Abstract:
A method of forming a package on package, semiconductor package arrangement is described. In one aspect, solder bumps on a lower surface of a first grid array package substrate are fused to corresponding unencapsulated solder bumps on an upper surface of a second grid array package substrate. The fused solder bumps form solder joints that electrically connect the first and second packages. The height of the resulting solder joints is greater than a height of a die that is flip chip mounted to the second substrate such that the first substrate does not contact any portion of the second package and an air gap is formed that separates the second die from the first package. Corresponding PoP packages structures are also described.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to semiconductor packaging. More particularly, improved package on package fabrication techniques and designs are described. 
         [0002]    Some semiconductor packaging designs contemplate stacking two or more packaged devices on top of one another. For example, a packaged memory device may be stacked on top of a packaged processor as shown in  FIGS. 1(   a ) and  1 ( b ). In the illustrated arrangement, the top package  100  has solder bumps  101  on its bottom surface  102  facing the bottom package  110 . The solder bumps  101  are formed on I/O pads (not shown). The bottom package has a BGA substrate  111  that supports a die  112 . The BGA substrate  111  has contact pads  113  on its top surface  115  that are complementary to the solder bumps on top package  100 . During assembly of a stacked package on package (PoP) device  120 , the top package  100  is placed on the bottom package  110  and the solder bumps  101  are reflowed to form contacts  124  between the I/O pads on the top package  100  and the contact pads  113  as illustrated in  FIG. 1(   b ). Although this approach works well in many applications, warping of either the top or bottom package can sometimes result in one or more contacts not being properly formed, resulting in an “open” contact type defect  125  as diagrammatically illustrated in  FIG. 1(   b ). 
         [0003]    Another package stacking approach is illustrated in  FIGS. 2(   a ) and  2 ( b ). In this approach, the top package  100  is generally similar to the top package illustrated in  FIGS. 1(   a ) and  1 ( b ). The bottom package  210  is somewhat similar to the bottom package illustrated in  FIGS. 1(   a ) and  1 ( b ) except that the contact pads  113  on the top surface  115  of BGA substrate  111  are bumped to form solder bumps  217  and the top side of the BGA substrate is overmolded. That is, a molding/encapsulating material  218  such as epoxy encapsulates the die  112  and solder balls  217  on the top surface of the BGA substrate  111  (the die may or may not be exposed). The molding material  218  helps stiffen the package thereby reducing warping. Laser ablation is then used to remove molding material around bumps  217  on the lower package as diagrammatically illustrated in  FIG. 2(   b ). In this state, the top package  100  is placed over the bottom package  210  and the facing solder bumps  101 ,  217  are reflowed such they join together to form solder joint contacts  224  between the I/O pads on the top package  100  and contact pads on the bottom package  210  as diagrammatically illustrated in  FIG. 2(   b ). Again, this approach works well in many applications. However, the ablation process tends to leave some molding material dust, which can contaminate some of the solder joints as illustrated by joint  224 ( a ) in  FIG. 2(   b ). Such contamination is another type of defect that can result in an open contact type defect or a poor connection. Another drawback of the molding/ablation approach is that the ablation forms troughs or well around the solder balls. Such recesses can be hard to clean and sometimes retain cleaning solution which can have corrosive effects on the solder ball surface and solder joints. Furthermore, a wall of molding material  218 (a) to the outside of the solder joints impairs visual inspection of the solder joints after reflow. Therefore, although current package stacking approaches work well in many applications, there are continuing efforts to develop more reliable, low cost packaging designs. 
       SUMMARY OF THE INVENTION 
       [0004]    A method of forming a package on package, semiconductor package arrangement is described. In one aspect, solder bumps on a lower surface of a first grid array package substrate are fused to corresponding unencapsulated solder bumps on an upper surface of a second grid array package substrate. The fused solder bumps form solder joints that electrically connect the first and second packages. The height of the resulting solder joints is greater than a height of a die that is flip chip mounted to the second substrate such that the first substrate does not contact any portion of the second package and an air gap is formed that separates the second die from the first package. 
         [0005]    The first grid array package has a first substrate, a first die mounted on the first substrate and a multiplicity of exposed solder bumps on a lower surface of the first substrate. The second grid array package has a second substrate, a second die flip chip mounted on the top surface of the second substrate, a multiplicity of lower solder bumps on a lower surface of the second substrate, and a multiplicity of upper solder bumps on the top surface of the second substrate. No molding material is provided on the top surface of the second substrate and therefore no molding material surrounds around any of the upper solder bumps on the second substrate. In this condition, the upper solder bumps on the second substrate are fused to corresponding solder bumps on the first grid array package. The resulting solder joints electrically couple the first and second grid array packages thereby forming a stacked package on package arrangement. The fused solder bumps are sized such that a height of the resulting solder joints is greater than a height of the second die and such that the first substrate does not contact any portion of the second package and an air gap is formed that separates the second die from the first package. Thus, after the fusing, no portion of the first grid array package physically contacts any portion of the second grid array package other than the solder joints. Preferably, no plastic molding material is provided on the top surface of the second substrate or the lower surface of the first substrate. 
         [0006]    In some preferred arrangements, the first grid array package is a memory package and the second die is a processor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The invention and the advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIGS. 1(   a ) and  1 ( b ) diagrammatically illustrate a previous package on package stacking approach. 
           [0009]      FIGS. 2(   a ) and  2 ( b ) diagrammatically illustrate another previous package on package stacking approach. 
           [0010]      FIGS. 3(   a ) and  3 ( b ) diagrammatically illustrate package on package stacking approach in accordance with one embodiment. 
           [0011]      FIGS. 4(   a ) and  4 ( b ) are diagrammatic perspective views of the top and bottom surfaces of a representative bottom package suitable for use in the embodiment of  FIG. 3 . 
       
    
    
       [0012]    In the drawings, like reference numerals are sometimes used to designate like structural elements. It should also be appreciated that the depictions in the figures are diagrammatic and not to scale. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    Referring now to  FIG. 3 , a stacked package arrangement and assembly method in accordance with the present invention will be described that is suitable for packaging integrated circuits. The illustrated arrangement is a form often referred to as a package on package (PoP) type semiconductor package. 
         [0014]    The top package  300  is a grid array package that may take most any desired grid array form. By way of example, in the illustrated embodiment, top package  300  takes substantially the same form as the top package  100  illustrated in  FIGS. 1 and 2  although this is not a requirement. In the illustrated embodiment, one or more dice (not shown) are mounted on and electrically connected to a substrate  305 . The substrate  305  may take the form of a conventional BGA substrate or any other suitable form and has solder bumps  301  on its bottom surface  302 . As will be appreciated by those familiar with the art, the substrate  305  typically has a number of I/O pads (not shown) on its lower surface and routing traces and vias that facilitate electrical connection of the encapsulated die (or dice) to the I/O pads. Solder bumps  301  are formed on I/O pads. In some specific applications, the top package  300  may be a commodity type memory package and therefore its specific design may vary widely. 
         [0015]    The bottom package  310  is a grid array type package having a substrate  311  that supports a flip chip mounted die  312 . The substrate  311  has contact pads  313  on its top surface  315  that are complementary to the solder bumps on top package  300 . Each contact pad  313  has an associated solder bump  317  thereon such that the solder bumps  317  may be positioned to face the solder bumps  301  on top package  300 . The substrate  311  also has a set of contact pads  340  on its lower surface  342 . The lower contact pads  340  are each bumped with corresponding solder balls  344  and are arranged to facilitate electrically coupling the bottom package  310  to an external device. Thus, the bottom package  310  is quite similar to the bottom package  110  illustrated in  FIG. 1 , except that solder bumps  317  are formed on the contact pads  313  on the top surface of substrate  311 . Therefore, the bottom package also has an appearance that it somewhat similar to bottom package  210  illustrated in  FIG. 2  with a significant exception that no encapsulant / molding material is deposited on the top surface of the substrate  111  and thus, there is no encapsulant that surrounds the solder bumps  317 . 
         [0016]    Like substrate  305 , substrate  311  may take the form of a BGA substrate and typically includes routing traces and vias (not shown) that electrically connect the flip chip mounted die  312  to the upper and lower contact pads. The substrate may be formed from any suitable material—by way of example, BT (Bismaleimide-Triazine) FR4 and other such materials are commonly used to form the substrate. 
         [0017]    During assembly of a stacked package on package (PoP) device  320 , the top package  300  is placed on the bottom package  310  and the facing solder bumps  301  and  317  on the top and bottom packages respectively are reflowed to form solder joints  324  between the I/O pads on the top package substrate  305  and the contact pads  313  on the bottom package substrate as illustrated in  FIG. 3(   b ). 
         [0018]    The volume of solder in the facing solder bumps  301 ,  317  and the solder reflow conditions are arranged so that the resulting solder joints  324  have a standoff height that is greater than the height of the flip chip mounted die  312 . With this arrangement, an air gap  327  will be formed between the die  312  and the bottom surface  302  of top package  300  (which is typically the bottom surface of substrate  305 ). By designing in a gap between the die  312  and bottom surface  302 , space is provided to accommodate warping of one or both of the packages  300 ,  310 . That is, even if one (or both) of the packages are warped somewhat, there is a much higher probability that each of the facing solder ball pairs  301 ,  317  will come into contact with one another to thereby form robust solder joints  324 . Thus, the air gap  327  provides sufficient tolerances so that the die  312  on the lower package  310  doesn&#39;t contact the bottom surface  302  of the top package  300  in a manner that prevents any of the facing solder ball pairs from reflowing together. It has been determined that this approach provides a significantly lower open contact defect rate than the conventional approach described above with respect to  FIG. 1 . The only extra step required in the formation of the bottom package  310  when compared to the approach of  FIG. 1  is the bumping of the contact pads  313  on the top surface of the lower package—which is a relatively inexpensive process. 
         [0019]    The described approach is more cost effective than the approach described above with respect to  FIG. 2  because the steps of encapsulating the top surface of the lower package substrate  311  and then laser ablating the regions around the solder balls can be eliminated. The described approach also substantially eliminates the risk that ablation dust contaminates a potential solder joint in a manner that causes an open or defective contact. 
         [0020]    The appropriate air gap between the top of die  312  and the adjacent bottom surface  302  of top package  300  will vary based on a number of factors including (a) the amount of warpage that might be expected in the component packages  300 ,  310 ; (b) package height constraints; (c) the positions of the solder bumps  301 ,  317 ; (d) the footprint size of the stacked packages; etc. It is noted that the term air gap distance is used because the actual standoff distance will vary somewhat based on the warpage of the specific components used, which will typically vary from component to component. The air gap would be expected if the actual components used had a reference warpage (which may be zero). 
         [0021]    In one particular application, the top package  300  takes the form of a memory package (e.g. RAM, FLASH memory, etc.) and the bottom package  310  takes the form of a processor arranged to utilize the memory in the memory package. As will be appreciated by those familiar with the art, memory tends to be a commodity and therefore the quality and susceptibility to warpage may vary significantly by supplier and/or product line. The described PoP packaging approach can help facilitate the successful use of memory products from a variety of different suppliers and/or having significantly different cost points and warpage susceptibilities without requiring PoP package redesign and without incurring an undue number defects. 
         [0022]      FIGS. 4(   a ) and  4 ( b ) illustrate a processor package  410  suitable for use as the bottom package  310  in the embodiment of  FIG. 3 . As seen in  FIG. 4(   a ), two rows of contact pads  313 /solder balls  317  are provided around and peripherally outside of the die  312 . The actual number of rows of contact pads/solder balls will vary in accordance with the design requirements of the processor—although 1-3 rows of contacts are most common In the illustrated embodiment the rows of contacts are continuous and symmetrical, but again, that is not a requirement.  FIG. 4(   b ) illustrates the bottom surface of processor package  410 . 
         [0023]    Although only a few embodiments of the invention have been described in detail, it should be appreciated that the invention may be implemented in many other forms without departing from the spirit or scope of the invention. For example, although a particular top package design has been illustrated, it should be appreciated that the form factor of the top package may be widely varied. In the illustrated embodiment a two device PoP design is shown. However, it should be appreciated that the same approach can readily be used in PoP designs that stack  3  or more devices on top of one another. Therefore, the present embodiments should be considered illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalents of the appended claims.