Although they are not exactly new, stacked packages have become a beneficiary of this handheld explosion, with these packages expected to grow from 130 million units in 1999 to more than one billion in 2004.
Stacks can be two or more die in a single package or individual packages vertically stacked.
Stacked TSOPs are an example of individual packages vertically stacked, while stacked CSPs vertically stack individual die within a single package. Stacked CSPs may feature flexible or rigid substrates. Many stacked packages integrate memory devices, such as flash and SRAM, but ASIC devices and MCMs have been integrated in stacks, as well. Most of the early stacked packages were designed for low-volume applications, such as military and aerospace. However, the stacked packages that are driving the volume projections cited combine a flash chip and an SRAM chip for the cellular handset market.