Here's a board that used "solder-mask defined pads" for the tiny EPC BGA GaN fets, with pad stack and mask like EPC recommends. Boards fabbed by Cirexx.
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Those pads are 0.3 mm diameter, a hair under 12 mils. We called out a
0.2 mm solder mask cutout, but the board house didn't do that. I kind of like them the way they are.