Abstract:
The invention provides a heater for flip chip bonding. The heater transfers more heat to the periphery of a die than to the center. This results in a more even temperature profile along the die and helps prevent epoxy voiding problems.

Description:
BACKGROUND  
     BACKGROUND OF THE INVENTION  
       [0001]      FIG. 1   a  is a cross sectional side view of a heating assembly  100  to attach a die  106  to a substrate  112 . A heating block  102  generates heat. A heat nozzle  104  transmits heat from the heating block  102  to a die  106 . The die  106  is positioned above a substrate  112 . Solder bumps  110 , once melted by the heat applied to the die  106  and then cooled, attach the die  106  to the substrate  112 . Underfill material  108 , such as epoxy resin, substantially fills the areas between the die  106  and the substrate  112 .  
         [0002]      FIG. 1   b  is a graph that shows the heat generated by different areas of the heating block  102 . The heating block  102  generates heat in a substantially uniform manner, as shown by the graph in  FIG. 1   b.  Heat put out at one point of the heating block&#39;s  102  surface is about equal to heat put out at another point of the heating block&#39;s  102  surface.  FIG. 1   c  is a graph that shows the thermal conductivity of the heat nozzle  104 . As shown in the graph of  FIG. 1   c,  the thermal conductivity of the heat nozzle  104  is substantially the same from the left edge  114  of the die to the right edge  116  of the die.  FIG. 1   d  illustrates a graph that shows the temperature of the solder bump  110  and the underfill material  108  beneath the die  106  that results from the heat generated by the heating block  102  as shown by the graph in  FIG. 1   b  and transmitted from the heating block  102  to the die  106  by the heat nozzle  104  as shown by the graph in  FIG. 1   c.  The graph in  FIG. 1   d  shows that the temperature at the die  106  is lower at the left  114  and right  116  edges of the die  106 , and has a higher temperature peak  118  approximately in the center.  
         [0003]     Since heat may be exchanged between the edges  114 ,  116  of the die  106  and the surrounding environment, some heat at the edges of the die is dispersed, leaving the center of the die  106  hotter. Applying enough heat to ensure that the temperature near the edges  114 ,  116  of the die  106  is hot enough to melt the solder bumps  110  to attach the die  106  to the substrate  112  may result in a higher peak  118  temperature near the center of the die  106  that may be too high and result in overheating the underfill material  108  and causing voids in the underfill  108  to occur.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1   a  is a cross sectional side view of a heating assembly.  
         [0005]      FIG. 1   b  is a graph that shows the heat generated by different areas of the heating block.  
         [0006]      FIG. 1   c  is a graph that shows the thermal conductivity of the heat nozzle.  
         [0007]      FIG. 1   d  illustrates a graph that shows the temperature of the underfill material beneath the die.  
         [0008]      FIG. 2  illustrates a graph that shows the temperature of underfill material between a die and a substrate from a left die edge to a right die edge according to one embodiment of the present invention.  
         [0009]      FIG. 3   a  is a cross sectional side view of one embodiment of a heating assembly that may provide more heat to the edges of a die than to the center of a die.  
         [0010]      FIG. 3   b  illustrates a graph that shows the heat generated by the heating block.  
         [0011]      FIG. 3   c  illustrates a graph that shows the thermal conductivity of the heat nozzle.  
         [0012]      FIG. 3   d  illustrates a graph that shows the temperature of the underfill material beneath the die.  
         [0013]      FIG. 3   e  is a cross sectional top view of a heating block that generates uneven heat according to one embodiment of the present invention.  
         [0014]      FIG. 3   f  is a cross sectional side view of one embodiment of a heating assembly that may provide more heat to the edges of a die than to the center of a die.  
         [0015]      FIG. 4   a  is a cross sectional side view of another embodiment of a heating assembly that may provide more heat to the edges of a die than to the center of a die.  
         [0016]      FIG. 4   b  illustrates a graph that shows the heat generated by the heating block.  
         [0017]      FIG. 4   c  illustrates a graph that shows the thermal conductivity of the heat nozzle.  
         [0018]      FIG. 4   d  illustrates a graph that shows the temperature of the underfill material beneath the die.  
         [0019]      FIG. 4   e  is a cross sectional top view of a heat nozzle that has a varying thermal conductivity according to one embodiment of the present invention.  
         [0020]      FIG. 4   f  is a cross sectional side view of one embodiment of a heating assembly that may provide more heat to the edges of a die than to the center of a die.  
     
    
     DETAILED DESCRIPTION  
       [0021]      FIG. 2  illustrates a graph  200  that shows the temperature of solder bumps (or other connectors) and underfill material between a die and a substrate from a left die edge  214  to a right die edge  216  according to one embodiment of the present invention. According to an embodiment, the underfill temperature may be substantially the same from the left die edge  214  through the middle of the die to the right die edge  216 . Since more heat is lost through the edges of a die than the center of the die, this temperature distribution may be achieved by applying more heat to the edges of the die than to the center of the die.  
         [0022]     In some embodiments, the graph  200  may not be flat as that shown in  FIG. 2 , and the temperature may vary some at various points along the die, but the highest temperature may still remain below a temperature that would cause voids in underfill to occur. For example, in one embodiment, the underfill material may comprise an epoxy resin. The temperature of the underfill may be hotter in the middle of the die than at the edges  214 ,  216 , but the temperature at the middle may remain lower than a temperature that would vaporize the underfill material to form voids. Various other embodiments may have a temperature that is cooler in the middle of the die than at the edges  214 ,  216 , temperatures that are coolest at the edges  214 ,  216 , hotter at a distance from the edges  214 ,  216 , and cooler again in the middle of the die, or other temperature distributions.  
         [0023]      FIG. 3   a  is a cross sectional side view of one embodiment of a heating assembly  300  that may provide more heat to the edges of a die than to the center of a die. The heating assembly  300  may include a heating block  202  and a heat nozzle  204 . The heating block  202  may be anything that can generate heat and transfer it to the heat nozzle  204 . In various embodiments, the heating block  202  may produce heat by passing through a conductive element that has resistance, by generating microwaves, through infrared radiation, or other methods. In an embodiment, the heating block  202  may be capable of heating itself, the heat nozzle  202 , and a die  206  to a temperature in a range from about 200 degrees Celsius to about 340 degrees Celsius at a rate in a range from about 10 degrees Celsius per second to about 50 degrees Celsius per second or higher. The heat nozzle  204  may receive heat generated by the heating block  202  and transmit that heat to a die  206 . In an embodiment, the heating block  202  and heat nozzle  204  may be two components that are coupled so that the heat nozzle  204  may receive the heat generated by the heating block  202  and transmit that heat to the die  206 . In another embodiment, the heating block  202  and heat nozzle  204  may comprise a single component. For example, the heating block  202  may be the part of the component that generates heat, and the heat nozzle  204  may be an area of the component adapted to transmit the heat to the die  206 . Together, the heating block  202  and the heat nozzle may be considered to comprise a heater.  
         [0024]     The die  206  may be an integrated circuit die such as a microprocessor. The die  206  may be positioned above a substrate  212 . Connectors, such as solder bumps  210  or other connectors, may be between the die  206  and the substrate  212 . The heater may operate to apply heat to the die  206 . This heat may melt the solder bumps  210 . When cooled, the solder bumps  210  may couple the die  206  to the substrate  212 . Underfill material  208  may substantially fill the areas between the die  206  and the substrate  212 . In an embodiment, the underfill material  208  may comprise an epoxy material.  
         [0025]     In the embodiment illustrated in  FIG. 3   a,  the heating block  202  may generate heat unevenly.  FIG. 3   b  illustrates a graph  350  that shows the heat generated by the heating block  202  in an embodiment. As shown in the graph  350 , the heating block  202  may generate more heat toward the edges  214 ,  216  of the die  206  than in the middle. The heat nozzle  204  may have a substantially uniform thermal conductivity, as shown by the graph  352  in  FIG. 3   c.  Since the heating block  202  may generate less heat in the middle of the die  206 , as shown by graph  350  in  FIG. 3   b,  the temperature of the underfill  208  may be substantially uniform from the left die edge  214  to the right die edge  216  as shown in graph  354  in  FIG. 3   d.  In other embodiments, the temperature of the solder bumps  210  (or other connectors) and the underfill  208  may not be substantially uniform as shown in graph  354 , but may vary somewhat between the left die edge  214  and the right die edge  216 . However, in an embodiment where the heating block  202  generates less heat in the middle of the die  206  as shown in graph  350  of  FIG. 3   b,  this variation may be less than in prior art systems, such as shown in  FIG. 1   d.  Generating less heat in the middle of the die  206  may result in the maximum underfill  208  temperature being low enough to substantially prevent formation of underfill voids.  
         [0026]      FIG. 3   e  is a cross sectional top view of a heating block  202  that generates uneven heat according to one embodiment of the present invention. In an embodiment, the heating block  202  may include a middle section  322  and a peripheral section  320 . The heating block  202  may generate more heat in the peripheral section  320  than in the middle section  322 . The die  206  may be positioned so that the middle section  322  is positioned over the middle of the die  206 . This may result in the heat generation graph  350  as shown in  FIG. 3   b,  and result in a more even solder bump  210  (or other connector) and underfill  208  temperature, as shown in the graph  354  of  FIG. 3   d.  The graph  354  indicates that the temperature of the solder bumps  210  (or other connectors) and the underfill  208  may be substantially uniform from the left die edge  214  to the right die edge  216  in an embodiment. In other embodiments, the temperature of the solder bumps  210  (or other connectors) and the underfill  208  may not be substantially uniform as shown in graph  354 , but may vary somewhat between the left die edge  214  and the right die edge  216 . However, this variation may be less than in prior art systems, such as shown in  FIG. 1   d.  While the illustrated embodiment includes a sharp boundary between the middle section  322  and the peripheral section  320 , in other embodiments there may be a gradual transition rather than a boundary. There may be progressively less heat generated at points closer to the middle of the heating block  202 .  
         [0027]      FIG. 3   f  is a cross sectional side view of one embodiment of a heating assembly  300  that may provide more heat to the edges of a die  206  than to the center of a die  206 . In the embodiment illustrated in  FIG. 3   f,  at least some heat that is generated by the heating block  202  is generated by heating elements  318 , which may comprise conductive or semi-conductive elements wherein heat is generated by the resistance of the heating element  318  as a current passes through it, within a matrix material of the heating block  202 . In an embodiment, there are more heating elements  318  per unit volume in the peripheral section  320  of the heating block  202  than in the middle section  322  of the heating block  202 . Since there are more heating elements  318  in the peripheral section  320 , the heating block  202  produces more heat in the peripheral section  320  than in the middle section  322 .  
         [0028]      FIG. 4   a  is a cross sectional side view of another embodiment of a heating assembly  400  that may provide more heat to the edges of a die  206  than to the center of a die  206 . The heating assembly  400  may be similar in most respects to the heating assembly  300  described above, and may include a heating block  402 , a heat nozzle  404 , a die  206 , a substrate  212 , solder bumps  210  or other connectors, and underfill material  208 . The heating block  402  may generate heat in a uniform manner, or in a non-uniform manner as described with respect to  FIG. 3   a.  The shape, size, and material of the heat nozzle  404  may vary. In an embodiment, the heat nozzle  404  may comprise a thermally conductive material such as silicon nitride, aluminum nitride, copper carbide, tungsten carbide, steel, or another material.  
         [0029]     In the embodiment illustrated in  FIG. 4   a,  the heat nozzle  404  may have a non-uniform thermal conductivity.  FIG. 4   b  illustrates a graph  450  that shows the heat generated by the heating block  402  in an embodiment. As shown in the graph  450 , the heating block  402  may generate heat in a substantially uniform manner. However, the heat nozzle  404  may have a non-uniform thermal conductivity, as shown by the graph  452  in  FIG. 4   c.  As shown by the graph  452  of  FIG. 4   c,  the heat nozzle  404  may have a higher thermal conductivity toward the edges  214 ,  216  of the die  206 , and a lower thermal conductivity toward the center of the die  206 . Less heat will be transmitted by the center of the heat nozzle  404  than the periphery. Such differences in thermal conductivity may mean that the temperature of the solder bumps  210  (or other connectors) and the underfill  208  may be substantially uniform from the left die edge  214  to the right die edge  216  as shown in graph  454  in  FIG. 4   d;  more heat is transmitted to the die  206  edges  214 ,  216  than to the die middle. In other embodiments, the temperature of the solder bumps  210  (or other connectors) and the underfill  208  may not be substantially uniform as shown in graph  454 , but may vary somewhat between the left die edge  214  and the right die edge  216 . However, in an embodiment where the heat nozzle  404  has a lower thermal conductivity in the middle, and therefore transmits less heat to the middle of the die  206 , this variation may be less than in prior art systems, such as shown in  FIG. 1   d.  Transmitting less heat to the middle of the die  206  may result in the maximum underfill  208  temperature being low enough to substantially prevent formation of underfill voids.  
         [0030]      FIG. 4   e  is a cross sectional top view of a heat nozzle  404  that has a varying thermal conductivity according to one embodiment of the present invention. In an embodiment, the heat nozzle  404  may include a middle section  422  and a peripheral section  420 . The heat nozzle  404  may have a higher thermal conductivity in the peripheral section  420  than in the middle section  422 . The die  206  may be positioned so that the middle section  422  is positioned over the middle of the die  206 . This may result in less heat being transmitted to the middle of the die  206 , and result in a more even underfill temperature, as shown in the graph  454  of  FIG. 4   d.  While the illustrated embodiment includes a sharp boundary between the middle section  422  and the peripheral section  420 , in other embodiments there may be a gradual transition rather than a boundary. There may be a progressively lower thermal conductivity at points closer to the middle of the heat nozzle  404 .  
         [0031]      FIG. 4   f  is a cross sectional side view of one embodiment of a heating assembly  400  that may provide more heat to the edges of a die  206  than to the center of a die  206 . In the embodiment illustrated in  FIG. 4   f,  there may be a cavity  424  in the middle section of the heat nozzle  404 . Such a cavity  424  means that the middle of the heat nozzle  404  is not in direct contact with the die  206  surface, creating an air gap that reduces the conduction of heat from the heat nozzle  404  to the die  206 . In an embodiment, this cavity  424  may be a portion of a spherical shape. The maximum distance from the die to the surface of the heat nozzle  404 , at the center of the spherical cavity  424 , may be in a range from about several microns to about 100 microns. The cavity may extend to about two-thirds of the surface of the heat nozzle  404  in an embodiment. Since the presence of the cavity  424  reduces conduction of heat to the center of the die  206 , the heat nozzle  404  transmits more heat in the peripheral section  420  than in the middle section  422 . This may result in a more uniform temperature in the solder bump  210  (or other connector) and the underfill material  208 , as illustrated in  FIGS. 2, 3   d,  and  4   d.  In other embodiments, the peripheral section  420  of the heat nozzle  404  may comprise different materials than the middle section  422 . For example, different materials in the peripheral section  420  and the middle section  422  may be deposited, laminated, or sintered together to form the heat nozzle  404  with varying thermal conductivities.  
         [0032]     In other embodiments, various combinations of heating blocks  202  that produce different amounts of heat in different areas and heat nozzles  404  that have non-uniform thermal conductivities may be used in a heating assembly. These different combinations can be used to ensure a more uniform temperature in the solder bumps  210  (or other connectors) and the underfill material  208 , as illustrated in  FIGS. 2, 3   d,  and  4   d.    
         [0033]     The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. This description and the claims following include terms, such as left, right, top, bottom, over, under, upper, lower, first, second, etc. that are used for descriptive purposes only and are not to be construed as limiting. The embodiments of a device or article described herein can be manufactured, used, or shipped in a number of positions and orientations. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above teaching. Persons skilled in the art will recognize various equivalent combinations and substitutions for various components shown in the Figures. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.