Abstract:
To provide a printed circuit board in which two wiring patterns having a width of 0.15 mm, which are to be passed through the area of the printed circuit board for mounting a 1608-size chip component, are formed by printing, thereby increasing the circuit packaging density, reducing the production cost and suppressing the yield reduction. In a range in which the distance between two lands  12  to which a surface-mount chip component is electrically connected (the distance between points  17   a  and  17   b ) allows formation of two wiring patterns having a line width of 0.25 mm (the distance is equal to or more than 1.25 mm), wiring patterns  15  and  16  having a line width of 0.25 mm are formed. In this way, parts of the wiring patterns having a line width of 0.15 mm (narrow parts  15   a  and  16   a ) are minimized, and thus, the reduction in production yield is suppressed.

Description:
[0001]     The present application is based on and claims priority of Japanese patent application No. 2004-238690 filed on Aug. 18, 2004, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a printed circuit board on which surface-mount chip components are mounted. In particular, it relates to a printed circuit board on which a wiring pattern to extend under surface-mount chip components is formed by printing.  
         [0004]     2. Description of the Related Art  
         [0005]     To reduce the size and weight of electronic apparatus, more electronic components for printed circuit boards are becoming manufactured as chips, and the packaging densities of such chip components on printed circuit boards are becoming higher. In order to increase the packaging density of electronic circuit on a printed circuit board, the layout of the wiring pattern that interconnects the electronic components is important, and the way of layout and the way of formation of the wiring pattern are investigated in various ways for increasing the packaging density of the circuit.  
         [0006]     Techniques for layout or formation of wiring patterns for increasing the circuit packaging density according to the prior art are disclosed in Japanese Patent Laid-Open Publication No. 3-233991, Japanese Patent Laid-Open Publication No. 5-57863, Japanese Patent Laid-Open Publication No. 9-18097 and Japanese Patent Laid-Open Publication No. 11-340590, for example.  
       SUMMARY OF THE INVENTION  
       [0007]     As a method for enhancing the circuit packaging density as described above, it is common practice to pass a wiring pattern under a surface-mount chip component. Standard sizes of surface-mount chip components include a 2125-size (a size of 2 mm by 1.25 mm) and a 1608-size (a size of 1.6 mm by 0.8 mm), and the longitudinal dimension of a 1608-size chip component is 1.6 mm. However, when mounting the chip component on a printed circuit boar, soldering of terminal parts has to be allowed for, and thus, the width of the space through which the wiring pattern can be passed under the mounted 1608-size chip component (that is, the distance between two lands to which the terminals of the chip component are electrically connected) is actually about 1.0 mm. In order to pass two wiring patterns through such a space having a width of 1.0 mm, it is desirable that the line width of the wiring patterns is 0.15 mm, considering the facts that the distance between each wiring pattern and the land opposing to the wiring pattern is desirably 0.25 mm or more (because the forming error (maximum printing deviation) of the openings for the lands in the applied solder resist is about 0.15 mm) and that the distance between the wiring patterns is desirably 0.2 mm or more (because the adjacent wiring patterns may come into contact with each other and the solder resist may be difficult to apply if the distance is equal to or smaller than 0.2 mm).  
         [0008]     In the case where wiring patterns are formed by photography (photo etching), a wiring pattern having a width of 0.15 mm can be formed without any problem, and thus, two wiring patterns can be passed through the space having a width of 1.0 mm. However, the photo etching has a problem that it requires high production cost and long production period. On the other hand, in the case where wiring patterns are formed by printing (screen printing), although the printing technique has recently become able to produce a wiring pattern having a width of 0.15 mm, there remains a problem that the production yield decreases (and, as a result, the cost increases) if a large number of wiring patterns having a width of 0.15 mm are formed.  
         [0009]     In view of such circumstances, an object of the present invention is to provide a printed circuit board in which two wiring patterns having a width of 0.15 mm, which are to be passed through the area of the printed circuit board for mounting a 1608-size chip component, are formed by printing, thereby increasing the circuit packaging density, reducing the production cost and suppressing the yield reduction.  
         [0010]     A printed circuit board according to a first implementation is a printed circuit board on which a 1608-size surface-mount chip component is to be mounted, comprising: two wiring patterns formed by printing between two lands to which terminals of the 1608-size surface-mount chip component are electrically connected.  
         [0011]     According to this implementation, two wiring patterns are formed by printing under the chip component mounted.  
         [0012]     A printed circuit board according to a second implementation is the printed circuit board according to the first implementation, in which the two wiring patterns have a line width of 0.15 mm, the distance between the two wiring patterns is 0.2 mm or more, and the distance between each wiring pattern and the land opposing the wiring pattern is 0.25 mm or more.  
         [0013]     A printed circuit board according to a third implementation is the printed circuit board according to the first or second implementation, in which the two wiring patterns each have a wide part that extends within a range where the distance between opposing two intersections of the intersections of a straight line parallel to the longitudinal direction of the 1608-size surface-mount chip and the two lands is 1.1 mm or more.  
         [0014]     The expression “the distance between opposing two intersections of the intersections of a straight line parallel to the longitudinal direction of the 1608-size surface-mount chip and the two lands” means the distance between the two lands at a particular y coordinate in a coordinate system whose x axis extends in the longitudinal direction of the chip component and whose y axis extends in the lateral direction of the chip component, or the distance between the intersection of a line that is parallel to the longitudinal direction of the chip component (that is, parallel to the x axis) and is moving in the lateral direction (that is, along the y axis) and one of the lands and the intersection of the same line and the other land (thus, for example, if rectangular lands are formed in parallel to each other, “the distance between the opposing two intersections of the line parallel to the longitudinal direction of the chip component and the two lands is constant within an area between the lands or infinite outside the area between the lands”). Therefore, according to this implementation, the wiring patterns extending under the chip component each have a wide part within a range where the distance between the two lands is 1.1 mm or more. The value of “1.1 mm” is a total of the sum of the widths of the two wiring patterns each having a width of 0.2 mm (0.2 mm×2=0.4 mm), which is greater than 0.15 mm, the distance of 0.2 mm between the wiring patterns, and the sum of the distances between the wiring patterns and their respective opposing lands (0.25 mm×2=0.5 mm) and represents the width that allows formation of the wiring patterns having a line width of 0.2 mm.  
         [0015]     A printed circuit board according to a fourth implementation is the printed circuit board according to the third implementation, in which the wide part of the wiring pattern has a line width of 0.2 mm.  
         [0016]     A printed circuit board according to a fifth implementation is the printed circuit board according to any one of the first to fourth implementations, in which the two wiring patterns each have a wide part that extends within a range where the distance between opposing two intersections of the intersections of a straight line parallel to the longitudinal direction of the 1608-size surface-mount chip and the two lands is 1.25 mm or more.  
         [0017]     The value of “1.25 mm” is a total of the sum of the widths of the two wiring patterns each having a width of 0.25 mm (0.25 mm×2=0.5 mm), which is greater than 0.15 mm and 0.2 mm, the distance of 0.25 mm between the wiring patterns, and the sum of the distances between the wiring patterns and their respective opposing lands (0.25 mm×2=0.5 mm) and represents the width that allows formation of the two wiring patterns having a line width of 0.25 mm with the distance between the wiring patterns being kept at 0.25 mm.  
         [0018]     A printed circuit board according to a sixth implementation is the printed circuit board according to the fifth implementation, in which the wide part of the wiring pattern has a line width of 0.25 mm.  
         [0019]     A printed circuit board according to a seventh implementation is a printed circuit board on which a surface-mount chip component is to be mounted, comprising: wiring patterns having a line width of 0.15 mm that are formed by printing between two lands to which terminals of the surface-mount chip component are electrically connected, in which the distance between the wiring patterns having a line width of 0.15 mm is 0.2 mm or more, the distance between each wiring pattern and the land opposing the wiring pattern is 0.25 mm or more, and each of the wiring patterns having a line width of 0.15 mm has a wide part that extends at least outside the area of the printed circuit board directly under the surface-mount chip component.  
         [0020]     According to this implementation, the wiring patterns having a line width of 0.15 mm are formed by printing under the surface-mount chip component with the distance between the wiring patterns being kept at 0.2 mm or more and the distances between the wiring patterns and their respective opposing lands being kept at 0.25 mm or more and each have a part that has a line width greater than 0.15 mm and extends at least outside the area of the printed circuit board directly under the surface-mount chip component. Here, the expression “the distance between the wiring patterns being kept at 0.2 mm or more” means that, if a plurality of wiring patterns are formed, the distance therebetween is kept at 0.2 mm or more, and therefore, the present invention is not limited to the case of a plurality of wiring patterns, and according to the present invention, only one wiring pattern may be formed.  
       ADVANTAGES OF THE PRESENT INVENTION  
       [0021]     According to the first implementation, a printed circuit board on which a 1608-size surface-mount chip component is to be mounted comprises: two wiring patterns formed by printing between two lands to which terminals of the 1608-size surface-mount chip component are electrically connected. In this case, since two wiring patterns extends under the 1608-type chip component, the circuit packaging density increases. In addition, since the wiring patterns are formed by printing, the production cost and period can be reduced compared to the case of using a photography technique.  
         [0022]     According to the third implementation, in the printed circuit board according to the first or second implementation, the two wiring patterns each have a wide part that extends within a range where the distance between opposing two intersections of the intersections of a straight line parallel to the longitudinal direction of the 1608-size surface-mount chip and the two lands is 1.1 mm or more. In this case, since the pattern size (line width) is increased within the range in which a clearance of 1.1 mm is assured between the lands and the wiring patterns having a line width of 0.2 mm can be formed, the range of formation of the narrow parts of the wiring patterns having a line width of 0.15 mm, which causes reduction in yield when the wiring patterns are formed by printing, can be minimized. Thus, the production cost and period can be advantageously reduced by using the printing technique, while suppressing the reduction in yield due to the use of the printing technique.  
         [0023]     According to the fifth implementation, in the printed circuit board according to any one of the first to fourth implementations, the two wiring patterns each have a wide part that extends within a range where the distance between opposing two intersections of the intersections of a straight line parallel to the longitudinal direction of the 1608-size surface-mount chip and the two lands is 1.25 mm or more. In this case, since the pattern size (line width) is increased within the range in which a clearance of 1.25 mm is assured between the lands and the wiring patterns having a line width of 0.25 mm can be formed, the reduction in production yield can be further suppressed.  
         [0024]     According to the seventh implementation, a printed circuit board on which a surface-mount chip component is to be mounted comprises wiring patterns having a line width of 0.15 mm that are formed by printing between two lands to which terminals of the surface-mount chip component are electrically connected, the distance between the wiring patterns having a line width of 0.15 mm is 0.2 mm or more, the distance between each wiring pattern and the land opposing the wiring pattern is 0.25 mm or more, and each of the wiring patterns having a line width of 0.15 mm has a wide part that extends at least outside the area of the printed circuit board directly under the surface-mount chip component. In this case, since the wiring patterns having a line width of 0.15 mm are formed by printing under the surface-mount chip component, the print circuit board whose circuit packaging density is improved can be produced at low cost in a short production period. In addition, since the part of the wiring pattern having the line width of 0.15 mm is minimized, the reduction in production yield can be suppressed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1   a  is a perspective view of apart of a printed circuit board in which wiring patterns according to the present invention are formed;  
         [0026]      FIG. 1   b  is a cross-sectional view taken along the line A-A in  FIG. 1   a;    
         [0027]      FIG. 2  is a top view of the part of the printed circuit board in which the wiring patterns according to the present invention are formed;  
         [0028]      FIG. 3  shows the same as  FIG. 2  except that a chip component is removed;  
         [0029]      FIG. 4  is a top view of a part of another printed circuit board according to the present invention in which wiring patterns are formed;  
         [0030]      FIG. 5  is a top view of a part of another printed circuit board according to the present invention in which wiring patterns are formed; and  
         [0031]      FIG. 6  shows two wiring patterns having a line width of 0.25 mm extending under a 1608-type chip. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]     In the following, a specific embodiment of the present invention will be described with reference to the drawings. However, it is to be noted that the embodiment described below is intended only for illustration and is not intended to limit the scope of the present invention.  
         [0033]      FIG. 1  contains enlarged views of a part of a printed circuit board according to this embodiment of the present invention in which wiring patterns according to the present invention is formed:  FIG. 1   a  is a perspective view thereof; and  FIG. 1   b  is a cross-sectional view thereof taken along the line A-A.  FIG. 2  is a top view of the same part, and  FIG. 3  is the same as  FIG. 2  except that a chip component is removed.  
         [0034]     As shown in  FIGS. 1 and 2 , on a printed circuit board  1  according to this embodiment, lands  12  made of a copper foil are formed, and a surface-mount chip component  11  of a size of 1608 (that is, a size of 1.6 mm by 0.8 mm) (referred to simply as 1608-type chip, hereinafter) is mounted on the printed circuit board  1  by soldering terminals  11   a  to the lands  12 . To prevent excessive solder from coming into contact with the other parts during soldering of the terminals  11   a  to the lands  12 , a solder resist  14  is applied to the area other than openings  14   a . As shown in  FIG. 2 , the lands  12  according to this embodiment are designed so that a surface-mount chip component  13  of a size of 2125 (that is, a size of 2 mm by 1.25 mm) (referred to simply as 2125-type chip, hereinafter) can be mounted on the printed circuit board  1 .  
         [0035]     As shown in  FIG. 3 , wiring patterns  15  and  16  are disposed (between the lands  12 ) so as to extend under the 1608-type chip  11  to be mounted. The wiring patterns  15  and  16  have narrow parts  15   a  and  16   a  having a line width of 0.15 mm and extending between the lands  12 , respectively, and the narrow parts  15   a  and  16   a  are spaced apart from each other by 0.2 mm and spaced apart from their respective opposing lands  12  by 0.25 mm. Wide parts  15   b  and  16   b  of the wiring patterns  15  and  16  have a line width of 0.25 mm and are spaced apart from each other by 0.25 mm. The wiring patterns on the printed circuit board  1  according to this embodiment are formed by etching a copper foil on an insulating plate into a predetermined pattern, and the resist pattern for the etching is formed by printing (screen printing).  FIG. 3  shows the narrow parts  15   a  and  16   a  and the wide parts  15   b  and  16   b  as being stacked one on another at the connections thereof. However, this is a CAD view of the wiring patterns under design, in which the 0.15-mm-wide pattern and the 0.25-mm-wide pattern are drawn separately.  FIG. 3  is drawn in this way solely to help understanding of the present invention, and the actual wiring patterns are as shown in  FIGS. 1 and 2 .  
         [0036]     The forming error (the maximum printing deviation) of the solder resist  14  (openings  14   a ) is about 0.15 mm, and thus, if the lands  12  (or the resist openings  14   a ) are too close to the wiring patterns  15   a  and  16   a , the wiring patterns  15   a  and  16   a  may overlap with the openings  14   a  (if the wiring pattern  15  or  16  overlaps with the opening  14   a , the solder resist  14  is not applied to the overlapping part of the wiring pattern, and the solder adheres to the wiring pattern to cause accidental short-circuit). The distance of 0.25 mm between the narrow parts  15   a  and  16   b  and their respective opposing lands  12  is intended to avoid this possibility. In this embodiment, the openings  14   a  are spaced apart from the respective lands  12  by 0.05 mm, and thus, setting the distance of the narrow parts  15   a  and  16   a  from their respective opposing lands  12  at 0.25 mm results in a distance of 0.2 mm between the wiring patterns  15   a  and  16   a  and their respective openings  14   a , and thus, a distance not less than the forming error of 0.15 mm is assured therebetween. The distance of 0.2 mm between the narrow parts  15   a  and  16   a  is intended to avoid the possibilities, which arise if the distance between the narrow parts  15   a  and  16   a  is less than 0.2 mm, that the adjacent wiring patterns may come into contact with each other, and that the solder resist  14  may be difficult to appropriately apply.  
         [0037]     The line width of the wide parts  15   b  and  16   b  of 0.25 mm and the distance between the wiring patterns of 0.25 mm are designed to assure that the wiring patterns be formed by screen printing with high yield (that is, with few defectives). For the same reason as described above, the wide parts  15   b  and  16   b  have to be spaced apart from the lands  12  by 0.25 mm or more, and thus, a “clearance” of 1.25 mm (=0.25*5) is needed to form the wide parts (having a width of 0.25 mm)  15   b  and  16   b . Thus, the wiring patterns  15  and  16  change from the narrow parts  15   a  and  16   a  to the wide parts  15   b  and  16   b , respectively, in a part where the distance between inner two intersections (points  17   a  and  17   b ) of the intersections of a straight line  17  parallel to the longitudinal direction of the 1608-type chip  11  and the two lands  12  is 1.25 mm or more.  
         [0038]      FIG. 6  shows a case where two wiring patterns  60  having a line width of 0.25 mm extend under the 1608-type chip  11 . The length of the 1608-type chip  11  is 1.6 mm. However, soldering of the terminals  11   a  for mounting the chip on the printed circuit board has to be allowed for, so that the width of the space in which the wiring patterns can pass through under the mounted 1608-type chip  11  (that is, the distance between the lands  12 ) is about 1.0 mm. As described above, since the distance between the wiring patterns has to be 0.2 mm or more, the distance between each wiring pattern  60  and the opposing land  12  cannot be greater than 0.15 mm, so that the distance between the wiring pattern  60  and the opposing solder resist opening  14   a  is equal to or smaller than 0.15 mm. Thus, as described above, the forming error of the solder resist cannot be accommodated. Therefore, considering that two wiring patterns are passed under the 1608-type chip  11 , it is desirable that the line width of the wiring patterns is 0.15 mm. However, if wiring patterns having a line width of 0.15 mm are formed by printing (screen printing), the fraction defective exponentially increases with the amount of the wiring patterns.  
         [0039]     In the printed circuit board  1  according to this embodiment, the wiring patterns have a line width of 0.15 mm only in a required minimum part thereof, so that the reduction in yield can be minimized even if the wiring patterns are formed by printing (screen printing). In addition, since printing is used for forming the wiring patterns, the production cost and period can be reduced.  
         [0040]     According to this embodiment, the line width of the wide parts  15   b  and  16   b  is 0.25 mm, and the distance between the wide parts is 0.25 mm. However, the present invention is not limited to these specific values, the wide parts may have any dimensions as far as they can be formed by printing with high yield. For example, the line width of the wide parts may be 0.2 mm, and the distance therebetween may be 0.2 mm. If layout of the wiring patterns permits, the distance between the wide parts  15   c  and  16   c  can be widened as shown in  FIG. 4 .  
         [0041]     In addition, according to this embodiment described above, the 1608-type chip is used as a mount-surface chip component. However, the present invention is not limited thereto. For example, as shown in  FIG. 5 , three wiring patterns  55  each having a narrow part having a width of 0.15 mm and a wide part having a width of 0.25 mm may be formed by printing under a surface-mount chip component  51  of a size of 2125 (a size of 2 mm by 1.25 mm) with the narrow parts being spaced apart from each other by 0.2 mm, the outer wiring patterns  55  being spaced apart from their respective opposing lands  52  by 0.25 mm, and the wide parts extending at least outside of the area of the printed circuit board directly under the surface-mount chip component  51  (that is, the area having a width allowing formation of the wiring patterns having a width of 0.25 mm).