DrDiettrich:
See Figure 22, the PD(max) is below 1.2W for the minimum pad size, less than 1.5W max.
Note that the chart specifies an ambient temperature of 50°C (122°F). I used 25°C (77°F) which seems more likely for hobby use.
There is a disconnect between that chart and the specifications. Let's try a 17.5 mm square copper pad where the two curves cross the scales neatly at: RJA = 50°C/W and PD(max) = 1.4W. We calculate the junction temperature as 50*1.4 = 70°C above ambient. That would put the junction temperature at 70+50 = 120°C which is 30°C below the maximum junction temperature. Are they using the wrong maximum junction temperature? Are they using the wrong ambient temperature (should be 80°C=176°F?) Does the datasheet have the maximum junction temperature wrong?