I was about to suggest SoMs as well.

The biggest challenge with BGAs from a hobbyist perspective isn't the soldering. There are lots of hobbyists successfully reflowing BGAs with toaster ovens. The bigger problem is manufacturing the PCBs to break out the BGA balls. TI has some BGAs with a proprietary ball pattern that are designed to be broken out with just 4 layers, but that's the exception, and you generally need very narrow traces and trace clearance to route between balls and vias. I just looked at OSH Park's current capabilities and they only do up to 4 layers and their DRC specifies 6 mm minimum trace width and 6 mm minimum spacing. I've heard of people using OSH Park and similar lower-end prototyping services to manufacture one-off PCBs for fine-pitch BGAs, but you'll be failing DRC by a massive margin, so expect terrible yield on the PCBs, assuming any of them work at all.

Unfortunately I think DRC will be similarly challenging for most SoM connectors since their pin spacing has a similar pitch to BGAs, so you'll probably need an off-the-shelf break-out board to work around that. Honestly, at some point you should consider whether a vanilla dev board with low-speed GPIO headers could meet your requirements.

Those OSHPark rules are 6mil, not mm - that's a little over 0.15mm. But it's still not really suitable for BGP modules. You can probably do 60-ball RAM modules, but a medium-sized processor seems like it would be off the table.

I think that dirtyPCBs does 6-layer boards, but they're comparatively pricey for small runs or prototypes.

Thanks for the correction; I thought 6 mm sounded way too high. Gotta love American measurement units!

There are Internet accessible Chinese PCB manufacturers that will do short runs for designs of almost any complexity, but they aren't really targeting hobbyists, though that seems to be changing.

Yeah, mils are a really strange measurement unit. I'm not sure who thought that 1/1000" would be a good standard.

I think it harks back to older manufacturing processes, via plastics (where mils are used) back to metalworking. Machinists will usually talk about small distances in "thou", as in thousandths of an inch.

Always cracks me up when AvE mentally converts 2.5/64" or some crazy shit into thou while thinking out loud, and then goes "metric fanboys be like whaaat?"

Huh, I hadn't heard of that sort of thing, besides the various dev boards offered by the microprocessor developers. Thanks!