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chengong
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So I polished my CPU's IHS, annnnddd

Fri Jul 31, 2015 4:58 pm

So far there doesn't seem to be much of an improvement, I can't quite do a test yet because it's too hot today. my estimation is ~3 degrees while idle and ~5 degrees under load?

The CPU in question is a 3930k OCed to 4.4 Ghz with max voltage of about 1.300v. It is cooled by an EK LTX block (maybe the thermal bottleneck was at the block?) into a 3x120mm pull only rad.
My IHS had been obviously not flat since the beginning, whenever I put coolers on it only like 60% of the surface is squishing the thermal paste significantly so that's how I know.
When I starded sanding I can also see part of the IHS starts to show the copper much sooner than other parts.

Now it's flattened, although not really a mirror polish, I finished with 3000grit sand paper. The contact seems to be decent as I can start to feel atmospheric pressure pushing them together when making bare contact.
I really wonder, wouldn't bare contact be better if you had two flat metal surfaces?
Pics coming soon.

Idle temp compared with GPU which has direct waterblock contact.
Image
Last edited by chengong on Fri Jul 31, 2015 6:44 pm, edited 1 time in total.
 
BobbinThreadbare
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Re: So I polished my CPU's IHS, annnnddd

Fri Jul 31, 2015 5:01 pm

5 degrees under load seems like rather drastic difference to me.
 
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Re: So I polished my CPU's IHS, annnnddd

Fri Jul 31, 2015 5:24 pm

Yeah, I'd put 5 degrees on top of an already decent waterblock up there at "amazing" in terms of improvement vs. cost/time.
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Re: So I polished my CPU's IHS, annnnddd

Fri Jul 31, 2015 5:34 pm

Yes, metal to metal contact is always preferable.

This might be a good time to mention that the IHS is not designed to be flat. It's designed to be higher at the edges, and for a good reason. With higher edges, you have a peripheral spring that guarantees at least some contact between the IHS and heatsink all around. If, instead, you lap both mating surfaces shiny-smooth, then the smallest bump on either surface, or the least little tilt of the heatsink, will mean the contact area is greatly diminished. You also are at greater risk of mechanically stressing the die when cinching down the heatsink.
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Re: So I polished my CPU's IHS, annnnddd

Fri Jul 31, 2015 5:46 pm

The purpose of thermal paste is to fill in the voids and other imperfections of two metal surfaces (that are quite rough if you use a microscope to look closer). Otherwise the thermally insulating air would fill the voids.

With two perfectly smooth surfaces, the thermal paste would actually perform worse, especially the thicker, gritter ones (although they perform the best with rough or uneven contacts).
 
chengong
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Re: So I polished my CPU's IHS, annnnddd

Fri Jul 31, 2015 6:31 pm

I still applied thermal paste, I believe it's GC Extreme as it came with my EK GPU block.

Old contact:
Image

Image

I think the unpolished IHS ruined my water block to some degree, it's larger and even more difficult to grind so the finish isn't as good....
Image
 
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Re: So I polished my CPU's IHS, annnnddd

Mon Aug 03, 2015 3:07 am

chengong wrote:
I think the unpolished IHS ruined my water block

Explain the logic underlying that conclusion?
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Re: So I polished my CPU's IHS, annnnddd

Mon Aug 03, 2015 7:21 am

Thicker goop is good for non-flat surface mating.

As already pointed out, goop is better than air and even mirror-finishes have enough air between them that goop (applied correctly) can lower the thermal resistance.

  • If surfaces are super-flat (regardless of the finish quality) you need a less viscous thermal compound to provide a thinner layer between the two surfaces.
  • If surfaces are not particularly flat you need something stickier to act as a bulking agent and hold the compound in place where the two non-flat surfaces don't make direct contact.

If you think your IHS and waterblock aren't particularly flat and you don't want to spend ages sanding them flat, then I'd just suggest more thermal compound; The difference between near-perfect, mirror-smooth surfaces using a thin layer of runny paste and uneven, coarse surfaces with thick goop (but no air bubbles) should be minimal - like 3-5 degrees at most. There's an old old test on Dan's data where he tested the difference between pastes and obscure stuff like toothpaste and vegemite spread. Other sites have since copied him to humourous effect, but the answer is all the same - properly applied paste is more important than what paste you use

Admittedly there's a lot of information on how to apply paste without actually discussing whether that's the best way or quantity to apply for any given combination of surfaces. Viscous paste is worse for smooth flat surfaces, very runny paste will not effectively fill the gaps in uneven surfaces, The correct amount of paste is going to involve a little trial and error, looking at the contact patch it makes, but the answer is "as little paste as possible to make contact with the entire IHS". As an engineer with a prior background in fluid dynamics, casting and injection moulding my education says that the ideal way to apply paste is in a small 'X' shape that fills the middle ninth (centre box if you imagine a tic-tac-toe grid on the IHS). That's the ideal theoretical method and in reality the paste application tests show that to do well, sometimes beaten by "pea" or "grain" because in practice it's not possible to attach the heatsink 100% vertically and 100% in the correct position on first contact. When in doubt, just press down and slide it around and fraction to aid the spreading!
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chengong
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Re: So I polished my CPU's IHS, annnnddd

Tue Aug 04, 2015 7:36 pm

Topinio wrote:
chengong wrote:
I think the unpolished IHS ruined my water block

Explain the logic underlying that conclusion?

well "ruined" is a bit too strong. it just got some dents and scratches on the surface that are actually a bit deep to grind out.

I've done all kinds of spreading methods, but haven't done any scientific test to see which works better. Obviously I would like to get a bit better cooling performance with some out of the ordinary methods but I don't think I'll start picking the ideal way to apply thermal paste any time soon. I did the x thing on my GPU even though it's perfectly flat and probably doesn't need that much, still worked great. This time for the polished IHS I did something new where I simply applied thermal paste and scrapped them away leaving the entire surface covered in a thin layer that naturally sticks on and you can't easily scrape off. I imagine this should be a good way to do it but I'm not totally convinced by the results, on close inspection some of my cores seem to be cooler than before but some other cores are not that difference, which could be an indication of a problematic contact.
Image
 
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Re: So I polished my CPU's IHS, annnnddd

Wed Aug 05, 2015 5:11 am

No, the thin layer doesn't work as well for non-flat surfaces and guarantees a lot of trapped air bubbles.

Pea, rice-grain, small cross - it doesn't really matter that much, what you need to do is put a blob in the centre of the die and use the cpu cooler to compress it outwards - this fills the voids between two non flat surfaces and introduces no air bubbles.

In the grand scheme of things, if there's enough paste to fill the voids between the two surfaces, the differences are minimal (3-5C)
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Re: So I polished my CPU's IHS, annnnddd

Wed Aug 05, 2015 5:30 am

chengong wrote:
Topinio wrote:
chengong wrote:
I think the unpolished IHS ruined my water block

Explain the logic underlying that conclusion?

well "ruined" is a bit too strong. it just got some dents and scratches on the surface that are actually a bit deep to grind out.

I seriously doubt your IHS did that. In order for the IHS to put dents and scratches in your water block, it would've had to have had bits of abrasive embedded in it. More likely it was like that to begin with, and you just didn't notice.
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