Abstract:
A back electrode type electronic part includes a main body including a circuit, and electrodes arranged for solder bumps on a back surface portion of the electronic part and connected to the circuit. Each of groups of the electrodes in portions of the electrode arrangement is provided for a single first solder bump which is larger than second solder bumps for the electrodes arranged other than the corner portions. Also, the group of electrodes includes electrodes having a substantially same potential level when the circuit operates.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a back electrode type electronic part and an electronic assembly with the same mounted on a printed circuit board. 
   2. Description of the Related Art 
   With appearance of a small size electronic appliance such as a portable information terminal, a back electrode type or a Ball Grid Array (BGA) type electronic part used in such an electronic appliance is made small in size. Also, a BGA electrode is made small. Therefore, a soldering connection section between the BGA type electronic part and a printed circuit board in the electronic appliance is made small so that the endurance to heat cycle stress and external stress is decreased. 
     FIG. 1  shows a cross sectional view of an electronic assembly with a printed circuit board on which a conventional BGA type electronic part is mounted. Referring to  FIG. 1 , solder balls  13  are provided between electrode lands  12  of the electronic part  11  and electrode lands  16  of the printed circuit board  15 . Especially, the solder balls on the corners are shown by a reference numerals  14 . 
   In Japanese Laid Open Patent Application (JP-A-Heisei 10-56093), a semiconductor device and an electric appliance in which the semiconductor device is incorporated are described. In this reference, a dummy electrode is provided in a corner section of a BGA electrode arrangement while one substrate electrode corresponds to one part electrode. Thus, even if a crack is generated in a solder connection section of the dummy electrode due to heat cycles, solder connection of a signal electrode is guaranteed. 
   and even if 
   A heat cycle stress is applied to the printed circuit board on which the BGA type electronic part is mounted, due to environment temperature change and change of the heating of BGA type electronic part itself. At this time, any warp is caused for the difference in thermal expansion percentage between the BGA type electronic part and the printed circuit board. This warp often centers on the solder connection sections in 4 peripheral corner sections of electrode arrangement of the BGA type electronic part. When the solder connection section is not endured for this warp, there is the high possibility that the crack is generated in the solder connection section. 
   Also, when an external stress is applied to the printed circuit board on which the BGA type electronic part is mounted, to bend the board, the 4 corner sections of the electrode arrangement of the BGA type electronic part are easiest to receive stress transformation most. This is because there are few neighbor electrodes by which the stress can be dispersed and the transformation of the printed circuit board due to the external stress becomes the largest. The destruction of the solder connection section through the stress transformation often progresses from the 4 corner sections toward the inside of the electrode arrangement. 
   In conjunction with the above description, a chip carrier is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 4-314355). In this reference, positioning pads are provided on a back surface of a chip carrier to have a larger area than connection pads. A conductive circuit is provided on a substrate to have a larger area than a connection conductive circuit corresponding to the connection pad. The chip carrier is preliminarily soldered and the chip carrier is positioned on the substrate. Then, reflow is carried out. Thus, the positioning is carried out by use of solder balls for the positioning pads and then the connection pads are connected to the substrate. 
   Also, a semiconductor device is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 9-330993). In this reference, a solder bump forming land  3  is separately formed as lands  3   a  and  3   b  in a BGA structure. After the semiconductor chip  1  is molded, a test is carried out by use of the lands  3   a  and  3   b . Thereafter, when a solder bump is formed, lands  3   a  and  3   b  are electrically connected by a single solder bump. 
   Also, a BGA semiconductor device is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 7-321247). In this reference, a pad  14  is formed to have a shape in which a length in a direction of a line passing through a transformation center  12  is larger than that in a direction orthogonal to the line. Thus, a contact angle in a direction of generation of thermal warp is made larger than a conventional device so that a solder life to the thermal warp is elongated. 
   Also, a surface mounting type semiconductor package is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 9-307022). In this reference, a semiconductor package  3  has a rectangular package body  15 . Solder balls  22  which are covered by a solder layer  23  are arranged in a matrix on a back surface  16   b  of the package body. The solder balls are soldered to pads  8  of a printed circuit board  2  by reflow. The solder balls  22   a  in the outermost of an arrangement area of the solder balls have a larger diameter than the other solder balls. 
   In addition, a BGA electronic part is disclosed in Japanese Registered Utility Model No. 3012948. In this reference, dummy terminals  8  are provided in a region of soldering sections where any crack is easy to be generated due to heat cycles. The dummy terminals are arranged in an outermost portion or corner portions of the terminal arrangement. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is a back electrode type electronic part in which strength of the solder connection sections at the 4 corner sections of the electrode arrangement is increased for improvement of the mount reliability. 
   Another object of the present invention is to provide an electronic assembly in which such a back electrode type electronic part is mounted on a printed circuit board by which the reliability of the assembly is increased. 
   In order to achieve an aspect of the present invention, a back electrode type electronic part includes a main body including a circuit, and electrodes arranged for solder bumps on a back surface portion of the electronic part and connected to the circuit. Each of groups of the electrodes in portions of the electrode arrangement is provided for a single first solder bump which is larger than second solder bumps for the electrodes arranged other than the corner portions. Also, the group of electrodes includes electrodes having a substantially same potential level when the circuit operates. 
   The electrodes may be arranged in a matrix, and the corner portions may be 4 corner portions. 
   Also, the group of electrodes may include a non-contact electrode which is not connected to the circuit. 
   Also, one of the electrodes of the group may be a signal electrode, a ground potential electrode, or a power supply potential electrode. 
   In order to another aspect of the present invention, an electronic assembly includes a back electrode type electronic part, a printed circuit board and solder bumps. The back electrode type electronic part includes electrodes provided on a back surface portion of the electronic part and connected to the circuit. Groups of the electrodes at corner portions of the electrode arrangement is groups of integrated electrodes, and the group of integrated electrodes includes the electrodes having a substantially same potential level when the circuit operates. The printed circuit board has substrate electrodes corresponding to the electrodes provided for the electronic part. One of the substrate electrodes as a first substrate electrode is provided for each of the groups of integrated electrodes, and the substrate electrodes as second substrate electrodes other than the first substrate electrodes are provided for the electrodes of the electronic part other than the integrated electrodes. Solder bumps including first solder bumps connected with the groups of integrated electrodes and the first substrate electrodes and second solder bumps connected with the second substrate electrodes and the electrodes of the electronic part other than the integrated electrodes. 
   Also, the electrodes of the electronic part may be arranged in a matrix, and the portions are 4 corner portions. 
   Also, one of the integrated electrodes of the group may be a non-contact electrode which is not connected to the circuit, a signal electrode or a power supply potential electrode. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross sectional view showing the state in which a conventional BGA type electronic part is mounted on a printed circuit board; 
       FIG. 2  is a back plan view showing the electrode arrangement of a BGA type electronic part according to a fist embodiment of the present invention; 
       FIG. 3  is a partial plan view showing the electrode arrangement of a printed circuit board on which the BGA type electronic part of  FIG. 2  is mounted; 
       FIG. 4  is a plan view showing the state in which the BGA type electronic part of  FIG. 2  is mounted on the printed circuit board of  FIG. 3 ; 
       FIG. 5  is a cross sectional view along the line I—I of  FIG. 4  when the BGA type electronic part of  FIG. 2  is mounted on the printed circuit board of  FIG. 3 ; 
       FIG. 6  is a plan view showing the electrode arrangement of the BGA type electronic part according to a second embodiment of the present invention; and 
       FIG. 7  is a plan view showing the state in which the BGA type electronic part of  FIG. 6  is mounted on the printed circuit board. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, a back electrode type electronic part and an assembly in which the back electrode type electronic part is mounted on a printed circuit board will be described 
     FIG. 2  is a back plan view showing a back electrode type or BGA type electronic part according to the first embodiment of the present invention. A reference numeral  1  in  FIG. 2  denotes the BGA type electronic part,  2  denotes integration possible specific electrodes, and  3  denotes a general electrodes. The electronic part includes a circuit (not shown) therein and the specific electrodes  2  and the general electrodes  3  are connected to the circuit. In the first embodiment, a set of four ground potential electrodes as the specific electrodes  2  are provided at each of four corner sections of the BGA type electronic part  1 . As seen from  FIG. 2 , the electrodes are arranged in a matrix with a constant distance between adjacent ones in a row direction or a column direction. However, a pitch between the specific electrodes may be shorter than a pitch between the general electrodes. Also, in  FIG. 3 , all the electrodes have the same size. However, the specific electrode may be have a different from the general electrode in size. For example, the specific electrode may be larger or smaller than the general electrode. 
   Although ground potential electrodes are used as the specific electrodes, other electrode may be used as the specific electrodes. For example, the following electrodes shown by {circle around (1)} to {circle around (7)} may be used as a set of integration possible electrodes. That is,
         {circle around (1)} the ground potential electrodes are gathered or are provided newly for a set;   {circle around (2)} non-contact electrodes which are not connected to the circuit of the electronic part  1  are gathered or are provided newly for a set;   {circle around (3)} signal electrodes which having a same signal level when the circuit operates are gathered or are provided newly for a set;   {circle around (4)} power supply potential electrodes are gathered or are provided newly for a set;   {circle around (5)} ground potential electrodes and non-contact electrode electrodes are gathered for a set;   {circle around (6)} same signal electrodes and non-contact electrode electrodes are gathered for a set; and   {circle around (7)} power supply electrodes and non-contact power supply electrodes are gathered for a set.       

     FIG. 3  shows a printed circuit board on which the BGA type electronic part  1  shown in  FIG. 2  is mounted. In  FIG. 3 , a reference numeral  4  denotes a substrate,  5  denotes an integration electrode and  6  denotes a general electrode. The substrate  4  in this embodiment has an integration electrode  5  for the specific electrodes  2  in the BGA type electronic part shown in  FIG. 2 and a  general electrode  6  for the general electrode  3 . The integration electrodes are provided at the four corner sections and the four specific electrode are integrated into the same integration electrode  5 . By this, the solder connection section is made large so that it is made possible to improve the connection strength between the soldered BGA type electronic part and the printed circuit board. 
     FIG. 4  shows the state in which the BGA type electronic part shown in  FIG. 1  is soldered to the printed circuit board  4  shown in FIG.  3 . The group of specific electrodes  2  of the BGA type electronic part  1  are soldered to the integration electrode  5  of the printed circuit board  4 . At this time, as shown in  FIG. 5 , a solder bump for the specific electrodes  2  of the BGA type electronic part and the integration electrode  5  is large. The solder bump for the general electrode  3  of the BGA type electronic part and the general electrode  6  of the printed circuit board  4  is small. 
   That is, the large solder connection section between the specific electrodes  2  of the BGA type electronic part and the integration electrode  5  of the printed circuit board  4 . As a result, the connection strength of the solder connection sections in the four corner sections is improves increasingly. Therefore, it becomes possible to prevent generation of any crack due to the above-mentioned heat cycle stress and destruction due to the external stress, resulting in improvement of the reliability of the mounted BGA type electronic part. 
   In the present invention, the number of specific electrodes gathered at each of the four corner sections is not limited to  4  electrodes, and may be an optional number. For example, as shown in  FIG. 6 , three specific electrodes  3 , namely, the specific electrode  2  situated in each of the four corner of the BGA type electronic part  1  and the two electrodes  2  arranged along the edge section of the BGA type electronic part  1  and located in the neighborhood to the above electrode  2  are set as the integration possible specific electrodes. Also, the number of integration electrodes is not limited to four. The number of integration electrodes may be an optional number. 
   Also, as shown in  FIG. 7 , a square integration electrode  10  is provided for the substrate  4  to mount the BGA type electronic part  1  shown in FIG.  6 . This integration electrode  10  is connected with three specific electrodes  2  of the BGA type electronic part  1  shown in  FIG. 6  with solder. 
   The integration electrode provided on the substrate  4  may be circular as shown in FIG.  3  and  FIG. 4 , rectangular as shown in  FIG. 7 , or optional shapes such as an ellipse and an oval shape. 
   According to the present invention, the group of specific electrodes is soldered to the integration electrode of the substrate while the general electrode of the BGA type electronic part is connected with the general electrode of the printed circuit board. The solder connection section of the specific electrodes of the BGA type electronic part is made large in each of the four corner sections. Therefore, the connection strength of the soldering section in the four corners is increased. Also, it is possible to prevent generation of the crack due to the above-mentioned heat cycle stress and the destruction due to the external stress, resulting in the improvement of the loaded reliability of the BGA type electronic part.