Total thermal resistance from junction to air is
Rja = Rjc + Rcs + Rsa.
Where: a = air, j = junction, c = case, s = (heat)sink
Rjc & Rsa are reasonably fixed by component choices.
Rcs is somewhat more variable - see below.
If your data sheet does not give junction to case thermal resistance (usually Rjc or similar)* then change manufacturers - this is one of the most fundamental thermal parameters and ALL manufacturers 'worth their salt' will supply it. You can look it up for a handful of TO220 packages to get a feel.
Your comment about varying with heatsink suggests you don't really understand the subject. Finding one of the many many good tutorials on the net would be advised.
Thermal resistances are like electrical resistance - they can be added in series.Heat flows from junction to case outer, from case to sink (cia washer and thermal paste, and from heatsink to air. So you get
Rthermal = Rjc + Rcs + Rsa
Trise jc = Rjc x Watts etc
Trise = Tjc + Tcs + Tsa
see prior paragraph for meanings.
Rjc is specified by the device manufacturer.
Rcs is set by thermal washer material, heatsink compound, pressure of mounting etc. It should be a minor contributor.
Rsa is set by heatink design and size, air flow etc.
Temperature rise = Rja x Watts.
Search eg www.digikey.com for TO220 and look at a few Tjc ratings.
This datasheeet
http://ww1.microchip.com/downloads/en/DeviceDoc/22057a.pdf
gives an unusually large number of Rjc and Rja ratings for various package versions of the same part and is a good starting point.
For the TO220 package Rjc is 2 C/W.
Rja here is from about 30 to 60 C/W but the heatsinks are "wimpy".
Large heatsinks with fans can be around 1 C/W - much less with much care.
A well heatsunk TO220 can manage Tja = 10C/W without too much effort.
But it can be far worse without due effort.
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Edited:
The initial paragraph originally read as follows. It is evident that I wrote "case to sink" but intended "junction to case".
If your data sheet dies not give case to sink thermal resistance (usually Rjc or similar) then change manufacturers - this is one of the most fundamental thermal parameters and ALL manufacturers 'worth their salt' will supply it. You can look it up for a handful of TO220 packages to get a feel.
Best Answer
How is this not just a straight forward math problem? Work it out:
You say the thermal resistance from junction to case is 4°C/W, and that the junction is producing 10 W of heat. It will therefore be 40° hotter than the case. The max allowed junction temperature is 150°C, so the max allowed case temperature is 110°C.
You say the worst case ambient temperature is 50°C, so that means 10 W of heat flow from the case to ambient can't drop more than 60°C. Overall the case to ambient path must have (60°C)/(10 W) = 6°C/W thermal resistance or less.
Of course in practise you'd want to make sure the case to ambient resistance is some margin below the absolute maximum of 6°C/W.