Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printed circuit board, and more particularly to a printed circuit board having high signal transmission quality. 
     2. Description of Related Art 
     Printed circuit boards (PCBs) typically consist of three principal components: a base, resin, and copper foil. The base for a PCB is often constructed from woven glass fiber cloth. In the process for 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 sized base which has a size of the PCB. 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 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 set of first fibers  11  of the base  10  disposed in the X-axis direction are interlaced with the set of 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, −45 degrees relative to the X-axis. In a PCB, some line segments of the signal traces  17  will 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 will be 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 ). 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 . However, in a PCB, if the impedances of different signal traces vary greatly, the delay times of signals on different signal traces will be very different. 
     SUMMARY OF THE INVENTION 
     A printed circuit board includes a base formed from a plurality of woven fibers, and signal traces laid on the base. Each of the signal traces includes at least a straight line segment. The signal traces are laid on the base in such a manner that the line segments of the signal traces mapped on the base cross the fibers at angles not equal to zero degrees. 
     Other advantages and novel features of the present invention will become more apparent from the following detailed description of preferred embodiment when taken in conjunction with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a conventional PCB including a first kind of base and a plurality of signal traces; 
         FIG. 2  is a cross sectional view taken along line II-II in  FIG. 1 ; 
         FIG. 3  is an enlarged view of an encircled portion III of  FIG. 1 ; 
         FIG. 4  is a schematic view of a PCB in accordance with a preferred embodiment of the present invention applied on the first kind of base of  FIG. 1 ; 
         FIG. 5  is a schematic view of another PCB in accordance with a preferred embodiment of the present invention applied on a second kind of base; 
         FIG. 6  is a schematic view of another PCB in accordance with another preferred embodiment of the present invention applied on the second kind of base of  FIG. 5 ; 
         FIG. 7  is a schematic view of a first manner to produce the PCB of the present invention; and 
         FIG. 8  is a schematic view of a second manner to produce the PCB of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 3 , an enlarged encircled portion III of the base  10  of  FIG. 1  is shown. Two adjacent parallel first fibers  11  and two adjacent parallel second fibers  12  perpendicular to the fibers  11 , together define a square area  15  (shown with dashed lines in  FIG. 3 ) that is L long along the fibers  11  and W wide along the fibers  12 , wherein L=17.86×10 −3  inches, and W=17.86×10 −3  inches. An angle φ between the first fiber  11  and a diagonal of the area  15  is 45 degrees. An angle θ between the second fiber  12  and the diagonal of the area  15  is also 45 degrees. 
     Referring to  FIG. 4 , when the signal traces  17  of the PCB  1  in  FIG. 1  is rotated an angle α which is equal to half of one of the angles φ, θ and is 22.5 degrees in this embodiment relative to the Y-axis in a counter-clockwise direction on the PCB  1  with other elements on the PCB  1  remaining unchanged we achieve PCB  1 ′ of a first embodiment of the present invention. On the PCB  1 ′, any line segment of the signal traces  17  is not coincident with the fibers  11 ,  12  and the diagonals of the areas  15  of base  10 . The angle between any line segment of the signal traces  17  and any fiber  11  or  12  is not smaller than 22.5 degrees. So the impedances of the signal traces  17  are kept in a small range to improve signal transmission quality. 
     It is to be understood that in the above embodiment the signal traces  17  also can be rotated in a clockwise direction, or rotates the base  10  relative to the signal traces  17 . Furthermore, the value of the rotated angle α is not limited to half of the angles φ, θ. Any angle can be used in the present invention as long as any line segments of the signal traces  17  are not coincident with the fibers  11 ,  12  and the diagonals of the areas  15  of base  10  after rotation. 
     Referring to  FIG. 5 , a second kind of base  30  is placed on a plane  5 . The plane  5  defines an X-axis extending in a horizontal direction, and a Y-axis perpendicular to the X-axis. The base  30  includes a set of first fibers  31  and a set of second fibers  32 . The set of first fibers  31  of the base  30  disposed in the X-axis direction are interlaced with the set of second fibers  32  of the base  30  disposed in the Y-axis direction. Two adjacent parallel first fibers  31  and two adjacent parallel second fibers  32  perpendicular to the fibers  31 , together define a rectangular area  35  (shown with dashed lines in  FIG. 5 ) that is L′ long and W′ wide, wherein L′=21.28×10 −3  inches, and W′=16.67×10 −3  inches. An angle φ′ between the first fiber  31  and a diagonal of the area  35  is arc tan L′/W′=52 degrees. An angle θ′ between the second fiber  32  and the diagonal of the area  35  is arc tan W′/L′=38 degrees. When the rectangular area  35  is not square-shaped, an angle α′ being equal to half of the smaller one of the angles φ′, θ′ is set, in the second embodiment angle α′=19 degrees. 
     In normal manner for laying signal traces on the base  30 , signal traces are laid on the base  31  to have one of the straight line segments of the signal traces been coincident with the X-axis of the base  30 , for example, the straight line segments of the signal traces arranged on the base  30  at an angle of 0, 45, 90, −45 degrees relative to the X-axis of the base  30 . However, to achieve the second embodiment of the present invention, each signal trace  37  laid on the base  31  in an abnormal manner is rotated 19 degrees relative to the original position of each signal trace  37  laid in the normal manner. Thus, the straight line segments of the signal traces  37  are arranged on the base  30  at an angle of 19, 64, 109, −26 degrees relative to the X-axis of the base  30 . So the line segments of the signal traces  37  are not coincident with the fibers  31 ,  32  and the diagonals of the areas  35  of the base  30 . The impedances of the signal traces  37  are kept in a small range thereby providing quality signal transmission. 
     Referring to  FIG. 6 , a set of parallel signal traces  47  are laid on the base  30 . The parallel signal traces  47  are laid in the abnormal manner as described above. Any line segment of the signal traces  47  is not coincident with the fibers  31 ,  32  and the diagonals of the areas  35  of the base  30 . The impedances of the set of signal traces  47  are substantially uniform, and the delay times of signals on the set of parallel signal traces  47  will be uniform. 
     To achieve the PCBs in the above embodiment, there are several different manners.  FIG. 7  shows a kind of manner. A large base  70  is prepared firstly. The base  70  is constructed from the same glass fiber cloth as the base  30  of  FIG. 5 . The base  70  also defines the X-axis along which the parallel first fibers extend, and the Y-axis perpendicular to the X-axis. The parallel second fibers extend along the Y-axis. The base  70  is then cut into four rectangular uniform sized bases  71 , which have the same size of the final PCB, in a normal manner which means two parallel edges of the sized bases  71  are parallel to the first fibers and the other two parallel edges of the sized bases  71  are parallel to the second fibers. In normal manner for laying signal traces, signal traces are laid on the sized base  71  to have one of the straight line segments of the signal traces been coincident with the X′-axis of the base  70 , for example, the straight line segments of the signal traces arranged on the sized base  71  at an angle of 0, 45, 90, −45 degrees relative to the X-axis of the base  70 . However, to achieve the embodiments of the present invention, each signal trace  73  is laid on the sized base  71  in an abnormal manner and is rotated an angle relative to the original position of each signal trace laid in the normal manner. In the base  70 , the rotated angle is 19 degrees as that in the base  30  of  FIG. 5 . Thus, the straight line segments of the signal traces  73  are arranged on the sized base  71  at an angle of 19, 64, 109, −26 degrees relative to the X′-axis of the base  70 . So substantially all line segments of substantially all signal traces  73  mapped on the base cross the fibers of the base  70  at angles not equal to zero degrees. 
       FIG. 8  shows another kind of manner to product the PCBs. The base  70  is cut into four rectangular sized bases  81  in an abnormal manner to have two parallel edges of each sized base  80  crossing the X-axis direction of the base  81  with an oblique angle. The oblique angle is 19 degrees. The sized base  81  defines an X′-axis angled relative to the X-axis with 19 degrees. Signal traces  83  are laid on the sized base  81  in the normal manner which means one of the straight line segments of the signal traces being coincident with the X″-axis of the sized base  80 , for example, the straight line segments of the signal traces  81  being arranged on the sized base  81  at angles of 0, 45, 90, or −45 degrees relative to the X″-axis of the sized base  81 . So substantially all line segments of substantially all signal traces  83  mapped on the base cross the fibers of the base  70  at angles not equal to zero degrees. 
     After the invention was made, applicants tested the invention to measure the signal delay times of different signal traces, and compare the data with measured delay times of a conventional PCB. A conventional PCB, using the base  30  of  FIG. 5  without rotation, was found to have delay times of 152.8×10 −12  seconds per inch when signals were transmitted through signal traces having line segments coincident with the fibers  31  or  32 . If line segments of the signal traces crossed over the fibers  31  or  32  of the base  30  at an angle, the delay times were found to be 148.6×10 −12  seconds per inch, a difference between two delay times of 4.2×10 −12  seconds per inch. On the PCB of the present invention (rotated as in  FIG. 6 ) however, signal traces with line segments extending 0 or 90 degrees relative to the X′ axis, had delay times of 150.3×10 −12  seconds per inch, and signal traces with line segments extending 45 or −45 degrees relative to the X′ axis, had delay times of 150.4×10 −12  seconds per inch, a difference of only 0.1×10 −12  seconds per inch. From the above data, it is apparent that signal transmission quality is improved when using the present invention. 
     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.

Technology Category: 5