Junction to board thermal resistance
\$R_{θJB}\$ is used when no heat sink is on board.   

If there is a heatsink, do not use above metric and instead use \$R_{θJA}\$ which is in parallel with \$R_{θJC}+R_{θCS}+R_{θSA}~~~~\$ as the net thermal resistance.


\$R_{θJB}\$ Junction-to-board thermal resistance 11.9 °C/W
\$R_{θJA}\$ Junction-to-ambient thermal resistance 32.5 °C/W
\$R_{θJC}\$ (top) Junction-to-case (top) thermal resistance 27 °C/W
\$R_{θJC}\$ (bot) Junction-to-case (bottom) thermal resistance 1.7 °C/W


\$ψ_{JT}\$ Junction-to-top characterization parameter 0.3 °C/W
\$ψ_{JB}\$ Junction-to-board characterization parameter 11.9 °C/W



Thus add 1.7 to your interfaces \$R_{θCS}+R_{θSA}~\$  then // with 32.5

For J= junction, C=case, S=sink, A=ambient, B=board and \$R_θ\$ means thermal resistance, although often just truncated to \$R_{JC}\$

The main revelation in the report is that the following analysis is old and obsolete, because the board does not dissipate 100% of the heat, with some cooling effect in the case to ambient.

\$R_{θJA} = R_{θJC} + R_{θCA}~\$  (obsolete)