A method for manufacturing a multilayer circuit board through-hole with prestress assistance

Abstract

The application relates to the field of circuit board manufacturing, and particularly provides a prestress-assisted multilayer circuit board via preparation method. The method comprises the following steps: S1, placing a multilayer circuit board on a drilling equipment machining table and fixing the multilayer circuit board through a presser foot; S2, determining the drilling coordinates of a to-be-prepared via and making the axis of a drill bit correspond to the drilling position; S3, driving the drill bit to rotate and feed in the direction perpendicular to the board surface, so as to form a via in the multilayer circuit board; S4, keeping the presser foot in the pressing state, and driving the drill bit to exit the via in the direction opposite to the feeding direction; S5, releasing the presser foot from the pressing and positioning, and taking out the multilayer circuit board with the completed via processing; and in the drilling process of the drill bit, a tensile prestress along the board surface is applied to the multilayer circuit board. Through the application of the tensile prestress along the board surface, the copper layer stability is enhanced, the resin medium layer constraint capability is improved, the resin flow, adhesion and drill dirt formation are reduced, and the via cleanliness and interlayer connection reliability are improved.

Description

Technical Field

[0001] This application relates to the field of circuit board manufacturing, and more specifically, to a method for preparing through-holes in a prestress-assisted multilayer circuit board. Background Technology

[0002] With the trend towards higher integration and miniaturization in electronic devices, multilayer circuit boards (PCBs) are widely used in communication equipment, industrial control equipment, and high-performance electronic products due to their high wiring density and space utilization. PCBs are typically formed by alternating layers of conductive and insulating dielectric layers, with through-holes providing electrical connections between different conductive layers. The manufacturing process of multilayer PCBs involves first creating through-holes that penetrate multiple layers on the board, and then using processes such as hole wall metallization and copper plating to establish stable conductive connections between different layers. Therefore, the processing quality of the through-holes, especially the integrity and cleanliness of the hole walls, as well as the reliability of the inner copper connection areas, directly affects the electrical connection performance and long-term service reliability of the multilayer PCB.

[0003] Multilayer circuit boards typically contain a large number of through-holes, and the diameter of these through-holes is usually larger than that of blind vias. To balance processing efficiency and manufacturing costs, mechanical drilling is commonly used in industrial production instead of laser drilling. However, multilayer circuit boards are usually formed by alternating layers of copper and resin dielectric. During mechanical drilling, the drill bit needs to continuously pass through different material regions, simultaneously cutting through metallic copper, resin, and glass fiber. Copper has good ductility and is prone to plastic deformation during drilling, while resin has poor thermal conductivity and limited heat resistance. The friction, compression, and heat generated by the drill bit and the board material, as well as the shear deformation of the material, can easily cause localized temperature rise and softening of the resin. Under the rotary cutting and axial feed of the drill bit, the softened resin is easily squeezed, dragged, and adheres to the hole wall and the edge of the inner copper layer, forming resin drilling sludge. Resin drilling sludge hinders the effective bonding between the metal plating layer and the inner copper layer during subsequent copper plating, reduces the metallization quality of the hole wall, and increases the risk of thermal cycling failure.

[0004] To address the resin contamination problem generated during mechanical drilling, existing technologies typically employ methods such as optimizing drilling parameters, reducing drilling temperature, or adding post-processing decontamination techniques. For example, reducing drilling speed, adjusting feed rate, using coolant assistance, or optimizing drill bit structure can reduce resin softening during drilling; alternatively, residual resin on the hole wall can be removed after drilling through chemical treatment, plasma treatment, or mechanical cleaning. However, these methods primarily target post-drilling residues or reduce resin softening by lowering cutting intensity. When the drill continuously cuts copper, resin, and fiberglass layers, differences in deformation behavior between different materials persist, especially in deep hole drilling, multi-layer material interfaces, and high-density drilling. Resin can still easily soften due to heat and be dragged by pressure, flowing and adhering to the hole wall. Therefore, existing methods are insufficient to effectively reduce resin contamination during drilling and cannot simultaneously meet the high-efficiency, high-quality, and high-reliability manufacturing requirements of multi-layer circuit board through-hole processing. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of the prior art by providing a prestress-assisted method for preparing through-holes in multilayer circuit boards. This method solves the problems in the prior art, such as the difficulty in effectively reducing resin drilling debris during drilling and the difficulty in simultaneously meeting the manufacturing requirements of high efficiency, high quality, and high reliability in the through-hole processing of multilayer circuit boards.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: This application provides a method for fabricating through-holes in a prestress-assisted multilayer circuit board, comprising the following steps: S1. Place the multilayer circuit board on the drilling machine's processing table and fix it in place by pressing and positioning it with pressure feet; S2. Determine the drilling coordinates of the through hole to be prepared, and make the drill bit axis correspond to the drilling position; S3. Drive the drill bit to rotate and feed in a direction perpendicular to the board surface to form through holes in the multilayer circuit board; S4. Keep the pressure foot pressed down and drive the drill bit to exit the through hole in the opposite direction of the feed direction; S5. Release the pressure foot clamping and positioning, and remove the multilayer circuit board with completed through-hole processing; During the drilling process, tensile prestress along the board surface is applied to the multilayer circuit board.

[0007] This application applies in-plane tensile prestress to a multilayer circuit board during drilling, placing the drilling area under tension and thus altering the deformation behavior of different materials in the multilayer circuit board under drilling loads. The multilayer circuit board is formed by alternating lamination of copper and resin dielectric layers. The copper layer has high stiffness and strong resistance to deformation, while the resin dielectric layer has lower stiffness and is viscoelastic. By applying in-plane tensile prestress, for the high-stiffness copper layer, the prestress keeps the periphery of the hole taut, improving the copper layer's ability to withstand drilling loads, reducing local warping, flanging, and plastic extrusion deformation of the copper layer during drill bit feed, and reducing the lateral extrusion force exerted by the copper layer on the adjacent resin dielectric layer. For the lower-stiffness resin dielectric layer, the pre-stretching effect changes the resin from a free deformation state to an in-plane tensile constraint state. The lateral plastic flow of the resin under drilling loads is restricted, and the resin forms short fragments that are discharged with the cutting process, rather than undergoing continuous plastic flow, stringing, or coating adhesion. Simultaneously, the reduced adhesion between the tensile-constrained resin and the drill bit sidewall weakens the dragging and adhesion of the softened resin during drill rotation, decreasing the likelihood of resin adhering to the drill bit surface and being re-applied to the hole wall during drill movement, thereby further reducing resin smudge formation. Furthermore, maintaining tension in the peri-hole area improves the contact between the drill bit and the hole wall, reducing continuous pressure and repeated friction between the drill bit sidewall and the resin, decreasing frictional heat accumulation during drilling, slowing the thermal softening of the resin dielectric layer, and maintaining high resin viscosity and anti-flow capability. Therefore, by applying in-plane tensile prestress during drilling, this application can simultaneously reduce the amount of resin smudge generated and the probability of softened resin migrating to the hole wall and inner copper layer regions, improving the cleanliness of the through-hole, and enhancing the bonding reliability between the subsequent copper plating layer and the inner copper layer, thereby improving the interlayer conductivity and long-term thermal cycling reliability of the multilayer circuit board.

[0008] Furthermore, the tensile prestress extends from the center of the pressure foot towards the edge of the multilayer circuit board. This tensile prestress, extending from the center to the edge, creates a uniform and symmetrical in-plane tensile stress field around the hole, avoiding stress concentration and uneven deformation of the board caused by localized unidirectional tension. This ensures uniform constraint in different directions throughout the drilling area. For the copper layer, the uniform radial tension keeps the copper layer around the hole flat and taut, improving its resistance to drill bit cutting loads and reducing local warping, edge curling, and plastic extrusion caused by uneven stress. This reduces the squeezing and dragging effect of the copper layer on the resin dielectric layer. For the resin dielectric layer, the symmetrical in-plane tensile stress uniformly constrains the resin, making the resin deformation around the drill bit cutting area more consistent. This prevents excessive lateral flow due to stress release in localized areas and increases the resin's fracture tendency under shearing. The resin is more likely to form debris and be discharged, reducing the likelihood of continuous softening flow and smear migration. Therefore, the radially uniform in-plane prestress enhances the overall constraint effect of the prestress on the drilling area, reducing the possibility of softening resin migrating to the hole wall and inner copper hole edge, forming drill smear.

[0009] Furthermore, the pressure foot is a columnar structure with an axial through-hole. The diameter of the through-hole gradually increases from top to bottom, while the edge of the lower surface of the pressure foot is planar. This gradual increase in the diameter of the through-hole creates an arc-shaped transition area on the inner side of the lower surface of the pressure foot and an outer planar area. The arc-shaped transition structure causes a change in the direction of the vertical load transmitted inside the pressure foot as it passes through the inclined arc-shaped contact area. Since the normal direction of the arc-shaped area has a horizontal component, when the pressure foot is subjected to a vertically downward load, the force is transmitted along the arc surface to the outer planar contact area, maintaining the vertical component while generating a horizontal component pointing outwards. The vertical component still serves to press the multilayer circuit board, improving the constraint stability during drilling; while the horizontal component pointing outwards along the board surface acts on the edge of the circuit board, causing the circuit board to expand outwards, thus creating tensile prestress within the board surface. Because the annular pressure foot is positioned around the area to be processed, this horizontal component can be evenly distributed along the periphery of the hole and transmitted outwards from the periphery, thereby creating radial in-plane tensile prestress near the hole to be processed. In this way, without the need for an additional independent lateral tensioning mechanism, simply by changing the contact surface shape of the traditional pressure foot, an in-plane tensioning effect can be generated simultaneously during the original clamping and positioning process, achieving both clamping and fixing functions and prestressing loading functions. In addition, local prestressing loading can avoid dimensional changes and interlayer stress accumulation caused by tension on the entire board, making it easier to control the range and magnitude of prestressing, improving the through-hole machining accuracy and the reliability of multilayer circuit board manufacturing.

[0010] Furthermore, the ratio of the presser foot's height to its outer diameter is 0.3-0.6. The presser foot has a short and stout structure, rather than a slender columnar structure. A smaller height-to-outer-diameter ratio reduces the bending deformation of the presser foot during vertical loading, allowing the load force to be transferred more directly to the annular contact area on the lower surface, reducing force loss and in-plane prestress shift caused by the presser foot's own deformation. At the same time, a larger outer diameter improves the circumferential continuity of the annular support area, making the radial expansion effect formed by the vertical pressure conversion more uniform and stably transferred to the multilayer circuit board along the hole periphery, enhancing the tensile constraint effect of the resin dielectric layer, and reducing the lateral plastic flow of resin and drill smudge formation during drilling. If the height-to-outer-diameter ratio is too large, the presser foot is prone to elastic deformation, leading to a decrease in prestress transfer efficiency; if it is too small, the presser foot's stiffness and structural strength are insufficient, making it difficult to withstand cyclic loading.

[0011] Furthermore, the magnitude of the force applied to the upper surface of the pressure foot varies periodically. This periodic variation in the force causes the vertical clamping force transmitted from the pressure foot to the multilayer circuit board, as well as the in-plane tensile prestress generated by the arc-shaped area, to change periodically simultaneously. Throughout this variation, a certain vertical clamping force is maintained to ensure that the multilayer circuit board does not loosen or warp. The periodically varying in-plane prestress keeps the material around the hole in a periodic stretching and recovery state. For the copper layer, the periodically enhanced stretching repeatedly increases the tension of the copper layer around the hole, reducing local plastic deformation, flaking, and compression of the resin dielectric layer during drill cutting. For the resin dielectric layer, the periodically varying tensile stress dynamically constrains its softening deformation process, limiting the lateral flow and migration of the resin under drilling pressure during the stretching enhancement stage, and releasing local stress concentration during the stress reduction stage, preventing the resin from continuously accumulating plastic deformation. Periodic stress disturbance can also reduce the stable adhesion between the softened resin and the drill bit sidewall and hole wall, making the resin more likely to break under the shearing action of the drill bit and be discharged with the chips, rather than being continuously dragged by the drill bit to form a smear-like drill smear. Therefore, compared with constant prestress, periodically varying in-plane prestress can dynamically control the deformation process of the material in the drilling area while maintaining the stability of the plate, further reducing the formation of resin drill smear.

[0012] Furthermore, a gripper is installed at the circumferential edge of the multilayer circuit board. Driven by a lead screw motor, the gripper applies in-plane tensile prestress to the multilayer circuit board. By installing the gripper at the circumferential edge of the multilayer circuit board and driving it with a lead screw motor to apply in-plane tensile prestress, precise control of the magnitude and direction of the prestress is achieved. The lead screw motor can adjust the tensile force according to the drilling stage, maintaining stable constraint while avoiding excessive stress that could cause board deformation.

[0013] Furthermore, in step S3, before drilling, the drilling area of ​​the multilayer circuit board is locally cooled. Before drilling, the area around the through-hole to be machined is locally pre-cooled, bringing the resin dielectric layer and the copper layer around the hole to a lower temperature. For the resin, lowering the temperature weakens the movement of resin molecular chains, increasing resin viscosity, elastic modulus, and shear resistance, thus giving the resin stronger load-bearing and restraining capacity when in-plane tensile prestress is applied subsequently. The pre-cooled resin is in a low-temperature tensile restraint state, allowing the in-plane tensile prestress to be more effectively transferred to the area around the hole, limiting the lateral flow of the resin under the pressure of the drill bit. For the copper layer, pre-cooling improves the yield strength and dimensional stability of the copper layer, reduces local softening and plastic deformation caused by frictional heat during drilling, and improves the support capacity of the hole-surrounding structure. When the drill bit enters the cutting zone, frictional heat causes a local temperature increase. However, because pre-cooling lowers the initial temperature, the resin needs to absorb more heat to reach a softened state. This reduces the softening area and shortens its duration, making it difficult for the resin to quickly enter a low-viscosity flow state. Instead, it is constrained by tensile prestress in a higher stiffness state. Therefore, local pre-cooling does not simply reduce the drilling temperature, but rather enhances the resin's initial mechanical properties, thereby increasing its response to in-plane tensile prestress. This prestress inhibits the migration of softened resin to the hole wall and inner copper hole edge, thus reducing resin smudge formation.

[0014] Furthermore, in step S4, after the drill bit withdraws from the through hole, a low-temperature gas is introduced into the through hole from bottom to top along the axial direction of the through hole. The low-temperature gas is dry nitrogen or compressed air with a temperature of -20 to 5°C. The upward airflow passes through the hole wall, rapidly cooling the drilling area and causing the resin medium layer, which has softened during drilling, to undergo rapid thermal shrinkage. Due to the difference in thermal expansion coefficients between the resin and the copper layer, thermal stress is generated at the interface between the resin and the inner copper layer during the cooling and shrinkage process. This weakens the bonding strength between the drill bit and the hole wall and the copper layer, making the weakly bonded resin more prone to microcracks and detachment from the hole wall. Here, the thermal stress and the in-plane prestress form a composite stress field. During the drilling stage, the in-plane tensile prestress keeps the resin around the hole in a tensile constraint state along the board surface, restricting the migration of softened resin towards the hole wall. During the drill withdrawal and cooling stage, the low-temperature gas causes the resin to thermally shrink, and since the resin is already subject to in-plane tensile constraint, its free shrinkage is restricted, thus generating additional thermal shrinkage stress. The combined effect of thermal stress and existing tensile prestress further destabilizes the internal stress state of the resin near the hole wall, promoting microcrack formation and reducing the interfacial bonding strength between the resin and the copper layer. Both factors mutually reinforce each other by altering resin deformation and interfacial bonding, thus enhancing the removal of drilling debris.

[0015] Furthermore, cryogenic gas is introduced into the via in a pulsed manner. This pulsed introduction avoids the uneven thermal stress caused by rapid cooling of the via wall surface due to continuous strong cooling. In pulsed cooling, the initial cryogenic gas flow rapidly lowers the resin temperature on the via wall, causing the softened resin to shrink and generate thermal shrinkage stress. Subsequent intermittent phases redistribute heat inside and outside the via wall, reducing local temperature differences. The subsequent introduction of cryogenic gas further expands the cooling area and temperature gradient, allowing thermal shrinkage stress to gradually accumulate. Because the in-plane tensile prestress during drilling already places the resin around the via in a tensile constraint state, resin shrinkage during cooling is restricted, resulting in a superimposed composite stress field. This makes the resin near the via wall more prone to microcracks, reducing the bonding strength between drill smudge and the via wall and copper layer, and promoting the detachment of weakly bonded drill smudge. Simultaneously, the pulsed method enhances thermal stress removal while reducing localized thermal shock caused by continuous strong cooling, improving drill smudge removal efficiency and via reliability.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) This application applies tensile prestress along the in-plane direction during drilling, thereby changing the stress state of the material around the hole, keeping the copper layer taut and reducing local deformation. This restricts the plastic flow, stringing, and migration of the resin dielectric layer under drilling pressure and heat, reducing the adhesion of softened resin to the hole wall and inner copper area. In addition, the tensile constraint reduces the pressure and friction between the drill bit and the hole wall, slows down the resin thermal softening process, and makes the resin generated during drilling easier to break down and be discharged. Therefore, this application can effectively reduce the formation of resin drilling debris, improve the cleanliness of the through hole and the reliability of the metallization bonding of the hole wall, and improve the interlayer conductivity and long-term reliability of multilayer circuit boards.

[0017] (2) No changes are needed to the drill bit, spindle, or basic drilling control process; only a prestressing loading structure is added, making it compatible with existing drilling processes. This application does not require changes to the basic processing flow of mechanical drilling of multilayer circuit boards. The existing drilling equipment is still used to complete the positioning, clamping, feeding, and retraction operations. Only an in-plane tensile prestressing application mechanism needs to be added during the drilling process. Prestressing can be achieved by adding a gripper or drive mechanism to the circumference of the existing processing table or by making adaptive improvements to the pressure foot structure. There is no need to replace the drill bit, spindle, or drilling control system, and the equipment modification difficulty is low. At the same time, the prestressing loading process is carried out synchronously with the original pressure foot fixing process, without affecting the drilling cycle and processing efficiency, making it easy to directly apply to existing multilayer circuit board through-hole manufacturing production lines.

[0018] (3) Low cost, easy maintenance, and good engineering application value. Compared with existing methods that remove resin drilling debris by reducing drilling speed, adding complex cooling systems, or using chemical post-treatment, this application mainly reduces drilling debris formation by changing the stress state during the drilling process, without the need for additional high-cost material processing equipment or complex processes. The prestressing mechanism mainly includes a mechanical clamping structure, a drive assembly, and a control assembly. It has a simple structure, high reliability, and is not prone to pollution or consumable wear during long-term operation, resulting in low maintenance costs. In addition, since this method can reduce resin migration and adhesion from the drilling source, it can reduce the load of subsequent drilling debris removal treatment, reduce the use of chemical treatment agents, and improve the economy and environmental protection of the multilayer circuit board manufacturing process. Attached Figure Description

[0019] Figure 1 A schematic diagram illustrating a prestress-assisted method for fabricating through-holes in a multilayer circuit board, provided by the present invention. Figure 2 This is a cross-sectional schematic diagram of a columnar press foot with an axial through hole used in a prestress-assisted multilayer circuit board through-hole fabrication method provided in Embodiment 2 of the present invention. Figure 3 The total deformation cloud diagram (left side is the color scale) of the pressure foot used in the prestress-assisted through-hole fabrication method of a multilayer circuit board provided in Embodiment 2 of the present invention is shown in the main view section direction of the static simulation using ANSYS Mechanical finite element simulation software. Figure 4 The equivalent stress distribution cloud diagram in the top view of the pressure foot used in the prestress-assisted through-hole fabrication method of a multilayer circuit board provided in Embodiment 2 of the present invention is a static simulation using ANSYS Mechanical finite element simulation software (the left side is a color scale). Figure 5 The stress distribution results of the inner arc-shaped region of the pressure foot used in the prestress-assisted through-hole fabrication method of a multilayer circuit board provided in Embodiment 2 of the present invention were obtained by static simulation using ANSYS Mechanical finite element simulation software. Figure 6 This is a schematic diagram of a groove structure on the upper surface of the pressure foot used in a prestress-assisted method for preparing through-holes in a multilayer circuit board according to Embodiment 2 of the present invention. Figure 7 This is a schematic diagram of an air inlet provided on the annular cavity on the underside of the pressure foot in a prestress-assisted multilayer circuit board through-hole preparation method provided in Embodiment 3 of the present invention. Detailed Implementation

[0020] To make the implementation process of this invention clearer, a detailed description will be provided below in conjunction with the accompanying drawings.

[0021] Example 1:

[0022] This application provides a method for fabricating through-holes in a prestress-assisted multilayer circuit board, such as... Figure 1 As shown, it includes the following steps: S1. Place the multilayer circuit board on the drilling machine's processing table and fix it in place by pressing it with pressure feet.

[0023] Specifically, the multilayer circuit board to be processed is placed on the processing table of the drilling equipment, ensuring the board surface is parallel to the table. A backing plate and a cover plate are respectively placed on the top and bottom sides of the multilayer circuit board to improve support stability during drilling. The backing plate, positioned between the multilayer circuit board and the processing table, bears the axial force generated during drilling and reduces damage to the processing table when the drill bit exits the multilayer circuit board. The cover plate, positioned above the multilayer circuit board, improves the stress state when the drill bit enters the board surface and reduces burr formation. The backing plate and cover plate can be made of resin, aluminum, or composite materials. After stacking the backing plate, multilayer circuit board, and cover plate in sequence, their positions are fixed using locating pins, locating holes, or limiting structures, ensuring the area to be processed on the multilayer circuit board corresponds to the processing area of ​​the drilling equipment.

[0024] A pressure foot is installed above the multilayer circuit board, acting on the cover plate. The pressure foot is a ring-shaped pressure foot used in drilling equipment, positioned around the location of the through hole to be drilled, and has a central through hole for the drill bit to pass through. The pressure foot is made of a material with a certain rigidity, such as hard alloy, aluminum alloy, stainless steel, or high-strength engineering plastic, to ensure that it does not easily deform under stress. Before drilling, a drive mechanism moves the pressure foot downwards in a direction perpendicular to the surface of the multilayer circuit board, and the lower end face of the pressure foot abuts against the surface of the cover plate, thereby applying a clamping force to the area around the hole to be drilled. The force of the pressure foot is tens to hundreds of Newtons. By locally clamping the area around the drilling area with the ring-shaped pressure foot, local vibration and warping of the multilayer circuit board below are limited during drill bit feed, ensuring stable transmission of the drilling load to the machining table and improving drilling position accuracy. Simultaneously, the ring-shaped pressure foot only acts on the area around the hole to be drilled, without excessive constraint on the entire multilayer circuit board, facilitating the application of in-plane tensile prestress from the center to the edge of the multilayer circuit board during subsequent drilling.

[0025] S2. Determine the drilling coordinates of the through hole to be prepared, and make the drill bit axis correspond to the drilling position.

[0026] Specifically, the position coordinates of the through-holes to be processed are determined according to the design documents of the multilayer circuit board, and these coordinates are input into the control system of the drilling equipment. The drilling equipment determines the position of the multilayer circuit board through a vision positioning system, mechanical positioning system, or program control system, ensuring that the position of the through-hole corresponds to the processing coordinate system of the drilling equipment. The control system of the drilling equipment is a commonly used control system in existing processes. By adjusting the position of the drill bit, the center axis of the drill bit is made to coincide with the center position of the through-hole, and the drill bit axis extends in a direction perpendicular to the surface of the multilayer circuit board. By setting the drill bit axis coaxially with the designed position of the through-hole, it is ensured that the cutting area of ​​the drill bit acts evenly on the material around the hole, avoiding uneven hole wall thickness, increased local extrusion, and increased resin drilling contamination caused by drill bit offset. After completing the drilling coordinate positioning, the positional relationship between the multilayer circuit board, cover plate, and backing plate remains unchanged, providing stable processing conditions for subsequent drilling processing.

[0027] S3. Drive the drill bit to rotate and feed in a direction perpendicular to the board surface to form through holes in the multilayer circuit board.

[0028] Specifically, the drilling equipment drives the drill bit to rotate at high speed, causing it to rotate around its own axis while simultaneously controlling its downward movement in a direction perpendicular to the surface of the multilayer circuit board. The drill bit first passes through the cover plate and enters the interior of the multilayer circuit board, continuously cutting the copper and resin dielectric layers. During rotation, the drill bit shears and cuts the copper layer with its cutting edges and generates cutting, extrusion, and friction on the resin dielectric layer, gradually removing material and forming a through-hole penetrating the thickness of the multilayer circuit board. During drilling, the chips generated are discharged through the drill bit's spiral grooves and auxiliary chip removal structure.

[0029] Furthermore, during the drilling process, a tensile prestress is applied to the multilayer circuit board along the in-plane direction. This prestress extends from the middle region of the multilayer circuit board towards its edge, creating a radially tensile state around the through-hole. The tensile prestress acts around the drilling area, placing the copper layer and resin dielectric layer around the hole under pre-tension constraint during drilling. By applying this prestress, the structural stability of the area around the hole is improved, limiting excessive lateral flow of the resin dielectric layer under drilling pressure and frictional heat, and reducing the possibility of softened resin migrating along the hole wall and forming drill smudge. The specific method of applying the tensile prestress can be achieved through a clamping mechanism located at the edge of the multilayer circuit board, an improved pressure foot structure, or other prestressing loading mechanisms.

[0030] S4. Keep the pressure foot pressed down and drive the drill bit out of the through hole in the opposite direction of the feed direction.

[0031] Specifically, after the drill bit completes its penetration through the multilayer circuit board and forms a through hole, the pressure foot maintains its clamping position on the multilayer circuit board, cover plate, and backing plate, keeping the multilayer circuit board in a stable constrained state. Subsequently, the drilling equipment is controlled to drive the drill bit to move in the opposite direction to the feed direction, gradually withdrawing the drill bit from the through hole. During the withdrawal process, the drill bit continues to rotate, but the rotation speed can be reduced to minimize secondary dragging of the hole wall during withdrawal. Because the pressure foot maintains its clamping action, and the tensile prestress still acts on the periphery of the hole, the re-adhesion of softening resin onto the hole wall caused by the helical groove during drill withdrawal is reduced, improving the hole wall quality.

[0032] S5. Release the pressure foot clamping and positioning, and remove the multilayer circuit board with completed through-hole processing.

[0033] Specifically, after the drill bit has completely withdrawn from the through hole, the drill bit rotation is stopped, and the tensile prestress is released. Then, the control drive mechanism moves the pressure foot away from the multilayer circuit board, separating the pressure foot from the cover plate and releasing the clamping and positioning of the multilayer circuit board. The cover plates and backing plates located on the top and bottom sides of the multilayer circuit board are removed sequentially, and the multilayer circuit board with the completed through hole is removed from the processing table. The removed multilayer circuit board can be cleaned according to subsequent manufacturing processes, such as using gas purging, ultrasonic cleaning, chemical cleaning, or plasma treatment to remove residual debris and small amounts of adhering substances from the hole opening, in preparation for subsequent processes such as hole wall roughening, copper plating, and electroplating.

[0034] By following the steps above, while maintaining the existing mechanical drilling process, in-plane tensile prestress is introduced during the drilling process to keep the material around the hole under tensile constraint, thereby reducing resin drilling fouling and improving the processing quality and reliability of through holes in multilayer circuit boards.

[0035] Example 2:

[0036] Unlike Embodiment 1, this embodiment improves the presser foot structure, enabling it to convert the vertical force on the upper surface of the presser foot into a vertical clamping force and an in-plane tensile force extending outward along the board surface while simultaneously clamping and fixing the multilayer circuit board. Specifically, the presser foot is a columnar structure with an axial through hole, the diameter of the upper end of the through hole being smaller than the diameter of the lower end; the diameter of the through hole gradually increases from top to bottom (or the diameter of the upper part can remain unchanged and then gradually increase), and the diameter of the lower end of the through hole is smaller than the outer diameter of the presser foot; the ratio of the height of the presser foot to its outer diameter is 0.3-0.6. Preferably, as follows... Figure 2 As shown, the change in the diameter of the through hole gradually decreases with increasing depth, meaning the diameter changes more rapidly near the upper region and gradually decreases towards the lower region. Specifically, the relationship between the through hole diameter D and the through hole depth h is as follows: D(h) = Where D0 represents the diameter of the region at the upper end of the through hole that remains unchanged, h0 represents the depth at the point where the diameter begins to increase, D is the increase in diameter at the maximum diameter of the through hole, and k represents the rate of change. The rate of change of the through hole diameter is... It can be seen that when h first enters the extended region, h0 has the largest rate of change. The diameter of the through hole increases rapidly, and as the depth of the through hole continues to increase... Gradually decrease, The pressure is gradually reduced. This allows for faster expansion near the upper region, enabling the inclined surface of the hole wall to quickly generate a radial component force when the pressure foot is subjected to vertical loading. This converts some of the vertical pressure into an outward force along the plate surface, rapidly establishing tensile prestress around the hole. As the depth increases, the hole diameter change gradually slows down, making the force transmission process smoother and avoiding excessive radial expansion and local stress concentration in deeper areas. This allows the tensile prestress to act evenly on the copper layer and resin dielectric layer around the hole, resulting in more uniform tensile stress. In Example 4, the inner annular cavity adopts a continuously varying curvature annular structure, causing the tangentially entering cold air to be continuously guided by bending during flow, preventing direct impact of airflow on local areas and avoiding turbulence and sudden velocity changes. The arc-shaped sidewall allows the airflow to smoothly change direction along the cavity wall, forming a stable circumferential vortex within the annular cavity. Simultaneously, the centrifugal force generated by the vortex allows the cold air to spread evenly circumferentially, reducing uneven cooling caused by local high-speed jets. Furthermore, the curved transition structure can reduce the energy loss caused by the collision between the airflow and the cavity wall, and maintain a high degree of flow continuity of the cold air, thereby improving the coverage and heat exchange efficiency of the cold air in the area to be processed, and achieving more uniform and stable local cooling in the drilling area.

[0037] Static simulation analysis of the improved pressure foot structure was performed using ANSYS Mechanical finite element simulation software to study the force transmission process and in-plane prestress transmission process of the pressure foot under vertical loading. In the simulation model, the pressure foot material was set to steel, and fixed constraints were applied to the base contact surface to restrict all translational and rotational degrees of freedom of the structure to simulate the actual fixed state of the pressure foot installed in the drilling equipment. A uniformly distributed pressure load was applied to the upper surface of the pressure foot, with the load direction perpendicular to the top surface of the pressure foot and the load magnitude set to 1 MPa, to simulate the vertical loading effect applied to the pressure foot by the drive mechanism during drilling. The mesh was generated using the ANSYS Mechanical automatic mesh generation method, and the mesh was refined by reducing the element size to obtain a high mesh density in the arc transition region, so as to accurately capture the stress changes and force transmission characteristics in this region. The static structural analysis module was used for solving the problem, and the default sparse matrix direct solver was used to solve the structural response. By analyzing the stress distribution inside the presser foot and the force state of the contact area on the lower surface, it is shown that the arc-shaped area of ​​the presser foot can cause the vertical loading force to change direction along the inclined contact surface, generating an outward component along the plate surface while maintaining the vertical pressing action.

[0038] Static finite element simulation was performed on the improved presser foot structure to analyze the deformation and stress distribution inside the presser foot under vertical loading. Figure 3 The total deformation contour plot along the main view section of the pressure foot shows that the upper region of the pressure foot exhibits larger deformation, represented by the red area. The deformation gradually decreases towards the outer plane contact area of ​​the lower surface, appearing as a green to blue area. This indicates that when the pressure foot is subjected to a vertical load, the deformation is mainly transmitted from the top to the bottom, forming a strong constraint effect in the lower surface plane area. Since the outer plane area of ​​the lower surface is in contact with the multilayer circuit board, its deformation is restricted, making this area the primary location for load transfer and stress concentration. Figure 4 The equivalent stress distribution cloud diagram of the pressure foot in a top-down view shows that the central area of ​​the pressure foot is mainly blue, with a low equivalent stress. The equivalent stress gradually increases towards the green area along the circumferential edge, indicating that the vertical load, after being transmitted through the arc-shaped transition area inside the pressure foot, gradually concentrates towards the outer edge. The simulation results demonstrate that the improved pressure foot structure enables the vertical force applied at the top to change direction along the arc-shaped area, forming a larger stress concentration at the outer edge. This generates an outward component along the board surface, realizing the conversion from vertical clamping force to in-plane tensile prestress. This provides a mechanical basis for applying radial tensile prestress to multilayer circuit boards during drilling using the pressure foot structure.

[0039] Figure 5The figure shows the stress distribution in the inner arc-shaped region of the pressure foot. Blue arrows represent the clamping force transmitted vertically, and red arrows represent the tensile force transmitted along the board surface. As can be seen, when the vertical load on the upper surface of the pressure foot is transmitted through the inner arc-shaped region, the continuous change in the normal direction of the arc surface causes the force, originally vertical, to generate a horizontal component during transmission. This horizontal component is transmitted away from the center of the pressure foot and ultimately acts on the outer planar region, causing it to bear not only the vertical clamping force but also an outward tensile force along the board surface. Since the outer planar region is in contact with the multilayer circuit board, the in-plane tensile force can be further transmitted to the multilayer circuit board, generating in-plane tensile prestress in the material around the drilling area from the center to the edge. Simulation results show that the improved pressure foot structure can effectively convert the vertical load into in-plane tensile force, providing stable radial tensile constraint for the drilling area while maintaining the original clamping and positioning function. The results verify the feasibility of achieving prestressing through curved transition structures and the transfer of surface forces.

[0040] The force applied to the upper surface of the pressure foot varies according to a preset cycle, causing the vertical clamping force and in-plane tensile prestress transmitted to the multilayer circuit board by the pressure foot to change periodically. Specifically, the pressure foot loading force can be controlled by a cylinder, hydraulic drive mechanism, or servo drive mechanism in the existing system, allowing the pressure foot loading force to be periodically adjusted based on the basic clamping force. For example, the initial loading force of the pressure foot is set to 50-500 N, and the loading force is periodically varied at a frequency of 0.5-20 Hz, with a variation amplitude of 10%-30% of the basic loading force, that is, the loading force periodically varies within the range of 1.1-1.3 times the basic value. Throughout the entire variation process, the loading force always remains greater than the minimum clamping force that can stably clamp the multilayer circuit board to avoid loosening of the board during drilling. The loading force varies at a frequency of 0.5-20 Hz, allowing the material around the hole to undergo periodic tensile strengthening and stress release processes. At lower frequencies, the resin medium layer can generate a sufficient stress response, effectively transferring the tensile constraint to the softened area. At higher frequencies, the resin deformation process can be continuously disturbed during drilling, reducing the stable adhesion between the softened resin and the drill bit. A variation amplitude of 10%-30% can cause significant fluctuations in the in-plane prestress without compromising the original clamping stability: too small an amplitude makes it difficult to change the resin flow state; too large an amplitude easily causes plate vibration and local stress concentration. Therefore, this range can balance dynamic adjustment effect and processing stability, allowing the periodically varying prestress to more effectively suppress resin flow, stringing, and drill sludge formation.

[0041] A radially extending groove structure is provided on the outer planar area of ​​the upper surface of the presser foot. The extension direction of the groove is consistent with the direction of in-plane prestress transmission generated by the presser foot. Figure 6 As shown, the groove width is set to 0.1-1 mm and the depth to 0.05-0.5 mm. The groove cross-sectional shape is arc-shaped to avoid local stress concentration of the pressure foot caused by sharp corners, while maintaining a large contact area. This not only improves the mechanical interlocking ability between the pressure foot and the multilayer circuit board, but also enhances the transmission of in-plane horizontal force to the interior of the board. When the arc-shaped area on the upper surface of the pressure foot converts the vertical loading force into a radially outward horizontal component, the radial groove can provide directional constraint on the horizontal component, forming a stronger anti-slip effect between the pressure foot and the multilayer circuit board, reducing energy loss caused by interface slippage during the transmission of horizontal force, and allowing more in-plane tensile force to be transmitted to the area around the drilled hole. At the same time, the radial groove forms a micro-scale interlocking structure in the contact area, improving the interface friction and shear bearing capacity, allowing the area around the hole to maintain a stable radial tensile state, and preventing relative displacement between the pressure foot and the circuit board during prestressing loading. Therefore, this structure can improve the loading efficiency and stability of in-plane prestress, keep the resin medium layer under tensile constraint during drilling, more effectively restrict the lateral flow and migration of softened resin, and reduce drilling fouling formation.

[0042] It should be noted that the pressure foot can act directly on the surface of the multilayer circuit board, or indirectly through the cover plate. When the pressure foot acts directly on the multilayer circuit board, the horizontal component generated by the pressure foot is directly transmitted to the board surface, causing in-plane tensile prestress in the peripheral area. When the pressure foot acts on the cover plate, the horizontal component generated by the pressure foot first acts on the cover plate, and is then transmitted to the board through the contact between the cover plate and the multilayer circuit board, causing the multilayer circuit board to be in a state of in-plane tension. Furthermore, when using the cover plate to transmit the force, the cover plate can be set as a thin plate structure, for example, with a thickness of 0.1-1 mm. Under the action of the horizontal component of the pressure foot, the thin cover plate can generate expansion deformation along the board surface direction, and drive the board to generate in-plane tension through frictional constraint with the multilayer circuit board. A micro-textured structure can also be set on the side of the cover plate that contacts the multilayer circuit board to enhance the in-plane shear transmission capability between the cover plate and the circuit board and reduce the slippage loss of the horizontal force of the pressure foot at the interface between the cover plate and the circuit board. The interfacial friction enhancement effect of the thin cover plate enables the horizontal load generated by the pressure foot to be more effectively converted into in-plane tensile prestress in the periphery area of ​​the multilayer circuit board, improving the prestress transfer efficiency and thus further enhancing the inhibition effect on resin drilling fouling.

[0043] Example 3:

[0044] Unlike Embodiment 1, this embodiment features multiple grippers along the circumferential edge of the multilayer circuit board. These grippers hold the stacked processing structure, consisting of a cover plate, the multilayer circuit board, and a backing plate, and apply in-plane tensile prestress to the stacked structure under the drive of a lead screw motor. Specifically, the cover plate, the multilayer circuit board, and the backing plate are sequentially attached and held relatively fixed by a clamping mechanism. Subsequently, multiple grippers are evenly distributed circumferentially along the edge of the stacked structure and connected to the lead screw motor. Before drilling, the lead screw motor drives each gripper to move synchronously away from the center of the stacked structure, causing the cover plate, multilayer circuit board, and backing plate to undergo in-plane tensile deformation from the center to the edge, thereby creating radial tensile prestress in the drilling area of ​​the multilayer circuit board. Specifically, the gripper travel distance can be set to 0.01-0.05 mm to avoid excessive deformation, and the synchronous drive of multiple grippers ensures that the tensile force in different directions remains relatively consistent, avoiding stress concentration caused by localized unidirectional tension.

[0045] Furthermore, a flexible clamping layer, such as polyurethane, rubber, or polyimide, can be provided at the contact point between the gripper and the edge of the cover plate, backing plate, or multilayer circuit board to improve clamping stability and avoid damage to the board edges. By simultaneously stretching the cover plate, backing plate, and multilayer circuit board, relative slippage between layers can be avoided, allowing the in-plane tensile force generated by the gripper to be effectively transferred to the drilling area. During drilling, the in-plane tensile prestress keeps the copper layer around the hole taut, improving the copper layer's ability to resist drilling loads and reducing copper layer warping, flanging, and compression of the resin dielectric layer; at the same time, it keeps the resin dielectric layer in an in-plane tensile constraint state, restricting its lateral flow under drill bit extrusion and frictional heat, reducing the possibility of softened resin migrating to the hole wall and forming drill smudge. Compared to passively generating prestress using a pressure foot structure, the screw motor-driven gripper can actively adjust the tensile displacement and loading force, allowing the prestress magnitude to be precisely matched according to the board size, thickness, and drilling parameters, improving the stability and controllability of prestress application.

[0046] Example 4:

[0047] Based on Example 1, in step S3, the drilling area of ​​the multilayer circuit board is locally cooled before drilling. Specifically, before drilling, the area around the through-hole to be processed is pre-cooled using a cold gas jet method. The cooling device is located near the drill bit, with the cooling nozzles facing the area of ​​the through-hole to be processed, and low-temperature gas is continuously sprayed onto the surface of the multilayer circuit board through the nozzles. The low-temperature gas is dry nitrogen to avoid water vapor condensation affecting the performance of the circuit board. The temperature of the low-temperature gas is -5 to 5°C, the gas flow rate is 5 to 50 L / min, and the spraying time is 10 to 120 s. Local pre-cooling before drilling lowers the temperature of the resin dielectric layer in the area to be processed. Since the viscoelastic properties of resin materials are significantly affected by temperature, lowering the temperature can increase the elastic modulus and viscosity of the resin, weaken the molecular chain movement, and enhance the resin's resistance to shear deformation. When in-plane tensile prestress is applied and drilling is performed subsequently, the resin in the low-temperature state can more effectively withstand the tensile constraint, reducing lateral flow and stringing caused by the drill bit's extrusion. Meanwhile, as the temperature of the copper layer decreases, the material strength and dimensional stability improve, reducing local softening and deformation of the copper layer and the squeezing effect on the resin during drilling.

[0048] Preferably, in Embodiment 2, an air inlet is provided on the sidewall of the inner annular cavity on the upper surface of the presser foot. The air inlet penetrates the sidewall of the outer planar area of ​​the presser foot and extends tangentially along the annular cavity, such as... Figure 7 As shown. In use, the air inlet is connected to the cold air supply device. Cold air enters the annular cavity tangentially through the air inlet and forms a circumferential rotating flow within the annular cavity, finally exiting downwards from the open area at the center of the annular cavity to the area to be processed. Because the tangentially entering airflow has a circumferential velocity component, the airflow forms a stable annular swirling field within the annular cavity, allowing the cold air to be evenly distributed around the hole to be processed, increasing the heat exchange area between the cold air and the surface of the multilayer circuit board, avoiding temperature unevenness caused by unilateral cooling, and thus achieving uniform local cooling of the drilling area. At the same time, the cold airflow can reduce the temperature of the pressure foot itself when passing through the annular cavity inside the pressure foot, keeping the pressure foot material with high rigidity and dimensional stability. Due to the reduced deformation of the pressure foot, when the pressure foot is subjected to vertical loading force, the arc-shaped area is more stable in the conversion of the force direction, reducing the force transmission deviation caused by the thermal deformation of the pressure foot, so that the vertical clamping force and in-plane tensile prestress can be applied to the multilayer circuit board more accurately and stably. In addition, at lower temperatures, the difference in thermal expansion between the pressure foot and the multilayer circuit board in the contact area decreases, which helps to maintain stable contact between the pressure foot and the board and improve the consistency of prestress loading.

[0049] Furthermore, two symmetrically arranged air inlets extending tangentially towards each other are positioned on the sidewall of the annular cavity. These two inlets introduce tangential airflows in opposite directions, creating a symmetrical rotating flow field within the annular cavity. This avoids airflow deviation and localized temperature gradients caused by unilateral air intake, resulting in a more uniform distribution of cool air within the annular cavity. Simultaneously, the opposing airflows enhance internal turbulence within the annular cavity, improving heat exchange efficiency between the cool air and the pressure foot and the area to be processed. This leads to a faster and more uniform temperature reduction in the drilling area, further enhancing the pre-cooling effect.

[0050] In use, first apply in-plane tensile prestress using the pressure foot to place the area to be processed under tensile constraint. Then, turn on the cooling air supply device for pre-cooling, continuing cooling for 10-60 seconds before starting the drill bit for drilling. During drilling, low-temperature gas can be continuously introduced to promptly remove the frictional heat generated during drilling, reducing the temperature rise of the resin around the hole. Through the above methods, the resin is placed in a low-temperature, high-viscosity, and tensile-constrained state at the beginning of drilling, enhancing the restrictive effect of prestress on resin flow. At the same time, continuous cooling during drilling further slows down resin softening, reducing the softened area and thus reducing resin drilling fouling formation.

[0051] Example 5:

[0052] Based on embodiments 1, 2, 3, or 4, in step S4, after the drill bit withdraws from the through hole, cryogenic gas is introduced into the through hole from bottom to top along the axial direction of the through hole. The cryogenic gas is dry nitrogen or compressed air, with a temperature of -20 to 5°C, and is introduced in a pulsed manner. The pulsed ventilation method includes a high-pressure ventilation stage and a low-pressure recovery stage, wherein the gas pressure in the high-pressure ventilation stage is 0.2-0.8 MPa, and the duration is 0.1-0.5 s; the gas pressure in the low-pressure recovery stage is 0.05-0.2 MPa, and the duration is 0.05-0.3 s. Through the alternating changes in high and low pressure, the gas flow rate and cooling intensity in the through hole are periodically changed. During the high-pressure ventilation phase, the high-velocity, low-temperature gas can rapidly enter the via, improving the heat exchange efficiency between the resin and the airflow near the via wall. This causes the softened resin to cool and shrink rapidly, and enhances the thermal stress between the resin and the copper layer due to the difference in thermal expansion. During the low-pressure recovery phase, the impact of the airflow is reduced, allowing for a redistribution of temperature inside and outside the via wall, preventing excessive surface shrinkage and localized thermal stress concentration caused by continuous rapid cooling in the via opening area. By setting a high-pressure pulse duration of 0.1-0.5 s, the cold gas can effectively act on the via wall area, generating an effective temperature drop, while avoiding damage to the via wall caused by prolonged high-pressure impact. By setting a recovery time of 0.05-0.3 s, heat can be transferred to the surrounding area, allowing the subsequent high-pressure cooling phase to further expand the temperature difference and accumulate thermal shrinkage stress. These parameter ranges enhance the resin's stripping effect while maintaining the uniformity of the via wall cooling process. Because the in-plane tensile prestress during drilling has put the resin around the hole in a tensile constrained state, the periodic thermal shrinkage stress generated by pulse cooling and the in-plane tensile stress work together to cause the internal stress state of the resin to change continuously, promote the generation of microcracks between the drill sludge and the hole wall and reduce the interfacial bonding strength, thereby improving the detachment effect of weakly bonded drill sludge.

[0053] Furthermore, a trace amount of dry ice particles is added to the cryogenic gas, and the cryogenic gas carrying the dry ice particles enters the through-hole from bottom to top along the axial direction of the through-hole. Specifically, the dry ice particles are fine particles with a particle size of 50-100μm, so as to facilitate their entry into the interior of the through-hole with the gas flow and full contact with the hole wall. The amount of dry ice particles added can be set to 0.1%-1% of the gas flow mass to avoid excessive particles causing blockage of the through-hole or increased gas flow resistance. After the cryogenic gas carrying dry ice particles enters the through-hole, the dry ice particles move at high speed under the action of the gas flow and impact the hole wall. On the one hand, this can generate micro-scale mechanical impact on the weakly bonded drill smudge attached to the hole wall, causing cracks in the drill smudge layer and reducing its bonding strength with the hole wall; on the other hand, the dry ice particles sublimate and absorb heat during contact with the hole wall, further absorbing heat from the resin drill smudge and the resin near the hole wall, causing the softened resin to cool down and shrink rapidly. Because the in-plane tensile prestress during drilling already places the resin around the hole under tensile constraint, the thermal contraction caused by the cooling of dry ice particles is limited, resulting in stronger local thermal stress. This makes the resin-copper interface and the drill smudge adhesion interface more prone to microcracks and peeling. Simultaneously, the small size of micron-sized dry ice particles allows them to enter the recessed areas of the hole wall with the airflow, improving the removal of residual drill smudge without causing significant mechanical damage to the hole wall. Therefore, by introducing a small amount of dry ice particles into the low-temperature airflow, the synergistic effect of mechanical impact, sublimation endothermics, and thermal stress peeling can be achieved, further improving the drill smudge removal effect.

[0054] When a backing plate is placed under a multilayer circuit board, a vent hole running through the thickness direction can be provided within the backing plate, and an air jet pipe communicating with the vent hole can be provided on the underside of the backing plate. Specifically, the vent hole is coaxially arranged with the through hole to be processed, and its diameter can be set to be greater than or equal to the diameter of the through hole to ensure that the cryogenic gas can smoothly enter the through hole. The air jet pipe is connected to an external cryogenic gas source, and the gas pressure and on / off state are adjusted by a control valve, so that the cryogenic gas passes through the air jet pipe, the vent hole of the backing plate, and the through hole of the multilayer circuit board from bottom to top, providing directional cooling to the hole wall. Furthermore, a sealing ring or flexible sealing layer can be provided in the contact area between the backing plate and the multilayer circuit board to reduce gas leakage between the backing plate and the circuit board, allowing more cryogenic gas flow into the through hole, improving cooling efficiency and drilling smudge removal effect. The backing plate serves both as a drilling support and as a cryogenic gas transmission channel, making the cooling structure compatible with existing drilling processes.

[0055] The above are merely preferred embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A method for fabricating through-holes in a prestressed multilayer circuit board, the method comprising the following steps: S1. Place the multilayer circuit board on the drilling machine's processing table and fix it in place by pressing and positioning it with pressure feet; S2. Determine the drilling coordinates of the through hole to be prepared, and make the drill bit axis correspond to the drilling position; S3. Drive the drill bit to rotate and feed in a direction perpendicular to the board surface to form a through hole in the multilayer circuit board; S4. Keep the pressure foot in a pressed state and drive the drill bit to exit the through hole in the opposite direction of the feed direction; S5. Release the clamping and positioning of the presser foot, and remove the multilayer circuit board with the through hole processed; Its features are: During the drilling process, tensile prestress along the surface of the multilayer circuit board is applied to the multilayer circuit board.

2. The method for fabricating through-holes in a prestressed multilayer circuit board according to claim 1, characterized in that: The tensile prestress extends from the center of the pressure foot toward the edge of the multilayer circuit board.

3. The method for preparing through-holes in a prestressed multilayer circuit board according to claim 2, characterized in that: The presser foot is a columnar structure with an axial through hole. The diameter of the through hole gradually increases from top to bottom, and the edge of the lower surface of the presser foot is a planar structure.

4. The method for preparing through-holes in a prestressed multilayer circuit board according to claim 3, characterized in that: The ratio of the height of the presser foot to the outer diameter of the presser foot is 0.3-0.

6.

5. The method for preparing through-holes in a prestressed multilayer circuit board according to claim 4, characterized in that: The magnitude of the force applied to the upper surface of the presser foot varies periodically.

6. The method for preparing through-holes in a prestressed multilayer circuit board according to claim 1, characterized in that: A gripper is provided on the circumferential edge of the multilayer circuit board, and the gripper applies in-plane tensile prestress to the multilayer circuit board under the drive of a lead screw motor.

7. The method for fabricating through-holes in a multilayer circuit board with prestress assistance according to any one of claims 1-6, characterized in that: In step S3, before drilling, the drilling area of ​​the multilayer circuit board is locally cooled.

8. The method for fabricating through-holes in a multilayer circuit board with prestress assistance according to any one of claims 1-6, characterized in that: In step S4, after the drill bit exits the through hole, low-temperature gas is introduced into the through hole from bottom to top along the axial direction of the through hole.

9. The method for preparing through-holes in a prestressed multilayer circuit board according to claim 8, characterized in that: The cryogenic gas is dry nitrogen or compressed air, with a temperature of -20 to 5°C.

10. The method for preparing through-holes in a prestressed multilayer circuit board according to claim 9, characterized in that: The cryogenic gas is introduced into the through hole in a pulsed manner.

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