After ordering two different 1 watt metal film resistors (without looking at the data sheet) and receiving what look like red 1/4 watt size resistors (and thought they must have sent the wrong part), I finally went to the data sheet:
It is hard for me to understand the power rating, given my experience with much larger (i.e. 1 watt carbon composition) resistors. I have seen many PCBs darkened and otherwise deteriorated from the heat from the full sized versions of 1 watt resistors that were dumping well below their rated power. From this data sheet (the details of the pre-bent version) it seems that 8 mm is about the recommended elevation of these tiny hot spots, with the hole to hole spacing for the 1 watt size being as small as 12.5mm.
There are lots of graphs of PCB and resistor temperature hot spot versus lead length and elevation, and different for iron and copper leads. The derating appears to go to zero power at 155C ambient, yet hot spot temperatures sometimes goes higher. I see lots of traps and pitfalls in using these without careful thermo design and production, though some of the peak power ratings are impressive for the size.
Anybody have any horror stories about these tiny fireballs?