So the i.MX 8M SoM package will be smaller and dissipate less heat in the final handset?
I wouldn't expect the same chip going into a smartphone to require a heatsink or any thermal consideration at all on a sparsely populated, open-air circuit board.
It could be that power management is not enabled yet, or perhaps other thermal protections are not fully enabled yet. I doubt the SoM package itself will get much smaller, but I'm just basically guessing since I'm not an employee of Purism.
