Abstract:
A printed circuit board includes a base and a signal trace laid on the base. The signal trace includes a plurality of straight line segments parallel to the first fibers. The signal trace is laid on the base in such a manner that the line segments of the signal trace mapped on the base partly superpose the first fibers and partly superpose gaps between two adjacent first fibers.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a printed circuit board, and more particularly to a printed circuit board having high signal transmission quality. 
     2. General Background 
     A printed circuit board (PCB) typically consists of a base, resin, and copper foil. The base for the PCB is often constructed from woven glass fiber cloth. During the process of manufacturing the PCB, the base is treated by passing it through a dip pan containing resin. The treated base is then passed through a set of squeeze rollers, and then a drying oven to cure. Then, the base is cut into a desired PCB size. Once the base has been prepared, copper foil is applied to one or two sides of the base, typically by electrodeposition. The copper foil is etched to form different signal traces on the PCB. 
     Referring to  FIGS. 1 and 2 , a PCB  1  includes a base  10  which includes a set of first fibers  11  and a second set of second fibers  12 . The PCB  1  defines an X-axis extending in a horizontal direction, and a Y-axis perpendicular to the X-axis. The first fibers  11  of the base  10  disposed in the X-axis direction are interlaced with the second fibers  12  of the base  10  disposed in the Y-axis direction. Resin is impregnated in gaps  13  among the fibers  11  and  12 . Generally, each straight line segment of a signal trace  17  is arranged on the PCB  1  at an angle of 0, 45, 90, or −45 degrees relative to the X-axis. In a PCB, some line segments of the signal traces  17  cross the fibers  11  or  12  of the base  10  (such as the line segment of the signal traces  17  arranged at the angles of 45, −45 degrees of  FIG. 1 ), and some line segments of signal traces are coincident with the fibers  11  or  12  of base  10  (such as the line segment of signal traces arranged at the angles of 0, 90 degrees of  FIG. 1 ). There is delay between signals on different signal traces. 
     What is needed, therefore, is a PCB having little delay time of signals on signal traces laid thereon. 
     SUMMARY 
     A printed circuit board includes a base and a signal trace laid on the base. The signal trace includes a plurality of straight line segments parallel to the first fibers. The signal traces are laid on the base in such a manner that the line segments of the signal trace mapped on the base partly superpose the first fibers and partly superpose gaps between two adjacent first fibers. 
     Other advantages and novel features will be drawn from the following detailed description of embodiments with attached drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a conventional PCB according to the prior art including a base and a plurality of signal traces; 
         FIG. 2  is a cross sectional view taken along line II-II in  FIG. 1  according to the prior art; 
         FIG. 3  is a schematic view of a PCB in accordance with a first exemplary embodiment of the present invention; 
         FIG. 4  is a schematic view of a PCB in accordance with a second exemplary embodiment of the present invention, the PCB including a set of parallel fibers each with a width P′ and a signal trace with two line segments with a distance of a variable X away from each other; 
         FIG. 5  is a schematic view of the PCB in accordance with the second exemplary embodiment of the present invention when the variable X is equal to P′/2; and; 
         FIG. 6  is a schematic view of a PCB in accordance with the third exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 and 2 , because the fibers  11 ,  12  and the resin have different dielectric constants, impedances of different signal traces  17  vary over a large range according to the locations and the angles of the line segments of the signal traces  17  relative to the base  10 . In a PCB, if the impedances of different signal traces vary greatly, delay time of signals on different signal traces will be very different. 
     Referring to  FIG. 3 , in a first exemplary embodiment of the present invention, a PCB  2  includes a base  20  which includes a set of parallel first fibers  21  and a set of parallel second fibers  22  perpendicular to the set of first fibers  21 . Two signal trace  27  are laid on the PCB  2 . The signal trace  27  includes a plurality of parallel line segments A 1 , B 1 , A 2 , B 2 , A 3 , and B 3 . The length of the line segment A 1  is equal to that of the line segment B 1 . The length of the line segment A 2  is equal to that of the line segment B 2 . The length of the line segment A 3  is equal to that of the line segment B 3 . The perpendicular distance between centerlines of two adjacent first fibers  21  is P. The perpendicular distance between the centerlines of the line segments A 1 , B 1  is equal to 3P/2. The perpendicular distance between the centerlines of the line segments B 1 , A 2  is equal to P/2. The perpendicular distance between the centerlines of the line segments A 2 , B 2  is equal to P/2. The perpendicular distance between centerlines of the line segments B 2 , A 3  is equal to P/2. The perpendicular distance between the centerlines of the line segments A 3 , B 3  is equal to P/2. A connecting line segment at an angle of 45, or −45 degrees relative to the first fibers  21  is connected between two adjacent line segments of the signal trace  27 . It is to be understood that in the above embodiment the signal trace  27  partly superposes the first fibers  21  and partly superposes gaps between two adjacent first fibers  21 . So there is little variance between the impedances of two signal traces  27 . Therefore, delay time of signals on two signal traces  27  is little, thus ensuring signal transmission quality. 
     According to the principle of the above embodiment, the perpendicular distance between two adjacent line segments of the signal trace  27  is designed equal to N*P/2, wherein N is an odd number. One of the line segments of the signal trace  27  is defined as a reference line segment. A first set of line segments is laid in the perpendicular distance of M*P away from the reference line segment, wherein M is an integer. A second set of line segments is laid in the perpendicular distance of R*P/2 away from the reference line segment, wherein R is an odd number. The length of the first set of line segments is equal to that of the second set of the line segments. So, in the above embodiment, the length of one of the line segments of the signal trace  27  can be designed equal to the adjacent line segments at both ends of the one of the line segments. 
     Referring to  FIG. 4 , in a second exemplary embodiment of the present invention, a PCB  4  includes a base  40  which includes a set of parallel first fibers  41  and a set of parallel second fibers  42  perpendicular to the set of first fibers  41 . Two signal traces  47  are laid on the PCB  4 . The signal trace  47  includes a plurality of parallel line segments C 1 , C 2 , C 3 , and C 4 . The length of the line segment C 1  is equal to that of the line segment C 2 . The length of the line segment C 3  is equal to that of the line segment C 4 . The perpendicular distance between centerlines of two adjacent first fibers  41  is P′. A width of the first fiber  41  is F. A width of the signal trace  47  is W. The perpendicular distance between centerlines of the line segments C 1 , C 2  is equal to P′/2. A variable X is defined as the perpendicular distance between the centerlines of the line segments C 3 , C 2 . The perpendicular distance between centerlines of the line segments C 4 , C 3 , as shown, is equal to P′/2. A connecting line segment at an angle of 45, or −45 degrees relative to the first fibers  41  is connected between two adjacent line segments of the signal trace  47 . The perpendicular distance between an upper edge of the line segment C 1  and a lower edge of the line segment C 3  is equal to P′/2−W−X, as well as the perpendicular distance between the upper edge of the line segment C 2  and the lower edge of the line segment C 4 . The perpendicular distance between a lower edge of the line segment C 2  and an upper edge of the line segment C 3  is equal to X−W. The perpendicular distance between a lower edge of the line segment C 1  and an upper edge of the line segment C 4  is equal to P′+W−X. 
     When the first fiber  41  is thin, the inequalities: P′/2−W−X&lt;F, X−W&lt;F, and P′+W−X&gt;P′−F are satisfied, which means P′ satisfies: 2(W+F)&lt;P′&lt;4(W+F), for enabling the signal trace  47  to be partly superposing the fibers  41  and partly superposing gaps between two adjacent first fibers  41 , wherein when the perpendicular distance between centerlines of the line segments C 2 , C 4  is equal to the perpendicular distance between centerlines of the line segments C 2 , C 3 , which means the equality: X−W=P′/2−W−X is satisfied, the variable X satisfies: X=P′/4, as shown in  FIG. 5 . 
     When the first fiber  41  is large, the inequalities: P′/2−W−X&lt;P′−F, X−W&lt;P′−F, and P′+W−X&gt;F are satisfied, which means P′ satisfies: 4(F−W)&lt;P′&lt;2(F−W), for enabling the signal trace  47  to be partly superposing the first fibers  41  and partly superposing gaps between two adjacent first fibers  41 . The perpendicular distance between the upper edge of the line segment C 1  and the lower edge of the line segment C 2  is equal to P′/2−W, and The perpendicular distance between the lower edge of the line segment C 1  and the upper edge of the line segment C 2  is equal to P′/2+W. If P′ satisfies: 2(F−W)&lt;P′&lt;2(F+W), namely P′/2−W&lt;F, and P′/2+W&gt;F, the line segments C 1 , C 2  partly superpose the fibers  41  and partly superpose gaps between two adjacent first fibers  41 . 
     In other words, when P′ satisfies: 4(F−W)/3&lt;P′&lt;4(F+W), the signal trace  47  partly superposes the fibers  41  and partly superposes gaps between two adjacent first fibers  41 . So there is little variance between the impedances of two signal traces  47 . Therefore, delay time of signals on two signal traces  47  is little, thus ensuring signal transmission quality. 
     Referring to  FIG. 6 , in a third exemplary embodiment of the present invention, a PCB  5  includes an base  50  which includes a set of parallel first fibers  51  and a set of parallel second fibers  52  perpendicular to the set of first fibers  51 . Two signal trace  57  are laid on the PCB  5 . The signal trace  57  includes a plurality of parallel line segments D 1 , D 2 , D 3 , D 4 , D 5 , and D 6 . The perpendicular distance between centerlines of two adjacent first fibers  41  is P′. A width of the first fiber  51  is F. A width of the signal trace  57  is W. The perpendicular distance between centerlines of the line segments D 1 , D 2  is equal to P′/2. A variable X is defined as the perpendicular distance between centerlines of the line segments D 2 , D 3 . The perpendicular distance between centerlines of the line segments D 4 , D 3  is equal to 2X. The perpendicular distance between centerlines of the line segments D 5 , D 4  is equal to X. The perpendicular distance between centerlines of the line segments D 6 , D 5  is equal to P′/2. The line segments D 1 , D 3  are on the same side of the line segment D 2 . The line segments D 4 , D 6  are on the same side of the line segment D 5 . A connecting line segments at an angle of 45 or −45 degrees relative to the first fibers  51  is connected between each two adjacent line segments of the signal trace  57 . The length of the line segment D 1  is equal to that of the line segment D 2 . The length of the line segment D 4  is equal to that of the line segment D 3 . The length of the line segment D 6  is equal to that of the line segment D 5 . The perpendicular distance between an upper edge of the line segment D 3  and a lower edge of line segment D 1  is equal to P′/2−W−X, as well as the perpendicular distance between an upper edge of the line segment D 6  and a lower edge of line segment D 4 . The perpendicular distance between a lower edge of the line segment D 3  and an upper edge of one of the line segments D 2 , D 5 , and the perpendicular distance between a lower edge of the line segment D 5  and an upper edge of the line segment D 4  are both equal to X−W. 
     When the first fiber  51  is thin, the inequalities: P′/2−W−X&lt;F, X−W&lt;F are satisfied, which means P′ satisfies: 2(W+F)&lt;P′&lt;4(F+W), for enabling the signal trace  57  to be partly superposing the first fibers  51  and partly superposing gaps between two adjacent first fibers  51 . When the first fiber  51  is large, the inequalities: P′/2−W−X&lt;P′−F, X−W&lt;P′−F are satisfied, which means P′ satisfies: 4(F−W)&lt;P′&lt;2(F−W), for enabling the signal trace  57  to be partly superposing the first fibers  51  and partly superposing gaps between two adjacent first fibers  51 . The perpendicular distance between the upper edge of the line segment D 1  and the lower edge of the line segment D 2  is equal to P′/2+W, and The perpendicular distance between the lower edge of the line segment D 1  and the upper edge of the line segment D 2  is equal to P′/2−W. If P′ satisfies: 2(F−W)&lt;P′&lt;2(F+W), namely P′/2−W&lt;F, and P′/2+W&gt;F, the line segments D 1 , D 2  partly superpose the first fibers  51  and partly superpose gaps between two adjacent first fibers  51 . 
     In other words, when P′ satisfies: 4(F−W)/3&lt;P′&lt;4(F+W), the signal trace  57  partly superposes the first fibers  51  and partly superposes gaps between two adjacent first fibers  51 . So there is little variance between the impedances of two signal traces  57 . Therefore, delay time of signals on two signal traces  57  is little, thus ensuring signal transmission quality. 
     It is to be understood, however, that even though numerous characteristics and advantages of the present invention have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.