Fanout is the first stage of routing a dense part: each pad gets a short trace and a via, or a via in the pad, so its signal can drop to an inner layer. For BGAs it is planned ring by ring, with outer balls routed on the surface and inner balls taken down through vias to successive layers, and the via grid sets how many layers the escape needs. A clean fanout makes the rest of the routing possible.
Do the fanout on the FPGA first and lock it, then we'll know how many signal layers we really need.
Also heard as
- fan-out
- BGA fanout
Related terms
Dog bone
Short trace from a surface mount pad to a nearby via, shaped like a cartoon bone, used to fan out parts.
Breakout
Drilled hole wandering past the edge of its pad, or the routing that escapes signals from a dense part.
Via-in-pad
Via placed directly in a component pad, filled and plated over so solder cannot drain down it.
Differential pair
Two traces carrying equal and opposite signals, routed tightly together at matched length and impedance.