Abstract:
A chip scale package (CSP) device includes a CSP having a semiconductor die electrically coupled to a plurality of solder balls. A can having an inside top surface and one or more side walls defines a chamber. The CSP is housed in the chamber and is attached to the inside top surface of the can. A printed circuit board is attached to the solder balls and to the one or more side walls to provide support to the CSP and to the can. The CSP may be a Wafer-Level CSP. The can may be built from a metallic substance or from a non-metallic substance. The can provides stress relief to the CSP during a drop test and during a thermal cycle test.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to semiconductor packaging, and more particularly the invention relates to a Chip Scale Package (CSP) with a can attachment. 
       BACKGROUND 
       [0002]    CSPs are used in a wide variety of consumer electronics products. In particular, Wafer-Level CSP (WL-CSP)s are used in mobile phones, PDAs, lap top computers, printers and other applications. The WL-CSP integrated circuit (IC) is attached to a printed circuit board (PCB) by, for example, a conventional surface mount technology. The pads of the IC connect directly to the pads of the PCB through individual solder balls and thus do not require underfill encapsulation material. 
         [0003]    The WL-CSP offers several advantages over other types of CSPs (e.g., ball grid array-type, laminate-type). The WL-CSP does not require bond wires or interposer connections between the IC and the solder balls. Since the WL-CSP IC pads connect directly to the PCB, the inductance between IC and the PCB is minimized thereby improving signal quality. 
         [0004]    After a device having a WL-CSP attached to a PCB is built, the device is subjected to a drop test to measure its durability. The drop test is a technique for measuring the durability of a component by subjecting it to a free fall from a predetermined height onto a surface under prescribed conditions. 
         [0005]      FIG. 1  illustrates a device  100  including a PCB  104  to which is attached a WL-CSP  108  by a plurality of solder balls  112 . When subjected to a drop test, the device  100  is deformed due to the impact from a mechanical shock.  FIGS. 2 and 3  illustrate the device  100  during a drop impact. As shown in  FIGS. 2 and 3 , depending on the orientation during the impact, the PCB  104  may be bent upward or downward. The WL-CSP  108  attached to the PCB  104  tends to follow the deformation of the PCB  104 , which leads to uneven loading of the solder balls  212 . 
         [0006]    As WL-CSPs increase in complexity and functionality, the size of the silicon die (not shown) in the WL-CSP  108  and I/O count also increase. As the size of the silicon die (not shown) in the WL-CSP  108  increases, the solder balls at the periphery are subjected to increased loading and deformation because they are farther away from a neutral axis  216 . An increase in the thickness of the silicon die (not shown) in the WL-CSP  108  also causes the WL-CSP  108  to become less compliant to bending, resulting in deformation. 
         [0007]    The device  100  may also be subjected to a thermal cycle test, which is used to thermally cycle a semiconductor device between hot and cold temperatures to determine the durability of the device.  FIG. 4  illustrates the effect on the device  100  during a thermal cycle test. Since the silicon die (not shown) in the WL-CSP  108  expands and contracts at a much lower rate than the PCB  104  during a thermal cycle test, the solder balls  112  at the periphery are subjected to high shear stress, hence resulting in poor test reliability. 
         [0008]    Furthermore, a recent trend toward smaller bump pitch of the solder balls  112  has resulted in smaller diameter of the solder balls  112 . A smaller bump pitch of the solder balls  112  also results in smaller Under Bump Metallurgy (UBM). Since the UBM opening defines the contact area of the solder balls  112  to the silicon die (not shown) in the WL-CSP  108 , a smaller contact area lowers a threshold limit of the solder balls  112  to withstand shear and tensile loads during the drop test and the thermal cycling test. 
         [0009]    Unlike a ball grid array (BGA) device that generally has some redundant solder balls as dummy balls to improve the BGA device&#39;s drop test and thermal cycling reliability, a WL-CSP device&#39;s solder balls are all functional because the WL-CSP device is not designed to accommodate high I/O counts like a BGA device. Consequently, the drop test and thermal cycle test performance of the WL-CSP device cannot generally be improved by adding redundant solder balls. 
       SUMMARY OF THE DISCLOSURE 
       [0010]    A chip scale package (CSP) device includes a CSP having a semiconductor die electrically coupled to a plurality of solder balls. The CSP is housed in a can having an inside top surface and one or more side walls defining a chamber. The CSP is attached to the inside top surface of the can. The CSP may be a Wafer-Level CSP or any other type of CSP. 
         [0011]    The can is made from a metallic substance or a non-metallic substance. The side walls of the can are spaced from the CSP by a selected distance. The inside top surface of the can may be square, rectangular or any other shape. In one implementation, an opening is formed on a top surface of the can. The can enhances a drop test performance and a thermal cycle test performance of the CSP device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    For a more complete understanding of the features, example embodiments and possible advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures and in which: 
           [0013]      FIG. 1  illustrates a device including a printed circuit board to which is attached a WL-CSP; 
           [0014]      FIGS. 2 and 3  illustrate a device during a drop impact; 
           [0015]      FIG. 4  illustrates a device during a thermal cycle test; 
           [0016]      FIG. 5  illustrates a device in accordance with one embodiment; 
           [0017]      FIG. 6  illustrates a plan view of a CSP device with a metal can attachment in accordance with one embodiment; 
           [0018]      FIG. 7  illustrates a perspective view of a CSP device attached to a metal can; 
           [0019]      FIG. 8  illustrates an embodiment in which two CSPs are attached to a single metal can; 
           [0020]      FIG. 9  illustrates another embodiment in which two CSPs are attached to a single metal can; 
           [0021]      FIG. 10  illustrates another embodiment in which a CSP is attached to a metal can having an opening; 
           [0022]      FIG. 11  illustrates a device built by attaching the embodiment shown in  FIG. 10 ; and 
           [0023]      FIGS. 12 and 13  illustrate an exemplary method of manufacturing a CSP device with a metal can attachment in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0024]      FIG. 5  illustrates a device  500  in accordance with one embodiment. The device  500  includes a WL-CSP  504 . The WL-CSP  504  includes a silicon die  506  on which a metal layer  510  is formed. A plurality of solder balls  512  are attached to the metal layer  510  and are electrically coupled to the silicon die  506 . 
         [0025]    A can  516  in the form of an open box has a top member  528  and a side member  524 . The can  516  is placed over, and attached to, the WL-CSP  504 . The can  516  and the WL-CSP  504  are then attached to a PCB  508 . As shown in  FIG. 5 , the WL-CSP  504  is attached to the PCB  508  via the solder balls  512 . The PCB  508  supports the WL-CSP  504  through the solder balls  512  and supports the can  516  through the side member  524 . The inside top  520  of the can  516  is attached to the WL-CSP  504 . Hence, the WL-CSP  504  is enclosed in a cavity  528  defined by the can  516  and the PCB  508 . 
         [0026]    The side member  524  of the can  516  is spaced from the WL-CSP  504  by a separation distance D. The separation distance D may be varied if desired. In one implementation, the separation distance D may be zero, in which case the side member  524  touches the WL-CSP  504 . While the side member  524  is shown to be perpendicular to the top member  528  of the can  516 , it will be understood by those skilled in the art the side member  524  may be at other desired angles with respect to the top member  528 . The can  516  is made from a metal substance or alternatively from a suitable non-metallic substance, such as a composite, a plastic or glass. 
         [0027]    It will be understood by those skilled in the art that while the device  500  is implemented with a WL-CSP, other types of CSPs may also be used with the can  516 . It will also be understood that BGA type packages in which a silicon die is encapsulated in an encapsulating compound may also be attached to a can to improve drop test and thermal cycle test performances. Also, any other type of semiconductor die may used instead of a silicon die. 
         [0028]    Referring back to  FIG. 5 , the can  516  alleviates shear and tensile stress experienced by the solder balls  512  at the edges or periphery during a drop test. The can  516  through its side  514  absorbs some of the shear and tensile stress generated during the drop test, thus improving the device  500 &#39;s drop test performance. Likewise, the can  516  alleviates shear and tensile stress experienced by the solder balls  512  at the edges during a thermal recycle test, thus improving the device  500 &#39;s performance during the thermal recycle test. 
         [0029]    Furthermore, the can  516  adds to the rigidity of the device  500 , thus preventing the device  500  from deforming excessively during a drop test. Because a portion of the shear and tensile stress is absorbed by the can  516  during the drop test, the solder balls  512  at the edges or the periphery are subjected to lesser out-of-plane deformation compared to a CSP device without a can attachment. 
         [0030]    Moreover, the presence of the can  516  restricts the movement of the silicon die  506  with respect to the PCB  508  during a thermal cycle test. For example, the thermal coefficient of expansion value of the PCB is much closer to the thermal coefficient of expansion value of a metal can (e.g., copper metal can) than the thermal coefficient of expansion value of the silicon die  506  in a WL-CSP. Consequently, the can  516  (i.e., a metal can) expands and contracts at a closer rate to the rate of expansion and contraction of the PCB  508 , thus subjecting the solder balls  512  at the edges to be subjected to less shear deformation and stress. Also, the WL-CSP  504  with the can  516  attachment improves the device  500 &#39;s drop test performance and thermal cycle test performance without requiring redundant solder balls or other expensive solutions such as utilizing a redistributed line (RDL). 
         [0031]      FIG. 6  illustrates a plan view of the WL-CSP  504  attached to the can  516 . The WL-CSP  504  resides inside the cavity  524  of the can  516 . While the can  516  is illustrated as having a square shape, it will be understood by those skilled in the art that the can  516  can take other shapes (e.g., rectangular, round).  FIG. 7  illustrates a perspective view of the WL-CSP  504  attached to the can  516 . 
         [0032]      FIG. 8  illustrates an embodiment in which two CSPs  804  and  808  are attached to a single can  816 . Thus, the two CSPs  804  and  808  reside in the cavity of the can  816 . The CSPs  804  and  808  may be WL-CSP or any other type of CSPs. It will be understood that more than two CSPS may be attached to a single can. The embodiment of  FIG. 8  enables multiple CSPs to have enhanced drop test and thermal cycle test performances. Also, the size of the can  816  may be enlarged to shield selected areas of a PCB (not shown) from electromagnetic interference. 
         [0033]      FIG. 9  illustrates yet another embodiment in which two CSPs  904  and  908  are attached to a single can  912  but the CSPs  904  and  908  are separated from each other by an isolation wall  916 . The isolation wall  916  provides EMI isolation of the enclosed CSPs. The CSPs may be WL-CSPs or any other type of CSPs. It will be understood by those skilled in the art that a single can may accommodate more than two CSPs by having additional isolation walls. 
         [0034]      FIG. 10  illustrates yet another embodiment in which a CSP  1004  is attached to a can  1016  having an opening  1020 . The opening  1020  is formed about the center of the can  1016 . 
         [0035]      FIG. 11  illustrates a device  1100  built by attaching the embodiment of  FIG. 10  to a PCB  1104 . The opening  1020  provides additional flexibility to the can  1016  enabling it to bend during a drop test. This additional flexibility due to the opening  1020  enables the device  1100  to withstand greater shear and stress, thus improving the device  1100 &#39;s drop test and thermal cycling test performances. 
         [0036]    There are several advantages of the above-described embodiments. The CSP devices with the can attachment exhibit superior drop test thermal cycle test performances compared to other designs. Various computer simulations were run for a WL-CSP with a metal can attachment of 3.6×6×0.6 mm, F 8×8, 0.4 pitch, 0.25 mm ball, SAC405 solder. The simulation results indicate that the WL-CSP with the metal can attachment reduces peeling stress in solder balls by approximately a factor 2×. Also, the metal can attachment increases characteristic life of a WL-CSP by a factor of 6× compared to other designs. The metal can attachment to the WL-CSP also functions as a heat sink, thus dissipating heat from the silicon die in the WL-CSP. Also, the metal can provides EMI isolation between a WL-CSP and other electronic component in a circuit board, thereby improving the electrical performance of the WL-CSP. The metal can attachment to the WL-CSP makes it unnecessary for a separate EMI shield, which reduces the overall cost of the device. 
         [0037]      FIGS. 12 and 13  illustrate an exemplary method of manufacturing a CSP device with a metal can attachment in accordance with one embodiment. As shown in  FIG. 12 , CSP devices  1204 ,  1208  and  1212  are each attached to a respective chamber  1220 ,  1224  and  1228  formed on a metal frame  1232 . The chambers are each sized to house a CSP device. The metal frame  1232  containing the CSP devices is then sawed or diced along the lines L 1 -L 2  to create singular devices each capped with a metal can. A CSP device  1300  with a metal can attachment created from the process shown in  FIG. 12  is illustrated in  FIG. 13 . 
         [0038]    It will also be appreciated that one or more of the elements depicted in the drawings/figures can also be implemented in a more separated or integrated manner, or even removed or rendered as inoperable in certain cases, as is useful in accordance with a particular application. 
         [0039]    As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
         [0040]    The foregoing description of illustrated embodiments of the present invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed herein. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes only, various equivalent modifications are possible within the spirit and scope of the present invention, as those skilled in the relevant art will recognize and appreciate. As indicated, these modifications may be made to the present invention in light of the foregoing description of illustrated embodiments of the present invention and are to be included within the spirit and scope of the present invention. 
         [0041]    Thus, while the present invention has been described herein with reference to particular embodiments thereof, a latitude of modification, various changes and substitutions are intended in the foregoing disclosures, and it will be appreciated that in some instances some features of embodiments of the invention will be employed without a corresponding use of other features without departing from the scope and spirit of the invention as set forth. Therefore, many modifications may be made to adapt a particular situation or material to the essential scope and spirit of the present invention. It is intended that the invention not be limited to the particular terms used in following claims and/or to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include any and all embodiments and equivalents falling within the scope of the appended claims. Thus, the scope of the invention is to be determined solely by the appended claims.