Abstract:
A method of forming BGA interconnections having improved fatigue life is disclosed. In particular, a combination of mask-defined and pad-defined solder joints are selectively positioned within the BGA package. The mask-defined solder joints possess a high equilibrium height, which forces the pad-defined solder joints to elongate, thereby making the pad-defined solder joints more compliant. Further, the pad-defined solder joints possess a slightly longer fatigue life because the stress concentrations found in the mask-defined solder joints are not present in the pad-defined solder joints. Therefore, the fatigue life of BGA packages is increased by implementing a majority of mask-defined solder joints to maintain a high equilibrium height, and selectively placing pad-defined solder joints in high stress areas of the BGA package.

Description:
This application is a divisional of Ser. No. 09/426,578, filed on Oct. 25, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates generally to Ball Grid Array (hereinafter “BGA”) packages. More particularly, the present invention relates to a method of forming interconnections having mixed solder joint profiles within BGA packages to increase fatigue life of the BGA interconnections. 
     2. Related Art 
     In the manufacture of BGA packages, differences in the coefficients of thermal expansion between a chip carrier or module and a board creates stresses, in particular, shear stress, within the interconnections, or solder joints. The stresses are typically the highest in the solder joints at the corners of the BGA package, and in the solder joints directly beneath the corners and edges of the chip. Frequently, the solder joints in these regions cannot withstand the stresses applied over many on/off cycles, resulting in fatigue failure of the BGA solder joints. It is well known that elongating the solder joints will extend the fatigue life. It was determined that elongated solder joints are more compliant and have lower shear stress than when compared to shorter solder joints having the same volume. 
     Several techniques have been used in the industry to produce elongated solder joints. For instance, spacers, high-melt solder columns, and other additional materials, have been placed between the module and the board to force the solder joints to elongate. Lifting forces have been applied to the BGA packages during solidification to extend the solder joints. Solder joints having increased volume have been placed at selected locations within the package thereby forcing the other solder joints to elongate, and so on. 
     However, some of these techniques are incompatible with the trend toward reducing the size of semiconductor packages. Other techniques entail a complicated assembly process which increases manufacturing costs and reduces production yields. Further, some techniques decrease the space on the substrate available for wiring. 
     Accordingly, there exists a need in the industry for a BGA package exhibiting an increased fatigue life, without increasing the pad size, solder volume, size of the board, etc., or raising the costs. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of forming BGA interconnections, or solder joints, using a combination of mask-defined and pad-defined solder joints to increase fatigue life of the solder joints. In particular, pad-defined solder joints lack the stress concentrations found within the mask-defined solder joints. Therefore, pad-defined solder joints are selectively placed in regions of high stress, particularly at the corners of the BGA package, and directly below the corners and edges of the chip. Mask-defined solder joints are located throughout the remainder of the BGA package to increase the equilibrium height of the pad-defined solder joints, thereby making the pad-defined solder joints more compliant. 
     The first general aspect of the present invention provides a method of forming Ball Grid Array (BGA) interconnections, comprising the steps of: providing a first substrate and a second substrate, each having a plurality of conductive pads mounted thereon; and applying a first mask to the first substrate and a second mask to the second substrate, wherein a first plurality of openings of the first and second masks expose selected conductive pads and have a diameter larger than a diameter of the conductive pads, and a second plurality of openings of the first and second masks expose selected conductive pads and have a diameter smaller than a diameter of the conductive pads. This aspect allows for a plurality of mask-defined solder joints designed to increase the equilibrium height of pad-defined solder joints. Further, this aspect selectively positions elongated pad-defined solder joints, having no stress concentrations therein, at the high stress regions of the BGA package, thereby increasing the fatigue life of the package. 
     The second general aspect of the present invention provides a semiconductor package having a series of Ball Grid Array (BGA) interconnections, wherein a plurality of the BGA interconnections are pad-defined solder joints and a plurality of the BGA interconnections are mask-defined solder joints. This aspect allows for similar advantages as those associated with the first aspect. 
     The third general aspect of the present invention provides a method of forming Ball Grid Array (BGA) interconnections having mixed solder profiles, comprising the steps of: providing a first substrate and a second substrate; applying a mask to at least one of the first and second substrates, wherein a plurality of openings in the mask produces pad-defined solder joints and a plurality of openings in the mask produces mask-defined solder joints. This aspect provides similar advantages as those mentioned with respect to the first aspect. 
     The fourth general aspect of the present invention provides a substrate having a series of Ball Grid Array (BGA) interconnections, wherein a plurality of the BGA interconnections are pad-defined solder joints and a plurality of the EGA interconnections are mask-defined solder joints. This aspect provides similar advantages as those mentioned with respect to the first aspect. 
     The fifth general aspect of the present invention provides a solder mask, adapted to be coupled to a substrate having conductive pads thereon, the solder mask having a first plurality of openings and a second plurality of openings, wherein the first plurality of openings are larger than the conductive pads and the second plurality of openings are smaller than the conductive pads. This aspect provides similar advantages as those mentioned with respect to the first aspect. 
     The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
     FIG. 1 depicts a cross-sectional view of a module in accordance with a preferred embodiment of the present invention; 
     FIG. 2 depicts a surface of the module in FIG. 1; 
     FIG. 3 depicts a printed circuit board in accordance with a preferred embodiment of the present invention; 
     FIG. 4 depicts an enlarged view of the surface of the module in FIG. 2 having a mask thereon in accordance with a preferred embodiment of the present invention; 
     FIG. 5 depicts an enlarged view of the board in FIG. 3 having a mask thereon in accordance with a preferred embodiment of the present invention; 
     FIG. 6 depicts the module and a graphite frame having solder balls therein in accordance with a preferred embodiment of the present invention; 
     FIG. 7 depicts the module having solder balls wetted thereto in accordance with a preferred embodiment of the present invention; 
     FIG. 8 depicts a BGA package having mixed profile interconnections in accordance with a preferred embodiment of the present invention; 
     FIG. 9 depicts a cross-sectional view of a mask-defined solder joint in accordance with a preferred embodiment of the present invention; and 
     FIG. 10 depicts a cross-sectional view of a pad-defined solder joint in accordance with a preferred embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although certain preferred embodiments of the present invention will be shown and described in detail, it should be understood that various changes and modifications may be made without departing from the scope of the appended claims. The scope of the present invention will in no way be limited to the number of constituting components, the materials thereof, the shapes thereof, the relative arrangement thereof, etc., and are disclosed simply as an example of the preferred embodiment. Although the drawings are intended to illustrate the present invention, the drawings are not necessarily drawn to scale. 
     Referring to the drawings, FIG. 1 shows a cross-sectional view of a module  10 . The module  10  has an integrated circuit chip  11  electrically and mechanically connected to a surface  13  of a chip carrier or substrate  12  in a first level assembly process, using a process well known in the art. A plurality of conductive pads  14  are affixed to a surface  15  of the substrate  12  (see also FIG. 2) by a process known in the art. The substrate  12  is preferably a “non-wettable” insulative material, such as ceramic, FR4, IBM&#39;s Dry-Clad™, LCP (Liquid Crystal Polymer) polyimide, etc. A non-wettable material is one that solder will not adhere to or “wet” to. 
     FIG. 3 shows a printed circuit card or board  16 , upon which the module  10  (shown in FIGS. 1 and 2) will be electrically mounted in a second level assembly process. The board  16  is preferably a non-wettable insulative material, i.e., ceramic, FR4, IBM&#39;s Dry-Clad™, LCP (Liquid Crystal Polymer) polyimide, etc. The board  16  has a plurality of conductive pads  18  mounted on a surface  17 , which correspond with the conductive pads  14  on surface  15  of the module  10  (see FIGS.  1  and  2 ). 
     The conductive pads  14 ,  18  of the module  10  and the board  16  are preferably copper, but may be any other suitable conductive material known and used in the art. The conductive pads  14 ,  18  of the module  10  and the board  16  are mounted on the surfaces  15 ,  17  of the substrate  12  and the board  16  using techniques well known in the art. It should be understood that the number and placement of the conductive pads  14 ,  18  are limited in the accompanying figures only to facilitate a clear explanation of the present invention, and are not intended to be limiting in any way. 
     FIG. 4 shows an enlarged view of the module  10 . A mask  20  is applied to the surface  15  (see FIGS. 1 and 2) of the substrate  12  of the module  10  using heat and pressure, or other processes well known in the industry. The mask  20  is made of a non-wettable material, such as an epoxy. The mask  20  covering the surface  15  of the substrate  12  has a plurality of openings  24  and  26  created by a photolithographic process, or other process known and used in the industry, e.g., laser ablation, etc. In this example, the openings  24  within mask  20 , located at the corners of the module  10  are larger than the conductive pads  14 . In contrast, the openings  26  coinciding with the location of the remaining conductive pads  14 , and are smaller than the conductive pads  14  (represented by dashed lines). 
     FIG. 5 shows an enlarged view of the board  16 . A mask  22  is applied to the surface  17  (see FIG. 3) of the board  16  using heat and pressure, or other processes well known in the industry. The mask  22  is made of a non-wettable material, such as an epoxy. The mask  22  covering the surface  17  of the board  16  has a plurality of openings  28  and  30 , which are similar to and coincide with the openings  24 ,  26  of the mask  20  covering the substrate  12 . In this example, the openings  28 , located at the corners of the board  16  are larger than the conductive pads  18  of the board  16 . The openings  30  coinciding with the location of the remaining conductive pads  18  are smaller than the conductive pads  18 . 
     Solder paste or flux (not shown) is deposited on the conductive pads  14 ,  18  of the substrate  12  and the board  16  within the openings  24 ,  26 ,  28 ,  30  of the masks  20 ,  22 . The solder paste may be deposited by a screening process, but other known methods may be used. After the solder paste has been deposited, the substrate  12  is placed mask side down in a graphite frame or “boat”  32  (shown in FIG.  6 ). The boat  32  contains solder balls  34 , each having the same diameter and volume. Heat is applied such that the solder balls  34  begin to soften and wet onto the surfaces of the conductive pads  14  exposed by the openings  24 ,  26  of the mask  20 . In particular, the solder balls  34  will adhere to or wet to the entire conductive pad  14  in regions of the substrate  12  exposed by opening  24 . Whereas, in regions of the substrate  12  in which the conductive pad  14  is exposed by opening  26 , the wettable area is limited to the size of the opening  26  (which is less than the size of the conductive pad  14 ), rather than the entire conductive pad  14 . The temperature is reduced causing the solder balls  34  to solidify. The module  10 , having solder balls  34  attached to the substrate  12 , is removed from the boat  32 , and is illustrated in FIG.  7 . 
     The module  10  is then placed on the board  16 , such that the conductive pads  18  of the board  16  are aligned with the corresponding conductive pads  14  of the substrate  12 . Heat is again applied causing a free end  35  (see FIG. 7) of the solder balls  34  to wet onto the wettable areas of the board  16 , i.e, the surfaces of the conductive pads  18  that are exposed by the openings  28 ,  30  of the mask  22 . 
     FIG. 8 shows a BGA package  36  formed after the solder balls  34  (shown in FIG. 7) wet onto the wettable areas of the conductive pads  18  of the board  16 . It should be understood that in this example the wettable areas of the board  16  correspond to and are similar to the wettable areas of the substrate  12 . The BGA package  36  has a plurality of interconnections or solder joints  38  which connect the module  10  to the board  16 . Pad-defined solder joints  40  are selectively placed at the high stress areas of the BGA package  36 , in this example at the corners, while mask-defined solder joints  42  are located therebetween. 
     FIG. 9 shows a cross-sectional view of a mask-defined solder joint  42 , having a diameter  56 . The masks  20 ,  22  partially covering the conductive pads  14 ,  18  have a thickness T greater than the thickness t of the conductive pads  14 ,  18 . Further, the openings  26 ,  30  of the masks  20 ,  22  have diameters  52  smaller than the diameters  50  of the conductive pads  14 ,  18 . These conductive pads  14 ,  18 , partially covered by the masks  20 ,  22  are referred to as “captured” pads. Captured pads have a wettable area defined by the diameter  52  of the openings  26 ,  30  in the masks  20 ,  22 , rather than by the diameter  50  of the conductive pads  14 ,  18 . As a result, the amount of solder that wets to the conductive pads  14 ,  18  in a mask-defined solder joint  42  is reduced. Captured pads develop a collar or cylindrical portion  48  at the top and bottom of the solder joint  42 . The cylindrical portion  48  at each end of the mask-defined solder joint  42  increases the equilibrium height H of the solder joint  42 . Equilibrium height of a solder joint is the natural height achieved as the solder wets to the conductive pads and forces the module  10  and the board  16  farther apart, or the height at which the pressure within solder acting on the contact area of the pad balances the weight of the BGA package applied to that solder joint. This is true because the height of a truncated sphere is increased as the diameter of its base is decreased. 
     FIG. 10 shows a cross-sectional view of a pad-defined solder joint  40 , having a diameter  58  (which is slightly less than the diameter  56  of the mask-defined solder joint). These “uncaptured pads” (the wettable area is not limited by the diameter of the mask opening) have a wettable area defined by the diameter  50  of the conductive pads  14 ,  18 . The solder balls  34  (shown in FIG. 7) adhere or wet to the entire diameter  50  of the conductive pads  14 ,  18 . As a result, the equilibrium height of the pad-defined solder joints  40  is slightly less than that of the mask-defined solder joints  42 . 
     It is important to note that the equilibrium height H of the mask-defined solder joints  42  is typically greater than that of the pad-defined solder joints  40 , for a given solder volume. Solder joints with greater equilibrium heights tend to be more compliant and less likely to fracture. However, stress concentrations occur at a plurality of discontinuities  54  of the mask-defined solder joints  42  (refer to FIG.  9 ), where the mid-section  46  abruptly meets the cylindrical portions  48 . These stress concentrations tend to reduce the fatigue life of mask-defined solder joints  42 . In contrast, pad-defined solder joints  40  do not have the discontinuities  54  found in mask-defined solder joints  42 , thereby eliminating stress concentrations within the pad-defined solder joints  40 . Therefore, in order to increase the fatigue life of solder joints  38  a combination of mask-defined solder joints  42  and pad-defined solder joints  40  are used. In particular, pad-defined solder joints  40  eliminate the stress concentrations of the mask-defined solder joints  42 . In addition, the mask-defined solder joints  42  increase the equilibrium height H of the pad-defined solder joints  40 , making the pad-defined solder joints  40  more compliant. Therefore, by combining the attributes of the pad-defined solder joints and the mask-defined solder joints the fatigue life of the solder joints  38  within the BGA package  36  is increased. 
     It is recommended that approximately 80-90% of the solder joints  38  be mask-defined solder joints  42 , while the remaining 10-20% be pad-defined solder joints  40 . A majority of the solder joints  38  are mask-defined solder joints  42  because the pad-defined solder joints  40  tend to compress the mask-defined solder joints  42 . This is attributable to the lower normal equilibrium height of the pad-defined solder joints. By utilizing a majority of mask-defined solder joints  42  the average height of the BGA package  36  is approximately equal to the equilibrium height H of a BGA package containing all mask-defined solder joints  42 . Since the pad-defined solder joints  40  lack the stress concentrations present within the mask-defined solder joints  42 , the pad-defined solder joints  40  should be selectively placed in the high stress regions, such as beneath the corners and edges of the chip  11  or at the corners or edges of the BGA package  36 . 
     It is important to note that the accompanying figures do not depict the placement of pad-defined solder joints selectively placed at the corners and edges of the chip  11  only for the simplicity of the disclosure. However, it should be understood that the present invention includes the use of pad-defined solder joints at the corners and edges of the chip  11 , as well as any other high stress locations within the BGA package. 
     It is important to note that while the present invention was described wherein both the module and the board had masks with identical openings, the invention is not limited to the example in this disclosure. It is possible that only one side of the BGA package, either the module or the board, be masked having a combination of pad-defined and mask-defined solder joint openings, while the other side uses solely pad-defined or mask-defined solder joints. Likewise, both sides may be masked but it is not necessary that the openings at corresponding conductive pads be identical, etc. There are vast combinations of solder joints within the BGA package  36  which are not to be considered limited by this disclosure. 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.