Patent Publication Number: US-2009224026-A1

Title: Electronic component mounting method

Description:
TECHNICAL FIELD 
     The present invention relates to an electronic component mounting method that is to mount an electronic component formed with solder bumps onto a board by soldering. 
     BACKGROUND ART 
     Along with the recent progress in the size reduction and functional improvement for electronic appliances, size and thickness reductions have been achieved for the electronic components including the semiconductor packages to be built in electronic appliances. In addition, further increase in the mounting density is also sought. There is a trend toward adopting a structure stacked with board modules whose electronic components are mounted on the board, as a mounting form coping with such dense mounting (see Patent document 1,for example). In the Patent Document, by mounting on a board a plurality of semiconductor packages formed with solder bumps, a component-mounted board is to be fabricated with density without increasing the board size. 
     [Patent Document 1] JP-A-2005-26648 
     DISCLOSURE OF INVENTION 
     In the meanwhile, the semiconductor package for use in a stack structure is thin and hence low in rigidity, thus having a nature that warp is readily caused upon a reflow heating for soldering. For this reason, the solder bumps might be floated by such a warp during solder reflow so that the solder bump cannot be normally soldered with the connection electrode of the board. This tends to cause a poor conductivity or a poor junction, e.g. insufficient soldering strength. The problem is encountered commonly where thin semiconductor packages are mounted by soldering without limited to the structure stacked with a plurality of semiconductor packages. 
     Therefore, it is an object of the present invention to provide an electronic component mounting method capable of preventing against a poor junction when to mount thin semiconductor packages by soldering. 
     An electronic component mounting method in the present invention is a method that is to mount onto a board an electronic component formed with solder bumps on a lower surface thereof, the method comprising: a solder transfer step of providing a solder paste onto the solder bump by transfer; a mounting step of placing the electronic component on the board and putting the solder bumps on connection electrodes of the board through the solder paste; and a reflow step of heating up the board together with the electronic component and fusing a solder ingredient of the solder bumps and solder paste thereby soldering the electronic component on the board. 
     According to the invention, the electronic component in a state a solder paste is provided on the solder bumps by transfer is mounted on the board so that the solder bumps are put on the connection electrodes through a solder paste. Due to this, even where there is a gap between the solder bump and the connection electrode, the fused portion of solder is increased in amount by the solder ingredient of the solder paste wherein the fused portion of solder is ensured to wettably spread. This can prevent a poor junction when to mount a thin semiconductor package by soldering. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram of a component-mounted board manufacturing line in one embodiment of the present embodiment. 
         FIG. 2  is plan view of an electronic-component placement apparatus in one embodiment of the invention. 
         FIG. 3  is a structure explanatory view of an electronic component to be mounted on a board in one embodiment of the invention. 
         FIG. 4  is a structure explanatory view of an electronic component to be mounted on a board in one embodiment of the invention. 
         FIGS. 5(   a ) to  5 ( e ) are process explanatory views of a board manufacturing method in one embodiment of the invention. 
         FIGS. 6(   a ) to  6 ( c ) are process explanatory views of a board manufacturing method in one embodiment of the invention. 
         FIGS. 7(   a ) to  7 ( c ) are soldering-process explanatory view in the electronic-component mounting method in one embodiment of the invention. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention will now be explained with reference to the drawings. 
     Referring first to  FIG. 1 , explanation is made on a component-mounted board manufacturing line. In  FIG. 1 , the component-mounted board manufacturing line is constructed with a screen printer M 1 , an electronic-component placement apparatus M 2  and a reflow apparatus M 3  that are arranged in series. The screen printer M 1  is to print a solder paste, for connecting thereon an electronic component, onto a board. The electronic-component placement apparatus M 2  is to mount an electronic component on a board printed with a solder paste. The reflow apparatus M 3  is to heat up the board mounted with electronic components and fuse a solder ingredient of the solder paste, thereby fixing the electronic component on the board. 
     Referring now to  FIG. 2 , explanation is made on the construction of the electronic-component placement apparatus M 2 . In  FIG. 2 , a transport path  2  is arranged in an X direction centrally in a base  1 . The transport path  2  is for transporting a board  3  on which electronic components are to be mounted, and for placing the board  3  in a position where to mount an electronic component. In front of the transport path  2 , first and second component supply parts  4 A and  4 B are arranged in parallel with respect to the X direction. The first and second component supply parts  4 A and  4 B have respective trays where first and second electronic components  11 ,  12  are contained. A third component supply part  4 C is arranged in back of the transport path  2 . The third component supply part  4 C is arranged with a tape feeder  5  which is to feed intermittently a tape held with third electronic components  13  (see  FIG. 5 ) up to a pickup position of the mount head, explained in the following. 
     A Y-axis table  6 A and a Y-axis guide  6 B are arranged at respective ends of the base  1  with respect to the X direction. An X-axis table is suspended between the Y-axis table  6 A and the Y-axis guide  6 B. The X-axis table  7  is arranged thereon with a mounting head  8 . The mounting head  8  is of a gang type having a plurality of unitary heads  8   a,  to move in unison with a board-recognition camera  9 . 
     By driving the X-axis table  7  and the Y-axis table  6 A, the mounting head  8  is moved in the X-axis direction. The unitary heads  8   a  have respective suction nozzles allowed to take a first electronic component  11  out of a first component supply part  4 A, a second electronic component  12  out of a second component supply part  4 B, and a third electronic component  13  out of a third component supply part  4 C, and then mount those on a board  3  placed on the transport path  2 . 
     Between the transport path  2  and the first and second component supply parts  4 A and  4 B, there are arranged a line camera  10 , a nozzle stocker  14  and a solder-paste transfer table  15 . Between the transport path  2  and the third component supply part  4 C, arranged are the line camera  10 , the nozzle stocker  14  and the solder-paste transfer table  15 . The mounting head  8 , picked up the electronic components from the component supplies, passes above the line camera  10  in the course moving to the board  3 . Due to this, recognition is made of the electronic components that are in a state held by the mounting head  8 . 
     The nozzle stocker  14  accommodates a plurality of types of nozzles suitable for the electronic components to be mounted on the board  3 . By accessing of the mounting head  8  to the nozzle stocker  14 , suction nozzles can be selectively attached depending upon the electronic component to mount. The paste transfer table  15  is to supply, onto the table, a solder paste, in a thin film state, rendered viscous by mixing a solder ingredient in a flux. By ascending and descending the mounting head held with the electronic components relative to the paste transfer table  15 , solder paste is provided onto the solder bumps formed on the underside of the electronic component. 
     Referring now to  FIGS. 3 and 4 , explanation is made on the first and second electronic components  11 ,  12 . The first electronic component  11  (electronic component) is a thin package formed by encapsulating a semiconductor element with a resin. As shown in  FIG. 3 , solder bumps  16  are formed on a lower surface  11   a,  in order for connection to the board  3 . Meanwhile, electrodes  17  (second connection electrode) are formed on an upper surface  11   b,  in order for connection to an electronic component to be mounted stacked on the first electronic component  11 . The second electronic component  12  is also a thin package formed by encapsulating a semiconductor element with a resin. As shown in  FIG. 4 , on a lower surface  12   a,  solder bumps  18  are formed in the same arrangement as the electrodes  17  of the first electronic component  12 , in order for connection to the first electronic component  11 . Those thin packages, low in rigidity, have a property to be readily warped during a reflow heating when connected through solder bumps. 
     Explanation is now made on an electronic-component mounting method to mount first and second electronic components  11 ,  12  onto a board  3 . By the electronic-component mounting method, first and second electronic components  11 ,  12  formed with solder bumps in their lower surfaces can be mounted stacked in plurality on a board  3 , to thereby form a densely-mounted board. 
     In  FIG. 5(   a ), electrodes  3   a,    3   b  (connection electrodes) are formed on the upper surface of the board  3 . The electrodes  3   a  are formed in the same arrangement as the bumps  16  of the first electronic component  11  while the electrodes  3   b  are in the same arrangement as the leads  13  a of the third electronic component  13 . The board  3  is first transported to the screen printer M 1  shown in  FIG. 1  where solder paste  19  is provided onto the electrodes  3   a,    3   b  of the board  3  as shown in  FIG. 5(   b ) (solder printing step). Then, the board  3  supplied with solder is transported to the electronic-component placement apparatus M 2  where it is placed in a mounting position on the transport path  2 . The mounting head  8  is moved to the above of the board  3  where the board-recognizing camera  9  takes an image of the board  3  thereby recognizing the position of the board  3  (first recognition step). 
     Thereafter, solder paste is transferred onto the first electronic component  11 . Namely, the first electronic component  11 , taken out of the first component supply  4 A by the mounting head  8 , is moved to the paste-transfer table, in a state held by the suction nozzle  20 . By ascending and descending the first electronic component  11  relative to the coat film of solder paste  19  as shown in  FIG. 5(   c ), solder paste  19  is supplied, by transfer, to the solder bumps  16  at their lower faces (solder transfer step). 
     Then, the electronic component transferred with solder paste is mounted onto the solder-printed board  3   b  by means of the mounting head  8 , as shown in  FIG. 5(   d ). At first, the first electronic component  11  (electronic component in the first level) is aligned with the electrodes  3  a of the board  3  depending upon a recognition result in the first recognition step, and then putting the solder bumps  16  on the electrodes  3   a  thus effecting a mounting (mounting step). In the mounting step, the third electronic component  13  are also mounted through aligning the leads  13   a  with the electrodes  3   b.    
     Thereafter, an electronic component is mounted in the second level. At first, positional recognition is made on the first electronic component  11  by the board-recognition camera  9 . Here, positional recognition is made by recognizing, as featuring points of the electronic component, the electrodes  16  formed in the outermost, diagonal positions of among the electrodes  15  formed on the upper surface  11   b  of the first electronic component  11  (second recognition step). 
     Then, the mounting head  8 , picked a second electronic component  12  out of the second component supply  4 B, moves to the solder-paste transfer table  15 . Here, the second electronic component  12  is ascended and descended relative to the coat film of solder paste  19 , as shown in  FIG. 6(   a ). This provides solder paste  19  to the lower faces of the solder bumps  18  by transfer (second solder transfer step). Then, the second electronic component  12  is aligned with the first electronic component  11  depending upon the recognition result in the second recognition step, and mounted thereon by putting the solder bumps  18  of the second electronic component  12  on the electrodes  17  formed on the upper surface of the first electronic component  12  (second mounting step). 
     Thereafter, the board  3  is transported into the reflow apparatus M 3 . In this case, the board  3  mounted with the first to third electronic components  11  to  13  is heated together with those electronic components up to a reflow temperature higher than a solder melt point. This causes a soldering of the solder bumps  16  of the first electronic component  11  with the electrodes  3   a  of the board  3 , the leads  13   a  of the third electronic component  13  with the electrodes  3   b,  and the solder bumps  18  of the second electronic component  12  with the electrodes  17  of the electronic component  11  (reflow step). The soldering is done by fusing the solder ingredients of the solder bumps  16 ,  18  and solder paste  19 . This completes a densely-mounted board stacked with the packages of the first and second electronic components  11 ,  12 , etc. formed by encapsulating semiconductor elements with resins. 
     The soldering behavior in the reflow step is explained while referring to  FIG. 7 . The present embodiment explained on the example the solder bumps  16  of the first electronic component  11  is soldered to the electrodes  3   a  of the board  3 . However, this is true for the case where the solder bumps  18  of the second electronic component  12  is soldered to the electrodes  17  of the first electronic component  11 . As stated before, because the first electronic component  11  is a thin semiconductor package, the solder bumps  16  tend to float due to an upward warp of the package body upon mounting the first electronic component  11  onto the board  3  and further during conducting a reflow. This possibly results in a gap d caused between the solder bump  16  and the electrode  3   a,  as shown in  FIG. 7(   a ). 
     Even where there is a gap between the solder bump  16  and the electrode  3  a in this manner, the present embodiment transfers a solder paste  19  additionally onto the solder bumps  16  prior to component mounting. Furthermore, solder paste  19  is also provided onto the electrodes  3   a.  Due to this, the electrode  3   a  for connection and the solder bump  16  are placed in a state covered around at their upper and lower surfaces by a sufficient amount of solder paste  19 . 
     Then, reflow is performed in such a state. Namely, in the process of fusing the solder due to heating, the solder paste  19  has a fused portion of solder  19   a,  in a sufficient amount, as a fused solder ingredient thereof This spreads wettably in the viscous liquid resin  19   b  in the state connecting between the lower end of the solder bump  16  and the electrode  3   a,  as shown in  FIG. 7(   b ). At this time, the surface tension of the fused portion of solder  19   a  causes a force acting to pull the solder bump  16  toward the electrode  3   a  and to narrow the initial existent gap d. 
     Thereafter, by continuing the heating furthermore, the solder bump  16  fuses into one body with the fused portion of solder  19   a.  As shown in  FIG. 7(   c ), a solder junction  1   6   a  is formed connecting between the first electronic component  11  and the electrode  3   a.  The solder junction  16   a  is thereafter cooled down and solidified thus completing a soldering of the first electronic component  11  to the board  3 . The solder connection  16   a  is in an amount of the fused solder of the solder bump  16  added with a solder of the solder paste  19 . Therefore, the first electronic component  11  and the board  3  are connected together by a sufficient amount of solder, thus ensuring a sufficient soldering strength and conductivity. 
     Incidentally, the embodiment showed the structure that the first and second electronic components  11 ,  12  are mounted stacked on the board  3 . Besides, the invention can be applied also to the general electronic-component mounting structure other than the stack structure provided that it is in such a mounting form that the package is thin and ready to warp wherein a gap is to be caused between the solder bump and connection electrode. Meanwhile, the embodiment showed the example to previously provide solder also to the connection electrode  3 A of the board  3  by printing. However, where the solder to be additionally provided to the solder bump is satisfactorily small in amount because the package warping degree is comparatively small, the provision of solder to the connection electrode may be omitted. 
     This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2005-243866 filed on Aug. 25, 2005, the contents of which are incorporated herein by reference in its entirety. 
     INDUSTRIAL APPLICABILITY 
     The electronic-component mounting method in the present invention has an effect that poor connection can be prevented in mounting a thin semiconductor packages by soldering, which is useful in the field to mount a thin electronic component formed with solder bumps onto a board by soldering.