Those white ceramic cased ones about 3 watt in the form of a rectangular block 15x5x12mm . No cracks in the ceramic but white powder laying around some of them. eg 6 power transistors in "parallel", one has a lot of powder around it on the pcb, one has some around it and the other 4 are clean. No fan air directed in that area to blow anything away. Both have the powder deposited not along the long sides but at the end away from any possible heat radiated from the transitor/ heatsink, perhaps just coincidence. Can anything be inferred from these deposits/ associated transistor?
I've seen failed o/c ones of this type with a crack in the casing, does it mean the powdery ones have been overheated and will fail imminently or an indicator that the associated transistor is taking too much of its share of the current?