Abstract:
An apparatus comprises a substrate including a surface and a plurality of bonding pads positioned on the surface. The apparatus also includes a material comprising a solder positioned on the bonding pads and extending a distance outward therefrom. A first of the bonding pads in a first location on the substrate surface includes the solder extending a first distance outward therefrom. A second of the bonding pads in a second location on the substrate surface includes the solder extending a second distance outward therefrom. The first distance is different than the second distance. Other embodiments are described and claimed.

Description:
BACKGROUND 
       [0001]    Ball grid array (BGA) packages are often used for connections such as second level interconnects, between a package substrate and a printed circuit board (PCB). As electronic devices become smaller and distances between adjacent connections become smaller, the risk of forming unreliable solder interconnects increases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]    Embodiments are described by way of example, with reference to the accompanying drawings, which are not necessarily drawn to scale. 
           [0003]      FIG. 1  illustrate a package substrate positioned above a PCB prior to heating to a reflow temperature, in accordance with certain embodiments. 
           [0004]      FIG. 2  illustrates the package substrate of  FIG. 1 , at reflow temperature but prior to joint formation, in accordance with certain embodiments. 
           [0005]      FIG. 3  illustrates the package substrate of  FIGS. 1-2 , after a joint has been formed, in accordance with certain embodiments; 
           [0006]      FIG. 4(A)-4(D)  illustrate certain processing operations, in accordance with certain embodiments. 
           [0007]      FIG. 5(A)-5(D)  illustrate certain processing operations, in accordance with certain embodiments. 
           [0008]      FIG. 6(   a )- 6 (B) illustrate certain processing operations, in accordance with certain embodiments. 
           [0009]      FIG. 7(A)-7(B)  illustrate certain processing operations, in accordance with certain embodiments. 
           [0010]      FIG. 8  illustrates a flow chart of process operations, in accordance with certain embodiments. 
           [0011]      FIG. 9  illustrates an electronic system arrangement in which embodiments may find application. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    In order to show features of various embodiments most clearly, the drawings included herein include a representation of various electronic and/or mechanical devices. The actual appearance of the fabricated structures may appear different while still incorporating the claimed structures of the illustrated embodiments. Moreover, the drawings may show only the structures necessary to understand the illustrated embodiments. For example, in certain figures only a small number of interconnection structures are illustrated, whereas in typical configurations, there are a large number of interconnection structures. Additional structures have not been included to maintain the clarity of the drawings. 
         [0013]    Certain surface mount technology (SMT) processes for forming interconnections such as BGA solder interconnections between a package substrate and a PCB include the positioning of solder balls onto substrate bonding pads. The solder balls are uniform in size so a uniform amount of solder is placed on each pad. However, thermal expansion mismatch between the substrate and a die structure positioned thereon may cause the substrate to warp and bend in a manner that bends the BGA surface into a concave shape. As the substrate is aligned with the PCB and heated, the substrate begins to flatten out with the increased temperature and at the reflow temperature the substrate converts to a convex shape on the BGA surface. This shape moves the solder bumps in the central portion of the substrate towards the PCB, and moves the solder bumps in the outer portion of the substrate away from the PCB. Such movement increases the risk of center portion solder bump bridging (adjacent bumps contacting each other, which can result in electrical shorting) and outer portion solder bump opens (no electrical connection between the substrate and PCB) during the SMT process. Such risks may be minimized in accordance with certain embodiments. 
         [0014]    Certain embodiments relating to the formation of interconnection structures will be discussed in connection with the Figures.  FIG. 1  illustrates a package including a substrate  10  to be positioned on a printed circuit board (PCB)  12 . The substrate  10  includes one or more die structures  14  positioned on a first surface  16  thereof. The die structure  14  may in certain embodiments be a semiconductor such as silicon. The substrate  10  will be coupled to the PCB  12  through solder connections, including solder regions  2 ,  4  positioned on a second surface  18  of the substrate  10 , and the solder regions  6  on the PCB. 
         [0015]    During processes for attaching the die structure  14  to the substrate  10 , thermal expansion mismatch stresses between the die structure  14  and the substrate  10  (which may be formed from an organic polymeric material) often lead to warpage of the substrate  10 , as illustrated in  FIG. 1 . As illustrated, the substrate  10  has a center portion at a higher vertical position than outer portions of the substrate  10 . 
         [0016]    In accordance with certain embodiments, the amount of solder positioned on the second surface  18  of the substrate  10  is varied in different locations, with a center portion including a smaller volume of solder, and an outer portion including a larger volume of solder.  FIG. 1  illustrates two central solder regions  2 , and two outer solder regions  4 . The PCB  12  includes solder regions  6 . The solder regions  2 ,  4  on the substrate  10  may be formed from solder balls and/or solder paste. The solder regions  6  may be formed from solder paste. Additional solder regions will typically be present on both the substrate  10  and PCB  12  but are not shown for clarity. As illustrated in  FIG. 1 , the central solder regions  2  on the substrate  10  have a smaller volume of solder than the outer solder regions  4 . In addition, the warpage causes the central solder regions  2  to be lifted up relative to the outer solder regions  4 . As a result, the central solder regions  2  are spaced a greater distance away from the PCB  12  than the outer solder regions  4 . 
         [0017]      FIG. 2  illustrates the package of  FIG. 1  at reflow temperature (for example, 230° C.) but just prior to the formation of the solder joint between the substrate  10  and PCB  12 . The change in temperature (from room temperature to reflow temperature) has caused the substrate  10  to warp in an opposite manner from the configuration in  FIG. 1 , with the center solder regions  2  being moved towards the PCB  12  and the outer solder regions  4  being moved away from the PCB  12 . The combination of the change in warpage and the melting of the solder have brought the center solder regions  2  just into contact with adjacent solder paste regions  6  on the PCB  12 . The outer solder regions  4  are also just in contact with adjacent solder paste regions  6  on the PCB  12 . 
         [0018]      FIG. 3  illustrates the package of  FIGS. 1-2  after the solder joint formation is complete and the package has been brought back to room temperature. The change in temperature has caused the substrate  10  to warp or bend back towards a more flat configuration than the warpage at reflow temperature. The interconnection includes solder joints  17  (each including the solder from a center solder region  2  and an adjacent solder paste region  6 ) and solder joints  15  (each including solder from an outer solder region  4  and an adjacent solder paste region  6 ). 
         [0019]    Embodiments include a number of processes for forming a different volume of solder at different locations on the surface  18  of the substrate  10 . Embodiments also include processes in which solder has a different height at different locations on the surface  18  of the substrate  10 . 
         [0020]      FIGS. 4(A)-4(D)  illustrate certain processing operations in accordance with certain embodiments.  FIG. 4(A)  illustrates a substrate  20  including bonding pads  22 . A stencil  30  having openings (apertures)  24 ,  26  is positioned adjacent to the substrate  20  for solder paste printing. Solder paste is printed onto the bonding pads  22  through the stencil  30  using any suitable process. A heating operation is carried out to reflow and form solder regions  28  bonded to the bonding pads  22 . The solder regions  28  are illustrated in  FIG. 4(B) . After the reflow operation, another solder paste print will be carried out using stencil  31 . The stencil  31  includes openings  32 ,  34 . The openings  32 ,  34  are different sizes, with outer opening  32  being larger than central opening  34 . This enables varying amounts of solder paste to be printed onto the substrate at different locations, depending on the presence of and sizes of the openings. In certain embodiments, certain of the opening(s) may not be necessary if no additional solder is needed in a particular location. 
         [0021]      FIG. 4(C)  illustrates the solder paste regions  36 ,  38  printed onto the reflowed solder regions  28  on the bonding pads  22 . The solder paste region  38  has a smaller volume than the solder paste region  36 , due to the smaller opening size of the central opening  34  in the stencil. Another heating operation is carried out to reflow the solder paste  36 ,  38  and combine it with the underlying reflowed solder  28  to form solder regions  40 ,  42 , as illustrated in  FIG. 4(D) . Solder region  42  has less solder and extends a smaller distance away from the bonding pad  22  than solder region  40 . Such a configuration is similar to that illustrated in  FIG. 1  and attachment of the substrate  20  to a PCB may proceed in accordance with the description above in connection with  FIGS. 1-3 . The solder regions  40 ,  42  are at vertical positions such that when a reflow operation is carried out to couple the substrate  20  to a PCB, the outer solder region  40  may move upwards and the central solder region  42  downwards, which leads to the formation of a solder joint with a minimized risk of solder bridging between adjacent solder regions and a minimized risk for forming open regions between the substrate and PCB. 
         [0022]      FIGS. 5(A)-5(D)  illustrate certain processing operations in accordance with certain embodiments.  FIG. 5(A)  illustrates a substrate  50  including bonding pads  52 . Solder balls  54  are positioned on the bonding pads  52 , using any suitable process. The solder balls  54  may be positioned on flux (not shown) on the solder pads  52  in order to hold the solder balls  54  in place prior to reflow. A reflow operation is then conducted to enable the solder to wet and form a good connection with the bonding pad  52 . A stencil  56  having openings  58 ,  60  is positioned adjacent to the substrate  50  for solder paste printing onto the reflowed solder balls  54 ′ that are bonded to the bonding pads  52 . The openings  58 ,  60  are different sizes, with outer opening  58  being larger than central opening  60 . This enables varying amounts of solder paste to be printed onto the substrate at different locations, depending on the presence of and size of the openings. 
         [0023]      FIG. 5(C)  illustrates the solder paste regions  62 ,  64  printed onto the reflowed solder balls  54 ′. The solder paste region  64  has a smaller volume than the solder paste region  62 , due to the smaller opening  60  in the center of the stencil  56 . Another heating operation is carried out to reflow the solder paste  62 ,  64  and combine it with the underlying reflowed solder  54 ′ to form solder regions  66 ,  68 , as illustrated in  FIG. 5(D) . Solder region  68  has less solder and extends a smaller distance away from the bonding pad  52  than solder region  66 . Such a configuration is similar to that illustrated in  FIG. 1  and attachment of the substrate  50  to a PCB may proceed in accordance with the description above in connection with  FIGS. 1-3 . The solder regions  66 ,  68  are at vertical positions such that when a reflow operation is carried out to couple the substrate  50  to a PCB, the outer solder region  66  may move upwards and the inner solder region  68  downwards, which leads to the formation of a solder joint with a minimized risk of solder bridging between adjacent solder regions and a minimized risk for forming open regions between the substrate and PCB. 
         [0024]      FIGS. 6(A)-6(B)  illustrate certain processing operations in accordance with certain embodiments.  FIG. 6(A)  illustrates a substrate  70  including bonding pads  72 ,  74 . Solder balls  76  are positioned on the bonding pads  72 ,  74  using any suitable process. The solder balls  74  may be positioned on flux (not shown) on the solder pads  72  in order to hold the solder balls  74  in place prior to reflow. The bonding pads  72 ,  74  may be formed using a solder resist layer  78  that is patterned to form openings that are filled with metal to form the bonding pads  72 ,  74 . The openings in the solder resist layer  78  may be varied in size at different locations so that different sized bonding pads are formed. As illustrated in  FIGS. 6(A)-6(B) , the central bonding pad  74  has a width that is greater than that of the outer bonding pad  72 . 
         [0025]    The solder balls  76  placed on the bonding pads  72 ,  74  may be of uniform size. A reflow operation is then conducted to enable the solder to wet and form a good connection with the bonding pads  72 ,  74 , as illustrated in  FIG. 6(B) . As a result of the outer bonding pad  72  having a smaller surface area to contact the solder than the central bonding pad  74 , the resultant solder region  80  will be more narrow and longer than resultant solder region  82 , even though the same size solder bumps  76  were used. Such a structure has a similar effect to positioning more solder material on the outer bonding pad in order to make it longer, as described in embodiments above. Attachment of the substrate  70  to a PCB may proceed in a similar manner as described above in connection with  FIGS. 1-3 . The solder regions  80 ,  82  are at vertical positions such that when a reflow operation is carried out to couple the substrate  70  to a PCB, the outer solder region  80  may move upwards and the inner solder region  82  downwards, which leads to the formation of a solder joint with a minimized risk of solder bridging between adjacent solder regions and a minimized risk for forming open regions between the substrate and PCB. 
         [0026]      FIGS. 7(A)-7(B)  illustrate certain processing operations in accordance with certain embodiments.  FIG. 7(A)  illustrates a substrate  90  including bonding pads  92 . The illustrated bonding pads  92  in this embodiment each have the same area. Solder balls  94 ,  96  are positioned on the bonding pads  92  using any suitable process. The solder balls  94 ,  96  may be positioned on flux (not shown) on the bonding pads  92  in order to hold the solder balls  94 ,  96  in place prior to reflow. The solder balls  94 ,  96  are formed to be different sizes. As illustrated in  FIG. 7(A) , the central region solder ball  96  has a smaller diameter than the outer region solder ball  94 . A reflow operation is conducted to wet the bonding pads  92  and form solder regions  98 ,  99 , as illustrated in  FIG. 7(B) . Due to the larger volume of the solder ball  94 , the solder region  98  is larger and longer than solder region  99 . Attachment of the substrate  90  to a PCB may proceed in a similar manner as described above in connection with  FIGS. 1-3 . The solder regions  98 ,  99  are at vertical positions such that when a reflow operation is carried out to couple the substrate  90  to a PCB, the outer solder region  98  may move upwards and the central solder region  99  downwards, which leads to the formation of a solder joint with a minimized risk of solder bridging between adjacent solder regions and a minimized risk for forming open regions between the substrate and PCB. 
         [0027]    It should be appreciated that in certain embodiments, the distance that the solder extends outward from the surface may sequentially change depending on the location on the surface and the expected warpage to be compensated for. 
         [0028]      FIG. 8  is a flowchart of operations in accordance with certain embodiments. Box  100  is providing bonding pads on a substrate. Box  102  is positioning a greater volume of solder on outer bonding pads than on inner bonding pads. The solder may include, but is not limited to, solder paste and solder balls. Box  104  is performing a reflow operation to bond the solder to the bonding pads. Box  106  is aligning the substrate pads having the solder thereon with PCB pads. The PCB pad will typically include solder paste thereon. Box  108  is performing another reflow operation to form a solder joint between the substrate and the PCB. 
         [0029]    Assemblies including components formed as described in embodiments above may find application in a variety of electronic components.  FIG. 9  schematically illustrates one example of an electronic system environment in which aspects of described embodiments may be embodied. Other embodiments need not include all of the features specified in  FIG. 4 , and may include alternative features not specified in  FIG. 9 . 
         [0030]    The system  200  of  FIG. 4  may include at least one die  202  positioned on a package substrate  204 , which is then coupled to a PCB  206 . The package substrate  204  coupled to the board  206  is an example of an electronic device assembly that may be formed in accordance with embodiments such as described above. A variety of other system components, including, but not limited to memory and other components discussed below, may also include structures formed in accordance with the embodiments described above. 
         [0031]    The system  200  may further include memory  208  and one or more controllers  210   a ,  210   b  . . .  210   n , which may also be disposed on the PCB  206 . The system may include a variety of other components, including, but not limited to, storage  212 , display  214 , and network connection  216 . The system  200  may comprise any suitable computing device, including, but not limited to, a mainframe, server, personal computer, workstation, laptop, tablet, netbook, handheld computer, handheld gaming device, handheld entertainment device (for example, MP3 (moving picture experts group layer-3 audio) player), PDA (personal digital assistant) smart phone or other telephony device (wireless or wired), network appliance, virtualization device, storage controller, network controller, router, etc. 
         [0032]    It should be appreciated that many changes may be made within the scope of the embodiments described herein. The term die as used herein refers to a workpiece that is transformed by various process operations into a desired electronic device. A die is usually singulated from a wafer, and may be made of semiconducting, non-semiconducting, or combinations of semiconducting and non-semiconducting materials. Terms such as “first”, “second”, and the like, if used herein, do not necessarily denote any particular order, quantity, or importance, but are used to distinguish one element from another. Terms such as “top”, bottom”, “upper”, “lower”, “over”, “under”, and the like are used for descriptive purposes and to provide a relative position and are not to be construed as limiting. Embodiments may be manufactured, used, and contained in a variety of positions and orientations. 
         [0033]    In the foregoing Detailed Description, various features are grouped together for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the invention require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter may lie in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate preferred embodiment. 
         [0034]    While certain exemplary embodiments have been described above and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative and not restrictive, and that embodiments are not restricted to the specific constructions and arrangements shown and described since modifications may occur to those having ordinary skill in the art.