Re: Thermal pads for transistor mounts

Serge Auckland wrote:
"Gareth Magennis" <sound.service@xxxxxxxxxxxxx> wrote in message news:JY6dnZyoCZOpnNranZ2dnUVZ8s2mnZ2d@xxxxxxxxx

"Serge Auckland" <serge.auckland@xxxxxxxxxxxxx> wrote in message news:--ydnebw35pTf9vaRVnyugA@xxxxxxxxx

"Gareth Magennis" <sound.service@xxxxxxxxxxxxx> wrote in message news:oJGdnYn1pdMfX9vaRVnyiAA@xxxxxxxxx

Am I right in my belief that putting heatsink compound on silicon pads gives less thermal conductivity than pads alone?


Cheers,



Gareth.


My understanding is that thermal compound couples the bottom of the device to the heatsink by filling-in the tiny interstices that would otherwise exist between the slightly rough device and the slightly rough heatsink. Consequently, heat is conducted better than it would be if the device were clamped directly to the heatsink.



My understanding was that silicon pads already have the optimum contact with both surfaces due to their squidginess. Putting 2 layers of thermal compound with less than 100% thermal conductivity between them is only going to make things worse.

I just seem to remember reading something about this somewhere, but that does not of course make it true.


Gareth.


Do you mean Silicon, or Silicone? Silicon is a pretty hard material and would behave as I have described. I have no idea about silicone, I thought it was used in car wax and breast implants.

The usual white heat transfer compound is a silicone compound.
At least some of those products exibit rather crappy heat transfer characteristics, plus it hardens and deteriorates with age.

While I haven't use this for audio, serious computer builders use products like Titan Silver Grease or Artic Silver, which contains very fine grained silver flakes in a non-silicone compound.
Note that some similar products are silicone based, to be avoided.

Of cause, for some applications electric isolation is needed; a silver based compound may be less applicable for this ;)
Maybe a hard thin mica or ceramic isolation shimmer with said compound centered (to avoid contact round edges) can be used. YMMV..

And as stated by others, the point with any compound is filling an uneven surface, i.e. as little as possible should be applied.
If two surfaces could have a perfect meet, any compund would only result in lesser contact. A Bit the same as with lipstick..

--
Kind regards,
Mogens V.

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