A method and system for bidirectional staggered riveting of multilayer printed circuit boards

Abstract

This invention discloses a bidirectional staggered riveting method and system for multilayer printed circuit boards, relating to the field of printed circuit board manufacturing technology. The method includes: placing the multilayer board on a worktable and activating the segmented pneumatic clamping assembly above; driving the upper punching module corresponding to the first diagonal target hole to perform unidirectional riveting from top to bottom; the rivet structure includes a flange end and a hollow tube coaxially connected; in the piercing stage, the rivet is first inserted into the target hole of the multilayer board, and the central positioning pin with a damping spring in the upper punching module first penetrates the internal channel of the rivet; in the flanging stage, the outer conical sleeve extrudes the hollow tube of the rivet in a high-dynamic micro-step forming mode; a set pressure holding pause period is introduced; and the lower punching module corresponding to the second diagonal target hole is driven to perform reverse unidirectional riveting from bottom to top. This invention can achieve bidirectional staggered riveting without flipping the multilayer board and has functions of venting, positioning alignment, and flexible stress release.

Description

Technical Field

[0001] This invention relates to the field of printed circuit board manufacturing technology, and in particular to a method and system for bidirectional staggered riveting of multilayer printed circuit boards. Background Technology

[0002] As printed circuit boards (PCBs) develop towards higher density, multilayering, and higher frequency and speed, the manufacturing process of PCBs with four or more layers is becoming increasingly complex. In the lamination process of multilayer boards, the inner core boards and prepregs need to be stacked together in sequence and then fed into a press for high temperature and high pressure curing.

[0003] To prevent the loosely stacked layers from sliding relative to each other (layer misalignment) during the pressing process and from warping of the board due to stress release during subsequent high-temperature cooling, rivets are usually driven into the target holes at the four corners of the board for fixation.

[0004] Most existing automatic riveting machines are unidirectional riveting devices, meaning all rivets (flange end upwards, hollow tube downwards) are inserted into the target hole from the same side of the multilayer board. During stamping, the upper punch applies instantaneous heavy pressure to the flange end of the rivet, forcing the hollow tube at the bottom to impact the forming protrusion of the base below and causing forced flanging. This traditional unidirectional riveting method has the following defects, which directly lead to warping and delamination risks during the pressing process: ① Unilateral stress concentration and asymmetric shape: The flange end before forming and the flange head after forming are completely different in thickness, roughness, and local extrusion pressure on the board surface. If all rivets are driven in from the same side, it means that one side of the multilayer board before pressing is a flat flange end, and the other side is a raised and stress-complex flange head. When the press applies hundreds of tons of pressure, this slight difference in thickness and asymmetry in shape will lead to uneven pressure distribution on the board surface by the pressure plate. In addition, the change from flow to solid state of the semi-cured sheet during high-temperature pressing can easily cause the unilateral concentrated release of mechanical stress to cause unilateral warping of the entire board. ② Interlayer air bubble trapping (potential for bulging): Existing clamping mechanisms typically use four cylinders working simultaneously to firmly press the edges of the multilayer board, followed by nailing. This instantaneous action of locking the four corners ignores the microscopic airflow distribution within the loosely stacked layers, easily causing air between the layers to be trapped and trapped in the center of the board. During subsequent high-temperature lamination, this air expands due to heat, which is one of the main causes of PCB board bursting or delamination. ③ Hole deviation and internal microcracks caused by violent stamping: Generally, a one-time instantaneous heavy pressure feeding method is used. When the hollow tube of the rivet is forcibly opened and the edge is turned over, the lateral tension of the metal will directly push the surrounding semi-cured sheet and inner core board, resulting in micron-level lateral slippage between layers (i.e., hole deviation).

[0005] To address the issue of uneven force distribution on one side, a solution combining unidirectional equipment with an automatic flipping mechanism has been explored. However, this approach carries extremely high technical risks. The reason is that before riveting, the multilayer boards (core board and prepreg) are loosely stacked without any securing. If a robotic arm clamps them and flips them 180 degrees in mid-air, the prepreg and inner core board are highly susceptible to misalignment or even scattering. Therefore, throughout the entire targeting and riveting process, the multilayer boards must be kept horizontal and flipping must be strictly prohibited.

[0006] Therefore, there is an urgent need for a riveting process that can achieve alternating forward and reverse nailing without flipping the multilayer board, and has functions such as air venting, positioning alignment and flexible stress release. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a bidirectional staggered riveting method and system for multilayer printed circuit boards, which can achieve staggered riveting in both directions without flipping the multilayer board, and has functions of air venting, positioning alignment, and flexible stress release.

[0008] In order to achieve the objective of this invention, the following solution is proposed: A bidirectional staggered riveting method for multilayer printed circuit boards is disclosed for fixing multilayer boards before lamination. The multilayer boards include at least one core board and one prepreg layer, and the multilayer boards have four corner target holes. The method includes the following steps: S1. Wave-type exhaust and clamping: Place the multilayer board on the workbench and start the segmented pneumatic clamping assembly above; by controlling the timing distribution of air pressure, the central pressure plate of the assembly first contacts the center of the multilayer board and applies pre-tightening force, and then the outer ring pressure plate descends, squeezing out the trapped gas between the layers from the center to the surrounding area, and finally clamping the riveting target hole area at the four corners. S2, First Diagonal Positioning and Riveting: The upper stamping module corresponding to the first diagonal target hole is driven to perform unidirectional riveting from top to bottom; the rivet structure includes a flange end and a hollow tube connected coaxially; in the piercing stage, the rivet is first placed into the target hole of the multilayer plate, and the central positioning pin with damping spring in the upper stamping module first penetrates the internal channel of the rivet; in the flanging stage, the tapered sleeve around the central positioning pin squeezes the hollow tube of the rivet in a high-dynamic micro-step forming mode, forcing its tube wall to expand outward and flanging; S3, Pressure Relief: With the first diagonal riveting completed and the tapered sleeve not retracted, a set pressure holding pause period is introduced to allow the mechanical stress generated inside the multilayer board due to the unidirectional force on the first diagonal to be released to the remaining unfixed areas. S4, Second Diagonal Reverse Compensation: After the pressure holding pause period ends, drive the lower punching module corresponding to the second diagonal target hole to perform reverse unidirectional riveting from bottom to top.

[0009] Furthermore, in step S1, the wave-like exhaust pressing action is achieved entirely by the same intake source through throttling air passages of different diameters, resulting in a purely mechanical delay. The time difference between the descent of the central pressure plate and the outer annular pressure plate is controlled between 0.2 seconds and 0.5 seconds.

[0010] Furthermore, in step S2, the high-dynamic micro-stepping molding mode refers to the following: the servo motor driving the conical sleeve does not continuously press down at a constant speed, but executes according to a step-like motion curve of "feed-pause-refeed"; the feed displacement is set to 0.1mm to 0.3mm each time, and the pause time after each feed is set to 10ms to 30ms, so as to utilize the physical creep characteristics of the prepreg polymer material to flexibly absorb the tension of the rivet expanding outward.

[0011] Furthermore, in step S3, the duration of the pressure holding pause is set to 0.3 to 0.8 seconds, during which the center positioning pin remains in the state of penetrating the rivet.

[0012] A bidirectional staggered riveting system for implementing the bidirectional staggered riveting method for multilayer printed circuit boards includes a frame, a workbench, a PLC main control module, and also includes a segmented pneumatic clamping assembly and a stamping module respectively connected to the PLC main control module. The segmented pneumatic clamping assembly is located above the workbench, including a central pressure plate driven by a cylinder and an annular pressure plate sleeved around it, with a one-way throttle valve assembly at each air inlet end; The stamping die includes two upper stamping dies located above the first diagonal of the multilayer board, and two lower stamping dies located below the second diagonal. Each stamping die is driven by an independent servo electric cylinder and has a nested central positioning pin and a tapered sleeve at its end. The central positioning pin is connected to a damping spring. In its natural state, the length of the central positioning pin extending outside the tapered sleeve is greater than the thickness of the multilayer board.

[0013] Furthermore, a high-precision grating ruler thickness measuring component is also installed on the workbench, which is used to measure the actual thickness of the multilayer board after the segmented pneumatic clamping component has completed its operation, and to feed the data back to the PLC main control module; the PLC main control module is used to dynamically adjust the starting coordinate position of the servo electric cylinder to execute the high dynamic micro-stepping molding mode according to the actual thickness of the multilayer board.

[0014] The beneficial effects of this invention are as follows: 1. The present invention adopts a segmented pneumatic pressing component, which utilizes physical air throttling to achieve a wave-like exhaust effect of pressing first in the center and then around the perimeter, just like pushing out air bubbles from the inside to the outside when applying a film, effectively avoiding the bursting of the pressing plate.

[0015] 2. The present invention adopts a nested stamping structure. Before the metal rivet expands under force, the central positioning pin passes through the target hole and rigidly locks the relative positions of all the plates in the radial direction. This can solve the problem of the lateral pushing force of the rivet when it is turned over affecting the alignment accuracy of the inner layer.

[0016] 3. This invention abandons the one-time violent impact of cylinders or hydraulics, and adopts the step-type feed curve of servo motor. Through the micro-stepping of "feed-pause-refeed", a time window for stress creep of polymer material in semi-cured sheet is left.

[0017] 4. Combining diagonal staggered pressure holding allows unidirectional stress to be fully released before reverse nailing compensation; the alternating layout of the flanges on both sides ensures that the physical thickness and mechanical internal stress of the upper and lower surfaces of the multilayer board are symmetrical, greatly improving the flatness of the final product. Attached Figure Description

[0018] Figure 1 A flowchart of a bidirectional staggered riveting method for multilayer printed circuit boards; Figure 2 A schematic diagram showing four target holes and four rivets; Figure 3 This is a schematic diagram of the stamping die and rivets. Detailed Implementation

[0019] Example 1 This embodiment provides a multi-layer printed circuit board bidirectional staggered riveting system, the main hardware of which includes a frame, a workbench located in the core processing area, a PLC main control module, and a segmented pneumatic pressing assembly and a stamping module connected to the PLC main control module.

[0020] Figure 2 This diagram illustrates the target holes and rivets. For the target holes at the four corners of the multilayer board, four sets of stamping dies are arranged diagonally on the worktable to drive the rivets into the corresponding target holes. First diagonal: Adopts a forward riveting structure, with two sets of upper stamping modules above the worktable and corresponding modules for feeding nails and supporting anvils below the worktable. The second diagonal section employs a reverse riveting configuration, with two sets of lower stamping modules located below the worktable, and corresponding modules for feeding rivets and supporting anvils located above the worktable. It should be noted that the feeding rivet and supporting anvil modules are existing technology, and their structure and principles will not be described in detail in this embodiment.

[0021] Each stamping die is driven by an independent servo electric cylinder and has a nested assembly center positioning pin and a tapered sleeve at the end (see...). Figure 3 The center positioning pin is connected to a damping spring. In its natural state, the length of the center positioning pin extending to the outside of the tapered sleeve is greater than the thickness of the multilayer board.

[0022] The segmented pneumatic clamping assembly is located above the worktable and includes a central pressure plate driven by a cylinder and an annular pressure plate surrounding it. Each air inlet end is equipped with a one-way throttle valve assembly. It should be noted that the annular pressure plate is not necessarily circular; it can also be U-shaped.

[0023] A high-precision grating ruler thickness measuring component is also installed on the worktable. This component measures the actual thickness of the multilayer board after the segmented pneumatic clamping assembly completes its operation and feeds the data back to the PLC main control module. The PLC main control module dynamically adjusts the starting coordinate position of the servo electric cylinder to execute the high-dynamic micro-stepping molding mode based on the actual thickness of the multilayer board. The high-precision grating ruler thickness measuring component is existing technology, and its structure and principle will not be described in detail in this embodiment.

[0024] Example 2 like Figure 1 As shown in the figure, this embodiment provides a bidirectional staggered riveting method for multilayer printed circuit boards, which includes the following steps: S1, Wave-type exhaust and compression Once the multi-layer board is placed on the workbench, the segmented pneumatic clamping assembly above the workbench is activated.

[0025] The high-pressure airflow first enters the cavity of the central pressure plate, and the central pressure plate drops instantly, pressing down on the physical center area of ​​the multilayer board with a preset pre-tightening force (such as 15kg).

[0026] The airflow must pass through a one-way throttle valve with a limited orifice before it can fill the cavity of the outer annular pressure plate. This purely mechanical fluid resistance spontaneously creates a mechanical action delay of approximately 0.3 seconds. During this 0.3 seconds, the air inside the centrally pressurized multilayer plate is forced to escape to the pressure-free edges. Subsequently, the outer annular pressure plate descends, compressing the four target hole areas at the edge of the multilayer plate. This wave-like action effectively expels interlayer trapped gas, reducing the risk of pressure-induced plate bursting.

[0027] S2, First diagonal positioning and riveting After the venting and compaction are completed, the rivet feeding and support anvil module under the worktable is activated. The feeding system pre-feeds T-shaped rivets into this module. The rivets consist of a hollow tube end and a flat flange end.

[0028] The top surface of the supporting anvil receives the flat flange end of the rivet, and then the cylinder drives the supporting anvil to rise upward, pushing the hollow tube end of the rivet into the target hole of the multilayer board from bottom to top.

[0029] After the rivet is inserted, the support anvil does not retract. During the subsequent rivet flanging process, the support anvil always provides strong back support for the rivet, ensuring that the hollow tube end of the rivet will not slide out of the target hole when it is squeezed.

[0030] The upper stamping die above the workbench begins its descent. Because the front end of the central locating pin is longer than the outer tapered sleeve, it moves first upon contact with the rivet, smoothly inserting directly into and penetrating the hollow channel inside the rivet. The very tip of the central locating pin can even further insert into the locating recess of the supporting anvil. At this point, the central locating pin, with an outer diameter equal to the inner diameter of the rivet, acts as a rigid inner core skeleton, connecting all the hole walls of the layers and achieving radial restriction. This ensures that when the hollow tube of the rivet expands and deforms laterally, no inner core plate or prepreg in the multilayer board can slip laterally, thus solving the hole misalignment problem.

[0031] After the central locating pin penetrates the hollow channel inside the rivet, the upper stamping module continues to descend, and the outer tapered sleeve begins to contact the hollow tube of the rivet. At this time, the servo motor driving the tapered sleeve starts the high-dynamic micro-stepping forming mode: the servo motor does not press down continuously at a constant speed, but performs a stepped feed of "downward 0.2mm - instantaneous stop 20ms - downward 0.2mm again".

[0032] In the stepped feed, the rivet's internal collapse is blocked by the central locating pin, forcing the hollow tube wall of the rivet to curl outwards and spread outwards. The 20-millisecond transient pause provides the prepreg polymer material inside the multilayer board with physical creep time to absorb the lateral expansion tension of the metal.

[0033] S3, Pressure Relief After the first diagonal riveting is completed, the upper stamping module remains in place without being immediately withdrawn, introducing a 0.5-second pressure holding pause. During this period, the local mechanical stress generated by the pressure along the first diagonal inside the multilayer board is released to the remaining unfixed areas and a new static stress equilibrium is reached.

[0034] S4, Second Diagonal Reverse Compensation After the pressure holding pause, the lower punching module corresponding to the second diagonal target hole is driven to perform reverse unidirectional riveting from bottom to top. This reverse riveting step not only compensates for the distribution of the rivet flange on both sides of the multilayer board in terms of physical thickness, but also neutralizes the residual deformation torque after the pressure is released from the first diagonal by the upward compressive stress. Ultimately, the forces on both sides of the multilayer board are symmetrical, greatly reducing the risk of warping and scrapping after entering the press.

[0035] The above embodiments are only used to illustrate the technical ideas and features of the present invention, and are not intended to be unique or to limit the present invention. Those skilled in the art should understand that various changes or equivalent substitutions made to the present invention without departing from its scope are all within the scope of protection of the present invention.

Claims

1. A method for bidirectional staggered riveting of multilayer printed circuit boards, used to fix multilayer boards before lamination, wherein the multilayer board comprises at least one core board and one prepreg layer, and the multilayer board has four corner target holes, characterized in that... The methods include: S1. Wave-type exhaust and clamping: Place the multilayer board on the workbench and start the segmented pneumatic clamping assembly above; by controlling the timing distribution of air pressure, the central pressure plate of the assembly first contacts the center of the multilayer board and applies pre-tightening force, and then the outer ring pressure plate descends, squeezing out the trapped gas between the layers from the center to the surrounding area, and finally clamping the riveting target hole area at the four corners. S2, First Diagonal Positioning and Riveting: The upper stamping module corresponding to the first diagonal target hole is driven to perform unidirectional riveting from top to bottom; the rivet structure includes a flange end and a hollow tube connected coaxially; in the piercing stage, the rivet is first placed into the target hole of the multilayer plate, and the central positioning pin with damping spring in the upper stamping module first penetrates the internal channel of the rivet; in the flanging stage, the tapered sleeve around the central positioning pin squeezes the hollow tube of the rivet in a high-dynamic micro-step forming mode, forcing its tube wall to expand outward and flanging; S3, Pressure Relief: With the first diagonal riveting completed and the tapered sleeve not retracted, a set pressure holding pause period is introduced to allow the mechanical stress generated inside the multilayer board due to the unidirectional force on the first diagonal to be released to the remaining unfixed areas. S4, Second Diagonal Reverse Compensation: After the pressure holding pause period ends, drive the lower punching module corresponding to the second diagonal target hole to perform reverse unidirectional riveting from bottom to top.

2. The method for bidirectional staggered riveting of multilayer printed circuit boards according to claim 1, characterized in that, In step S1, the wave-like exhaust pressing action is achieved by the same intake source through throttling air passages of different diameters, which is a purely mechanical delay. The time difference between the descent of the central pressure plate and the outer annular pressure plate is controlled between 0.2 seconds and 0.5 seconds.

3. The method for bidirectional staggered riveting of multilayer printed circuit boards according to claim 1, characterized in that, In step S2, the high-dynamic micro-stepping molding mode means that the servo motor driving the conical sleeve does not press down continuously at a constant speed, but executes according to a step-like motion curve of "feed-pause-refeed". The feed displacement is set to 0.1mm to 0.3mm each time, and the pause time after each feed is set to 10ms to 30ms, so as to utilize the physical creep characteristics of the prepreg polymer material to flexibly absorb the tension of the rivet expanding outward.

4. The method for bidirectional staggered riveting of multilayer printed circuit boards according to claim 1, characterized in that, In step S3, the duration of the pressure holding pause is set to 0.3 to 0.8 seconds. During this period, the center positioning pin remains in the state of penetrating the rivet.

5. A bidirectional staggered riveting system for implementing the bidirectional staggered riveting method for multilayer printed circuit boards according to any one of claims 1 to 4, comprising a frame, a workbench, and a PLC main control module, characterized in that, It also includes a segmented pneumatic clamping assembly and a stamping module, which are respectively connected to the PLC main control module; The segmented pneumatic clamping assembly is located above the workbench, including a central pressure plate driven by a cylinder and an annular pressure plate sleeved around it, with a one-way throttle valve assembly at each air inlet end; The stamping die includes two upper stamping dies located above the first diagonal of the multilayer board, and two lower stamping dies located below the second diagonal. Each stamping die is driven by an independent servo electric cylinder and has a nested central positioning pin and a tapered sleeve at its end. The central positioning pin is connected to a damping spring. In its natural state, the length of the central positioning pin extending outside the tapered sleeve is greater than the thickness of the multilayer board.

6. The bidirectional staggered riveting system according to claim 5, characterized in that, The workbench is also equipped with a high-precision grating ruler thickness measuring component, which is used to measure the actual thickness of the multilayer board after the segmented pneumatic clamping component has completed its operation, and to feed the data back to the PLC main control module; the PLC main control module is used to dynamically adjust the starting coordinate position of the servo electric cylinder to execute the high dynamic micro-stepping molding mode according to the actual thickness of the multilayer board.

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