Method for improving uneven local thickness of rigid-flex board and rigid-flex board

Abstract

The invention relates to the technical field of circuit boards, in particular to a method for improving local thickness unevenness of a rigid-flex board and the rigid-flex board. The method comprises the following steps: S1, extracting residual copper rate distribution, setting a residual copper rate threshold value, and identifying a glue filling area and a non-glue filling area of a rigid-flex board plane according to the residual copper rate threshold value; s2, windowing parameters are obtained according to the hole ring height and diameter of the non-glue-filling area, and windowing is conducted on the cover film according to the windowing parameters; after windowing is completed, the covering film is attached to the corresponding position, the hole ring is sleeved with the window, and the local thickness of the rigid-flex board is consistent; or dividing a high-voltage region and a low-voltage region in the non-glue filling region according to the voltage level and the signal characteristic in the circuit design; the copper is thickened in the high-voltage area, and a step is formed by the thickened copper layer in the high-voltage area; when the glue filling area is filled with glue, pressure difference is formed, the glue layer flows towards the low-pressure area, uniform filling of the glue layer is achieved, and the local thickness of the rigid-flex board is consistent. According to the invention, uneven local thickness of the rigid-flex board is improved.

Description

Technical Field

[0001] This invention relates to the field of circuit board technology, and in particular to a method for improving local thickness unevenness in rigid-flex boards and a rigid-flex board. Background Technology

[0002] A rigid-flex PCB is a composite circuit board formed by bonding a rigid circuit board (PCB) and a flexible circuit board (FPC) through processes such as lamination and stacking. A typical structural example is shown below. Figure 1 As shown, the core structure includes: Rigid area: Utilizes rigid substrates such as FR4 (glass fiber reinforced epoxy resin) to provide mechanical support and circuit stability; Flexible area: Using polyimide (PI) or polyester (PET) as the substrate, it gives the circuit flexibility to adapt to complex spatial layout; Bonding layer: Seamless bonding of rigid and flexible materials is achieved through epoxy resin or acrylic adhesive, ensuring reliable electrical connection between layers.

[0003] This design enables three-dimensional cabling, replacing traditional cable connections and reducing assembly complexity. While such products optimize space and improve integration, complex manufacturing processes (such as buried vias, filled vias, and resin plugging) can lead to uneven dielectric thickness in certain areas. The IPC-6013 standard points out a discrepancy between the current industry-allowed thickness deviation (±10μm) and the actual requirements for high-speed signals (±3μm). Furthermore, uneven thickness results in a yield loss of 18%-22% for 5G millimeter-wave antenna boards (Prismark 2024 report). If this uneven dielectric thickness occurs at blind via locations, it directly leads to different dielectric thicknesses for blind vias of the same specification, easily causing quality issues. Uneven dielectric thickness also directly results in stress concentration in the product, causing impedance fluctuations due to dielectric thickness variations.

[0004] Its failure stack schematic diagram Figure 2 As shown, the reason for this situation is that during the pattern plating of through holes in the flexible circuit board area, the hole ring is too high. This causes stress concentration in the protruding hole ring area when laminating the upper cover film and PP insulation, squeezing the adhesive in the PP insulation and cover film to one side. This results in the thickness of the squeezed area being significantly higher than that of the large copper area (such as buried via 205, the third blind via 203, and the fourth blind via 204). In some cases, unreasonable cover film design can also lead to uneven dielectric thickness in certain areas (such as...). Figure 2 The first blind hole 201 and the second blind hole 202 are shown.

[0005] The problems caused by this type of localized thickness unevenness are as follows: 1. If blind holes exist in areas with uneven thickness, residual adhesive will appear; 2, thickness fluctuation will change the axial stress distribution when drilling, resulting in the difference of hole wall roughness (such as the hole wall roughness of uneven thickness area can reach 3.5 μm, and the normal area is 1.2 μm). In the chemical copper plating link, the hole in the uneven thickness area is easy to appear "dog bone effect", that is, the difference between the hole copper thickness (such as 15 μm) and the hole copper thickness (such as 25 μm) is too large, which affects the conduction reliability; 3, resulting in characteristic impedance fluctuation. For example, when the PI substrate thickness deviation of soft board area is ± 5 μm, the actual value of 50 Ω impedance line may deviate to 45-55 Ω, which increases the reflection loss of high-speed signal (such as 10 Gbps differential pair) by 20%, 4, the current density distribution difference of the area with thickness difference can reach 3-5 times, which causes the further differentiation of plating layer thickness. For example, the copper thickness of dense circuit in hard board area may reach 35 μm, while the adjacent soft board area is only 18 μm, which causes the difference of resistivity and the risk of local overheating. SUMMARY

[0006] In view of the shortcomings of the prior art, the present application discloses a method for improving the local uneven thickness of soft and hard combination board and a soft and hard combination board.

[0007] The technical scheme adopted by the present application is as follows: In a first aspect, a method for improving the local uneven thickness of soft and hard combination board is provided, comprising the following steps: S1, extracting residual copper rate distribution, setting residual copper rate threshold and identifying glue filling area and non-glue filling area of soft and hard combination board plane according to residual copper rate threshold; S2, obtaining windowing parameters according to the hole ring height and diameter of non-glue filling area, and performing windowing processing on the cover film according to the windowing parameters; after windowing is completed, the cover film is attached to the corresponding position, so that the hole ring is sleeved into the window, the height difference of the hole ring is compensated through the window depth of the cover film, and the remaining area of the non-glue filling area is filled by the cover film, so that the local thickness of the soft and hard combination board is consistent; Alternatively, according to the voltage level and signal characteristics in circuit design, high voltage area and low voltage area are divided in the non-glue filling area; the copper thickness of the high voltage area is increased to 1.2-1.5 times of the design value, and the stepped copper layer is formed by using the thickened copper layer of the high voltage area; when the glue is filled in the glue filling area, the pressure difference is formed, and the glue layer will flow to the low voltage area, so that the glue layer is uniformly filled, and the local thickness of the soft and hard combination board is consistent.

[0008] In an embodiment of the present application, in step S1, the residual copper rate threshold ≤5% is set as the glue filling area, and the residual copper rate threshold ≥30% is set as the non-glue filling area.

[0009] In one embodiment of the present application, in step S2, after the hole ring area of the non-adhesive area is selected, the actual height and the actual diameter of the hole ring are scanned hole by hole to generate a height and diameter parameter table for each hole; the detection data is screened to retain valid hole ring parameters; the window diameter is set according to the actual diameter of the hole ring and the window depth is set according to the actual height of the hole ring to obtain the windowing parameters.

[0010] In one embodiment of the present application, the window diameter = the actual diameter of the hole ring + (0.02-0.05) mm.

[0011] In one embodiment of the present application, the window depth = the actual height of the hole ring ± 5 μm.

[0012] In one embodiment of the present application, in step S2, for the hole rings with a height difference ≤10 μm in the same area, they can be classified into the same windowing group and the same window depth is used.

[0013] In one embodiment of the present application, the cover film is a polyimide film.

[0014] In one embodiment of the present application, in step S2, the bonding temperature is 130℃±5℃; the heating rate is 5℃ / min; the bonding pressure is 0.8MPa-1.2MPa and the pressure holding time is 30s-60s.

[0015] In one embodiment of the present application, it further comprises the following steps: S3, detecting the hole ring step difference or the medium thickness of the rigid-flexible combined board; the influencing factor of the hole ring step difference is the hole ring height; the influencing factor of the medium thickness is the residual copper rate, the copper thickness or the cover film windowing area; If the hole ring step difference is out of tolerance, the hole ring height is re-detected and the windowing depth is adjusted; If the medium thickness unevenness is out of tolerance due to the residual copper rate, the etching is re-performed; If the medium thickness unevenness is out of tolerance due to the insufficient copper thickness, the copper is supplemented by electroplating; If the medium thickness unevenness is out of tolerance due to the deviation of the cover film windowing area, the windowing area is supplemented.

[0016] In the second aspect, a rigid-flexible combined board is provided, which is prepared by the method for improving the local thickness unevenness of the rigid-flexible combined board.

[0017] The above technical solution of the present application has the following advantages compared with the prior art: The method for improving the local thickness unevenness of the rigid-flexible combined board can accurately identify the adhesive area and the non-adhesive area and use the windowing compensation or copper layer thickening technology accordingly, effectively solves the problem of local thickness deviation caused by the hole ring height difference and the uneven flow of the adhesive layer, and has the advantages of significantly improving the uniformity of the medium layer, ensuring the integrity of signal transmission and improving the reliability of the product. Attached Figure Description

[0018] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0019] Figure 1 This is a typical structural diagram of an existing rigid-flex PCB.

[0020] Figure 2 This is a schematic diagram of an existing rigid-flex PCB that exhibits localized thickness unevenness.

[0021] Figure 3 This is a schematic diagram of the rigid-flex plate in this invention.

[0022] Explanation of reference numerals on the accompanying drawings: 101. First electroplated copper layer; 102. First substrate copper layer; 103. First insulating layer; 104. First cover film layer; 105. Second electroplated copper layer; 106. Second substrate copper layer; 107. Flexible base layer; 108. Third substrate copper layer; 109. Third electroplated copper layer; 110. Second cover film layer; 111. Second insulating layer; 112. Fourth substrate copper layer; 113. Fourth electroplated copper layer; 201. First blind hole; 202. Second blind hole; 203. Third blind hole; 204. Fourth blind hole; 205. Buried hole; 301, First window area; 302, Second window area. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0024] In existing technologies, rigid-flex PCBs are prone to uneven dielectric thickness during lamination due to structural differences between the rigid and flexible regions. Traditional methods attempt to mitigate this issue by uniformly filling the adhesive layer or adjusting lamination parameters, but these methods cannot precisely address the localized stress concentration caused by differences in copper thickness between the via ring region and the via itself. When dielectric thickness fluctuations exist in blind or buried via regions, they can easily lead to abnormal via wall roughness, impedance mismatch, and plating differentiation, severely impacting the stability of high-speed signal transmission.

[0025] To solve the above problems, different processing strategies need to be taken for different areas of physical property differences. The existing method does not consider the influence of non-rubber filling area hole ring height on the flow of rubber layer, and does not use copper thickness adjustment to form pressure gradient to promote uniformity of filling. By analyzing the correlation between residual copper rate distribution and circuit function partition, it is found that there are essential differences in material accumulation characteristics between the rubber filling area and the non-rubber filling area. Based on this, a regional processing mechanism needs to be established to guide the flow of rubber layer through structural compensation or pressure difference to achieve local thickness uniformization.

[0026] Therefore, the present application provides a method comprising the following steps: extracting the residual copper rate distribution and setting a threshold to divide the rubber filling area and the non-rubber filling area; generating windowing parameters according to the hole ring parameters of the non-rubber filling area, and compensating for the height difference through film windowing; or dividing the high-voltage area and the low-voltage area according to the voltage level, forming a step difference by thickening the copper layer in the high-voltage area, and guiding the flow of rubber layer by using the pressure difference. Embodiment

[0027] The present embodiment provides a method for improving the local thickness uniformity of a rigid-flex board, comprising the following steps: S1, extract the residual copper rate distribution, set the residual copper rate threshold, and identify the rubber filling area and the non-rubber filling area of the rigid-flex board plane according to the residual copper rate threshold; S2, obtain the windowing parameters according to the hole ring height and diameter of the non-rubber filling area, and perform windowing processing on the cover film according to the windowing parameters; after windowing is completed, the cover film is attached to the corresponding position, the hole ring is fitted into the window, the height difference of the hole ring is compensated by the window depth of the cover film, and the remaining area of the non-rubber filling area is filled by the cover film, so that the local thickness of the rigid-flex board is consistent; S3, detect the hole ring step difference or the intermediate thickness of the rigid-flex board; the influencing factor of the hole ring step difference is the hole ring height; the influencing factor of the intermediate thickness is the residual copper rate, the copper thickness or the cover film windowing area; If the hole ring step difference is out of tolerance, the hole ring height is re-detected and the windowing depth is adjusted; If the intermediate thickness is out of tolerance due to the residual copper rate, re-etching is performed; If the intermediate thickness is out of tolerance due to insufficient copper thickness, supplement the copper plating; If the intermediate thickness is out of tolerance due to the deviation of the cover film windowing area, supplement the windowing area.

[0028] The residual copper rate threshold refers to a key indicator for distinguishing the need for adhesive layer filling. Specifically, it can be achieved by image recognition technology, such as using a high-resolution CCD camera (5 million pixels) with a telecentric lens (detection speed up to 100 holes per second, accuracy ±0.5 μm), extracting copper layer distribution data and calculating the area residual copper rate. The threshold divides the board surface into high copper density areas that need to be filled with adhesive and low copper density areas that do not need to be filled with adhesive. The windowing parameter refers to the geometric parameters of the window on the cover film. Specifically, it can be achieved by selecting an ultraviolet picosecond laser (wavelength 355 nm), window edge roughness ≤0.1 μm, and repeat positioning accuracy ±0.5 μm. According to the actual hole ring height and diameter, adjust the window size to match the window depth with the hole ring height to compensate for the thickness difference.

[0029] Specifically, first, the board surface is divided into adhesive filling area and non-adhesive filling area through residual copper rate analysis. For the non-adhesive filling area, after measuring the hole ring height and diameter, customized windowing parameters are generated to make the cover film window accurately accommodate the hole ring and fill the surrounding area.

[0030] The embodiment further proposes that in step S1, residual copper rate threshold ≤5% is set as the adhesive filling area, and residual copper rate threshold ≥30% is set as the non-adhesive filling area.

[0031] The adhesive filling area refers to the area where the residual copper rate is less than or equal to 5%. This area needs to be filled with adhesive to supplement the medium thickness due to low copper foil coverage. The non-adhesive filling area refers to the area where the residual copper rate is greater than or equal to 30%. This area does not need additional adhesive due to high copper foil coverage.

[0032] Specifically, during the processing of rigid-flexible combined boards, first, the copper layer distribution data of the board surface is obtained through optical detection equipment, and the board surface is divided into adhesive filling area and non-adhesive filling area based on the pre-set residual copper rate threshold. For the adhesive filling area with residual copper rate ≤5%, subsequent medium thickness uniformization is achieved through cover film windowing or adhesive layer flow compensation. For the non-adhesive filling area with residual copper rate ≥30%, differential treatment is performed by adjusting the cover film parameters or copper layer thickness. By accurately dividing the two types of areas, the problem of uneven adhesive distribution caused by ambiguous residual copper rate definition can be avoided.

[0033] The embodiment further proposes that in step S2, after selecting the hole ring area of the non-adhesive filling area, the actual height and actual diameter of the hole ring are scanned hole by hole to generate a height and diameter parameter table for each hole. The detection data is filtered to retain valid hole ring parameters. The window diameter is set according to the actual diameter of the hole ring, and the window depth is set according to the actual height of the hole ring to obtain the windowing parameters.

[0034] The hole-by-hole scanning refers to independent measurement of each hole ring, which can be achieved by laser range finder or optical profilometer to obtain the three-dimensional topography data of the hole ring.

[0035] The effective hole ring parameter refers to the parameter retained after data screening and meeting the preset tolerance range, and can be specifically realized by a standard deviation algorithm or a quartile deviation method, and is used to exclude abnormal data caused by measurement error or hole ring deformation.

[0036] The window diameter is set based on the actual diameter of the hole ring, and can be specifically realized by a numerical superposition compensation mechanism, and is used to ensure that the window of the cover film forms a gap fit with the edge of the hole ring after being opened.

[0037] The window depth is set based on the actual height of the hole ring, and can be specifically realized by a height mapping algorithm, and is used to match the vertical space relationship between the cover film and the hole ring.

[0038] Specifically, after selecting the hole ring area in the non-rubber filling area, the actual height and diameter data of each hole ring are obtained by hole-by-hole scanning to form an original data set containing all hole ring parameters. Then, the data is screened to eliminate abnormal values caused by measurement error or hole ring structure defects, and the effective parameters are retained. According to the effective parameters after screening, a compensation amount is added to the actual diameter of each hole ring to determine the window diameter, and the window depth is adjusted according to the actual height, and finally the opening window parameters accurately matching the geometric characteristics of the hole ring are generated.

[0039] The embodiment further provides a technical scheme that the window diameter is equal to the actual diameter of the hole ring plus 0.02-0.05 mm.

[0040] The window diameter refers to the size of the circular opening formed by opening the window of the cover film, and can be specifically realized by a laser cutting process, and the value needs to match the outer diameter of the hole ring to realize accurate fitting. The actual diameter of the hole ring refers to the measured outer diameter of the annular conductive structure formed by the copper layer on the surface of the soft and hard combination plate, and can be specifically realized by a non-contact detection by an optical measuring instrument or a laser scanner. This parameter directly reflects the space occupation of the hole ring on the substrate surface.

[0041] Specifically, during the opening window process of the cover film, the actual diameter data of the hole ring in the non-rubber filling area is first obtained by hole-by-hole scanning, and then the window diameter is dynamically adjusted according to the measured diameter value of each hole ring. For example, when it is detected that the actual diameter of a hole ring is 0.3 mm, the window diameter can be set to 0.32-0.35 mm. By accurately controlling the gap amount between the window edge and the outer wall of the hole ring, it is avoided that the hole ring is deformed due to too small window, and it is also avoided that the boundary between the rubber filling area and the non-rubber filling area is blurred due to too large window. This dynamic matching mechanism enables the cover film window to completely wrap the hole ring structure, and eliminates the phenomenon of blocked rubber flow or local stress concentration caused by window size deviation.

[0042] The embodiment further provides a technical scheme that the window depth is set to the actual height of the hole ring plus or minus 5 μm.

[0043] The window depth refers to the size of the recessed area formed by the windowing of the cover film in the vertical direction, which can be precisely controlled by using a laser micro-processing device, and the value thereof needs to be matched with the height of the hole ring to compensate for the flow of the adhesive layer during the pressing process. The actual height of the hole ring refers to the vertical distance of the copper ring structure on the substrate surface relative to the plane of the base material, which can be measured non-contact by using a three-dimensional laser scanner or a white light interferometer, and the average value of the height data obtained at multiple points is taken as the reference parameter.

[0044] Specifically, in the windowing process of the cover film, the actual height data of each hole ring is first obtained by a high-precision measuring device, and then the window depth is set to the measured height value plus or minus 5 pm. This setting method allows a micron-level gap to be formed between the bottom of the window and the top of the hole ring when the cover film is attached, which avoids both the deformation of the hole ring due to the window being too shallow and the insufficient filling of the adhesive layer due to the window being too deep. When the cover film is combined with the substrate by the hot pressing process, the difference in the window depth can guide the uniform distribution of the adhesive in the non-adhesive area, thereby eliminating the local thickness fluctuation caused by the difference in the height of the hole ring.

[0045] The embodiment further proposes that in step S2, for hole rings with a height difference of ≤10 pm in the same area, they can be classified into the same windowing group, and the same window depth is used.

[0046] The same area refers to a local range with the same process conditions or similar structural characteristics, which can be divided by circuit design zoning or physical location adjacency, and is used to define the spatial boundary of the hole ring grouping. The height difference of ≤10 pm refers to the difference in the vertical distance between the top of the hole ring and the reference surface, which can be measured by a laser height meter or an optical interferometer. This threshold setting ensures that the height difference of the hole rings in the same group is within the compensation capability of the cover film. The same windowing group refers to a set of hole rings that share the same processing parameters, which can be realized by database marking or processing file grouping, and is used for batch processing of hole rings with similar characteristics. The same window depth refers to the vertical size of the recessed area formed by the cover film at the corresponding hole ring position, which is determined based on the statistical average or median of the height of the hole rings in the group, and the matching error between the cover film and the hole ring is reduced by using uniform processing parameters.

[0047] Specifically, in the windowing process of the cover film, the height of the hole rings in the non-adhesive area is first measured and the data is recorded. According to the measurement results, the hole rings that are spatially adjacent and have a height difference of ≤10 pm are classified into the same processing group, and the average value of the height of the hole rings in each processing group is calculated. Based on this average value, the depth parameter of the cover film window is set, and the same processing parameter is used to window all the hole rings in the group. In this way, batch matching of the cover film window depth and the hole ring height is achieved, avoiding the loss of efficiency and the fluctuation of matching accuracy caused by adjusting the processing parameters one by one.

[0048] The embodiment further proposes that the cover film is a polyimide film, preferably a modified polyimide film. The modified polyimide film refers to a polyimide substrate treated by chemical modification, which can be achieved by adding inorganic fillers or adjusting the molecular chain structure, for example, by adding silica nanoparticles or introducing fluorine-containing groups into the polyimide precursor solution. This material can improve the high-temperature dimensional stability while maintaining the basic flexibility, and reduce the window deformation by reducing the thermal expansion coefficient during the lamination process.

[0049] Specifically, in the cover film lamination process, the modified polyimide film is used as a key material to compensate for the thickness of the medium layer. When the cover film is covered on the non-rubber filling area after laser windowing, the modified components in the molecular chain can enhance the material's anti-creep ability and inhibit the window edge warping caused by thermal stress during the high-temperature pressing stage. For example, the introduction of fluorine-containing groups can control the linear shrinkage rate of the film to be less than 0.05% at 130℃, ensuring the fitting accuracy of the hole ring and the window. At the same time, the uniform dispersion of inorganic fillers can improve the compression resilience of the film, so that the cover film can fully fill the gap around the hole ring during the pressure maintaining stage.

[0050] The embodiment further proposes that in step S2, the lamination temperature is 130℃±5℃; the heating rate is 5℃ / min; the lamination pressure is 0.8MPa-1.2MPa, and the pressure maintaining time is 30s-60s.

[0051] The lamination temperature refers to the temperature control range of the heating area when the cover film is combined with the substrate, which can be achieved by using a segmented temperature control system. By monitoring the temperature of the heating plate in real time and adjusting the heating power, the temperature fluctuation is ensured to be within the specified range. This temperature range can ensure that the modified polyimide film reaches the molten state, and can also avoid premature curing of the glue layer caused by high temperature.

[0052] The heating rate refers to the heating speed from room temperature to the target lamination temperature, which can be achieved by using a gradient heating program. By setting the duration and temperature increment of the heating stage, the uniformity of heat conduction is controlled to avoid local overheating and material deformation.

[0053] The lamination pressure refers to the pressure applied between the cover film and the substrate during the pressing process, which can be achieved by using a hydraulic servo system. By linking the pressure sensor and the proportional valve, the pressure distribution is uniform and covers the entire lamination area. This pressure range can not only expel the interface bubbles, but also prevent excessive extrusion from causing deformation of the hole ring structure.

[0054] The pressure maintaining time refers to the duration of constant pressure after reaching the target lamination pressure, which can be achieved by using a timer and a pressure maintaining module to work together. This ensures that the glue layer completes the flow and curing process under constant pressure, and avoids the separation of the bonding interface caused by premature pressure release.

[0055] Specifically, in the process of laminating the cover film, the cover film is first pre-aligned with the substrate and then sent to the pressing equipment. The heating plate is heated to 130℃ at a rate of 5℃ per minute and then enters the constant temperature stage, with temperature fluctuation controlled within ±5℃. When the temperature is stable, the hydraulic system contacts the cover film at an initial pressure of 0.8 MPa, and then gradually increases to 1.2 MPa within 10 seconds and maintains the pressure. During the pressure application process, the cover film is embedded in the hole ring area in a molten state through the window structure, and the adhesive layer fills the gap in the non-glue filling area under the driving force of pressure. After the pressure maintaining stage lasts for 30-60 seconds, the system automatically depressurizes and enters the cooling stage. The entire process is precisely controlled by the temperature gradient and pressure curve to match the flowability and solidification speed of the adhesive layer with the hole ring height compensation requirement, achieving a balance between the interfacial bonding strength and the uniformity of the thickness.

[0056] The embodiment also provides a rigid-flexible printed circuit board prepared by the method for improving local thickness unevenness of a rigid-flexible printed circuit board.

[0057] As shown in Figure 3 The rigid-flexible printed circuit board comprises, from top to bottom, a first rigid substrate, a flexible substrate 107, and a second rigid substrate. The first rigid substrate comprises a first electroplated copper layer 101, a first substrate copper layer 102, a first insulating layer 103, a first cover film layer 104, a second electroplated copper layer 105, and a second substrate copper layer 106. The second rigid substrate comprises a third substrate copper layer 108, a third electroplated copper layer 109, a second cover film layer 110, a second insulating layer 111, a fourth substrate copper layer 112, and a fourth electroplated copper layer 113. The first rigid substrate is provided with a first blind hole 201, a second blind hole 202, and a first windowed area 301. The second rigid substrate is provided with a third blind hole 203, a fourth blind hole 204, and a second windowed area 302. The second substrate copper layer 106, the flexible substrate 107, and the third substrate copper layer 108 are provided with a buried hole 205. Embodiment

[0058] The embodiment is basically the same as Embodiment One, except that step S2 is different.

[0059] The embodiment provides a method for improving local thickness unevenness of a rigid-flexible printed circuit board, comprising the following steps: S1, extracting a residual copper rate distribution, setting a residual copper rate threshold, and identifying a glue filling area and a non-glue filling area of a plane of the rigid-flexible printed circuit board according to the residual copper rate threshold; S2, dividing a high-voltage area and a low-voltage area in the non-glue filling area according to voltage levels and signal characteristics in circuit design; the copper thickness of the high-voltage area is increased to 1.2-1.5 times of the design value, and a step is formed by thickening the copper layer in the high-voltage area; when the glue is filled in the glue filling area, a pressure difference is formed, the adhesive layer flows to the low-voltage area, the adhesive layer is uniformly filled, and the local thickness of the rigid-flexible printed circuit board is consistent; S3, detect the hole ring step difference or the medium thickness of the soft and hard combination board; the influencing factor of the hole ring step difference is the hole ring height; the influencing factor of the medium thickness is the residual copper rate and the copper thickness; If the hole ring step difference exceeds the standard, the hole ring height is detected again, and the windowing depth is adjusted; If the medium thickness is uneven due to the residual copper rate, the etching is re-performed; If the medium thickness is uneven due to insufficient copper thickness, the copper is supplemented by electroplating.

[0060] Among them, the copper thickness thickening in the high voltage area refers to selective electroplating of high voltage signal lines, which can be realized by mask shielding and secondary electroplating process, forming a stepped copper layer structure to generate pressure difference driven adhesive layer flow.

[0061] Specifically, when the voltage partition strategy is adopted, the copper layer thickness is increased in the high voltage line area, forming a height difference to make the adhesive layer preferentially flow to the low voltage area when filling the adhesive.

[0062] As shown in Figure 3 The embodiment also provides a soft and hard combination board prepared by the method for improving local thickness unevenness of the soft and hard combination board.

[0063] From the embodiment one and the embodiment two, it can be seen that both methods change the adhesive layer distribution state by physical structure regulation, and eliminate the local thickness difference caused by hole ring protrusion or uneven copper thickness.

[0064] Obviously, the above embodiments are only examples for clear illustration, and are not limitations to the embodiments. For ordinary skilled in the art, other different forms of changes or variations can be made on the basis of the above description. Here, it is not necessary and impossible to enumerate all the embodiments. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A method for improving local thickness unevenness in rigid-flex PCBs, characterized in that, Includes the following steps: S1. Extract the residual copper rate distribution, set the residual copper rate threshold, and identify the glue-filled area and non-glue-filled area of ​​the rigid-flex board plane based on the residual copper rate threshold. S2. Obtain the window opening parameters based on the height and diameter of the hole ring in the non-filled area, and perform window opening processing on the cover film according to the window opening parameters; after the window is opened, the cover film is attached to the corresponding position so that the hole ring fits into the window. The height difference of the hole ring is compensated by the window depth of the cover film. The remaining area of ​​the non-filled area is filled by the cover film so that the local thickness of the rigid-flex board is consistent. Alternatively, based on the voltage level and signal characteristics in the circuit design, the non-filled area can be divided into high-voltage and low-voltage areas; the copper thickness in the high-voltage area can be increased to 1.2-1.5 times the design value, and the increased copper layer in the high-voltage area can be used to form a step; when filling the filler area, a pressure difference is formed, and the adhesive layer will flow to the low-voltage area to achieve uniform filling of the adhesive layer and make the local thickness of the rigid-flex board consistent.

2. The method for improving local thickness unevenness of a rigid-flex PCB according to claim 1, characterized in that, In step S1, a residual copper rate threshold of ≤5% is defined as a glue-filled area, and a residual copper rate threshold of ≥30% is defined as a non-glue-filled area.

3. The method for improving local thickness unevenness of a rigid-flex PCB according to claim 1, characterized in that, In step S2, after selecting the hole ring area in the non-filled area, the actual height and actual diameter of the hole ring are scanned hole by hole to generate a height and diameter parameter table for each hole; the detection data are filtered to retain valid hole ring parameters; the window diameter is set according to the actual diameter of the hole ring and the window depth is set according to the actual height of the hole ring to obtain the window opening parameters.

4. The method for improving local thickness unevenness of a rigid-flex PCB according to claim 3, characterized in that, Window diameter = actual diameter of the hole ring + (0.02-0.05) mm.

5. The method for improving local thickness unevenness of a rigid-flex PCB according to claim 3, characterized in that, Window depth = actual height of the hole ring ± 5μm.

6. The method for improving local thickness unevenness of a rigid-flex PCB according to claim 1, characterized in that, In step S2, hole rings with a height difference of ≤10μm within the same region can be classified into the same window group and use the same window depth.

7. The method for improving local thickness unevenness of a rigid-flex PCB according to claim 1, characterized in that, The covering film is a polyimide film.

8. The method for improving local thickness unevenness of a rigid-flex PCB according to claim 1, characterized in that, In step S2, the bonding temperature is 130℃±5℃; the heating rate is 5℃ / min; the bonding pressure is 0.8MPa-1.2MPa; and the pressure holding time is 30s-60s.

9. The method for improving local thickness unevenness of a rigid-flex PCB according to claim 1, characterized in that, It also includes the following steps: S3. Detect the hole ring step difference or dielectric thickness of the rigid-flex PCB; the influencing factor of the hole ring step difference is the hole ring height; the influencing factor of the dielectric thickness is the residual copper ratio, copper thickness, or cover film opening area. If the step difference of the hole ring exceeds the standard, the height of the hole ring should be re-inspected and the opening depth adjusted. If the uneven dielectric thickness exceeds the standard due to residual copper content, then re-etch; If the uneven dielectric thickness exceeds the standard due to insufficient copper thickness, then additional copper plating should be performed. If the uneven thickness exceeds the standard due to deviation in the opening area of ​​the covering film, the opening area should be increased.

10. A rigid-flex PCB, characterized in that, It is prepared using the method for improving local thickness unevenness of the rigid-soft bonded plate as described in any one of claims 1-9.

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