[Amps] Ldmos heat spreaders

[Steve Thompson]

Manfred wrote:

> I do agree on the interfaces being extremely important, and the
> most important one is that between the FET and the spreader.
> Soldering the FET to the spreader is essential when the
> application has high power dissipation, like 600 watts from
> each device, which is usual in class AB linear service. Thermal
> paste, even the best one, just isn't good enough.

I'll add a caveat - soldering has the potential to be the best 
option, provided that the joint is complete. I worked with power 
rf where die carriers were soldered to flanges in a fancy oven 
with nitrogen atmosphere etc. and voids still happened. With a 
large contact area, in open air with flux around I see 
opportunities for it to go wrong.

Soldering becomes the second best option if you blow a device.

If you work on getting the flange properly flat, clamping does 
give good results but it takes work. Transistors out of the packet 
are nowhere near good enough.

With water cooling, why not sit the transistor over a pocket and 
run the coolant directly against the flange? That's a method I saw 
in production test at the Philips (as it was) factory.

NEC had an ill fated TV broadcast tx which cooled the transistors 
with compressed air blown directly onto the flange. In terms of 
cooling I was told it worked just fine, the problem came when the 
nozzles blocked with dust.

Steve