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Re: [Amps] Ldmos heat spreaders

Subject: Re: [Amps] Ldmos heat spreaders
From: Steve Thompson <>
Date: Mon, 22 Aug 2016 07:44:16 +0100
List-post: <">>
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.

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