A PCB structure and a manufacturing method thereof, and an electronic device

Abstract

The present application provides a PCB structure, including a first core board, an adhesive layer, and a second core board that are stacked, the second core board and the first core board are bonded by the adhesive layer, the thermal expansion coefficient of the second core board is smaller than the thermal expansion coefficient of the first core board, a circuit layer is provided on the surface of the first core board, and there is no circuit layer between the adhesive layer and the second core board. The present application changes the thermal expansion coefficient of the entire PCB structure by providing at least one second core board with a smaller expansion coefficient than the first core board among the multiple first core boards, so as to improve the overall rigidity of the PCB structure and solve the warping problem of the PCB structure during the reflow soldering process. At the same time, the present application also provides a method for manufacturing a PCB structure and an electronic device.

Description

Technical Field

[0001] The present invention relates to the technical field of printed circuit boards (PCBs), and in particular to a PCB structure and a manufacturing method thereof, and an electronic device. Background Art

[0002] PCB (Printed Circuit Board) is one of the most important and key components in electronic products in the ICT industry. It plays the role of physical support for different components (including chips, capacitors, resistors, etc.) in the system and signal transmission between components. When components are installed on the PCB, they often need to go through reflow soldering, which is to melt the lead-free solder to solder the components to the pads on the surface of the PCB. During the reflow soldering process of the PCB, the PCB will generally warp and deform due to heat, and the flatness of the board surface will deteriorate, that is, the warpage will increase. As the ICT industry systems become more and more complex, the chip packages used are getting larger and larger, the weight is getting heavier, and the number of solder balls is increasing. The local area of ​​the PCB where the chip is soldered is more sensitive to the warpage of the PCB. Therefore, how to improve the rigidity of the PCB board and reduce the warpage of the PCB board after reflow soldering has become the focus of research and development in the industry. Summary of the invention

[0003] The technical problem to be solved by the present invention is to provide a PCB structure, which improves the overall rigidity of the PCB structure by adding a second core board design with a small thermal expansion coefficient, and solves the warping problem of the PCB structure during the reflow soldering process.

[0004] In order to achieve the above object, the embodiments of the present invention provide the following technical solutions:

[0005] In the first aspect, the present invention provides a PCB structure, including a first core board, an adhesive layer and a second core board arranged in layers, the second core board and the first core board are bonded by the adhesive layer, the thermal expansion coefficient of the second core board is smaller than the thermal expansion coefficient of the first core board, a metal layer is provided on the surface of the first core board, and there is no metal layer between the adhesive layer and the second core board. The first core board and the second core board here are copper-clad boards, which are plate-like composite structures with resin glue and glass fiber cloth as raw materials, and are covered with copper foil on one or both sides of the outermost layer through pre-impregnation, pre-drying, heating, pressurization and other operations. The adhesive sheet is a semi-cured sheet, which is a thin sheet formed by impregnating degreased glass fiber cloth with resin glue and then pre-drying, and its function is to bond different copper-clad boards, such as the first core board and / or the second core board mentioned in the embodiment. It can be understood that the PCB structure is formed by stacking and bonding a first core board and a second core board, which is different from the design in the traditional scheme in that the copper-clad board in the PCB structure in this embodiment includes a second core board with a thermal expansion coefficient smaller than the first core board in addition to the first core board. It is understandable that, since the thermal expansion coefficient of the second core plate is smaller than that of the first core plate, in the process of reflow soldering, when the first core plate and the second core plate absorb heat and deform, since the expansion of the second core plate is smaller than that of the first core plate, and since the second core plate is bonded to the first core plate as a whole, the second core plate with a small expansion will suppress the first core plate with a large expansion, thereby reducing the expansion of the entire PCB structure to reduce the warping deformation caused by reflow soldering. For the second core plate of the present application, the purpose of its design is to improve the thermal expansion performance and rigidity of the PCB structure, and the first core plate and the second core plate are integrated into one by the bonding layer, so that when the first core plate is deformed by thermal expansion, it can be effectively suppressed by the second core plate bonded thereto. It is understandable that the larger the bonding range between the second core plate and the first core plate, the more obvious the improvement effect of the corresponding second core plate. At the same time, it should also be noted that the production of the circuit pattern in the embodiment is achieved by selectively etching the copper foil on the copper clad plate to obtain the required circuit pattern. The raw materials of the first core board and the second core board in the present application are both copper-clad plate structures, and copper foil is provided on the surface thereof, the purpose of which is to obtain the required circuit pattern on the copper foil by selective etching, and for the embodiments of the present application, the purpose of the second core board design is to improve the thermal expansion performance and rigidity of the entire PCB structure, and it is not necessary to participate in the circuit design in the PCB structure. Therefore, for the second core board, the surface of the copper-clad plate is chemically cleaned, and the copper foil is removed by film exposure, development etching and other processing to ensure that there is no circuit layer on the second core board. The advantage of such a design is that the functions of the first core board and the second core board are clear, and the residual copper foil of the second core board is prevented from affecting the circuit layer in the first core board.Another reason for etching all the copper foil on the surface of the second core board is that reducing the copper foil on the second core board will increase the overall thermal expansion coefficient of the PCB structure in disguise, affecting the overall thermal expansion performance of the PCB structure.

[0006] In a possible implementation, the expansion coefficient of the second core board is less than or equal to 10ppm / ℃, and the expansion coefficient of the first core board ranges from 15ppm / ℃ to 20ppm / ℃. In the embodiment, the first core board is a conventional copper-clad board, and its thermal expansion coefficient is generally between 15ppm / ℃ and 20ppm / ℃. As for the second core board in the embodiment, its design purpose is to adjust the thermal expansion performance of the entire PCB structure. For this reason, the thermal expansion coefficient of the second core board is less than or equal to 10ppm / ℃.

[0007] In one possible implementation, the elastic modulus of the second core board is greater than or equal to 25Gpa. In the embodiment, the elastic modulus of the second core board is greater than or equal to 25Gpa to enhance the rigidity of the second core board, so that when the PCB structure is heated and expands, the deformation of the second core board with a higher elastic modulus will be smaller. For the entire PCB structure, the overall rigidity of the PCB structure will be improved due to the presence of the second core board with a higher elastic modulus. It can be understood that when the elastic modulus of the second core board is greater than or equal to 25Gpa, the second core board will significantly improve the rigidity of the entire PCB structure.

[0008] In a possible implementation, the thickness of the second core board ranges from 0.3mm to 1mm. In the embodiment, the role of the second core board is to improve the thermal expansion performance and elastic modulus of the entire PCB structure. If the thickness of the second core board is too low, for example, less than 0.3mm, the improvement effect will not be obvious enough, and the thermal expansion performance of the entire PCB structure cannot be adjusted; if the thickness is too large, the most direct impact is to increase the overall thickness of the PCB structure. It can be understood that the role of the second core board is limited to improving the thermal expansion performance and elastic modulus of the PCB structure, and does not participate in the circuit layout. Therefore, if the thickness of the second core board is too large, for example, more than 1mm, it will inevitably lead to an increase in the thickness of the PCB structure, which is not conducive to miniaturized design.

[0009] In a possible embodiment, the second core board includes a first surface and a second surface, the number of the first core boards and the number of the bonding layers are both two, one of the bonding layers is stacked between the first surface and one of the first core boards, and the other bonding layer is stacked between the second surface and the other first core board. In this embodiment, the second core board is sandwiched between two first core boards, and the second core board and the first core board are bonded by the bonding layer. Such a design ensures that the first surface and the second surface of the second core board are bonded to the first core board, ensuring that the second core board effectively suppresses the thermal expansion deformation of the first core boards on both sides thereof, thereby improving the thermal expansion performance of the entire PCB structure.

[0010] In a possible implementation, the adhesive layer fully covers the first surface and the second surface. The second core plate is bonded to the first core plates on both sides as a whole through the adhesive layer. The second core plate suppresses the thermal expansion and deformation of the first core plates on both sides through the adhesive force. When the adhesive layer fully covers the first surface and the second surface, it can ensure the maximization of the adhesive force under the same size, which is beneficial to the influence of the second core plate on the thermal expansion and deformation of the first core plate; at the same time, the enhancement of the adhesive force can also avoid cracking between different layer structures.

[0011] In a possible implementation, there are multiple first core boards, which are stacked and symmetrically distributed on both sides of the second core board. In the embodiment, there is one second core board, and the corresponding number of first core boards is multiple. Since the thermal expansion coefficient and elastic modulus of the second core board are different from those of the first core board, the second core board is set at the center of the entire PCB structure, and multiple first core boards are symmetrically arranged on both sides of the second core board, which can reduce the influence of stress concentration caused by the reflow process and improve the warping problem of the entire PCB structure.

[0012] In a possible embodiment, the PCB structure includes a central layer, and the number of the second core plates is multiple, and the multiple second core plates are symmetrically distributed on both sides of the central layer. In the embodiment, the PCB structure includes a central layer, and the central layer can be a first core plate or a second core plate, and the specific situation is selected according to actual needs. The role of the second core plate in the embodiment is to improve the thermal expansion performance and rigidity of the entire PCB structure, so when the second core plate is selectively added to the PCB structure, regardless of whether the central layer of the PCB structure is the first core plate or the second core plate, the preferred multiple second core plates need to be symmetrically distributed. The advantage of the symmetrically distributed second core plate is that it can more effectively improve the thermal expansion performance and rigidity of the PCB structure, and it can also alleviate the problem of stress concentration. In the existing multi-layer PCB structure, copper clad plates with the same thermal expansion coefficient are mostly selected. The purpose of this design is to reduce the stress problem caused by thermal expansion deformation between different copper clad plates. For this application, the problem to be overcome is the warping of the PCB board after heating. Therefore, after introducing the second core plate with different thermal expansion coefficients, it is necessary to reduce the stress problem caused by different thermal expansion coefficients as much as possible. The symmetrical distribution here can effectively distribute the stress between the first core board and the second core board, thereby avoiding the stress concentration in a certain area of ​​the PCB structure.

[0013] In a possible implementation manner, when the number of the second core boards is an odd number, the center layer of the PCB structure is the second core board. In this embodiment, the number of the second core boards is an odd number, such as 3, 5, etc. At this time, in order to ensure the symmetrical distribution of the second core boards in the PCB structure, the center layer of the corresponding PCB structure is the second core board, and the two sides of the second core board located in the center layer are the first core boards bonded by the bonding layer.

[0014] In a possible embodiment, when the number of the second core boards is an even number, the central layer of the PCB structure is the first core board or the bonding layer, and a plurality of the second core boards are symmetrically distributed on both sides of the central layer. The number of the second core boards in the embodiment is an even number, such as 2 / 4 / 6, etc. At this time, in order to ensure the symmetrical distribution of the second core boards in the PCB structure, the central layer of the corresponding PCB structure is the first core board or the bonding layer. When the central layer is the first core board, second core boards bonded by bonding layers are respectively provided on both sides of the first core board, and then a first core board bonded by bonding layers is provided on the outer side of the second core board to ensure that each second core board is sandwiched by two first core boards. When the central layer is the bonding layer, first core boards are respectively provided on both sides of the bonding layer, and a second core board bonded by bonding layers is provided on the outer side of the first core board, and at the same time, a first core board bonded by bonding layers is provided on the outer side of the second core board to ensure that each second core board is sandwiched by two first core boards.

[0015] In a possible embodiment, the thickness of the second core plate accounts for more than or equal to 10% of the thickness of the PCB structure. In the embodiment, the thickness of the PCB structure is mainly determined by the thickness of the first core plate, the thickness of the second core plate and the thickness of the bonding layer. For the entire PCB structure, the first core plate is responsible for the work of the high-speed circuit, so the multiple first core plates are the main part of the entire PCB structure, and the second core plate plays an auxiliary role as an intermediate layer structure for adjusting the thermal expansion performance and rigidity of the PCB structure. Therefore, for the entire PCB structure, the thickness of the second core plate accounts for less than the thickness of the first core plate in the PCB structure, but in order to ensure that the second core plate improves the thermal expansion performance and rigidity of the entire PCB structure, the thickness of the second core plate in the PCB structure accounts for more than or equal to 10%. If it is lower than this percentage, the improvement effect of the second core plate is not obvious, and the warping problem of the PCB structure after the reflow soldering process cannot be overcome. It should be noted here that in the embodiment, regardless of whether the number of the second core plates is one or more, the thickness ratio here refers to the thickness ratio of all second core plates in the entire PCB structure.

[0016] In a possible embodiment, the surface of the second core plate is provided with a micro-protrusion structure, and the micro-protrusion structure is used to increase the bonding force between the second core plate and the adhesive layer. The design purpose of the micro-protrusion structure in the embodiment is to increase the contact area between the second core plate and the adhesive layer, thereby increasing the bonding force between the second core plate and the adhesive layer and avoiding warping and cracking.

[0017] In a possible embodiment, the second core board includes an edge area and a central area, the edge area surrounds the central area, and the distribution density of the micro-protrusion structure in the edge area is greater than the distribution density of the micro-protrusion structure in the central area. In the embodiment, the micro-protrusion structure is mainly distributed in the edge area of ​​the second core board to improve the bonding force of the edge area during the pressing process. It can be understood that for the PCB structure, the location where warping deformation occurs mostly occurs at the edge of the board. This embodiment increases the distribution density of the micro-protrusion structure in the edge area of ​​the second core board to further increase the contact area between the second core board and the adhesive layer in the edge area, thereby improving the bonding force between the two and avoiding warping and cracking.

[0018] In the second aspect, the present invention also provides a method for manufacturing a PCB structure, comprising the following steps: providing a first core board, the first core board is a copper-clad board, etching the copper foil on the first core board to obtain a circuit layer; providing a second core board, the second core board is a copper-clad board, etching the copper foil on the second core board, so that the surface of the second core board has no circuit layer, and the thermal expansion coefficient of the second core board is smaller than the thermal expansion coefficient of the first core board; using an adhesive layer to press and bond the first core board and / or the second core board. In the embodiment, the first core board and the second core board are both made of resin glue as raw materials, and after adding glass fiber cloth, they are pre-dried, heated, pressed, etc., to form a plate-like composite structure with copper foil on one or both sides of its outermost layer. Similar to the traditional PCB structure manufacturing process, an adhesive sheet (semi-cured sheet) is used, and a plurality of first core boards and at least one second core board stacked in a high-temperature press are pressed together to obtain the required PCB structure. The difference is that the thermal expansion coefficient of the second core board is smaller than that of the first core board, so that the thermal expansion performance and rigidity of the entire PCB can be improved by the second core board, so as to obtain a PCB structure that can overcome the warping caused by reflow soldering. At the same time, in the process of making the first core board, the circuit layer is obtained by etching the copper foil on the first core board, and when making the second core board, the copper foil on the second core board is etched off to ensure that there is no circuit layer on the surface of the second core board. The purpose of this process operation is to avoid the influence of the addition of the second core board on the original first core board circuit layout, and at the same time, etching the copper foil on the second core board helps to further reduce the thermal expansion coefficient of the second core board.

[0019] In the third aspect, the present invention also provides an electronic device, which includes components and the above-mentioned PCB structure, wherein the PCB structure includes a pad, and the components are welded on the pad. The components here can be devices such as chips, capacitors, resistors, etc. The pad refers to the welding structure exposed by the metal layer on the outermost side of the PCB structure. During the etching process of the metal layer, on the one hand, etching will be performed according to the needs of the circuit layout, and at the same time, suitable pads will be provided for the components according to the design of the circuit layout to achieve the matching of components and circuits. These components are welded to the PCB structure through the pads arranged on the surface of the PCB structure, thereby achieving fixed installation of the components and the PCB structure. In the embodiment, the components are welded to the PCB structure by reflow soldering. Compared with the traditional PCB structure, after reflow soldering, the thermal expansion of the copper-clad board will cause warping and deformation, which will cause the risk of desoldering. The electronic device provided by the present application has a second core board in its PCB structure to improve the thermal expansion performance, which can overcome the warping problem of the PCB structure after reflow soldering. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0021] Figure 1 is a schematic diagram of the structure of an electronic device in one embodiment of the present invention;

[0022] Figure 2 It is a schematic diagram of the warping of the PCB structure in the prior art;

[0023] Figure 3 is a cross-sectional view of a first PCB structure according to an embodiment of the present invention;

[0024] Figure 4 is a cross-sectional view of a second PCB structure in an embodiment of the present invention;

[0025] Figure 5 is a cross-sectional view of a third PCB structure in an embodiment of the present invention;

[0026] Figure 6 is a cross-sectional view of a fourth PCB structure in an embodiment of the present invention;

[0027] Figure 7 is a performance comparison diagram of the PCB structure in the prior art and the embodiment of the present application;

[0028] Figure 8 is a comparison chart of the maximum warpage of the PCB structure in the prior art and the embodiment of the present application;

[0029] Fig. 9 is a comparison diagram of the warpage fluctuation of the PCB structure in the prior art and the embodiment of the present application;

[0030] Fig.10a It is a test cloud diagram of the PCB structure at 30°C in the prior art;

[0031] Fig.10b It is a test cloud diagram of the PCB structure at 250°C in the prior art;

[0032] Fig.10c It is a test cloud diagram of the PCB structure at 130°C in the prior art;

[0033] Fig.11a It is the test cloud diagram of the PCB structure in this application at 30°C;

[0034] Fig.11b It is the test cloud diagram of the PCB structure in this application at 250°C;

[0035] Fig.11c It is the test cloud diagram of the PCB structure in this application at 130°C;

[0036] Fig.12 is a cross-sectional view of a second core plate provided by the present invention;

[0037] Fig.13 is a top view of a second core plate provided by the present invention;

[0038] Fig.14 This is a flow chart of PCB structure manufacturing provided by the present invention. DETAILED DESCRIPTION

[0039] The technical solutions in the embodiments of the present invention will be clearly described below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0040] In the present invention, the terms "first", "second", etc. are used to distinguish different devices, components or parts, and are not used to indicate or imply the relative importance and quantity of the indicated devices, components or parts. Unless otherwise specified, "plurality" means two or more.

[0041] Figure 1 This is a specific application of the PCB structure 100 provided in this application in an electronic device 200. The electronic device 200 here can be an electronic product such as a radio, a player, a communication device, a computer, etc. In this application scenario, the PCB structure 100 is used to assume the role of physical support and signal transmission of different components 110 in the electronic device 200. Figure 1 The installation relationship between the PCB structure 100 and the components is described by taking the chip as a specific component 110. Figure 1 As shown, a pad 120 is provided on one side surface of the PCB structure 100. Here, the pad 120 refers to a welding structure formed on the metal layer (not shown) outside the PCB structure. During the etching process of the metal layer, etching will be performed according to the needs of the circuit layout, and suitable pads will be provided for components according to the design of the circuit layout to achieve matching between components and circuits. The chip (component 110) is soldered on the pad 120 by reflow soldering, thereby achieving the installation of the chip (component 110) on the PCB structure 100. In the specific implementation process, lead-free solder 150 is coated on the opposite surface of the package base plate 115 corresponding to the component 110 and the pad 120 on the PCB structure 100, and then the lead-free solder 150 is melted by the reflow soldering process, and then cooled and solidified to complete the fixed installation.

[0042] In the process of installing the chip on the PCB structure by reflow soldering, Figure 2As shown, the traditional PCB structure 300 may expand and warp due to local heating, resulting in open solder joints and bridging between the PCB structure 300 and the packaging board 115, thereby affecting the stability of the performance of the entire electronic device.

[0043] In response to the above problems, Figure 3 As shown, the present application provides a PCB structure 100, which includes a first core board 10, an adhesive layer 30 and a second core board 20 which are stacked, the second core board 20 and the first core board 10 are bonded by the adhesive layer 30, the thermal expansion coefficient of the second core board 20 is less than the thermal expansion coefficient of the first core board 10, the surface of the first core board 10 is provided with a metal layer 12, and there is no metal layer 12 between the adhesive layer 30 and the second core board 20. The PCB structure 100 in the embodiment is a multi-layer core board structure formed by stacking a first core board 10, an adhesive layer 30 and a second core board 20 in sequence, and each first core board 10 and the second core board 20 are bonded by the adhesive layer 30 to form a whole. The first core board 10 and the second core board 20 here are copper clad laminates (Copper Clad Laminate, CCL), which are plate-shaped composite structures with resin glue and glass fiber cloth as raw materials, and are covered with copper foil on one or both sides of the outermost layer through pre-impregnation, pre-drying, heating, pressurization and other operations. The adhesive sheet 30 (Prepreg, PP) is a semi-cured sheet, which is a thin sheet formed by dipping the degreased glass fiber cloth in resin glue and then pre-drying. Different from the copper clad laminate (the first core board 10 and the second core board 20), there is no copper foil on both sides of the adhesive sheet 30. The role of the adhesive sheet 30 is to bond different copper clad laminates, such as the first core board 10 and / or the second core board 20 mentioned in the embodiment. It can be understood that the copper clad laminate (the first core board 10 and the second core board 20) is a resin substrate with copper foil bonded on the surface. Through etching technology, the required circuit layout can be engraved on the copper foil, and then the first core board 10 and / or the second core board 20 with different circuit layouts are bonded by using the adhesive sheet 20, and then the PCB structure 100 is obtained through drilling, electroplating and other processes.

[0044] In the present embodiment, the PCB structure 100 is formed by stacking and bonding a first core board 10 and a second core board 20. Different from the conventional solution, the copper clad board in the PCB structure 100 of the present application includes, in addition to the first core board 10, a second core board 20 having a thermal expansion coefficient smaller than that of the first core board 10. It can be understood that, since the thermal expansion coefficient of the second core board 20 is smaller than that of the first core board 10, in the process of reflow soldering, when the first core board 10 and the second core board 20 absorb heat and deform, since the expansion of the second core board 20 is smaller than that of the first core board 10, and the second core board 20 is bonded to the first core board 10 as a whole, the second core board with a small expansion will suppress the first core board with a large expansion, thereby reducing the expansion of the entire PCB structure, so as to reduce the warping deformation caused by reflow soldering.

[0045] It is understandable that in the production of the PCB structure 100, three kinds of substrates are mainly included: the first core board 10, the second core board 20 and the adhesive sheet 30. The first core board 10 and the second core board 20 are both copper-clad boards, that is, glass fiber cloth is impregnated with a certain resin glue, and dried to form a pre-impregnated material (adhesive sheet 30), and then multiple pre-impregnated materials are stacked together according to certain thickness requirements, and copper foil is covered on one or both sides of the outermost layer, and a plate-shaped composite material is formed after heating and pressurizing. The copper-clad board made of glass fiber cloth has good electrical properties and a high operating temperature, which meets the working requirements of high-speed circuits. Similarly, the adhesive sheet 30 is a thin sheet of pre-impregnated material made of degreased glass fiber cloth impregnated with a certain resin glue and pre-dried, and its two side surfaces are not covered with copper foil. By comparing the manufacturing methods of the first core board 10, the second core board 20 and the bonding sheet 30, it can be seen that the first core board 10 and the second core board 20 can be understood as the main body layer of the PCB structure 100, and the bonding sheet 30 is the connecting layer between these main body layers (playing a bonding role). It should be noted here that, taking the first core board 10 as an example, in its manufacturing process, in order to improve the bonding effect between the copper foil and the resin material, the bonding surface is usually micro-etched and oxidized (also called browning treatment) to obtain a microscopically rough bonding surface.

[0046] In the present embodiment, the circuit layer 12 is produced by selectively etching the copper foil on the copper clad board. The raw materials of the first core board 10 and the second core board 20 in the present application are both copper clad board structures, and copper foil is provided on the surface thereof. The purpose is to obtain the required circuit pattern on the copper foil by selective etching. However, for the embodiment of the present application, the purpose of the design of the second core board 20 is to improve the thermal expansion performance of the entire PCB structure 100, and it does not need to participate in the circuit design in the PCB structure 100. Therefore, for the second core board 20, the surface of the copper clad board is chemically cleaned, film-exposed, developed and etched to remove the copper foil on the surface, thereby ensuring that there is no circuit layer 12 on the second core board 20. The advantage of such a design is that the functions of the first core board 10 and the second core board 20 are clear, and the residual copper foil of the second core board 20 is prevented from affecting the circuit layer 12 in the first core board 10. Another reason for completely etching the copper foil on the surface of the second core board 20 is to avoid the influence of the copper foil on the second core board 20 on the overall thermal expansion coefficient of the PCB structure 100. It can be understood that the thermal expansion coefficient of the copper foil is larger than that of the copper clad laminate. Therefore, in order to reduce the overall thermal expansion coefficient of the PCB structure 100, unnecessary copper foil can be removed.

[0047] In a specific embodiment, Figure 3 As shown, the thermal expansion coefficient of the second core board 20 is less than 10ppm / ℃. In the embodiment, the first core board 10 is a conventional copper-clad board. Considering the electrical performance of the PCB structure 100, its thermal expansion coefficient is generally between 10ppm / ℃ and 20ppm / ℃. For example, the thermal expansion coefficient of the copper-clad board made of S7439HW material is about 12ppm / ℃ to 14ppm / ℃. For the second core board 20 in the embodiment, its design purpose is to adjust the thermal expansion performance of the entire PCB structure 100. Considering this aspect, the thermal expansion coefficient of the second core board 20 needs to be less than or equal to 10ppm / ℃.

[0048] In a specific embodiment, the elastic modulus of the second core board 20 is greater than or equal to 25Gpa. In this embodiment, the elastic modulus of the second core board 20 is greater than or equal to 25Gpa to enhance the rigidity of the second core board 20, so that when the PCB structure 100 is heated and expands, the deformation of the second core board 20 with a higher elastic modulus will be smaller. For the entire PCB structure 100, due to the presence of the second core board 20 with a higher elastic modulus, the overall rigidity of the PCB structure 100 will be improved. It can be understood that when the elastic modulus of the second core board 20 is greater than or equal to 25Gpa, the second core board 20 will significantly improve the rigidity of the entire PCB structure 100.

[0049] It is understandable that there are two main factors affecting the warping problem of the PCB structure 100, one is the thermal expansion coefficient of the material, and the other is the elastic modulus of the material. For the solution of the present application, since the warping problem of the PCB structure 100 originates from the heating problem in the reflow soldering, the thermal expansion of the material caused by the temperature change is the root cause of the warping problem. Therefore, first of all, in the implementation scheme of the present application, the thermal expansion coefficient of the second core board 20 is first limited. On the basis of this limitation, the elastic modulus of the material is also improved to further reduce the occurrence of the warping problem after heating.

[0050] In a possible implementation, the thickness of the second core board ranges from 0.3mm to 1mm. In the embodiment, the role of the second core board is to improve the thermal expansion performance and elastic modulus of the entire PCB structure. If the thickness of the second core board is too low, for example, less than 0.3mm, the improvement effect will not be obvious enough, and the thermal expansion performance of the entire PCB structure cannot be adjusted; if the thickness is too large, the most direct impact is to increase the overall thickness of the PCB structure. It can be understood that the role of the second core board is limited to improving the thermal expansion performance and elastic modulus of the PCB structure, and does not participate in the circuit layout. Therefore, if the thickness of the second core board is too large, for example, more than 1mm, it will inevitably lead to an increase in the thickness of the PCB structure, which is not conducive to miniaturized design.

[0051] In some possible embodiments, Figure 4 , Figure 5 and Figure 6 As shown, the second core panel 20 includes a first surface 21 and a second surface 22, the number of first core panels 10 and the number of bonding layers 20 are both two, one bonding layer 20 is stacked between the first surface 21 and one of the first core panels 22, and the other bonding layer 20 is stacked between the second surface 22 and the other first core panel 10. Figures 4 to 6In the PCB structure 100 shown, the number of the second core plates 20 is 1, 2 and 3 respectively. For the different number of second core plates 20 in different embodiments, each second core plate 20 is sandwiched between two first core plates 10 and bonded by the bonding layer 30. For the second core plates 20 of the present application, the purpose of its design is to improve the thermal expansion performance and rigidity of the PCB structure 100. In order to better play its role, it is preferred to sandwich each second core plate 20 between different first core plates 10, so that when the first core plate 10 is deformed by thermal expansion, it can be effectively suppressed by the second core plate 20 bonded thereto. It can be understood that the larger the bonding range between the second core plate 20 and the first core plate 10, the more obvious the improvement effect of the corresponding second core plate 20. Therefore, by bonding the first surface 21 and the second surface 22 of each second core plate 20 to the first core plate 10, the improvement effect of the second core plate 20 can be maximized. The second core board 20 is sandwiched between two first core boards 10, and is bonded to the first core board 10 through an adhesive layer 20. This design ensures that the first surface 21 and the second surface 22 of the second core board 20 are bonded to the first core board 10, so that the second core board 20 can effectively suppress the thermal expansion deformation of the first core boards 10 on both sides thereof, thereby improving the thermal expansion performance of the entire PCB structure 100.

[0052] In a specific embodiment, Figure 5 As shown, there are multiple first core plates 10 , which are stacked and symmetrically distributed on both sides of the second core plate 20 . Figure 5 The number of second core boards 20 in the PCB structure 100 is one, and the corresponding number of first core boards 10 is multiple. Since the thermal expansion coefficient and elastic modulus of the second core board 20 are different from those of the first core board 10, the second core board 20 is arranged at the center of the entire PCB structure 100, and multiple first core boards 10 are symmetrically arranged on both sides of the second core board 20. This can reduce the impact of stress concentration caused by the reflow soldering process and improve the warping problem of the entire PCB structure 100.

[0053] In some specific embodiments, Figure 5 and Figure 6 As shown, the PCB structure 100 includes a central layer 50, a plurality of second core boards 20, and the plurality of second core boards 20 are symmetrically distributed on both sides of the central layer 50. Figure 5 as well as Figure 6 As can be seen from the figure, the number of the second core boards 20 is 2 and 3 respectively. For different numbers of the second core boards 20, the second core boards 20 in the embodiment are always symmetrically distributed about the central layer 50 in the PCB structure 100. Figure 5In the embodiment, the number of the second core plates 20 is 2. At this time, the central layer 50 of the PCB structure 100 is the bonding layer 20, and the corresponding two second core plates 20 are symmetrically distributed on both sides of the central layer 50. It can be understood that the role of the second core plate 20 is to improve the thermal expansion performance and rigidity of the entire PCB structure 100. Therefore, when the second core plates 20 are selectively added between the multiple first core plates 10, the preferred second core plates 20 need to be symmetrically distributed. The advantage of the symmetrically distributed second core plates 20 is that they can more effectively improve the thermal expansion performance and rigidity of the PCB structure, and can also alleviate the problem of stress concentration. In the existing multi-layer PCB structure 100, copper clad plates with the same thermal expansion coefficient are mostly selected. The purpose of this design is to reduce the stress problem caused by thermal expansion and deformation between different copper clad plates. For this application, the problem to be overcome is the warping of the PCB structure 100 after heating. Therefore, after introducing the second core plates 20 with different thermal expansion coefficients, it is necessary to reduce the stress problem caused by different thermal expansion coefficients as much as possible. The symmetrical distribution here can effectively distribute the stress between the first core board 10 and the second core board 20 , thereby avoiding the stress concentration in a certain area of ​​the PCB structure 100 .

[0054] In some specific embodiments, Figure 6 As shown, when the number of the second core boards 20 is an odd number, the central layer of the PCB structure 100 is the second core board 20 . Figure 6 Corresponding to the three second core boards 20, in order to ensure the symmetrical distribution of the second core boards 20 in the PCB structure 100, the central layer 50 corresponding to the PCB structure 100 is the second core board 20, and both sides of the second core board 20 belonging to the central layer 50 are the first core boards 10 bonded by the adhesive layer 30.

[0055] In some specific embodiments, Figure 5 As shown, when the number of second core boards is an even number, the central layer 50 of the PCB structure 100 is the first core board 10 or the bonding layer 30, and the plurality of second core boards 20 are symmetrically distributed on both sides of the central layer 50. In the embodiment, the number of second core boards is an even number, such as 2 / 4 / 6, etc. At this time, in order to ensure the symmetrical distribution of the second core boards in the PCB structure, the central layer of the corresponding PCB structure is the first core board or the bonding layer. Figure 5 In the embodiment shown, the central layer of the PCB structure 100 is the bonding layer 30 , and at this time, the number of the second core boards 20 is two, and the two second core boards 20 are symmetrically distributed on both sides of the bonding layer 30 serving as the central layer 50 .

[0056] In order to better understand the beneficial effects of the technical solution of the present application, three specific embodiments are described below. Figure 4 , Figure 5 and Figure 6 As shown, there are cases where the number of the second core boards 20 is 1, 2 and 3.

[0057] like Figure 4 As shown, in this embodiment, the PCB structure 100 includes 4 first core boards 10 and a second core board 20 and 6 adhesive layers 30 for bonding the first core board 10 and the second core board 20, wherein the second core board 20 is located between the middle two of the 4 first core boards 10. Specifically, the PCB structure 100 is, from top to bottom, adhesive layer 30, first core board 10, adhesive layer 30, first core board 10, adhesive layer 30, second core board 20 (center layer), adhesive layer 30, first core board 10, adhesive layer 30, first core board 10, adhesive layer 30. In this specific embodiment, the second core board 20 is made of 8 sheets of 2116 glass fiber cloth pressed together, and its thermal expansion coefficient is less than 10ppm / ℃ and its elastic modulus is greater than or equal to 25Gpa. The corresponding 4 first core boards 10 and 6 adhesive layers 30 still use M4S material, which is a conventional substrate material.

[0058] In order to better see the beneficial effects of the present application, a conventional PCB structure without the second core board 20 and a PCB structure 100 including the second core board 20 are tested. Figure 7 As shown in the figure, the bar graphs represent the thermal expansion coefficient and elastic coefficient numerical diagrams of the traditional PCB structure (white shadow) and the PCB structure provided by the application document (black shadow). Figure 6 It can be seen that the thermal expansion coefficient of the PCB structure 100 including the second core board 20 is 11.66ppm / °C and the elastic modulus is 26.27GPa. Compared with the conventional PCB structure without the second core board 20, its thermal expansion coefficient is reduced by about 25% and its elastic modulus is increased by about 14%. Thus, the CTE of the overall PCB structure is reduced and the rigidity of the overall PCB structure is improved.

[0059] In order to better illustrate the beneficial effect, the Shadow Moire device is used to actually test the thermal deformation of simulated reflow soldering of two PCB structures (PCB structure 100 including the second core board 20 and the traditional PCB structure without the second core board 20) with the same outer dimensions (420mm*400mm) and the same chip position dimensions (75*60mm). Figure 8 and Fig. 9 As shown, Figure 8 The bar graphs in the figure represent the maximum values ​​of the warpage of the conventional PCB structure (white shadow) and the PCB structure provided in the application document (black shadow). Fig. 9 The bar graphs in the figure represent the warpage fluctuations of the conventional PCB structure (white shadow) and the PCB structure provided by the application document (black shadow). Fig.10a , Fig.10b and Fig.10c ,in Fig.10a , Fig.10b and Fig.10c Schematic diagram of thermal deformation of a conventional PCB structure without the second core board 20 at 30°C, 250°C and 130°C respectively. Fig.11a , Fig.11b and Fig.11c It is a schematic diagram of the thermal deformation of the PCB structure 100 including the second core board 20 at 30°C, 250°C and 130°C respectively. The conditions of 30 degrees, 250 degrees and 130 degrees are selected here in order to restore the changes of the PCB structure 100 after heating up and cooling down during the reflow soldering process. It can be clearly seen that the maximum warpage of the PCB structure 100 with the second core board 20 is 42% lower than the maximum warpage of the traditional PCB structure, and the fluctuation of the warpage is also improved by 22%. At the same time, the second core board 20 also changes the deformation direction of the warpage corresponding to the projection area of ​​the PCB structure and the components (taking the chip as an example here) (from convex to concave), so that it can better match the deformation of the components.

[0060] like Figure 5 As shown, in this embodiment, the PCB structure 100 includes 4 first core boards 10 and 2 second core boards 20 and 7 adhesive layers 30 for bonding the first core boards 10 and the second core boards 20, wherein each second core board 20 is located between two first core boards 10, and the central layer of the PCB structure 100 is the adhesive layer 30. Specifically, the PCB structure 100 is, from top to bottom, the adhesive layer 30, the first core board 10, the adhesive layer 30, the second core board 20, the adhesive layer 30, the first core board 10, the adhesive layer 30 (central layer), the first core board 10, the adhesive layer 30, the second core board 20, the adhesive layer 30, the first core board 10, the adhesive layer 30. In this specific embodiment, the second core board 20 is made of 8 sheets of 2116 glass fiber cloth pressed together, and its thermal expansion coefficient is less than 10ppm / ℃ and its elastic modulus is greater than or equal to 25Gpa. The corresponding 4 first core boards 10 and the 7 adhesive layers 30 still use M4S material, which is a conventional substrate material. Through the design of the second core board 20 , the thermal expansion coefficient (reduction) and elastic modulus (increase) of the entire PCB structure 100 can be greatly improved.

[0061] like Figure 6As shown, in this embodiment, the PCB structure 100 includes 4 first core boards 10 and 3 second core boards 20 and 8 adhesive layers 30 for bonding the first core boards 10 and the second core boards 20, wherein each second core board 20 is located between two first core boards 10, and the central layer of the PCB structure 100 is the second core board 20. Specifically, the PCB structure 100 is, from top to bottom, adhesive layer 30, first core board 10, adhesive layer 30, second core board 20, adhesive layer 30, first core board 10, adhesive layer 30, second core board 20 (middle layer), adhesive layer 30, first core board 10, adhesive layer 30, second core board 20, adhesive layer 30, first core board 10, adhesive layer 30. In this specific embodiment, the second core board 20 is formed by pressing together 8 sheets of 2116 glass fiber cloth, whose thermal expansion coefficient is less than 10ppm / ℃ and elastic modulus is greater than or equal to 25Gpa. The corresponding four first core boards 10 and eight adhesive layers 30 still use M4S material, which is a conventional substrate material. Through the design of three second core boards 20, the thermal expansion coefficient (reduction) and elastic modulus (increase) of the entire PCB structure 100 can be greatly improved.

[0062] In the above three embodiments, the outermost side of the PCB structure 100 is provided with an adhesive layer 20, and then a metal layer 12 is provided on the outer surface of the adhesive layer 20. The design of the metal layer 12 is to prepare for the production of the pad. It can be understood that in the conventional production process of the PCB structure 100, an adhesive layer 20 is attached to the outermost side of the board, which plays a role in bonding the outermost metal layer 12 on the one hand, and plays a role in saving costs on the other hand. If the outermost layer is designed as the first core board 10, the production cost will increase.

[0063] In a specific embodiment, the second core board 20 accounts for more than or equal to 10% of the thickness of the PCB structure 100, and the expansion coefficient of the first core board 10 is in the range of 15ppm / ℃ to 20ppm / ℃. In the embodiment, the first core board is a conventional copper clad board, and its thermal expansion coefficient is generally in the range of 15ppm / ℃ to 20ppm / ℃. Figure 3It can be seen that the thickness of the PCB structure 100 in the embodiment is mainly determined by the thickness of the first core board 10, the thickness of the second core board 20 and the thickness of the bonding layer 30. For the entire PCB structure 100, it is the first core board 10 (copper foil is covered on both sides) that undertakes the work of the high-speed circuit. Therefore, multiple first core boards 10 are the main part of the entire PCB structure 100, and the second core board 20 plays an auxiliary role as an intermediate layer structure for adjusting the thermal expansion performance and rigidity of the PCB structure. Therefore, for the entire PCB structure, the thickness of the second core board accounts for a smaller proportion of the thickness of the PCB structure than the thickness of the first core board. However, in order to ensure that the second core board improves the thermal expansion performance and rigidity of the entire PCB structure, the thickness of the second core board in the PCB structure accounts for more than or equal to 10%. If it is lower than this proportion, the improvement effect of the second core board 20 is not obvious, and the warping problem of the PCB structure 100 after the reflow soldering process cannot be overcome.

[0064] Specifically, for design considerations of the PCB structure 100, the thickness of the second core board 20 is 0.3 mm to 1.0 mm, and the thickness of the PCB structure 100 excluding the second core board 20 is 2.0 mm to 3.6 mm. Figure 3 As shown, the second core board 20 is formed by pressing eight sheets of 2116 glass fiber cloth together with a thickness of 0.8 mm, and the corresponding thickness of the four first core boards 10 and the bonding layer 30 is 2.0 mm. Therefore, it can be seen that the thickness of the second core board 20 in the entire PCB structure 100 accounts for 28%.

[0065] It is understandable that in a specific implementation, the thickness of the four first core boards 10 is not kept consistent, and the relative thickness can be selected according to specific needs, such as the design of the circuit pattern and the need for drilling. Generally speaking, conventional copper clad laminates include non-standard specifications of 0.1mm, 0.2mm, 0.3mm, 0.5mm and standard specifications of 0.8mm, 1.0mm, 1.2mm, 1.6mm, 2.0mm, etc.

[0066] In a specific embodiment, Fig.12 As shown, the surface (first surface 21 and second surface 22) of the second core plate 20 is provided with a micro-protrusion structure 25, and the micro-protrusion structure 25 is used to increase the bonding force between the second core plate 20 and the adhesive layer 30. The design purpose of the micro-protrusion structure 25 in the embodiment is to increase the contact area between the second core plate 20 and the adhesive layer 30, thereby increasing the bonding force between the second core plate 20 and the adhesive layer 30 and avoiding warping and cracking.

[0067] Specifically, Fig.13As shown, the second core board 20 includes an edge area 23 (outside the dotted line) and a central area 24 (inside the dotted line), the edge area 24 surrounds the central area 23, and the distribution density of the micro-protrusion structure 25 in the edge area 24 is greater than the distribution density of the micro-protrusion structure 25 in the central area 23. In the embodiment, the micro-protrusion structure 25 is mainly distributed in the edge area 24 of the second core board 20, so as to improve the bonding force of the edge area 24 during the pressing process. It can be understood that for the PCB structure 100, the position where the warping deformation occurs mostly occurs at the edge of the board. In this embodiment, by increasing the distribution density of the micro-protrusion structure 25 of the second core board 20 at the edge area 24, the contact surface between the second core board 20 and the bonding layer 30 at the edge area 24 is further increased, thereby improving the bonding force between the two and avoiding the occurrence of warping and cracking.

[0068] like Fig.14 As shown, the present application also provides a method for manufacturing a PCB structure, comprising the following steps:

[0069] S110, providing a first core board, the first core board being a copper-clad board, and etching the copper foil on the first core board to obtain a circuit layer;

[0070] Among them, the first core board is a conventional copper-clad board. In order to ensure the overall electrical performance of the PCB structure, a plate-like substrate is selected, which is made of glass fiber cloth impregnated with resin glue and then covered with copper foil. It should be noted that the first core board provided here is suitable for building the circuit layout of the entire PCB structure. Therefore, the copper foil on the first core board needs to be etched to obtain a suitable circuit layer.

[0071] S120, providing a second core board, wherein the second core board is a copper-clad board, and etching away the copper foil on the second core board so that there is no circuit layer on the surface of the second core board, and the thermal expansion coefficient of the second core board is smaller than the thermal expansion coefficient of the first core board;

[0072] Among them, the second core board here is also a plate-shaped substrate obtained by impregnating glass fiber cloth with resin glue and then covering the surface with copper foil. Different from the first core board, the thermal expansion coefficient of the second core board in the embodiment is smaller than that of the first core board, and the purpose is to improve the thermal expansion performance of the entire PCB structure. It should be noted that the copper foil on both sides of the second core board is deliberately etched to remove the copper foil on both sides of the second core board to avoid the copper foil from affecting the circuit pattern on the first core board. In the manufacturing process of the PCB structure, the second core board is processed by ready-made copper-clad boards, and these copper-clad boards are already covered with copper foil on both sides when purchased. Therefore, in order to avoid the influence of the copper foil on the circuit pattern on the first core board, it needs to be etched away. Specifically, the second core board can be composed of 8 sheets of 2116 glass fiber cloth.

[0073] It should be noted that there is no causal relationship between step S110 and step S120 , and the two can be selectively performed one after the other or simultaneously, and the specific situation can be selected according to processing needs.

[0074] S130, using the adhesive layer to press and bond the first core board and / or the second core board.

[0075] The bonding layer is a thin sheet formed by impregnating degreased glass fiber cloth with resin glue and then pre-drying it. Its function is to bond different copper clad laminates. The function of the bonding layer is to bond adjacent copper clad laminates, which can be the bonding between the first core board and the second core board, or between the first core board and the first core board. The specific bonding object is determined according to the actual stacking of the PCB structure.

[0076] In a specific embodiment, after multiple first core boards, at least one second core board and multiple adhesive layers are stacked and arranged, they only need to be pressed together through a high-temperature press to form a complete PCB structure, and then go through necessary processing steps such as drilling, electroplating, outer layer graphics, solder mask, characters, surface treatment, and appearance processing to obtain a finished PCB structure.

[0077] The following will Figure 5 The PCB structure 100 shown is used to illustrate the PCB manufacturing method provided above.

[0078] First, four first core boards 10 are provided. The first core boards 10 are copper-clad boards made of M4S material. Then, the copper foils on both sides of the first core boards 10 are etched to obtain the required circuit patterns 12.

[0079] Similarly, a second core board 20 is provided, which is a copper-clad board formed by pressing 8 sheets of 2116 glass fiber cloth. The specific board model is MCL-E-705G, and the corresponding thermal expansion coefficient of the second core board 20 is in the range of 5ppm / ℃ to 7ppm / ℃, and the elastic modulus is in the range of 32Gpa to 34Gpa. Then, the copper foil on both sides of the copper-clad board is fully etched to ensure that there is no copper foil on both sides of the second core board 20, so as to avoid the residual copper foil on the second core board 20 affecting the normal operation of the high-speed circuit of the entire PCB structure 100.

[0080] Then, four first core boards 10 and one second core board 20 are stacked, with the second core board 20 placed in the center. At the same time, adhesive sheets 30 are set on both sides of each copper clad board (first core board or second core board), for a total of six sheets, thus forming a stacked structure consisting of six adhesive sheets 30, four first core boards 10 and one second core board 20.

[0081] Then, a hot press is used for high-temperature pressing to obtain a PCB structure with a stable structure. Finally, the PCB structure is subjected to necessary processing steps such as drilling, electroplating, outer layer graphics, solder mask, characters, surface treatment, and shape processing to obtain the final PCB structure product.

[0082] It is understandable that the manufacturing method of the PCB structure provided by the present application is different from the traditional manufacturing method in the design of the second core board. By adding a second core board with a smaller thermal expansion coefficient and a larger elastic modulus to the original PCB structure composed of the first core board and the bonding layer, the thermal expansion coefficient and elastic modulus of the entire PCB structure are changed. Figure 6 The simulation experimental data shown in the figure shows that in the traditional PCB structure without adding the first core board, the thermal expansion coefficient is 15.55ppm / ℃ and the elastic modulus is 22.98Gpa. By adding a second core board at the center of the PCB structure, the thermal expansion coefficient of the new PCB structure is 11.66ppm / ℃ and the Young's modulus is 26.27Gpa. By comparison, it can be seen that after adding the second core board, the thermal expansion coefficient of the PCB structure is reduced by about 25%, and the elastic modulus is increased by about 14%. Thereby reducing the thermal expansion coefficient of the overall PCB structure, improving the rigidity of the overall PCB structure, and avoiding the phenomenon of open welding and continuous welding caused by warping of the PCB structure during reflow soldering.

[0083] The above are preferred embodiments of the present invention. It should be noted that, for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A PCB structure, characterized in that: The invention comprises a first core board, an adhesive layer and a second core board which are stacked, wherein the second core board and the first core board are bonded to each other through the adhesive layer, the thermal expansion coefficient of the second core board is smaller than the thermal expansion coefficient of the first core board, circuit layers are provided on the surfaces of both sides of the first core board, and no copper foil is provided on the surface of the second core board; The expansion coefficient of the second core board is less than or equal to 10 ppm / °C, the expansion coefficient of the first core board ranges from 15 ppm / °C to 20 ppm / °C, the thickness of the second core board accounts for less than the thickness of the first core board in the PCB structure, the thickness of the second core board accounts for greater than or equal to 10% of the PCB structure, and the elastic modulus of the second core board is greater than or equal to 25 Gpa.

2. The PCB structure according to claim 1, characterized in that: The thickness of the second core plate is in the range of 0.3 mm to 1 mm.

3. The PCB structure according to claim 1, characterized in that: The second core panel includes a first surface and a second surface, the number of the first core panels and the number of the bonding layers are both two, one of the bonding layers is stacked between the first surface and one of the first core panels, and the other bonding layer is stacked between the second surface and another of the first core panels.

4. The PCB structure according to claim 3, characterized in that: The adhesive layer fully covers the first surface and the second surface.

5. The PCB structure according to claim 1, characterized in that: There are multiple first core plates, which are stacked and symmetrically distributed on both sides of the second core plate.

6. The PCB structure according to claim 1, characterized in that: The PCB structure includes a central layer, and the number of the second core boards is multiple, and the multiple second core boards are symmetrically distributed on both sides of the central layer.

7. The PCB structure according to claim 6, characterized in that: When the number of the second core plates is an odd number, one of the second core plates is the central layer.

8. The PCB structure according to claim 7, characterized in that: When the number of the second core boards is an even number, the central layer of the PCB structure is the first core board or the bonding layer.

9. The PCB structure according to claim 1, characterized in that: A micro-protrusion structure is provided on the surface of the second core plate, and the micro-protrusion structure is used to increase the bonding force between the second core plate and the adhesive layer.

10. The PCB structure according to claim 9, characterized in that: The second core plate includes an edge region and a central region, the edge region surrounds the central region, and the distribution density of the micro-protrusion structures in the edge region is greater than the distribution density of the micro-protrusion structures in the central region.

11. A method for manufacturing a PCB structure, characterized in that: The following steps are involved: Providing a first core board, wherein the first core board is a copper-clad board, etching copper foils on surfaces of both sides of the first core board to obtain a circuit layer; Providing a second core board, wherein the second core board is a copper-clad board, etching away the copper foil on the second core board so that there is no circuit layer on the surface of the second core board, and the thermal expansion coefficient of the second core board is smaller than the thermal expansion coefficient of the first core board; Using the adhesive layer, pressing and bonding the first core plate and the second core plate; Among them, the expansion coefficient of the second core board is less than or equal to 10 ppm / ℃, the expansion coefficient of the first core board ranges from 15 ppm / ℃ to 20 ppm / ℃, the thickness of the second core board accounts for less than the thickness of the first core board in the PCB structure, the thickness of the second core board accounts for greater than or equal to 10% of the PCB structure, and the elastic modulus of the second core board is greater than or equal to 25 Gpa.

12. An electronic device, comprising components and a PCB structure as claimed in any one of claims 1 to 10, characterized in that: The PCB structure includes a pad, and the components are welded on the pad.

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