Patent Publication Number: US-2023164927-A1

Title: Method for fabricating asymmetric board

Description:
The present application claims priority of Chinese patent application, with Application No. 202010297206.0, titled “a method for fabricating an asymmetric board”, filed on Apr. 15, 2020 to CNIPA, the entire content of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present application relates to the technical field of circuit board fabricating, and more particularly to a method for fabricating an asymmetric board. 
     BACKGROUND 
     The statements here only provide background information related to the present application, and do not necessarily constitute prior art. 
     With the widespread application of automotive radars, the mixed pressure of different materials and asymmetric laminated structures are more and more widely used in the design of radar boards. Different from the asymmetric laminating of conventional materials, the asymmetric laminating of the radar board is mainly composed of high-frequency materials and ordinary FR4 materials, and generally, formed by a high-frequency core board combined with multiple FR4 core boards. In addition, in order to ensure signal transmission efficiency and reduce loss, the outermost layer where the asymmetric structure is located is usually a high-frequency core board, and the high-frequency core board usually contains PTFE (polytetrafluoroethylene) material. In addition, the assembly is based on impositions, and there is a connection positions between one unit and another unit. According to the normal fabricating process, the circuit board needs to be reflow soldered during assembly, and the temperature is as high as 260° C. In the case of high temperature and large X, Y axis expansion coefficients of PTFE materials, mixed pressure asymmetric laminated high-frequency board warps after pressing. Furthermore, due to thermal stress during assembly, the shrinkage of the high-frequency surface leads to more serious warping, which in turn leads to virtual welding and desoldering after assembly, which seriously affects the assembly effect and product reliability. 
     The asymmetric laminated circuit board of conventional materials can be drilled in the prepreg on the layer where the asymmetric structure is located to cut off the prepreg arm on the asymmetric side, to reduce torque and release stress. However, in the mixed-pressure asymmetric laminated circuit board of the radar board, the high-frequency core layer where the asymmetric structure is located contains PTFE as the filler, which cannot be made by pressing the prepreg and copper foil. Therefore, it is impossible to achieve the purpose of improving the degree of warping by drilling and releasing the stress in the prepreg where the asymmetric layer is located. 
     Therefore, for radar boards with mixed materials of different materials and asymmetrical laminating, the problems of warping after pressing and assembling still need to be improved. 
     TECHNICAL PROBLEM 
     An object of one of embodiments of the present application is to provide a method for fabricating an asymmetric board, aimed to solve the problem of warping of existing asymmetric circuit boards. 
     SUMMARY 
     In order to solve above technical problem, the technical solution adopted in an embodiment of the present application is: 
     A method for fabricating an asymmetric board is provided, the method includes: 
     fabricating a master board; wherein the master board includes at least one first sub-board, each first sub-board includes a first insulating layer, and a first copper layer and a second copper layer disposed on opposite sides of the first insulating layer; 
     fabricating a second sub-board; wherein the second sub-board includes a second insulating layer, and a third copper layer and a fourth copper layer disposed on opposite sides of the second insulating layer, and material of the first insulating layer is different from that of the second insulating layer; 
     thermal compression bonding; laminating the master board and the second sub-board and placing a prepreg between the master board and the second sub-board, wherein the third copper layer is located on a side of the second sub-board away from the master board, the second copper layer is located on a side of the first sub-board away from the second sub-board, and the asymmetric board is formed by thermal compression bonding the master board and the second sub-board; a plurality of impositions are formed on the asymmetric board, and each imposition includes a plurality of units, the first copper layer to the fourth copper layer are correspondingly formed connection positions between one imposition and another imposition, between one unit and another unit, and between the unit and a board edge of the asymmetric board; and 
     milling a finished board; performing milling at the connection positions between one imposition and another imposition, the connection positions between the imposition and the board edge, and the connection positions between one unit and another unit, to obtain the plurality of impositions, and each imposition includes a plurality of units connected to each other; 
     the method for fabricating the asymmetric board further includes at least one of the following three steps: 
     laying copper; in the step of fabricating the master board, laying copper on the connection positions of the master board except for the second copper layer of an outermost layer to obtain laying copper areas; 
     digging copper; in the step of fabricating the second sub-board, digging copper on the connection positions of the third copper layer; and 
     depth control milling; after the step of milling the finished board, on each of the impositions, performing depth control milling at the connection positions between one unit and another unit from a side of the second sub-board to obtain a depth control groove. 
     In one embodiment, the laying copper areas between one unit and another unit are arranged apart from each other, and lengths of two adjacent laying copper areas between one unit and another unit are different; the laying copper areas between one imposition and another imposition are continuously arranged, and the laying copper areas between the unit and the board edge are continuously arranged. 
     In one embodiment, in the multiple laying copper areas between one unit and another unit, and the two laying copper areas spaced apart have a same shape. 
     In one embodiment, an unilateral indentation distance of the laying copper areas between one unit and another unit relative to the connection positions is greater than or equal to 0.15 mm; a distance from the laying copper areas between one imposition and another imposition to the unit is greater than or equal to 0.2 mm, and a distance from the laying copper areas between the unit and the board edge to the unit is greater than or equal to 0.2 mm. 
     In one embodiment, a width of the digging copper area is smaller than a width of a position for milling finished board in the step of milling a finished board. 
     In one embodiment, a distance between the digging copper area and the position for milling finished board in the step of milling a finished board is greater than or equal to 0.1 mm. 
     In one embodiment, the step of depth control milling is configured for controlling to mill penetrating the second sub-board and not damage the first copper layer of the master board. 
     In one embodiment, the method includes the step of digging copper and the step of depth control milling, in the step of digging copper, forming a digging copper area and connection portions located outside the digging copper area on the connection positions of the third copper layer, the step of thermal compression bonding further includes removing the connection portion, and the depth control groove having a minimum depth D 0   min =H 1 −H 4 +ΔX and a maximum depthD 0   max =H 1 −H 4 +H 3 −ΔX, wherein the H 1  represents a thickness of the third copper layer, the H 3  represents a thickness of the prepreg, the H 4  represents a thickness of the third copper layer, and the ΔX represents a precision tolerance of a depth control milling machine. 
     In one embodiment, the method includes the step of laying copper, in the step of milling a finished board, the position for milling finished board corresponds to a position of part of the laying copper areas. 
     In one embodiment, the method includes the step of digging copper, in the step of digging copper, forming a digging copper area and connection portions located outside the digging copper area on the connection positions of the third copper layer, and the connection portion corresponds to positions of remaining laying copper areas. 
     In one embodiment, the method further includes the step of depth control groove, a width of the depth control groove is larger than a width of the layer copper areas in a connection positions of an inner circuit of the second sub-board. 
     In one embodiment, a distance of a single side of the depth control groove exceeding a single side of the laying copper area in the connection positions in each of the impositions is greater than or equal to 0.075 mm. 
     In one embodiment, in the step of depth control milling, the depth control milling being performed on each of the connection portions, and in each of the impositions, a number of the depth control grooves is multiple. 
     In one embodiment, a distance between an edge of the depth control groove and an edge of the unit adjacent to the depth control groove is greater than or equal to 0.075 mm. 
     In one embodiment, in the step of thermal compression bonding, a temperature drop rate is ranged from 2° C./min˜3° C./min. 
     BENEFICAL EFFECTS 
     The beneficial effect of the method for fabricating the asymmetric board provided by the embodiment of the present application is that: the method for fabricating the asymmetric board provided by the embodiment of the present application includes the steps of fabricating a master board, fabricating a second sub-board, thermal compression bonding the master board and the second sub-board, and milling a finished board; further includes at least one of the following three steps: laying copper on the connection positions of the master board except for the second copper layer of an outermost layer to obtain laying copper area, digging copper on the connection positions of the third copper layer, and after the step of milling the finished board, on each of the impositions, performing depth control milling at the connection positions from a side of the second sub-board on each imposition to obtain a depth control groove, any one of the three steps is beneficial to reduce the warping of the asymmetrical board after heating, so as to solve the problem of virtual soldering and desoldering after the circuit board is assembled, and improve the reliability of the product. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order to explain the embodiments of the present application more clearly, a brief introduction regarding the accompanying drawings that need to be used for describing the embodiments of the present application or the prior art is given below; it is obvious that the accompanying drawings described as follows are only some embodiments of the present application, for those skilled in the art, other drawings can also be obtained according to the current drawings on the premise of paying no creative labor. 
         FIG.  1    is a flow chart of steps of a method for fabricating an asymmetric board provided by an embodiment of the present application; 
         FIG.  2    is a schematic diagram of a laminated structure of an asymmetric board; 
         FIG.  3    is a schematic diagram of laying copper of the connection positions of the inner circuit layer in a master board; 
         FIG.  4    is an enlarged view of part A in  FIG.  3   ; 
         FIG.  5    is a schematic longitudinal cross-sectional view of a second sub-board; 
         FIG.  6    is a schematic diagram of digging copper of a third copper layer of a second sub-board; 
         FIG.  7    is an enlarged view of part B in  FIG.  6   ; 
         FIG.  8    is a structural schematic diagram of connection positions of the imposition before depth control milling; and 
         FIG.  9    is a structural schematic diagram of connection positions of the imposition after depth control milling. 
     
    
    
     DETAILED DESCRIPTION 
     In order to make the purpose, the technical solution and the advantages of the present application be clearer and more understandable, the present application will be further described in detail below with reference to accompanying figures and embodiments. It should be understood that the specific embodiments described herein are merely intended to illustrate but not to limit the present application. 
     It is noted that when a component is referred to as being “fixed to” or “disposed on” another component, it can be directly or indirectly on another component. When a component is referred to as being “connected to” another component, it can be directly or indirectly connected to another component. Terms such as “up”, “down” “left”, “right” and so on are the directions or location relationships shown in the accompanying figures, which are only intended to describe the present application conveniently and simplify the description, but not to indicate or imply that an indicated device or component must have specific locations or be constructed and manipulated according to specific locations; therefore, these terms shouldn&#39;t be considered as any limitation to the present application. In addition, terms “the first” and “the second” are only used in describe purposes, and should not be considered as indicating or implying any relative importance, or impliedly indicating the number of indicated technical features. As such, technical feature(s) restricted by “the first” or “the second” can explicitly or impliedly comprise one or more such technical feature(s). In the description of the present application, “a plurality of” means two or more, unless there is additional explicit and specific limitation. 
     In order to illustrate the technical solutions provided by the present application, detailed descriptions are given below in conjunction with specific drawings and embodiments. 
     Referring to  FIG.  2   , the asymmetric board  100  includes a master board  1  having a first sub-board  11  or a master board formed by thermal compression bonding a plurality of first sub-boards  11  together, and a second sub-board  2 . The first sub-board  11  includes a first insulating layer  112 , and a first copper layer  111  and a second copper layer  113  arranged on opposite sides of the first insulating layer  112 . The second sub-board  2  may be a high-frequency core board, and the second sub-board  2  includes a second insulating layer  22 , the third copper layer  21  and the fourth copper layer  23  arranged on opposite sides of the second insulating layer  22 , the material of the first insulating layer  112  of the first sub-board  11  and the material of the second insulating layer  22  of the second sub-board  2  are different. The master board  1  and the second sub-board  2  are laminated and thermal compression bonded together. Since the material of the first insulating layer  112  of the first sub-board  11  and the material of the second insulating layer  22  of the second sub-board  2  are different, therefore, the resulting laminated structure is asymmetric in form, that is, the aforementioned asymmetric board  100  is obtained. 
     The asymmetric board  100  tends to warp in the direction with a large expansion coefficient when subjected to high temperature. Generally, the material of the second insulating layer  22  in the high-frequency core board is PTFE, and the material of the first sub-board  11  is a commonly used FR4 core board. The FR4 material used in the first insulating layer  112  has a smaller expansion coefficient than that of the second insulating layer  22 . Therefore, after the master board  1  and the second sub-board  2  are laminated and thermal compression bonded together, the asymmetric board  100  is likely to warp in the direction of the second sub-board  2 . 
     Referring to  FIG.  2    together, arranging the thickness of the second sub-board  2  to be H 1 , where the thickness of the third copper layer  21  is H 4 , the thickness of the second insulating layer  22  is H 5 , and the thickness of the fourth copper layer  23  is H 6 , then H 1 =H 4 +H 5 +H 6 , and arranging the thickness of master board  1  to be H 2 . 
     Referring to  FIG.  1   , an embodiment of the present application provides a method for fabricating an asymmetric board, to solve the above-mentioned warping problem of the asymmetric board  100 . Specifically, the method for fabricating the asymmetric board includes: fabricating a master board  1 : referring to  FIG.  2   , the master board  1  includes at least one first sub-board  11 ; 
     fabricating a second sub-board  2 : referring to  FIG.  2    in combination; 
     thermal compression bonding: laminating the master board  1  and the second sub-board  2 , where the third copper layer  21  is located on a side of the second sub-board  2  away from the master board  1 , and at least one prepreg is placed between master board  1  and second sub-board  2 , the master board  1  and the second sub-board  2  are placed into the pressing machine to perform thermal compression bonding to obtain the asymmetric board  100 ; as shown in  FIGS.  3  and  6   , a plurality of impositions  3  are formed on the asymmetric board  100 , and each imposition  3  includes a plurality of units  31  (refer to  FIGS.  3  and  6    in combination, PCS 1 ˜PCS 4  represent four units  31  in one imposition  3 , of course, the number of units  31  in one imposition  3  is not limited to four), the first copper layer  111  to the fourth copper layer  23  of the asymmetric board  100  corresponding to the area between one imposition  3  and another imposition  3 , the area between one unit  31  and another unit  31 , and the area between the unit  31  and the board edge  35  are all used as the connection locations  30 ; and 
     milling a finished board; performing milling at the connection positions 30  between one imposition  3  and another imposition  3 , the connection positions between the imposition  3  and the board edge  35 , and the connection positions  30  between one unit  31  and another unit  31 , to obtain the plurality of impositions  3 , and each imposition  3  includes a plurality of units  31 ; after milling the finished board, the units  31  in each imposition  3  are connected to each other. 
     The method for fabricating the asymmetric board provided by embodiments of the present application includes at least one of the following three steps: 
     laying copper: in the step of fabricating the master board  1  further includes laying copper on the connection positions  30  of the circuit layer of the first sub-board  11 . Specifically, except for the second copper layer  113  of an outermost layer of the master board  1 , the copper is partially reserved in the connection positions  30  on the other first copper layer  111  and the second copper layer  113 , to form a plurality of laying copper areas  32  spaced apart from each other, as shown in  FIG.  3   : 
     digging copper: in the step of fabricating the second sub-board  2  further includes digging copper on the connection positions  30  of the protective layer of the second sub-board  2 . Specifically, on the side of the second sub-board  2  away from the master board  1 , that is, von the part of the third copper layer  21  corresponding to the connection positions  30 , to form a plurality of digging copper areas  33  spaced apart from each other, and connection portions  34  of copper material is formed between the digging copper areas  33 , as shown in  FIG.  6   ; and 
     depth control milling: after the step of milling the finished board, on each of the impositions  31 , performing depth control milling at the connection positions 30  between one unit  31  and another unit  31  from a side of the second sub-board  2  to obtain a depth control groove  36 , as shown in  FIG.  9   . 
     It should be noted that after depth control milling, one unit  31  and another unit  31  on each imposition  3  are still connected, and they can be shipped as one imposition  3 . 
     The method for fabricating the asymmetric board provided by the embodiments of the present application includes at least one of the following three steps: digging copper on the connection positions  30  of the protective layer of the second sub-board  2 , laying copper on the connection positions  30  of the circuit layer of the first sub-board  11 , and after milling the finished board, performing depth control milling at the connection positions 30  from a side of the second sub-board  2 , any one of the three steps is beneficial to reduce the warping of the asymmetrical board  100  after heating, so as to solve the problem of virtual soldering and desoldering after the circuit board is assembled, and improve the reliability of the product. 
     It should be noted that the above steps of fabricating the master board  1  and the second sub-board  2  are independent of each other and do not affect each other. Therefore, the steps of fabricating the master board  1  and fabricating the second sub-board  2  can be carried out sequentially in time or at the same time. The following is an example of fabricating the master board  1  and the second sub-board  2  as an example. That is, as shown in  FIG.  1   , in the embodiment, step S 1  is to fabricate the master board  1 , step S 2  is to fabricate the second sub-board  2 , and step S 3  is thermal compression bonding between master board  1  and second sub-board  2 , step S 4  is the milling the finished board, and step S 5  is depth control milling. 
     Specifically, in step S 1 , the following detailed steps are included: 
     Step S 1 - 1 , cutting: cutting an entire large copper clad laminate into a required work board according to design requirements, and pass the cut work board through the tunnel furnace to reduce the internal stress of the board; the work board includes an insulating layer and copper layers on both sides 
     Step S 1 - 2 , transferring an inner layer pattern of the work board: coating a layer of photosensitive polymer on the copper layer on at least one side of the work board, irradiating the layer of photosensitive polymer with an exposure machine (such as a 4CCD alignment lens semi-automatically), and transferring the inner layer pattern required by the master board  1  to the photosensitive polymer; 
     Step S 1 - 3 , developing: placing the work board covered with photosensitive polymer into the developer, the photosensitive polymer that has not undergone polymerization reaction is developed, and the photosensitive polymer that undergoes polymerization reaction will not be developed (in negative Photopolymer as an example), the copper material that does not need to be retained on the copper layer of the work board is exposed; 
     Step S 1 - 4 , etching: etching the exposed copper material by an etching solution, and the copper material protected by the photosensitive polymer is retained to obtain the desired inner layer pattern; 
     Step S 1 - 5 , film stripping: stripping the photosensitive polymer on the copper layer by a stripping liquid to obtain the first sub-board  11 ;the copper layers on both sides of the first sub-board  11  are the first copper layer  111  and the second copper layer  113  respectively, and the first insulating layer  112  is located between the first copper layer  111  and the second copper layer  113 ; 
     Step S 1 - 6 , optical inspecting: optically inspecting the completed first sub-board  11  to confirm the quality; 
     Step S 1 - 7 , punching, use a punching machine to punch out multiple positioning holes on the first sub-board  11  (for example, 8 positioning holes, including 4 fusion positioning holes and 4 riveting positioning holes); 
     Step S 1 - 8 , browning: browning the first copper layer  111  and the second copper layer  113  on the opposite sides of the first sub-board  11  with a browning liquid to roughen the surface of the copper conductor; 
     Step S 1 - 9 , first thermal compression bonding (correspondingly, based on the time sequence, the aforementioned thermal compression bonding between the master board  1  and the second sub-board  2  is named second thermal compression bonding): laminating the browned first sub-board  11  with the prepreg(herein, the first prepreg  4 , as shown in  FIG.  2   ) according to the laminating requirements of the customer, riveting according to the positioning holes, and fully illuminating the interlayer alignment ring with an inspecting machine, and then thermal compression bonding under the high temperature environment in the pressing machine; 
     Step S 1 - 10 , post-compression bonding processing procedures: including target shooting, edge milling, drilling, copper sinking, board powering, outer circuit, inner layer etching, inner layer automatic optical inspecting, etc. 
     It should also be noted that in the master board  1  (the following are examples of the master board  1  being thermal compression bonded by multiple first sub-boards  11 ), referring to  FIG.  2   , the second copper layer  113  of the outermost layer on the side of the second sub-board  2  is used as one of the protective layers of the asymmetric board  100  during the second thermal compression bonding process, and only the various tool holes required for the subsequent processes are fabricated on the protective layer. Therefore, in the above step S 1 , at least one first sub-board  11  has only one side of the copper layer to fabricate the inner layer pattern according to the above steps S 1 - 2  to S 1 - 5 , that is, in the above step S 1 , at least one first sub-board  11  has only the copper layer on one side of the first sub-board  11  to fabricate e the inner layer pattern according to the above steps S 1 - 2  to S 1 - 5 . 
     Specifically, the inner layer pattern should include: the circuit pattern (not shown) in the unit  31 , the pattern corresponding to the laying copper area  32  on the connection positions  30  between one unit  31  and another unit  31 , and the pattern corresponding to the laying copper area  32  on the connection positions  30  between one imposition  3  and another imposition  3 , and the pattern corresponding to the laying copper area  32  on the connection positions  30  between the unit  31  and the board edge  35 . Therefore, in the above step S 1 - 5 , the inner layer patterns obtained are that on the first copper layer  111  (or the first copper layer  111  and the second copper layer  113 ) of the first sub-board  11 , on the connection positions  30  between one unit  31  and another unit  31 , on the connection positions  30  between one imposition  3  and another imposition  3 , and on the connection positions  30  between the unit  31  and the board edge  35 . 
     Optionally, as shown in  FIG.  3   , the copper is intermittently laid on the connection positions  30  between one unit  31  and another unit  31 , forming a plurality of laying copper areas  32  sequentially spaced apart by a certain distance; and the laying copper areas on the connection positions between one imposition  3  and another imposition  3 , and between the unit  31  and the board edge  35  are continuously arranged. This is because the gap between the laying copper areas  32  arranged at intervals between one unit  31  and another unit  31  is beneficial to release the internal stress of the first sub-board  11 , thereby reducing the warping of the first sub-board  11  and the asymmetric board  100 ; and the connection positions  30  between one imposition  3  and another imposition  3 , as well as the connection positions  30  between the unit  31  and the board edge  35 , need to be removed in the subsequent steps of milling the finished board. Therefore, the laying copper areas  32  do not need to be arranged at intervals. The complexity of production is lower, and the production costs can be reduced. 
     As shown in  FIG.  2   , taking a six-layer asymmetric board as an example, the layer L 1  and layer L 2  are the third copper layer  21  and the fourth copper layer  23  of the second sub-board  2  respectively, and the layer L 1  is the protective layer on the side of the second sub-board  2 , layer L 3  to layer L 6  are located on the master board  1  composed of two first sub-boards  11  laminated, and the layer L 6  is the protective layer on the side of the master board  1  during the second thermal compression bonding process. In step S 1 , only the various tool holes required for the subsequent process are fabricated on the second copper layer  113  used as the layer L 6 , and the first copper layer  111  and second copper layer  113  used as the layer L 3  to layer L 5  respectively are normally produced the above-mentioned inner layer patterns according to the above steps S 1 - 2  to Step S 1 - 5 . In this way, the laying copper areas  32  arranged at intervals can reduce the warping of the master board  1 , thereby reducing the warping of the asymmetric board  100  caused by the master board  1  and the second sub-board  2  during the second thermal compression bonding process, and thereby reduce the warping of the finished circuit board caused by heat during the customer&#39;s assembly process; in addition, the arrangement of multiple laying copper areas  32  can enhance the rigidity of the master board  1 . 
     In addition, as shown in  FIG.  4   , the shapes of the multiple laying copper areas  32  in the connection positions  30  between one unit  31  and another unit  31  may not necessarily be completely consistent. In this embodiment, among the multiple laying copper areas  32  in the connection positions  30  between one unit  31  and another unit  31 , two adjacent laying copper areas  32  spaced apart have the same shape. As shown in  FIG.  4   , 
     the length and the width of one laying copper area  32  are close or even equal, and the length of one laying copper area  32  adjacent to it is obviously larger than its width and in a shape of elongated (for this specific effect, please combine the following step S 4 ). 
     Specifically, in step S 2 , the step S 2  includes: 
     Step S 2 - 1 , cutting: cutting an entire large copper clad laminate into a required work board according to design requirements, and pass the cut work board through the tunnel furnace to reduce the internal stress of the board; 
     Step S 2 - 2 , transferring a pattern of the work board: coating a layer of photosensitive polymer on the copper layer on at least one side of the work board, irradiating the layer of photosensitive polymer with an exposure machine (such as a 4CCD alignment lens semi-automatically), and transferring the inner layer pattern required by the second sub-board  2  to the photosensitive polymer; 
     Step S 2 - 3 , developing: placing the work board covered with photosensitive polymer into the developer, the photosensitive polymer that has not undergone polymerization reaction is developed, and the photosensitive polymer that undergoes polymerization reaction will not be developed (in negative Photopolymer as an example), the copper material that does not need to be retained on the copper layer of the work board is exposed; 
     Step S 2 - 4 , etching: etching the exposed copper material by an etching solution, and the copper material protected by the photosensitive polymer is retained to obtain the desired patterns on the copper layers of both sides; 
     Step S 2 - 5 , film stripping: stripping the photosensitive polymer on the copper layer by a stripping liquid to obtain the second sub-board  2 ;the copper layers on both sides of the second sub-board  2  are the third copper layer  21  and the fourth copper layer  23  respectively, and the second insulating layer  22  is located between the third copper layer  21  and the fourth copper layer  23 ; 
     Step S 2 - 6 , optical inspecting: optically inspecting the completed second sub-board  2  to confirm the quality; 
     Step S 2 - 7 , punching, use a punching machine to punch out multiple positioning holes on the second sub-board  2  (for example, 8 positioning holes, including 4 fusion positioning holes and 4 riveting positioning holes). 
     It should be noted here that in the step S 2 , since the layer L 1 , that is, the third copper layer  21 , is used as another protective layer of the asymmetric board  100 , the inner layer circuit pattern is normally fabricated on the layer L 2 , that is, on the fourth copper layer  23 . 
     Specifically, the patterns of the second sub-board  2  should include: pattern on the connection positions  30  between one unit  31  and another unit  31  formed on the third copper layer  21  that correspond to pattern other than the digging copper area  33  (that is, the pattern of the connection portion  34 ), the connection positions  30  between one imposition  3  and another imposition  3  corresponds to the pattern other than the digging copper area  33 , and the connection positions  30  between the unit  31  and the board edge  35  corresponds to the pattern other than the digging copper area  33 , the inner layer circuit pattern(not shown, and at this time, the inner layer circuit pattern is not fabricated in the unit  31  of the layer L 1  to serve as a protective layer) of the unit  31  is formed on the fourth copper layer  23 . Therefore, in the above step S 2 - 5 , the obtained patterns are: the third copper layer  21  of the second sub-board  2  is provided with the multiple connection portions  34  spaced apart on the connection positions  30  between one unit  31  and another unit  31 , the multiple connection portions  34  spaced apart on the connection positions  30  between one imposition  3  and another imposition  3 , and the multiple connection portions  34  spaced apart on the connection positions  30  between the unit  31  and the board edge  35 , as shown in  FIG.  6   , and the fourth copper layer  23  is provided with the inner circuit pattern of the unit  31 , as shown in  FIG.  5   . 
     By partially digging copper on the connection positions  30  of the layer L 1 , part of the stress of the second insulating layer  22  of the second sub-board  2  can be released, thereby reducing the degree of warping of the second sub-board  2  after being heated, and further, reduce the degree of warping of the finished circuit board after assembly. 
     In an optional embodiment, in the step S 2 , the connection portions  34  are arranged to correspond to the positions of the partial laying copper areas  32  in step S 1 . In this way, in the thickness direction of the asymmetric board  100 , the connection portions  34  always have corresponding laying copper areas  32 , which is more convenient for thermal compression bonding between the connection portions  34  and the laying copper areas  32  (for details, please continue to combine the following step S 3 ). 
     In step S 3 , the specific steps of the second thermal compression bonding are: laminating at least one prepreg (here defined as the second prepreg  5 , referring to  FIG.  2   ) between the master board  1  and the second sub-board  2  obtained above according to laminating requirements of the customer, positioning through the positioning holes, fully illuminating the interlayer alignment ring with an inspecting machine, and thermal compression bonding in a high temperature environment. 
     In an optional embodiment, in the second thermal compression bonding, the temperature drop rate is ranged from 2° C./min-3° C./min, for example, keep 2° C./min. Due to the low temperature drop rate, the internal stress of the asymmetric board  100  can be further reduced, thereby reducing its warping after heating. 
     Referring to  FIG.  3    and  FIG.  6    together, in a specific embodiment, the connection portions  34  on the layer L 1  correspond to the two laying copper areas  32  arranged at intervals on the layer L 3  to the layer L 5 . That is, corresponding to the laying copper area  32  with a smaller length. In this way, during the second thermal compression bonding, there will be no case where there is no layer copper area  32  under the connection portion  34 , and the deformation of the connection portion  34  during the thermal compression bonding process and the deformation of the asymmetric board  100  can be avoided. 
     In addition, in the step S 3 , it also includes browning, laser drilling, plasma treatment, copper immersion and board powering, thickening of the board, and outer layer circuit fabricating (fabricating the outer layer circuit on units of the layer L 1  and the layer L 6 , and after the outer layer circuit is fabricated, the connection portion  34  on the layer L 1  is removed, and the connection positions  30  of the layer L 6  can be left with the copper pattern in consistent with the laying copper of the layers L 3 ˜L 5 , or in other words, a laying copper area  32  is also formed on the connection positions of the layer L 6 ), pattern plating, outer layer etching, solder resist ink printing, character printing, and testing and so on. Referring to  FIGS.  8  and  9    together, the outermost layer of the imposition  3  is the solder resist layer  6  formed by solder resist ink printing. 
     After step S 3 , go to step S 4 , the milling finished board: performing milling on the obtained asymmetric board  100 , that is, performing milling the connection positions  30  between one imposition  3  and another imposition  3 , the connection positions  30  between the imposition  3  and the board edge  35 , and the connection positions  30  between one unit  31  and another unit  31 , respectively; cutting off the connection between one imposition  3  and another imposition  3 , the connection between the imposition  3  and the board edge  35 , and partial connection between one unit  31  and another unit  31 , to obtain a plurality of impositions  3 , and each imposition  3  includes a plurality of units  31  connected to each other. 
     In each imposition  3  obtained, there is no connection portion  34  on the connection positions  30  of the layer L 1  between one unit  31  and another unit  31 , and laying copper areas  32  are provided on the connection positions  30  of the layer L 3  to the layer L 6 . That is, after the step S 4 , in each imposition  3 , laying copper areas  32  are reserved between one unit  31  and another unit  31  on the connection positions  30  between the layer L 3  to the layer L 6 . 
     In a specific embodiment, referring to  FIG.  3    together, it may be that the long-length and elongated laying copper areas  32  in the connection positions  30  are milled, and the other laying copper areas  32  are reserved. Of course, this is only an example here to show that position for milling finished board can be performed according to the position of a part of the laying copper areas  32 , and the shape of the laying copper areas  32  are not particularly limited. In practical applications, the laying copper areas  32  are arranged according to the connection requirements between one unit  31  and another unit  31  after milling finished board. 
     In an optional embodiment, referring to  FIG.  6    and  FIG.  7    in combination, in order to prevent the alignment deviation during milling board in the step S 4 , in the above step S 2 , a sufficient distance should be reserved between the copper cutting area  33  and the position for milling finished board, that is the width of the digging copper area  22  should be smaller than the width of the position for milling finished board. Specifically, the distance between the digging copper area  33  and the position for milling finished board is D 3 , and D 3 ≥0.1 mm, that is, the edge of the copper digging area  33  close to the unit  31  is retracted by at least 0.1 mm relative to the edge of the position for milling finished board adjacent to the unit  31 . 
     Referring to  FIG.  1   , in one embodiment, after the milling finished board of step S 4  is completed, then go to step S 5 , in the connection positions  30  between one unit  31  and another unit  31  of each imposition  3 , depth control milling from a side of the second sub-board  2 , to obtain the depth control groove  36 . In this way, on the one hand, the second core board  2  is partially removed, which can reduce its expansion after the second thermal compression bonding and the heating process of the subsequent assembly process of the customer, thereby reducing the warping of the second sub-board  2  and the asymmetric board  100 ;on the other hand, the connection positions  30  between one unit  31  and another unit  31  is partially removed in height, and the depth control groove  36  can make room for the expansion of the unit  31  and reduce the thermal stress of the unit  31 , especially on the side of the second sub-board  2 , the warping of the second sub-board  2  and the asymmetric board  100  can also be reduced. 
     Optionally, the depth control milling is arranged to perform depth control milling on the asymmetric board  100  from the side of the third copper layer  21  to the connection positions  30  between the units  31 . There is no doubt that, since one unit  31  and another unit  31  in each imposition  3  are connected together by a part of the second insulating layer  22 , a part of the second prepreg  5 , the laying copper area  32 , and a part of the first prepreg  4  between the laying copper areas  32 . Therefore, depth control milling is implemented correspondingly to above the remaining laying copper areas  32 , that is, the position of the deep control groove  36  corresponds to the position of the remaining laying copper area  32  until it reaches the second prepreg  5  between the master board  1  and the second sub-board  2 . The result of this is that depth control milling removes part of the second sub-board  2  in connection positions  30  and part of the second prepreg  5 . That is, the depth control milling is configured for controlling to mill penetrating the second sub-board  2  and does not damage the first copper layer  111  on the side of the first sub-board  11  closest to the second sub-board  2 . The advantage is that it can avoid that the depth D 0  of the deep control groove  36  is too large, which will cause the unit  31  to break and affect the shipment. 
     Therefore, the minimum depth of the depth control groove  36  is D 0   min =H 1 −H 4 +ΔX, and the maximum depth of the depth control groove  36  is D 0   max =H 1 −H 4 +H 3 −ΔX, where H 3  represents the thickness of the second prepreg  5 , and ΔX represents the precision tolerance of the depth control milling machine. Among them, the thickness of the solder resist layer  6  is relatively small compared to other layers and can be ignored. 
     For example, in a specific embodiment, H 1  is 0.186 mm, H 4  is 0.018 mm, H 3  is 0.08 mm, and the precision tolerance ΔX of the depth control milling machine is 0.025 mm. Therefore, the minimum depth D 0   min  of the depth control groove  36  is 0.193 mm, and the maximum depth D 0   max  of the depth control groove  36  is 0.223 mm. This is only an example, in other optional embodiments, depending on the laminating structure of the specific asymmetric board  100  and the depth control milling machine, the depth D 0  of the depth control groove  36  is allowed to have other ranges, which are not particularly limited. 
     In practical applications, in order to ensure the release effect of thermal stress, the depth D 0  of the depth control groove  36  should be as close to D 0   max  as possible. 
     In an optional embodiment, in each imposition  3 , the number of the depth control grooves  36  is multiple, and the positions of the depth control grooves  36  correspond to the positions of the connection portions  34  one by one. That is, in step S 5 , depth control milling is started at each connection portion  34 . 
     In an optional embodiment, referring to  FIG.  8    and  FIG.  9    in combination, the width of the depth control groove  36  is greater than the width of the laying copper area  32  in the connection positions  30  of the inner layer circuit (layer L 3  to layer L 5 ). Specifically, the distance of the single side of the depth control groove  36  exceeds the single side of the laying copper area  32  of the inner layer circuit of the connection positions (the area filled with diagonal lines in  FIGS.  8  and  9    represents the laying copper area  32  in the connection position) is D 5 , and D 5 ≥0.075 mm. 
     In an optional embodiment, referring to  FIGS.  8  and  9    in combination, the width of the depth control groove  36  must be smaller than the width of the connection positions  30 . Specifically, the distance between the edge of the depth control groove  36  and the edge of the unit  31  is D 6 , and D 6 ≥0.075 mm. 
     Optionally, due to the alignment error in step S 4  and step S 5 , the depth control milling machine and other equipment used also have precision tolerances, in order to avoid exposing the copper laid on the edges of the inner layer circuit layers (layer L 3  to layer L 5 ) of the imposition  3  and the edges of the depth control grooves, as shown in  FIG.  4   , a unilateral indentation distance of the laying copper areas 32  between one unit  31  and another unit  31  relative to the connection positions  30  is D 1 , that is, the distance between the edge of the laying copper area  32 between one unit  31  and another unit  31  and the edge of the connection positions  30  is D 1 , and D 1 ≥0.15 mm. 
     According to the different forms of the units  31 , this includes that the unilateral indentation distance of the laying copper areas  32  in the connection positions  30  along the short side direction of the unit  31  between one unit  31  and another unit  31  relative to the connection positions  30  is greater than or equal to 0.15 mm, the unilateral indentation distance of the laying copper areas 32  in the connection positions  30  along the long side direction of the unit  31  between one unit  31  and another unit  31  relative to the connection positions  30  is greater than or equal to 0.2 mm. Further optionally, the unilateral indentation distance of the laying copper areas  32  in the connection positions  30  along the short side direction of the unit  31  between one unit  31  and another unit  31  relative to the connection positions  30  is greater than or equal to 0.15 mm, the unilateral indentation distance of the laying copper areas  32  in the connection positions  30  along the long side direction of the unit  31  between one unit  31  and another unit  31  relative to the connection positions  30  is greater than or equal to 0.2 mm. It should be noted here that the aforementioned long side direction is only based on the rectangular unit  31  shown in  FIG.  3    and the position shown by the reference symbol “D 1 ” in  FIG.  4   , and the long side of the unit  31  is the up and down directions of the paper surface shown in  FIG.  3   , the short side of the unit  31  is the left and right directions of the paper surface shown in  FIG.  3   . However, it is not limited to this. In actual production, the long side direction and the short side direction of the unit  31  can be set according to specific needs. 
     Therefore, in the embodiment, the unilateral indentation distance of the laying copper areas 32  in the connection positions  30  along the long side direction of the unit  31  between one unit  31  and another unit  31  relative to the connection positions  30  is the gap distance between the laying copper area  32  between one unit  31  and another unit  31  and the unit  31 , and the gap distance is greater than or equal to 0.2 mm; the unilateral indentation distance of the laying copper areas  32  in the connection positions  30  along the short side direction of the unit  31  between one unit  31  and another unit  31  relative to the connection positions  30  is the gap distance between the two adjacent laying copper areas  32  in the connection positions  30  between one unit  31  and another unit  31  and the unit  31 , and the gap distance is greater than or equal to 0.15 mm. 
     Further, in an optional embodiment, the gap distance between the laying copper area  32  between one unit  31  and another unit  31  and the unit  31  is greater than or equal to 0.20 mm. 
     The distances between the laying copper area  32  between one imposition  3  and another imposition  3 , the laying copper area  32  between the unit  31  and the board edge  35  and the adjacent units  31  are all D 2 , and D 2 ≥0.2 mm. 
     The aforementioned embodiments are only preferred embodiments of the present application, and should not be regarded as being limitation to the present application. Any modification, equivalent replacement, improvement, and so on, which are made within the spirit and the principle of the present application, should be included in the protection scope of the present application.