OK, I don't know too much about heat dissipation.
For semiconductors that are designed to take heat sinks I can do the arithmetic of adding thermal resistances from the semiconductor datasheets + thermal resistance of the thermal pad + thermal resistance of the heat sink just fine.
For some medium-power surface mount components (semis with tiny body, big leads) I know that dissipation is done primarily by conducting heat away from the device via the leads and/or a thermal pad on the bottom and then it is radiated via the PCB. I know this because the datasheets/appnotes all emphasize PCB layout and copper thickness etc. to achieve the rated dissipation.
But for small leaded components (say 1/8W resistors or TO-92's without heatsinks) it must be the case that the bulk of heat dissipation is done not through the tiny package, but that most of the heat is dissipated through the component leads into the PCB which then does the radiating. Yet... the datasheets give no guidance into how lead lengths or PCB layout reduces dissipation ratings.
To some extent things must be self-compensating: If you use longer leads (reducing conduction to the PCB) then the device is presumably sitting in free air (away from a hot PCB) and the body itself is going to be better at dissipating heat. But I'm looking for basic arithmetic with thermal resistances to turn common sense into something quantifiable.
Back in the late 90's, HP had some decent appnotes with summaries (including the basic thermal arithmetic and example PCB layouts) related to dissipation in T1-3/4 LED arrays but I cannot locate these appnotes anymore. I am looking for something similar that discusses numbers (maybe with arithmetic examples) for other small leaded components (say 1/4W and 1/8W resistors, TO-92's, etc.) The best I can find is recommendations to derate by factors of a few to several when you have a lot of these components in close proximity. Nothing talks about PCB/copper layout.
Any hints?
Tim.