Starting initial testing to see how in the world I might try to fan out the GTYs of an UltraScale+ B676 package on OSHPark 4 layer.
I don't have the full schematic done, just doing initial layout feasibility checks.
The goal is to hook up "as much as reasonably possible". First non-negotiable requirement is that it has to fit on an OSHPark 4L board and it has to provide minimum power and JTAG connections so I can verify the reballed chip is alive.
After that, anything is a bonus.
Here's an initial quadrant dogbone fanout on 0.5mm legs for all of the power/ground balls in one corner of the GTY area. This will likely not be the final via location.