Definition of Thermal Impedance;


Graph of Maximum Thermal Impedance for 2N3439, 2N3439L, 2N3440, and 2N3440L in a TO-5 or TO-39 package


Definition. What is Thermal Impedance: The application of an
electrical stress sufficient to pass heat through the interface of dissimilar materials, primarily to determine the quality
of attachment by measuring the electrical characteristics of the temperature sensitive parameter. Thermal
impedance measurement is performed by quantifying changes to the heat sensitive parameter caused by current flow
and its induced heating. It is used to detect interface voids between die, attachment medium and the header or heat
sink.

Definition. What is Thermal Resistance: Thermal resistance is the temperature rise, per unit power dissipation, of a junction
above the temperature of a stated external reference point under conditions of thermal equilibrium.

Related topics:
How to Derate a 2N3439 NPN Transistor.
Note the curve is the same for a 2N3440 transistor also.
Linked page; Technical Engineering Dictionary.