Abstract:
The present invention relates to a method and a device for wetting the bumps of a semiconductor chip with a soldering flux, in which a container, which accommodates the soldering flux and is open on the bottom, and a base plate, which contains at least one cavity, are moved back-and-forth in relation to one another on one side of the cavity to the other side of the cavity. The viewed in the movement direction front wall of the container is lifted up during the relative movement, so that it is located at a distance above the base plate. The distance is somewhat greater than the height difference by which the soldering flux projects above the level of the surface of the base plate. This measure has the effect that the front wall of the container does not convey any soldering flux onto the base plate, which has caused the loss of this soldering flux until now.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is related to and claims priority of the PCT patent application no. PCT/EP2007/059545 entitled “Method And Device For Wetting Bumps Of A Semiconductor Chip With Soldering Flux”, filed Sep. 11, 2007, which in turn claims priority of Swiss patent application no. 1506/06, filed on Sep. 21, 2006, the disclosure of which is herein incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a method and a device for wetting the bumps of a semiconductor chip with soldering flux. 
       BACKGROUND OF THE INVENTION 
       [0003]    For mounting semiconductor chips a technology is widespread in which the semiconductor chips are initially attached to a substrate using a so-called die bonder. Subsequently, the electrical connection areas of the semiconductor chip are wired to the substrate using a so-called wire bonder. Another technology is flip chip technology, in which the connection areas of the semiconductor chip are provided with a so-called bump. During mounting on a substrate, the semiconductor chip is turned over, which is referred to in technical jargon as the “flip”. The bumps of the semiconductor chip are then wetted using a soldering flux. For this purpose, the bumps of the semiconductor chip are immersed in a cavity filled with soldering flux, for example. Subsequently, the semiconductor chip is placed on the substrate, the bumps contacting electrical connection areas of the substrate. The semiconductor chip and the substrate are then soldered in a furnace. 
         [0004]    A device for wetting the bumps of a semiconductor chip with a soldering flux is known from U.S. Pat. No. 6,293,317. This device contains a base plate having at least one cavity, which is filled with soldering flux and is refilled by the back-and-forth movement of a container, which is open on the bottom, after each wetting of the bumps of a semiconductor chip. The level of the mean soldering flux layer in the cavity is typically in a range from 0 to 200 micrometers. 
         [0005]    Further devices for wetting the bumps of a semiconductor chip are known from EP 789391, WO 01/35709, and JP 8-340175. 
         [0006]    All of these devices share the disadvantage that the loss of soldering flux is relatively large. 
         [0007]    The present invention is based on the object of developing a device of this type whose loss of soldering flux is significantly less. 
       SHORT DESCRIPTION OF THE INVENTION 
       [0008]    The present invention relates to a device in which a container receiving the soldering flux, which is open on the bottom, and a base plate, which contains at least one cavity, are moved back-and-forth in relation to one another from one side of the cavity to the other side of the cavity. The named object is achieved by a method in which the front wall of the container, viewed in the movement direction, is lifted up during the relative movement, so that it is located at a distance A above the base plate. The distance A is somewhat greater than the height difference by which the soldering flux projects beyond the level of the surface of the base plate. Because this height difference is typically only a few micrometers, the front wall of the container also has to be lifted up only very little. The distance A is typically in the range from 20 to 200 micrometers. This measure brings about that the front wall of the container does not convey any soldering flux out of the cavity onto the base plate, which until now has caused the loss of this soldering flux. 
         [0009]    According to the present invention, a device capable of performing the method has means for lifting up the front wall of the container viewed in the movement direction. The container is preferably mounted in such a way that the friction arising during the relative movement between the container and the base plate exerts a torque on the container, so that the container automatically tilts around the lower edge, which rests on the base plate, of the rear wall of the container viewed in the movement direction during the relative movement. On the other hand, an additional active drive may be provided, which tilts the container around the lower edge of the rear wall in each case before the beginning of the movement. The active drive may be of a mechanical, electromechanical, pneumatic or hydraulic nature. 
         [0010]    Exemplary embodiments of devices which are capable of performing the method according to the present invention are explained in greater detail in the following on the basis of the drawing. The figures are not to scale. 
     
    
     
       DESCRIPTION OF THE DRAWING FIGURES 
         [0011]      FIGS. 1 ,  2  show a top view and a lateral view of a device for wetting the bumps of a semiconductor chip according to CH 694634, 
           [0012]      FIG. 3  shows a base plate of this device having a cavity, 
           [0013]      FIG. 4  shows a first exemplary embodiment of a device for wetting the bumps of a semiconductor chip according to the present invention, 
           [0014]      FIG. 5  shows a second exemplary embodiment of a device according to the present invention, 
           [0015]      FIG. 6  shows a third exemplary embodiment of a device according to the present invention, and 
           [0016]      FIG. 7  shows a fourth exemplary embodiment of a device according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIGS. 1 and 2  show a top view and a lateral view of a device  1 , known from CH 694634, for wetting the bumps of a semiconductor chip with soldering flux which may have the form of a liquid substance, electrically conductive epoxy, or soldering paste. The device  1  comprises an oblong base plate  2 , into which at least one cavity  3  is incorporated, and a container  4 , which is open on the bottom, for accommodating the liquid substance. In operation, the container  4  slides back-and-forth at a predefined velocity on the base plate  2  between two positions P 1  and P 2 , which are located on the left and right of the cavity  3 . The container  4  has two walls  5  and  6 , which alternately represent the front or rear wall in relation to the movement direction. 
         [0018]    The container  4  is driven using a slide, for example, to which the container  4  is removably attached. The slide comprises a bottom and a top slide part  8   a  and  8   b.  The container  4  has two pins  10 , which are mounted in a circular recess in the top slide part  8   b.  The top slide part  8   b  is drawn by means of a spring against the bottom slide part  8   a  in the direction toward the base plate  2 , so that the lower edge of the container  4  is pressed against the base plate  2  with a predetermined force. The slide  8  itself is moved back-and-forth using a pneumatic drive (not shown), for example, along a guide rail  9  running parallel to the base plate, thereby moving also the container  4  sliding on the base plate  2 . 
         [0019]      FIG. 3  shows the cavity  3  filled with the soldering flux  12 . The cavity  3  has a depth t. The soldering flux  12  extends along the circumference of the cavity  3  up to the upper edge  13  of the cavity  3 . It frequently occurs that the soldering flux projects beyond the level of the surface of the base plate  2  enclosing the cavity because of surface tension. The cavity  3  is filled homogeneously with flux  12 , the viscosity of the flux  12  not playing a role in a broad range from at least 8 to 45 Ns/m 2  (8000 to 45000 cp). Therefore, all bumps are uniformly wetted with soldering flux upon immersion of a semiconductor chip provided with bumps in the cavity  3 . 
         [0020]    The present invention may be implemented in a device of this type in that the drive of the container  4  is altered in such way that the front wall of the container  4  viewed in the movement direction is lifted up somewhat during the back-and-forth movement, so that the lower edge of the front wall is moved over the base plate  2  at a distance above the surface of the soldering flux. The lower edge of the rear wall of the container  4  contacts the base plate  2  and slides on the surface of the base plate  2 , so that it strips the soldering flux uniformly like a spatula. 
         [0021]    In the following, various solutions are shown for how the front wall of the container  4  may be lifted up during the movement along the base plate  2 . A steel plate is frequently used as the base plate  2 , which has been milled and mechanically processed from solid material to obtain an even surface having the required fineness. The cavity is subsequently formed by milling. Instead of such a base plate  2 , however, a carrier plate may also be used, into which a steel sheet having an incorporated cavity, preferably produced by etching, is inserted. Therefore, the term base plate is also to be understood to mean such a steel sheet. 
       Example 1 
       [0022]    In this example, the drive known from CH 694634 is modified in such a way that the drive exerts a torque on the container  4 , so that the front wall is automatically lifted up from the base plate  2  during the movement. As described above, the container  4  is driven by the slide formed by the bottom and the top slide parts  8   a  and  8   b,  in which the container  4  is removably mounted. The modifications of the drive are obvious from  FIG. 4 , which shows the modified drive in a lateral view. A plate  14  (not visible in  FIG. 4 , but implemented as in the example of  FIG. 5 ), is attached in each case to the two side walls, which projects downward laterally adjacent the base plate  2 . The plates  14  contain pins  10 , which engage in a recess  15  in the top slide part  8   b  situated below the level of the top side of the base plate  2 . The spring pulls the top slide part  8   b  downward against the guide rail  9 . When the bottom slide part  8   a  is moved along the guide rail  9 , the top slide part  8   b  exerts a force directed in the movement direction on the plates  14 . This force causes the container  4  to also perform the movement of the slide on one hand, and generates a torque acting on the container  4  on the other hand, which causes the container  4  to be tilted around the lower edge of the rear wall. Because the rear wall rests on the base plate  2 , the front wall lifts up from the base plate  2 . The strength of the torque and thus the degree of tilting is essentially a function of three factors, namely the distance of the pins  10  from the top side of the base plate  2 , the strength of the force of the spring, and the viscosity of the soldering flux. The distance of the lower slide part  8   a  to the guide rail  9  is therefore advantageously adjustable. If this distance is increased, the force exerted by the spring increases and thus also the torque. Pins  10  and recess  15  may be exchanged, of course, i.e., the pins  10  may also be fastened to the slide part  8   b  and the recess  15  may be applied in the plates  14 . 
       Example 2 
       [0023]    This example is based on the preceding example, but the lower edges of the walls  5  and  6  of the container  4 , which are each alternately the front or rear wall, are implemented in such a way that the container  4  comes to rest on a surface  16  upon tilting.  FIG. 5  shows a possible solution. If the drive moves the container  4  in the direction identified by the arrow  19 , the force being transmitted at the pin  10 , the wall  6  is the front wall and the wall  5  is the rear wall. The lower edge of such a wall  5 ,  6  comprises an inner edge  17  and an outer edge  18 , which delimit the cited surface  16 . In the rest state, the container  4  lies on the inner edges  17  of the front and rear wall. The surface  16  runs slightly diagonally upward at a predetermined angle a originating from the inner edge  17  to the outer edge  18 , so that the outer edge  18  does not contact the base plate  2  in the rest state of the container  4 . During the movement, the container  4  tilts as a result of the force engaging at the pin  10  and the torque thus generated around the inner edge  17  of the rear wall. If the generated torque exceeds a first value M 1 , the container  4  tilts so far that it comes to rest on the surface  16 . This surface  16  is so wide that the container  4  only tilts around the outer edge  18  if the torque exceeds a second value M 2 &gt;M 1 . With this design of the lower edge, the distance of the lower edge of the front wall from the base plate  2  is independent of the torque in the torque range from M 1  to M 2 . Such a design offers the advantage that it provides a robust operating range in the event of changing operating conditions and unpredictably changing external conditions, within which the front wall of the container  4  is lifted up by a precisely defined distance from the top side of the base plate  2 .  FIG. 5  is not drawn to scale, in particular, the angle α is illustrated much larger than it actually is. 
       Example 3 
       [0024]    In this example, the force generated by the drive engages above the base plate  2 . A top slide part  8   b  is also provided here.  FIG. 6 , which is not drawn to scale, shows the front and rear walls  6 ,  5  of the container  4  in a sectional view. The section plane runs perpendicularly to the base plate  2  and parallel to the movement direction of the container  4  represented by an arrow  19 . The front and the rear walls have a lower edge which is narrower than the thickness of the wall. The walls have a surface  21  running diagonally upward on their exterior side  20 . The top slide part  8   b  has surfaces diametrically opposite this surface  21  having the same angle of inclination. The top slide part  8   b  additionally contains a projection  23  having a groove  24 , into which the top end of the front or rear wall projects. If the slide is moved in the direction of the arrow, the top slide part  8   b  exerts a torque on the container  4 , which causes the container  4  to tilt around the lower edge of the rear wall. The top end of the front wall comes to a stop on the delimitation surface of the groove  24 . In this example as well, the distance of the lower edge of the front wall from the base plate  2 , which results during the back-and-forth movement, is largely independent of the operating conditions. 
       Example 4 
       [0025]    In this example, the container  4  is tilted around the rear edge using an active drive. The active drive may be of a mechanical, electromechanical, pneumatic or hydraulic nature. In this example, the drive is of a mechanical nature.  FIG. 7 , which is not drawn to scale, again shows the top slide part  8   b,  which is expanded by a simple mechanical drive  25 , which is attached to the top slide part  8   b . The top slide part  8   b  contains two guides  26 , into which the top end of the front and the rear walls  6 ,  5  (in relation to the movement direction) of the container  4  project. A first rod  27  is fastened to the top end of the front wall and a second rod  28  is fastened to the top end of the rear wall via a joint. The other ends of the two rods  27  and  28  are also attached via a joint to an eccentric  29 . In the position shown, the eccentric  29  presses the rear wall  5  of the container  4  against the base plate  2  and pulls the front wall  6  of the container  4  away from the base plate  2 . Upon the direction change of the container  4  at the locations P 1  and P 2  ( FIG. 1 ), the eccentric  29  is rotated into another position, so that it now presses the particular other wall of the container  4  against the base plate  2  or lifts it therefrom, respectively. 
       Further Examples 
       [0026]    In the devices described on the basis of  FIGS. 4 through 7 , the base plate  2  is situated fixed in place and the slide is used as a drive to move the container  4  back-and-forth. However, it is also possible for all of these devices to fix the slide in place and to move the base plate  2  in relation to the container  4  using a drive. If the slide is fixed in the device shown in  FIG. 4  and the drive moves the base plate  2  back-and-forth, the slide is used on the one hand as a mechanism to be able to take out the container easily. On the other hand, the friction force arising between the base plate  2  and the container  4  during the movement of the base plate  2  generates a torque acting on the container  4 , because the coupling point between the slide part  8   b  and the container  4  (in the form of the recess  15  and the pin  10 ) lies below the level of the base plate  2 . The torque causes the container  4  to tilt around the lower edge of the rear wall, viewed in the movement direction of the base plate  2 . 
         [0027]    This is also the case for the devices shown in  FIGS. 5 and 6 , if the base plate  2  is moved back-and-forth and the container is situated fixed in place. Here as well, the friction arising between the base plate  2  and the container  4  causes a torque which tilts the container  4  around the rear wall, viewed in the movement direction of the base plate  2 . 
         [0028]    The container  4  may also be situated fixed in place and the base plate  2  may be moved back-and-forth in the device shown in  FIG. 7 . 
         [0029]    The device according to the present invention offers multiple advantages:
   The loss of soldering flux is much less than in the prior art.   The wear of the lower edges of the two walls is halved.