Your desktop PC case probably has a volume of 50 litres or more-but the CPU, GPU, RAM, and handful of other chips that actually constitute the computer probably account for less than 1 percent of that volume.
In recent years we’ve seen the rise of one method of increasing density: stacking one die or chip on top of another.
Despite fairly regular announcements from various research groups and semiconductor companies, and the maturation of 3D stacking and packaging technologies, multi-storey logic chips are rare beasts indeed.
First, as chips get smaller, there is less surface area that makes contact with the heatsink/water block/etc. Second, as chips get smaller, hot spots-clusters of transistors that see more action than other parts of the chip-become denser and hotter.
Did you know that, on a modern CPU-an LGA 1155 Ivy Bridge chip, for example-the majority of those 1155 pins are used for power delivery? Take a look at the diagram above.
If you want to stack one CPU on top of another, you not only have to increase the number of pins at the bottom of the chip, but you also have to find a way of connecting those power lines to the upper chip through the lower chip.