Micro-hole back drilling processing method, micro-hole back drilling processing system and printed circuit board

Abstract

The present application discloses a micro-hole back drilling processing method, a micro-hole back drilling processing system and a printed circuit board. The processing method is used for a printed circuit board, and includes: providing a printed circuit board, on which at least one via is provided; flash plating the printed circuit board to form a first copper layer on the inner wall of the via to obtain a metallized via; performing automatic optical detection on the metallized via and obtaining back drilling information of the metallized via; back drilling the metallized via according to the back drilling information to form a back drilled hole. In this way, high-precision back drilling of printed circuit boards of high-frequency communication equipment can be achieved, thereby reducing the loss during the high-speed and high-frequency signal transmission of the hole link and improving the integrity of the transmission signal.

Description

Technical Field

[0001] The present application relates to the technical field of circuit boards, and in particular to a micro-hole back drilling processing method, a micro-hole back drilling processing system and a printed circuit board. Background Art

[0002] In the field of special circuit boards such as high-speed backplanes, back drilling is increasingly used. Back drilling refers to the use of a larger drill bit to control the depth of the metallized via hole on the circuit board to remove part of the metallized inner wall of the metallized via hole to avoid reflection, scattering, delay, etc. of high-speed signal transmission. In the prior art, the hole diameter accuracy of the back drilling hole can only be controlled to about 0.2 mm.

[0003] With the continuous development of communication technology and the emergence of 5G communication, high-frequency communication backplanes require smaller and smaller hole spacing to meet the unit requirements. However, due to the poor accuracy of back drilling in existing technology, it is impossible to perform fine processing on the above-mentioned circuit boards, resulting in high-frequency communication backplanes being prone to reflection, scattering, and delays in high-speed signal transmission. Summary of the invention

[0004] The present application provides a micro-via back drilling processing method, a micro-via back drilling processing system and a printed circuit board, which realize high-precision back drilling of printed circuit boards of high-frequency communication equipment, thereby reducing the loss during high-speed and high-frequency signal transmission of the hole link and improving the integrity of the transmission signal.

[0005] On the one hand, an embodiment of the present application provides a micro-hole back drilling processing method, which is used for a printed circuit board, and the processing method includes: providing a printed circuit board, on which at least one via is arranged; flash plating the printed circuit board to form a first copper layer on the inner wall of the via hole to obtain a metallized via hole; performing automatic optical detection on the metallized via hole and obtaining back drilling information of the metallized via hole; back drilling the metallized via hole according to the back drilling information to form a back drilled hole.

[0006] On the other hand, the embodiment of the present application also provides a micro-hole back drilling processing system for executing the micro-hole back drilling processing method as described above, and the processing system includes: an automatic optical detection component, a coating machine, a processor and a CNC laser drilling machine, one end of the processor is connected to the automatic optical detection component, and the other end is connected to the CNC laser drilling machine; the CNC laser drilling machine is used to mechanically drill a printed circuit board to form at least one via; the coating machine is used to flash plate the printed circuit board to form a first copper layer on the inner wall of the via to obtain a metallized via; the automatic optical detection component is used to perform automatic optical detection on the metallized via; the processor is used to analyze the detection results of the automatic optical detection to obtain back drilling information of the metallized via; the CNC laser drilling machine is also used to back drill the metallized via according to the back drilling information to form a back drilled hole.

[0007] On the other hand, the embodiment of the present application further provides a printed circuit board, which is manufactured by the micro-hole back drilling processing method as described above, and the printed circuit board includes: a first outer metal layer, n inner circuit layers and a second outer metal layer stacked together; at least one via hole is opened on the printed circuit board, and the via hole includes a back-drilled hole reaching a cutoff surface from the first outer metal layer, and a metallized via hole reaching the second outer metal layer from the cutoff surface, wherein the cutoff surface is between the mth layer and the m+1th inner circuit layer; m is less than n and m and n are both positive integers; the difference between the hole diameter of the back-drilled hole and the hole diameter of the via hole is 0.02-0.1 mm.

[0008] Through the above method, the present application adopts flash copper plating before back drilling to reduce the copper thickness at the metallized via hole. The copper thickness is greatly reduced, and the back drilling drill is not easy to wear, and it is not easy to have drilling deviation or drill breakage. Furthermore, the metallized via hole is measured by an automatic optical detection component to determine the back drilling position of the back drilling hole, and then back drilling is performed according to the back drilling position, so as to achieve high-precision back drilling of the printed circuit board of high-frequency communication equipment, thereby reducing the loss in the process of high-speed and high-frequency signal transmission of the hole link and improving the integrity of the transmission signal. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for describing the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0010] Figure 1 It is a flow chart of an embodiment of a micro-hole back drilling processing method of the present application;

[0011] Figure 2 It is a flow chart of another embodiment of the micro-hole back drilling processing method of the present application;

[0012] Figure 3 It is a flow chart of another embodiment of the micro-hole back drilling processing method of the present application;

[0013] Figure 4 yes Figure 1 Schematic diagram of the process of step S14;

[0014] Figure 5 It is a structural schematic diagram of an embodiment of a micro-hole back drilling processing system of the present application;

[0015] Figure 6 It is a structural schematic diagram of an embodiment of a printed circuit board of the present application;

[0016] Figure 7 It is a structural schematic diagram of another embodiment of the printed circuit board of the present application. DETAILED DESCRIPTION

[0017] The present application is further described in detail below in conjunction with the accompanying drawings and examples. It is particularly noted that the following examples are only used to illustrate the present application, but are not intended to limit the scope of the present application. Similarly, the following examples are only some embodiments of the present application rather than all embodiments, and all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present application.

[0018] See also Figure 1 , Figure 1 The figure is a flow chart of an embodiment of a micro-hole back drilling processing method of the present application. The processing method is used for a printed circuit board, and the processing method comprises:

[0019] Step S11: providing a printed circuit board.

[0020] Wherein, at least one conducting hole is arranged on the printed circuit board.

[0021] Specifically, in step S11, according to actual needs, via holes can be drilled on the printed circuit board. The position and diameter of the via holes are determined according to actual requirements. The via holes can be used to achieve mutual conduction between the layers of the printed circuit board, facilitate the soldering of electronic components on the printed circuit board, and provide positioning points or alignment points for the subsequent processing steps. Optionally, the drilling processing method may include at least one of laser drilling, punching holes, and milling holes by a gong machine. In other embodiments, the via holes can also be opened in the insulating area of ​​each layer of the printed circuit board.

[0022] Step S12: flash plating the printed circuit board to form a first copper layer on the inner wall of the via hole to obtain a metallized via hole.

[0023] Specifically, the metallization of the via hole is realized, and each inner circuit layer of the printed circuit board is connected through the copper layer. The current density of the flash plating can be 1.0-1.4ASD, and the flash plating time can be 10-30 minutes. In step S12, the purpose of the flash plating is to form a first copper layer with a thickness of 0.2-8 microns (for example, 0.2 microns, 0.5 microns, 1.0 microns, 5.0 microns or 8.0 microns) in the through hole in a relatively short time, and at the same time, it can avoid the problem of too small hole diameter of the metallized via hole on the printed circuit board caused by too thick copper layer.

[0024] In other embodiments, the via holes may also be opened in the insulating areas of each layer of the printed circuit board. The metallized inner wall of the processed metallized via hole is not connected to the internal circuits of the internal circuit layer of the printed circuit board. During back drilling, the drill bit only cuts the non-metallic layer and does not cut these circuit layers, thereby preventing a large amount of copper chips from being generated to cause back drilling hole blockage.

[0025] Step S13: performing automatic optical inspection on the metallized via hole and obtaining backdrilling information of the metallized via hole.

[0026] Specifically, the functions to be realized by high-frequency communication equipment are becoming more and more complex, and the degree of integration is getting higher and higher. Correspondingly, the printed circuit boards of high-frequency communication equipment need more layers and greater thickness to meet their functional design requirements. Multiple cameras, light source controllers, light sources, programmable logic controllers, computers and other equipment form an automatic optical detection component, and multiple cameras are used to simultaneously shoot multiple surfaces of the printed circuit board, and the original images obtained by shooting are collected and then image processing and hole position detection are performed, and then the hole position detection results and processed image data are output, and the actual back drilling information of the back drilling hole is calculated according to the hole position deviation to obtain the back drilling information of the metallized via hole. Among them, the back drilling information includes at least one of the thickness of the printed circuit board, the spacing between adjacent inner circuit layers in the printed circuit board, the back drilling position (for example, the hole center spacing between adjacent metallized via holes, the hole wall spacing between adjacent metallized via holes), the back drilling accuracy, the hole diameter of the metal via hole, the back drilling depth, the back drilling hole diameter (for example, the hole diameter of the metal via hole, the hole diameter of the back drilling hole), the drill nozzle speed or the back drilling speed.

[0027] Step S14: back-drilling the metallized via hole according to the back-drilling information to form a back-drilled hole.

[0028] Specifically, after the backdrilling information is obtained, the metallized via hole to be backdrilled can be backdrilled. The metallized via hole can be backdrilled from the first outer metal layer of the printed circuit board. The backdrilled hole diameter is larger than the hole diameter of the metallized via hole so as to grind off the first copper layer at the hole wall of the metallized via hole. Since the backdrilling information is obtained by the automatic optical detection component, the error is very small, which can improve the hole position accuracy of the backdrilling.

[0029] Different from the prior art, the present application uses flash copper plating before back drilling to reduce the copper thickness at the metallized via hole. The copper thickness is greatly reduced, and the back drilling tool is not easy to wear, and it is not easy to drill off or break. Furthermore, the metallized via hole is measured using an automatic optical detection component to determine the back drilling position of the back drilling hole, and then back drilling is performed according to the back drilling position, which can improve the back drilling hole position accuracy, so as to realize back drilling on a high-frequency communication backplane, thereby avoiding reflection, scattering, delay, etc. of high-speed signal transmission on the high-frequency communication backplane.

[0030] Before step S13, the method further includes:

[0031] Step S15: Tinning the printed circuit board to form a tinned protective layer on the inner wall of the metallized via hole, and forming a tinned protective film on the circuit pattern area of ​​the first outer metal layer and the second outer metal layer of the printed circuit board.

[0032] Specifically, after the metallized via hole is processed, the printed circuit board can be immersed in a tin plating solution for electroplating, so that a tin-plated protective layer is formed on the inner wall of the metallized via hole, and a tin-plated protective film is formed in the circuit pattern area of ​​the first outer metal layer and the second outer metal layer. The tin-plated protective layer can protect the first copper layer on the inner wall of the metallized via hole, prevent the first copper layer from being damaged in subsequent etching or copper scrap removal operations, and can also be used to reduce the generation of copper scraps. The tin-plated protective film can be used as an anti-corrosion film.

[0033] See also Figure 2 , Figure 2 1 is a flow chart of another embodiment of the micro-via back drilling processing method of the present application. After back drilling the metallized via hole according to the back drilling information, the processing method further includes:

[0034] Step S21: removing the tin-plated protective layer.

[0035] Specifically, the tin-plated protective layer on the inner wall of the metallized via hole is removed by a conventional process method, for example, an acid solution is used to remove the tin-plated protective layer or laser energy is used to burn off the tin-plated protective layer.

[0036] Step S22: electroplating is performed on the printed circuit board to form a second copper layer at the back-drilled holes.

[0037] Specifically, copper is electroplated according to the final product requirements. By controlling the copper thickness in the hole before back drilling, copper is plated twice before and after back drilling, which effectively reduces the back drilling copper wire and the risk of back drilling hole blockage.

[0038] Step S23: plugging the back-drilled holes with resin.

[0039] Specifically, what is formed after back drilling is a stepped hole. For the same through hole, the diameter of one end of the back drilled hole is large, and the diameter of the other end of the metallized via hole is small. Extend the resin injection head from one end of the metallized via hole into one end of the back drilled hole, align the lower edge of the resin injection head with the end face between the metallized via hole and the back drilled hole, first fill the back drilled hole with resin, and after the back drilled hole is filled with resin, lift the resin injection head and continue to fill the metallized via hole with resin. Put the printed circuit board after resin injection into the oven, keep baking at 70-75℃ for 30 minutes, then keep baking at 80-85℃ for 30 minutes, and then keep baking at 120-140℃ for 60 minutes to complete the resin plugging of the printed circuit board.

[0040] In one embodiment, the difference between the diameter of the back-drilled hole and the diameter of the via hole is 0.02-0.1 mm (e.g., 0.02 mm, 0.04 mm, 0.06 mm, 0.08 mm, 0.10 mm). The thickness of the first copper layer is in the range of 2-8 microns (e.g., 2 microns, 4 microns, 6 microns, 8 microns). The precision of the back-drilling process is 0.001-0.01 mm (e.g., 0.001 mm, 0.004 mm, 0.007 mm, 0.01 mm). The thickness of the second copper layer is in the range of 5-30 microns (e.g., 5 microns, 10 microns, 20 microns, 30 microns).

[0041] See also Figure 3 , Figure 3 FIG. 2 is a flow chart of another embodiment of the micro-hole back drilling processing method of the present application. In one embodiment, before step S11, the processing method further includes:

[0042] Step S31: providing a plurality of sub-board layers.

[0043] Specifically, a plurality of sub-board layers constitute a circuit board, wherein the plurality of sub-board layers refers to two or more sub-board layers, and the sub-board layers may be double-sided copper-clad board layers.

[0044] Step S32: Laminating a plurality of sub-board layers to form a printed circuit board.

[0045] Step S33: mechanically drilling a preset drilling area on the printed circuit board to form a printed circuit board provided with at least one conducting hole.

[0046] Specifically, it can be understood that the metal layer in the preset drilling area on at least one of the multiple sub-boards is etched away, for example, via holes are drilled on the second sub-board layer and the third sub-board layer that need to be conductive on the circuit board. If the second sub-board layer and the third sub-board layer do not have a metal layer in the area of ​​the via hole, then in the area of ​​the via hole, the metal layer corresponding to the via hole on other sub-boards other than the second sub-board layer and the third sub-board layer, such as the first sub-board layer and the fourth sub-board layer, is etched away, and the second sub-board layer and the third sub-board layer are processed normally.

[0047] See also Figure 4 , Figure 4 yes Figure 1 Schematic diagram of the process of step S14 in FIG. In one embodiment, step S14 includes:

[0048] Step S41: Send the back drilling information to the CNC laser drilling machine.

[0049] Step S42: the CNC laser drilling machine selects corresponding drilling parameters according to the back drilling information and performs back drilling on the metallized via hole.

[0050] The backdrilling information includes at least one of the thickness of the printed circuit board, the spacing between adjacent inner circuit layers in the printed circuit board, the backdrilling position (e.g., the spacing between the centers of adjacent metallized via holes, the spacing between the walls of adjacent metallized via holes), the backdrilling accuracy, the hole diameter of the metal via hole, the backdrilling depth, the backdrilling hole diameter (e.g., the hole diameter of the metal via hole, the hole diameter of the backdrilled hole), the drill speed or the retracting speed. The drilling parameters may include at least one of the working time, the emission position, the laser energy and the laser hole diameter. In other embodiments, the drilling parameters may also include at least one of the spindle speed, the feed speed, the tool hole diameter and the tool length compensation.

[0051] See also Figure 5 , Figure 5 It is a structural schematic diagram of an embodiment of a processing system for micro-hole back drilling of the present application. The processing system 50 is used to execute the processing method for micro-hole back drilling in the above-mentioned embodiment.

[0052] The processing system 50 includes: an automatic optical detection component 51 , a coating machine 52 , a processor 53 and a CNC laser drilling machine 54 .

[0053] One end of the processor 53 is connected to the automatic optical inspection component 51, and the other end is connected to the CNC laser drilling machine 54. The CNC laser drilling machine 54 is used to mechanically drill holes in the printed circuit board to form at least one via hole. The coating machine 52 performs flash plating on the printed circuit board to form a first copper layer on the inner wall of the via hole to obtain a metallized via hole. The automatic optical inspection component 51 is used to perform automatic optical inspection on the metallized via hole obtained by the coating machine 52. The processor 53 is used to analyze the inspection results of the automatic optical inspection to obtain the back drilling information of the metallized via hole. The CNC laser drilling machine 54 is also used to back drill the metallized via hole according to the back drilling information to form a back drill hole.

[0054] Specifically, the automatic optical inspection component may include multiple cameras, light source controllers, light sources, programmable logic controllers, computers and other equipment. The multiple cameras simultaneously shoot multiple surfaces of the printed circuit board, and collect the original images obtained by shooting and perform image processing and hole position detection. The processor may collect the original images obtained by shooting and perform image processing and hole position detection, and then output the hole position detection results and processed image data, and calculate the actual back drilling information of the back drilling hole according to the hole position deviation to obtain the back drilling information of the metallized via hole.

[0055] The coating machine is also used to tin the printed circuit board, forming a tin-plated protective layer on the inner wall of the metallized through hole, and forming a tin-plated protective film on the circuit pattern area of ​​the first outer metal layer and the second outer metal layer of the printed circuit board.

[0056] The processor is used to send the back drilling information to the CNC laser drilling machine.

[0057] The CNC laser drilling machine is used to select corresponding drilling parameters according to the back drilling information and perform back drilling on the metallized via hole.

[0058] The plater is also used to electroplate printed circuit boards to form a second copper layer at the back-drilled holes.

[0059] The CNC laser drilling machine is used to mechanically drill a preset drilling area on a printed circuit board to form a printed circuit board provided with at least one conducting hole.

[0060] Different from the prior art, the present application uses flash copper plating before back drilling to reduce the copper thickness at the metallized via hole. The copper thickness is greatly reduced, and the back drilling tool is not easy to wear, and it is not easy to drill off or break. Furthermore, the metallized via hole is measured using an automatic optical detection component to determine the back drilling position of the back drilling hole, and then back drilling is performed according to the back drilling position, which can improve the back drilling hole position accuracy, so as to realize back drilling on a high-frequency communication backplane, thereby avoiding reflection, scattering, delay, etc. of high-speed signal transmission on the high-frequency communication backplane.

[0061] See also Figure 6 , Figure 6 1 is a schematic diagram of the structure of an embodiment of a printed circuit board of the present application. The printed circuit board 100 is prepared by using the micro-hole back drilling processing method in the above embodiment.

[0062] The printed circuit board 100 comprises: a first outer metal layer 11, n inner circuit layers 13 and a second outer metal layer 12 which are stacked. At least one via 10 is provided on the printed circuit board 100, and the via comprises a back-drilled hole 101 from the first outer metal layer 11 to a cut-off surface 14, and a metallized via 102 from the cut-off surface 14 to the second outer metal layer 12. The cut-off surface 14 is between the mth and m+1th inner circuit layers 13, where m is less than n and both m and n are positive integers.

[0063] Specifically, Figure 6 As shown in the figure, the parameters are as follows: H: thickness of circuit board 100; H1: depth of back-drilled hole 101; H2: spacing between circuit layer 13 in m-th layer and m+1-th layer; H3: distance between circuit layer 13 in m+1-th layer and cut-off surface 14; D1: hole diameter of metallized via 102; D2: hole diameter of back-drilled hole 101; L1: distance between outer metal 11 and back-drilled hole 101; L2: distance between inner circuit 13 and back-drilled hole 101; L3: spacing between hole walls of adjacent back-drilled holes 101; L: spacing between hole center axes of adjacent vias 10.

[0064] The back-drilled hole 101 region of the 1st to mth inner circuit layers 13 may have a non-metal layer (not shown) surrounding the back-drilled hole 13, and the multiple inner circuit layers 13 that need to be interconnected between layers in the m+1st to nth inner circuit layers 13 are connected to each other through the metallized vias 102. Among them, the back-drilled hole 101 is a via section that does not play a connecting or transmitting role in the step via, and the metallized via 102 is a via section that plays a connecting or transmitting role in the step via.

[0065] The difference between the hole diameter D2 of the back-drilled hole 101 and the hole diameter L1 of the metallized via 102 is 0.02-0.1 mm (e.g., 0.02 mm, 0.04 mm, 0.06 mm, 0.08 mm, 0.1 mm). A first copper layer 15 is disposed on the inner wall of the metallized via 102, and the thickness of the first copper layer 15 is in the range of 2-8 μm (e.g., 2 μm, 4 μm, 6 μm, 8 μm).

[0066] See also Figure 7 , Figure 7 In one embodiment, a second copper layer 16 is disposed on the inner wall of the back-drilled hole 101, and the thickness of the second copper layer 16 is in the range of 5-30 microns (e.g., 5 microns, 10 microns, 20 microns, 30 microns), and the back-drilled hole is filled with resin 17.

[0067] The above descriptions are only some embodiments of the present application, and do not limit the protection scope of the present application. Any equivalent device or equivalent process transformation made using the contents of the present application specification and drawings, or directly or indirectly used in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A method for processing micro-hole back drilling, characterized in that: The processing method is used for a printed circuit board, and the processing method comprises: Providing a printed circuit board, wherein at least one conducting hole is provided on the printed circuit board; Flash plating the printed circuit board to form a first copper layer on the inner wall of the via hole to obtain a metallized via hole; Tinning the printed circuit board to form a tinned protective layer on the inner wall of the metallized via hole, and forming a tinned protective film on the circuit pattern area of ​​the first outer metal layer and the second outer metal layer of the printed circuit board; Performing automatic optical inspection on the metallized via hole and obtaining backdrilling information of the metallized via hole; Backdrilling the metallized via hole according to the backdrill information to form a backdrilled hole; Among them, the automatic optical inspection of the metallized via hole and obtaining the back drilling information of the metallized via hole include: photographing multiple surfaces of the printed circuit board at the same time, collecting the original images obtained by the photography, performing image processing and hole position detection, outputting the hole position detection results and the processed image data, and calculating the back drilling information of the metallized via hole according to the hole position deviation.

2. The processing method according to claim 1, characterized in that: After back-drilling the metallized via hole according to the back-drill information, the processing method further includes: removing the tin-plated protective layer; Plating the printed circuit board to form a second copper layer at the back-drilled hole; The back-drilled holes are plugged with resin.

3. The processing method according to claim 2, characterized in that: The difference between the diameter of the back-drilled hole and the diameter of the via hole is 0.02-0.1 mm; The thickness of the first copper layer ranges from 0.2 to 8 microns; The back drilling process has an accuracy of 0.001-0.01 mm; The thickness of the second copper layer is in the range of 0.5-30 microns.

4. The processing method according to claim 1, characterized in that: Before providing a printed circuit board, the processing method further includes: Provide multiple daughterboard layers; Laminating the plurality of sub-board layers to form the printed circuit board; Mechanical drilling is performed on a preset drilling area on the printed circuit board to form the printed circuit board provided with at least one of the vias.

5. The processing method according to claim 4, characterized in that: The backdrilling the metallized via hole according to the backdrill information includes: Sending the back drilling information to a CNC laser drilling machine; The CNC laser drilling machine selects corresponding drilling parameters according to the back drilling information, and performs back drilling on the metallized via hole; Wherein, the backdrilling information includes at least one of the depth, spacing, hole diameter or position of the backdrilled holes; The drilling parameters include at least one of working time, emission position, laser energy and laser hole diameter.

6. A micro-hole back drilling processing system, characterized in that: A processing method for performing micro-hole back drilling as described in any one of claims 1 to 5, wherein the processing system comprises: an automatic optical detection component, a coating machine, a processor, and a CNC laser drilling machine, wherein one end of the processor is connected to the automatic optical detection component, and the other end is connected to the CNC laser drilling machine; The CNC laser drilling machine is used to mechanically drill a printed circuit board to form at least one via hole; The coating machine is used to perform flash plating on the printed circuit board to form a first copper layer on the inner wall of the via hole to obtain a metallized via hole; The automatic optical inspection component is used to perform automatic optical inspection on the metallized vias; wherein the automatic optical inspection component at least includes a plurality of cameras, a light source controller, a light source, a programmable logic controller and a computer; The processor is used for analyzing the detection result of the automatic optical detection to obtain the back drilling information of the metallized via hole; The numerically controlled laser drilling machine is also used to perform back drilling on the metallized via hole according to the back drilling information to form a back drilled hole.

7. A printed circuit board, characterized in that: The printed circuit board is manufactured by the micro-hole back drilling processing method as described in any one of claims 1 to 5, wherein the printed circuit board comprises: a first outer metal layer, n inner circuit layers and a second outer metal layer which are stacked; At least one via is provided on the printed circuit board, the via comprises a back-drilled hole from the first outer metal layer to the cut-off surface, and a metallized via from the cut-off surface to the second outer metal layer, wherein the cut-off surface is between the m-th layer and the m+1-th layer of inner circuit layers; m is less than n and both m and n are positive integers; The difference between the diameter of the back-drilled hole and the diameter of the via hole is 0.02-0.1 mm; Wherein, the printed circuit board is formed by laminating a plurality of sub-board layers, and the sub-board layers are double-sided copper-clad board layers.

8. The printed circuit board according to claim 7, characterized in that: A first copper layer is disposed on the inner wall of the metallized via hole, and the thickness of the first copper layer is in the range of 2-8 microns.

9. The printed circuit board according to claim 7, characterized in that: A second copper layer is disposed on the inner wall of the back-drilled hole, and the thickness of the second copper layer is in the range of 5-30 microns; The back-drilled hole is filled with resin.

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