Abstract:
A printed wiring board unit includes an electronic circuit component, a printed wiring board, a plurality of first conductive terminals disposed between the electronic circuit component and the printed wiring board, at least one of the first conductive terminals arranged along a quadrangular outline, and a plurality of second conductive terminals disposed between the electronic circuit component and the printed wiring board, the second conductive terminals arranged at a corner of the quadrangular outline, and the second conductive terminals contacting at least one of the printed wiring board and the electronic circuit component in a relatively displaceable manner.

Description:
CROSS-REFERENCE TO APPLICATION 
       [0001]    This application is based upon and claims the benefit of priority of prior Japanese Patent Application No. 2008-24786 filed on Feb. 5, 2008, the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field 
         [0003]    Aspects of the invention relate to a printed wiring board unit comprising a printed wiring board and an electronic part mounted on the printed wiring board. 
         [0004]    2. Description of the Related Art 
         [0005]    Mother boards have printed wiring boards. Packages of electronic circuit components such as LSI chip packages are mounted on the printed wiring board. The electronic circuit components are mounted with a ball grid array (BGA). The BGA is a package of solder bumps arranged in a matrix between printed wiring boards and a package substrate included in an LSI chip package. The package of solder bumps includes an array of bumps arranged along a rectangular outline of the printed wiring board. 
       [Patent Document 1] 
       [0000]    
       
         Japanese Laid-open Patent Publication No. 2004-241594 
       
     
       [Patent Document 2] 
       [0000]    
       
         Japanese Laid-open Patent Publication No. 2003-31614 
       
     
         [0008]    The printed wiring boards are made of, for example, resin boards. Package substrates of the electronic circuit components are made of, for example, rectangular ceramic substrates. Thus, a thermal expansion rate of the package substrate is greatly different from that of the printed wiring board. Different thermal expansion rates occur between the printed wiring board and the package substrate during temperature changes. The difference of the thermal expansion rates peaks on diagonal lines of the package substrate. Consequently, the greatest stress is produced at a solder bump arranged at a corner of the rectangular package substrate. The stress repeatedly subjected to the solder bump eventually causes a solder crack. 
         [0009]    Aspects of the present invention solve the issue described above. An object of the present invention is to provide a printed wiring board unit, a semiconductor package, and a connector for the semiconductor package for maintaining a connection between a printed wiring board and an electronic circuit component regardless of such stress. 
       SUMMARY 
       [0010]    According to an aspect of an embodiment, a printed wiring board unit includes an electronic circuit component, a printed wiring board, a plurality of first conductive terminals disposed between the electronic circuit component and the printed wiring board, some of the first conductive terminals arranged along a quadrangular outline, and a plurality of second conductive terminals disposed between the electronic circuit component and the printed wiring board, the second conductive terminals being arranged at a corner of the quadrangular outline, and the second conductive terminals contacting at least one of the printed wiring board and the electronic circuit component in a relatively displaceable manner. 
         [0011]    The object and advantages of the aspects in accordance with the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 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 THE DRAWINGS 
         [0012]      FIG. 1  is a perspective view of an appearance of an embodiment of an electronic device; 
           [0013]      FIG. 2  illustrates a structure of a printed wiring board unit according to the first embodiment in accordance with aspects of the present invention; 
           [0014]      FIG. 3  is a vertical sectional view schematically illustrating a part of a structure of the printed wiring board unit according to the first embodiment in accordance with aspects of the present invention; 
           [0015]      FIG. 4  is an enlarged sectional view of the printed wiring board unit, which is sectioned with 4-to-4 line shown in  FIG. 3 ; 
           [0016]      FIG. 5  is an enlarged sectional view of the printed wiring board unit sectioned with 5-to-5 line shown in  FIG. 4 ; 
           [0017]      FIG. 6  is a vertical sectional view of the printed wiring board on which a semiconductor package and a connector are laid out; 
           [0018]      FIG. 7  is a vertical sectional view of the printed wiring board on which the semiconductor package and the connector are laid out; 
           [0019]      FIG. 8  is a vertical sectional view schematically illustrating a structure of a printed wiring board unit according to the second embodiment in accordance with aspects of the present invention; 
           [0020]      FIG. 9  is a vertical sectional view illustrating the printed wiring board on which a connector is laid out; 
           [0021]      FIG. 10  is an enlarged sectional view schematically illustrating a structure of a printed wiring board according to the third embodiment in accordance with aspects of the present invention; 
           [0022]      FIG. 11  is a vertical sectional view schematically illustrating the printed wiring board on which a semiconductor package is laid out; 
           [0023]      FIG. 12  is a vertical sectional view schematically illustrating the printed wiring board and the package substrate between which connector is inserted; and 
           [0024]      FIG. 13  is an enlarged sectional view of a structure of a printed wiring board unit according to a variation of the embodiments in accordance with aspects of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0025]    The first embodiment in accordance with aspects of the present invention will be disclosed with reference to the accompanying drawings. 
         [0026]      FIG. 1  illustrates the appearance of the embodiment of the electronic device, server computer  11 , schematically. Server computer  11  has enclosure  12 . Enclosure  12  encloses a storage space therein. The storage space houses a printed wiring board unit called a mother board, which will be described in more detail later. A semiconductor package called large-scale integration circuit (LSI) chip package and a main memory are mounted on the mother board. 
         [0027]    The LSI chip package executes, for example, various operations with software programs according to data stored in the main memory temporary. The software programs and the data may be stored in a hard disk drive (HDD) housed in the storage space together with the other components. Server computer  11  is mounted in, for example, a rack. 
         [0028]      FIG. 2  illustrates the structure of mother board  13  according to the first embodiment in accordance with aspects of the present invention, schematically. Mother board  13  has printed wiring board  14 . The printed wiring board  14  is made from, for example, resin. The electronic circuit components, such as an LSI chip package  15 , are mounted on the printed wiring board  14 . The LSI chip package  15  defines, for example, a square underside. Semiconductor package connector  16  is inserted between the LSI chip package  15  and the printed wiring board  14 . LSI chip package  15  and connector  16  are described later in detail. 
         [0029]    Heat sink  17  is mounted on LSI chip package  15 . Heat sink  17  has base plate  17   a,  which extends over the top surface of LSI chip package  15  in parallel. Multiple fins  17   b  stand perpendicularly from base plate  17   a.  Fins  17   b  extend horizontally to each other. Flow passages formed between fins  17   b  produce flows of air. Heat sink  17  sandwiches LSI chip package  15  with printed wiring board  14 . Heat sink  17  may be made from, for example, metallic material such as aluminum or copper. 
         [0030]    Bolster plate  18  is connected with heat sink  17 . Bolster plate  18  is affixed on the underside of printed wiring board  14 . Heat sink  17  and bolster plate  18  are attached on printed wiring board  14  with four bolts  19 . Each bolt  19  penetrates base plate  17   a  and printed wiring board  14 . Each bolt  19  is situated outside of a corner of LSI chip package  15 . Each bolt  19  may be situated on extensions of the diagonal lines of the underside of LSI chip package  15 . 
         [0031]    Elastic member  21  is inserted between bolt head  19   a  of bolt  19  and base plate  17   a  included in heat sink  17 . Elastic members  21  may be, for example, helical springs. The helical spring is biased between bolt head  19   a  and base plate  17   a.  Thus, heat sink  17  and base plate  17   a  are pressed against printed wiring board  14  with a certain pressure. 
         [0032]    LSI chip package  15  has package substrate  25  as shown in  FIG. 3 . Package substrate  25  is made from, for example, ceramic. A semiconductor chip, such as LSI chip  26 , is mounted on package substrate  25 . A stiffening member, stiffener  27 , is affixed around LSI chip  26  on package substrate  25 . Stiffener  27  surrounds, for example, LSI chip  26  continuously. Stiffener  27  is made from metallic material such as copper. 
         [0033]    A thermal conducting member, head spreader  28 , is mounted on LSI chip  26  and stiffener  27 . Heat spreader  28  is made from, for example, metallic material such as copper. Other thermal conducting members such as thermal conduction paste may be inserted between LSI chip  26  and heat spreader  28 . These thermal conducting members increase a contact area between LSI chip  26  and heat spreader  28 . 
         [0034]    Conductive pads  29  are arranged in a matrix on printed wiring board  14 . Likewise, conductive pads  31  are arranged in a matrix on an underside of package substrate  25 . Each conductive pad  29  arranged on printed wiring board  14  corresponds to one of conductive pads  31  arranged on the underside of package substrate  25  in a one-to-one relation. Conductive pad  29  is connected with conductive pad  31  through the first conductive terminal  32 . In this manner, the first conductive terminals  32  are connected with printed wiring board  14  and package substrate  25   b.    
         [0035]    Conductive pads  29  and  31  are arranged in a matrix with 5 rows and 5 columns as shown in  FIG. 4 . The first conductive terminals  32  include conductive terminals arranged along the outline C formed in a quadrangle. In this embodiment, outline C is in a square shape. The first conductive terminals  32  are arranged in a matrix excluding corners of the quadrangle. The first conductive terminals  32  are made of, for example, solder bumps. The solder bumps are made from alloy of, for example, tin, silver and copper. 
         [0036]    Connector  16  has connector frame  33 . Connector frame  33  has support frame  34  extending along an outline of package substrate  25 . Support frame  34  supports support plate  35  with its inner edge. Support plate  35  extends parallel to printed wiring board  14  and package substrate  25 . Aperture  36 , which is formed in a shape modeled after the quadrangle outline C, is provided on support plate  35 . Support plate  35  supports the second conductive terminals  37 . The second conductive terminals  37  are situated at positions corresponding to the corners of the square aperture. The second conductive terminals  37  are situated at each corner of the quadrangle outline. The both ends of the second conductive terminal  37  are held with conductive pads  31  and  29 , respectively. 
         [0037]    Support frame  34  holds the underside of package substrate  25  with bearing face  38  formed around aperture  36  flatly. The outer edge of bearing face  38  is defined with outline L modeled after the outline of package substrate  25 . Positioning frame  39  extending along the outline of package substrate  25  is attached to support frame  34 . Positioning frame  39  may be integrated with support frame  34 . Positioning frame  39  defines restriction wall  41  standing from bearing face  38  along outline L of bearing face  38 . Restriction wall  41  surrounds the outer edge of package substrate  25  continuously. Package substrate  25  is guided into place on connector frame with restriction wall  41 . 
         [0038]    In this embodiment, a given clearance may be kept between the underside of support frame  34  and the surface of printed wiring board  14 . Support frame  34 , support plate  35  and positioning frame  39  are made from, for example, resin material having a high thermal resistance excluding thermoplastic resin. The resin material described above keeps its original form when heated at a melting temperature of the first conductive terminal  32  in a reflow furnace later described. Thus, melting or deformation of support frame  34 , support plate  35  and positioning frame  39  may be avoided. Representative of the resin material is epoxy resin. Support plate  35  may have flexibility. 
         [0039]    The second conductive terminal  37  is made of an elastic member, in other words, a plate spring as shown in  FIG. 5 . The second conductive member  37  is inserted into insulation  42  made from elastic material such as gum. Insulation  42  is inserted into through hole  43  formed on support plate  35 . The second conductive terminal  37  is bended at a given angle so as to stand from the surface and underside of support plate  35 . Therefore, the each end of the second conductive terminal  37  is pressed against conductive pad  29  arranged on printed wiring board  14  and conductive pad  31  arranged on package substrate  25 , respectively, with a given pressure. 
         [0040]    For a mother board  13 , as described above, an LSI chip produces heat during operation. The heat produced by the LSI chip  26  is conducted to the package substrate  25  and the printed wiring board  14 . Ceramic substrates are used for package substrates  25 , whereas resin substrates are used for printed wiring boards  14 . The thermal expansion rates of the two materials are very different. Consequently, a stress forms between package substrate  25  and printed wiring board  14  during temperature changes. This stress reaches its peak at the corners of the quadrangle. 
         [0041]    As described earlier, the second conductive terminals  37  are situated at the corners of the quadrangle. The ends of the second conductive terminal  37  are pressed against conductive pads  29  and  31 , respectively, with the given pressure. Thus, the ends of the second conductive terminal  37  are relatively flexible against conductive pads  29  and  31  when there is the difference of thermal expansion rates between printed wiring board  14  and package substrate  25 . Therefore, the second conductive terminal  37  maintains with conductive pads  29  and  31  throughout temperature changes. Thus, a connection between LSI chip package  15  and printed wiring board  14  is maintained. 
         [0042]    Next, an assembling process of mother board  13  will be described. As shown in  FIG. 6 , an LSI chip package  15  may be used. Solder balls  44  are first applied to conductive pads  31  excluding the conductive pads situated at the corners. Package substrate  25  is then fitted into support frame  34  included in package substrate  25 . The package substrate  25  is fitted into a support frame  34  included in the connector frame  33 . Restriction wall  41  guides LSI chip package  15  into place on the connector frame  33 . Therefore, the end of the second conductive terminal  37  are held with conductive pad  31  on a surface of support plate  35 . Thus, connector  16  is integrated with LSI chip package  15 . 
         [0043]    As shown in  FIG. 7 , LSI chip package  15  and connector  16  are mounted at a given position on printed wiring board  14 . Solder balls  44  and the second conductive terminals  37  are held with conductive pads  29  arranged on printed wiring board  14  under a weight of, for example, LSI chip package  15 . LSI chip package  15  and connector  16  are put in a reflow furnace. If the elasticity of the ends of the second conductive terminal  37  is high, a given weight is loaded on LSI chip package  15 . 
         [0044]    A temperature in the reflow furnace is set to 260 degree Centigrade temperature. While solder balls  44  melt at such a temperature, support frame  34  and support plate  35  having a higher thermal resistances are not deformed or melted in any degree. After heating, solder balls  44  become hardened by cooling. In this manner, solder balls  44  adhere to conductive pads  31 . Therefore, conductive pads  29  and  31  are connected through the first conductive terminals  32  reciprocally. Then, heat sink  17  is attached on the LSI chip package  15 . 
         [0045]      FIG. 8  schematically illustrates the structure of mother board  13   a  according to a second embodiment in accordance with aspects of the present invention. For mother board  13   a,  positioning members, positioning pins  46 , are coupled to support frame  34 . Positioning pins  46  are projected from the underside of support frame  34 . Positioning pins  46  are provided, for example, at four corners of the underside of support frame  34 . Positioning pins  46  are inserted into receiving holes  47  formed on the surface of the printed wiring boards  14 . Components and a structure identical to those of mother board  13  are given the same reference numbers. Mother board  13   a  may function similarly to mother board  13  described early. 
         [0046]    Connector  16  is provided on printed wiring board  14  in assembling mother board  13   a  as shown in  FIG. 9 . Positioning pins  46  are inserted into receiving holes  47  formed on printed wiring board  14 . With positioning pins  46 , connector  16  is positioned on printed wiring board  14 . At the same time, the ends of the second conductive terminals  37  are held with conductive pads  29  arranged on printed wiring board  14 . Then, LSI chip package  15  is fitted into support frame  34 . As describe above, LSI chip package  15  may be mounted on printed wiring board  14  by reflowing. In this manner, mother board  13   a  is assembled. 
         [0047]      FIG. 10  illustrates the structure of mother board  13   b  according to the third embodiment in accordance with aspects of the present invention schematically. For mother board  13   b  shown in  FIG. 10 , the first support frame  48  and the second support frame  49  are equivalent to support frame  34  described previously. The first and second support frames  48  and  49  extend along the outline corresponding to two sides of package substrate  25 , which oppose each other. Each support plate  35  provides two of the second conductive terminals  37 , respectively. Components and a structure identical to those of mother board  13  described before are given the same reference numbers. Mother board  13   b  described above may function similarly to mother board  13 . 
         [0048]    LSI chip package  15  is mounted at a given position on printed wiring board  14  in assembling mother board  13   b  as shown in  FIG. 11 . Solder balls  44  are first applied to conductive pads  31  arranged on package substrate  25  excluding ones situated at the corners of package substrate  25 . Then, the LSI chip package  15  is held with conductive pads  31  arranged on the underside of LSI chip package  15  and conductive pads  29  arranged on the top face of printed wiring board  14   b.  Then, the solder balls  44  are melted in the reflow furnace. In this manner, LSI chip package  15  is mounted on printed wiring board  14  through the first conductive terminals  32 . 
         [0049]    Then, the first support frame  48  and the second support frame  49  are set as shown in  FIG. 12 . The second conductive terminals  37  are clipped with clips  51 . Clip  51  may be, for example, a pair of plate springs. The plate springs may be affixed each other at one side. The first and second support frames  48  and  49  slide on a surface of printed wiring board  14 . Clips  51  are situated between printed wiring board  14  and substrate  25 . The first and second support frames  48  and  49  are slid until their restriction walls  41  hold the outer edge of package substrate  25  and then clips  51  are removed. Each end of the second conductive terminals  37  is pressed against conductive pads  29  and  31 , respectively, with its elasticity. In this manner, mother board  13   b  is assembled. 
         [0050]    For mother boards  13 ,  13   a  and  13   b  disclosed above, a plurality of the second conductive terminals  37  may be arranged in corners of quadrangular outline C. The shape of support plate  35  may be determined according to the arrangement of the second conductive terminals  37 . The second conductive terminals  37  are arranged in each corner of the quadrangular outline, for example, symmetrical with respect to the center of an arrangement of the first conductive terminals  32 . Arranged as describe above, the second conductive terminals  37  are reliably provide contact with conductive pads  29  and  31 . Alternatively, the second conductive terminals  37  may be arranged along the outline of the quadrangular outline continuously. 
         [0051]    Accordingly, aspects of the present invention provide the printed wiring board unit, semiconductor package, and the connector for the semiconductor package for maintaining a connection between a printed wiring board and electric circuit components regardless of thermally produced stresses. 
         [0052]    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 in accordance with aspects 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.