Patent Publication Number: US-2023164908-A1

Title: Wiring board and electronic device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2021-190262, filed on Nov. 24, 2021, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiment discussed herein is related to a wiring board and an electronic device. 
     BACKGROUND 
     In general, a flexible substrate may be used as a wiring board in an electronic device, such as a wearable device, for example. The flexible substrate is a wiring board that has flexibility and that is usable in a bent state. The flexible substrate as described above is formed by laminating a wiring pattern, such as copper, on flexible insulating resin. In this case, if the wiring pattern is laminated on an entire surface of the insulating resin, bendability of the wiring board decreases, and therefore, for example, the wiring pattern is arranged in a predetermined shape with an interval to ensure the bendability of the wiring board. 
     Patent Literature 1: Japanese Laid-open Patent Publication No. 2020-088005 
     However, in the conventional flexible substrate, a remaining copper rate is reduced to ensure the bendability, so that it becomes difficult to control a thickness of the insulating resin and dimensional stability decreases, which is a problem. Specifically, with a decrease in the remaining copper rate that is a ratio of an area that is covered by the wiring pattern in the surface of the insulating resin, an area of the insulating resin that is exposed without being covered by the wiring pattern increases, so that the flexible substrate becomes easily bendable. In contrast, in a portion in which the insulating resin is exposed, it is difficult to keep a constant thickness due to fluidity of the insulating resin. As a result, a substrate thickness of the flexible substrate may vary or positional accuracy of the wiring pattern may be reduced, so that the remaining copper rate decreases and the dimensional stability also decreases. 
     SUMMARY 
     According to an aspect of an embodiment, a wiring board includes a plurality of insulating layers each being made of flexible insulating resin; and a conductor layer that is laminated on the plurality of insulating layers and that has a conductor pattern, wherein the conductor layer includes a conductor pattern that has a certain shape in which a plurality of unit patterns are connected in plan view, and each of the unit patterns includes a U-shaped pattern; an inverted U-shaped pattern that is arranged such that an opening side is located away from an opening side of the U-shaped pattern; and a straight line pattern that connects center portions of the U-shaped pattern and the inverted U-shaped pattern. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating an external appearance of an electronic device according to one embodiment; 
         FIG.  2    is a cross-sectional view illustrating a wiring board according to one embodiment; 
         FIG.  3 A  is a diagram for explaining a shape of a ground pattern; 
         FIG.  3 B  is a diagram for explaining a shape of a ground pattern; 
         FIG.  4    is a diagram for explaining dimensions of the ground pattern; 
         FIG.  5    is a plan view illustrating a specific example of a conductor layer; 
         FIG.  6 A  is a diagram illustrating a specific example of another ground pattern; 
         FIG.  6 B  is a diagram illustrating a specific example of another ground pattern; 
         FIG.  7 A  is a diagram illustrating stress distributions of unit patterns; and 
         FIG.  7 B  is a diagram illustrating stress distributions of unit patterns. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     One embodiment of a wiring board and an electronic device disclosed in the present application will be described in detail below with reference to the drawings. The present invention is not limited by the embodiment below. 
       FIG.  1    is a diagram illustrating an external appearance of an electronic device  100  according to one embodiment. The electronic device  100  illustrated in  FIG.   1    is, for example, a wearable device that is wearable on a human body, and can be wound around an arm or a leg by being bent in a longitudinal direction, for example. The electronic device  100  is constructed by mounting an electronic component on a wiring board that is configured with a flexible substrate. Specifically, the wiring board includes mounting units  110  and connecting units  120 , and electronic components  115  are mounted on the mounting units  110 . 
     The mounting units  110  are regions that are spread in a planar manner so as to have predetermined areas and that have surfaces on which the electronic components  115  are mountable. In the example illustrated in  FIG.  1   , the wiring board includes the plurality of mounting units  110 , and the mounting units  110  are connected to one another by the connecting units  120 . 
     The electronic components  115  include, for example, a semiconductor device, a chip part (a chip condenser, a chip resistor, a chip inductor, or the like), a crystal oscillator, and the like. The electronic components  115  are mounted on the mounting units  110  and electrically connected to a conductor layer of the wiring board (to be described later). 
     The connecting units  120  connect the mounting units  110  and are formed in a bendable manner. Specifically, the connecting units  120  connect the adjacent mounting units  110  and bend such that the mounting units  110  approach each other, so that the entire electronic device  100  bends in a longitudinal direction. In other words, the wiring board includes at least the two mounting units  110  and the connecting unit  120  that connects the two mounting units  110 , and the wiring board bends in a direction in which the mounting units  110  are connected by a straight line. 
       FIG.  2    is a cross-sectional view of the wiring board cut along a line I-I in  FIG.  1   . As illustrated in  FIG.  2   , the wiring board is a multilayer substrate in which insulating layers and conductor layers are laminated. Specifically, the wiring board includes insulating layers  211  to  215 , conductor layers  221  to  224 , and conductor layers  231  to  234 . 
     The insulating layers  211  to  214  are formed by using, for example, flexible insulating resin, such as epoxy resin or polyimide resin, and thicknesses of the insulating layers  211  to  214  may be set to, for example, about 20 to 45 micrometers (um). 
     The conductor layers  221  to  224  are conductor layers that are formed on the mounting units  110 , and formed on upper surfaces of the insulating layers  211  to  214 , respectively. The conductor layers  221  to  224  include wiring patterns for transmitting electrical signals, and ground patterns connected to a ground power supply (not illustrated). The wiring patterns of the different conductor layers  221  to  224  are electrically connected by vias  241  and  242  that penetrate through the insulating layers  211  to  214 . Further, the ground patterns of the conductor layers  221  to  224  are spread in a planar manner, and cover upper surfaces of the insulating layers  211  to  214  in a planar manner. The conductor layers  221  to  224  are formed by using, for example, copper or a copper alloy, and thicknesses of the conductor layers  221  to  224  may be set to, for example, about 10 to 20 µm. 
     The conductor layers  231  to  234  are conductor layers that are formed on the connecting units  120 , and formed on upper surfaces of the insulating layers  211  to  214 , respectively. The conductor layers  231  to  234  include wiring patterns for transmitting electrical signals, and ground patterns connected to a ground power supply (not illustrated). In the connecting units  120 , the wiring patterns of the different conductor layers  231  to  234  are not connected by a via, but the wiring patterns of the conductor layers  231  to  234  are electrically connected to wiring patterns of the conductor layers  221  to  224  in the adjacent mounting units  110 . Further, in the connecting units  120 , the ground patterns of the conductor layers  231   to  234  do not completely cover the upper surfaces of the insulating layers  211  to  214 , but are formed in shapes with intervals. Shapes of the ground patterns of the conductor layers  231  to  234  will be described in detail later. The conductor layers  231  to  234  are formed by using, for example, copper or a copper alloy, and thicknesses of the conductor layers  231  to  234  may be set to, for example, about 10 to 20 µm. 
     The insulating layer  215  is a solder resist layer, and is formed by using flexible insulating resin, such as phenolic resin or polyimide resin, for example. The insulating layer  215  may be formed by using the same insulating resin as the insulating layers  211  to  214 , and a thickness of the insulating layer  215  may be set to, for example, about 15 to 35 µm. 
     The insulating layer  211  is a layer that forms a lowermost layer of the wiring board, and the conductor layer  221  and the conductor layer  231  are formed on an upper surface thereof. The conductor layer  221  and the conductor layer  231  are formed on the upper surface of the insulating layer  211  by, for example, the subtractive method or the semi-additive method. The conductor layer  231  is mainly configured with a ground pattern, and formed in a certain shape that exposes the upper surface of the insulating layer  211  so as to maintain the bendability of the connecting unit  120 . 
     The insulating layer  212  is a layer that covers the conductor layer  221  and the conductor layer  231 , and the conductor layer  222  and the conductor layer  232  are formed on an upper surface thereof. The conductor layer  222  and the conductor layer  232  are formed on the upper surface of the insulating layer  212  by, for example, the subtractive method or the semi-additive method. The conductor layer  232  is mainly configured with a ground pattern, and formed in a certain shape that exposes the upper surface of the insulating layer  212  so as to maintain the bendability of the connecting unit  120 . 
     The insulating layer  213  is a layer that covers the conductor layer  222  and the conductor layer  232 , and the conductor layer  223  and the conductor layer  233  are formed on an upper surface thereof. The conductor layer  223  and the conductor layer  233  are formed on the upper surface of the insulating layer  213  by, for example, the subtractive method or the semi-additive method. A wiring pattern of the conductor layer  223  is electrically connected to, for example, a wiring pattern of the conductor layer  222  via the via  241 . Specifically, the via  241  penetrate through the insulating layer  213  and connects the wiring patterns of the different layers. The conductor layer  233  is mainly configured with a ground pattern, and formed in a certain shape that exposes the upper surface of the insulating layer  213  so as to maintain the bendability of the connecting unit  120 . 
     The insulating layer  214  is a layer that covers the conductor layer  223  and the conductor layer  233 , and the conductor layer  224  and the conductor layer  234  are formed on an upper surface thereof. The conductor layer  224  and the conductor layer  234  are formed on the upper surface of the insulating layer  214  by, for example, the subtractive method or the semi-additive method. A wiring pattern of the conductor layer  224  is electrically connected to, for example, a wiring pattern of the conductor layer  223  via the via  242 . Specifically, the via  242  penetrates through the insulating layer  214  and connects the wiring patterns of the different layers. The conductor layer  234  is mainly configured with a ground pattern, and formed in a certain shape that exposes the upper surface of the insulating layer  214  so as to maintain the bendability of the connecting unit  120 . 
     The insulating layer  215  covers the conductor layer  224  and the conductor layer  234 . In the mounting unit  110 , an opening portion  251  is formed in a part of the insulating layer  215  and exposes the upper surface of the conductor layer  224 . It is possible to electrically connect a terminal of the electronic component  115  to the conductor layer  224  that is exposed from the opening portion  251 . 
     Shapes of the ground patterns of the conductor layers  231  to  234  in the connecting unit  120  will be described below with reference to  FIG.  3 A  to  FIG.  5   .  FIG.  3 A  is a plan view illustrating the shape of the conductor layer  231  that is formed on the upper surface of the insulating layer  211  in the connecting unit  120 . In  FIG.  3 A , a left-right direction corresponds to the longitudinal direction of the electronic device  100  and serve as a bending direction of the connecting unit  120 . 
     As illustrated in  FIG.  3 A , the ground pattern of the conductor layer  231  has a certain shape in which a plurality of unit patterns  310  are connected in plan view. Meanwhile, the plan view is a viewpoint at which the surface of the insulating layer  211  on which the conductor layer  231  is formed is viewed from a vertical direction. As illustrated in  FIG.  3 B , each of the unit patterns  310  has a certain shape in which center portions of a U-shaped pattern  311  and an inverted U-shaped pattern  312  that are arranged such that respective opening sides are located away from each other are connected by a straight line pattern  313 . The bending direction of the connecting unit  120  is perpendicular to the straight line pattern  313  included in the unit pattern  310 . Here, “perpendicular” indicates not only a state in which the bending direction of the wiring board and the straight line pattern  313  cross each other at a right angle, but also a state in which the bending direction of the wiring board and the straight line pattern  313  cross each other at an angle in a predetermine range including the right angle. 
     In the ground pattern of the conductor layer  231 , one end 311a of a U-shaped pattern  311  of a first unit pattern  310  is connected to one end 312b of an inverted U-shaped pattern  312  of a second unit pattern  310  that is located in the upper left, and one end 311b of the U-shaped pattern  311  of the first unit pattern  310  is connected to one end 312a of an inverted U-shaped pattern  312  of a third unit pattern  310  that is located in the upper right. Similarly, one end 312a of an inverted U-shaped pattern  312  of the first unit pattern  310  is connected to one end 311b of a U-shaped pattern  311  of a fourth unit pattern  310  that is located in the lower left, and one end 312b of the inverted U-shaped pattern  312  of the first unit pattern  310  is connected to one end 311a of a U-shaped pattern  311  of a fifth unit pattern  310  that is located in the lower right. 
     In this manner, by adopting the shape in which the unit patterns  310  are connected, the ground pattern of the conductor layer  231  has a certain shape in which a conductor pattern that extends in a top-bottom direction perpendicular to the bending direction alternately includes a portion that protrudes to the right and a portion that protrudes to the left. Further, the portion that protrudes to the right in the conductor pattern extending in the top-bottom direction is connected to a portion that protrudes to the left in a conductor pattern that is located adjacently on the right side, and the portion that protrudes to the left in the conductor pattern extending in the top-bottom direction is connected to a portion that protrudes to the right in a conductor pattern that is located adjacently on the left side. 
     Furthermore, in the conductor layer  231 , the insulating layer  211  is exposed from vertically long cross-shaped regions surrounded by the ground pattern. By providing the regions in which the insulating layer  211  is exposed, it is possible to ensure the bendability of the connecting unit  120  due to the flexibility of the insulating resin. Moreover, a region in which the insulating layer  211  is exposed is divided into small regions that are surrounded by the ground pattern, so that it is possible to maintain a constant thickness of the insulating layer  212  that covers the conductor layer  231  for each of the small regions. Furthermore, in this shape, a relatively large number of conductor patterns are arranged in the vicinity of the straight line pattern  313   of the unit pattern  310 , so that it is possible to increase the remaining copper rate that is a ratio of an area that is covered by the conductor layer  231  in the upper surface of the insulating layer  211 . As a result, it is possible to prevent variation in a substrate thickness of the wiring board, and it is possible to improve positional accuracy of the wiring pattern. In other words, it is possible to maintain the bendability and ensure dimensional stability. 
     Here, as illustrated in  FIG.  4   , a width L1 of the conductor pattern that forms the ground pattern of the conductor layer  231  may be set to, for example, about 25 to 100 µm. Further, a width L2 of a narrow portion of the vertically long cross-shaped region in which the insulating layer  211  is exposed may be set to, for example, about 50 to 150 µm. Furthermore, a width L3 of a wide portion of the vertically long cross-shaped region in which the insulating layer  211  is exposed may be set to, for example, about  100  to 350 µm. A height L4 of the vertically long cross-shaped region in which the insulating layer  211  is exposed may be set to, for example, about  500  to 700 µm. 
     Meanwhile, in the connecting unit  120 , only the ground pattern is formed in the conductor layer  231 , but wiring patterns for transmitting electrical signals between the adjacent mounting units  110  may be formed in the conductor layers  231  to  234 . Specifically, as illustrated in  FIG.  5    for example, a wiring pattern  235  is formed on the conductor layer  232  that is formed on the upper surface of the insulating layer  212 , in addition to the same ground pattern as the ground pattern of the conductor layer  231  as described above. 
     The wiring pattern  235  transmits an electrical signal between the adjacent mounting units  110 . Specifically, the wiring pattern  235  connects the conductor layers  222  of the two mounting units  110  that are connected to each other by the connecting unit  120 , and an electrical signal is transmitted between the conductor layers  222  of the adjacent mounting units  110 . In the conductor layer  232 , a ground pattern is formed in a region in which the wiring pattern  235  is not formed, similarly to the conductor layer  231 . The ground pattern may be connected to the ground patterns of the conductor layers  222  of the adjacent mounting units  110 . 
     A ground pattern is formed on each of the entire conductor layers  231  to  234  of the connecting unit  120  in the same manner as the conductor layer  231  as described above, or a wiring pattern is formed in a part of each of the conductor layers  231  to  234  on an as-needed basis in the same manner as the conductor layer  232 . It is preferable to form the wiring patterns in the conductor layers  232  and  233  except for the conductor layer  231  that is the lowermost layer and the conductor layer  234  that is the uppermost layer. 
     The bendability of the ground pattern according to one embodiment will be described below by using a specific example. 
     As described above, the ground pattern in each of the conductor layers  231  to  234  of the connecting unit  120  according to the present embodiment has a certain shape in which the unit patterns  310  are connected. Each of the unit patterns  310  has a certain shape in which the center portions of the U-shaped pattern  311  and the inverted U-shaped pattern  312  that are arranged such that the respective opening sides are located away from each other are connected by the straight line pattern  313 . 
     In contrast, as illustrated in  FIG.  6 A  for example, it may be possible to form a ground pattern that has a certain shape in which H-shaped unit patterns  320  are connected.  FIG.  6 B  illustrates verification results that are obtained by a bendability analysis tool with respect to the ground pattern in which the unit patterns  310  according to the present embodiment are connected, and the ground pattern in which the unit patterns  320  are connected as illustrated in  FIG.  6 A . 
     In  FIG.  6 B , the remaining copper rate is set to the same value of 44.6% for the case in which the unit patterns  310  are adopted and the case in which the unit patterns  320  are adopted. Even if the remaining copper rate is the same as described above, in the case in which the unit patterns  310  are adopted, the bending modulus is 352 megapascal (MPa), which is smaller than the bending modulus of 407 MPa in the case in which the unit patterns  320  are adopted. At the same time, a reaction force and a maximum stress value are reduced in the case in which the unit patterns  310  are adopted, as compared to the case in which the unit patterns  320  are adopted. 
     This indicates that, when the unit patterns  310  are adopted, it is possible to bend the connecting unit  120  with a reduced force and it is possible to ensure good bendability of the connecting unit  120  according to the present embodiment. 
       FIGS.  7 A and  7 B  are diagrams illustrating a specific example of stress distributions of the unit patterns  310  and  320 . Specifically,  FIG.  7 A  illustrates the stress distribution of the unit pattern  310 , and  FIG.  7 B  illustrates the stress distribution of the unit pattern  320 . In  FIGS.  7 A and  7 B , darker portions represent higher stress. 
     As illustrated in  FIGS.  7 A and  7 B , in the unit pattern  320 , the stress is concentrated in a center portion in the H-shape, whereas the stress is relatively distributed without being concentrated in the straight line pattern  313  in the unit pattern  310 . This may occur because if the unit pattern  310  is pulled in the left-right direction when being bent in the bending direction, the U-shaped pattern  311  and the inverted U-shaped pattern  312  are relatively easily stretched in the left-right direction, so that the stress is distributed. 
     In this manner, the stress at the time of bending is distributed due to the shape of the unit pattern  310 , so that if the ground pattern of the connecting unit  120  is formed into the shape in which the unit patterns  310  are connected, it is possible to ensure the preferable bendability of the connecting unit  120 . 
     As described above, according to the present embodiment, in the conductor layer that is formed on the upper surface of the insulating layer of the multilayer wiring board, the ground pattern in which the plurality of unit patterns are connected is formed. Further, each of the unit patterns has a certain shape in which the U-shaped pattern and the inverted U-shaped pattern are arranged such that the respective opening sides are located away from each other and the center portions of the U-shaped pattern and the inverted U-shaped pattern are connected by the straight line pattern. Therefore, even if a ratio of the area that is covered by the conductor layer in the upper surface of the insulating layer is increased, it is possible to distribute the stress that is applied to each of the unit patterns when the wiring board bends, so that it is possible to maintain the flexibility and ensure the dimensional stability. 
     Meanwhile, in one embodiment as described above, the four-layer wiring board including the insulating layers  211  to  214 , the conductor layers  221  to  224 , and the conductor layers  231  to  234  is described as an example, but the number of layers of the wiring board is not limited to this example. As long as the wiring board has a configuration in which the conductor layer is laminated on the insulating layer, it is possible to maintain the bendability of the wiring board and ensure the dimensional stability regardless of the number of the layers by forming the conductor pattern into a certain shape in which the unit patterns  310  are connected. 
     According to one embodiment of the wiring board and the electronic device disclosed in the present application, it is possible to maintain the bendability and ensure the dimensional stability. 
     All examples and conditional language recited herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.