Silicon Interposer

A silicon interposer is a passive slab of silicon that sits between one or more active dies and the package substrate, carrying ultra-fine wiring to interconnect those dies. Because it is made of silicon, it can be patterned with the same lithography used for chips, achieving routing densities and line pitches far beyond what an organic substrate can offer. It is the foundation of most 2.5D packaging.

How it connects everything

Dies are attached to the top of the interposer with tiny micro-bumps, roughly ten microns across. Signals travel horizontally through the interposer's fine wiring between neighbouring dies, and vertically through through-silicon vias that pass entirely through the interposer to reach the package substrate and the board below. This lets a processor die and high-bandwidth memory stacks sit side by side and exchange data over thousands of short, dense connections.

Benefits and trade-offs

Because both the interposer and the dies are silicon, their thermal expansion matches closely, which reduces the mechanical stress that plagues mixed-material packages. The result is excellent signal integrity, very high bandwidth, and lower power per bit moved. The cost is a complex, relatively expensive process, so silicon interposers are reserved for high-value parts such as data-center GPUs, AI accelerators, and HBM-equipped processors where bandwidth justifies the expense.

See Through-Silicon Via (TSV) for the vertical connections it relies on and Chiplet for the dies it commonly hosts.