Package substrate

A package substrate is a small, thin circuit board that sits between a chip's die and the board it's soldered to. The die's connections are tiny and packed close together; the board's are much larger and wider apart. The substrate bridges the two, fanning the connections out through several layers of fine wiring.

It's built much like a printed circuit board, only finer. The core is woven glass fiber cloth soaked in epoxy resin or a similar resin and cured into a thin, stiff sheet. Thin copper foil is bonded on, and wiring patterns are formed with light-sensitive masks, plating and etching. More layers are added on top, each a thin insulating film carrying copper lines, connected through holes drilled by laser and filled with copper. In advanced substrates, lines can be around ten micrometers wide, far finer than on a normal circuit board. Pads on top match the die; pads underneath will carry the solder balls.

In the system-on-chip, the die is flipped and bonded face-down straight onto the substrate. In memory and storage chips, the dies are usually connected with bond wires. The exact resins vary by manufacturer; BT resin and epoxy build-up films are common.