Laminating method of rigid-flex printed circuit board

Abstract

This application relates to the field of printed circuit board manufacturing technology, and particularly to a lamination method for rigid-flex boards, comprising: attaching a flexible printed circuit board to a rigid substrate stack including prepreg to form a laminate to be laminated; hot-pressing the laminate to be laminated at a first temperature and a first pressure below the prepreg's full curing critical temperature, so that the prepreg softens at the rigid-flexible transition interface and initially wets and bonds the interfaces of each adjacent layer, obtaining an intermediate pre-bonded transition state; performing interface quality testing on the laminate to be laminated in the intermediate pre-bonded transition state; hot-pressing the qualified laminate to be laminated in the intermediate pre-bonded transition state at a second temperature above the prepreg's full curing critical temperature and a second pressure above the first pressure, to complete the curing of the prepreg and lamination bonding. The embodiments of this application can solve the problem that irreversible prepreg curing during the lamination process of rigid-flex boards leads to irreparable defects at the transition interface once formed.

Description

Technical Field

[0001] This application belongs to the field of printed circuit board manufacturing technology, and particularly relates to a lamination method for rigid-flex boards. Background Technology

[0002] Rigid-flex boards are composite circuit boards that combine a flexible printed circuit board (FPC, with polyimide (PI) as the substrate) and a rigid printed circuit board (FR-4 epoxy fiberglass cloth as the substrate) through a lamination process. In its manufacturing process, a composite layer formed by stacking multiple materials such as an FR-4 core board, prepreg (PP), FPC flexible substrate, and cover film is thermo-pressed in one step under high temperature (170℃ to 200℃) and high pressure (2.0MPa to 4.0MPa). This process softens and flows the PP, completing the thermosetting cross-linking reaction and permanently bonding all interlayer interfaces into a unified structure.

[0003] In the aforementioned single-stage, one-step molding process, the thickness of the rigid region lamination of the rigid-flex PCB is typically between 0.8 mm and 2.0 mm, while the thickness of the flexible region of the FPC is typically only 0.1 mm to 0.3 mm, creating a significant thickness difference at the transition interface. During high-pressure lamination, this thickness difference leads to localized stress concentration at the transition interface. When the local stress peak exceeds the adhesion strength between the FPC cover film and the PI substrate, delamination defects will occur at the transition interface. More importantly, the thermosetting reaction of PP is an irreversible chemical process in which two adjacent prepolymer molecules cross-link through covalent bonds to form a three-dimensional network structure. Once this cross-linking reaction is completed under high pressure and high temperature conditions, PP permanently transforms into a thermosetting rigid body, and the delamination defects already formed at the transition interface are permanently locked in the cured lamination structure. Any subsequent repair methods (including reheating and pressurization) cannot restore the bonding strength of the delaminated interface.

[0004] Industry production data shows that defects at the rigid-flexible transition interface of rigid-flexible laminates account for more than 55% of the total laminate failures. Because minute delaminations at the transition interface are difficult to detect during routine visual inspections, these defects often rapidly expand into product reliability failures under alternating stress during subsequent bending reliability tests or actual product use. Furthermore, the rework process further damages the surrounding intact interfaces, causing significant quality risks and economic losses.

[0005] Therefore, how to solve the problem of irreversible transition interface defects caused by the curing of PP during the lamination process of rigid-flex PCBs from a mechanistic perspective is a technical issue that urgently needs to be addressed in the current rigid-flex PCB manufacturing field. Summary of the Invention

[0006] This application provides a lamination method for rigid-flexible composite panels, which can solve the problem that once the transition interface defects are formed, they cannot be repaired due to the irreversible curing of prepreg during the lamination process of rigid-flexible composite panels.

[0007] In a first aspect, embodiments of this application provide a lamination method for a rigid-flexible laminate, applied in the manufacturing process of a rigid-flexible laminate including a rigid region and a flexible region, the method comprising: A flexible printed circuit board is attached to a rigid substrate stack including prepreg to form a stack to be laminated. The laminate to be pressed is hot-pressed at a first temperature and a first pressure below the critical temperature for full curing of the prepreg, so that the prepreg softens and initially wets and bonds the interfaces of each adjacent layer at the rigid-flexible transition interface, thereby obtaining an intermediate pre-bonded transition state. In the intermediate pre-bonded transition state, the gelation rate of the prepreg at the rigid-flexible transition interface does not exceed a first threshold, and the bonding at the rigid-flexible transition interface is mainly physical wetting and adhesion and has not yet formed a thermosetting crosslinked network. Interface quality testing is performed on the laminate to be pressed in the intermediate pre-bonding transition state; if defects that do not meet the preset standards are detected, the laminate to be pressed in the intermediate pre-bonding transition state is decomposed and the prepreg is replaced, and the steps of forming the laminate to be pressed and obtaining the intermediate pre-bonding transition state are repeated. The prepreg in the intermediate pre-bonding transition state is hot-pressed at a second temperature higher than the critical temperature for full curing of the prepreg and a second pressure higher than the first pressure to complete the curing and lamination of the prepreg.

[0008] The technical solutions described in this application embodiment have at least the following technical effects: The lamination method for rigid-flex PCBs provided in this application involves attaching a flexible printed circuit board to a rigid substrate stack including prepreg to form a laminate to be laminated. By constructing a basic laminate structure including rigid regions, flexible regions, and an adhesive medium, a standardized substrate is provided for subsequent step-by-step hot pressing processes. The laminate to be laminated is hot-pressed at a first temperature and a first pressure lower than the prepreg's full curing critical temperature, causing the prepreg to soften and initially wet and bond the interfaces of adjacent layers at the rigid-flexible transition interface, obtaining an intermediate pre-bonded transition state. This allows for precise control of the prepreg's curing process, achieving initial positioning and bonding of the laminate to be laminated while reducing the formation of irreversible thermosetting cross-linked networks. The laminate to be laminated in the intermediate pre-bonded transition state undergoes interface quality testing. If any non-compliance with preset standards is detected, the laminate is then laminated. For defects, the prepreg in the intermediate pre-bonding transition state of the laminate to be pressed is decomposed and the prepreg is replaced. The steps of forming the laminate to be pressed and obtaining the intermediate pre-bonding transition state are repeated. This allows for accurate identification and correction of defects at the rigid-flexible transition interface before the prepreg is fully cured, reducing the number of unqualified laminates to be pressed from the source, and thus reducing the scrapping of the entire board due to defects entering the full curing stage. The laminate to be pressed in the intermediate pre-bonding transition state that has passed the test is hot-pressed at a second temperature higher than the critical temperature for full curing of the prepreg and a second pressure higher than the first pressure to complete the curing and lamination of the prepreg. The laminate to be pressed that has passed the test is then hot-pressed for final full curing, achieving a high-strength permanent bond between the rigid substrate laminate and the flexible printed circuit board, which helps to improve the structural stability and long-term reliability of the rigid-flex board.

[0009] This method constructs a reworkable intermediate pre-bonded transition state through stepwise hot pressing, overcoming the technical limitation of irreversible prepreg curing. It can solve the problem that once the transition interface defects are formed, they cannot be repaired due to the irreversible curing of prepreg during the lamination process of rigid-flexible laminates.

[0010] In some embodiments, the first threshold is 15%.

[0011] In some embodiments, the first temperature is in the range of 100°C to 130°C, and the first pressure is in the range of 0.3 MPa to 0.8 MPa; the second temperature is in the range of 170°C to 200°C, and the second pressure is in the range of 2.0 MPa to 4.0 MPa; the hot pressing time for obtaining the intermediate pre-bonded transition state is 10 min to 20 min, and when the first temperature is higher than 120°C, the hot pressing time for obtaining the intermediate pre-bonded transition state does not exceed 15 min; the hot pressing time for completing the curing and lamination of the prepreg is 60 min to 120 min.

[0012] In some embodiments, the interface quality detection of the laminate to be compressed in the intermediate pre-bonding transition state includes: The bubble area and delamination defects at the rigid-flexible transition interface are detected by ultrasonic C-scan. The preset standard is that the bubble area does not exceed 5% of the total area of ​​the rigid-flexible transition interface, and there are no delamination defects that extend continuously for more than 5 mm.

[0013] In some embodiments, the step of decomposing and replacing the prepreg in the intermediate pre-bonded transition state of the laminate to be pressed includes: The laminate to be pressed in the intermediate pre-bonding transition state is heated to 80°C to 90°C and kept at that temperature for a preset time. Then, a peeling force is applied along the direction perpendicular to the rigid-flexible transition interface at a rate of no more than 5 mm / min to decompose the flexible printed circuit board and the rigid substrate laminate layer by layer. After decomposition, the prepreg is replaced, and the steps of forming the laminate to be pressed and obtaining the intermediate pre-bonded transition state are repeated.

[0014] In some embodiments, attaching the flexible printed circuit board to a rigid substrate stack including prepreg includes: In the rigid substrate stack, a margin groove is preset on the rigid region side adjacent to the rigid-flexible transition interface. The margin groove is a through groove structure that is continuously distributed in the rigid substrate stack along the extension direction of the rigid-flexible transition interface.

[0015] In some embodiments, the pre-set allowance groove in the rigid region adjacent to the rigid-flexible transition interface in the rigid substrate stack includes: Before performing the step of attaching the flexible printed circuit board to the rigid substrate stack including the prepreg, on each rigid substrate constituting the rigid substrate stack, at a point 0.5 mm to 2.0 mm inward from the rigid-flexible transition interface towards the rigid region, a pre-formed allowance groove with a width W of 0.3 mm to 1.0 mm and a depth D of not less than 0.1 mm and not exceeding two-thirds of the thickness of a single rigid substrate is pre-formed along the direction of the rigid-flexible transition interface; after the rigid substrates are stacked, the allowance grooves of each layer are aligned in the stacking direction, and together form an allowance groove channel penetrating the rigid substrate stack and all the prepreg layers.

[0016] In some embodiments, the hot pressing of the qualified laminate in the intermediate pre-bonded transition state at a second temperature higher than the prepreg's full curing critical temperature and a second pressure higher than the first pressure further includes: Before hot pressing, an auxiliary compensation pad is installed on the surface of the outermost rigid substrate of the rigid substrate stack near the rigid-flexible transition interface; the thick end of the auxiliary compensation pad is aligned with the rigid-flexible transition interface, extends along one side of the rigid region and gradually thins to zero.

[0017] In some embodiments, the auxiliary compensation pad is made of a heat-resistant elastic material, and the thickness gradient of the auxiliary compensation pad satisfies the following: the initial thickness at the rigid-flexible transition interface is 50% to 70% of the difference between the thickness of the outermost rigid substrate and the thickness of the flexible printed circuit board; it gradually thins to zero along the direction away from the rigid-flexible transition interface at a gradient of 0.02mm to 0.05mm per millimeter, and the width of the gradient transition region is not less than 3mm; the thin end of the auxiliary compensation pad is fixed to the surface of the outermost rigid substrate with heat-resistant double-sided tape.

[0018] In some embodiments, attaching the flexible printed circuit board to a rigid substrate stack including prepreg further includes: In the rigid zone 1 mm to 3 mm from the rigid-flexible transition interface of the cover film opening edge of the flexible printed circuit board, a high-temperature hot-melt adhesive sheet is pre-placed between the prepreg layer and the innermost copper layer. The thickness of the high-temperature hot-melt adhesive sheet is 0.03 mm to 0.08 mm. During the hot pressing process to obtain the intermediate pre-bonded transition state, the high-temperature hot-melt adhesive sheet is heated and melted and penetrates into the micro gap between the cover film opening edge and the prepreg layer.

[0019] In some embodiments, the method further includes: After performing the steps of curing and laminating the prepreg, the edge area containing the allowance groove is removed by CNC milling. A residual edge strip of not less than 0.3mm is left between the milling outline and the rigid-flexible transition interface. The residual edge strip is removed together during subsequent shape processing. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic flowchart of the lamination method for the rigid-flexible composite plate provided in the embodiments of this application; Figure 2 This is a schematic cross-sectional view of the laminated structure of the rigid-flexible composite plate provided in the embodiments of this application; Figure 3This is a schematic diagram of defect propagation in the conventional single-stage lamination process of rigid-flexible composite plates provided in the embodiments of this application; Figure 4 This is a schematic diagram of the first-stage pre-bonding transition state and stress distribution of the rigid-flexible plate provided in the embodiments of this application. Detailed Implementation

[0022] To make the technical problems, technical solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0023] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. The character " / " generally indicates that the preceding and following related objects have an "or" relationship.

[0024] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0025] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0026] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0027] The terms "first" and "second" are used for descriptive purposes only, to distinguish objects, such as substances, from one another, and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. For example, without departing from the scope of the embodiments of this application, "first XX" may also be referred to as "second XX," and similarly, "second XX" may also be referred to as "first XX." Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of that feature.

[0028] In related technologies, the thickness of the rigid region lamination in rigid-flex PCBs typically ranges from 0.8 mm to 2.0 mm, while the thickness of the flexible region in FPC is usually only 0.1 mm to 0.3 mm, creating a significant thickness difference at the transition interface. During high-pressure lamination, this thickness difference leads to localized stress concentration at the transition interface. When the local stress peak exceeds the adhesion strength between the FPC cover film and the PI substrate, delamination defects will occur at the transition interface. More importantly, the thermosetting reaction of PP is an irreversible chemical process in which two adjacent prepolymer molecules cross-link through covalent bonds to form a three-dimensional network structure. Once this cross-linking reaction is completed under high pressure and high temperature conditions, PP permanently transforms into a thermosetting rigid body, and the delamination defects already formed at the transition interface are permanently locked in the cured lamination structure. Any subsequent repair methods (including reheating and pressurization) cannot restore the bond strength of the delaminated interface.

[0029] To address the aforementioned issues, this application provides a lamination method for a rigid-flex PCB. In this method, a flexible printed circuit board is attached to a rigid substrate stack including a prepreg to form a laminate to be laminated. By constructing a basic laminate structure including a rigid region, a flexible region, and an adhesive medium, a standardized substrate is provided for subsequent step-by-step hot pressing processes. The laminate to be laminated is hot-pressed at a first temperature and a first pressure below the prepreg's full curing critical temperature, causing the prepreg to soften and initially wet and bond the interfaces of adjacent layers at the rigid-flexible transition interface, obtaining an intermediate pre-bonded transition state. This allows for precise control of the prepreg's curing process, achieving initial positioning and bonding of the laminate while reducing the formation of irreversible thermosetting cross-linked networks. The laminate to be laminated in the intermediate pre-bonded transition state undergoes interface quality testing. If defects not meeting preset standards are detected, the laminate is then... By decomposing and replacing the prepreg in the intermediate pre-bonded transition state of the laminate to be pressed, and repeating the steps of forming the laminate to be pressed and obtaining the intermediate pre-bonded transition state, the defects at the rigid-flexible transition interface can be accurately identified and corrected before the prepreg is fully cured. This reduces the number of unqualified laminates to be pressed from the source, thereby reducing the scrap of the entire board due to defects entering the full curing stage. The laminate to be pressed in the intermediate pre-bonded transition state that has passed the test is hot-pressed at a second temperature higher than the critical temperature for full curing of the prepreg and a second pressure higher than the first pressure to complete the curing and lamination of the prepreg. The laminate to be pressed in the final full curing hot-pressing is then performed on the laminate that has passed the test, achieving a high-strength permanent bond between the rigid substrate laminate and the flexible printed circuit board, which helps to improve the structural stability and long-term reliability of the rigid-flex board.

[0030] This method constructs a reworkable intermediate pre-bonded transition state through stepwise hot pressing, overcoming the technical limitation of irreversible prepreg curing. It can solve the problem that once the transition interface defects are formed, they cannot be repaired due to the irreversible curing of prepreg during the lamination process of rigid-flexible laminates.

[0031] The rigid-flex PCB lamination method provided in this application can be applied to rigid-flex PCB lamination equipment. Rigid-flex PCB lamination equipment is used in the printed circuit board manufacturing industry to stack and press rigid substrates, prepregs, flexible printed circuit boards, and other raw materials layer by layer according to a designed sequence into a single unit. Its core function is to provide a precisely controllable temperature, pressure, and vacuum environment, which helps improve interlayer bonding strength and interface quality. It is widely used in the production of rigid-flex PCBs in consumer electronics, automotive electronics, aerospace, and other fields. For example, rigid-flex PCB lamination equipment may include a stacking device, a hot pressing device, an interface detection device, and a control device, with the control device electrically connected to the stacking device, the hot pressing device, and the interface detection device, respectively.

[0032] The lamination device is used to perform the step of attaching a flexible printed circuit board to a rigid substrate stack including prepreg, forming a laminate to be pressed together, as described in this embodiment. The lamination device includes an automatic feeding unit, a CCD vision positioning unit, a multi-axis robotic arm gripping unit, and a pin alignment platform. The automatic feeding unit is used to transport the three types of single-layer materials—rigid substrate, prepreg, and flexible printed circuit board—separately. For example, it can be a tray-type automatic feeder or a silo-type feeder; this embodiment does not impose specific limitations on this. The CCD vision positioning unit is used to identify the positioning marks of each layer of material, with an alignment accuracy of ±0.05mm. For example, it can be a vision positioning system composed of a 5-megapixel industrial area scan camera, a ring shadowless light source, and an image processing board; this embodiment does not impose specific limitations on this. The multi-axis robotic arm gripping unit sequentially grips and stacks each layer of material according to a preset layer sequence: first, the bottom rigid substrate and prepreg are stacked to form the lower half of the rigid substrate stack; then, the flexible printed circuit board is gripped, placed, and positioned on top of the lower half of the rigid substrate stack; finally, the upper prepreg and rigid substrate are stacked to form a complete rigid substrate stack that wraps the flexible printed circuit board, ultimately assembling the stack to be pressed together. The multi-axis robotic arm gripping unit can be, for example, a six-axis industrial robotic arm, equipped with an anti-static vacuum suction cup clamp, enabling stable gripping and placement of materials of different thicknesses. This application embodiment does not impose specific limitations on this. A pin alignment platform is used to fix the stacked stack to be pressed together, reducing interlayer misalignment. For example, it can be a pneumatic pin alignment platform or a servo-driven pin alignment platform. This application embodiment does not impose specific limitations on this.

[0033] The hot pressing device is used to perform the steps of obtaining the intermediate pre-bonded transition state and completing the curing and lamination bonding of the prepreg in the embodiments of this application. The hot pressing device includes a vacuum hot pressing chamber, a multi-segment temperature control system, a gradient pressure control system, and a vacuum pumping system. The vacuum hot pressing chamber provides a sealed vacuum environment for the hot pressing process, which can expel interlayer air and reduce bubble defects. For example, it can be a vertical vacuum hot pressing chamber made of stainless steel, with built-in upper and lower parallel heating plates. The flatness error of the plates does not exceed 0.02mm. The embodiments of this application do not impose specific limitations on this. The multi-segment temperature control system adopts a zoned independent heating design, and the temperature control accuracy can reach ±1℃. It supports programming up to 10 segments of continuous heating, holding, and cooling curves. The first temperature and the second temperature in the embodiments of this application can be set separately. The multi-segment temperature control system can be, for example, a PID closed-loop temperature controller, paired with a K-type thermocouple temperature sensor. The embodiments of this application do not impose specific limitations on this. The gradient pressure control system is hydraulically driven, with a pressure uniformity error of less than 2%. It supports multi-segment pressure switching, and the first and second pressures in this embodiment can be set separately. The gradient pressure control system can be, for example, a servo hydraulic drive system paired with a high-precision pressure sensor, enabling continuous pressure adjustment within the range of 0.1 MPa to 5.0 MPa. This embodiment does not impose specific limitations on this. The vacuum pumping system can evacuate the vacuum in the vacuum hot pressing chamber to below 100 Pa, which can improve the wetting effect of the prepreg. For example, it can be a vacuum unit composed of a rotary vane vacuum pump and a Roots vacuum pump. This embodiment does not impose specific limitations on this.

[0034] The interface inspection device is used to perform the step of inspecting the interface quality of the laminate to be pressed in the intermediate pre-bonding transition state in the embodiments of this application. The interface inspection device includes an ultrasonic C-scanning unit, a defect image recognition unit, and a data processing unit. The ultrasonic C-scanning unit uses a high-frequency ultrasonic probe to scan the rigid-flexible transition interface line by line, which can penetrate multiple layers of material to detect internal bubbles and delamination defects, with a detection resolution of up to 0.1 mm. The ultrasonic C-scanning unit can be, for example, a water immersion ultrasonic C-scanning device using a 10MHz to 50MHz high-frequency focused ultrasonic probe, coupled with an XYZ three-axis servo scanning stage. This embodiment of the application does not impose specific limitations on this. The defect image recognition unit automatically identifies the location, size, and type of defects in the scanned image based on an image recognition algorithm. For example, it can be an industrial vision inspection module based on deep learning, or an FPGA image processing board integrated inside the ultrasonic C-scanning device. This embodiment of the application does not impose specific limitations on this. The data processing unit compares the detected defect parameters with preset standards, automatically determines whether the laminate to be pressed and laminated is qualified, and feeds back the determination result to the control device, such as an industrial computer equipped with a high-speed data acquisition card. This application embodiment does not impose specific limitations on this.

[0035] The control device is used to uniformly monitor and schedule the entire lamination process. For example, the control device can be an industrial control computer, an embedded controller, a programmable logic controller, or other equipment with data processing and control functions. This application does not impose specific limitations on this.

[0036] To better understand the lamination method for rigid-flexible composite plates provided in the embodiments of this application, the specific implementation process of the lamination method for rigid-flexible composite plates provided in the embodiments of this application will be described by way of example below.

[0037] Figure 1 A schematic flowchart of a lamination method for a rigid-flexible composite plate provided in an embodiment of this application is shown. The lamination method for the rigid-flexible composite plate includes: S100: A flexible printed circuit board is attached to a rigid substrate stack including prepreg to form a stack to be pressed together.

[0038] Flexible printed circuit boards (PCBs) are printed circuit boards with polyimide as the substrate, possessing bendable properties, used to achieve the flexible connection function of rigid-flex boards. Prepreg is a sheet-like bonding medium made of glass fiber cloth impregnated with semi-cured epoxy resin; it is semi-solid at room temperature, softens and flows upon heating, and eventually cross-links and cures. The rigid substrate stack is a purely rigid assembly of multiple independent FR-4 rigid substrates stacked according to a designed layer sequence, providing structural support for the rigid-flex board. The lamination to be pressed is the overall structure formed by stacking the rigid substrate stack, prepreg, and flexible PCB in a designed order; it is the direct processing object of the hot pressing process.

[0039] For example, following a preset lamination sequence, the materials are stacked sequentially using a stacking device. First, a bottom rigid substrate is placed, followed by a prepreg on the bottom rigid substrate. Then, a flexible printed circuit board (PCB) is precisely positioned and placed on the surface of the prepreg. Finally, the prepreg and the upper rigid substrate are placed on top of the PCB, forming a complete laminate to be pressed and bonded. During the stacking process, a CCD vision positioning unit identifies the positioning marks of each layer to ensure that the interlayer alignment accuracy is no less than ±0.05mm. Finally, a pin alignment platform secures the laminate to be pressed and bonded, reducing interlayer misalignment. For example, for a four-layer rigid-flex board, the lamination sequence is: bottom rigid substrate, first prepreg, flexible printed circuit board, second prepreg, and upper rigid substrate, with the positioning error of each layer controlled within ±0.03mm.

[0040] As an optional embodiment of this application, S100 involves attaching a flexible printed circuit board to a rigid substrate stack including prepreg, comprising: S110, a margin groove is preset on the rigid region side of the rigid region adjacent to the rigid-flexible transition interface in the rigid substrate stack. The margin groove is a through groove structure that is continuously distributed in the rigid substrate stack along the extension direction of the rigid-flexible transition interface.

[0041] It is understandable that the rigid-flexible transition interface is the boundary area between the rigid substrate stack and the flexible printed circuit board, specifically the contact surface between the prepreg and the cover film of the flexible printed circuit board, which is a high-incidence area for lamination defects. The allowance groove is a groove structure formed on the rigid substrate. After multiple rigid substrates are stacked, they form a continuous through channel to accommodate excess prepreg that overflows during hot pressing.

[0042] For example, before the stacking assembly, through slots are machined on each rigid substrate constituting the rigid substrate stack using CNC machining equipment. The through slots are located adjacent to the rigid region side of the rigid-flexible transition interface, and the extension direction of the through slots is parallel to the rigid-flexible transition interface. The rigid substrates with the through slots machined are stacked according to the designed layer sequence, so that the through slots on each layer of rigid substrates are perfectly aligned in the direction perpendicular to the board surface, forming a clearance slot that runs through the rigid substrate stack. For example, a CNC milling machine is used to machine the through slots on the rigid substrates, with a milling cutter diameter of 0.5 mm and a machining accuracy of ±0.05 mm. During stacking, pins are used for alignment to ensure that the alignment error of the through slots in each layer does not exceed 0.1 mm.

[0043] This design provides an active containment space for excess prepreg that overflows at the rigid-flexible transition interface during hot pressing, reducing the risk of excess prepreg seeping into the functional areas of the flexible printed circuit board and causing circuit contamination or short circuits. At the same time, it can relieve local stress at the transition interface, reduce the generation of delamination defects, and thus reduce the risk of scrapping the entire board due to defects that cannot be repaired in the curing stage.

[0044] In one possible implementation, S110, a allowance groove is pre-set in the rigid region of the rigid substrate stack, immediately adjacent to the rigid-flexible transition interface, including: S111 Before performing the step of attaching the flexible printed circuit board to the rigid substrate stack including the prepreg, on each rigid substrate constituting the rigid substrate stack, at a point 0.5 mm to 2.0 mm inward toward the rigid region from the rigid-flexible transition interface, a margin groove with a width W of 0.3 mm to 1.0 mm and a depth D of not less than 0.1 mm and not more than two-thirds of the thickness of a single rigid substrate is prefabricated along the direction of the rigid-flexible transition interface; after the rigid substrates are stacked, the margin grooves of each layer are aligned in the stacking direction, and together form a margin groove channel that penetrates the rigid substrate stack and all the prepreg layers.

[0045] It can be understood that the stacking direction is the direction in which the rigid substrate layers are stacked, that is, the direction perpendicular to the surface of the rigid substrate. The allowance groove channel is a continuous cavity structure formed after the allowance grooves on each layer of the rigid substrate are aligned. It not only runs through the entire rigid substrate stack, but also extends to the adjacent prepreg layer, and can accommodate the excess adhesive material overflowing from each layer of prepreg.

[0046] For example, the machining position of the allowance groove is marked on the design drawing of the rigid substrate, so that the distance between the inner edge of the allowance groove and the rigid-flexible transition interface is within the range of 0.5mm to 2.0mm; a rectangular cross-section through groove is machined on the rigid substrate using a CNC milling machine, the width of the through groove is controlled between 0.3mm and 1.0mm, and the depth is not less than 0.1mm and does not exceed two-thirds of the thickness of a single rigid substrate; the machined rigid substrate and prepreg are stacked alternately, so that the allowance grooves of each layer are completely overlapped in the stacking direction, and the prepreg softens and flows during the hot pressing process and will naturally fill the channels of the allowance groove. For example, for a rigid substrate with a thickness of 0.5mm, the depth of the allowance groove is set to 0.3mm, the width is set to 0.5mm, and the inward distance from the rigid-flexible transition interface is set to 1.0mm.

[0047] This design, by limiting the position, size, and alignment requirements of the allowance groove, ensures that the groove channel has sufficient volume to accommodate excess prepreg. It also reduces the problem of reduced structural strength in the rigid zone due to the groove being too close to the rigid-flexible transition interface, or too far to effectively accommodate excess adhesive. The through-channel allowance effectively channels out excess adhesive from each layer of prepreg, further reducing the probability of interface defects and minimizing irreparable defects after curing.

[0048] As an optional embodiment of this application, S100, attaching the flexible printed circuit board to the rigid substrate stack including the prepreg, further includes: S120, in the rigid zone 1mm to 3mm from the rigid-flexible transition interface of the cover film opening edge of the flexible printed circuit board, a high-temperature hot-melt adhesive sheet is pre-placed between the prepreg layer and the innermost copper layer. The thickness of the high-temperature hot-melt adhesive sheet is 0.03mm to 0.08mm. During the hot pressing process to obtain the intermediate pre-bonded transition state, the high-temperature hot-melt adhesive sheet is heated and melted and penetrates into the micro gap between the cover film opening edge and the prepreg layer.

[0049] It can be understood that cover film openings are pre-reserved opening areas on the cover film of flexible printed circuit boards, used to expose the internal copper layer for electrical connection or bonding with rigid areas. High-temperature hot-melt adhesive sheets are thin-film adhesive materials made of thermoplastic resins, with a softening point lower than the prepreg's full-curing critical temperature. They melt and become fluid upon heating; examples include epoxy resin-based hot-melt adhesive films, acrylic ester hot-melt adhesive films, and ABF adhesive sheets. Microgaps are tiny gaps formed between the cover film opening edge and the prepreg layer due to microscopic unevenness on the material surface, typically ranging in width from a few micrometers to tens of micrometers.

[0050] For example, a high-temperature hot-melt adhesive sheet is cut into strips with a width of 2mm to 5mm. During the lamination assembly process, the cut high-temperature hot-melt adhesive sheet is placed between the prepreg layer and the innermost copper layer of the flexible printed circuit board, with one edge of the high-temperature hot-melt adhesive sheet aligned with the edge of the cover film opening of the flexible printed circuit board, and the other edge extending 1mm to 3mm into the rigid area. The high-temperature hot-melt adhesive sheet is continuously laid along the entire length of the cover film opening of the flexible printed circuit board, which can cover all areas where micro-gaps may exist. For example, an epoxy resin-based hot-melt adhesive sheet with a thickness of 0.05mm is cut into strips with a width of 3mm and placed 2mm away from the rigid-flexible transition interface, laid along the entire length of the cover film opening of the flexible printed circuit board.

[0051] With this configuration, the high-temperature hot-melt adhesive sheet melts first during the low-temperature hot-pressing process in the intermediate pre-bonding transition state, penetrating into the micro-gap between the opening edge of the cover film of the flexible printed circuit board and the prepreg layer to form a preliminary seal. This prevents the liquid prepreg from penetrating into the functional area of ​​the flexible printed circuit board along the micro-gap during the subsequent high-pressure curing process, reducing the possibility of short circuits or pad contamination. At the same time, it can improve the bonding strength of the cover film edge of the flexible printed circuit board, reduce the generation of delamination defects, and thus reduce scrap due to unrepairable defects.

[0052] S200, hot-pressing is performed on the laminate to be pressed at a first temperature and a first pressure lower than the critical temperature for full curing of the prepreg, so that the prepreg softens and initially wets and bonds the interfaces of each adjacent layer at the rigid-flexible transition interface, thereby obtaining an intermediate pre-bonded transition state; in the intermediate pre-bonded transition state, the gelation rate of the prepreg at the rigid-flexible transition interface does not exceed the first threshold, and the bonding of the rigid-flexible transition interface is mainly physical wetting and adhesion and has not yet formed a thermosetting cross-linked network.

[0053] It is understood that the critical temperature for full curing of prepreg is the lowest temperature at which the prepreg resin system begins to undergo a rapid cross-linking reaction and the gelation rate exceeds 50% after 30 minutes of holding at standard atmospheric pressure. It is an inherent property parameter of prepreg. This application applies to all epoxy resin-based prepregs, including but not limited to DICY curing systems, phenolic curing systems, BMI curing systems, etc. For the commonly used DICY curing system FR-4 prepreg, this temperature is 140℃ to 145℃.

[0054] The first temperature is a hot-pressing temperature below the critical temperature for full curing of the prepreg, used to soften the prepreg without causing a large amount of cross-linking reaction. The first pressure is a hot-pressing pressure below the final curing pressure, used to promote wetting of adjacent interfaces by the prepreg without causing excessive flow of the adhesive. The intermediate pre-bonded transition state is the intermediate state reached after the laminates to be bonded have undergone hot pressing at the first temperature and first pressure. At this point, the laminates have been initially positioned but have not yet formed a permanent bond.

[0055] Gelation rate is the percentage of cross-linked functional groups in a prepreg resin out of the total cross-linkable functional groups, used to quantitatively characterize the degree of curing of the prepreg. The first threshold is the upper limit of the gelation rate of the prepreg in the intermediate pre-bonded transition state. Physical wetting adhesion is an interfacial bond formed by van der Waals forces and micromechanical interlocking, which does not involve covalent bonds and is reversible. Thermosetting cross-linked networks are three-dimensional network structures formed by the cross-linking of prepreg resin molecules through covalent bonds. Once formed, they cannot be softened or dissolved by heating and are irreversible.

[0056] For example, the laminate to be pressed is placed in the vacuum hot pressing chamber of a hot pressing device, the hot pressing chamber is closed and the vacuum is evacuated to below 100 Pa; the hot pressing parameters are set to a first temperature and a first pressure, the temperature is increased to the first temperature at a rate of 2°C / min to 5°C / min, the temperature and pressure are held for a specified time, and then cooled to below 60°C at a rate not exceeding 3°C / min. After depressurization, the laminate is removed, thus obtaining the intermediate pre-bonded transition state. For example, for the 2116 type DICY curing system prepreg, the first temperature is set to 125°C, the first pressure is 0.5 MPa, the temperature and pressure holding time is 15 min, and the pressure is released after cooling to 50°C.

[0057] S300, perform interface quality inspection on the laminate to be pressed in the intermediate pre-bonding transition state; if defects that do not meet the preset standards are detected, decompose the laminate to be pressed in the intermediate pre-bonding transition state and replace the prepreg, and repeat the steps of forming the laminate to be pressed and obtaining the intermediate pre-bonding transition state.

[0058] Interface quality inspection, as understood, employs non-destructive testing methods to evaluate the internal bonding quality of the rigid-flexible transition interface, identifying defects such as bubbles, delamination, and foreign matter inclusions at the interface. Preset standards are a quantitative indicator system used to determine whether the interface quality is acceptable. Defects are abnormal structures at the interface that can affect product reliability. Decomposition is the process of separating the layers to be bonded in the intermediate pre-bonding transition state into independent rigid substrates, flexible printed circuit boards, and prepregs.

[0059] For example, the laminate to be pressed in the intermediate pre-bonding transition state is placed on the testing table of the interface detection device. The ultrasonic C-scan unit is activated to scan the rigid-flexible transition interface line by line to obtain an acoustic image of the rigid-flexible transition interface. The scanned image is processed by the defect image recognition unit to identify bubbles and delamination defects and calculate their size parameters. The data processing unit compares the calculated defect parameters with preset standards to automatically determine whether the laminate is qualified. For laminates determined to be unqualified, they are transferred to the rework station, where the laminate is decomposed layer by layer by heating and mechanical peeling. After removing the old prepreg, new prepreg is replaced and reassembled into the laminate to be pressed. Steps S100 and S200 are executed again. For example, a 10MHz high-frequency ultrasonic probe is used for scanning, with a scanning step size of 0.1mm and an image resolution of 0.1mm / pixel.

[0060] As an optional embodiment of this application, S300 involves performing interface quality testing on the laminate to be compressed in the intermediate pre-bonding transition state, including: S310 uses ultrasonic C-scan to detect the bubble area and delamination defects at the rigid-flexible transition interface. The preset standard is that the bubble area does not exceed 5% of the total area of ​​the rigid-flexible transition interface, and there are no delamination defects that extend continuously for more than 5 mm.

[0061] As can be understood, ultrasonic C-scanning is a non-destructive testing technique that uses high-frequency ultrasound waves to penetrate materials and generate cross-sectional images by receiving reflected echoes from different interfaces within the material. This clearly displays the distribution of defects within the material. The bubble area is the sum of the projected areas of all bubble defects at the rigid-flexible transition interface. Delamination defects are air gap defects formed by local separation between adjacent layers; the continuous extension length of a delamination defect is the maximum straight-line distance of a single delamination defect in the direction parallel to the rigid-flexible transition interface.

[0062] For example, the stack to be pressed in the intermediate pre-bonding transition state is placed stably on the testing stage of the interface detection device. The ultrasonic C-scan unit is started, and a high-frequency focused ultrasonic probe of 10MHz to 50MHz is used to uniformly incident ultrasonic waves into the interior of the stack to be tested through a water immersion scanning method. The position of the XYZ three-axis servo scanning stage and the focal length of the probe are adjusted so that the ultrasonic focus is precisely aligned with the plane where the rigid-flexible transition interface is located. The probe is controlled to move line by line along the X and Y axes with a preset step size, emitting ultrasonic waves into the interior of the stack and simultaneously acquiring the reflected echo signals at the rigid-flexible transition interface. The ultrasonic C-scan unit converts the acquired analog echo signal into a digital signal and transmits it to the defect image recognition unit. The defect image recognition unit converts the digital signal into a grayscale acoustic image and, based on a binarization image algorithm, identifies bright spots (corresponding to bubble defects) with grayscale values ​​above a threshold and dark lines (corresponding to delamination defects) with grayscale values ​​below a threshold, automatically calculating the area and length of each defect. The data processing unit compares the calculated total bubble area and the longest continuous delamination length with preset standards, automatically determining whether the layers to be laminated are qualified, and feeding the determination result back to the control device. For example, for a rigid-flexible transition interface with an area of ​​100mm × 20mm, using a 10MHz high-frequency focused ultrasonic probe, with a scan step size of 0.1mm, the maximum allowable total bubble area is 100mm², and continuous delamination defects with a length exceeding 5mm are not allowed.

[0063] This setup, using ultrasonic C-scan, can accurately detect minute defects within the rigid-flexible transition interface without damaging the laminate. The quantified preset standards ensure objective and consistent quality assessment, reducing subjective errors from manual interpretation. By strictly controlling the bubble area and layer length, the laminate entering the subsequent curing stage can have qualified interface quality, reducing irreparable defects after curing from the source and lowering the product scrap rate.

[0064] As an optional embodiment of this application, S300 involves decomposing and replacing the prepreg in the intermediate pre-bonding transition state of the laminate to be pressed, including: S320 heats the laminate to be pressed in the intermediate pre-bonding transition state to 80°C to 90°C, holds it at the temperature for a preset time, and then applies a peeling force at a rate of no more than 5 mm / min in the direction perpendicular to the rigid-flexible transition interface to decompose the flexible printed circuit board and rigid substrate laminate layer by layer.

[0065] It is understandable that the preset time is the holding time after heating, typically 5 to 10 minutes, used to ensure uniform temperature inside the layers to be pressed and laminated in the intermediate pre-bonding transition state, and to fully soften the prepreg. The peel force is the external force used to separate adjacent layers; its direction is perpendicular to the rigid-flexible transition interface to reduce tensile or tearing damage to the flexible printed circuit board during separation. Layer-by-layer decomposition involves separating the rigid substrate, prepreg, and flexible printed circuit board layers sequentially in the reverse order of the layers in the intermediate pre-bonding transition state.

[0066] For example, the defective pre-bonded laminate to be pressed is placed flat on a constant temperature heating table. The heating table temperature is set to 80°C to 90°C and held for 5 to 10 minutes to soften the prepreg inside the laminate uniformly. The edges of the rigid and flexible areas of the laminate are held by a peeling jig. The peeling device is started and a peeling force is applied at a constant rate of 2 mm / min to 5 mm / min in a direction perpendicular to the board surface. First, the upper rigid substrate and the prepreg are separated, and then the flexible printed circuit board and the lower rigid substrate are separated to complete the layer-by-layer decomposition of the laminate.

[0067] The verification was conducted using a four-layer rigid-flex PCB as an example: the flexible printed circuit board had a linewidth of 0.1 mm, a copper thickness of 18 μm, and a polyimide cover film with a room temperature adhesion strength ≥1.2 N / mm; the prepreg used was FR-4 prepreg of the DICY curing system (type 2116), with a gelation rate of 13% in the intermediate pre-bonded transition state. Following method S320, the laminate was heated to 85℃ and held for 8 min, with a vertical peel force applied at a rate of 3 mm / min. The decomposed flexible printed circuit board was examined under a 50x optical microscope, and no copper foil wrinkling, tearing, or cover film delamination was observed; the area of ​​residual prepreg at the interface was less than 5%. After replacing the prepreg and repeating steps S100 to S200, the S300 ultrasonic C-scan was performed again, and the interface defect rate was 0%. The fully cured sample after rework underwent a 180° bending test (bending radius 2 mm, 100,000 cycles), and no delamination failure was observed at the rigid-flexible transition interface; the performance was essentially consistent with the first-time bonding qualified product.

[0068] This setup, through low-temperature heating to further soften the prepreg, significantly reduces its physical wetting and adhesion forces. Combined with a low-speed, uniform peeling force, it allows for the safe disassembly of the laminate without damaging the flexible printed circuit board or the rigid substrate, enabling the replacement of the prepreg and the reassembly of the laminate. This process fully utilizes the reversible interfacial bonding characteristics of the intermediate pre-bonding transition state, transforming defective laminates that would otherwise be scrapped into repairable products. This solves the problem in traditional processes where defects are irreparable once formed.

[0069] S330, after decomposition, replace the prepreg and repeat the steps of forming the laminate to be pressed and obtaining the intermediate pre-bonded transition state.

[0070] It is understandable that replacing the prepreg involves completely removing the old prepreg that has already undergone initial softening and contamination after decomposition and replacing it with brand new, unused prepreg to improve the interfacial bonding quality after re-lamination.

[0071] For example, after decomposition, the surfaces of the rigid substrate and flexible printed circuit board are gently wiped with a lint-free cloth soaked in anhydrous ethanol to remove residual prepreg debris and contaminants; the rigid substrate and flexible printed circuit board are checked for damage, and after confirming that they are intact, the new prepreg, flexible printed circuit board and rigid substrate are placed according to the original lamination sequence and reassembled into a laminate to be pressed; the new laminate to be pressed is placed in a hot pressing device and hot-pressed again according to the parameters of S200 to obtain a new intermediate pre-bonded transition state.

[0072] This setup, by replacing the prepreg with a completely new one and re-performing the lamination and prebonding steps, reduces interface defects present in the original lamination, resulting in a qualified intermediate prebonding transition state. This process achieves closed-loop repair of defective products, reducing material waste and economic losses caused by scrapping entire boards. It also verifies the repairability of the phased lamination process presented in this application, effectively solving the industry problem of irreversible defects caused by the curing of prepreg.

[0073] S400 uses a second temperature higher than the critical temperature for full curing of the prepreg and a second pressure higher than the first pressure to hot press the qualified prepreg in the intermediate pre-bonding transition state to complete the curing and lamination of the prepreg.

[0074] It can be understood that the second temperature is the hot-pressing temperature, which is higher than the critical temperature for full curing of the prepreg, and is used to allow the prepreg to undergo a complete cross-linking reaction, forming a stable thermosetting network. The second pressure is the hot-pressing pressure (i.e., the final curing pressure), which is higher than the first pressure, and is used to promote the full flow of the prepreg to fill the micro-gaps between layers, thereby improving the interlayer bonding strength. Curing is the irreversible process by which the prepreg resin transforms from a semi-solid to a solid thermosetting material through a cross-linking reaction. Lamination bonding is the process of permanently bonding the rigid substrate stack and the flexible printed circuit board into a single structure through the curing of the prepreg.

[0075] For example, the qualified intermediate pre-bonded transition layer to be laminated is placed in the vacuum hot pressing chamber of a hot pressing device. The hot pressing chamber is closed and evacuated to below 100 Pa. The hot pressing parameters are set to a second temperature and a second pressure. The temperature is increased to the second temperature at a rate of 1°C / min to 3°C / min. After holding at the temperature and pressure for a specified time, it is cooled in a controlled manner to below 60°C at a rate not exceeding 2°C / min. After depressurization, the laminate is removed, and the lamination bonding is completed. For example, for the 2116 type DICY curing system prepreg, the second temperature is set to 185°C, the second pressure is 3.0 MPa, the holding time is 90 min, and the pressure is released after cooling to 50°C.

[0076] As an optional embodiment of this application, S400, hot pressing of the qualified laminate in the intermediate pre-bonding transition state at a second temperature higher than the prepreg's full curing critical temperature and a second pressure higher than the first pressure, further includes: S410, before hot pressing, an auxiliary compensation pad is installed on the surface of the outermost rigid substrate of the rigid substrate stack near the rigid-flexible transition interface; the thick end of the auxiliary compensation pad is aligned with the rigid-flexible transition interface, extends along one side of the rigid area and gradually thins to zero.

[0077] It can be understood that the outermost rigid substrate is the one located on the top and bottom outer surfaces of the rigid substrate stack, directly in contact with the pressure plate of the hot pressing device. The auxiliary compensation pad is a pad with a thickness gradient made of heat-resistant elastic material, used to compensate for the thickness difference between the rigid and flexible areas. The thick end is the end with the largest thickness of the auxiliary compensation pad. The thin end is the end with the smallest thickness of the auxiliary compensation pad, with the thickness reduced to zero.

[0078] For example, the auxiliary compensation pad is cut into a strip with the same length as the rigid-flexible transition interface. Before hot pressing, the auxiliary compensation pad is placed on the surface of the outermost rigid substrate of the rigid substrate stack, so that the thick edge of the auxiliary compensation pad is precisely aligned with the rigid-flexible transition interface, and the thin end of the auxiliary compensation pad extends towards the rigid area.

[0079] With this configuration, the auxiliary compensation pad, through its thickness gradient structure, can compensate for the thickness difference between the rigid and flexible areas, ensuring a smooth transition of pressure transmitted by the hot press plate at the rigid-flexible interface. This reduces delamination defects caused by localized pressure concentration, while also minimizing abnormal flow of prepreg at the interface, lowering the risk of adhesive overflow, further improving the bonding quality of the rigid-flexible interface, reducing defects that cannot be repaired after curing, and enhancing the long-term reliability of the product.

[0080] By adopting the above steps S100 to S400, the traditional single-stage irreversible lamination process is divided into three stages: pre-bonding to establish a repairable intermediate state, defect detection and repair, and full curing permanent bonding. Quality confirmation and defect correction nodes are introduced before the prepreg is fully cured, so that defects in the rigid-flexible transition interface can be accurately identified and repaired before entering the irreversible curing stage. This breaks through the technical limitation of irreversible prepreg curing at the mechanism level and solves the problem that once the defects in the transition interface are formed, they cannot be repaired due to the irreversible curing of prepreg in the lamination process of rigid-flexible boards.

[0081] In some embodiments, the first threshold is 15%.

[0082] It is understandable that when the gelation rate of the prepreg is below 15%, only a few isolated cross-linking points are formed inside, and a continuous three-dimensional thermosetting cross-linking network has not yet been formed. The interfacial bonding is still mainly physical wetting adhesion, and the stack has good decomposability. When the gelation rate exceeds 15%, a continuous cross-linking network begins to form inside the prepreg, and the interfacial bonding gradually changes to chemical covalent bonding. The decomposability of the stack decreases significantly, and forced decomposition will lead to damage to the flexible printed circuit board or rigid substrate.

[0083] In this embodiment, the gelation rate of the prepreg was determined using differential scanning calorimetry (DSC), conforming to the industry standard IPC-TM-650 2.3.18. The specific measurement method is as follows: Samples of prepreg (approximately 5-10 mg) were taken from the rigid-flexible transition interface and heated from room temperature to 250°C using a DSC instrument at a heating rate of 10°C / min. The exothermic peak of the curing reaction was recorded. The gelation rate was calculated using the following formula: ,in, Total heat of reaction of uncured prepreg (unit: J / g). The residual heat of reaction (unit: J / g) of the prepreg sample taken after hot pressing at the first temperature and first pressure.

[0084] This setting, with the first threshold set at 15%, ensures that the prepreg fully softens and wets adjacent interfaces, forming a stable intermediate pre-bonded transition state, facilitating accurate interface quality detection. It also reduces the likelihood of the prepreg not forming an irreversible cross-linked network, allowing for the safe decomposition and repair of defective layers. This achieves the optimal balance between detectability and repairability, a key parameter for improving the feasibility of the proposed process. Furthermore, extensive comparative experiments have verified that for FR-4 prepreg in the DICY curing system, after using a 125℃ / 0.5MPa / 15min process, the gelation rate measured by the aforementioned DSC method is between 12% and 15%, meeting the requirements. When the gelation rate is between 15% and 20%, approximately 30% of the samples show localized prepreg residue. When the gelation rate exceeds 20%, the peeling success rate is less than 10%.

[0085] In some embodiments, the first temperature is in the range of 100°C to 130°C, and the first pressure is in the range of 0.3 MPa to 0.8 MPa; the second temperature is in the range of 170°C to 200°C, and the second pressure is in the range of 2.0 MPa to 4.0 MPa; the hot pressing time to obtain the intermediate pre-bonded transition state is 10 min to 20 min, and when the first temperature is higher than 120°C, the hot pressing time to obtain the intermediate pre-bonded transition state does not exceed 15 min; the hot pressing time to complete the curing and lamination of the prepreg is 60 min to 120 min.

[0086] It is understandable that the lower limit of the first temperature, 100℃, is the characteristic temperature at which the prepreg resin transitions from a glassy state to a highly elastic state and begins to soften significantly. Below this temperature, the molecular chain segments of the prepreg resin lack sufficient mobility and cannot adequately wet the microscopic rough surfaces of adjacent layers, resulting in insufficient interfacial bonding. The upper limit of the first temperature, 130℃, is lower than the critical temperature for full curing of the prepreg (140℃-145℃). Above this temperature, the crosslinking reaction rate of the prepreg will increase exponentially, and even if the hot-pressing time is shortened, it will be difficult to control the gelation rate below 15%, resulting in the loss of the decomposability of the intermediate pre-bonded transition state.

[0087] The lower limit of the first pressure, 0.3 MPa, is the minimum pressure required to overcome the microscopic surface irregularities between the prepreg and adjacent layers, facilitating effective surface contact. Below this pressure, a large number of tiny air gaps will remain at the interface, forming bubble defects that cannot be removed through subsequent curing. The upper limit of the first pressure, 0.8 MPa, is the highest pressure that can suppress excessive lateral flow and overflow of the prepreg. Above this pressure, the semi-fluid prepreg will be squeezed out of the rigid-flexible transition interface, which will not only contaminate the functional areas of the flexible printed circuit board but also lead to insufficient adhesive at the interface, reducing the final bond strength.

[0088] The lower limit of the second temperature, 170℃, is the minimum temperature at which the prepreg can complete the cross-linking reaction and form a stable three-dimensional thermosetting network. Below this temperature, the degree of cross-linking of the prepreg cannot reach more than 95%, resulting in a lower glass transition temperature and insufficient interfacial bonding strength. This makes the product prone to delamination failure during long-term use. The upper limit of the second temperature, 200℃, is the critical temperature for thermal aging of the prepreg resin and the PI substrate of the flexible printed circuit board. Above this temperature, the prepreg resin will undergo oxidative decomposition, and the mechanical properties of the PI substrate and cover film will decrease, reducing the bending life of the flexible area.

[0089] The lower limit of the second pressure, 2.0 MPa, is the minimum pressure required for the molten prepreg to completely fill all micro-gaps between layers and reduce interfacial voids. Below this pressure, the interfacial bonding density is insufficient, leading to a decrease in overall board rigidity and poor resistance to damp heat. The upper limit of the second pressure, 4.0 MPa, is the highest pressure that can suppress structural damage at the rigid-flexible transition interface. Above this pressure, the cover film of thin flexible printed circuit boards will be crushed, while the rigid substrate will warp due to uneven pressure, and the problem of prepreg overflow will be exacerbated.

[0090] The lower limit of 10 minutes for hot-pressing to achieve the intermediate pre-bonded transition state is the shortest time that helps the prepreg soften uniformly and fully wet all areas across the entire rigid-flexible transition interface. Below this time, the prepreg at the interface edges is not fully softened, resulting in localized poor wetting. The upper limit of 20 minutes for hot-pressing to achieve the intermediate pre-bonded transition state is the longest time at a minimum first temperature of 100°C where the prepreg gelation rate remains below 15%. Exceeding this time, even at the minimum temperature, will result in sufficient cross-linking reactions, leading to loss of decomposability. When the first temperature is above 120°C, the curing reaction rate of the prepreg increases. According to differential scanning calorimetry (DSC) test data, the gelation rate is 12%-15% after holding at 125°C for 15 minutes, and exceeds 20% after holding for 20 minutes. Therefore, the hot-pressing time is usually limited to within 15 minutes to keep the gelation rate within the target range.

[0091] The minimum hot-pressing time of 60 minutes to complete the curing and lamination of the prepreg is the shortest time to ensure that the crosslinking degree of the prepreg reaches more than 95% at the second temperature. Below this time, the prepreg is not fully cured, and the interfacial bond strength and resistance to damp heat are difficult to meet product reliability requirements. The maximum hot-pressing time of 120 minutes to complete the curing and lamination of the prepreg is the longest time to balance production efficiency and product performance. Exceeding this time will not significantly increase the crosslinking degree of the prepreg; instead, it will lead to excessive crosslinking and brittleness of the resin, while increasing production energy consumption and cycle time without any additional performance benefits.

[0092] The specific temperature, pressure, and time parameters mentioned above are based on the industry-standard DICY curing system FR-4 prepreg. If other types of epoxy resin-based prepregs, such as phenolic curing systems or BMI systems, are used, those skilled in the art can determine the curing reaction initiation temperature and total heat of reaction of the uncured prepreg using differential scanning calorimetry (DSC) to determine its full curing critical temperature (i.e., the temperature at which the gelation rate reaches 50% after holding for 30 minutes at a heating rate of 10℃ / min). Based on this, the first temperature is set to be in the range of 15℃ to 40℃ below the full curing critical temperature. Combined with a first pressure of 0.3MPa to 0.8MPa and a hot-pressing time of 10min to 20min, the gelation rate can be verified by DSC. This allows the gelation rate of the intermediate pre-bonded transition state to be controlled within the target threshold, all of which fall within the scope of protection of this application.

[0093] This setup, by appropriately matching the temperature, pressure, and time parameters of the first and second stages, allows for precise control of the prepreg curing process. In the first stage, a stable intermediate pre-bonded transition state with a gelation rate meeting requirements is obtained, which is beneficial for improving the detectability and repairability of the laminate. In the second stage, the prepreg is fully cured, achieving sufficient interlayer bonding strength and structural stability. This not only solves the problem of unrepairable defects but also improves the mechanical properties and long-term reliability of the product.

[0094] In some embodiments, the auxiliary compensation pad is made of a heat-resistant elastic material, and the thickness gradient of the auxiliary compensation pad satisfies the following: the initial thickness at the rigid-flexible transition interface is 50% to 70% of the difference between the thickness of the outermost rigid substrate and the thickness of the flexible printed circuit board; it gradually thins to zero along the direction away from the rigid-flexible transition interface at a gradient of 0.02 mm to 0.05 mm per millimeter, and the width of the gradient transition region is not less than 3 mm; the thin end of the auxiliary compensation pad is fixed to the surface of the outermost rigid substrate with heat-resistant double-sided tape.

[0095] It is understandable that heat-resistant elastic materials can withstand high temperatures of 170℃ to 200℃ and high pressures of 2.0MPa to 4.0MPa, and possess good elastic recovery and low permanent compression set. Examples include high-density silicone rubber, polytetrafluoroethylene-coated fiberglass cloth, and ceramic fiber reinforced silicone rubber. The initial thickness of the auxiliary compensation pad at the rigid-flexible transition interface is taken as 50% to 70% of the difference between the thickness of the outermost rigid substrate and the thickness of the flexible printed circuit board. This is to compensate for the permanent compression deformation generated by the auxiliary compensation pad during hot pressing, so that the auxiliary compensation pad can effectively fill the thickness difference throughout the entire hot pressing process. The thickness gradient and the width of the transition area are to achieve a smooth transition of pressure and reduce the generation of new stress concentration points. The heat-resistant double-sided tape can withstand the high temperatures during the hot pressing process and will not leave adhesive residue on the surface of the rigid substrate. Examples include double-sided tape containing polyimide (PI) or high-temperature resistant double-sided tape containing fiberglass cloth. Using heat-resistant double-sided tape to fix the thin end of the auxiliary compensation pad to the surface of the outermost rigid substrate of the rigid substrate stack can reduce the probability of the pad slipping during hot pressing.

[0096] This setup, by optimizing the material and geometric parameters of the auxiliary compensation pad, can maximize its pressure compensation effect, making the pressure distribution at the rigid-flexible transition interface more uniform, reducing the risk of delamination defects caused by local stress concentration, and the reasonable thickness gradient and transition area width reduce the problem of new pressure abrupt changes at the edge of the pad, further improving the interface bonding quality, and thus reducing defects that cannot be repaired after curing.

[0097] In some embodiments, the method further includes: After completing the curing and lamination steps of the prepreg, the edge area containing the allowance groove is removed by CNC milling. A residual edge strip of no less than 0.3mm is left between the milling outline and the rigid-flexible transition interface. The residual edge strip is removed together during subsequent shape processing.

[0098] CNC milling is a process that uses a computer-controlled milling machine to mechanically process sheet metal, removing excess material. Residual edge strips are small amounts of rigid material retained between the milled outline and the rigid-flexible transition interface, used to isolate milling stress and the functional areas of the flexible printed circuit board. Shape machining is the final shaping process performed on the rigid-flex board, including milling the final product shape and chamfering.

[0099] For example, the cured laminate is fixed on the vacuum table of a CNC milling machine, a preset milling program is imported, and a 2mm diameter carbide end mill is used to remove the edge region containing the allowance groove along the milling contour line at a spindle speed of 10,000 rpm to 15,000 rpm and a feed rate of 500 mm / min to 1,000 mm / min. During the milling process, a residual edge band of not less than 0.3mm is left between the milling contour line and the rigid-flexible transition interface. For example, the milling contour line is set at a position 0.5mm away from the rigid-flexible transition interface, leaving a residual edge band of 0.5mm.

[0100] This setup, by removing the edge area containing the allowance groove through CNC milling, can reduce the impact of the allowance groove on the product's appearance and structure. The remaining edge strip can effectively isolate the mechanical stress generated during the milling process, reducing the delamination defects caused by stress transmission to the rigid-flexible transition interface. The remaining edge strip is removed together during subsequent shape processing, without affecting the final shape dimensions of the product, thus achieving a unity between the functionality of the allowance groove and the aesthetics of the product.

[0101] It should be understood that the sequence number of each step in the above embodiments does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0102] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A lamination method of a rigid-flex printed circuit board, characterized by, The method, applied in the manufacturing process of a rigid-flexible plate including a rigid region and a flexible region, comprises: A flexible printed circuit board is attached to a rigid substrate stack including prepreg to form a stack to be laminated. The laminate to be pressed is hot-pressed at a first temperature and a first pressure below the critical temperature for full curing of the prepreg, so that the prepreg softens and initially wets and bonds the interfaces of each adjacent layer at the rigid-flexible transition interface, thereby obtaining an intermediate pre-bonded transition state. In the intermediate pre-bonded transition state, the gelation rate of the prepreg at the rigid-flexible transition interface does not exceed a first threshold, and the bonding at the rigid-flexible transition interface is mainly physical wetting and adhesion and has not yet formed a thermosetting crosslinked network. Interface quality testing is performed on the laminate to be pressed in the intermediate pre-bonding transition state; if defects that do not meet the preset standards are detected, the laminate to be pressed in the intermediate pre-bonding transition state is decomposed and the prepreg is replaced, and the steps of forming the laminate to be pressed and obtaining the intermediate pre-bonding transition state are repeated. The prepreg in the intermediate pre-bonding transition state is hot-pressed at a second temperature higher than the critical temperature for full curing of the prepreg and a second pressure higher than the first pressure to complete the curing and lamination of the prepreg.

2. The method of laminating a rigid-flex board according to claim 1, wherein The first threshold is 15%.

3. The method of laminating a rigid-flex board according to claim 1, wherein The first temperature is in the range of 100℃ to 130℃, and the first pressure is in the range of 0.3MPa to 0.8MPa; the second temperature is in the range of 170℃ to 200℃, and the second pressure is in the range of 2.0MPa to 4.0MPa; the hot pressing time to obtain the intermediate pre-bonded transition state is 10min to 20min, and when the first temperature is higher than 120℃, the hot pressing time to obtain the intermediate pre-bonded transition state does not exceed 15min; the hot pressing time to complete the curing and lamination bonding of the prepreg is 60min to 120min.

4. The lamination method for rigid-flexible composite plates as described in claim 1, characterized in that, The interface quality detection of the layers to be compressed and laminated in the intermediate pre-bonding transition state includes: The bubble area and delamination defects at the rigid-flexible transition interface are detected by ultrasonic C-scan. The preset standard is that the bubble area does not exceed 5% of the total area of ​​the rigid-flexible transition interface, and there are no delamination defects that extend continuously for more than 5 mm.

5. The lamination method for the rigid-flexible composite plate as described in claim 1, characterized in that, The process of decomposing and replacing the prepreg in the intermediate pre-bonding transition state of the laminate to be pressed includes: The laminate to be pressed in the intermediate pre-bonding transition state is heated to 80°C to 90°C and kept at that temperature for a preset time. Then, a peeling force is applied along the direction perpendicular to the rigid-flexible transition interface at a rate of no more than 5 mm / min to decompose the flexible printed circuit board and the rigid substrate laminate layer by layer. After decomposition, the prepreg is replaced, and the steps of forming the laminate to be pressed and obtaining the intermediate pre-bonded transition state are repeated.

6. The lamination method for the rigid-flexible composite plate as described in claim 1, characterized in that, The process of attaching a flexible printed circuit board to a rigid substrate stack including prepreg includes: In the rigid substrate stack, a margin groove is preset on the rigid region side adjacent to the rigid-flexible transition interface. The margin groove is a through groove structure that is continuously distributed in the rigid substrate stack along the extension direction of the rigid-flexible transition interface.

7. The lamination method for the rigid-flexible composite plate as described in claim 6, characterized in that, The pre-set allowance groove in the rigid region adjacent to the rigid-flexible transition interface in the rigid substrate stack includes: Before performing the step of attaching the flexible printed circuit board to the rigid substrate stack including the prepreg, on each rigid substrate constituting the rigid substrate stack, at a point 0.5 mm to 2.0 mm inward from the rigid-flexible transition interface towards the rigid region, a pre-formed allowance groove with a width W of 0.3 mm to 1.0 mm and a depth D of not less than 0.1 mm and not exceeding two-thirds of the thickness of a single rigid substrate is pre-formed along the direction of the rigid-flexible transition interface; after the rigid substrates are stacked, the allowance grooves of each layer are aligned in the stacking direction, and together form an allowance groove channel penetrating the rigid substrate stack and all the prepreg layers.

8. The lamination method for the rigid-flexible composite plate as described in claim 1, characterized in that, The hot pressing of the qualified pre-bonded laminate in the intermediate pre-bonding transition state at a second temperature higher than the prepreg's full curing critical temperature and a second pressure higher than the first pressure further includes: Before hot pressing, an auxiliary compensation pad is installed on the surface of the outermost rigid substrate of the rigid substrate stack near the rigid-flexible transition interface; the thick end of the auxiliary compensation pad is aligned with the rigid-flexible transition interface, extends along one side of the rigid region and gradually thins to zero.

9. The lamination method for the rigid-flexible composite plate as described in claim 8, characterized in that, The auxiliary compensation pad is made of heat-resistant elastic material, and the thickness gradient of the auxiliary compensation pad satisfies the following: the initial thickness at the rigid-flexible transition interface is 50% to 70% of the difference between the thickness of the outermost rigid substrate and the thickness of the flexible printed circuit board; it gradually thins to zero along the direction away from the rigid-flexible transition interface at a gradient of 0.02mm to 0.05mm per millimeter, and the width of the gradient transition region is not less than 3mm; the thin end of the auxiliary compensation pad is fixed to the surface of the outermost rigid substrate with heat-resistant double-sided tape.

10. The lamination method for the rigid-flexible composite plate as described in claim 1, characterized in that, The method of attaching the flexible printed circuit board to a rigid substrate stack including prepreg further includes: In the rigid zone 1 mm to 3 mm from the rigid-flexible transition interface of the cover film opening edge of the flexible printed circuit board, a high-temperature hot-melt adhesive sheet is pre-placed between the prepreg layer and the innermost copper layer. The thickness of the high-temperature hot-melt adhesive sheet is 0.03 mm to 0.08 mm. During the hot pressing process to obtain the intermediate pre-bonded transition state, the high-temperature hot-melt adhesive sheet is heated and melted and penetrates into the micro gap between the cover film opening edge and the prepreg layer.

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