Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printed circuit board and a method of production of an electronic apparatus. 
     2. Description of the Related Art 
     In the past, integrated circuits, resistors, capacitors, and numerous other electronic devices have been surface mounted on printed circuit boards by reflow soldering. 
     Normally, for surface mounting an electronic device on a printed circuit board by reflow soldering, first a printing plate for printing solder paste is arranged over the board electrodes of the printed circuit board and cream-like solder paste is coated on the printing plate to coat the board electrodes with solder paste. Next, the printing plate is removed, the electronic device is placed on the printed circuit board, and solder melted by hot air heating or infrared heating called “reflow” is used to bond the electronic device electrodes and the board electrodes. 
     A printed circuit board carrying an electronic device produced by reflow soldering in this way is inspected by an inspection for the appearance or an electrical test connecting the board electrodes and device electrodes and then used to produce an electronic apparatus. However, if the solder melted by the reflow does not stop at between the electronic device electrodes and the board electrodes and leaks outside of the electronic device, sometimes the warping of the electronic device and the printed circuit board at the time of reflow will result in soldering failure etc. and the problem of a drop in yield of the electronic apparatuses. 
     Using  FIGS. 1A to 1C , an example of soldering the board electrodes provided on a printed circuit board and device bottom electrodes provided at an electronic device will be explained.  FIG. 1A  is a side view showing a device electrode arranged at the bottom of an electronic device  20   a , that is, a device bottom electrode  22   a , a board electrode  12   a  of the printed circuit board  10   a , and a solder joint  24   a ,  FIG. 1B  is a top view showing the positional relationship between the device bottom electrode  22   a  and the board electrode  12   a , and  FIG. 1C  shows a mode of mounting an electronic device  20   b  on the printed circuit board  10   b.    
     As shown in  FIG. 1A , at the solder joint  24   a  between the electronic device electrode  22   a  and board electrode  12   a , the solder melted at the time of the reflow did not stop between the electrodes and leaked to the outside of the electronic device. There was less solder between the electrodes and a low level of height of solder between the electrodes. 
     As shown in  FIG. 1A , if the amount of solder between the electrodes is small, as shown in  FIG. 1C , when warping of the electronic device  20   b  and printed circuit board  10   b  occurs due to the thermal stress at the time of reflow, unless the molten solder is high enough to allow for that deformation, the device bottom electrode  22   b  and board electrode  12   b  will end failing to bond. 
     Japanese Laid-open Patent Publication No. 2005-228959 proposes a method of using electrodes at the side surface of a device and firmly bonding with the electrodes at the side surfaces of the device so as to avoid the phenomenon of such bonding failure between electrodes at the time of reflow. 
     The method proposed requires that electrodes of an electronic device be made device side electrodes. However, along with the higher density of electronic apparatuses, electronic devices are being made smaller in size. It is therefore difficult to form side electrodes at the outer edges of an electronic device. Further, device side electrodes are arranged to cover the outer edges of the electronic device. The board electrodes are extended in the direction of the outer edges of the electronic device and the area occupied by the electronic device on the printed circuit board is increased, so there is the problem that this impairs the increased density of electronic apparatuses. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a printed circuit board and/or a method of production of an electronic apparatus for carrying an electronic device having device bottom electrodes by soldering on a printed circuit board so as to prevent bonding failure between the electronic device and the printed circuit board. 
     According to an aspect of the present invention, there is provided a printed circuit board provided with board electrodes, wherein each the board electrode is provided with: a board electrode base, for carrying by soldering a bottom electrode arranged at a bottom of an electronic device inside from outer edges of the electronic device, arranged inside from the outer edges of the electronic device and a projection projecting out from the board electrode base, narrower in width than the board electrode base, and connected to an interconnect of the printed circuit board. 
     Preferably, the board electrode base is larger than the bottom electrode. In one embodiment, a surrounding of the board electrode can be treated with a solder resist. 
     According to another aspect of the present invention, there is provided a method of production of an electronic apparatus comprising: forming, on a printed circuit board, board electrodes, each provided with a board electrode base, for carrying by soldering a bottom electrode arranged at a bottom of an electronic device inside from outer edges of the electronic device, arranged inside from the outer edges of the electronic device and a projection projecting out from the board electrode base, narrower in width than the board electrode base, and connected to an interconnect of the printed circuit board; arranging on the board electrode a printing plate having patterns from which the board electrodes are exposed; coating the printing plate with a solder paste to coat the exposed board electrodes with the solder paste; removing the printing plate and placing the bottom electrodes on the board electrode bases of the board electrodes coated with the solder paste so as to carry the electronic device; and soldering together the board electrode bases and the bottom electrodes. 
     Preferably, the patterns of the printing plate from which the board electrodes are exposed are rectangular, circular, elliptical, semicircular, or fan-shaped. In one embodiment, a surrounding of the board electrodes can be treated with solder resist. 
     According to the present invention, a projection is provided at a board electrode, so the molten solder remains between the electrodes, the height of the molten solder between the electrodes is raised, and that molten solder absorbs warping of the electronic device and printed circuit board at the time of reflow, so it is possible to prevent bonding failure between an electronic device having device bottom electrodes and a printed circuit board and possible to prevent a drop in yield of the electronic apparatuses surface mounting electronic devices having device bottom electrodes. 
     Furthermore, according to the present invention, it is possible to prevent bonding failure of device bottom electrodes, which occupy a smaller area on a printed circuit board than device side electrodes, at a printed circuit board and thereby provide a higher density electronic apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be more clearly understood from the description of the preferred embodiments as set forth below with references to the accompanying drawings, wherein: 
         FIG. 1A  is a side view showing a device bottom electrode, a board electrode of a board, and a solder joint,  FIG. 1B  is a top view showing the positional relationship between device bottom electrode and board electrode, and  FIG. 1C  is a side view showing the state where a device bottom electrode and a board electrode fail to bond; 
         FIG. 2A  is a side view showing the state before reflow where a board electrode is coated with solder paste,  FIG. 2B  is a side view showing the shape of solder paste, coated on the board electrode, at the time of reflow,  FIG. 2C  is a top view showing the positional relationship between a device bottom electrode and board electrode, and  FIG. 2D  is a side view showing the state where a device bottom electrode and a board electrode are bonded; 
         FIG. 3A  is a top view of a printing plate,  FIG. 3B  is a top view of a printed circuit board having the printing plate arranged at its top surface and coated with solder paste,  FIG. 3C  is a top view of a printed circuit board, from which the printing plate has been removed and coated on its board electrode with solder paste, in the state before reflow, and  FIG. 3D  is a top view of a printed circuit board on which molten solder after reflow moves on to the board electrode; and 
         FIG. 4A  is a top view of a circular printing plate opening,  FIG. 4B  of an elliptical printing plate opening,  FIG. 4C  of a semicircular printing plate opening, and  FIG. 4D  of a fan-shaped printing plate opening. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Below, embodiments of the present invention will be explained with reference to the drawings. 
     Using  FIGS. 2A to 2D , in an electronic apparatus, an example of a board electrode provided on a printed circuit board, a device bottom electrode provided on an electronic device, and a solder joint will be explained. 
       FIG. 2A  is a side view showing the state before reflow where a board electrode  12   c  is coated with a solder paste,  FIG. 2B  is a side view showing the shape of the solder paste, coated on the board electrode  12   c , at the time of reflow,  FIG. 2C  is a top view showing the positional relationship between a device bottom electrode  22   c  and a board electrode  12   c  having a board electrode base  14   c  and a projection  13   c , and  FIG. 2D  is a side view showing the state where a device bottom electrode  22   c  of an electronic device  20   c  and a board electrode  12   c  of the printed circuit board  10   c  are bonded through a solder joint  24   c  in an electronic apparatus  1 . 
     The board electrode  12   c  shown in  FIG. 2C  has a board electrode base  14   c  and projection  13   c . The projection  13   c  is reduced in area, whereby the flow of molten solder outside of the electronic device is prevented, the molten solder is kept between the electrodes, and a fillet shape solder joint  24   c  shown by  FIG. 2D  is formed. By forming such board electrodes and arranging them on the printed circuit board, fillet shape solder joints  24   c  are formed at the reflow soldering and the level of the molten solder between the electrodes becomes higher, so it is possible to absorb warping of the electronic device  20   c  and printed circuit board  10   c  at the time of reflow and possible to avoid the bonding failure phenomenon. 
     Note that the projections  13   c  are connected to not shown conductors on the printed circuit board so as to electrically connect the electronic device  20   c  to other electronic devices etc. on the printed circuit board. 
     Furthermore, the longitudinal dimension L 2  of the board electrode base  14   c  of the board electrode  12   c  shown in  FIG. 2C  is longer than the longitudinal dimension L 1  of the device bottom electrode  12   c  and furthermore is shorter than the dimension L 3  from the end of the inside of the electronic device on the board electrode base  14   c  to the outer periphery of the electronic device  20   c . Further, the width dimension L 7  of the projection  13   c  is shorter than the width dimension L 6  of the board electrode base  14   c  for making the molten solder move. 
     In this way, the board electrode bases  14   c  are arranged at the inside from the outer periphery of the electronic device  20   c , so the board electrodes are prevented from being extended in the direction of the outer edge of the electronic device and the area occupied by an electronic device having device bottom electrodes on the printed circuit board is reduced. 
       FIG. 2A  is a side view showing the state before reflow where a board electrode  12   c  is coated with solder paste  24   c - 1 . Since this is before reflow, the state is shown where the solder paste is coated on the printed circuit board  12   c  by exactly the thickness of the opening of the not shown printing plate. 
       FIG. 2B  is a side view showing the molten solder  24   c - 2  coated on the board electrode  12   c  at the time of reflow. This shows the state where reflow heating causes the solder paste to melt and the surface tension of the molten solder acting between the device bottom electrode  22   c  and board electrode  12   c  causes the solder to move to the device bottom electrode  22   c  side. In this way, the solder paste  24   c - 1  shown in  FIG. 2A  melts, then, as shown in  FIG. 2B , the molten solder  24   c - 2  moves between the bottom electrode  22   c  and board electrode base  14   c  whereby the level of the molten solder becomes higher. 
     Using  FIGS. 3A to 3D , an example of the method of reflow soldering an electronic device on to a printed circuit board will be explained. 
       FIG. 3A  is a top view of a printing plate  32  having an opening  34  for coating a board electrode with a solder paste,  FIG. 3B  is a top view of a printed circuit board  10   d  over which a printing plate  32  is placed and on which a solder paste  36  is coated,  FIG. 3C  is a top view of printed circuit board  10   c , from which a printing plate  32  is removed and on the board electrode  13   d ,  14   d  of which a solder paste  36  is coated, in the state before reflow, and  FIG. 3D  is a top view of a printed circuit board  10   d  in the state with the reflowed molten solder  38  moved on to the board electrode  13   d ,  14   d.    
     Note that in  FIGS. 3A to 3D , for explanation, the reflow soldering of a single board electrode was explained, but an actual printed circuit board has a large number of board electrodes. 
     As shown in  FIG. 3A , the printing plate  32  has an opening  34  for coating a board electrode  13   d ,  14   d  of the printed circuit board  10   d  with solder paste. 
       FIG. 3B  shows a printed circuit board  10   d  on the top surface of which a printing plate  32  has been arranged and a solder paste  36  coated on the printed circuit board  10   d . By coating a solder paste on the printing plate  32 , the solder paste  36  enters through the opening  34  and the solder paste  36  is coated on the board electrode comprised of the board electrode base  14   d  and projection  13   d . Note that the printed circuit board  10   d  is treated at its surface at portions other than the board electrode so as to be resistant to deposition of molten solder, that is, is given a solder resist  33 . 
     Note that the opening  34  has a rectangular shape not having any projecting shape matching with the shape of the projection  13   d  of the board electrode, having an area larger than the board electrode comprised of the board electrode base  14   d  and projection  13   d , and covering the board electrode. The opening  34  is made a rectangular shape in this way since if the opening  34  has a projecting shape matching the projection  13   d , the projecting shape of the printing plate  32  will snag the squeegee etc. for coating the solder paste. Therefore, bending of the projecting shape of the printing plate  32  can be avoided. 
       FIG. 3C  shows the state where the printing plate  32  is removed from the printed circuit board  10   d  coated with the solder paste  36  shown in  FIG. 3B . As illustrated, the solder paste  36  is coated on the board electrode base  14   d  and projection  13   d  in accordance with the rectangular shape of the opening  34  of the printing plate  32 . 
       FIG. 3D  shows the state where the printed circuit board  10   d  coated with the solder paste shown in  FIG. 3C  is reflowed to make the solder paste  34  melt and where the molten solder  38  moves on to the board electrode  13   d ,  14   d . The printed circuit board  10   d  is treated on its surface with the solder resist  33 , so as shown by the arrows, the molten solder moves from the projection  13   d  on to the board electrode base  14   d  due to the surface tension of the molten solder  38  caused by the reflow. 
     Note that in the above explanation, the opening of the printing plate was made a rectangular shape, but the opening is not necessarily limited to this shape. It is sufficient that it be a shape not snagging a squeegee etc. for coating the solder paste and covering the board electrode. 
     Using  FIGS. 4A to 4D , examples of an opening of a printing plate having a shape other than a rectangle will be explained. 
       FIG. 4A  is a top view showing a circular printing plate opening  34   a ,  FIG. 4B  an elliptical printing plate opening  34   b ,  FIG. 4C  a semicircular printing plate opening  34   c , and  FIG. 4D  a fan-shaped printing plate opening  34   d.    
     The illustrated openings having circular, elliptical, semispherical, fan-shaped, and other shapes do not have projecting shapes and do not snag squeegees etc. for coating the solder paste. In the step of coating the paste, it is possible to avoid part of the printing plate  32  from being bent. Further, the state is shown where the printed circuit board  10   d  coated with the paste is reflowed so that the solder paste  34  melts and where the molten solder  38  moves on the board electrode. The printed circuit board  10   d  is treated on its surface with solder resist  33 , so the molten solder moves from the projection  13   d  on to the board electrode base  14   d  due to the surface tension of the molten solder  38 . 
     In this way, in the method of production of an electronic apparatus according to the present invention, a printing plate having a rectangular, circular, elliptical, semicircular, or fan-shaped opening is used for reflow soldering while avoiding the formation of solder balls leading to short-circuit accidents etc. between the electronic device electrodes on the printed circuit board. 
     Many different embodiments of the present invention may be constructed without departing from the scope of the present invention, and it should be understood that the present invention is not limited to the specific embodiments described in this specification, except as defined in the appended claims.

Technology Category: h