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taz1004 (Jun 18, 2001 02:21 p.m.):
I've seen people lapping the heck out of their heatsinks. Poor heatsinks. Lapping with higher than 300 grit sandpaper isn't recommended because if it's too smooth, there's no place for thermal compounds to go in. As long as the surface is flat, (not necessarily smooth) thermal compound will do the rest.
I've seen guys lapping for hours trying to get it to mirror like finish. Ends up shaving up to 1mm off the heatsink. This is insane. If you sand the bottom of the heatsink too much, it will likely decrease the thickness of the heatsink and therefore decrease the pressure on the core which the clamp is designed for. And might cause ineffective cooling.
Phil (Jun 18, 2001 04:45 p.m.):
Your right that you can never achieve 100% contact, but the arguement over how smooth you need it is not one thats can just be settled by saying 'this is how it is' I would expect the temp differances between a surface lapped with say up to 2000 grit and then polished with brasso to be almost the same as those of one only lapped to about 1200 grit. I have mine lapped with 1200 grit not to a mirror polish but it actually came out quite dull and milky which is odd as I have lapped the same sort of heatsink but only gone to 800 grit and got a silvery smooth but not shiny surface. I would recomend going up to around 1000 grit though, it would be an interesting comparrison to take a load of heatsinks and compare the differances each grade of lapping makes to them.
taz1004 (Jun 18, 2001 04:31 p.m.):
"the entire reason of lapping is to make the surface falt, thermal paste fills in the small holes that you can not see so that contact is made with what ever you are trying to coo."
These thermal paste molecules are not that fine as you'd expect. Silver is even larger if you use AS2. You need some degree of roughness so the holes can be filled with thermal compounds. Mirror like finish wont let the compounds in. Generally 300~400 grit sandpaper is right size. Try it. Apply some thermal compounds on a mirror and scrape it off with a razor. How much of it is staying on the mirror?
"Now a lapped surface, doesnt have as many holes, and much more metal will touch the surface of what you are trying to cool ,better contact, better cooling"
I didn't say lapping is bad. I said excessive lapping is bad. Yes there wont be as many holes if you lap with say 4000 grit. Problem is, holes are too small for thermal compound molecules.
"Now less thermal paste is needed, and that is good because thermal paste is not anyware close to efficient at transferring heat as heatsink to core(providing 100% contact)"
You can never achieve 100% contact even with a mirror.
Few web sites did testing on this. I can't seem to dig it up.