Abstract:
An area area package includes a plurality of solder balls not used as electrical connectors. These non-connected solder balls, or “dummy balls,” provide protection to solder balls connected to live pins and therefore increase reliability of the area array package. The dummy balls may be placed in the corners, along the diagonals or in other high stress location on the area array package. To further increase reliability, a continuous copper ball land pad may be used to connect each group of corner dummy balls. Continuous copper pads help to reduce stress on the dummy balls. For center-depopulated BGA packages, an array of dummy balls may be used in the center of the package to prevent substrate bending and improve drop test reliability.

Description:
RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 60/443,817, filed Jan. 30, 2003 and entitled “AREA ARRAY PACKAGE WITH IMPROVED RELIABILITY OF SOLDER JOINTS.” 
    
    
     TECHNICAL FIELD 
     The present application relates to area array packages, and more particularly to increasing the reliability of solder joints of an area array package subjected to environmental stresses. 
     BACKGROUND 
     Semiconductor integrated circuit chips have to be connected in order to enable them to interact electrically with the outside world. A ball grid array (BGA) semiconductor chip package employs a plurality of solder balls as external terminals. A BGA package is widely employed because it allows a multi-pin structure over a limited area. 
     Chip devices typically have a coefficient of thermal expansion (CTE) of about 3 ppm/C. These devices are relatively stiff and can fracture in a brittle manner if stressed by excessive bending. An area array package includes a chip, a package substrate, and optional molding. The CTE of the area array package is affected by each component of the package. 
     An epoxy-glass printed circuit board can have a CTE in the range of about 16 to 21 ppm/C, depending on the glass cloth, resin system, and copper content. It is necessary to provide a physical connection between the area array package and the printed circuit board in order to obtain a useful electrical connection. 
     A mismatch in CTE that exists between an area array package and a printed circuit board contributes to thermally driven stress and can affect package reliability in many ways. In some manner, all electronics packaging schemes involving an area array package and a printed circuit board are affected by this fundamental mismatch in CTE. Measurements on semiconductor packages of the ball-grid array (BGA) type and chips-size (CSP) type found that solder joints located close to package corners are under particularly heavy strain due to mismatch in CTE. 
     As the land pad pitch and size shrinks, solder joint reliability is typically decreased. The area array package requires a packaging scheme to form and maintain electrical contacts between the package and a printed circuit board over the entire face of a device. Temperature-dependent shear strains exist between the area array package and the printed circuit board. There is a very predictable, finite fatigue life of solder ball connections. Additionally, area array packages are also subject to mechanical stresses not related to the CTE. These stresses include mechanical bending and other environmental stresses. 
     Stresses between the area array package and the printed circuit board leads to the need for protection of solder balls that provide electrical connection. The protection of the solder balls that provide electrical connection should provide sufficient relief such that strains on solder connections are reduced to acceptable levels so that fatigue life improvement is realized. 
     SUMMARY 
     An area array package includes a plurality of solder balls not used as electrical connectors. These non-connected solder balls, or “dummy balls,” provide protection to solder balls connected to live pins and therefore increase reliability of the area array package. The dummy balls may be placed in the corners, along the diagonals or in other high stress locations on the area array package. To further increase reliability, a continuous copper ball land pad may be used to connect each group of corner dummy balls. Continuous copper pads help to reduce stress on the dummy balls. For center-depopulated BGA packages, an array of dummy balls may be used in the center of the package to prevent substrate bending and improve drop test reliability. 
     Improving the reliability of area array packages using dummy balls and advantages of the embodiments will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates an area array package connected to a printed circuit board. 
     FIG. 2 illustrates dummy ball configuration on the area array package. 
     FIG. 3 illustrates an optional center dummy ball configuration for center-depopulated area array packages. 
     FIG. 4 is a flowchart illustrating how to prepare a connection between the area array package and the printed circuit board to increase reliability. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 illustrates an area array package  100 . The area array package  100  includes a substrate  105 , a die  106 , and optional molding  108 . The substrate  105  may be composed of different materials, including ceramic, laminate, or polyimide tape. The die  106  is typically a silicon chip and is connected to the substrate  105 . The die  106  may be wire bonded or be flip chip attached to the substrate  105 . Molding  108  may be placed around the die  106  to protect the die  106 . Molding  108  is typically used when the die  106  is wire bonded to the substrate  105 , but may also be used even if the die  106  is flip chip attached. 
     The area array package  100  is mounted on a printed circuit board  110  or similar component. The printed circuit board  110  is typically a laminate. The area array package  100  is connected to the printed circuit board  110  by a plurality of solder balls  115 . The solder balls  115  may provide both a mechanical and electrical connection between the area array package  100  and the printed circuit board  110 . 
     FIG. 2 illustrates the solder ball connections  115  between the area array package  100  and the printed circuit board  110 . The solder ball connections  115  are typically placed on the bottom of the substrate  105  of the area array package  100 . In the present invention, two types of solder ball connections  115  are used. The first type of solder ball connection  205  provides both a mechanical and electrical interface between the area array package  100  and the printed circuit board  110 . Thus, in addition to physically holding the area array package  100  to the printed circuit board  110 , the solder ball connections  205  permit the transmission of electrical signals between the area array package  100  and the printed circuit board  110 . This type of solder ball connection  205  is typical of those found in BGA applications. The second type of solder ball connection is a dummy ball  210 . Dummy balls  210  provide only mechanical connection between the area array package  100  and the printed circuit board  110 . The dummy balls  210  are not connected to any electrical signal lines. 
     In one embodiment of the invention, the area array package  100  includes solder ball pads  208 . Each solder ball pad  208  is adapted to receive a solder ball. In FIG. 2, only a sampling of solder ball pads  208  are illustrated for clarity, although a solder ball pad  208  is typically located in each position adapted to receive a solder ball. The solder ball pads  208  in each corner  215 ,  220 ,  225 ,  230  of the area array package  100  and along the diagonals  235 ,  240  connecting the corners  215 ,  220 ,  225 ,  230  of the area array package  100  do not have electrical connections to the area array package  100 . These solder ball pads  208  without electrical connections are configured to use dummy balls  210 . 
     Measurements on semiconductor packages found that the solder joints located close to the package corners are under particularly heavy strain. By designing an area array package  100  to not have electrical connections in the corners  215 ,  220 ,  225 ,  230  and along the diagonals  235 ,  240  connecting the corners  215 ,  220 ,  225 ,  230  of the area array package  100 , standard solder joints may be replaced with the dummy balls  210 . Replacing standard solder joints with dummy balls  210  allows the heavy strain experienced in the corners and diagonals to be absorbed by the dummy balls  210 . Because the dummy balls  210  do not contain any electrical connections, the strain applied to the dummy balls  210  cannot cause a sensitive electrical connection to break. In addition, designing an area array package  100  with no electrical connections along the diagonals  235 ,  240  also allows dummy balls  210  to be placed along the diagonals  235 ,  240  to further absorb the stresses and strains without breaking any electrical connections. 
     Empirical tests conducted with the dummy balls  210  show the dummy balls  210  failing long before the regular solder balls in both the temperature cycles before failure and the bend to fail tests. Because the dummy balls  210  do not have the electrical connections, their failure does not cause the connection between the area array package  100  and the printed circuit board  110  to fail. By locating the dummy balls  210  in the high stress positions, the overall life of the array package  100  is increased. Table 1 illustrates the testing. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Sample 
                 I/O Circuit 
                 Dummy Balls 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Temperature Cycle 
                   
               
               
                   
                 (cycles to failure) 
               
             
          
           
               
                 1 
                 1455 
                 379 
               
               
                 2 
                 2065 
                 140 
               
               
                 3 
                 1759 
                 1070 
               
               
                 4 
                 1750 
                 467 
               
               
                 5 
                 426 
                 189 
               
               
                 6 
                 1923 
                 329 
               
               
                 7 
                 916 
                 176 
               
               
                 8 
                 1184 
                 134 
               
               
                 9 
                 1797 
                 121 
               
               
                 10 
                 1425 
                 211 
               
               
                 11 
                 1898 
                 1269 
               
               
                 12 
                 2315 
                 417 
               
               
                 13 
                 1805 
                 737 
               
               
                 14 
                 768 
                 218 
               
               
                 15 
                 1849 
                 1075 
               
               
                 16 
                 1809 
                 171 
               
               
                 17 
                 1946 
                 375 
               
               
                 18 
                 1075 
                 589 
               
               
                 19 
                 2259 
                 387 
               
               
                 20 
                 2300 
                 125 
               
               
                 Avg 
                 1636 
                 429 
               
             
          
           
               
                   
                 Bend To Fail 
                   
               
               
                   
                 (mm deflection) 
               
             
          
           
               
                 1 
                 14.5 
                 10 
               
               
                 2 
                 15.5 
                 15 
               
               
                 3 
                 15 
                 14.5 
               
               
                 4 
                 19 
                 14.5 
               
               
                 5 
                 15 
                 14.5 
               
               
                 6 
                 16 
                 13.5 
               
               
                 7 
                 14 
                 12.5 
               
               
                 8 
                 15.5 
                 13 
               
               
                 9 
                 18.5 
                 18 
               
               
                 10 
                 18 
                 14 
               
               
                 11 
                 14 
                 8 
               
               
                 12 
                 16 
                 7.5 
               
               
                 13 
                 15 
                 14 
               
               
                 Avg 
                 16 
                 13 
               
               
                   
               
             
          
         
       
     
     To further reduce the stress on the dummy balls  210 , a continuous copper ball land pad  250  may be placed under the dummy balls  210  in one or more of the corners  215 ,  220 ,  225 ,  230 . The copper pad  250  is placed between the dummy balls  210  and the substrate  105 . The copper pad  250  provides a uniform landing area and further reduces the stress on the corner dummy balls  210 . Modeling data conducted with the copper pads  250  show an increase in the number of temperature cycles before failure. Table 2 illustrates the testing. 
     
       
         
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Temp Cycles to Failure 
                   
               
             
          
           
               
                 Dummy Ball 
                 Without Copper Pad 
                 With Copper Pad 
               
               
                   
               
             
          
           
               
                 1 
                 920 
                 987 
               
               
                 2 
                 3216 
                 3750 
               
               
                 3 
                 3099 
                 3793 
               
               
                 4 
                 2475 
                 2617 
               
               
                   
               
             
          
         
       
     
     FIG. 3 illustrates an alternative arrangement of the solder ball connections  115  between the area array package  100  and the printed circuit board  110  for a center depopulated package. In this embodiment, an array of dummy balls  350  is positioned in the center of the area array package  100  in the depopulated region. The array of dummy balls  350  positioned in the center prevents substrate bending during electrical testing and provides more solder joints to improve drop-test reliability. The array of dummy balls  350  positioned in the center may also be combined with the use of dummy balls on the corners and diagonals to enhance reliability. 
     FIG. 4 is a flowchart illustrating how to prepare a connection between the area array package  100  and the printed circuit board  110  to increase reliability. The process begins in a START block  405 . Proceeding to block  410 , a package outline is created for the area array package  100 . The package outline includes a plurality of solder balls  115  that exceed the number of the input/output (I/O) requirements of the chip device  106 . By including a high number of solder balls  115 , not every solder ball  115  needs to be used as an I/O connection. This allows some of the solder balls  115  to be used as dummy balls  210 . 
     Proceeding to block  415 , high stress locations along the connection between the area array package  100  and the printed circuit board  110  are identified. The high stress locations are those locations where failure of the solder balls  115  is likely to first occur when subjected to environmental or other stresses. High stress locations may be identified by modeling data (computer simulation), empirical tests, or other appropriate technique. 
     Proceeding to block  420 , for each of the high stress locations, an appropriate number of solder balls  115  are selected to be dummy balls  210 . The number of dummy balls  210  to use for each area is selected based on target performance levels and the required number of I/O connections required. As described above, high stress areas are typically found in the corners and along the diagonals of the area array package  100 . Thus, dummy balls  210  are placed in these areas, as illustrated in FIG.  2 . The configuration in FIG. 2 is illustrative only, and the number of dummy balls  210  to use in each area of high stress may vary based on the target performance levels and the required number of I/O connections required When a center-depopulated package is used, an array of dummy balls may optionally be placed in the center of the area array package  100 . The center dummy balls may be used with or without the dummy balls in the corners and the diagonals. The center dummy balls may be placed in a rectangular or square layout, but other types of layouts may also be used. 
     Proceeding to block  425 , the substrate is routed so the die pads are only connected to locations containing live I/O solder balls. No electrical connections are made between the die pads and the dummy balls. 
     Proceeding to block  430 , a continuous copper or other pad may be placed between the dummy balls in the corners and the area array package  100 . The continuous copper pads improve the resistance of the corner dummy balls to environmental stresses. The use of the copper pads is optional. 
     After the package is designed and the dummy balls are placed, empirical testing may be performed to determine if the package meets the target stress levels. If the target levels are not met, further modifications of the design may be made. Once the target levels are met, the process terminates in END block  435 . 
     Although the present device has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the present device as defined by the appended claims.