PCB cavity processing method and PCB

Abstract

The application discloses a PCB cavity processing method, comprising the following steps: S1: attaching a PI protective film on a white core of a PCB; S2: after the PI protective film is attached, performing fast pressing treatment to realize preliminary solidification of a glue layer of the PI protective film; S3: performing a PCB processing procedure to produce the PCB to an outer layer, adopting a depth control milling device to mill the outer frame of a cover opening area, and milling off the peripheral connection between the cover and the substrate; S4: removing the milled-off outer frame, peeling off the cover along the interface between the PI protective film and the upper and lower layers, and exposing the cavity. The application opens a large-area cavity area on the PCB, facilitates subsequent burying of electronic components into the PCB, and reduces the thickness; during the inner layer pressing stage of the PCB, the PI protective film is used to separate the cover and the PP of the substrate layer, so that the cover can be easily peeled off.

Description

Technical Field

[0001] This invention relates to the field of PCB processing technology, and in particular to a PCB cavity processing method and a PCB. Background Technology

[0002] A PCB (Printed Circuit Board) is a flat panel on an insulating substrate (such as glass fiber reinforced epoxy resin, polyimide, etc.) with copper conductors formed through processes such as etching and printing. It is used for mechanical support and electrical connection of electronic components. To reduce equipment thickness, electronic components are typically embedded within the PCB, requiring the creation of cavities—recessed areas on the PCB. Currently, these cavities are processed using controlled-depth drilling (bag drilling), with processing paths (such as…)… Figure 7 (As shown) and the production time is long, while the flatness and depth of the cavity bottom are difficult to guarantee. Summary of the Invention

[0003] The purpose of this invention is to solve the technical problems of long production time and difficulty in ensuring flatness and depth when the existing PCB cavity size is large.

[0004] To achieve the objectives of this invention, the following technical solution is adopted:

[0005] A PCB cavity processing method includes the following steps:

[0006] S1: Apply a protective layer to the white core of the PCB;

[0007] S2: After the protective layer is applied, a rapid pressing process is performed to achieve the initial curing of the protective layer;

[0008] S3: Perform PCB processing steps to produce the PCB to the outer layer. Use a controlled depth milling machine to mill the outer frame of the cover area and cut off the outer connection between the cover and the substrate.

[0009] S4: Remove the milled outer frame, peel off the cover along the interface between the protective layer and the upper and lower layers to expose the cavity.

[0010] In some embodiments, the protective layer is a PI protective film or an inner copper layer.

[0011] In some embodiments, the PI protective film is a three-layer composite structure, comprising, from top to bottom, an adhesive layer, a PI film, and an acrylic adhesive layer, wherein the adhesive layer is bonded to the cover plane and the acrylic adhesive layer is bonded to the PP.

[0012] In some embodiments, the total thickness of the PI protective film is 27.5 μm, wherein the adhesive layer thickness is 15 μm, the PI film thickness is 12.5 μm, and the release agent is <1 μm.

[0013] In some embodiments, the PI protective film has an upper PP layer and a lower PP layer above and below it, respectively, the thickness of the lower PP layer is set to be more than 150 μm, and the machining accuracy of the controlled depth milling is ±75 μm.

[0014] In some embodiments, the specific surface area of ​​the inner copper layer is between 1 and 5.

[0015] In some embodiments, in step S2, if the manual film application method is used, a pre-pressing process is required before fast pressing to remove air bubbles and fix the position of the PI protective film.

[0016] A PCB is manufactured using the PCB cavity processing method described above.

[0017] The PCB cavity processing method and PCB provided by this invention have the following advantages:

[0018] This invention employs a method of embedding a protective layer within the inner layer of the PCB and then removing the outer frame and cover during the outer layer processing. This creates a large cavity area on the PCB, facilitating the subsequent embedding of electronic components and reducing thickness. During the inner layer lamination stage, the protective layer separates the cover from the substrate layer (PP), forming a separable interface. Subsequently, the cover can be easily peeled off simply by milling off the outer frame of the cavity area. Simultaneously, depth control milling and height detection ensure the depth and flatness of the cavity. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a flowchart of a PCB cavity processing method provided in an embodiment of the present invention.

[0021] Figure 2 This is a schematic diagram of the PI protective film provided in an embodiment of the present invention located on a PCB board.

[0022] Figure 3 This is a schematic diagram of the PI protective film provided in an embodiment of the present invention.

[0023] Figure 4 This is a schematic diagram of a PCB provided in an embodiment of the present invention.

[0024] Figure 5 This is a schematic diagram of the outer frame of the opening area provided in an embodiment of the present invention.

[0025] Figure 6This is a schematic diagram of the inner copper sheet provided in an embodiment of the present invention.

[0026] Figure 7 This is a schematic diagram of the prior art of this invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments.

[0028] In this embodiment, "several" and "more than" refer to two or more. In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0029] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0030] This embodiment of a PCB cavity processing method includes the following steps: S1: attaching a protective layer to the white core of the PCB; S2: performing a fast pressing process after attaching the protective layer to achieve preliminary curing of the protective layer; S3: performing PCB processing steps to produce the PCB to the outer layer, using a controlled-depth milling machine to mill the outer frame of the cover area, severing the peripheral connection between the cover and the substrate; S4: removing the milled outer frame, peeling the cover along the interface between the protective layer and the upper and lower layers to expose the cavity. Through the above technical solution, this embodiment pre-embeds a protective layer during the PCB inner layer lamination stage. This protective layer acts as a sacrificial layer to effectively isolate the cover from the PP of the substrate layer, forming a clear and separable interface. When the PCB is processed to the outer layer, only controlled-depth milling of the outer frame of the cover area is needed to sever the peripheral mechanical connection between the cover and the substrate. Subsequently, by removing the outer frame and peeling the cover along the interface of the protective layer, a large-area cavity structure can be quickly formed. This processing method avoids the tedious process of machining the entire cavity area point by point using traditional controlled depth drilling, significantly shortening production time. At the same time, due to the presence of the protective layer interface, the flatness of the bottom of the cavity is directly determined by the surface quality of the protective layer, while the depth accuracy is ensured by the machining accuracy of controlled depth milling and the thickness setting of the lower PP layer. This effectively solves the problem of difficulty in controlling flatness and depth in the machining of large-size cavities.

[0031] Specifically, in step S1, the "white core" refers to the core substrate that has not undergone pattern transfer and etching. Its high surface cleanliness facilitates uniform adhesion of the protective layer. The adhesion operation can be performed using automated film-applying equipment or manually. When using manual film-applying, a pre-pressing process is required before fast pressing. This utilizes low temperature and low pressure conditions to eliminate air bubbles between the protective film and the core substrate, and to initially fix the position of the protective film to prevent displacement in subsequent processes. Step S2's fast pressing process employs a short-time, high-temperature, and high-pressure process to induce a preliminary cross-linking reaction in the adhesive layer of the protective layer, achieving a semi-cured state. This ensures reliable adhesion between the protective layer and the core substrate while retaining a certain degree of interfacial peelability. The fast pressing parameters need to be optimized based on the characteristics of the protective layer material.

[0032] Step S3 of the PCB manufacturing process covers conventional processes such as inner layer pattern making, lamination, drilling, electroplating, and outer layer pattern making, up to the outer layer stage of PCB production. At this point, the structure of the uncovered area from top to bottom consists of outer copper foil, upper PP layer, cover, protective layer, lower PP layer, and substrate core material. The milling path is designed along the outer edge of the uncovered area, and the edge width is usually set to 0.5 to 10 mm, which ensures effective connection strength between the cover and the substrate and facilitates subsequent removal operations.

[0033] In step S4, the outer frame can be removed using mechanical clamping or vacuum adsorption. When peeling off the cap, a uniform peeling force is applied along the interface between the protective layer and the upper and lower layers. Since the bonding strength between the protective layer and PP is much lower than the bonding strength between PP layers, the cap can be completely peeled off without damaging the bottom structure of the cavity. The bottom of the cavity after peeling is the surface of the protective layer, and its flatness is determined by the quality of the protective layer itself.

[0034] The selection of protective layer materials requires comprehensive consideration of interfacial peelability, heat resistance, chemical stability, and cost. PI protective film possesses excellent high-temperature resistance and mechanical strength, making it suitable for high-reliability applications. In its three-layer composite structure, the adhesive layer provides peelable adhesion to the cover, the PI film acts as a support carrier to ensure flatness, and the acrylic adhesive layer forms a moderate bond with the PP. An inner copper layer serves as an alternative, utilizing the difference in adhesive properties between copper and PP to achieve interfacial separation. The copper's specific surface area is controlled between 1 and 5, and the bonding strength is adjusted through surface roughening treatment. Too low a specific surface area leads to peeling difficulties, while too high an area increases costs and may leave copper shavings.

[0035] <Example 1>

[0036] like Figures 1 to 5 As shown, this embodiment provides a PCB cavity processing method, which includes the following steps:

[0037] S1: Apply a PI protective film to the white core of the PCB. The white core is the original inner core board (substrate is glass fiber + epoxy resin) that has not undergone browning and circuit etching. It serves as the base substrate for cavity processing, and its clean surface is suitable for film application. The PI protective film is a multi-layer composite film with polyimide (PI) as its core, which has the functions of adhesion, isolation, and insulation. In this embodiment, the PI protective film has a three-layer composite structure, which includes, from top to bottom, an adhesive layer, a PI film, and an acrylic adhesive layer. The adhesive layer is bonded to the plane of the cover to ensure that the cover and the PI film are firmly bonded during pressing and to prevent the cover from shifting. The acrylic adhesive layer is bonded to the substrate, which is made of PP material. The acrylic adhesive layer is bonded to the PP to fix the PI film on the substrate and prevent the PI film from shifting during pressing. The PI film provides support and insulation, and serves as the separation interface between the cover and the substrate to ensure that only the PI film is separated when the cover is opened. The "cover" refers to a temporary covering layer applied over the cavity area to be processed during the PCB lamination stage. Essentially, it's a "partial board material" that matches the PCB substrate structure and is subsequently removed by peeling to expose the cavity underneath. In this embodiment, the total thickness of the PI protective film is 27.5 μm, including a 15 μm adhesive layer, a 12.5 μm PI film thickness, and a release agent thickness of <1 μm, resulting in a relatively small PI protective film thickness.

[0038] S2: After attaching the PI protective film, a rapid pressing process is performed to achieve initial curing of the PI protective film adhesive layer. The above steps allow the PI film bonding sheet and acrylic adhesive to initially cure, temporarily fixing the cover, PI film, and substrate to prevent displacement. "Rapid pressing process" refers to a short period of high temperature and high pressure to initially cure the adhesive layer.

[0039] S3: Perform PCB processing steps, bringing the PCB to the outer layer and completing routine processes such as outer layer circuitry and solder mask installation. At this point, the cavity area is still covered by the "lid". Use depth-controlled milling equipment to mill the outer frame of the cavity area (e.g., Figure 5 (As shown) Perform milling to sever the outer perimeter connection between the cover and the substrate. Mill off the outer border of the cavity area according to the design scope, and cut off the connection between the cover and the substrate (leaving only the PI film adhesive).

[0040] S4: Remove the milled outer frame, peel off the cover along the interface between the PI protective film and the upper and lower layers to expose the cavity.

[0041] Furthermore, the PI protective film has an upper PP layer and a lower PP layer above and below it, respectively. The thickness of the lower PP layer is set to be greater than 150μm. The machining accuracy of the controlled-depth milling is ±75μm, which is adapted to the controlled-depth milling accuracy error and avoids damage to the substrate or the bottom of the cavity during milling. In addition, if the cavity area is large, the height of the entire surface needs to be detected and a virtual plane needs to be established before controlled-depth milling. Specifically, a laser height detection device is used to scan the entire cavity area to obtain the height data of several evenly distributed measurement points in the area. Based on the height data, a virtual reference plane is established by software. The virtual reference plane is imported into the controlled-depth milling device, and the milling depth is set with this plane as the reference to make the depth difference in the large cavity area smaller. PP (Prepreg) is usually composed of epoxy resin and fiberglass cloth. Before pressing, it is in a "semi-dry" state. After heating and pressurizing, it will be completely cured, which not only bonds the core board and copper foil together, but also plays a role in interlayer insulation.

[0042] Furthermore, in step S2, if manual film application is used, pre-pressing is required before fast pressing to remove air bubbles and fix the position of the PI protective film. If mechanical film application is used, fast pressing is performed directly. If manual film application is used, pre-pressing (to remove air bubbles) is performed before fast pressing.

[0043] <Example 2>

[0044] In this embodiment, the parts that are the same as in Embodiment 1 are given the same reference numerals, and the same text descriptions are omitted.

[0045] like Figure 6As shown, the main difference between this embodiment and Embodiment 1 is that the inner copper layer is used as a peelable cover structure.

[0046] S1: Attach an inner copper layer (Cu) to the white core of the PCB. Specifically, the inner copper layer is located below the white core, and the inner copper layer is selected with a specific surface area between 1 and 5.

[0047] S2: After attaching the inner copper layer, a fast lamination process is performed. The PCB undergoes conventional etching and pickling processes before lamination to bond the inner copper layer to the underlying layer. This ensures the copper layer remains in a stable position during subsequent processing, preventing displacement or warping during lamination. The temperature and time parameters for the fast lamination process are optimized based on the copper layer thickness and PP characteristics to ensure effective bonding while preventing over-curing that could affect subsequent peel performance.

[0048] S3: Perform the PCB processing steps to produce the PCB up to the outer layer. Use a controlled depth milling machine to mill the outer frame of the cavity area, severing the connection between the inner copper layer and the outer perimeter of the substrate. During controlled depth milling, ensure that the outer frame is completely cut off. The milling path is designed along the outer contour of the cavity area, leaving appropriate process margins to facilitate subsequent removal operations.

[0049] S4: Remove the milled outer frame and peel off the inner copper layer along the interface between the inner copper layer and the lower PP layer to expose the cavity. Because the surface of the inner copper layer has undergone special treatment to reduce roughness, its bonding strength with the PP is significantly lower than the bonding strength between the copper layer and the adhesive layer. During peeling, the copper layer separates entirely from the interface, leaving no adhesive residue or copper shavings at the bottom of the cavity. Furthermore, the surface of the inner copper layer facing the substrate is roughened. This roughness range allows the copper layer and PP to form a suitable mechanical fit, ensuring accurate pressing and positioning, while preventing excessively strong chemical bonding, ensuring smooth subsequent peeling.

[0050] <Example 3>

[0051] In this embodiment, the parts that are the same as in Embodiment 1 and Embodiment 2 are given the same reference numerals, and the same text descriptions are omitted.

[0052] like Figure 4 As shown, compared with Embodiment 1 and Embodiment 2, this embodiment provides a PCB, which is manufactured using the above-described PCB cavity processing method.

[0053] This invention employs a process of embedding a PI protective film in the inner layer of a PCB and then removing the outer frame and cover during the outer layer processing. This creates a large cavity area on the PCB, facilitating the subsequent embedding of electronic components and reducing thickness. During the inner layer lamination stage, the PI protective film separates the cover from the PP of the substrate layer, forming a separable interface. Subsequently, the cover can be easily peeled off simply by milling off the outer frame of the open area. At the same time, depth control milling and height detection ensure the depth and flatness of the cavity.

[0054] In the above embodiments one to three, during the working process, depending on the different working environments, some of the technical implementation methods of embodiments one to three can be combined or replaced.

[0055] The technical principles of the present invention have been described above in conjunction with specific embodiments. However, it should be noted that these descriptions are merely for explaining the principles of the present invention and should not be construed as limiting the scope of protection of the present invention in any way. Based on this explanation, those skilled in the art can conceive of other specific embodiments or equivalent substitutions of the present invention without creative effort, and all such embodiments will fall within the scope of protection of the present invention.

Claims

1. A method for processing a PCB cavity, characterized in that, Includes the following steps: S1: Apply a protective layer to the white core of the PCB; S2: After the protective layer is applied, a rapid pressing process is performed to achieve the initial curing of the protective layer; S3: Perform PCB processing steps to produce the PCB to the outer layer. Use a controlled depth milling machine to mill the outer frame of the cover area and cut off the outer connection between the cover and the substrate. S4: Remove the milled outer frame, peel off the cover along the interface between the protective layer and the upper and lower layers to expose the cavity.

2. The PCB cavity processing method according to claim 1, characterized in that, The protective layer is a PI protective film or an inner copper foil.

3. The PCB cavity processing method according to claim 2, characterized in that, The PI protective film has a three-layer composite structure, which includes an adhesive layer, a PI film, and an acrylic adhesive layer from top to bottom. The adhesive layer is bonded to the surface of the cover, and the acrylic adhesive layer is bonded to the PP.

4. The PCB cavity processing method according to claim 3, characterized in that, The total thickness of the PI protective film is 27.5 μm, of which the adhesive layer thickness is 15 μm, the PI film thickness is 12.5 μm, and the release agent is <1 μm.

5. The PCB cavity processing method according to claim 4, characterized in that, The PI protective film has an upper PP layer and a lower PP layer above and below it, respectively. The thickness of the lower PP layer is set to be more than 150μm, and the machining accuracy of the controlled depth milling is ±75μm.

6. The PCB cavity processing method according to claim 1, characterized in that, The specific surface area of ​​the inner copper layer is between 1 and 5.

7. The PCB cavity processing method according to claim 1, characterized in that, In step S2, if the manual film application method is used, pre-pressing is required before fast pressing to remove air bubbles and fix the position of the PI protective film.

8. A PCB, characterized in that, It is manufactured using the PCB cavity processing method described in any one of claims 1 to 7.

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