no mystery to Tj
far too much poo over
really basic stuff
http://www.relexsoftware.com/resourc..._heatsinks.asp
http://www.electronics-cooling.com/h...alccorner.html
(MANY additional sources of the same calcs)
when Tcase is known, so too is Tjunction in the electronics packaging world
WCing is no different, just done by people outside the industry lacking a value for θjc
(or either of the others in reality, several test benches are capable however)
θjc = Thermal resistance between the die and the package case (the IHS for us)
θcs = Thermal resistance between the case and the heat sink (or waterblock)
θsa = Thermal resistance between the heat sink and ambient (or waterblock and coolant)
a rational discussion focuses on the uncertainty associated with each of the 3 above independent variables;
this uncertainty discussion relates to ONLY test equipment and methodology issues - nothing else
- if the same source, which includes θjc, is used to test various components then the absolute value of θjc is moot; the results include θcs + θsa and this 'we' call comparative testing (a bit sloppy eh ?)
- if different sources are used to test the same device then the variation will be due to the sum of θjc + θcs (with that same slop due again to the inclusion of θcs)
- if θcs is known or can be measured, which it can be, then θjc can be deduced
no magic, simple arithmetic will suffice (with very high tech testing)
so as a practical matter for WCing or hsf designers, the assumption of a value for θjc is quite reasonable;
Tj is a 'feel good' number for sink designers - it does not define the efficacy of a cooling solution
consider it now from a different perspective:
which is greater; the variation in θjc or that due to θcs ? - and by what order of magnitude ?
I will not insult the readership by stating the obvious.
science works better without polemics
EDIT
wrong terms included in second example, now corrected - sorry
RE-EDIT
jeez, had it correct initially; changed back (need to drop this whole thing !)