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The great CPU paste method debate

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I was looking at videos about methods of cpu paste, when I came across one of those videos that use glass or plastic plates, and they managed to do this:

415877


The video this came form is here, around the 2:00 minute mark:

If you watch the video, they're using the "spread it like peanut butter" method, and things start out fine (around the 2:00 minute mark), but then as he presses harder and harder it all goes sideways and makes this pretty fractal pattern. All the dark parts are air pockets (or perhaps even a complete vacuum). I'd estimate that IF this ever happened in the real world, your thermal results would be massively horrible. It looks like only about 30% coverage over the surface, and horrible coverage over the hot center.

CPUs and CPU coolers are not made of glass or plastic or whatever he used here, but at the same time, all things flex under pressure mechanically. So if your cooler screws down, as some do, with a flat plate bolted at four corners, which is larger than the CPU heat spreader, then the tighter you go the more you're going to flex the cooler into a concave shape above the CPU heat spreader, and potentially open up a gap. Does this ever happen in the real world? No way to be sure, but there are certainly people who report horribly bad thermal performance that's cured by a simple reinstallation.

The traditional center dot method, especially in a thicker paste, is going to be much safer, as even if you warp the interface a bit, the dot of paste will remain as the fulcrum and you'll maintain full contact in the middle of the two surfaces (at least up to a point). But you will leave all the edges our your heat spreader out of contact, and while some say that doesn't matter, I think of it as a giant air bubble all the way around the edges. Yes most of the heat goes through the center, so this is clearly the safer choice, but not perfect. After all the heat spreader... spreads heat.

I certainly believe that pressure matters (in theory there should be a linear relationship between pressure, and number of molecules in direct contact), and I certainly also believe that ideally you'd cover the entire surface of your heat spreader, and not just the middle. So I think we have a situation where the best possible paste application — the thinnest possible layer between two perfectly flat mated surfaces and mounted at very high pressure — is also the riskiest, as the moment you warp the cooler bottom into a concave shape, you risk popping the two surfaces apart and losing the vast majority of your conductivity, especially in the center.

It's also interesting to note that some coolers do NOT mount at the four corners, for example, the Noctua coolers I've looked at use a bracket at pairs of corners that lets the cooler itself screw down on two center posts. In one dimension it can still warp upwards, but not in both dimensions.

This rambling does however hint at a method one could use to optimize your performance:
  • Start by spreading a thin layer across the entire surface
  • Add a very small center dot on top of that.
  • Install with low pressure.
  • Monitoring your temperature, give all the mounting screws a quarter or eight turn tighter ever hour or so until the temperature skyrockets (or some very expensive part breaks - disclaimer: I didn't make you do this).
  • if you didn't crack your CPU, back the screws off until the temperature comes back down.
Second disclaimer: I haven't actually tried this yet.
 

Adrian B

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You don't need to spread paste, just apply it to the top of the CPU and add the cooler. The paste is spread as pressure is applied as the screws for the cooler are tightened down. Both surfaces are not totally flat - which is why a thermal layer of paste is a common method of connecting both surfaces.

As long as you apply enough paste any excess is forced out from between the cooler and the CPU.

 

UtterDisbelief

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I was looking at videos about methods of cpu paste, when I came across one of those videos that use glass or plastic plates, and they managed to do this:

View attachment 415877

The video this came form is here, around the 2:00 minute mark:

If you watch the video, they're using the "spread it like peanut butter" method, and things start out fine (around the 2:00 minute mark), but then as he presses harder and harder it all goes sideways and makes this pretty fractal pattern. All the dark parts are air pockets (or perhaps even a complete vacuum). I'd estimate that IF this ever happened in the real world, your thermal results would be massively horrible. It looks like only about 30% coverage over the surface, and horrible coverage over the hot center.

CPUs and CPU coolers are not made of glass or plastic or whatever he used here, but at the same time, all things flex under pressure mechanically. So if your cooler screws down, as some do, with a flat plate bolted at four corners, which is larger than the CPU heat spreader, then the tighter you go the more you're going to flex the cooler into a concave shape above the CPU heat spreader, and potentially open up a gap. Does this ever happen in the real world? No way to be sure, but there are certainly people who report horribly bad thermal performance that's cured by a simple reinstallation.

The traditional center dot method, especially in a thicker paste, is going to be much safer, as even if you warp the interface a bit, the dot of paste will remain as the fulcrum and you'll maintain full contact in the middle of the two surfaces (at least up to a point). But you will leave all the edges our your heat spreader out of contact, and while some say that doesn't matter, I think of it as a giant air bubble all the way around the edges. Yes most of the heat goes through the center, so this is clearly the safer choice, but not perfect. After all the heat spreader... spreads heat.

I certainly believe that pressure matters (in theory there should be a linear relationship between pressure, and number of molecules in direct contact), and I certainly also believe that ideally you'd cover the entire surface of your heat spreader, and not just the middle. So I think we have a situation where the best possible paste application — the thinnest possible layer between two perfectly flat mated surfaces and mounted at very high pressure — is also the riskiest, as the moment you warp the cooler bottom into a concave shape, you risk popping the two surfaces apart and losing the vast majority of your conductivity, especially in the center.

It's also interesting to note that some coolers do NOT mount at the four corners, for example, the Noctua coolers I've looked at use a bracket at pairs of corners that lets the cooler itself screw down on two center posts. In one dimension it can still warp upwards, but not in both dimensions.

This rambling does however hint at a method one could use to optimize your performance:
  • Start by spreading a thin layer across the entire surface
  • Add a very small center dot on top of that.
  • Install with low pressure.
  • Monitoring your temperature, give all the mounting screws a quarter or eight turn tighter ever hour or so until the temperature skyrockets (or some very expensive part breaks - disclaimer: I didn't make you do this).
  • if you didn't crack your CPU, back the screws off until the temperature comes back down.
Second disclaimer: I haven't actually tried this yet.
You should mount the heatsink the same way the cylinder-head on an auto engine is - small increments of tightening, one corner then opposite corner, then next along, then opposite corner, then repeat.

I think the most flex that occurs anywhere is actually the motherboard. That's why they put a steel plate on the back behind the CPU socket. The board will flex way before a solid heatsink. The CPU heat spreader already helps mitigate any possible "tilt" between contacting surfaces.

Must admit I tend to apply thermal paste evenly all over except perhaps the last millimeter around the edge, then slowly and gently "twist" the heatsink into place. I don't want the silver paste I use oozing out anywhere. Experience tells me to use enough that there are no likely air-pockets and not too much that reduces thermal conductivity and makes a mess. It's actually quite a thin layer.

Also it's worth noting that when the CPU heats up the paste thins and spreads. Not sure how air-tight that makes any potentially trapped air pockets.

Basically I think I'm saying take your time rather than rushing as these videos always do. It's an important step.

:)
 
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