Hi,
The physics of static heating isnt that hard to understand really, for simple cases. Heat flow is a little more involved.
Basically the material being heated has a property called the specific heat capacity. This along with how much material the object contains tells you how fast the temperature will rise with a given drive wattage. For example, with 500 watts you can heat a cup of water from room temperature to 100 degrees C in about 2.5 minutes if there are not too many heat losses.
For a smaller quantity, it will be faster. for half a cup 1.25 minutes, a quarter cup 0.625 minutes, an eighth of a cup 0.3125 minutes, etc.
I went down to an eighth of a cup for a reason, to show the minutes would be 0.3125 (19 seconds), and metal like copper has a specific heat capacity roughly 1/8 that of water.
A cup of water weighs in at about 236 grams, so 1/8 of that would heat up in about
19 seconds, so 240 grams of copper would heat up to 100 C in about 19 seconds.
Assuming we have 1/10 of that (24 grams of copper) it would take about 2 seconds
to heat up to 100 C.
That's at 500 watts, however. At 100 watts, 24 grams of copper in 10 seconds.
That is very roughly about the size of the transistor metal, or maybe half of that,
so say around 5 seconds. We could look closer at the actual volume to see how it fairs.
You can also add a heatsink but then we get into heat flow which is a little more involved:
Ut=k*D^2*U
where Ut is the partial of U with respect to t, and D^2 is the Laplacian, and U is the function, or simplified in one dimension:
Ut=k*Uxx
where Uxx is the second partial of the function U.
So you see how simple this is without the heat flow part, and with the heat flow part we'd have to know all the dimensions, but one thing is clear, that the heat flow does not get from the junction to the tips of the heat sink in zero time, which means the static case does not hold (junction gets much hotter than the tips of the heat sink over a short time period).
What i would suggest is some experimentation. Heat up the device without a heat sink to learn it's characteristics, then use a heat sink and take more measurements and compare.