Although it’s not presented at a 3D story to the public, the Raspberry Pi Foundation’s $25 alpha board based on a 700MHz ARM 11 processor is very much 3D because there’s an SDRAM stacked on top of the processor. Why? It’s for at least two of the reasons that drive 3D: BOM cost and product size. The alpha version of the Raspberry Pi board (see below) is a bit larger than the planned credit-card or business-card size planned for the final product.
The alpha board also clearly carries the Broadcom logo, suggesting that the 700MHz ARM 11 processor is incorporated into a Broadcom Mobile Multimedia Processor SoC, designed for mobile applications such as smartphones and media players. What makes the story 3D is that the 128 or 256 Mbytes of SDRAM on the Raspberry Pi board is said to be mounted on top of the processor SoC, according to this video:
So the SDRAM is likely to be mounted as a package-on-package 3D stacked device, which is what it appears to be in the photo above. Mounting the SDRAM in this manner saves board area (which costs money). If the processor SoC and SDRAM had been designed for 3D SIP or full 3D stacked-chip assembly, with appropriately sized I/O drivers, then this configuration would also save energy as well as cost and board size. That’s the Triple Crown of the 3D advantage.