A method for back drilling of printed circuit boards

Abstract

This invention discloses a method for back-drilling printed circuit boards (PCBs). The method includes: forming a first metallized via and a second metallized via on the PCB; obtaining the actual board thickness at the location of the first metallized via; controlling a drill bit to drill stepwise from a first surface conductive layer to a first distance from the upper surface of a reference conductive layer at the location of the first metallized via, based on the actual board thickness; controlling the drill bit to drill down to the reference conductive layer at the first distance from the upper surface of the reference conductive layer; and obtaining a continuity or disconnection signal of a continuity detection circuit based on the drill bit; setting a back-drilling depth based on the actual board thickness at the location of the first metallized via; and controlling the drill bit to drill down to the back-drilling depth after obtaining a disconnection signal of the continuity detection circuit to form a back-drilled hole. The technical solution of this invention improves the back-drilling accuracy and the quality of the back-drilled hole.

Description

Technical Field

[0001] This invention relates to the field of printed circuit board manufacturing technology, and in particular to a method for back-drilling printed circuit boards. Background Technology

[0002] With the widespread application of high-speed signal transmission in electronic products, excess copper plating in the metallized vias of printed circuit boards (PCBs) can easily form antenna-like structures, generating signal radiation and interfering with surrounding signals, severely compromising signal transmission integrity. Therefore, back-drilling is widely adopted to remove excess copper plating and mitigate its adverse effects on signal transmission through the vias. Simultaneously, with the continuous increase in PCB thickness and size, the demand for back-drilling of high aspect ratio PCBs is becoming increasingly urgent, and the requirements for high-speed signal transmission are also increasing the precision requirements for the dimensions of the back-drilled studs. Fluctuations in PCB thickness uniformity are becoming more pronounced, and accurately obtaining the back-drilling depth is a prerequisite for achieving precise depth control in back-drilling.

[0003] Currently, the mainstream back-drilling depth control method in the industry mainly utilizes resistive or capacitive sensing systems. The drill bit, printed circuit board, and machine tool table form a capacitive or resistive circuit. When the drill bit contacts the surface aluminum cover plate, the sensing system is triggered to record grating ruler data to obtain height information. Back-drilling is then performed based on a preset drilling depth. However, this method is significantly affected by fluctuations in board thickness uniformity and cannot accurately compensate for board thickness deviations. This results in back-drilling residual pile accuracy failing to meet high standards, leading to insufficient depth control accuracy and poor signal transmission stability. Existing technologies also involve detecting and superimposing the actual thickness of each copper layer and dielectric layer between the back-drilling surface and the signal layer, along with relevant parameters such as electroplated copper thickness and aluminum sheet thickness, and a preset safety distance, to determine the back-drilling depth and perform back-drilling processing. However, this solution is cumbersome to operate and has low detection efficiency, significantly reducing back-drilling processing efficiency and making it difficult to meet the needs of large-scale production of high aspect ratio back-drilling. On the other hand, this solution is still affected by factors such as the uniformity of copper plating thickness and aluminum sheet thickness, and still cannot achieve precise control of back-drilling depth. It still fails to fundamentally solve a series of technical pain points such as insufficient accuracy of back-drilled residual piles, poor signal transmission stability, and poor back-drilled hole processing quality. At the same time, using deep drilling for direct back-drilling processing is prone to process problems such as poor chip removal, cutting heat accumulation, and tool breakage in the processing of high aspect ratio printed circuit boards, making it difficult to guarantee the overall processing quality of back-drilled holes. Summary of the Invention

[0004] This invention provides a back-drilling method for printed circuit boards to improve the control accuracy of back-drilling depth and the quality of back-drilling processing.

[0005] According to one aspect of the present invention, a method for back-drilling a printed circuit board is provided. The method for back-drilling a printed circuit board according to an embodiment of the present invention includes: A first metallized via and a second metallized via are formed on a printed circuit board; wherein, the printed circuit board includes, in sequence along its thickness direction, a first surface conductive layer, a reference conductive layer, a signal layer, and a second surface conductive layer; the first metallized via is conductive to the first surface conductive layer, the reference conductive layer, the signal layer, and the second surface conductive layer, and the second metallized via is conductive to the reference conductive layer but not to the signal layer; the second metallized via is electrically connected to the first metallized via only through the reference conductive layer; Obtain the actual plate thickness at the location of the first metallized through-hole; At the first metallized through-hole location, based on the actual plate thickness, the drill bit is controlled to drill step by step from the first surface conductive layer to a first distance from the upper surface of the reference conductive layer; wherein, the tip of the drill bit is made of conductive material, the inside of the drill bit body is made of conductive material, and the outer surface of the drill bit body is insulated. The drill bit is controlled to drill down to the reference conductive layer at a first distance from the upper surface of the reference conductive layer, and a continuity or disconnection signal of the continuity detection circuit is obtained based on the drill bit; wherein, the continuity detection circuit includes a conductive circuit formed by a first metallized via, the reference conductive layer, and a second metallized via; The back drill depth is set based on the actual plate thickness at the location of the first metallized through hole. Once the disconnection signal of the continuity detection circuit is received, the drill bit is controlled to drill down to the back drilling depth to form a back drill hole.

[0006] Optionally, the reference conductive layer is the layer closest to the signal layer, and the reference conductive layer is a ground layer or power layer with a continuous conductive area.

[0007] Optionally, at the location of the first metallized via, based on the actual board thickness, the drill bit is controlled to drill stepwise from the first surface conductive layer to a first distance from the upper surface of the reference conductive layer, including: Obtain the theoretical plate thickness at the location of the first metallized via, the theoretical thickness from the first surface conductive layer to the reference conductive layer, and the first distance; The error parameter is obtained based on the ratio of the actual plate thickness to the theoretical plate thickness at the location of the first metallized through hole; The initial depth from the first surface conductive layer to the reference conductive layer is determined based on the error parameters and the theoretical thickness from the first surface conductive layer to the reference conductive layer. The first drilling depth is determined based on the initial depth and the first compensation parameter; wherein, the first compensation parameter is calculated based on the angle between the drill bit tip and the horizontal plane; The drilling depth is calculated based on the first drilling depth and the first distance, and the step drilling is completed according to the drilling depth.

[0008] Optionally, based on the actual board thickness at the location of the first metallized through-hole, the back drill depth is set, including: Obtain the theoretical plate thickness at the location of the first metallized via and the theoretical thickness from the reference conductive layer to the signal layer at the location of the first metallized via; Based on the ratio of the actual board thickness to the theoretical board thickness, and the theoretical thickness from the reference conductive layer to the signal layer at the location of the first metallized via, the initial back-drilling depth from the reference conductive layer to the signal layer is determined. By combining the initial back-drilling depth with the second compensation parameter, the corresponding back-drilling depth is obtained; wherein, the second compensation parameter is calculated based on at least one of the angle between the drill bit tip and the horizontal plane and the safety distance.

[0009] Optionally, step-by-step drilling can be performed according to drilling depth, including: At the first metallized through-hole position, when the drill bit contacts the first surface conductive layer during drilling, the conduction signal of the continuity detection circuit is obtained; Based on the conduction signal, the initial drilling height of the drill bit is obtained; Drill in stages from the initial drilling height to the first distance from the upper surface of the reference conductive layer.

[0010] Optionally, drilling in stages based on the initial drilling height to a first distance from the upper surface of the reference conductive layer includes: During the process of drilling step by step from the initial drilling height to the first distance from the upper surface of the reference conductive layer, no on / off signals of the continuity detection circuit are collected.

[0011] Optionally, acquiring the on / off signal of the continuity detection circuit includes: Before the drill bit drills from the first distance on the upper surface of the reference conductive layer to the lower surface of the reference conductive layer, the conduction signal of the continuity detection circuit is obtained; When the drill bit penetrates the reference conductive layer, a disconnection signal is obtained from the continuity detection circuit.

[0012] Optionally, after forming the first and second metallized vias on the printed circuit board, the method further includes: Isolation rings are formed at the upper and lower hole rings of the first metallized through hole to be back-drilled, and at the upper and lower hole rings of the second metallized through hole, respectively, so that the first metallized through hole and the second metallized through hole are disconnected from the first surface conductive layer and the second surface conductive layer, respectively; wherein, the isolation rings are formed by chemical etching, laser cutting or mechanical cutting. After the isolation ring is formed, it also includes: The cold-stamped cover plate, the printed circuit board to be back drilled, and the pad are stacked on the drilling machine worktable from top to bottom. A window is made in the cold-stamped cover plate above the second metallized through hole to expose the second metallized through hole.

[0013] Optionally, after forming the back drill hole, the following steps are also included: According to the circuit network connectivity requirements, the isolation ring at the lower orifice ring of the first metallized through hole is filled by copper plating.

[0014] Optionally, before controlling the drill bit to drill stepwise from the first surface conductive layer to a first distance from the upper surface of the reference conductive layer, the method further includes: Determine whether the back-drilling process of the printed circuit board requires step-by-step drilling; if not, at the first metallized through-hole position, control the drill bit to back-drill from the first surface conductive layer to the reference conductive layer, and obtain the on or off signal of the continuity detection circuit based on the drill bit. The back drill depth is set based on the actual plate thickness at the location of the first metallized through hole. Once the disconnection signal of the continuity detection circuit is received, the drill bit is controlled to drill down to the back drilling depth to form a back drill hole.

[0015] The technical solution of this invention, during back drilling, does not acquire the on / off signal of the continuity detection circuit before the drill bit reaches the reference conductive layer. Simultaneously, a special drill bit with a conductive tip and interior body and an insulating outer surface is used for back drilling. This avoids redundant current signals and signal distortion caused by random metal chip discharge and contact between the drill bit and the conductive layer of the hole wall during step-by-step drilling and retraction. It also prevents the inability to accurately capture the critical signal of the continuity detection circuit breaking when the drill bit penetrates the reference conductive layer due to signal interference, thus providing a reliable signal reference for subsequent back drilling depth control.

[0016] Meanwhile, the technical solution of this invention relies on the optimized back-drilling signal processing logic described above. Within the processing range from the conductive layer on the first surface of the printed circuit board to the first distance from the upper surface of the reference conductive layer, a step-by-step drilling method is adopted. This effectively alleviates the process difficulties such as poor chip removal, heat accumulation, and tool breakage in the back-drilling process of high aspect ratio printed circuit boards, significantly improving the quality of the back-drilled holes. Furthermore, at the moment the drill penetrates the reference conductive layer, the disconnection signal of the continuity detection circuit can be accurately captured. Using this signal as the control basis, the drill is driven to complete the back-drilling process according to the preset drilling depth. Compared to traditional back-drilling methods, the technical solution of this invention uses the reference conductive layer closest to the signal layer as the starting point for back-drilling towards the signal layer. This results in a significantly smaller back-drilling depth than the traditional process that starts from the surface of the printed circuit board. This effectively weakens the processing deviation caused by the overall thickness variation tolerance of the printed circuit board, fundamentally optimizing the depth control logic, achieving high-precision control of the residual length, and greatly improving the back-drilling accuracy.

[0017] It should be understood that the description in this section is not intended to identify key or essential features of the embodiments of the present invention, nor is it intended to limit the scope of the invention. Other features of the invention will become readily apparent from the following description. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 A flowchart of a back-drilling method for printed circuit boards provided in an embodiment of the present invention; Figure 2 A cross-sectional schematic diagram of a printed circuit board provided in an embodiment of the present invention; Figure 3 This is a schematic diagram illustrating a back-drilling process according to a printed circuit board back-drilling method provided in an embodiment of the present invention. Figure 4 A cross-sectional schematic diagram of a drill bit provided in an embodiment of the present invention; Figure 5 This is a schematic diagram illustrating another back-drilling method for printed circuit boards provided in an embodiment of the present invention. Figure 6 This is a schematic diagram illustrating another back-drilling method for printed circuit boards provided in an embodiment of the present invention. Figure 7 A flowchart for controlling a drill bit to drill stepwise from a first surface conductive layer to a first distance from the upper surface of a reference conductive layer, provided in an embodiment of the present invention; Figure 8 A flowchart illustrating how to set the back drill depth based on the actual plate thickness at the location of the first metallized through hole, as provided in this embodiment of the invention. Figure 9 A flowchart of another printed circuit board back-drilling method provided in an embodiment of the present invention; Figure 10 A flowchart of another printed circuit board back-drilling processing method provided in an embodiment of the present invention. Detailed Implementation

[0020] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0021] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0022] Figure 1 This is a flowchart of a back-drilling method for printed circuit boards provided in an embodiment of the present invention. Figure 2 This is a cross-sectional schematic diagram of a printed circuit board provided in an embodiment of the present invention. Figure 3 This is a schematic diagram illustrating a back-drilling process according to a printed circuit board back-drilling method provided in an embodiment of the present invention. Figure 4 This is a cross-sectional schematic diagram of a drill bit provided in an embodiment of the present invention. The embodiments of the present invention can... Figure 2 or Figure 3 The printed circuit board shown uses Figure 4 The drill bit shown performs back drilling according to the printed circuit board back drilling method. This embodiment of the invention is applicable to scenarios where back drilling is performed on printed circuit boards, and can be implemented by software and / or hardware. For example... Figure 1 As shown, the printed circuit board back-drilling processing method of this invention includes: S101. Form a first metallized through-hole and a second metallized through-hole on the printed circuit board.

[0023] refer to Figure 2The printed circuit board includes, along its thickness direction, a first surface conductive layer 3, a reference conductive layer 4, a signal layer 5, and a second surface conductive layer 6 in sequence. The first metallized via 1 is conductive to the first surface conductive layer 3, the reference conductive layer 4, the signal layer 5, and the second surface conductive layer 6. The second metallized via 2 is conductive to the reference conductive layer 4 but not to the signal layer 5. The second metallized via 2 is electrically connected to the first metallized via 1 only through the reference conductive layer 4.

[0024] In this embodiment of the invention, the first surface conductive layer 3 is an outer conductive layer disposed on the back drilling surface. Typically, during back drilling, the drill bit starts drilling down on the back drilling surface to remove the copper foil of the first surface conductive layer 3 at the location of the first metallized through hole 1, forming the back drilling entrance.

[0025] The reference conductive layer 4 is an inner conductive copper foil layer located between the first surface conductive layer 3 and the signal layer 5. The reference conductive layer 4 is a layer that must be drilled through. A ground layer or power layer with a continuous conductive area and already physically connected to the first metallized via 1 and the second metallized via 2 can be used as the reference conductive layer 4. Alternatively, during the fabrication of the printed circuit board, a ground layer, power layer, or other signal layer that was not originally connected to the first metallized via 1 and the second metallized via 2 can be physically connected to the first metallized via 1 and the second metallized via 2 to serve as the reference conductive layer 4, thereby achieving a conductive connection between the first metallized via 1 and the second metallized via 2 through the reference conductive layer 4.

[0026] Signal layer 5 is the conductive layer on the printed circuit board that actually carries high-speed signal traces and can achieve interlayer conductive connections through vias. In the back-drilling process of the printed circuit board, the purpose is to remove the copper pillars between the first surface conductive layer 3 and the signal layer 5. Signal layer 5 is a non-drill-through layer. Retaining signal layer 5, as well as the copper pillars between signal layer 5 and the second surface conductive layer 6, reduces high-speed signal reflection and improves signal integrity.

[0027] The second surface conductive layer 6 is an outer conductive layer that is not back-drilled. The copper foil on the second surface conductive layer 6 can be designed based on the outer layer pattern and cooperate with the signal layer 5 to design signal routing and transmission performance. It can provide a return current and impedance control environment for the signal layer 5 by arranging chips, high-speed interfaces, key traces, etc.

[0028] The inner walls of both the first metallized via 1 and the second metallized via 2 are coated with copper, allowing the first metallized via 1 to be electrically connected to the first surface conductive layer 3, the reference conductive layer 4, the signal layer 5, and the second surface conductive layer 6. Similarly, the second metallized via 2 can be electrically connected to the first surface conductive layer 3, the reference conductive layer 4, and the second surface conductive layer 6. The first surface conductive layer 3 is a discontinuous conductive layer, and the second surface conductive layer 6 is also a discontinuous conductive layer. This means that although both the first metallized via 1 and the second metallized via 2 are electrically connected to the first surface conductive layer 3 and the second surface conductive layer 6, there is no conductive connection between the first metallized via 1 and the second metallized via 2 through the first surface conductive layer 3 or the second surface conductive layer 6.

[0029] S102. Obtain the actual plate thickness at the location of the first metallized through hole.

[0030] refer to Figure 2 The actual thickness of the printed circuit board (PCB) at the back-drilling location can be obtained through contact measurement methods, such as using a thickness gauge; it can also be obtained through non-contact measurement methods, such as using a laser thickness gauge, optical interferometric thickness gauge, or capacitance / eddy current thickness gauge; or it can be obtained indirectly through a drilling rig, such as using a drilling rig table reference method. This invention does not impose specific limitations on the method used to obtain the actual thickness of the PCB at the back-drilling location.

[0031] S103. At the first metallized through-hole location, based on the actual plate thickness, control the drill bit to drill stepwise from the first surface conductive layer to a first distance from the upper surface of the reference conductive layer.

[0032] refer to Figure 3 and Figure 4 Based on the thickness-to-diameter ratio of the first metallized through-hole 1 to be back-drilled, the material characteristics of the printed circuit board, and the processing accuracy and quality requirements, a step-by-step drilling method can be adopted. Step-by-step drilling can avoid process problems such as poor chip removal, heat accumulation, and tool breakage during back drilling, significantly improving the hole wall processing quality. Step-by-step drilling refers to drilling in at least two segments based on the total drilling depth. The drilling depth of each segment can be equal or unequal, and this invention does not impose specific limitations. For example, when the first metallized through-hole 1 has a small diameter and a relatively deep total drilling depth, step-by-step drilling can be used. At least two segments are set for the drilling stroke corresponding to the total drilling depth of the first metallized through-hole 1. After drilling each segment, the drill bit 7 first retracts to remove the drill chips in the first metallized through-hole 1, and then the drill bit 7 moves down to drill the next segment, repeating this process until the drilling is completed.

[0033] In some embodiments, the tip 71 of the drill bit 7 is made of a conductive material, the interior 73 of the drill bit 7 is made of a conductive material, and the outer surface 72 of the drill bit 7 is insulated. The diameter of the drill bit 7 is larger than the diameter of the first metallized through-hole 1, allowing the drill bit 7 to drill away the copper pillars on the inner wall of the first metallized through-hole 1. A conductive connection is maintained between the interior 73 of the drill bit 7 and the tip 71. The outer surface 72 of the drill bit 7 can be insulated by coating with an insulating material or depositing an insulating film; this embodiment of the invention does not impose specific limitations on this.

[0034] In this context, the upper surface of the reference conductive layer refers to the side of the reference conductive layer 4 closest to the back drill face. The first distance 'a' refers to the distance range from which the drilling rig begins to acquire the on / off signal of the continuity detection circuit. This is to accurately capture the critical signal of the continuity detection circuit being disconnected when the drill bit 7 penetrates the reference conductive layer 4 after reaching the first distance 'a' range. Simultaneously, the first distance 'a' is a safe distance to avoid directly penetrating the reference conductive layer 4 during step-by-step drilling, ensuring a reliable initial position reference for subsequent back drill depth control. The first distance 'a' can be set based on the minimum distance required for the maximum step-by-step drilling depth without penetrating the reference conductive layer 4, or it can be set according to actual conditions.

[0035] It is understood that the continuity detection circuit includes a conductive circuit formed by the first metallized through-hole 1, the reference conductive layer 4, and the second metallized through-hole 2. When the drill bit 7 drills at the position of the first metallized through-hole 1, the tip 71 contacts the inner wall of the first metallized through-hole 1. Before drilling through the reference conductive layer 4, the drill bit 7, the first metallized through-hole 1, the reference conductive layer 4, and the second metallized through-hole 2 sequentially form a conductive connection, constituting a conductive circuit. After drilling through the reference conductive layer 4, due to the insulation of the outer surface 72 of the drill body, the first metallized through-hole 1 and the second metallized through-hole 2 can no longer maintain a conductive connection through the reference conductive layer 4, and the conductive circuit is broken. For example, one end of the continuity detection circuit can be connected to the drill rig conductive contact 8 as an electrode, and the drill bit 7 can be connected to the other end of the continuity detection circuit as another electrode. For example, the drill rig conductive contact 8 can be a drill rig mushroom head, with the drill rig conductive contact 8 as the positive electrode and conductively connected to the second metallized through-hole 2; the drill bit 7 is the negative electrode and is mounted on the drill rig. Before drilling through the reference conductive layer 4, the drill rig conductive contact 8, the first metallized through hole 1, and the second metallized through hole 2 form a conductive circuit. The drill rig can collect the conduction signal of the continuity detection circuit. When drilling through the reference conductive layer 4, the conductive circuit formed by the drill rig conductive contact 8, the first metallized through hole 1, and the second metallized through hole 2 is disconnected. The drill rig can collect the disconnection signal of the continuity detection circuit.

[0036] For example, the total drilling depth using step drilling can be the difference between the thickness of the first surface conductive layer 3 to the reference conductive layer 4 and the first distance a.

[0037] For example, when back drilling is performed using a step-by-step drilling method, the drill bit 7 needs to drill down and back up multiple times before drilling through the reference conductive layer 4. During the drilling process, due to the random discharge of metal chips, redundant current signals are generated when the metal chips come into contact with the drill bit 7. At the same time, if the outer surface of the drill bit 7 is made of conductive material, redundant current signals are generated when the outer surface of the drill bit 7 comes into contact with the first surface conductive layer 3 and other conductive layers in the hole wall during the multiple drilling down and back-up processes, causing signal interference. Therefore, in order to prevent the failure to accurately capture the critical signal of the disconnection signal of the continuity detection circuit when the drill bit 7 drills through the reference conductive layer 4 due to signal interference, and to provide a reliable signal reference for subsequent back drilling depth control, and at the same time, to avoid directly drilling through the reference conductive layer 4 in step-by-step drilling, which would prevent the provision of a reliable initial position for subsequent back drilling depth control, a method is needed. Optionally, a first distance a can be set on the upper surface of the reference conductive layer 4, and step drilling can be performed on the portion (i.e., La) outside the first distance a of the thickness of the first surface conductive layer 3 to the reference conductive layer 4, and the on or off signal of the continuity detection circuit is not collected within the La distance.

[0038] S104. Control the drill bit to drill down to the reference conductive layer at a first distance from the upper surface of the reference conductive layer, and obtain the on or off signal of the on / off detection circuit based on the drill bit.

[0039] refer to Figure 3 and Figure 4 The continuity detection circuit includes a conductive circuit formed by the first metallized via 1, the reference conductive layer 4, and the second metallized via 2. Specifically, when the drill bit 7 is controlled to drill down using a step-by-step drilling method, it needs to drill down and retract multiple times before reaching the first distance 'a' of the upper surface of the reference conductive layer 4. When the drill bit 7 is drilling down, the continuity detection circuit is in a conducting state; when it retracts, it is in a disconnected state. Therefore, to avoid generating redundant current signals that would prevent the capture of the crucial signal indicating the disconnection of the continuity detection circuit when the drill bit 7 penetrates the reference conductive layer 4, the continuity detection circuit's conduction or disconnection signal is not acquired before reaching the first distance 'a' (i.e., 'La') of the upper surface of the reference conductive layer 4. Once the drill bit reaches the first distance 'a' from the upper surface of the reference conductive layer 4, the continuity detection circuit's conduction or disconnection signal is recorded and acquired. Furthermore, step-by-step drilling is not used within the first distance 'a', and the first distance 'a' can be set to a relatively short distance. By continuously recording and acquiring the on / off signal of the continuity detection circuit within a first distance a on the upper surface of the reference conductive layer 4, the key signal of the disconnection signal of the continuity detection circuit when the drill bit 7 drills through the reference conductive layer 4 can be accurately captured, providing a reliable signal reference for subsequent back-drilling depth control.

[0040] In some embodiments, acquiring the on / off signal of the continuity detection circuit includes: Before the drill bit drills from the first distance on the upper surface of the reference conductive layer to the lower surface of the reference conductive layer, the conduction signal of the continuity detection circuit is obtained; When the drill bit penetrates the reference conductive layer, a disconnection signal is obtained from the continuity detection circuit.

[0041] Before drilling through the reference conductive layer, the drill bit, the first metallized through-hole, the reference conductive layer, the second metallized through-hole, and the drill rig's conductive contact maintain a conductive connection, forming a conductive circuit, allowing the acquisition of a continuity detection signal. When drilling through the reference conductive layer, the drill bit tip does not contact the reference conductive layer, but the drill body does. Due to the insulation of the drill body's outer surface, the first and second metallized through-holes cannot continue to maintain a conductive connection through the reference conductive layer. At the instant the reference conductive layer is drilled through, the conductive circuit breaks, allowing the acquisition of a continuity detection signal.

[0042] S105. Based on the actual plate thickness at the location of the first metallized through hole, set the back drill depth.

[0043] In this process, the tip of the drill bit forms an angle with the sides of the first metallized through hole. The junction of the two angles forms an ineffective cutting line, resulting in ineffective drilling depth. Therefore, with the same tool diameter, a drill bit with a larger tip angle can be selected for machining to reduce ineffective drilling depth.

[0044] The lamination process of printed circuit boards (PCBs) is affected by multiple factors, including differences in residual copper content between layers, differences in resin flow properties, and fluctuations in lamination conditions, which can easily lead to uneven board thickness after lamination. Not only is there a deviation between the theoretical and actual thickness of the PCB, but the thickness also varies at different locations on the same PCB, resulting in a certain thickness tolerance for PCBs.

[0045] Figure 5 This is a schematic diagram illustrating another back-drilling process according to the printed circuit board back-drilling method provided in an embodiment of the present invention, with reference to... Figure 5If the back-drilling depth is directly set according to the theoretical thickness H (i.e., the theoretical back-drilling depth) of the reference conductive layer 4 to the signal layer 5, poor back-drilling depth adaptability is likely to occur. If the plate thickness is too thick, the back-drilling depth will be insufficient, resulting in excessively long residual piles, which in turn will cause signal reflection, impedance shift, and abnormal high-frequency signal transmission. If the plate thickness is too small, the back-drilling will be too deep, which can easily damage or even penetrate the signal layer, causing product scrap. Therefore, by obtaining the actual plate thickness T at the first metallized through hole 1, and then dividing the actual plate thickness T by the theoretical plate thickness to obtain the relative error, the initial back-drilling depth is obtained by multiplying the relative error by the theoretical thickness H of the reference conductive layer 4 to the signal layer 5. The back-drilling depth is then compensated for to obtain the back-drilling depth. Back-drilling processing based on the back-drilling depth can improve the accuracy of back-drilling.

[0046] S106. After receiving the disconnection signal of the continuity detection circuit, control the drill bit to drill down to the back drilling depth to form a back drill hole.

[0047] refer to Figure 5 Specifically, when the drill bit 7 penetrates the reference conductive layer 4, that is, at the instant the drill bit 7 penetrates the reference conductive layer 4, a disconnection signal of the continuity detection circuit can be obtained, and the initial position of the back drilling depth control can be accurately obtained. Based on this, drilling continues according to the back drilling depth to complete the back drilling, thereby improving the accuracy of the back drilling.

[0048] Figure 6 This is a schematic diagram illustrating another back-drilling method for printed circuit boards provided in this embodiment of the invention. Drilling continues to proceed to the specified depth. Figure 6 Complete the back drilling at the indicated position.

[0049] The technical solution of this invention, during back drilling, does not acquire the on / off signal of the continuity detection circuit before the drill bit reaches the reference conductive layer. Simultaneously, a special drill bit with a conductive tip and internal body and an insulating outer surface is used for back drilling. This avoids redundant current signals and signal distortion caused by random metal chip discharge and contact between the drill bit and the conductive layer of the hole wall during step-by-step drilling and retraction. It also prevents the inability to accurately capture the critical signal of the continuity detection circuit breaking when the drill bit penetrates the reference conductive layer due to signal interference, providing a reliable signal reference for subsequent back drilling depth control. Furthermore, based on the optimized back drilling signal processing logic, this invention employs a step-by-step drilling method within the processing range from the first conductive layer of the printed circuit board to the first distance from the upper surface of the reference conductive layer. This effectively alleviates process difficulties such as poor chip removal, heat accumulation, and tool breakage in the back drilling process of high aspect ratio printed circuit boards, significantly improving the quality of back drilled holes. Furthermore, the moment the drill bit penetrates the reference conductive layer, the disconnection signal of the continuity detection circuit can be accurately captured. Using this continuity signal as the control basis, the drill bit is driven to complete the back-drilling process according to the preset back-drilling depth. Compared to traditional back-drilling methods, the technical solution of this invention uses the reference conductive layer closest to the signal layer as the back-drilling starting point to perform back-drilling towards the signal layer. This results in a significantly smaller back-drilling depth than the traditional process that starts from the surface of the printed circuit board. This effectively mitigates the processing deviation caused by the overall thickness variation tolerance of the printed circuit board, fundamentally optimizes the depth control logic, achieves high-precision control of the residual pile length, and greatly improves the back-drilling accuracy.

[0050] Optionally, the reference conductive layer is the layer closest to the signal layer, and the reference conductive layer is a ground layer or power layer with a continuous conductive area.

[0051] The core objective of back-drilling is to minimize the length of excess copper in the first metallized via beyond the signal layer (i.e., the length of the residual hole). The shorter this residual length, the less interference it causes to the signal transmission performance of the printed circuit board (PCB). Back-drilling accuracy is directly related to board thickness tolerance, back-drilling depth control deviation, and the machining accuracy of the back-drilling equipment itself. The deviation between the theoretical and actual board thickness of the PCB is called board thickness tolerance. Given a fixed inherent machining tolerance of the back-drilling equipment, reducing the PCB board thickness tolerance is an effective way to improve back-drilling accuracy. Furthermore, without changing the overall PCB manufacturing process, shortening the back-drilling depth can significantly improve overall back-drilling accuracy. In other words, the smaller the distance between the back-drilling starting position and the signal layer, the lower the impact of board thickness tolerance on back-drilling depth control, and the higher the final back-drilling accuracy. Therefore, using the conductive layer closest to the signal layer as the reference conductive layer, and the reference conductive layer as the back-drilling starting position, can effectively shorten the distance between the back-drilling starting position and the signal layer, improving back-drilling accuracy. Preferably, the reference conductive layer is a ground layer or power layer with a continuous conductive area. This reduces the need for additional steps to physically connect the reference conductive layer with the first metallized via and the second metallized via. The ground layer or power layer that is already physically connected to the first metallized via can be used directly as the reference conductive layer without affecting the design function of the printed circuit board.

[0052] Figure 7 A flowchart illustrating how to control a drill bit to drill stepwise from a first surface conductive layer to a first distance from the upper surface of a reference conductive layer, as provided in this embodiment of the invention, is shown below. Figure 7 As shown, in some embodiments, S103, at the location of the first metallized through-hole, based on the actual board thickness, controls the drill bit to drill stepwise from the first surface conductive layer to a first distance from the upper surface of the reference conductive layer, including: A1. Obtain the theoretical plate thickness at the location of the first metallized via, the theoretical thickness from the first surface conductive layer to the reference conductive layer, and the first distance.

[0053] The theoretical board thickness at the location of the first metallized via and the theoretical thickness from the first surface conductive layer to the reference conductive layer can be obtained from the printed circuit board design drawings. The first distance can be set according to actual conditions; for example, the first distance can be 0.2 mm.

[0054] A2. Obtain the error parameter based on the ratio of the actual plate thickness to the theoretical plate thickness at the location of the first metallized through hole.

[0055] In the process of back-drilling printed circuit boards (PCBs), after lamination and copper plating, there may be a difference between the actual and theoretical thickness of the PCB at the location of the first metallized via to be back-drilled. To improve back-drilling accuracy, error compensation can be performed by assessing the degree of difference between the actual and theoretical thickness at the location of the first metallized via. For example, an error parameter can more intuitively reflect the deviation of the actual thickness T at the location of the first metallized via from the theoretical thickness T1. For instance, the error parameter k is: .

[0056] A3. Determine the initial depth from the first surface conductive layer to the reference conductive layer based on the error parameters and the theoretical thickness from the first surface conductive layer to the reference conductive layer.

[0057] The error parameter is a deviation ratio, which more intuitively reflects the degree of deviation between the actual plate thickness T and the theoretical plate thickness T1, and more objectively and accurately reflects the range of plate thickness variation, making drilling depth compensation more scientific. For example, the actual thickness L is obtained by multiplying the theoretical thickness L1 from the first surface conductive layer to the reference conductive layer by the error parameter k. For instance, the actual thickness L from the first surface conductive layer to the reference conductive layer is: The initial depth is the actual thickness of the first surface conductive layer to the reference conductive layer.

[0058] A4. Determine the first drilling depth based on the initial depth and the first compensation parameter.

[0059] The first compensation parameter is calculated based on the angle between the drill bit tip and the horizontal plane.

[0060] In this process, the two cutting edges of the drill bit tip intersect to form an angle, and the intersection of these angles creates an ineffective cutting area, resulting in ineffective drilling depth. The first compensation parameter can compensate for the deviation in drilling depth caused by the drill bit tip angle. The first drilling depth can be the distance from the first surface conductive layer to the reference conductive layer after compensation based on the initial depth. By compensating for the initial depth of the step-by-step drilling, a more accurate first drilling depth can be obtained.

[0061] A5. Calculate the drilling depth based on the first drilling depth and the first distance, and complete the step-by-step drilling according to the drilling depth.

[0062] The total drilling depth can be calculated based on the difference between the first drilling depth and the first distance. A more accurate first drilling depth allows for a more precise total drilling depth. By ensuring accurate acquisition of the critical signal indicating the disconnection of the continuity detection circuit when drilling through the reference conductive layer, setting the total drilling depth stroke for step-by-step drilling can avoid process problems such as poor chip removal, heat buildup, and tool breakage during drilling, thus improving the hole wall machining quality.

[0063] In some embodiments, step drilling is performed according to drilling depth, including: At the first metallized through-hole position, when the drill bit drills down and contacts the first surface conductive layer, the continuity signal of the continuity detection circuit is obtained.

[0064] The drill bit's tip is made of a conductive material. During the drill bit's downward feed, when the tip just touches the first surface conductive layer, the continuity detection circuit is electrically connected and generates a current signal. At this time, the drilling rig can obtain the continuity signal of the continuity detection circuit.

[0065] The initial drilling height of the drill bit is obtained based on the conduction signal.

[0066] The drilling rig identifies and determines the position where the cutting tip contacts the first surface conductive layer as the initial position for drilling. The initial drilling height is determined by the initial drilling position and used as the reference starting point for the subsequent step drilling depth.

[0067] Drill in stages from the initial drilling height to the first distance from the upper surface of the reference conductive layer.

[0068] After determining the initial drilling position, depth control machining is performed based on the principle of depth control. The initial drilling height is used as a reference to control the drill bit to drill according to the drilling depth. The drill spindle uses a servo motor and a grating ruler to provide feedback on the drill bit position, which can accurately control the effective drilling depth and accurately grasp the drilling endpoint.

[0069] Optionally, drilling in stages based on the initial drilling height to a first distance from the upper surface of the reference conductive layer includes: During the process of drilling step by step from the initial drilling height to the first distance from the upper surface of the reference conductive layer, no on / off signals of the continuity detection circuit are collected.

[0070] Specifically, only the conduction signal when the drill bit first contacts the first surface conductive layer is collected to determine the initial drilling position. After that, the entire drilling depth is controlled through step-by-step drilling, and the drilling endpoint is accurately determined. At the same time, redundant current signals during multiple drilling and retraction processes in the step-by-step drilling process are avoided from interfering with the determination of the reference conductive layer as the starting point of back drilling.

[0071] Figure 8 A flowchart illustrating how to set the back drill depth based on the actual plate thickness at the location of the first metallized through-hole, as provided in this embodiment of the invention, is shown below. Figure 8 As shown, S105, based on the actual plate thickness at the location of the first metallized through hole, sets the back drill depth, including: B1. Obtain the theoretical plate thickness at the location of the first metallized via and the theoretical thickness from the reference conductive layer to the signal layer at the location of the first metallized via.

[0072] The theoretical board thickness at the location of the first metallized via and the theoretical thickness from the reference conductive layer to the signal layer at the location of the first metallized via can both be obtained based on the printed circuit board design drawings.

[0073] B2. Based on the ratio of the actual plate thickness to the theoretical plate thickness, and the theoretical thickness of the reference conductive layer to the signal layer at the location of the first metallized via, determine the initial back-drilling depth from the reference conductive layer to the signal layer.

[0074] The printing circuit board lamination process is affected by multiple factors, including differences in residual copper content between layers, differences in resin flow properties, and fluctuations in lamination conditions, which can easily lead to uneven board thickness after lamination. Not only is there a deviation between the theoretical and actual board thickness, but the thickness also varies at different locations on the same printed circuit board, resulting in a certain thickness tolerance for printed circuit boards.

[0075] The ratio of the actual plate thickness T to the theoretical plate thickness T1 at the location of the first metallized via more intuitively reflects the change in plate thickness after lamination, and the degree of difference between the actual and theoretical plate thickness. This ratio is used to calculate the actual thickness of the lamination from the reference conductive layer to the signal layer, making the drilling depth compensation more scientific. For example, an error parameter is obtained based on the ratio of the actual to the theoretical plate thickness at the location of the first metallized via. The theoretical thickness from the reference conductive layer to the signal layer is then multiplied by the error parameter to obtain the actual thickness. The actual thickness from the reference conductive layer to the signal layer is the initial back-drilling depth.

[0076] B3. Combine the initial back drill depth with the second compensation parameter to obtain the corresponding back drill depth.

[0077] The second compensation parameter is calculated based on at least one of the angle between the drill bit tip and the horizontal plane and the safety distance.

[0078] The safety distance refers to the length of the copper post remaining at the signal layer during back-drilling, which requires a safety margin. This effectively avoids damage to the signal layer circuitry due to excessive back-drilling depth, preventing quality defects such as open circuits in the inner signal layer. This safety distance can be determined comprehensively based on the back-drilling machine's own machining accuracy tolerance and the copper layer loss caused by subsequent processes. Additionally, the intersection of the two cutting edges of the drill bit creates an angle, and the resulting ineffective cutting area at the intersection will result in ineffective drilling depth. Compensating for the initial drilling depth based on the drill bit tip's angle to the horizontal plane can effectively improve the accuracy of post remaining dimensions. Simultaneously, comprehensive compensation of the initial drilling depth using the safety distance ensures sufficient copper post allowance while maintaining back-drilling machining accuracy, preventing damage to the inner signal layer circuitry. Compensating the initial drilling depth using a second compensation parameter yields a more precise back-drilling depth, thereby improving the accuracy of the back-drilled post remaining.

[0079] The initial drilling depth is compensated using the second compensation parameter to obtain the back drill depth, and drilling continues at the back drill depth until... Figure 6 At the position shown, complete the back drilling.

[0080] Figure 9 A flowchart of another printed circuit board back-drilling method provided by an embodiment of the present invention is shown below. Figure 9 As shown, the steps of the back-drilling process for printed circuit boards are as follows: S201. Form a first metallized through-hole and a second metallized through-hole on the printed circuit board.

[0081] S202. Isolation rings are formed at the upper and lower opening rings of the first metallized through hole to be back drilled, and at the upper and lower opening rings of the second metallized through hole, respectively, so that the first metallized through hole and the second metallized through hole are disconnected from the first surface conductive layer and the second surface conductive layer, respectively.

[0082] like Figure 2 , Figure 3 or Figure 5 As shown, the discontinuous positions of the first surface conductive layer 3 and the second surface conductive layer 6 around the first metallized via 1 and the second metallized via 2 represent isolation rings. These isolation rings can be formed by chemical etching, laser cutting, or mechanical cutting. By setting the isolation rings, it is ensured that the first metallized via 1 and the second metallized via 2 are electrically connected only through the reference conductive layer 4. Furthermore, when there are at least two first metallized vias 1 on the printed circuit board, the isolation rings prevent different first metallized vias 1 from conducting to each other, ensuring that different first metallized vias 1 form a conductive loop only with the second metallized via 2. This allows different first metallized vias 1 at the same drilling depth to be back-drilled through the same second metallized via 2, reducing the number of second metallized vias 2, lowering design complexity, and enabling independent depth control and precision control for each first metallized via 1 to be back-drilled, significantly improving back-drilling efficiency.

[0083] S203. Place the cold-stamped cover plate, the printed circuit board to be back drilled, and the pad on the drilling machine worktable from top to bottom.

[0084] The cold-stamped cover plate can be a stamped material for electronic and electrical insulation, while the backing plate provides stable support for the printed circuit board to be back-drilled. Placing the cold-stamped cover plate, the printed circuit board to be back-drilled, and the backing plate on the drilling machine from top to bottom can help stabilize the drilling, prevent scratches on the material, reduce burrs and off-center holes, improve the quality of the drilled hole opening, and enhance the drilling quality.

[0085] Understandably, existing back-drilling technologies typically place an aluminum sheet above the back-drilling surface of the printed circuit board. Besides improving drilling quality, the aluminum sheet also acts as a conductive medium, contacting the drill's conductive contacts for back-drilling depth detection and depth control. Compared to existing technologies, the technical solution of this invention uses the reference conductive layer closest to the signal layer as the back-drilling starting point to perform back-drilling towards the signal layer. This eliminates the need to detect the thickness from the first surface conductive layer to the signal layer—that is, the thickness of the entire drilled layer—effectively eliminating the impact of aluminum sheet thickness deviation on back-drilling accuracy. Simultaneously, it reduces the use of high-cost auxiliary materials, saving production costs.

[0086] S204. A window is made in the cold stamping cover plate above the position corresponding to the second metallized through hole to expose the second metallized through hole.

[0087] For example, by opening a window in the cold-stamped cover plate, the second metallized through hole under the cold-stamped cover plate is exposed, thereby enabling the conductive contact to be electrically connected to the second metallized through hole.

[0088] S205. Obtain the actual plate thickness at the location of the first metallized through hole.

[0089] S206. At the first metallized through-hole location, based on the actual plate thickness, control the drill bit to drill stepwise from the first surface conductive layer to a first distance from the upper surface of the reference conductive layer.

[0090] S207. Control the drill bit to drill down to the reference conductive layer at a first distance from the upper surface of the reference conductive layer, and obtain the on or off signal of the continuity detection circuit based on the drill bit.

[0091] S208. Based on the actual plate thickness at the location of the first metallized through hole, set the back drill depth.

[0092] S209. When the disconnection signal of the continuity detection circuit is obtained, the drill bit is controlled to drill down to the back drilling depth to form a back drill hole.

[0093] S210. According to the circuit network connectivity requirements, the isolation ring at the lower hole ring of the first metallized through hole is filled by copper plating.

[0094] The lower orifice ring of the first metallized via is located in the second surface conductive layer. The circuit network connectivity requirement can be the electrical connectivity requirement between the signal layer and the second surface conductive layer, or between the signal layer and other inner signal layers through the second surface conductive layer.

[0095] If the printed circuit board requires the circuit of the signal layer to be electrically connected to the second reference conductive layer or the signal layer to other inner signal layers through the second surface conductive layer via the first metallized via, after back drilling is completed, the isolation ring at the lower hole ring of the first metallized via can be subjected to copper plating and electroplating. By filling with copper, the surface of the hole ring after filling is flush with the second surface conductive layer, and the circuit pattern is completed based on the graphic design and loop network connection requirements without affecting the original design of the printed circuit board.

[0096] Figure 10 A flowchart of another printed circuit board back-drilling processing method provided by an embodiment of the present invention is shown below. Figure 10 As shown, the steps of the back-drilling process for printed circuit boards are as follows: S301. A first metallized through-hole and a second metallized through-hole are formed on the printed circuit board.

[0097] S302. Isolation rings are formed at the upper and lower opening rings of the first metallized through hole to be back drilled, and at the upper and lower opening rings of the second metallized through hole, so that the first metallized through hole and the second metallized through hole are disconnected from the first surface conductive layer and the second surface conductive layer, respectively.

[0098] The isolation ring is formed by chemical etching, laser cutting, or mechanical cutting.

[0099] S303. Place the cold stamping cover plate, the printed circuit board to be back drilled, and the pad on the drilling machine worktable from top to bottom.

[0100] S304. A window is made in the cold-stamped cover plate above the second metallized through hole to expose the second metallized through hole.

[0101] S305. Determine if it is a step-by-step drilling process; if not, proceed to S306; if yes, proceed to S307.

[0102] S306, at the first metallized through-hole position, control the drill bit to back-drill from the first surface conductive layer to the reference conductive layer, and obtain the on or off signal of the continuity detection circuit based on the drill bit.

[0103] When back drilling is not performed in stages, the drill bit does not need to make multiple cuts and retractions before penetrating the reference conductive layer when back drilling at the first metallized through-hole position. Therefore, during back drilling, it is not necessary to determine the first distance and control the drill bit to drill down into the first surface conductive layer until penetrating the reference conductive layer. The continuity or disconnection signal of the continuity detection circuit is continuously recorded and acquired during the back drilling process, which facilitates the accurate capture of the critical signal of the continuity detection circuit disconnection when the drill bit penetrates the reference conductive layer.

[0104] S307. Obtain the actual plate thickness at the location of the first metallized through hole.

[0105] S308. At the first metallized through-hole location, based on the actual plate thickness, control the drill bit to drill stepwise from the first surface conductive layer to a first distance from the upper surface of the reference conductive layer.

[0106] S309, Control the drill bit to drill down to the reference conductive layer at a first distance from the upper surface of the reference conductive layer, and obtain the on or off signal of the continuity detection circuit based on the drill bit.

[0107] S310. Based on the actual plate thickness at the location of the first metallized through hole, set the back drill depth.

[0108] S311. When the disconnection signal of the continuity detection circuit is obtained, the drill bit is controlled to drill down to the back drilling depth to form a back drill hole.

[0109] S312. According to the circuit network connectivity requirements, the isolation ring at the lower hole ring of the first metallized through hole is filled by copper plating.

[0110] It should be understood that the various forms of processes shown above can be used, with steps reordered, added, or deleted. For example, the steps described in this invention can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution of this invention can be achieved, and this is not limited herein.

[0111] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A method for back-drilling a printed circuit board, characterized in that, include: A first metallized via and a second metallized via are formed on a printed circuit board; wherein, the printed circuit board includes, along its thickness direction, a first surface conductive layer, a reference conductive layer, a signal layer, and a second surface conductive layer in sequence; the first metallized via is conductive to the first surface conductive layer, the reference conductive layer, the signal layer, and the second surface conductive layer; the second metallized via is conductive to the reference conductive layer but not to the signal layer; the second metallized via is electrically connected to the first metallized via only through the reference conductive layer; Obtain the actual plate thickness at the location of the first metallized through-hole; At the first metallized through-hole location, based on the actual plate thickness, the drill bit is controlled to drill stepwise from the first surface conductive layer to a first distance from the upper surface of the reference conductive layer; wherein, the tip of the drill bit is made of conductive material, the inside of the drill bit body is made of conductive material, and the outer surface of the drill bit body is insulated; The drill bit is controlled to drill down to the reference conductive layer at a first distance from the upper surface of the reference conductive layer, and a continuity or disconnection signal of the continuity detection circuit is obtained based on the drill bit; wherein, the continuity detection circuit includes a conductive circuit formed by the first metallized via, the reference conductive layer and the second metallized via; The back drill depth is set based on the actual plate thickness at the location of the first metallized through hole. Upon receiving the disconnection signal of the on / off detection circuit, the drill bit is controlled to drill down to the back drill depth to form a back drill hole.

2. The printed circuit board back-drilling method according to claim 1, characterized in that, The reference conductive layer is the layer closest to the signal layer, and the reference conductive layer is a ground layer or a power layer with a continuous conductive region.

3. The printed circuit board back-drilling method according to claim 1, characterized in that, The step of controlling the drill bit to drill stepwise from the first surface conductive layer to a first distance from the upper surface of the reference conductive layer at the location of the first metallized through hole, based on the actual plate thickness, includes: Obtain the theoretical plate thickness at the location of the first metallized via, the theoretical thickness from the first surface conductive layer to the reference conductive layer, and the first distance; The error parameter is obtained based on the ratio of the actual plate thickness at the location of the first metallized through hole to the theoretical plate thickness; The initial depth from the first surface conductive layer to the reference conductive layer is determined based on the error parameters and the theoretical thickness from the first surface conductive layer to the reference conductive layer; The first drilling depth is determined based on the initial depth and the first compensation parameter; wherein the first compensation parameter is calculated based on the angle between the drill bit tip and the horizontal plane; The drilling depth is calculated based on the first drilling depth and the first distance, and the step drilling is completed according to the drilling depth.

4. The printed circuit board back-drilling method according to claim 1, characterized in that, The step of setting the back drill depth based on the actual plate thickness at the location of the first metallized through hole includes: Obtain the theoretical plate thickness at the location of the first metallized via and the theoretical thickness from the reference conductive layer to the signal layer at the location of the first metallized via; Based on the ratio of the actual plate thickness to the theoretical plate thickness, and the theoretical thickness from the reference conductive layer to the signal layer at the location of the first metallized via, the initial back-drilling depth from the reference conductive layer to the signal layer is determined. By combining the initial back-drilling depth with the second compensation parameter, the corresponding back-drilling depth is obtained; wherein, the second compensation parameter is calculated based on at least one of the angle between the drill bit tip and the horizontal plane and the safety distance.

5. The printed circuit board back-drilling method according to claim 3, characterized in that, The step-by-step drilling is completed according to the stated drilling depth, including: At the first metallized through-hole position, when the drill bit is controlled to drill down and contact the first surface conductive layer, the conduction signal of the continuity detection circuit is obtained; Based on the conduction signal, the initial drilling height of the drill bit is obtained; Based on the initial drilling height, drill in stages to a first distance from the upper surface of the reference conductive layer.

6. The printed circuit board back-drilling method according to claim 5, characterized in that, The step-by-step drilling based on the initial drilling height to a first distance from the upper surface of the reference conductive layer includes: During the process of drilling step by step from the initial drilling height to a first distance from the upper surface of the reference conductive layer, the on / off signal of the continuity detection circuit is not collected.

7. The printed circuit board back-drilling method according to claim 1, characterized in that, The process of acquiring the on / off signal of the continuity detection circuit includes: Before the drill bit drills from a first distance from the upper surface of the reference conductive layer to the lower surface of the reference conductive layer, the continuity signal of the continuity detection circuit is acquired; When the drill bit penetrates the reference conductive layer, a disconnection signal is obtained from the continuity detection circuit.

8. The printed circuit board back-drilling method according to claim 1, characterized in that, After forming the first metallized via and the second metallized via on the printed circuit board, the method further includes: Isolation rings are formed at the upper and lower hole rings of the first metallized through hole to be back-drilled, and at the upper and lower hole rings of the second metallized through hole, respectively, so that the first metallized through hole and the second metallized through hole are disconnected from the first surface conductive layer and the second surface conductive layer, respectively; wherein, the isolation rings are formed by chemical etching, laser cutting or mechanical cutting. After forming the isolation ring, the method further includes: The cold-stamped cover plate, the printed circuit board to be back drilled, and the pad are stacked on the drilling machine worktable from top to bottom. A window is made in the cold-stamped cover plate above the position corresponding to the second metallized through hole to expose the second metallized through hole.

9. The method for back-drilling printed circuit boards according to claim 8, characterized in that, After forming the back drill hole, the method further includes: According to the circuit network connectivity requirements, the isolation ring at the lower opening of the first metallized through hole is filled by copper plating.

10. The printed circuit board back-drilling method according to claim 1, characterized in that, Before the controlled drill bit drills stepwise from the first surface conductive layer to a first distance from the upper surface of the reference conductive layer, the method further includes: Determine whether the back-drilling process of the printed circuit board requires step-by-step drilling; if not, at the first metallized through-hole position, control the drill bit to back-drill from the first surface conductive layer to the reference conductive layer, and obtain the on or off signal of the continuity detection circuit based on the drill bit. The back drill depth is set based on the actual plate thickness at the location of the first metallized through hole. Upon receiving a disconnection signal from the continuity detection circuit, the drill bit is controlled to drill down to the specified back-drilling depth to form a back-drilled hole.

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