Abstract:
An electrical package and a method of forming the electrical package, where the electrical package has a substrate with a frontside, an intergrated circuit coupled to the frontside of the substrate, and at least one non-collapsible metal connector created on the frontside of the first substrate.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority and the benefit of U.S. Provisional application, Ser. No. 61/389,731 filed Oct. 5, 2010. 
     
    
     FIELD OF DISCLOSURE 
       [0002]    This disclosure relates generally to integrated circuit packaging, and in particular to package on package systems. 
       BACKGROUND 
       [0003]    Package on Package (“POP”) is a packaging system that allows one IC package to be coupled to another IC package providing more functionality in less space. Signals may be routed through each package. 
         [0004]    Coupling of IC packages is desirable and may reduce the size of the end user device.  FIG. 1A  shows an exemplary POP system  100 . A first die  104  is mounted onto a first package substrate  108 . The package substrate  108  has solder balls  112  on a front side  116  of the package substrate  108 . The solder balls  112  provide electrical connectivity to a second substrate package  120  through conducting pads  128 . A mold compound  124  is formed over the solder balls  112 . A second package  126  is formed over the solder balls and die  104  to form a package on package system. 
         [0005]    Disadvantages exist in shrinking the thickness of the bottom package  122 . For example, if the height of the solder ball  112  is reduced, the pitch between the solder balls decreases. The pitch is the distance between each solder ball  112 . As the pitch decreases, bridging problems with the solder balls occur during reflow of the solder balls  112 . Reflow process is applied so that the solder balls  112  provide an attachment mechanism between package to package stacks. 
         [0006]      FIG. 1B  shows an exemplary POP system  100  after a reflow process is applied. After reflow, bridging may result in electrical shorts  114  and reduce signaling capability between the packages and between the IC die and other surrounding circuits, reducing reliability of the POP system. Another disadvantage of the POP system  100  is that the solder ball attach is generally performed after the IC die  104  is attached to the substrate  108 . Because the package system  100  cannot be tested for functionality until after the IC die  104  is attached, the IC die may need to be discarded if the package is found to be not functioning properly. This results in increased expenses. 
         [0007]    Therefore, it would be desirable to develop an improved electronic package-on-package system without these disadvantages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1A  is a graphical illustration of a cross-sectional view of a package-on-package system; 
           [0009]      FIG. 1B  is a graphical illustration of a package-on-package system illustrating solder ball bridging; 
           [0010]      FIG. 2  is a flow chart illustrating an exemplary packaging process employing non-collapsible metal connectors that provide electrical connections between packages; 
           [0011]      FIG. 3A-3J  is a graphical illustration of a cross-sectional view of an exemplary packaging process employing copper cylinder connectors that provide electrical connections between packages; 
           [0012]      FIG. 4  is a block diagram showing an exemplary wireless communication system in which it may be advantageous to use a package-on-package system. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Inventive aspects are disclosed in the following description and related drawings directed to specific embodiments. Alternate embodiments may be devised without departing from the scope of the invention. Additionally, well-known elements may not be described in detail or may be omitted so as not to obscure relevant details. 
         [0014]    The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. Likewise, the term “embodiments of the invention” does not require that all embodiments include the discussed feature, advantage or mode of operation. The terminology used herein is for the purpose of describing particular embodiments. The described embodiments are to illustrate the teachings of the invention and are not intended to limit the embodiments of the invention described. 
         [0015]    Package on package (“POP”) systems may be accomplished through the use of non-collapsible metal connectors in place of solder balls for electrical connection between POP systems. Examples of non-collapsible metals include tin, gold, nickel, chrome and copper. The non-collapsible metals may be formed into three dimensional connector shapes including cylindrical, rectangular, or eliptical shapes. For example, a copper cylinder may be used in place of solder balls for electrical connection between POP systems. However, embodiments of the invention are not so limited to these formations. 
         [0016]    Use of non-collapsible metal connectors allows the height of the bottom package to be reduced without reduction of electrical connection reliability. This is because non-collapsible metal may not require the common solder ball reflow process which may spread or widen when reflow occurs. Therefore because non-collapsible metal does not require reflow and does not spread, there is no bridging. Thus electrical reliability is increased. Additionally, non-collapsible metal connectors may allow greater flexibility and reduced costs because whereas solder ball fabrication is generally performed by an assembly house, non-collapsible metal connectors may be formed by the substrate manufacturer. This allows the package to be tested prior to attaching the die and avoids having to discard the die where the package is found not to be functioning properly. 
         [0017]      FIG. 2  is a flow chart illustrating an exemplary POP process. A POP process begins at block  202  by receiving a package substrate.  FIG. 3A  is an illustration of a cross sectional view of an exemplary first package substrate  300 . The substrate may be formed from any suitable material, including organic material or inorganic material or a combination of both. For example, the substrate may be formed of glass or silicon or ceramic. At block  204  of  FIG. 2 , one or more non-collapsible metal connectors are formed, for example, by a lithographic mask and electrolytic plating.  FIG. 3A  illustrates as an example, copper cylinder connectors  302  formed on the substrate  300  The copper cylinder connectors are formed on the frontside  301  of the substrate  300 .  FIGS. 3B and 3C  illustrate an optional solder cap  304  which may be formed over the copper cylinders  302 . 
         [0018]    Because the copper cylinder connectors  302  do not themselves reflow, the width, diameter and pitch of the copper cylinder connectors  302  are more easily maintained. Pitch is the distance between each copper cylinder. The height of the copper cylinder connector may be designed independent of pitch considerations. That is, the height of the copper cylinder may be reduced without consideration of the bridging problems suffered by the solder ball connect method. Therefore the height of the copper cylinder may be reduced, even if a tight pitch is desired. 
         [0019]    In one embodiment, the copper cylinder connector is designed to have reduced height in order to minimize the size of the package height. For example, the height of the copper cylinder may be at about the same height of an IC die which may be later attached to the substrate. In an alternative embodiment, where the IC die is embedded within the first substrate  300 , the height of the copper cylinder may be reduced to the height necessary to make the connection between a first package and a second package. The pitch may be designed independently to the desired number of inputs/outputs between each stacked package. In an exemplary embodiment, the pitch may vary between the copper cylinders  302  as may the actual diameters of the copper cylinders. In another exemplary embodiment, the pitch may be constant between copper cylinders  302 . As the pitch shrinks, the diameter of the copper cylinders may shrink to further accommodate a smaller package and maintain a minimum number of input/outputs. 
         [0020]    The exemplary process continues at block  206  an IC die is attached to the substrate  300 . The term IC die is defined to include any type of IC device, chip or logic device. Any method of die attach may be used, including for example a flip chip, direct die attach, or wire bonding.  FIG. 3D  is a cross sectional view of an exemplary IC die shown as a flip chip  308  attached to the substrate  300 . To provide additional stability to the flip chip  308  attached to the substrate  300 , underfill molding  310  may be applied as shown in  FIG. 3E . However, the underfill molding  310  may be omitted if not desirable. For example, if wire bonding is used as the IC die attach method at block  206 , the underfill molding  310  may be omitted. 
         [0021]    After the IC die attach is performed  206 , at block  209  either option  1  or option  2  may be selected. If option  1  is selected, at block  210  an overmold  312  may be formed over the copper cylinders  302  and over the optional solder cap  304  to encapsulate the assembly.  FIG. 3F  is a cross sectional view of the overmold  312  formed over the copper cylinders  302  and IC die  308 . If at block  204  the optional solder cap  304  had been formed over the copper cylinder  302 , then overmold  312  would also encapsulate the solder cap  304 . An alternative embodiment known as flange POP may be used instead. In the flange POP alternative embodiment, the overmold is formed over the IC die  308  only and not the copper cylinders  302  or the optional solder cap  304 . 
         [0022]    As an alternative embodiment after the IC die attach is performed  206 , the overmold  210  process may be omitted and a bare die methodology used instead. Bare die methodology means that no overmold is formed over the IC die  208 . This alternative embodiment is illustrated at block  209  by selecting option  2  which bypasses blocks  210  and  212  and leads directly to block  214  which is described later. 
         [0023]    At block  212 , a through mold via process is performed, exposing the copper cylinder  302  either through grinding or laser. This process allows an electrical input/output connection to be made through the molding on the bottom package  350  in  FIG. 3H  to a second package which will be formed later in the process at block  218 .  FIG. 3G  is a cross sectional view of the exposed copper cylinders  302  as a result of the through mold via process. If at block  204  the optional solder cap was formed over the copper cylinder, then block  212  will involve the step of exposing the top of the optional solder cap  304 . 
         [0024]    After block  206  option  1  or option  2 , at block  214 , solder balls are attached at the bottom of the substrate  300 .  FIG. 3H  is a cross sectional view of the packaging system after block  214  is performed with solder balls  316 . Generally the overall process  200  thus described is performed on either a package strip or singulated package. A package strip consists of many units of packages such as the one described. Therefore at block  216 , singulation of the strip may occur. Singulation is the process of cutting the strip into single packages, resulting in the individual separation of a single package  350  shown in  FIG. 3H . Singulation may occur prior to package on package formation at block  218 . 
         [0025]    At block  218 , a second package is coupled to the first package  350  as illustrated in  FIGS. 3I and 3J .  FIGS. 3I and 3J  shows two exemplary second packages. Note that  FIGS. 3I and 3J  shows the first package  350  and the second package  370  or  380  respectively, as being slightly apart in order to clearly illustrate the exemplary embodiments. However it should be understood that the first package  350  and the second package  370  or  380  are physically connected.  FIG. 3I  shows an exemplary second package  370  having solder balls  368  which may connect to the copper cylinders  302  on the first package, thereby electrically connecting the first package  350  and the second package  370 . Alternatively, the solder balls  368  on the second package  370  may connect to the optional solder balls  304  on the first package  350 . 
         [0026]      FIG. 3J  shows an exemplary second package  380  having copper cylinders  372  placed on the backside  361  of the package substrate  360  which may connect to the copper cylinders  302  on the first package  350 , thereby electrically connecting the first package  350  and the second package  380 . 
         [0027]    The exemplary embodiments as disclosed herein are used to illustrate the inventive teachings. Other embodiments may be practiced without departing from the spirit and scope of the invention. For example, package  350  may be comprised of any type of IC die. Instead of a flip chip  308 , there may be vertically stacked IC die or IC die located horizontally adjacent or elsewhere within the same plane. Similarly, the second package  370  may be comprised of any set of IC die. Additionally the embodiments disclosed herein are not limited to two stacked packages, but may include additional stacked packages or other packages within the same plane. The signaling through the copper cylinders  302  is not limited by the exemplary embodiments disclosed herein. For example, signaling may occur between two stacked packages  350  and  370  through the copper cylinders  302 . Alternatively, signaling communication may occur between a bottom package  350  having copper cylinders  302  which is in communication with a PCB, mother board or with an IC die directly. 
         [0028]      FIG. 4  shows an exemplary wireless communication system  400  in which an embodiment of an electronic package-on-package system may be advantageously employed. For purposes of illustration,  FIG. 4  shows three remote units  420 ,  430 , and  450  and two base stations  440 . It should be recognized that typical wireless communication systems may have many more remote units and base stations. Any of remote units  420 ,  430 , and  450 , as well as the base stations  440 , may include an electronic package-on-package system such as disclosed herein.  FIG. 4  shows forward link signals  480  from the base stations  440  and the remote units  620 ,  630 , and  650  and reverse link signals  490  from the remote units  420 ,  430 , and  450  to base stations  440 . 
         [0029]    In  FIG. 4 , remote unit  420  is shown as a mobile telephone, remote unit  430  is shown as a portable computer, and remote unit  450  is shown as a fixed location remote unit in a wireless local loop system. For example, the remote units may be cell phones, hand-held personal communication systems (PCS) units, portable data units such as personal data assistants, or fixed location data units such as meter reading equipment. Although  FIG. 4  illustrates certain exemplary remote units that may include an electronic package-on-package system as disclosed herein, the package is not limited to these exemplary illustrated units. Embodiments may be suitably employed in any electronic device in which an electronic package-on-package system is desired. 
         [0030]    While exemplary embodiments incorporating the principles of the present invention have been disclosed hereinabove, the present invention is not limited to the disclosed embodiments. Instead, this application is intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.