The BF561 datasheet has the following chart of thermal resistance of the BC-256-1 package:
I notice thermal resistance of junction-to-case is less than the overall junction-to-ambient. This is expected since air is poor conductor of heat.
How much does heat is conducted to the ground and power pins? My reason for asking is I have a board on which other parts are dumping power into the ground plane, heating it up. The vias escaping the BGA are thermals (another engineer's general preference; not related to thermal issues on this board). I am guessing there is modest thermal resistiance from the plane, through the escape via, into the ball, and onto the die. What I'm trying to work out is whether heating of the plane could be heating the part. If the junction-to-ambient thermal resistance figure include a significant amount of heat dissipated to the plane, then my heated plane could be an issue. Do you have any info on junction-to-ground plane thermal resistance?