Microscope view of FFVB676 substrate seen edge on
https://files.ioc.exchange/media_attachments/files/111/700/895/804/531/137/original/c318ef6f8a40eb8f.jpg
Side view of the package substrate. It's a fourteen layer board!
The overall construction is two sets of seven layers using very thin, light colored dielectric with no evident glass reinforcement (so likely some kind of fancy microwave laminate) on either side of a thick core using a more conventional woven glass-resin structure.
The glass in the core looks to be spread but it's otherwise unremarkable.
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