Thanks for the info!I was being a tad bit generic when talking about stacking. Basically, to fit things inside that small of a package, there are some things like DRAM stacked to make space for more radios, and the PCB is thicker. I'll let Anandtech get specific: AnandTech - LG Optimus 2X & NVIDIA Tegra 2 Review: The First Dual-Core SmartphoneGranted I've never installed or even held a 4G LTE radio in my hand, but I've built computers for over 15 years and I have a hard time believing they stack 4G LTE radios to save space. So honestly, these are all just guesses, as I've yet to see someone post a picture of a 4G LTE radio.
Another thing, they most likely built 4G LTE radios for phones first, before the tablets. Since building a new one just for the tablet would cost millions in R&D, wouldn't it be more likely that they would use the 4G LTE radio they had already developed for phones in their tablets to save money?
If we look at the Thunderbolt's thickness, it's because they packed in dual radios (for 3G data & voice) plus an LTE radio:In the smartphone space, the integration is even more pronounced. With physical space and power as major constraints, smartphone chip makers have been forced to further trade performance for integration. The level of integration is so high within a smartphone that you almost never hear about what CPU a phone uses, but rather what Application Processor it uses otherwise known as an SoC (System on Chip or System on a Chip).
Package on Package (DRAM on top, SoC on bottom)ósource: statschippack.com
Integrate a CPU, GPU, memory controller, video decoder, audio decoder, video encoder (sometimes), camera processor, system memory and maybe even a modem onto a single chip and youíve got something that can only be described as a System on a Chip. Itís a single physical chip that integrates nearly all of the functions of the entire computer. Nearly all of the aforementioned components are on a single piece of silicon, with the exception of any integrated memory. To save board real estate and enable smaller form factors, itís not uncommon to stack DRAM on top of the SoC package instead of beside it. The SoC in a PoP (Package on Package) stack has contacts on its top surface that line up with the balls on the DRAM for power and signaling. PoP stacks work because the SoC underneath doesnít dissipate much heat and thus doesnít mind being insulated by some DRAM up top.
So remember how the Thunderbolt is pretty thick? Itís because the PCB runs the span of the entire phone and adds thickness. You can see just how much of the board is covered with stuff if you look at the FCC disassembly photo. For comparison, most other smartphones lately use a PCB that runs perhaps half the device height, then packs the battery in beside it.
So the PCB had to be longer, making the device have to be thicker in order to include the battery over the top of the PCB, instead of alongside it.