To continue Moore's Law trend toward smaller, sleeker handset devices, designers can leverage advanced packaging and interconnect methods to meet the miniaturization requirements.
PoP (package-on-package) stacking technology has found its killer application in the cellular handset. Mobile smartphone handsets require both state-of-the-art semiconductors and packaging technology to meet increasing demands on size and functionality while meeting strict manufacturing cost and reliability requirement goals.
The advantages of PoP stacking of components extend beyond area savings. Many handset designs can be created with a basic platform approach similar to that of a PC by combining the pc-board, chip set and various peripheral components to make a phone with specific feature set.
Chip sets in these devices are often designed to accommodate package stacking, which helps make derivative products from a base design. One commonly used method varies the amount of memory to differentiate one product from another.
When the memory is placed in PoP stacks, it is a simple matter to substitute one or more memory components in the stack for different components. Each of these contains a different amount of memory or a different set of features. In this way, a family of products can be quickly and easily implemented.
Changing the number and density of memory components in the common stack footprint easily differentiates a variety of product offerings in a family. Apple, for instance, introduced the iPhone in 4- and 8- GByte configurations as an assembly-time, build-option by Apple.
The NAND flash TSOP packages in the iPhone each contained stacks of four NAND flash die, with the packages stacked on top of each other. This gives Apple the flexibility to build a 4- or 8-Gb configuration by selecting the appropriate stack of memory. Apple can also select the configuration as a last decision before handset assembly. This permits the manufacturer to react quickly to changing market conditions in the component or handset markets.
Die stacking alone requires stacking variants be made when die are assembled into packages rather than during the final assembly of the end product. This increases the amount of time needed to make a differentiated product. Using PoP stacking simplifies and streamlines the process. It allows end-product manufacturers to stack different component sets together in packages, allowing changes late in the process.