Chris,
I know that Manfred and others on this list have advanced
thermodynamics knowledge
Well, mine isn't really advanced, it's just somewhat better than that of
most hams. I still need to use online calculators for more complex
things such as calculating the thermal resistance of a heat spreader.
There is prevalent thought that unless one can significantly derate
device power handling, bolted device to copper heat spreader
connections are quite inferior - soldering is best. The device case
to copper connection seems the weakest link.
It's not the weakest link, but it can indeed contribute a very
significant amount of thermal resistance, while a good solder joint is a
lot better. In a really "serious" amplifier, the weakest link is
actually the internal thermal resistance, in those few millimeters from
the silicon chip to the mounting surface of the FET. When using a good
spreader, heatsink, fans, or specially if using a good water-cooled
block, the device's internal thermal resistance can be higher than the
sum of all other thermal resistances!
In a rather casual setup, such as bolting a transistor onto a generous
aluminium heatsink, the internal thermal resistance of the device can
still be about a third of the total, at least in the case of smaller
transistors.
Why don't the manufacturers add another $3.00 worth of copper to the
case size (and mass) to better spread the heat over more area (and
slow the thermal time constant)?
I can only take a guess at this, as I'm not a manufacturer. My guess is
this: The datasheet looks better when the internal thermal resistance is
lower, since that determines the official datasheet power dissipation
rating! And the internal thermal resistance would INCREASE rather than
decrease, if the manufacturer added more copper between the silicon and
the mounting surface, due to the longer path!
So, mounting the silicon chip on a thicker copper base is clearly a bad
idea, from the ratings point of view. Instead using a copper base with a
larger area, but not being any thicker, would slightly improve the power
dissipation rating. But the improvement provided by this would be rather
small.
So the manufacturers opt for using a very small copper base, make it
thin so that the thermal path is short and thus the internal thermal
resistance is low (and the power dissipation rating in the datasheet is
high), and they leave it to the system designer to keep that small
mounting surface cool.
Such transistors are really designed to be soldered to the cooling
system. The manufacturers provide two versions of the cases: One that
MUST be soldered, and another that has screw notches, so you have the
choice of soldering it or mounting it with bolts and thermal grease.
The latter option really only makes sense in high efficiency
applications, or in pulsed service at low duty cycle, so that the power
dissipation is only a small fraction of the value rated for the FET.
And once we accept the fact that these FETs are intended to be soldered
down, it suddenly makes sense that the manufacturer provides just that
minimal, small and thin piece of copper: The thermal resistance of a
solder joint is very low, and now the system designer has total
flexibility in designing the optimal cooling system! This is good,
because these FETs can work into the VHF and even UHF range, and they
work at very low impedances, and the mounting surface is also the source
connection. So it's necessary to optimize the heat spreader or cooling
block not only for cooling, but also to provide the lowest possible
impedance between the source connection and those parts of the circuit
where it matters. If the FETs were provided with a big copper base built
in, this would interfere with such optimization!
It's certainly not that the manufacturers are trying to save on copper.
Three dollars currently buys you more than a pound of pure copper! The
added cost for a bigger copper base would be insignificant.
Wouldn't this greatly improve the functioning? Wouldn't bolted
mounting then work better?
Yes, a bigger mounting surface would indeed improve dissipation when
using bolted mounting. But it wouldn't make a significant difference in
the thermal aspect when using a soldered mounting, and it would make it
harder to get low impedance connections to the source.
Manfred
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