Translation - Japanese: QFNパッケージとDFNパッケージの熱データ（ΘJB) - KBA88214 - Community Translated (JA)
What is the thermal data (ΘJB) used for QFN and DFN packages?
The traditional ΘJC is not a useful parameter for QFN and DFN packages, because the heat flow is primarily (> 90%) through the solder-attached paddle to the board, and not through the top of the package. The thermal resistance from the junction to the bottom of the package is more useful. The thermal resistance of the board becomes insignificant when the board material is copper.
Therefore, ΘJB = ΘJC Bottom.