Abstract:
A printed circuit board (PCB) includes two layers, two signal transmission traces, and a vertical interconnect access (via). The signal transmission traces are respectively arranged on the layers. The signal transmission traces are electrically connected to each other through the via. A centerline of the via with a vertical line of the layers form an acute angle θ, the angle θ is less than cos −1 [(Lv 2 −Lt 2 )/(Lv 2 +Lt 2 )]. Wherein Lt is loss of the two signal transmitting traces in a unit length, and Lv is loss of the via in a unit length.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a printed circuit board. 
     2. Description of Related Art 
     Via stands for “vertical interconnect access” which is a vertical electrical connection between different layers of conductors in printed circuit board (PCB) design. Vias are pads with plated holes that provide electrical connections between copper traces on different layers of the PCB. Generally speaking, vias are vertical to the layers of the PCB, if a via is designed to a slantwise angle with the layers of the PCB, the signal transmission distance of the via and the corresponding copper traces will be reduced, which may reduce signal transmission loss. Therefore, there is room for improvement in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present embodiments 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 embodiments. Moreover, in the drawings, all the views are schematic, and like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a cross-sectional, schematic view of an embodiment of a printed circuit board including a vertical interconnect access (via). 
         FIGS. 2-4  are equivalent, schematic diagrams for three different designs of the via of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure, including the accompanying drawings, is illustrated by way of example and not by way of limitation. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
     Referring to  FIG. 1 , an embodiment of a printed circuit board (PCB)  100  includes a first signal layer  10 , a second signal layer  30 , and a dielectric layer  20  sandwiched between the first signal layer  10  and the second signal layer  30 . The first signal layer  10  includes a first signal transmission trace  12 . The second signal layer  30  includes a second signal transmission trace  32 . The first signal transmission trace  12  is electrically connected to the second signal transmission trace  32  through a vertical interconnect access (via)  40 . A centerline  42  of the via  40  with a vertical line  110  of the PCB  100  form an acute angle θ. The following paragraphs will describe design requirements of the acute angle θ. 
     Referring to  FIGS. 2-4 , there are three different equivalent, schematic views for three different designs of the via  40 , which are designed between a fixed point “A” of the first signal layer  10  and a fixed point “B” of the second signal layer  30 . The sum of a horizontal distance between the fixed point “A” and the via  40  and a horizontal distance between the fixed point “B” and the via  40  is l. In a first design of  FIG. 2 , “AC” section stands for a part of the first signal transmission trace  12  and the length of the “AC” section is ½. “BD” section stands for a part of the second signal transmission trace  32  and the length of the “BD” section is ½. “CD” section stands for the via  40  perpendicularly connected between points C and D of the first and second signal transmission traces  12  and  32 , and the length of the “CD” section is h (the perpendicular relationship of the “CD” section and the “AC” section just for calculating the design requirements of the acute angle θ). There is a formula for signal transmission loss α 1  of “ACDB” section: α 1 =Lt*l+Lv*h, where Lt is signal transmission loss in the first and second signal transmitting traces  12  and  32  in a unit length, and Lv is signal transmission loss in the via  40  in a unit length. In the PCB  100 , Lv&gt;Lt. 
     In a second design of  FIG. 3 , “AB” section stands for the via  40  slantingly connected between points A and B of first and second transmission traces  12  and  32 . There is a formula for signal transmission loss α 2  of “AB” section: α 2 =Lv*h*secθ. 
     In a third design of  FIG. 4 , “AF” section stands for a part of the first signal transmission trace  12 . “GB” section stands for a part of the second signal transmission trace  12 . “FG” section stands for the via  40 . Suppose an angle θc is an optimum value of the angle θ, and the loss of the signal transmitting traces  12  and  32  and the via  40  is at a lowest value. A loss difference between the signal transmission loss α 1  and α 2  is α(θ). See the following formulas: 
                     α   ⁡     (   θ   )       =       ⁢       α   ⁢           ⁢   1     -     α   ⁢           ⁢   2                   =       ⁢       (       Lt   *   1     +     Lv   *   h       )     -     Lv   *   h   *   sec   ⁢           ⁢   θ                     =       ⁢       Lv   *   h   *     (     1   -     sec   ⁢           ⁢   θ       )       +     Lt   *   h   *   tan   ⁢           ⁢   θ         ,               
to differentiate α(θ) and make the corresponding differential coefficient equal to zero.
 
α(θ)′=0
 
− Lt*h *sec2θ+ Lv*h *secθ*tan θ=0
 
 Lt/Lv =sin θ
 
θ c =sin −1 ( Lt/Lv )
 
     Sometimes, the angle θ may not be exactly designed to the optimum value sin −1 (Lt/Lv). For example, the angle θ may be adjusted according to requirements. However, the loss difference α(θ) cannot be less than zero. Suppose an angle θe is a maximal value of the angle θ. The angle θe can be calculated according to following formulas:
 
α(θ)=α1−α2=0
 
( Lt*l+Lv*h )− Lv*h* secθ=0
 
θ e =cos −1 [( Lv   2   −Lt   2 )/( Lv   2   +Lt   2 )]
 
Where, Lt and Lv can be measured by some measuring devices or calculated according to following formulas:
 
 Lt= 2.3 f*DF *√{square root over (∈ eff )}+35.36*√{square root over ( f )}/( Z 0 W )
 
 Lv= 0.11( R/Zv+G*Zv )
 
Where, “DF” stands for loss tangent, “f” stands for signal frequency, “W” stands for the width of the signal transmitting traces  12  and  32 , “Z 0 ” stands for characteristic impedance of the signal transmitting traces  12  and  32 , “Zv” stands for characteristic impedance of the via  40 , “R” stands for resistance of an equivalent circuit of the via  40  in a unit length, “G” stands for conductance of an equivalent circuit of the via  40  in a unit length, ∈ eff  stands for effective dielectric constant of the signal transmitting traces  12  and  32 .
 
     In actual design, if some conditions are satisfied, the angle θ equals to sin −1 (Lt/Lv). Therefore, the loss of the signal transmitting traces  12  and  32 , and the via  40  is a lowest value, which can increase signal transmission quality. If some conditions are not satisfied to design the angle θ to sin 31 1 (Lt/Lv), the angle θ must be less than cos −1 [(Lv 2 −Lt 2 )/(Lv 2 +Lt 2 )]. 
     It is to be understood, however, that even though numerous characteristics and advantages of the embodiments have been set forth in the foregoing description, together with details of the structure and function of the embodiments, the disclosure is illustrative only, and changes may be made in details, especially in matters of shape, size, and arrangement of parts within the principles of the embodiments to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.