Icepak compact model Rjb
 

In order to find steady state junction temperatures using compact models like 2R we generally need Rjc, Rjb and power. I can find the Rjc and power data. But as Rjb is boundary condition dependent, how would you approximate the value. I don't have option to do experiment to find Rjb.

Are you doing a heatsink solution ?? if so, a first approximation will be to limit all the heat to the case, ie Rjc <<< Rjb. This will provide a worst case scenario.

In any case, Rjc and Rjb should be used with caution in thermal modeling. They are more useful when comparing packages, but when you want to predict Tj, more details will be better.