A physical interface that sits between a high-density component and a standard circuit board. It takes very fine, tight connections from a silicon die or complex package and spreads them out to a wider pitch that a standard PCB can handle. It acts as a translation layer for signals and power.
The new BGA has a 0.4mm pitch, so we need an interposer to mount it on our standard FR-4 test boards.
Interposers are heavily used in advanced packaging to stack memory and processors together before they ever touch the main PCB.
Related terms
Ball grid array
A surface-mount package type that uses a grid of solder balls beneath the component for electrical connections.
Fanout
Short connections that bring each pin of a dense part out to a via so it can be routed on other layers.
Breakout
Drilled hole wandering past the edge of its pad, or the routing that escapes signals from a dense part.