Known good die KGD
An unpackaged piece of silicon that has been fully tested and guaranteed to work. A concept that barely mattered until people started stacking nine of them on top of each other.
In a classic chip you test after packaging, and if something is wrong you throw away one chip. In a package with eight stacked memory dies and a logic die, a single fault means all nine go in the bin along with the packaging work — yield losses multiply rather than add. So the entire economics of advanced packaging hangs on being able to determine before assembly that each individual piece works. That is harder than it sounds: a bare die has no package to handle it by, the contact pads are microscopic, and full at-speed testing normally relies on infrastructure that lives in the package. Chiplet development has therefore made wafer probing and built-in self-test central concerns.