2.5D 3D Packaging Naming
"2.5D" and "3D" packaging are named by counting how many silicon-to-silicon connection paths exist between dies, not by literally counting spatial dimensions. In a 2.5D package, multiple dies sit side by side on top of a shared silicon or organic interposer, and every die-to-die signal takes exactly one path: up through the die, across the interposer's routing, and back up into the neighboring die — a single horizontal hop dressed up with some vertical plumbing underneath it, which is why it only earns the "half" in 2.5D rather than a full third dimension. In a true 3D package, dies are stacked directly on top of one another and connected by through-silicon vias (TSVs) running straight down through the silicon itself, giving a genuinely vertical signal path with no interposer detour required.
A few things explain why the fractional "2.5" survived as a permanent industry term instead of being replaced by something cleaner:
The interposer is the reason 2.5D cannot claim a full third dimension. TSMC's CoWoS and similar schemes place a GPU logic die and several HBM memory stacks next to each other on top of a passive or active silicon interposer, which carries the fine-pitch wiring needed to connect them at much tighter spacing than the organic package substrate below could achieve alone. Every die still sits in essentially one horizontal plane; the interposer only adds a thin routing and via layer underneath that plane, so the package gains far denser interconnect without actually stacking the dies' active silicon on top of each other — hence "half" a dimension, not a whole one.
True 3D stacking is named for removing the interposer's detour entirely, at the cost of a much harder manufacturing problem. Stacking an HBM die directly on top of a logic die and connecting them with TSVs shortens the electrical path dramatically and allows far more vertical interconnects per unit area than any interposer's lateral wiring could match, but it also means every stacked die must tolerate thermal and mechanical stress from its neighbors directly above and below it, and TSV formation (etching, lining, and filling a narrow deep hole straight through a wafer) is a substantially harder process step than the interposer's conventional wiring.
"Chiplet" names the die itself, independent of which dimensionality scheme joins it to its neighbors. A chiplet is simply one of the separately manufactured pieces of silicon — a logic die, an I/O die, an HBM stack — that gets combined with others into one package; AMD's MI300 and similar products mix both schemes in the same part, using 3D stacking for some die pairs (such as logic under memory) and 2.5D interposer routing for others (such as logic chiplets arranged side by side), which is why a single modern product's datasheet can legitimately use both terms in the same sentence.
Siblings worth distinguishing: this entry covers package-level dimensionality naming and chiplet terminology specifically; it does not cover transistor architecture naming (planar, FinFET, GAA, covered elsewhere), which names gate-wrap geometry inside a single die rather than how separate dies are joined together, nor does it cover damascene or CMP naming, which describe on-die interconnect fabrication steps that happen long before any packaging decision is made.
Read "2.5D" and "3D" as a direct statement about the electrical path length and manufacturing difficulty a package accepted, not as a literal geometry count: 2.5D means an interposer detour with easier fabrication, 3D means a direct TSV path with harder fabrication, and a real product's choice between — or combination of — the two is a deliberate trade between interconnect density, thermal management, and yield risk, named precisely enough that the fraction in "2.5D" is doing real engineering work rather than sitting there as marketing shorthand.