Interposer

An interposer is an intermediate layer placed between one or more chip dies and the package substrate, providing dense, short interconnects that let multiple separate dies communicate with each other as if they were part of a single larger chip.

Interposers exist because a standard package substrate's wiring simply isn't fine or dense enough to let multiple separate chip dies communicate with the very high bandwidth and low latency that many modern multi-die designs need. As chip designs increasingly combine multiple separate dies into a single package rather than building everything onto one large die, an interposer addresses the resulting communication challenge by sitting between the dies and the substrate, providing much finer, denser interconnect wiring than the substrate could offer on its own, letting the separate dies function together with performance approaching that of a single integrated chip.

Interposers can be built from different materials, including silicon, which provides the finest wiring density but at higher cost, and organic materials, which cost less but offer somewhat coarser wiring. Because silicon interposers can be manufactured using semiconductor fabrication techniques, they achieve very fine wiring density, though at higher cost than organic interposer materials, which are less expensive but provide comparatively coarser interconnect density — the right choice depends on how much interconnect density a given multi-die design actually needs.

Interposers are a key enabling technology for chiplet-based designs, where a complex chip is built from multiple smaller dies rather than one large monolithic die. Because chiplet-based designs specifically rely on multiple separate dies working together as a cohesive system, interposers, along with related advanced packaging technologies, have become an increasingly important enabler of this design approach, letting chipmakers combine dies from different processes or even different manufacturers into one package.

Interposers can also route power and provide other supporting functions beyond pure data interconnect, making them a more capable layer than a simple wiring bridge. Beyond just carrying data signals between dies, an interposer can also help distribute power and, in some designs, embed passive components, meaning it often serves multiple supporting roles within the overall package rather than functioning purely as a data interconnect layer.

Read the interposer through a local-transit-hub lens: rather than routing every connection through the comparatively slow, low-density infrastructure of the wider package substrate, the interposer acts like a dense local transit hub sitting right between the dies, letting them exchange enormous amounts of traffic directly and efficiently.