But BGA is a terrible idea for all the reasons we've listed extensively. I hope it doesn't become the standard, especially if string topology does become the standard. But the CPU model like what Spondoolies has mastered (and everyone else that's tried has gone bankrupt or made terrible hardware or both) has a definite efficiency ceiling and complexity floor which leave a lot of room for improvement. Most of which improvement can be made quite easily by using a matrix of small chips. This also makes design and manufacture a lot easier too, since you don't need a friggin' X-Ray machine to inspect your solder joints. The kind of chips Bitmain and Avalon are doing already are the best option and, unless the cost savings from chip fab outweigh the cost increases and efficiency hits from every other aspect of the process, I don't see a good reason to do any different.

If we do a backplane, it'd be be basically a PCB and edge connector version of PCIe cable + USB cable. The only thing a backplane might do differently is integrated USB hub chips so there's only one cabled connection between the backplane and a controller. All our boards will have USB connectivity, so making a box with a bunch of 'em in it would be possible by sticking a several-port hub in there and getting a bunch of short cables.