Package substrate
Beginner
The small circuit board inside a chip’s case. It spreads the chip’s tiny connections out to bigger ones that solder onto the main board.
Novice
A small multilayer circuit board inside the package. It carries signals and power from the chip’s tiny connections out to the much larger solder balls that attach the package to the main board.
Expert
Its line width, via size and layer count limit how many of the die’s connections can be routed out and how well power is delivered. When it can’t reach the wiring density two dies need between them, a silicon interposer or bridge goes between the dies and the substrate.