The board I made was just a quick and dirty 30x2 .6" wide DIP adaptor.
The trace/space was 5mil. At 6.25mil it would end with 28 traces unrouted, 5 mil would route almost immediately. The board was 4 layer as shown.
The price penalty of using PSRAM might not be worth it. SDRAM would be harder, but pay for itself in the end.
A lot of the commercial designs out there are able to use 6 layers on small embedded boards.
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