Total Power Dissipation - Design Margin
If you have never done MTBF calculations before there is a general failure rate acceleration factor that often doubles for every 10'C rise (Arhennius Law).  

My criteria for a commercial design when I managed this function, was 85'C for junction temperature at maximum ambient of 40'C normally operated at room temp.


package thermal resistance, Rjs = 230'C/W from junction to solder point  
FR4 Rsa= 338'C/W   from solder point ambient with free moving air, while sealed air is worse and adding 1 cm^2 exposed copper to collectors reduces by 20'C/W per cm² area of collector copper