Hi m.c.
please refer to FC-PBGA appnote p.42
Comparison of Lid vs. No-Lid FC-PBGA Thermal Resistances
http://cache.freescale.com/files/32bit/doc/package_info/FC-PBGAPRES.pdf
Best regards
igor
Please use corresponding Datasheet(s), Table "Package Thermal Resistance Data"
Regards,
Yuri.