Laser drilling method

Abstract

To process a through hole having a hole diameter substantially the same as the diameter of a liquid column.SOLUTION: According to this laser drilling method, a blind hole 49, which reaches a depth 53 from a first face 1a of a work-piece 1 to a center of a thickness 1c of the work-piece 1 and has a bottom 51, is processed, a water column 26, into which laser beam 36 is guided toward the bottom 51 from a second face 1b which is a rear face of the first face 1a of the work-piece 1, is brought into contact with the second face 1b, hole drilling is performed to penetrate the blind hole 49 from the second face 1b, thereby forming a through hole 58.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for drilling holes using a laser directed into a water column. [Background technology]

[0002] A method has been proposed for forming a hole in a substrate by moving a liquid column and a laser beam on the substrate along the closed shape of the hole (JP-T No. 2023-536600, hereinafter referred to as "Patent Document 1"). Summary of the Invention [Problem to be solved by the invention]

[0003] In Patent Document 1, the diameter of the hole that is opened is much larger than the diameter of the liquid column. The present invention aims to process a through-hole with a hole diameter that is approximately the same as the diameter of the liquid column. [Means for solving the problem]

[0004] A first aspect of the present invention is A blind hole is machined from a first surface of the workpiece to a depth at the center of the thickness of the workpiece and having a bottom; A water column with a laser guided inside is directed from a water nozzle perpendicularly to the second surface so as to extend from a second surface, which is the back surface of the first surface of the workpiece, toward the bottom. With the water nozzle stationary relative to the workpiece, a hole is drilled from the second surface until it penetrates through to the blind hole, thereby forming a through hole. This is a laser drilling method.

[0005] The workpiece is made of metal or ceramic. Metals include stainless steel and tool steel. Ceramics include alumina, zirconia, silicon carbide, gallium nitride, and diamond.

[0006] The water column collects water in the blind hole drilled from the first surface, scattering the laser and inhibiting the progress of the machining. If the blind hole becomes too deep, machining may hardly progress at all. The water column collects water in the blind hole drilled from the second surface, scattering the laser and inhibiting the progress of the machining. When a hole from the second surface penetrates a blind hole drilled from the first surface, the water that has collected inside the hole escapes to the first surface side. This suppresses turbulence in the water column. As the turbulence in the water column decreases, laser scattering from the water column is suppressed.

[0007] When high machining accuracy is required for the hole to be machined, the surface of the workpiece that requires high machining accuracy is referred to as the second surface. The second surface is, for example, the surface of the product.

[0008] The lower limit of the nozzle hole diameter is 0.04 mm, 0.05 mm, and 0.09 mm. The upper limit of the nozzle hole diameter is 0.09 mm and 0.1 mm.

[0009] The lower limits of the diameter of the through-hole are 0.08 mm, 0.1 mm, and 0.12 mm. The upper limits of the diameter of the through-hole are 0.19 mm, 0.2 mm, 0.25 mm, and 0.3 mm. The lower limit of the range of the ratio of the workpiece thickness to the through-hole diameter is 5 or 6. The upper limit of the range of the ratio of the workpiece thickness to the through-hole diameter is 16. The diameter of the through hole may be the diameter of the opening of the through hole. When the blind hole is machined by grinding or cutting from the first surface, the thickness from the bottom of the blind hole to the second surface is 2.5 times or more, preferably 3 times or more, or 4 times or more, the diameter of the through hole.

[0010] The diameter of the through hole is 10 times or less the diameter of the nozzle hole. The diameter of the through hole is slightly larger than the diameter of the nozzle hole. The ratio varies depending on the material and processing conditions. The diameter of the through hole is less than 10 times the diameter of the nozzle hole. The lower limits of the ratio of the diameter of the through hole to the diameter of the nozzle hole are 0.95, 1, 1.05, and 1.1. The upper limits of the ratio of the diameter of the through hole to the diameter of the nozzle hole are 1.2, 1.5, and 2.

[0011] The laser is preferably a pulsed laser. The laser may be a blue to green laser. The laser may be an ultraviolet laser. [Effects of the Invention]

[0012] According to the present invention, it is possible to machine a through hole having a diameter approximately equal to the diameter of the liquid column. [Brief explanation of the drawings]

[0013] [Figure 1] Laser processing device of embodiment 1 [Figure 2] Flowchart of the laser drilling method of embodiment 1 [Figure 3] Schematic diagram of the laser head on the first surface side during positioning in the first embodiment. [Figure 4] Cross-sectional view of the first embodiment when drilling a hole from the first surface. [Figure 5] Cross-sectional view of the first embodiment when drilling a hole from the first surface. [Figure 6] Schematic diagram of the laser head on the second surface side during positioning in the first embodiment. [Figure 7] Cross-sectional view of embodiment 1 when drilling a hole from the second surface [Figure 8] Cross-sectional view of embodiment 1 when penetrated [Figure 9] Cross-sectional view of the final stage of drilling in the first embodiment [Figure 10] Photo of Example 1 after drilling [Figure 11] Flowchart of the laser drilling method of embodiment 2 [Figure 12] Cross-sectional view of the second embodiment when drilling a hole from the first surface. [Figure 13] Cross-sectional view of the second surface of the second embodiment when drilling holes. [Figure 14] Cross-sectional view of embodiment 2 when penetrated [Figure 15] Cross-sectional view of the final stage of drilling in embodiment 2 [Figure 16] Flowchart of the laser drilling method of embodiment 3 [Figure 17] Cross-sectional view of embodiment 3 when drilling a hole from the second surface. [Figure 18] Cross-sectional view of the penetration stage of embodiment 3 [Figure 19] Cross-sectional view of the final stage of drilling in embodiment 3 DETAILED DESCRIPTION OF THE INVENTION

[0014] <Embodiment 1> The laser processing apparatus 10 of this embodiment has a laser head 11, a water nozzle 13, a head cover 15, a laser oscillator 35, a water pump 37, a gas supply source 39, and an optical system (not shown). The water nozzle 13 has a nozzle 13a. The water nozzle 13 is fastened to the laser head 11. This forms a liquid storage chamber 12. The liquid storage chamber 12 is cylindrical and is located above the water nozzle 13. The liquid storage chamber 12 is connected to a water pump 37. The water pump 37 supplies water 38 to the liquid storage chamber 12.

[0015] The head cover 15 has a water column outlet 19 and a gas supply chamber 18. The head cover 15 is fastened to the laser head 11. This forms a gas supply path 17. The gas supply path 17 is annular. The gas supply path 17 is connected to a gas supply source 39 and the gas supply chamber 18. A gas supply 39 supplies a gas 40 to the laser head 11. The gas may be, for example, compressed air or helium. The laser oscillator 35 oscillates a laser 36. The laser 36 is, for example, a second harmonic Nd:YAG pulse laser.

[0016] As shown in Fig. 2, the drilling method of this embodiment includes the following steps: First, the laser head 11 is positioned at a specific position on the first surface (step S1). Next, a blind hole is drilled from the first surface (step S2). Next, the workpiece 1 is turned over (step S3). Next, the laser head 11 is positioned at a specific position on the second surface (step S4). Then, a hole is drilled from the second surface to penetrate the workpiece (step S5). 4, the workpiece 1 has a first surface 1a and a second surface 1b. The workpiece 1 also has a thickness 1c. The workpiece 1 is, for example, in the shape of a flat plate.

[0017] As shown in FIG. 3, in step S1, the laser head 11 is positioned at a predetermined drilling position (specific position) 45 on the first surface 1a of the workpiece 1. The drilling position 45 is identified, for example, by its distance from a reference point 1d of the workpiece 1. The reference point 1d may be, for example, the center, a corner, or the vertex of the orifice flat. For example, the position of the laser head 11 is identified by an X coordinate 46 and a Y coordinate 47. When viewed from the front of the machine, the X coordinate extends in the left-right direction. In FIG. 3, the direction in which the laser head 11 moves to the right is defined as the +X direction. When viewed from the front of the machine, the Y coordinate extends in the front-to-back direction. In FIG. 3, the direction in which the laser head 11 moves to the rear is defined as the +Y direction. The distance from the workpiece 1 to the laser head 11 is set within a range that allows the laser 36 to be sufficiently guided. The laser head 11 is positioned with an accuracy of at least half the nozzle diameter 23 of the water nozzle 13.

[0018] As shown in FIG. 1, in step S2, water supplied to the liquid storage chamber 12 is ejected from the nozzle 13a as a water column 26. The water column diameter 24 of the water column 26 is substantially the same as the nozzle diameter 23. For example, the water column diameter 24 is in the range of 90% to 105% of the nozzle diameter 23. Gas 40 supplied to the laser head 11 from a gas supply source 39 is ejected from the water column outlet 19. A laser 36 is focused on the upper surface of the nozzle 13a by an optical system (not shown). The laser 36 then propagates inside the water column 26. The water column 26 through which the laser 36 propagates is referred to as a water laser 27.

[0019] As shown in FIG. 4, the water laser 27 from the laser head 11 collides with the workpiece 1, and a blind hole 49 is drilled in the first surface 1a of the workpiece 1. The laser head 11 remains stationary while drilling the blind hole 49. The workpiece 1 also remains stationary. In other words, the laser head 11 does not move relative to the workpiece 1. The blind hole 49 has a bottom 51. As time passes from the start of drilling, the hole depth 53 increases.

[0020] As shown in FIG. 5, when hole depth 53 relative to hole diameter 54 becomes large, water accumulates inside blind hole 49 due to water column 26. This causes diffuse reflection of laser 36 inside blind hole 49, reducing the machining speed of blind hole 49. Furthermore, as indicated by reference numeral 28, water bounces off the outer periphery of blind hole 49 toward laser head 11. The bouncing water 28 disrupts water column 26. This causes laser 36 to be emitted from water column 26, reducing the energy of laser 36 reaching workpiece 1. This reduces the machining speed of blind hole 49. When the depth 53 of the blind hole 49 reaches about 50% of the thickness 1c of the workpiece 1, the water laser 27 is stopped. Specifically, the laser oscillator 35 stops oscillating the laser 36. Next, the water pump 37 may stop supplying water.

[0021] In step S3, the workpiece 1 is turned over so that the second surface 1b faces the laser head 11. 6, in step S4, the laser head 11 is accurately positioned at the drilling position 45. The rest of step S4 is substantially the same as step S1.

[0022] In step S5, the laser head 11 forms a water laser 27. The water laser 27 forms a hole 55 from the second surface 1b. Step S5 is substantially the same as step S2. In step S5, the laser head 11 remains stationary while machining the hole 55. The workpiece 1 also remains stationary. That is, the laser head 11 does not move relative to the workpiece 1. The hole 55 is drilled toward the bottom 51 of the blind hole 49 machined in step S2.

[0023] As shown in FIG. 7, the blind hole 55 drilled from the second surface 1b is a tapered hole whose diameter decreases as the depth 56 increases. The blind hole 55 is substantially coaxial with the blind hole 49. As shown in FIG. 8, as the drilling continues, the hole from the second surface 1b penetrates into the hole in the first surface 1a. At this time, a narrow through-hole 57 is created in the center of the thickness 1c of the workpiece 1. At this time, the water that had accumulated in the hole escapes toward the first surface 1a. At the same time, the water column 26 escapes toward the first surface 1a. At this time, the water column 26 becomes undisturbed. The intensity of the laser 36 inside the water column 26 increases. The water laser 27 processes the through-hole 57 so that it rapidly expands almost simultaneously with the hole penetration. As a result, a cylindrical through-hole 58 is formed, as shown in FIG. 9. The diameter of the through-hole 58 is substantially constant throughout its entire depth. The laser head 11 then stops the water laser 27 .

[0024] Conventional drilling methods using drills and grindstones have made it difficult to drill small holes in metals and ceramics. In particular, it has been difficult to drill holes with a diameter of 0.1 mm or less in metals. It has also been difficult to drill holes with a diameter of 0.2 mm or less in ceramics. In particular, it was extremely difficult to drill small holes with a ratio (L / d) of thickness (L) to diameter (d) of more than 5.

[0025] The laser drilling method of this embodiment can drill through holes 58 of 0.1 mm or less in metal. The laser drilling method of this embodiment can drill through holes 58 of 0.2 mm or less in ceramic. Furthermore, the laser drilling method of this embodiment can drill through holes 58 with an L / d ratio of more than 5 times in metal and ceramic. When the liquid column traces a circular or spiral trajectory, as in Patent Document 1, the diameter of the hole that can be drilled in the workpiece is much larger than the diameter of the liquid column. The laser drilling method of this embodiment makes it possible to drill a hole with a diameter that is substantially the same as the diameter of the liquid column. The diameter of the liquid column can be controlled by the nozzle diameter and spraying conditions. The diameter of the liquid column is one to several times the nozzle diameter. The nozzle diameter is, for example, 0.04 mm to 0.08 mm. This allows for drilling small holes with diameters of 0.06 mm to 0.09 mm in ceramics and metals.

[0026] When drilling small holes in difficult-to-machine materials using a water column guided by a laser, the cross-section of the hole sometimes shrinks as the hole gets deeper. Also, depending on the drilling conditions and material, the cross-section of the hole sometimes expands as the hole gets deeper. Also, the cross-sectional shape of the hole sometimes becomes distorted as the hole gets deeper. According to the laser drilling method of this embodiment, the change in hole diameter with respect to the depth is suppressed, and the deformation of the cross-sectional shape of the hole with respect to the depth is also suppressed. [Example]

[0027] A workpiece 1 made of SUS316 stainless steel and having a thickness of 1 mm was drilled using the laser drilling method of this embodiment under the following processing conditions. Laser: Second harmonic Nd:YAG laser, 7W Injection pressure: 40MPa Water nozzle diameter: 0.07 mm As a result, a through hole 58 with a diameter of 0.1 mm was machined. A photograph of the machined through hole 58 is shown in Figure 10. The upper side of Figure 10 is a photograph of through hole 58b taken from the second surface 1b. The lower side of Figure 10 is a photograph of through hole 58a taken from the first surface 1a. The hole diameter was almost the same on the second surface 1b side and the first surface 1a side. [Example]

[0028] A workpiece 1 made of 90% alumina and having a thickness of 1.1 mm was drilled using the laser drilling method of this embodiment under the following processing conditions. Laser: Second harmonic Nd:YAG laser, 60W Injection pressure: 15MPa Water nozzle diameter: 0.07 mm As a result, a through hole with a diameter of 0.075 mm was successfully machined. As in Example 1, a photograph of the machined through hole was examined and it was found that the hole diameter was almost the same on the second surface 1b side and the first surface 1a side.

[0029] Second Embodiment As shown in Fig. 11, the drilling method of this embodiment includes the following steps. First, a blind hole is drilled from the first surface 1a by polishing (step S12). Next, the workpiece 1 is turned over (step S3). Next, the laser head 11 is positioned at a specific position on the second surface 1b (step S4). Then, a hole is drilled from the second surface 1b to penetrate the workpiece (step S15). In the following description, the workpiece 1 is made of ceramic (for example, alumina).

[0030] As shown in FIG. 12, in step S12, the workpiece 1 is placed with the first surface 1a facing up. First, the polishing spindle 65, to which the polishing tool 66 is attached, is positioned in the XY direction at the position where the hole is to be drilled. Next, the polishing spindle 65 is rotated. Then, the polishing spindle 65 is fed in the -Z direction to drill a blind hole 67. The polishing tool 66 is in the shape of a right cylinder.

[0031] The blind hole 67 has a bottom 68. The diameter 69 of the blind hole 67 is sufficiently larger than the nozzle diameter 23 of the water nozzle 13. For example, the diameter 69 is 10 times or more the nozzle diameter 23. For example, the diameter 69 is 1 mm or more. The bottom 68 is, for example, circular. The depth 72 of the blind hole 67 is, for example, larger than half the thickness 1c of the workpiece 1. The bottom 68 reaches the center of the thickness 1c. A distance 71 between the bottom 68 and the second surface 1b is, for example, smaller than half the thickness 1c of the workpiece 1. The distance 71 is set to be smaller than the depth of a hole that can be penetrated in one go by the water laser 27. The depth of a hole that can be penetrated in one go by the water laser 27 is determined in advance by conducting a drilling test. For example, the distance 71 is set so that it is smaller than the reachable hole depth 53 when processing step 2 of embodiment 1 is performed. The positional accuracy of the base 68 in the X and Y directions does not require a particularly high level of accuracy.

[0032] Steps S3 and S4 are substantially the same as those in the first embodiment. As shown in FIG. 13, in step S15, drilling is performed using the water laser 27. While step S15 is being performed, the laser head 11 is stationary relative to the workpiece 1. As the drilling progresses, a hole 77 is formed. The drilling is performed toward the bottom 68. Step S15 in this embodiment is substantially the same as step S5 in the first embodiment. Finally, the hole 77 penetrates into the blind hole 67. At this time, a narrow through-portion 78 is formed, as shown in FIG. 14.

[0033] Water accumulated in the hole 77 being drilled escapes through the through hole 78 to the back side of the laser head 11. The water laser 27 also escapes to the back side of the workpiece 1. This reduces the turbulence of the water column 26. The intensity of the laser 36 guided into the water column 26 increases. The through column 78 then receives the energy of the laser 36 and rapidly expands. As a result, a cylindrical through hole 79 with a nearly constant cross section in the depth direction is formed, as shown in FIG. 15. When viewed from the Z direction, the entire cross section of the through hole 79 should be contained within the plane of the bottom 68.

[0034] If the workpiece 1 is made of metal, a cutting tool can be used instead of the polishing tool 66. The blind hole 67 can be formed by cutting.

[0035] According to the laser drilling method of this embodiment, it is possible to drill a through hole 79 having a small diameter portion on the second surface 1b side. Furthermore, the required positional accuracy of the blind hole 67 is lower than that of the laser drilling method of embodiment 1. The laser processing device 10 using the water laser 27 is a specialized machine and is highly rare. On the first surface 1a side, the blind hole 67 can be pre-drilled by polishing, so the time required for small diameter laser drilling on the second surface 1b side can be shortened.

[0036] The aspect ratio of the small diameter portion machined with the water laser 27 (the ratio of the distance 71 to the diameter 81 of the entrance of the through hole 79) is 2.5 or more. Depending on the material, holes with an aspect ratio of 4 or more can be machined. Even with the drilling method of the first embodiment, there is a limit to the thickness 1c of the workpiece 1 that can be drilled. According to this embodiment, even if the workpiece 1 has a large thickness 1c, by increasing the depth of the blind hole 67, it is possible to drill a hole with a smaller diameter on the second surface 1b side.

[0037] <Embodiment 3> As shown in FIG. 16, the drilling method of this embodiment includes the following steps: First, the laser head 11 is positioned at a specific position on the first surface 1a (step S1). Next, a blind hole is drilled from the first surface 1a (step S2). Next, the workpiece 1 is turned over (step S3). Next, a hole is drilled at a specific position on the second surface 1b by polishing (step S24). Next, the first surface 1a is polished to a flat surface (step S25).

[0038] FIG. 17 shows the workpiece 1 turned upside down in step S2. As shown in FIG. 17, in step S1, the depth of the bottom 51a of the blind hole 49a from the first surface 1a may vary greatly depending on the position. The depth 53a of the shallowest part of the blind hole 49a is approximately 20% to 50% of the thickness 1c. In step S1, drilling is performed for a time period that does not allow the blind hole 49a to penetrate all the way to the second surface 1b. In step S1, the opening of the blind hole 49a may become rounded or slightly widen.

[0039] In step S24, the polishing tool 66 is attached, and the rotating polishing spindle 65 is positioned at the drilling position 45. The polishing spindle 65 is lowered perpendicular to the second surface 1b to drill a hole. The drilled depth 92 is greater than the difference between the thickness 1c and the depth 53a. Then, as shown in FIG. 18, a hole 91 from the second surface 1b penetrates the blind hole 49a.

[0040] As shown in FIG. 19, in step S25, the first surface 1a is flat-polished by a removal thickness 93. The removal thickness 93 is, for example, 0.5 mm to 1 mm. This removes irregular portions of the openings of the blind holes 49a. As a result, the openings of the blind holes 49a become closer to a perfect circle. Furthermore, the variation in the diameter of the openings of the blind holes 49a becomes smaller. The diameter of the blind holes 49a is 0.2 mm or less.

[0041] After repeatedly performing laser drilling, the inventors discovered the following problems: When drilling with the water laser 27 stopped, it is difficult to control the depth of the hole. The drilling speed may vary depending on the position. The hole may be tilted, the hole diameter may expand inside the hole, or the opening of the hole may be deformed. When blind hole 49a is drilled with water laser 27, the hole depth may change significantly inside blind hole 49a. However, the interior of blind hole 49a rarely expands, and blind hole 49a rarely tilts. Furthermore, the opening of the blind hole 49a may be chipped, rounded, or out of round.

[0042] According to this embodiment, it is possible to machine a through hole 94 having a small diameter portion on the first surface 1a side. In addition, the axis is less likely to tilt, and internal variations in the hole diameter can be suppressed. Furthermore, since the first surface 1a is flat-polished, variations in the diameter of the opening are suppressed. Furthermore, the hole shape of the opening tends to approach a perfect circle. The aspect ratio of the small diameter portion machined with the water laser 27 is approximately 2.5 to 4. According to this embodiment, even if the workpiece 1 has a large thickness 1c, it is possible to machine a hole with a small diameter on the first surface 1a side.

[0043] In the first and second embodiments, the first surface 1a may be polished to a plane by a removal thickness 93 as shown in FIG.

[0044] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the gist of the present invention, and all technical matters included in the technical ideas described in the claims are subject to the present invention. The above-described embodiments are preferred examples, but a person skilled in the art can realize various alternatives, modifications, variations, or improvements from the contents disclosed in this specification, and these are included in the technical scope described in the appended claims. [Explanation of symbols]

[0045] 1 Work 1a 1st page 1b 2nd side 1c thickness 11 Laser Head 26 water column 36 Laser 49,67 blind hole 51,68 bottom 53,72 depth

Claims

1. A blind hole is machined from a first surface of the workpiece to a depth at the center of the thickness of the workpiece and having a bottom; A water column with a laser guided into it is directed from a water nozzle perpendicularly to the second surface so as to extend from a second surface, which is the back surface of the first surface of the workpiece, toward the bottom. With the water nozzle stationary relative to the workpiece, a hole is drilled from the second surface until it penetrates through to the blind hole, thereby forming a through hole. Laser drilling method.

2. forming a hole whose diameter decreases from the second surface toward the first surface; When penetrating from the second surface to the first surface, the water column (26) penetrates to the first surface side, forming a cylindrical through hole from the second surface to the first surface. The laser drilling method of claim 1.

3. When the water column penetrates to the first surface side, a narrow penetration portion is formed, and as the water column passes through to the first surface side, turbulence of the water column is reduced, and the intensity of the laser is increased, the penetration portion is enlarged. The laser drilling method according to claim 1 or 2.

4. When the water column generated from the water nozzle is directed at the second surface and hole drilling progresses, the water column collides with the hole being drilled and rebounds toward the water nozzle, causing the water column to become turbulent and attenuating the laser. The laser drilling method according to claim 1 or 2.

5. The water column is directed from the water nozzle perpendicularly to the first surface at a drilling position on the first surface, and the blind hole is drilled from the first surface while the water nozzle is kept stationary with respect to the workpiece. The laser drilling method according to claim 1 or 2.

6. the water column generated from the water nozzle is directed at the first surface, and as the blind hole processing progresses, the water column collides with the blind hole and rebounds toward the water nozzle, causing the water column to become turbulent and attenuating the laser. The laser drilling method of claim 5.

7. The blind hole is machined from the first surface by grinding or cutting. The laser drilling method according to claim 1 or 2.

8. The blind hole has a larger diameter than the through hole. The laser drilling method of claim 7.

9. the workpiece is metal, The hole has a diameter of 0.10 mm or less. The laser drilling method according to claim 1 or 2.

10. The workpiece is made of ceramics, The holes have a diameter of 0.2 mm or less. The laser drilling method according to claim 1 or 2.

11. The through hole has a diameter that is 1 to 3 times the diameter of the water nozzle. The laser drilling method according to claim 1 or 2.

12. The ratio of the thickness of the workpiece to the diameter of the through hole is 5 or more and 16 or less. The laser drilling method according to claim 1 or 2.

13. A water column with a laser guided inside is directed at a first surface of the workpiece, and while the water nozzle is stationary relative to the workpiece, a blind hole is machined that reaches from the first surface to a depth at the center of the thickness of the workpiece and has a bottom; A through hole is formed by drilling a hole from a second surface, which is the reverse side of the first surface of the workpiece. Laser drilling method.

14. A hole is drilled from the second surface by grinding or cutting.

14. The laser drilling method of claim 13.

15. The workpiece is made of ceramics, The blind hole has a diameter of 0.2 mm or less.

15. The laser drilling method of claim 13 or 14.

16. The through hole has a diameter that is 1 to 3 times the diameter of the water nozzle.

15. The laser drilling method of claim 13 or 14.

17. The ratio of the thickness of the workpiece to the diameter of the blind hole is 5 or more and 16 or less.

15. The laser drilling method of claim 13 or 14.

18. polishing the surface of the first surface in which the through holes are formed; 15. The laser drilling method according to claim 1, 2, 13 or 14.

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