[Amps] Heatsinks for SSPAs

[Bill Turner]

ORIGINAL MESSAGE: 
On Sat, 2 Mar 2013 18:53:42 +0000 (UTC), you wrote:

>
>It sounds great in theory and maybe in practice too.  How do you mount the transistors to the chassis without a through hole into the water jacket?
>
>If you look at the NXP BFL578XR demo board, they are using a 3/8" solid copper heat spreader which then has a water inlet and outlet for cooling the heat spreader.  Similar idea except the transistor mounting does not penetrate the water jacket and there is no chance it will leak due to thermal expansion and contraction.
>
>A second note you might find interesting is that many of the parts are supplied as solder mount and are mounted directly to the copper heat spreader via a solder mask.  That may be one way to mount to the chassis bottom, but I'm not so sure it's easy to solder to Aluminum or what two very dissimilar metals will do when bonded.
>
>More food for thought.
>
>Paul, kg7hf

REPLY: 
Excellent points, Paul. As far as through holes are concerned, just drill
the hole in the usual way, mount the part and then cover the nut with RTV
silicon to make it waterproof. RTV is easy to remove if the need should
ever arise. Some RTV is corrosive, so I'd use the kind sold in pet shops
for sealing aquariums. Permanent and waterproof. 

And good point about soldering to aluminum. In that case, I'd drill a hole
through the flange and bolt it too the aluminum with a small nut and bolt.
Or possibly, solder it to a small piece of copper material and bolt that to
the aluminum.  Either way, I would not trust anything soldered to aluminum.
I've tried that in the past and had very poor results The bond is very weak
and easily broken. Maybe there are better techniques these days, don't know
about that. 

And of course, use thermal heat sink compound as necessary. 

Thanks for your thoughts. 

Bill, W6WRT