Well my electronics teacher went to Cambridge University.
He got the sack and is now a gardener. He was once very rude to me, and now when I drive past and see him raking up leaves in the pissing rain, I beep my horn and give him the V's. He probably doesn't even know who I am, but it feels good.
Agent (05-11-2007)
Agent is correct (and the lapping guide says this) that metal to metal contact offers the best heat transfer - however a mirror surface may not be flat - and two mirror surfaces are unlikely to be perfectly parallel so there will be voids between them. (as someone else said, the purpose of lapping is to get the surfaces flat, as well as smooth). Given that it is impossible to get the two surfaces perfectly flat (if it was, there would be no need for thermal compound) then tyhere will be a requirement for thermal compound to fill the voids.
I would suggest that with a mirror surface it would be difficult to apply the compound thinly enough to prevent it making heat transfer worse thatn it would with a slightly rougher surface. where the tim can be squeezed into the voids.
Again, the lapping guide that agent referenced points out that lapping to beyond the smoothness provided by 600 grit paper is unlikely to provide any worthwhile improvement.
However I would be interested to hear of any experiments where someone has polished a heatsink and cpu to a mirror finish and mounted the two without thermal compound as theory would suggest that would give the best transfer - provided the surfaces are flat.
Incidentally - a quick Google search brought up this brief introduction the thermal transfer:
Heat Transfer
Last edited by peterb; 04-11-2007 at 10:32 PM.
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Agent (04-11-2007)
For maximum contact area and minimal thermal resistance you'll want both parts surfaces as smooth as reasonably possible. Of course it would be practically impossible to make the surface so flat and smooth that you do not need thermal compound at all, but generally less is more.
rosaline and agent make the most sense to me with their explanations and reasoning. it is quite tricky to explain all the nuances and nothing seems as straightforward as it initially did!
so basically flat without curvature is ideal
then minimise the little ridges by using fine wet n dry (both surfaces)
use the minimum of a good thermal compound
its interesting to think how flat and how fine a grade of sanding grit would be required to not need thermal compound. I would imaging that would have to be factory level...but who knows?!
has anyone tried using a burnisher? it's a rounded hard steel tool used in jewellery to polish stones, wood and the like and for other things i'm not too sure off. when using it on wood it slightly compresses to top fibres making it flat and hard. i was thinking it might be used on a lapped heat sink (i wouldnt want to put too much pressure on a cpu die) to flatten the ridges. though peterb states finer than '600 grit paper is unlikely to provide any worthwhile improvement'
Here's a couple of youtubes for you all
YouTube - This is Why We Lap by TheInformationator
Clean Lapped cpu & heatsink sticking together
YouTube - Big Typhoon Modification by TheInformationator
Pay attention to the last few seconds of that one
Great vids. I can tell you that lapping a Big Typhoon is a real pain. It's top heavy and has some nasty ridges above the copper base that dig into you as you grind. It takes time to get a good finish without putting bevels in or grinding it level. (There's a difference between level and flat - a surface can be flat but not level). I've also done the styrofoam down the sides and that does lower temps which is quite surprising.
It is very important that when you apply thermal paste that you apply the minimum amount necessary. I've now decided that I am going to go the whole hog to a 1µ professional finish on my next build just because I can. I think I'll take some pics as well next year along with pre and post temps.
"Reality is what it is, not what you want it to be." Frank Zappa. ----------- "The invisible and the non-existent look very much alike." Huang Po.----------- "A drowsy line of wasted time bathes my open mind", - Ride.
From the guid/thread he did How To: Extensively Modify Your Thermaltake Big Typhoon - Overclock.net - Overclocking.net
He uses 1500 as the final grade then polishing cloth for a final shine and remove the scratches.
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