Printed wiring board

Abstract

This printed wiring board comprises: a first conductor layer; an insulating layer disposed on the first conductor layer; a second conductor layer disposed on the insulating layer; and a connection conductor layer. The insulating layer has formed therein a through-hole which penetrates the insulating layer to expose the first conductor layer. The opening edge of the through-hole has, between the upper end of the through-hole and the lower end of the through-hole, a polygonal shape or a shape having one or more protrusions in a cross-sectional view perpendicular to a direction from the upper end toward the lower end. The radii of curvature of the corners of the polygonal shape or the tips of the protrusions are greater at the lower end than at the upper end. The connection conductor layer is disposed on the inner wall surface of the through-hole and on the first conductor layer exposed from the through-hole such that the first conductor layer and the second conductor layer are electrically connected.

Description

Printed wiring board 【0001】 The present disclosure relates to a printed wiring board. 【0002】 WO 2019 / 039237 (Patent Document 1) describes a printed wiring board. The printed wiring board described in Patent Document 1 includes a first conductor layer, an insulating layer disposed on the first conductor layer, a second conductor layer disposed on the insulating layer, and a connection conductor layer. 【0003】 Through holes are formed in the insulating layer. The connection conductor layer is formed on the inner wall surface of the through hole and on the first conductor layer so as to electrically connect the first conductor layer and the second conductor layer. The opening edge of the through hole has an arc and a plurality of protrusions in a cross-sectional view perpendicular to the direction from the upper end to the lower end of the through hole. The protrusions are arranged at intervals along the circumferential direction and protrude in the opposite direction to the center of the arc. 【0004】 WO 2019 / 039237 【0005】 The printed wiring board of the present disclosure includes a first conductor layer, an insulating layer disposed on the first conductor layer, a second conductor layer disposed on the insulating layer, and a connection conductor layer. Through holes are formed in the insulating layer to expose the first conductor layer through the insulating layer. The opening edge of the through hole has a shape having a polygon or at least one protrusion extending between the upper end and the lower end in a cross-sectional view perpendicular to the direction from the upper end of the through hole to the lower end of the through hole. The radius of curvature of the corner of the polygon or the tip of the protrusion is larger at the lower end than at the upper end. The connection conductor layer is disposed on the inner wall surface of the through hole and on the first conductor layer exposed from the through hole so as to electrically connect the first conductor layer and the second conductor layer. 【0006】Figure 1 is a cross-sectional view of the printed circuit board 100. Figure 2 is a cross-sectional view taken along line II-II in Figure 1. Figure 3 is a cross-sectional view taken along line III-III in Figure 1. Figure 4 is a manufacturing process diagram of the printed circuit board 100. Figure 5 is a cross-sectional view illustrating the substrate attachment process S2. Figure 6 is a cross-sectional view illustrating the drilling process S3. Figure 7 is a cross-sectional view illustrating the connection conductor layer formation process S4. Figure 8 is a cross-sectional view illustrating the resist pattern formation process S5. Figure 9 is a cross-sectional view illustrating the conductor layer formation process S6. Figure 10 is a cross-sectional view illustrating the resist pattern removal process S7. Figure 11 is a cross-sectional view of the printed circuit board 100 according to a first modified example. Figure 12 is a cross-sectional view of the printed circuit board 100 according to a second modified example. Figure 13 is a cross-sectional view of the printed circuit board 100 according to a third modified example. Figure 14A is a cross-sectional view of the printed circuit board 100 according to a fourth modified example. Figure 14B is a cross-sectional view of the printed circuit board 100 according to a fifth modified example. Figure 14C is a cross-sectional view of the printed circuit board 100 according to a sixth modified example. Figure 14D is a cross-sectional view of the printed circuit board 100 according to the seventh modified example. 【0007】 [Problems this disclosure aims to solve] In the printed circuit board described in Patent Document 1, air remaining in the through-holes can easily escape before the start of plating (when the plating solution is poured in). However, in the printed circuit board described in Patent Document 1, stress concentration is likely to occur at the lower end of the through-holes, and there is room for improvement in the reliability of the connecting conductor layer. This disclosure provides a printed circuit board with improved reliability of the connecting conductor layer. 【0008】 [Effects of this disclosure] According to the printed circuit board of this disclosure, the reliability of the connecting conductor layer is improved. 【0009】 [Description of Embodiments of the Disclosure] First, embodiments of the disclosure will be listed and described. 【0010】(1) The printed circuit board according to the embodiment comprises a first conductor layer, an insulating layer disposed on the first conductor layer, a second conductor layer disposed on the insulating layer, and a connecting conductor layer. The insulating layer has through holes formed therein that penetrate the insulating layer and expose the first conductor layer. The opening edge of the through hole has a polygonal shape or at least one projection extending between the upper and lower ends in a cross-sectional view perpendicular to the direction from the upper end to the lower end of the through hole. The radius of curvature of the corners of the polygon or the tip of the projection is larger at the lower end than at the upper end. The connecting conductor layer is disposed on the inner wall surface of the through hole and on the first conductor layer exposed from the through hole so as to electrically connect the first conductor layer and the second conductor layer. According to the printed circuit board of (1) above, the reliability of the connecting conductor layer is improved. 【0011】 (2) In the printed circuit board described in (1) above, the number of corners may be 3 or more and 8 or less. (3) In the printed circuit board described in (1) above, the number of protrusions may be 1 or more and 8 or less. 【0012】 (4) In the printed circuit board described in (1) above, the length of the protruding portion may be 10 μm or more and 150 μm or less. 【0013】 (5) In the printed circuit board described in (1) above, the width of the protruding portion may be 1 / 24 or more and 1 / 5 or less of the circumference of the opening edge of the through hole. 【0014】 (6) In the printed circuit boards described in (1) to (5) above, the radius of curvature of the corner or tip at the upper end may be 2.5 μm or more and 35 μm or less. 【0015】 (7) In the printed circuit boards described in (1) to (6) above, the radius of curvature of the corner or tip at the lower end may be 5 μm or more and 50 μm or less. 【0016】 (8) In the printed circuit boards described in (1) to (7) above, the aspect ratio of the through-holes may be 0.5 or more and 3.0 or less. 【0017】 (9) In the printed circuit boards described in (1) to (8) above, the diameter of the through-holes may be 20 μm or more and 300 μm or less. 【0018】(10) In the printed circuit boards described in (1) to (9) above, the depth of the through-hole may be 5 μm or more and 500 μm or less. 【0019】 (11) In the printed circuit boards described in (1) to (10) above, the insulating layer may include multiple layers. 【0020】 (12) In the printed circuit boards described in (1) to (11) above, the insulating layer may include at least one fluororesin layer. 【0021】 (13) In the printed circuit boards described in (1) to (12) above, the connecting conductor layer may be an electroless plating layer. 【0022】 [Details of Embodiments of the Disclosure] Details of embodiments of the disclosure will be described with reference to the drawings. In the following drawings, the same or corresponding parts will be denoted by the same reference numerals, and redundant descriptions will not be repeated. The printed circuit board according to the embodiment will be referred to as printed circuit board 100. 【0023】 (Configuration of the printed circuit board 100) The configuration of the printed circuit board 100 is described below. 【0024】 Figure 1 is a cross-sectional view of the printed circuit board 100. As shown in Figure 1, the printed circuit board 100 includes a substrate 10, an adhesive layer 20, a base material 21, an adhesive layer 22, a base material 23, a conductor layer 30, a conductor layer 31, and a connecting conductor layer 40. 【0025】 The substrate 10 has a base material 11 and a plurality of conductive layers 12. The base material 11 has a main surface 11a and a main surface 11b. The main surface 11b is the opposite surface of the main surface 11a. The base material 11 is made of an electrically insulating material. For example, the base material 11 is made of glass epoxy. That is, the substrate 10 is, for example, a rigid substrate. 【0026】In the example shown in Figure 1, there are four conductive layers 12. Of these, the conductive layer 12 located on the main surface 11a is designated as conductive layer 12a, the conductive layer 12 located on the main surface 11b is designated as conductive layer 12b, and the conductive layers 12 located within the substrate 11 are designated as conductive layer 12c and conductive layer 12d. Conductive layer 12c is located closer to conductive layer 12a than conductive layer 12d within the substrate 11. Each of the conductive layers 12 is made of a conductor. Each of the conductive layers 12 is made of, for example, copper or a copper alloy. Each of the conductive layers 12 is patterned to form wiring on the substrate 10. 【0027】 The adhesive layer 20 is positioned on the main surface 11a so as to cover the conductive layer 12a. The adhesive layer 20 is made of an adhesive. The substrate 21 is positioned on the adhesive layer 20. In other words, the substrate 21 is attached to the substrate 10 by the adhesive layer 20. The substrate 21 is made of, for example, a fluororesin. 【0028】 The adhesive layer 22 is positioned on the main surface 11b so as to cover the conductive layer 12b. The adhesive layer 22 is made of an adhesive. The substrate 23 is positioned on the adhesive layer 22. In other words, the substrate 23 is attached to the substrate 10 by the adhesive layer 22. The substrate 23 is made of, for example, a fluororesin. 【0029】 Through holes 50 are formed in the base material 11, adhesive layer 20, base material 21, adhesive layer 22, and base material 23. The through holes 50 penetrate through the base material 11, adhesive layer 20, base material 21, adhesive layer 22, and base material 23. The through holes 50 have an upper end 50a and a lower end 50b. The lower end 50b is the end opposite to the upper end 50a. The back surface of the conductor layer 31 is exposed from the lower end 50b. 【0030】 Conductor layer 30 is placed on base material 21. Conductor layer 31 is placed on base material 23. Each of conductor layer 30 and conductor layer 31 is made of a conductor. Each of conductor layer 30 and conductor layer 31 is made of, for example, copper or a copper alloy. In the through hole 50, a through hole 30a is formed in conductor layer 30 that penetrates conductor layer 30. 【0031】The connecting conductor layer 40 is arranged on the inner wall surface of the through hole 50 and on the back surface of the conductor layer 31 exposed from the through hole 50 (lower end 50b) so as to electrically connect the conductor layer 30 and the conductor layer 31. The connecting conductor layer 40 is made of a conductor. The connecting conductor layer 40 is made of, for example, copper or a copper alloy. The connecting conductor layer 40 is, for example, an electroless plating layer (a layer formed by electroless plating). 【0032】 The connecting conductor layer 40 is also located on the surface of the conductor layer 30 and on the surface of the conductor layer 31. Furthermore, the connecting conductor layer 40 is also located on the inner wall surface of the through hole 30a. A conductor layer 41 is located on the connecting conductor layer 40. The conductor layer 41 is made of a conductor. For example, the conductor layer 41 is made of copper or a copper alloy. The conductor layer 41 is, for example, an electroplated layer (a layer formed by electroplating). The conductor layer 30, the connecting conductor layer 40 located on the conductor layer 30, and the conductor layer 41 located on the connecting conductor layer 30 form wiring located on the base material 21. The conductor layer 31, the connecting conductor layer 40 located on the conductor layer 31, and the conductor layer 41 located on the connecting conductor layer 31 form wiring located on the base material 23. 【0033】 Figure 2 is a cross-sectional view taken along line II-II in Figure 1. Figure 3 is a cross-sectional view taken along line III-III in Figure 1. Figures 2 and 3 show cross-sections perpendicular to the direction from the upper end 50a to the lower end 50b. Also, in Figures 2 and 3, the connecting conductor layer 40 and the conductor layer 41 are not shown. As shown in Figures 2 and 3, the through hole 50 is polygonal in cross-sectional view, extending from the upper end 50a to the lower end 50b. In the examples shown in Figures 2 and 3, the opening edge of the through hole 50 is quadrilateral (square). Here, even if the corners of the polygon are rounded, it is still considered a polygon. 【0034】The polygon described above has multiple angles 50c. In the example shown in Figure 2, the polygon has four angles 50c. The radius of curvature of the angle 50c at the lower end 50b is greater than the radius of curvature of the angle 50c at the upper end 50a. The radius of curvature of the angle 50c at the upper end 50a is, for example, 2.5 μm or more and 35 μm or less. The radius of curvature of the angle 50c at the upper end 50a may be 2.5 μm or more and 15 μm or less. The radius of curvature of the angle 50c at the lower end 50b is, for example, 5 μm or more and 50 μm or less. The radius of curvature of the angle 50c at the lower end 50b may be 5 μm or more and 25 μm or less. 【0035】 The number of angles 50c in the above polygon may be 3 or 5 or more. The number of angles 50c in the above polygon may be 3 or more and 8 or 3 or more and 6 or less. 【0036】 Let the diameter of the through hole 50 be denoted as hole diameter D. Hole diameter D is the diameter of the outer circumference of the opening edge of the through hole 50. Hole diameter D is, for example, 20 μm or more and 300 μm or less. Hole diameter D may also be 100 μm or more and 200 μm or less. 【0037】 The depth of the through hole 50 is denoted as depth DT (see Figure 1). Depth DT is the distance between the upper end 50a and the lower end 50b. Depth DT is, for example, 10 μm or more and 500 μm or less. Depth DT may also be 100 μm or more and 300 μm or less. The aspect ratio of the through hole 50 is, for example, 0.5 or more and 3.0 or less. The aspect ratio of the through hole 50 may also be 1 or more and 2 or less. The aspect ratio of the through hole 50 is the value obtained by dividing depth DT by the hole diameter D. 【0038】 (Method for manufacturing the printed circuit board 100) The method for manufacturing the printed circuit board 100 will be described below. 【0039】 Figure 4 is a diagram illustrating the manufacturing process of the printed circuit board 100. As shown in Figure 4, the manufacturing method of the printed circuit board 100 includes a preparation step S1, a substrate attachment step S2, a drilling step S3, a connecting conductor layer formation step S4, a resist pattern formation step S5, a conductor layer formation step S6, a resist pattern removal step S7, and an etching step S8. 【0040】 In preparation step S1, the substrate 10 is prepared. Following preparation step S1, the substrate attachment step S2 is performed. 【0041】 Figure 5 is a cross-sectional view illustrating the substrate attachment process S2. As shown in Figure 5, in the substrate attachment process S2, the substrate 21 is attached to the main surface 11a by adhesive layer 20, and the substrate 23 is attached to the main surface 11b by adhesive layer 22. In the substrate attachment process S2, firstly, the substrates 21 and 23 are prepared. At this stage, an uncured adhesive layer 20 is placed on one surface of the substrate 21, and an uncured adhesive layer 22 is placed on one surface of the substrate 23. In addition, a conductive layer 30 is placed on the other surface of the substrate 21, and a conductive layer 31 is placed on the other surface of the substrate 23. 【0042】 Secondly, the substrate 21 is positioned so that the adhesive layer 20 covers the conductive layer 12a, and the substrate 23 is positioned so that the adhesive layer 22 covers the conductive layer 12b. Thirdly, the substrate 21 and the substrate 23 are heat-pressed toward the substrate 10. This hardens the adhesive layers 20 and 22, and the substrates 21 and 23 are attached to the substrate 10. After the substrate attachment process S2, a hole-punching process S3 is performed. 【0043】 Figure 6 is a cross-sectional view illustrating the drilling process S3. As shown in Figure 6, in the drilling process S3, through holes 50 are formed in the base material 11, adhesive layer 20, base material 21, adhesive layer 22, and base material 23, and through holes 30a are formed in the conductor layer 30, for example, by laser processing. By adjusting the focal position and laser power in the laser processing, the radius of curvature of the angle 50c at the lower end 50b can be made larger than the radius of curvature of the angle 50c at the upper end 50a. After the drilling process S3, the connecting conductor layer formation process S4 is performed. 【0044】FIG. 7 is a cross-sectional view for explaining the connection conductor layer forming step S4. As shown in FIG. 7, in the connection conductor layer forming step S4, for example, electroless plating is performed to form a connection conductor layer 40 on the inner wall surface of the through hole 50 and on the back surface of the conductor layer 31 exposed from the through hole 50. At this time, the connection conductor layer 40 is also formed on the surface of the conductor layer 30 and on the surface of the conductor layer 31, and is also formed on the inner wall surface of the through hole 30a. After the connection conductor layer forming step S4, a resist pattern forming step S5 is performed. 【0045】 FIG. 8 is a cross-sectional view for explaining the resist pattern forming step S5. As shown in FIG. 8, in the resist pattern forming step S5, a resist pattern 60 is formed on the connection conductor layer 40. The resist pattern 60 is formed, for example, by attaching a dry film resist and exposing and developing the dry film resist. The resist pattern 60 has an opening 61 penetrating therethrough. After the resist pattern forming step S5, a conductor layer forming step S6 is performed. 【0046】 FIG. 9 is a cross-sectional view for explaining the conductor layer forming step S6. As shown in FIG. 9, in the conductor layer forming step S6, for example, by an electrolytic plating method, a conductor layer 41 is formed on the connection conductor layer 40 exposed from the opening 61. After the conductor layer forming step S6, a resist pattern removing step S7 is performed. FIG. 10 is a cross-sectional view for explaining the resist pattern removing step S7. As shown in FIG. 10, in the resist pattern removing step S7, the resist pattern 60 is removed from above the connection conductor layer 40. After the resist pattern removing step S7, an etching step S8 is performed. 【0047】 In the etching step S8, etching is performed to remove the connection conductor layer 40 under the resist pattern 60, and the conductor layer 30 or the conductor layer 31 thereunder is also removed. Thus, the structure of the printed wiring board 100 shown in FIGS. 1 to 3 is formed. 【0048】(First and Second Modified Examples) FIG. 11 is a cross-sectional view of the printed wiring board 100 according to the first modified example. A cross-section corresponding to FIG. 1 is shown in FIG. 11. As shown in FIG. 11, the through-hole 50 may be formed in the base material 11, the adhesive layer 20, and the base material 21. That is, the lower end 50b may be located within the base material 11. In this case, for example, the conductor layer 12c is exposed from the through-hole 50. Although not shown, the conductor layer 12b or the conductor layer 12d may be exposed from the through-hole 50. 【0049】 FIG. 12 is a cross-sectional view of the printed wiring board 100 according to the second modified example. A cross-section corresponding to FIG. 1 is shown in FIG. 12. As shown in FIG. 12, the through-hole 50 may be formed in the adhesive layer 20 and the base material 21. In this case, the conductor layer 12a is exposed from the through-hole 50. That is, in the printed wiring board 100, an insulating layer (in the example of FIG. 1, the base material 11, the adhesive layer 20, the base material 21, the adhesive layer 22, and the base material 23 correspond to the insulating layer; in the example of FIG. 11, the base material 11, the adhesive layer 20, and the base material 21 correspond to the insulating layer; in the example of FIG. 12, the adhesive layer 20 and the base material 21 correspond to the insulating layer) is formed, another conductor layer is formed on the insulating layer, and the through-hole 50 may be formed in the insulating layer. 【0050】 (Third Modified Example) FIG. 13 is a cross-sectional view of the printed wiring board 100 according to the third modified example. A cross-section corresponding to FIG. 2 is shown in FIG. 13. The opening edge of the through-hole 50 does not have to be a convex polygon in a cross-sectional view. That is, as shown in FIG. 13, the opening edge of the through-hole 50 may be a concave polygon in a cross-sectional view. Also in this case, the hole diameter D is the diameter of the circumscribed circle of the concave polygon. 【0051】 (Fourth to Seventh Modified Examples) FIG. 14A is a cross-sectional view of the printed wiring board 100 according to the fourth modified example. FIG. 14B is a cross-sectional view of the printed wiring board 100 according to the fifth modified example. FIG. 14C is a cross-sectional view of the printed wiring board 100 according to the sixth modified example. FIG. 14D is a cross-sectional view of the printed wiring board 100 according to the seventh modified example. Cross-sections corresponding to FIG. 2 are shown in FIGS. 14A to 14D. 【0052】The opening edge of the through hole 50 does not have to be polygonal in cross-sectional view. That is, as shown in Figures 14A to 14D, the opening edge of the through hole 50 may have an arc 50d and a projection 50e protruding from the arc 50d in cross-sectional view. The number of projections 50e may be one or two or more. The number of projections 50e may be between one and eight, for example. The number of projections 50e may be between one and six. The projections 50e may protrude toward the center (center C) of the arc 50d, or they may protrude in the opposite direction from the center C. 【0053】 In these cases, the hole diameter D is considered to be the diameter of a circle centered at the center C and overlapping with the arc 50d. The width of the projection 50e is denoted as width W. Width W is the width of the projection 50e in the circumferential direction. Width W is measured at the base end of the projection 50e. The length of the projection 50e is denoted as length L. Length L is the distance between the tip of the projection 50e and the circle centered at the center C and overlapping with the arc 50d. Width W is, for example, 1 / 5 to 1 / 24 of the circumference of the through hole 50. The circumference of the through hole 50 is considered to be the circumference of a circle centered at the center C and overlapping with the arc 50d. Length L is, for example, 10 μm to 150 μm. 【0054】 The radius of curvature of the tip of the projection 50e at the lower end 50b is larger than the radius of curvature of the tip of the projection 50e at the upper end 50a. The radius of curvature of the tip of the projection 50e at the upper end 50a is, for example, 2.5 μm or more and 35 μm or less. The radius of curvature of the tip of the projection 50e at the upper end 50a may be 2.5 μm or more and 15 μm or less. The radius of curvature of the tip of the projection 50e at the lower end 50b is, for example, 5 μm or more and 50 μm or less. The radius of curvature of the tip of the projection 50e at the lower end 50b may be 5 μm or more and 25 μm or less. 【0055】 (Effects of the printed circuit board 100) The effects of the printed circuit board 100 are explained below. 【0056】Hydrogen gas is generated during the reaction when the connecting conductor layer 40 is formed by electroless plating. If the bubbles of the generated hydrogen gas remain on the inner wall surface of the through hole 50 or on the back surface of the conductor layer 31 exposed from the through hole 50, the connecting conductor layer 40 becomes thinner in the areas where the bubbles remain, and the reliability of the connecting conductor layer 40 decreases due to the thinned connecting conductor layer 40. In particular, if the fluororesin layer (substrate 21, substrate 23) forms part of the inner wall surface of the through hole 50, the surface roughness tends to increase in that part, making it even more difficult for bubbles to escape. 【0057】 In the printed circuit board 100, the opening edge of the through hole 50 has a polygonal shape with a corner 50c extending between the upper end 50a and the lower end 50b in a cross-sectional view, or a shape with a protrusion 50e. Air bubbles can easily escape along the corner 50c and protrusion 50e, and the connecting conductor layer 40 is less likely to become partially thinned, thus improving the reliability of the connecting conductor layer 40. 【0058】 In a cross-sectional view passing through the lower end 50b, the corners 50c and the tips of the protrusions 50e become points of stress concentration. However, in the printed circuit board 100, the radius of curvature at the tips of the corners 50c and protrusions 50e is larger at the lower end 50b than at the upper end 50a. Therefore, the above-mentioned stress concentration is mitigated in the printed circuit board 100. From this perspective as well, the reliability of the connecting conductor layer 40 is improved in the printed circuit board 100. 【0059】 As the radius of curvature of the corner 50c and protrusion 50e increases, stress concentration is reduced, but it becomes more difficult for air bubbles to escape. Therefore, by setting the radius of curvature of the tip of the corner 50c and protrusion 50e at the upper end 50a to 2.5 μm or more and 35 μm or less, and the radius of curvature of the tip of the corner 50c and protrusion 50e at the lower end 50b to 5 μm or more, it is possible to achieve both ease of air bubble escape and reduction of stress concentration. 【0060】When the opening edge of the through-hole 50 is polygonal in cross-sectional view, the number of corners 50c increases, which increases the number of paths through which air bubbles can escape. On the other hand, if the number of corners 50c is too large, the shape of the through-hole 50 in cross-sectional view approaches that of a circle, and air bubbles tend to remain on the inner wall surface of the through-hole 50. From this viewpoint, by setting the number of corners 50c to 3 or more and 8 or less, it is possible to ensure that air bubbles can escape easily. 【0061】 When the opening edge of the through hole 50 has an arc 50d and a projection 50e in a cross-sectional view, the number of paths through which air bubbles can escape increases as the number of projections 50e increases. On the other hand, if the number of projections 50e is too large, the projections 50e become sharp, raising concerns about stress concentration. From this viewpoint, by setting the number of projections 50e to between 1 and 8, it is possible to achieve both ease of air bubble escape and mitigation of stress concentration. 【0062】 The larger the aspect ratio of the through-hole 50, the more likely air bubbles are to remain on the inner wall surface of the through-hole 50. On the other hand, the larger the aspect ratio of the through-hole 50, the more likely stress concentration is to occur at the lower end 50b. From this viewpoint, by setting the aspect ratio of the through-hole 50 to 0.5 or more and 3.0 or less, it is possible to ensure that air bubbles can escape easily without excessive stress concentration. 【0063】 (Example) Samples 1 to 22 were prepared to evaluate the reliability of the connecting conductor layer 40. In samples 1 to 8 and samples 17 to 22, the shape of the opening edge of the through hole 50 in cross-sectional view was polygonal (quadrilateral or octagonal). In samples 9 to 16, the shape of the opening edge of the through hole 50 in cross-sectional view was a shape having an arc 50d and a protrusion 50e. "Outer protrusion" means a shape in which the protrusion 50e protrudes away from the center C (see Figures 14A and 14B), and "inner protrusion" means a shape in which the protrusion 50e protrudes toward the center C. Details of samples 1 to 22 are shown in Tables 1 and 2. 【0064】 【0065】 【0066】The reliability of the connecting conductor layer 40 was evaluated by a heat cycle test. The heat cycle test involved raising the temperature from room temperature to 130°C, then lowering it to -70°C, and then returning it to room temperature. This cycle was repeated 1000 times over one hour. After 1000 cycles, samples whose electrical resistance increased by 10 percent or more were considered defective samples, and the reliability of the connecting conductor layer 40 was evaluated by dividing the number of defective samples by the total number of samples (defect rate). The results of the heat cycle test for samples 1 to 22 are also shown in Table 1. 【0067】 Samples 2 and 3 had lower failure rates compared to Sample 1, and Sample 6 had lower failure rates compared to Samples 4 and 5. Sample 8 had lower failure rates compared to Sample 7. Sample 18 had lower failure rates compared to Sample 17, and Sample 20 had lower failure rates compared to Sample 19. Sample 22 had lower failure rates compared to Sample 21. From these comparisons, it became clear that the reliability of the connecting conductor layer 40 is improved by making the radius of curvature of the corner 50c at the lower end 50b larger than the radius of curvature of the corner 50c at the upper end 50a. 【0068】 Sample 10 had a lower defect rate compared to Sample 9, and Sample 12 had a lower defect rate compared to Sample 11. Furthermore, Sample 14 had a lower defect rate compared to Sample 13, and Sample 16 had a lower defect rate compared to Sample 15. From these comparisons, it became clear that the reliability of the connecting conductor layer 40 is improved by making the radius of curvature of the tip of the protrusion 50e at the lower end 50b larger than the radius of curvature of the tip of the protrusion 50e at the upper end 50a. 【0069】 The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the embodiments described above, and all modifications within the meaning and scope of equivalents of the claims are intended to be included. 【0070】10 Substrate, 11 Base material, 11a, 11b Main surface, 12, 12a, 12b, 12c, 12d Conductor layer, 20 Adhesive layer, 21 Base material, 22 Adhesive layer, 23 Base material, 30 Conductor layer, 30a Through hole, 31 Conductor layer, 40 Connecting conductor layer, 41 Conductor layer, 50 Through hole, 50a Upper end, 50b Lower end, 50c Corner, 50d Arc, 50e Protrusion, 60 Resist pattern, 61 Opening, 100 Printed wiring board, C Center, D Hole diameter, DT Depth, L Length, S1 Preparation process, S2 Base material attachment process, S3 Drilling process, S4 Connecting conductor layer formation process, S5 Resist pattern formation process, S6 Conductor layer formation process, S7 Resist pattern removal process, S8 Etching process, W Width.

Claims

1. A printed circuit board comprising: a first conductor layer; an insulating layer disposed on the first conductor layer; a second conductor layer disposed on the insulating layer; and a connecting conductor layer, wherein the insulating layer has a through hole formed therein that penetrates the insulating layer and exposes the first conductor layer; the opening edge of the through hole has a polygonal shape or at least one protrusion extending between the upper end and the lower end in a cross-sectional view perpendicular to the direction from the upper end to the lower end of the through hole; the radius of curvature of the corners of the polygon or the tip of the protrusion is larger at the lower end than at the upper end; and the connecting conductor layer is disposed on the inner wall surface of the through hole and on the first conductor layer exposed from the through hole so as to electrically connect the first conductor layer and the second conductor layer.

2. The printed circuit board according to claim 1, wherein the number of corners is 3 or more and 8 or less.

3. The printed circuit board according to claim 1, wherein the number of protrusions is 1 or more and 8 or less.

4. The printed circuit board according to claim 1, wherein the length of the protrusion is 10 μm or more and 150 μm or less.

5. The printed circuit board according to claim 1, wherein the width of the protrusion is 1 / 24 or more and 1 / 5 or less of the circumference of the opening edge of the through hole.

6. The printed circuit board according to any one of claims 1 to 5, wherein the radius of curvature of the corner or tip is 2.5 μm or more and 35 μm or less at the upper end.

7. The printed circuit board according to any one of claims 1 to 6, wherein the radius of curvature of the corner or tip is 5 μm or more and 50 μm or less at the lower end.

8. The printed circuit board according to any one of claims 1 to 7, wherein the aspect ratio of the through hole is 0.5 or more and 3.0 or less.

9. The printed circuit board according to any one of claims 1 to 8, wherein the diameter of the through hole is 20 μm or more and 300 μm or less.

10. The printed circuit board according to any one of claims 1 to 9, wherein the depth of the through hole is 5 μm or more and 500 μm or less.

11. The printed circuit board according to any one of claims 1 to 10, wherein the insulating layer comprises a plurality of layers.

12. The printed circuit board according to any one of claims 1 to 11, wherein the insulating layer comprises at least one fluororesin layer.

13. The printed circuit board according to any one of claims 1 to 12, wherein the connecting conductor layer is an electroless plating layer.

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