flip-chip

Flip-chip is a way of attaching a bare silicon die to its package by flipping it face-down and landing it directly onto matching pads — the opposite of the older style where the chip sat face-up and was wired out from its edges. Imagine resting a Lego brick studs-down onto a baseplate: every stud meets a socket at once, all across the underside. Instead of studs, a flip-chip has a grid of tiny solder bumps that simultaneously touch down on the substrate when the assembly is heated and the solder melts ('reflows').

Because the connections come straight off the whole face of the chip rather than only its perimeter, flip-chip offers thousands of I/O in parallel, far shorter electrical paths (less inductance, faster signalling), and better heat removal from the now-exposed silicon back. A modern CPU or GPU package may carry tens of thousands of these bumps. The trade-off is that you cannot inspect the joints by eye (they are hidden under the die) and the silicon and substrate expand at different rates with heat, so an underfill epoxy is usually injected to glue them together and spread that mechanical stress.