and my last BGA was of 0.8 mm pitch. I understand that there are some 0.5mm packages coming out in the high-pin-count versions. That will be interesting!

Initially, TI and others (we were using a TI part) recommended that infamous "dog-bone" land pattern in order to ensure that the via didn't suck the solder off the BGA contact. Unfortunately, U.S. PCB houses couldn't hold the required tolerances in drilling, registration, and track width, particularly as related to solder mask. The result was that one had either to give up on BGA or use an Asian board house. The problem with the U.S. houses is the low yield at each step, not to mention the cost of x-ray inspection, while the risk with Asian houses is loss of tight control over one's IP.

Even with properly manufactured (in Asia) boards, the U.S. assembly houses waste a considerable share of the boards in the assembly process.

Using a thin outer layer, one can either plug or pre-solder the vias in the BGA site, thereby allowing the BGA to be soldered to the feed-throughs that route the signal to other layers. That reduces the outer-layer track density, which helps with power supply bypass.

The use of additional processing steps, however, increases the cost so much, well, that it's prohibitive. Perhaps the pre-soldering or plugging can be done in Asia ...

Now ... about those adapters ... you're not soldering the adapters to the board, but, rather, relying on compression contacts via that elastomeric pad?

RE