Patent Document

FIELD OF THE PRESENT INVENTION 
       [0001]    This invention relates to an apparatus for heating a substrate during die bonding. In particular, the apparatus includes a heating device for heating a plurality of bond pads on the substrate before semiconductor dies are bonded thereto. The invention also relates to a method of heating a substrate during die bonding. 
       BACKGROUND OF THE INVENTION 
       [0002]    Eutectic die bonding requires heating a die solder layer or a die solder bump of a semiconductor die to a required temperature, before the semiconductor die is bonded to a substrate (e.g. a lead frame) via an interface between the heated die solder layer or bump and a substrate bond pad. The substrate is typically placed on a heated anvil to transfer heat from the heated anvil through the heat-conducting substrate to the substrate, to thereby heat the die solder layer or bump to the required temperature for eutectic die bonding. 
         [0003]      FIG. 1  shows a conventional indexing apparatus  100  for heating a substrate  102 , which comprises an array of substrate bond pads (not shown) positioned relative to a die collet  104  of a die bonder for bonding along a bonding line  106 . Specifically, the indexing apparatus  100  comprises: i) an anvil  108  for supporting the substrate  102 ; and ii) a workholder base  110  for supporting the anvil  108 . In particular, the anvil  108  is maintained at a uniform temperature (e.g. 280° C. if the die solder layer or bump is made up of a gold-tin composite) throughout the eutectic die bonding process to heat the substrate bond pads. The apparatus  100  is also configured to index the substrate  102  along direction A shown in  FIG. 1 , so that successive rows of the substrate bond pads are aligned with the bond line  106  to receive semiconductor dies from the die collet  104 . Since the entire substrate  102  is placed on the heated anvil  108  throughout the eutectic die bonding process, a pre-bond heating time and a post-bond heating time for each substrate bond pad will thus vary depending on its location on the substrate  102 . This means that the substrate bond pads that receive the semiconductor dies earlier than other substrate bond pads would have a shorter pre-bond heating time and longer post-bond heating time. This, however, increases the likelihood of internal damage of thermally vulnerable semiconductor dies, such as LED devices, during eutectic bonding. 
         [0004]    Thus, it is an object of the present invention to seek to propose an apparatus for indexing a substrate that at least reduces the likelihood of damage to semiconductor dies during die bonding. 
       SUMMARY OF THE INVENTION 
       [0005]    A first aspect of the present invention is an apparatus for heating a substrate during die bonding. The apparatus comprises: a substrate carrier configured to hold the substrate; a heating device configured to heat the substrate; a first actuator for effecting relative motion between the substrate carrier and the heating device such that the substrate is relatively indexed with respect to the heating device; a second actuator for effecting relative motion between the substrate carrier and the heating device such that the heating device contacts the substrate to heat different portions of the substrate. In particular, the second actuator is operative to separate the heating device from the substrate in order for the first actuator to relatively index the substrate across the heating device. 
         [0006]    Some preferred but optional features of the apparatus are defined in the dependent claims. 
         [0007]    A second aspect of the invention is a method of heating a substrate during die bonding. The method comprises the steps of: contacting a heating device against the substrate to heat at least a portion of the substrate; separating the heating device from the substrate; and indexing the substrate relatively across the heating device after the heating device has been separated from the substrate. 
         [0008]    Some preferred but optional steps of the method are also defined in the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: 
           [0010]      FIG. 1  shows a conventional indexing apparatus for heating a substrate during die bonding; 
           [0011]      FIG. 2  shows an apparatus for heating a substrate during die bonding, according to an embodiment of the invention; 
           [0012]      FIG. 3  is a top view of the apparatus of  FIG. 2  without showing a top cover, while  FIGS. 3   b  and  3   c  are different side views of the apparatus of  FIG. 2  showing the top cover; 
           [0013]      FIG. 4  shows a simplified version of the apparatus of  FIG. 2 ; 
           [0014]      FIGS. 5   a  and  5   b  are top and side views of the apparatus of  FIG. 4  respectively; and 
           [0015]      FIGS. 6   a - 6   d  show an operation of the apparatus of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]      FIG. 2  shows a substrate-indexer  200  for indexing a substrate along an indexing direction B during die bonding. Specifically, the substrate-indexer  200  comprises: i) a substrate carrier  202  configured to hold a substrate; ii) a heating device (shown as a heating anvil block  206 ) configured to heat the substrate; iii) a cooling device (shown as a cooling anvil block  208 ), arranged adjacent to the heating anvil block  206 , for cooling the substrate  204 ; and iv) a positioning device—in the form of first and second actuators—configured to index the substrate carrier  202  along direction B through a plurality of indexing positions first across the heating anvil block  206 , and subsequently, across the cooling anvil block  208 . In particular, the positioning device is operative to separate the substrate carrier  202  from the heating and cooling anvil blocks  206 ,  208 , before the substrate carrier  202  is indexed to a new position. The positioning device is also operative to press the heating anvil block  206  and/or the cooling anvil  208  against the substrate  204  at the plurality of indexing positions, so as to achieve the desired effects of heating and/or cooling different portions of the substrate  204  during die bonding. 
         [0017]    To index the substrate carrier  202  through the indexing positions, the first actuator includes: i) a horizontal linear guide  210  for guiding the substrate carrier  202 ; ii) an indexing mechanism  212 , which is coupled to the horizontal linear guide  210  and the substrate carrier  202 , for indexing the substrate  204  along the indexing direction B on a horizontal plane; iii) and a linear motor  214  operative to drive the indexing mechanism  212 —and, thereby, the substrate carrier  202 —along the horizontal linear guide  210 . 
         [0018]    To separate the substrate carrier  202  from the heating and cooling anvil blocks  206 ,  208 , the second actuator includes: i) a movable anvil platform  216  coupled to the heating and cooling anvil blocks  206 ,  208 ; ii) and a motor  217  for driving the anvil platform  216  vertically between a top position to press the heating and/or cooling anvil blocks  206 ,  208  against different portions of the substrate  204 , and a bottom position to separate the heating and/or cooling anvil blocks  206 ,  208  from the substrate  204 . 
         [0019]    Moreover, the substrate carrier  202  is connected to a shielding gas supply (not shown) and the substrate carrier  202  comprises carrier openings for introducing a shielding gas (e.g. an inert gas or a forming gas) onto the substrate  204  during die bonding. By introducing the shielding gas, the substrate  204  may be protected against oxidation and stronger bonds may be formed between the semiconductor dies and the respective substrate bond pads. To retain the volume of the shielding gas that envelopes the substrate  204 , the substrate indexer  200  additionally comprises a top cover  222  for retaining the shielding gas within a space between the top cover  222  and the substrate carrier  202 . 
         [0020]      FIG. 3   a  is a top view of the substrate-indexer  200  without showing the top cover  222 . It is seen that each of the opposite sides of a surface of the substrate carrier  202  parallel to the indexing direction B comprises a plurality of carrier openings  220  for introducing the shielding gas onto the substrate  204  during die bonding. In particular, the surface of the substrate carrier  202  is the surface for holding the substrate  204 . 
         [0021]      FIGS. 3   b  and  3   c  are different side views of the substrate-indexer  200  showing the top cover  222 . It can be seen that the top cover  222  retains the shielding gas  301  within the space  224  between the top cover  222  and the substrate carrier  202 . This retains the volume of shielding gas  301  that envelopes the top surface of the substrate  204 , and inhibits the ingress of ambient air into the space  224 . Advantageously, oxidation of the substrate  204  and the semiconductor dies bonded thereto can be minimised. 
         [0022]    It is also seen from  FIG. 3   b  that the substrate carrier  202  includes clamps  218  for clamping the substrate  204 . By securing the substrate  204  in place as the anvil platform  216  is driven to press the heating and/or the cooling anvil blocks  206 ,  208  against different portions of the substrate  204 , the desired effects of heating and/or cooling different portions of the substrate  204  could be more effectively achieved. 
         [0023]    Optionally, the top cover  222  may include a slot, which is aligned with successive rows of the substrate bond pads as the substrate carrier  202  holding the substrate  204  is driven by the indexing mechanism  212  through the plurality of indexing positions. By providing the slot in the top cover  222 , a die collet of a die bonder is movable through the top cover to reach the corresponding row of substrate bond pads for bonding thereto. Hence, the slot of the top cover  222  is aligned with a bonding line of the die bonder. 
         [0024]      FIG. 4  shows a simplified version of the substrate-indexer  200 , comprising: i) the heating anvil block  206  for heating the substrate  204 ; ii) the cooling anvil block  208  for cooling the substrate  204 ; and iii) the substrate carrier  202  for indexing the substrate  204  first across the heating anvil block  206 , and subsequently, across the cooling anvil block  208  in the indexing direction B. 
         [0025]      FIGS. 5   a  and  5   b  are top and side views of the substrate-indexer  200  of  FIG. 4  respectively. To minimise unnecessary heat transfer between the heating and cooling anvil blocks  206 ,  208 , an insulation medium  500  is provided between them. Examples of the suitable insulation media  500  include silica, ceramic fiber, and air. 
         [0026]    In addition, it is seen from  FIGS. 5   a  and  5   b  that the substrate-indexer  200  is configured such that the bonding line  300  is positioned above the heating anvil block  206 , and in close proximity to the cooling anvil block  208 . It should be noted that the bonding line  300  is fixed and does not vary throughout the die bonding process. This advantageously simplifies the construction and operation of the die bonder. 
         [0027]      FIGS. 6   a  to  6   d  show an operation of the substrate-indexer  200 . 
         [0028]      FIG. 6   a  shows the heating and cooling anvil blocks  206 ,  208  being driven to a top position to press against respective base portions of the substrate  204 . 
         [0029]    At this relative position between the substrate carrier  202  and the heating and cooling anvil blocks  206 ,  208 , the bonding line  300  is aligned with a particular row of the substrate bond pad array. The substrate bond pads belonging to this particular row of substrate bond pad array would have already been subject to a desired duration of pre-bond heating by the heating anvil block  206  in the course of the substrate  204  being indexed by the indexing mechanism  212  across the heating anvil block  206  through a plurality of indexing positions. Accordingly, the temperature of each of these substrate bond pads would have already been elevated to the required temperature T1 (e.g. 280° C. if the die solder layer or bump is made of a gold-tin composite) at the time of bonding. 
         [0030]    Regarding the rows of substrate bond pads located to the right of the bonding line  300  with reference to  FIG. 6   a , whether they have been sufficiently heated by the heating anvil block  206  for die bonding depends on their respective distances from the bonding line  300 . Whereas one or more successive rows of substrate bond pads located to the right of the bonding line  300  may have reached the required temperature T1 for die bonding, one or more successive rows of the substrate bond pads from the extreme right of the substrate  204  in  FIG. 6   a  may require further heating by the heating anvil block  206  to reach the required temperature T1 for die bonding. 
         [0031]    Likewise, whether the rows of substrate bond pads located to the left of the bonding line  300  with reference to  FIG. 6   a  have been sufficiently cooled by the cooling anvil block  208  after die bonding depends on their respective distances from the bonding line  300 . Whereas one or more successive rows of substrate bond pads further away from the heating anvil block  206  may have already been sufficiently cooled to a post-bond cooling temperature T2 such that T2&lt;T1 (e.g. below 200° C.) by the cooling anvil block  208 , one or more successive rows of substrate bond pads nearer to the heating anvil block  206  may require further cooling by the cooling anvil block  208  to reach the required post-bond cooling temperature T2. 
         [0032]      FIG. 6   b  shows that after the bonding of semiconductor dies has been completed along that particular row of substrate bond pads, the heating and cooling anvil blocks  206 ,  208  are driven to a bottom position to separate from the substrate carrier  202 —and, hence, the substrate  204 . This thus terminates contact between the substrate  204  and the heating and cooling anvil blocks  206 ,  208 . 
         [0033]    Subsequently, the substrate carrier  202  is indexed by the indexing mechanism  212  by a distance of a bond pad pitch d along the indexing direction B shown in  FIG. 6   c . Thus, the substrate  204  that is held by the substrate carrier  202  is accordingly indexed through a distance of one bond pad pitch d. It should be appreciated that this distance of one bond pad pitch d corresponds to the distance between adjacent rows of substrate bond pads of the substrate  204 . 
         [0034]    After the substrate carrier  202  has been indexed by the distance of one bond pad pitch d, the heating and cooling anvil blocks  206 ,  208  are driven back to the top position to press against the respective base portions of the substrate  204 , as shown in  FIG. 6   d . This aligns the bonding line  300  with another row of substrate bond pads for die bonding. 
         [0035]    It should be appreciated the cycle of the substrate-indexer&#39;s  200  operation described with reference to  FIGS. 6   a - 6   d  is successively performed, starting with an initial position of the substrate carrier  202  at the right of the heating anvil block  206  to heat a first row of substrate bond pads, and ending with a final position of the substrate carrier  202  at the left of the cooling anvil block  208  to cool a final row of substrate bond pads. 
         [0036]    By using the substrate-indexer  200  to index the substrate  204  during die bonding, the pre-bond heating time of each substrate bond pad is kept consistent for heating to the desired temperature T1 (e.g. 280° C. if the die solder layer or bump is made of a gold-tin composite) for die bonding. Similarly, the post-bond cooling time of each substrate bond pad is also kept consistent for cooling the substrate bond pad to a temperature T2 such that T2&lt;T1 (e.g. below 200° C.) after a semiconductor die has been bonded thereto. Thus, the substrate-indexer  200  provides control of a temporal temperature profile for each of the substrate bond pads. This advantageously reduces the likelihood of damage to the internal mechanical structure of the semiconductor dies, as may be the case with the conventional indexing apparatus  100  in which the entire substrate  102  is placed on a heated anvil throughout the entire die bonding process. Moreover, by separating the heating and cooling anvil blocks  206 ,  208  from the substrate carrier  202  before indexing the substrate carrier  202  to a new position, the substrate-indexer  200  also prevents the substrate  204  from being scratched by the heating and cooling anvil blocks  206 ,  208 , especially if the substrate  204  is made of ceramic material. In addition, the substrate carrier  202  advantageously provides a convenient means of indexing the substrate  204  across the heating and cooling anvil blocks  206 ,  208 , compared with the use of indexing pins. This is because the use of indexing pins is usually package-specific, and their construction is typically complex in terms of various design parameters such as the pitch, the position and the stroke of the indexing pins. 
         [0037]    The substrate-indexer  200  is particularly, but not exclusively, applicable for indexing the substrate  204  for eutectic die bonding. Various embodiments of the substrate-indexer  200  can be envisaged without departing from the scope of the claimed invention. For instance, the substrate-indexer  200  may comprise a plurality of heating anvil blocks  206  at different heating temperatures for heating the substrate  204 , and/or a plurality of cooling anvil blocks  208  at different cooling temperatures for cooling the substrate  204 . In addition, cooling fluids (e.g. cooling air or cooling water) may be used in place of the cooling anvil block  208  for cooling the substrate  204 . Moreover, the positioning device of the substrate-indexer  200  may instead be configured to index the heating and cooling anvil blocks  206 ,  208  in an opposite direction to the indexing direction B, and to move the substrate carrier  202  vertically between top and bottom positions during die bonding. In this instance, however, the position of the bonding line  300  is not fixed but has to move together with the heating and cooling anvil blocks  206 ,  208  as the semiconductor dies are bonded to successive rows of the substrate bond pads.

Technology Category: 5