Abstract:
There is provided a substrate that includes a base substrate, a socket that has a step where the step has a first surface and a second surface, the socket being electrically coupled with the base substrate at the first surface; and a connection substrate that is disposed between the second surface and the base substrate, where the connection substrate is electrically coupled with the socket at the second surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2010-122864, filed on May 28, 2010 the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments discussed herein are related to a substrate and an IC socket. 
       BACKGROUND 
       [0003]    Server systems have required an improvement in increasing transmission rate for interfaces used to connect peripheral circuits with a central processing unit or units (CPU or CPUs) together with demand for an increased processing speed. However, when a signal in electric form transmits between the peripheral circuits and the CPU, the transmission rate or a transmission range will be limited owing to the deterioration of waveforms caused by the transmission lines, connectors, and the like on printed circuit substrates which provided therebetween. For improvement in these problems, optical interconnection technology has been introduced in the transmission of signals in the server systems. The optical interconnection technology is a way of connecting a path for optical signal which is converted from responsive electric signal by an optical module. 
         [0004]      FIG. 1  illustrates an example of application of the optical interconnection technology to a substrate which includes an IC for transmitting electric signal and an optical module for converting the electric signals to optical signals to be sent to a periphery circuit. Electric signals are transmitted from an IC  11  such as a CPU including an interface operable at high speed so as to respond to high speed data processing of electric signals. The electric signals is electrically transferred to a vertical cavity surface emitting laser (VCSEL)  18  mounted on a substrate  16  to be converted resultantly to optical signals. The electric signals from the IC  11  propagates to the VCSEL  18  through a land grid array (LGA) type IC package  12 , an IC socket  13 , a substrate  14 , a socket  15 , a substrate  16 , and a driver IC  17 . The IC package  12  electrically connects and secures the IC  11  to IC socket  13  mounded on the substrate  14  which also has electrical paths for transferring the electric signals from the IC socket  13  to the socket  15 . The electric signals further transfer to the driver IC  17  via the substrate  16 . The electric signals are amplified by the driver IC  17  and applied to the VCSEL  18 , which are disposed in an optical module  19  are disposed in an optical module  19 . 
         [0005]    An example of electrical socket for a device such as an IC is disclosed in Japanese Laid-open Patent Publication No. 7-130438 to improve an electrical characteristics of the device. 
       SUMMARY 
       [0006]    According to an aspect of the invention, a substrate includes a base substrate, a socket that has a step, the step having a first surface and a second surface, the socket being electrically coupled with the base substrate at the first surface; and a connection substrate disposed between the second surface and the base substrate, the connection substrate being electrically coupled with the socket at the second surface. 
         [0007]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0008]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0009]      FIG. 1  is a diagram illustrating an existing example; 
           [0010]      FIG. 2  is a diagram illustrating a whole configuration of a substrate according to an embodiment of the present invention; 
           [0011]      FIG. 3  is a diagram illustrating a magnified view of a cross section of a first embodiment; 
           [0012]      FIG. 4  is a diagram illustrating a perspective view of an example of a flexible cable used in the first embodiment; 
           [0013]      FIG. 5  is a diagram illustrating a magnified view of a cross section of a second embodiment; and 
           [0014]      FIG. 6  is a diagram illustrating a magnified view of a cross section of a third embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0015]    When transmitting a high transmission rate electric signal having a transmission rate exceeding 20 Gb/s, for example, it is necessary to form impedance matched transmission lines. When there is an impedance mismatch, a signal is reflected, resulting in deterioration of the signal. In  FIG. 1 , there are many connection portions, for example, between the IC socket  13  and the substrate  14 , between the substrate  14  and the socket  15 , and between the socket  15  and the substrate  16 , and hence, impedance mismatching may occur at the connection portions, resulting in deterioration of a signal. In addition, a printed circuit substrate having high-frequency characteristics inferior to those of the substrate  16  is generally used as the substrate  14  owing to cost, for example. Hence in  FIG. 1 , the waveform of the electric signal of the IC  11 , which passes through a transmission line on the substrate  14 , deteriorates. 
         [0016]    Further, in  FIG. 1 , the IC socket  13  and the socket  15  for the optical module  19  are mounted on the substrate  14 . This results in a problem in that a necessary mounting space increases. 
         [0017]    It is an object of the present application to provide a substrate and an IC socket which allow the number of connection portions to be decreased, allow an IC to be connected to the substrate without deterioration of the high-frequency characteristics, and allow a mounting space to be reduced. 
         [0018]    A substrate disclosed in the present application is a substrate having an IC package and an IC socket on which the IC package is mounted. The IC socket includes a step formed of a first surface and a second surface facing the substrate. Another substrate is provided in such a manner as to be sandwiched between the second surface and the substrate. The IC socket is connected to the other substrate at the second surface, and the IC socket is connected to the substrate at the first surface. 
         [0019]      FIG. 2  illustrates the whole configuration of a substrate including an exemplary embodiment. An IC  1  is mounted on the upper surface of an IC socket  3  using, for example, an LGA IC package  2 . The IC socket  3  has a step having a higher and lower surfaces between which the level difference is corresponding to the thickness of a flexible cable  5 , provided on the lower surface of the IC socket  3 . The IC socket  3  has also pins in contact with a substrate  4 , and pins in contact with the flexible cable  5 . Thereby, the IC socket  3  connects the IC package  2  to both the substrate  4  and the flexible cable  5 . The substrate  4  is, for example, a glass-epoxy printed circuit substrate. The flexible cable  5  is a flexible substrate made of a material having good high-frequency characteristics, and has a driver IC  6 , a VCSEL  7 , and the like mounted thereon. 
         [0020]    By employing the configuration described above, an electric signal generated in IC  1  output from the IC package  2  is transmitted over the flexible cable  5  through the IC socket  3 . The electric signal from the IC  1  is amplified by the driver IC  6 , converted into an optical signal by the VCSEL  7 , and transmitted through an optical waveguide  8 . Accordingly, the signals from the optical waveguide  8  may be input to the IC  1  through the path described above. 
         [0021]    Thus, in the present embodiment, the electric signal generated in the IC  1  output from the IC package  2  is transmitted over the flexible cable  5  having good high-frequency characteristics without passing through the substrate  4 . Similarly, the IC  1  may receive the signals from the driver IC  6  with the flexible cable  5 . Thus, compared with the existing example illustrated in FIG.  1 , the number of connection portions is decreased and the IC  1  may be connected to the flexible cable  5  without deterioration or with little deterioration of the high-frequency characteristics. Hence, the output and the received waveforms of the signal of the IC  1  are prevented from deteriorating. 
         [0022]    In the existing example illustrated in  FIG. 1 , two sockets, that is the IC socket  13  and the socket  15 , are used. In the present embodiment, however, using only the IC socket  3 , the IC package  2  is connected to the substrate  4  and the flexible cable  5 . This configuration of the present embodiment allows the mounting space on the substrate  4  to be reduced. Hence, cost reduction is also realized owing to a reduction in the area of the substrate and the number of components. 
         [0023]    Further, since the IC socket  3  is employed in the configuration of the present embodiment, the configuration may provide high flexibility for combining the IC  1  with the optical module. Since components may be arranged below the flexible cable  5 , thereby the flexibility may be provided for arranging components. 
         [0024]    A specific example for realizing the connection between the IC  1  and the flexible cable  5  in the present embodiment will now be described.  FIG. 3  illustrates a magnified view of a cross section of the portion enclosed by a dotted line illustrated in  FIG. 2  in the first embodiment.  FIG. 4  is a perspective view of the flexible cable  5 .  FIG. 3  is a cross-sectional view taken along line III-III of  FIG. 4 . 
         [0025]    First, the flexible cable  5  is described with reference to  FIG. 4 . As illustrated in  FIG. 4 , the flexible cable  5  has a micro strip line structure in which signal lines  53  through which the electric signal from the IC  1  passes are formed on the upper surface of a dielectric substrate  54  that has a ground pattern  55  formed on the lower surface thereof. A high-speed electric signal passes through the flexible cable  5 , which functions as a transmission line with a characteristic impedance of, for example, 50Ω. Note that  FIG. 4  illustrates the case in which the electric signal from the IC  1  is a differential signal and a pair of signal lines  53  are provided. 
         [0026]    The flexible cable  5  has through holes  51 . The through holes  51  are holes extending through the flexible cable  5  in the thickness direction from the upper surface on which the signal lines  53  are formed to the lower surface on which the ground pattern  55  is formed. The flexible cable  5  has vias  52  provided thereon, and also has a ground pattern formed on the upper surface thereof on which the signal lines  53  are formed. 
         [0027]    Referring to  FIG. 3 , a first embodiment will now be described. The IC socket  3  (shaded area) includes a plurality of types of pin formed as springs. A pin  31  provided on a surface in contact with the substrate  4  connects a pad of the IC package  2  to a pad of the substrate  4 . Three types of pins  32 ,  33 , and  34  are provided on a surface that is in contact with the flexible cable  5  and that is formed with a step, corresponding to the thickness of the flexible cable  5 , between itself and the surface in contact with the substrate  4 . The pin  32 , extending through the corresponding through hole  51  of the flexible cable  5 , connects a pad of the IC package  2  to a pad of the substrate  4 . The pin  33 , in contact with the corresponding via 52, connects a ground pad of the IC package  2  to the ground pattern  55  of the flexible cable  5 . The pin  34 , in contact with the signal line  53 , transmits the electric signal from the IC  1  output from the IC package  2  over the flexible cable  5 . 
         [0028]    The flexible cable  5 , which is positioned in such a manner as to be sandwiched between the IC socket  3  and the substrate  4 , is pressed toward the substrate  4  side by the pins  33  and  34  provided on the IC socket  3 . In this case, it is necessary to perform positioning of the flexible cable  5 . In the first embodiment with the configuration described above, the IC package  2  and the IC socket  3  are pressed from above, thereby positioning the flexible cable  5  using the pin  32  and the through hole  51 , and signal connection and the like are made using the pins  33  and  34 . This configuration realizes connection of the IC  1  to the flexible cable  5  without deterioration of the high-frequency characteristics, while reducing the number of connection portions. 
         [0029]      FIG. 5  is a magnified view of a cross section of a second embodiment. In the second embodiment, the flexible cable  5  as illustrated in  FIG. 5  includes, instead of the through hole  51  in  FIG. 3 , an opening  56  for exposing the ground pattern  55  formed on the lower surface, the opening  56  extending through the upper surface having the signal lines  53  formed thereon. A pin  35  provided on a surface of the IC socket  3  in contact with the flexible cable  5  is in contact with the ground pattern  55  exposed through the opening  56 , thereby connecting the ground pad of the IC package  2  to the ground pattern  55  of the flexible cable  5 . In the second embodiment, the flexible cable  5  is positioned using the pin  35  and the opening  56 . In other words, the pin  35  has functions of both ground connection and positioning. 
         [0030]    When focusing on the ground connection made by the IC socket  3  in the first embodiment illustrated in  FIG. 3 , there are two discontinuous portions, at which potential electrical reflection may be caused, between the pin  33  and the via 52 and between the via 52 and the ground pattern  55 , since the connection is through the via 52. On the other hand, in the second embodiment, there is only a single discontinuous portion between the pin  35  and the ground pattern  55 , and hence, deterioration of the high-frequency characteristics may be further reduced. 
         [0031]    The rest of the points are the same as those of the first embodiment. Accordingly, components in  FIG. 5  corresponding to those in  FIG. 3  are denoted by the same references, and the description thereof is omitted. The configuration of the second embodiment also enables connection of the IC  1  to the flexible cable  5  without deterioration or with a little deterioration of the high-frequency characteristics, while reducing the number of connection portions. 
         [0032]      FIG. 6  is a magnified view of a cross section of a third embodiment. In the first and second embodiments, the IC socket  3  has a step corresponding to the thickness of the flexible cable  5  provided on the surface thereof in contact with the substrate  4 . On the other hand, as illustrated in  FIG. 6 , the IC socket  3  in the third embodiment has a step corresponding to the thickness of the flexible cable  5  on a side thereof opposite the surface in contact with the substrate  4 , in other words, on the surface on which the IC package  2  is mounted. 
         [0033]    Two types of pins  32  and  36  of the IC socket  3  are provided on a surface that is in contact with the flexible cable  5 . A step corresponding to the thickness of the flexible cable  5  is formed on a side of the surfaces of the IC socket  3 , on which the IC package  2  is mounted. Similarly to the first embodiment, the pin  32 , extending through the through hole  51  of the flexible cable  5 , connects a pad of the IC package  2  to a pad of the substrate  4 . The pins  36 , in contact with the ground pattern  55 , connect ground pads of the substrate  4  to the ground pattern  55  of the flexible cable  5 . The ground pattern  55  is connected to the ground pad of the IC package  2  through the via 52. The signal lines  53  are in contact with the signal pads of the IC package  2 . This allows connection for the electric signal from the IC  1  output from the IC package  2  to be made. 
         [0034]    The rest of the points are the same as those of the first and second embodiments. Accordingly, components in  FIG. 6  corresponding to those in  FIGS. 3 and 5  are denoted by the same reference symbols, and the description thereof is omitted. In the third embodiment, the flexible cable  5 , which is positioned in such a manner as to be sandwiched between the IC socket  3  and the IC package  2 , is pressed toward the IC package  2  side by the pins  36  provided on the IC socket  3 . Also in this case, the flexible cable  5  is positioned using the pin  32  and the through hole  51 . The configuration of the third embodiment also enables connection of the IC  1  to the flexible cable  5  without deterioration or with a little deterioration of the high-frequency characteristics, while reducing the number of connection portions. 
         [0035]    As described above in detail, according to the first to third embodiments, the electric signal generated by the IC  1  output from the IC package  2  is transmitted over the flexible cable  5  having good high-frequency characteristics without passing through the substrate  4 . Similarly, the IC  1  may receive signals through the flexible cable  5 . This allows the number of connection portions to be decreased and the IC  1  to be connected to the flexible cable  5  via the IC package  2  without deterioration or with a little deterioration of the high-frequency characteristics. Hence, the waveform of the signal from the IC  1  is prevented from deteriorating and the IC  1  may receive the signals prevented from deteriorating. In addition, the flexible cable  5  may be arranged accurately by being positioned using the pin  32  and the through hole  51 , or using the pin  35  and the opening  56 . 
         [0036]    By connecting the IC package  2  to the substrate  4  and the flexible cable  5  using only the IC socket  3 , the mounting space  15  in  FIG. 1  may be reduced according the embodiments 1 to 3. Hence, cost reduction is realized owing to a reduction in the area of the substrate and the number of components. 
         [0037]    As described above, the embodiments according to the present invention may become possible to mount a space-saving optical interconnect module which leads to prevent the increase of deterioration of electric signals in a system such as a server system in which introduction of optical interconnect technology has been needed in accordance with increased speed and/or increased transfer rate of electric signals. 
         [0038]    Note that the present invention is not limited to the embodiments described above, and various improvements or modifications are possible within the scope of the invention. 
         [0039]    For example, although an example of connection using the flexible cable  5  has been described in the embodiments, the present invention may be applied to an optical interconnect module that uses a printed circuit substrate (rigid substrate) having good high-frequency characteristics, instead of the flexible cable  5 . 
         [0040]    In addition, the IC socket  3  may be made to connect the IC package  2  to three or more substrates. 
         [0041]    In the first and third embodiments, the pin  32  that extends through the through hole  51  and positions the flexible cable  5  connects a pad of the IC package  2  and a pad of the substrate  4 . However, not limited to this, the pin  32  may be used only for positioning and not for connection. 
         [0042]    Note that the flexible cable  5  is an example of another substrate; the pin  31  is an example of a first pin; the pins  33  to  36  are examples of second pins; the pin  32  is an example of a third pin; the pin  34  is an example of a signal pin; and the pin  35  is an example of a ground pin. 
         [0043]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.