Patent Publication Number: US-2011051351-A1

Title: Circuit board, semiconductor device including the same, memory module, memory system, and manufacturing method of circuit board

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a circuit board and a semiconductor device including the circuit board, and more particularly relates to a circuit board in which it is possible to reduce the number of board terminals to be formed on a main surface, and a semiconductor device including the circuit board. The present invention also relates to a memory module and a memory system including the circuit board and a manufacturing method of the circuit board. 
     2. Description of Related Art 
     DRAM (Dynamic Random Access Memory) is widely used as a main memory in personal computers and servers. In a personal computer or a server, instead of mounting a DRAM directly on a motherboard, it is a common to attach the DRAM to a socket (a memory slot) provided on the motherboard in the form of a memory module mounted onto a module board (see Japanese Patent Application Laid-open No. 2006-324326). 
     In recent years, the amount of input/output data or number of addresses in DRAMs have continued to increase. Therefore, there has been a concern that the number of terminals to be provided on a memory module may reach a number that cannot be accommodated within the area that has been set by the standards. The number of the board terminals on a memory module can be increased by decreasing a pitch and a width of the board terminals. However, the contact reliability of the memory module with the socket disadvantageously decreases when the pitch and the width of the board terminals is decreased. Particularly, in recent years, there has been a strong demand for lowering power consumption and increasing operation speed. To achieve this, it is necessary to avoid an increase in the electric resistance that arises due to downsizing of the board terminals. 
     The above problem is not limited to memory modules, and the same problem arises in circuit boards having board terminals and semiconductor devices including the circuit board. 
     SUMMARY 
     In one embodiment, there is provided a circuit board that includes: a main surface; a back surface parallel to the main surface; a side surface positioned between edges of the main surface and the back surface; and first and second board terminals covering a portion of the main surface and a portion of the side surface, respectively. 
     In another embodiment, there is provided a semiconductor device that includes: a circuit board including a main surface, a back surface parallel to the main surface, a side surface positioned between edges of the main surface and the back surface, and first and second board terminals covering a portion of the main surface and a portion of the side surface, respectively; and a semiconductor chip mounted on the main surface of the circuit board and having a plurality of chip terminals, wherein the first and second board terminals of the circuit board are electrically connected to corresponding ones of the chip terminals of the semiconductor chip via an internal wiring provided inside the circuit board. 
     In still another embodiment, there is provided a manufacturing method of a circuit board that includes: forming a hole that communicates from a main surface to a back surface of a board; forming a metal film on a surface of the board including an internal wall of the hole; forming a first board terminal on the main surface and forming a second board terminal on the internal wall of the hole by patterning the metal film; and cutting the board along the hole. 
     According to the present invention, because board terminals are provided not only on the main surface but also on a side surface of the circuit board, the total number of the board terminals can be increased while maintaining sufficient pitch and width of the board terminals. Furthermore, the board terminals that are provided on the side surface are formed by not only exposing the internal wiring, but by covering the side surface of the circuit board with the board terminals. As a result, in contrast to the semiconductor device described in Japanese Patent Application Laid-open No. 2006-324326, reliable electric connection can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and advantages of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic plan view of a configuration of a memory module according to a first embodiment of the present invention; 
         FIG. 2  is an enlarged view of a region  108  shown in  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along a line A-A′ shown in  FIG. 2 ; 
         FIG. 4  is a schematic plan view showing a state where the memory module according to the first embodiment is attached to a socket; 
         FIG. 5  is a schematic partial cross-sectional view showing a state where the memory module according to the first embodiment is attached to a socket; 
         FIGS. 6A to 6F  are process diagrams for explaining the manufacturing method of the memory module according to the first embodiment; 
         FIGS. 7A and 7B  are diagrams for explaining the manufacturing method of the memory module according to a modification example of the first embodiment; 
         FIG. 8  is a schematic plan view of a configuration of a memory module according to the second embodiment of the present invention; 
         FIGS. 9A and 9B  are diagrams showing a configuration of a memory module according to a third embodiment of the present invention, where  FIG. 9A  is a schematic perspective view of the memory module and  FIG. 9B  is a cross-sectional view of the memory module taken along a line B-B′ shown in  FIG. 9A ; and 
         FIG. 10  is a modification example of the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will be explained below in detail with reference to the accompanying drawings. 
       FIG. 1  is a schematic plan view of a configuration of a memory module according to a first embodiment of the present invention. 
     The memory module according to the first embodiment is a so-called SO-DIMM, and more specifically it is a semiconductor device in which a plurality (four in this example) of DRAMs  200  are mounted on a main surface  100   a  of a module board (a circuit board)  100 . It is needless to say that the target of the present invention is not limited to SO-DIMMs, and the present invention can be also applied to various types of DIMMs (including Unbuffered-DIMM or FB-DIMM). Moreover, the semiconductor chip to be mounted is not limited to the DRAM, but can be other semiconductor memories (such as a SRAM, a flash memory, and a PRAM). Furthermore, the semiconductor chip to be mounted on the circuit board does not need to be a memory, but can be a device such as a CPU or a microcomputer. 
     The main surface  100   a  of the module board  100  is substantially rectangular with a long side extending in an X direction and a short side extending in a Y direction. The DRAMs  200  are arrayed in the X direction in an upper part of the main surface  100   a  and board terminals (first board terminals)  102  are arrayed in the X direction in a lower part of the main surface  100   a.  Although not shown in  FIG. 1 , a back surface of the module board  100  that is parallel to the main surface  100   a  has the same configuration as the main surface  100   a.    
     The board terminals  102  provided on the main surface and the back surface of the module board  100  are mainly used as signal terminals. The signal terminal can be an address terminal to which an address signal is input, a command terminal to which a command signal is input, a clock terminal to which a clock signal is input, or a data terminal to which data is input or from which data is output. 
     As shown in  FIG. 1 , in the first embodiment, a part of the lower side of the module board  100  is cut out and board terminals (second board terminals)  101  are provided on a side surface of the cutout part. The board terminals  101  are not only provided on the side surface of the module board  100  but a portion of the board terminals  101  is turned toward the main surface and the back surface. In the first embodiment, the board terminals  101  are provided at two places. One of the board terminals  101  is used as a power terminal to which a power potential (VDD) is supplied and the other board terminal  101  is used as a power terminal to which a ground potential (GND) is supplied. As a result, power terminals to which the power potential (VDD) or the ground potential (GND) is supplied are not included in the board terminals  102  that are provided on the main surface or the back surface. Accordingly, it is supposed that a larger number of signal terminals can be arranged on the main surface and the back surface. This does not mean that not a single power terminal is included in the board terminals  102 ; some of the board terminals  102  are used as terminals to which a reference power is supplied. Moreover, it also does not mean that no power terminal to which the power potential (VDD) or the ground potential (GND) is supplied is included in the board terminals  102 ; these potentials can be supplied to some of the board terminals  102 . 
     As shown in  FIG. 1 , the terminal width of the board terminals  101  in the X direction is significantly wider than the terminal width of the board terminals  102 . As a result, the electric resistance of the power terminal is advantageously reduced as compared to that in a conventional technology. 
       FIG. 2  is an enlarged view of a region  108  shown in  FIG. 1 . 
     As shown in  FIG. 2 , an overall width of the board terminal  101  in the X direction is L 1  and an overall width of a flat portion provided inside the cut out portion in the X direction is L 2 . The value of the width L 1  can vary depending on the length of the module board  100  in the X direction or the number of board terminals  101 . For example, in the case of an SO-DIMM having a width of 65 mm, the board terminals  101  having the width L 1  of 30 mm can be provided at as many as two places. Although the board terminals  101  are provided at two places in the first embodiment, the board terminals  101  can be provided at four or eight places when there is a need to provide more board terminals when there are a number of different types of power sources or upon considering the arrangement balance of the power sources. This feature is explained in detail later as a second embodiment of the present invention. 
     The width L 2  also corresponds to a width of a flat portion at which a socket (a conductor) makes a contact. Note that a configuration is allowable in which the socket achieves conductance by engaging even with portions (curved portions on both sides of the width L 1 ) other than the flat portion and such a configuration is not excluded from the scope of the present invention. A width W 1  is a width of the portion of the board terminals  101  that is formed so as to be turned toward the main surface and the back surface of the module board  100 . The reason for providing such a wrapping portion is that there is a need to secure a margin so that the terminals on the side surface are not etched away during the etching process for forming terminals. 
       FIG. 3  is a cross-sectional view taken along a line A-A′ shown in  FIG. 2 . 
     As shown in  FIG. 3 , the board terminal  101  includes a first part  101   c  formed on a side surface  100   c  over a total width W 2  between the edges of the main surface  100   a  and a back surface  100   b,  a second part  101   a  formed on the main surface  100   a,  and a third part  101   b  formed on the back surface  100   b . The first part  101   c  and the second part  101   a  are mutually connected via the edge of the main surface  100   a.  Similarly, the first part  101   c  and the third part  101   b  are mutually connected via the edge of the back surface  100   b.    
     The side surface  100   c  of the module board  100  is, as shown in  FIG. 3 , a surface located between the edges of the main surface  100   a  and the back surface  100   b,  and the total width W 2  between the edges corresponds to a length in a Z direction. The main surface  100   a  and the back surface  100   b  of the module board  100  are mutually parallel surfaces and the width W 1  corresponds to a length in the Y direction in  FIG. 3 . 
     The board terminals  101  having such a configuration are connected to a plurality of power wirings  301  provided within the module board  100 . Thus, an important feature of the first embodiment is not that the power wirings  301  are exposed on the side surface  100   c  of the module board  100 , but that the board terminals  101  are formed so as to cover the side surface  100   c  of the module board  100 . Consequently, it is possible to achieve higher connection reliability. On the other hand, if only the power wirings  301  are exposed on the side surface  100   c  of the module board  100 , it is difficult to achieve reliable conductance with the socket. Another important feature is that each of the board terminals  101  is connected to a plurality of the power wirings  301 . With this configuration, a reliable electrical connection can be achieved between each of the power wirings  301  and the board terminal  101 . 
     The board terminals  102  provided on the main surface  100   a  and the back surface  100   b  are connected to signal wirings  304  provided on the main surface  100   a  and the back surface  100   b  of the module board  100 . The signal wiring  304  is connected via a through hole electrode  303  to the signal wiring  302  provided within the module board  100 . The power wiring  301  and the signal wiring  302  are each connected to corresponding terminals of the DRAMs  200 . 
       FIG. 4  is a schematic plan view and  FIG. 5  is a schematic partial cross-sectional view showing a state where the memory module according to the first embodiment is attached to a socket. 
     As shown in  FIG. 4  and  FIG. 5 , when the memory module according to the first embodiment is attached to a socket  400 , wires  401  of the socket  400  contact the board terminals  102  and wires  402  of the socket  400  contact the board terminals  101 . The socket  400  is arranged in a motherboard  500  that forms a memory system. The wires  401  are connected to a memory controller (not shown) mounted on the motherboard  500  and the wires  402  are connected to a power supply apparatus (not shown) that is mounted on the motherboard  500 . As a result, signal transmission and reception is performed between the DRAMs  200  and the memory controller and power is supplied to the DRAMs  200  from the power supply apparatus. 
     Thus, according to the first embodiment, because the board terminals are provided not only on the main surface and the back surface but also on the side surface of the module board  100 , the total number of board terminals that are required to be provided on the main surface and the back surface can be reduced. Further, because the board terminals  101  provided on the side surface are wider than the board terminals  102 , and the wide board terminals  101  are used as the power terminals, there is no need to provide many power terminals that are required in the ordinary memory modules. For example, in a case of an ordinary 240-pin memory module, a VDD terminal having about 20 pins and a GND terminal having about 60 pins are provided. On the other hand, in the first embodiment, because all or some of the power terminals can be shifted from the main or back surface to the side surface, space becomes available in the area where terminals are provided on the main or back surface of the board so that more terminals can be provided on the main or back surface without reducing the size of the terminals. It is preferable that the surface area of the board terminal  101  that is used as the GND terminal is about three times the surface area of the board terminal  101  that is used as the VDD terminal. 
     A manufacturing method of the memory module according to the first embodiment is explained next. 
       FIGS. 6A to 6F  are process diagrams for explaining the manufacturing method of the memory module according to the first embodiment. 
     As shown in  FIG. 6A , the module board  100  having a multilayered wiring structure is prepared first. As explained in connection with  FIG. 3 , the power wirings  301  and the signal wirings  302  are provided within the module board  100  and the through hole electrode  303  is used to connect the power wiring  301  to the signal wiring  302  of different layers. 
     Next, as shown in  FIG. 6B , holes  109  that communicate from the main surface to the back surface are made in the module board  100  using a router and the like at places of the side surface where the board terminals  101  are to be formed. In this state, as shown in  FIG. 6C , a metal film  103  is formed on the entire surface of the module board  100  including the internal walls of the holes  109 . As a concrete method, electroless copper plating or electrolyte copper plating can be used to form the metal film  103 . As a result, all the portions of the power wirings  301  and the signal wirings  302  that are exposed on the surface are connected to the metal film  103 . 
     Next, the places where the board terminals  101  and  102  are to be formed are masked and the metal film  103  is subjected to patterning. As a result, as shown in  FIG. 6D , the board terminals  101  and  102  are formed. At this time, to prevent the metal film  103  formed on the internal walls of the holes  109  from being removed, the patterning is performed in such a manner that the metal film  103  remains around the holes  109 . 
     As shown in  FIG. 6E , gold metal plating  104  is formed on the board terminals  101  and  102  and, as shown in  FIG. 6F , a redundant portion  105  of the module board  100  is cut in a traversed manner at the holes  109  with a router and the like. With this process, the module board  100  is completed. Thereafter, when the DRAMs  200  are mounted on the main surface and the back surface, the memory module according to the first embodiment is completed. 
     When the module board  100  is cut in a traversed manner at the holes  109 , as shown in  FIG. 1 , it leads to a configuration in which the board terminals  101  are provided inside the cut out portion; however, this feature is not essential in the present invention. For example, as shown in  FIG. 7A , a substantially step-less structure can be obtained by cutting the module board  100  along a line  109   a  that runs along the internal walls of the holes  109 . Besides, as shown in  FIG. 7B , a structure in which the board terminals  101  are formed on a salient can be obtained by cutting the module board  100  along a line  109   b  that detours from above the holes  109 . 
       FIG. 8  is a schematic plan view of a configuration of a memory module according to the second embodiment of the present invention. 
     As shown in  FIG. 8 , in the second embodiment, four board terminals  101  are provided on the side surface. Among the four board terminals  101 , a board terminal  101 -V 1  is a terminal for providing the power potential VDD to DRAMs  200 - 1  and  200 - 2  and a board terminal  101 -G 1  is a terminal for providing the ground potential GND to the DRAMs  200 - 1  and  200 - 2 . On the other hand, a board terminal  101 -V 2  is a terminal for providing the power potential VDD to DRAMs  200 - 3  and  200 - 4  and a board terminal  101 -G 2  is a terminal for providing the ground potential GND to the DRAMs  200 - 3  and  200 - 4 . 
     In this manner, in the second embodiment, a plurality of the DRAMs  200  are divided into groups and the power terminals are allocated per group. With this configuration, it is possible to make the power supply efficiency uniform for each of the DRAMs  200 . It is needless to say that the power terminals can be allocated to individual DRAMs  200 . When such a configuration is employed, it is possible to make the power supply efficiency almost perfectly uniform for each of the DRAMs  200 . 
       FIGS. 9A and 9B  show a configuration of a memory module according to a third embodiment of the present invention, where  FIG. 9A  is a schematic perspective view of the memory module and  FIG. 9B  is a cross-sectional view of the memory module taken along a line B-B′ shown in  FIG. 9A . 
     As shown in  FIGS. 9A and 9B , in the third embodiment, three board terminals  101  are provided on the side surface. Among the three board terminals  101 , board terminals  101 -G 1  and  101 -G 2  arranged on either sides are the terminals to which the ground potential GND is supplied and a board terminal  101 -V arranged in between is a terminal to which the power potential VDD is supplied. Within the module board  100 , as shown in  FIG. 9B , a wide ground wiring  301 G and a plurality of data wirings  302 DQ are provided along each other so as to overlap in a laminating direction (the Z direction) of the module board  100  and, similarly, a wide VDD wiring  301 V and a plurality of command address wirings  302 CA are provided along each other so as to overlap in the laminating direction (the Z direction) of the module board  100 . 
     With this configuration, the wide ground wiring  301 G functions as a reference plate for the data wirings  302 DQ and the wide VDD wiring  301 V functions as a reference plate for the command address wirings  302 CA. This configuration is similar to the wiring configuration on the motherboard. That is, even on the motherboard, a wide VSS wiring is provided as a reference plate for data wirings DQ and a wide VDD wiring is provided as a reference plate for command address wirings CA. With this configuration, the impedance of the signal wirings on the motherboard and the impedance of the signal wirings on the module board  100  can be made equal and the signal quality can be improved. 
     The wide ground wirings  301 G and the wide VDD wiring  301 V can be drawn simply from the board terminals  101  provided on the side surface so that the signal wirings and the power wirings do not need to be drawn wastefully inside the module board  100 . As a result, lowering of the electric resistance of the signal wiring and the power wiring as well as simplification of the wiring layout on the module board  100  can be achieved. 
       FIG. 10  is a modification example of the third embodiment. In the example shown in  FIG. 9B , the wide ground wirings  301 G and the wide VDD wirings  301 V are formed in the same wiring layer; however, in the example shown in  FIG. 10 , these wirings are formed in different wiring layers. Similar effects can be achieved even with this layout. 
     It is apparent that the present invention is not limited to the above embodiments, but may be modified and changed without departing from the scope and spirit of the invention. 
     For example, in the above embodiments, there have been explained examples in which the present invention is applied to a memory module; however, the target of the present invention is not limited to memory modules, and the invention can be also applied to various types of modules having a semiconductor chip mounted thereon or even semiconductor devices other than these modules. In addition, the present invention can be also applied to a circuit board on which a semiconductor chip has not been mounted yet. 
     In addition, while not specifically claimed in the claim section, the applicant reserves the right to include in the claim section of the application at any appropriate time the following apparatus and method: 
     A1. A memory module connectable to a socket, comprising: 
     a circuit board including a main surface, a back surface parallel to the main surface, a side surface positioned between edges of the main surface and the back surface, and first and second board terminals covering a portion of the main surface and a portion of the side surface, respectively; and 
     a semiconductor memory mounted on the main surface of the circuit board and having at least a signal terminal and a power terminal, wherein 
     the first board terminal of the circuit board is electrically connected to the signal terminal of the semiconductor memory via a signal wiring provided on the circuit board, and 
     the second board terminal of the circuit board is electrically connected to the power terminal of the semiconductor memory via a power wiring provided on the circuit board. 
     B1. A manufacturing method of a circuit board comprising: 
     forming a hole that penetrates from a main surface to a back surface of a board; 
     forming a metal film on a surface of the board including an internal wall of the hole; 
     forming a first board terminal on the main surface and forming a second board terminal on the internal wall of the hole by patterning the metal film; and 
     cutting the board along the hole.