Question why do we need thermal glue?

velocci

Distinguished
Dec 10, 2005
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HI all, I know this is a stupid question, but why do we need thermal glue between the cpu and heatsink? the bottom of the heatsink is flat and so is the cpu. why not just put the HSF directly on the cpu?
 

kanewolf

Titan
Moderator
First you don't want thermal glue. You want to be able to disassemble things in the future. Why do you use a little oil when cooking? Because the heat transfer is more efficient. Same concept with thermal paste. It fills all the microscopic flaws in the metals and makes them conduct heat better.
 
The surfaces are not perfectly flat. If they were they'd shine like a mirror with a completely undistorted reflection (what you do when grinding telescope mirrors and camera lenses - it is expensive and hard to get right).

So when you mate the two together, there are small microscopic air gaps. Metal on metal contact is ideal. Thermal paste is about 100x worse at transferring heat than metal on metal (which is why the metal pastes perform better). But air is about 100x worse at transferring heat than thermal paste. So thermal paste is preferable to no paste (air).

Incidentally, because of the huge difference between the thermal conductivity of metal vs paste vs air, there actually isn't much difference between the different pastes. People have tested toothpaste, peanut butter, nutella, denture cream, etc as substitutes for paste. All of them work, just slightly worse than thermal paste (and you may end up with ants in your PC). The heated debate over which brand paste is best, is really just splitting hairs.
 

AllanGH

Commendable
Mar 10, 2019
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If it flows like toothpaste, it'll work. If you have a standard, flat heatsink surface, just use a pea-sized blob, right in the center of the processor top, and you'll be fine. If you have a series of copper heat tubes (usually 4) running across the surface that lands on top of the processor, use a line along each heat tube.
 
Reactions: DMAN999
May 16, 2019
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Some people still enjoy lapping their CPU and heatsink to improve thermal conductivity between the two. It does void your CPU warranty though.

Lapping is the process of polishing a surface until it's as flat and shiny as possible. The process involves using fine sandpaper in progressively finer grades lubricated with thin oil and stuck to a flat surface, often a thick glass mirror, then gently rubbing the CPU lid and heatsink base until a flat mirror finish is achieved.

Thermal compound is still needed, but the amount needed is often miniscule compared to an un-lapped pairing.

I used to do it all the time with the old Intel C2D CPUs as the lids were often seriously concave. I had an E4300 that benefitted by 12°C under load from lapping it and the heatpipe cooler I was using. That allowed an overclock from a stock 1.8GHz to 2.93GHz with core temperatures still well within safe limits. Not too shabby for a couple of evening's work.

DISCLAIMER: DON'T DO IT UNLESS YOU DON'T WANT A WARRANTY ON YOUR CPU AND ARE CONFIDENT THAT YOU KNOW WHAT YOU'RE DOING.
 
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DMAN999

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Apr 17, 2019
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Velocci,
There are multiple thoughts/methods for applying thermal paste.
I have been working on PC's since the mid to late 1980's and IMO any of the most recommended methods (rice grain size dot at center of CPU, one thin line, or two thin lines in an X or using a credit card to apply a thin layer) work fine. I personally check the recommended methods of my cooler manufacturer and my CPU manufacturer and choose the method that common sense tells me will work best for my current configuration.
 
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