Abstract:
A circuit board structure, comprising: a first metal layer, including at least one cavity; and a plurality of first pads, for connecting at least one differential signal transmission line; wherein at least part of a vertical projection of the first pad, which is projected on the first metal layer, is overlapped with the cavity.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a circuit board structure, and particularly relates a circuit board structure that can provide low capacitance when it is connected to a connector. 
         [0003]    2. Description of the Prior Art 
         [0004]      FIG. 1  illustrates how a circuit board is welded to a connector, in the prior art. In this case, the circuit board  103  is welded to an SMD (Surface Mount Device) connector  101  with pins welded to a surface layer of the circuit board  103 , via surface welding technology. Accordingly, when the circuit board  103  is layout, suitable pads (or named footprints) are provided as welding interface between the SMD connector  101  and the circuit board  103 , according to the contact interface required by the pins. The characteristic impedance for the transmission line on the circuit board  103  can be controlled by different transmission line widths and substrate height. However, the SMD connector  101  presents conductance characteristic with high impedance for a signal transmission line of a single end channel. In order to avoid such characteristic causing multi-reflection for the transmission signal in the channel, many related invention provides low impedance capacitance loading to the pad. By this way, inductance/capacitance conjugate cancellation can be acquired to reach excellent impedance control. For example, American patent with patent numbers disclose such technique. 
         [0005]    However, a single end signal has low noise reduction ability for the channel noise. Accordingly, differential signal transmission such as SATA, PCIe, USB are commonly utilized for high speed transmission protocol. However, differential impedance is not sure to be high impedance, and differential signal transmission line may cause mutual inductance effect to each other. Equation 1 indicates the differential impedance Z diff  of the SMD connector  101 . Ls and Lm respectively indicate the self inductance and the mutual inductance of the SMD connector  101 , Cs and Cm are respectively the self capacitance and the mutual capacitance of the SMD connector  101 . Normally, Ls and Cs are constant values, such that Z diff  is inversely proportion to Lm and Cm. Therefore, the impedance of the SMD connector  101  will be different corresponding to different design and coupling methods. Thus, prior art can not well control impedance variation of the differential channel. 
         [0000]    
       
         
           
             
               
                 
                   
                     Z 
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                             L 
                             s 
                           
                           - 
                           
                             L 
                             m 
                           
                         
                         
                           
                             C 
                             s 
                           
                           + 
                           
                             C 
                             m 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
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                   1 
                 
               
             
           
         
       
     
         [0006]    Besides, the capacitance value is an important parameter when the signal is transmitted.  FIG. 2  illustrates the capacitance value when the signal is transmitted between the circuit board and the connector, in the prior art. As shown in  FIG. 2 , different capacitance values are distributed between the connector, the connector pad, the circuit board and the package. Different return loss is caused corresponding to different capacitance value when the signal is transmitted. Accordingly, desired return loss is hard to be acquired when high capacitance exists. 
       SUMMARY OF THE INVENTION 
       [0007]    One embodiment of the present invention discloses a circuit board structure, comprising: a first metal layer, including at least one cavity; and a plurality of first pads, for connecting at least one differential signal transmission line; wherein at least part of a vertical projection of the first pad, which is projected on the first metal layer, is overlapped with the cavity. 
         [0008]    Another embodiment of the present invention discloses a circuit board structure, comprising: a first metal layer, including at least one low capacitance region; and a plurality of first pads, for connecting at least one differential signal transmission line; wherein at least part of a vertical projection of the first pad, which is projected on the first metal layer, is overlapped with the low capacitance region. 
         [0009]    Via above-mentioned embodiments, the capacitance of the connection point for the connector an the circuit board can be decreased. 
         [0010]    These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the along detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates how a circuit board is welded to a connector, in the prior art. 
           [0012]      FIG. 2  illustrates the capacitance value when the signal is transmitted between the circuit board and the connector, in the prior art. 
           [0013]      FIG. 3  illustrates a circuit board structure according to a first embodiment of the present invention. 
           [0014]      FIG. 4  ( a ),  FIG. 4  ( b ),  FIG. 5 ,  FIG. 6  respectively illustrates cross section view diagrams of the circuit board structure. 
           [0015]      FIG. 7  illustrates a circuit board structure according to a second embodiment of the present invention. 
           [0016]      FIG. 8  ( a ),  FIG. 8  ( b ),  FIG. 9  ( a ) and  FIG. 9  ( b ) respectively illustrates cross section view diagrams of the circuit board structure shown in  FIG. 7 . 
           [0017]      FIG. 10  illustrates a circuit board structure  1000  according to a third embodiment of the present invention. 
           [0018]      FIGS. 11  ( a ),  11  ( b ) illustrate the cross section view diagram of the circuit structure shown in  FIG. 10 . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Certain terms are used throughout the description and along claims to refer to particular components. As one skilled in the art will appreciate, electronic equipment manufacturers may refer to a component by different names. This document does not intend to distinguish between components that differ in name but not function. In the along description and in the claims, the terms “include” and “comprise” are used in an open-ended fashion, and thus should be interpreted to mean “include, but not limited to . . .”. Also, the term “couple” is intended to mean either an indirect or direct electrical connection. Accordingly, if one device is coupled to another device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
         [0020]      FIG. 3  illustrates a circuit board structure  300  according to a first embodiment of the present invention. The circuit board structure  300  includes a plurality of pads  301 , differential signal lines  303 , a first grounding copper foil  305 , a grounding welding point  307 , a grounding pad  309  (i.e. above mentioned footprint), and a contact  311 . Additionally, the circuit board structure  300  further includes a cavity  313 . Detail structure relation of these devices will be described hereafter. The pad  301  is coupled to the differential signal lines  303 , and the layout layer thereof, which is normally regarded as a signal line layer, locates at a surface copper foil of a PCB (printed circuit board). The impedance of the differential signal lines  303  can be between 80-100 ohn, thus a grounding plane thereof can be a copper foil under the signal layer, that is, the first grounding copper foil  305 . The grounding welding point  307  is a grounding footprint/pad of a connector. The grounding welding point  307  couples a connector grounding signal and the first grounding copper foil  305 , via the grounding pad  309  and the contact  311 , to make a grounding level of the circuit board the same with which of the connector. 
         [0021]      FIG. 4  ( a ),  FIG. 4  ( b ),  FIG. 5 ,  FIG. 6  respectively illustrates cross section view diagrams of the circuit board structure  300 .  FIGS. 4  ( a ) and  4  ( b ) illustrate a cross section view along the X direction,  FIG. 5  illustrates a cross section view along the Y direction, and  FIG. 6  illustrates a cross section view along the Z direction. As shown in  FIG. 4  ( a ) , the pad  301  and the grounding welding point  307  are above the first grounding copper foil  305 , and a part of the first grounding copper foil  305  is removed torm the cavity  313 . In the embodiment shown in  FIG. 4  ( a ), the cavity  313  is just under the pad  301  and has a region larger than which of the pad  301 . Accordingly, if looks down at the embodiment shown in  FIG. 3 , the pad  301  is above the cavity  313 . Alternatively, it can be presented that a vertical projection of the first pad  301 , which is projected on the first grounding copper foil  305 , is included by the cavity  313 . Besides, non conductive material  304  can be filled into the cavity  313  and filled between the pad  301  and the grounding welding point  307 . Besides the first grounding copper foil  305 , the circuit board structure  300  can further comprise a second grounding copper foil  402 , such that the grounding layer can be changed corresponding to different circuit design. Via above mentioned structures, the capacitance value near the pad  301  can be decreased, such that desired return loss can be acquired. Besides utilizing the cavity  313  to decrease capacitance, the cavity  313  can be filled with other material to form a low capacitance region  403 , as shown in  FIG. 4  ( b ). The low capacitance region  403  can decrease the capacitance near the pad  301 , the same as the cavity  313 . Additionally, the non conductive material  304  can be filled into a space between the pad  301 , the grounding welding point  307 , the first grounding copper foil  305  and the low capacitance region  403 . For example, material of the low capacitance region  403  can be chemical compounds FR-5, FR-4 and R04003, which are composed of epoxy resin, phenolic resin, paraformaldehyde, silicone and Teflon, and can be enhance by glass fiber, insulation paper and linen. 
         [0022]      FIG. 5  is a cross section view of the circuit board  300  along the Y axis. As shown in  FIG. 5 , the differential signal line  303  is above the first grounding copper layer  305 , the second grounding copper layer  402  and the non conductive material  304 .  FIG. 6  is a cross section view of the circuit board  300  along the Z axis. As shown in  FIG. 6 , the grounding pad  309  is connected to the first grounding copper layer  305  via the contact  311 . Therefore, the connector is coupled to the first grounding copper layer  305  as well when it is coupled to the grounding pad  309 , and the ground level of the connector and the circuit board  300  will be at the same level. If the circuit board  300  further comprise the second grounding copper layer  402 , the contact  311  also makes the second grounding copper layer  402  electrically coupled to the differential signal line  303  and the first grounding copper layer  305 . 
         [0023]      FIG. 7  illustrates a circuit board structure  700  according to a second embodiment of the present invention. The difference between the circuit board structure  700  and the circuit board structure  300  is: the cavity in the circuit board structure  300  is larger than the pad, but the cavity or the low capacitance region of the circuit board structure  700  is smaller or equals to the pad. If looks down to the circuit board structure  700 , the cavity or the low capacitance region is covered by the pad, thus the cavity  313  is presented by dot lines.  FIG. 8  ( a ),  FIG. 8  ( b ),  FIG. 9  ( a ) and  FIG. 9  ( b ) respectively illustrates cross section view diagrams of the circuit board structure  700  shown in  FIG. 7 .  FIGS. 8  ( a ) and  8  ( b ) illustrate a cross section view along the P direction, and  FIGS. 9  ( a ) and  9  ( b ) illustrate a cross section view along the Q direction. 
         [0024]    As shown in  FIG. 8  ( a ) , sizes of the cavity  313  and the pad  301  are the same. Such structure can also be presented as: a vertical projection of the pad  301 , which is projected on the first grounding copper foil  305 , is totally overlapped with the cavity  313 .  FIG. 8  ( b ) illustrates the situation that the cavity  313  in  FIG. 8  ( a ) is replaced by the low capacitance region  403 . In  FIG. 9  ( a ), the cavity  313  is smaller than the pad  301 . Such structure can be presented as: a vertical projection of the pad  301 , which is projected on the first grounding copper foil  305 , includes the cavity  313 .  FIG. 9  ( b ) illustrates the situation that the cavity  313  in  FIG. 9  ( a ) is replaced by the low capacitance region  403 . 
         [0025]      FIG. 10  illustrates a circuit board structure  1000  according to a third embodiment of the present invention. The difference between the circuit board structure  1000  and the circuit board structure  300  is: the cavity in the circuit board structure  300  is larger than the pad, smaller than two pads, but the cavity of the circuit board structure  1000  can comprise more than two pads. Accordingly, if looks down on the circuit board structure  1000 , the same cavity  313  contains more than two pads  301 . 
         [0026]      FIGS. 11  ( a ),  11  ( b ) illustrate the cross section view diagram of the circuit structure shown in  FIG. 10 . As shown in  FIG. 11  ( a ), the cavity  313  includes a plurality of pads  301 . Alternatively, it can be presented that each of the cavities  313  includes a plurality of vertical projections of the pad  301 , which are projected on the first grounding copper foil  305 .  FIG. 11  ( b ) illustrates the situation that the cavity  313  in  FIG. 9  ( a ) is replaced by the low capacitance region  403 . 
         [0027]    Via above-mentioned embodiments, the capacitance of the connection point for the connector an the circuit board can be decreased. 
         [0028]    Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.