Package
Beginner
The protective case around the silicon chip that connects it to the circuit board and helps carry heat away.
Novice
The case that holds the bare silicon chip, connects its signals and power to solder balls or pins on the circuit board, and carries heat away. In a wire-bond package fine wires join pads on the chip’s edge to the package; in a flip-chip package the chip sits face-down on tiny solder bumps.
Expert
Package choice sets the number of balls, the inductance of the power connections, the thermal resistance (and so the TDP), the cost, and whether several dies can share one package. It is chosen at the spec stage because it limits die size, pad placement and the number of signals.
Explained in Specification (Design Flow).
See also: Flip-chip bumps, Pad-limited vs. core-limited.