As mobile devices add features with every new release, the memory requirements for all those fancy functions continues to grow. Memory speed can be the overall system bottleneck. Using separate processor and memory chips is not an option -- modern device speeds simply can't tolerate the delays in routing memory bus signals on a PCB between the memory chip and processor chip.
Apple has long been at the forefront of this development, using 3D stacked package technologies to reduce distances to enable operation at higher frequencies and lower power. As early as 2010, the A4 processor integrated memory into a 3D 'silicon sandwich' (Link).
A recent patent application shows that Apple continues to push the limits of what can be done in modern packaging technologies. US Patent Publication 2020/00176431 ("DOUBLE SIDE MOUNTED LARGE MCM PACKAGE WITH MEMORY CHANNEL LENGTH REDUCTION") enables an impressive number of separate die on a single package.
Shown below is a cross-section from the patent application. The processor is indicated as '111', and individual memory die, 124, are packaged into memory modules indicated by 120. These memory die are packaged on both sides of a substrate, 102, which includes connections between the memory packages and the processor. This is all molded together into a single MCM or Multi-chip Module.
This illustration is impressive, but Apple is clearly thinking beyond what's shown in this figure. Their patent application contemplates many more memory packages alongside the processor. Shown below is a possible implementation that includes as many as 88 separate memory packages in a single MCM.
This a top-down view of the entire MCM layout, and remember, those memory packages (120) are mounted on both sides of the substrate, so there are twice as many as shown in this figure.
Apple is also patenting in other areas of silicon packaging, including recent applications on interconnecting individual die (US Patent Publication 2020/0075497), and thin-film coatings for isolation and shielding (US Patent Publication 2020/0154561).
It is one of the great triumphs of packaging technology that as products get more and more complex, the circuit boards can appear simpler and simpler. Don't believe us? Look at this picture of an iPhone 5 board:
Now compare that to the iPhone X, which is just a processor and a couple other devices.
Innovations from Apple and others have moved us from an era with hundreds of individual PCB components, to devices that can pack those hundred components into a single little square of plastic. We'll continue looking at their latest patent publications to see where they are heading next.
(Patent figures from published patent applications, photos from published teardown reports from iFixit)