Abstract:
The present invention relates to a printed circuit board. In one embodiment, a printed circuit board includes a dielectric layer and a conductive trace formed on the dielectric layer. The conductive layer includes a first conductive portion, a connecting portion and a second conductive portion. The connecting portion includes a first end and a second end. The first end is connected to the first conductive portion; the second end is connected to the second conductive portion. A width of the connecting portion gradually decreases from the first end to the second end. Reflection and cross talk of signals transmitted in the presented printed circuit board can be reduced.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to printed circuit boards and, particularly, to printed circuit boards with reduced impedance mismatch. 
     2. Discussion of Related Art 
     Typically, printed circuit boards include at least one non-conductive sheet with at least one dielectric layer formed on a surface of the non-conductive sheet. Many types of conductive traces, for example, transmission lines and circuit terminals, can be formed in the conductive layer. The transmission lines and the circuit terminals may be interconnected. 
       FIG. 6  shows a typical structure of conductive trace, in which a first transmission line  41  and a second transmission line  42  having different widths are directly connected with each other. Generally, the depth of the first transmission line  41  equals to the depth of the second transmission line  42 . Due to the difference in widths, the first transmission line  41  and the second transmission line  42  have different impedances. In other words, the different widths cause impedance mismatch between the first transmission line  41  and the second transmission line  42 . The impedance mismatch in printed circuit boards, especially in high-speed printed circuit boards may cause various problems, such as signal reflection, cross talk, signal retardance, etc. 
     Therefore it is desired to develop a printed circuit board that can reduce impedance mismatch between transmission lines and circuit terminals or other transmission lines. 
     SUMMARY 
     In one embodiment, a printed circuit board includes a dielectric layer and a conductive trace formed on the dielectric layer. The conductive layer includes a first conductive portion, a connecting portion and a second conductive portion formed therein. The connecting portion includes a first end and a second end. The first end is connected to the first conductive portion; the second end is connected to the second conductive portion. A width of the connecting portion gradually decreases from the first end to the second end. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present printed circuit board can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present printed circuit board. 
         FIG. 1  is a schematic view showing a printed circuit board in accordance with a first embodiment; 
         FIG. 2  is a schematic view showing a printed circuit board in accordance with a first embodiment; 
         FIG. 3  is a schematic view showing a printed circuit board in accordance with a second embodiment; 
         FIG. 4  is a schematic view showing a printed circuit board in accordance with a third embodiment; 
         FIG. 5  is a schematic view showing a printed circuit board in accordance with a third embodiment; and 
         FIG. 6  is a schematic view showing a printed circuit board in accordance with related art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a printed circuit board  10  in accordance with a first preferred embodiment includes a dielectric layer  11  and a conductive trace  12  formed on the dielectric layer  11 . The conductive trace  12  includes a first conductive portion  121 , a connecting portion  123 , and a second conductive portion  122 . The connecting portion  123  includes a first end  1231  and a second end  1232 . The first end  1231  is connected to the first conductive portion  121  and the second end  1232  is connected to the second conductive portion  122 . 
     In the first preferred embodiment, the first conductive portion  121  and the second conductive portion  122  are transmission lines. The first conductive portion  121 , the second conductive portion  122 , and the connecting portion  123  define a central axis  1211 , a central axis  1221 , and a central axis  1233  respectively. The conductive trace  12  is substantially axially symmetrical. In other words, the central axes  1211 ,  1221 ,  1233  lie in a same line. 
     The first conductive portion  121  has a width of W 1  and the second conductive portion  122  has a width of W 2 . The width W 1  of the first conductive portion  121  is larger than the width W 2  of the second conductive portion  122 . A width of the first end  1231  of the connecting portion  123  substantially equals to W 1  and a width of the second end  1232  of the connecting portion  123  substantially equals to W 2 . A width of the connecting portion gradually decreases from the first end  1231  to the second end  1232 . Specifically, in the first preferred embodiment, W 1 =4 mils (1 mil= 1/1000 inches) and W 2 =2 mils. 
     The connecting portion  123  includes two sidewalls  1234 ,  1235  adjacent to the dielectric layer  11 . The sidewalls  1234 ,  1235  are on opposite sides of the connecting portion  123 . Each of the sidewalls  1234 ,  1235  extends directly (straightly) from the first conductive portion  121  to the second conductive portion  122 . Furthermore, each of the sidewalls  1234 ,  1235  extends from the first conductive portion  121  to the second conductive portion  122  at an acute angle α relative to the central axis  1233 . The acute angle α is in a range from about 37° to about 54.5°. Preferably, the acute angle α is in the range from about 40° to about 49°. 
     The connecting portion  123  has a length of L 1  along a direction of the central axis  1233 , i.e., the length L 1  equals to the shortest distance between the first end  1231  and the second end  1232 . The length L 1  of the connecting portion  123  can be calculated using the width W 1  of the first conductive portion  121  and the width W 2  of the second conductive portion  122 . Specifically, L 1  is in the range from 1.6*(W 1 -W 2 ) to 2.4*(W 1 -W 2 ). Preferably, L 1  is in the range from 1.8*(W 1 -W 2 ) to 2.2*(W 1 -W 2 ). 
     In the first preferred embodiment, L 1  equals to 2.2*(W 1 -W 2 ), that is, L 1 =4.4 mils. Because the width of the connecting portion  123  gradually decreases from the first end  1231  to the second end  1232 , a variable W 3  is used to represent the width of the connecting portion  123  at any point of the connecting portion  123 . The width W 3  of the connecting portion  123  at a predetermined point can be calculated using a formula W 2 +2X*tan(α), wherein X represents a distance from the second end  1232  to the predetermined point of the connecting portion  123 . 
     In these embodiments, the first conductive portion  121  and the second conductive portion  122  are connected to the first end  1231  and the second end  1232  of the connecting portion respectively. Furthermore a width of the connecting portion  123  gradually decreases form the first end  1231  to the second end  1232 . As a result, impedance mismatch between the first conductive portion  121  and the second conductive portion  122  can be reduced/minimized, in addition, signal reflection and cross talk can also be reduced. 
     Referring to  FIG. 3 , a printed circuit board  20  in accordance with a second embodiment is similar to that of the first embodiment. The printed circuit board  20  includes a first conductive portion  221 , a connecting portion  223 , and a second conductive portion  222 . The connecting portion  223  includes a first end  2211 , and a second end  2221 . A width of the connecting portion  223  gradually decreases from the first end  2211  to the second end  2221 . A width of the first conductive portion  221  is represented by a variable W 4 . A width of the second conductive portion  222  is represented by a variable W 5 . A length of the connecting portion  223  is represented by a variable L 2 . In the second embodiment, W 4 =6 mils, W 5 =2 mils, and L 2 =1.9*(W 4 -W 5 ), that is, L 2 =7.6 mils. 
     Referring to  FIG. 4 , a printed circuit board  30  in accordance with a third embodiment is similar to that of the first embodiment. The printed circuit board  30  includes a first conductive portion  321 , a second conductive portion  322 , and a connecting portion  323 . The first conductive portion  321  is a circular shaped welding pad, which has an arc-shaped sidewall  3211 . The second conductive portion  322  is a transmission line, which has two sidewalls  3221 ,  3222  on opposite sides of the second conductive portion  322 . The connecting portion  323  includes two sidewalls  3233 ,  3234  on opposite sides of the connecting portion  323 . The sidewall  3233  of the connecting portion  322  extends smoothly from the sidewall  3221  of the second conductive portion  322  to the sidewall  3121  of the first conductive portion  321 . The sidewall  3234  of the connecting portion  322  extends smoothly from the sidewall  3222  of the second conductive portion  322  to the sidewall  3121  of the first conductive portion  321 . Preferably, each of the sidewalls  3233 ,  3234  extends smoothly from a tangential direction of the sidewall  3211  correspondingly. 
     In the third preferred embodiment, the sidewalls  3233 ,  3234  are curved shaped. Referring to  FIG. 5 , the first conductive portion  321  has a diameter of D 1 . The second conductive portion  322  has a width of W 6 . The connecting portion  323  has a length of L 3 . The diameter D 1  of the first conductive portion  321  is larger than the width W 6  of the second conductive portion  322 . The length L 3  of the connecting portion  323  is in the range from 1.6*(D 1 -W 6 ) to 2.4*(D 1 -W 6 ). Preferably, L 3  is in the range from 1.8*(D 1 -W 6 ) to 2.2*(D 1 -W 6 ). 
     It is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention.