Abstract:
The present invention relates to a circuit board in which signal lines transmitting a signal are wired, and which is capable of increasing the speed of signal transmission. There is provided a circuit board in which signal lines are wired, including: a signal pad which is formed at the tip of the signal line and has a signal via in the center thereof; and plural ground vias which are formed in positions to surround the signal pad and transmit a ground potential over plural wiring layers.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a circuit board in which signal lines transmitting a signal are wired.  
         [0003]     2. Description of the Related Art  
         [0004]     According to the tendency to increase the speed of circuit operation in recent years, a signal wiring pattern on a circuit board is also devised so as to enable the high-speed transmission of a signal to be performed.  
         [0005]     Further, since there is a limit when only the signal wiring pattern is devised, a device of ground pad and the like is also proposed (see Japanese Patent Laid-Open No. 2001-24084, Japanese Patent Laid-Open No. 2000-100814).  
         [0006]     However, there is an increasing need for even higher transmission speed, and hence, a device for realizing the even higher transmission speed is desired.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention has been made in view of the above circumstances and provides a circuit board which is improved in the signal transmission speed.  
         [0008]     To this end, a circuit board according to the present invention in which signal lines are wired includes: a signal pad which is formed at the tip of the signal line and has in the center thereof a signal via to connect the signal lines with each other over plural wiring layers; and plural ground vias which are formed in positions to surround the signal pad and transmit a ground potential over the plural wiring layers.  
         [0009]     In a printed circuit board for which the high-speed transmission performance is required, as represented by a LSI package board and the like, the performance is evaluated by using indexes referred to as S parameters. In order to improve the transmission performance, the signal quality with reduced loss is required. At present, the frequency band in which S 11  representing the reflection characteristic satisfies the level from −100 dB up to −20 dB is about 5 GHz, but in order to effect the higher speed transmission, the frequency band needs to be expanded to at least 10 GHz or more. Conventionally, fine design values in relation to the signal transmission path have been specified in consideration of the high-speed transmission, but particular consideration has not been paid for the ground via. However, in order to realize the higher speed transmission, it is necessary to take into account the ground via, as well.  
         [0010]     According to the present invention, a circuit board whose transmission characteristic is greatly improved is realized by arranging plural ground vias around a signal via.  
         [0011]     Here, the circuit board according to the present invention, may have a ground pattern spreading to surround a signal pad with a predetermined clearance formed between the ground pattern and the signal pad, and have the ground vias formed in the ground pattern at positions adjacent to the clearance.  
         [0012]     Further, in the circuit board according to the present invention, it is preferred that distances between the center of the signal via and each center of the plural ground vias formed in the positions to surround the signal pad, are equal to each other for the plural ground vias.  
         [0013]     By arranging the signal via at the same distance from each of the plural ground vias, the simulation of performance and the pattern design are facilitated.  
         [0014]     As described above, according to the present invention, it is possible to obtain a circuit board whose high-speed transmission characteristic of a signal is improved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a perspective view of a circuit board in which signal lines are wired;  
         [0016]      FIG. 2  is a partial cross-sectional view of the circuit board shown in  FIG. 1 ;  
         [0017]      FIG. 3  is a figure showing an example of a positional relationship between signal pads and ground pads;  
         [0018]      FIG. 4  is a figure showing another example of a positional relationship between signal pads and ground pads;  
         [0019]      FIG. 5  is a figure showing a positional relationship between signal pads and ground pads as a comparison example for the present invention;  
         [0020]      FIG. 6  is a figure showing a simulation result of the frequency characteristic when a ground pad (a ground via) is arranged at a position adjacent to a signal pad;  
         [0021]      FIG. 7  is a figure showing a simulation result of the frequency characteristic when a ground pad (a ground via) is arranged at a position adjacent to a signal pad;  
         [0022]      FIG. 8  is a figure showing a simulation result of the frequency characteristic when two ground pads (two ground vias) are arranged at a position surrounding a signal pad, as shown in  FIG. 3 ;  
         [0023]      FIG. 9  is a figure showing a simulation result of the frequency characteristic when three ground pads (three ground vias) are arranged at a position surrounding a signal pad, as shown in  FIG. 4 ; and  
         [0024]      FIG. 10  is a figure showing another example of a positional relationship of signal pads and ground vias. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]     In the following, embodiments according to the present invention will be described.  
         [0026]      FIG. 1  is a perspective view showing a circuit board in which signal lines are wired.  
         [0027]     On a board  11  of a circuit board  10 , signal lines  20  for high-speed transmission of a signal are wired, and signal pads  30  are formed at the tip of each of the signal lines  20 . In the circuit board  10  shown in  FIG. 1 , wirings and ground patterns are formed on the front surface, the rear surface and further on the intermediate layer of the board  11 .  
         [0028]     Note that the ground pattern is not shown in  FIG. 1 .  
         [0029]      FIG. 2  is a partial cross-sectional view of the circuit board shown in  FIG. 1 .  
         [0030]     On the front surface of the board  11  of the circuit board  10 , a signal line  20  is wired and a signal pad  30  is formed at the tip of the signal line  20 . Further, signal pads  31 ,  32  are also respectively formed on the intermediate layer and the rear surface of the board  11  at positions facing the signal pad  30 . The signal pad  30  on the front surface of the board  11  and the signal pads  31 ,  32  on the intermediate layer and the rear surface of the board  11  are connected with each other via a signal via  33 . The signal pad  31  on the intermediate layer of the board  11  is connected with a signal line  21  formed on the intermediate layer, and the signal pad  32  on the rear surface is connected with a signal line  22  formed on the rear surface.  
         [0031]     Further, ground pads  40 ,  41 ,  42  are formed on the front surface, the intermediate layer, the rear surface at positions adjacent to the signal pads  30 ,  31 ,  32 , respectively. The ground pads  40 ,  41 ,  42  are connected with each other via a ground via  43 . Further, on the intermediate layer, a ground pattern  51  connected to the ground pad  41  is expanded, and on the rear surface, a ground pattern  52  connected to the ground pad  42  is expanded.  
         [0032]      FIG. 3  is a figure showing an example of a positional relationship between the signal pads and the ground pads.  
         [0033]     In  FIG. 3 , there are shown two signal lines  20  and signal pads  30  formed at the tip of each of the two signal lines  20 . Signal vias  33  are formed at the center of each of the signal pads  30 . The signal lines  20  are connected with the signal lines of other wiring layers (the intermediate layer and the rear surface) by the signal vias  33 .  
         [0034]     Further, in  FIG. 3 , two ground pads  40  are formed for each one of the signal pads  30  at positions to surround the each one of the signal pads  30 . At the center of each of the two ground pads  40 , a ground via  43  for effecting connection with the ground pattern of other wiring layers is formed.  
         [0035]     In this way, by arranging the plural ground vias  43  (two in this case) at positions to surround the signal pad  30  having the signal via  33  at the center thereof, it is possible to significantly improve the high-speed signal transmission characteristic, as described later.  
         [0036]      FIG. 4  is a figure showing another example of a positional relationship between the signal pads and the ground pads.  
         [0037]     In contrast to  FIG. 3 , three ground pads  40 , each having a ground via  43 , are formed for each one of signal pads  30  with a signal via  33  formed at the center thereof, at positions to surround the each one of the signal pads  30 .  
         [0038]     In this way, by arranging the three ground pads  40 , each having a ground via  43 , at positions to surround the signal pad  30  having the signal via  33 , the high-speed signal transmission characteristic can be further improved.  
         [0039]      FIG. 5  is a figure showing a positional relationship between the signal pads and the ground pads as a comparison example for the present invention.  
         [0040]     In the case of  FIG. 5 , for each one of signal pads  30 , one ground pad  40  is formed at a position adjacent to the signal pad  30 .  
         [0041]     For purposes of the following explanation, a distance between the center of a signal via  33  formed at the center of the signal pad  30  and the center point of a ground via  43  formed at the center of the ground pad  40  formed at a position adjacent to the signal pad  30  is expressed by d. The distance “d” is applied not only to the case shown in  FIG. 5 , but also to the case where two or three ground pads  40  are formed so as to surround one signal pad  30 , as shown in  FIG. 3  and  FIG. 4 . Note that in the cases of  FIG. 3  and  FIG. 4 , the distance “d” is the same for each of the plural ground pads arranged at positions to surround one of the signal pads  30 .  
         [0042]      FIG. 6  is a figure showing a simulation result of the frequency characteristic when one ground pad (one ground via) is arranged at a position adjacent to one signal pad, as shown in  FIG. 5 . Further,  FIG. 6  shows the case where the distance “d” (see  FIG. 5 ) is set as d=1.6 mm.  
         [0043]     Here, S 21  of the S parameters is related to the transmittance of the signal, S 11  is related to the reflectance of the signal, and the horizontal axis shows frequency (GHz).  FIG. 6  means that the frequency characteristic is better when the transmittance (S 21 ) is higher and the reflectance (S 11 ) is lower.  
         [0044]     In the case shown in  FIG. 6 , the level of the reflectance S 11  of −20 dB or less is attained in the frequency range up to 2.5 GHz, and hence, the reflectance is desired to be lowered up to a frequency higher than this frequency.  
         [0045]      FIG. 7  is a figure showing a simulation result of the frequency characteristic when one ground pad (one ground via) is arranged at a position adjacent to one signal pad, as in the case shown in  FIG. 6 . However, unlike the case shown in  FIG. 6 ,  FIG. 7  shows the case where the distance “d” (see  FIG. 5 ) is set as d=0.7 mm.  
         [0046]     By bringing the ground via close to the signal via up to the distance “d”=0.7 mm, the level of the reflectance S 11  of −20 dB or less is attained in the frequency range up to 8 GHz, and hence, the frequency characteristic is further improved as compared with the case shown in  FIG. 6 .  
         [0047]      FIG. 8  is a figure showing a simulation result of the frequency characteristic when two ground pads (two ground vias) are arranged at a position to surround each one of the signal pads, as shown in  FIG. 3 . The distance “d” (see  FIG. 5 ) is arranged to be the same for the two ground vias, and is set as d=0.7 mm as in the case shown in  FIG. 7 .  
         [0048]     It is seen from  FIG. 8  that the level of the reflectance S 11  of −20 dB or less is maintained in the frequency range up to 17.6 GHz, and that the upper limit of the frequency range is extended to a frequency twice or higher than the frequency shown in  FIG. 7 .  
         [0049]      FIG. 9  is a figure showing a simulation result of the frequency characteristic when three ground pads (three ground vias) are arranged at a position surrounding each one of the signal pads, as shown in  FIG. 4 . The distance “d” (see FIG.  5 ) is arranged to be the same for the three ground vias, and is set as d=0.7 mm as in the case shown in  FIG. 7  and  FIG. 8 .  
         [0050]     It is seen from  FIG. 9  that the level of the reflectance S 11  of −20 dB or less is maintained in the frequency range up to 27.8 GHz, and that the frequency characteristic is further improved as compared even with the case shown in  FIG. 8 .  
         [0051]      FIG. 10  is a figure showing another example of a positional relationship between signal pads and ground vias.  
         [0052]     A signal pad  30  is formed at the tip of each signal line  20 , and a signal via  33  (see  FIG. 2 ) is formed at the center of each of the signal pads  30 . Further, a ground pattern  50  is expanded around each of the signal lines  20  and the signal pads  30  with a predetermined clearance formed between the ground pattern  50  and the signal lines  20 , and between the ground pattern  50  and the signal pads  30 . In the position of the ground pattern adjacent to the clearance surrounding the signal pad  30 , two ground vias  43  are formed for each one of the signal pads. These ground vias  43  connect the ground pattern  50  shown in  FIG. 10  with a ground pad  51  formed in another layer, which is different from the layer shown in  FIG. 10 , in the circuit board  10  (see  FIG. 1 ,  FIG. 2 ).  
         [0053]     Also in the case shown in  FIG. 10 , plural ground vias  43  (two in this case) are formed in positions to surround each one of the signal via  33 , so that the high-speed transmission characteristic of a signal is significantly improved.  
         [0054]     Note that examples in which two or three ground vias are arranged for one signal pad (one signal via) are shown here, but four or more ground vias may be arranged for one signal pad (one signal via).