have a look here [0], Fig 1; the spacer serves really as a mechanical spacer (if dies are same or similar size); also nice for heat spreading. [0] https://www.

have a look here [0], Fig 1; the spacer serves really as a mechanical spacer (if dies are same or similar size); also nice for heat spreading. [0] https://www.semanticscholar.org/paper/Productivity-improvement-of-stack-package-line-die-Jin-Li/bd8bc2ea571d8e898ce74aa1d107d20fd035dbff https://www.semanticscholar.org/paper/Productivity-improveme...