Microelectronic devices generally have a die (i.e., a chip) that includes integrated circuitry having a high density of very small components. In a typical process, a large number of dies are manufactured on a single wafer using many different processes that may be repeated at various stages (e.g., implanting, doping, photolithography, chemical vapor deposition, plasma vapor deposition, plating, planarizing, etching, etc.). The dies typically include an array of very small bond-pads electrically coupled to the integrated circuitry. The bond-pads are the external electrical contacts on the die through which the supply voltage, signals, etc., are transmitted to and from the integrated circuitry. The dies are then separated from one another (i.e., singulated) by dicing the wafer and backgrinding the individual dies. After the dies have been singulated, they are typically “packaged” to couple the bond-pads to a larger array of electrical terminals that can be more easily coupled to the various power supply lines, signal lines, and ground lines.
An individual die can be packaged by electrically coupling the bond-pads on the die to arrays of pins, ball-pads, or other types of electrical terminals, and then encapsulating the die to protect it from environmental factors (e.g., moisture, particulates, static electricity, and physical impact). In one application, the bond-pads are electrically connected to contacts on an interposer substrate that has an array of ball-pads. FIG. 1A schematically illustrates a conventional packaged microelectronic device 10 including an interposer substrate 20 and a microelectronic die 40 attached to the interposer substrate 20. The microelectronic die 40 has been encapsulated with a casing 30 to protect the die 40 from environmental factors.
Electronic products require packaged microelectronic devices to have an extremely high density of components in a very limited space. For example, the space available for memory devices, processors, displays, and other microelectronic components is quite limited in cell phones, PDAs, portable computers, and many other products. As such, there is a strong drive to reduce the surface area or “footprint” of the microelectronic device 10 on a printed circuit board. Reducing the size of the microelectronic device 10 is difficult because high performance microelectronic devices 10 generally have more bond-pads, which result in larger ball-grid arrays and thus larger footprints. One technique used to increase the density of microelectronic devices 10 within a given footprint is to stack one microelectronic device 10 on top of another.
FIG. 1B schematically illustrates the packaged microelectronic device (identified as 10a) of FIG. 1A stacked on top of a second similar microelectronic device 10b. The interposer substrate 20 of the first microelectronic device 10a is coupled to the interposer substrate 20 of the second microelectronic device 10b by large solder balls 50. One drawback of the stacked devices 10a-b is that the large solder balls 50 required to span the distance between the two interposer substrates 20 use valuable space on the interposer substrates 20, which increases the footprint of the microelectronic devices 10a-b. 
FIG. 2 schematically illustrates another packaged microelectronic device 60 in accordance with the prior art. The device 60 includes a first microelectronic die 70a attached to a substrate 80 and a second microelectronic die 70b attached to the first die 70a. The first and second dies 70a-b are electrically coupled to the substrate 80 with a plurality of wire-bonds 90. The device further includes a casing 95 encapsulating the dies 70a-b and wire-bonds 90. One drawback of the packaged microelectronic device 60 illustrated in FIG. 2 is that if one of the dies 70a-b fails a post-encapsulation quality control test because it is not properly wire bonded to the substrate 80 or for some other reason, the packaged device 60, including the good die 70, is typically discarded. Accordingly, encapsulating multiple dies together reduces the yield of the resulting packages.