On 8/25/2011 1:53 AM, Ian White GM3SEK wrote:
> Another observation: if you have ever disassembled a CPU from its
> heatsink, another thing to note is that the film of heat transfer
> compound is extremely thin. The problem for manual application is to
> replicate this, which usually means smearing on the compound and then
> wiping almost all of it away. As Carl mentioned earlier, there's at
> least 20 years of good experience with water cooling of tube amps
> among serious 1.3GHz operators. Typically these amps use 3CX100 tubes
> in singles, pairs, fours, sixes or even eight at a time. The key
> article, which covers all the relevant points about water conductivity
> and monitoring leakage current, was by N6CA; it was in the ARRL
> Handbook for many years and can surely be found on the web. Regarding
> the size of the water reservoir, another 'data point' is a moonbouncer
> who uses a large plastic garbage can with no other secondary heat
> exchanger. Starting from room temperature, he can operate intensively
> for a whole weekend before the water is becoming a little warm. There
> are rest periods of 8-10 hours between moonset and the next moonrise,
> but this experience does show that secondary heat exchangers aren't
> always necessary. What all this adds up to is: let's not discuss this
> topic as if we were the first people ever to think of it - of course
> we aren't! There's loads of information and experience around, and
> most of it is readily available on the web.
My company recently quoted a project where we had multiple power
transistors that were attached to a LARGE and heavy (150#) aluminum
heatsink. I don't believe we used anything other than normal thermal
compound. To apply the compound, we are planning on silkscreening
directly on to the heatsink. Just like ink on a T-shirt. This is
pretty much the same process we use to screen solder paste on to PC
boards to reflow surface mount devices. You control the amount of
compound applied by the thickness of the metal silkscreen and the
pressure of the metal squeegee wiping across the screen. It would
probably be pretty easy to make a silkscreen at home using a wood or
metal frame and some very thin metal sheet. The screens we use are
laser cut stainless steel. If you're interested I can take some photos.
The PC boards that the devices are attached to are then bolted to the
heatsink and the devices themselves are bolted to the heatsink over the
screened on thermal compound. I'll have to check with some of my
process engineers but I think we've also used this technique to apply
thermal compound directly to PC boards as well.
--
73,
Gary K9GS
Check out K9NS on the web: http://www.k9ns.com
Greater Milwaukee DX Association: http://www.gmdxa.org
Society of Midwest Contesters: http://www.w9smc.com
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