BGA Confusion

[guenther_jena]

Started working on a design using the Lattice ECP5. Looking at the device LFE5U-25 there are package options with BGA (0.8mm pin-to-pin distance) having 256 or 381 pins. Looking at the available IO pins, I would've guessed the more available pins the more IO pins the chips will have (but probably not growing proportional for reasons). But the number of IO pins are the same for the 256 and 381 pin BGA - why does the bigger package even exist?

See on page 11 here in the datasheet: https://www.latticesemi.com/-/media/LatticeSemi/Documents/DataSheets/ECP5/FPGA-DS-02012-1-9-ECP5-ECP5G-Family-Data-Sheet.ashx?document_id=50461 .

[hamster_nz]

Footprint compatible with a larger/smaller part?

Thermal consideration?

Cost? (low pinout part in lower cost packaging for low I/O use cases).

[SiliconWizard]

Pin-out compatibility is the most likely reason, for an easy upgrade/downgrade from the -12 to the -45 variant for the 256-pin package. For the 381-pin package, it's from the -12 to -25 (not up to -45, probably for marketing reasons).
I haven't looked at the footprints to see if the 256 BGA and 381 BGA have common pins (so a common footprint could be used).

Thermal reasons could play a role maybe, just look at the number of IOs between the 256, 285, 381 and even 554 BGA. The very large 554 BGA has few IOs relatively.
But otherwise it's often just subtle marketing decisions.

[Someone]

256 caBGA (14 x 14 mm) no parts with transceivers
381 caBGA (17 x 17 mm) smallest parts with full transceiver count (identical IO count to 256 caBGA parts)

Pretty common for FPGAs