A method and system for etching open a cover of a multilayer rigid-flex circuit board having an inner layer pad

Abstract

The application discloses a multilayer soft and hard combined circuit board etching uncovering processing method and system, and relates to the technical field of printed circuit board manufacturing. The disclosed multilayer soft and hard combined circuit board etching uncovering processing method and system realize precise uncovering through prepositioning of a TPI adhesive layer and a segmented hot pressing process, and the protective layer is removed through chemical dissolution after etching, so that damage to the solder pad caused by traditional processes is avoided, and the method has the advantages of simplifying the processing flow, improving uncovering precision and product yield.

Description

Technical Field

[0001] This application relates to the field of printed circuit board manufacturing technology, and in particular to a method and system for etching and opening a multilayer circuit board with inner pads. Background Technology

[0002] Currently, the design scheme for placing pads at the cover-up point of multilayer rigid-flex PCBs generally adopts a post-cover-up process. This process choice is mainly to avoid damage to the inner layer circuit pads caused by subsequent processes such as copper plating and etching. However, the post-cover-up process has obvious limitations: First, its processing flow is complex, requiring multiple steps; second, the precise cutting process has extremely high requirements, making it difficult to ensure consistency in actual operation, resulting in low product yield. While the front-cover-up process can avoid some of the problems of the post-cover-up process, it requires pre-printing photosensitive ink or applying a protective film to the pads. This not only increases the process complexity, but the performance of the protective material directly affects the reliability of the final product, also leading to difficulties in controlling yield. Neither of these traditional processes can guarantee both processing efficiency and product quality, especially when processing multilayer rigid-flex PCBs with inner layer pads, where existing technologies struggle to achieve stable and efficient cover-up processing. Furthermore, the protective measures for the pad area in traditional processes often affect the precision of subsequent circuit formation processes, further restricting product performance improvement.

[0003] The above content is only used to help understand the technical solution of this application and does not represent an admission that the above content is prior art. Summary of the Invention

[0004] The main purpose of this application is to provide a method and system for etching and opening a multilayer circuit board with inner pads, which aims to simplify the processing flow, improve the opening accuracy and product yield.

[0005] To achieve the above objectives, this application proposes a method for etching and opening a multilayer rigid-soft bonded circuit board with inner pads. The method includes:

[0006] On the surface of the pad area of ​​the circuit board substrate, a prepreg layer and an FRCC layer are stacked sequentially; wherein, the prepreg layer has a first opening at the position corresponding to the pad area, and the FPCC is composed of copper foil and a TPI adhesive layer directly covered thereon, the TPI adhesive layer being disposed close to the prepreg layer.

[0007] The prepreg layer and the FRCC layer are hot-pressed together to cure the prepreg layer and the FRCC layer onto the substrate.

[0008] The copper foil of FRCC is etched to form the circuit.

[0009] After the line etching is completed, a preset capping solution is injected into the FRCC layer to dissolve the TPI adhesive layer and complete the capping process.

[0010] In one embodiment, the step of hot-pressing the prepreg layer and the FRCC layer to co-cure the prepreg layer and the FRCC layer on the substrate includes:

[0011] Hot pressing is performed using a three-stage pressure control method, wherein:

[0012] The heating section uses the first pressure for hot pressing to remove air bubbles between the semi-cured sheet and the FRCC layer;

[0013] The insulation section uses a second pressure for hot pressing, so that the prepreg layer and FRCC layer are bonded together and co-cured on the substrate;

[0014] The cooling section uses a third pressure for hot pressing to ensure that the circuit board maintains a stable structure during the cooling process; wherein the first pressure is less than the third pressure, and the third pressure is less than the second pressure.

[0015] In one embodiment, the method further includes:

[0016] Before stacking the semi-cured sheets, a high-temperature release film with a melting point ≥250°C is covered on the surface of the pads and removed after hot pressing.

[0017] In one embodiment, the preset opening liquid is an alkaline solution containing 5-8 wt% tetramethylammonium hydroxide solution or 5-8 wt% KOH solution.

[0018] In one embodiment, the substrate includes a substrate body and a cover film, the pad area is disposed between the substrate body and the cover film, and the cover film has a second opening corresponding to the pad area.

[0019] In one embodiment, the diameter of the second window is smaller than the diameter of the first window.

[0020] In one embodiment, the prepreg, the cover film, and the pad area form a stepped structure.

[0021] Furthermore, to achieve the above objectives, this application also proposes a multi-layer rigid-flex PCB etching and capping system with inner pads. The system includes: a memory, a processor, and a multi-layer rigid-flex PCB etching and capping program stored in the memory and executable on the processor. The multi-layer rigid-flex PCB etching and capping program is configured to perform the steps of the multi-layer rigid-flex PCB etching and capping method.

[0022] This application provides a method and system for etching and opening a multilayer rigid-soft integrated circuit board with inner pads. It achieves precise opening by pre-applying a TPI adhesive layer and a segmented hot pressing process. After etching, the protective layer is removed by chemical dissolution, which avoids damage to the pads caused by traditional processes. It has the advantages of simplifying the processing flow, improving the opening accuracy and the yield of circuit board products. Attached Figure Description

[0023] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a flowchart illustrating an embodiment of the etching and opening process for a multilayer rigid-soft bonded circuit board with inner pads according to this application.

[0026] Figure 2 This is a schematic diagram of a multilayer rigid-flex circuit board with inner pads according to an embodiment of the present application.

[0027] Figure 3 This is a schematic diagram of another embodiment of the multilayer rigid-flex circuit board with inner pads provided in this application;

[0028] Figure 4 This is a schematic diagram of a structure provided for an embodiment of the circuit board etching and opening system with inner pads that incorporates multiple layers of rigid and flexible bonding, as described in this application.

[0029] Explanation of icon numbers:

[0030] 1. Copper foil; 2. TPI adhesive layer; 3. Prepreg layer; 4. Pad area; 5. Cover film; 6. Substrate body; 7. Memory; 8. Processor.

[0031] The purpose, features, and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0032] The technical solutions of this application will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are merely some embodiments of this application, and not all embodiments. The components of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely represents selected embodiments of this application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0033] It should be understood that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures. Furthermore, in the description of this application, the terms "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0034] In existing technologies, designs with pads at the cover opening of multilayer rigid-flex PCBs typically employ a rear-opening process. This requires complex steps to protect the inner layer pads from copper plating or etching, but this method involves precise cutting and has a low yield. Another front-opening method requires printing photosensitive ink or covering the pad surface with a protective film, resulting in a cumbersome processing flow and difficulty in improving yield. When circuit boards need to operate in high-temperature or high-precision environments, existing processes struggle to balance structural stability and processing efficiency.

[0035] To address the aforementioned issues, a method is needed that avoids mechanical cutting processes and simplifies the protective layer fabrication. Considering the susceptibility of traditional protective films to failure at high temperatures, an attempt is made to achieve self-protection through material combinations. Furthermore, the use of a soluble adhesive layer as a temporary isolation layer is explored, allowing for capping via chemical dissolution after circuit etching, thereby reducing mechanical processing steps.

[0036] Therefore, this application proposes a method for etching and opening the cover of a multilayer rigid-soft bonded circuit board with inner pads, referring to... Figure 1 and Figure 2 The method includes steps S100 to S400, wherein:

[0037] Step S100: On the surface of the pad area 4 of the circuit board substrate, a prepreg layer 3 and an FRCC layer are sequentially stacked; wherein, the prepreg layer 3 has a first opening at the position corresponding to the pad area 4, and the FRCC is composed of copper foil 1 and TPI adhesive layer 2 directly covered thereon, and the TPI adhesive layer 2 is disposed close to the prepreg layer.

[0038] Step S200: Hot-press the prepreg layer 3 and the FRCC layer to co-cure the prepreg layer 3 and the FRCC layer on the substrate;

[0039] Step S300: Etch the copper foil 1 of FRCC to form a circuit;

[0040] Step S400: After the line etching is completed, a preset capping solution is injected into the FRCC layer to dissolve the TPI adhesive layer 2 and complete the capping.

[0041] In this embodiment, the circuit board includes a substrate, which includes a substrate body 6 and a cover film 5. The pad area is disposed between the substrate body 6 and the cover film 5 for connecting electronic components. The prepreg layer 3 refers to an incompletely cured resin material layer, which can be made of glass fiber reinforced epoxy resin prepreg. Its first opening can be formed by laser cutting or chemical etching to expose the pad area 4 in subsequent steps. The FRCC layer is directly composited with copper foil 1 and TPI adhesive layer 2. The TPI adhesive layer 2 is a thermoplastic polyimide material, which can be a type that is heat-resistant and can be dissolved by alkaline solutions, such as polyimide containing ester bonds. Its solubility allows the opening liquid to selectively remove the adhesive layer. Co-curing refers to the simultaneous curing and cross-linking of the prepreg layer 3 and the FRCC layer by hot pressing. Specifically, a segmented temperature control process can be used to achieve the interfacial bonding strength. The opening liquid is an alkaline solution, such as tetramethylammonium hydroxide solution, and its concentration range can be controlled within 5-8 wt% to balance the dissolution rate and material compatibility.

[0042] Specifically, firstly, a prepreg layer 3 is covered on the surface of the substrate pad area 4, with the first opening of this layer aligned with the pad position to reserve space for capping. Then, an FRCC layer is laminated, with its TPI adhesive layer 2 facing the prepreg layer 3. A hot-pressing process is then used to bond and cure the two layers, forming a stable laminated structure (see reference). Figure 2 During the etching process, all the copper foil 1 in the areas requiring capping is etched away, exposing the TPI adhesive layer 2. Because there is another layer of TPI adhesive underneath for protection, the pads are not affected by the etching solution, while the outer layer of circuitry can still be etched and formed. After the circuit etching is complete, the capping solution contacts the TPI adhesive layer 2 through the first opening, dissolving the TPI adhesive layer 2 and exposing the pads on the substrate, thus achieving non-mechanical capping (see reference). Figure 3 In this process, the first opening size of the semi-cured layer 3 needs to be larger than the pad area 4 to ensure that the opening liquid is fully wetted, while the thermal stability of the TPI adhesive layer 2 ensures that it maintains its structural integrity during the etching process.

[0043] Compared to existing technologies, traditional rear-opening capping processes rely on mechanical cutting, which limits processing accuracy, while front-opening capping requires additional steps for coating and removing a protective film. This solution replaces mechanical cutting with the chemical dissolution properties of the TPI adhesive layer 2, avoiding the risk of physical damage to the pads. Simultaneously, the co-curing structure of the prepreg layer 3 and the FRCC layer provides self-protection for the pads during the etching process, eliminating the need for a separate temporary protective layer and simplifying the process flow.

[0044] Through the above technical solution, this application realizes the replacement of mechanical processing with chemical dissolution in the circuit board capping process, effectively reducing process complexity and improving processing accuracy. The stability and controllable dissolution of the TPI adhesive layer 2 during hot pressing ensure accurate capping position, while the first window design of the semi-cured sheet layer 3 avoids the erosion of the surrounding structure by the capping liquid. This method is particularly suitable for high-density circuit board manufacturing, improving overall production yield while ensuring pad integrity.

[0045] In one feasible implementation, the step of hot-pressing the prepreg layer 3 and the FRCC layer to co-cure the prepreg layer 3 and the FRCC layer on the substrate includes hot-pressing using a three-stage pressure control method, wherein: the heating stage uses a first pressure to hot-press, so as to expel air bubbles between the prepreg layer 3 and the FRCC layer; the heat preservation stage uses a second pressure to hot-press, so that the prepreg layer 3 and the FRCC layer bond and co-cure on the substrate; and the cooling stage uses a third pressure to hot-press, so as to keep the circuit board stable during the cooling process; wherein the first pressure is less than the third pressure, and the third pressure is less than the second pressure.

[0046] In this embodiment, the three-stage pressure control method refers to dividing the hot-pressing process into three stages where temperature and pressure change in tandem. This can be achieved by linking a pressure control module with a temperature sensor, adjusting the pressure value in stages to match the material state at different stages. The first pressure is the initial pressure applied during the heating stage, which can be a value lower than the standard lamination pressure. Its function is to promote the escape of gas between materials through moderate pressurization. The second pressure is the maximum pressure applied during the heat preservation stage, which can be a value higher than the conventional lamination pressure. Its function is to promote resin flow and interfacial bonding through high pressure. The third pressure is the maintaining pressure applied during the cooling stage, which can be a value between the first and second pressures. Its function is to counteract the internal stress generated by material shrinkage. For example, the first pressure can be set between 0.1 MPa and 0.3 MPa, the second pressure between 0.5 MPa and 0.8 MPa, and the third pressure between the first and second pressures, such as 0.3 MPa to 0.5 MPa. In this embodiment, the first pressure, the second pressure, and the third pressure can be flexibly set according to actual process requirements to achieve the best lamination effect; their specific values ​​are not limited here.

[0047] Specifically, during the heating phase, a lower pressure is applied to gradually soften the semi-cured sheet 3 and the FRCC layer as the temperature rises. This allows residual gas between the layers to escape more easily in the softened state. Once the set temperature is reached, the heat preservation phase begins. At this stage, maximum pressure is applied to ensure the resin in the semi-cured sheet 3 flows fully and impregnates the TPI adhesive layer 2 of the FRCC layer. Simultaneously, the sustained high temperature causes a co-curing reaction between the two materials, forming a stable bond. During the cooling phase, the pressure is adjusted to an intermediate value. This avoids excessive pressure leading to structural deformation after curing, while also preventing excessive pressure causing interlayer separation. The pressure maintenance counteracts the stress generated by the material's cooling shrinkage.

[0048] Compared to existing technologies, traditional hot pressing processes often employ single-pressure or two-stage pressure control, failing to simultaneously address the requirements of bubble removal, interfacial bonding, and structural stability. Existing technologies, such as direct high-pressure pressing, easily lead to bubble residue, while simple low-pressure pressing struggles to guarantee interlayer bonding strength. This solution utilizes a three-stage pressure gradient, matching corresponding pressure parameters at different material states, thereby eliminating interlayer bubbles, ensuring bonding strength, and preventing structural deformation during cooling.

[0049] Through the above technical solution, this application effectively solves the problem of poor bonding caused by residual air bubbles during the lamination process of multilayer circuit boards, overcomes the structural deformation defects caused by material shrinkage in traditional processes, improves the reliability of interlayer bonding and the overall structural stability of the circuit board, and provides a high-quality lamination substrate for subsequent etching and capping processes.

[0050] In one feasible implementation, the method further includes covering the surface of the pads with a high-temperature release film before stacking the semi-cured sheet 3, the high-temperature release film having a melting point ≥250°C, and removing it after hot pressing.

[0051] In this embodiment, the high-temperature release film refers to an isolation material that maintains physical stability during high-temperature hot pressing. Specifically, it can be achieved using polyimide film or polytetrafluoroethylene coating material. Its function is to prevent the semi-cured layer 3 from adhering to the pad surface during hot pressing. A melting point ≥250℃ means that the material will not melt or deform within the hot pressing process temperature range. This can be achieved by selecting polymer materials with high thermal stability, such as glass fiber reinforced silicone-based composite materials. Its function is to ensure that the release film maintains structural integrity during hot pressing. The removal operation refers to peeling or dissolving the release film after the hot pressing process. This can be achieved using mechanical peeling or chemical dissolution methods. Its function is to eliminate interference from the covering material on subsequent etching and capping processes.

[0052] Specifically, after a high-temperature resistant release film is applied to the surface of the pad area 4 of the substrate, the prepreg layer 3 and the FRCC layer are bonded to the substrate via a hot-pressing process. Due to the presence of the release film, the resin material of the prepreg layer 3 will not penetrate into the pad surface at high temperatures, preventing contamination of the pad area 4 by non-conductive materials. After hot pressing, the release film is removed by physical peeling or chemical dissolution, leaving the pad surface clean and providing an unobstructed interface for subsequent circuit etching and capping fluid injection.

[0053] Compared to existing technologies, traditional front-opening capping processes require printing photosensitive ink or applying a protective film to the pad surface. However, once the ink has cured, it is difficult to completely remove and residues are easily left behind. Furthermore, the protective film may curl or crack at high temperatures. This solution, by using a high-temperature resistant release film, effectively isolates the resin material from contact with the pads during hot pressing and allows for complete removal of the cover after the process with a simple operation. This prevents abnormal circuit impedance or obstructed penetration of the capping fluid due to residues.

[0054] Through the above technical solution, this application solves the problem that the pad protection material is difficult to completely remove in the front opening process, avoids poor circuit conductivity or failure of the opening liquid due to the residue of the protective layer, and simplifies the processing steps, thereby improving the process reliability of the pad area 4 of the inner layer of the multilayer rigid-flex board.

[0055] In one feasible implementation, the preset opening liquid is an alkaline solution containing 5-8 wt% tetramethylammonium hydroxide solution or 5-8 wt% KOH solution.

[0056] In this embodiment, the alkaline solution refers to a chemical liquid with a pH value greater than 7. Specifically, it can be an aqueous solution of tetramethylammonium hydroxide or potassium hydroxide to dissolve the TPI adhesive layer 2 while avoiding excessive corrosion of the copper foil 1 circuit. The 5-8 wt% tetramethylammonium hydroxide solution or 5-8 wt% KOH solution refers to a specific concentration range of 5% to 8% of the solute by mass. This concentration range can be achieved by adjusting the ratio of solute to solvent. This concentration range ensures that the TPI adhesive layer 2 is fully dissolved while controlling the reaction rate to avoid damage to the surrounding circuit structure.

[0057] Specifically, after the circuit etching is completed, the decapping solution is injected into the interface area between the FRCC layer and the semi-cured film layer 3 through a pre-designed injection port. Because the TPI adhesive layer 2 has selective dissolution characteristics to alkaline solutions, tetramethylammonium hydroxide or potassium hydroxide solution can directionally decompose the adhesive layer material within a concentration range of 5-8 wt%, while the copper foil 1 circuit remains intact due to its strong alkali resistance. This concentration range has been experimentally verified to ensure that the decapping operation is completed within a reasonable time while avoiding the problems of copper foil 1 surface oxidation caused by high-concentration solutions or insufficient dissolution efficiency caused by low-concentration solutions.

[0058] Compared to existing technologies, traditional capping processes often employ acidic solutions or mechanical cutting. The former easily causes circuit corrosion, while the latter suffers from low positioning accuracy. However, using an alkaline solution of a specific concentration not only reduces the risk of chemical corrosion to the copper foil 1 but also achieves precise capping through chemical dissolution, avoiding pad damage caused by physical cutting. Through this technical solution, this application effectively solves the problem of balancing circuit damage and process efficiency during capping. While ensuring the integrity of the inner layer pad structure, it significantly improves capping accuracy and yield, providing a reliable solution for inner layer pad processing of multilayer rigid-flex boards.

[0059] In one feasible embodiment, the substrate includes a substrate body 6 and a cover film 5, the pad area 4 is disposed between the substrate body 6 and the cover film 5, and the cover film 5 has a second opening corresponding to the pad area 4.

[0060] In this embodiment, the substrate body 6 refers to the rigid or semi-rigid base material that carries the circuitry and pad area 4. Specifically, it can be implemented using FR4 epoxy resin substrate or polyimide substrate, providing structural support and an electrical connection base. The cover film 5 refers to the flexible insulating layer covering the surface of the substrate body 6. Specifically, it can be implemented using polyimide film or modified epoxy resin film, protecting the pad area 4 from external contamination or mechanical damage. The second window refers to the through-hole structure opened in the cover film 5, which can be implemented using laser drilling or chemical etching processes, exposing the pad area 4 for subsequent cover opening operations.

[0061] Specifically, the pad area 4 is sandwiched between the substrate body 6 and the cover film 5, with the cover film 5 partially exposing the pad area 4 through a second opening. During the capping process, the second opening of the cover film 5 corresponds to the first opening of the prepreg layer 3. By controlling the size difference between the two, for example, making the diameter of the second opening smaller than the diameter of the first opening, it can be ensured that the capping liquid only acts on the TPI adhesive layer 2 in the target area. The bonding between the cover film 5 and the substrate body 6 can be achieved through a hot-pressing process, for example, by coating the surface of the substrate body 6 with adhesive and then laminating the cover film 5, and curing is completed within the temperature range of 120-150℃.

[0062] Compared to existing technologies, where the pad area 4 is directly exposed to the subsequent processing environment and is susceptible to corrosion from copper plating or etching solutions, this solution, through the coordinated design of the cover film 5 and the second window, physically isolates the pad area 4 during the processing stage, exposing it only directionally through the window structure during the capping stage. Furthermore, existing technologies using photosensitive ink or protective film to temporarily cover the pads require multiple printing and removal steps, while this solution, through the integrated structure of the cover film 5 and the substrate body 6, reduces process complexity. Through these technical solutions, this application achieves stable protection of the pad area 4 during processing, avoiding the risk of capping solution penetrating into non-target areas. Simultaneously, the pre-bonded structure of the cover film 5 and the substrate body 6 simplifies the process flow and improves the consistency and yield of the circuit board capping process.

[0063] In one feasible implementation, the diameter of the second opening is smaller than the diameter of the first opening. In this embodiment, the diameter of the second opening of the cover film 5 is smaller than the diameter of the first opening of the prepreg layer 3. After the prepreg layer 3 and the cover film 5 are stacked during the hot pressing process, the edges of their openings form a misaligned structure. When the opening liquid is subsequently injected, the stepped structure between the prepreg layer 3 and the cover film 5 can prevent the opening liquid from laterally penetrating into the pad area 4, avoiding corrosion of the pad surface. At the same time, the difference in diameter between the first and second openings provides a vertical channel for the opening liquid, ensuring that the TPI adhesive layer 2 is fully dissolved.

[0064] Compared to existing technologies, where the cover film 5 and the semi-cured layer 3 use windows of the same size, the opening liquid can easily seep into the pad area 4 along the edge of the window, requiring an additional protective layer or adjustment of process parameters. This solution uses the size difference to create a physical barrier structure, eliminating the need for an additional protective film or changes in the opening liquid concentration, achieving reliable protection under conventional process conditions. Through this technical solution, this application effectively solves the problem of pad corrosion caused by lateral penetration of the opening liquid. By replacing complex protection processes with structural design, it simplifies the processing flow while ensuring the opening effect, improving the yield and reliability of circuit board production.

[0065] In one feasible implementation, the prepreg layer 3, the cover film 5, and the pad area 4 form a stepped structure. In this embodiment, the prepreg layer 3 refers to an adhesive layer composed of incompletely cured resin material, specifically glass fiber reinforced epoxy resin prepreg, which achieves interlayer bonding through hot-pressing curing. The cover film 5 refers to an insulating protective layer covering the surface of the substrate body 6, specifically polyimide film, used to protect the substrate body 6 and form a window structure. The pad area 4 refers to conductive connection points on the substrate surface, specifically formed by etching copper foil 1, used to achieve electrical connections. The stepped structure refers to the stepped layers formed vertically by the prepreg layer 3, the cover film 5, and the pad area 4, specifically achieved by controlling the size difference between the first window of the prepreg layer 3 and the second window of the cover film 5. This structure can prevent colloid overflow during hot pressing and enhance the stability of interlayer bonding.

[0066] Specifically, the second opening diameter of the cover film 5 is smaller than the first opening diameter of the prepreg layer 3, resulting in a stepped exposure area in the vertical direction of the pad region 4. When the prepreg layer 3 and the cover film 5 are bonded by thermocompression, the difference in opening size between the two causes the resin material to form a uniformly distributed filling area at the edge of the pad. This stepped structure restricts the flow direction of the colloid, preventing it from covering the pad surface, while simultaneously creating a mechanical interlock between the contact surfaces of the prepreg layer 3 and the cover film 5, enhancing the interlayer adhesion. During subsequent capping fluid injection, the stepped structure guides the liquid along a predetermined path to dissolve the TPI adhesive layer 2, preventing corrosion of the pad.

[0067] Compared to existing technologies, the current front-opening process uses an aligned window design between the cover film 5 and the semi-cured layer 3. During hot pressing, the adhesive easily overflows onto the pad surface, leading to pad contamination or short circuits in subsequent etching processes. This solution uses a stepped structure to create physical isolation, avoiding the adhesive overflow problem and improving interlayer bonding strength through a stepped layer distribution. This simplifies the protective film application process and reduces processing difficulty. Through the above technical solution, this application solves the pad contamination problem caused by adhesive overflow in existing technologies. Simultaneously, by enhancing interlayer bonding stability, it reduces the risk of interlayer delamination during the opening process, thereby improving the yield and reliability of circuit board processing.

[0068] It should be noted that the above examples are only for understanding this application and do not constitute a limitation on the etching and opening process of the circuit board with inner pads in the multilayer hard and soft bonding of this application. Any simple modifications based on this technical concept are within the protection scope of this application.

[0069] This application also provides a multi-layer rigid-flex PCB etching and decapsulation system with inner pads. Please refer to [reference needed]. Figure 4The system includes: a memory 7, a processor 8, and a multi-layer rigid-flex PCB etching and decapsulation process stored on the memory 7 and executable on the processor 8. The multi-layer rigid-flex PCB etching and decapsulation process is configured to implement the steps of the multi-layer rigid-flex PCB etching and decapsulation method.

[0070] The multi-layer rigid-flex PCB etching and capping system provided in this application adopts the multi-layer rigid-flex PCB etching and capping method described in the above embodiments, which simplifies the processing flow, improves capping accuracy, and increases product yield. Compared with the prior art, the beneficial effects of the multi-layer rigid-flex PCB etching and capping system provided in this application are the same as those of the multi-layer rigid-flex PCB etching and capping method described in the above embodiments. Furthermore, other technical features of the multi-layer rigid-flex PCB etching and capping system are the same as those disclosed in the methods of the above embodiments, and will not be repeated here.

[0071] The above description is only a part of the embodiments of this application and does not limit the patent scope of this application. All equivalent structural transformations made under the technical concept of this application and using the contents of the specification and drawings of this application, or direct / indirect applications in other related technical fields, are included in the patent protection scope of this application.

Claims

1. A method for etching and opening a multilayer rigid-flex PCB with inner pads, characterized in that, The method includes: On the surface of the pad area of ​​the circuit board substrate, a prepreg layer and an FRCC layer are sequentially stacked; wherein, the prepreg layer has a first opening at the position corresponding to the pad area, the substrate includes a substrate body and a cover film, the pad area is disposed between the substrate body and the cover film, and the cover film has a second opening corresponding to the pad area, the diameter of the second opening is smaller than the diameter of the first opening, the FRCC is composed of copper foil and a TPI adhesive layer directly coated thereon, the TPI adhesive layer is disposed close to the prepreg layer, and the material of the TPI adhesive layer is a thermoplastic polyimide material that is resistant to high temperature and can be dissolved by alkaline solution; The prepreg layer and the FRCC layer are hot-pressed together to co-cure the prepreg layer and the FRCC layer on the substrate; The copper foil of FRCC is etched to form the circuit. After the line etching is completed, a preset capping solution is injected into the FRCC layer to dissolve the TPI adhesive layer and complete the capping. The preset capping solution is an alkaline solution containing 5-8 wt% tetramethylammonium hydroxide solution or 5-8 wt% KOH solution. Also includes: Before stacking the semi-cured sheets, a high-temperature release film with a melting point ≥250℃ is covered on the surface of the pads and removed after hot pressing.

2. The etching and capping method for multilayer rigid-flex PCBs with inner pads as described in claim 1, characterized in that, The step of co-curing the prepreg and FRCC layers on the substrate by hot pressing includes: Hot pressing is performed using a three-stage pressure control method, wherein: The heating section uses the first pressure for hot pressing to remove air bubbles between the semi-cured sheet and the FRCC layer; The insulation section uses a second pressure for hot pressing, so that the prepreg layer and FRCC layer are bonded together and co-cured on the substrate; The cooling section uses a third pressure for hot pressing to ensure that the circuit board maintains a stable structure during the cooling process; wherein the first pressure is less than the third pressure, and the third pressure is less than the second pressure.

3. The etching and capping method for multi-layer rigid-flex PCBs with inner pads as described in claim 1, characterized in that... The prepreg, the cover film, and the pad area form a stepped structure.

4. A multi-layer rigid-flex PCB etching and capping system with inner pads, characterized in that, The system includes: a memory, a processor, and a multi-layer rigid-flex PCB etching and decapsulation process stored on the memory and executable on the processor, wherein the multi-layer rigid-flex PCB etching and decapsulation process is configured to implement the steps of the multi-layer rigid-flex PCB etching and decapsulation method as described in any one of claims 1 to 3.

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