Package-on-Package (POP)


Package-on-Package (POP), as its name implies, is a semiconductor packaging innovation that involves the stacking of two or more packages on top of one another.  Signals are routed between the packages through standard package interfaces.  Obviously, one advantage of this vertical combination of different packages is board space savings.  POP is a good packaging solution for applications that require more features in less space, such as digital cameras, PDA's, MP3 players, and mobile gaming devices.


POP assemblies today usually consist of just two packages, such as a memory device mounted on top of a logic device. Most companies that are developing POP assembly capabilities are leveraging well-established assembly processes and infrastructure (such as those used in CSP, BGA and flip-chips), so that little or no development will be required for the top package.  The bottom or base package may likewise employ die stacking in order to allow the combination of analog functionality or flash memory to the logic chip.


Figure 1. Examples of Package-on-Package (POP) Structures

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According to Amkor's website, POP is an enabling technology that offers the following benefits:  1)  it provides OEM's and EMS with a platform for effective 3-D integration of logic and memory;  2) it simplifies the business logistics of stacking; 3) it allows integration control at the system level, thereby facilitating the optimization of stack combinations; 4) it eliminates margin stacking and expands technology reuse; 5) it helps mitigate the impact of high costs usually associated with multimedia processing; and 6) it makes mass customization of systems for various usage requirements possible.  Figure 1 shows an example of a package-on-package (POP) structure.


As of this writing (2006), the top package of a typical POP may have an I/O interface consisting of hundreds of pins, with an I/O pitch of 0.65 mm.  The industry may soon move to 0.5 mm pitch though.  The bottom package of a typical POP employs a finer I/O pitch, i.e., it currently uses a pitch of 0.5 mm that may soon be replaced by a 0.4 mm pitch. Wafer thinning is an essential part of POP assembly, in order to make up for the increased height requirements of vertical integration.  POP package heights of 1.2 mm to 1.6 mm are now available in the industry in various configurations.  


See Also:   CSP;  Ball Grid Array;  Flip ChipDie Stacking




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