Patent Publication Number: US-2012043113-A1

Title: Printed circuit board

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a printed circuit board (PCB). 
     2. Description of Related Art 
     Generally, a PCB is mounted to a chassis of an electronic device, such as a computer, with screws. The PCB includes a number of mount holes electrically connected to ground layers of the PCB. Two ends of each mount hole are connected to two tin rings located on a top surface and a bottom surface of the PCB respectively. When screws are engaged in corresponding screw holes of the chassis through the mount holes, the PCB is electrically connected to the chassis by the screws, the tin rings, and the mount holes, to be grounded through the chassis. Therefore, most electrical noise of the PCB will flow into ground through the chassis, and electromagnetic radiation of the PCB caused by the noise will be decreased. However, the chassis functions as an antenna for high frequency noise caused by imperfect layout of the PCB. Once flowing into the chassis, the high frequency noise will be radiated out of the chassis in the form of electromagnetic wave. 
    
    
     
       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 schematic diagram of a printed circuit board (PCB) in accordance with an embodiment. 
         FIG. 2  is a schematic diagram of the PCB of  FIG. 1  assembled in a chassis of an electronic device. 
         FIG. 3  is a partially exploded view of the PCB and the chassis of  FIG. 2 . 
     
    
    
     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  FIGS. 1 and 2 , an embodiment of a printed circuit board (PCB)  100  may be assembled in a chassis  220  of an electronic device  200 . The PCB  100  includes a first signal layer  102 , a second signal layer  108 , and a ground layer  106  sandwiched between the first signal layer  102  and the second signal layer  108 . A number of conductive mount holes  120  extend through the PCB  100 , and each mount hole  120  is lined with a conductive material to electrically connect to the ground layer  106 . A number of circular electromagnetic wave absorption elements  160  are located on a top surface  110  of the PCB  100  and align with corresponding mount holes  120 . Each electromagnetic wave absorption element  160  is electrically connected to and around a corresponding mount hole  120 . It may be understood that the PCB  100  also includes other layers, such as a power layer, for example. These other layers fall within well-known technologies, and are therefore not described here. 
     Each electromagnetic wave absorption element  160  includes a conductive layer  162  electrically connected to a corresponding mount hole  120 , and an absorption layer  166  for absorption high-frequency noise, and sandwiching the conductive layer  162  with the top surface  110  of the PCB  100 . The conductive layer  162  of each electromagnetic wave absorption element  160  is electrically connected to the ground layer  106  through the conductive material of the corresponding mount hole  120 . At a bottom surface of the PCB  100 , and around each mount hole  120 , a tin ring (not shown) is set. Each tin ring is electrically connected to a corresponding mount hole  120 , and is electrically connected to the ground layer  106  through the conductive material of the corresponding mount hole  120 . It may be understood that noise of the PCB  100  flows into the ground layer  106 . In one embodiment, a frequency of the high-frequency noise is more than 1 gigahertz (GHz). Each electromagnetic wave absorption element  160  is ring-shaped, and outer radius and inner radius of each electromagnetic wave absorption element  160  are about 10 millimeters (mm) and 5 mm respectively. In other embodiments, the shape, the outer radius, and the inner radius of each electromagnetic wave absorption element  160  can be adjusted according to actual need. 
     The conductive layer  162  of each electromagnetic wave absorption element  160  is made of conductive material. The absorption layer  166  of each electromagnetic wave absorption element  160  is made of electromagnetic wave absorption material. The electromagnetic wave absorption material includes polymer medium and magnetic loss absorption material. Weight of the polymer medium accounts for the total weight of the electromagnetic wave absorption material in a range from about 5% to about 15%, and weight of the magnetic loss absorption material accounts for the total weight of the electromagnetic wave absorption material in a range from about 85% to about 95%. In one embodiment, the conductive material may be tin or copper. The electromagnetic wave absorption material is insulated. The polymer medium is chlorinated polyethylene, the magnetic loss absorption material is a mixture including iron, aluminum, and silicon. In other embodiments, proportion and ingredient of the polymer medium and the electromagnetic wave absorption material in the electromagnetic wave absorption material can be adjusted according to actual need. 
     Referring to  FIGS. 2 and 3 , a number of conductive posts  222  defining screw holes (not labeled) are formed on the chassis  220  of the electronic device  200 , corresponding to the mount holes  120 . In assembly, a number of screws  300  are employed to mount the PCB  100  to the chassis  220 . In detail, each screw  300  extends through a corresponding mount hole  120  from the top surface  110  of the PCB  100 , and is engaged in the screw hole of a corresponding post  222 . Each tin ring, at the bottom surface of the PCB  100 , contacts a corresponding post  222 . In one embodiment, the electronic device  200  may be a computer or a server. 
     In use, noise of the PCB  100  with frequency no more than 1 GHz, flows to ground through the ground layer  106 , the conductive material of the mount holes  120 , the tin rings, the posts  222 , and the chassis  220 . Noise of the PCB  100  with frequency more than 1 GHz, that is, the high-frequency noise, flows into the conductive layers  162  of the electromagnetic wave absorption elements  160  through the ground layer  106  and the conductive material of the mount holes  120 , in addition, is absorbed by the absorption layers  166  of the electromagnetic wave absorption elements  160 , therefore, preventing the high frequency noise from being radiated out of the chassis in the form of electromagnetic wave. 
     As detailed above, assembling the electromagnetic wave absorption element  160  on the top surface  110  of the PCB  100 , to absorb the high-frequency noise of the PCB  100 , can prevent the high frequency noise from being radiated out of the chassis in the form of electromagnetic wave. Therefore, function of the electronic device  200  to prevent electromagnetic radiation is improved. 
     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.