Abstract:
Method for mounting an electronic component on a circuit board firstly flip-chip bonding positioning bumps of the electronic component to pads of the circuit board by applying ultrasonic vibrations to the electronic component. Bonding bumps, whose diameters are shorter than those of the positioning bumps and whose projecting lengths are shorter than those of the positioning bumps, of the electronic component are flip-chip bonded to electrodes of the circuit board by applying ultrasonic vibrations to the electronic element. In this manner, a center of each of the bonding bumps is previously relatively displaced, with respect to a center of each of the electrodes in a width direction, in a direction parallel to a direction to the ultrasonic vibrations.

Description:
This application is a divisional of U.S. patent application Ser. No. 11/041,292, filed Jan. 25, 2005, now U.S. Pat No. 7,355,285 which is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-316499 filed on Oct. 29, 2004. The entire contents of these applications are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to a structure and a method of mounting an electronic component, more precisely relates to a structure and a method of mounting an electronic component, in which the electronic component is bonded by ultrasonic waves. 
   When a semiconductor chip, which is an example of the electronic component, is mounted on a circuit board by flip chip bonding, bumps of the semiconductor chip are headed for the circuit board, then the bumps, which are made of gold or solder, are respectively bonded to electrodes of the circuit board. A space between the semiconductor chip and the circuit board is filled with synthetic resin so as to protect a circuit face of the semiconductor chip, prevent the bumps from corrosion and improve bonding strength therebetween. 
   These days, the semiconductor chip is flip-chip-bonded by applying ultrasonic vibrations. By applying ultrasonic vibrations, bonding sections between the bumps and the electrodes are alloyed and bonded or bonded. 
   These days, semiconductor chips are miniaturized and have many pins. Therefore, separations between bums must be very short. When the small semiconductor chip having many pins is bonded to a circuit board by ultrasonic vibrations, the bumps of the semiconductor chip are easily displaced from electrodes of the circuit board, so that bonding reliability between the bums and the electrodes is lowered. 
     FIGS. 7A and 7B  show a conventional structure of mounting a semiconductor chip on a circuit board.  FIG. 7A  is a plan view showing positional relationships between bumps  12   a  and  12   b  of the semiconductor chip  10  and electrodes  22   a  and  22   b  of the circuit board;  FIG. 7B  is a partial front view of bonding sections between the semiconductor chip  10  and the circuit board  20 . The semiconductor chip  10  is flip-chip-bonded to the circuit board  20  by ultrasonic vibrations. Bumps  12  of the semiconductor chip  10  are displaced from electrodes  22  of the circuit board  20 . 
   By applying ultrasonic vibrations to the semiconductor chip  10 , the semiconductor chip  10  reciprocally moves in a direction of the ultrasonic vibrations, e.g., right and left in  FIG. 7A . Each of the electrodes  22  is formed into a thin rectangular. Therefore, even if the bumps  12   a  are displaced from the rectangular electrodes  22   a , whose longitudinal axis is parallel to the direction of the ultrasonic vibrations, and bonded to the electrodes  22   a , the bonding sections occur no problems. On the other hand, if the bumps  12   b  are displaced from the rectangular electrodes  22   b , whose transverse axis is parallel to the direction of the ultrasonic vibrations, the bumps  12   b  are bonded at positions displaced from centers of the electrodes  22   b , so that bonding reliability between the bums  12   b  and the electrodes  22   b  must be lowered. 
   The inventors measured displacement of semiconductor chips with respect to circuit boards. The results are shown in  FIG. 8 . Three groups ( 1 ), ( 2 ) and ( 3 ) of samples were prepared. Semiconductor chips of the samples were flip-chip-bonded to the circuit boards. In each group ( 1 ), ( 2 ) and ( 3 ), three samples were flip-chip-bonded with loads of 5, 10 and 15 gf/bump respectively. No ultrasonic vibrations were applied to the samples of the group ( 1 ); ultrasonic vibrations of 200 kHz were applied to the group ( 2 ); and ultrasonic vibrations of 50 kHz were applied to the group ( 3 ). According to  FIG. 8 , in the case of applying no ultrasonic vibrations, displacement of the semiconductor chips were small without reference to variation of the loads. On the other hand, in the case of applying ultrasonic vibrations, the displacement was easily occurred, and the displacement was increased with increasing frequency of ultrasonic vibrations. 
   In each group ( 2 ) and ( 3 ), ultrasonic vibrations having the same frequency were applied to all of the samples. The displacement was increased with reducing the load. Therefore, the load should be small so as not to damage the semiconductor chip, but the displacement of the semiconductor chip must be increased. 
   SUMMARY OF THE INVENTION 
   The present invention was invented to solve the problems of the conventional technology. 
   An object of the present invention is to provide a structure of mounting an electronic component on a circuit board, which is capable of securely flip-chip-bonding the electronic component having bumps, whose separations are very short, to the circuit board without displacement. Another object is to provide a method of producing the mounting structure of the present invention. 
   To achieve the objects, the present invention has following structures. Namely, the structure of mounting an electronic component on a circuit board is characterized in that bumps of the electronic component are respectively flip-chip-bonded to electrodes of the circuit board by applying ultrasonic vibrations to the electronic component, and that a center of -each of the bumps is previously relatively displaced, with respect to a center in a width direction of each of the electrodes, in a direction parallel to a direction of the ultrasonic vibrations. 
   In the structure, the bumps may be arranged in the electronic component at regular separations, and the electrodes may be arranged in the circuit board at regular separations. 
   Note that, the electronic component is not limited to a semiconductor chip having bumps, other electronic component having bumps for electric bonding may be applied. 
   Another structure of mounting an electronic component on a circuit board is characterized in that bumps of the electronic component are respectively flip-chip-bonded to electrodes of the circuit board by applying ultrasonic vibrations to the electronic component, that the electronic component further includes positioning bumps, whose diameters are larger than those of the bumps and whose projecting lengths are longer than those of the bumps, and that the circuit board further includes pads, on which the positioning bumps of the electronic component are respectively bonded. 
   With this structure, the positioning bumps of the electronic component are bonded by the pads of the circuit board, so that displacement of the electronic component with respect to the circuit board can be prevented. Therefore, the bumps of the electronic component can be securely bonded to the electrodes of the circuit board when the electronic component is mounted on the circuit board by ultrasonic vibrations. 
   Further, the method of mounting an electronic component on a circuit board comprises the steps of: firstly flip-chip-bonding positioning bumps of the electronic component to pads of the circuit board by applying ultrasonic vibrations to the electronic component; and secondly flip-chip-bonding bumps, whose diameters are shorter than those of the positioning bumps and whose projecting lengths are shorter than those of the positioning bumps, of the electronic component to electrodes of the circuit board by applying ultrasonic vibrations to the electronic component. 
   With this method, the positioning bumps of the electronic component are firstly bonded to the pads of the circuit board, so that the electronic component can be correctly mounted on the circuit board without displacement. 
   Another method of mounting an electronic component on a circuit board comprises the steps of: flip-chip-bonding positioning bumps of the electronic component to pads of the circuit board by applying ultrasonic vibrations to the electronic component; and flip-chip-bonding bumps, whose diameters are shorter than those of the positioning bumps and whose projecting lengths are shorter than those of the positioning bumps, of the electronic component to electrodes of the circuit board by applying ultrasonic vibrations to the electronic component, wherein the positioning bumps are firstly tentatively flip-chip-bonded to the pads by applying ultrasonic vibrations to the electronic component, displacement of the electronic component, which has been tentatively bonded to the circuit board, with respect to the regular position in the circuit board is measured, and the electronic component is completely bonded to the circuit board, by applying ultrasonic vibrations to the electronic component, with correcting the position of the electronic component on the basis of the measured displacement. 
   With this method, the electronic component is tentatively bonded to the circuit board, then the displacement of the electronic component is measured. Finally, the displacement is corrected on the basis of the measured displacement. Therefore, the electronic component can be precisely completely mounted on the circuit board. 
   To securely correct the displacement of the electronic component, a position and/or inclination of a table supporting the circuit board may be controlled when the electronic component is completely bonded to the circuit board, a -load applying to the electronic component may be controlled when the electronic component is completely bonded to the circuit board, and a direction of the ultrasonic vibrations may be asymmetrically controlled when the electronic component is completely bonded to the circuit board. 
   In the structures and the methods of the present invention, the electronic component can be securely flip-chip-bonded to the circuit board without displacement when the electronic component is mounted on the circuit board by applying ultrasonic vibrations. Therefore, even an electronic component, whose bump separations are very short, can be correctly mounted on the circuit board. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will now be described by way of examples and with reference to the accompanying drawings, in which: 
       FIG. 1  is an explanation view of a mounting machine for flip-chip-bonding a semiconductor chip by the method of the present invention; 
       FIG. 2  is an explanation view showing an arrangement of bumps of the semiconductor chip and electrodes of a circuit board; 
       FIG. 3  is an explanation view showing an arrangement of the bumps of the semiconductor chip and the electrodes of the circuit board; 
       FIG. 4  is an explanation view showing an arrangement of the bumps of the semiconductor chip and the electrodes of the circuit board; 
       FIG. 5  is an explanation view showing a planar arrangement of the bumps of the semiconductor chip and the electrodes of the circuit board; 
       FIG. 6  is an explanation view showing a method including a tentatively bonding step and a completely bonding step; 
       FIG. 7A  is a plan view showing the displacement of the bumps and the electrodes in the conventional mounting structure; 
       FIG. 7B  is a partial front view of bonding sections between the semiconductor chip and the circuit board in the conventional mounting structure; and 
       FIG. 8  is a graph showing displacement of the semiconductor chips with respect to the circuit boards. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
     FIG. 1  shows a mounting machine  50  for flip-chip-bonding a semiconductor chip  10 , which is an example of the electronic component, onto a circuit board  20 . 
   The mounting machine  50  mainly comprises a ultrasonic head side and a supporting table side. The ultrasonic head side includes: a ultrasonic head  52  air-sucking and holding the semiconductor chip  10 ; a ultrasonic wave oscillator  54  applying ultrasonic vibrations to the ultrasonic head  52 ; a press mechanism  56  pressing the semiconductor chip  10  toward the circuit board  10 ; and a press mechanism control section  58  controlling a load of the press mechanism  56 . 
   On the other hand, the supporting table side includes: an alignment mechanism  60  positioning and moving a supporting table  21  in X- Y- and θ-directions; and an alignment mechanism control section  62  controlling the alignment mechanism  60  so as to adjust mutual positions of the semiconductor chip  10  and the circuit board  20  and correctly flip-chip-bond the semiconductor chip  10  to the circuit board  20 . Further, the supporting table side includes: an imaging unit  64 , which acts as a detecting mechanism, detecting the mutual positions of the semiconductor chip  10  and the circuit board  20 ; an image processing section  66  processing image data sent from the imaging unit  64 ; a drive mechanism  68  moving the imaging unit  64 ; and a drive mechanism control section  69  controlling the drive mechanism  68  so as to move the imaging unit  64  to a prescribed position. 
   A main controller  70  controls the alignment mechanism control section  62 , on the basis of results detected by the image processing section  66 , so as to correctly position the semiconductor chip  10  and the circuit board  20 . Then, the main controller  70  controls the press mechanism control section  58  and the ultrasonic wave oscillator  54 , so that the semiconductor chip  10  can be flip-chip-bonded or mounted on the circuit board  20  with a suitable load. 
     FIGS. 2 and 3  show embodiments of the structure of mounting the semiconductor chip  10  on the circuit board  20 . An arrangement of bumps  12  of the semiconductor chip  10  and electrodes  22  of the circuit board  20  is shown in each of the drawings. 
   Conventionally, when the semiconductor chip  10  is flip-chip-bonded to the circuit board  20 , the bumps  12  of the semiconductor chip  10  and the electrodes  22  of the circuit board  20  are mutually corresponded. On the other hand, in the present embodiment, a center of each bump  12  is slightly relatively displaced, with respect to a center in a width direction of each electrode  22 . With this arrangement, the bumps  12  are properly bonded to the electrodes  22  when the semiconductor chip  10  is bonded to the circuit board  20  by applying ultrasonic vibrations. 
   In  FIG. 2 , the bumps  12  are provided to the semiconductor chip  10  at regular separations. On the other hand, the electrodes  22   c ,  22   d ,  22   e  and  22   f  are alternately displaced rightward and leftward with respect to the centers of the corresponded bumps  12 . By previously displacing the electrodes  22  in the width direction with respect to the centers of the bumps  12 , serious displacement of the bumps  12  and the electrodes  22  can be prevented on the whole even if the vibrated bumps  12  displace from the electrodes  22  when the semiconductor chip  10  is bonded to the circuit board  20  by applying ultrasonic vibrations. 
   Namely, if the centers of the bumps  12  and the centers in the width direction of the electrodes  22  are previously corresponded, all of the bumps  12  are moved in one direction with respect to the electrodes  22  by applying ultrasonic vibrations to the semiconductor chip  10 . If the bumps  12  is moved in one direction, all of the bumps  12  disbond from the electrodes  22 . In the present embodiment, the bumps  12  and the electrodes  22  are previously displaced, so all of the bumps  12  do not disbond from the electrodes  22  even if all of the bumps  12  are moved in one direction. Some of the electrodes  22  hold the bumps  12 , so that the bumps  12  never disbond from the electrodes  22  on the whole. Therefore, bondability of the bumps  12  and the electrodes  22  can be improved. 
   A distance of displacing each electrode  22  with respect to each bump  12  is within an allowable variation range of mounting accuracy of mounting semiconductor chips on circuit boards. If the displacing distance is within the allowable range, the bumps  12  can be suitably bonded to the electrodes  22 . 
   For example, if a diameter of each bump  12  is 20 μm and a separation between the centers of the adjacent bumps  12  is 40 μm, the maximum displacement (displacing distance) between the center of the bump  12  and the center of the electrode  22  is about 10 μm. Note that, a range of ultrasonic vibrations is about 1 μm. 
   In the above described embodiment, the bumps  12  are provided to the semiconductor chip  10  at regular separations, and the bumps  22  of the circuit board  20  are displaced. On the other hand, in an embodiment shown in  FIG. 3 , the bumps  12   c ,  12   d ,  12   e  and  12   f  are alternately displaced rightward and leftward with respect to the centers in the width direction of the corresponded electrodes  22 . 
   In the present invention, the center of each bump  12  may be relatively displaced with respect to the center in the width direction of each electrode  22 . The bumps  12  or the electrodes  22  may be arranged at regular separations. Further, if the centers of the bumps  12  and the electrodes  22  are relatively displaced, the bumps  12  and the electrodes  22  need not be arranged at regular separations. In this case too, the same effects can be gained. 
   In the above described embodiments, a direction parallel to the direction of ultrasonic vibrations is regarded as a standard direction. Each of the bumps  12  is relatively displaced with respect to each of the electrodes  22  in the standard direction. The bumps  12  may be alternately or randomly displaced toward one side and the other side of each of the electrodes  22 . 
   Generally, the bumps  12  are arranged along edges of the rectangular semiconductor chip  10  (see  FIG. 7A ). The bumps  12  and/or the electrodes  22  arranged in the standard direction are displaced. Note that, if the electrodes  22  are not formed into the thin rectangles, it is effective to displace the electrodes  22  arranged perpendicular to the standard direction, with respect to the centers of the bumps  12  as well as the electrodes  22  arranged parallel to the standard direction. 
   Further, another embodiment is shown in  FIG. 4 . In the present embodiment, the semiconductor chip  10  has not only the bumps  12  for electric bonding but also bumps  13  for positioning the semiconductor chip  10 . 
   As shown in  FIG. 4 , each positioning bump  13  has a diameter larger than that of each bump  12  and a projection length longer than that of each bump  12 . On the other hand, the circuit board  20  has positioning pads  23 , to which the positioning bumps  13  are respectively bonded. To bond the positioning bumps  13 , the pads  23  are broader than the electrodes  22 . 
   When the semiconductor chip  10  is mounted onto the circuit board  20 , the positioning bumps  13  firstly contact the pads  23 , then the bumps  12  contact the electrodes  22 . Since the positioning bumps  13  contact the pads  23  of the circuit board  20 , the semiconductor chip  10  is exceedingly displaced, with respect to the circuit board  20 , by ultrasonic vibrations, so that the semiconductor chip  10  can be correctly mounted on the circuit board  20  without relatively displacing the bumps  12  from the electrodes  22 . 
   In  FIG. 4 , the positioning bumps  13  are provided to ends of each line of the bumps  12 , but the positions of the positioning bumps  13  are not limited to an example shown in  FIG. 4 . The positioning bumps  13  may be provided to, for example, intermediate positions of each line of the bumps  12 . 
   A method of mounting the semiconductor chip  10  having the positioning bumps  13  on the circuit board  20  will be explained with reference to  FIG. 5 . In  FIG. 5 , the positioning bumps  13  are arranged in a direction perpendicular to the direction of ultrasonic vibrations (standard direction). The longitudinal direction of the rectangular positioning bumps  13  are parallel to the standard direction. On the other hand, the bumps  12  for electric bonding are arranged parallel to the standard direction. By arranging the positioning bumps  13  perpendicular to the standard direction, the positioning bumps  13  firstly contact the pads  23 , then the bumps  12  contact the electrodes  12  when the semiconductor chip  10  is mounted onto the circuit board  20 . Therefore, the positioning bumps  13  act as stoppers to restrain displacing the bumps  12  from the electrodes  12 , so that the bumps  12  can be securely bonded to the electrodes  12 . 
   Note that, in  FIG. 5 , all of the bumps  13  arranged perpendicular to the standard direction are positioning bumps, but some of them may be used as the bumps  12  for electric bonding. Further, some of the bumps  12  arranged parallel to the standard direction may be used as the positioning bumps  13 . 
   When the semiconductor chip  10  having the positioning bumps  13  is mounted onto the circuit board  20 , the positioning bumps  13  firstly contact the pads  23 . Thus, the semiconductor chip  10  may be tentatively bonded to the circuit board  20  by the positioning bumps  13 . In this case, displacement of the semiconductor chip  10  with respect to the circuit board  20  is measured, then the alignment mechanism  60  supporting the circuit board  20  is controlled on the basis of the measured displacement so as to completely bond the semiconductor chip  10  to the circuit board  20 . 
   The method including the tentatively bonding step and the completely bonding step will be explained with reference to  FIG. 6 . As described above, firstly the semiconductor  10  is tentatively bonded to the circuit board  20  by the positioning bumps  13  (Step  80 ). Then, a calibration step is performed. Namely, the drive mechanism control section  69  controls the drive mechanism  68  so as to move the imaging unit  64  to a position above the supporting table and scan image data of the semiconductor chip  10 , which has been tentatively bonded to the circuit board  20  (Step  82 ). If an infrared image scanner or camera is used as the imaging unit  64 , the bonded position of the semiconductor chip  10  can be detected as if it is seen fluoroscopically. 
   In Step  84 , the image processing section  66  measured the displacement of the semiconductor chip  10  with respect to the regular position on the circuit board  20  on the basis of the image data sent from the imaging unit  64 . 
   Next, the main controller  70  calculates correction data, which will be used for correcting actions of the alignment mechanism  60 , the ultrasonic wave oscillator  54  and the press mechanism  56 , on the basis of the displacement measured in Step  84  (Step  86 ). Then, the semiconductor chip  10  is completely bonded to the circuit board  20  (Step  88 ). 
   The correction data calculated in Step  86 , e.g., the displaced distance and the displaced direction of the semiconductor chip  10  with respect to the regular position, are fed back to correctly position the semiconductor chip  10 . With this feedback control, the semiconductor chip  10  can be bonded at regular position on the circuit board  20  in the completely bonding step (Step  88 ). 
   There are several means to correct the displacement of the semiconductor chip  10 . For example, the supporting table, which constitutes the alignment mechanism  60  and supports the circuit board  20 , may be slightly moved in X- Y- and θ-directions to correct the displacement and bond at the regular position; inclination of the supporting table may be controlled to correct the displacement in the completely bonding step; a load applied to the semiconductor chip  10  may be controlled; and ultrasonic vibration may be asymmetrically applied to the semiconductor chip  10  to correct the displacement. Further, a plurality of above means may be combined. Note that, the word “asymmetrically” means that a speed of ultrasonic vibrations in one direction is greater than that in the other direction. Namely, a range of ultrasonic vibrations in one direction is much greater than that in the other direction. 
   In this method, the semiconductor chip  10  is mounted on the circuit board  20  by the two bonding steps: the tentative bonding step and the completely bonding step, the displacement of the semiconductor chip  10  with respect to the regular position on the circuit board  20  is measured in the tentative bonding step, and the displacement is corrected by the feedback control in the completely bonding step. Therefore, the semiconductor chip  10  can be flip-chip-bonded to the circuit board  20  with high accuracy. Since the high precision mounting can be realized, yield of the flip chip bonding can be improved and very small electronic components, which have been difficult to mount by flip chip bonding, can be securely mounted. 
   As described in background of the invention, the displacement of the semiconductor chip is increased with increasing frequency of ultrasonic vibrations. However, in the present invention, the semiconductor chip  10  can be securely mounted on the circuit board  20  with high mounting accuracy even if the frequency of ultrasonic vibrations is increased. By increasing the frequency of ultrasonic vibrations, required time for flip chip bonding can be shortened, so the secure flip chip mounting can be efficiently performed. 
   The invention may be embodied in other specific forms without departing from the spirit of essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.