Tube Time on Twitter: "annotated cross section (closeup) of a modern BGA-packaged SoC. you can see the individual fiberglass bundles embedded in epoxy that make up the PC board substrate! also check
integrated circuit - How are BGA dies constructed? - Electrical Engineering Stack Exchange
CPCORE Structure Type | Organic Package | KYOCERA
BGA Failure Analysis on Static Transfer Switch - Gideon Labs