Abstract:
A method for defining a stepped hole in a printed circuit board (PCB) by drilling layers of the PCB by different sized drill bits along a same axis. The stepped hole in the layers of the PCB are decreased in diameters sequentially.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to circuit boards, and more particularly to a printed circuit board (PCB) with stepped holes, and a method for drilling the stepped holes. 
         [0003]    2. Description of Related Art 
         [0004]    In a multilayer PCB, vias may be defined to provide electrical connections between copper traces on different layers of the PCB. A via may include a cylindrical plated hole, and pads encircling the plated hole in the different layers of the PCB to solder connections of components of the PCB. A proper proportion of a diameter of each of the pads to a diameter of the cylindrical plated hole needs to satisfy required impedance. Sometimes, the proper proportion may not be achieved because the diameters of the pads are changeable to suit components in different sizes, while the diameters of the cylindrical plated holes are constant. Therefore, the required impedance may be not acquired. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a cross-sectional view of an embodiment of a printed circuit board (PCB) with a stepped hole. 
           [0006]      FIG. 2  is a flowchart diagram of an embodiment of a method for drilling the stepped hole of  FIG. 1 . 
           [0007]      FIGS. 3A-3F  are cross-sectional views of the PCB of  FIG. 1  in different states of drilling the stepped hole. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    Referring to  FIG. 1 , an embodiment of a printed circuit board (PCB)  1  includes at least two layers and a via. In this embodiment, the at least two layers include a plurality of layers L 1 -L 6 . The layers L 1 -L 6  are laminated together. The via includes a stepped hole  10  defined through the layers L 1 -L 6 . A conductive sheet  12  plates the sidewall bounding the stepped hole  10 . The conductive sheet  12  may be made of copper, aluminum, or other metal. 
         [0009]    In the illustrated embodiment, the diameter of the stepped hole  10  is different at each of the layers L 1 -L 6  and is denoted as diameters D 1 -D 6 , respectively. Relationships of the diameters D 1 -D 6  are expressed as D 1 &gt;D 2 &gt;D 3 &gt;D 4 &gt;D 5 &gt;D 6 , i.e., they are decreased sequentially. 
         [0010]    Referring to  FIG. 2 , an embodiment of a method for defining the stepped hole  10  in the PCB  1  of  FIG. 1  includes the following steps. 
         [0011]    In step  1 , a hole  10 A as shown in  FIG. 3A  is defined in the PCB  1  through the plurality of layers L 1 -L 6 , via drilling through the layer L 1  down to the layer L 6  with a first drill bit. A diameter of the first drill bit is D 6 , therefore, a diameter of the hole  10 A is D 6 . 
         [0012]    In step  2 , a hole  10 B as shown in  FIG. 3B  is defined in the PCB  1  through the layers L 1 -L 5 , via drilling though the layer L 1  down to the layer L 5  coaxially aligned with the hole  10 A, using a second drill bit. A diameter of the second drill bit is D 5 . Therefore, a diameter of the hole  10 B through the layers L 1 -L 5  is D 5 . 
         [0013]    In step  3 , a hole  10 C as shown in  FIG. 3C  is defined in the PCB  1  through the layers L 1 -L 4 , via drilling though the layer L 1  down to the layer L 4  coaxially aligned with the hole  10 B, by a third drill bit. A diameter of the third drill bit is D 4 . Therefore, a diameter of the hole  10 C in the layers L 1 -L 4  is D 4 . 
         [0014]    In step  4 , a hole  10 D as shown in  FIG. 3D  is defined in the PCB  1  through the layers L 1 -L 3 , via drilling though the layer L 1  down to the layer L 3  coaxially aligned with the hole  10 C, by a fourth drill bit. A diameter of the fourth drill bit is D 3 . Therefore, a diameter of the hole  10 D in the layers L 1 -L 3  is D 3 . 
         [0015]    In step  5 , a hole  10 E as shown in  FIG. 3E  is defined in the PCB  1  through the layers L 1  and L 2 , via drilling though the layer L 1  down to the layer L 2  coaxially aligned with the hole  10 D, by a fifth drill bit. A diameter of the fifth drill bit is D 2 . Therefore, a diameter of the hole  10 E in the layers L 1  and L 2  is D 2 . 
         [0016]    In step  6 , a hole  10 F as shown in  FIG. 3F  is drilled in the layer L 1  of the PCB  1 , coaxially aligned with the hole  10 E, by a sixth drill bit. A diameter of the sixth drill bit is D 1 . Therefore, a diameter of the hole  10 F in the layer L 1  is D 1 . The hole  10 F in the layer L 1 , the hole  10 E in the layer L 2 , the hole  10 D in the layer L 3 , the hole  10 C in the layer L 4 , the hole  10 B in the layer L 5 , and the hole  10 A in the layer L 6  communicate in that order, to form the stepped hole  10 . 
         [0017]    Holes at different depths can be defined in the PCB  1  according to need, for example, a stepped hole may be defined only in the layers L 1 -L 4  without drilling the layers L 5  and L 6 . The diameters D 1 -D 6  of the stepped hole  10  can be predetermined according to need, such as required impedance of the via in the layers L 1 -L 6 . For example, the diameters D 1  and D 6  of the stepped hole  10  can be predetermined according to sizes of pads which are respectively formed on the layers L 1  and L 6  to provide an electronic connection between the layers L 1  and L 6  via the stepped hole  10 . 
         [0018]    The foregoing description of the exemplary embodiments of the disclosure has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Many modifications and variations are possible in light of the above everything. The embodiments were chosen and described in order to explain the principles of the disclosure and their practical application so as to enable others of ordinary skill in the art to utilize the disclosure and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those of ordinary skills in the art to which the present disclosure pertains without departing from its spirit and scope. Accordingly, the scope of the present disclosure is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.