Is there any difference between SMD and TH parts?

[aandy]

i agree

 

[darian]

Well, the main factor influencing the changes of temperature at the junction is the thermal inertia of the dye... You agree to that at least I hope? So yes you can dissipate a lot in the tracks with SMTs but the problem is that it's only smoothing the process of temperature fluctuation because it's physically far from the junstion and the connection is not that direct anyway, with some metal to metal interfaces. Once again, you can only measure the temperature of the case but not the junction. Do you agree with this principle of thermal inertia at least? I am not sure you can reach the same thermal inertia with a SMT would it be with the added dissipation of the tracks. Maybe I am wrong, maybe all the designers who thinks that using a slightly oversized transistor for the VAS of a power amp are wrong, who knows?

Regards

 

[Conrad Hoffman]

Agreed. My point is that you have to look at specific parts to know what thru-hole or SMT is doing. I'm always suspicious of blanket rules. My guess, and it's only a guess, is that the conditions surrounding the die itself are very similar between the two when you're talking about instantaneous temperature changes. Everything beyond the leadframe is thermally distant. I don't know how serious the matter is. If you look at SOA curves, the pulse power is usually high. It's only the longer pulses and DC where the current has to be limited because the die current hogs in small areas and breaks down. If rapid temperature changes were a big factor, I'd expect a different SOA curve. But, I'm out of my element here.

 

[darian]

So finally we agree, everything but the rest of the dye is very remote from the junction thermally speaking. So what is prevailing to define the thermal inertia is the dye itself and not all the dissipators you can bound to the case or big fat leads to the outside world. So a bigger dye means more thermal inertia around the junction. I am very happy we can agree on that. Now I feel better about my knowledge of thermodynamic.

Regards