Back drilling method, back drilling depth test structure and circuit board
By setting up a back drill depth test structure on the circuit board, using the probe to detect the line on and off relationship, and accurately determine the back drill depth, the problem of difficult to control the back drill depth in the existing technology is solved, and high-precision back drilling operation is achieved, improving the integrity of signal transmission.
Patent Information
- Application Number
- CN202510006924.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-01-02
- Publication Date
- 2025-04-25
AI Technical Summary
In the prior art, the back drill depth is difficult to accurately control, resulting in the problem of too deep or too shallow back drilling, affecting signal integrity and circuit board reliability.
The back drill depth test structure is provided on the circuit board, including a reference hole, a first detection hole, a second detection hole, a first conduction pattern and a second conduction pattern. The line on-off relationship is detected by the probe, and the depth of the layer that must be cut and the layer that cannot be cut is determined, thereby accurately determining the depth of the drilling during back drilling.
The precision control of the depth of the back drill is achieved, the excess metal in the through holes to be drilled is fully removed, the accuracy of the back drill is improved, the long back drill piles are avoided affecting signal transmission and improving the integrity of signal transmission.
Smart Images
Figure CN119893822A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of printed circuit boards, and more particularly to a back drilling method, a back drilling depth testing structure and a circuit board. Background Art
[0002] In the field of modern electronic manufacturing, the performance and reliability of printed circuit boards (PCBs) are crucial to the final product as they are the basic platform for carrying and connecting various electronic components. As electronic products develop towards smaller size and higher integration, the manufacturing process of PCBs is also facing higher requirements. Back drilling refers to a drilling process performed in existing vias (PTH, also known as plated through holes) during the PCB production process. It is mainly used to eliminate or reduce the signal integrity problems that may be caused by conductors in vias in high-speed digital circuits, thereby improving the quality of high-speed signal transmission.
[0003] Traditionally, backdrilling depth control mostly uses a fixed drilling depth setting, that is, backdrilling is performed according to a pre-calculated theoretical value. However, due to factors such as changes in material thickness, the backdrilling depth in actual operation is often difficult to accurately control, resulting in a certain deviation between the backdrilling result and actual demand. This deviation may cause the backdrilling to be too deep or too shallow: too deep may damage the inner layer circuit or cause other structural damage; too shallow may not completely remove the unnecessary parts, affecting signal integrity. Therefore, how to achieve more accurate and reliable backdrilling depth control has become a technical problem that needs to be solved urgently. Summary of the invention
[0004] In view of this, embodiments of the present invention provide a backdrilling method, a backdrilling depth testing structure and a circuit board, which are beneficial to solving at least part of the above problems existing in the prior art.
[0005] In a first aspect, an embodiment of the present invention provides a back-drilling depth test structure, which is arranged on a circuit board, wherein the circuit board has multiple internal conductive layers, the multiple internal conductive layers include a must-cut layer and a must-not-cut layer adjacent to the must-cut layer, the back-drilling depth test structure includes a reference hole, a first detection hole, a second detection hole, a first conductive pattern and a second conductive pattern, the reference hole is a metallized hole, the first detection hole and the second detection hole are non-metallized holes, the first conductive pattern is arranged on the must-cut layer and extends from the metal hole wall of the reference hole to the first detection hole, and the second conductive pattern is arranged on the must-not-cut layer and extends from the metal hole wall of the reference hole to the second detection hole.
[0006] In a second aspect, an embodiment of the present invention further provides a circuit board, the circuit board comprising a circuit area and a test area arranged outside the circuit area, the test area being provided with a back-drilling depth test structure, the circuit area having a via hole to be back-drilled, the via hole to be back-drilled being a metallized hole; the circuit board having a front side and a back side arranged opposite to each other, and having a plurality of internal conductive layers between the front side and the back side, an insulating layer being arranged between every two adjacent internal conductive layers, the plurality of internal conductive layers comprising a must-cut layer and a must-not-cut layer adjacent to the must-cut layer, the distance from the must-cut layer to the back side being less than the distance from the must-not-cut layer to the back side; the back-drilling depth test structure comprising a reference hole, a first detection hole, a second detection hole, a first conductive pattern and a second conductive pattern, the reference hole, the first detection hole and the second detection hole being arranged at intervals, the reference hole being a metallized hole, the first detection hole and the second detection hole being non-metallized holes, the first conductive pattern being arranged in the must-cut layer and extending from the metal hole wall of the reference hole to the first detection hole, the second conductive pattern being arranged in the must-not-cut layer and extending from the metal hole wall of the reference hole to the second detection hole.
[0007] Further, the first conductive pattern includes a first conductive ring surrounding the first detection hole, and the second conductive pattern includes a second conductive ring surrounding the second detection hole.
[0008] Furthermore, the thickness of the insulating layer between the layer that must not be cut and the layer that must be cut is greater than or equal to 4 mils.
[0009] Furthermore, the first conductive pattern avoids the second detection hole.
[0010] Further, the backdrilling depth test structure is arranged close to the to-be-backdrilled via hole.
[0011] In a third aspect, an embodiment of the present invention further provides a back drilling method, the back drilling method comprising the following steps: providing a circuit board, the circuit board being the circuit board described in the second aspect; extending a test probe into the first detection hole from the back side of the circuit board, and determining the depth of the layer that must be cut according to the on-off relationship between the test probe and the reference hole; extending the test probe into the second detection hole from the back side of the circuit board, and determining the depth of the layer that must not be cut according to the on-off relationship between the test probe and the reference hole; determining the down drilling depth according to the depth of the layer that must be cut and the depth of the layer that must not be cut; and causing a drill bit to back drill the via hole to be back drilled from the back side of the circuit board to form a back drilled hole, the back drilling depth of the drill bit being the down drilling depth.
[0012] Further, the method of determining the depth of the layer that must be cut according to the on-off relationship between the test probe and the reference hole includes: when the test probe and the reference hole are connected through the first conductive pattern, the depth of the test probe extending into the first detection hole is determined as the depth of the layer that must be cut; the method of determining the depth of the layer that must not be cut according to the on-off relationship between the test probe and the reference hole includes: when the test probe and the reference hole are connected through the second conductive pattern, the depth of the test probe extending into the second detection hole is determined as the depth of the layer that must not be cut.
[0013] Furthermore, the test probe is arranged on a drill bit, and the test probe is extended into the first detection hole by drilling the drill bit into the first detection hole, and the test probe is extended into the second detection hole by drilling the drill bit into the second detection hole.
[0014] In a fourth aspect, an embodiment of the present invention further provides a circuit board, and the circuit board is obtained by the method described in the third aspect.
[0015] The embodiment of the present invention provides a backdrilling method, a backdrilling depth test structure and a circuit board. The backdrilling depth test structure is set on the circuit board. The backdrilling depth test structure includes a reference hole, a first detection hole, a second detection hole, a first conductive pattern and a second conductive pattern. The first conductive pattern is set in the layer that must be cut and extends from the metal hole wall of the reference hole to the first detection hole. The second conductive pattern is set in the layer that must not be cut and extends from the metal hole wall of the reference hole to the second detection hole. Therefore, the positions of the layer that must be cut and the layer that must not be cut can be determined by inserting a probe into the first detection hole and the second detection hole to detect the continuity of the circuit, and then the drilling depth during backdrilling can be determined more accurately. The technical solution of the embodiment of the present invention can fully remove excess metal in the conductive hole to be backdrilled during backdrilling, improve the backdrilling accuracy, avoid the backdrilling residual pile being too long to affect the signal transmission, and help improve the integrity of the signal transmission. BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The above and other objects, features and advantages of the present invention will become more apparent through the following description of the embodiments of the present invention with reference to the accompanying drawings, in which:
[0017] Figure 1 is a schematic cross-sectional view of a circuit board according to an embodiment of the present invention;
[0018] Figure 2 is a schematic top view of a circuit board according to an embodiment of the present invention;
[0019] Figure 3 It is a structural schematic diagram of a back drilling depth test structure in a necessary cutting layer according to an embodiment of the present invention;
[0020] Figure 4 It is a structural schematic diagram of a back drilling depth test structure in a non-cutting layer according to an embodiment of the present invention;
[0021] Figure 5 is a schematic flow chart of a back drilling method according to an embodiment of the present invention;
[0022] Figure 6 is a schematic diagram of a detection method for determining the depth of a layer that must be cut according to an embodiment of the present invention;
[0023] Figure 7 It is a detection schematic diagram of determining the depth of a layer that cannot be cut according to an embodiment of the present invention;
[0024] Figure 8 is a schematic diagram of a drill bit according to an embodiment of the present invention back-drilling a via hole to be back-drilled on a circuit board;
[0025] Fig. 9 FIG. 1 is a schematic diagram of a circuit board after back drilling according to an embodiment of the present invention.
[0026] Description of reference numerals:
[0027] 1-circuit board; 11-front; 12-back; 13-internal conductive layer; 131-must-cut layer; 132-must-cut layer; 14-through hole to be back-drilled; 15-insulating layer; 16-back-drilled hole; 17-circuit area; 18-test area; 2-back-drilling depth test structure; 21-reference hole; 211-metal hole wall; 22-first detection hole; 23-second detection hole; 24-first conductive pattern; 241-first conductive loop; 25-second conductive pattern; 251-second conductive loop; 3-test probe; 4-drill bit. DETAILED DESCRIPTION
[0028] The present application is described below based on embodiments, but the present application is not limited to these embodiments. In the detailed description of the present application below, some specific details are described in detail. It is possible for those skilled in the art to fully understand the present application without the description of these details. In order to avoid confusing the essence of the present application, known methods, processes, flows, components and circuits are not described in detail.
[0029] In addition, persons of ordinary skill in the art will appreciate that the drawings provided herein are for illustration purposes and are not necessarily drawn to scale.
[0030] Unless the context clearly requires otherwise, the words "include", "comprising" and similar words throughout the application should be interpreted as including rather than exclusive or exhaustive; that is, the meaning is "including but not limited to".
[0031] In the description of this application, it should be understood that the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance. In addition, in the description of this application, unless otherwise specified, the meaning of "plurality" is two or more.
[0032] In an embodiment of the present invention, a circuit board 1 and a backdrill depth test structure 2 provided on the circuit board 1 are provided. Another aspect of the embodiment of the present invention further provides a backdrilling solution that can be applied to backdrill the circuit board 1. After backdrilling and other subsequent processing procedures, the circuit board 1 can obtain a circuit board product with predetermined performance.
[0033] In the embodiment of the present invention, refer to Figure 1 , the circuit board 1 includes a multilayer structure of multiple insulating layers 15 and conductive layers stacked, wherein the circuit board 1 includes multiple internal conductive layers 13. The circuit board 1 has a front side 11 and a back side 12 arranged opposite to each other, multiple internal conductive layers 13 are located between the front side 11 and the back side 12, and an insulating layer 15 is provided between every two adjacent internal conductive layers 13. The circuit board 1 includes a circuit area 17, and the circuit area 17 is provided with conductive circuits required for the circuit board. Among them, the circuit area 17 has a via hole 14 to be back-drilled, and the via hole 14 to be back-drilled is a metallized hole. In some application scenarios, the via hole 14 to be back-drilled only needs to conduct the pattern of the front side 11 of the circuit board 1 to one or some internal conductive layers 13, and does not need to conduct the front side 11 and the back side 12 of the circuit board 1. Therefore, in the process of processing the circuit board 1, it is necessary to back-drill the via hole 14 to be back-drilled to remove a certain length of the metal layer of the via hole 14 to be back-drilled.
[0034] In this embodiment, the multiple internal conductive layers 13 include a must cut layer 131 (Must Cut Layer) and a must not cut layer 132 (Must Not Cut Layer), wherein the must cut layer 131 is an internal conductive layer 13 farthest from the front side 11 of the circuit board 1 required for conducting the back-drilled via 14, and the must not cut layer 132 is an internal conductive layer 13 adjacent to the must cut layer 131, and the distance from the must cut layer 131 to the back side 12 is smaller than the distance from the must not cut layer 132 to the back side 12.
[0035] Reference Figure 1 and Figure 2The circuit board 1 also includes a test area 18 arranged outside the circuit area 17, and the test area 18 is provided with a back-drilling depth test structure 2. The back-drilling depth test structure 2 includes a reference hole 21, a first test hole 22, a second test hole 23, a first conductive pattern 24 and a second conductive pattern 25. Among them, the reference hole 21, the first test hole 22 and the second test hole 23 are arranged at intervals, the reference hole 21 is a plated through hole (PTH, also known as a plated through hole), and the first test hole 22 and the second test hole 23 are non-plated through holes (NPTH, also known as non-plated through holes). Optionally, the reference hole 21, the first test hole 22 and the second test hole 23 can be through holes that penetrate the front side 11 and the back side 12 of the circuit board 1. Preferably, the back-drilling depth test structure 2 is arranged close to the via hole 14 to be back-drilled, so that the back-drilling depth test structure 2 can better simulate the conditions at the via hole 14 to be back-drilled, and reduce the layer thickness difference caused by the difference in residual copper rate in different areas of the circuit board 1.
[0036] Figure 3 is a schematic diagram of the structure of the back drilling depth test structure 2 in the required cutting layer 131 according to an embodiment of the present invention, referring to Figure 3 The first conductive pattern 24 is provided in the required cutting layer 131 and extends from the metal hole wall 211 of the reference hole 21 to the first detection hole 22. When the conductor contacts the portion of the first conductive pattern 24 extending to the first detection hole 22, the conductor can be conductively connected to the reference hole 21 through the first conductive pattern 24. Figure 4 is a schematic diagram of the structure of the back drilling depth test structure 2 in the non-cutting layer 132 according to an embodiment of the present invention, referring to Figure 4 , the second conductive pattern 25 is provided in the layer that must not be cut 132 and extends from the metal hole wall 211 of the reference hole 21 to the second detection hole 23. When the test probe 3 contacts the portion of the second conductive pattern 25 extending to the second detection hole 23, the test probe 3 can be conductively connected with the reference hole 21 through the second conductive pattern 25. Therefore, it is possible to determine whether the test probe 3 has reached the layer that must be cut 131 or the layer that must not be cut 132 through the conductive state between the test probe 3 and the reference hole 21, and further determine the depth of the layer that must be cut 131 and the depth of the layer that must not be cut 132. In the embodiment of the present invention, the depth of the layer that must be cut 131 refers to the distance from the layer that must be cut 131 to the back side 12 of the circuit board 1, and the depth of the layer that must not be cut 132 refers to the distance from the layer that must not be cut 132 to the back side 12 of the circuit board 1.
[0037] In some embodiments, reference Figure 3, the first conductive pattern 24 may include a first conductive ring 241 surrounding the first detection hole 22, and the inner circle of the first conductive ring 241 extends to the inner wall of the first detection hole 22. Therefore, when the test probe 3 is inserted into the first detection hole 22, as long as it fits with the inner wall of the first detection hole 22, it can be ensured that when it reaches the necessary cutting layer 131, it contacts the first conductive pattern 24 and is conductive with the detection hole, which is conducive to ensuring the accuracy of the depth detection result of the necessary cutting layer 131. In the embodiment of the present invention, the shape of the first conductive ring 241 is not limited to a circular ring, and the outer contour shape of the first conductive ring 241 can be any shape. Preferably, the first conductive pattern 24 avoids the second detection hole 23, which can make the depth detection result of the necessary cutting layer 131 more accurate.
[0038] In some embodiments, the second conductive pattern 25 may include a second conductive ring 251 surrounding the second detection hole 23, and the inner circle of the second conductive ring 251 extends to the inner wall of the second detection hole 23. Therefore, when the test probe 3 is inserted into the second detection hole 23, as long as it fits the inner wall of the second detection hole 23, it can be ensured that when it reaches the layer that must not be cut 132, it contacts the first conductive pattern and is connected to the detection hole, which is conducive to ensuring the accuracy of the depth detection result of the layer that must not be cut 132. In the embodiment of the present invention, the shape of the second conductive ring 251 is not limited to a circular ring, and the outer contour shape of the second conductive ring 251 can be any shape. Preferably, the second conductive pattern 25 avoids the first detection hole 22, which can make the depth detection result of the layer that must be cut 131 more accurate.
[0039] In some embodiments, the thickness of the insulating layer 15 between the must-cut layer 132 and the must-cut layer 131 is greater than or equal to 4 mils (mil, milliinches), which helps to reduce the difficulty of backdrilling depth control and signal crosstalk.
[0040] Another aspect of the embodiments of the present invention also relates to a back drilling method, which can be used to back drill the circuit board 1 in at least some of the above embodiments. Figure 5 is a schematic diagram of a back drilling method according to an embodiment of the present invention, referring to Figure 5 The back drilling method includes the following steps S100 to S500:
[0041] Step S100: providing a circuit board.
[0042] The circuit board 1 adopts the circuit board 1 described in at least some of the above embodiments.
[0043] Step S200: insert a test probe into a first detection hole from the back side of the circuit board, and determine the depth of the layer that must be cut according to the on-off relationship between the test probe and the reference hole.
[0044] The end of the test probe 3 is gradually extended into the hole wall of the first detection hole 22 from the back side 12 of the circuit board 1, and the other end of the test probe 3 is electrically connected to the reference hole 21. When the test probe 3 has not yet reached the necessary cutting layer 131, the circuit between the test probe 3 and the reference hole 21 is open. Figure 6 When the test probe 3 gradually extends to the position where the layer 131 that must be cut is located, the test probe 3 contacts the first conductive pattern 24, so that the test probe 3 is connected to the reference hole 21 through the first conductive pattern 24 to form a loop. At this time, the depth of the test probe 3 extending into the first detection hole 22 can be determined as the depth of the layer 131 that must be cut. In an optional embodiment, the diameter of the first detection hole 22 is set according to the size of the test probe 3, so that the test probe 3 can be in close contact with the inner wall of the first detection hole 22 to ensure the accuracy of the test result. In an optional embodiment, the test probe 3 can be set on the drill bit 4, and the test probe 3 is inserted into the first detection hole 22 by drilling the drill bit 4 into the first detection hole 22. The diameter of the drill bit 4 can be slightly larger than the diameter of the first detection hole 22, which is conducive to ensuring that the test probe 3 can be in contact with the first conductive pattern 24 when it moves to the layer 131 that must be cut.
[0045] Step S300: insert a test probe into the second detection hole from the back side of the circuit board, and determine the depth of the layer that cannot be cut according to the on-off relationship between the test probe and the reference hole.
[0046] The end of the test probe 3 is gradually extended into the hole wall of the second detection hole 23 from the back side 12 of the circuit board 1, and the other end of the test probe 3 is electrically connected to the reference hole 21. When the test probe 3 has not yet reached the non-cutting layer 132, the circuit between the test probe 3 and the reference hole 21 is open. Figure 7 When the test probe 3 gradually extends to the position of the layer 132 that must not be cut, the test probe 3 contacts the second conductive pattern 25, so that the test probe 3 is conductively connected to the reference hole 21 through the second conductive pattern 25 to form a loop. At this time, the depth of the test probe 3 extending into the second detection hole 23 can be determined as the depth of the layer 132 that must not be cut. In an optional embodiment, the diameter of the second detection hole 23 is set according to the size of the test probe 3, so that the test probe 3 can be in close contact with the inner wall of the second detection hole 23 to ensure the accuracy of the test result. In an optional embodiment, the test probe 3 can be set on the drill bit 4, and the test probe 3 is inserted into the second detection hole 23 by drilling the drill bit 4 into the second detection hole 23. The diameter of the drill bit 4 can be slightly larger than the diameter of the second detection hole 23, which is conducive to ensuring that the test probe 3 can be in contact with the second conductive pattern 25 when it moves to the layer 131 that must be cut.
[0047] Step S400: Determine the drilling depth according to the depth of the layer that must be cut and the depth of the layer that must not be cut.
[0048] The drilling depth when back-drilling the via hole 14 to be back-drilled can be determined based on the depth of the layer 131 that must be cut and the depth of the layer 132 that must not be cut. When determining the drilling depth, the drilling depth can also be calculated in combination with the copper coverage on the back side 12 of the circuit board 1 between the location of the back-drilling depth test structure 2 and the location of the via hole 14 to be back-drilled, so as to eliminate the interference of the copper coverage difference. When there is no obvious difference in the surface residual copper at the location of the back-drilling depth test structure 2 and the location of the via hole 14 to be back-drilled, the drilling depth can be determined as a value between the depth of the layer 131 that must be cut and the depth of the layer 132 that must not be cut in combination with the shape of the end of the drill bit 4.
[0049] Step S500: enabling a drill bit to back-drill the via hole to be back-drilled from the back side of the circuit board to form a back-drilled hole, and the back-drilling depth of the drill bit is the drilling depth.
[0050] Reference Figure 8 , a drill bit 4 having a diameter larger than the diameter of the via hole 14 to be back-drilled is used to back-drill the via hole 14 to be back-drilled from the back side 12 of the circuit board 1, and the drill bit 4 stops back-drilling when the drilling depth is reached. Thus, the circuit board 1 after back-drilling forms a back-drilled hole 16 (refer to Fig. 9 ), the position of the residual pile can be better controlled near the necessary cutting layer 131.
[0051] It is easy to understand that the execution order of the above-mentioned step S300 and step S400 can be exchanged, that is, step S300 can be executed first and then step S400, or step S400 can be executed first and then step S300.
[0052] An embodiment of the present invention also relates to a circuit board, which is obtained by processing through the above-mentioned back drilling method. After back drilling, a resin filling treatment can be performed after the back drilling hole 16 to avoid the occurrence of CAF (Conductive Anodic Filament) phenomenon, which causes electrical short circuit or other failures and affects the reliability and performance of the circuit board. When the back drilling depth test structure 2 is set at the edge of the circuit board 1, this part of the circuit board 1 can also be removed by cutting or the like as needed, which is conducive to reducing the size of the circuit board.
[0053] The technical solution of the embodiment of the present invention is to set a backdrilling depth test structure 2 on the circuit board 1. The backdrilling depth test structure 2 includes a reference hole 21, a first detection hole 22, a second detection hole 23, a first conductive pattern 24 and a second conductive pattern 25. The first conductive pattern 24 is arranged on the layer 131 that must be cut and extends from the metal hole wall 211 of the reference hole 21 to the first detection hole 22. The second conductive pattern 25 is arranged on the layer 132 that must not be cut and extends from the metal hole wall 211 of the reference hole 21 to the second detection hole 23. Therefore, the positions of the layer 131 that must be cut and the layer 132 that must not be cut can be determined by inserting a probe into the first detection hole 22 and the second detection hole 23 to detect the continuity of the circuit, thereby more accurately determining the drilling depth during backdrilling. The technical solution of the embodiment of the present invention can fully remove the excess metal in the conductive hole 14 to be backdrilled during backdrilling, improve the backdrilling accuracy, avoid the backdrilling residual pile being too long to affect the signal transmission, and help improve the integrity of the signal transmission. In addition, this method can save the process of extracting samples for slice measurement after back drilling, thereby improving efficiency and reducing product waste.
[0054] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.
Claims
1. A back drilling depth test structure, provided on a circuit board, wherein the circuit board has a plurality of internal conductive layers, wherein the plurality of internal conductive layers include a must-cut layer and a must-not-cut layer adjacent to the must-cut layer, characterized in that: The back-drilling depth test structure includes a reference hole, a first detection hole, a second detection hole, a first conductive pattern and a second conductive pattern, the reference hole is a metallized hole, the first detection hole and the second detection hole are non-metallized holes, the first conductive pattern is arranged on the must-cut layer and extends from the metal hole wall of the reference hole to the first detection hole, and the second conductive pattern is arranged on the must-cut layer and extends from the metal hole wall of the reference hole to the second detection hole.
2. A circuit board, characterized in that: The circuit board comprises a circuit area and a test area arranged outside the circuit area, the test area is provided with a backdrill depth test structure, the circuit area has a via hole to be backdrilled, and the via hole to be backdrilled is a metallized hole; The circuit board has a front side and a back side that are arranged opposite to each other, and has a plurality of internal conductive layers located between the front side and the back side, an insulating layer is arranged between every two adjacent internal conductive layers, the plurality of internal conductive layers include a must-cut layer and a must-not-cut layer adjacent to the must-cut layer, and a distance from the must-cut layer to the back side is smaller than a distance from the must-not-cut layer to the back side; The back-drilling depth test structure includes a reference hole, a first detection hole, a second detection hole, a first conductive pattern and a second conductive pattern. The reference hole, the first detection hole and the second detection hole are arranged at intervals. The reference hole is a metallized hole, and the first detection hole and the second detection hole are non-metallized holes. The first conductive pattern is arranged on the must-cut layer and extends from the metal hole wall of the reference hole to the first detection hole. The second conductive pattern is arranged on the must-cut layer and extends from the metal hole wall of the reference hole to the second detection hole.
3. The circuit board according to claim 2, characterized in that: The first conductive pattern includes a first conductive ring surrounding the first detection hole, and the second conductive pattern includes a second conductive ring surrounding the second detection hole.
4. The circuit board according to claim 2, characterized in that: The thickness of the insulating layer between the layer that must not be cut and the layer that must be cut is greater than or equal to 4 mils.
5. The circuit board according to claim 2, characterized in that: The first conductive pattern avoids the second detection hole.
6. The circuit board according to claim 2, characterized in that: The backdrilling depth testing structure is disposed close to the to-be-backdrilled via hole.
7. A back drilling method, characterized in that: The back drilling method comprises: Providing a circuit board, wherein the circuit board is a circuit board according to any one of claims 2 to 6; Insert a test probe into the first detection hole from the back side of the circuit board, and determine the depth of the layer that must be cut according to the on-off relationship between the test probe and the reference hole; Inserting a test probe into the second detection hole from the back side of the circuit board, and determining the depth of the non-cutting layer according to the on-off relationship between the test probe and the reference hole; Determining the drilling depth according to the depth of the layer that must be cut and the depth of the layer that must not be cut; and The drill bit is used to back-drill the via hole to be back-drilled from the back side of the circuit board to form a back-drilled hole, and the back-drilling depth of the drill bit is the drilling depth.
8. The back drilling method according to claim 7, characterized in that: The step of determining the depth of the required cutting layer according to the on-off relationship between the test probe and the reference hole comprises: When the test probe is electrically connected to the reference hole through the first conductive pattern, the depth of the test probe extending into the first detection hole is determined as the depth of the layer that must be cut; The step of determining the depth of the non-cutting layer according to the on-off relationship between the test probe and the reference hole comprises: When the test probe is electrically connected to the reference hole through the second conductive pattern, the depth of the test probe extending into the second detection hole is determined as the depth of the non-cutting layer.
9. The back drilling method according to claim 7, characterized in that: The test probe is arranged on a drill bit. The test probe is inserted into the first detection hole by drilling the drill bit into the first detection hole, and the test probe is inserted into the second detection hole by drilling the drill bit into the second detection hole.
10. A circuit board, characterized in that: The circuit board is obtained by the method according to any one of claims 7-9.