a silicon interposer
If chiplets are several small chips you want to assemble into one product, you need something to mount them on that lets them talk to each other with very fine, dense, fast wiring. An ordinary circuit board is far too coarse — its wires are huge and widely spaced, like country roads, fine for connecting whole components but hopeless for the thousands of tight connections chiplets need. A silicon interposer is a special base layer, itself made of silicon, that sits underneath several chiplets and carries dense, chip-quality wiring between them. The chiplets sit on top of the interposer, which sits on the package: the chips are still side by side, not stacked, which is why this is often called '2.5D' — more than flat 2D, but not the full vertical tower of 3D.
Why silicon specifically? Because you can etch wires on silicon at the same incredibly fine pitch as on a chip itself — far finer and far more numerous than a circuit board allows. An interposer can therefore carry tens of thousands of short, dense connections between neighboring chiplets, giving them almost the bandwidth they would have if they were one monolithic die, while still being separate, separately manufactured pieces. The interposer also routes power up to the chiplets and signals down to the package below, often using through-silicon vias to pass connections vertically through itself.
The interposer is the quiet hero of the chiplet era: it is what makes disaggregation practical. Without a way to wire chiplets together densely, splitting a chip into pieces would cripple the communication between those pieces. The honest tradeoff is cost — a large silicon interposer is itself an expensive piece of silicon to manufacture, and it adds area and assembly steps. This is why the field also explores cheaper alternatives, such as small silicon 'bridges' embedded only where two chiplets meet, instead of one big interposer under everything. 2.5D on an interposer and 3D stacking are complementary tools in the same advanced-packaging toolbox.
A GPU and its high-bandwidth memory stacks are both mounted side by side on a silicon interposer. The interposer carries thousands of dense wires between the GPU and the memory stacks just a few millimeters apart, delivering the huge bandwidth the GPU needs — something an ordinary circuit board, with its coarse wiring, could never provide.
An interposer is a silicon base with chip-fine wiring that links side-by-side chiplets — 2.5D, between flat and stacked.
2.5D (chiplets side by side on an interposer) is not the same as 3D (chiplets stacked vertically). The interposer keeps chips flat but wires them densely; it solves the communication problem of disaggregation, but a large interposer is itself a costly piece of silicon.