the first embodiment, bubbles (or small cavities) may be present inside the portion 410 of the metal wiring 400.

[0079] 9, a printed circuit board using the laminated structure 10β may include metal wiring 400 on both the first surface 25 and the second surface 25β of the resin substrate 20. That is, the resin substrate 20 includes a second metal wiring on the second surface 25β opposite the first surface 25 of the resin substrate 200, and the resin substrate 20 has a second recess 24β opening on the second surface 25β. In the second wiring-covered region of the second surface 25β covered with the second metal wiring, a portion of the second metal wiring extends into the second recess 24β. In the second surface-exposed region exposed from the second metal wiring, a stress relaxation structure (narrow groove 27) may be formed on the inner surface of the second recess 24β. In such a printed circuit board, metal wiring is included on both surfaces, and the metal wiring is less likely to peel off from the resin substrate 20 on either surface.

[0080] The printed circuit board 100 of embodiment 3 can be manufactured by using the laminated structure 10 of embodiment 1 and patterning the metal layer 40 by a conventionally known method (e.g., photolithography) to produce metal wiring 400.

[0081] Various tests were carried out using samples prepared under the following conditions: The samples were prepared by the second manufacturing method (Embodiment 2-2).

[0082] [Example 1] (Sample Preparation) A 50 mm long x 50 mm wide x 0.1 mm thick LCP (liquid crystal polymer) sheet (melting point Tm (catalog value) = 310°C) was used as the resin substrate 20, and a low-pressure mercury lamp (MHU-110BK manufactured by Multiply Corporation, dominant wavelength: 254 nm) was used to produce an illuminance of 9.54 mW / cm ２The resin substrate 20 was exposed to light for 10 minutes. Thereafter, a metal precursor 72 consisting of a single plating layer as shown in FIG. 7A was produced. The first metal element was Ni, and the second metal element was Cu. According to the conditions in Table 1, pretreatment of the resin substrate 20 before producing the metal precursor 72 (Process Nos. 1 to 5) and electroless plating (Process No. 6) for producing the metal precursor 72 were performed, thereby producing 10 samples of the precursor laminate 70. The obtained precursor laminate 70 samples were preheated at 200°C for 60 minutes (Process No. 7), and then further heated at 300°C for 10 minutes (Process No. 8).

[0083] The metal layer 40 was removed from the obtained laminated structure 10, and the inner surface 24a of the recess 24 was observed by SEM from the first surface 25 side of the resin substrate 20. The conditions for the SEM observation were as follows. Pretreatment: Using a JEC-3000FC manufactured by JEOL, pretreatment was performed with Pt coating at 20 mA for 30 seconds. Scanning electron microscope (SEM) / energy dispersive X-ray fluorescence analyzer (EDX): Hitachi High-Tech Corporation, ultra-high resolution field emission scanning electron microscope SU8230, accelerating voltage 3 kV, magnification 100,000 times (×100k) / Bruker A-X Co., Ltd., EDX for SEM XFlash 5060FQ

[0084] The width of the narrow grooves 27 was measured using the length measurement function of the imaging software (Hitachi SU8200 Scanning Electron Microscope) built into the scanning electron microscope. When the narrow grooves 27 on the inner surface 24a of the recesses 24 were difficult to see, the brightness, contrast, and the like were adjusted using the imaging software or other image processing software (e.g., Photo 2019 manufactured by Microsoft). A schematic diagram of one of the obtained SEM images is shown in FIG. 5.

[0085] 5, multiple crack-like narrow grooves 27 were observed on the inner surface 24a of the recess 24. The narrow grooves 27 had an average width of approximately 1.25 nm. When the printed circuit board 100 is formed from this laminated structure 10, it is believed that the stress relaxation ability is high and peeling of the metal wiring can be sufficiently suppressed.

[0086] [Comparative Example 1] A commercially available copper-clad laminate (glass epoxy copper-clad laminate R1705, manufactured by the former Panasonic Electric Works Co., Ltd.) was prepared as Comparative Example 1. After removing the copper foil from the copper-clad laminate, the surface of the glass epoxy substrate was observed using an SEM. The conditions for SEM observation were the same as those for Example 1.

[0087] From the SEM image, numerous spherical depressions of different sizes were observed on the surface of the glass epoxy substrate. The depressions were roughly divided into three categories based on their dimensions. The depressions of roughly three sizes are thought to correspond to depressions formed by primary particles (largest particles), secondary particles (medium-sized particles), and tertiary particles (smallest particles) adhering to the roughened surface of the copper foil. The dimensions (diameter of the opening) of each of the three depressions were as follows: Diameter of the largest depression (corresponding to primary particles): approximately 3.0 μm Diameter of the medium-sized depression (corresponding to secondary particles): approximately 1.0 to 1.5 μm Diameter of the smallest depression (corresponding to tertiary particles): approximately 25 to 50 nm

[0088] The opening of each recess was circular, and no recess corresponding to the "narrow groove 27" of the present invention was present. Therefore, when a printed circuit board is formed, it is thought that the stress relaxation ability is low and peeling of the metal wiring cannot be sufficiently suppressed.

[0089]

[0090] The disclosure of this specification may include the following aspects: <1> A laminated structure comprising a resin substrate and a metal layer provided on a first surface of the resin substrate, wherein the resin substrate has a recess that opens to the first surface and has a stress relaxation structure on an inner surface of the recess, and a portion of the metal layer extends into the recess.

[0091] <2> The laminated structure according to <1>, wherein the stress relaxation structure is a narrow groove provided on an inner surface of the recess.

[0092] <3> The stacked structure according to <2>, wherein the average width of the narrow grooves on the inner surface of the recess is 0.1 nm or more and 100 nm or less.

[0093] <4> The laminate structure according to any one of <1> to <3>, further comprising a second metal layer on a second surface of the resin substrate opposite to the first surface, wherein the resin substrate has a second recess that opens to the second surface and has a stress relaxation structure on an inner surface of the second recess, and a portion of the second metal layer extends into the second recess.

[0094] <5> The laminate structure according to any one of <1> to <4>, wherein, in a cross-sectional view of the resin substrate in a thickness direction, a width of a part of the metal layer at the position of the first surface of the resin substrate is smaller than a maximum width of the part of the metal layer inside the resin substrate.

[0095] <6> A printed circuit board comprising: a resin base material; and metal wiring provided on a first surface of the resin base material, wherein the resin base material has a recess that opens to the first surface, and in a wiring-covered region of the first surface that is covered with the metal wiring, a portion of the metal wiring extends into the recess, and in a surface-exposed region that is exposed from the metal wiring, a stress relaxation structure is provided on an inner surface of the recess.

[0096] <7> The printed circuit board according to <6>, wherein the stress relaxation structure is a narrow groove provided on the inner surface of the recess.

[0097] <8> The printed circuit board according to <7>, wherein the average width of the narrow grooves on the inner surface of the recess is 0.1 nm or more and 100 nm or less.

[0098] <9> The printed circuit board according to any one of <6> to <8>, further comprising a second metal wiring on a second surface of the resin base material opposite to the first surface, wherein the resin base material has a second recess that opens to the second surface, wherein in a second wiring-covered region of the second surface that is covered with the second metal wiring, a part of the second metal wiring extends into the second recess, and in a second surface-exposed region that is exposed from the second metal wiring, a stress relaxation structure is provided on an inner surface of the second recess.

[0099] <10> The printed circuit board according to any one of <6> to <9>, wherein, in a cross-sectional view in a thickness direction of the resin base material, a width of a part of the metal wiring at the position of the first surface of the resin base material is smaller than a maximum width of a part of the metal wiring inside the resin base material.

[0100] This application claims priority based on Japanese Patent Application No. 2023-058789, filed on March 31, 2023, the entire contents of which are incorporated herein by reference.

[0101] 10, 10β Laminated structure 20 Resin substrate 24 Recess 24a Inner surface of recess 25 First surface of resin substrate 25β Second surface of resin substrate 26 Opening of recess 27 Stress relaxation structure (narrow groove) 27a Opening of narrow groove 40 Metal layer 40x First metal layer (barrier film) 40y Second metal layer 41 Part of metal layer 100 Printed circuit board 400 Metal wiring 410 Part of metal wiring 251 Wiring-covered region 252 Surface-exposed region

Claims

1. a resin substrate and a metal layer provided on a first surface of the resin substrate; the resin substrate has a recessed portion that opens on the first surface, and a stress relaxation structure on an inner surface of the recessed portion; the metal layer includes a first metal layer disposed on the first surface and a second metal layer disposed on the first metal layer; A laminated structure, wherein a portion of the second metal layer extends through the first metal layer into the recess.

2. The laminated structure according to claim 1 , wherein the stress relaxation structure is a narrow groove provided on the inner surface of the recess.

3. 3. The laminated structure according to claim 2, wherein an average width of the narrow grooves on the inner surface of the recess is 0.1 nm or more and 100 nm or less.

4. a second metal layer on a second surface of the resin substrate opposite to the first surface, the resin substrate has a second recess that opens to the second surface, and a stress relaxation structure on an inner surface of the second recess; The laminated structure of claim 1 , wherein a portion of the second metal layer extends into the second recess.

5. 2. The laminated structure according to claim 1, wherein, in a cross-sectional view of the resin substrate in the thickness direction, a width of a portion of the metal layer at the position of the first surface of the resin substrate is smaller than a maximum width of a portion of the metal layer inside the resin substrate.

6. a resin substrate and metal wiring provided on a first surface of the resin substrate; the resin substrate has a recess that opens on the first surface, the metal wiring includes a first metal layer disposed on the first surface and a second metal layer disposed on the first metal layer; Of the first aspect, In the wiring covered region covered with the metal wiring, a portion of the second metal layer penetrates the first metal layer and extends into the recess; The printed circuit board has a stress relaxation structure on the inner surface of the recess in the surface exposed area exposed from the metal wiring.

7. The printed circuit board according to claim 6 , wherein the stress relaxation structure is a narrow groove provided on the inner surface of the recess.

8. 8. The printed circuit board according to claim 7, wherein an average width of the narrow grooves on the inner surface of the recess is 0.1 nm or more and 100 nm or less.

9. further comprising a second metal wiring on a second surface of the resin base material opposite to the first surface, the resin substrate has a second recess that opens to the second surface, Of the second aspect, In the second wiring covered region covered with the second metal wiring, a portion of the second metal wiring extends into the second recess; The printed circuit board according to claim 6 , wherein the second exposed surface region exposed from the second metal wiring has a stress relaxation structure on the inner surface of the second recess.

10. 7. The printed circuit board according to claim 6, wherein, in a cross-sectional view in a thickness direction of the resin base material, a width of a portion of the metal wiring at the position of the first surface of the resin base material is smaller than a maximum width of a portion of the metal wiring inside the resin base material.