Rigid silicon requires rigid interconnects. Flexible ICs allow flexible packaging, or different packaging. Instead of building from the printed circuit board up, build from the heatsink up. Use a precision pick and place system to glue the thin, wimpy, inexpensive silicon to the strong massive heatsink. Then mask on the solder balls. Then apply a thin, wimpy, inexpensive circuit "board". Attach all the old style surface mount components to the other side of the circuit "board". "Board" is in quotes because it would get all of its mechanical strength from heatsink. It might be so thin, it is no longer board like.
The big win here, is that one wafer is good for at least 5 sets of circuits. The lose is the grid of holes etched through the silicon as part of the pealing process. Assuming the grid of holes doesn't use up a significant portion of the surface area, the factory is getting close to 5 times as many devices out of each ingot of silicon.