Abstract:
In a multilayer printed wiring board having a plurality of laminated resin layers, a plurality of wiring patterns formed on the interfacial surface of the resin layers, and a plurality of lands formed on the outermost layer of the resin layers and on which the solder is provided, at least one of the wiring patterns has a plurality of openings in the form of a mesh, the size of openings of the wiring patterns in a region corresponding to the position of solder in which a stress generated in the solder provided on the lands becomes a value larger than a desired value due to thermal deformation of the semiconductor device and the multilayer printed wiring board is larger than that of openings in the other regions.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a wiring pattern of a multilayer printed wiring board mounting a semiconductor device.  
         [0003]     2. Related Background Art  
         [0004]     A printed circuit board in which a semiconductor device is mounted on a printed wiring board has been used so far for various products. Because wirings of a printed wiring board are increased in density, the BGA method and the CSP method for connecting a semiconductor device with a printed wiring board by a plurality of solder balls arranged like an array have been frequently used. Moreover, a multilayer printed wring board is generally used.  
         [0005]     It is requested for a solder ball for connecting a semiconductor device with a printed wiring board to keep a desired joint strength. To keep the joint strength, it is necessary to minimize a relative deformation between the printed wiring board and the semiconductor device.  
         [0006]     However, the printed wiring board is exposed to a high temperature of 200° C. or higher in a heating-cooling process such as a solder reflow process. Moreover, the coefficient of thermal expansion of epoxy resin or glass epoxy resin for forming the printed wiring board is larger than the coefficient of thermal expansion of an IC chip comprising silicon forming a semiconductor device by approximately 10 times. Therefore, in the solder reflow process, the elongation of the printed wiring board becomes larger than the elongation of a semiconductor device including an IC chip having a small coefficient of thermal expansion. Because of the difference between the coefficients of thermal expansion, the printed wiring board greatly warps and thereby, a large stress is applied to a solder ball. Therefore, a crack occurs on the solder ball or the joint between the semiconductor device and the printed wiring board is disconnected.  
         [0007]     As a means for reducing the warp, it is known to minimize the coefficient of thermal expansion of the printed wiring board. Japanese Patent Application Laid-Open No. 2002-100880 (Patent Document 1) discloses a method for preventing the warp of a multilayer printed wiring board due to thermal expansion and reducing a stress applied to a solder joint portion. That is, a plurality of lands are formed on the surface of a multilayer printed wiring board, a semiconductor device is mounted by solder, and a solid ground layer made of a metal, such as copper, of an inner layer of the multilayer printed wiring board is formed by punching a circular or hexagonal continuous pattern. Thereby, the thermal expansion coefficient of the printed wiring board is decreased and the warp of the printed wiring board is prevented.  
         [0008]     Moreover, Japanese Patent Application Laid-Open No. H05-343820 (Patent Document 2) discloses that a solid ground layer of an inner layer of a printed wiring board as in Patent Document 1 is formed into a shape a continuous pattern of a circle or polygon such as a hexagon obtained by punching. Moreover, its purpose is to adjust the characteristic impedance of a signal wiring set to a wiring layer adjacent to the ground layer. That is, the characteristic impedance value of the signal wiring set to the adjacent wiring layer is adjusted in accordance with the size or shape of a circular or hexagonal continuous pattern. Thereby, it is possible to realize impedance matching of a wiring and obtain the propagation characteristic of a high-speed stable electric signal.  
         [0009]     In recent years, because a product is downsized and has an advanced function, the wiring of a printed wiring board is further increased in density and the size of a solder ball for connecting a semiconductor device with the printed wiring board is further decreased. Therefore, how to maintain a joint strength by small solder ball is a large problem. Moreover, the thickness of a printed wiring board is decreased and the number of wiring pattern layers made of a metal such as a copper foil is increased. Therefore, the coefficient of thermal expansion and the stiffness of a printed wiring board are raised and the difference between coefficients of thermal expansion of a semiconductor device and a printed wiring board is increased.  
         [0010]     In the case of a method for forming a continuous pattern of a circle or polygon such as a hexagon on a solid ground layer of an inner layer disclosed in Patent Documents 1 and 2, to stabilize the wiring impedance of the whole wiring layer, it is effective to minimize the size of the continuous pattern of circle or polygon such as a hexagon. However, when the size of the continuous pattern of circle or polygon such as a hexagon is decreased, the coefficient of thermal expansion of a printed wiring board increases and the above problem of warp is actualized.  
       SUMMARY OF THE INVENTION  
       [0011]     It is an object of the present invention to provide a multilayer printed wiring board capable of reducing the stress of the solder joint portion with a semiconductor device generated due to thermal expansion and stabilizing the impedance of a wiring.  
         [0012]     To achieve the above object, a multilayer printed wiring board of the present invention has a plurality of laminated resin layers, a plurality of wiring patterns formed on a interfacial surface between the resin layers, and a plurality of lands which are formed on an outermost layer of the resin layers and on which a solder is provided. At least one layer of the wiring patterns has a plurality of openings in the form of a mesh, and a size of the openings of the wiring patterns in a region corresponding to a solder position at which a stress generated in the solder provided on the lands due to thermal deformation of a semiconductor device and the multilayer printed wiring board becomes larger than a desired value is smaller than a size of the openings in the other regions.  
         [0013]     Moreover, in a multilayer printed wiring board of the present invention, the wiring patterns having a plurality of openings in the form of a mesh are wiring patterns adjacent to the outermost layer of the resin layers on which the lands are formed.  
         [0014]     Furthermore, in a multilayer printed wiring board of the present invention, the lands are formed in the form of an array and the region corresponding to a solder position where the generated stress becomes larger than a desired value is a region located at four corners of the lands.  
         [0015]     Furthermore, in a multilayer printed wiring board of the present invention, the semiconductor device includes an IC chip sealed in a package, and the region corresponding to a solder position at which the generated stress becomes larger than a desired value is a region located at four corners of the lands and a region located at four corners of the IC chip.  
         [0016]     The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a schematic view of a multilayer printed wiring board of Embodiment 1;  
         [0018]      FIGS. 2A and 2B  are schematic views of a semiconductor device of the Embodiment 1;  
         [0019]      FIG. 3  is a top view showing a ground layer of the multilayer printed wiring board of the Embodiment 1;  
         [0020]      FIG. 4  is a top view showing a ground layer of a multilayer printed wiring board of Embodiment 2;  
         [0021]      FIG. 5  is a top view showing a ground layer of a multilayer printed wiring board of Embodiment 3; and  
         [0022]      FIG. 6  is a top view showing a ground layer of a multilayer printed wiring board of Embodiment 4. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Embodiments of the present invention are described below by referring to the accompanying drawings.  
       Embodiment 1  
       [0024]      FIG. 1  is a sectional view of a multilayer printed wiring board of the Embodiment 1. In  FIG. 1 , a multilayer printed wiring board P 1  is formed of laminated four resin layers  1   a ,  1   b ,  1   c , and  1   d . A first wiring pattern  2  is formed on the interfacial surface between the first resin layer  1   a  and the second resin layer  1   b , a second wiring pattern  3  is formed on the interfacial surface between the second resin layer  1   b  and the third resin layer  1   c , and a third wiring pattern  4  is formed on the interfacial surface between the third resin layer  1   c  and the fourth resin layer  1   d . The first wiring pattern  2  is a ground layer, the second wiring pattern  3  is a signal wiring layer, and the third wiring pattern  4  is a power supply layer. The resin layers  1   a ,  1   b ,  1   c , and  1   d  are respectively formed of a glass epoxy resin obtained by reinforcing an epoxy resin with glass cloth.  
         [0025]     Moreover, a plurality of lands  5  are formed on the surface of the first resin layer  1   a  in the form of an array and a solder resist  6  for insulation is applied to portions other than surfaces of the lands  5 . The lands  5  are electrically connected through a semiconductor device  10  and a plurality of solder balls  13 . The wiring patterns  2 ,  3 , and  4  are connected to the lands  5  formed on the surface of the multilayer printed wiring board P 1  through wirings not illustrated.  
         [0026]      FIGS. 2A and 2B  are conceptual views of a semiconductor device  10  mounted on the multilayer printed wiring board P 1  shown in  FIG. 1 .  FIG. 2A  is a top view of the device  10  viewed from the solder ball  13  side and  FIG. 2B  is a sectional view of the device  10 . In  FIGS. 2A and 2B , reference numeral  11  denotes an IC chip which is sealed with a package  12  made of an epoxy resin. A plurality of solder balls  13  are formed at the bottom surface of the package  12  in the form of an array. The IC chip is formed of silicon, the package  12  is formed of an epoxy resin, and the solder balls are formed of a lead-free solder.  
         [0027]      FIG. 3  is a top view obtained by enlarging a part of the first wiring pattern  2  of the multilayer printed wiring board shown in  FIG. 1 . To simplify the description, the contour of the semiconductor device  10  and positions of the solder balls  13  are shown by a dotted line. A plurality of rectangular openings are formed on the first wiring pattern  2  in the form of a mesh.  
         [0028]     Normally, a place on which stresses generated due to thermal expansion of a multilayer printed wiring board are concentrated can be previously determined in accordance with the arrangement of a multilayer printed wiring board, semiconductor device, and solder balls. Therefore, by lowering the thermal expansion coefficient of the printed wiring board at the place on which stresses are concentrated, stress concentration is eliminated and a crack does not occur on a joint portion.  
         [0029]     In the solder balls arranged in the form of an array shown in  FIG. 3 , a joint portion where a stress is maximized due to thermal expansion is a solder ball located at four corners of the arrayed solder balls. Therefore, the solder balls located at four corners of the arrayed solder balls are formed of a mesh pattern constituted of an opening  8  having a large opening area. Moreover, other regions are formed of a mesh pattern constituted of an opening  7  having a small opening area. Thereby, it is achieved to decrease only the thermal expansion coefficient nearby the opening  8  having a large opening area of a multilayer printed wiring board.  
         [0030]     Furthermore, the region where the opening  8  having a large opening area is formed is very small compared to the region where the opening  7  having a small opening area is formed. Therefore, for the first wiring pattern  2  as a whole, the characteristic impedance is stabilized, so that it is possible to stably keep the wiring impedance on the adjacent second wiring pattern  3 .  
         [0031]     A wiring pattern to be formed by a mesh pattern is not restricted to the first wiring pattern  2  of the multilayer printed wiring board P 1 . It is also allowed to use the second wiring pattern  3  or third wiring pattern  4 . Moreover, it is allowed to use not only a ground layer but also a power supply layer or another signal wiring layer.  
         [0032]     The mesh size of the first wiring pattern  2  is preferably such that the size of the square hole of the opening  8  having a large opening area is 1.5 to 3 times larger than that of the opening  7  having a small opening area. It is preferable that the size of the square hole of the opening  7  is about 0.2 mm×0.2 mm square, and that of the opening  8  is about 0.4 mm×0.4 mm square. In this case, it is possible to form twenty-five openings  7  and nine openings  8  in an area of 1.4 mm×1.4 mm square. That is, the area of the first wiring pattern  2  is about ½ for the opening  7  and about ⅓ for the opening  8  compared to the case of a conductive material whose whole surface is solid. The thermal expansion coefficient of the first wiring pattern  2  made of a metal such as copper is proportional to the area of the metal. The thermal expansion coefficient of the first wiring pattern  2  corresponding to solder balls located at four corners of an arrayed solder ball on which the opening  8  is formed is about ⅓ compared to a solid ground. Thereby, it is possible to moderate joint stresses applied to the solder balls located at four corners.  
       Embodiment 2  
       [0033]      FIG. 4  is a top view showing a first wiring pattern  2  of a multilayer printed wiring board P 2  of the Embodiment 2. To simplify the description, the contour of a semiconductor device  10 , contour of an IC chip  11 , and position of a solder ball  13  are shown by a dotted line. Rectangular openings are formed on the first wiring pattern  2  in the form of a mesh.  
         [0034]     In  FIG. 4 , a joint portion where a maximum stress due to thermal expansion is generated is a solder ball  13   a  located at four corners of the solder balls arranged in the form of an array on the semiconductor device  10  shown in  FIG. 3  as well as a solder ball  13   b  located at four corners of the IC chip  11 . This is because the thermal expansion coefficient of a semiconductor device is greatly influenced by the IC chip  11  made of silicon, so that a large joint stress is generated also in the solder ball  13   b  located at four corners of the IC chip  11 . In the first wiring pattern  2 , a region corresponding to joint portions formed on solder balls  13   a  at four corners of the semiconductor device  10  and a region corresponding to joint portions formed on the solder balls  13   b  at four corners of the IC chip  11  are constituted of an opening  8  having a large opening area and other regions are constituted of an opening  7  having a small opening area.  
         [0035]     Thereby, a stress due to thermal expansion is reduced. Moreover, the first wiring pattern  2  is stabilized as a whole as in  FIG. 3  and it is possible to stably keep the wiring impedance on the adjacent second wiring pattern  3 .  
       Embodiment 3  
       [0036]      FIG. 5  is a top view showing a first wiring pattern  2  of a multilayer printed wiring board P 3  of the Embodiment 3. To simplify the description, the contour of a semiconductor device  10 , contour of an IC chip  11 , and positions of solder balls  13   a  are shown by a dotted line. Rectangular openings are formed on the first wiring pattern  2  in the form of a mesh.  
         [0037]     In  FIG. 5 , a portion at which a joint portion to generate a maximum stress due to thermal expansion is formed is a solder ball located at four corners of the semiconductor device  10  similarly to  FIG. 3 . In  FIG. 5 , in the first wiring pattern  2 , a mesh opening  8  formed by a plurality of rectangular openings  9   a  is formed only on a portion corresponding to solder balls located at four corners of the semiconductor device  10 . The other regions are constituted of a solid ground pattern  9   b.    
         [0038]     Thereby, a stress due to thermal expansion is reduced. Moreover, by forming the rectangular opening  9   a  into a fine mesh pitch, it is possible to reduce the difference in copper ratio for unit area compared to the case of a copper-foil solid portion  9   b . Therefore, the first wiring pattern  2  is stabilized as a whole and it is possible to stably keep the wiring impedance on the adjacent second wiring pattern  3 .  
       Embodiment 4  
       [0039]      FIG. 6  is a top view showing a first wiring pattern  2  of a multilayer printed wiring board P 4  of Embodiment 4. To simplify the description, the contour of a semiconductor device  10 , contour of an IC chip  11 , and positions of solder balls  13  are shown by a dotted line. Rectangular openings are formed on the first wiring pattern  2  in the form of a mesh.  
         [0040]     In  FIG. 6 , a joint portion where a maximum stress due to thermal expansion is generated is not only solder balls  13   a  located at four corners of the arrayed solder ball of the semiconductor device  10  shown in  FIG. 5  but also solder balls  13   b  located at four corners of the IC chip  11 . This is because the thermal expansion coefficient of the semiconductor device is greatly influenced by the IC chip  11  made of silicon, so that a large joint stress is generated also in the solder balls  13   b  located at four corners of the IC chip  11 . In the first wiring pattern  2 , a rectangular mesh opening  9   a  is formed on the solder balls  13   a  at four corners of the semiconductor device  10  and the region corresponding to joint portions formed at solder balls  13   b  at four corners of the IC chip  11 .  
         [0041]     Thereby, a stress due to thermal expansion is reduced. Moreover, similarly to  FIG. 5 , the first wiring  2  is stabilized as a whole and it is possible to stably keep the wiring impedance on the adjacent second wiring pattern  3 .  
         [0042]     According to the present invention, it is possible to reduce only the joint stress generated in a solder ball to which a maximum stress is applied due to the difference between thermal expansion coefficients among a plurality of solder balls for connecting a semiconductor device with a multilayer printed wiring board. That is, it is possible to reduce only the thermal expansion coefficient of a predetermined portion of a ground layer of an inner layer of the multilayer printed wiring board corresponding to a solder ball to which a maximum stress is applied. Moreover, at the same time, it is possible to keep small the variation of the impedance of a signal wiring of the multilayer printed wiring board and to secure high-speed stable electric signal propagation characteristics.  
         [0043]     This application claims priority from Japanese Patent Application Nos. 2004-156991 filed on May 27, 2004 and 2005-141087 filed on May 13, 2005, which are hereby incorporated by reference herein.