Abstract:
A semiconductor packaging system includes a semiconductor die and a solder pillar on a side of the semiconductor die extending outwardly from a side of the semiconductor die. The solder pillar electrically couples to an electrical contact of a packaging substrate, even when access to the electrical contact is limited by a mask.

Description:
TECHNICAL FIELD 
       [0001]    The present description generally relates to flip chip assembly and, more specifically, to the use of new shapes for portions of solder material. 
       BACKGROUND 
       [0002]    Flip chip assembly is a process in use today for integrated circuit (IC) packaging. A semiconductor die is created with metallized contact pads on a surface of the die. A mask is then laid down and solder is plated over the mask. The mask is removed and small solder balls are then formed on the metallized contact pads by reflowing the solder material. Depending on the process, the die is then cut and flipped over so the solder balls align with metal contacts on a packaging substrate. The die is placed on the packaging substrate, and the solder is then reflowed to ensure that the solder makes sufficient electrical contact with the metal contacts on the packaging substrate. Insulating underfill is then applied to the package. The result is a semiconductor package where the inputs and outputs of the die are in electrical communication with the packaging substrate. An overall system may have other components thereon, such as processors, passive components, power components, and the like, which are then interfaced with the semiconductor die through, for example, traces on the package. 
         [0003]      FIG. 1  illustrates a conventional assembly technique employing solder balls. A semiconductor die  101  includes a metallized contact pad  102 , which is in contact with a solder ball  103  (also known as a “flip chip bump”). A package substrate  104  includes a copper contact  105  and the mask  106 . As seen in  FIG. 1 , the mask  106  prevents the solder ball  103  from making electrical contact with the copper contact  105 , even after the solder ball  103  is reflowed.  FIG. 1  illustrates a defect that happens from time to time during flip chip techniques that use solder balls. Such a lack of contact is sometimes caused by a shift in solder mask registration or other kind of misalignment. 
         [0004]    Currently there are two solutions available, one of which is shown in  FIG. 2 . The assembly technique of  FIG. 2  includes filling the solder mask opening of the package substrate  104  with the solder  107  to facilitate contact with the solder ball  103 . The assembly technique is known as solder on pad (SOP). The SOP technique has several disadvantages. SOP is relatively hard to control for both coplanarity and quality. SOP involves an additional thermal cycle, and precautions are necessary to maintain surface quality for good solder attachment. Furthermore, in practice, current SOP processes can only be used for pitches of 150 μm and larger. 
         [0005]    Another solution uses copper posts, as shown in  FIG. 3 .  FIG. 3  shows a process that uses a copper post  301  and a solder cap  302  to make electrical contact between the copper contact  105  and the semiconductor die  101 . The copper post technique of  FIG. 3  has several disadvantages, as well. For instance, the copper post technique is relatively expensive when compared to the technique of  FIGS. 1 and 2 . Furthermore, copper is quite rigid, and some materials within the semiconductor die  101  are somewhat brittle, so that when stress is applied to the assembly, the semiconductor die  101  can be mechanically damaged. 
       BRIEF SUMMARY 
       [0006]    According to one embodiment, a semiconductor package system includes a semiconductor die and a solder pillar on a side of the semiconductor die extending outwardly from a side of the semiconductor die. 
         [0007]    According to another embodiment, a method for packaging a semiconductor die includes disposing photo resist upon a die, the die having a first metal contact, and the photo resist defining a volume that is substantially pillar-shaped and aligned with the first metal contact. The method also includes providing solder material within the volume, reflowing the solder material within the volume, and removing the photo resist to expose the solder material. 
         [0008]    According to yet another embodiment, a semiconductor die has multiple conductive pads, each of the conductive pads providing an interface to circuitry within the semiconductor die. The die also has means for facilitating electrical communication with contacts on a package substrate, each of the means for facilitating corresponding to, and in contact with, one of the conductive pads and having a pillar shape and being formed of solder material. 
         [0009]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description that follows may be better understood. Additional features and advantages will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the technology of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    For a more complete understanding of the present disclosure, reference is now made to the following description taken in conjunction with the accompanying drawings. 
           [0011]      FIG. 1  is a schematic illustrating a conventional assembly technique employing solder balls. 
           [0012]      FIG. 2  is a schematic illustrating a conventional SOP assembly technique employing solder balls. 
           [0013]      FIG. 3  is a schematic illustrating a conventional assembly technique employing copper posts. 
           [0014]      FIG. 4  is an illustration of an exemplary system adapted according to one embodiment of the disclosure. 
           [0015]      FIGS. 5A and 5B  are illustrations of an exemplary technique, according to one embodiment of the disclosure, for creating a cylindrical solder bump and making preliminary contact with a package substrate. 
           [0016]      FIG. 6  is an illustration of three different basic shapes that can be used for solder pillars according to various embodiments. 
           [0017]      FIG. 7  is a schematic illustrating an exemplary wireless communication system in which an embodiment may be advantageously employed. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 4  is an illustration of an exemplary system  400  adapted according to one embodiment. The system  400  includes a semiconductor die  401  and under bump metallurgy (UMB)  402 , which provides an electrical contact between the solder bump  403  and circuitry (not shown) within the die  401 . While not shown in  FIG. 4  for simplicity, it is understood that the solder bump  403  is aligned and moved relative to a metal contact  405  so as to make electrical contact therewith. 
         [0019]    In the present example, the solder bump  403  is shaped substantially as a cylinder. That is, in this example, the solder bump  403  conforms to a basic cylinder shape but deviates from a true cylinder shape at the interface with the UBM  402 . The circumference of the cylinder shape can be smaller than that of the solder ball of  FIG. 1 , while the elongated dimension allows for electrical contact to be made between the UBM  402  and the metal contact  405  of the package substrate  404 . In short, the cylindrical shape of the solder bump  403  facilitates making preliminary contact with the metal contact  405  through the opening in the mask  406 . 
         [0020]      FIGS. 5A and 5B  are illustrations of an exemplary technique  500  for creating a cylindrical solder bump and making preliminary contact with a package substrate. In block  501 , the UBM  402  is created on the semiconductor die  401  by, e.g., sputtering. In an exemplary embodiment, the die pad may be coated with the UBM. The UBM  402  creates an electrical interface with circuitry inside of the semiconductor die  401 . 
         [0021]    In block  502 , photo resist is applied in a pattern to create a mask  550 . In block  502 , the view of the photo resist is a cut-away view, and it is understood that the photo resist creates a substantially cylindrical volume  551 . The technique uses the pattern of the mask  550  to provide the shape of the solder bump (as explained in more detail below). Any of a variety of materials, such as color photoresists, can be used to create the mask  550 . In one particular example, polyamide is used as a material for the mask  550  because its relatively high heat resistance allows for a reflow process to be performed before the mask  550  has been removed (as explained below with respect to the block  504 ). 
         [0022]    In block  503 , the solder material  552  is applied. In one example, the solder is plated as a eutectic mixture on the mask  550 . The mask  550  allows the solder material  552  into the volume  551 , thereby creating a cylindrical shape with a cap on top. 
         [0023]    Further in block  503 , the solder material  552  is reflowed. For instance, the structure can be heated beyond the melting point of the solder but not so high as to melt or char the mask  550  or the die  401 . Reflowing after plating is used in this embodiment to cause the different layers of solder material to coalesce. In some embodiments, a reflow profile is used, wherein the structure is slowly heated up and cooled down. The solder material  552  is constrained by boundaries of the volume  551 , and the shape of the eventual solder bump is dictated, at least in part, by the shape of the volume  551 . 
         [0024]    In block  504 , the mask  550  is removed after the solder material  552  has returned to a solid state. Any of a variety of techniques can be used to remove the mask  550 , such as, for example, stripping the mask  550  with a solvent, e.g., acetone. 
         [0025]    In block  505  buffing is performed to make a distal surface  553  substantially flat. Thus, in this example, the shape of the solder bump  403  is defined by the mask pattern, the plating process, and the buffing process. Buffing can be used to achieve a greater degree of coplanarity than could be achieved otherwise. In this context, coplanarity refers to the property of the surface  553  as it relates spatially to similar surfaces of other solder bumps (not shown) on the die  401 . In many embodiments, the distal surface  553  and similar surfaces of other solder bumps are substantially coplanar so that contact is made by all solder bumps to their respective package substrate contacts. For many applications, coplanarity on the order of a few microns is sufficient. Buffing, such as by chemical mechanical polishing, can be performed either before or after mask removal. 
         [0026]    In block  506  ( FIG. 5B ), the structure that includes the die  401  and the solder bump  403  is brought into proximity with the package substrate  404  and aligned with the metal contact  405 . In block  506 , some embodiments (as seen in  FIG. 5B ) include flipping the structure that includes the die  401  and the solder bump  403  so that it is spatially located above the package substrate  404 . Preliminary contact is made between the solder bump  403  and the metal contact  405 , for example by solder reflow. 
         [0027]    While the technique  500  is shown as a series of specific processes, various embodiments are not limited thereto. In fact, other embodiments may add, omit, modify, or rearrange various processes. For instance, after the block  506 , some embodiments perform an additional reflow process followed by an underfill process. The resulting package substrate assembly, including the die  401 , can be used in further manufacturing processing, such as disposing other components onto the package substrate  404  and installing the package substrate assembly in a device (e.g., a mobile device or other processor-based device). Furthermore, while the technique is illustrated with respect to a single solder bump  403 , it is noted that many embodiments will perform the technique for a multitude of solder bumps on a die (e.g., 800 solder bumps). Furthermore, while the embodiments above have been described with reference made to specific materials for the mask and the solder bump, it is noted that various embodiments may use any suitable solder or mask material. 
         [0028]    Additionally, while solder pillars have been shown above as substantially cylindrical in shape, other embodiments may include pillars of different geometries.  FIG. 6  is an illustration of three different basic shapes that can be used for solder pillars according to various embodiments, and  FIG. 6  is intended to be non-exclusive.  FIG. 6  includes a rectangular volume  601 , a triangular volume  602 , and a cylindrical volume  603 , though a variety of arbitrary shapes, such as octagonal volumes, are adaptable to various embodiments. 
         [0029]    Various embodiments of the invention provide advantages over prior art solutions. For instance, some embodiments offer more control of the structure than was provided with solder balls. The diameter of the pillar can be adjusted to easily fit within the aperture provided by the mask on the package substrate. In fact, the diameter of the column can be adjusted to compensate for alignment tolerance, thereby helping to ensure contact with the metal pad. Also, solder pillars offer the alignment benefits of copper posts while avoiding the rigidity of copper posts that can lead to damage to the semiconductor die when stress is applied to the structure. 
         [0030]    Furthermore, some embodiments offer better coplanarity than the SOP solution shown in  FIG. 2 , especially when buffing or another shaping process is performed on the pillars. Coplanarity generally becomes a greater issue as the number of bumps and contacts increases, and greater coplanarity can help increase yield. 
         [0031]      FIG. 7  shows an exemplary wireless communication system  700  in which an embodiment of the invention may be advantageously employed. For purposes of illustration,  FIG. 7  shows three remote units  720 ,  730 , and  740  and two base stations  750 ,  760 . It will be recognized that wireless communication systems may have many more remote units and base stations. The remote units  720 ,  730 , and  740  and the base stations  750 ,  760  can include any of a variety of components, such as memory units, Analog to Digital Converters (ADCs), Digital to Analog Converters (DACs), processors, delta sigma data converters, and the like (and the components can be manufactured from semiconductor dies such as the die  401  of  FIGS. 4 and 5 ). Embodiments can utilize package assemblies that include components wherein the components have been mounted on the package assemblies using solder pillar techniques described above.  FIG. 7  shows forward link signals  780  from the base stations  750 ,  760  to the remote units  720 ,  730 , and  740  and the reverse link signals  790  from the remote units  720 ,  730 , and  740  to the base stations  750 ,  760 . 
         [0032]    Generally, remote units may include cell phones, hand-held personal communication systems (PCS) units, portable data units such as personal data assistants, fixed location data units such as meter reading equipment, and/or the like. In  FIG. 7 , the remote unit  720  is shown as a mobile telephone, the remote unit  730  is shown as a portable computer, and the remote unit  740  is shown as a fixed location remote unit in a wireless local loop system. The base stations  750 ,  760  can be any of a variety of wireless base stations, including, e.g., cellular telephone base stations, wireless network access points (e.g., IEEE 802.11 compliant access points), and the like. Although  FIG. 7  illustrates remote units and base stations, the disclosure is not limited to these exemplary illustrated units. 
         [0033]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the technology as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.