I am going to use a nMOSFET (NTMFS5C670NL) which will a varied power dissipation. The current going from drain to source is determined by a control signal. The maximum power dissipation for the MOSFET is 1.29 W.
The junction-to-case thermal resistance Rjc is 2.4 ℃/W, and the junction-to-ambient thermal resistance Rja is 41 ℃/W.
The actual junction temperature formula is Tja = (Rja*Pd)+Ta. If the ambient temperature is 25 ℃, then Tja will be equal to 77.89.
Case temperature Tc will be (-Rjc*Pd) + Tj = 75 ℃.
These values are only valid if the MOSFET is surface mounted using a 650 mm^2, 2 Oz Cu pad. This is the only scenario the datasheet for the MOSFET gives.
Is there any way to give an educated guess on what the case temperature will be with a bigger or smaller Cu pad? I have failed to find any white papers that can give a decent estimation.