Coreless substrate package and fabrication method thereof

A coreless substrate package, comprising:

• a coreless substrate;
• a package device mounted on a coreless substrate, wherein the package device comprises a function silicon die, wherein the function silicon die has an active front side facing downwardly to the coreless substrate, and an exposed, passive rear side opposite to the active front side, wherein the package device further comprises an encapsulant covering peripheral sidewalls of the function silicon die;
• an underfill material filling into a space between the package device and the coreless substrate;
• a stiffener ring disposed on a top surface of the coreless substrate along perimeter of the coreless substrate;
• and
• a gap fill material disposed in a gap between the stiffener ring and the package device.