Back drilling method for reducing circuit board via hole stub

Abstract

The invention relates to a back drilling method for reducing circuit board via hole stubs, which comprises the following steps: S1, blind hole pre-processing: performing laser pre-processing at the tail end of a target via hole stub to form a positioning blind hole; s2, back drilling, wherein a drill bit with an impedance monitoring module is adopted to conduct tracking drilling on the positioning blind hole; s3, monitoring a dynamic resistance value, and collecting an electrical impedance value of the impedance monitoring module in real time; and S4, tool retracting is conducted, and when it is monitored that the electrical impedance value reaches a preset value, drilling is stopped immediately, and the drill bit is retracted. According to the method, the laser preprocessing technology and the impedance feedback technology are combined, dependence and uncertainty on empirical judgment are reduced, the production process is simplified, the machining cost is reduced, and accurate control over the back drilling via hole stub can be achieved. The length of the via hole stub can be effectively reduced, reflection, scattering and delay in high-speed signal transmission are reduced, signal integrity is improved, and the method is particularly suitable for design of high-frequency and high-density interconnected circuit boards.

Description

Technical Field

[0001] The present invention relates to the field of circuit boards, and more particularly, to a back drilling method for reducing via stubs on a circuit board. Background Art

[0002] In the manufacturing of multi-layer circuit boards, a via stub refers to the redundant part of a via that is not connected to the signal path. This phenomenon is particularly critical in high-frequency or high-speed signal transmission because it may cause problems such as signal reflection and impedance discontinuity. Back drilling is commonly used to remove redundant via stubs on the signal transmission path to reduce signal reflection and loss. Traditional back drilling processes generally cut via stubs through secondary drilling, but there are the following problems: 1. Unstable control of stub length: Affected by drill bit positioning accuracy and material hardness differences, the stub length fluctuates greatly. 2. Low processing efficiency: The drill bit parameters need to be adjusted multiple times, and the process is complex. 3. High cost: It is highly dependent on the operator's experience, and the yield is insufficient. Therefore, traditional via stub removal processes have problems such as low removal efficiency and high defect rates. Summary of the Invention

[0003] In view of the above, the present invention provides a back drilling method for reducing via stubs on a circuit board that can accurately remove redundant via stubs and effectively improve the removal efficiency.

[0004] The object of the present invention is achieved by the following technical solutions: A back drilling method for reducing via stubs on a circuit board, comprising the following steps: S1: Blind hole preprocessing, laser preprocessing is performed at the end of the target via stub to form a positioning blind hole; S2: Back drilling, using a drill bit with an impedance monitoring module to perform tracking drilling on the positioning blind hole; S3: Monitoring dynamic resistance value, the resistance value of the impedance monitoring module is collected in real time; S4: Retracting the tool, when the monitored resistance value reaches a preset value, immediately stop drilling and retract the drill bit.

[0005] In the above technical solution, a positioning blind hole is formed through preprocessing, providing a physical reference for subsequent back drilling, reducing the stub tolerance caused by drill bit deviation in the traditional back drilling process, and ensuring that the drilling path is precisely aligned with the end of the target stub. The impedance monitoring module is used to monitor the change of the resistance value in real time, directly reflecting the contact state between the drill bit and the conductive layer through electrical parameters, avoiding errors relying on theoretical board thickness or empirical compensation values, and improving the judgment accuracy of the termination drilling time. In addition, when the resistance value reaches the preset threshold, the tool is immediately retracted, which can effectively prevent over-drilling or under-drilling, reduce problems such as hole wall damage or excessive stub length caused by over-processing, and improve the yield.

[0006] Therefore, by combining laser pre-processing and impedance feedback technology, the present invention reduces the dependence on empirical judgment and uncertainty, simplifies the production process and reduces the processing cost, and can achieve precise control of the back-drilled via stub. It can effectively reduce the length of the via stub, reduce reflection, scattering and delay in high-speed signal transmission, thereby improving signal integrity, and is particularly suitable for high-frequency, high-density interconnect circuit board designs.

[0007] Optionally, in a possible implementation, the diameter of the positioning blind hole matches the diameter of the original via, or is 1.1 - 1.3 times the diameter of the original via.

[0008] In the above technical solution, the via stub is removed by two-stage drilling, where the laser-drilled blind hole is the first stage. Therefore, when the diameter of the blind hole is the same as or slightly larger than the diameter of the original via, it can ensure effective removal of part of the stub and provide a larger alignment redundancy space for the drill bit in the follow-up, effectively alleviating the tiny position deviation caused by pre-processing errors or equipment vibration, and enhancing process stability.

[0009] Optionally, in a possible implementation, the diameter of the drill bit is greater than or equal to the diameter of the positioning blind hole, and the end of the drill bit has a chamfer structure of 20° - 30°.

[0010] In the above technical solution, the chamfer structure is integrated with the drill bit, and the edge of the hole is automatically chamfered while the back drilling is completed, eliminating the drawback of the traditional process that requires step-by-step operations for drilling and chamfering, reducing the number of tool replacements, and significantly improving the processing efficiency. In addition, the chamfer angle design of 20° - 30° can effectively disperse the stress at the edge of the hole, reduce the risk of microcracks, and at the same time reduce the burrs caused by the acute angle during the drilling process, improving the surface quality of the hole wall and the structural reliability.

[0011] Optionally, in a possible implementation, the impedance monitoring module is a capacitive sensor.

[0012] In the above technical solution, the capacitive sensor has extremely high sensitivity to tiny impedance changes, can accurately capture the impedance jump caused by material removal during the drilling process, and ensures timely triggering of the retraction mechanism at the preset threshold point to avoid over-drilling or excessive stub residue.

[0013] Optionally, in a possible implementation, in step S1, the laser pre-processing is performed using a laser drilling machine, and the depth of the positioning blind hole is 50% - 80% of the length of the original via stub.

[0014] In the above technical solution, by limiting the pre-processing depth to 50%-80% of the length of the stub pile, it can effectively guide the subsequent back-drilling path and retain sufficient structural strength to avoid substrate damage caused by over-processing. The high-precision energy focusing characteristic of the laser drilling machine can accurately achieve the control of this depth threshold, ensuring the balance between the stub pile removal efficiency and the processing safety.

[0015] Optionally, in a possible implementation manner, the rotational speed of the laser drilling machine during the processing of the positioning blind hole is 30-50KRPM, and the feed rate is 5-8mm / s. The rotational speed of the drill bit during the tracking drilling is 10-30KRPM, and the feed rate is 3-4mm / s.

[0016] In the above technical solution, the high rotational speed of 30-50KRPM and the feed rate of 5-8mm / s are adopted in the laser pre-processing stage, which can improve the material removal efficiency while ensuring the geometric accuracy of the positioning blind hole, and avoid carbonization of the hole wall or substrate damage caused by heat accumulation. In the tracking drilling stage, the rotational speed is reduced to 10-30KRPM and the feed rate is adjusted to 3-4mm / s, effectively reducing the interference of mechanical vibration on the impedance monitoring signal and ensuring the real-time capture accuracy of the impedance change when the chamfer structure at the end of the drill bit contacts the hole wall.

[0017] Optionally, in a possible implementation manner, infrared thermal imaging data is synchronously collected in the step S3. When the temperature change rate in the drilling area is monitored to exceed 5℃ / ms, the auxiliary cooling system is started to perform physical and chemical cooling on the drill bit.

[0018] In the above technical solution, a high-sensitivity infrared thermal imaging system is used to monitor the temperature gradient in the drilling area at the millisecond level. When the monitored temperature change rate exceeds the preset value, the physical and chemical cooling system intervenes precisely. This real-time feedback mechanism can effectively inhibit material carbonization or structural deformation caused by local overheating of the drill bit, ensuring the thermal stability of the back-drilling process.

[0019] Optionally, in a possible implementation manner, the auxiliary cooling system adopts pulsed gas-liquid two-phase cooling, where the gas phase is compressed air and the liquid phase is deionized water atomized particles.

[0020] In the above technical solution, compressed air and deionized water atomized particles form a two-phase medium, and dynamic heat exchange is achieved through pulsed spraying. The gas phase accelerates the diffusion coverage range of the liquid phase particles, enhancing the contact uniformity between the coolant and the drill bit surface; the liquid phase absorbs the instantaneous high temperature through the high specific heat capacity of deionized water, while avoiding oxidation or scaling of the metal surface caused by water quality impurities.

[0021] Optionally, in a possible implementation manner, in the step S4, when the impedance value of the monitoring module reaches the preset value, the drill bit retracts the tool within 0.1ms.

[0022] In the above technical solution, the retraction action is triggered by the real-time threshold judgment of the impedance value, and the instantaneous response at the 0.1 ms level can effectively avoid problems such as over-cutting at the bottom of the hole and burr growth caused by continuous feeding after the drilling penetrates, ensuring that the machining dimensional accuracy meets the micron-level control requirements.

[0023] Optionally, in a possible implementation manner, the length of the via stub after machining is less than or equal to 150 μm.

[0024] In the above technical solution, shortening the length of the via stub to less than 150 μm can significantly reduce the parasitic capacitance effect in high-speed signal transmission and reduce the impedance mutation of the signal path. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0026] Figure 1 FIG. is a schematic structural diagram of a via stub after laser pre-processing in an embodiment.

[0027] Figure 2 FIG. is a schematic structural diagram of a via stub after back drilling in an embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] To make the objectives, technical solutions, and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. The components of the embodiments of the present application described and illustrated herein can be arranged and designed in various different configurations.

[0029] Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the present application claimed, but merely represents selected embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts fall within the scope of protection of the present application.

[0030] Please refer to Figure 1 and Figure 2 , this embodiment provides a back drilling method for reducing the via stub 2 of a circuit board, including the following steps: S1: Pre-machining of blind holes. A positioning blind hole 1 is formed by laser pre-machining at the end of the target via stub 2. Among them, the positioning of the positioning blind hole 1 can be realized by a vision positioning system. The aperture of the positioning blind hole 1 matches the original via aperture, such as 0.2 mm, or is 1.1 - 1.3 times the original via aperture. S2: Back drilling. The positioning blind hole 1 is drilled by tracking with a drill bit 3 equipped with an impedance monitoring module. The diameter of the drill bit 3 is greater than or equal to the aperture of the positioning blind hole 1, and the end of the drill bit 3 has a chamfer structure of 20° - 30°. For example, in this embodiment, the diameter of the drill bit 3 is 0.25 mm, and the chamfer is 25°. S3: Monitoring dynamic resistance value. The resistance value of the impedance monitoring module is collected in real time. S4: Retracting the tool. When the monitored resistance value reaches the preset value, immediately stop drilling and retract the drill bit 3.

[0031] In this embodiment, the positioning blind hole 1 is formed by pre-machining, providing a physical reference for subsequent back drilling, reducing the stub tolerance caused by the offset of the drill bit 3 in the traditional back drilling process, and ensuring the precise alignment of the drilling path with the end of the target stub. The impedance monitoring module is used to monitor the change of the resistance value in real time, directly reflecting the contact state between the drill bit 3 and the conductive layer through electrical parameters, avoiding the error of relying on the theoretical board thickness or empirical compensation value, and improving the judgment accuracy of the termination drilling time. In addition, when the resistance value reaches the preset threshold, immediately retracting the tool can effectively prevent over-drilling or under-drilling, reduce the problem of hole wall damage or stub length exceeding the standard caused by excessive processing, and improve the yield rate.

[0032] Therefore, by combining laser pre-machining and impedance feedback technology, this embodiment reduces the dependence on empirical judgment and uncertainty, simplifies the production process and reduces the processing cost, and can achieve precise control of the back-drilled via stub 2. It can effectively reduce the length of the via stub 2, reduce reflection, scattering and delay in high-speed signal transmission, thereby improving signal integrity, and is especially suitable for high-frequency, high-density interconnect circuit board design.

[0033] Specifically, the via stub 2 is removed by two-stage drilling. The laser-drilled blind hole is the first stage. Therefore, when the aperture of the blind hole is the same as or slightly larger than the original via aperture, it can ensure the effective removal of part of the stub, and provide a larger alignment redundancy space for the drill bit 3 subsequently, effectively alleviating the small position deviation caused by pre-machining error or equipment vibration, and enhancing the process stability.

[0034] In addition, the chamfering structure is integrated with the drill bit 3, and the edge of the hole is automatically chamfered while the back drilling is completed, eliminating the disadvantage of drilling and chamfering in traditional processes that require separate operations, reducing the number of tool changes, and significantly improving processing efficiency. In addition, the chamfering angle design of 20°-30° can effectively disperse the stress on the edge of the hole, reduce the risk of microcracks, and reduce burrs caused by sharp angles during drilling, thereby improving the surface quality of the hole wall and structural reliability. At the same time, the diameter of the drill bit 3 matches or is slightly larger than the aperture of the positioning blind hole 1, ensuring that the drilling trajectory completely covers the pre-processing area, avoiding the formation of residual materials or secondary stumps due to insufficient diameter of the drill bit 3, and ensuring the consistency of the back drilling depth.

[0035] It should be noted that the impedance monitoring module of this embodiment is a capacitive sensor. Capacitive sensors are extremely sensitive to tiny impedance changes and can accurately capture impedance jumps caused by material removal during the drilling process, ensuring that the tool retraction mechanism is triggered in time at the preset threshold point to avoid over-drilling or excessive residual piles. In addition, it has excellent dynamic response characteristics and can achieve millisecond-level real-time monitoring under high-speed drilling conditions, significantly improving the timeliness of process control and reducing processing errors caused by signal delays.

[0036] In this embodiment, the laser pre-processing in step S1 is performed by a laser drilling machine, and the depth of the blind hole 1 is positioned to be 50%-80% of the length of the original via stump 2. The laser drilling machine is an existing conventional equipment, which uses a visual positioning system to locate the position of the via stump 2, and outputs the via positioning coordinates to a corresponding file. The laser drilling machine determines the coordinate system according to the output file, and at this time, the laser drilling machine is driven by a mobile platform to the coordinates of the corresponding via to perform laser drilling operations.

[0037] By limiting the pre-processing depth to 50%-80% of the length of the stump, it can effectively guide the subsequent back-drilling path while retaining sufficient structural strength to avoid damage to the substrate caused by over-processing. The high-precision energy focusing characteristics of the laser drilling machine can accurately achieve this depth threshold control, ensuring a balance between stump removal efficiency and processing safety.

[0038] It should be noted that the monitoring principle of the via stump 2 in this embodiment is as follows: when the back-drilling drill bit 3 drills downward, after the drill bit 3 drills through the metal layer according to the set depth, according to the length of the remaining via stump 2, the capacitive sensing circuit in the capacitive sensor inside the spindle of the drill bit 3 starts to work, and the capacitance resistance changes. The resistance corresponds to the impedance generated by the via stump 2, and the length of the via stump 2 can be determined according to the resistance change. That is, when the drill bit 3 starts to contact the via stump 2 and starts to cut off the via stump 2, the resistance of the capacitive sensor inside it will begin to change gradually, and the value of the via stump 2 is determined according to this changed value.

[0039] Among them, impedance refers to the following: the existence of the via stub 2 will cause impedance discontinuity, resulting in reflections, resonances, etc. As the frequency increases, the impact becomes greater, and the impact is quantified by the impedance value.

[0040] The length of the via stub 2 is a technology for optimizing signal transmission. In high-frequency and high-speed circuit boards, vias are used to connect circuits between different layers. Signals will be transmitted on the redundant copper pillars. The existence of the via stub 2 will cause signal reflection, scattering, and delay, thus affecting signal integrity.

[0041] In this embodiment, after drilling through the positioning blind hole 1 by laser drilling, using the impedance monitoring module built into the drill bit 3, that is, the capacitive sensor, to connect the control system and set the back drilling depth, and start back drilling the remaining via stub 2. When the drill bit 3 drills downward, the impedance value of the capacitive sensor increases. As the remaining value of the via stub 2 decreases, its impedance value will also decrease in turn until the impedance value reaches a stable state and reaches the preset target value. For example, the impedance threshold set in this embodiment is 200 ohms. Among them, different lengths of the via stub 2 obtain corresponding insertion losses (the following table shows the loss value data corresponding to different stub lengths), which are converted into losses according to the actual impedance data after work. When the value of the via stub 2 is ≤ 150um, a tool retraction command is triggered to terminate drilling. Therefore, it is also possible to determine the length of the via stub 2 according to different preset back drilling depths, which can meet the simultaneous operation of different back drilling depths and convert the loss data corresponding to different stub lengths.

[0042]

[0043] In this embodiment, the rotational speed of the laser drilling machine when processing the positioning blind hole 1 is 30 - 50 KRPM, and the feed rate is 5 - 8 mm / s. The rotational speed of the drill bit 3 when performing tracking drilling is 10 - 30 KRPM, and the feed rate is 3 - 4 mm / s. For example, in this embodiment, the laser drilling machine cuts the stub body at 45 KRPM and a feed rate of 6 mm / s, and the drill bit 3 trims the end at 12 KRPM and 1.5 mm / s.

[0044] In the laser preprocessing stage, a high rotational speed of 30 - 50 KRPM is combined with a feed rate of 5 - 8 mm / s, which can improve the material removal efficiency while ensuring the geometric accuracy of the positioning blind hole 1, and avoid carbonization of the hole wall or substrate damage caused by heat accumulation. In the tracking drilling stage, the rotational speed is reduced to 10 - 30 KRPM and the feed rate is adjusted to 3 - 4 mm / s, effectively reducing the interference of mechanical vibration on the impedance monitoring signal and ensuring the real-time capture accuracy of the impedance change when the chamfer structure at the end of the drill bit 3 contacts the hole wall. By matching the process parameters of the two stages, a processing gradient of "high-speed rough positioning + low-speed fine trimming" is formed, so that the thickness of the transition region between the bottom of the positioning blind hole 1 and the end of the target stub is accurately controlled within the micron-level tolerance range.

[0045] In step S3 of this embodiment, infrared thermal imaging data is synchronously collected. When the temperature change rate in the drilling area is monitored to exceed 5 °C / ms, the auxiliary cooling system is activated to perform physical and chemical cooling on the drill bit 3. A high-sensitivity infrared thermal imaging system is used to monitor the millisecond-level temperature gradient in the drilling area. When the monitored temperature change rate exceeds the preset value, the physical and chemical cooling system intervenes precisely. This real-time feedback mechanism can effectively suppress the material carbonization or structural deformation caused by local overheating of the drill bit 3, ensuring the thermal stability of the back drilling process. In addition, the infrared thermal imaging technology provides a non-contact full-area temperature field distribution image, enabling the operator to intuitively identify the diffusion path of the drilling heat effect. Combining with the temperature change rate threshold trigger mechanism, a dynamic matching closed-loop of processing parameters and cooling intensity is formed to avoid the hysteresis problem of traditional empirical cooling.

[0046] Specifically, the auxiliary cooling system adopts pulsed gas-liquid two-phase cooling, where the gas phase is compressed air and the liquid phase is deionized water atomized particles. The compressed air and the deionized water atomized particles form a two-phase medium, and dynamic heat exchange is achieved through pulsed injection. The gas phase accelerates the diffusion coverage range of the liquid phase particles and enhances the contact uniformity between the coolant and the surface of the drill bit 3; the liquid phase absorbs the instantaneous high temperature through the high specific heat capacity of deionized water, and at the same time avoids oxidation or scaling of the metal surface due to water quality impurities.

[0047] In step S4 of this embodiment, when the impedance value of the monitoring module reaches the preset value, the drill bit 3 retracts the tool within 0.1 ms. The retraction action is triggered by the real-time threshold judgment of the impedance value. The instantaneous response at the 0.1 ms level can effectively avoid problems such as bottom overcutting and burr growth caused by continuous feeding after the drilling penetrates, ensuring that the machining dimensional accuracy meets the micron-level control requirements.

[0048] It should be noted that the length of the via stub 2 after processing is less than or equal to 150 μm. The traditional process can only achieve 200 - 300 μm. Shortening the length of the via stub 2 to less than 150 μm can significantly reduce the parasitic capacitance effect in high-speed signal transmission and reduce the impedance mutation of the signal path.

[0049] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by terms such as "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the present invention.

[0050] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of such features. In the description of the present invention, "a plurality of" means two or more unless otherwise specifically defined.

[0051] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made therein without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A back drilling method for reducing via hole stumps in a circuit board, characterized in that: The following steps are involved: S1: Blind hole pre-processing: laser pre-processing is performed at the end of the target via stump to form a positioning blind hole; S2: back drilling, using a drill bit with an impedance monitoring module to track and drill the positioning blind hole; S3: monitor the dynamic resistance value and collect the electrical impedance value of the impedance monitoring module in real time; S4: retract the drill bit. When the electrical impedance value reaches a preset value, the drilling is stopped immediately and the drill bit is retracted.

2. The back drilling method for reducing via hole stumps in a circuit board according to claim 1, characterized in that: The aperture of the positioning blind hole matches the aperture of the original through hole, or is 1.1-1.3 times the aperture of the original through hole.

3. The back drilling method for reducing via hole stumps in a circuit board according to claim 2, characterized in that: The diameter of the drill bit is greater than or equal to the diameter of the positioning blind hole, and the end of the drill bit has a chamfered structure of 20°-30°.

4. The back drilling method for reducing via hole stumps in a circuit board according to claim 1, characterized in that: The impedance monitoring module is a capacitive sensor.

5. The back drilling method for reducing via hole stumps in a circuit board according to claim 1, characterized in that: The laser pre-processing in step S1 is performed by a laser drilling machine, and the depth of the positioning blind hole is 50%-80% of the length of the original through-hole stump.

6. The back drilling method for reducing via hole stumps in a circuit board according to claim 5, characterized in that: The rotation speed of the laser drilling machine when processing the positioning blind hole is 30-50KRPM, and the feed speed is 5-8mm / s. The rotation speed of the drill bit when performing tracking drilling is 10-30KRPM, and the feed speed is 3-4mm / s.

7. The back drilling method for reducing via hole stumps in a circuit board according to claim 1, characterized in that: In the step S3, infrared thermal imaging data is collected synchronously, and when it is monitored that the temperature change rate of the drilling area exceeds 5°C / ms, the auxiliary cooling system is started to perform physical and chemical cooling on the drill bit.

8. The back drilling method for reducing via hole stumps in a circuit board according to claim 7, characterized in that: The auxiliary cooling system adopts pulsed gas-liquid two-phase cooling, wherein the gas phase is compressed air and the liquid phase is deionized water atomized particles.

9. The back drilling method for reducing via hole stumps in a circuit board according to claim 1, characterized in that: In the step S4, when the electrical impedance value of the monitoring module reaches a preset value, the drill bit is retracted within 0.1 ms.

10. The back drilling method for reducing via hole stumps in a circuit board according to claim 1, characterized in that: The length of the via stump after processing is less than or equal to 150 μm.

Images